MONTE-CARLO STUDIES OF THE TEMPERATURE-DEPENDENT SIZE OF POLYELECTROLYTE CHAINS

We have performed off-lattice Monte Carlo simulations of isolated, sho rt (N=40), fully ionized polyelectrolytes in the presence of a low mol ecular mass, monovalent salt in the concentration range 0.01 less than or equal to C less than or equal to 1.0 mol dm(-3). The polyelectroly te is modeled as a freely jointed chain of N hard spherical beads of r adius a = 2.0 Angstrom. The mean-square end-to-end distance and the ra dius of gyration have been calculated as functions of the Bjerrum leng th Lambda, where Lambda = e(2)/epsilon(0) epsilon(r)kT. 1/Lambda is th us proportional to the temperature. The results show an interesting te mperature dependence; at high temperatures the polyion size decreases with increasing temperature, which is to be expected from simple consi derations of the energy/entropy balance. On lowering the temperature, however, we have found that the polyion reaches a maximum size at a ce rtain temperature, which depends on the salt concentration. Further co oling then results in a contraction of the chain. For low salt concent rations, the maximum size represents a rodlike configuration, and the polymer shows a coil-to-rod-to-coil transition as the temperature is i ncreased. We suggest that this behavior is due to the increased screen ing at low temperatures. The Debye-Huckel approximation does not take into account the fact that for Lambda/2a > 1 Manning condensation will reduce the effective charge of the chain. We have therefore also inco rporated this phenomenon into the model in an ad hoc fashion by reduci ng the charge of each band according to the Manning fraction.