Amphiphilic Molecules and Liquid Crystals

A brief description of the essential features of amphiphillic molecules will be presented. This will be followed by a discussion of specific physical phenomena and their relation to a number of contemporary research areas. Foremost amongst these is the possible application of amphiphillic monolayers to the physics of two-dimensional systems. Specific experimental and theoretical examples will largely be drawn from studies of synthetic phosphatidylcholine (e.g., lecithin) type lipids. Recent experimental results from our laboratory will be mentioned. 1 . Introduction. The intention in preparing this article is to provide a brief survey of some of the salient features of amphiphillic materials and then illustrate how the physical properties of macroscopic systems formed from amphiphillics relate to phenomena of fundamental importance to a variety of other subjects. Hopefully, this superficial discussion of diverse phenomena from disparate disciplines will enable the reader that is already an expert in one discipline to better appreciate how his work bears on the current activities in other disciplines. The reader should also be aware that this paper was originally prepared for oral presentation and, in the manner of oral presentations, there are fewer literature citations than would have been the case had this article been prepared as a written review article. Nevertheless those citations that are listed should provide the reader with entry points into the literature of different fields. One of the classical textbooks on amphiphillic materials is Adamsons' [l], however, Tanford has recently prepared an excellent and concise review [2] of hydrophobic effects and his own book [3] is certainly relevant. 2. Amphiphillic molecules. The saturated fatty acid CH,(CH,),COOH can be considered as the prototype of all amphiphillic molecules in that it consists of two parts which separately have very different solubilities in water, or other solvents. In this case the polar part, HCOOH or formic acid, is infinitely soluble in water while the alkane or hydrocarbon part CH,(CH,),-,CH, is much less soluble in water than in most non-aqueous solvents. For simple alkanes, with less than 10 20 carbon atoms, the solubility in water decreases by roughly one order of magnitude with each increase in the carbon number by two. Conventional terminology attributes this decrease with increasing size of the hydrocarbon to a hydrophobic interaction. Unfortunately this is somewhat of a misnomer since the effect does not derive from a repulsion between water and the hydrocarbon [2, 31. Rather it is well known that when a hydrocarbon is in contact with water the network of hydrogen bonds between water molecules reconstructs itself to avoid the region occupied by the hydrocarbon. This constraint on the local structure of water decreases the entropy of the water near the hydrocarbon and results in a larger free energy for the total system, i.e., the water plus the solvated hydrocarbon. The hydrophobic effect thus arises more from the self-attraction of water for itself, which tends to squeeze the hydrocarbon out, rather than a repulsion between water and hydrocarbon. A corollary of this is that for hydrophobic groups more complicated than the simple saturated normal alkanes mentioned above, i.e., those containing branched chains or those with large enough carbon numbers that the chain can fold over upon itself, the solubility decreases in proportion to the surface area rather than the carbon number. Amphiphillic molecules are often termed surfactants or surface active molecules. Figure 1 illustrates that if one dissolves a low concentration of amphiphillics in a water solution, by virtue of the hydroArticle published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979385 C3-424 P. S. PERSHAN