Shear-dependent slip of an amphiphilic n-hexanol self-assembled monolayer in a calcite nanopore.

Abstract

Amphiphilic molecules at solid-liquid interfaces can substantially modify molecular transport under nanoscale confinement, yet the coupling between the self-assembled monolayer (SAM) structure and flow remains poorly understood. Here, molecular dynamics simulations are used to investigate n-hexanol confined between calcite surfaces and to determine how its interfacial self-assembly influences pressure-driven transport. n-Hexanol spontaneously forms densely packed SAMs through strong interactions between hydroxyl headgroups and the calcite surface, producing a hydrophobic methyl-terminated interface. The SAM remains anchored under flow, while its molecular orientation becomes progressively broader with increasing driving force. Most notably, the slip length exhibits a non-monotonic dependence on the shear rate: it initially increases with shear rate and subsequently decreases at higher shear rates. We interpret this behavior as a competition between low interfacial friction associated with an ordered SAM and enhanced momentum dissipation associated with shear-induced conformational disorder and tail reorientation. Thus, the SAM acts not as a passive hydrophobic coating but as a shear-responsive molecular boundary layer. In-plane molecular diffusion remains Fickian, whereas motion normal to the surfaces is strongly restricted and becomes sub-diffusive as a consequence of confinement and interfacial ordering. Under comparable driving conditions, n-hexanol also exhibits higher streaming velocities than several simple hydrocarbons reported for calcite nanopores, despite the reduction in effective pore aperture caused by SAM formation. These results demonstrate that interfacial amphiphilic organization can provide a molecularly tunable boundary condition for confined transport and establish a mechanistic framework for shear-responsive lubrication in mineral nanopores.

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