Electrostatic curvature contributions to interfacial tension of micellar and microemulsion phases

Enhanced solubility of methyl ester sulfonates below their Krafft points in mixed micellar solutions

HYPOTHESIS

Methyl ester sulfonates (MES) show limited water solubility at lower temperatures (Krafft point). One way to increase their solubility below their Krafft points is to incorporate them in anionic surfactant micelles. The electrostatic interactions between the ionic surfactant molecules and charged micelles play an important role for the degree of MES solubility.

EXPERIMENTS

The solubility and electrolytic conductivity for binary and ternary surfactant mixtures of MES with anionic sodium alpha olefin sulfonate (AOS) and sodium lauryl ether sulfate with two ethylene oxide groups (SLES-2EO) at 5 °C during long-term storage were measured. Phase diagrams were established; a general phase separation theoretical model for their explanation was developed and checked experimentally.

FINDINGS

The binary and ternary phase diagrams for studied surfactant mixtures include phase domains: mixed micelles; micelles + crystallites; crystallites, and molecular solution. The proposed general phase separation model for ionic surfactant mixtures is convenient for construction of such complex phase diagrams and provides information on the concentrations of all components of the complex solution and on the micellar electrostatic potential. The obtained maximal MES mole fraction of transparent micellar solutions could be of interest to increase the range of applicability of MES-surfactants.

Micelle-monomer equilibria in solutions of ionic surfactants and in ionic-nonionic mixtures: a generalized phase separation model

On the basis of a detailed physicochemical model, a complete system of equations is formulated that describes the equilibrium between micelles and monomers in solutions of ionic surfactants and their mixtures with nonionic surfactants. The equations of the system express mass balances, chemical and mechanical equilibria. Each nonionic surfactant is characterized by a single thermodynamic parameter--its micellization constant. Each ionic surfactant is characterized by three parameters, including the Stern constant that quantifies the counterion binding. In the case of mixed micelles, each pair of surfactants is characterized with an interaction parameter, β, in terms of the regular solution theory. The comparison of the model with experimental data for surfactant binary mixtures shows that β is constant--independent of the micelle composition and electrolyte concentration. The solution of the system of equations gives the concentrations of all monomeric species, the micelle composition, ionization degree, surface potential and mean area per head group. Upon additional assumptions for the micelle shape, the mean aggregation number can be also estimated. The model gives quantitative theoretical interpretation of the dependence of the critical micellization concentration (CMC) of ionic surfactants on the ionic strength; of the CMC of mixed surfactant solutions, and of the electrolytic conductivity of micellar solutions. It turns out, that in the absence of added salt the conductivity is completely dominated by the contribution of the small ions: monomers and counterions. The theoretical predictions are in good agreement with experimental data.

Determination of the aggregation number and charge of ionic surfactant micelles from the stepwise thinning of foam films

The stepwise thinning (stratification) of liquid films, which contain micelles of an ionic surfactant, depends on the micelle aggregation number, N(agg), and charge, Z. Vice versa, from the height of the step and the final film thickness one can determine N(agg), Z, and the degree of micelle ionization. The determination of N(agg) is based on the experimental fact that the step height is equal to the inverse cubic root of the micelle concentration. In addition, Z is determined from the final thickness of the film, which depends on the concentration of counterions dissociated from the micelles in the bulk. The method is applied to micellar solutions of six surfactants, both anionic and cationic: sodium dodecylsulfate (SDS), cetyl trimethylammonium bromide (CTAB), cetylpyridinium chloride (CPC), sodium laurylethersulfates with 1 and 3 ethylene oxide groups (SLES-1EO and SLES-3EO), and potassium myristate. The method has the following advantages: (i) N(agg) and Z are determined simultaneously, from the same set of experimental data; (ii) N(agg) and Z are determined for each given surfactant concentration (i.e. their concentration dependence is obtained), and (iii) N(agg) and Z can be determined even for turbid solutions, like those of carboxylates, where the micelles coexist with acid-soap crystallites, so that the application of other methods is difficult. The results indicate that the micelles of greater aggregation number have a lower degree of ionization, which can be explained with the effect of counterion binding. The proposed method is applicable to the concentration range, in which the films stratify and the micelles are spherical. This is satisfied for numerous systems representing scientific and practical interest.

Electrostatic stretching of a charged vesicle

We present a closed-form solution of electrostatic potential self-induced by a uniformly charged micro/nanovesicle and the corresponding elastic deformation of the vesicle membrane due to Maxwell stress. At equilibrium, the electrostatic force induced on both sides of the membrane is balanced by the elastic force of the stretched membrane. We develop differential and integral solutions of the coupled Poisson-Boltzmann system for a spherical vesicle and demonstrate that the integral solution is relatively flexible in formulating asymmetric configurations. Analytical results are formulated in terms of vesicle size, Debye length, and the surface charge density. The membrane stretching is characterized by the dimensionless group that defines the relative strength of the net electric force with respect to the membrane stiffness. We found that the self-induced electrostatic interaction will lead to a pre-stressed membrane although the small displacement is often negligible compared with the vesicle size. Quantitative analysis also reveals that the electric force can assist the vesicle in recovering its opening pore.

Derivation of Analytical Expressions for the Electrical Potential Distribution in Lipid Structures

The electrical potential inside a lipid structure, which is described by a modified Poisson-Boltzmann equation in the literature (Borukhov et al. Electrochim. Acta 2000, 46, 221), is solved, taking into account the effects of ionic sizes. Here, a micelle comprises an ionic surfactant layer and an aqueous core; the dissociation of the former yields a charged surface. The governing equation, which was solved numerically in a previous study for spherical geometry (Hsu et al. J. Phys. Chem. B 2003, 107, 14429), is solved analytically in this study for planar, cylindrical, and spherical geometries. The analytical results obtained are readily applicable for the evaluation of the spatial distributions of counterions inside a lipid structure. We show that if the linear size of a reverse micelle is fixed, the degree of dissociation of the surfactant layer follows the order planar > cylindrical > spherical.

Growth of linear charged micelles

The electrostatics of micellar growth is reviewed and extended for solutions containing excess salt. In dilute solution the expansion of a linear micelle with increasing salt concentration is explained for a wide range of ionic strength. When the micellar charge density is very high, counterions condense nonuniformly onto the micellar rod. In that case the micelle may contract upon the addition of salt. In semidilute solutions the excluded-volume effect is an additional factor complicating the ionic strength dependence of micellar growth.