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Effect of inorganic and organic counterions on interfacial properties of oleic acid-based gemini surfactants
Accepted Manuscript Title: Effect of Inorganic and Organic Counterions on Interfacial Properties of Oleic Acid-Based Gemini Surfactants Authors: Tadashi Sugahara, Yuichiro Takamatsu, Masaaki Akamatsu, Kenichi Sakai, Masahiko Abe, Hideki Sakai PII: DOI: Reference:
S0927-7757(17)30934-2 https://doi.org/10.1016/j.colsurfa.2017.10.035 COLSUA 21993
To appear in:
Colloids and Surfaces A: Physicochem. Eng. Aspects
Received date: Revised date: Accepted date:
11-8-2017 12-10-2017 17-10-2017
Please cite this article as: Tadashi Sugahara, Yuichiro Takamatsu, Masaaki Akamatsu, Kenichi Sakai, Masahiko Abe, Hideki Sakai, Effect of Inorganic and Organic Counterions on Interfacial Properties of Oleic Acid-Based Gemini Surfactants, Colloids and Surfaces A: Physicochemical and Engineering Aspects https://doi.org/10.1016/j.colsurfa.2017.10.035 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Effect of Inorganic and Organic Counterions on Interfacial Properties of Oleic Acid-Based Gemini Surfactants
Tadashi Sugahara,1 Yuichiro Takamatsu,2 Masaaki Akamatsu,1 Kenichi Sakai,1,3* Masahiko Abe3 and Hideki Sakai1,3* 1. Department of Pure and Applied Chemistry, Faculty of Science and Technology, Tokyo University of Science, 2641 Yamazaki, Noda, Chiba 278-8510, Japan. 2. Miyoshi Oil & Fat Co. Ltd., 4-66-1 Horikiri, Katsushika, Tokyo 124-8510, Japan. 3. Research Institute for Science and Technology, Tokyo University of Science, 2641 Yamazaki, Noda, Chiba 278-8510, Japan. Corresponding authors E-mail: [email protected], [email protected]
Graphical abstract
Abstract We studied the effect of counterion species of oleic acid-based gemini surfactants with carboxylate headgroups. Adsorption and self-aggregation properties were investigated by static surface tension, pyrene fluorescence, dynamic light scattering and phase diagram measurements. We found that, in the case of inorganic counterions (Li+, Na+, K+, and Cs+), a decreased ion size leads to a decreased critical micelle concentration (cmc) as well as a decreased area per surfactant molecule adsorbed at the air/aqueous solution interface (Acmc). The smaller inorganic counterions (hard acid) can interact with the carboxylate headgroups (hard base), and hence reduce the head-to-head electrostatic repulsion. The smaller counterions also induced the lower phase transition concentration from an optically isotropic solution (L1) phase to a hexagonal liquid crystal (H1) phase. In the case of organic counterions (monoethanolamine (MEA) and diethanolamine (DEA)), the micellization behavior was predominantly affected by the hydrophobicity of the counterions, whereas the hydrated ion size was a key parameter determining the molecular packing at the air/solution interface. In addition, we found a structural transformation from larger aggregates to smaller ones (micelles) to occur for the organic counterion systems at dilute surfactant concentrations.
Keywords Gemini surfactant, Oleic acid, Counterion, Surface tension, Liquid crystal, Acid soap
1. Introduction Gemini surfactants are a dimer of monomeric surfactants linked by a spacer at the level of hydrophilic headgroups [1,2]. In general, they have greater interfacial properties such as low critical micelle concentration (cmc) and high capability in reducing aqueous surface tension when compared to the corresponding monomeric surfactants. On the basis of such interfacial properties, structure-performance relationship in gemini surfactants has been of interest from various aspects including e.g., a structure and symmetry of hydrophobic chains [3–6], and a structure of spacer units [7–13]. The adsorption and micellization properties are also affected by the species of counterions and electrolytes in the case of ionic surfactants. It has been reported that the cmc is decreased as Li+ > Na+ > K+ > Cs+ in monomeric dodecyl sulfate salts [14–16], whereas an opposite tendency is observed in carboxylate salts (i.e., decreased cmc as Cs+ > K+ > Na+ > Li+ [17,18]). These results have been rationalized by the hard and soft acids and bases (HSAB) theory [19]. The relativity soft sulfate headgroup can interact with soft acids such as Cs+. This leads to the reduced head-to-head repulsion and hence to the decreased cmc. In contrast, the relativity hard carboxylate headgroups can interact with hard acids such as Li+, and hence the decreased cmc is observed for the hard-type counterions. Organic counterions also affect not only the interfacial properties but also the self-aggregation structures of surfactants. Oda et al. [20] reported that the Krafft temperature of cationic gemini surfactants is affected by a few factors of organic counterions such as polarizability, dehydrated ion size, and ionic morphology. Rodriguez-Abreu et al. [21] found that the formation of a liquid crystal occurs at a lower concentration for a monoethanolamine (MEA) salt than for a triethanolamine (TEA) salt in anionic gemini surfactants with carboxylate headgroups. Furthermore, Oda et al. [22,23] reported the formation of chiral ribbons in cationic gemini surfactants with chiral organic counterions. In this study, we have studied the effect of inorganic and organic counterions on the adsorption and self-aggregation properties of oleic acid-based gemini surfactants with carboxylic acid headgroups (CC-9,9-EsH10). Chemical structures of CC-9,9-EsH10-X are shown in Fig. 1, where X denotes the counterion species. We have developed the oleic acid-based surfactants and studied their aqueous solution properties [24–28]. We note that comprehensive knowledge about the effect of inorganic and organic counterions on the solution properties in gemini surfactants with carboxylate headgroups (including our oleic acid-based gemini surfactants) has not been reported,
yet.
