Chapter 4 - Phospholipid Monolayers

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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 myself 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.

Section snippets

Thermodynamic measurements

The very first and most simple measurement to characterize a surfactant monolayer is that of the lateral pressure π as a function of molecular area A. π is measured as the difference in surface pressure comparing the value in the absence (π0) and presence (π1) of surfactant at the surface [4] π=π0π1.

Since π is the derivative of the surface free energy with respect to the intrinsic variable A it is a thermodynamic variable.

Measuring π versus A, the so-called pressure/area (π, A)-isotherms, one

Theoretical calculations

Stimulated by the possibilities of more refined structural analysis and by the interest in different aspects of low-dimensional systems there has been an increasing activity to theoretically describe surfactant films. These shall be summarized below classified not according to the tools used but with respect to the aims.

Thermodynamic equilibrium?

Attempts to describe a monolayer by equilibrium thermodynamics are frequently criticized since in most cases the system is not in thermodynamic equilibrium:

  • (i)

    The equilibrium spreading pressure generally amounts to at most a few mN/m, often to the pressure corresponding to the main phase transition [152]. Hence for higher lateral pressures the monolayer would prefer to form a three-dimensional crystal on the water surface. From this follows that generally the ordered monolayer phases are

System diversity

Above we have concentrated on showing basic features of well-defined phospholipid monolayers and will subsequently try to conclude on more complex systems. For these there do not exist too many structural data and the extrapolation therefore is not based on many facts.

Future outlook

This contribution has concentrated on our understanding of some of the most simple phospholipid systems, and I have to apologize with those colleagues working on complexer systems who are therefore not well represented here. The latter is unfortunate since the biologically more relevant systems are not the simple ones. However, lack of space and partly also of knowledge prevented me from simply listing the wealth of nature. Instead I wanted to spend more effort to elaborate on physical

Acknowledgement

In writing this chapter I have profited from numerous experimental contributions from my skillful and eager co-workers. However, this review would never have been completed without the competent and elaborate literature research by Andrea Dietrich and Ulrike Höhne, by the patient and careful preparation of this manuscript by Helga Resch.

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