Hydrolysis driven surface activity of esterquat surfactants

Accepted Manuscript Hydrolysis Driven Surface Activity of Esterquat Surfactants Grażyna Para, Jacek Łuczyński, Jerzy Palus, Ewelina Jarek, Kazimiera A. Wilk, Piotr Warszyński PII: DOI: Reference:

S0021-9797(15)30366-0 http://dx.doi.org/10.1016/j.jcis.2015.11.056 YJCIS 20907

To appear in:

Journal of Colloid and Interface Science

Received Date: Revised Date: Accepted Date:

3 September 2015 18 November 2015 21 November 2015

Please cite this article as: G. Para, J. Łuczyński, J. Palus, E. Jarek, K.A. Wilk, P. Warszyński, Hydrolysis Driven Surface Activity of Esterquat Surfactants, Journal of Colloid and Interface Science (2015), doi: http://dx.doi.org/ 10.1016/j.jcis.2015.11.056

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Hydrolysis Driven Surface Activity of Esterquat Surfactants Grażyna Para,† Jacek Łuczyński,‡ Jerzy Palus,‡ Ewelina Jarek,† Kazimiera A. Wilk,‡ Piotr Warszyński*,† †

J. Haber Institute of Catalysis and Surface Chemistry, Polish Academy of Sciences,

ul. Niezapominajek 8, 30-239 Kraków, Poland, [email protected] ‡

Faculty of Chemistry, Wrocław University of Technology, ul. Wybrzeże Wyspiańskiego 27,

50-370 Wrocław, Poland

*Corresponding author, tel. +48 126395223 E-mail address: [email protected] (Warszyński)

KEYWORDS: cationic amphiphiles, esterquats, hydrolysis, cleavable surfactants, adsorption, water/air interface

KEYWORDS: cationic amphiphiles, esterquats, hydrolysis, cleavable surfactants, adsorption, water/air interface Hypothesis Surface activity of selected cleavable esterquat cationic surfactants is determined by the synergistic effect of surface active products of their hydrolysis. Experiments Interfacial behavior of two classes of esterquat surfactants, quaternary alkylammmonium esters and amino acid betaine (trimethylglycine) esters of fatty acids were examined both experimentally and theoretically. The surface tension measurements at air/water interface were performed by the pendant drop shape analysis method, then the obtained isotherms were theoretically described by the model of adsorption of ionic/non-ionic surfactants mixtures taking into account the presence of surface active products of surfactant hydrolysis. Findings

We found that surface activity of the mixture of surface active compounds resulting from the esterquat basic hydrolysis increases with time and it is higher when the ester carbonyl group is connected with the quaternary amine by bridging oxygen than in the inverted (betaine ester type) arrangement. That is, in the first case, the consequence of strong synergistic effect between the cationic esterquat surfactant and the anionic product of its hydrolysis - dodecanoate ion, while in the second case, the non-ionic hydrolysis product – dodecanol exhibits much weaker synergy. The addition of side CH3 group into the esterquat head-group slows down the hydrolysis that leads to the lower surface activity of the resulting mixture.

INTRODUCTION

Search for environmentally-friendly surfactants and basic understanding of their aggregation and adsorption behavior are of current interest due to the forthcoming applications, such as new cosmetic formulations and drug delivery systems, engineering nanostructures or functional interfaces [1-6]. Surface active agents with hydrophobic and hydrophilic parts linked by structural labile motifs as the acetal, amide, amine, ester, disulphide, ether or thioether moieties, may possess new profound physicochemical and biological functionalities. Commonly, the labile group is called cleavable, but in the literature the terms - chemodegradable, destructible, hydrolysable and acid (alkali) sensitive , can also be found [1,4,7-9]. Chemically and/or enzymatically induced cleavage of a bond will cause the separation of the polar part and the hydrophobic tail and, as a consequence, change of surface activity. Consequently, such surfactants can be readily biodegradable and show lower toxicity [4,10-14] in comparison to classical ones.- However, their properties vary widely depending on the type, number, and positions of the inserted functional groups [4,8,15]. The most well-known, and probably the most commercially viable example of cleavable surfactants comprises the family of cationic esterquat-type surfactants (abbreviated often as esterquats) with the ester bond -CO-O- or -O-CO- located between the quaternary ammonium head group(s) (Qm) and the hydrocarbon tail(s) (Rn). Grafting of the hydrolysable ester moiety onto the hydrophobic backbone of the surfactant structure allows decomposing of the molecule into fragments, lowering the environmental exposure levels, and furthermore, making it possible to improve the rate of biodegradation and to obtain high quality, environmentally friendly products for various applications [9,15-18]. Moreover, using hydrolysable surfactants for drug nanocarriers formation opens the possibility of designing new controlled delivery systems that