Fig. 1 Chemical structures of CC-9,9-EsH10-X. (X+ = Li+, Na+, K+, Cs+, MEA+, DEA+, and TEA+).
2. Materials and Methods 2-1. Materials CC-9,9-EsH10 was synthesized according to the procedure reported in our previous paper[26]. CC-9,9-EsH10-X (X = Li+, Na+, K+, and Cs+) was obtained at pH 9 through the addition of an aqueous XOH solution. The aqueous CC-9,9-EsH10-X solution obtained here was dried through freeze-drying. CC-9,9-EsH10-X (X = MEA+, diethanolamine (DEA+), and TEA+) was obtained via a reaction between CC-9,9-EsH10 (1 eq) and MEA, DEA, or TEA (2 eq) in ethanol for 48 h at room temperature. Then the solvent was evaporated under a reduced pressure. We note that XOH and XCl (X = Li+, Na+, K+, and Cs+), MEA, DEA and TEA were purchased from Tokyo Chemical Industry and used without further purification. Pyrene was purchased from Kanto Chemical Co. The water used in this study was deionized with a Barnstead NANO Pure Diamond UV system and filtered with a Millipore membrane filter (pore size 0.22 m). 2-2. Methods Static surface tension of CC-9,9-EsH10-X (X+ = Li+, Na+, K+, Cs+, MEA+, and DEA+) was measured by the Wilhelmy method using a Krüss K100 auto surface tensiometer with a platinum plate. These measurements were performed at pH 9. In the inorganic counterion systems, 10 mmol dm-3 XCl was added as a background electrolyte. The surface tension was assumed to be equilibrated when the standard deviation in the surface tension became less than 0.1 mN m-1 over 1000 s. For pyrene I1/I3 measurements, 1 mmol dm-3 pyrene ethanol solution (20 L) was
added into CC-9,9-EsH10-X (X = MEA+ and DEA+) aqueous solutions (5 cm3). Then the sample solutions were shaken for 24 h in a water-shaker bath. The fluorescence spectra were recorded between 360 and 390 nm with an excitation wavelength of 330 nm using a Shimadzu RF-5300PC fluorescence spectrophotometer. These measurements were performed at pH 9. The hydrodynamic diameter of CC-9,9-EsH10-X (X = MEA+ and DEA+) assemblies was estimated using an IBC Nicomp 380ZLS particle size analyzer. These dynamic light scattering (DLS) measurements were performed at pH 9. Before each measurement, all sample solutions were filtered with a 1 μm cellulose acetate membrane filter. The phase diagrams of CC-9,9-EsH10-X (X+ = Li+, Na+, K+, Cs+, MEA+, and DEA+) were characterized by small angle X-ray scattering (SAXS) and polarized optical microscopy (POM) measurements. The SAXS measurements were performed using an Anton Paar SAXSess. The apparatus was operated at 40 kV and 50 mA using Cu-Kα X-rays (wavelength of 0.154 nm). The X-ray irradiation time was fixed at 20 min. The POM measurements were performed using an Olympus IMT-2 microscope. The measurement temperature was always maintained at 25 ºC.
3.Results and Discussion 3-1. Interfacial properties of CC-9,9-EsH10-X Fig. 2 shows the static surface tension of aqueous solutions of CC-9,9-EsH10-X (X = Li+, Na+, K+, and Cs+) as a function of their concentrations. For all of the surfactants, the surface tension (γ) decreased with increasing surfactant concentration (c) and became constant above a critical concentration. This concentration corresponds to the cmc of each surfactant. The surface excess concentration estimated at the cmc (Γcmc) and the occupied area per surfactant molecule adsorbed at the air/aqueous solution interface (Acmc) were calculated using the following equations [29].
𝛤𝑐𝑚𝑐 = −
1 𝜕𝛾 ( ) ・・・(1) 2.303𝑛𝑅𝑇 𝜕𝑙𝑜𝑔𝑐
𝐴𝑐𝑚𝑐 =
1 ・・・(2) 𝑁𝐴 𝛤𝑐𝑚𝑐
Here, T is the absolute temperature, NA is Avogadro’s constant, and R is the gas
constant. The number of adsorption species (n in equation 1) is assumed to be 1 in the presence of 10 mmol dm-3 XCl. The calculation results are shown in Table 1.
(a)
(b)
(c)
(d)
Fig. 2 Static surface tensions of (a) CC-9,9-EsH10-Li, (b) CC-9,9-EsH10-Na, (c) CC-9,9-EsH10-K, and (d) CC-9,9-EsH10-Cs aqueous solutions as a function of their concentrations. These measurements were performed at pH 9 in the presence of 10 mmol dm-3 XCl. The CC-9,9-EsH10-Na data were referred from reference 26.
