acase@tulane.edu / The statistical mechanical theory of hydrophobic interactions was
initiated decades ago for purely repulsive hydrophobic species, in fact,
originally for hard-sphere solutes in liquid water. Systems which treat
only repulsive solute-water interactions obviously differ from the real
world situation. The issue of the changes to be expected from
inclusion of realistic attractive solute-water interactions has been of
specific interest also for decades. We consider the local molecular
field (LMF) theory for the effects of solute attractive forces on
hydrophobic interactions. The principal result of LMF theory is
outlined, then tested by obtaining radial distribution functions (rdfs)
for Ar atoms in water, with and without attractive interactions
distinguished by the Weeks-Chandler-Andersen (WCA) separation. Change
from purely repulsive atomic solute interactions to include realistic
attractive interactions substantially diminishes the strength of
hydrophobic bonds. Since attractions make a big contribution to
hydrophobic interactions, Pratt-Chandler theory, which did not include
attractions, should not be naively compared to computer simulation
results with general physical interactions, including attractions. Lack
of general appreciation of this point has lead to mistaken
comparisons throughout the history of this subject. The
rdfs permit evaluation of osmotic second virial coefficients B2.
Those B2 are consistent with the conclusion that incorporation of
attractive interactions leads to more positive (repulsive) values. In
all cases here, B2 becomes more attractive with increasing
temperature below T = 360K, the so-call inverse temperature
behavior.
In 2010, the Gulf of Mexico Macondo well (Deepwater Horizon) oil spill
focused the attention of the world on water-oil phase equilibrium. In
response to the disaster, chemical dispersants were applied to break oil
slicks into droplets and thus to avoid large-scale fouling of beaches and to
speed up biodegradation. Eventually the dispersant COREXIT 9500 was
used predominantly in responding to this accident. The formulation of
COREXIT dispersants is somewhat complicated and the various constituents
(and their interactions) deserve exhaustive study. Here we focus on
sorbitan monooleate (SPAN80), one important component of COREXIT 9500,
and we investigate its behavior in oil-water-surfactant systems.
Extensive all-atom molecular dynamics calculations on the water-squalane
interface for nine different loadings with SPAN80, at T=300K, are
analyzed for the surface tension equation of state, desorption free
energy profiles as they depend on loading, and to evaluate escape times
for absorbed SPAN80 into the bulk phases. These results suggest that
loading only weakly affects accommodation of a SPAN80 molecule by this
squalane-water interface. Specifically, the surface tension equation of
state is simple from conditions of low loading (high tension) to high loading
(lower tension)
studied, and the desorption free energy profiles are weakly dependent on
loading here. The perpendicular motion of the centroid of the SPAN80
head-group ring is well-described by a diffusional model near the
minimum of the desorption free energy profile. Lateral diffusional
motion is weakly dependent on loading. Escape times evaluated on the
basis of a diffusional model and the desorption free energies are
0.07~s (into the squalane) and 300~h (into the
water). The latter value is consistent with irreversible absorption
observed by related experimental work. / 1 / Liang Tan
| Identifer | oai:union.ndltd.org:TULANE/oai:http://digitallibrary.tulane.edu/:tulane_77157 |
| Date | January 2017 |
| Contributors | Tan, Liang (author), Pratt, Lawrence (Thesis advisor), School of Science & Engineering Chemical and Biomolecular Engineering (Degree granting institution) |
| Publisher | Tulane University |
| Source Sets | Tulane University |
| Language | English |
| Detected Language | English |
| Type | Text |
| Format | electronic, 129 |
| Rights | No embargo, Copyright is in accordance with U.S. Copyright law. |
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