Molecular Configurations and Solvation Forces in Confined Alkane Films
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isobaric liquid bulk or droplet is explicitly simulated to equilibrate the confined fluid to a known state without referring to the chemical potential. This approach is not only time-consuming because many extra molecules have to be simulated outside the confined region of interest, but is also likely to suffer from finite-size effects because the area of the confining surface is significantly limited. Therefore, it is clearly desirable to have a new method, which combines the advantages and avoids the difficulties of current methods. For this purpose, a new ensemble is proposed [2], which replaces the isothermal-isobaric reservoir with constant temperature (7) and constant parallel pressure (P1 ) constraints so as to simulate only the confined region. One reason to believe the validity of the approximation that P1, is uniform throughout the confined region and equal to the bulk pressure in the SFA is that the external interaction field created by the configuration of the confining surfaces varies extremely slowly from the center of the confined region to the liquid bulk. Since the variation in P,, between the bulk and the confined fluid is proportional to the gradient of the molecule-surface interaction parallel to the surface [11], the error associated with this approximation is expected to be small in simulating the SFA. In the new ensemble method, PH and Tare specified and fixed to bypass the chemical potential and enable the use of a fixed number of confined molecules (N). To control PV 1, one can adjust either surface area (A) or separation (h). The latter is preferred since varying A differentially is not possible when atomically structured surfaces are used in simulations. The new method can therefore be best described as an NPI4T ensemble method. With this new method, from each MD simulation with a fixed N, one can calculate a time-averaged separation (h) and solvation force, which is defined as (fo) =A(P. -Pb). Repeating for different N's, a solvation-force profile as a function of surface separation, as well as the dependence of the number of confined molecules on surface separation, can be obtained. MODEL AND METHODS Our model systems for free-standing films in this paper are 360 n-decane and t-butyl-hexane molecules on a model fcc( 111) surface at 300 K. Since detailed descriptions of our models can be found elsewhere [1,12], only a brief summary is given here. The surface is represented by a fivelayer slab with 22x22 atoms in each layer and rigidly fixed to their bulk positions. The methyl and methylene groups and carbon atoms in the alkanes are modeled as united atoms (UAs) with interaction centers placed at the positions of the C atoms. Each C-C bond length is constrained to its equilibrium liquid-phase value by applying the SHAKE algorithm. The internal C-C-C bond bending and C-C-C-C torsional motions are also considered by using a simple harmonic potential for the former and a 6-term cosine polynomial potential for the latter. A new torsional potential is also developed for the torsional motion o
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