m-s-m Cationic Gemini and Zwitterionic Surfactants-Spacer-Dependent Synergistic Interactions

Properties and Applications of Quaternary Ammonium Gemini Surfactant 12-6-12: An Overview

Surfactants are amphiphilic molecules and one of the most versatile products of the chemical industry. They can be absorbed at the air-water interface and can align themselves so that the hydrophobic part is in the air while the hydrophilic part is in water. This alignment lowers the surface or interfacial tension. Gemini surfactants are a modern variety of surfactants with unique properties and a very wide range of potential applications. Hexamethylene-1,6-bis(N-dodecyl-N,N-dimethylammonium bromide) is one such representative compound that is a better alternative to a single analogue. It shows excellent surface, antimicrobial, and anticorrosion properties. With a highly efficient synthetic method and a good ecological profile, it is a potential candidate for numerous applications, including biomedical applications.

The partitioning of primary alcohols into the aggregates of gemini amphiphiles determined from diffusion NMR experiments

The partition constants (p-values) of primary alcohols in solutions containing aggregates of symmetric gemini surfactants of the family N,N'-dimethyl, N-dialkyl-α,ω-alkanediammonium dibromide (m-s-m = symmetric gemini surfactants) have been computed from the measured values of their diffusion coefficients obtained from NMR-diffusion experiments. From the p-values, both mole-fraction and concentration-based partition coefficients and Gibbs energies of transfer for the alcohols from the bulk D2O phase to the gemini aggregate phase have been calculated. As expected, the Gibbs energy of transfer decreased linearly with an increase in the alcohol carbon length for each of the primary alcohol/gemini amphiphile series studied. The Gibbs transfer energy increment per CH2 for the alcohols was consistent for all the alcohol/gemini amphiphile series and was in excellent agreement with the values measured for the same primary alcohol series in conventional single-headed, single-tailed surfactants. Surprisingly, the partition coefficients of the alcohols in the symmetric gemini aggregates exhibited little, if any, dependence on the spacer length of the gemini amphiphiles and were remarkably consistent as the length of the main surfactant chain increased at constant spacer length. When these results are compared to the partition coefficients of the same alcohols in corresponding monomeric surfactants, we observe little difference in the thermodynamic driving forces governing the transfer of alcohols from water to the aggregates of either monomeric or symmetric gemini surfactants.

Improved Green Synthesis and Crystal Structures of Symmetrical Cationic Gemini Surfactants

Gemini surfactants are composed of two hydrocarbon tails with corresponding polar headgroups, linked via a covalent spacer. The synthesis of these surfactants is a very active area of research due to their application as catalysts and other applied areas of study. The modification of green microwave techniques developed in our research on ionic liquids has resulted in the significant improvement of the synthesis of N,N'-bis(dimethylalkyl)-α,ω-alkanediammonium dibromide (m-s-m type) symmetrical gemini surfactants. This approach utilizes a remarkably more economical, green, and sustainable methodology for the production of symmetrical gemini surfactants that can be utilized in numerous commercial applications. The improved synthetic approach of these gemini surfactants has led to the characterization of their crystalline packing for the first time ever using X-ray crystallographic analysis.

Adsorption and Aggregation Behavior of Mixtures of Quaternary-Ammonium-Salt-Type Amphiphilic Compounds with Fluorinated Counterions and Surfactants

