Salt effect on p-nitrophenol at the water surface: A surface second harmonic generation study

The effect of lithium perchlorate (LiClO4), guanidinium hydrochloride (GdmCl) and lithium chloride (LiCl) on the resonance surface second harmonic generation (SSHG) of p-nitrophenol (PNP) at the air/water interface is reported. On addition of GdmCl and LiClO4 the intensity of the SSH signal decreases appreciably, which is attributed to the decrease in surface population of PNP. Addition of LiCl causes an increase in the intensity of SSHG due to increase in surface population. The orientation of the PNP molecules remains unchanged on addition of all the salts. The free energy of adsorption (Delta(ad)G) of PNP increases upon addition of the salting-in agents, but decreases for the salting-out agent. The results indicate that the factors which increase solubility of organic solutes in the bulk decrease the surface population of organic molecules at the surface and vice versa.