Investigating the effective interaction between silica colloidal particles near the critical point of a binary solvent by small angle neutron scattering.

An effective attractive potential can be introduced between colloidal particles dispersed in a binary solvent when the solvent condition approaches its demixing temperatures. Despite the debate of the physical origins of this effective attraction, it is widely termed as the critical Casimir force and is believed to be responsible for the colloidal stability in a wide range of particle concentration at both critical and near-critical solvent concentrations. Here, we study the effective attraction and equilibrium phase transition of charged spherical silica particles in the binary solvent of 2,6-lutidine and water as a function of the particle volume fraction and temperature at the critical solvent concentration. By analyzing our small angle neutron scattering (SANS) data, we found that at a relatively small particle volume fraction, the density fluctuation introduced attraction between silica particles can be satisfactorily explained by the function form commonly used for the critical Casimir interaction. However, at large silica particle volume fractions, an additional long range attraction has to be introduced to satisfactorily fit our SANS data and explain the large shift of the phase transition temperature. Therefore, while at relatively low volume fractions, the solvent introduced attraction may be dominated by the critical Casimir force, the physical mechanism of the effective attraction at large particle volume fractions seems to be different from the critical Casimir force. Furthermore, the range of this long range attraction is consistent with a recently proposed new theory, where the attraction can be introduced by the solvent capillary condensation between particles. We also demonstrate that the reduced second virial coefficient close to the particle phase transition is similar to the values of the binodal transition of the sticky hard sphere system.