Issue 1, 2011

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 Cm+Cn with varied chain length mismatch (mn), using light microscopy, DSC, turbidity, surface tension, SAXS and SANS. The compounds consist of alkyltrimethylammonium alkylsulfonates, denoted by TAmSon. Depending on the asymmetry between both ions, three regimes can be identified: (i) weakly asymmetric; (ii) approximately symmetric; and (iii) highly asymmetric. For the TA16Son 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 TAmSo8 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.

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

Article information

Article type
Paper
Submitted
05 Jun 2010
Accepted
23 Aug 2010
First published
11 Oct 2010

Soft Matter, 2011,7, 225-236

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

B. F. B. Silva, E. F. Marques and U. Olsson, Soft Matter, 2011, 7, 225 DOI: 10.1039/C0SM00477D

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