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

Abstract

The driving forces involved in self-association of propan-1-ol in binary and ternary water–propanol–urea systems were investigated quantitatively by using Monte Carlo (MC) simulations. By choosing propanol as a model the roles of both hydrophobic and hydrophilic interactions were studied through the calculation of the potential of mean force (PMF). All the MC simulations were performed at 25°C and 1 atm, representing the solutes and urea via optimised potential for liquid simulation (OPLS) potential functions and the TIP4P water model. The reaction coordinate, r_c, was defined as the distance between the centres of the central sites (C1 atom of propanol) of two propan-1-ol molecules. Inspection of spatial views of different configurations 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 ternary system. The presence of urea in the vicinity of each alkyl group causes 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 Å) a bridge is formed by a water molecule. This supports the idea that the formation of a bridge by a water molecule could be an effective driving force for the association of hydrophilic groups.