engleski

Unusual Catanionic Surfactant Mixtures: Dodecylpyridinium Chloride/Sodium Cholate

Mixtures of oppositely charged surfactants (catanionic surfactant mixtures) exhibit a strong synergism of their bulk and surface properties mainly due to the electrostatic interactions between the surfactant head groups. In these mixtures, the catanionic surfactant precipitates at equimolar concentrations and various coexisting and single, nano- and/or microstructures form when solution composition shifts from the stoichiometric range. In dodecylpyridinium chloride (DPC) and sodium cholate (NaC) mixtures no precipitation occurs at stoichiometric concentration. However, DPC/NaC mixture exhibits strong synergism showing high surface activity and significant shift of the critical micelle concentration toward lower total surfactant concentration. In this study, surface tension, conductivity, light scattering and microelectrophoretic measurements were utilized to assess the formation of mixed monolayer at the air/solution interface and small mixed micelles in the bulk phase. The non-ideal interactions between surfactant molecules in the adsorbed and micellar state were interpreted according to the regular solution theory. Relatively high negative values of the molecular interaction parameters indicated a strong attraction between DPC and NaC in both mixed monolayer and mixed micelles. This phenomenon could be attributed to the strong electrostatic attractions between the oppositely charged surfactants. However, the less negative surface molecular interaction parameter than the micellar molecular interaction parameter indicated slightly weaker interaction between two surfactants in the adsorbed state. The compositions of mixed monolayer and mixed micelles were not equal to the bulk solution composition suggesting enrichment in the cationic surfactant. The asymmetry of the mixed monolayer composition decreased with the increasing mole fraction of DPC in solution. The composition of mixed micelles was only moderately changed. The mole fraction of DPC in mixed micelles ranged from 0.45 to 0.55 with varying mole fraction of DPC in the mixture from 0.25 to 0.75. The activity coefficients, which reflect the intensity of intermolecular interactions, were << 1 indicating a significant deviation from the ideal mixing in both adsorbed and micellar state. The interplay between electrostatic effects, geometry of molecules and dissimilar separation of hydrophobic and hydrophilic moieties in surfactant molecules dictated the unusual behaviour of DPC/NaC mixture with small and narrowly distributed mixed micelles being formed throughout the entire concentration range evaluated. It is likely that the bulky pyridinium head groups of DPC combined with the bulkiness of the planar steroid non-planar part of NaC created sterically unfavourable conditions for the precipitation, larger micelles and/or vesicles formation to take place in DCP/NaC mixtures.

catanionic surfactant; association; adsorption at interface

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