Aqueous Hexadecyltrimethylammonium Acetate Solutions:  pH and Critical Micelle Concentration Evidence for Dependence of the Degree of Micelle Ionic Dissociation on Acetate Ion Concentration

Abstract

The counterion binding parameters (β) of aqueous hexadecyltrimethylammonium acetate (C16TAOAc) are estimated from pH data at various C16TAOAc-AcOH concentrations with or without added sodium acetate buffer. In contrast to what is usually reported for ionic amphiphiles associated with less hydrophilic counterions, β varies from ca. 0.4 to ca. 0.7 and depends upon the acetate ion concentration in the aqueous phase. The critical micelle concentrations (cmc) for C16TAOAc, C16TAOAc-NaOAc, C16TAOAc-AcOH, and C16TAOAc-NaOAc-AcOH solutions, measured from extinction data for fluorescence of acridine, are also examined in terms of AcOconcentration dependence. The plot of the logarithm of the amphiphile activity at cmc vs the logarithm of acetate ion activity is markedly curved, as expected for significant variations in β.

Growth of Cetyltrimethylammonium Chloride and Acetate Micelles with Counterion Concentration

“…In this context, specific counterion effects on micellar aggregation numbers are known to accompany trends in counterion binding selectivities (1, 2, 5-7), and the acetate ion is known to be more weakly bound to cationic micelle surfaces than the chloride ion (15)(16)(17)(18)(19). Many of the properties of CTAOAc solutions, in particular the variation of ␣ with detergent concentration (23), are reminiscent of those of CTA micelles with highly hydrated counterions such as fluoride (33,34) and hydroxide (33)(34)(35). The aggregation numbers of CTAOAc micelles are indeed quite similar to those reported for CTAOH micelles (7,35).…”

Effects of Polyelectrolytes on the Structures and Interactions of Surfactant Aggregates

“…But this cannot explain the result that the PA ions actually dominate (x Br < 0.5) the initial dilute phases that separate out. We attributed the latter effect to the intrinsically high ‘chemical‘ affinity of bromide ions to the surfactant aggregates. − However, Figure also shows that, as the mixtures are progressively diluted with water, the phase compositions “swing over” more and more toward the alternative mixing plane. This we interpret 19 in the following way: As the system becomes more and more dilute, the translational entropy per charge of the ions becomes more and more important.…”

Phase behavior of gemini surfactant hexylene-1,6-bis(dodecyldimethylammonium bromide) and polyelectrolyte NaPAA

“…In this regard, the phase separation process can be described as an ion-exchange process [31]. Nevertheless, Br − has a stronger affinity with the surfactant surface than PAA − [33][34][35][36], and hence its concentration increases with increasing surfactant concentration. When c s,total is increased to a certain value, the gel phase transforms into a birefringent phase which is probably nematic, hexagonal or lamellar due to a decreased curvature in the surfactant aggregates by increasing bromide ions.…”