can be activated by internal or external triggering mechanism, as they are stable in aqueous solutions for a certain period of time only in some pH range.. Under alkaline conditions, all esters are susceptible to hydrolysis. Esterquats, due to the quaternary ammonium group adjacent to the ester group, are even more reactive than normal esters [19-22]. The nitrogen atom attracts electrons from the neighboring atoms resulting in the electron deficiency of the carbonyl carbon that favors a nucleophilic attack by hydroxyl ions. Hydrolysis of betaine esters exhibits a very strong pH dependence. Under alkaline conditions, the rate of hydrolysis of these esterquats is much higher than for esters in general, moreover, they are hydrolyzed at a significant rate by the base-catalyzed mechanism even at neutral pH. On the contrary, the betaine esters are more stable than esters in an acidic environment [16, 23-25]. Similarly, the regular esterquats are very stable in acidic environment but they undergo hydrolysis under neutral, and in particular, under basic conditions [24-27].In the present paper, as a continuation of our studies on multifunctional surfactants, their structure design, and synthetic methodology along with adsorption description at interfaces [17,28-30] we consider, on both the experimental, and theoretical level, the nonequivalent adsorption of soft cationic surfactants of the esterquat character, taking into account the effect of their hydrolysis. Structures and abbreviations of the synthesized, cationic ester-containing surfactants, i.e., N,N,N-trimethyl-2(dodecanoyloxy)ethaneammonium bromide (DMM-11) and N,N,N-trimethyl-2-(dodecanoyloxy)1-methylethaneammonium bromide (DMP2M-11) as well as dodecyloxycarbonylmethyl-N,N,Ntrimethylammonium

bromide

(DMGM-12)

and

dodecyloxycarbonyl-1-ethyl-N,N,N-

trimethylammonium bromide (DMALM-12) are presented in Scheme 1. Therefore, we investigated two classes of esterquats, fatty acid aminoesters (DMM-11 and DMP2M-11), briefly regular esterquats, with the carbonyl carbon of the ester bond Q1-O-CO-R11 facing the hydrophobic tail [9], and the amino acid betaine (trimethylglycine) esters (DMGM-12 and DMALM-12), briefly betaine esters, with the opposite orientation of the ester bond, i.e., with the bridging oxygen of Q2-CO-O-R12 facing the tail (cf. Scheme 1) [16, 19].

Scheme 1. Schematic illustration of the main concept of the model of the hydrolysis driven adsorption of the studied esterquats.

The chemical hydrolysis and biodegradation rate of DMM-11 as dodecyl regular esterquat and DMGM-12 as dodecyl betainate was investigated by Tehrani – Bagha and coworkers [24] and compared with the rates for gemini surfactants consisting of those monomers. Using 1 H NMR for the D2O surfactant solution they found that at pD 7.5 the alkaline hydrolysis could be described by the first-order reaction kinetics and that the rate for the dodecyl betainate was much faster than that of dodecyl regular esterquat [24]. Lindstedt et al. determined the rate of hydrolysis for the family of DMGM-n surfactants. They found that for DMGM-14 the rate of hydrolysis changed from a fraction of percent per hour for pH 3 to above 50% per hour for pH 8 [25]. Lundberg and Holmberg [31] investigated the kinetics of hydrolysis and micellar growth in solution of surface-active alkyl (n=10,12,14 and 18) betaine esters together with ethyl betainate as non surface-active reference compound. To study base-catalysed hydrolysis they used 1H nuclear magnetic resonance (NMR). They demonstrated that the hydrolysis rate was concentration dependent and the degradation products induced micellar growth. In other work of Lundberg's et al. [32] the phase behavior of two relevant systems for the utilization of dodecyl betainate as a pharmaceutical excipient, dodecyl betainate in combination with its degradation products and with phosphatidylcholine (PC), were studied by using different techniques: 2H NMR, small angle X-ray spectroscopy, optical microscopy and laser diffraction for dilute dodecyl betainate/PC dispersions. It was shown that introduction of relatively small amounts of the hydrolysis products of dodecyl betainate, i.e., dodecanol and betaine (used in the form of betaine hydrochloride), had strong effect on the phase behavior of the binary dodecyl betainate/D2O system [32].

There exists only a limited number of papers dedicated to adsorption and aggregation behavior of studied alkyl betainates and regular esterquats. Różycka-Roszak et al. [33] investigated the micelization of some amphiphilic betaine ester derivatives by the titration calorimetry. Dopierala et al. [34-35] measured the static and dynamic surface tension of DMM-n (n=9, 13), DMP2M-n (n = 9, 11, 13), DMGM-12, DMALM-12 and also other esterquat surfactants. The authors tried to describe the surface tension isotherm using Frumkin and reorientation adsorption models, however, they did not take into account the effect of hydrolysis. El Achouri et al. [36] synthesized surfactants of the esterquat type, (dodecanoyloxy)propyl-nalkyldimethylammonium bromide (with the hydrocarbon chain lengths of alkyl group 8, 12 and 14) and characterized

