Properties of materials - Biomedical Engineering Desk Reference - page 143

relatively fewer unmatched Lewis acid-base pairings than
in this reference state. Conversely, in less-associated
water, the network is relatively incomplete and there are
more unmatched Lewis acid-base pairings than in the
reference state. These unmatched pairings in less associ-
ated water are readily available to participate in other
chemical reactions, such as dissolving a solute molecule or
hydrating a water-contacting surface. Therefore, it can be
generally concluded that less-associated water is a stron-
ger solvent than more-associated water because it has
a greater potential to engage in reactions other than self-
association. In chemical terminology, less self-associated
water has a greater chemical potential than more self-as-
sociated water. Interestingly, more self-associated water
with a relatively more complete 3D network of hydrogen
bonds must be less dense (greater partial molar volume)
than less self-associated water because formation of lin-
early directed hydrogen bonds takes up space ( Fig. 3.1.5-
1 C), increasing free volume in the liquid. This is why
water ice with a complete crystalline network is less dense
than liquid water and floats upon unfrozen water, a phe-
nomenon with profound environmental impact. Thus,
less associated water is not only more reactive but also
more dense. These inferred relationships between water
structure and reactivity are summarized in Table 3.1.5-1 ,
which will be a useful aid to subsequent discussion.
A variety of lines of evidence ranging from molecular
simulations ( Lum et al. , 1999; Robinson et al. , 1996 )to
experimental studies of water's solvent properties in
porous media ( Qi and Soka, 1998; Wiggins, 1988 ) sug-
gest that water expands and contracts in density (molar
volume) with commensurate changes in chemical po-
tential to accommodate presence of imposed solutes and
surfaces. The word ''imposed'' is specifically chosen here
to emphasize that a solute (e.g., an ion or a macromole-
cule) or an extended surface (e.g., the outer region of
a biomaterial) must in some way interfere with self-
association. Simply stated, the solute or surface gets in
the way and water molecules must reorient to maintain
as many hydrogen bonds with neighbors as is possible in
this imposed presence of solute or surface. Water may
not be able to maintain an extensive hydrogen-bond
network in certain cases and this has important and
measurable effects on water solvency. The next sections

will first consider ''hydrophobic'' and ''hydrophilic''
solute molecules and then extend the discussion to hy-
drophobic and hydrophilic biomaterial surfaces, at least
to the extent possible within the current scientific
knowledge base.

The hydrophobic effect

The hydrophobic effect is related to the insolubility of
hydrocarbons in water and is fundamental to the orga-
nization of lipids into bilayers, the structural elements of
life as we know it ( Tanford, 1973 ). Clearly then, the
hydrophobic effect is among the more fundamental, life-
giving phenomena attributable to water. Hydrocarbons
are sparingly soluble in water because of the strong self-
association of water, not the strong self-association of
hydrocarbons as is sometimes thought. Thus water
structure is seen to be directly related to solvent prop-
erties in this very well-known case.
The so-called ''entropy of hydrophobic hydration''
( DS) has received a great deal of research attention from
the molecular-simulation community because it domi-
nates the overall (positive) free energy of hydrophobic
hydration ( DG ) at ambient temperatures and pressures.
The rather highly negative entropy of hydration of small
hydrocarbons ( DS z 20 e.u.; see Kauzmann, 1959 , for
discussion related to lipids and proteins) turns out to
be substantially due to constraints imposed on water-
molecule orientation and translation as water attempts to
maintain hydrogen-bond neighbors near the solute mol-
ecule ( Paulaitis et al, 1996 ). Apparently, there are no
structural ''icebergs'' with enhanced self-association
around small hydrocarbons ( Besseling and Lyklema,
1995 ) as has been invoked in the past to account for DS
( Berendsen, 1967 ). Instead, water surrounding small
solutes such as methane or ethane may be viewed as
spatially constrained by a ''solute-straddling'' effect that
maximizes as many hydrogen-bonded neighbors as possi-
ble at the expense of orientational flexibility. Interestingly,
while these constraints on water-molecule orientation do
not significantly promote local self-association (i.e., in-
crease structure), this lack of flexibility does have the
effect of reducing repulsive, non-hydrogen-bonding in-
teractions between water-molecule neighbors, account-
ing for a somewhat surprisingly exothermic (z 2 kcal/
mol) enthalpy of hydration (DH) of small hydrocarbons
( Besseling and Lyklema, 1995 ). The strong temperature
sensitivities of these entropic and enthalpic effects are
nearly equal and opposite and compensate in a way
that causes the overall free energy of hydration ( D G ¼
DH TDS) to be essentially temperature insensitive.
Increasing temperature expands the self-associated net-
work of water, creating more space for a hydrophobic
solute to occupy, and DS becomes more positive ( d TDS

Table 3.1.5-1 Relationships among water structure and solvent
properties

Extent of water
self-association

Density Partial
molar
volume

Chemical potential
(number of available
hydrogen bonds)

More Less More Less

Less More Less More