DCI Transport through Dodecyl Sulfate Films on Salty Glycerol: Effects of Seawater Ions on Gas Entry

Gas-liquid scattering experiments were employed to measure the entry and dissociation of the acidic gas DCl into salty glycerol coated with dodecyl sulfate ions (DS- = CH3(CH2)(11)OSO3-). Five sets of salty solutions were examined: 0.25 and 0.5 M NaCl, 0.25 M MgCl2, 0.25 M CaCl2, and artificial sea salt. DS- bulk concentrations were varied from 0 to 11 mM, generating DS- surface coverages of up to 34% of a compact monolayer, as determined by surface tension and argon scattering measurements. DS- surface segregation is enhanced by the dissolved salts in the order MgCl2 approximate to CaCl2 > sea salt > NaCl. We find that DCl penetration through the dodecyl chains decreases at first gradually and then sharply as more chains segregate to the surface, dropping from 70% entry on bare glycerol to 11% for DS- surface concentrations of 1.8 x 10(14) cm(-2). When plotted against DS- surface concentration, the DCl entry probabilities fall within a single band for all solutions. These observations imply that the monovalent Na+ and divalent Ca2+ and Mg2+ ions do not bind differently enough to the ROSO3- headgroup to significantly alter the diffusive passage of DCl molecules through the dodecyl chains at the same DS- chain density. The chief difference among the salts is the greater propensity for the divalent salts to expel the soluble ionic surfactant to the surface.