them by

the

spectroscopic methods (e.g., NMR,

IR, MS). The

physicochemical properties of surfactants were investigated by surface tension measurements using the Wilhelmy plate method. The adsorption parameters were determined from the adsorption isotherms using the Gibbs equation without applying any particular model isotherm. The critical micelle concentration (CMC), the degree of counterion binding of micelles in water, and the standard Gibbs free energy of micellization were determined from the conductometric measurements and compared with ones for conventional alkyltrimethylammonium bromide surfactants. However, in their analysis of surface activity the authors did not account for the hydrolysis. Similarly, in the paper of Bing Han et al. [37] the influence of hydrolysis on the adsorption isotherms of di-chain esterquats was not discussed, the same as in our previous paper where we used the extended “surface quasi two-dimensional electrolyte” (STDE) model of adsorption of ionic surfactants proposed by Warszynski et al. [38,39], in order to describe the adsorption behavior of cationic gemini type surfactants ethylene-bis-[N,N-dimethyl-N-(nalkyl)oxycarbonyl(methyl)methylammonium] dibromides (n=10, 12) [17]. Consequently, to our best knowledge, the exact analysis of the effect of hydrolysis on surface activity of regular esterquats and betaine esters has so far neither been discussed nor described theoretically. Therefore, in the present paper we try to bridge the gap, and to analyse the surface tension isotherms of the cleavable surfactants solutions at different pH values, i.e., at different stages of hydrolysis. Experimentally measured surface tension data were interpreted using the extended “surface quasi two-dimensional electrolyte” (STDE) model of adsorption of ionic/non-ionic surfactant mixtures [40]. This novel approach allows accounting for the effect of surface active hydrolysis products.

THEORETICAL BACKGROUND

Basic hydrolysis of esterquats. The mechanism of basic hydrolysis of ester-containing surfactants is well understood [22-24]. For the case of studied dodecyl regular esterquats (DMM11 and DMP2M-11) and dodecyl betainates (DMGM-12 and DMALM-12) it is illustrated detailed in the Scheme S1 the “Supplementary Material” for this paper. The hydrolysis of regular dodecyl

esterquats

results

in

formation

of

non-surface

active

salt,

choline

(2-

hydroxyethyltrimethylammonium cation) - in the case of DMM-11 and β-methylcholine (2hydroxypropyltrimethylammonium cation) - in the case of DMP2M-11 and surface active dodecanoate anion, which at low pH can be protonated to get dodecanoic acid. For dodecyl betainates the basic hydrolysis results in non-surface active, zwitterionic amino acid, betaine (N,N,N-trimethylglycine) for DMGM-12 and propiobetaine (1-carboxyethyltrimethylammonium betaine) for DMALM-12 and surface active dodecanol.

Model of adsorption of surfactants and surface active hydrolysis products. The main concept of the model is illustrated in Scheme 1. The hydrolysis of surfactants containing ester bonds results in the formation of surface active products, either neutral (dodecanol or undissociated dodecanoic acid) or negatively charged (dodecanoate anion) (see Scheme S1, where the hydrolysis products are denoted by 0/-), thus, the model of adsorption for the surfactant mixture needs to be used to describe the experimental results for the surface tension. For that purpose we applied the extended model of adsorption of ionic/non-ionic surfactant mixtures [40,41] that is the extension of the surface two dimensional electrolyte (STDE) model proposed earlier by Warszyński et al. [38,39] to describe adsorption of ionic surfactants. We consider the system containing cationic surfactant and anionic or neutral hydrolysis product. The system of adsorption equation derived from the equilibrium condition for the transfer from the solution to the Stern layer at the solution surface can be formulated as:

     eψ   φ  exp  − s  1 − θ s − ∑θ h,i − ∑θ a ,i  = θ s exp  −2 H s  θ s + ∑ θh,i   exp  s  αs  kT    kT  i i   i   as

(1)

for the cationic surfactant, ah,i

α h ,i

 z eψ    exp  − h,i s  1 − θ s − ∑ θ h,i − ∑ θ a ,i  kT   i i  

g h ,i

 z φ    = θ h,i exp  −2 H s  θ s + ∑ θ h ,i   exp  h ,i h ,i  i    kT   (2)

for the hydrolysis products (
, = 0for neutral, -1 for anionic), aa , i

α a,i

  eψ   exp  s 1 − θ s − ∑ θ h,i − ∑ θ a,i   kT   i i 

g a ,i

φ  = θ a,i exp  a,i   kT 

(3)

for the non-surface active anions of the electrolyte (Br-and OH-). The symbols in the above equations denote:  , , ,  ,

- the activities of the respective

components that can be calculated from the extended Debye – Hückel theory of strong electrolyte solutions - for the neutral species equal to their concentrations,  = Γ ⁄Γ is the relative surfactant

surface concentration, whereΓ surfactant

surface

is its surface (excess) concentration and Γ is the limiting concentration

at

the

maximal

coverage, , = Γ, ⁄Γ , ,  , = Γ , ⁄Γ , ,  = Γ ⁄Γ , Γ, , Γ , , Γ , Γ and Γ are the same quantities for hydrolysis products and electrolyte anions;  , , , and  , are the ratios of the size of surfactant cations, surface active hydrolysis products and electrolyte anions relatively to the size of the adsorption site ( = 1 for the sake of simplicity),  is the surface interaction parameter accounting mainly for the attractive lateral interactions among the adsorbed surfactant hydrophobic tails, is the "surface activity" of surfactant ion, being a measure of the standard free energy of

adsorption after separating the contribution of the electric

component, , is the same parameter for the respective surface active hydrolysis products and  , are the "surface activities" electrolyte anions that are a measure of their affinity to the

surface layer, φ , φ, , φ , , are the corrections for the activity of the two dimensional electrolyte in the surface layer accounting for the lateral interaction between ions. In the derivation of equations 1-3 we assumed that non-surface active cations resulting from the hydrolysis (see Scheme 1) and H3O+ (HCl) or Na+ (NaOH) added to adjust pH, do not penetrate Stern layer due to strong electrostatic repulsion of the positively charged interface. The electric potential of the Stern layer, , can be found from:

