Ms. Johal et al., COADSORPTION OF TRI-CHAIN SURFACTANTS AND DODECANOL AT A HYDROPHOBIC SURFACE STUDIED BY SUM-FREQUENCY SPECTROSCOPY, Journal of physical chemistry, 100(1), 1996, pp. 274-279
In situ sum-frequency vibrational spectra have been recorded of monola
yer films adsorbed onto a planar hydrophobic surface from mixed aqueou
s solutions of tri-n-alkylsulfotricarballylate (tri-chain) surfactants
and dodecanol. The change in composition of the film with concentrati
on 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 dodecan
ol is gradually displaced by the tri-chain surfactant. At a concentrat
ion of one third of the tri-chain critical micelle concentration (cmc)
, when the dodecanol coverage is approximately two-thirds of a full mo
nolayer, 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 co
ntrast to the highly orientated dodecanol, This points to minimal mixi
ng between the allkyl chains of the dodecanol and tri-chain molecules.
The simplest interpretation of this behavior is that the tri-chain mo
lecules are adsorbing individually or as clusters, around which dodeca
nol molecules are able to pack.