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Polyoxyethylene alkyl ether carboxylic acids: An overview of a neglected class of surfactants with multiresponsive properties
Polyoxyethylene alkyl ether carboxylic acids: an overview of a neglected class of surfactants with multiresponsive properties Leonardo Chiappisi PII: DOI: Reference:
S0001-8686(17)30356-1 doi: 10.1016/j.cis.2017.10.001 CIS 1834
To appear in:
Advances in Colloid and Interface Science
Received date: Revised date: Accepted date:
10 August 2017 7 October 2017 10 October 2017
Please cite this article as: Chiappisi Leonardo, Polyoxyethylene alkyl ether carboxylic acids: an overview of a neglected class of surfactants with multiresponsive properties, Advances in Colloid and Interface Science (2017), doi: 10.1016/j.cis.2017.10.001
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Polyoxyethylene alkyl ether carboxylic acids: an overview of a neglected class of surfactants with multiresponsive properties
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Leonardo Chiappisia,b,∗ a Technische
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Universität Berlin, Stranski Laboratorium für Physikalische Chemie und Theoretische Chemie, Institut für Chemie, Straße des 17. Juni 124, Sekr. TC7, D-10623 Berlin, Germany b Institut Max von Laue - Paul Langevin, Large Scale Structures Group, 71 avenue des Martyrs - 38042 Grenoble Cedex 9, France
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Abstract
In this work, an overview on aqueous solutions of polyoxyethylene alkyl ether carboxylic acids is given. Unique properties arise from the combination of the nonionic, temperature-responsive polyoxyethylene block with the weakly
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ionic, pH-responsive carboxylic acid termination in a single surfactant headgroup. Accordingly, this class of surfactant finds broad application across very different sectors. Despite their large use on an industrial and a technical scale, the literature lacks a systematic and detailed characterization of their physico-chemical properties which is provided
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herein. In addition, a comprehensive overview is given of their self-assembly and interfacial behavior, of their use as colloidal building blocks and for large-scale applications.
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Contents
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Keywords: polyoxyethylene alkyl ether carboxylic acids, surfactant science, self-assembly
1
Introduction
2
2
Synthetic procedure and availability
4
3
Physico-chemical properties
4
3.1
pH titrations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5
3.2
Surface tension . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8
3.3
Small-angle neutron scattering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
12
3.4
Temperature responsiveness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
16
4
Colloidal building blocks and technical uses
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4.1
Building blocks for colloidal supramolecular structures . . . . . . . . . . . . . . . . . . . . . . . . .
18
4.2
Practical applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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∗ Corresponding
Author. Email address: [email protected] (Leonardo Chiappisi)
Preprint submitted to Advances in Colloid and Interface Science
October 10, 2017
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5
Conclusions
21 22
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References
1. Introduction
Surfactants are among the most versatile compounds in the colloidal playground. The properties of their aque-
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ous solutions, e.g., critical micelle concentration (cmc), surface activity, solubilization capacity, foamability, foam stability, depend on the molecular structure of the lyophobic and lyophilic groups. Given their amphiphilic nature,
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surfactants are employed as solubilizing agents and detergents, to reduce surface and interfacial tensions, to stabilize (or destabilize) foams and emulsions, to disperse or flocculate solids or liquids in liquids, etc.[1]. Optimizing surfactants for their function is a complex and tedious task, however great efforts have been pursued by the scientific and
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industrial communities towards this goal. Recent progress has been well reviewed[2–12]. Carb. Acid termination
Ethyleneoxide units
Alkyl Chain
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O
OH
O
i
j
O
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Figure 1: Schematic representation of the chemical formula of polyoxyethylene alkyl ether carboxylic acids, composed of a hydrophobic alkyl chain, hydrophilic ethylene oxide units and terminated by a carboxy-methyl unit.
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This work is focussed on polyoxyethylene alkyl ether carboxylic acids (chemical formula given in Fig. 1), often abbreviated as AECs. For the sake of simplicity, three main parts of the surfactant can be identified: the lipophilic alkyl chain, which can vary in length and degree of saturation; the polyoxyethylene (EO) block, and the terminal carboxylic acid, whose degree of ionization depends on the pH of the solution. Hereafter, the surfactant chemical structure will be abbreviated as Ci Ej CH2 COOH, with i being the usual nomenclature for fatty acids, j representing the number of ethylene oxide units, and CH2 COOH is the carboxymethyl termination. Depending on pH, mixed systems of acids and sodium salts are present, but for the sake of simplicity we name them "acids", keeping in mind the importance of the degree of ionization. The surfactant structure strongly resembles that of nonionic polyoxyethylene alkyl ethers Ci Ej or fatty acids (Ci COOH). Polyoxyethylene alkyl ether carboxylic acids, however, present some major advantages with respect to the above-mentioned compounds, which arise from the combination of ionic and nonionic surfactant properties in a single molecule. Fatty acids – or soaps, when in their salt form – are the oldest man-made surfactants and are easily prepared from the alkaline treatment of natural fats and oils. However, their use is essentially limited by their high Krafft point, i.e., the temperature at which the free energy of the fatty acid molecule is lower in the crystal than in the micellar 2
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aggregate, and the low solubility in neutral and acidic medium. Optically clear solutions of medium and long-chain fatty acids are obtained only in very alkaline solutions, e.g. over pH ∼ 10.5 for sodium oleate[13, 14]. A simple
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approach for increasing the solubility of fatty acids consists of exchanging their counterion, usually sodium, with a
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bulkier one with a lower tendency to crystallize[15–18]. Similarly, the Krafft point can be reduced and the tolerance to multivalent cations increased by the use of branched chain fatty acids[19, 20]. Moreover, the low solubility of fatty acid salts of multivalent cations, in particular calcium and magnesium, represents a strong limitation to their use in
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everyday consumer products. Although different parameters affect the solubility of fatty acids – pH, temperature, or the presence of multivalent cations – the problem has a common origin: the high stability of the fatty acid crystal and
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the low hydrophilicity of the nonionized species. Both factors are addressed when some ethylene oxide units (EO) are inserted between the carboxylic headgroup and the aliphatic chain. The bulky and amorphous EO units prevent AECs from crystallizing and confer a remarkable hydrophilicity even in acidic media, where solubilities and cmcs are close
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to the corresponding nonionic polyoxyethylene alkyl ether surfactants[21]. The strategy of introducing some ethylene oxide units between hydrophonic tail and surfactant headgroup is widely applied to alkyl sulfates. Despite they are permanently charged, alkyl sulfates are characterized by high Krafft points and a low solubility in the presence of
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multivalent ions[22–24]. Both limitations are, at least partially, overcome, by the insertion of few – generally two to four – ethylene oxide units.
In contrast to their ionic counterparts, nonionic polyoxyethylene alkyl ether are generally water soluble (provided
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a sufficient number of EO units are present), their behavior is pH independent and they are resistant to hard water conditions, i.e., in the presence of multivalent cations[4, 25]. Polyoxyethylene alkyl ethers exhibit a rich phase diagram
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in aqueous solution as a function of temperature and concentration[26]. With increasing temperature the hydrophilicity of the EO block is reduced, resulting in a gradual dehydration of the EO units. Above a certain temperature, called the cloud point (CP), phase separation into a surfactant-rich and a surfactant-poor phase occurs[4, 27]. This phenomenon is common to all polyoxyethylene alkyl ethers, but the CP varies strongly with: the length of the surfactant tail, the number of EO units, and the presence of additives, e.g. inorganic salts or alcohols[28–30]. The complex physics underlying this phenomenon was recently reviewed[4, 27]. The continuous dehydration of the surfactant headgroup has remarkable consequences not only on the phase boundaries, but also on other properties such as the cmc, the adsorption at interfaces, the spontaneous curvature of the surfactant film, and is the basis for several surfactantmediated extraction methods[31–33]. Fatty acids and polyoxyethylene alkyl ethers are both highly relevant classes of surfactants, each exhibiting characteristic properties. However, the combination of the aqueous solubility and the thermoresponsive behavior of nonionic polyoxyethylene alkyl ethers with a pH-responsive carboxylic headgroup opens up a multitude of possibilities, as will be shown hereafter. Current literature lacks in systematic studies of the physico-chemical properties of AECs. To compensate for this lack, a comprehensive physico-chemical characterization of aqueous solutions of polyoxyethylene alkyl ether carboxylic acids is provided in this work. Experimental details are given in the supporting information. In particular, their behavior is rationalized in terms of their chemical structure and experimental conditions such as 3
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pH, ionic strength, and temperature. These new results are analyzed and discussed jointly with the available literature data, therefore providing a comprehensive overview on the self-assembly properties of AECs. The behavior of
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polyoxyethylene alkyl ether carboxylic acids will be compared with that of the nonionic polyoxyethylene alkyl ethers
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and strong anionic polyoxyethylene alkyl ether sulfates. Based on this exhaustive characterization, recent progress in fundamental and applied research, demonstrating the potential of these surfactants in different fields, is discussed.
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2. Synthetic procedure and availability
AECs are prepared from polyoxyethylene alkyl ethers[34–40]. Polyoxyethylene alkyl ethers are mainly prepared
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by the addition of ethylene oxide to fatty alcohols in the presence of a strong base, resulting in a rather broad distribution of EO units[41, 42]. Polyoxyethylene alkyl ether carboxylic acids with branched alkyl chain can be prepared from Guerbet alcohols[43–45]. Two main methods are available for the final modification leading to the carboxymethyl ter-
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mination. The first method consists of the reaction of the alcohol with monochloroacetic acid in the presence of sodium hydroxide, i.e. the Williamson’s ether synthesis[34, 38, 46–48].The reaction of the fatty alcohol with chloroacetic acid presents some drawbacks: the low yield of ca. 90 %, the use of the toxic chloroacetic acid, and the formation of a large
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amount of sodium chloride and other byproducts as diglycolic and glycolic acids. To avoid the formation of these short chain carboxylic acids, a bulky base such as a secondary or tertiary alkoxide can be used[34, 38, 46]. The second
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method consists of the direct oxidation of the terminal alcohol[34, 49–51]. The reaction is mostly performed using oxygen as the oxidizing agent in the presence of a platinum or palladium-based catalyst[34, 49, 50]. The efficiency of the catalyst can be improved by the introduction of Pb, Bi, or Cd salts[34, 52–54], or by the use of a Au-based
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catalyst[40, 51, 55–57]. Synthetic routes using H2 O2 as oxidizing agent are also reported[51]. Although the oxidation reaction has a yield close to 100%, the main route for the industrial preparation of AECs is the Williamson’s synthesis, as it has proven to be more cost-effective[46, 51, 55]. A further limitation to the implementation of the large-scale oxidation reaction, is the long diffusion time of oxygen into the highly viscous surfactant phase. In contrast, the higher yield and simpler reaction conditions make the oxidation reaction more suitable for academic research purposes. Technical grade polyoxyethylene alkyl ether carboxylic acids are readily available in large quantities and at low cost. The alkyl chain length generally varies between 4 and 20 carbon atoms, and the degree of ethoxylation between 2 and 20. Some of the key manufacturers of AECs are Kao Chemicals (AKYPO R series), Huntsman (EMPICOL R C series), Clariant (Emulsogen R series), Uniqema (Atpol series), Nippon Shokubai, and BASF SE[58].
3. Physico-chemical properties As mentioned previously, the surfactant can be subdivided in three main parts: the lipophilic alkyl chain, the hydrophilic polyoxyethylene part, and the ionizable carboxylic termination. This subdivision helps in rationalizing properties such as the cmc, surface tension, size and shape of the aggregates. Some studies dealing with the self-assembly and interfacial properties of AEC solutions are available in the literature[21, 36, 59–69], but a systematic study is 4
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missing. To provide a more exhaustive characterization of their properties, the behavior of aqueous solutions of a set of AECs with variable alkyl chain length and number of ethylenoxide units was probed as a function of pH, ionic
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strength, and temperature. In particular, three technical surfactants, represented by the formulæ C12 E10 CH2 COOH,
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C18:1 E9 CH2 COOH, and C12 E4.5 CH2 COOH were chosen to highlight alkyl chain length and headgroup size effects. This choice was limited to surfactants having a sufficient number of EO units to be water soluble over a large pH range, and an alkyl chain large enough to form well-defined self-assembled aggregates. The shorter alkyl chains denoted by
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C12 are in fact 2:1 mixtures of C12 and C14 and the longer alkyl chain denoted as C18:1 is a 3:1 mixture of C18:1 and C16:0 . The number of EO units show a roughly Gaussian distribution, with a distribution width of approximately three
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for the C12 E4.5 CH2 COOH surfactant[65]. Further details on the materials and on the techniques used are given in the experimental section in the appendix of this work and in the supporting information. The use of technical surfactants presents two major advantages: their low cost and availability in large quantities, and the increase in stability of the
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easy-to-process isotropic micellar and hexagonal phases over that of the lamellar Lα one[26]. Despite small deviation of the physico-chemical properties such as cmc, surface tension, and cloud point of nonionic polyoxyethylene alkyl ether surfactant solutions are reported between pure and non-pure components, no difference is found between the
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trends as a function of the average alkyl chain length and number of EO units remains unchanged[26, 70, 71]. This similarities allows generaling the findings of this work on technical surfactants also on their pure counterparts.
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3.1. pH titrations
Potentiometric and turbidimetric titrations of 1 wt% solutions of C12 E10 CH2 COOH, C18:1 E9 CH2 COOH, and
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C12 E4.5 CH2 COOH were conducted at different ionic strengths. All results are reported in Fig. 2. The turbidity of the solution (first row of Fig. 2) reflects the weight average molar mass of the self-assembled surfactant aggregates, the smaller the aggregate the more transparent the solution. C12 E10 CH2 COOH solutions were completely transparent throughout the titration, the C18:1 E9 CH2 COOH were slightly translucent for pH < 3.0, and the C12 E4.5 CH2 COOH turned from turbid to transparent during the titration. Clearly, the larger the number of EO units and the higher the pH, the smaller the aggregates are in solution. Similar turbidity curves were reported earlier for C12 E4.5 CH2 COOH by Kunz and coworkers[65], showing a transition from vesicles to globular micelles. Structural details will be discussed in section 3.3, based on the small-angle neutron scattering (SANS) data. Potentiometric titrations, through the charge balance, directly provide the degree of ionization α of the surfactant, i.e., the fraction of nonprotonated carboxylic units (second row of Fig. 2). The effective acidity constant is then calculated as: pKa (α) = pH − log
α . 1−α
(1)
A monotonic increase in pKa (α) with pH is observed for all surfactants at each ionic strength (third row of Fig. 2). The pKa (α) values are found to be between 3.5 and 5.0, depending on the pH and ionic strength of the solution. In all cases, the acidity of the surfactant increases upon addition of sodium chloride. The strong dependence of the pKa from the experimental conditions makes a direct comparison with literature values difficult. Extrapolating the pKa 5
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C12E10CH2COOH
C18:1E9CH2COOH
0.8
0.4
NaCl 0.0 M 0.15 M 1.0 M
0.2
NaCl 0.0 M 0.15 M 1.0 M
0.8
NaCl 0.0 M 0.15 M 1.0 M
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0.6 0.4 0.2
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0.0 5.0 4.5 4.0
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pKa( )
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0.6
0.0
3.5 0 -20
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!0( ) / mV
C12E4.5CH2COOH
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Transmission
1.0
-40 -60
2
3
4
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-80 5
6
2
3
4 pH
5
6
2
3
4
5
6
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Figure 2: From top to bottom: Light transmission (wavelength λ = 520 nm, path 2.5 cm), degree of ionization α, effective pKa , and electrostatic surface potential Ψ0 as a function of pH, obtained from turbidimetric and potentiometric titrations of 1 wt% solutions of (from left to right) C12 E10 CH2 COOH, C18:1 E9 CH2 COOH, and C12 E4.5 CH2 COOH with the initial addition of different amounts of NaCl. See text for full details.
values to α → 0 allows for the determination of the intrinsic acidity constant pKa (0) (reported in Table 1). The pKa (0) values are approximately constant, and slightly decrease with increasing number of EO units of the headgroup. This is a result of the electron withdrawing effect of the ethylene oxide units[60]. To a first approximation, the effective acidity constant of a generic polyacid (weak electrolytes, dendrimers, aggregated weak acids/bases, etc.) is determined by two main factors: the intrinsic binding affinity pKa (0), and the electrostatic interaction between the charged headgroups and can be described as[73–76]: ! dGel 1 pKa (α) = pKa (0) + 2.303RT dα
(2)
where Gel , R, and T are the headgroup interaction contribution to the change in Gibbs free energy of ionization, the ideal gas constant, and the temperature, respectively. More complex models, taking into account conformational changes of the colloid as a function of pH or ionic strength, are also available[77–79]. The main contribution to the free energy of ionization arises from the work needed to create an additional charge on the micellar interface, and is 6
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Table 1: Results from pH titrations obtained at different initial NaCl concentrations (in mol L−1 ).
0.0
3.49 ± 0.03
0.15
3.47 ± 0.01
1.0 C18:1 E9 CH2 COOH
0.0
3.68 ± 0.05
3.50 ± 0.02
1.0
3.67 ± 0.02
0.0
4.00 ± 0.03
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C12 E4.5 CH2 COOH
3.60 ± 0.02
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0.15
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pKa (0)
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C12 E10 CH2 COOH
[NaCl]
0.15
3.90 ± 0.01
1.0
3.73 ± 0.02
0.0
3.8±0.2
C12 E4.5 CH2 COOH
0.0
4.67
C12 E10 CH2 COOHc at α = 0.5
0.0
4.0
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C8 E5 CH2 COOHa
b
from Ref. [60]; b from Ref. [65]; c from Ref. [72]
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a
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directly related to the surface potential Ψ0 [74]:
dGel = −e0 NA Ψ0 (α) dα
(3)
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where e0 and NA are the elementary charge and the Avogadro constant, respectively. The electrostatic potential at the micellar headgroup/water interface can be obtained by combining Eqs. 2 and 3 (fourth row of Fig. 2). The surface potential decreases from 0 mV at low pH to -80 mV in neutral and basic water. Values of up to -50 mV are found in saline alkaline solutions.
