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

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 [CnH2n+1(CH3)2N+(CH2)6N+(CH3)2CH2CH2CH2SO3−]Br− (CnC6Tri, n = 12, 14, and 16) and CnH2n+1(CH3)2N+CH2CH2CH2SO3− (n = 12, 14, and 16), respectively. Series II are [C14H29(CH3)2N+CsN+(CH3)2CH2CH2CH2SO3−]Br– (C14CsTri, Cs = –(CH2)2–, –(CH2)6–, –(CH2)8–, –(CH2)10–, p–xylyl), where Cs 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 (C14C2Tri and C14CpxTri), 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 (C14C6Tri, C16C6Tri, C14C8Tri and C14C10Tri), 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 (C12C6Tri).