Second Harmonic Generation (SHG) Spectroscopy is a well established technique for the investigation of planar interfaces. Recently it has also been applied to investigate the surface properties of colloidal interfaces [1]. In contrast to linear optical methods, where the separation of bulk and surface signals is often difficult if not impossible, second order nonlinear effects are exclusively surface sensitive. The signal generation depends on the nonlinear susceptibility (χ⁽²⁾) of the materials which vanishes for centrosymmetric materials. Interfaces between two such materials represent a symmetry break and do therefore possess a nonlinear surface susceptibility, which depends on the number of surface molecules, their orientation and present electro-magnetic fields [2,3]. The method can in general be applied to an interface which is formed by two centrosymmetric bulk media, and which is optically accessible. The poster will present some theoretical background of the method focusing on the application for particle surface charge measurements. Further, experimental results of surface charge measurements on silica particles will be presented. In this context the influence of particle concentration [4] as well as of the pH and the bulk electrolyte concentration will is illustrated. Electrophoretic zeta potential measurements will be presented as a reference. All experimental results will reveal that the SHG signal provides information on the potential at the Stern layer which is situated closer to the particle surface than the shear plane, where the zeta potential is measured. Therefore, SHG can be applied to directly monitor the ion displacements by adding ions to the suspension (with a higher affinity to the Silica surface than the already present ions) while the zeta-potential measurements do not reveal these phenomena. In this context we will investigate the Hofmeister series from a new perspective, a view which has not been experimentally accessible yet.

[1] H. Wang, E. Yan, E. Borguet, K. Eisenthal: Second harmonic generation from the surface of centrosymmetric particles in bulk solution Chem. Phys. Lett., 1996, 259, 15-20

[2] K. Eisenthal: Second Harmonic Spectroscopy of Aqueous Nano- and Microparticle Interfaces Chem. Rev., 2006, 106, 1462-1477

[3] L. Schneider, W. Peukert: Review: Second Harmonic Generation Spectroscopy as a Method for In-Situ and Online Characterization of Particle Surface Properties Particle and Particle Systems Characterization, 2007, 23, 351-359

[4] L. Schneider, H.-J. Schmid, W. Peukert: Influence of Particle Size and Concentration on the Second Harmonic Signal generated at Colloidal Surfaces Appl. Physics.B, 2007, 87, 333-339