(604e) Angle-Resolved Second-Harmonic Light Scattering of Colloidal Interfaces

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 task is, however, highly challenging since with many experimental techniques, interfacial molecules cannot be unambiguously separated from molecules in the bulk. Nonlinear optical techniques 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 are strongly dependent on the particle size as well as on the polarization and orientation of molecules at the particle interface^1,2. The SHS intensity is determined by the molecular hyper-polarizabilites of the interfacial molecular structure and thus contains information about molecular species, number densities and orientations of surface adsorbed molecules. For charged particles, the interfacial electric field causes polar ordering of water molecules within the particles electrical double layer and gives rise to additional second-harmonic light scattering. The field strength of this contribution is linearly dependent on the interfacial electric field and can, thus, be used to determine the electric potential of the particle surface via the so-called Χ⁽³⁾method.

In our study we have used second-harmonic light scattering to investigate the interfacial molecular structure and the surface charge of amphoteric particles. The surface potential of these particles changes its polarity when the isoelectric point is being crossed. The total second-harmonic field generated by the charged particles is composed of contributions directly from the interface and of 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. Additional charge screening experiments at conditions where the particles carry a strongly positive, negative or no net charge corroborate our conclusions and have allowed us to determine the surface charge densities of the particles by applying the &Chi⁽³⁾method.

[1]   Schürer, B., Wunderlich, S., Sauerbeck, C., Peschel, U., Peukert, W.,Phys. Rev. B, 82 (2010) 241404

[2]   Schürer, B., Hoffmann, M., Wunderlich, S., Harnau, L., Peschel, U., Ballauff, M., Peukert, W., J. Phys. Chem. C, 115 (2011) 18302–18309