Experimental compressibilities and average intermolecular distances of poly(ethylene glycol) molecular masses 2000–8000 Da in aqueous solution

Small-Angle Neutron Scattering (SANS) is a powerful, nondestructive technique that can measure simultaneously macroscopic compressibilities of solutions and overall shapes of macromolecular solutes, as well as their intermolecular structural correlations. We have conducted SANS experiments on aqueous solutions of polyethylene glycols (PEGs) with nominal molecular masses 2000, 4000, and 8000 Da over the q-range 0.03–0.30 Å−1 [q = (2π/λ)sin θ]. By incorporating accurate background subtraction and short extrapolations of the intermolecular structure factor S(q) down to q = 0, the isothermal compressibility can be measured. The results indicate a significant and systematic dependence of the solutionsʹ compressibility on both molecular mass and concentration of PEG, unlike the solutionsʹ osmotic pressures and activity of the water. This implies that the structure of water in the vicinity of PEG is considerably altered relative to the bulk state even though the activity coefficient of water remains nearly invariant in this range. Graphs of S(q) for 3% w/w to 17% w/w solutions each show a gradual rise from the low-q side to a broad plateau, which indicates weak intermediate-range correlations between oligomers that are probably associated with soft, repulsive, solvent-mediated PEG–PEG interactions. Since both the water and PEG change structures from their neat forms, any quantitative assignment of changes in partial volumes must necessarily be arbitrary. However, the linear change in compressibility with PEG concentration below ∼7% w/v can be said to indicate a composite solution, which parallels the behavior of composite solids.