Ψ0 can also be obtained by the one-dimensional Poisson-Boltzmann equation for spherical particles immersed in an electrolyte solution[74]:
! 8πe0 I e0 Ψ(r) d 2 dΨ(r) r = sinh dr ε0 εr kB T r2 dr
(4)
with r being the distance from the center of the spherical particle, I the ionic strength of the solution, ε0 εr the dielectric constant of the solvent, and kB the Boltzmann constant. The equation can be solved using the boundary conditions that: Ψ(r → ∞) = 0 and dΨ/dr = −4πσgeo /ε0 εr at r = R, with R being the distance of the headgroup-water interface from the micelle center, and σgeo the geometrical charge density at the headgroup-water interface. σgeo and the radius of the micelle can be estimated from the SANS data reported in section 3.3 of this work. In particular, the geometric charge density is obtained from the degree of ionization α, the aggregation number Nagg , and the micellar radius R as σgeo = αNagg /4πR2 . Eq. 4 was solved iteratively using the Fortran routine made available by Lund University[80] using the values of Nagg and R determined from the SANS patterns (see supporting information for further details). 7
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The surface electrostatic potential was also determined from the charged hard-sphere structure factor used for the interpretation of the SANS data. The values of the surface potential directly determined from the potentiometric
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titrations, from the solution of the Poisson-Boltzmann equation (Eq. 4), and from the analysis of the SANS patterns, pH
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S (q) respectively denoted hereafter as Ψ0 , ΨPB 0 , and Ψ0 , are reported in Fig. 3. All values and detailed calculations are
given in the supporting information. Briefly, the electrostatic potential predicted by solving the Poisson-Boltzmann
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0 -20
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-60
0
pH
/ mV
-40
-100
0
-120
PB
pH
0
S(q)
z
-120
-100
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-140 -140
0
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-80
-80 0
0
PB
0
S(q)
C18:1E9CH2COOH C18:1E9CH2COOH+NaCl C12E10CH2COOH C12E10CH2COOH+NaCl C12E4.5CH2COOH
S(q)
-60 and
0
PB
-40
-20
0
/ mV pH
Figure 3: Plot reporting the values of the surface potential at the micelle headgroup/water interface determined from pH titrations (Ψ0 ) as a
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function of the ones obtained by the solution of the Poisson-Boltzmann equation ΨPB 0 (empty points) and as a function of the values determined S (q)
by the analysis of the structure factor of the SANS patterns Ψ0
(solid points). Exact values used for the calculations are given in Table S3. The
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dashed line represents f (x) = x and is a guide for the eyes. If the different methods would result in the same surface potential values, all points would fall on the dashed line. The inset at the bottom-left corner schematically shows the micelle/water interface as a function of the distance from the micelle center z, with the location of the plane at which the potentials are calculated/determined. See text for further details.
equation is significantly more negative than the one found from pH titrations. In contrast, slightly less negative values are found for Ψ0S (q) . This discrepancy is thought to arise from the effect of the micelle geometry on the calculation S (q) of ΨPB 0 and Ψ0 . In both cases, the micelles are modeled as rigid, spherical particles. However, analysis of the
SANS revealed that the micelles are in fact slightly anisotropic. ΨPB 0 is calculated at the surface of a rigid sphere with a volume equivalent of that of the globular micelle; Ψ0S (q) is calculated at the surface of the sphere with a radius equal to the hard sphere radius, close to the largest dimension of the globular micelle. The ζ-potential values, i.e., the electrostatic potential at the slipping plane, are in the range -20 – -30 mV for C18:1 E9 CH2 COOH at pH 4 to 5[68], in line with the trend of decreasing potential with increasing distance from the micelle headgroup/water interface. Moreover, due to the flexibility of the ethylene oxide block, and its size distribution, the charges of the micelles are distributed in an interfacial region rather than on a sharp plane, allowing for more spatial freedom to minimize the electrostatic repulsion. Despite the substantial approximations, a good agreement is found between the different values. This demonstrates that the dependence of acidity constant on pH for the micellized AECs is predominantly governed by the electrostatic repulsion and counterion condensation, and that the shape and size of the self-assembled 8
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aggregates play only a minor role. The effects of the addition of sodium chloride to a surfactant solution at constant pH can be rationalized accordingly. Due to counterion condensation and charge screening, the electrostatic potential
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at the micelle/water interface is reduced, essentially lowering the repulsion between the charged headgroups. This
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screening is observed in the decrease in the pKa (α) and, in turn, in an increase in the degree of ionization α. 3.2. Surface tension
50 45 40 35
10-7
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30 25
pH = 2.5 4.1 5.0 8.0
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/ mN m-1
55
pH = 2.5 4.1 5.0 8.0
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pH = 2.5 4.1 5.0 8.0 8.0 + 0.2M NaCl
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Surface tension
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65
10-6 10-5 10-4 10-3 10-2 10-7 10-6 10-5 10-4 10-3 10-2 10-7 10-6 10-5 10-4 10-3 10-2 [C12E10CH2COOH] / mol kg-1 [C18:1E9CH2COOH] / mol kg-1 [C12E4.5CH2COOH] / mol kg-1
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Figure 4: Equilibrium surface tension of (from left to right) C12 E10 CH2 COOH, C18:1 E9 CH2 COOH, and C12 E4.5 CH2 COOH solutions in water of varying acidity as a function of the molality of surfactant.
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The equilibrium surface tension of C12 E10 CH2 COOH, C18:1 E9 CH2 COOH, and C12 E4.5 CH2 COOH solutions at different surfactant concentration and pH was measured and is shown in Fig. 4. The recorded curves are clearly different from typical titration curves. A clear minimum in the surface tension curve is observed, which is deeper the higher the pH of the solution, i.e., at higher degree of ionization of the surfactant. Technical surfactants, where a large spectrum of molecules with variable surface activity coexist, usually – but not necessarily – show a minimum in their surface tension vs. log C curves[81]. Few examples of surface tension curves with the same shape were reported also for similar, monodisperse surfactant systems[61, 82]. In addition to the polydisperse nature of the surfactant, the equilibrium between the charged and uncharged forms adds a certain degree of complexity to the data interpretation. Different theoretical models for the descriptions of the adsorption of ionic/nonionic surfactant at interfaces are available, some of them explicitly consider electrostatic interactions while others work within the thermodynamic concept of a by-definition-neutral Gibbs dividing interface[83]. Given the complexity of the studied system, the surface tension titration curves were interpreted within the Gibbsian framework. At constant pressure and temperature, the Gibbs adsorption isotherm is given as: dγ = −Γ s RT d ln a s − Γc RT d ln ac
9
(5)
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where γ is the surface tension, Γ s , a s , Γc , and ac are the surface excess and the activity of the surfactant and its coun-
and
χc = αχ s
(6)
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Γc = αi Γ s
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terion, respectively. To preserve the electroneutrality of interface and solution, the following relations are introduced:
where αi , χ s , and χc are the degree of ionization of the surfactant at the interface, and the molar fractions of the surfactant and the counterion, respectively. αi is not to be confused with the degree of ionization of the surfactant
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in solution α determined from potentiometric titrations. Small difference of 0.2[84] and 1 unit[85] are reported between the pKa values of fatty acids determined in bulk solution or at the air-water interface. This differences arises from from the electrostatic potential at the air-water interface established upon the adsorption of charged species[84].
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Assuming that the potential at the micelle-water interface is similar to that at the air-water interface, it is reasonable to approximate that αi ≃ α. At least for the more dilute region of the titration curves, where the surfactant is present
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in its nonmicellized form, ideal behavior can be assumed, such that the activities of the surfactant and its counterion may be substituted with their concentrations. Eq. 5 can be rewritten accordingly:
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dγ ≈ −Γ s (1 + αi )RT d ln χ s − αi Γ s d ln αi
(7)
As the titrations are performed at constant pH, it is reasonable to neglect the second term of Eq. 7. Finally, the
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surfactant surface excess can be obtained as:
Γs = −
dγ 1 (1 + α)RT d ln χ s
(8)
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When working in excess of an inert salt, i.e., a salt presenting no specific interaction with the surfactant, the second term of Eq. 5 is zero and Eq. 7 simplifies to that of a nonionic surfactant. The minimum area per molecule at the air-water interface Ah is calculated from Γ s as Ah = (NA Γ s )−1 , where NA is the Avogadro constant. Within the same thermodynamic framework, the Gibbs free energy of micellization ∆G0mic can be obtained. The aggregation of n partly charged surfactant molecules S α− and their counterions C + into a micelle Mn can be expressed with the following equilibrium:
nS α− + nα(1 − β)C + ⇋ Mnnαβ−
(9)
where (1-β) is the fraction of condensed counterions. The ∆G0mic can then be calculated as: ∆G0mic 1 = ln a M − ln a s − α(1 − β) ln ac . RT n
(10)
Substituting the activities with the molar fractions, the following relation is obtained: ∆G0mic ≈ 1 + α(1 − β) ln χ s (cmc) + α(1 − β) ln α. RT
(11)
Eq. 11 reduces to the classical equation of ionic surfactants[90] for α = 1: ∆G0mic ≈ (2 − β) ln χ s (cmc) RT 10
(12)
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Table 2: Summary of parameters obtained from surface tension titrations and of those used for its analysis. α is the degree of ionization obtained from potentiometric titrations, 1 − β is the fraction of condensed counterions, determined from SANS experiments. The cmc is given in µmol L−1 , the headgroup area Ah in Å2 , the surface excess Γ in µmol m−2 , the surface tension dependence from the concentration dγ/d ln(C) in 10−3 N m−1 , the surface tension at high surfactant concentration γ(c → ∞) and at the cmc γ(c = cmc) in 10−3 N m−1 , the Gibbs free energy of micellization
SC
∆G0mic in 103 J mol−1 . Values for nonionic surfactants are reported for comparison at the bottom of the table. Values in italics are from literature.
a
Values determined in 0.2 M acetic acid/sodium acetate buffer, from Ref. 21. Other values are taken from: Ref. 86; Ref. 87; Ref. 61 (chemically pure polyoxyethylene alkyl ether carboxylic acid); e Ref. 88 f Ref. 89; g Ref. 24.
d
γ(c → ∞)
γ(c = cmc)
∆G0mic
1.9
-5.3
31.1
30.3
-40.7
—
—
33.5
31.9
—
β
cmc
Ah
Γ
2.5
0.09
0.0
20.5
86
4.0a
—
—
29.0
—
4.1
0.49
0.45
5.0
0.78
8.0 NaCl
C18:1 E9 CH2 COOH
102
1.6
33.5
31.1
-44.2
0.45
100
110
1.5
-6.8
37.5
31.8
-47.3
1.00
0.45
MA
-6.0
180
130
1.3
-6.2
40.7
35.5
-48.7
8.0
1.00
1.0
75
75
2.2
-5.5
37.7
35.7
-47.6
2.5
0.05
0.0
2.0
51
3.3
-8.5
31.3
31.8
-45.1
—
5.5
—
—
—
33.6
32.0
—
0.3
0.4
14
96
1.7
-6.0
31.1
32.7
-45.1
0.7
0.4
24
97
1.7
-7.2
34.7
33.8
-52.1
a
4.0
4.1 5.0
—
PT
−1
ED
54
AC CE
NU
α
+ 0.2 mol L
c
dγ/d ln(C)
pH C12 E10 CH2 COOH
b
8.0
1.0
0.4
43
124
1.3
-6.7
39.2
37.0
-56.0
2.5
0.05
0.0
5.5
58
2.8
-7.4
28.0
27.9
-42.5
4.0
—
—
15
—
—
—
28.8
30.5
—
4.1
0.30
0.15
26
68
2.4
-7.8
28.2
28.7
-45.9
5.0
0.60
0.15
150
81
2.0
-8.1
30.4
28.6
-48.5
∼ 7d
—
—
400
—
—
—
39.1
30.5
—
8.0
1.00
0.25
450
123
1.4
-6.7
35.4
34.0
-51.0
C12 E10 (at 303 K)b
—
—
—
12.7
—
—
—
—
—
-38.5
C16 E9
—
—
—
2.0
53
—
—
36
36
-42.5
—
—
—
6.6
—
—
—
31
30
-39.5
—
1.0
—
2200
52
—
—
—
43
—
—
1.0
—
1300
—
—
—
—
—
—
C12 E4.5 CH2 COOH
a
c
C12 E5 e C12 E3 OSO3 Na
f
C12 E10 OSO3 Nag
11
ACCEPTED MANUSCRIPT
It should be noted that by introducing the relation χc ≈ αχ s from Eq. 6, the contribution of hydroxide ions to the charge balance of the system is neglected. However, this approximation holds in the range of pH (2.5 to 8) and
T
surfactant concentration to which Eqs. 7 and 11 are applied. The critical micelle concentration is taken either at the
RI P
abrupt change in slope of the surface tension titration curves or, if present, at the minimum value. The values are summarized in Table 2.
For each surfactant, increasing the degree of ionization α causes an increase of the cmc of up to two orders of
SC
magnitude. The values of β used for the calculation of ∆G0mic were obtained from the analysis of the SANS patterns given in Fig. 5. The values of β ≃ 0.3 - 0.5 found for C12 E10 CH2 COOH and C18:1 E9 CH2 COOH are relatively
NU
high when compared to the typical values of 0.1-0.2 for weak and strong ionic surfactants in water[91–94]. Such low counterion condensation arises from the presence of the EO units, which “dilute” the charge of the surfactant headgroup[23, 60, 95], as also supported by the lower values of β found for C12 E4.5 CH2 COOH. The consequences of
MA
the presence of the EO units are also evident when comparing the cmc values found in solutions of C12 E10 CH2 COOH and C12 E4.5 H2 COOH at different pH. At low pH, the cmc is higher for the surfactant with the larger headgroup, as expected by the hydrophilic nature of EO units. In contrast, at high pH, where the ionic form of the surfactant
ED
dominates, the cmc of C12 E10 CH2 COOH is a factor 2 lower than that of C12 E4.5 CH2 COOH. A similar reduction of the cmc with increasing number of EO units was also reported for polyethylene oxide alkyl ether sulfates[24, 89, 96]. effects of the EO units.
PT
This counterintuitive relation of increasing hydrophilicity and decreasing cmc can be ascribed to the charge-dilution Moreover, the values for the cmc and the ∆G0mic are close to those reported for similar nonionic Ci Ej [61, 86–88].
AC CE
With increasing pH a decrease in the free energy of micellization is observed. This trend is in contrast to an expected increase due to the electrostatic repulsion between the charged headgroups, which should become more important the higher the degree of ionization of the surfactant. One explanation lies in high sensitivity of the calculation of ∆G0mic via Eq. 11 on the values of β. The β values used were determined from the analysis of the SANS patterns at surfactant concentrations much higher than the cmc and may be underestimated, in particular when compared with the high values of 0.7-0.85, determined by conductometric experiments close to the cmc for the sodium salts of several AECs in pure water (resulting pH of 8)[60, 69]. In addition to strongly increasing the cmc of the surfactant, the ionization of the carboxylic headgroup causes a decrease of the maximal surfactant surface excess Γ s and, accordingly, an increase of the minimal area occupied by the surfactant at the air-water interface. This effect is particularly pronounced for C12 E4.5 CH2 COOH, the surfactant with the smallest number of EO units. However, the reported values must be treated with caution given the technical nature of the surfactant used and the uncertainty arising from the approximation of α ≃ αi . All surfactants show a minimum in the surface tension curves, at least at higher pH values. The technical nature of the surfactant can only partially explain this behavior, as the polydispersity in alkyl chain length and number of EO units is pH independent. However, the difference in surface activity between the main component and impurities, e.g., the non-carboxymethylated polyoxyethylene alkyl ether, increases with increasing pH. In addition, at pH 2.5 12
ACCEPTED MANUSCRIPT
and 8.0 the surfactant can be regarded as a pure nonionic or pure ionic compound, respectively. Sakai et al., however, recorded the surface tension for the chemically pure C12 E4 CH2 COOH surfactant at pH ∼ 7, reporting a similar, well-
T
pronounced minimum in the surface tension close to the cmc[61]. The authors justify the observed minimum with
RI P
a vesicle-to-micelle structural transition in the proximity of the cmc. Zana et al. reported a continuous increase in surfactant aggregation number for C12 E5 CH2 COOH, C12 E7 CH2 COOH, and C12 E9 CH2 COOH close to the cmc[60]. A similar effect was found in another pure, weakly-anionic surfactant[82]. In both cases, the authors assumed structural
SC
rearrangements to justify the shape of the surface tension curves. A common assumption is that the concentration and therefore the activity of the free surfactant remains constant or increases slightly after the cmc. However, it was
NU
demonstrated that the activity of free surfactant can decrease with increasing total surfactant concentration above the cmc[97, 98], due to the increased ionic strength which reduces the repulsion between the surfactant headgroups and the micelles. To address the effect arising from the increasing ionic strength, a surface tension curve of C12 E10 CH2 COOH
MA
at a pH value of 8 in the presence of 0.2 mol L−1 NaCl was recorded. The minimum in the surface tension curve is reduced but is still present, in line with the findings by Svanedal et al.[82]. In summary, the uncommon shape of the surface tension curves arises from a number of different factors: the polydisperse nature of the surfactant, a
ED
likely change in the micelle geometry at concentrations higher than the cmc and, more importantly, a reduction of the activity coefficient of the surfactant, resulting in a well-pronounced minimum in the surface tension curve at high pH.
100
C18:1E9CH2COOH pD = 2.0 2.8 4.4 4.1 6.1 11.7
104
102 101
0.1
104 103 102 101
100 10-1
C12E4.5CH2COOH pD = 2.0 2.7 4.3 4.7 5.0 5.3 6.6 11.7
106
I(q) / cm-1
103 I(q) / cm-1
101
C12E10CH2COOH pD = 2.0 2.8 4.2 4.8 5.2 5.9 11.4
AC CE
I(q) / cm-1
102
PT
3.3. Small-angle neutron scattering
100
10-1 1 q / nm-1
10-1 0.1
q / nm-1
1
0.1
q / nm-1
1
Figure 5: SANS patterns arising from ca. 1 wt% solution of (from left to right) C12 E10 CH2 COOH, C18:1 E9 CH2 COOH, and C12 E4.5 CH2 COOH. Solid lines show best fits according to different models described in detail in the main text and supporting information. The models are schematically represented by the sketches, not to scale, in the figure.
