POLARIZED INFRARED ATTENUATED TOTAL-REFLECTION FOR THE IN-SITU DETERMINATION OF THE ORIENTATION OF SURFACTANT ADSORBED AT THE SOLID SOLUTION INTERFACE/

Polarized infrared attenuated reflection (IR-ATR) has been used to det ermine the average orientation of the methylene tail of cetyltrimethyl ammonium bromide (CTAB) adsorbed at the silica/solution interface unde r both equilibrium and nonequilibrium conditions, in situ. The equilib rium orientation was measured over the pH range of 2-10. It was found that the equilibrium orientation of the surfactant was larger at highe r pH values owing to an increase in the packing density with increasin g surface excess. The evolution of the orientation of the surfactant a t the interface during the adsorption process was monitored at a pH of 9.2. To the authors' knowledge, this is the first time that nonequili brium changes in the molecular orientation of surfactants adsorbing at the solid/solution interface have been determined in situ. During the initial stage of adsorption, there was no preferred orientation of th e surfactant, but as the surface excess increased with time, the surfa ctant began to orient in a direction more normal to the surface. This change in orientation was rapid. Desorption of this CTAB layer by addi tion of poly(styrenesulfonate) was slow and led to a 90% desorption. T he residual 10% of surfactant retained the orientation of the original highly packed layer and relaxed slowly to a more random configuration only after an extended period of time. This suggests that the extract ion of the surfactant by poly(styrenesulfonate) occurs in localized ar eas and that the surfactant remaining on the surface is found in highl y packed patches or clusters.