Monte Carlo simulation of the influence of urea on the self-association ofpropan-1-ol in water

The driving forces involved in self-association of propan-1-ol in binary an d ternary water-propanol-urea systems were investigated quantitatively by u sing Monte Carlo (MC) simulations. By choosing propanol as a model the role s of both hydrophobic and hydrophilic interactions were studied through the calculation of the potential of mean force (PMF). All the MC simulations w ere performed at 25 degrees C and 1 atm, representing the solutes and urea via optimised potential for liquid simulation (OPLS) potential functions an d the TIP4P water model. The reaction coordinate, I,, was defined as the di stance between the centres of the central sites (C1 atom of propanol) of tw o propan-1-ol molecules. Inspection of spatial views of different configura tions of the association phenomenon and the shape of the PMF indicate that association in the binary system is essentially hydrophobic. In contrast to the binary system the alkyl groups are far apart from each other in the te rnary system. The presence of urea in the vicinity of each alkyl group caus es a greater structural adaptability and therefore more complete solvation. For the ternary system at a suitable geometry (in this case at r(c) = 5.2 Angstrom) a bridge is formed by a water molecule. This supports the idea th at the formation of a bridge by a water molecule could be an effective driv ing force for the association of hydrophilic groups.