The size and shape of three representative surfactants, C18:1 E9 CH2 COOH, C12 E10 CH2 COOH, and C12 E4.5 CH2 COOH were studied in D2 O as a function of pD via small-angle neutron scattering (SANS). The scattering patterns could be 13
ACCEPTED MANUSCRIPT
described using different models, reported in detail in the supporting information. In all cases, a homogeneous coreshell structure was assumed, where the core is formed by the anhydrous hydrophobic blocks, and the shell is formed
T
by the hydrated EO units and the carboxymethyl unit. Depending on the nature of the surfactant and on the acidity
RI P
of the solution, the data were described using vesicle, rod or ellipsoid models[99]. A charged hard-sphere structure factor was used for the ellipsoids[100]. The obtained parameters are given in Tables 3 and 4, and the analytical expressions used for the description of the SANS patterns are given in the ESI. The fits were constrained by imposing
SC
the condition that at least one characteristic length of the aggregate core should be 1.6 and 2.1 nm, as determined by the maximum length of the C14 and C16 alkyl chains of the surfactants, respectively (see supporting information for
NU
further details). The thickness of the shell depends on the conformation of the ethylene oxide units[101–103], and its limits can be estimated assuming different conformations: 0.35 nm per EO unit for the fully extended chain as in polyoxyethylene crystals, 0.19 nm per EO unit for the meander conformation.
MA
Scattering patterns of ca. 1 wt% aqueous solutions are shown in Fig. 5, additional curves determined in a 150 mM NaCl solution are given in the supporting information. At high pD, the scattering behavior of the different surfactants is similar, and a peak is found at q p = 0.3−0.4 nm−1 due to the electrostatic repulsion between the micelles. This value
ED
corresponds to a most probable distance of 2π/q p ∼ 15 − 20 nm and a dehydrated micelle volume of ν = 8π3 φ/q p of 40-100 nm3 , with φ being the surfactant volume fraction. At lower pD, the surfactants self-assemble into very different structures.
PT
C12 E10 CH2 COOH is the surfactant with the largest head-to-tail size ratio investigated in this work and, accordingly, it forms the aggregates with the largest curvature. The micelles formed are slightly anisotropic at low pD but
AC CE
become increasingly spherical as the pD is increased. With increasing pD, the shell thickness decreases from ca. 1.6 to 1.4 nm with an approximately constant hydration of 60-70 vol% D2 O in the shell. The micelles at low pD are structurally very similar to those formed by the nonionic C12 E10 [104]. The addition of 150 mM NaCl appears to have no effect on the micelle size and shape. C18:1 E9 CH2 COOH has a similar headgroup to C12 E10 CH2 COOH but a longer alkyl chain and the self-assembled structures vary with pD. In particular, at low pD, when the surfactant is found in its nonionic form, cylindrical micelles are found, as evidenced by the large q−1 region in the SANS patterns. Increasing the degree of ionization of the surfactant causes a transition from rodlike to ellipsoidal micelles. The transition from rods to ellipsoids causes a reduction of the shell thickness from 1.8 to 1.5 nm, while the shell hydration increases from 50 to 70 vol% D2 O. The structures show only minimal changes upon addition of 150 mM NaCl or 1 mM CuCl2 [66]. C12 E4.5 CH2 COOH features the smallest headgroup of the investigated surfactants. The surfactant self-assembles into planar structures up to pD ∼ 5 as evidenced by the large q−2 region in the SANS spectrum. The SANS patterns can be described using a model of large polydisperse unilamellar vesicle at pD < 5. The overall size of the vesicle is outside of the probed q-range and the value given for the radius (200 nm) has no physical meaning. The addition of 150 mM NaCl at pD 4.3 triggers the formation of small vesicles with low polydispersity, and a core radius of ca. 18 nm. Globular oblate ellipsoidal micelles appear at pD > 4.3, which become increasingly spherical with increasing pD. 14
ACCEPTED MANUSCRIPT
Table 3: Summary of fit parameters used for the description with an ellipsoidal model of the scattering curves reported in Fig. 5. χ is the fraction of surfactant in the globular micellar state; the ionic strength I is given in 10−3 mol L−1 , the ellipsoidal rotational semi-axis A, equatorial semi-axis B,
T
shell thickness T, and the hard sphere radius RHS are given in nm; Z is the charge per micelle, given in elementary charge units determined from the
RI P
structure factor, Nagg is the number of molecules per aggregate; αS (q) is given by Z/Nagg ; β = Z/αNagg is the fraction of noncondensed counterions,
B
T
RHS
1
8
2.0
3.7
1.6
1.6
3.5
1
7
2.8
3.6
1.6
1.6
1
6
4.2
2.8
1.6
1.6
1
4
4.8
2.2
1.6
1
4
5.2
2.1
1.6
1
4
5.9
1.6
1.6
1
5
11.4
1.6
1
153
2.0
1
149
4.8
1
150
+NaCl
φwshell
αS (q)
β
Acs h
Ahsw
0.0
117
0.58
0.00
—
53
161
3.7
0.0
114
0.58
0.00
0.00
54
163
4.3
20.0
88
0.62
0.23
0.56
56
183
1
8
1
6
NU 4.4
19.0
69
0.66
0.27
0.46
58
209
1.6
4.2
19.5
66
0.67
0.29
0.43
59
215
1.6
4.2
21.3
50
0.71
0.42
0.47
64
255
1.5
1.4
4.0
20.8
44
0.67
0.47
0.47
67
244
3.8
1.6
1.6
3.5
—
47
0.77
—
—
82
159
2.7
1.6
1.6
3.7
—
85
0.63
—
—
56
187
MA
1.6
2.3
1.5
1.5
3.5
—
64
0.64
—
—
61
210
2.0
500
2
1.8
—
0.0
18000
0.49
0.00
—
54
103
2.8
60
2
1.7
—
0.0
2200
0.47
0.00
0.00
54
103
5
4.4
4.0
2.0
1.5
5.3
28.2
145
0.58
0.20
0.56
59
149
4
5.1
3.8
2.0
1.5
5.0
26.8
137
0.59
0.20
0.39
60
152
1
5
6.1
3.0
2.0
1.5
4.9
25.4
108
0.63
0.23
0.27
62
170
1
7
11.7
2.9
2.0
1.5
5.1
30.5
105
0.63
0.29
0.29
63
173
1
152
2.0
500
2.0
1.8
0.0
—
18000
0.49
—
—
54
103
1
149
4.4
8.5
2.0
1.7
3.6
—
307
0.57
—
—
56
127
1
152
11.7
3.3
2.0
1.7
4.8
—
119
0.68
—
—
61
179
0
8
2.0
—
—
—
—
—
—
—
—
—
—
—
0
6
2.7
—
—
—
—
—
—
—
—
—
—
—
0.6
5
4.3
1.6
6.0
0.9
—
—
710
0.06
—
—
37
52
0.85
4
4.8
1.6
3.8
0.9
5.8
24.0
285
0.28
0.08
0.12
41
69
0.925
4
5.0
1.6
3.5
0.8
5.6
27.1
241
0.21
0.11
0.14
43
70
0.985
4
5.3
1.6
3.1
0.8
4.5
27.4
189
0.27
0.14
0.17
45
76
1
5
6.6
1.6
2.3
0.8
3.8
24.4
104
0.42
0.23
0.23
51
99
1
7
11.7
1.6
2.2
0.8
3.8
25.6
95
0.44
0.27
0.27
53
103
0
151
2.0
—
—
—
—
—
—
—
—
—
—
—
0
148
4.3
—
—
—
—
—
—
—
—
—
—
1
152
11.7
1.6
— 15 2.6 0.8
4.3
—
133
0.36
—
—
24
54
AC CE
1
C12 E4.5 CH2 COOH
Nagg
11.4
1
+NaCl
Z
SC
A
prolate ellipsoid −→ cylinder
C18:1 E9 CH2 COOH
pD
ED
+NaCl
I
PT
C12 E10 CH2 COOH
χ
oblate ellipsoid −→ bilayers/vesicles
respectively, given in Å2 .
sphere −→ prolate ellipsoid
and Ahsw are the areas per surfactant molecule at the core-shell and at the shell-water interface, φwshell is the volume fraction of D2 O in the shell; Acs h
ACCEPTED MANUSCRIPT
Table 4: Summary of fit parameters used for the description with a vesicle model of the scattering curves arising from 1 wt% solutions of C12 E4.5 CH2 COOH. 1-χ is the fraction of surfactant in the vesicular state; the ionic strength I is given in 10−3 mol L−1 ; the core radius Rc ,
mixture of large (Rc >200 nm) and small (Rc = 6.5 nm) vesicles.
I
pD
Rc
σ
1.00
0.2
The pattern recorded at pD 2.7 was described as a
TEO
TALK
hRi
φwshell
1.2
1.7
210
0.29
1.2
1.7
210
0.29
2.0
>200
0.67
6
2.7
>200
0.2
0.33(a)
6
2.7
8.1
0.2
1.2
1.7
8.3
0.29
0.28
5
4.3
>200
0.2
1.2
1.7
210
0.29
0.15
4
4.8
>200
0.2
1.2
1.7
210
0.29
0.08
4
5.0
>200
0.2
1.2
1.7
210
0.29
0.02
4
5.3
>200
0.2
1.2
1.7
210
0.29
0.00
5
6.6
—
—
—
—
—
—
0.00
7
11.7
—
—
—
—
—
—
1.00
151
2.0
>200
0.2
1.2
1.7
204
0.29
1.00
148
4.3
19
0.2
1.2
1.7
19.9
0.29
0.00
152
11.7
—
—
—
—
—
—
NU
PT
ED
+NaCl
SC
8
(a)
MA
C12 E4.5 CH2 COOH
1-χ
(a)
RI P
bution; hRi is the mean core radius in nm; φwshell is the volume fraction of D2 O in the shell.
T
thicknesses of the hydrophilic TEO and hydrophobic layer TALK are given in nm; σ is the standard deviation of the core radius lognormal distri-
Until pD < 5 they coexist with vesicular aggregates, in agreement with the previous studies of Kunz et al.[64, 65]. It
AC CE
should be remarked that, due to the coexistence of different types of structures, the SANS analysis is not univocal for intermediate pD regions, and the sizes and relative amounts of the different structures should be treated with caution. The gradual transition from large vesicular aggregates to small micelles however, is unequivocal. Aggregation number and micelle size of C12 E4 SO4 Na or C12 E6 SO4 Na are similar to those found for C12 E4.5 CH2 COOH at high pH, i.e., where the surfactant headgroup is fully charged.[105] In summary, AECs self-assemble in dilute solutions into a wide range of morphologies, depending on the pD of the solution and on the size ratio of their hydrophilic and hydrophobic parts. In particular, the spontaneous formation of vesicles by a single anionic surfactant at such mild pD values is a rare finding. By addition of appropriate amounts of dodecanoate, the pD range in which vesicles are found can be extended[64]. The structures formed by the investigated AECs are graphically summarized in Fig. 6 as a function of the volume fraction of the hydrophilic part of the surfactant and the degree of ionization. On the one hand, VEO /Vtot is directly related to the spontaneous curvature of nonionic Ci Ej surfactants and was already successfully employed to rationalize their the phase behavior[106–108]. On the other hand, the degree of ionization determined by potentiometric titrations quantifies the electrostatic contributions to the headgroup requirement and, accordingly, its influence on size and shape of the self-assembled aggregates. It should be noted here, that the choice of these two variables does not do justice to the complexity of system. Nevertheless, 16
ACCEPTED MANUSCRIPT
T
0.8
RI P
0.6 0. 6 0.4 0. 0.2
0.5
0.55
0.6
0.65 0
NU
0.0 0. 0 0.45 45
SC
degree of ioni de ionization nization on
1.0
VEO/Vtot
Figure 6: Summary of SANS-models used for the description of the self-assembled aggregates of polyoxyethylene alkyl ether carboxylic acids as
MA
a function of degree of ionization α and volume fraction of the hydrophilic part of the surfactant. For details of the model used see Tables 3 and 4. Dotted lines are guide for the eye. , N, and • represent samples with anisotropy values less than 1.5, between 1.5 and 3, and larger than 3, respectively. H represents samples forming bilayers. represents samples where the coexistence of vesicles and globular micelle was observed.
ED
Sketches are not to scale.
it offers a simple approach to rationalize the two nonorthogonal variables to predict the shape of the self-assembled
PT
surfactant aggregates.
Few studies on concentrated AECs systems are reported in litterature[36, 69]. The investigation of the phase behavior of polydisperse C12,13 E4 CH2 COOH and C12-15 E6 CH2 COOH revealed the presence of extended gel regions[36].
AC CE
A more detailed investigation by polarized optical microscopy and small-angle X-ray scattering was performed on the monodisperse sodium salts of C14 E4 CH2 COOH and C14 E7 CH2 COOH in water at 25 ◦ C[69]. The results are summarized in Fig. 7 together with the phase behavior of the strongly anionic C12 E2 OSO3 Na[109] and sodium dodecyl sulfate C12 OSO3 Na, often abbreviated as SDS or SLS[110], and the nonionic C14 E8 [111] and C12 E3 and C12 E5 [26] surfactants. The sodium salts of the AECs show a similar behavior, with the presence of an isotropic micellar solution L1 at concentrations . 25 wt%, followed by a hexagonal H1 phase, which is more extended for the surfactant with the larger headgroup. Both surfactants present a rather extended lamellar Lα phase at concentrations between ∼ 40 and 80 wt%, followed by a sponge L3 phase at high concentration. Only for C14 E4 CH2 COOH a reverse cubic phase V2 at high concentration is reported, although it should be noted that this phase can be easily overseen. In contrast and despite the fewer number of ethylene oxide units, the strongly anionic C12 E2 OSO3 Na forms phases with a larger curvature over the whole concentration range, with the boundaries between the L1 and Lα phases and the Lα and H1 being located at higher surfactant concentration. It is however evident, that the introduction of the 2 ethylene oxide units between the dodecyl chain and the sulfate headgroup enriches the phase behavior, mainly by preventing the crystallization of the surfactant. To be noted, this surfactant is of technical purity. Moreover, a comparison between C14 E4 CH2 COONa and C12 E5 reveals that the ionic headgroup stabilizes the hexagonal phases at low to moderate 17
ACCEPTED MANUSCRIPT
L1
S+L1
L1
H1
L1 + L
C12E3
L1
C14E4CH2COONa
L
C14E7CH2COONa
H1
25
50
V2
L2
L3
V1
SC
0
L H1
L1
C14E8
L3
L
L1 L1
L2
L H1
C12E5
L
T
C12E2OSO3Na
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C12OSO3Na
L
75
L
+S 100
Surfactant concentration wt%
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Figure 7: Simplified representation of the phase behavior of alkyl sulfates (green color), alkyl carboxylates (blue color) and nonionic alkyl ethoxylates (red color) sorted by number of EO units, taken from different studies performed at 25 ◦ C in water: the sodium salts of C14 E4 CH2 COOH and C14 E7 CH2 COOH from Ref. 69, the strongly anionic C12 E2 OSO3 Na (technical grade) from Ref. 109, and C12 OSO3 Na from Ref. 110 and the
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nonionic C14 E8 from Ref. 111 and C12 E3 and C12 E5 from Ref. 26. Phase notations are: L1 for the isotropic micellar solution; H1 for the hexagonal phase; Lα for the lamellar phase; L2 for the hydrated liquid surfactant phase; V1 for the cubic phase; L3 for the sponge phase; V2 for the reverse cubic phase, and S the for solid surfactant phase.
