Molecular dynamics study of the molecular weight dependence of surface tensions of normal alkanes and methyl methacrylate oligomers

Surface tensions (gamma) of normal alkanes and methyl methacrylate (MMA) oligomers at various molecular weights in the low molecular weight range were computed using a newly proposed molecular dynamics (MD) simulation strategy which was developed based on the definition of gamma = (partial derivative U/partial derivative sigma)(n,V,S). The MD simulations, even with the use of a generic force field, reproduced the experimentally observed molecular weight dependence of gamma (i.e., gamma proportional to M-n(-2/3), where M-n is the number-average molecular weight) for both series of oligomers. Analysis of the data reveals that solvent accessible surface area, one of the key input variables used for the calculation of gamma, exhibits an M-n(2/3) (rather than M-n(1)) dependence. The reason for such dependence is that solvent accessible surface area formed by the chainlike small molecules depends, to a larger extent, on their orientations rather than their size. However, this is not the case for high molecular weight molecules as solvent accessible surface area of such surfaces are determined by the orientations of their segments which are determined by the conformations of the molecules. This may explain why surface tension of polymers experimentally exhibits an M-n(-1) dependence. It is inferred that the corresponding molecular weight dependence of the entropy changes associated with molecules in the low and high molecular weight ranges would be different.