X-ray evanescent wave-induced fluorescence study of adsorption of a sulfonated polystyrene ionomer from dimethyl sulfoxide to the solution / vapor interface

We have studied the adsorption of an ionomer, poly(styrene-ran-manganese styrene sulfonate), from dimethyl sulfoxide solution to the solution/vapor interface at 20°C. The polymer sulfonation level was 15 mol% and the solution concentration studied was 1.00% (wt/wt). The adsorption was followed over time (4 days) by X-ray evanescent wave-induced fluorescence (XEWIF), which is a non-destructive probe of the polymer adsorption profile. We found that a state of apparent equilibrium was reached after a 4-day period and that the amount of polymer adsorbed was rather larger than anticipated from surface tension measurements. This anomaly is explained by the influence of image charge forces, which increase the surface energy and must be balanced with the surface energy reduction of placing the non-polar repeat segments at the surface. We suggest that this effect may be generalized to aqueous polyelectrolytes.