Tuning of physico-chemical characteristics of charged micelles by controlling head group interactions via hydrophobically and sterically modified counter ions

Abstract

Different tetraalkylammoniums, viz., N⁺(CH₃)₄, N⁺(C₂H₅)₄, N⁺(C₃H₇)₄, N⁺(C₄H₉)₄ along with simple ammonium salts of dodecylbenzene sulphonic acid were prepared by an ion-exchange technique. The cmcs of dodecylbenzene sulfonate salts with various counterions were determined by electrical conductivity and surface tension measurements within the temperature range 283–313 K. The counterion ionization constant, α, the surface parameters Γ_max and A_min and also the thermodynamic parameters of the micellization process, viz., ΔG⁰_m, ΔH⁰_m and ΔS⁰_m in aqueous solution have been determined by using a pseudo-phase model. The order of cmcs in aqueous solution is found to be NH₄⁺ > N⁺(CH₃)₄ > N⁺(C₂H₅)₄ > N⁺(C₃H₇)₄ > N⁺(C₄H₉)₄ at any given temperature. On the other hand, the aggregation number increases with alkyl chain length first due to increasing hydrophobic interactions and then decreases as a function of counterion size passing through a maximum for N⁺(C₂H₅)₄. Spontaneity of micellization in aqueous solution is supported by large negative ΔG⁰_m as well as the positive entropy change for the micellization process for all the above counterions. At a given temperature, ΔG⁰_m for a surfactant with different counterions followed the order N⁺(CH₃)₄ > NH₄⁺ > N⁺(C₂H₅)₄ > N⁺(C₃H₇)₄ > N⁺(C₄H₉)₄. Electrostatic interaction along with effective charge screening and hydrophobicity of the surfactant head group together may give an explanation for the observed variation of aggregation behaviour and the energetics as a function of the nature of the counterion.