Patchy Colloidal Particles Via Surfactant Adsorption: Interactions and Gels of Tunable Structure

The presence of charged molecules attached on the surface of Brownian particles can dramatically affect their mutual interaction as well as their interactions with foreign surfaces. With respect to aggregation, the coexistence of domains of charged adsorbed molecules and hydrophobic domains on polymer colloids opens up the possibility of tuning the interactions in a wide range from homogeneously hydrophobic surfaces to completely hydrophilic repulsive surfaces with strong hydration forces. In a well characterized system made of styrene-acrylate copolymer particles and two different ionic surfactants, aliphatic C-18 carboxylate and aliphatic C-15 sulfonate, we have shown experimentally by means of laser light scattering 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 prior to reaching full coverage of the particle surface. In the low salt limit, by shearing the dispersion at very high shear-rate in a microchannel, it is shown that the surfactant domains on two particles can fuse/adhere leading to aggregation as long as an even small-sized uncovered polymer patch is present and aggregation is always possible on the free hydrophobic polymer patches. In the case of fully developed films, by analyzing the mechanism of shear aggregation in the low-salt limit theoretically, we show that short-range hydration repulsive forces dominate over DLVO forces and adhesion/aggregation can never be achieved even upon application of extremely high collision energies. We can also provide evidence that gels obtained by shearing the dispersion at high-shear rate at low-salt exhibit a structure that is strongly affected by the degree of coverage of surfactant, i.e. by the relative extension of charged-hydrophilic to hydrophobic patches. The fractal dimension of the gel can indeed vary from 2.1 at high surfactant coverage where only a few small patches are available for aggregation (valence-limited case) to 2.8 at low surfactant coverage where the gel is made of very compact clusters. This finding unfolds new possibilities for making engineered mesoscopic disordered materials by tuning the surface properties at microscopic level. Zaccone, Wu, Lattuada and Morbidelli, Journal of Physical Chemistry B, 112, 1976 (2008) Zaccone, Wu, Lattuada and Morbidelli, Journal of Physical Chemistry B, in press