Dependence of alkyl chain conformation of simple ionic surfactants on head group functionality as studied by vibrational sum-frequency spectroscopy

The conformational order of sodium dodecyl sulfate (SDS), sodium dodecylsulfonate (DDS), dodecyltrimethylammonium chloride (DTAC), and dodecylammonium chloride (DAC) adsorbed at the D2O/CCl4 interface has been examined by total internal reflection sum-frequency vibrational spectroscopy (VSFS). The vibrational sum-frequency spectra indicate the presence of gauche conformations in the hydrocarbon chains of SDS, DDS, DTAC, and DAC at all surface coverages examined. An increase in the surface concentration of surfactant at the D2O/CCl4 interface results in the reduction of gauche defects in the hydrocarbon chain as determined from the intensity ratio of the methyl to methylene symmetric stretch vibrational modes. Analysis of the SF vibrational spectra suggest significantly different alkyl chain conformations for the cationic and anionic surfactants examined. The allyl chains of the cationic surfactants DTAC and DAC possess the fewest gauche defects, while SDS and DDS display more disorder in the hydrocarbon chains at similar surface concentrations. Mixed surfactant monolayers are also examined and are observed to display the least number of gauche defects, attributed to a reduction in head group repulsion.