ψ s =ψ d +

σδ ε 0ε s

(4)

while the diffuse layer potential at the boundary between the Stern layer and the diffuse part of electric double layer can be determined from the formula:

ψd =

 σe  2 kT sinh −1   e  2ε 0ε s kT κ 

(5)

where:  is the elementary charge, is the Boltzmann constant,  is the vacuum dielectric permittivity,  is the dielectric constant of the solution,  is the Debye - Hückel reciprocal length 



σ = F  Γ s − ∑ Γ h ,i − ∑ Γ a ,i  

i

i



(6)

is the surface charge density,  is the Faraday constant,  is the thickness of the Stern layer and  is the dielectric constant in the Stern layer. We assumed that adsorption of non-ionic surface active molecules does not influence the electric properties of the Stern layer. The procedure of solving the system of eqs 1-6 and the detailed interpretation of parameters are described elsewhere [40]. By the numerical solution of this system of equations, the surface concentration of all components in the Stern layer can be determined directly. Total surface excess concentrationΓ of all components has to include adsorption of all electrolyte and surfactant ions in the diffuse part of the electric double layer, where the distribution of ions has to be found using the solution of Poisson-Boltzmann equation. The surface tension of the solution can be predicted by integration of the Gibbs equation for the mixture of ionic–nonionic surfactant:

d γ = − RT (∑ ΓTj d ln a j )

(7)

j

From the fit of the calculated isotherm to the experimental data the parameters of the model for investigated system can be obtained.

EXPERIMENTAL SECTION

Synthesis of surfactants

Materials,

methodology

and

details

of

synthesis

of

N,N,N-trimethyl-2-

(dodecanoyloxy)ethaneammonium bromide (DMM-11), dodecyloxycarbonylmethyl-N,N,Ntrimethylammonium methylethaneammonium

bromide bromide

(DMGM-12) (DMP2M-11)

N,N,N-trimethyl-2-(dodecanoyloxy)-1and

dodecyloxycarbonyl-1-ethyl-N,N,N-

trimethylammonium bromide (DMALM-12), as well as the methods to obtain the spectroscopic and analytical data of cationic ester-containing surfactants, the results concerning the elemental analysis and the NMR 1H chemical shifts are collected in the “Supplementary Material” for this paper.

Surface tension measurements.

The equilibrium surface tension measurements were performed using the pendant drop shape analysis method. The experimental setup for the dynamic and equilibrium surface tension determination was described in details elsewhere [42]. The method is based on fitting the solution of the Young-Laplace equation of capillarity to the shape of the pendant drop [43], which is recorded by the digital camera. The measured surface tension value, being the only unknown

parameter in that equation, corresponds to the best-fit value. The dynamic surface tension measurements were performed every 5 sec. If not stated otherwise the measured surface tension corresponded to the steady state reached after time depending on the surfactant concentration and extent of hydrolysis. All surface tension measurements were performed at room temperature 295 K. The water used in all experiments was purified by means of a Millipore (Bedford, MA) Milli– Q purification system. If not stated otherwise all solutions were freshly prepared before experiments.

RESULTS AND DISCUSSION

In our previous work [29] we demonstrated that adsorption of cationic surfactants; monomeric, dimeric and gemini type ammonium salts, at air/solution interface is strongly influenced by their structure. In this paper we examined the adsorption behaviour, at water/air interface, of selected quaternary ammonium surfactants with ester link between hydrophobic part and hydrophilic headgroup (DMM-11, DMP2M-11, DMGM-12 and DMALM-12). We analysed the effect of ester bond orientation, Q1-O-CO-R11 or Q2-CO-O-R12, and the presence of side CH3 between amine and ester group on their surface activity influenced by the hydrolysis process. Figures 1 and 2 illustrate the dependencies of the surface tension on the concentration of esterquats in aqueous solutions, measured directly after their preparation without any pH adjustment, together with the surface tension isotherms for the surface active hydrolysis products, undissociated dodecanoic acid or dodecanoate anion (fully dissociated sodium dodecanoate) for DMM-11 and DMP2M-11 (Fig 1.) and dodecanol for DMGM-12 and DMALM-12. For the sake of comparison we also present the isotherm for simple quaternary ammonium surfactant with the same number of carbon atoms in the hydrophobic chain, dodecyltrimethylammonium bromide (DTABr).

Figure 1. Left - the dependence of surface tension on the concentration of DMM-11 and DMP2M-11 solutions, together with isotherms of: DTABr [39], C11H23COOH [44] and C11H23COONa. Symbols denote experimental data, lines represent fits of the Frumkin (dodecanoic acid), STDE (DTABr, dodecanoate) and extended STDE adsorption model (DMM11 and DMP2M-11) to the experimental data. Experimental error of surface tension measurements is below the size of a symbol. Right – the corresponding dependence of pH on the concentration of DMM-11 and DMP2M-11 solutions, lines represent the results of model calculations (see text).

