Measurement of forces between hydroxypropylcellulose polymers: Temperature favored assembly and salt exclusion

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 Angstrom separation and a longer-ranged force that varies exponentially vs distance with a decay length of about 3-4 Angstrom. 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 similar to 40 degreesC, 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 Angstrom 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 Angstrom decay length as the force and entropy.