Viscosity of Nanoconfined Branched-Chain Fatty Acids Studied by Resonance Shear Measurements

Lubricant performance can be improved using additives such as organic friction modifiers (OFMs) and is influenced by their conformation and properties in the space confined between the substrate surfaces, rendering the detailed property analysis of confined OFMs and lubricants a matter of high practical significance. To date, studies on fatty acids as confined OFMs have mainly focused on linear- and unsaturated-chain molecules, leaving branched-chain structures underexplored. To bridge this gap, we used resonance shear measurements in this study to probe the viscosity of two branched-chain C₁₈ fatty acids (isostearic acid T and isostearic acid) confined between mica surfaces at different applied normal loads (L) and surface separation distances (D). The viscosity parameter (b_s) of both acids significantly increased at D < ∼4 nm because of structuring and was lower for isostearic acid than that for isostearic acid T at L > ∼0.6 mN. This reversal of bulk viscosity order under nanoconfinement was ascribed to the ability of the bulky methyl-substituted side chain of isostearic acid to prevent ordering in the nanospace between the mica surfaces and thus preserve fluidlike properties. The obtained results provide fundamental insights into the lubricity of branched-chain fatty acids and are expected to promote the development of novel high-performance OFMs.