Page 322 - Handbook Of Multiphase Flow Assurance

Computer modeling of interaction between a hydrate surface and an inhibitor 321

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monomer although it used the DREIDING forcefield. In Cerius hydrogen bonding was
modeled by the charge-charge interaction. This is a common approach for simulation of
water-based systems (Jorgensen et al., 1983; Rodger, 1994b). However, this approach does not
give a complete set of water-water interactions and results in lower ice melting temperature
(e.g., 200 K for SPC water). The major reasons for writing our own code were to improve the
potential model by adding the hydrogen bonding interaction and to be able to sample any
variable at any time during the simulation neither of which could be explicitly done using
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Cerius .
The main Monte-Carlo routine was written by Professor James Haile of the Clemson
University while on sabbatical at the CSM. Hydrogen bonding interaction of up to 5 kcal/mol
was included in the hand-written code for the simulation of chemicals' interactions with a
SPC-water-based (Berendsen et al., 1981) hydrate surface along with the dispersion (Lennard-
Jones) and electrostatic (Coulombic) interactions. The SPC water potential were validated
along with other water potentials by Jorgensen et al. (1983) and is widely used for water
simulations. Relative magnitudes of all interactions used in the hand-written code are shown
in Fig. 10.76. The hydrogen bonding was implemented as a distance and angle dependent in-
teraction based on the Lennard-Jones 12–10 potential. The donor-acceptor distance cutoff was
set to 4 Å and a spline function was applied to the hydrogen-bonding potential in the interval
between 3.5 and 4 Å to avoid an abrupt potential cutoff. A hydrogen bond was strongest at
a 180° donor-hydrogen-acceptor angle and at a 2.8 Å donor-acceptor distance. More specific
details of the simulation can be found in the Computer Code section which contains the poly-
mer adsorption simulation code.

Testing the simulation code
Prior to using the code, its components were tested:
(1) The interaction potentials were calculated by hand for a test pair of molecules and were
found to be same as calculated by the program.

FIG. 10.76 Relative magnitudes of the Lennard-Jones, electrostatic and hydrogen-bond potentials.