Effect of urea and surfactant on p-nitrophenol at the water surface: a surface second-harmonic generation study

Abstract

The effect of salting-in denaturant, urea and a water-soluble surfactant, cetyltrimethylammonium bromide (CTAB), on the resonance surface second-harmonic generation (SSHG) of p-nitrophenol (PNP) is reported. On addition of urea to an aqueous PNP solution the intensity of the SSH signal decreases markedly. CTAB on the other hand produces a 10-fold increase in the SSH signal of PNP at a concentration one-quarter of the critical micellar concentration. Addition of urea reduces the intensity of the SSH signal in the presence of CTAB also. The orientation of the PNP molecule at the surface remains unchanged in both cases. The free energy of adsorption (ΔadG) of PNP does not change on addition of CTAB. However, the urea-induced decrease both in the absence and in the presence of CTAB is accompanied by an increase in ΔadG.