Xiaolin Zhao

Polarization of water molecules at a charged interface: second harmonic studies of the silica/water interface

Abstract Surface second harmonic generation was used to probe the silica/water interface. At pH values where the silica surface is charged, we found that the water molecules near the interface are polarized by the interfacial electric field and are responsible for the observed second harmonic light. Two types of silanol sites having pKa values of 4.5 and 8.5 were inferred from the data. A promising result of the studies was the development of a new method for obtaining electric potentials at charged liquid interfaces.

Polarization of water molecules at a charged interface. Second harmonic studies of charged monolayers at the air/water interface

Abstract The second harmonic generation from charged monolayers at air/water interfaces is shown to be linearly related to the interface electric potential. This dependence is due to the polarization of water molecules in the electrostatic field of the charged monolayer. The observed linear dependence of the second harmonic field on the interface electric potential serves as the basis of a proposed method for obtaining the interface potential, which it should be noted is a nonintrusive optical method. The work presented here also indicates that the Gouy—Chapman model is valid at the air/water charged interface up to at least a 1 M total electrolyte concentration.

New method for determination of surface pKa using second harmonic generation

Abstract A new method to determine the p K a of chemical species at aqueous insulating interfaces is based on the discovery that the interface potential is linearly proportional to the measured second harmonic electric field. The proportionality constant is the effective third-order susceptibility and is the same for all aqueous interfaces studied, which has its origin in the polarization of bulk water induced by the charged form of the acid or base at the interface. We have determined that CH 3 (CH 2 ) 21 NH + 3 at the air/water interface has a p K a of 9.9±0.2 versus 10.6 in bulk water.

Determination of pKa at the air/water interface by second harmonic generation

Abstract We have used second harmonic generation to directly observe chemical changes in a lipid monolayer. We observed that p K a of p -hexadecyl aniline at the air/water interface is 3.6 ± 0.2 versus 5.3 ± 0.1 in the bulk. This fiftyfold increase in acidity is due to the higher free energy of a charged species at the air/water interface, which favors a shift in the equilibrium towards the neutral form.

Monolayer orientational fluctuations and a new phase transition at the air water interface detected by second harmonic generation

Fluctuations in the second harmonic intensity were observed in a number of insoluble amphilphiles spread at an air/aqueous interface. The fluctuations, which depend on the polarization of both the incident light and the second harmonic light generated, indicate a new phase transition for which the orientation of the head group is the order parameter. It is found that the phase transition occurs in certain long chain aromatic species but not in others of the same chain length with different head groups. The phase transition is identified as a weak first order transition. Agreement is obtained with a modified Landau–Ginzburg equation in which the chain–chain interactions provide a friction for the motions of the head groups.