Surfactants at the solid–liquid interface: Measurements at higher concentrations using optical reflectometry

• A simple additive two-part model is used to analyse optical reflectivity data.
• Applied to simple mixed system (alcohol–water).
• Applied to strongly adsorbing layer (CTAB on hydrophilic silica).
• Non-adsorbing (depletion) system tested (SDS on hydrophilic silica).
• Quantitative analysis of depletion effects may be possible by optical reflectivity.

Both adsorbing and non-adsorbing surfactant solutions have been investigated by optical reflectivity (OR) on a hydrophilic silica surface over a wide range of solution concentrations. The use of the OR technique is tested at surfactant concentrations well above those usually employed with this approach. To establish a correlation with the OR response expected from these solutions, a simple additive two-part model is introduced comprising an interfacial term and a bulk solution term. The adsorbing system demonstrated the expected regular adsorption isotherm behaviour at lower solution concentrations, and at higher solution concentrations more closely fitted the bulk solution response. When this bulk response was subtracted from the total OR signal, a constant adsorbed amount was found, validating our two-part model approach. Whilst data obtained from alcohol–water mixtures also showed the expected non-adsorbing system response with the signal arising exclusively from the bulk solution, other systems (sucrose solutions and non-adsorbing sodium dodecylsulphate) showed more complex behaviour even after the solution refractive index changes had been subtracted. The implications of these data for probing depletion effects for these systems are discussed.

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