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concentrations. The transition between lamellar and reverse phases takes place at similar concentrations. In addition, the comparison between C14 E7 CH2 COONa and C14 E8 shows that AECs form hexagonal and lamellar phases at lower surfactant concentration than their nonionic counterpart. This observation is further supported by a comparison with
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the extended phase diagram of polyoxyethylene dodecyl- and oleyl- ethers, which shows that the lowest concentration at which Ci Ej form hexagonal phases in water at 25 ◦ C is ca. 40 wt% for dodecyl ethers[108] and 25 wt% for
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oleyl ethers[107]. It should be kept in mind that the comparison between the different classes of surfactants is only indicative and a direct correlation cannot be made, as experimental parameters such as temperature, ionic strength, pH, strongly affect the behavior of the surfactants in a very different fashion. 3.4. Temperature responsiveness
The thermoresponsive properties of 1 wt% aqueous solutions of C12 E10 CH2 COOH, C18:1 E9 CH2 COOH, and C12 E4.5 CH2 COOH at different pH were probed by following the turbidity of the solution by recording the light scattering intensity at an angle of 90◦ (see Fig. 8). Solutions of C12 E10 CH2 COOH show a cloud point temperature of 68 ◦
C at pH 2.5, while no micelle growth can be observed at higher pH values, when the surfactant is partially or fully
charged. C18:1 E9 CH2 COOH, with a more hydrophobic alkyl chain, exhibits its cloud point at pH 2.5 at 47 ◦ C. At pH 4.1 (α = 0.4) a micelle growth is observed to start at ca. 55 ◦ C and terminate with phase separation at 92 ◦ C. The structures are not sensitive to temperature at higher pH values. Solutions of C12 E4.5 CH2 COOH can be seen to exhibit a different behavior. Turbid solutions are found at low pH, and a precipitate slowly forms at 25 ◦ C, 1 wt%, and pH < 3.5, as reported by Kunz et al.[65]. When heated, the solution shows constant turbidity up to 55 ◦ C, at which point all surfactant precipitates leaving a clear solution. At pH 4.1 a constant turbidity is recorded over the whole probed temperature range. Interestingly, at pH 5.0, a bluish solution at low temperature becomes clear for T > 20 ◦ C. A further 18
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pH = 2.5 4.1 5.0 8.0
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103
102
101
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Mean count rate / 103 cps
pH = 2.5 4.1 5.0 8.0
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pH = 2.5 4.1 5.0 8.0
104
100 20
40
60
80
100 0
20
40 60 Temperature / °C
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0
80
100 0
20
40
60
80
100
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Figure 8: Scattering intensity determined at a scattering angle of 90◦ as function of temperature for 1 wt% solutions of (from left to right) C12 E10 CH2 COOH, C18:1 E9 CH2 COOH, and C12 E4.5 CH2 COOH at different pH values.
increase in temperature causes a slight micellar growth. Finally, at high pH, C12 E4.5 CH2 COOH self-assembles into
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small micelles whose size is temperature independent. The cloud points determined at pH 2.5, where α ∼ 0.0, is lower by ca. 20-30 ◦ C than that of their nonionic alkyl ether analogs, as shown in Table 5. A similar reduction in cloud point temperature was found for nonionic
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alkyl ethers where the terminal OH group is substituted with an OCH3 moiety[112–114]. The lower cloud point temperature of methyl-capped alkyl ethers with respect to the conventional, hydroxy-terminated ones, is ascribed to
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different hydration properties, with the methyl-capped ones showing smaller hydration numbers[114]. Table 5: Cloud point temperatures recorded for 1 wt% aqueous solutions of C12 E10 CH2 COOH, C18:1 E9 CH2 COOH, and C12 E4.5 CH2 COOH at different pH values. The values for the corresponding nonionic alkyl ether surfactants, which are pH-independent, are reported for comparison and were measured in water at a concentration of 1 wt%.
pH = 2.5
pH = 4.1
pH = 5.0
pH = 8.0
C12 E10 CH2 COOH
68◦ C
—
—
—
C18:1 E9 CH2 COOH
47◦ C
>92◦ C
—
—
C12 E4.5 CH2 COOH
<5◦ C
<5◦ C
—
—
C12 E10 from Refs. 1, 33, and 115:
77-88 ◦ C
C18:1 E10 from Refs. 116, 31, and 117:
72-85 ◦ C
C12 E5 from Refs. 118, 119, and 27:
30-32 ◦ C
The molecular mechanism underlying the clouding phenomenon in dilute nonionic surfactant solutions has been intensively discussed[4, 25, 27, 120]. For what concerns nonionic, polyethylene oxide-based surfactants, the phaseseparation is ascribed to the reduced hydrophilicity and progressive dehydration of the EO units with increasing 19
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temperature. However, the nature of the critical fluctuations giving rise to the turbidity of the dilute solutions of Ci Ej solution is less evident and still a matter of debate. Different mechanisms apply as a function of the surfactant
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headgroup size: on the one hand, amphiphiles with a smaller headgroup were reported to continuously grow into long,
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wormlike micelles[119, 121]; on the other hand, this growth is limited for alkyl ethers with a larger headgroup and small, globular micelles tend to associate in large clusters near the cloud point[121, 122]. However, the mechanisms are not mutually exclusive, and both micelle growth and clustering, albeit to different extents, take place in dilute Ci Ej
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solutions near the cloud temperature. With this in mind, it is likely that the cylindrical micelles found at 25 ◦ C and low pH in dilute solutions of C18:1 E9 CH2 COOH behave similarly to C16 E6 or C12 E5 aggregates, growing in length with
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increasing temperature and forming an extended network of wormlike micelles at the cloud temperature[121, 123]. Differently, the cloud behavior of C12 E10 CH2 COOH, with a larger headgroup as compared to the hydrophobic chain, is likely to resemble to that of C12 E8 , whose aggregation numbers are constant over an extended range of temperatures
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and micelle growth and clustering takes place only close to the cloud point[121, 122].
4. Colloidal building blocks and technical uses
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4.1. Building blocks for colloidal supramolecular structures In the previous section, it was shown that polyoxyethylene alkyl ether carboxylic acids can self-assemble into
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diverse structures, depending on pH, ionic strength, and length of the hydrophilic and hydrophobic blocks. Moreover, for surfactants with a sufficiently large headgroup, e.g., C12 E10 CH2 COOH, the charge density of the micelle can be varied with minimal effects on its curvature. This is rarely the case, as the electrostatic contribution to the headgroup
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area is, for most ionic surfactants, the dominant term[41, 124, 125]. Typically, strong variations in size and shape are observed when the ionic strength of the solution is increased, or upon addition of counterions which condense at the micellar interface[126–129]. This effect is even more pronounced when the counterion takes the form of a polyelectrolyte chain[130–132], which triggers the transition from globular to cylindrical aggregates[133–136]. When AECs are admixed to the oppositely charged polycation chitosan (poly d-glucosamine) they co-assembly into hierarchical, complex structures[21, 63]. However, the shape and size of the micellar aggregate is only minimally affected by the presence of chitosan for the investigated C8 E5 CH2 COOH, C12 E10 CH2 COOH, C12 E4.5 CH2 COOH, and C18:1 E9 CH2 COOH surfactants. The presence of a non-negligible steric contribution to the headgroup area Ah therefore allows to systematically investigate the effects of charge density and curvature of the micellar aggregate on the co-assembled structures with oppositely charged polyelectrolytes. The effect of the degree of ionization was investigated in detail on chitosan complexes with C18:1 E9 CH2 COOH[63], while the effect of curvature at constant ionization degree was studied on chitosan complexes with C12 E10 CH2 COOH, C12 E4.5 CH2 COOH, and C18:1 E9 CH2 COOH at pH 4.0[21]. A structural phase diagram for the chitosan-C18:1 E9 CH2 COOH complexes, determined as a function of pH and mixing ratio Z = [surfactant molecules]/[chitosan chargeable units], is shown in Fig. 9. Three main co-assembled structures can be identified: in the bottom-left corner, at low mixing ratio 20
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0.5
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0.4 0.3 0.2 0.1 0 3.6
3.9
4.2
4.5
4.8
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3.3
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-
+
[Surf ]/[Chitosan ]
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5.1
pH
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Figure 9: Structural phase diagram depicting the different structures found in mixtures of C18:1 E9 CH2 COOH and chitosan as a function of pH and mixing ratio Z. Experiments were performed at a constant chitosan concentration of 0.3 wt%, in an acetic acid/sodium acetate buffer of 0.2 mol L−1 . The different symbols represent sample composition investigated by small-angle neutron scattering. See Ref. 63 for full details. Adapted with
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permission from Ref. 63. Copyright 2017 American Chemical Society.
and low surfactant content, the micelles are randomly distributed in solution, and the chitosan network is preserved; in the central region of the phase diagram, one-dimensional aggregates are formed, where aligned micelles are glued
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together by chitosan chains. An appropriate scattering form factor for such object is available[137]. Finally, in the top-right corner, at high degree of ionization and high surfactant content, the aggregates are collapsed into a core-
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corona suprastructure, with a core formed of densely packed micelles surrounded by a stabilizing chitosan corona. Similar structures are often found in complexes of polyelectrolyte-b-neutral block copolymers and oppositely charged surfactants as a consequence of microphase-separation[138–141], but only rarely when homo-polyelectrolytes are involved[142, 143].
The structural evolution observed in chitosan - C18:1 E9 CH2 COOH is also found in complexes with other alkyl ether carboxylic acids self-assembling into globular micelles, such as C8 E5 CH2 COOH or C12 E10 CH2 COOH[21]. However, when AECs assemble into vesicles, e.g., C12 E4.5 CH2 COOH at pH ∼ 4, the formation of ordered multilayered vesicles with a well-defined interlayer distance has been observed[21]. The average number of layers forming each vesicles depends on the mixing ratio. Such structures have a significant potential as drug delivery agents and for cosmetic applications, as they may be loaded with a hydrophilic cargo in the internal lumen, a charged one in the PE-surfactant layer, and a lipophilic load within the hydrophobic alkylic layer. AEC - chitosan complexes exhibit an exceptional structural variety in solution, but they also exhibit a remarkable high solubility when compared to mixtures of chitosan with strong anionic surfactants, like sodium dodecyl sulfate[144–146]. The high solubility and the mildness of the surfactant combined with the beneficial properties of chitosan make the chitosan - alkyl ether carboxylic acid complexes highly interesting systems for detergency and body care, as demonstrated by diverse patents in the field[147–149]. Similarly, highly soluble catanionic surfactant 21
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systems with unique properties can be expected from mixtures of AECs with oppositely charged, ethoxylated cationic surfactants.
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AECs are not only very versatile with respect to their non-covalent interaction properties, but the broad reactivity
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of the carboxylic acid termination allows for simple chemical modifications. For instance, nonionic, thermoresponsive surfactants with a chelating headgroup were prepared by coupling alkyl ether carboxylic acids with an amino acid derivative for the recovery of metal ions[150–152]. Amino acid surfactants are often prepared by coupling a single
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or a short sequence of amino acids with fatty acids, resulting in interesting, green surfactants[153–155]. The use of AECs enables the insertion of some ethylene oxide units between the hydrophobic chain and the amino acid surfactant,
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conferring upon them a more hydrophilic character and thermoresponsive properties. The same concept applies to covalent coupling with the carboxylic headgroup of fatty acids with polymers[156–159], nanoparticles[160–162], or surfaces[163, 164]. In particular, the covalent conjugation of carboxylic acids to an amine-functionalized surface
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via 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC) activation, allows for a simple and very versatile surface modification[164, 165]. AEC-modified surfaces are potentially attractive as supports for tethered lipid membranes, separated from the solid surface by a soft, hydrated spacer of tunable size. Such bilayers, deposited on a hydrated,
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soft cushion, represent excellent model systems to study cell membranes[166–169]. They are also the active elements of highly sensitive biosensors[170, 171].
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4.2. Practical applications
In the previous sections, a comprehensive characterization of AECs has been given. The properties of AEC-water
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solutions depend on a large number of factors, most notably pH and temperature, as well as the size of the hydrophilic and hydrophobic blocks of the surfactant. In addition of being highly tunable, their high solubility in hard water, high salinity, and acidic as well as alkaline conditions, makes them suitable for a wide scope of applications[34, 67]. A comprehensive summary of industrial applications of polyoxyethylene alkyl ether carboxylic acids is given – in german language only – in a review by Czichocki et al.[67] and in a more recent book chapter[34]. The most notable field of application is as household and industrial cleaning agents. This is due to their remarkable mildness, resistance to hard water conditions, high temperatures, the presence of bleaching and oxidizing agents, in addition to their good foaming, wetting, and emulsification properties[34, 36, 172–180]. Moreover, their high compatibility with cationic polyelectrolytes makes them a valuable ingredient for the formulations of shampoos, and skin care, hair conditioning and strengthening component in cosmetic preparations[147–149, 173, 179]. In particular, with respect to alkyl ether sulfates, AECs were reported to cause lower swelling of human stratum corneum and a minor denaturation of the skin proteins[173, 181]. The studies also show that optimal properties are achieved by mixing of alkyl ether carboxylates and alkyl ether sulfates. These mixtures show similar mildness, but better foaming and cleansing properties[173, 181, 182]. Another important field of application of AECs is in enhanced oil recovery[67, 183–188]. In order to extract residual oil from small pores, a sufficiently low interfacial tension between water and oil must be achieved[189]. 22
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Values as low as 10−2 − 10−3 mN m−1 are reported for C12 E3 CH2 COOH[190]. Even lower values are obtained in mixtures with nonionic surfactants[191]. The reason for the use of AECs in oil recovery, is their ability to form stable
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emulsions also in the presence of large amounts of monovalent and divalent cations and high temperature[44, 186,
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190], conditions at which many efficient sulfated and nonionic surfactants fail[185]. Improved performance can be obtained using branched alkyl chains, for instance, based on Guerbet alcohols[44, 192].
This multifunctional class of surfactants finds application beyond oil recovery and home and body care products.
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For instance, they are used as additives during textile processing[193, 194], as lubricants[195–197], or as dispersant for dyes, inks, and pesticides[198–201]. AECs with low ethylene oxide content were shown to be promising scale
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inhibitor agents[202]. As for the previously mentioned applications, their use is justified by their good wettability, emulsification, and solubilization capacity also in conditions where many other surfactants fail. For a more detailed description of the large-scale use of AECs we address the reader to the work of Meijer et al.[34].
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In addition to their widespread on an industrial scale, academic research on the use of AECs for a variety of purposes is being carried out. Very promising results have been obtained regarding their use for pollutant recovery[62, 66, 144, 203, 204]. C18:1 E9 CH2 COOH has been shown to be particularly effective for the removal of heavy metal
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cations in micellar enhanced ultrafiltration (MEUF)[66, 204]. MEUF is a membrane-based separation technique, in which the wastewater, enriched with surfactants, is filtered through a membrane which enables the separation of the surfactant micelles and associated components from the solution[205, 206]. A high efficiency is obtained
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when surfactants with a low cmc and a high affinity towards the pollutant are used. A widely used surfactant for the removal of heavy metal ions is sodium dodecyl sulfate (SDS). However, to decrease the amount of surfactant
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required it is a common procedure to add a nonionic surfactant to the mixture, with the aim of reducing the cmc[207– 209]. Polyoxyethylene alkyl ether carboxylic acids have three major advantages, when compared to other commonly employed systems: (i) surfactants with a long alkyl chain can be used, exhibiting very low cmc values; (ii) the affinity between surfactant and metal ion is pH dependent and can be exploited for a selective separation of complex metal ion mixtures[204]; and (iii) surfactant – metal pairs can be easily separated and the surfactant recovered for the next cycle by simply decreasing the pH of the solution and increasing the temperature above the cloud point of the surfactant[66]. Metal ions can also be extracted from solution by ion foam flotation[210]. The procedure consists of bubbling a gas, usually air, into a surfactant solution and separating the resultant foam, enriched in surfactant and its associated counterions. Studies on C18:1 E9 CH2 COOH have demonstrated its high efficiency for ion foam flotation applications[62, 203].
5. Conclusions Polyoxyethylene alkyl ether carboxylic acids are highly versatile, multiresponsive surfactants, which combine in a single molecule, peculiarities of pH-responsive fatty acids, and temperature responsive nonionic polyoxyethylene alkyl ethers. This gives rise to a complex, multiresponsive behavior in aqueous solutions. The surfactants properties 23
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change strongly with pH, as the magnitude of the charge of its headgroup is varied. At low pH, the behavior of AECs is similar to nonionic alkyl ethers, exhibiting similar cmc values and temperature-responsive behavior. With increasing
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pH, the surfactant headgroup is increasingly charged, thus affecting the properties of the surfactant solutions. The
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extent of the structural changes on pH variation depends on the number of ethylene oxide units in the headgroup. The transition from lamellar aggregates to globular micelles via well-defined vesicles was shown for C12 E4.5 CH2 COOH, while only minor changes in shape and size were observed for C12 E10 CH2 COOH. Soft, globular particles, whose
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degree of charge can be systematically varied without affecting their shape and size form a rare and valuable model system for studying charge-driven assembly processes[63]. The surfactants exhibit pKa values around 4, which allows
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for the fine-tuning of the surfactants properties in a pH range particularly useful for cosmetic application[64, 65]. The high reactivity of the carboxylic acid termination enables the preparation of surfactants tailored to specific purposes, e.g., the thermoresponsive and chelating amino-acid modified AECs for the recovery of uranyl ions[150,
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151]. However, the possibilities offered by the chemical conjugation of AECs, e.g., (i) with single amino acids or peptides, for the preparation of temperature-responsive functional surfactants; (ii) with solid surfaces, to prepare tethered lipid membranes with a variable hydrophilic cushion; (iii) with polymers, for the preparation of hydrophilic
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and temperature-responsive drug carriers, have scarcely been explored. AECs, in addition to being very interesting components for colloidal scientists, are widely used in large-scale, industrial applications. Due to the simultaneous presence of the ethylene oxide units and the carboxylic acid termina-
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tion, clear solutions of surfactants with long alkyl chains can be obtained in alkaline, as well as acidic solutions which are remarkably stable in hard water and high salinity conditions. Further advantages are given by their mildness, low
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toxicity, stability to hydrolysis and oxidizing agents and high temperatures. For these reasons, in addition to their low production cost, they find application in different fields, most notably as detergents and in home and body-care products, for enhanced oil recovery and as additives in the textile and metal processing industry. In summary, polyoxyethylene alkyl ether carboxylic acids are highly interesting components in colloid and surfactant science. The complex and highly tunable morphologies of the self-assembled micelles can be exploited to formulate supramolecular aggregates with a degree of diversity rarely found in colloid science. From the perspective of large-scale applications, their mildness and high solubility have opened the door to numerous consumer products and industrial uses. This work provides a comprehensive overview of the physico-chemical properties of this class of surfactants, of their use as building blocks in colloidal chemistry, and of their industrial application, with the aim of facilitating their spread in current research activities.
Acknowledgments Sylvain Prévost, Ralf Schweins, and Pierre Bauduin are gratefully acknowledged for the useful discussions and suggestions. The partnership for soft condensed matter (PSCM) is acknowledged for providing the light scattering apparatus and the tensiometer, the Institut Laue-Langevin (ILL) for the allocation of SANS beamtime. LC acknowledges 24
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the TU-Berlin and the PSCM at the ILL for postdoctoral funding through a three-year cooperation agreement.
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Supporting Information
Additional details and characterization of the AECs used in this work; detailed description of the expressions used for the analysis of the SANS patterns and additional SANS curves; details on the calculations of the surface
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electrostatic potential of the surfactant micelles.
