Randomly Charged Polymers: An Exact Enumeration Study

We perform an exact enumeration study of polymers formed from a (quenched) random sequence of charged monomers $\pm q_0$. Such polymers, known as polyampholytes, are compact when completely neutral and expanded when highly charged. Our exhaustive search included all spatial conformations and quenched sequences for up to 12--step (13--site) walks. We investigate the behavior of the polymer as a function of its overall excess charge $Q$, and temperature $T$. At low temperatures there is a phase transition from compact to extended configurations when the charge exceeds $Q_c\approx q_0 \sqrt{N}$. There are also indications of a transition for small $Q$ between two compact states on varying temperature. Numerical estimates are provided for the condensation energy, surface tension, and the critical exponent $\nu$.