Figure 2. Left - the dependence of surface tension on the concentration of DMGM-12 and DMALM-12 solutions, together with isotherms for DTABr [39] and dodecanol [45]. Symbols denote experimental data, Lines represent fits of the Frumkin (dodecanol), STDE (DTABr) adsorption and extended STDE adsorption model (DMGM-12, DMALM-12) to the experimental data. Experimental error of surface tension measurements is below the size of a symbol. Right the corresponding dependence of pH on the concentration of DMGM-12 and DMALM-12 solutions, lines represent the results of model calculations (see text).

One can observe that the surface activity of the investigated surfactants is higher, i.e. surfactant concentration inducing comparable decrease of surface tension is lower, than for the classical

quaternary ammonium surfactant - DTABr that is in agreement with the earlier findings [33] that the CMC for a betaine esters with a hydrocarbon chain of n carbons is close to the value for an alkyltrimethylammonium surfactant with a hydrocarbon chain of n + 2 carbons. However, there are distinct differences between surfactant pairs with two arrangements of the ester linkage dodecyl betainate Q2-CO-O-R12 and ester Q1-O-CO-R11. The DMM-11 having the same chemical formula as DMGM-12 (C17H36NO2Br) is almost one order of magnitude more surface active. The critical micelle concentration (CMC) for DMM-11 is 7x10-4M, whereas for DMGM12 is about 5x10-3M. The same effect, however, less pronounced can be observed for the DMP2M-11 and DMALM-12 pair (C18H38NO2Br) with CMC 3x10-3M and 8x10-3M respectively. On the other hand, the structure of the headgroup has also a strong effect on the surface activity. Analysing the surface tension of pairs DMM-11, DMP2M-11 and DMGM-12 and DMALM-12 one can conclude that the addition of the hydrophobic CH3- side group between nitrogen atom of amine group and ester linker (c.f. Scheme S1 in the “Supplementary Material”) makes the surfactant less surface active. The theoretical model of adsorption of surfactant mixtures was applied to describe experimental data. It was assumed that, in the observed range of pH, the rate of hydrolysis was slow so the concentration of the surface active hydrolysis products - dodecanol for DMGM-12 and DMALM-12 and dodecanoic acid for DMM-11 and DMP2M-11 was given by, = ,  and concentration of unhydrolysed surfactant by  = 1 − . # , where, is the degree of hydrolysis and is the total concentration of surfactant. Since the results of the surface tension dependency on surfactant concentration were obtained for the solutions with natural pH (i.e. without pH adjustment), the decrease in pH for more concentrated solutions, as illustrated in Figs 1 and 2, could be used as the measure of the degree of basic hydrolysis. As the $% ( $% = −&'% , where % is acid dissociation constant) value of dodecanoic acid is 5.3, one needs

additionally to consider its dissociation and formation of dodecanoate anions [41]. For the pH calculations the solubilibity of CO2 in water and the equilibrium between hydrocarbonate and carbonate anions were taken into account. Lines in Figures 1 and 2 denote fits of the theoretical model to experimental data for surface tension and pH simultaneously. The best fit parameters are collected in Table 1, while other model parameters were determined previously for the CnTABr surfactants [39]. The isotherm parameters for the hydrolysis products were determined separately using the isotherms illustrated in Figs 1 and 2. One can observe that the experimental data can be satisfactorily described by the theoretical model.

Interpreting results of surface tension measurements in terms of the extended STDE model we conclude that the process of hydrolysis has a profound effect on the surface activity of the resulting mixture of original cationic surfactants and the reaction products. The hydrolysis of the DMM-11 surfactant seems to be the most progressed  = 0.04#. Moreover, one of the reaction products (cf. Scheme S1 in “Suplementary Material”) is dodecanoate, surface active anion. Therefore, in the bulk and in particular at the interface it can form, with cationic DMM-11, catanionic complex with very high surface activity. In the case of DMM-11 we also noted the lowest value of surface tension at CMC, equal to ca. 25 mN/m, which was not observed for other investigated surfactants. This value is similar to the minimum value of surface tension of investigated previously by us N,N-(didodecyl)-N,N-dimethylammonium bromide (dDDDABr) [29], having one quaternary ammonium group with two dodecyl - hydrocarbon chain in the molecule. It is much lower than that for typical cationic surfactant (eg. CTABr or DTABr) [39]. The structure of the obtained DMM-11 +- dodecanoate- complex is similar, therefore, its adsorption produces such low surface tension at CMC. This behavior is also confirmed by low surface tension values of di-chain esterquat surfactants [37] and fresh unbuffered aqueous solutions of gemini dodecyl esterquats – betaine compounds, in which the initial hydrolysis takes place [12]. The DMP2M-11 surfactant is much less hydrolysed  = 0.004#, thus, the surface activity of the resulting DMP2M-11 - dodecanoic acid (dodecanoate-) mixture is significantly lower and the surface tension at CMC, higher than for DMM-11. As it can observed in Figs 1 and 2 the DMM-11 (CMC - 7x10 -4M) is more surface active than DMGM-12 (CMC - 5x10-3M) . That is partly due to less hydrolysed DMGM-12  = 0.01# but, what is more important, the surface active product of the reaction is a non-ionic surface active alkanol - dodecanol. Therefore, much lower synergy with the cationic surfactant can be expected. Nevertheless, the results of our theoretical model calculations suggest that already at  = 0.01 dodecanol dominates at the interface and the surface tension values coincide with the

