STUDY OF COLLOIDAL INTERACTIONS MEDIATED BY BIOMOLECULAR RECOGNITION AND BY ELECTROKINETIC FLOWS.

This PhD thesis focuses on new types of interparticle interactions in colloidal dispersions. In particular, it considers interactions mediated by two different phenomena - biomolecular recognition and electrokinetic flows. The general purpose of this investigation is twofold: while we investigate new mechanisms providing new forms of colloidal interactions, we also exploit the measured interactions to better describe the phenomena at their origin. Indeed, the observed colloidal interactions enable to detect both biomolecular recognition and electrokinetic flows with sensitivity hardly achievable in other ways. In all cases, interactions have been investigated by optical techniques: static and dynamic light scattering, electric transmitted light intensity and electric birefringence. The first part of the thesis reports a systematic study of colloidal interactions performed on a dispersion of charged spherical latex particles having two peculiar properties: their surface can be functionalized with various surfactant molecules and their refractive index is very close to that of water. We have quantified interparticle interactions through the measurement of the second virial coefficients extracted from static and dynamic light scattering measurements. We have studied particles coated with various surfactants and in presence of various ionic strengths. In particular we have studied particles coated with glyco-lipids, so that the colloids are effectively coated with sugar groups. In agreement with previous literature we have found attractive interaction in carbohydrate-coated colloidal dispersions when calcium ions are present in the solution. The second part of the thesis reports a previously undescribed interaction found when colloidal mixture of large and small colloids charged of the same sign are under the effect of an electric field at low (sub-kHz) frequencies. Measurements of the intensity of the transmitted polarized light as a function of field strength and frequency on rods-spheres mixtures have revealed that the previously observed negative torque acting on the rods at low frequency, is accompanied by a field-induced clearing of the dispersion. Measurements performed on mixtures of large (dilute) and small (semidilute) spherical colloids show that such clearing effect is universal and due to an anisotropic redistribution of the small spheres around the larger ones. O(E2) electro-osmotic flows, greatly enhanced by the presence of the small spheres, can be the responsible mechanism of the phenomena observed in rod-sphere and sphere-sphere binary mixture of charged colloidal particles.