Effect of pH, surface charge and counter-ions on the adsorption of sodium dodecyl sulfate to the sapphire/solution interface

TL;DR

This study investigates how pH, surface charge, and counter-ions affect SDS surfactant adsorption on sapphire surfaces, revealing charge-dependent adsorption, layer structure, and ion-specific effects using neutron reflection.

Contribution

It provides detailed insights into the structure and composition of SDS layers on sapphire, highlighting ion-specific influences and the role of surface charge in adsorption behavior.

Findings

Adsorption decreases with increasing pH due to charge reversal.

Layer thickness is approximately 24 Å with 20% water content.

Cesium dodecyl sulfate forms denser, less hydrated layers.

Abstract

The role of ionic interactions between sodium dodecyl sulfate, SDS, and sapphire surfaces have been studied using specular neutron reflection to determine the structure and composition of adsorbed surfactant layers. Increasing the pH of the solution from 3 to 9 reduces the adsorption by reversing the charge of the alumina. This occurs at lower pH for the R-plane (1 -1 0 2) than the C-plane (0 0 0 1), corresponding to the different points of zero charge. The largest surface excess is about 6.5 micromol m-2, the thickness of the adsorbed layer is about 24 Angstrom and it contains roughly 20% water. The hydrocarbon tails of the surfactant molecules clearly interpenetrate rather than form an ordered bilayer. The structure is similar in either pure water or in 0.1 M NaCl when the surfactant is at the respective critical micelle concentration. Different structures were seen with lithium and cesium dodecyl sulfate. The CsDS forms dense layers with little or no hydration and a surface excess of about 10.5 micromol m-2. The metal cation strongly influences the hydration of the adsorbed surfactant. An overall picture of 'flattened micelles' for the structure of the adsorbed layer is observed