Biomedical Engineering Reference
In-Depth Information
chemical work). At the same time, in order to decrease
water chemical potential in the distant compartment
relative to that in the closer one; water density must de-
crease (see row 2 of
Table 3.1.5-1
, fewer molecules/unit
volume available for chemical work). This thinking gives
rise to the notion of contiguous regions of variable water
density within a polyelectrolyte solution. Here again, it is
evident that adjustment of water chemical potential to
accommodate the presence of a large solute molecule
appears to be a necessary mechanism to account for
commonly observed hydration effects. The next section
will explore how these same effects might account for
surface wetting effects.
that water in direct contact with a hydrophobic surface is
less dense than bulk water some distance away from the
hydrophobic surface.
This reasoning has been recently corroborated theo-
retically through molecular simulations of water near
hydrophobic surfaces (
Besseling and Lyklema, 1995;
Lum
et al.,
1999
; Silverstein
et al.,
1998) and experi-
mentally by application of sophisticated vibrational
spectroscopies (
Du
et al.,
1994
;
Gragson and Richmond,
1997
). Although there is no precise uniformity among all
investigators using a variety of different computational
and experimental approaches, it appears that density
variations propagate something of the order of 5 nm from
a hydrophobic surface, or about 20 water layers.
There are at least two classes of hydrophilic surfaces
that deserve separate mention here because these rep-
resent important categories of biomaterials as well
(
Hoffman, 1986
). One class includes surfaces that
ad
sorb water through the interaction with surface-
resident Lewis acid or base groups. These water-surface
interactions are constrained to the outermost surface
layer, say the upper 1 nm or so. Examples of these bio-
materials might include polymers that have been surface
treated by exposure to gas discharges, use of flames, or
reaction with oxidative reagents as well as ceramics,
metals, and glass. Another category of hydrophilic sur-
faces embraces those that significantly
ab
sorb water.
Examples here are hydrogel polymers such as poly(vinyl
alcohol) (PVA), poly(ethylene oxide) (PEO), or hydrox-
yethylmethacrylate (HEMA) that can visibly swell or
even go into water solution, depending on the molecular
weight and extent of crosslinking. Modern surface engi-
neering can create materials that fall somewhere between
water-adsorbent and -absorbent by depositing very thin
films using self-assembly techniques (
P.-Grosdemange
et al.,
1991; Prime and Whitesides, 1993
), reactive gas
plasma deposition (
Lopez
et al.,
1992
), or radiation
grafting (
Hoffman and Harris, 1972; Hoffman and Kraft,
1972; Ratner and Hoffman, 1980
) as examples. Here,
oligomers that would otherwise dissolve in water form
a thin-film surface that cannot swell in the usual, mac-
roscopic application of the word. In all of the mentioned
cases, however, water hydrogen bonds with functional
groups that may be characterized as either Lewis acid or
base. In the limit of very strong (energetic) surface
acidity or basicity, water can become ionized through
proton or hydroxyl abstraction.
The subject of water structure near hydrophilic sur-
faces is considerably more complex than water struc-
turing at hydrophobic surfaces just discussed, which
itself is no trivial matter. This extra complexity is due to
three related features of hydrophilic surfaces. First, each
hydrophilic surface is a unique combination of both type
and surface concentration of water-interactive Lewis acid
or base functional groups
The surface wetting effect
It is a very common observation that water wets certain
kinds of surfaces whereas water beads up on others,
forming droplets with a finite ''contact angle.'' This and
related wetting phenomena have intrigued scientists for
almost three centuries, and the molecular mechanisms of
wetting are still an important area of research to this day.
The reason for such continued interest is that wetting
phenomena probe the various intermolecular forces and
interactions responsible for much of the chemistry and
physics of everyday life. Some of the remaining open
questions are related to water structure and solvent
properties near different kinds of surfaces.
Although surfaces on which water spreads are com-
monly termed hydrophilic and those on which water
droplets form hydrophobic, the definitions employed in
preceding sections based on presence or absence of Lewis
acid/base groups that can hydrogen bond with water will
continue to be used here, as this is a somewhat more
precise way of categorizing biomaterials. Thus, hydro-
phobic surfaces are distinguished from hydrophilic by
virtue of having no Lewis acid or base functional groups
available for water interaction.
Water near hydrophobic surfaces finds itself in a pre-
dicament similar to that briefly mentioned in the pre-
ceding section on the hydration of large hydrophobic
solutes in that there are no configurational options
available to water molecules closest to the surface that
allow maintenance of nearest-neighbor hydrogen bonds.
These surface-contacting water molecules are conse-
quently in a less self-associated state and, according to
row 2 of
Table 3.1.5-1
, must temporarily be in a state of
higher chemical potential than bulk water. The key word
here is temporarily, because chemical potential gradients
cannot exist at equilibrium. At constant temperature and
pressure, the only recourse available to the system
toward establishment of equilibrium is decreasing local
water density by increasing the extent of water self-
association (row 1,
Table 3.1.5-1
). Thus it is reasoned
(amine, carboxyl, ether,
