Aqueous phase behavior of salt-free catanionic surfactants: the influence of solubility mismatch on spontaneous curvature and balance of forces

Abstract

In this paper, we investigate the phase behavior and microstructure for a series of salt-free catanionic surfactants of the type C_m⁺C_n⁻ with varied chain length mismatch (m ≠ n), using light microscopy, DSC, turbidity, surface tension, SAXS and SANS. The compounds consist of alkyltrimethylammonium alkylsulfonates, denoted by TA_mSo_n. Depending on the asymmetry between both ions, three regimes can be identified: (i) weakly asymmetric; (ii) approximately symmetric; and (iii) highly asymmetric. For the TA₁₆So_n compounds, with n = 8 and 9 (weakly asymmetric), the surfactant forms a lamellar phase in water, however, with a striking miscibility gap. This miscibility gap is a consequence of the concentration dependent bilayer charge density. For n = 8, also a temperature-dependent vesicle-to-micelle transition at low surfactant concentration is observed. When the mismatch is low (n = 10) only a non-swelling lamellar phase is formed (approximately symmetric regime). For high mismatch, (n = 6 and 7) an extensive micellar phase is obtained – highly asymmetric regime. Conversely, for the TA_mSo₈ compounds, where m = 12 and 14, the unconventional lamellar miscibility gap and vesicle-micelle transition are again present. These findings are rationalized by considering the effect of film charge density—arising from the chain solubility difference—on the spontaneous curvature and balance of colloidal forces. The type of phase behavior reported here should be extensive to other families of salt-free catanionic amphiphiles, where an appropriate tuning of the solubility mismatch can allow the control of self-assembly.