A proposed general structure for condensed monomolecular films

Abstract

A very brief review is given of the theories put forward by earlier workers to explain the observed characteristics of condensed monolayers. From the detailed structures of numerous condensed films as discussed in a previous publication, certain generalizations are now suggested. For the least compressible region ( YZ ) evidence based upon surface potential measurements, upon the X-ray structure of hydrocarbons near the melting-point, and upon the compressibility, strongly support the statement that in this region the long chains are vertically orientated and close-packed, but not as tightly as is possible in the three-dimensional crystal. In the more compressible region ( XY ) the behaviour appears to be intermediate between that in the YZ region (determined only by the hydrocarbon chain packing), and in the expanded state (determined only by the head-group dipole orientation). The most general statement possible is that the configuration at the limiting area X is determined both by the orientation of the head-group dipole with respect to the surface, and by the long-chain packing. All known classes of condensed monolayers are grouped according to the factors thought to be primarily responsible for determining the limiting area X , namely, the ‘size’ of the head group, hydrogen bond formation between the head groups, and the hydrocarbon chain packing. The physical states of monolayers, and the factors by which they are influenced, are briefly discussed. Forces both between the polar head groups and between the hydrophobic tails have to be considered.