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Graphical abstract OH O
O
O
C
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O
Multiresponsive properties
Industrial Applications
Complex self-assembly
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Colloidal building blocks
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O pH-responsive
degree of io ionization on
Temperature-sensitive
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Headgroup volume fraction
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Highlights
• pH- and Temperature-responsive behavior is reported
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• Overview of physico-chemical properties of multiresponsive surfactant
• Extremely versatile building blocks for colloidal supramolecular assemblies
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• Review of technical and large-scale applications of the surfactants
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S0001-8686(17)30356-1 doi: 10.1016/j.cis.2017.10.001 CIS 1834
To appear in:
Advances in Colloid and Interface Science
Received date: Revised date: Accepted date:
10 August 2017 7 October 2017 10 October 2017
Please cite this article as: Chiappisi Leonardo, Polyoxyethylene alkyl ether carboxylic acids: an overview of a neglected class of surfactants with multiresponsive properties, Advances in Colloid and Interface Science (2017), doi: 10.1016/j.cis.2017.10.001
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Polyoxyethylene alkyl ether carboxylic acids: an overview of a neglected class of surfactants with multiresponsive properties
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Leonardo Chiappisia,b,∗ a Technische
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Universität Berlin, Stranski Laboratorium für Physikalische Chemie und Theoretische Chemie, Institut für Chemie, Straße des 17. Juni 124, Sekr. TC7, D-10623 Berlin, Germany b Institut Max von Laue - Paul Langevin, Large Scale Structures Group, 71 avenue des Martyrs - 38042 Grenoble Cedex 9, France
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Abstract
In this work, an overview on aqueous solutions of polyoxyethylene alkyl ether carboxylic acids is given. Unique properties arise from the combination of the nonionic, temperature-responsive polyoxyethylene block with the weakly
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ionic, pH-responsive carboxylic acid termination in a single surfactant headgroup. Accordingly, this class of surfactant finds broad application across very different sectors. Despite their large use on an industrial and a technical scale, the literature lacks a systematic and detailed characterization of their physico-chemical properties which is provided
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herein. In addition, a comprehensive overview is given of their self-assembly and interfacial behavior, of their use as colloidal building blocks and for large-scale applications.
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Contents
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Keywords: polyoxyethylene alkyl ether carboxylic acids, surfactant science, self-assembly
1
Introduction
2
2
Synthetic procedure and availability
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3
Physico-chemical properties
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3.1
pH titrations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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3.2
Surface tension . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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3.3
Small-angle neutron scattering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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3.4
Temperature responsiveness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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4
Colloidal building blocks and technical uses
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4.1
Building blocks for colloidal supramolecular structures . . . . . . . . . . . . . . . . . . . . . . . . .
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4.2
Practical applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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∗ Corresponding
Author. Email address: [email protected] (Leonardo Chiappisi)
Preprint submitted to Advances in Colloid and Interface Science
October 10, 2017
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5
Conclusions
21 22
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References
1. Introduction
Surfactants are among the most versatile compounds in the colloidal playground. The properties of their aque-
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ous solutions, e.g., critical micelle concentration (cmc), surface activity, solubilization capacity, foamability, foam stability, depend on the molecular structure of the lyophobic and lyophilic groups. Given their amphiphilic nature,
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surfactants are employed as solubilizing agents and detergents, to reduce surface and interfacial tensions, to stabilize (or destabilize) foams and emulsions, to disperse or flocculate solids or liquids in liquids, etc.[1]. Optimizing surfactants for their function is a complex and tedious task, however great efforts have been pursued by the scientific and
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industrial communities towards this goal. Recent progress has been well reviewed[2–12]. Carb. Acid termination
Ethyleneoxide units
Alkyl Chain
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O
OH
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i
j
O
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Figure 1: Schematic representation of the chemical formula of polyoxyethylene alkyl ether carboxylic acids, composed of a hydrophobic alkyl chain, hydrophilic ethylene oxide units and terminated by a carboxy-methyl unit.
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This work is focussed on polyoxyethylene alkyl ether carboxylic acids (chemical formula given in Fig. 1), often abbreviated as AECs. For the sake of simplicity, three main parts of the surfactant can be identified: the lipophilic alkyl chain, which can vary in length and degree of saturation; the polyoxyethylene (EO) block, and the terminal carboxylic acid, whose degree of ionization depends on the pH of the solution. Hereafter, the surfactant chemical structure will be abbreviated as Ci Ej CH2 COOH, with i being the usual nomenclature for fatty acids, j representing the number of ethylene oxide units, and CH2 COOH is the carboxymethyl termination. Depending on pH, mixed systems of acids and sodium salts are present, but for the sake of simplicity we name them "acids", keeping in mind the importance of the degree of ionization. The surfactant structure strongly resembles that of nonionic polyoxyethylene alkyl ethers Ci Ej or fatty acids (Ci COOH). Polyoxyethylene alkyl ether carboxylic acids, however, present some major advantages with respect to the above-mentioned compounds, which arise from the combination of ionic and nonionic surfactant properties in a single molecule. Fatty acids – or soaps, when in their salt form – are the oldest man-made surfactants and are easily prepared from the alkaline treatment of natural fats and oils. However, their use is essentially limited by their high Krafft point, i.e., the temperature at which the free energy of the fatty acid molecule is lower in the crystal than in the micellar 2
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aggregate, and the low solubility in neutral and acidic medium. Optically clear solutions of medium and long-chain fatty acids are obtained only in very alkaline solutions, e.g. over pH ∼ 10.5 for sodium oleate[13, 14]. A simple
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approach for increasing the solubility of fatty acids consists of exchanging their counterion, usually sodium, with a
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bulkier one with a lower tendency to crystallize[15–18]. Similarly, the Krafft point can be reduced and the tolerance to multivalent cations increased by the use of branched chain fatty acids[19, 20]. Moreover, the low solubility of fatty acid salts of multivalent cations, in particular calcium and magnesium, represents a strong limitation to their use in
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everyday consumer products. Although different parameters affect the solubility of fatty acids – pH, temperature, or the presence of multivalent cations – the problem has a common origin: the high stability of the fatty acid crystal and
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the low hydrophilicity of the nonionized species. Both factors are addressed when some ethylene oxide units (EO) are inserted between the carboxylic headgroup and the aliphatic chain. The bulky and amorphous EO units prevent AECs from crystallizing and confer a remarkable hydrophilicity even in acidic media, where solubilities and cmcs are close
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to the corresponding nonionic polyoxyethylene alkyl ether surfactants[21]. The strategy of introducing some ethylene oxide units between hydrophonic tail and surfactant headgroup is widely applied to alkyl sulfates. Despite they are permanently charged, alkyl sulfates are characterized by high Krafft points and a low solubility in the presence of
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multivalent ions[22–24]. Both limitations are, at least partially, overcome, by the insertion of few – generally two to four – ethylene oxide units.
In contrast to their ionic counterparts, nonionic polyoxyethylene alkyl ether are generally water soluble (provided
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a sufficient number of EO units are present), their behavior is pH independent and they are resistant to hard water conditions, i.e., in the presence of multivalent cations[4, 25]. Polyoxyethylene alkyl ethers exhibit a rich phase diagram
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in aqueous solution as a function of temperature and concentration[26]. With increasing temperature the hydrophilicity of the EO block is reduced, resulting in a gradual dehydration of the EO units. Above a certain temperature, called the cloud point (CP), phase separation into a surfactant-rich and a surfactant-poor phase occurs[4, 27]. This phenomenon is common to all polyoxyethylene alkyl ethers, but the CP varies strongly with: the length of the surfactant tail, the number of EO units, and the presence of additives, e.g. inorganic salts or alcohols[28–30]. The complex physics underlying this phenomenon was recently reviewed[4, 27]. The continuous dehydration of the surfactant headgroup has remarkable consequences not only on the phase boundaries, but also on other properties such as the cmc, the adsorption at interfaces, the spontaneous curvature of the surfactant film, and is the basis for several surfactantmediated extraction methods[31–33]. Fatty acids and polyoxyethylene alkyl ethers are both highly relevant classes of surfactants, each exhibiting characteristic properties. However, the combination of the aqueous solubility and the thermoresponsive behavior of nonionic polyoxyethylene alkyl ethers with a pH-responsive carboxylic headgroup opens up a multitude of possibilities, as will be shown hereafter. Current literature lacks in systematic studies of the physico-chemical properties of AECs. To compensate for this lack, a comprehensive physico-chemical characterization of aqueous solutions of polyoxyethylene alkyl ether carboxylic acids is provided in this work. Experimental details are given in the supporting information. In particular, their behavior is rationalized in terms of their chemical structure and experimental conditions such as 3
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pH, ionic strength, and temperature. These new results are analyzed and discussed jointly with the available literature data, therefore providing a comprehensive overview on the self-assembly properties of AECs. The behavior of
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polyoxyethylene alkyl ether carboxylic acids will be compared with that of the nonionic polyoxyethylene alkyl ethers
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and strong anionic polyoxyethylene alkyl ether sulfates. Based on this exhaustive characterization, recent progress in fundamental and applied research, demonstrating the potential of these surfactants in different fields, is discussed.
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2. Synthetic procedure and availability
AECs are prepared from polyoxyethylene alkyl ethers[34–40]. Polyoxyethylene alkyl ethers are mainly prepared
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by the addition of ethylene oxide to fatty alcohols in the presence of a strong base, resulting in a rather broad distribution of EO units[41, 42]. Polyoxyethylene alkyl ether carboxylic acids with branched alkyl chain can be prepared from Guerbet alcohols[43–45]. Two main methods are available for the final modification leading to the carboxymethyl ter-
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mination. The first method consists of the reaction of the alcohol with monochloroacetic acid in the presence of sodium hydroxide, i.e. the Williamson’s ether synthesis[34, 38, 46–48].The reaction of the fatty alcohol with chloroacetic acid presents some drawbacks: the low yield of ca. 90 %, the use of the toxic chloroacetic acid, and the formation of a large
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amount of sodium chloride and other byproducts as diglycolic and glycolic acids. To avoid the formation of these short chain carboxylic acids, a bulky base such as a secondary or tertiary alkoxide can be used[34, 38, 46]. The second
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method consists of the direct oxidation of the terminal alcohol[34, 49–51]. The reaction is mostly performed using oxygen as the oxidizing agent in the presence of a platinum or palladium-based catalyst[34, 49, 50]. The efficiency of the catalyst can be improved by the introduction of Pb, Bi, or Cd salts[34, 52–54], or by the use of a Au-based
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catalyst[40, 51, 55–57]. Synthetic routes using H2 O2 as oxidizing agent are also reported[51]. Although the oxidation reaction has a yield close to 100%, the main route for the industrial preparation of AECs is the Williamson’s synthesis, as it has proven to be more cost-effective[46, 51, 55]. A further limitation to the implementation of the large-scale oxidation reaction, is the long diffusion time of oxygen into the highly viscous surfactant phase. In contrast, the higher yield and simpler reaction conditions make the oxidation reaction more suitable for academic research purposes. Technical grade polyoxyethylene alkyl ether carboxylic acids are readily available in large quantities and at low cost. The alkyl chain length generally varies between 4 and 20 carbon atoms, and the degree of ethoxylation between 2 and 20. Some of the key manufacturers of AECs are Kao Chemicals (AKYPO R series), Huntsman (EMPICOL R C series), Clariant (Emulsogen R series), Uniqema (Atpol series), Nippon Shokubai, and BASF SE[58].
3. Physico-chemical properties As mentioned previously, the surfactant can be subdivided in three main parts: the lipophilic alkyl chain, the hydrophilic polyoxyethylene part, and the ionizable carboxylic termination. This subdivision helps in rationalizing properties such as the cmc, surface tension, size and shape of the aggregates. Some studies dealing with the self-assembly and interfacial properties of AEC solutions are available in the literature[21, 36, 59–69], but a systematic study is 4
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missing. To provide a more exhaustive characterization of their properties, the behavior of aqueous solutions of a set of AECs with variable alkyl chain length and number of ethylenoxide units was probed as a function of pH, ionic
T
strength, and temperature. In particular, three technical surfactants, represented by the formulæ C12 E10 CH2 COOH,
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C18:1 E9 CH2 COOH, and C12 E4.5 CH2 COOH were chosen to highlight alkyl chain length and headgroup size effects. This choice was limited to surfactants having a sufficient number of EO units to be water soluble over a large pH range, and an alkyl chain large enough to form well-defined self-assembled aggregates. The shorter alkyl chains denoted by
SC
C12 are in fact 2:1 mixtures of C12 and C14 and the longer alkyl chain denoted as C18:1 is a 3:1 mixture of C18:1 and C16:0 . The number of EO units show a roughly Gaussian distribution, with a distribution width of approximately three
NU
for the C12 E4.5 CH2 COOH surfactant[65]. Further details on the materials and on the techniques used are given in the experimental section in the appendix of this work and in the supporting information. The use of technical surfactants presents two major advantages: their low cost and availability in large quantities, and the increase in stability of the
MA
easy-to-process isotropic micellar and hexagonal phases over that of the lamellar Lα one[26]. Despite small deviation of the physico-chemical properties such as cmc, surface tension, and cloud point of nonionic polyoxyethylene alkyl ether surfactant solutions are reported between pure and non-pure components, no difference is found between the
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trends as a function of the average alkyl chain length and number of EO units remains unchanged[26, 70, 71]. This similarities allows generaling the findings of this work on technical surfactants also on their pure counterparts.
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3.1. pH titrations
Potentiometric and turbidimetric titrations of 1 wt% solutions of C12 E10 CH2 COOH, C18:1 E9 CH2 COOH, and
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C12 E4.5 CH2 COOH were conducted at different ionic strengths. All results are reported in Fig. 2. The turbidity of the solution (first row of Fig. 2) reflects the weight average molar mass of the self-assembled surfactant aggregates, the smaller the aggregate the more transparent the solution. C12 E10 CH2 COOH solutions were completely transparent throughout the titration, the C18:1 E9 CH2 COOH were slightly translucent for pH < 3.0, and the C12 E4.5 CH2 COOH turned from turbid to transparent during the titration. Clearly, the larger the number of EO units and the higher the pH, the smaller the aggregates are in solution. Similar turbidity curves were reported earlier for C12 E4.5 CH2 COOH by Kunz and coworkers[65], showing a transition from vesicles to globular micelles. Structural details will be discussed in section 3.3, based on the small-angle neutron scattering (SANS) data. Potentiometric titrations, through the charge balance, directly provide the degree of ionization α of the surfactant, i.e., the fraction of nonprotonated carboxylic units (second row of Fig. 2). The effective acidity constant is then calculated as: pKa (α) = pH − log
α . 1−α
(1)
A monotonic increase in pKa (α) with pH is observed for all surfactants at each ionic strength (third row of Fig. 2). The pKa (α) values are found to be between 3.5 and 5.0, depending on the pH and ionic strength of the solution. In all cases, the acidity of the surfactant increases upon addition of sodium chloride. The strong dependence of the pKa from the experimental conditions makes a direct comparison with literature values difficult. Extrapolating the pKa 5
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C12E10CH2COOH
C18:1E9CH2COOH
0.8
0.4
NaCl 0.0 M 0.15 M 1.0 M
0.2
NaCl 0.0 M 0.15 M 1.0 M
0.8
NaCl 0.0 M 0.15 M 1.0 M
SC
0.6 0.4 0.2
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0.0 5.0 4.5 4.0
MA
pKa( )
T
0.6
0.0
3.5 0 -20
ED
!0( ) / mV
C12E4.5CH2COOH
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Transmission
1.0
-40 -60
2
3
4
PT
-80 5
6
2
3
4 pH
5
6
2
3
4
5
6
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Figure 2: From top to bottom: Light transmission (wavelength λ = 520 nm, path 2.5 cm), degree of ionization α, effective pKa , and electrostatic surface potential Ψ0 as a function of pH, obtained from turbidimetric and potentiometric titrations of 1 wt% solutions of (from left to right) C12 E10 CH2 COOH, C18:1 E9 CH2 COOH, and C12 E4.5 CH2 COOH with the initial addition of different amounts of NaCl. See text for full details.
values to α → 0 allows for the determination of the intrinsic acidity constant pKa (0) (reported in Table 1). The pKa (0) values are approximately constant, and slightly decrease with increasing number of EO units of the headgroup. This is a result of the electron withdrawing effect of the ethylene oxide units[60]. To a first approximation, the effective acidity constant of a generic polyacid (weak electrolytes, dendrimers, aggregated weak acids/bases, etc.) is determined by two main factors: the intrinsic binding affinity pKa (0), and the electrostatic interaction between the charged headgroups and can be described as[73–76]: ! dGel 1 pKa (α) = pKa (0) + 2.303RT dα
(2)
where Gel , R, and T are the headgroup interaction contribution to the change in Gibbs free energy of ionization, the ideal gas constant, and the temperature, respectively. More complex models, taking into account conformational changes of the colloid as a function of pH or ionic strength, are also available[77–79]. The main contribution to the free energy of ionization arises from the work needed to create an additional charge on the micellar interface, and is 6
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Table 1: Results from pH titrations obtained at different initial NaCl concentrations (in mol L−1 ).
0.0
3.49 ± 0.03
0.15
3.47 ± 0.01
1.0 C18:1 E9 CH2 COOH
0.0
3.68 ± 0.05
3.50 ± 0.02
1.0
3.67 ± 0.02
0.0
4.00 ± 0.03
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C12 E4.5 CH2 COOH
3.60 ± 0.02
SC
0.15
T
pKa (0)
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C12 E10 CH2 COOH
[NaCl]
0.15
3.90 ± 0.01
1.0
3.73 ± 0.02
0.0
3.8±0.2
C12 E4.5 CH2 COOH
0.0
4.67
C12 E10 CH2 COOHc at α = 0.5
0.0
4.0
MA
C8 E5 CH2 COOHa
b
from Ref. [60]; b from Ref. [65]; c from Ref. [72]
ED
a
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directly related to the surface potential Ψ0 [74]:
dGel = −e0 NA Ψ0 (α) dα
(3)
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where e0 and NA are the elementary charge and the Avogadro constant, respectively. The electrostatic potential at the micellar headgroup/water interface can be obtained by combining Eqs. 2 and 3 (fourth row of Fig. 2). The surface potential decreases from 0 mV at low pH to -80 mV in neutral and basic water. Values of up to -50 mV are found in saline alkaline solutions.
