Formation and study of monolayers in the air-water interface

The formation of monolayers on the air-water interface is based on the insolubility of the molecules that form the layers, and above all, their amphiphilic properties. That is to say, they must possess a hydrophilic part (e.g. an alcohol, acid or amine like functional group) and a hydrophobic part (with one or two aliphatic chains) [13]. Using the Langmuir technique, as depicted in figure 3.3, the preparation of thin films is carried out by adding a certain quantity of amphiphilic molecules dissolved in a volatile solvent which is itself immiscible in water and therefore spreads all over the water surface, occupying the entire available surface. The selection of the solvent or mixture of solvents is important, because it must favour the maximum dispersion of the dissolved molecules on the water [14]. The volatile solvent must have a high volatility, so it could be easily evaporated at room temperatures. Once the solvent has evaporated, the molecules in the monolayer will lie on the surface with their polar groups immersed in the water and the hydrophobic chains outside it. This is the most favourable energetic situation for the system [15]. At this moment, the surface tension (𝛾𝛾) of the area covered by the monolayer is reduced with respect to surface tension of the clean water surface (𝛾𝛾0). The surface pressure (π) is defined as the measure of this reduction.

Π = 𝛾𝛾0−𝛾𝛾 Eq. 3.2

Figure 3.3: Steps for the preparation of a Langmuir monolayer.

In principle, any method that determines the surface tension can be used to measure the surface pressure. In practice, two systems are widely used: the Langmuir [5] and the Wilhemy [16, 17] types. After the evaporation of the solvent, the monolayer is compressed by moving the barrier, reducing the available surface area and increasing the surface density of molecules. This causes a reduction in 𝛾𝛾 and an increase in π. The representation of the surface pressure versus the area per molecule (A) on the water surface is a two dimensional analogy of a pressure-volume isotherm, and is referred to as the surface pressure – area isotherm orπ-A isotherm for short.

3.1.4 π-A Isotherms

A surface pressure-area (π-A) isotherm is a continual measurement of the surface pressure with changing monolayer confinement area. π-A isotherms give information about the stability of the monolayer on the air-water interface, the organization of the molecules and the interaction between them.

Figure 3.4 shows the isotherms of two compounds and the transitions exhibited as the surface pressure rises. Isotherms of fatty acids such as stearic acid, have been studied extensively because the phases are well defined. Other molecules such as phospholipids have a more complicated π-A isotherms with more than three phases and orientational conformations. The phase behaviour of the monolayer is mainly determined by the chemical and physical properties of the amphiphile, the subphase composition and the subphase temperature.

Figure 3.4: Two typical π-A isotherms. The dark blue line is an isotherm of a phospholipid with two hydrocarbon chains and the pale blue line is an isotherm of a fatty acid with a single hydrocarbon chain.

Two important parameters can be extracted from the π-A isotherms: the value of the lower limit of the area per molecule which is obtained by extrapolating the last stretch of the 54

isotherm to the x-axis. This corresponds to the minimum occupied area per molecule in a situation of maximum packaging (discontinuous lines in figure 3.4); and the maximum surface pressure in which the monolayer loses its stability and uniformity, also known as collapse surface pressure (πc in figure 3.4).

The π-A isotherms show different regions that corresponds to different phases and states of organization of the monolayer, as well as the regions in which two phases coexist [15]. In figure 3.4, the different phases are ideally represented. The phases can be described as follows:

Gaseous (G): At very low surface pressures, the molecules are far away from each other’s, and its interaction between them are weak. At this point, collisions are rare, just like molecules in a gas, building a two dimensional gas phase.

Liquid: When the surface pressure increases, a compressible fluid phase is reached, in which the molecules experiment attractive forces that makes them adopt a compact structure, forming a liquid expanded (Le) phase.

Between the two described phases a similar process to condensation of a gas takes place, i.e. a region in which both phases coexist: Le-G. In there, the surface pressure is constant, which is consistent with the transaction from the gaseous phase to the liquid expanded phase [18, 19].

Further pressure increase results in a less compressible and more ordered phase, also known as liquid condensed (Lc). Here, the organization of the monolayer is compact and the hydrophobic part of the molecule orient perpendicular to the interface. Once again, a transition is presented between two phases, Le-Lc. First order transitions between phases should appear as constant pressure areas in the π-A isotherms. Transitions between the G phase and the Lc, Le or S phases appear to show constant surface pressure. This last state can only be observed only for the phospholipid due to its more branched structure (it has two hydrophobic tails).

Solid (S): When the monolayer is further compressed and before collapsing, the films enters the solid state (S), in which the film is rigid and the hydrophobic chains forms a compact stacking.

The number and the complexity of the phases observed in an isotherm vary depending of the system. The morphology of the coexistence of two phases in a monolayer can be visualized using Brewster Angle Microscope (BAM) [20].

3.2 Langmuir Blodgett film deposition