Adsorption of Sodium Dodecyl Sulfate in the Presence of Poly(ethylenimine) and Sodium Chloride Studied Using Sum Frequency Vibrational Spectroscopy

The ordering of sodium dodecyl sulfate, SDS, at the hydrophobic solid/aqueous solution interface has been studied as a function of the addition of the cationic polyelectrolyte poly(ethylenimine), PEI, and the univalent electrolyte sodium chloride (NaCl) to solution. The nonlinear optical technique of sum frequency vibrational spectroscopy (SFS) was employed to determine the ordering of the alkyl chain of the adsorbed surfactant. Sodium chloride was found to promote the onset of ordered SDS adsorption at lower bulk concentrations, a result interpreted primarily in terms of electrostatic shielding, with a minor contribution from salt-induced headgroup counterion condensation. Introduction of PEI to electrolyte-free SDS solutions was found to promote synergistic SDS adsorption through polyelectrolyte/surfactant complexation. Adsorption not only occurred at lower surfactant concentrations but also was more highly ordered than resulted from the addition of NaCl. The addition of NaCl and PEI together produced cooperative complexation of SDS and PEI and maximum ordering of the surfactant, which was independent of SDS concentration. This result is attributed to salt-induced enhancement of the strength of lateral surfactant alkyl chain interactions.