Coadsorption of tri-chain surfactants and dodecanol at a hydrophobic surface studied by sum-frequency spectroscopy

In situ sum-frequency vibrational spectra have been recorded of monolayer films adsorbed onto a planar hydrophobic surface from mixed aqueous solutions of tri-n-alkylsulfotricarballylate (tri-chain) surfactants and dodecanol. The change in composition of the film with concentration is similar for all the tri-chain surfactants investigated. At very low tri-chain concentrations the monolayer spectra arise entirely from dodecanol. As the tri-chain concentration increases, adsorbed dodecanol is gradually displaced by the tri-chain surfactant. At a concentration of one third of the tri-chain critical micelle concentration (cmc), when the dodecanol coverage is approximately two-thirds of a full monolayer, the tri-chain has the same packing density as in the absence of dodecanol. Above the cmc, only the tri-chain adsorbs significantly which is explained by the uptake of dodecanol in bulk phase micelles. The structure of the tri-chain and dodecanol surfactants in the mixed film is similar to that in the pure tri-chain and dodecanol monolayers, with the tri-chain showing significant conformational disorder in contrast to the highly orientated dodecanol, This points to minimal mixing between the allkyl chains of the dodecanol and tri-chain molecules. The simplest interpretation of this behavior is that the tri-chain molecules are adsorbing individually or as clusters, around which dodecanol molecules are able to pack.