Abstract
Self-assembly is a characteristic property of soft matter. Understanding the factors which assist or perturb this process is of great importance in many biological and industrial processes. Amphiphiles self-assemble and order into a variety of structures including well-ordered lamellar phases. The present work uses neutron reflectometry and neutron scattering to explore the effects of both interface roughness and temperature on the lamellar-phase structure of a non-ionic surfactant at a solid/liquid interface. The structure of concentrated solutions of tetraethyleneglycol dodecyl ether has been compared against a smooth surface and one with a roughness of the order of the lamellar spacing. This has been done in order to investigate the role perturbations have on the overall lamellar order, when these have length scales of the order of the interactions between neighboring lamellae. The results showed that the surfactant forms a well-ordered and aligned structure at a smooth surface, extending to a depth of several micrometers from the interface. Increasing the temperature of the sample and subsequent cooling promotes alignment and increases the number of oriented layers at the surface. The same sample forms a significantly less aligned structure against a rough surface that does not align to the same extent, even after heating. The perturbation of the structure caused by thermal fluctuations was found to be much less than that imposed by a small surface roughness.
Highlights
Amphiphiles with hydrophilic head groups and hydrophobic hydrocarbon tail regions occur widely as detergents, in biological membranes and in formulations for personal care and pharmaceutical products, coatings, etc. [1,2,3,4]
The present work uses neutron reflectometry and neutron scattering to explore the effects of both interface roughness and temperature on the lamellar-phase structure of a non-ionic surfactant at a solid/liquid interface
We explore the influence of surface roughness on multilayer structures to gain understanding of the extent that small geometric perturbations, with length scales approximately the same as the lamellar spacing, influence the long-range ordering of surfactants
Summary
Amphiphiles with hydrophilic head groups and hydrophobic hydrocarbon tail regions occur widely as detergents, in biological membranes and in formulations for personal care and pharmaceutical products, coatings, etc. [1,2,3,4]. The structure formed by amphiphiles can be perturbed by temperature, physical and chemical properties of surfaces, and the addition of other components such as solvents Understanding these different factors that induce or disturb the order of amphiphiles at interfaces is a major challenge, especially distinguishing between the effects when several perturbations are combined. We explore the influence of surface roughness on multilayer structures to gain understanding of the extent that small geometric perturbations, with length scales approximately the same as the lamellar spacing, influence the long-range ordering of surfactants. For this purpose, we have chosen a non-ionic surfactant, with known properties as a model system in a concentration range where ordered multilayer structures can form. The structure of the same surfactant dispersion is studied against a smooth and a rough surface at different temperatures, so that the effect of surface roughness can be distinguished from the perturbation due to thermal fluctuations
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