Table 1. Surface properties of CC-9,9-EsH10-X calculated from the static surface tension measurements. cmc
γcmcb)
Γcmc
Acmc
(mol dm-3)
(mN m-1)
(μmol m-2)
(nm2)
CC-9,9-EsH10-Li
3.2
29
3.0
0.56
CC-9,9-EsH10-Na a)
3.7
31
2.6
0.63
CC-9,9-EsH10-K
4.2
32
2.6
0.64
CC-9,9-EsH10-Cs
6.8
32
2.3
0.73
a) From reference 26 b) Surface tension measured at cmc
One can see in Table 1 that the cmc and Acmc values increased with increasing radius of the counterions. The same tendency has been reported for the surfactants with the carboxylate headgroups [17,18]. As mentioned in Introduction, the HSAB theory predicts that the increased counterion radius leads to the decreased degree of the counterion binding to the relativity hard carboxylate headgroups. This necessarily results in the lower capability in forming micelles as well as the looser packing at the air/aqueous solution interface, as observed in Fig. 2 and Table 1. We also studied the effect of organic counterions (MEA+, DEA+, and TEA+) on the surfactant solution properties. Under the current experimental conditions, the TEA salt was insoluble in dilute aqueous solutions, whereas the MEA and DEA salts were observed to be soluble. This phenomenon has been similarly observed in the previous papers [21,30]. In the case of the TEA salt, the carboxylic acid headgroups are not fully dissociated because of the weak basicity of TEA+ [21]. Hereafter we only discuss the aqueous solution properties of the MEA and DEA salts. Fig. 3 shows the surface tension and pyrene fluorescence I1/I3 results. Interestingly, the surface tension data showed a minimum as a function of surfactant concentration. This behavior was further studied by the pyrene fluorescence I1/I3 measurements. The
I1/I3 value started to decrease at a concentration far below the minimum concentration, and attained a first plateau (Region II). Then the I1/I3 value was again decreased from the minimum concentration (Region III), and finally, attained a second plateau (Region IV). On the basis of these results, we assume a concentration-dependent growth of the surfactant aggregates to occur. It is interesting to note that the self-aggregation structure is changed at such dilute concentrations only for the organic salts. We call the two surfactant concentrations, attained to the first and second break points in the I1/I3 results, “cac1” and “cac2”, respectively.
(a)
(b)
Fig. 3 Static surface tension and pyrene fluorescence I1/I3 data of (a) CC-9,9-EsH10-MEA and (b) CC-9,9-EsH10-DEA aqueous solutions as a function of their concentrations. These measurements were performed at pH 9. The error bars in the I1/I3 data were estimated through three repeated measurements.
We studied the self-aggregation structure by means of the DLS technique. The DLS measurements were performed at the surfactant concentrations of 0.01 and 1 mmol dm-3. These concentrations lie in Region II (cac1 < 0.01 mmol dm-3 < cac2) and Region IV (cac2 < 1 mmol dm-3) in Fig. 3. The DLS results (Fig. 4) demonstrate that two types of aggregates coexist at 0.01 mmol dm-3 for the MEA and DEA salts; one is the aggregates
in ca. 100 nm hydrodynamic diameter and the other is smaller aggregates in ca. 10 nm diameter. The latter hydrodynamic diameter is consistent with the size of aggregates formed by CC-9,9-EsH10-Na, reported in our previous paper [26]. Interestingly, at 1 mmol dm-3 the smaller aggregates make a larger impact on the DLS results. This suggests that the larger aggregates, formed above the cac1, experience a structural transformation into the smaller aggregates at the concentration above the cac2.
(a)
(b)
Fig. 4 DLS results of (a) CC-9,9-EsH10-MEA and (b) CC-9,9-EsH10-DEA aqueous solutions. These measurements were performed at pH 9.
A similar structural transformation has been reported in aqueous surfactant systems with carboxylate [31] and phosphate headgroups [32], where the formation of “acid soap” is suggested. Acid soap is a complex (or a dimer) formed by protonated and deprotonated surfactants under a neutral or weak alkaline condition [33–35]. In Region I (shown in Fig. 3), it is hypothesized that both the monomer and the acid soap coexist in the aqueous solution [31]. Under this situation, the acid soap may predominantly adsorb to the air/aqueous solution interface because of its higher hydrophobicity. In Region II (above cac1), the acid soap starts to form their relatively large self-aggregates and the equilibrium between the acid soap and the aggregate is achieved in the solution phase [31].
The
monomer
concentration
also
increases
with
increasing surfactant
concentration at a given equilibrium constant between the acid soap and the monomer [31]. Therefore, the surface tension continuously decreases with increasing surfactant concentration. In Region III (above cac2), the monomer can form micelles [31]. The monomers are consumed by the micelle formation, so that the equilibrium between the monomer and the acid soap shift to the monomer [31]. Therefore, the static surface tension increases in Region III. In Region IV, the molecules at the air/aqueous solution interface intrinsically consist of their monomers, and hence the surface tension becomes constant. Here, we discuss the interfacial parameters calculated from the surface tension data (Table 2). The cac1 was lower for the DEA salt than for the MEA salt, whereas the cac2 was almost the same with each other. The lower cac1 of the DEA salt (compared with the MEA salt) suggests that the higher hydrophobicity of DEA+ induces greater association nature of the surfactant molecules, as was similarly reported by Warnheim et al. [30] and Moreno et al. [36]. The values of Γcac and Acac were calculated using equations 1 and 2. These values were calculated at cac2 in this case, and we abbreviate these parameters as Γcac2 and Acac2. These interfacial parameters are also shown in Table 2. The Acac2 value of the DEA salt is slightly larger than that of the MEA salt. Gao et al. [37] suggested that two effects of organic counterions cooperatively contribute to the degree of molecular packing at the air/solution interface; one is the hydrophobicity and the other is the hydration size of the organic counterions. They found that a larger Acac2 value (or a smaller Γcac2 value) is observed for the organic counterions having lesser hydrophobicity and larger hydration size. In our case, DEA+ is more hydrophobic and has larger hydration size than MEA+. From this point, the larger Acac2 value for the DEA salt primarily results from the hydration size of DEA+ rather than the difference in the hydrophobicity of the two counterions.
