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The thermodynamic forces between hydroxypropylcellulose (HPC) molecules at close separation have been measured using the osmotic stress method coupled with X-ray scattering. Two force regimes are apparent:  a very short ranged, temperature insensitive force that dominates interactions within the last 2.5 Å separation and a longer-ranged force that varies exponentially vs distance with a decay length of about 3−4 Å. The longer-ranged force characteristics are strikingly similar to those found for many other macromolecules. We have previously argued that these characteristics are due to a hydration or water structuring force. The amplitude of the longer ranged force in these condensed arrays decreases linearly with temperature. The force switches from repulsive to attractive at ∼40 °C, about the same temperature at which HPC precipitates from dilute solution. The entropy of the HPC condensed array, derived from the temperature dependence of the force, also varies exponentially vs spacing with a 3−4 Å decay length. Measured forces are also surprisingly sensitive to added salt. Salt acts by its exclusion from the HPC phase. The salt concentration gradient within the space between polymers, inferred from the salt concentration dependence of the force curves, is itself apparently exponential with the about same 3−4 Å decay length as the force and entropy.

Electrostatically stabilized aqueous dispersions of nanometric-sized silica particles have been characterized by light scattering, small angle X-ray scattering, and osmotic pressure measurements. All three studies yield similar values for the particle size, molecular weight, and particle surface charge. In addition, good agreement is found between directly measured osmotic pressure values and those calculated from scattering studies. Using the particle properties as determined from the three experiments, the osmotic pressure as a function of volume fraction is compared to liquid state theory models with no adjustable parameters. Finally, small angle X-ray scattering studies indicating the presence of long-range order in moderately to highly concentrated dispersions are reported.

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