Determination of both wet and dry mass of water-soluble polymers adsorbed on planar silica using a quartz crystal microbalance

It is well-established that polymer adsorption at a model planar interface can be studied using a quartz crystal microbalance (QCM). Normally, this technique reports both the adsorbed mass of polymer chains plus any bound or entrained solvent molecules. Thus the total adsorbed amount significantly exceeds that reported by optical reflectometry or determined from adsorption isotherms obtained for colloidal substrates using a supernatant depletion assay. Herein we report a new QCM approach whereby the dry adsorbed amount, Γdry, is obtained directly from the wet (solvated) adsorbed amount, Γwet, by switching from a liquid flow to a flow of nitrogen gas. The latter conditions lead to complete removal of the solvent, leaving only the desolvated adsorbed polymer chains. This strategy is exemplified for the adsorption of two well-known nonionic water-soluble polymers, poly(ethylene glycol) (PEG) and poly(N-vinylpyrrolidone) (PNVP), from aqueous solution onto a model planar substrate (silica). These two systems were selected to facilitate direct comparison with the literature, which validates this new approach.