MD Simulation of Liquid Pentane Isomers Bull. Korean Chem. Soc. 1999, Vol. 20, No. 8 897 Molecular Dynamics Simulation Studies of Physico Chemical Properties of Liquid Pentane Isomers Seng Kue Lee and Song Hi Lee* Department of Chemistry, Kyungsung University, Pusan 608-736, Korea Received January 15, 1999 We have presented the thermodynamic, structural and dynamic properties of liquid pentane isomers - normal pentane, isopentane, and neopentane - using an expanded collapsed atomic model. The thermodynamic prop­ erties show that the intermolecular interactions become weaker as the molecular shape becomes more nearly spherical and the surface area decreases with branching. The structural properties are well predicted from the site-site radial, the average end-to-end distance, and the root-mean-squared radius of gyration distribution func­ tions. The dynamic properties are obtained from the time correlation functions - the mean square displacement (MSD), the velocity auto-correlation (VAC), the cosine (CAC), the stress (SAC), the pressure (PAC), and the heat flux auto-correlation (HFAC) functions - of liquid pentane isomers. Two self-diffusion coefficients of liq­ uid pentane isomers calculated from the MSD's via the Einstein equation and the VAC's via the Green-Kubo relation show the same trend but do not coincide with the branching effect on self-diffusion. The rotational re­ laxation time of liquid pentane isomers obtained from the CAC's decreases monotonously as branching increas­ es. Two kinds of viscosities of liquid pentane isomers calculated from the SAC and PAC functions via the Green-Kubo relation have the same trend compared with the experimental results. The thermal conductivity calculated from the HFAC increases as branching increases. Introduction covalent bonds. Those bonds either join two atoms of the same kind and hence are non-polar, or join two atoms that The family of organic compounds called hydrocarbons differ very little in electronegativity and hence are only can be divided into several groups based on the type of bond slightly polar. Furthermore, these bonds are directed in a that exists between the individual carbon atoms. Those very symmetrical way, so that the slight bond polarities tend hydrocarbons in which all the carbon-carbon bonds are sin­ to cancel out. As a result an alkane molecule is either non­ gle bonds are called alkanes. They have the general formula, polar or very weakly polar. CnH2n+2. Straight chain alkanes are called normal alkanes, or The forces holding non-polar molecules together (van der simply n-alkanes. The straight chain alkanes constitute a Waals force) are weak and effective in a very short range. family of hydrocarbons in which a chain of -CH2- groups is They act only between the portions of different molecules terminated at both ends by a hydrogen and they have the that are in close contact, that is, between the surfaces of mol­ general formula, H-(CH2)n-H. ecules. Therefore one would expect that within a family, the Normal pentane is an example of straight chain alkanes. larger the molecule, and hence the larger its surface area, the One glance at its three-dimensional structure shows that stronger the intermolecular forces. Thus, the boiling and because of the tetrahedral carbon atoms their chains are zig­ melting points of liquid alkanes rise as the number of carbon zagged and completely straight. Indeed, the structure is the atoms increases. Boiling and melting require overcoming the straightest possible arrangements of the chains, for rotations intermolecular forces of a liquid or solid. Branched chain about the carbon-carbon single bonds produce arrangements hydrocarbons are more compact than their straight chain iso­ that are less straight. The better description is unbranched. mers. For this reason, branched hydrocarbons tend to have This means that each carbon atom within the chain is bonded lower boiling points and higher densities than their straight to no more than two other carbon atoms and that unbranched chain isomers. alkanes contain only primary and secondary carbon atoms. Constitutional isomers have different physical properties, Isopentane and neopentane are examples of branched although they have the same carbon number. The difference chain alkanes. In neopentane the central carbon atom is may not be large, but they have different melting points, bonded to four carbon atoms. Normal pentane, isopentane boiling points, densities, indexes of refraction, and so forth. and neopentane have the same molecular formula: C5H12. In As mentioned above a branched chain hydrocarbon has a three compounds, the atoms are connected in a different lower boiling point than its straight chain isomer. Thus nor­ order. They