Issue 47, 2019

Understanding the structure and dynamics of cationic surfactants from studies of pure solid phases

Abstract

A homologous series of n-alkyl trimethylammonium bromide surfactants, H(CH2)nN+(CH3)3 Br, from C10TAB to C18TAB have been studied systematically in the bulk over a wide range of temperatures. Common features in the structures are identified, with packing dominated by the co-ordination of the cationic head groups with bromide anions and interdigitation of the hydrocarbon chains. This arrangement provides an explanation for the thin adsorbed bilayers that have been observed at various hydrophilic surfaces from aqueous solutions in previous studies. The molecular volumes and arrangement are comparable with structures of a number of different self-assembled amphiphiles. For these surfactants with bromide counter-ions, formation of crystal hydrates was not observed. The alkyl chains are highly mobile and at high temperatures a plastic phase is found for all materials with a transition enthalpy that is similar to the melting enthalpy of many long alkyl chains. Other unexpected phase transitions depend more markedly on the hydrocarbon chain length and evidently depend on delicate balances of the various contributions to the free energy.

Graphical abstract: Understanding the structure and dynamics of cationic surfactants from studies of pure solid phases

Supplementary files

Article information

Article type
Paper
Submitted
13 aug 2019
Accepted
24 sep 2019
First published
25 sep 2019
This article is Open Access
Creative Commons BY license

Phys. Chem. Chem. Phys., 2019,21, 25945-25951

Understanding the structure and dynamics of cationic surfactants from studies of pure solid phases

J. K. Cockcroft, A. Shamsabadi, H. Wu and A. R. Rennie, Phys. Chem. Chem. Phys., 2019, 21, 25945 DOI: 10.1039/C9CP04486H

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