Rheology of mixed solutions of sulfonated methyl esters and betaine in relation to the growth of giant micelles and shampoo applications

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.

Tensiometric and rheological investigations of single and mixed systems consisting of cocamidopropyl betaine (CAPB) and sodium dodecyl benzene sulfonate (SDBS) in aqueous solutions

The surface tension (σ), critical micelle concentration (CMC), surface excess (Γ), minimum area occupied by a surfactant molecule (A min), the viscosity and oscillatory rheological studies of aqueous solutions containing cocamidopropyl betaine (CAPB) and sodium dodecylbenzene sulfonate (SDBS) at molar fractions of 0.00, 0.25, 0.50, 0.75 and 1.00 and 25 °C were presented. CAPB and SDBS were not found to interact synergistically in water at any of the molar fractions studied. This is due to the fact that the critical micelle concentrations of these mixtures were higher than those predicted by Clint’s equation, indicating an antagonism that rarely occurs in mixed amphoteric/anionic surfactant systems. The minimum area occupied by a surfactant molecule (A min) was reduced in CAPB/SDBS mixtures compared to unmixed surfactants. In contrast, the viscosity of the mixed CAPB/SDBS system increased significantly from 1.0 mPa s to a maximum of 36.0 Pa s at higher CAPB mole fractions (0.5–0.8). The rheograms obtained from the oscillation measurements of the viscous CAPB/SDBS solutions are characteristic of wormlike micelles (WLMs) according to the Maxwell model. The results of this surprising binary CAPB/SDBS surfactant mixing system are presented and discussed.

Gradual transformation of anionic/zwitterionic wormlike micelles from viscous to elastic domains: Unravelling the effect of anionic surfactant chain length

HYPOTHESIS

Ultra-long tailed zwitterionic surfactants often form aqueous wormlike elastic micelles, whereas the shorter ones mainly exhibit spherical viscous micelles. Anionic surfactants are widely used to tune the micellar morphology from spherical into wormlike. Systematic investigations in the molecular level are insightful to understand the viscoelasticity regulative mechanism.

EXPERIMENTS

Anionic/zwitterionic hybrid wormlike micelles are composed of sodium alkylsulfate (SAS) homologues and dodecyl dimethyl amidopropyl hydroxyl sulfobetaine (DHSB). The formation of wormlike micelles was studied by employing rheometer, cryogenic transmission electron microscopy (cryo-TEM) and small angle X-ray scattering (SAXS) techniques. The effects of surfactant concentration, molar ratio, anionic surfactant chain length and temperature were investigated systematically.

FINDINGS

SAS promoted the formation of SAS/DHSB hybrid wormlike micelles. The increase in both chain length and molar ratio (x_SAS) of SAS are advantageous in the enhancement of viscosity. Interestingly, sodium hexadecylsulfate (SHS) endowed elastic wormlike micelles with thermally insensitive viscosity below its Krafft temperature (T_k), which was distinguished from the viscous ones formed by sodium octylsulfate (SOS). SAXS results showed that the size of SAS/DHSB wormlike micelles was primarily determinate by surfactants with longer hydrophobic tails.

Analytical modeling of micelle growth. 5. Molecular thermodynamics of micelles from zwitterionic surfactants

HYPOTHESIS

The critical micelle concentration, aggregation number, shape and length of spherocylindrical micelles in solutions of zwitterionic surfactants can be predicted by knowing the molecular parameters and surfactant concentrations. This can be achieved by upgrading the quantitative molecular thermodynamic model with expressions for the electrostatic interaction energy between the zwitterionic dipoles and micellar hydrophobic cores of spherical and cylindrical shapes.

THEORY

The correct prediction of the mean micellar aggregation numbers requires precise calculations of the free energy per molecule in the micelles. New analytical expressions for the dipole electrostatic interaction energy are derived based on the exact solutions of the electrostatic problem for a single charge close to a boundary of spherical and cylindrical dielectric media. The obtained general theory is valid for arbitrary ratios between dielectric constants, radii of spheres and cylinders, positions, and orientations of dipoles.

FINDINGS

The detailed numerical results show quantitatively the effects of the micelle curvature and dielectric properties of the continuum media on the decrease of the dipole electrostatic interaction energy. Excellent agreement was achieved between the theoretical predictions and experimental data for the critical micelle concentration, size and aggregation number of zwitterionic surfactant micelles. This study can be extended to mixed micelles of zwitterionic and ionic surfactants in the presence of salt to interpret and predict the synergistic effect on the rheology of solutions.

Strong Viscosity Increase in Aqueous Solutions of Cationic C22-Tailed Surfactant Wormlike Micelles

The viscoelastic properties and structure parameters have been investigated for aqueous solutions of wormlike micelles of cationic surfactant erucyl bis(hydroxyethyl) methylammonium chloride with long C22 tail in the presence inorganic salt KCl. The salt content has been varied to estimate linear to branched transition conditions due to screening of the electrostatic interaction in the networks. The local cylindrical structure and low electrostatic repulsion was obtained by SANS data. The drastic power law dependencies of rheological properties on surfactant concentrations were obtained at intermediate salt content. Two power law regions of viscosity dependence were detected in semi-dilute solutions related to “unbreakable” and “living” micellar chains. The fast contour length growth with surfactant concentration demonstrated that is in good agreement with theoretical predictions.