Table 2. Surface properties of CC-9,9-EsH10-X (X = MEA+ and DEA+) estimated by the static surface tension and pyrene fluorescence I1/I3 measurements. cac1
cac2
γcac2a)
Γcac2
Acac2
(mol dm-3)
(mol dm-3)
(mN m-1)
(μmol m-2)
(nm2)
CC-9,9-EsH10-MEA
6.3
25
33
3.1
0.53
CC-9,9-EsH10-DEA
4.0
25
33
2.8
0.60
a) The minimum surface tension is determined as γcac2.
In summary, the degree of molecular packing at the air/aqueous solution interface is determined by the balance between the hydrophobicity and hydration size of organic counterions. We found that, for the two organic counterions (MEA+ and DEA+), the hydration size (i.e., steric effect) is the predominant factor. In contrast, the ion size is the key parameter in the inorganic counterion systems. The hydration size of the inorganic counterions is relatively smaller than that of the organic counterions, and hence the steric factor does not make large impact on the packing degree in the inorganic systems.
3-2. Liquid crystal phase behavior Phase diagrams of CC-9,9-EsH10-X (X = Li+, Na+, K+, Cs+, MEA+ and DEA+) with water as a function of surfactant concentration are shown in Fig. 5. We note that the phase states were characterized on the basis of the SAXS and POM results, as shown in supporting information, Fig. S1. A few phases, including isotropic solution (L1) phase, hexagonal liquid crystal (H1) phase, and lamellar liquid crystal (Lα) phase, were observed. In the H1 phase region, the ratio of scattering vector q value was calculated as 1 : √ 3 : 2 : √7, and the fan-like texture was observed in the POM images. In the Lα phase region, the ratio of q value was calculated as 1 : 2, and the mosaic or maltese-cross texture was observed. In addition, the two patterns sometimes coexisted in the SAXS results at certain surfactant concentrations. The relative intensity of the observed peaks was changed as a function of surfactant concentration, and the pattern resembled with that of the H1 and Lα phases at the phase transition concentrations. In the series of the inorganic counterion systems, the L1-H1 phase transition occurs at higher surfactant concentrations with increasing ion radius. As mentioned in the previous section, the HSAB theory predicts that the increased ion radius induces the reduced interaction with the carboxylate headgroups, and hence the electrostatic head-to-head repulsion is not screened effectively. The observed concentration shift is
reflective of this difference in the interaction between the headgroups and the counterions. In the series of the organic counterion systems, the L1-H1 phase transition concentration was observed to be higher for the larger DEA salt than for the smaller MEA salt. A similar behavior has been reported for the monomeric and gemini surfactants having MEA+ or TEA+ as counterions [21]. The larger hydration size of the DEA salt disrupts the molecular packing within the aggregates, leading to a difficulty in forming the H1 phase.
(a)
(b)
Fig. 5 Phase diagrams of CC-9,9-EsH10-X (X = Li+, Na+, K+, Cs+, MEA+ and DEA+) with water as a function of surfactant concentration; (a) inorganic counterions and (b) organic counterions.
The structure of the H1 phase was further studied. We calculated the d-spacing values of the H1 phase, using the following equation; 𝑑 = 2𝜋⁄𝑞1 ・・・(3) where q1 is the value of the first scattering vector. Fig. 6 shows the d-spacing as a function of surfactant concentration. In the series of the inorganic counterion systems, the increased ion size resulted in the larger d-spacing at a given surfactant concentration. We hypothesize that the cylinder radius becomes larger with increasing ion radius as a result of the increased head-to-head electrostatic repulsion. In addition, the d-spacings of the MEA and DEA salts were almost the same with that of the Cs salt.
Fig. 6 d-spacing of CC-9,9-EsH10-X (X = Li+, Na+, K+, Cs+, MEA+ and DEA+) in the H1 phase as a function of surfactant concentration.
4. Conclusions In this study, we have discussed the effect of counterion species of oleic acid-based gemini surfactants on their adsorption and self-aggregation properties. In the inorganic counterion systems, the decreased counterion size resulted in the decreased cmc and
Acmc value. This indicates that the smaller counterion (hard acid) can interact with the carboxylate headgroups (hard base) more strongly, as predicted by the HSAB theory. This also leads to the lower L1-H1 transition concentration in the smaller inorganic counterion system. The effect of the organic counterions is more complicated because we have to take the following factors into consideration; one is the hydrophobicity and the
other is the hydrated counterion size. The experimental results indicate that the micellization behavior is primarily affected by the hydrophobicity of the counterions, whereas the packing at the air/solution interface is largely dependent on the hydrated ion size. In addition, we suggested that the structural transformation from larger aggregates into smaller aggregates (i.e., micelles) occurs at dilute surfactant concentrations in the organic counterion systems.