ones corresponding to its concentration in the mixture (squares in Figure 2). Degree of hydrolysis of DMALM-12  = 0.004#, having the same surface active hydrolysis product, is lower than that for DMGM-12, therefore, its surface activity is the lowest among the investigated surfactants. It is worth to note that in the model calculation, the dissociation of trimethylglycine ($% = 1.84 [46]) and propiobetaine ($% = 4.5 - fitted parameter, as its value could not be found in the literature) was also considered. Since the hydrolysis progresses with time, we measured surface activity of esterquats after storing stock solutions for three months and diluting them to the required concentrations before

experiments. The results of surface tension measurements are collected in Figures 3 and 4 and compared with the values obtained for the freshly prepared solutions.

Figure 3. The dependence of surface tension (Left) and pH (Right) on the concentration of DMM11 and DMP2M-11 solutions right after preparation and after three months storage. Symbols denote experimental data, lines - fits of the theoretical model to the experimental data (both surface tension and pH). Experimental error of surface tension measurements is below the size of a symbol.

For DMM-11 and DMP2M-11 solutions one can observe increase of surface activity and decrease of pH after three months storage. Applying the STDE model of adsorption of surfactant mixtures with only one adjustable parameter the degree of hydrolysis, with other parameters as in Table 1, we concluded that for DMM-11 it increased to  = 0.12 and for DMP2M-11 to  = 0.02. The peculiar behavior of the surface tension could be observed for DMGM-12. The

very well pronounced minimum in the dependence of dynamic surface tension on time could be observed for the solution stored for three months (see the inset in Figure 4). Then after c.a. 5000 seconds the surface tension reached a steady value. In Figure 4 the values of the surface tension corresponding to the minimum are denoted by up-pointing triangles, while the steady state values by down-pointing triangles. It is worth to note that the latter correspond to the respective surface tension values of the freshly prepared solution. Dotted line in Figure 4 marks the prediction of the surface tension and pH dependence on the concentration assuming the degree of hydrolysis  = 0.05, while the measured pH values are significantly lower and correspond to  = 0.15

(dashed line in Fig. 4). We attribute that behavior to the limited solubility of dodecanol in water 2.2x10-5M in room temperature [47]. During storage of the DMGM-12 solution the hydrolysis reaction progresses. When the concentration of dodecanol reaches the solubility limit it precipitates. After dilution some of the precipitated alcohol is redissolved that leads to a decrease

of the surface tension, then the equilibrium between the bulk and adsorption layer is attained. For DMALM-12 solution we did not observe any significant changes in surface activity in time.

Figure 4. The dependence of surface tension (Left) and pH (Right) on the concentration of DMGM-12 solutions. Symbols denote experimental data (explanation in the text), lines - fits of the theoretical model to the experimental data (both surface tension and pH). Experimental error of surface tension measurements is below the size of a symbol. Inset - The dependence of dynamic surface tension on time for 3x10 -4M DMGM-12 solution just after its preparation (solid symbols) and after three months storage (hollow symbols).

As it has been explained before, pH value of the esterquats solutions has a profound effect on the rate of their hydrolysis. Therefore, we investigated the surface activity of DMM-11, DMP2M11, DMGM-12 and DMALM-12 surfactants when pH of their solution was adjusted to 10 and compare it with determined in natural conditions and for pH 4. The resulting dependencies of the surface tension on surfactant concentration are collected in Figures 5-7.

Fig. 5. The dependence of surface tension on the concentration of DMM-11 solutions right after preparation without pH adjustment and after adjusting pH to 4 and 10. Symbols denote experimental data, lines - results of the theoretical model. Experimental error of surface tension measurements is below the size of a symbol.

Fig. 6. The dependence of surface tension on the concentration of DMP2M-11 solutions right after preparation without pH adjustment and after adjusting pH to 4 and 10. Symbols denote experimental data, lines - results of the theoretical model. Experimental error of surface tension measurements is below the size of a symbol

Applying the STDE model of adsorption of surfactant mixtures with the degree of hydrolysis as the only adjustable parameter and others as in Table 1, we concluded that for pH 10 it

increased: for DMM-11 to  = 0.12 and for DMP2M-11 to = 0.02 . A peculiar behavior of the dynamic surface tension of DMGM-12 surfactant with time could again be observed. An example is illustrated in inset graph in Figure 7. The value of the surface tension first rapidly drops down then irregularly but systematically increases to some steady state value with the large scatter of surface tension values. We attribute that behavior to a very rapid hydrolysis, in particular at the interface where cationic charge of adsorbed surfactant locally increases the concentration of OHanions. The surface is oversaturated with dodecanol that needs to diffuse out but its concentration is limited by solubility so it precipitates, which can be evidenced by the turbidity of the solution. The hypothetical hydrolysis degree corresponding to the minima of the DMGM-12 surface tension at pH 10 (up-pointing triangles in Fig. 7) was calculated using the theoretical model as  = 0.25 . The final, steady state values of the surface tension (marked in Fig 7 by down

pointing triangles) were never below the minimum surface tension of dodecanol at the solubility limit, 44 mN/m. Therefore, we can conclude that, in agreement with the earlier findings [24], the rate of basic hydrolysis is faster for the betaine arrangement (DMGM-12) than for the opposite regular esterquat arrangements (DMM-11) but the solubility of the reaction product, dodecanol, limits the final surface activity.