Ψ0 can also be obtained by the one-dimensional Poisson-Boltzmann equation for spherical particles immersed in an electrolyte solution[74]:
! 8πe0 I e0 Ψ(r) d 2 dΨ(r) r = sinh dr ε0 εr kB T r2 dr
(4)
with r being the distance from the center of the spherical particle, I the ionic strength of the solution, ε0 εr the dielectric constant of the solvent, and kB the Boltzmann constant. The equation can be solved using the boundary conditions that: Ψ(r → ∞) = 0 and dΨ/dr = −4πσgeo /ε0 εr at r = R, with R being the distance of the headgroup-water interface from the micelle center, and σgeo the geometrical charge density at the headgroup-water interface. σgeo and the radius of the micelle can be estimated from the SANS data reported in section 3.3 of this work. In particular, the geometric charge density is obtained from the degree of ionization α, the aggregation number Nagg , and the micellar radius R as σgeo = αNagg /4πR2 . Eq. 4 was solved iteratively using the Fortran routine made available by Lund University[80] using the values of Nagg and R determined from the SANS patterns (see supporting information for further details). 7
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The surface electrostatic potential was also determined from the charged hard-sphere structure factor used for the interpretation of the SANS data. The values of the surface potential directly determined from the potentiometric
T
titrations, from the solution of the Poisson-Boltzmann equation (Eq. 4), and from the analysis of the SANS patterns, pH
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S (q) respectively denoted hereafter as Ψ0 , ΨPB 0 , and Ψ0 , are reported in Fig. 3. All values and detailed calculations are
given in the supporting information. Briefly, the electrostatic potential predicted by solving the Poisson-Boltzmann
SC
0 -20
NU
-60
0
pH
/ mV
-40
-100
0
-120
PB
pH
0
S(q)
z
-120
-100
ED
-140 -140
0
MA
-80
-80 0
0
PB
0
S(q)
C18:1E9CH2COOH C18:1E9CH2COOH+NaCl C12E10CH2COOH C12E10CH2COOH+NaCl C12E4.5CH2COOH
S(q)
-60 and
0
PB
-40
-20
0
/ mV pH
Figure 3: Plot reporting the values of the surface potential at the micelle headgroup/water interface determined from pH titrations (Ψ0 ) as a
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function of the ones obtained by the solution of the Poisson-Boltzmann equation ΨPB 0 (empty points) and as a function of the values determined S (q)
by the analysis of the structure factor of the SANS patterns Ψ0
(solid points). Exact values used for the calculations are given in Table S3. The
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dashed line represents f (x) = x and is a guide for the eyes. If the different methods would result in the same surface potential values, all points would fall on the dashed line. The inset at the bottom-left corner schematically shows the micelle/water interface as a function of the distance from the micelle center z, with the location of the plane at which the potentials are calculated/determined. See text for further details.
equation is significantly more negative than the one found from pH titrations. In contrast, slightly less negative values are found for Ψ0S (q) . This discrepancy is thought to arise from the effect of the micelle geometry on the calculation S (q) of ΨPB 0 and Ψ0 . In both cases, the micelles are modeled as rigid, spherical particles. However, analysis of the
SANS revealed that the micelles are in fact slightly anisotropic. ΨPB 0 is calculated at the surface of a rigid sphere with a volume equivalent of that of the globular micelle; Ψ0S (q) is calculated at the surface of the sphere with a radius equal to the hard sphere radius, close to the largest dimension of the globular micelle. The ζ-potential values, i.e., the electrostatic potential at the slipping plane, are in the range -20 – -30 mV for C18:1 E9 CH2 COOH at pH 4 to 5[68], in line with the trend of decreasing potential with increasing distance from the micelle headgroup/water interface. Moreover, due to the flexibility of the ethylene oxide block, and its size distribution, the charges of the micelles are distributed in an interfacial region rather than on a sharp plane, allowing for more spatial freedom to minimize the electrostatic repulsion. Despite the substantial approximations, a good agreement is found between the different values. This demonstrates that the dependence of acidity constant on pH for the micellized AECs is predominantly governed by the electrostatic repulsion and counterion condensation, and that the shape and size of the self-assembled 8
ACCEPTED MANUSCRIPT
aggregates play only a minor role. The effects of the addition of sodium chloride to a surfactant solution at constant pH can be rationalized accordingly. Due to counterion condensation and charge screening, the electrostatic potential
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at the micelle/water interface is reduced, essentially lowering the repulsion between the charged headgroups. This
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screening is observed in the decrease in the pKa (α) and, in turn, in an increase in the degree of ionization α. 3.2. Surface tension
50 45 40 35
10-7
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30 25
pH = 2.5 4.1 5.0 8.0
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/ mN m-1
55
pH = 2.5 4.1 5.0 8.0
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pH = 2.5 4.1 5.0 8.0 8.0 + 0.2M NaCl
60
Surface tension
SC
65
10-6 10-5 10-4 10-3 10-2 10-7 10-6 10-5 10-4 10-3 10-2 10-7 10-6 10-5 10-4 10-3 10-2 [C12E10CH2COOH] / mol kg-1 [C18:1E9CH2COOH] / mol kg-1 [C12E4.5CH2COOH] / mol kg-1
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Figure 4: Equilibrium surface tension of (from left to right) C12 E10 CH2 COOH, C18:1 E9 CH2 COOH, and C12 E4.5 CH2 COOH solutions in water of varying acidity as a function of the molality of surfactant.
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The equilibrium surface tension of C12 E10 CH2 COOH, C18:1 E9 CH2 COOH, and C12 E4.5 CH2 COOH solutions at different surfactant concentration and pH was measured and is shown in Fig. 4. The recorded curves are clearly different from typical titration curves. A clear minimum in the surface tension curve is observed, which is deeper the higher the pH of the solution, i.e., at higher degree of ionization of the surfactant. Technical surfactants, where a large spectrum of molecules with variable surface activity coexist, usually – but not necessarily – show a minimum in their surface tension vs. log C curves[81]. Few examples of surface tension curves with the same shape were reported also for similar, monodisperse surfactant systems[61, 82]. In addition to the polydisperse nature of the surfactant, the equilibrium between the charged and uncharged forms adds a certain degree of complexity to the data interpretation. Different theoretical models for the descriptions of the adsorption of ionic/nonionic surfactant at interfaces are available, some of them explicitly consider electrostatic interactions while others work within the thermodynamic concept of a by-definition-neutral Gibbs dividing interface[83]. Given the complexity of the studied system, the surface tension titration curves were interpreted within the Gibbsian framework. At constant pressure and temperature, the Gibbs adsorption isotherm is given as: dγ = −Γ s RT d ln a s − Γc RT d ln ac
9
(5)
ACCEPTED MANUSCRIPT
where γ is the surface tension, Γ s , a s , Γc , and ac are the surface excess and the activity of the surfactant and its coun-
and
χc = αχ s
(6)
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Γc = αi Γ s
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terion, respectively. To preserve the electroneutrality of interface and solution, the following relations are introduced:
where αi , χ s , and χc are the degree of ionization of the surfactant at the interface, and the molar fractions of the surfactant and the counterion, respectively. αi is not to be confused with the degree of ionization of the surfactant
SC
in solution α determined from potentiometric titrations. Small difference of 0.2[84] and 1 unit[85] are reported between the pKa values of fatty acids determined in bulk solution or at the air-water interface. This differences arises from from the electrostatic potential at the air-water interface established upon the adsorption of charged species[84].
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Assuming that the potential at the micelle-water interface is similar to that at the air-water interface, it is reasonable to approximate that αi ≃ α. At least for the more dilute region of the titration curves, where the surfactant is present
MA
in its nonmicellized form, ideal behavior can be assumed, such that the activities of the surfactant and its counterion may be substituted with their concentrations. Eq. 5 can be rewritten accordingly:
ED
dγ ≈ −Γ s (1 + αi )RT d ln χ s − αi Γ s d ln αi
(7)
As the titrations are performed at constant pH, it is reasonable to neglect the second term of Eq. 7. Finally, the
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surfactant surface excess can be obtained as:
Γs = −
dγ 1 (1 + α)RT d ln χ s
(8)
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When working in excess of an inert salt, i.e., a salt presenting no specific interaction with the surfactant, the second term of Eq. 5 is zero and Eq. 7 simplifies to that of a nonionic surfactant. The minimum area per molecule at the air-water interface Ah is calculated from Γ s as Ah = (NA Γ s )−1 , where NA is the Avogadro constant. Within the same thermodynamic framework, the Gibbs free energy of micellization ∆G0mic can be obtained. The aggregation of n partly charged surfactant molecules S α− and their counterions C + into a micelle Mn can be expressed with the following equilibrium:
nS α− + nα(1 − β)C + ⇋ Mnnαβ−
(9)
where (1-β) is the fraction of condensed counterions. The ∆G0mic can then be calculated as: ∆G0mic 1 = ln a M − ln a s − α(1 − β) ln ac . RT n
(10)
Substituting the activities with the molar fractions, the following relation is obtained: ∆G0mic ≈ 1 + α(1 − β) ln χ s (cmc) + α(1 − β) ln α. RT
(11)
Eq. 11 reduces to the classical equation of ionic surfactants[90] for α = 1: ∆G0mic ≈ (2 − β) ln χ s (cmc) RT 10
(12)
T
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Table 2: Summary of parameters obtained from surface tension titrations and of those used for its analysis. α is the degree of ionization obtained from potentiometric titrations, 1 − β is the fraction of condensed counterions, determined from SANS experiments. The cmc is given in µmol L−1 , the headgroup area Ah in Å2 , the surface excess Γ in µmol m−2 , the surface tension dependence from the concentration dγ/d ln(C) in 10−3 N m−1 , the surface tension at high surfactant concentration γ(c → ∞) and at the cmc γ(c = cmc) in 10−3 N m−1 , the Gibbs free energy of micellization
SC
∆G0mic in 103 J mol−1 . Values for nonionic surfactants are reported for comparison at the bottom of the table. Values in italics are from literature.
a
Values determined in 0.2 M acetic acid/sodium acetate buffer, from Ref. 21. Other values are taken from: Ref. 86; Ref. 87; Ref. 61 (chemically pure polyoxyethylene alkyl ether carboxylic acid); e Ref. 88 f Ref. 89; g Ref. 24.
d
γ(c → ∞)
γ(c = cmc)
∆G0mic
1.9
-5.3
31.1
30.3
-40.7
—
—
33.5
31.9
—
β
cmc
Ah
Γ
2.5
0.09
0.0
20.5
86
4.0a
—
—
29.0
—
4.1
0.49
0.45
5.0
0.78
8.0 NaCl
C18:1 E9 CH2 COOH
102
1.6
33.5
31.1
-44.2
0.45
100
110
1.5
-6.8
37.5
31.8
-47.3
1.00
0.45
MA
-6.0
180
130
1.3
-6.2
40.7
35.5
-48.7
8.0
1.00
1.0
75
75
2.2
-5.5
37.7
35.7
-47.6
2.5
0.05
0.0
2.0
51
3.3
-8.5
31.3
31.8
-45.1
—
5.5
—
—
—
33.6
32.0
—
0.3
0.4
14
96
1.7
-6.0
31.1
32.7
-45.1
0.7
0.4
24
97
1.7
-7.2
34.7
33.8
-52.1
a
4.0
4.1 5.0
—
PT
−1
ED
54
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NU
α
+ 0.2 mol L
c
dγ/d ln(C)
pH C12 E10 CH2 COOH
b
8.0
1.0
0.4
43
124
1.3
-6.7
39.2
37.0
-56.0
2.5
0.05
0.0
5.5
58
2.8
-7.4
28.0
27.9
-42.5
4.0
—
—
15
—
—
—
28.8
30.5
—
4.1
0.30
0.15
26
68
2.4
-7.8
28.2
28.7
-45.9
5.0
0.60
0.15
150
81
2.0
-8.1
30.4
28.6
-48.5
∼ 7d
—
—
400
—
—
—
39.1
30.5
—
8.0
1.00
0.25
450
123
1.4
-6.7
35.4
34.0
-51.0
C12 E10 (at 303 K)b
—
—
—
12.7
—
—
—
—
—
-38.5
C16 E9
—
—
—
2.0
53
—
—
36
36
-42.5
—
—
—
6.6
—
—
—
31
30
-39.5
—
1.0
—
2200
52
—
—
—
43
—
—
1.0
—
1300
—
—
—
—
—
—
C12 E4.5 CH2 COOH
a
c
C12 E5 e C12 E3 OSO3 Na
f
C12 E10 OSO3 Nag
11
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It should be noted that by introducing the relation χc ≈ αχ s from Eq. 6, the contribution of hydroxide ions to the charge balance of the system is neglected. However, this approximation holds in the range of pH (2.5 to 8) and
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surfactant concentration to which Eqs. 7 and 11 are applied. The critical micelle concentration is taken either at the
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abrupt change in slope of the surface tension titration curves or, if present, at the minimum value. The values are summarized in Table 2.
For each surfactant, increasing the degree of ionization α causes an increase of the cmc of up to two orders of
SC
magnitude. The values of β used for the calculation of ∆G0mic were obtained from the analysis of the SANS patterns given in Fig. 5. The values of β ≃ 0.3 - 0.5 found for C12 E10 CH2 COOH and C18:1 E9 CH2 COOH are relatively
NU
high when compared to the typical values of 0.1-0.2 for weak and strong ionic surfactants in water[91–94]. Such low counterion condensation arises from the presence of the EO units, which “dilute” the charge of the surfactant headgroup[23, 60, 95], as also supported by the lower values of β found for C12 E4.5 CH2 COOH. The consequences of
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the presence of the EO units are also evident when comparing the cmc values found in solutions of C12 E10 CH2 COOH and C12 E4.5 H2 COOH at different pH. At low pH, the cmc is higher for the surfactant with the larger headgroup, as expected by the hydrophilic nature of EO units. In contrast, at high pH, where the ionic form of the surfactant
ED
dominates, the cmc of C12 E10 CH2 COOH is a factor 2 lower than that of C12 E4.5 CH2 COOH. A similar reduction of the cmc with increasing number of EO units was also reported for polyethylene oxide alkyl ether sulfates[24, 89, 96]. effects of the EO units.
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This counterintuitive relation of increasing hydrophilicity and decreasing cmc can be ascribed to the charge-dilution Moreover, the values for the cmc and the ∆G0mic are close to those reported for similar nonionic Ci Ej [61, 86–88].
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With increasing pH a decrease in the free energy of micellization is observed. This trend is in contrast to an expected increase due to the electrostatic repulsion between the charged headgroups, which should become more important the higher the degree of ionization of the surfactant. One explanation lies in high sensitivity of the calculation of ∆G0mic via Eq. 11 on the values of β. The β values used were determined from the analysis of the SANS patterns at surfactant concentrations much higher than the cmc and may be underestimated, in particular when compared with the high values of 0.7-0.85, determined by conductometric experiments close to the cmc for the sodium salts of several AECs in pure water (resulting pH of 8)[60, 69]. In addition to strongly increasing the cmc of the surfactant, the ionization of the carboxylic headgroup causes a decrease of the maximal surfactant surface excess Γ s and, accordingly, an increase of the minimal area occupied by the surfactant at the air-water interface. This effect is particularly pronounced for C12 E4.5 CH2 COOH, the surfactant with the smallest number of EO units. However, the reported values must be treated with caution given the technical nature of the surfactant used and the uncertainty arising from the approximation of α ≃ αi . All surfactants show a minimum in the surface tension curves, at least at higher pH values. The technical nature of the surfactant can only partially explain this behavior, as the polydispersity in alkyl chain length and number of EO units is pH independent. However, the difference in surface activity between the main component and impurities, e.g., the non-carboxymethylated polyoxyethylene alkyl ether, increases with increasing pH. In addition, at pH 2.5 12
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and 8.0 the surfactant can be regarded as a pure nonionic or pure ionic compound, respectively. Sakai et al., however, recorded the surface tension for the chemically pure C12 E4 CH2 COOH surfactant at pH ∼ 7, reporting a similar, well-
T
pronounced minimum in the surface tension close to the cmc[61]. The authors justify the observed minimum with
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a vesicle-to-micelle structural transition in the proximity of the cmc. Zana et al. reported a continuous increase in surfactant aggregation number for C12 E5 CH2 COOH, C12 E7 CH2 COOH, and C12 E9 CH2 COOH close to the cmc[60]. A similar effect was found in another pure, weakly-anionic surfactant[82]. In both cases, the authors assumed structural
SC
rearrangements to justify the shape of the surface tension curves. A common assumption is that the concentration and therefore the activity of the free surfactant remains constant or increases slightly after the cmc. However, it was
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demonstrated that the activity of free surfactant can decrease with increasing total surfactant concentration above the cmc[97, 98], due to the increased ionic strength which reduces the repulsion between the surfactant headgroups and the micelles. To address the effect arising from the increasing ionic strength, a surface tension curve of C12 E10 CH2 COOH
MA
at a pH value of 8 in the presence of 0.2 mol L−1 NaCl was recorded. The minimum in the surface tension curve is reduced but is still present, in line with the findings by Svanedal et al.[82]. In summary, the uncommon shape of the surface tension curves arises from a number of different factors: the polydisperse nature of the surfactant, a
ED
likely change in the micelle geometry at concentrations higher than the cmc and, more importantly, a reduction of the activity coefficient of the surfactant, resulting in a well-pronounced minimum in the surface tension curve at high pH.
100
C18:1E9CH2COOH pD = 2.0 2.8 4.4 4.1 6.1 11.7
104
102 101
0.1
104 103 102 101
100 10-1
C12E4.5CH2COOH pD = 2.0 2.7 4.3 4.7 5.0 5.3 6.6 11.7
106
I(q) / cm-1
103 I(q) / cm-1
101
C12E10CH2COOH pD = 2.0 2.8 4.2 4.8 5.2 5.9 11.4
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I(q) / cm-1
102
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3.3. Small-angle neutron scattering
100
10-1 1 q / nm-1
10-1 0.1
q / nm-1
1
0.1
q / nm-1
1
Figure 5: SANS patterns arising from ca. 1 wt% solution of (from left to right) C12 E10 CH2 COOH, C18:1 E9 CH2 COOH, and C12 E4.5 CH2 COOH. Solid lines show best fits according to different models described in detail in the main text and supporting information. The models are schematically represented by the sketches, not to scale, in the figure.
