Chemistry Reference
In-Depth Information
group—the lyophilic or hydrophilic group, in aqueous systems—and the ''tail''
refers to the lyophobic or hydrophobic group in water:
CH
3
ð
CH
2
Þ
n
CH
2
S
''
tail
''
''
head
''
In water, the hydrophobic group may be, for example, a hydrocarbon, fluorocarbon,
short polymeric chain, or siloxane chain of sufficient size to produce the desired
solubility characteristics when bound to a suitable hydrophilic group. In aqueous
systems, the hydrophilic group (the ''head'') will be ionic or highly polar, so that
it can act as a solubilizing functionality. In a nonpolar solvent such as hexane the
same groups will, in theory, function in the opposite sense. As the temperature,
pressure, or solvent environment of a surfactant (e.g., cosolvent addition, pH
changes, or the addition of electrolytes in aqueous systems) varies, significant
alterations in the solution and interfacial properties of the surfactant may occur.
As a result, modifications in the chemical structure of the surfactant may be needed
to maintain a desired degree of surface activity. It cannot be overemphasized that a
given surfactant effect will be intimately tied to the specific solvent environment in
use. Any change in that environment may significantly alter the effectiveness of a
surfactant and require major structural changes to retain the desired surface effects.
Therefore, for surface activity in a given system, the prospective surfactant mole-
cule must possess a chemical structure that is amphiphilic in the desired solvent
under the proposed conditions of use. But how can one determine the best chemical
structure for use in a given system?
For some time, a goal of surfactant
related research has been to devise a quan-
titative way to relate the chemical structure of surface
active molecules directly to
their physicochemical activity in use. One of the earliest attempts to correlate sur-
face activity and chemical structure came from the cosmetics industry and is known
as the ''hydrophile-lipophile balance'' (HLB) system. Described more fully in
Chapter 9, the HLB system relates the molecular composition of a surfactant (as
mol% of hydrophile) to its surfactant properties. Although it is not a quantitative
panacea for designing surfactant molecules, it continues to be an important tool
in the practical arsenal of surfactant formulation technology. In more recent
years, attempts have been made to use more theoretically ''satisfying'' tools such
as the concepts of cohesive energy densities
d
(also called solubility or Hildebrand
parameters) and molecular geometry to correlate such relationships as surfactant
chemical structure, the nature of the solvent, and surfactant activity on a more fun-
damental atomic and molecular level. Those and other schemes for predicting sur-
factant activity based on the specifics of molecular architecture will be addressed in
more detail in subsequent chapters.
2.1. BASIC SURFACTANT BUILDING BLOCKS
One way to approach the concept of building a useful surfactant molecule is to look
at the process much as a child building a boat with Leggo building blocks. The first