Acknowledgements We thank Dr. Avinash Bhadani and Dr. Koji Tsuchiya (Tokyo University of Science) for their helpful comments.
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version.
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S0927-7757(17)30934-2 https://doi.org/10.1016/j.colsurfa.2017.10.035 COLSUA 21993
To appear in:
Colloids and Surfaces A: Physicochem. Eng. Aspects
Received date: Revised date: Accepted date:
11-8-2017 12-10-2017 17-10-2017
Please cite this article as: Tadashi Sugahara, Yuichiro Takamatsu, Masaaki Akamatsu, Kenichi Sakai, Masahiko Abe, Hideki Sakai, Effect of Inorganic and Organic Counterions on Interfacial Properties of Oleic Acid-Based Gemini Surfactants, Colloids and Surfaces A: Physicochemical and Engineering Aspects https://doi.org/10.1016/j.colsurfa.2017.10.035 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Effect of Inorganic and Organic Counterions on Interfacial Properties of Oleic Acid-Based Gemini Surfactants
Tadashi Sugahara,1 Yuichiro Takamatsu,2 Masaaki Akamatsu,1 Kenichi Sakai,1,3* Masahiko Abe3 and Hideki Sakai1,3* 1. Department of Pure and Applied Chemistry, Faculty of Science and Technology, Tokyo University of Science, 2641 Yamazaki, Noda, Chiba 278-8510, Japan. 2. Miyoshi Oil & Fat Co. Ltd., 4-66-1 Horikiri, Katsushika, Tokyo 124-8510, Japan. 3. Research Institute for Science and Technology, Tokyo University of Science, 2641 Yamazaki, Noda, Chiba 278-8510, Japan. Corresponding authors E-mail: [email protected], [email protected]
Graphical abstract
Abstract We studied the effect of counterion species of oleic acid-based gemini surfactants with carboxylate headgroups. Adsorption and self-aggregation properties were investigated by static surface tension, pyrene fluorescence, dynamic light scattering and phase diagram measurements. We found that, in the case of inorganic counterions (Li+, Na+, K+, and Cs+), a decreased ion size leads to a decreased critical micelle concentration (cmc) as well as a decreased area per surfactant molecule adsorbed at the air/aqueous solution interface (Acmc). The smaller inorganic counterions (hard acid) can interact with the carboxylate headgroups (hard base), and hence reduce the head-to-head electrostatic repulsion. The smaller counterions also induced the lower phase transition concentration from an optically isotropic solution (L1) phase to a hexagonal liquid crystal (H1) phase. In the case of organic counterions (monoethanolamine (MEA) and diethanolamine (DEA)), the micellization behavior was predominantly affected by the hydrophobicity of the counterions, whereas the hydrated ion size was a key parameter determining the molecular packing at the air/solution interface. In addition, we found a structural transformation from larger aggregates to smaller ones (micelles) to occur for the organic counterion systems at dilute surfactant concentrations.
Keywords Gemini surfactant, Oleic acid, Counterion, Surface tension, Liquid crystal, Acid soap
1. Introduction Gemini surfactants are a dimer of monomeric surfactants linked by a spacer at the level of hydrophilic headgroups [1,2]. In general, they have greater interfacial properties such as low critical micelle concentration (cmc) and high capability in reducing aqueous surface tension when compared to the corresponding monomeric surfactants. On the basis of such interfacial properties, structure-performance relationship in gemini surfactants has been of interest from various aspects including e.g., a structure and symmetry of hydrophobic chains [3–6], and a structure of spacer units [7–13]. The adsorption and micellization properties are also affected by the species of counterions and electrolytes in the case of ionic surfactants. It has been reported that the cmc is decreased as Li+ > Na+ > K+ > Cs+ in monomeric dodecyl sulfate salts [14–16], whereas an opposite tendency is observed in carboxylate salts (i.e., decreased cmc as Cs+ > K+ > Na+ > Li+ [17,18]). These results have been rationalized by the hard and soft acids and bases (HSAB) theory [19]. The relativity soft sulfate headgroup can interact with soft acids such as Cs+. This leads to the reduced head-to-head repulsion and hence to the decreased cmc. In contrast, the relativity hard carboxylate headgroups can interact with hard acids such as Li+, and hence the decreased cmc is observed for the hard-type counterions. Organic counterions also affect not only the interfacial properties but also the self-aggregation structures of surfactants. Oda et al. [20] reported that the Krafft temperature of cationic gemini surfactants is affected by a few factors of organic counterions such as polarizability, dehydrated ion size, and ionic morphology. Rodriguez-Abreu et al. [21] found that the formation of a liquid crystal occurs at a lower concentration for a monoethanolamine (MEA) salt than for a triethanolamine (TEA) salt in anionic gemini surfactants with carboxylate headgroups. Furthermore, Oda et al. [22,23] reported the formation of chiral ribbons in cationic gemini surfactants with chiral organic counterions. In this study, we have studied the effect of inorganic and organic counterions on the adsorption and self-aggregation properties of oleic acid-based gemini surfactants with carboxylic acid headgroups (CC-9,9-EsH10). Chemical structures of CC-9,9-EsH10-X are shown in Fig. 1, where X denotes the counterion species. We have developed the oleic acid-based surfactants and studied their aqueous solution properties [24–28]. We note that comprehensive knowledge about the effect of inorganic and organic counterions on the solution properties in gemini surfactants with carboxylate headgroups (including our oleic acid-based gemini surfactants) has not been reported,
yet.
