Page 255 - Environmental Nanotechnology Applications and Impacts of Nanomaterials

240 Principles and Methods

subsequently provides a preferential orientation of water through hydro-
gen bonding. The adsorption of water at functional sites on a surface may
be associated with the net release of a proton or hydroxide to the solu-
tion. Thus, the surface may act as an acid or a base. Layer(s) of relatively
ordered water on surfaces may impede particle attachment or aggre-
gation due to the additional energy required to “squeeze” water out as
separation distances become small. In contrast, hydrophobic surfaces do
not interact with water and are essentially pushed toward one another
as water molecules preferentially bond with one another.
This latter phenomenon can be interpreted as an attractive interac-
tion referred to as the hydrophobic effect [3, 21]. The interactions
between particles and the bulk water are a function of the size of the
particles or hydrophobic surfaces [21]. Hydrophobic nanoparticles will
perturb water structuring to a much smaller degree than will larger par-
ticles with a similar chemistry [9]. In the case of small nanoparticles or
other hydrophobic molecules, water may form a cage structure around
the nanoparticle or molecular core resulting in a relatively stable con-
figuration referred to as a clathrate. Methane clathrates in deep ocean
waters are thought to represent an enormous reservoir of hydrocarbons
on our planet. Clathrate formation has been suggested as a mechanism
behind the hydration of otherwise hydrophobic C nanoparticles (which
60
have a diameter of a little over 1 nm) and may explain how these
fullerenes acquire a charge and interact with water to form stable col-
loidal suspensions of C aggregates.
60
As particle size becomes larger than 2 nm, an energetically unfavor-
able condition results. To compensate for a larger “disruption” between
hydrogen bonds, such as hydrophobic particle, the surrounding water
molecules must still restructure themselves to maintain their hydrogen-
bonding network with other water molecules.This water depletion or
drying between two interacting surfaces results a net attraction between
hydrophobic surfaces. It is important to bear in mind that the surfaces
themselves do not actually attract each other; in fact, the water simply
“likes” itself too much to allow the surface to remain exposed. The bal-
ance between particle-solvent and solvent-solvent interactions and
hydrogen bonding energies ultimately determines whether cavitation or
strict microscopic dewetting will occur. Therefore, for all but the small-
est size fraction of nanoparticles (1 nm < d < 2 nm) hydrophobic inter-
actions will be significant and will tend to favor particle aggregation.
In contrast with hydrophobic surfaces, hydrophilic surfaces possess
surface groups that may coordinate water molecules through hydration
[3, 4]. Subsequent layers of water molecules may hydrogen bond with
the hydrated water, resulting in several layers of relatively ordered
water extending from the surface. The kinetic energy of the water
molecules will tend to break hydrogen bonds. Thus, lower temperatures