Mechanistic insights into modification of clay minerals by detergent-derived surfactants and their impact on petroleum hydrocarbon uptake

Understanding how surfactants interact with clay minerals is essential for controlling organoclay properties and improving hydrocarbon adsorption performance. However, distinguishing between surfactant adsorption on external surfaces and within interlayer galleries remains challenging using conventional techniques. In this study, we introduce a combined Washburn-immersion methodology that enables both quantification of total surfactant adsorption and localization of adsorbed species. Bentonite and kaolinite were modified using three commercial detergent formulations at concentrations below their experimentally determined critical micelle concentrations (CMC). The resulting changes in wettability, interlayer accessibility, and hydrocarbon uptake were systematically examined. For the first time, this study provides a quantitative framework to distinguish between external-surface adsorption and true interlayer incorporation of surfactants in clay minerals and directly links adsorption location to hydrocarbon uptake behavior. Bentonite exhibited significantly higher surfactant uptake (2.83-3.10 mg·g^-1) accompanied by substantial interlayer incorporation, resulting in enhanced organophilicity and a 13-33% increase in diesel and kerosene adsorption. In contrast, kaolinite showed lower surfactant uptake (2.46-2.70 mg·g^-1) dominated by surface and edge adsorption, leading to a 10-30% reduction in persistent diesel uptake, while kerosene adsorption remained essentially unchanged. Interlayer density analysis further confirmed that surfactant intercalation promotes denser hydrocarbon packing in bentonite but has negligible influence on kaolinite. This work introduces a robust and scalable diagnostic approach for distinguishing surface and interlayer adsorption in surfactant-clay systems, offering a practical framework for screening and optimizing surfactant-modified clays intended for hydrocarbon sorption under controlled conditions.