Fig. 1 Chemical structures of CC-9,9-EsH10-X. (X+ = Li+, Na+, K+, Cs+, MEA+, DEA+, and TEA+).
2. Materials and Methods 2-1. Materials CC-9,9-EsH10 was synthesized according to the procedure reported in our previous paper[26]. CC-9,9-EsH10-X (X = Li+, Na+, K+, and Cs+) was obtained at pH 9 through the addition of an aqueous XOH solution. The aqueous CC-9,9-EsH10-X solution obtained here was dried through freeze-drying. CC-9,9-EsH10-X (X = MEA+, diethanolamine (DEA+), and TEA+) was obtained via a reaction between CC-9,9-EsH10 (1 eq) and MEA, DEA, or TEA (2 eq) in ethanol for 48 h at room temperature. Then the solvent was evaporated under a reduced pressure. We note that XOH and XCl (X = Li+, Na+, K+, and Cs+), MEA, DEA and TEA were purchased from Tokyo Chemical Industry and used without further purification. Pyrene was purchased from Kanto Chemical Co. The water used in this study was deionized with a Barnstead NANO Pure Diamond UV system and filtered with a Millipore membrane filter (pore size 0.22 m). 2-2. Methods Static surface tension of CC-9,9-EsH10-X (X+ = Li+, Na+, K+, Cs+, MEA+, and DEA+) was measured by the Wilhelmy method using a Krüss K100 auto surface tensiometer with a platinum plate. These measurements were performed at pH 9. In the inorganic counterion systems, 10 mmol dm-3 XCl was added as a background electrolyte. The surface tension was assumed to be equilibrated when the standard deviation in the surface tension became less than 0.1 mN m-1 over 1000 s. For pyrene I1/I3 measurements, 1 mmol dm-3 pyrene ethanol solution (20 L) was
added into CC-9,9-EsH10-X (X = MEA+ and DEA+) aqueous solutions (5 cm3). Then the sample solutions were shaken for 24 h in a water-shaker bath. The fluorescence spectra were recorded between 360 and 390 nm with an excitation wavelength of 330 nm using a Shimadzu RF-5300PC fluorescence spectrophotometer. These measurements were performed at pH 9. The hydrodynamic diameter of CC-9,9-EsH10-X (X = MEA+ and DEA+) assemblies was estimated using an IBC Nicomp 380ZLS particle size analyzer. These dynamic light scattering (DLS) measurements were performed at pH 9. Before each measurement, all sample solutions were filtered with a 1 μm cellulose acetate membrane filter. The phase diagrams of CC-9,9-EsH10-X (X+ = Li+, Na+, K+, Cs+, MEA+, and DEA+) were characterized by small angle X-ray scattering (SAXS) and polarized optical microscopy (POM) measurements. The SAXS measurements were performed using an Anton Paar SAXSess. The apparatus was operated at 40 kV and 50 mA using Cu-Kα X-rays (wavelength of 0.154 nm). The X-ray irradiation time was fixed at 20 min. The POM measurements were performed using an Olympus IMT-2 microscope. The measurement temperature was always maintained at 25 ºC.
3.Results and Discussion 3-1. Interfacial properties of CC-9,9-EsH10-X Fig. 2 shows the static surface tension of aqueous solutions of CC-9,9-EsH10-X (X = Li+, Na+, K+, and Cs+) as a function of their concentrations. For all of the surfactants, the surface tension (γ) decreased with increasing surfactant concentration (c) and became constant above a critical concentration. This concentration corresponds to the cmc of each surfactant. The surface excess concentration estimated at the cmc (Γcmc) and the occupied area per surfactant molecule adsorbed at the air/aqueous solution interface (Acmc) were calculated using the following equations [29].
𝛤𝑐𝑚𝑐 = −
1 𝜕𝛾 ( ) ・・・(1) 2.303𝑛𝑅𝑇 𝜕𝑙𝑜𝑔𝑐
𝐴𝑐𝑚𝑐 =
1 ・・・(2) 𝑁𝐴 𝛤𝑐𝑚𝑐
Here, T is the absolute temperature, NA is Avogadro’s constant, and R is the gas
constant. The number of adsorption species (n in equation 1) is assumed to be 1 in the presence of 10 mmol dm-3 XCl. The calculation results are shown in Table 1.
(a)
(b)
(c)
(d)
Fig. 2 Static surface tensions of (a) CC-9,9-EsH10-Li, (b) CC-9,9-EsH10-Na, (c) CC-9,9-EsH10-K, and (d) CC-9,9-EsH10-Cs aqueous solutions as a function of their concentrations. These measurements were performed at pH 9 in the presence of 10 mmol dm-3 XCl. The CC-9,9-EsH10-Na data were referred from reference 26.
