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| - Adsorption of Sodium Dodecyl Sulfate at the Hydrophobic Solid/Aqueous Solution Interface in the Presence of Poly(ethylene glycol): Dependence upon Polymer Molecular Weight
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| - The polar orientation and degree of conformational order of sodium dodecyl sulfate (SDS) adsorbed at the hydrophobicoctadecanethiol/aqueous solution interface in the presence of poly(ethylene glycol) (PEG) has been investigated asa function of the surfactant concentration and the molecular weight of the polymer. Sum frequency generation (SFG)vibrational spectroscopy was employed to obtain spectra of interfacial surfactant; weak SFG signals from interfacialpolymer were also detected for polymer molecular weights of 900 and above. The phase of the SFG spectra indicatedthat both the surfactant and polymer had a net orientation of their CH2 and/or CH3 groups toward the hydrophobicsurface. Spectra of SDS in the presence of mixed polymer/surfactant solutions showed increasing conformational orderas the surfactant concentration was raised. At the lowest surfactant concentrations, the spectra of SDS were weakerin the presence of the polymer than in its absence. All PEG molecular weights investigated, with the exception ofPEG 400, gave rise to significant inhibition of ordered surfactant adsorption below the critical micelle concentration.The greatest inhibitory effect was noted for PEG 900. Probing interfacial PEG specifically through the use of perdeuteratedSDS revealed that the polymer spectral intensity decreased monotonically as the surfactant concentration was increasedfor all polymer molecular weights where a PEG spectrum was apparent. These findings are interpreted in terms ofthe displacement of preadsorbed polymer as the surfactant concentration increases. This result is compatible withobservations of adsorption from SDS/PEG solutions at solid/solution and solution/air interfaces made using othertechniques.
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