against the surface tension forces. This work actually represents the
surface free energy of the expansion of the interface.
7.1.2 Surface free energy of adsorbed monolayers
The measurement of surface tension is perhaps the oldest method used to
characterize a nanofilm floating on the surface of water. One class of
molecules that can form nanofilms on water, and which will be discussed
further later in the chapter, are known as amphiphiles, and are capable of
self-assembling into nanostructures of remarkable complexity. Amphiphilic molecules, such as lipids and surfactants, have a tendency to
accumulate on the surface of water. The driving forces behind this selfassembly process involve a combination of factors, such as the lack of
solubility of the hydrocarbon portion of the molecule in water and the
tendency of the charged or polar headgroup to point toward the aqueous
phase. The net result is the formation of a film composed of a monolayer of highly oriented molecules. The closely packed monolayer is in
dynamic equilibrium with individual molecules in the bulk aqueous
phase, and so the packing density of the film increases as the concentration of molecules in the bulk phase increases. However, structural and
thermodynamic constraints often limit the packing density, and a saturated monolayer is usually formed when the bulk concentration is sufficiently high. The thickness of such a film is approximately the length
of the amphiphilic molecule (nanoscale). Due to this short dimension, the monomolecular film is often described as a two-dimensional
nanoassembly.
Figure 7.2 shows a monolayer of the surfactant sodium dodecyl sulfate
(SDS) confined to the air–water interface. Later in the book, we show that
nanofilms at the air–water interface are of vital importance to biological
processes and that their properties can be exploited in a number of
important applications. Thus, it is often crucial to know how well the film
is packed within both the monolayer and the cross-sectional area occupied by each molecule on the liquid surface. These factors often determine the physicochemical properties of the film.
Surface tension measurements provide a simple but powerful method for
determining a variety of characteristics of adsorbed monolayers, including the density of the monolayer. Surface tension measures the stability of
a surface. If the surface is relatively unstable, it has a large surface tension
value and is considered a “high energy surface.” For example, liquid water
has a high surface energy because the molecules prefer not to be on the
FUNDAMENTALS OF SURFACE SCIENCE 219
surface free energy of the expansion of the interface.
7.1.2 Surface free energy of adsorbed monolayers
The measurement of surface tension is perhaps the oldest method used to
characterize a nanofilm floating on the surface of water. One class of
molecules that can form nanofilms on water, and which will be discussed
further later in the chapter, are known as amphiphiles, and are capable of
self-assembling into nanostructures of remarkable complexity. Amphiphilic molecules, such as lipids and surfactants, have a tendency to
accumulate on the surface of water. The driving forces behind this selfassembly process involve a combination of factors, such as the lack of
solubility of the hydrocarbon portion of the molecule in water and the
tendency of the charged or polar headgroup to point toward the aqueous
phase. The net result is the formation of a film composed of a monolayer of highly oriented molecules. The closely packed monolayer is in
dynamic equilibrium with individual molecules in the bulk aqueous
phase, and so the packing density of the film increases as the concentration of molecules in the bulk phase increases. However, structural and
thermodynamic constraints often limit the packing density, and a saturated monolayer is usually formed when the bulk concentration is sufficiently high. The thickness of such a film is approximately the length
of the amphiphilic molecule (nanoscale). Due to this short dimension, the monomolecular film is often described as a two-dimensional
nanoassembly.
Figure 7.2 shows a monolayer of the surfactant sodium dodecyl sulfate
(SDS) confined to the air–water interface. Later in the book, we show that
nanofilms at the air–water interface are of vital importance to biological
processes and that their properties can be exploited in a number of
important applications. Thus, it is often crucial to know how well the film
is packed within both the monolayer and the cross-sectional area occupied by each molecule on the liquid surface. These factors often determine the physicochemical properties of the film.
Surface tension measurements provide a simple but powerful method for
determining a variety of characteristics of adsorbed monolayers, including the density of the monolayer. Surface tension measures the stability of
a surface. If the surface is relatively unstable, it has a large surface tension
value and is considered a “high energy surface.” For example, liquid water
has a high surface energy because the molecules prefer not to be on the
FUNDAMENTALS OF SURFACE SCIENCE 219