Fig. 7. The dependence of surface tension on the concentration of DMGM-12 solutions. Symbols denote experimental data (explanation in the text), lines - fits of the theoretical model to the experimental data. Inset - The dependence of dynamic surface tension on time for DMGM-12 1x10-4M and 1x10-3M solutions with pH adjusted to 10 just after its preparation.

The decrease of surface activity for pH 4 for DMM-11 and DMP2M-11 can be attributed to the increased concentration of neutral, undissociated dodecanoic acid at the expense of dodecanoate ion. As the consequence the strongly surface active catanionic complex can be formed to the lesser extent, and the resulting surface activity of the mixture is decreased. To confirm that hypothesis we measured the surface tension of surfactants dissolved in the solution with pH 3 (10-3M HCl). In those conditions: i) the hydrolysis should be strongly retarded, ii) the dodecanoic acid resulting from hydrolysis of DMM-11 and DMP2M-11 should be predominantly in the non-dissociated form.

Figure 8. The dependence of steady-state surface tension on esterquat surfactant concentration (dissolved at pH 3). Lines represent predictions of the theoretical model. Inset – the dynamic surface tension of esterquats for the concentration 10 -3M.

Analysing kinetic curves at the concentration 10-3M for all surfactants (inset graph in Fig.8), we observe slow adsorption kinetics, at the timescale of 1000 seconds, whereas, at the concentration 10 -3M the adsorption equilibrium should be attained after a few minutes. If the hydrolysis in acidic conditions is strongly retarded, the plausible explanation of slow kinetics is that the surfactants stock has already been partly hydrolysed during the period after their synthesis and the degree of that hydrolysis is the highest for DMM-11. Figure 8 illustrates the dependence of the steady state surface tension on surfactants concentration. Comparing the results presented in this Figure with the ones illustrated in Figures 1 and 2 for the surface tension

of esterquat surfactants in natural conditions one can observe much lower differences between their surface activities when they are dissolved in the solution with pH 3. Therefore, that shows that the difference in adsorption properties between esterquats having the same chemical formula but reversed arrangement of the ester linker between the quaternary ammonium headgroup and hydrophobic tail, e.g., DMM-11 and DMGM-12, stems mainly from their hydrolysis. Lines in Figure 8 represent predictions of the theoretical model using parameters given in Table 1 and calculated for pH 3. Good reproduction of experimental results confirms correctness of the applied model of adsorption. For DMALM-12 we did not observe any measurable changes of surface activity neither in basic nor acidic conditions. That result together with one obtained for DMP2M-11 vs. DMM-11 shows that the addition of the hydrophobic CH3 - side group between nitrogen atom of amine group and ester linker slows down the hydrolysis and thus decreases the surface activity of esterquats.

CONCLUSIONS

We investigated the adsorption of soft cationic surfactants of the esterquat-type character, mainly, fatty acid quaternary ammonium esters (DMM-11 and DMP2M-11), with the carbonyl carbon of the ester bond Q1-O-CO-R11 facing the hydrophobic tail and the amino acid betaine (trimethylglycine) esters (DMGM-12 and DMALM-12), with the opposite orientation of the ester bond, i.e., with the bridging oxygen of Q2-CO-O-R12 facing the tail. Our measurements show that two concurrent processes should be taken into account in the quantitative description of their adsorption behavior at the air/solution interface, i.e., the adsorption of surfactant molecules and hydrolysis processes occurring in the bulk and inside the surface layer. The resulting surface activity is increased due to formation surface active hydrolysis products – dodecanol or dodecanoic acid and the synergistic effect between them and surfactant ions. The cationic regular esterquat surfactants with the Q1-O-CO-R11 ester bond orientation exhibit higher surface activity as they can form catanionic dimers with dodecanoate anions. That effect can be substantially reduced in low pH of the solution when undissociated dodecanoic acid is formed. In those conditions the surface activity is similar as surfactants with the Q2-CO-O-R12 orientation of the ester bond (betaine ester type), which hydrolysis results in formation of non-ionic, surface active alcohol – dodecanol. In the alkaline solutions (pH 10) the surfactants solutions exhibit stronger decrease of surface tension due to largely increased rate of hydrolysis. According to the previous finding the amino acid betaine ester (DMGM-12) is fast hydrolysed in basic conditions [24] but

the surface activity is limited by the solubility of the reaction product, dodecanol. Presence of side CH3 between quaternary amine group and ester bond strongly decreases the hydrolysis rate that lowers the resulting surface activity. The results of preliminary ab-initio calculations confirm that effect and that will be the subject of further studies. The applied model of adsorption can correctly describe the observed behavior of the surface tension and allows evaluation of the degree of hydrolysis, which was not accounted for in previous investigations [34,35, 48-52]. Developing better understanding of adsorption properties of cleavable esterquats is a matter of some importance because of their potential use for the formation of environment sensitive microcapsules. Our results show that it is impossible to formulate general statement concerning surface activity of esterquats (as in [33]) as it depends on both their molecular structure and degree of hydrolysis.

