Effect of branches on the structure of narrowly confined alkane fluids: n-hexadecane and 2,6,11,15-tetramethylhexadecane

Using realistic molecular models and accurately calculating the state condi tion of the fluids of linear and branched alkanes confined between two stro ngly adsorbing solid surfaces, we obtained strikingly different structure f or the two confined fluids. The structure of the n-hexadecane is solid-like with the molecules clearly forming a layered structure. Within each molecu lar layer, the molecules are packed predominantly parallel to each other. T he structures of adjacent layers are correlated such that the main chain ax es of the molecules pack into a local hexagonal array. In contrast, the str ucture of the branched alkane (2,6,11,15-tetramethylhexadecane) fluid remai ns liquid-like. Although exhibiting density oscillations normal to the conf ining surfaces, the structure of the confined fluid of the branched alkane is disordered and the molecules are not confined to a particular layer. The amplitude of the density oscillations is much smaller than for the n-hexad ecane. The in-plane positional correlation shows isotropic structure charac teristic of liquid, and the orientation of the molecules shows very little correlation. (C) 2001 American Institute of Physics.