Conformation of Jojoba oil esters at the air-water interface

Abstract

Wax esters (WE) are lipids highly abundant in nature (providing energy storage, buoyancy or preventing dessication) and a constituent of many technological applications (foods, cosmetics, paintings). However, their interfacial behavior has been scarcely described and focusing on those with saturated chains (thus in solid state) which don’t form stable monolayers. Previously, we evaluated the configuration of the WE from ”jojoba oil” (J) at the air-water interface and we associated the ability of the long chains of WE to form monomolecular layers with its physical state. Thus, we proposed that, in the solid phase, WE exhibited a linear configuration whereas in the fluid state it adopted a hairpin structure with an amphipathic-driven orientation, being this one the structure organizable like compressible monolayers. Such hypothetic configurations were derived just from molecular area measurements. In the present work we contributed with further experimental evidences by combining Langmuir isotherms, measurements of surface potential, PM-IRRAS analysis of J and atomic-scale molecular dynamic simulations of an ether representative of WE. The surface potential (SP) of the monolayers during compression exhibited a pattern similar to that of most glycerophospholipids. The maximal SP, derived from a model that considered two populations of oriented water, was very close to the experimental value. The orientation of the ester group that was assumed in that calculation was coherent with the PM-IRRAS behavior of the carbonyl group with the C=O oriented towards the water and the C-O lying along the surface and were in accordance with their orientational angles ( 45° and 90°, respectively) determined by atomic-scale molecular dynamic simulations. Taken together the present results confirm a hairpin configuration of WE at the air-water interface.