Compaction of single large dsDNA chains in aqueous solution in the presence of primary alcohols, acetone, and ethylene glycol has been studied experimentally with the use of a fluorescence microscopy technique. It is found that in the presence of all studied organic solvents single DNA molecules exhibit a discrete phase transition from an elongated coiled to a compacted globular conformation. Interestingly, DNA phase transition occurred at various weight fractions of organic solvents in aqueous solution, but at similar dielectric constants of mixed solvent for all studied primary alcohols and acetone. On the other hand, the dielectric constant of ethylene glycol−water mixtures corresponding to the collapsing transition in single DNA differed from that for the other studied systems. The explanation of this phenomenon comes through consideration of the existence of ethylene glycol conformers with various polarities in aqueous solution. Thus, the dielectric permittivity of the solvent is a key factor that determines the conformational behavior of DNA in solution. The compaction of a single DNA molecule when the dielectric permittivity constant is lowered is thought to be due to the increased importance of ion−ion correlation. Monte Carlo simulations for a single polyelectrolyte chain also show that the dimensions of the chain diminish when the electrostatic coupling is increased, i.e., by decreasing the dielectric constant. The experimental result can be rationalized with a simple free energy model balancing the counterion entropy and the ion−ion correlation energy.

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The condensation of a single DNA can be induced by multivalent counterions and is thought to be due to ion−ion correlations. Both Monte Carlo simulations and fluorescence microscopy experiments confirm this picture. The effect is less pronounced for chain counterions (polyamines) than for simple spherical counterions with the same charge. When simple salt is added to the system, the polyelectrolyte unfolds. MC simulations show that this is due to a competition between the different small ion species. The multivalent ions in the vicinity of the macroion are exchanged for univalent ones, decreasing the effect of ion−ion correlations. It is also found that the temperature effect on the macroion conformation is quite insignificant in the studied interval (5.0−65.0 °C).

The force between two parallel charged flat surfaces, with discrete surface charges, has been calculated with Monte Carlo simulations for different values of the electrostatic coupling. For low electrostatic coupling (small counterion valence, small surface charge, high dielectric constant, and high temperature) the total force is dominated by the entropic contribution and can be described by mean field theory, independent of the character of the surface charges. For moderate electrostatic coupling, counterion correlation effects lead to a smaller repulsion than predicted by mean field theory. This correlation effect is strengthened by discrete surface charges and the repulsive force is further reduced. For large electrostatic coupling the total force for smeared out surface charges is known to be attractive due to counterion correlations. If discrete surface charges are considered the attractive force is weakened and can even be turned into a repulsive force. This is due to the counterions being strongly correlated to the discrete surface charges forming effective, oppositely directed, dipoles on the two walls.

The adsorption of flexible polyampholytes to charged surfaces has been investigated. The electrostatic interactions are included in a mean-field manner, while the chain connectivity is treated by Metropolis Monte Carlo (MC) simulations. For large enough surface charge densities, adsorption is found both for neutral polyampholytes and for polyampholytes carrying the same net charge as the surfaces. The simulation results are used to check the reliability of previously proposed analytical theories. The exponents predicted in those scaling relations are not reproduced by MC simulations. The simulations show that the size of the adsorbed polyampholyte initially grows with increasing surface charge density, but for sufficiently charged surfaces it reaches a maximum and starts to decrease. This general behavior is qualitatively captured by scaling arguments. The MC simulations do not bear any evidence for the existence of three disparate regimes where the polyampholyte adsorbs in different conformations.