The use of contact angle measurements to quantify the adsorption density and thickness of organic molecules on hydrophilic surfaces

Abstract

A relationship between the alkyl chain length of molecules separating hydrophilic surfaces from hydrophilic liquids and the resulting contact angle is established by considering both the molecular packing of the alkane chains and equations developed to describe contact angles on heterogeneous surfaces. A model is developed for the ideal packing of alkane molecules with particular reference to alcohol- and silane-coated silica surfaces. The model works reasonably well in describing the contact angle for alkylthioether-coated gold surfaces and alkyltrichlorosilane-treated, oxidised silicon surfaces. However, it does not adequately describe the contact angles for n-alcohol-coated, alkyldimethylsilane-coated or branched-alkylsilane-coated silica surfaces. The results are not a failing of the model; rather these molecules do not form close packed monolayers on silica surfaces. The limitations of this simple approach are recognised, but the approach does offer an acceptable explanation of much of the experimental contact angle data.