The effect of surface topography on the micellisation of hexadecyltrimethylammonium chloride at the silicon-aqueous interface

Abstract

Amphiphilic aggregation at solid-liquid interfaces can generate mesostructured micelles that can serve as soft templates. In this study we have simulated the self-assembly of hexadecyltrimethylammonium chloride (C16TAC) surfactants at the Si(1 0 0)- and Si(1 1 1)-aqueous interfaces. The surfactants are found to form semicylindrical micelles on Si(1 0 0) but hemispherical micelles on Si(1 1 1). This difference in micelle structure is shown to be a consequence of the starkly different surface topographies that result from the reconstruction of the two silicon surfaces, and reveals that micelle structure can be governed by epitaxial matching even with non-polar substrates.

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“…Such micelle morphology and orientation with respect to the protruding ridges on the reconstructed Si(100) surface maximizes the strength of the hydrophobic interactions between CTAC and the substrate. 45 AFM data confirm the formation of a highly oriented surfactant structure on the Si(100) surface (see Figure 3d and Figure S4). The 0.7 nm height and 6.5 nm spacing between the neighboring stripes in the AFM images are in-line with those previously observed for CTAB on graphite, an almost identical surfactant on a hydrophobic surface; this pattern was interpreted to be that of hemicylindrical structures.…”

“…Moreover, these micelles follow the topography of the Si substrate, resulting from the c (4 × 2) reconstruction of the (100) surface, and are arranged in a regular array with the long axis along Si[100] direction (Figure d). Such micelle morphology and orientation with respect to the protruding ridges on the reconstructed Si(100) surface maximizes the strength of the hydrophobic interactions between CTAC and the substrate . AFM data confirm the formation of a highly oriented surfactant structure on the Si(100) surface (see Figure d and Figure S4).…”

“…On this surface, surfactants do not retain their hexagonal micelle structure. Instead, as revealed by our AFM data (Figure d and Figure S4) and molecular dynamics simulations, surface symmetry dictates the formation of hemispherical CTAC micelles . These micelles are randomly arranged on Si(111) and do not support the formation of a well-ordered three-dimensional template.…”

“…In the concentration range of 45–70 wt %, CTAC is known to form cylindrical micelles in aqueous solution at room temperature . Our molecular simulations predicted that such cylindrical micelle geometry is retained upon micelle adsorption on the Si(100) surface . Moreover, these micelles follow the topography of the Si substrate, resulting from the c (4 × 2) reconstruction of the (100) surface, and are arranged in a regular array with the long axis along Si[100] direction (Figure d).…”

“…44 Our molecular simulations predicted that such cylindrical micelle geometry is retained upon micelle adsorption on the Si(100) surface. 45 Moreover, these micelles follow the topography of the Si substrate, resulting from the c(4 × 2) reconstruction of the (100) surface, and are arranged in a regular array with the long axis along Si[100] direction (Figure 3d). Such micelle morphology and orientation with respect to the protruding ridges on the reconstructed Si(100) surface maximizes the strength of the hydrophobic interactions between CTAC and the substrate.…”