The presence of charged molecules attached on the surface of Brownian particles can dramatically affect the mutual interaction of the latter ones as well as their interactions with foreign surfaces. Therefore, detailed knowledge about how molecules adsorb and assemble on the surface of Brownian particles is required in order to improve functionalization and targeting. We present a study on the relation between the surface states of surfactant molecules on sub-micron particles and the resulting colloidal interactions. To this aim, styrene-acrylate copolymer particles have been used and two different ionic surfactants, aliphatic C-18 carboxylate and aliphatic C-15 sulfonate, have been selected. By means of a combination of static light scattering (measurements and theory), SEM and tensiometry, we were able, with a novel methodology, to quantify the evolution of surfactant layer thickness on colloidal particles with nanometer resolution, as a function of the surfactant concentration in the system. A phase diagram of the surface states, in terms of the spreading pressure as a function of the molecular area, is obtained from the adsorption isotherms. The results indicate that an initial, gas-like state of noninteracting, adsorbed molecules laying down to the particle surface is followed, with increasing surfactant concentration, by the formation of condensed domains or hemimicelles prior to reaching full coverage of the particle surface. Seeking an independent validation of this model by other means is difficult. For example, the commonly used surface imaging techniques (e.g., AFM) are not well-suited to this aim, given the intense Brownian motion of the particles. Therefore, a novel protocol has been devised by which the colloidal system, at varying surfactant concentration, is sheared at very high shear-rate in a microchannel and the extent of aggregation is quantified by small-angle light scattering. With no added salt, it is shown that the surfactant films on two particles can fuse/adhere leading to aggregation as long as an even small-sized uncovered polymer patch is present. In the opposite case of fully developed films, by analyzing the mechanism of shear aggregation in the low-salt limit theoretically, we show that, whenever hydrophobic patches are completely absent, short-range hydration repulsive forces dominate over DLVO forces and adhesion/aggregation can never be achieved even upon application of extremely high collision energies. Consistently, a lower-limiting boundary for the hydration interaction is calculated.

Zaccone, Wu, Lattuada and Morbidelli, Journal of Physical Chemistry B, 112, 1976 (2008)

Zaccone, Wu, Lattuada and Morbidelli, Journal of Physical Chemistry B, submitted