We have measured the equation of state for DNA solutions at different ionic strengths covering 5 orders of magnitude in DNA osmotic pressure. At high osmotic pressures the equation of state is independent of the ionic strength and is dominated by exponentially decaying hydration repulsion. At lower pressures the equation of state is dominated by screened electrostatics. We also found a fluctuation-enhanced repulsion with a decay length of 4 times the Debye screening length. We present arguments that the form of the fluctuation part of the osmotic pressure is due to the coupling between bending fluctuations and the compressibility of the nematic array normal to the average director.

• Received 30 September 1996

DOI:https://doi.org/10.1103/PhysRevLett.78.895