ACKNOWLEDGEMENTS

This work was partially financed by the National Science Centre project UMO2011/03/B/ST4/01217, by a statutory activity subsidy from the Polish Ministry of Science and Higher Education (PMSHE) for the Faculty of Chemistry of Wrocław University of Technology as well as by the Marian Smoluchowski Krakow Research Consortium - a Leading National Research Centre KNOW supported by the PMSHE.

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FIGURE CAPTIONS Figure 1. Left - the dependence of surface tension on the concentration of DMM-11 and DMP2M-11 solutions, together with isotherms of: DTABr [39], C11H23COOH [44] and C11H23COONa. Symbols denote experimental data, lines represent fits of the Frumkin (dodecanoic acid), STDE (DTABr, dodecanoate) and extended STDE adsorption model (DMM11 and DMP2M-11) to the experimental data. Experimental error of surface tension measurements is below the size of a symbol. Right – the corresponding dependence of pH on the concentration of DMM-11 and DMP2M-11 solutions, lines represent the results of model calculations (see text). Figure 2. Left - the dependence of surface tension on the concentration of DMGM-12 and DMALM-12 solutions, together with isotherms for DTABr [39] and dodecanol [45]. Symbols denote experimental data, Lines represent fits of the Frumkin (dodecanol), STDE (DTABr) adsorption and extended STDE adsorption model (DMGM-12, DMALM-12) to the experimental data. Experimental error of surface tension measurements is below the size of a symbol. Right the corresponding dependence of pH on the concentration of DMGM-12 and DMALM-12 solutions, lines represent the results of model calculations (see text). Figure 3. The dependence of surface tension (Left) and pH (Right) on the concentration of DMM-11 and DMP2M-11 solutions right after preparation and after three months storage. Symbols denote experimental data, lines - fits of the theoretical model to the experimental data (both surface tension and pH). Experimental error of surface tension measurements is below the size of a symbol. Figure 4. The dependence of surface tension (Left) and pH (Right) on the concentration of DMGM-12 solutions. Symbols denote experimental data (explanation in the text), lines - fits of the theoretical model to the experimental data (both surface tension and pH). Experimental error of surface tension measurements is below the size of a symbol. Inset - The dependence of dynamic surface tension on time for 3x10 -4M DMGM-12 solution just after its preparation (solid symbols) and after three months storage (hollow symbols). Fig. 5. The dependence of surface tension on the concentration of DMM-11 solutions right after preparation without pH adjustment and after adjusting pH to 4 and 10. Symbols denote experimental data, lines - results of the theoretical model. Experimental error of surface tension measurements is below the size of a symbol. Fig. 6. The dependence of surface tension on the concentration of DMP2M-11 solutions right after preparation without pH adjustment and after adjusting pH to 4 and 10. Symbols denote experimental data, lines - results of the theoretical model. Experimental error of surface tension measurements is below the size of a symbol Fig. 7. The dependence of surface tension on the concentration of DMGM-12 solutions. Symbols denote experimental data (explanation in the text), lines - fits of the theoretical model to the experimental data. Inset - The dependence of dynamic surface tension on time for DMGM-12 1x10-4M and 1x10-3M solutions with pH adjusted to 10 just after its preparation. Figure 8. The dependence of steady-state surface tension on esterquat surfactant concentration (dissolved at pH 3). Lines represent predictions of the theoretical model. Inset – the dynamic surface tension of esterquats for the concentration 10 -3M.

Table 1 Best fit parameters of STDE adsorption model Surfactant/

DMM-11

DMP2M-11

Dodecanoic

DMGM-12

DMALM-12

Dodecanol

acid

Model parameter

Γ ,-.//1-23

5.4x10-10

5.2x10-10

6.1x10-10

5.4x10-10

5.2x10-10

6.4x10-10

 45'&/6578

8.0x10-5

8.0x10-5

2.1x10-5

7.0x10-5

7.0x10-5

2.0x10-5

 4.0

4.0

4.5

4.5

4.5

4.5

0.04

4x10-3

$% = 5.3

0.01

4x10-3

-



Other model parameters 44

α Br

−



0.64

[ mol / dm 3 ]

2800

:;< 45'&/65

78

/< 45'&/657 8

17000

:; < '= >< 4?58

0.35

δ 4?58

0.35

εs

24

Highlights • • • • •

We synthesized the selected four esterquat-type surfactants We developed model of adsorption of surfactant undergoing hydrolysis We proved that the surface activity depends on progress of hydrolysis We showed synergistic effect between surfactants and products of their hydrolysis We explained the observed peculiarities in the dynamic surface tension