Studying the Structure of Sodium Lauryl Ether Sulfate Solutions Using Dissipative Particle Dynamics

Abstract

Sodium lauryl ether sulfate (SLES) is a common anionic surfactant used in a large number of personal care products. Commercial products typically contain a distribution in the number of ethoxy groups; despite this, there is limited existing work studying the effect of the ethoxy groups on the phase formation and structure. This is particularly important for the effect the structure has on the viscosity, an important consideration for commercial products. Dissipative particle dynamics is used to simulate the full phase diagram of SLES in water, including both micellar and lyotropic liquid crystal phases. Phase transitions occur at locations which are in good agreement with experimental data, and we find that these boundaries can shift as a result of varying the number of ethoxy groups. Varying the ethoxy groups has a significant effect on the micellar shape and crystalline spacing, with a reduction leading to more nonspherical micelles and decreased periodic spacing of the hexagonal and lamellar phases. Finally, while typical commercial products contain a distribution of ethoxy groups, computational work tends to focus on simulations containing a single chain length. We show that it is valid to use monodisperse simulations to infer behavior about solutions with a polydisperse chain length, based on its mean molecular length.

Investigating anionic surfactant phase diagrams using dissipative particle dynamics: development of a transferable model

“…At higher concentrations, a transition is seen to a lamellar phase at c ≃ 60%. 49 The phase progression of AES, as determined using DPD, is described in Fig. 11, and is found to be in quite good agreement with experimental data, with simulations showing the expected L 1 –H 1 –L α phase sequence.…”

“…11 Phases assigned to simulations of AES. Experimental phase boundaries are taken from Hendrikse et al 49 at 25 1C (data unavailable at higher temperatures).…”

“…We note that for perfect lamellar phase structures, then it is also possible to resolve g ( r ) parallel and perpendicular to the layer normal but when layers form spontaneously with defects in them, this approach is less robust. The periodic boundary conditions and finite box size restrict the lamellar layer spacing such that spacing d must satisfy 49 where κ i are the number of layers that have formed in dimension i , where i = x , y , z .…”

“…Due to the periodic boundary conditions, the layers must satisfy L cos θ = κd , where κ is the number of layers in the simulation box. 49 The combination of two approaches to calculating d allows us to be confident in our calculation of the lamellar spacing.…”

“…SDS and LAS molecules have been previously studied at a range of levels from all-atom studies of micellisation [36][37][38][39] and interfacial behaviour, 40,41 to coarse grain studies investigating mesophase structure, energetics, and interfacial properties. [42][43][44] While SDS and LAS have been studied previously using DPD and other coarse-grained methods, 43,[45][46][47][48][49] only a handful of these previous studies 46,47,49 have attempted to reproduce the experimental phase diagram across a range of concentrations. In this paper, we present a parametrised DPD model, which shows a high degree of transferability, not seen in previous works, which performs well across a wide concentration range and readily applies to different molecules.…”