The surface adsorption and aggregation behavior of a mixture of quaternary-ammonium-salt-type amphiphilic monomeric compounds (C₄ FSA, C₈ FSA, and C₄ NTf₂) or gemini compounds (C₁₀-2-C₄ FSA) and various surfactants (nonionic hexaoxyethylene dodecyl ether (C₁₂EO₆), anionic sodium dodecyl sulfate (SDS), cationic dodecyltrimethylammonium bromide (C₁₂TAB), and zwitterionic N-dodecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate (C₁₂Sb)) was investigated. Both types of compounds contained alkyl chains of nonidentical lengths that used bis(fluorosulfonyl)imide (FSA^-) or bis(trifluoromethanesulfonyl)imide (NTf₂^-) as counterions. The mixtures were analyzed for surface tension, viscosity, electrical conductivity, and pyrene fluorescence, in addition to evaluation by cryogenic transmission electron microscopy, small-angle X-ray scattering, and dynamic light scattering. Our results showed that the surface tension depended on the surfactant structure. For the mixture of C₈ FSA and SDS, as the SDS concentration increased, the surface tension first decreased and became constant at the critical micelle concentration (CMC). In this concentration range, C₈ FSA and SDS were approximately equimolar (2.5 mmol dm^-3), the mixture adsorbed efficiently at the air-water interface, and vesicles and linear-type micelles were formed in the solution owing to the decreased electrostatic repulsion between the hydrophilic groups. As the SDS concentration further increased, the surface tension increased and reached another constant value. The C₈ FSA at the interface was replaced by SDS and the aggregates transformed into spherical micelles. The surface tension plot of the mixture of the amphiphilic compounds and C₁₂Sb showed a minimum at the CMC. The lowest CMC and surface tension were observed for C₁₀-2-C₄ FSA, indicating that the gemini compounds offer excellent adsorption and orientation at the air-water interface. It was revealed that the quaternary-ammonium-salt-type amphiphilic compounds in this study acted as ionic liquids on their own and as surfactants in aqueous solution. Further, they could improve the surface activity of conventional ionic surfactants.

Can the Isothermal Calorimetric Curve Shapes Suggest the Structural Changes in Micellar Aggregates?

Inspired by the unusual shapes of the titration curve observed for many surfactants and mixed colloidal systems, we decided to extend the analysis to isothermal titration calorimetric curves (ITC) by paying special attention to potential structural changes in micellar aggregates. In this paper, we used isothermal titration calorimetry in conjunction with Scanning Transmission Electron Microscopy (STEM), Small-Angle Neutron Scattering (SANS) and X-ray Scattering (SAXS) methods support by Monte Carlo and semiempirical quantum chemistry simulations to confirm if the isothermal calorimetric curve shape can reflect micelle transition phenomena. For that purpose, we analysed, from the thermodynamic point of view, a group of cationic gemini surfactants, alkanediyl-α,ω-bis(dimethylalkylammonium) bromides. We proposed the shape of aggregates created by surfactant molecules in aqueous solutions and changes thereof within a wide temperature range. The results provide evidence for the reorganization processes and the relationship (dependence) between the morphology of the created aggregates and the conditions such as temperature, surfactant concentration and spacer chain length which affect the processes.

m-s-m cationic gemini and zwitterionic surfactants - a thermodynamic analysis of their mixed micelle formation

Micelle formation enthalpies (Δ_micH values) have been calorimetrically determined at 298 K for three sets of mixed zwitterionic/cationic gemini systems consisting of N-dodecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate (ZW3-12) and a series of structurally related gemini surfactants, the N,N'-bis(dimethyldodecyl)-α,ω-alkanediammonium dibromide (12-s-12) systems. From the experimental and the estimated ideal micelle formation enthalpies, the excess enthalpies were obtained. The degrees of nonideality of the interaction in the mixed micelle (β^m) from our previous work was used along with the excess enthalpy values to determine excess thermodynamic quantities of the surfactants in the mixed system according to Regular Solution Theory (RST) and Motomura's theory. The excess enthalpies for the ZW3-12/12-4-12 were positive in magnitude and rose sharply when small amounts of the zwittergent were distributed into the gemini micelles. The excess enthalpies for the ZW3-12/12-5-12 and the ZW3-12/12-6-12 systems were also >0 kJ mol⁻¹, and as a function of zwittergent composition, were quite different to those of the ZW3-12/12-4-12 mixed micelles. These results indicate that the heat of mixed micelle formation is strongly dependent on electrostatic interactions and the structure of the surfactants involved, specifically, the length of the tether group for the 12-s-12 gemini surfactants. From the calorimetric data and the application of RST and Motomura's theory, we have obtained estimates of the excess Gibbs energy and entropy of mixing. An analysis of the three thermodynamic properties suggests that the relative contributions of enthalpic and entropic effects to nonideal behavior for mixed micelles involving gemini surfactants are strongly dependent on the gemini structure.

Excess enthalpies of micelle formation for the ZW3-12/12-s-12 system as a function of the mole fraction of zwittergent (α_ZW3-12).