many different methods have developed, each with its own benefits and
drawbacks. As more applications for superhydrophobic surfaces appear—
clothing, photoactive devices, windows, and more—the drive is for a
variety of superhydrophobic materials that can be customized to fit these
multiple needs.
7.2 ADSORPTION PHENOMENA:
SELF-ASSEMBLED MONOLAYERS
A molecule approaching a surface may experience a net attractive force
and consequently become trapped or confined at the surface. Such a
species is called the adsorbate, and adsorption is the physical process by
which adsorbate molecules accumulate onto a solid surface. Desorption
is the opposite process in which molecules leave the surface and enter the
bulk phase. The solid surface in question is referred to either as the
substrate or adsorbent. The former term is typically used for planar
surfaces and the latter term often refers to high-surface-area porous
solids. The sticking of a reactant molecule to a surface was first proposed
by Michael Faraday in 1834 as the initial step of a surface-catalyzed
reaction. In this section we are interested in the kind of adsorption that
results in a monolayer and has a thickness on the order of nanometers.
The surface itself does not need to be flat but may be rough or even
porous. The extent of adsorption depends not only on the types of
intermolecular forces involved (van der Waals, electrostatic, hydrogen
bonding), but also on the surface area; the greater the surface area of the
substrate, the greater the extent of adsorption.
The best adsorbents are those with large total surface areas, such as silica
gel (SiO 2 , surface area >1000 m
2
/g) and activated carbon. Silica gel is
commonly used in chromatographic columns to enhance the separation of solute mixtures by taking advantage of the different degrees of
adsorption of the various components. Another interesting example of
an adsorption phenomenon occurs in polar stratospheric clouds in the
upper atmosphere, in which highly porous particles of ice act as substrates for the adsorption of gases such as HCl.
In this and the next section we focus on the adsorption of molecules from
solution or gaseous phase onto a solid substrate. However, it is important to appreciate that adsorption is a surface phenomena and can occur
at any interface. For example, molecules may adsorb from an aqueous
CHAPTER 7: Fundamentals of Surface Nanoscience
230
drawbacks. As more applications for superhydrophobic surfaces appear—
clothing, photoactive devices, windows, and more—the drive is for a
variety of superhydrophobic materials that can be customized to fit these
multiple needs.
7.2 ADSORPTION PHENOMENA:
SELF-ASSEMBLED MONOLAYERS
A molecule approaching a surface may experience a net attractive force
and consequently become trapped or confined at the surface. Such a
species is called the adsorbate, and adsorption is the physical process by
which adsorbate molecules accumulate onto a solid surface. Desorption
is the opposite process in which molecules leave the surface and enter the
bulk phase. The solid surface in question is referred to either as the
substrate or adsorbent. The former term is typically used for planar
surfaces and the latter term often refers to high-surface-area porous
solids. The sticking of a reactant molecule to a surface was first proposed
by Michael Faraday in 1834 as the initial step of a surface-catalyzed
reaction. In this section we are interested in the kind of adsorption that
results in a monolayer and has a thickness on the order of nanometers.
The surface itself does not need to be flat but may be rough or even
porous. The extent of adsorption depends not only on the types of
intermolecular forces involved (van der Waals, electrostatic, hydrogen
bonding), but also on the surface area; the greater the surface area of the
substrate, the greater the extent of adsorption.
The best adsorbents are those with large total surface areas, such as silica
gel (SiO 2 , surface area >1000 m
2
/g) and activated carbon. Silica gel is
commonly used in chromatographic columns to enhance the separation of solute mixtures by taking advantage of the different degrees of
adsorption of the various components. Another interesting example of
an adsorption phenomenon occurs in polar stratospheric clouds in the
upper atmosphere, in which highly porous particles of ice act as substrates for the adsorption of gases such as HCl.
In this and the next section we focus on the adsorption of molecules from
solution or gaseous phase onto a solid substrate. However, it is important to appreciate that adsorption is a surface phenomena and can occur
at any interface. For example, molecules may adsorb from an aqueous
CHAPTER 7: Fundamentals of Surface Nanoscience
230