Interfacial analysis

Article By:

Allen, Heather C. Department of Chemistry, Ohio State University, Columbus, Ohio.

Last reviewed:2002

DOI:https://doi.org/10.1036/1097-8542.YB020470

Whether regarding a glass of water, a pond, or an ocean, at the liquid surface there is a boundary between the liquid water and the air. This boundary or interface is a transition region between the air (gas phase) and the water (liquid phase). A gradient with respect to the water density exists in this region and can sometimes be quite complex. There also exists a surface potential. As the water molecules move about in the bulk, there is randomness to their overall structure. The water molecules that are in the surface region have a net orientation with respect to the surface. The structural organization of an interface is unique because it lacks inversion symmetry about the interface. This lack of symmetry gives rise to an increase in the second-order nonlinear response of the interface to electromagnetic radiation and is the basis for second-harmonic, sum-frequency, and difference-frequency generation.

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