EFFECT OF UREA AND SURFACTANT ON P-NITROPHENOL AT THE WATER-SURFACE -A SURFACE 2ND-HARMONIC GENERATION STUDY

The effect of salting-in denaturant, urea and a water-soluble surfacta nt, cetyltrimethylammonium bromide (CTAB), on the resonance surface se cond-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 i ncrease in the SSH signal of PNP at a concentration one-quarter of the critical micellar concentration. Addition of urea reduces the intensi ty 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 f ree energy of adsorption (Delta(ad)G) of PNP does not change on additi on of CTAB. However, the urea-induced decrease both in the absence and in the presence of CTAB is accompanied by an increase in Delta(ad) G.