Transition temperatures and contact angles in the sequential-wetting scenario of n-alkanes on (salt) water

TL;DR

This paper investigates the complex wetting transitions of medium-chain alkanes on water and brine, combining theoretical calculations of Hamaker constants with modified Cahn theory to predict transition temperatures and analyze contact angle variations.

Contribution

It introduces a detailed theoretical framework to accurately predict first-order wetting transition temperatures for n-alkanes on water and brine, incorporating both long-range and short-range force effects.

Findings

First-order transition temperatures can be accurately predicted using the modified Cahn theory.

The Hamaker constant's sign change correlates with the critical transition point.

Contact angles vary significantly with temperature, reflecting wetting state changes.

Abstract

Alkanes of medium chain length show an unusual wetting behavior on (salt) water, exhibiting a sequence of two changes in the wetting state. When deposited on the water surface at low temperatures, the liquid alkane forms discrete droplets that are interconnected only by a molecularly thin film. On increase of the temperature, there occurs a sudden jump of the film thickness and, at this first-order transition, a mesoscopically thick layer of liquid alkane is formed. Heating the system further leads to a divergence of the film thickness in a continuous manner. While the long-range forces between substrate and adsorbate are responsible for the critical transition, which occurs at the temperature at which the Hamaker constant changes sign, it is primarily the short-range components of the forces that bring about the first-order transition. We calculate the Hamaker constant of the system and show how, within a modified Cahn theory, accurate predictions of the first-order transition temperatures can be obtained for n-alkanes (pentane and hexane) on water and even on brine. Furthermore, we study the variation of the contact angle as a function of temperature.