We have used IR-visible sum-frequency generation (SFG) spectroscopy to demonstrate that water dipoles at a liquid/solid interface flip by 180° when the pH of the aqueous solution crosses the isoelectric point of the surface (IEPS). We have also shown, for the first time, that the SFG signal intensity and thus the nonlinear polarizability of a water/solid interface depends strongly on the hydroxyl number density of the solid surface. A new methodology for the determination of the IEPS of a nonconductive, low-surface area material was introduced.

• Received 1 December 1998

DOI:https://doi.org/10.1103/PhysRevLett.83.1179