Page 254 - Handbook Of Multiphase Flow Assurance

Molecular modeling 253

The first hypothesis was based upon a similarity to the inhibition of ice growth. Polypeptides
present in the blood of the winter flounder allows it to survive the Arctic sea temperatures of
270.9 K. The structure of this polypeptide is described later in this chapter. It was shown (Knight
et al., 1993) that polypeptide adsorption on the ice surface prevents further crystal growth. We
hypothesized that adsorption on the hydrate surface might prevent hydrate growth just as in
ice. This hypothesis is addressed in Section “Docking of macromolecules on hydrate and ice”.
The second hypothesis was that hydrate inhibition is not the result of a crystal surface
interaction, but that of interaction with the bulk liquid. The hypothesis was that the poly-
mer changes the structure of water, making hydrate formation entropically unfavorable.
This hypothesis is addressed in Section “Studying of kinetic inhibitor interaction with water:
Solvation of the polymer in the bulk water”.

Docking of macromolecules on hydrate and ice

Introduction
The first portion of the computer study was directed to provide evidence for (or against) the
adsorption hypothesis. Evidence was obtained that some of the polymeric hydrate inhibitors
with relatively simple structures like polyvinylpyrrolidone (PVP) and polyvinylcaprolactam
(PVCap) might adsorb on the surface of hydrate crystal. This adsorption was hypothesized to
be similar to adsorption of the winter flounder polypeptide on a plane of growing ice.
Method of research
The molecular simulation program SYBYL® (version 6.01), a product of Tripos Associates,
Inc., was used to perform the docking studies. Docking of the winter flounder polypeptide
was simulated on three different surfaces of ice-I, and on crystal surfaces of sI and sII hy-
drate. The sequence of the aminoacids chemical formula of the winter flounder polypeptide
is shown here. Docking of the hydrate inhibitor polymers VC-713, PVP, PVCap shown in
Fig. 10.20 was performed on crystal surfaces of sI and sII hydrate. The crystal surface and
each macromolecule were docked in vacuo and the intermolecular energy was estimated for
different conformations.
Adsorption energy was the main parameter studied in this calculation. The energy of ad-
sorption is a measure of the interaction between the inhibitor and the water crystal (ice or
gas hydrate). Both the polymer (inhibitor macromolecule) and the site (hydrate or ice crys-
tal) were positioned by SYBYL® in the two separate molecular areas. The interaction of the
polymer with hydrate was calculated on an atom-to-atom and charge-to-charge basis which
accounted for each water molecule in the hydrate lattice. The interaction energy accounts for
the steric (van der Waals) and electrostatic (Coulombic) interactions.
The energy of interaction is zero at infinite separation of the site and the polymer, and it is
a potential function of distance between interacting sites. A review of potential functions was
presented by Prausnitz et al. (1986, sec. 5.5). The lowest interaction energies obtained in this
study cannot be viewed as absolute minima. The number of possible configurations of poly-
mer on the crystal surface is nearly infinite, only limited by computer precision. The energies
obtained energies represent local minima in energy and the best attempt of the author within
the available resources.