Introduction

Surfactants are a kind of amphiphilic organic compounds, which have the ability of forming ordered aggregates and reducing the surface tension in aqueous solutions^[1-4]. In most practical applications, mixtures of surfactants, rather than individual surfactants, are used in industries and life, because surfactant mixtures with proper additives can make the critical micelle concentration (CMC) values decrease and improve the surface activity, which have better performances and called "synergistic effect"^[5-7]. The regular solution theory developed by Rubingh^[8] has been widely used in most mixed binary surfactant systems^[9-15]. But this theory lacks accuracy for its dependence on "the CMC of the mixed surfactant solution", which is not appropriate, because each surfactant of the mixtures has its own individual behavior. Multi-component systems which consist of nonionic, cationic and anionic surfactants are widely used as detergents, emulsifiers, chemical flooding agents, etc. Triton X-100 (TX-100), quaternary ammonium salt surfactants and sulfate surfactants are typical of these three categories of surfactants, because they are accessible with simple structures and good performances^[16-24].

In comparison with the other experimental techniques available for studying the surfactant mixtures, nuclear magnetic resonance (NMR) spectroscopy has some advantages of not only providing microscopic information at molecular and atomic scale but also giving a chance to observe separately behaviors of each individual surfactant in the mixture at the same time^[25-30].

In this work, an NMR approach is utilized to study the binary ionic/nonionic surfactant mixtures that consist of non-ionic surfactants, quaternary ammonium salt surfactants and sulfate surfactants, to evaluate the essence of the interaction in mixing the surfactants and provide a reference for choosing proper kinds and proportions of surfactants to be mixed, to get giving the best performance needed.

1 Experimental section 1.1 NMR experiments

All NMR experiments were performed at 298.0 K on a Bruker AVANCE spectrometer with proton resonance frequency of 500.13 MHz. A small pulse flip-angle 30˚ was used rather than 90˚ in the conventional single pulse sequence, to reduce the time and get better signal-to-noise ratio. The NMR resonance peaks were ascertained through the peak of water (δ 4.70) as reference.

1.2 Reagents and materials

D₂O (with a deuteration of 99.9%) was the product of Sigma-Aldrich, surfactants cetyltrimethylammonium bromide [CTAB, molecular weight (MW)=364.45], tetradecyltrimethylammonium bromide (TTAB, MW=336.39), dodecayltrimethylammonium bromide (DTAB, MW=308.34), TX-100 (MW=646.86), sodium hexadecyl sulfate (SHS, MW=344.49), and sodium dodecyl sulfate (SDS, MW=288.38) and polyethylene glycol (23) lauryl ether (Brij-35, MW=1 199.54) were the products of Acros Organics, TCI, Sinopharm Chemical Reagent, Nacalai Tesque, Adamas-beta, Alfa Aesar and J. T. Baker, respectively. The above mentioned reagents were used without further purification as received. Chemical structures and proton numbering of TX-100, Brij-35, CTAB, TTAB, DTAB, SHS, and SDS are shown in Fig. 1. The high concentration solutions of single surfactants were prepared by D₂O and mixed in some certain proportions, then the mixed solutions are diluted by D₂O to get different concentrations of binary systems.

2 Results and discussion

According to the variation of chemical shift (δ) of characteristic peaks H-1, J-4, T-6, B-6, D-6, C-1 and S-1 against the reciprocal of concentrations, CMC values for TX-100, Brij-35, CTAB, TTAB, DTAB, SHS, and SDS in various mixed solutions were calculated, respectively (The calculation method was shown in Supporting Information that is available on the CJMR website at doi: 10.11938/cjmr20172610). The results of which have been depicted in Fig. 2 and 3.

From Fig. 2(a)~2(f), it can be found that the CMC values of ionic CTAB, TTAB, DTAB, SHS or SDS decrease with the decrease in their mole factions (with the increase in the mole factions of nonionic TX-100 or Brij-35). At the same time, CMC values of TX-100 or Brij-35 decrease with the increase in the mole factions of ionic surfactants. It suggests that mixing two surfactants results in a decrease in the CMC values of the mixed surfactants by their partners, respectively. As well known, a part of surfactant molecules in aqueous solutions are adsorbed at the interface between the aqueous and the air phase, the adsorbed molecules exchange fast with the molecules staying in the solution. They are in dynamic equilibrium with a definite equilibrium adsorption constant^{[5, 31]}. If a second surfactant is added, it would also be adsorbed at the interface, thus will replace a part of the already adsorbed molecules of the first surfactant, resulting in a decrease in the first surfactant molecules per unit area of the adsorption area. In order to reestablish the adsorption equilibrium, the concentration of the first surfactant in the solution should be decreased, so the rest molecules of the first surfactant in the bulk solution are caused to aggregate forming micelles. This is the reason why the CMC values of mixed surfactants decreased. This fact clearly reveals the essence of "synergistic effect" simply.

Through carefully examining, one finds that the extents of CMC values decreased by their partners are different in different surfactant pairs. Mixing with anionic surfactant, SDS and SHS, the decrease in CMC of TX-100 is more pronounced than when mixing with the cationic ones which have the same hydrophobic chains, respectively [Fig. 3(a) and 3(b)]. It can be understood that the electron cloud of the orbital of the phenyl group of TX-100 attracts the positively charged head group of surfactants, resulting in a decrease in space between cationic surfactants and TX-100 molecules. In other words, the area occupied by this couple of molecules, becomes smaller than the sum of the area of their individual molecules. On the contrary, the π orbital electron cloud of the phenyl group of TX-100 repels the negatively charged head group of surfactants, resulting in an increase in space between anionic surfactants and TX-100 molecules. The area occupied by this couples of molecules, becomes larger than the sum of the area of their individual molecules. Consequently, SDS or SHS replaces more TX-100 molecules adsorbed on the interface, it shows more pronounced effect in decreasing the CMC of TX-100. The more TX-100 adsorbed molecules are repelled from the interface adsorbed monolayer by mixing with anionic surfactants, the more obvious decrease in CMC of TX-100.

It should be mentioned that the electro-interaction is not the only factor influencing the degree of the CMC values decreased in binary surfactants. The length of the hydrophobic chains also shows some influence [Fig. 3(c) and 3(d)]. The longer the hydrophobic chains of the ionic surfactants are, the more obvious the CMC values of the other surfactant decreased, which suggests a steric effect between the adsorbed molecules.

3 Conclusion

Because the macroscopic measurement methods (tensiometry, spectrophotomentry, conductometry, etc.) are not able to provide individual behavior of each component in the surfactant mixture, models with complicated function fitting have to be built to analyze the observed data. On the contrary, the use of NMR provides microscopic information about the behavior of individual surfactant in the mixture. The results show that mixing surfactants induce the decrease in CMC of each surfactant by their partners in the binary mixtures. Through the surface adsorption theory of surfactant in combination with concerning the electro-characteristics and the hydrophobic chain length of the mixed binary surfactants, the essence of the traditional term, often called "synergistic effect", is successfully clarified.