Determining the Surface Potential of Charged Aqueous Interfaces Using Nonlinear Optical Methods

Second-order nonlinear optical techniques have recently been established as sensitive probes of charged interfaces through the nonlinear susceptibility of water and provide an attractive route to elucidate the surface potential. We discuss methods that have been proposed using electronic second-harmonic generation and vibrational sum-frequency generation. A detailed comparison is provided, using a unified notation and including a discussion of the assumptions that are either convenient or necessary in order to arrive at the surface potential. We then illustrate that, when using the full solution of the Poisson–Boltzmann equation that is applicable to a wide range of surface potentials, several benefits may be realized. The first is that, when using off-resonance phase measurements, the surface potential can be determined from the phase alone, without any additional information or calibrated intensity schemes. Next, we illustrate a scheme for surface potential measurement using intensity-only data off-resonance by changing the angle of incidence. Finally, we discuss the possibility of measurements that target the vibrational resonance with water and illustrate how the surface potential can be isolated from such data.