are, therefore, constitutional isomers.1 The three mal pentane has a boiling point of 36 oC, isopentane with a geometries of pentane isomers are shown in Figure 1. single branch 28 oC, and neopentane with two branches 9.5 The physical properties of the alkanes follow the pattern oC. The effect of branching on boiling point is observed laid down by methane and are consistent with the alkane within all families of organic compounds. It is reasonable structure.2 An alkane molecule is held together entirely by that branching should lower the boiling point: with branch­ 898 Bull. Korean Chem. Soc. 1999, Vol. 20, No. 8 Seng Kue Lee and Song Hi Lee ing the molecular shape tends to approach that of a sphere, Table 1. Potential parameters for liquid pentane isomers and as this happens the surface area decreases, as a result the LJ parameters o (nm) £ (kJ/mol) intermolecular forces become weaker and are overcome at a C-C 0.3923 0.5986 lower temperature. torsional c°(kJ/mol) c1 2 3 4 In the present paper, we perform equilibrium molecular c c c c5 C-C-C-C 9.279 12.156 -13.120 -3.060 26.240 -31.495 dynamics simulations to investigate the thermodynamic, structural, and dynamic properties and to calculate transport bond stretching re (nm) K°(kJ/mol • nm2) coefficients, namely the self-diffusion coefficient Ds, the C-C 0.153 132600 shear viscosity n, the bulk viscosity K, and the thermal con­ bond angle bending 0e (deg) K1(kJ/mol • deg2)K2(kJ/mol • deg3) ductivity 爲 of liquid pentane isomers at 273.15 K using an C-C-C 109.47 0.05021 0.000482 expanded collapsed atomic model. The model includes C-C bond stretching and C-C-C bond angle bending potentials in Table 2. Molecular dynamics simulation parameters for model of addition to Lennard-Jones potential and C-C-C-C torsional liquid pentane isomers rotational potential. Mass of Temper­ In Section II we present the molecular models and molec­ Pentane Number of Density Length of site ature isomers molecules (g/cc) box (nm) ular dynamics simulation method. We discuss our simulation (g/mole) (K) results in Section III and present concluding remarks in Sec­ tion IV. n-pen 64 14.42996 273.15 0.645 2.2823 i-pen 64 14.42996 273.15 0.639 2.2894 ne o-pen 64 14.42996 273.15 Molecular Models and Molecular Dynamics Simulations 0.613 2.3213 The expanded collapsed atomic model includes the C-C and 0.61455 at 298.15 K. The usual periodic boundary con­ bond stretching and C-C-C bond angle bending potential in dition in the x-, y-, and z- directions and minimum image addition to the Lennard-Jones (LJ) and torsional rotational convention for pair potential were applied. A spherical cut­ potentials of the original Rykaert Bellemans' (RB) collapsed off of radius Rc = 2.5 b, where(J is the Lennard-Jones param­ atomic model:3 eter, was employed for the pair interactions. Gaussian isokinetics9 was used to keep the temperature of the system 2 Vb (七)=K0(七-r)2 (1) constant. For the integration over time, we adopted Gear's fifth 2 3 Va=Ki(0-飢 )2-K2(0- ee)3 (2) order predictor-corrector algorithm10 with a time step of 0.0005 ps. MD runs of about 1,000,000 time steps were The equilibrium bond length (re) is fixed at 1.53 A and the needed for each liquid pentane isomer system to reach equi­ bond angles (0e) are also fixed, at 109.47 degrees. The force librium. The equilibrium properties were then averaged over constants (Ko, K1, and K2) are those used by Lee et al.4 and 10 blocks of 100,000 time steps for a total of 1,000,000 time Chynoweth et al.,5-7 which were originally provided by the steps, and the configurations of molecules were stored every work of White and Boville.8 They are given in Table 1. 10 time steps for analyses of structural and dynamic proper­ In the original RB model,3 monomeric units are treated as ties. As the systems approached the equilibrium states, the single spheres, with their masses given in Table 2. They molecules were moving continually in the simulation box. interact through an LJ potential between the spheres in dif­ The transport properties of liquid pentane isomers were cal- ferent molecules and between the spheres more than three places apart on the same molecule. The C-C-C-C torsional rotational potential is given as 2345 V(0) =c0 + c 1cos0+c2cos 8+c3cos ©+c4cos e+c5cos 0 (3) where 0 is the C-C-C-C dihedral angle. The Lennard-Jones parameters and ci values are listed in Table 1. Unlike in nor­ mal pentane, there are multiply imposed torsional rotational potentials in isopentane. That is to say, one doubly imposed dihedral state on the C2-C3 bond in isopentane exists as shown in Figure 1. The molecular dynamics simulation parameters such as number of molecules, mass of site, temperature, and length of simulation box are listed in Table 2.