Nikolai Brilliantov
University of Leicester
Lomonosov MSU
I will open the lecture with the discussion, how a dimension of a polyelectrolyte chain depends on the electrostatic interactions strength. Namely, I will consider the dependence of the gyration radius of a chain on the reduced Bjerrum length. In particular, I will focus on strongly charged polyelectrolytes in dilute salt-free solutions. I will mention that as the Bjerrum length increases, a polyelectrolyte chain undergoes a series of conformation changes, arriving at a final state of a compact globule. I will discuss the available experimental and numerical studies where the electrostatic collapse of a polyelectrolyte has been addressed; different mechanisms of this phenomenon, proposed in literature, will be also discussed.
In the second part of the lecture I will report the results of our recent MD simulations of the polyelectrolyte collapse. I will explain that two conformation regimes have been revealed: A regime of a "weak electrostatic collapse" and that of a "strong electrostatic collapse", which differ by the functional form of the dependence of the gyration radius on the Bjerrum length. In the former regime the globular state of a chain has a relatively loose packing and non-negligible fraction of free counterions. In the latter one all counterions are condensed within the globule and the
globule itself is significantly more dense.
In the final part of the lecture I will compare the predictions of different theories of electrostatic collapse with the simulation results. I will emphasize that the only theory which explains the observed
dependence of the gyration radius of a chain on the Bjerrum length, as well as the crossover between two regimes, is the so-called counterion-fluctuation theory. I will discuss in detail this theory and clarify the physical mechanism responsible for the chain collapse. In conclusion I will consider a possible extension and generalization of the counterion-fluctuation theory and analyse the application range of other, alternative theories of the polyelectrolyte collapse.
