Computer simulations of catanionic surfactants adsorbed at air/water interfaces. II. Full coverage

Abstract

We extend our previous molecular dynamics experiments ͓Rodriguez et al., J. Phys. Chem. B 109, 24427 ͑2005͔͒ to the analysis of the adsorption of catanionic surfactants at water/air interfaces, at a surfactant coverage close to that of the saturated monolayer: 30.3 Å 2 per headgroup. The mixture of surfactants investigated corresponds to equal amounts of dodecytrimethylammonium ͑DTA͒ and dodecylsulfate ͑DS͒. The structure of the interface is analyzed in terms of the local densities and orientational correlations of all relevant interfacial species. In accordance with experimental evidence, the DTA headgroups penetrate deeper into the aqueous substrate than the DS ones, although the average positions of all headgroups, with respect to the interface, lie in positions somewhat more external than the ones observed at lower coverages. Average tail tilts are close to 45°. The characteristics of the headgroup-water substrate correlations are also analyzed using a tessellation procedure of the interface. The density and polarization responses of the interfacial domains closest to the DS headgroups are enhanced, compared to those adjacent to the DTA detergents. Dynamical aspects related to the diffusion and to the orientational correlations of different water layers in close contact with the surfactant are also investigated.

Physicochemical studies on ion-pair amphiphiles: Solution and interfacial behaviour of systems derived from sodium dodecylsulfate and n-alkyltrimethylammonium bromide homologues

“…Moreover, limited studies on the interfacial behaviour of the IPAs at the air/water interface are available in literature. [15][16][17][26][27][28][29][30] Literature survey has been revealed that investigations on IPAs with systematic variation of the chain length of one of the components are only limited. 31,32 The study of Estoe et al 31 of aqueous solutions containing asymmetric chain catanionics of the homologous series n-propylammonium to n-heptylammonium dodecylsulphate (C 3 -C 12 to C 7 -C 12 ) has shown that the CMC of the catanionics decrease with increasing amine chain length where as the mean area per surfactant molecule and the mean area per hydrocarbon chain in the monolayer increase.…”

“…[15][16][17] Although different phases can be identified at the interface but their characteristic features and structures are still the subject of intensive research. Different techniques have been employed to study the phase behaviours and the interfacial morphologies of the IPAs; they are epifluorescence, polarization and Brewster angle microscopy, etc.…”

Diffusion at the liquid-vapor interface

“…[8][9][10][11] Similar systems of interest include aqueous solutions [16][17][18]46 ͑the latter is a useful review on ion solvation at liquid surfaces͒ and surfactants at interfaces. [13][14][15]21 Surfactants will be usually located at the surface and most will reach the diffusive regime. The relationship between structure and dynamics is specially interesting in systems with strong profiling, such as liquid metals, 20 confined water, 22,23,25,27,28 and liquid-solid interfaces.…”

Tristearin bilayers: structure of the aqueous interface and stability in the presence of surfactants

“…MD simulations have been widely used to investigate surfactant systems, however, sodium dodecyl sulphate (SDS) [28][29][30][31][32][33][34] or hexadecyltrimethylammonium bromide (CTAB) 29,35-38 have been investigated more extensively than sodium dodecylbenzene sulphonate (SDBS). Because of this, our chosen FF must be able to capture both the TS and SDBS and their mutual interactions, in the presence of water.…”

“…Because of this, our chosen FF must be able to capture both the TS and SDBS and their mutual interactions, in the presence of water. MD simulations have been widely used to investigate surfactant systems, however, sodium dodecyl sulphate (SDS) [28][29][30][31][32][33][34] or hexadecyltrimethylammonium bromide (CTAB) 29,[35][36][37][38] have been investigated more extensively than sodium dodecylbenzene sulphonate (SDBS). 33,39,40 Part of the reason for this may be ascribed to the very low critical micelle concentration (CMC) of SDBS, #3.0 mM under standard conditions, 41,42 compared with $8.3 mM for SDS, 42 meaning that the study of SDBS via simulation typically requires the simulation of systems comprising a large number of water molecules.…”