Table 1. Surface properties of CC-9,9-EsH10-X calculated from the static surface tension measurements. cmc
γcmcb)
Γcmc
Acmc
(mol dm-3)
(mN m-1)
(μmol m-2)
(nm2)
CC-9,9-EsH10-Li
3.2
29
3.0
0.56
CC-9,9-EsH10-Na a)
3.7
31
2.6
0.63
CC-9,9-EsH10-K
4.2
32
2.6
0.64
CC-9,9-EsH10-Cs
6.8
32
2.3
0.73
a) From reference 26 b) Surface tension measured at cmc
One can see in Table 1 that the cmc and Acmc values increased with increasing radius of the counterions. The same tendency has been reported for the surfactants with the carboxylate headgroups [17,18]. As mentioned in Introduction, the HSAB theory predicts that the increased counterion radius leads to the decreased degree of the counterion binding to the relativity hard carboxylate headgroups. This necessarily results in the lower capability in forming micelles as well as the looser packing at the air/aqueous solution interface, as observed in Fig. 2 and Table 1. We also studied the effect of organic counterions (MEA+, DEA+, and TEA+) on the surfactant solution properties. Under the current experimental conditions, the TEA salt was insoluble in dilute aqueous solutions, whereas the MEA and DEA salts were observed to be soluble. This phenomenon has been similarly observed in the previous papers [21,30]. In the case of the TEA salt, the carboxylic acid headgroups are not fully dissociated because of the weak basicity of TEA+ [21]. Hereafter we only discuss the aqueous solution properties of the MEA and DEA salts. Fig. 3 shows the surface tension and pyrene fluorescence I1/I3 results. Interestingly, the surface tension data showed a minimum as a function of surfactant concentration. This behavior was further studied by the pyrene fluorescence I1/I3 measurements. The
I1/I3 value started to decrease at a concentration far below the minimum concentration, and attained a first plateau (Region II). Then the I1/I3 value was again decreased from the minimum concentration (Region III), and finally, attained a second plateau (Region IV). On the basis of these results, we assume a concentration-dependent growth of the surfactant aggregates to occur. It is interesting to note that the self-aggregation structure is changed at such dilute concentrations only for the organic salts. We call the two surfactant concentrations, attained to the first and second break points in the I1/I3 results, “cac1” and “cac2”, respectively.
(a)
(b)
Fig. 3 Static surface tension and pyrene fluorescence I1/I3 data of (a) CC-9,9-EsH10-MEA and (b) CC-9,9-EsH10-DEA aqueous solutions as a function of their concentrations. These measurements were performed at pH 9. The error bars in the I1/I3 data were estimated through three repeated measurements.
We studied the self-aggregation structure by means of the DLS technique. The DLS measurements were performed at the surfactant concentrations of 0.01 and 1 mmol dm-3. These concentrations lie in Region II (cac1 < 0.01 mmol dm-3 < cac2) and Region IV (cac2 < 1 mmol dm-3) in Fig. 3. The DLS results (Fig. 4) demonstrate that two types of aggregates coexist at 0.01 mmol dm-3 for the MEA and DEA salts; one is the aggregates
in ca. 100 nm hydrodynamic diameter and the other is smaller aggregates in ca. 10 nm diameter. The latter hydrodynamic diameter is consistent with the size of aggregates formed by CC-9,9-EsH10-Na, reported in our previous paper [26]. Interestingly, at 1 mmol dm-3 the smaller aggregates make a larger impact on the DLS results. This suggests that the larger aggregates, formed above the cac1, experience a structural transformation into the smaller aggregates at the concentration above the cac2.
(a)
(b)
Fig. 4 DLS results of (a) CC-9,9-EsH10-MEA and (b) CC-9,9-EsH10-DEA aqueous solutions. These measurements were performed at pH 9.
A similar structural transformation has been reported in aqueous surfactant systems with carboxylate [31] and phosphate headgroups [32], where the formation of “acid soap” is suggested. Acid soap is a complex (or a dimer) formed by protonated and deprotonated surfactants under a neutral or weak alkaline condition [33–35]. In Region I (shown in Fig. 3), it is hypothesized that both the monomer and the acid soap coexist in the aqueous solution [31]. Under this situation, the acid soap may predominantly adsorb to the air/aqueous solution interface because of its higher hydrophobicity. In Region II (above cac1), the acid soap starts to form their relatively large self-aggregates and the equilibrium between the acid soap and the aggregate is achieved in the solution phase [31].
The
monomer
concentration
also
increases
with
increasing surfactant
concentration at a given equilibrium constant between the acid soap and the monomer [31]. Therefore, the surface tension continuously decreases with increasing surfactant concentration. In Region III (above cac2), the monomer can form micelles [31]. The monomers are consumed by the micelle formation, so that the equilibrium between the monomer and the acid soap shift to the monomer [31]. Therefore, the static surface tension increases in Region III. In Region IV, the molecules at the air/aqueous solution interface intrinsically consist of their monomers, and hence the surface tension becomes constant. Here, we discuss the interfacial parameters calculated from the surface tension data (Table 2). The cac1 was lower for the DEA salt than for the MEA salt, whereas the cac2 was almost the same with each other. The lower cac1 of the DEA salt (compared with the MEA salt) suggests that the higher hydrophobicity of DEA+ induces greater association nature of the surfactant molecules, as was similarly reported by Warnheim et al. [30] and Moreno et al. [36]. The values of Γcac and Acac were calculated using equations 1 and 2. These values were calculated at cac2 in this case, and we abbreviate these parameters as Γcac2 and Acac2. These interfacial parameters are also shown in Table 2. The Acac2 value of the DEA salt is slightly larger than that of the MEA salt. Gao et al. [37] suggested that two effects of organic counterions cooperatively contribute to the degree of molecular packing at the air/solution interface; one is the hydrophobicity and the other is the hydration size of the organic counterions. They found that a larger Acac2 value (or a smaller Γcac2 value) is observed for the organic counterions having lesser hydrophobicity and larger hydration size. In our case, DEA+ is more hydrophobic and has larger hydration size than MEA+. From this point, the larger Acac2 value for the DEA salt primarily results from the hydration size of DEA+ rather than the difference in the hydrophobicity of the two counterions.