Heads or tails? The synthesis, self-assembly, properties and uses of betaine and betaine-like surfactants

Betaines are a key class of zwitterionic surfactant that exhibit particularly favorable properties, making them indispensable in modern formulation. Due to their composition, betaines are readily biodegradable, mild on the skin and exhibit some antimicrobial activity. Vital to their function, these surfactants self-assemble into diverse micellar geometries, some of which contribute to increased solution viscosity, and their surface activity results in strong detergency and foaming. As such, their behavior has been exploited in various applications from personal care (including shampoos and liquid soaps) to specific industrial fields (such as enhanced oil recovery). This review aims to inform the reader of the diverse range of different betaine and betaine-like surfactants that have been actively researched over the past three decades. Synthesis as well as both chemical and physical characterization of betaine surfactants are discussed, including small-angle scattering studies that indicate self-assembly structures and rheological data that demonstrates texture and flow. Stimulus responsive systems and exotic betaine analogs with enhanced functionality are also covered. Crucially, the connection between surfactant molecular architecture and function are highlighted, exemplifying precisely why zwitterionic betaine and related surfactants are so uniquely functional.

pH-Responsive Rheological Properties and Microstructure Transition in Mixture of Anionic Gemini/Cationic Monomeric Surfactants

Surfactant aggregates have long been considered as a tool to improve drug delivery and have been widely used in medical products. The pH-responsive aggregation behavior in anionic gemini surfactant 1,3-bis(N-dodecyl-N-propanesulfonate sodium)-propane (C₁₂C₃C₁₂(SO₃)₂) and its mixture with a cationic monomeric surfactant cetyltrimethylammonium bromide (CTAB) have been investigated. The spherical-to-wormlike micelle transition was successfully realized in C₁₂C₃C₁₂(SO₃)₂ through decreasing the pH, while the rheological properties were perfectly enhanced for the formation of wormlike micelles. Especially at 140 mM and pH 6.7, the mixture showed high viscoelasticity, and the maximum of the zero-shear viscosity reached 1530 Pa·s. Acting as a sulfobetaine zwitterionic gemini surfactant, the electrostatic attraction, the hydrogen bond and the short spacer of C₁₂C₃C₁₂(SO₃)₂ molecules were all responsible for the significant micellar growth. Upon adding CTAB, the similar transition could also be realized at a low pH, and the further transformation to branched micelles occurred by adjusting the total concentration. Although the mixtures did not approach the viscosity maximum appearing in the C₁₂C₃C₁₂(SO₃)₂ solution, CTAB addition is more favorable for viscosity enhancement in the wormlike-micelle region. The weakened charges of the headgroups in a catanionic mixed system minimizes the micellar spontaneous curvature and enhances the intermolecular hydrogen-bonding interaction between C₁₂C₃C₁₂(SO₃)₂, facilitating the formation of a viscous solution, which would greatly induce entanglement and even the fusion of wormlike micelles, thus resulting in branched microstructures and a decline of viscosity.

Solubility of ionic surfactants below their Krafft point in mixed micellar solutions: Phase diagrams for methyl ester sulfonates and nonionic cosurfactants

HYPOTHESIS

Many ionic surfactants with wide applications in personal-care and house-hold detergency show limited water solubility at lower temperatures (Krafft point). This drawback can be overcome by using mixed solutions, where the ionic surfactant is incorporated in mixed micelles with another surfactant, which is soluble at lower temperatures.

EXPERIMENTS

The solubility and electrolytic conductivity for a binary surfactant mixture of anionic methyl ester sulfonates (MES) with nonionic alkyl polyglucoside and alkyl polyoxyethylene ether at 5 °C during long-term storage were measured. Phase diagrams were established; a general 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 methodology, which utilizes the equations of molecular thermodynamics at minimum number of experimental measurements, is convenient for construction of such phase diagrams. The results could increase the range of applicability of MES-surfactants with relatively high Krafft temperature, but with various useful properties such as excellent biodegradability and skin compatibility; stability in hard water; good wetting and cleaning performance.

Adsorption of Equimolar Mixtures of Cationic and Anionic Surfactants at the Water/Hexane Interface

In mixed solutions of anionic and cationic surfactants, called catanionics, ion pairs are formed which behave like non-ionic surfactants with a much higher surface activity than the single components. In equimolar mixtures of NaCnSO4 and CmTAB, all surface-active ions are paired. For mixtures with n + m = const, the interfacial properties are rather similar. Catanionics containing one long-chain surfactant and one surfactant with medium chain length exhibit a strong increase in surface activity as compared with the single compounds. In contrast, catanionics of one medium- and one short chain surfactant have a surface activity similar to that of the medium-chain surfactant alone. Both the Frumkin model and the reorientation model describe the experimental equilibrium data equally well, while the adsorption kinetics of the mixed medium- and short-chain surfactants can be well described only with the reorientation model.