Specific ion effects on the self-assembly and interfacial properties of double- and single-chain cationic amphiphiles

Abstract

Understanding the specific ion effects (SIE) on the self-assembly of ionic amphiphiles is a key unresolved issue in colloid and interface chemistry. We investigated the SIE on interfacial properties, aggregation behaviours, and interfacial molarities of dioctyldimethylammonium (DiC8X) and tetradecyltrimethylammonium (TTAX) (X = Br, Cl, and Ac) surfactants. Our results demonstrate strong SIE on the aggregation behaviours in both single-chain and double-chain surfactants. The aggregation behaviour depends on the net effect of nonspecific interactions and SIE. Vesicle-micelle transitions are observed exclusively in the double-chained surfactants studied in this work, regardless of their tail symmetry, and are absent in the single-chained surfactants examined. These transitions are counterion-dependent, occurring only with bromide, while the surfactants bearing acetate consistently form vesicles. The chemical trapping (CT) results reveal several key insights on interfacial molarities: 1) double-chain surfactants exhibit higher interfacial water molarity compared to single-chain surfactants, with counterion molarity showing an inverse trend; 2) within both TTAX and DiC8X classes, the interfacial water molarity is highest for surfactants with bromide, followed by chloride, and lowest for acetate; 3) for surfactants undergoing vesicle-micelle transitions, both interfacial water and bromide molarity increase with rising surfactant concentration at low concentration ranges, indicating looser interfacial packing with increasing concentration; and 4) the combined CT and molecular dynamic simulations indicate that acetate penetration into interfacial regions significantly increases with surfactant concentration in TTAAc and DiC8Ac aggregates, explaining the consistent vesicle formation of amphiphiles with acetate. Overall, this study provides critical insights into the delicate balance-of-forces controlling the morphologies of aggregates composed of ionic amphiphiles.