Table 2. Surface properties of CC-9,9-EsH10-X (X = MEA+ and DEA+) estimated by the static surface tension and pyrene fluorescence I1/I3 measurements. cac1
cac2
γcac2a)
Γcac2
Acac2
(mol dm-3)
(mol dm-3)
(mN m-1)
(μmol m-2)
(nm2)
CC-9,9-EsH10-MEA
6.3
25
33
3.1
0.53
CC-9,9-EsH10-DEA
4.0
25
33
2.8
0.60
a) The minimum surface tension is determined as γcac2.
In summary, the degree of molecular packing at the air/aqueous solution interface is determined by the balance between the hydrophobicity and hydration size of organic counterions. We found that, for the two organic counterions (MEA+ and DEA+), the hydration size (i.e., steric effect) is the predominant factor. In contrast, the ion size is the key parameter in the inorganic counterion systems. The hydration size of the inorganic counterions is relatively smaller than that of the organic counterions, and hence the steric factor does not make large impact on the packing degree in the inorganic systems.
3-2. Liquid crystal phase behavior Phase diagrams of CC-9,9-EsH10-X (X = Li+, Na+, K+, Cs+, MEA+ and DEA+) with water as a function of surfactant concentration are shown in Fig. 5. We note that the phase states were characterized on the basis of the SAXS and POM results, as shown in supporting information, Fig. S1. A few phases, including isotropic solution (L1) phase, hexagonal liquid crystal (H1) phase, and lamellar liquid crystal (Lα) phase, were observed. In the H1 phase region, the ratio of scattering vector q value was calculated as 1 : √ 3 : 2 : √7, and the fan-like texture was observed in the POM images. In the Lα phase region, the ratio of q value was calculated as 1 : 2, and the mosaic or maltese-cross texture was observed. In addition, the two patterns sometimes coexisted in the SAXS results at certain surfactant concentrations. The relative intensity of the observed peaks was changed as a function of surfactant concentration, and the pattern resembled with that of the H1 and Lα phases at the phase transition concentrations. In the series of the inorganic counterion systems, the L1-H1 phase transition occurs at higher surfactant concentrations with increasing ion radius. As mentioned in the previous section, the HSAB theory predicts that the increased ion radius induces the reduced interaction with the carboxylate headgroups, and hence the electrostatic head-to-head repulsion is not screened effectively. The observed concentration shift is
reflective of this difference in the interaction between the headgroups and the counterions. In the series of the organic counterion systems, the L1-H1 phase transition concentration was observed to be higher for the larger DEA salt than for the smaller MEA salt. A similar behavior has been reported for the monomeric and gemini surfactants having MEA+ or TEA+ as counterions [21]. The larger hydration size of the DEA salt disrupts the molecular packing within the aggregates, leading to a difficulty in forming the H1 phase.
(a)
(b)
Fig. 5 Phase diagrams of CC-9,9-EsH10-X (X = Li+, Na+, K+, Cs+, MEA+ and DEA+) with water as a function of surfactant concentration; (a) inorganic counterions and (b) organic counterions.
The structure of the H1 phase was further studied. We calculated the d-spacing values of the H1 phase, using the following equation; 𝑑 = 2𝜋⁄𝑞1 ・・・(3) where q1 is the value of the first scattering vector. Fig. 6 shows the d-spacing as a function of surfactant concentration. In the series of the inorganic counterion systems, the increased ion size resulted in the larger d-spacing at a given surfactant concentration. We hypothesize that the cylinder radius becomes larger with increasing ion radius as a result of the increased head-to-head electrostatic repulsion. In addition, the d-spacings of the MEA and DEA salts were almost the same with that of the Cs salt.
Fig. 6 d-spacing of CC-9,9-EsH10-X (X = Li+, Na+, K+, Cs+, MEA+ and DEA+) in the H1 phase as a function of surfactant concentration.
4. Conclusions In this study, we have discussed the effect of counterion species of oleic acid-based gemini surfactants on their adsorption and self-aggregation properties. In the inorganic counterion systems, the decreased counterion size resulted in the decreased cmc and
Acmc value. This indicates that the smaller counterion (hard acid) can interact with the carboxylate headgroups (hard base) more strongly, as predicted by the HSAB theory. This also leads to the lower L1-H1 transition concentration in the smaller inorganic counterion system. The effect of the organic counterions is more complicated because we have to take the following factors into consideration; one is the hydrophobicity and the
other is the hydrated counterion size. The experimental results indicate that the micellization behavior is primarily affected by the hydrophobicity of the counterions, whereas the packing at the air/solution interface is largely dependent on the hydrated ion size. In addition, we suggested that the structural transformation from larger aggregates into smaller aggregates (i.e., micelles) occurs at dilute surfactant concentrations in the organic counterion systems.
Acknowledgements We thank Dr. Avinash Bhadani and Dr. Koji Tsuchiya (Tokyo University of Science) for their helpful comments.
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version.
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