Study of the Coadsorption of an Anionic Surfactant and an Uncharged Polymer at the Aqueous Solution/Hydrophobic Surface Interface by Sum Frequency Spectroscopy

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Abstract

The polar orientation and degree of conformational order of sodium dodecyl sulfate (SDS) adsorbed at the hydrophobic octadecane thiol/aqueous solution interface has been investigated as a function of the surfactant concentration in the presence and absence of a fixed concentration of poly(ethylene oxide) (PEO). Sum frequency vibrational spectroscopy, based on the nonlinear optical phenomenon of sum frequency generation, was employed to obtain spectra of the interfacial surfactant; weak signals from PEO at the interface were also obtained. The phase of the spectra indicated that both the surfactant and the polymer had a net orientation of their CH2 and/or CH3 groups toward the hydrophobic surface. Spectra of SDS in the presence of mixed polymer/surfactant solutions showed increasing conformational order as the surfactant concentration was raised. At the lowest surfactant concentration, the spectra of SDS were weaker in the presence of the polymer than in its absence. Probing interfacial PEO specifically through the use of per-deuterated SDS revealed that the polymer spectral intensity decreased as the surfactant concentration was increased above a threshold concentration. These findings are interpreted in terms of competitive polymer/surfactant adsorption at low SDS concentrations. In both the presence and the absence of added electrolyte, the sum frequency signal attributable to the polymer disappeared as the surfactant concentration approached the critical aggregation concentration. This finding may potentially be attributed to continued PEO desorption, to the formation of surface-bound SDS/PEO complexes, or to a combination of the two processes.

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