Measurement of forces between hydroxypropylcellulose polymers: Temperaturefavored 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 exponentiall y 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 du e to a hydration or water structuring force. The amplitude of the longer ra nged force in these condensed arrays decreases linearly with temperature. T he force switches from repulsive to attractive at similar to 40 degreesC, a bout the same temperature at which HPC precipitates from dilute solution. T he entropy of the HPC condensed array, derived from the temperature depende nce 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 gra dient within the space between polymers, inferred from the salt concentrati on dependence of the force curves, is itself apparently exponential with th e about same 3-4 Angstrom decay length as the force and entropy.