The size and shape of three representative surfactants, C18:1 E9 CH2 COOH, C12 E10 CH2 COOH, and C12 E4.5 CH2 COOH were studied in D2 O as a function of pD via small-angle neutron scattering (SANS). The scattering patterns could be 13
ACCEPTED MANUSCRIPT
described using different models, reported in detail in the supporting information. In all cases, a homogeneous coreshell structure was assumed, where the core is formed by the anhydrous hydrophobic blocks, and the shell is formed
T
by the hydrated EO units and the carboxymethyl unit. Depending on the nature of the surfactant and on the acidity
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of the solution, the data were described using vesicle, rod or ellipsoid models[99]. A charged hard-sphere structure factor was used for the ellipsoids[100]. The obtained parameters are given in Tables 3 and 4, and the analytical expressions used for the description of the SANS patterns are given in the ESI. The fits were constrained by imposing
SC
the condition that at least one characteristic length of the aggregate core should be 1.6 and 2.1 nm, as determined by the maximum length of the C14 and C16 alkyl chains of the surfactants, respectively (see supporting information for
NU
further details). The thickness of the shell depends on the conformation of the ethylene oxide units[101–103], and its limits can be estimated assuming different conformations: 0.35 nm per EO unit for the fully extended chain as in polyoxyethylene crystals, 0.19 nm per EO unit for the meander conformation.
MA
Scattering patterns of ca. 1 wt% aqueous solutions are shown in Fig. 5, additional curves determined in a 150 mM NaCl solution are given in the supporting information. At high pD, the scattering behavior of the different surfactants is similar, and a peak is found at q p = 0.3−0.4 nm−1 due to the electrostatic repulsion between the micelles. This value
ED
corresponds to a most probable distance of 2π/q p ∼ 15 − 20 nm and a dehydrated micelle volume of ν = 8π3 φ/q p of 40-100 nm3 , with φ being the surfactant volume fraction. At lower pD, the surfactants self-assemble into very different structures.
PT
C12 E10 CH2 COOH is the surfactant with the largest head-to-tail size ratio investigated in this work and, accordingly, it forms the aggregates with the largest curvature. The micelles formed are slightly anisotropic at low pD but
AC CE
become increasingly spherical as the pD is increased. With increasing pD, the shell thickness decreases from ca. 1.6 to 1.4 nm with an approximately constant hydration of 60-70 vol% D2 O in the shell. The micelles at low pD are structurally very similar to those formed by the nonionic C12 E10 [104]. The addition of 150 mM NaCl appears to have no effect on the micelle size and shape. C18:1 E9 CH2 COOH has a similar headgroup to C12 E10 CH2 COOH but a longer alkyl chain and the self-assembled structures vary with pD. In particular, at low pD, when the surfactant is found in its nonionic form, cylindrical micelles are found, as evidenced by the large q−1 region in the SANS patterns. Increasing the degree of ionization of the surfactant causes a transition from rodlike to ellipsoidal micelles. The transition from rods to ellipsoids causes a reduction of the shell thickness from 1.8 to 1.5 nm, while the shell hydration increases from 50 to 70 vol% D2 O. The structures show only minimal changes upon addition of 150 mM NaCl or 1 mM CuCl2 [66]. C12 E4.5 CH2 COOH features the smallest headgroup of the investigated surfactants. The surfactant self-assembles into planar structures up to pD ∼ 5 as evidenced by the large q−2 region in the SANS spectrum. The SANS patterns can be described using a model of large polydisperse unilamellar vesicle at pD < 5. The overall size of the vesicle is outside of the probed q-range and the value given for the radius (200 nm) has no physical meaning. The addition of 150 mM NaCl at pD 4.3 triggers the formation of small vesicles with low polydispersity, and a core radius of ca. 18 nm. Globular oblate ellipsoidal micelles appear at pD > 4.3, which become increasingly spherical with increasing pD. 14
ACCEPTED MANUSCRIPT
Table 3: Summary of fit parameters used for the description with an ellipsoidal model of the scattering curves reported in Fig. 5. χ is the fraction of surfactant in the globular micellar state; the ionic strength I is given in 10−3 mol L−1 , the ellipsoidal rotational semi-axis A, equatorial semi-axis B,
T
shell thickness T, and the hard sphere radius RHS are given in nm; Z is the charge per micelle, given in elementary charge units determined from the
RI P
structure factor, Nagg is the number of molecules per aggregate; αS (q) is given by Z/Nagg ; β = Z/αNagg is the fraction of noncondensed counterions,
B
T
RHS
1
8
2.0
3.7
1.6
1.6
3.5
1
7
2.8
3.6
1.6
1.6
1
6
4.2
2.8
1.6
1.6
1
4
4.8
2.2
1.6
1
4
5.2
2.1
1.6
1
4
5.9
1.6
1.6
1
5
11.4
1.6
1
153
2.0
1
149
4.8
1
150
+NaCl
φwshell
αS (q)
β
Acs h
Ahsw
0.0
117
0.58
0.00
—
53
161
3.7
0.0
114
0.58
0.00
0.00
54
163
4.3
20.0
88
0.62
0.23
0.56
56
183
1
8
1
6
NU 4.4
19.0
69
0.66
0.27
0.46
58
209
1.6
4.2
19.5
66
0.67
0.29
0.43
59
215
1.6
4.2
21.3
50
0.71
0.42
0.47
64
255
1.5
1.4
4.0
20.8
44
0.67
0.47
0.47
67
244
3.8
1.6
1.6
3.5
—
47
0.77
—
—
82
159
2.7
1.6
1.6
3.7
—
85
0.63
—
—
56
187
MA
1.6
2.3
1.5
1.5
3.5
—
64
0.64
—
—
61
210
2.0
500
2
1.8
—
0.0
18000
0.49
0.00
—
54
103
2.8
60
2
1.7
—
0.0
2200
0.47
0.00
0.00
54
103
5
4.4
4.0
2.0
1.5
5.3
28.2
145
0.58
0.20
0.56
59
149
4
5.1
3.8
2.0
1.5
5.0
26.8
137
0.59
0.20
0.39
60
152
1
5
6.1
3.0
2.0
1.5
4.9
25.4
108
0.63
0.23
0.27
62
170
1
7
11.7
2.9
2.0
1.5
5.1
30.5
105
0.63
0.29
0.29
63
173
1
152
2.0
500
2.0
1.8
0.0
—
18000
0.49
—
—
54
103
1
149
4.4
8.5
2.0
1.7
3.6
—
307
0.57
—
—
56
127
1
152
11.7
3.3
2.0
1.7
4.8
—
119
0.68
—
—
61
179
0
8
2.0
—
—
—
—
—
—
—
—
—
—
—
0
6
2.7
—
—
—
—
—
—
—
—
—
—
—
0.6
5
4.3
1.6
6.0
0.9
—
—
710
0.06
—
—
37
52
0.85
4
4.8
1.6
3.8
0.9
5.8
24.0
285
0.28
0.08
0.12
41
69
0.925
4
5.0
1.6
3.5
0.8
5.6
27.1
241
0.21
0.11
0.14
43
70
0.985
4
5.3
1.6
3.1
0.8
4.5
27.4
189
0.27
0.14
0.17
45
76
1
5
6.6
1.6
2.3
0.8
3.8
24.4
104
0.42
0.23
0.23
51
99
1
7
11.7
1.6
2.2
0.8
3.8
25.6
95
0.44
0.27
0.27
53
103
0
151
2.0
—
—
—
—
—
—
—
—
—
—
—
0
148
4.3
—
—
—
—
—
—
—
—
—
—
1
152
11.7
1.6
— 15 2.6 0.8
4.3
—
133
0.36
—
—
24
54
AC CE
1
C12 E4.5 CH2 COOH
Nagg
11.4
1
+NaCl
Z
SC
A
prolate ellipsoid −→ cylinder
C18:1 E9 CH2 COOH
pD
ED
+NaCl
I
PT
C12 E10 CH2 COOH
χ
oblate ellipsoid −→ bilayers/vesicles
respectively, given in Å2 .
sphere −→ prolate ellipsoid
and Ahsw are the areas per surfactant molecule at the core-shell and at the shell-water interface, φwshell is the volume fraction of D2 O in the shell; Acs h
ACCEPTED MANUSCRIPT
Table 4: Summary of fit parameters used for the description with a vesicle model of the scattering curves arising from 1 wt% solutions of C12 E4.5 CH2 COOH. 1-χ is the fraction of surfactant in the vesicular state; the ionic strength I is given in 10−3 mol L−1 ; the core radius Rc ,
mixture of large (Rc >200 nm) and small (Rc = 6.5 nm) vesicles.
I
pD
Rc
σ
1.00
0.2
The pattern recorded at pD 2.7 was described as a
TEO
TALK
hRi
φwshell
1.2
1.7
210
0.29
1.2
1.7
210
0.29
2.0
>200
0.67
6
2.7
>200
0.2
0.33(a)
6
2.7
8.1
0.2
1.2
1.7
8.3
0.29
0.28
5
4.3
>200
0.2
1.2
1.7
210
0.29
0.15
4
4.8
>200
0.2
1.2
1.7
210
0.29
0.08
4
5.0
>200
0.2
1.2
1.7
210
0.29
0.02
4
5.3
>200
0.2
1.2
1.7
210
0.29
0.00
5
6.6
—
—
—
—
—
—
0.00
7
11.7
—
—
—
—
—
—
1.00
151
2.0
>200
0.2
1.2
1.7
204
0.29
1.00
148
4.3
19
0.2
1.2
1.7
19.9
0.29
0.00
152
11.7
—
—
—
—
—
—
NU
PT
ED
+NaCl
SC
8
(a)
MA
C12 E4.5 CH2 COOH
1-χ
(a)
RI P
bution; hRi is the mean core radius in nm; φwshell is the volume fraction of D2 O in the shell.
T
thicknesses of the hydrophilic TEO and hydrophobic layer TALK are given in nm; σ is the standard deviation of the core radius lognormal distri-
Until pD < 5 they coexist with vesicular aggregates, in agreement with the previous studies of Kunz et al.[64, 65]. It
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should be remarked that, due to the coexistence of different types of structures, the SANS analysis is not univocal for intermediate pD regions, and the sizes and relative amounts of the different structures should be treated with caution. The gradual transition from large vesicular aggregates to small micelles however, is unequivocal. Aggregation number and micelle size of C12 E4 SO4 Na or C12 E6 SO4 Na are similar to those found for C12 E4.5 CH2 COOH at high pH, i.e., where the surfactant headgroup is fully charged.[105] In summary, AECs self-assemble in dilute solutions into a wide range of morphologies, depending on the pD of the solution and on the size ratio of their hydrophilic and hydrophobic parts. In particular, the spontaneous formation of vesicles by a single anionic surfactant at such mild pD values is a rare finding. By addition of appropriate amounts of dodecanoate, the pD range in which vesicles are found can be extended[64]. The structures formed by the investigated AECs are graphically summarized in Fig. 6 as a function of the volume fraction of the hydrophilic part of the surfactant and the degree of ionization. On the one hand, VEO /Vtot is directly related to the spontaneous curvature of nonionic Ci Ej surfactants and was already successfully employed to rationalize their the phase behavior[106–108]. On the other hand, the degree of ionization determined by potentiometric titrations quantifies the electrostatic contributions to the headgroup requirement and, accordingly, its influence on size and shape of the self-assembled aggregates. It should be noted here, that the choice of these two variables does not do justice to the complexity of system. Nevertheless, 16
ACCEPTED MANUSCRIPT
T
0.8
RI P
0.6 0. 6 0.4 0. 0.2
0.5
0.55
0.6
0.65 0
NU
0.0 0. 0 0.45 45
SC
degree of ioni de ionization nization on
1.0
VEO/Vtot
Figure 6: Summary of SANS-models used for the description of the self-assembled aggregates of polyoxyethylene alkyl ether carboxylic acids as
MA
a function of degree of ionization α and volume fraction of the hydrophilic part of the surfactant. For details of the model used see Tables 3 and 4. Dotted lines are guide for the eye. , N, and • represent samples with anisotropy values less than 1.5, between 1.5 and 3, and larger than 3, respectively. H represents samples forming bilayers. represents samples where the coexistence of vesicles and globular micelle was observed.
ED
Sketches are not to scale.
it offers a simple approach to rationalize the two nonorthogonal variables to predict the shape of the self-assembled
PT
surfactant aggregates.
Few studies on concentrated AECs systems are reported in litterature[36, 69]. The investigation of the phase behavior of polydisperse C12,13 E4 CH2 COOH and C12-15 E6 CH2 COOH revealed the presence of extended gel regions[36].
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A more detailed investigation by polarized optical microscopy and small-angle X-ray scattering was performed on the monodisperse sodium salts of C14 E4 CH2 COOH and C14 E7 CH2 COOH in water at 25 ◦ C[69]. The results are summarized in Fig. 7 together with the phase behavior of the strongly anionic C12 E2 OSO3 Na[109] and sodium dodecyl sulfate C12 OSO3 Na, often abbreviated as SDS or SLS[110], and the nonionic C14 E8 [111] and C12 E3 and C12 E5 [26] surfactants. The sodium salts of the AECs show a similar behavior, with the presence of an isotropic micellar solution L1 at concentrations . 25 wt%, followed by a hexagonal H1 phase, which is more extended for the surfactant with the larger headgroup. Both surfactants present a rather extended lamellar Lα phase at concentrations between ∼ 40 and 80 wt%, followed by a sponge L3 phase at high concentration. Only for C14 E4 CH2 COOH a reverse cubic phase V2 at high concentration is reported, although it should be noted that this phase can be easily overseen. In contrast and despite the fewer number of ethylene oxide units, the strongly anionic C12 E2 OSO3 Na forms phases with a larger curvature over the whole concentration range, with the boundaries between the L1 and Lα phases and the Lα and H1 being located at higher surfactant concentration. It is however evident, that the introduction of the 2 ethylene oxide units between the dodecyl chain and the sulfate headgroup enriches the phase behavior, mainly by preventing the crystallization of the surfactant. To be noted, this surfactant is of technical purity. Moreover, a comparison between C14 E4 CH2 COONa and C12 E5 reveals that the ionic headgroup stabilizes the hexagonal phases at low to moderate 17
ACCEPTED MANUSCRIPT
L1
S+L1
L1
H1
L1 + L
C12E3
L1
C14E4CH2COONa
L
C14E7CH2COONa
H1
25
50
V2
L2
L3
V1
SC
0
L H1
L1
C14E8
L3
L
L1 L1
L2
L H1
C12E5
L
T
C12E2OSO3Na
RI P
C12OSO3Na
L
75
L
+S 100
Surfactant concentration wt%
NU
Figure 7: Simplified representation of the phase behavior of alkyl sulfates (green color), alkyl carboxylates (blue color) and nonionic alkyl ethoxylates (red color) sorted by number of EO units, taken from different studies performed at 25 ◦ C in water: the sodium salts of C14 E4 CH2 COOH and C14 E7 CH2 COOH from Ref. 69, the strongly anionic C12 E2 OSO3 Na (technical grade) from Ref. 109, and C12 OSO3 Na from Ref. 110 and the
MA
nonionic C14 E8 from Ref. 111 and C12 E3 and C12 E5 from Ref. 26. Phase notations are: L1 for the isotropic micellar solution; H1 for the hexagonal phase; Lα for the lamellar phase; L2 for the hydrated liquid surfactant phase; V1 for the cubic phase; L3 for the sponge phase; V2 for the reverse cubic phase, and S the for solid surfactant phase.
