Chemistry Reference
In-Depth Information
4
Surfactants in Solution:
Monolayers and Micelles
The amphiphilic nature of surfactants causes them to exhibit many properties that
appear on first sight, to be contradictory. Because of their special molecular struc-
tures, they possess something of a ''love-hate'' relationship in most solvents,
resulting in a tug of war among competing forces striving for a comfortable (ener-
getically speaking) accommodation within a given environment. Surfactants, one
might say, appear to feel to some extent that the grass is greener on the other side
of the fence, and as a result, they spend much of their time sitting on that ''fence''
between phases. Some of the basic characteristics of those ''fences'' were intro-
duced in Chapter 3. This chapter will begin the process of expanding on the speci-
fics of how surfactant molecular structures affect their surface activity. Specific
topics on the adsorption of surfactants at specific interfaces will be discussed in
later chapters. At this point, it is important to understand some of the more import-
ant aspects of the solution behavior of surfactants and some of the circumstances
that can affect that behavior.
In their energetic ''need'' to minimize unfavorable interactions or to maximize
favorable interactions with their environment, surfactants spend much of their time
at interfaces or associating with others of their own kind. The purpose of the dis-
cussions here is to introduce some of the more important and useful fundamental
concepts of surfactants in solution, as we currently understand them. A basic under-
standing of these concepts can help guide a prospective surfactant user in under-
standing a given phenomenon and choosing a material that may suit a particular
need. This chapter is concerned primarily with the more simple—if that term
can be applied in the present context—aspects of surfactant activity in terms of
self-assembled or spontaneous, thermodynamically driven aggregate structures in
solution. As is usually the case with surfactant-related discussions, the primary
emphasis will be placed on aqueous systems. Chapter 5 gives a broad introduction
to the more complex and highly ordered self-assembled structures such as vesicle,
bi- and multiplayer membranes, and the new darlings of the field, continuous bilayer
systems. All of those areas are becoming more important in current and potential
technological and research applications of surfactants and other amphiphiles.
