Chemistry Reference
In-Depth Information
disagreement within the surfactant literature as to the fine points of the definition of
solubilization, particularly as the surfactant : additive ratio decreases and one
approaches the nebulous frontier between swollen micellar systems and the
microemulsions discussed in Chapter 5 and emulsion regimes to be discussed
in Chapter 9. For now, the discussion is limited to systems in which the micelle
is clearly the primary vehicle for the observed phenomenon.
For present purposes, solubilization is defined as a spontaneous process leading
to a thermodynamically stable, isotropic solution of a substance (the additive) nor-
mally insoluble or only slightly soluble in a given solvent produced by the addition
of one or more amphiphilic compounds, including polymers, at or above their cri-
tical micelle concentration. Using such a definition, we can cover a broad area that
includes both dilute and concentrated surfactant solutions, aqueous and nonaqueous
solvents, all classes of surfactants and additives, and the effects of complex inter-
actions such as mixed micelle formation. It does not, however, limit the phenom-
enon to any single mechanism of action.
The history of solubilization research in the first half of the twentieth century has
been extensively reviewed, and several pertinent references are listed in the Biblio-
graphy. A discussion of some important results can be found in the work of
Elworthy, et. al., which also includes a description of many of the experimental
techniques that have been developed for investigations into the factors affecting
the process.
Although there are many aspects to understanding solubilization phenomena,
this discussion is concerned primarily with the correlations that can be made
between the molecular structure of a surfactant and its activity and capacity as a
solubilizing agent, the related effects of the chemical nature of the additive, and
the role of the solution environment. For a specified solvent system, water or aqu-
eous solutions, for example, two variables must be considered in the solubilization
process: (1) the molecular nature, purity, and homogeneity of the surfactant and (2)
the chemical nature of the additive. From a technological viewpoint, it is important
to understand exactly what surfactant structural features serve to maximize the
desired solubilizing effect, and the best way to achieve that understanding is
through a fundamental knowledge of the molecular and thermodynamic processes
involved. In addition, since most technological applications of solubilization (e.g.,
detergent action) involve complex multicomponent systems, such factors as tem-
perature, electrolyte content, and the presence of polymeric species and other
solutes must be examined. Obviously, for such applications as cleaning and deter-
gency, it is not possible to completely specify the system with anticipation; there-
fore, consideration must always be given to attaining broad solubilizing
capabilities, often at the expense of the optimum for a specific ''model'' set of cir-
cumstances.
Before addressing some of the specific aspects of the influence of surfactant
structure on solubilization, it will be useful to understand the ''geography'' of solu-
bilization—that is, the possible positions in (or on) the micelle that can serve as
host sites for the additive molecules and the factors that determine exactly where
solubilization will occur.
