Chapter 4. Phospholipid monolayers

H. Möhwald
Max-Planck-Institut f ür Kolloid- und Grenzflächenforschung,
Rudower Chaussee 5, D-12489 Berlin, Germany

1. Introduction

Why study phospholipid monolayers? For most of the readers of this book a convincing motivation may be the membrane biophysical aspect. The monolayer, being half of a membrane, is a very well-defined planar system to study intermolecular interactions between lipids and also between lipids and proteins. This was also my perspective when entering the field a decade ago. However, I then realized the many interesting aspects of physics in two dimensions as well as some technological relevance. Hopefully the reader will also grasp some of these newer aspects which are related to areas of future research interest.

Most of the basic principles of a phospholipid monolayer are typical for other insoluble monolayers [1] and hence one may find many ideas now becoming fashionable already in Langmuir's earlier work [2, 3]. However, whereas these original ideas were based only on indirect experimental observation and thus were close to speculations there has been a tremendous development of experimental tools to investigate monolayer structure. These techniques have been to some extent applied to the best defined monolayers of glycerophosphatidyls with saturated aliphatic tails, and therefore this chapter will concentrate mostly on these. The discussion of these results will hopefully help the reader to conclude at least tentatively on other systems which are not explicitly mentioned here.

This chapter is organized as follows: The main body will contain a description of experimental and theoretical techniques. It will be shown how they were applied to phospholipids, what one could learn and where the limitations are. Being my-self involved in some very recent developments I feel competent also to comment on future directions and improvement of our understanding. The next chapter will then briefly discuss theoretical developments. In a separate chapter the reader will
find an extraction of our present knowledge on phases, phase transitions and on the structure at length scales between molecular and macroscopic dimensions. I will also try to correlate results on phospholipid monolayers with those on other surfactant films which will lead to the elaboration of general physical principles and also suggest extrapolation to other phospholipids and complexer systems not yet studied as extensively.

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