Chemistry Reference
In-Depth Information
molecular aggregation in solution eventually began to develop a significant follow-
ing and today everyone accepts it as fact. Micelles have now been studied with
almost every technique devised by modern science, including X-ray diffraction
(XRD), nuclear magnetic resonance (NMR), electron spin resonance (ESR),
small-angle neutron scattering (SANS), light scattering, fluorescence, calorimetry,
and many other solution and spectroscopic techniques. Despite being probed,
prodded, and picked apart, however, micelles have still refused to yield the ultimate
data, the interpretation of which is universally accepted and that unequivocally
defines the true nature and characteristics of the aggregated species. It is possible,
of course, that the diversity of surfactant structures and micellar and related aggre-
gate species (vesicles, bilayers, microemulsions, etc.) are such that only very gen-
eral laws will be found to be applicable to all; perhaps each system will have its
specific twists, which preclude the existence of a ''universal theory of everything''
for surfactant aggregation, although in the generally ordered scheme of natural phe-
nomena, such a prospect is unlikely. However, in science, as in many other human
endeavors, it is as much the thrill of the hunt as the final capture that supplies the
driving force for our activities.
It is generally accepted that most surface-active molecules in aqueous solution
can aggregate into structures or clusters averaging 30-200 monomeric units in such
a way that the hydrophobic portions of the molecules are closely associated and
mutually protected from extensive contact with the bulk of the water phase. Not
so universally accepted are some of the ideas concerning micellar shapes, the nature
of the interior of a micelle, the ''roughness'' of the aggregate surface, the sites of
adsorption of additional solutes into (or onto) micelles, and the size distribution of
micelles in a given system. Although sophisticated experimental techniques con-
tinue to provide new insights into the nature of micelles, we still have things to
learn. Given the inherent tendency of scientists to question and refine experimental
procedures and to offer alternative interpretations for the results, it seems likely that
questions concerning the theory of micelle formation and a complete model of the
molecular nature of micelles will remain ''fair game'' for some time to come.
4.3.1. Manifestations of Micelle Formations
Early in the study of the solution properties of surface-active materials, it became
obvious that the bulk solution properties of such materials were unusual and could
change dramatically over very small concentration ranges. The measurement of
properties such as surface tension, electrical conductivity, or light scattering as a
function of surfactant concentration produces property curves that normally exhibit
relatively sharp discontinuities at comparatively low concentration (Figure 4.5).
The sudden change in a measured property is interpreted as indicating a significant
change in the nature of the solute species affecting the measured quantity. In the
case of the measurement of equivalent conductivity (top curve), the break may
be associated with an increase in the mass per unit charge of the conducting species.
For light scattering (bottom curve), the change in solution turbidity indicates the
appearance of a scattering species of significantly greater size than the monomeric
