In many colloids macroscopic properties such as stability are controlled by the inherent interfaces and interactions of the particle surface with the solvent. It is, therefore, of great importance to gain a detailed understanding of the interfacial molecular structure. This is, however, highly challenging since the interface cannot unambiguously be separated from the bulk in most experimental techniques that do not require other experimental constraints. Nonlinear optical techniques and in particular second-harmonic generation (SHG), are inherently surface sensitive and can be powerful tools in interface science. For colloidal particles, second-harmonic scattering (SHS) patterns exhibit distinct angular features that strongly dependent on particle size as well as on the polarization and orientation of molecules at the particle interface. In our study we have applied SHS to address double-layer charging and the orientation of water molecules at the surface of amphoteric particles. The surface potential of these particles can be adjusted by pH-dependent changes in the protonation state of surface groups. The total second-harmonic field generated by the charged particles comprises contributions directly from the interface and contributions whose origin is the surface charge. Both contributions interfere depending on the sign of the surface potential. Hence the second-harmonic field strength depends not only on the absolute value of the surface charge, but also on the direction of the resulting static electric field. The observed interference in the second-harmonic field allows us to determine the net orientation of interfacial water molecules. Additionally, charge screening experiments corroborate our conclusion and have allowed us to determine the surface charge densities of the particles by applying the χ(3)method.1
[1] Sauerbeck, C., Braunschweig, B., Peukert, W., J. Phys. Chem. C. (2014) DOI:10.1021/jp412295j
