Molecular arrangements and conformations of liquid <i>n</i>-tridecane chains confined between two hard walls

Liquid n-tridecane (C13H28) chains confined between two parallel hard walls separated by 5.0 nm have been studied by Monte Carlo simulations, employing united CH2 atoms linked with fixed bond lengths and angles and continuously varying torsional angles subject to appropriate interaction energies. Molecules located in the central region are in remarkable agreement with ideal unperturbed chains. Significant influences of walls persist over a distance of ∼1.5 nm, exhibiting progressively less-dense and less-pronounced segmental layers of ∼0.4 nm thickness. Segmental orientations preferentially aligned along the surface are found only in the first layer, accompanied by little perturbations in the fraction of trans-conformations. Furthermore, nearly all the chain units in the first segmental layer adjacent to the walls belong to two-dimensional chains, and exhibit considerable orientational correlations between neighboring segments of different chains. The terminal portions of some of the molecules in the succeeding layer are also located in the first layer, thus increasing the fraction of methyl chain ends in contact with the walls. These findings match very closely the experimental results of surface forces and viscosities of liquid n-alkanes confined between two mica plates, measured as a function of the plate separation.