Order in Langmuir Monolayers and in the Aqueous Subphase

Pulak Dutta    Dept. of Physics & Astronomy, Northwestern University, Evanston IL 60208, USA

A variety of recent experiments have revealed a wide range of phases and phase transitions in simple amphiphilic monolayers on the surface of water. This paper describes some of the ordering phenomena revealed by X-ray diffraction. Recent experiments probing order in the underlying subphase are also described.

1 INTRODUCTION

Floating monolayers of insoluble amphiphilic molecules---Langmuir films---are deceptively easy to make and to manipulate. A grainy back-and-white 1940’s newsreel from General Electric shows Irving Langmuir casually putting these monolayers through their paces using tools available in every kitchen. (The monolayers were made ‘visible’ by the use of a macroscopic floating film of oil that is displaced by the amphiphiles.) It can be a shock to realize that, in this movie, Langmuir is playing with materials at the molecular scale---he is manipulating molecules.

In other words, it’s not necessarily difficult or expensive to do nanoscience. Until recently, however, a rather basic question about Langmuir monolayers remained: exactly what are the molecules doing when we manipulate the monolayer (e.g. change its pressure, density or temperature)? Hypotheses had been made and some were generally accepted in the field, but they had not been proved. It is reasonable that at the highest possible areal densities, the molecules are oriented so as to occupy the smallest area (simple linear-chain molecules would be oriented vertically). But how are they packed within the monolayer plane, and what happens as the temperature and pressure change? These monolayers are named after Langmuir because he was the first to see isotherm discontinuities indicating phase transitions, but pressure and density are macroscopic averages and do not tell us what is happening to the molecules at these phase transitions. This was not known until new experimental techniques (including X-ray diffraction and a variety of microscopies) were applied to this system starting in the 1980’s and especially in the 1990’s.

It is probably safe to say that, of the various techniques, synchrotron X-ray diffraction has had the most impact in this area. In general one should not overestimate the power of X-ray scattering: it does not generate real-space pictures, and it is not useful in disordered systems such as monolayers on very rough surfaces or monolayers with no lateral order or tilt order. But in the case of Langmuir monolayers of simple, rod-like molecules (e.g. saturated fatty acids and alcohols), X-ray scattering has provided important and vivid new information that has explicitly contradicted the conventional wisdom. Many phases of these monolayers turned out not to be liquids (‘liquid expanded’, ‘liquid condensed’, ‘superliquid’ etc); rather, they are mesophases with some degree of positional order and are therefore characterizable by X-ray diffraction.

A 1999 review [1] describes the general features of the fatty acid/alcohol Langmuir monolayer phase diagram in some detail, and contains much more information than can be included here. The following section contains a brief overview of X-ray results. In subsequent sections some newer information is surveyed that was not available at the time Ref. 1 was published, regarding the ordering of the molecular backbones and positional order in the underlying aqueous subphase.

2 THE PHASE DIAGRAM OF FATTY ACID AND ALCOHOL LANGMUIR MONOLAYERS

In 1945, Ställberg-Stenhagen and Stenhagen [2] published an unexpectedly complex pressure-temperature phase diagram for fatty acid monolayers, containing a variety of phase boundaries and not explainable by the popular solid/ liquid expanded/ liquid condensed hierarchy of phases. These results were considered dubious and widely ignored; most studies continued to be performed only at room temperature. The 1966 book by Gaines [3], which was the bible of this field through the 1980’s, says that “In view of the experimental difficulties attendant on the demonstration of these effects, it seems that further study is needed before any of them can be considered well understood. No detailed discussion of them is given here…”

When such further studies were finally conducted [4], it was found that the phase diagram of ref. 2 was largely accurate. A ‘modern’ phase diagram is shown in Fig.1; the labels used for the phases have historical origins and don’t really mean anything. All phase boundaries except the Ov-L2 boundary can be seen as discontinuities in isotherms. The phase diagram is applicable to all insoluble fatty acids irrespective of chain length if the temperature range is appropriately shifted; the pressures change slightly as well, but the topology is not changed. The L2 phase is not seen in saturated fatty alcohols.

X-ray diffraction can also be used to locate the phase boundaries, but it is much easier to do this with isotherms, Brewster angle microscopy, etc. Once the boundaries are located, and the next step is to identify what these phases and phase transitions are, that is when X-rays become invaluable. An explanation of how this technique is used to determine not just the lattice but also the tilt magnitude and direction is given in Ref. 1. The schematic diagrams in Fig. 1 show the structures thus determined. There are

(a) two phases with distorted hexagonal (DH) i.e. centered rectangular lattices and vertical molecules: CS and S;

(b) one phase with a hexagonal lattice and vertical molecules (LS);

(c) two phases with DH lattices and chains tilted towards a near neighbor (L2, L2”);

(d) two phases with DH lattices and tilts towards a next-nearest neighbor (L2’, Ov).

All these phases have at least medium-range order. The high-pressure low-temperature CS phase appears to be long-range-ordered. The L2 phase is anisotropically ordered (better-ordered in one direction than in the direction normal to it). At higher temperatures there is finally a liquid phase (not shown). Another phase not shown in Fig. 1 is a thin sliver of the ‘I’ phase between the L2 and L2’ phases, in which the tilt direction is intermediate between the near-neighbor and next-nearest-neighhbor directions. A simple Landau theory [5] perfectly reproduces the details of the phase transition between the L2 and I phases (first order) and between the I and L2’ phases (continuous).

The story of the recently identified Ov phase illustrates the strengths and weaknesses of the various experimental techniques and the advantages of bringing multiple techniques to bear on this problem. The L2/Ov phase boundary is not seen in isotherms; in other words, there is no discontinuity in the density and in that narrow sense there is no phase transition at all. This phase boundary was located by Overbeck and Möbius [6] using Brewster Angle Microscopy: they saw changes in texture when crossing specific points in the isotherm. X-ray scattering had never been used in this region to look for phase boundaries, since this is time-consuming and there was no reason to explore this area. Once the mysterious phase boundary was detected, however, we rather quickly established [7] using X-rays that there was a distinct change in the diffraction pattern upon crossing the phase boundary (Fig. 2). The tilt direction changes, from towards a nearest-neighbor (L2 phase) below the phase boundary to towards a next-nearestneighbor (Ov phase) above the phase boundary. We also discovered why isotherm studies had seen nothing: the tilt angle and the packing of the tilted molecules remain unchanged even though the tilt direction (relative to the lattice) changes. Thus there is no change in the areal density, but there is a change of symmetry. The phase transition is first-order even though there is no flat coexistence region in the isotherm.

3 BACKBONE ORDERING IN FATTY ACID MONOLAYERS

The various phases listed above differ in the lattice symmetry; tilt or lack thereof; tilt direction; and range of positional order. However, there is one more parameter. The alkane chains are not cylinders with circular cross-sections; to fully specify how the molecules are arranged, one needs to also specify the orientation of the molecule about its long axis. Since the carbon atoms in the alkane tail define a plane, specifying the orientation of this backbone plane is a convenient way to specify the orientation of the entire molecule.

In single crystals where a large number of diffraction peaks can be observed, it is possible to deduce, from the peak intensities, the molecular...