ED
concentrations. The transition between lamellar and reverse phases takes place at similar concentrations. In addition, the comparison between C14 E7 CH2 COONa and C14 E8 shows that AECs form hexagonal and lamellar phases at lower surfactant concentration than their nonionic counterpart. This observation is further supported by a comparison with
PT
the extended phase diagram of polyoxyethylene dodecyl- and oleyl- ethers, which shows that the lowest concentration at which Ci Ej form hexagonal phases in water at 25 ◦ C is ca. 40 wt% for dodecyl ethers[108] and 25 wt% for
AC CE
oleyl ethers[107]. It should be kept in mind that the comparison between the different classes of surfactants is only indicative and a direct correlation cannot be made, as experimental parameters such as temperature, ionic strength, pH, strongly affect the behavior of the surfactants in a very different fashion. 3.4. Temperature responsiveness
The thermoresponsive properties of 1 wt% aqueous solutions of C12 E10 CH2 COOH, C18:1 E9 CH2 COOH, and C12 E4.5 CH2 COOH at different pH were probed by following the turbidity of the solution by recording the light scattering intensity at an angle of 90◦ (see Fig. 8). Solutions of C12 E10 CH2 COOH show a cloud point temperature of 68 ◦
C at pH 2.5, while no micelle growth can be observed at higher pH values, when the surfactant is partially or fully
charged. C18:1 E9 CH2 COOH, with a more hydrophobic alkyl chain, exhibits its cloud point at pH 2.5 at 47 ◦ C. At pH 4.1 (α = 0.4) a micelle growth is observed to start at ca. 55 ◦ C and terminate with phase separation at 92 ◦ C. The structures are not sensitive to temperature at higher pH values. Solutions of C12 E4.5 CH2 COOH can be seen to exhibit a different behavior. Turbid solutions are found at low pH, and a precipitate slowly forms at 25 ◦ C, 1 wt%, and pH < 3.5, as reported by Kunz et al.[65]. When heated, the solution shows constant turbidity up to 55 ◦ C, at which point all surfactant precipitates leaving a clear solution. At pH 4.1 a constant turbidity is recorded over the whole probed temperature range. Interestingly, at pH 5.0, a bluish solution at low temperature becomes clear for T > 20 ◦ C. A further 18
ACCEPTED MANUSCRIPT
pH = 2.5 4.1 5.0 8.0
RI P
103
102
101
SC
Mean count rate / 103 cps
pH = 2.5 4.1 5.0 8.0
T
pH = 2.5 4.1 5.0 8.0
104
100 20
40
60
80
100 0
20
40 60 Temperature / °C
NU
0
80
100 0
20
40
60
80
100
MA
Figure 8: Scattering intensity determined at a scattering angle of 90◦ as function of temperature for 1 wt% solutions of (from left to right) C12 E10 CH2 COOH, C18:1 E9 CH2 COOH, and C12 E4.5 CH2 COOH at different pH values.
increase in temperature causes a slight micellar growth. Finally, at high pH, C12 E4.5 CH2 COOH self-assembles into
ED
small micelles whose size is temperature independent. The cloud points determined at pH 2.5, where α ∼ 0.0, is lower by ca. 20-30 ◦ C than that of their nonionic alkyl ether analogs, as shown in Table 5. A similar reduction in cloud point temperature was found for nonionic
PT
alkyl ethers where the terminal OH group is substituted with an OCH3 moiety[112–114]. The lower cloud point temperature of methyl-capped alkyl ethers with respect to the conventional, hydroxy-terminated ones, is ascribed to
AC CE
different hydration properties, with the methyl-capped ones showing smaller hydration numbers[114]. Table 5: Cloud point temperatures recorded for 1 wt% aqueous solutions of C12 E10 CH2 COOH, C18:1 E9 CH2 COOH, and C12 E4.5 CH2 COOH at different pH values. The values for the corresponding nonionic alkyl ether surfactants, which are pH-independent, are reported for comparison and were measured in water at a concentration of 1 wt%.
pH = 2.5
pH = 4.1
pH = 5.0
pH = 8.0
C12 E10 CH2 COOH
68◦ C
—
—
—
C18:1 E9 CH2 COOH
47◦ C
>92◦ C
—
—
C12 E4.5 CH2 COOH
<5◦ C
<5◦ C
—
—
C12 E10 from Refs. 1, 33, and 115:
77-88 ◦ C
C18:1 E10 from Refs. 116, 31, and 117:
72-85 ◦ C
C12 E5 from Refs. 118, 119, and 27:
30-32 ◦ C
The molecular mechanism underlying the clouding phenomenon in dilute nonionic surfactant solutions has been intensively discussed[4, 25, 27, 120]. For what concerns nonionic, polyethylene oxide-based surfactants, the phaseseparation is ascribed to the reduced hydrophilicity and progressive dehydration of the EO units with increasing 19
ACCEPTED MANUSCRIPT
temperature. However, the nature of the critical fluctuations giving rise to the turbidity of the dilute solutions of Ci Ej solution is less evident and still a matter of debate. Different mechanisms apply as a function of the surfactant
T
headgroup size: on the one hand, amphiphiles with a smaller headgroup were reported to continuously grow into long,
RI P
wormlike micelles[119, 121]; on the other hand, this growth is limited for alkyl ethers with a larger headgroup and small, globular micelles tend to associate in large clusters near the cloud point[121, 122]. However, the mechanisms are not mutually exclusive, and both micelle growth and clustering, albeit to different extents, take place in dilute Ci Ej
SC
solutions near the cloud temperature. With this in mind, it is likely that the cylindrical micelles found at 25 ◦ C and low pH in dilute solutions of C18:1 E9 CH2 COOH behave similarly to C16 E6 or C12 E5 aggregates, growing in length with
NU
increasing temperature and forming an extended network of wormlike micelles at the cloud temperature[121, 123]. Differently, the cloud behavior of C12 E10 CH2 COOH, with a larger headgroup as compared to the hydrophobic chain, is likely to resemble to that of C12 E8 , whose aggregation numbers are constant over an extended range of temperatures
MA
and micelle growth and clustering takes place only close to the cloud point[121, 122].
4. Colloidal building blocks and technical uses
ED
4.1. Building blocks for colloidal supramolecular structures In the previous section, it was shown that polyoxyethylene alkyl ether carboxylic acids can self-assemble into
PT
diverse structures, depending on pH, ionic strength, and length of the hydrophilic and hydrophobic blocks. Moreover, for surfactants with a sufficiently large headgroup, e.g., C12 E10 CH2 COOH, the charge density of the micelle can be varied with minimal effects on its curvature. This is rarely the case, as the electrostatic contribution to the headgroup
AC CE
area is, for most ionic surfactants, the dominant term[41, 124, 125]. Typically, strong variations in size and shape are observed when the ionic strength of the solution is increased, or upon addition of counterions which condense at the micellar interface[126–129]. This effect is even more pronounced when the counterion takes the form of a polyelectrolyte chain[130–132], which triggers the transition from globular to cylindrical aggregates[133–136]. When AECs are admixed to the oppositely charged polycation chitosan (poly d-glucosamine) they co-assembly into hierarchical, complex structures[21, 63]. However, the shape and size of the micellar aggregate is only minimally affected by the presence of chitosan for the investigated C8 E5 CH2 COOH, C12 E10 CH2 COOH, C12 E4.5 CH2 COOH, and C18:1 E9 CH2 COOH surfactants. The presence of a non-negligible steric contribution to the headgroup area Ah therefore allows to systematically investigate the effects of charge density and curvature of the micellar aggregate on the co-assembled structures with oppositely charged polyelectrolytes. The effect of the degree of ionization was investigated in detail on chitosan complexes with C18:1 E9 CH2 COOH[63], while the effect of curvature at constant ionization degree was studied on chitosan complexes with C12 E10 CH2 COOH, C12 E4.5 CH2 COOH, and C18:1 E9 CH2 COOH at pH 4.0[21]. A structural phase diagram for the chitosan-C18:1 E9 CH2 COOH complexes, determined as a function of pH and mixing ratio Z = [surfactant molecules]/[chitosan chargeable units], is shown in Fig. 9. Three main co-assembled structures can be identified: in the bottom-left corner, at low mixing ratio 20
T
0.5
RI P
0.4 0.3 0.2 0.1 0 3.6
3.9
4.2
4.5
4.8
NU
3.3
SC
-
+
[Surf ]/[Chitosan ]
ACCEPTED MANUSCRIPT
5.1
pH
MA
Figure 9: Structural phase diagram depicting the different structures found in mixtures of C18:1 E9 CH2 COOH and chitosan as a function of pH and mixing ratio Z. Experiments were performed at a constant chitosan concentration of 0.3 wt%, in an acetic acid/sodium acetate buffer of 0.2 mol L−1 . The different symbols represent sample composition investigated by small-angle neutron scattering. See Ref. 63 for full details. Adapted with
ED
permission from Ref. 63. Copyright 2017 American Chemical Society.
and low surfactant content, the micelles are randomly distributed in solution, and the chitosan network is preserved; in the central region of the phase diagram, one-dimensional aggregates are formed, where aligned micelles are glued
PT
together by chitosan chains. An appropriate scattering form factor for such object is available[137]. Finally, in the top-right corner, at high degree of ionization and high surfactant content, the aggregates are collapsed into a core-
AC CE
corona suprastructure, with a core formed of densely packed micelles surrounded by a stabilizing chitosan corona. Similar structures are often found in complexes of polyelectrolyte-b-neutral block copolymers and oppositely charged surfactants as a consequence of microphase-separation[138–141], but only rarely when homo-polyelectrolytes are involved[142, 143].
The structural evolution observed in chitosan - C18:1 E9 CH2 COOH is also found in complexes with other alkyl ether carboxylic acids self-assembling into globular micelles, such as C8 E5 CH2 COOH or C12 E10 CH2 COOH[21]. However, when AECs assemble into vesicles, e.g., C12 E4.5 CH2 COOH at pH ∼ 4, the formation of ordered multilayered vesicles with a well-defined interlayer distance has been observed[21]. The average number of layers forming each vesicles depends on the mixing ratio. Such structures have a significant potential as drug delivery agents and for cosmetic applications, as they may be loaded with a hydrophilic cargo in the internal lumen, a charged one in the PE-surfactant layer, and a lipophilic load within the hydrophobic alkylic layer. AEC - chitosan complexes exhibit an exceptional structural variety in solution, but they also exhibit a remarkable high solubility when compared to mixtures of chitosan with strong anionic surfactants, like sodium dodecyl sulfate[144–146]. The high solubility and the mildness of the surfactant combined with the beneficial properties of chitosan make the chitosan - alkyl ether carboxylic acid complexes highly interesting systems for detergency and body care, as demonstrated by diverse patents in the field[147–149]. Similarly, highly soluble catanionic surfactant 21
ACCEPTED MANUSCRIPT
systems with unique properties can be expected from mixtures of AECs with oppositely charged, ethoxylated cationic surfactants.
T
AECs are not only very versatile with respect to their non-covalent interaction properties, but the broad reactivity
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of the carboxylic acid termination allows for simple chemical modifications. For instance, nonionic, thermoresponsive surfactants with a chelating headgroup were prepared by coupling alkyl ether carboxylic acids with an amino acid derivative for the recovery of metal ions[150–152]. Amino acid surfactants are often prepared by coupling a single
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or a short sequence of amino acids with fatty acids, resulting in interesting, green surfactants[153–155]. The use of AECs enables the insertion of some ethylene oxide units between the hydrophobic chain and the amino acid surfactant,
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conferring upon them a more hydrophilic character and thermoresponsive properties. The same concept applies to covalent coupling with the carboxylic headgroup of fatty acids with polymers[156–159], nanoparticles[160–162], or surfaces[163, 164]. In particular, the covalent conjugation of carboxylic acids to an amine-functionalized surface
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via 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC) activation, allows for a simple and very versatile surface modification[164, 165]. AEC-modified surfaces are potentially attractive as supports for tethered lipid membranes, separated from the solid surface by a soft, hydrated spacer of tunable size. Such bilayers, deposited on a hydrated,
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soft cushion, represent excellent model systems to study cell membranes[166–169]. They are also the active elements of highly sensitive biosensors[170, 171].
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4.2. Practical applications
In the previous sections, a comprehensive characterization of AECs has been given. The properties of AEC-water
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solutions depend on a large number of factors, most notably pH and temperature, as well as the size of the hydrophilic and hydrophobic blocks of the surfactant. In addition of being highly tunable, their high solubility in hard water, high salinity, and acidic as well as alkaline conditions, makes them suitable for a wide scope of applications[34, 67]. A comprehensive summary of industrial applications of polyoxyethylene alkyl ether carboxylic acids is given – in german language only – in a review by Czichocki et al.[67] and in a more recent book chapter[34]. The most notable field of application is as household and industrial cleaning agents. This is due to their remarkable mildness, resistance to hard water conditions, high temperatures, the presence of bleaching and oxidizing agents, in addition to their good foaming, wetting, and emulsification properties[34, 36, 172–180]. Moreover, their high compatibility with cationic polyelectrolytes makes them a valuable ingredient for the formulations of shampoos, and skin care, hair conditioning and strengthening component in cosmetic preparations[147–149, 173, 179]. In particular, with respect to alkyl ether sulfates, AECs were reported to cause lower swelling of human stratum corneum and a minor denaturation of the skin proteins[173, 181]. The studies also show that optimal properties are achieved by mixing of alkyl ether carboxylates and alkyl ether sulfates. These mixtures show similar mildness, but better foaming and cleansing properties[173, 181, 182]. Another important field of application of AECs is in enhanced oil recovery[67, 183–188]. In order to extract residual oil from small pores, a sufficiently low interfacial tension between water and oil must be achieved[189]. 22
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Values as low as 10−2 − 10−3 mN m−1 are reported for C12 E3 CH2 COOH[190]. Even lower values are obtained in mixtures with nonionic surfactants[191]. The reason for the use of AECs in oil recovery, is their ability to form stable
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emulsions also in the presence of large amounts of monovalent and divalent cations and high temperature[44, 186,
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190], conditions at which many efficient sulfated and nonionic surfactants fail[185]. Improved performance can be obtained using branched alkyl chains, for instance, based on Guerbet alcohols[44, 192].
This multifunctional class of surfactants finds application beyond oil recovery and home and body care products.
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For instance, they are used as additives during textile processing[193, 194], as lubricants[195–197], or as dispersant for dyes, inks, and pesticides[198–201]. AECs with low ethylene oxide content were shown to be promising scale
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inhibitor agents[202]. As for the previously mentioned applications, their use is justified by their good wettability, emulsification, and solubilization capacity also in conditions where many other surfactants fail. For a more detailed description of the large-scale use of AECs we address the reader to the work of Meijer et al.[34].
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In addition to their widespread on an industrial scale, academic research on the use of AECs for a variety of purposes is being carried out. Very promising results have been obtained regarding their use for pollutant recovery[62, 66, 144, 203, 204]. C18:1 E9 CH2 COOH has been shown to be particularly effective for the removal of heavy metal
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cations in micellar enhanced ultrafiltration (MEUF)[66, 204]. MEUF is a membrane-based separation technique, in which the wastewater, enriched with surfactants, is filtered through a membrane which enables the separation of the surfactant micelles and associated components from the solution[205, 206]. A high efficiency is obtained
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when surfactants with a low cmc and a high affinity towards the pollutant are used. A widely used surfactant for the removal of heavy metal ions is sodium dodecyl sulfate (SDS). However, to decrease the amount of surfactant
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required it is a common procedure to add a nonionic surfactant to the mixture, with the aim of reducing the cmc[207– 209]. Polyoxyethylene alkyl ether carboxylic acids have three major advantages, when compared to other commonly employed systems: (i) surfactants with a long alkyl chain can be used, exhibiting very low cmc values; (ii) the affinity between surfactant and metal ion is pH dependent and can be exploited for a selective separation of complex metal ion mixtures[204]; and (iii) surfactant – metal pairs can be easily separated and the surfactant recovered for the next cycle by simply decreasing the pH of the solution and increasing the temperature above the cloud point of the surfactant[66]. Metal ions can also be extracted from solution by ion foam flotation[210]. The procedure consists of bubbling a gas, usually air, into a surfactant solution and separating the resultant foam, enriched in surfactant and its associated counterions. Studies on C18:1 E9 CH2 COOH have demonstrated its high efficiency for ion foam flotation applications[62, 203].
5. Conclusions Polyoxyethylene alkyl ether carboxylic acids are highly versatile, multiresponsive surfactants, which combine in a single molecule, peculiarities of pH-responsive fatty acids, and temperature responsive nonionic polyoxyethylene alkyl ethers. This gives rise to a complex, multiresponsive behavior in aqueous solutions. The surfactants properties 23
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change strongly with pH, as the magnitude of the charge of its headgroup is varied. At low pH, the behavior of AECs is similar to nonionic alkyl ethers, exhibiting similar cmc values and temperature-responsive behavior. With increasing
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pH, the surfactant headgroup is increasingly charged, thus affecting the properties of the surfactant solutions. The
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extent of the structural changes on pH variation depends on the number of ethylene oxide units in the headgroup. The transition from lamellar aggregates to globular micelles via well-defined vesicles was shown for C12 E4.5 CH2 COOH, while only minor changes in shape and size were observed for C12 E10 CH2 COOH. Soft, globular particles, whose
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degree of charge can be systematically varied without affecting their shape and size form a rare and valuable model system for studying charge-driven assembly processes[63]. The surfactants exhibit pKa values around 4, which allows
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for the fine-tuning of the surfactants properties in a pH range particularly useful for cosmetic application[64, 65]. The high reactivity of the carboxylic acid termination enables the preparation of surfactants tailored to specific purposes, e.g., the thermoresponsive and chelating amino-acid modified AECs for the recovery of uranyl ions[150,
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151]. However, the possibilities offered by the chemical conjugation of AECs, e.g., (i) with single amino acids or peptides, for the preparation of temperature-responsive functional surfactants; (ii) with solid surfaces, to prepare tethered lipid membranes with a variable hydrophilic cushion; (iii) with polymers, for the preparation of hydrophilic
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and temperature-responsive drug carriers, have scarcely been explored. AECs, in addition to being very interesting components for colloidal scientists, are widely used in large-scale, industrial applications. Due to the simultaneous presence of the ethylene oxide units and the carboxylic acid termina-
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tion, clear solutions of surfactants with long alkyl chains can be obtained in alkaline, as well as acidic solutions which are remarkably stable in hard water and high salinity conditions. Further advantages are given by their mildness, low
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toxicity, stability to hydrolysis and oxidizing agents and high temperatures. For these reasons, in addition to their low production cost, they find application in different fields, most notably as detergents and in home and body-care products, for enhanced oil recovery and as additives in the textile and metal processing industry. In summary, polyoxyethylene alkyl ether carboxylic acids are highly interesting components in colloid and surfactant science. The complex and highly tunable morphologies of the self-assembled micelles can be exploited to formulate supramolecular aggregates with a degree of diversity rarely found in colloid science. From the perspective of large-scale applications, their mildness and high solubility have opened the door to numerous consumer products and industrial uses. This work provides a comprehensive overview of the physico-chemical properties of this class of surfactants, of their use as building blocks in colloidal chemistry, and of their industrial application, with the aim of facilitating their spread in current research activities.
Acknowledgments Sylvain Prévost, Ralf Schweins, and Pierre Bauduin are gratefully acknowledged for the useful discussions and suggestions. The partnership for soft condensed matter (PSCM) is acknowledged for providing the light scattering apparatus and the tensiometer, the Institut Laue-Langevin (ILL) for the allocation of SANS beamtime. LC acknowledges 24
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the TU-Berlin and the PSCM at the ILL for postdoctoral funding through a three-year cooperation agreement.
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Supporting Information
Additional details and characterization of the AECs used in this work; detailed description of the expressions used for the analysis of the SANS patterns and additional SANS curves; details on the calculations of the surface
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electrostatic potential of the surfactant micelles.
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Graphical abstract OH O
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Multiresponsive properties
Industrial Applications
Complex self-assembly
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Colloidal building blocks
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O pH-responsive
degree of io ionization on
Temperature-sensitive
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Headgroup volume fraction
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Highlights
• pH- and Temperature-responsive behavior is reported
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• Overview of physico-chemical properties of multiresponsive surfactant
• Extremely versatile building blocks for colloidal supramolecular assemblies
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• Review of technical and large-scale applications of the surfactants
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