Coil Dimensions of Polystyrene Chains in Colloid−Polymer Mixtures at the Protein Limit: A SANS Study

A new combination of polymer chains and small colloids is presented in order to investigate the behavior of colloid−polymer mixtures in solution (CP) in the protein limit. To this end, hydroxy-functionalized silica particles (OHSil) were prepared according to a procedure [ J. Am. Chem. Soc. 2003, 125, 3712], which led to colloid radii as small as 1.2 nm. Compared to this small size, the coil dimensions of the polystyrene (PS) chains used as the polymer component were larger by more than an order of magnitude. Investigation of the CP focused on the dimension of the PS chains in the presence of a varying amount of OHSil colloids. Chain size and shape were characterized by means of light scattering, viscosity experiments, and small-angle neutron scattering (SANS). Two solvents have been used. A mixture of dimethylformamide (DMF) with toluene was isorefractive to the OHSil particles, giving access to single chain behavior of PS. Pure DMF, on the other hand, allowed an efficient contrast matching of OHSil suitable for SANS experiments on fully deuterated PS. All characterization techniques provided clear evidence for a shrinking of the coil dimensions with increasing OHSil content in the solution. Particle scattering factors from SANS yielded a detailed picture of this chain compaction. The smallest coil dimensions were recorded close to the liquid−liquid phase boundary, occurring at an OHSil volume fraction of ∼0.17. This is considered to be a first experimental evidence for a shift of the liquid−liquid phase boundary toward lower colloid volume fractions if the colloid size increases. Altogether, present data extend earlier measurements on two chemically different CP in the protein limit, thus establishing the observed shrinking to be a universally valid feature of these systems. The results are in qualitative agreement with theoretical predictions.