Aggregation properties of zwitterionic surfactants with different ionic headgroups, hydrophobic chain length and inter-charge spacers

Abstract

Three series of zwitterionic surfactants with different ionic headgroups, various hydrophobic chain length, and flexible or rigid inter-charge spacers have been synthesized and their aggregation behaviors in aqueous solution have been studied. Series I and III represent [C n H 2 n+1 (CH 3) 2N +(CH 2) 6N +(CH 3) 2CH 2CH 2CH 2SO 3 −]Br − (C n C 6Tri, n = 12, 14, and 16) and C n H 2 n+1 (CH 3) 2N +CH 2CH 2CH 2SO 3 − ( n = 12, 14, and 16), respectively. Series II are [C 14H 29(CH 3) 2N +C sN +(CH 3) 2CH 2CH 2CH 2SO 3 −]Br – (C 14C sTri, C s = –(CH 2) 2–, –(CH 2) 6–, –(CH 2) 8–, –(CH 2) 10–, p–xylyl), where C s stands for the inter-charge spacer connecting two quaternary ammonium groups. The critical micelle concentrations ( CMC) of the surfactants with two cationic ammonium groups and one anionic sulfonate group (Series I and II) are much larger than those of the corresponding surfactants with one cationic ammonium group and one anionic sulfonate group (Series III). These surfactants exhibit obviously different aggregate morphologies. For Series III, only micelles are observed. For Series I and II, if the surfactants have short or rigid inter-charge spacer (C 14C 2Tri and C 14C pxTri), the aggregates also appear as micelles and have more exothermic micellization enthalpy change. However, even for Series I and II, if the surfactants have longer and flexible inter-charge spacer and longer hydrocarbon chains (C 14C 6Tri, C 16C 6Tri, C 14C 8Tri and C 14C 10Tri), vesicles are observed due to the reduced hydrophilic moiety size and the strong hydrophobic interaction, whereas vesicles cannot be formed if the surfactants have longer and more flexible inter-charge spacer but without enough longer hydrocarbon chain (C 12C 6Tri).