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

We have performed off-lattice Monte Carlo simulations of isolated, short (N=40), fully ionized polyelectrolytes in the presence of a low molecular 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 polyelectrolyte is modeled as a freely jointed chain of N hard spherical beads of radius a = 2.0 Angstrom. The mean-square end-to-end distance and the radius of gyration have been calculated as functions of the Bjerrum length Lambda, where Lambda = e(2)/epsilon(0) epsilon(r)kT. 1/Lambda is thus proportional to the temperature. The results show an interesting temperature dependence; at high temperatures the polyion size decreases with increasing temperature, which is to be expected from simple considerations of the energy/entropy balance. On lowering the temperature, however, we have found that the polyion reaches a maximum size at a certain temperature, which depends on the salt concentration. Further cooling then results in a contraction of the chain. For low salt concentrations, the maximum size represents a rodlike configuration, and the polymer shows a coil-to-rod-to-coil transition as the temperature is increased. We suggest that this behavior is due to the increased screening 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 incorporated this phenomenon into the model in an ad hoc fashion by reducing the charge of each band according to the Manning fraction.