Chain Length Effect on the Configurational Properties of an n-Alkane Chain in Solution

摘要

Dynamic and equilibrium properties of n-alkane chains immersed in solvent molecules have been investigated by a molecular dynamics method. The n-alkane chain is assumed to be a chain of elements (CH2) interconnected by bonds having a fixed bond length and bond angle, but each bond of the chain is allowed to execute hindered internal rotation. We studied the effect of the number of the chain elements (Nc = 10, 15 and 20) on the equilibrium properties of the system, e.g., the pair correlation functions between a chain element and solvent molecules, gcs(r), and between the chain elements, gcc(r), and the configurational properties such as the mean-square end-to-end distance < R2 >, the mean-square radius of gyration < S2 >, and the eigenvalues of the moment-of-inertia tensor < Si2 > / < S2 > (i = 1, 2 and 3). We also studied the dynamic properties of the system, e.g., the autocorrelation function C(A;t) where A = R2(t), = S2(t), or = V(t) (V = velocity of the center of mass), and the diffusion coefficient D. The gcs(r)`s are almost equal irrespective of the change of Nc while gcc(r) becomes larger as Nc increases; The MD computed configurational properties < R2 > and < S2 > were found to be a little different from the values calculated from the statistical equations of < R2 > and < S2 >, it may be due to the fact that our model for the MD simulations includes a long-range volume effect. From the < Si2 > / < S2 >, it is found that the chain molecule has a nearly spherical shape irrespective of the variation of Nc. For the dynamic properties we found that the C(R2;t) and C(S2;t) of lower Nc decay faster than those of higher Nc, while the C(V;t) of the center of mass in the chain is weakly dependent on the Nc. The center of mass diffusion coefficient Dc decreases as Nc increases while the end point diffusion coefficient De is nearly equal irrespective of the change of Nc.