Lipid Monolayers at the Air–Water Interface

A Tool for Understanding Electrostatic Interactions and Rheology in Biomembranes

Natalia Wilke1    Centro de Investigaciones en Química Biológica de Córdoba (CIQUIBIC), Dpto. de Química Biológica, Facultad de Ciencias Químicas, Universidad Nacional de Córdoba, Pabellón Argentina, Ciudad Universitaria, X5000HUA Córdoba, Argentina
1 Corresponding author: email address: wilke@mail.fcq.unc.edu.ar, natiwilke@gmail.com

1 Introduction

When amphiphiles are dissolved in an organic solvent and deposited on a water surface, the solution spreads rapidly to occupy the available area. While the solvent evaporates, the surfactant orientates to minimize contact of its nonpolar regions with water, but maximizes the water contact of its polar region, resulting in a one-molecule-thick surfactant film named “Langmuir monolayer.” These self-structured thin films have been the subject of study for many decades from a fundamental point of view in the fields of biophysics and biology as model biomembranes [1–7] and also as bottom-up 2D-patterning of molecularly thin films for different uses [8–14].

Langmuir monolayers are extremely valuable models for membranes [2,6,7,15,16] since experiments can easily be performed in which the molecular area, surface pressure, temperature, and chemical nature of the subphase are varied, and by this means, a broad set of thermodynamic parameters that characterize the monolayer can be accurately determined [1,17,18]. Although transmembrane processes cannot be studied in monolayers, this system is well suited for studying lateral mixing and structuring mediated by a variety of lipids and proteins which constitute biomembranes [5,19–23]. Using this model membrane, a broad spectrum of techniques can be applied, some of which are detailed in Section 2. These lipid films also enable the diffusing species to be followed over a relatively large area and for a long time. Furthermore, the environment of such molecules can be controlled and also varied in a controlled manner (see Section 5).

All of this makes Langmuir lipid monolayers a convenient system for analyzing the influence of domains on the mechanical properties of membranes. The results found using Langmuir monolayers and other model membranes are similar in some cases but not in others, raising the interesting question of which model system is more suitable, the answer to which may depend on the parameter under study (see Section 6).

2 Experimental Approaches on Monolayers

Once Langmuir monolayers are formed, these films can be compressed while the area of the interface and the surface tension are determined, which is usually performed using a Wilhelmy plate made of platinum or paper. The surface pressure “π” can then be calculated as the surface tension of the bare interface minus that of the interface modified by the lipid layer, with the plots of π as a function of the mean molecular area “MMA” (interface area divided by the number of molecules at the interface) being referred to as “compression isotherms.” These experiments have been detailed elsewhere (see, e.g., Refs. [1,4,6]). For most lipid monolayers, the slope of the compression isotherm indicates the film's response under expansion or compression since shear can be neglected [24]. However, for some protein monolayers [13,25] and very cohesive lipids [26,27], the slope of the isotherm depends on the position of the sensor (a rectangular Wilhelmy plate) since the usual compression mode is asymmetric [28] (see Fig. 2.1), and thus the mechanical perturbation made in the film is both a compression–expansion and a shape perturbation. In the case of highly cohesive films, the response under the asymmetric perturbation will be affected by both the shear (G*) and the compressibility (E*) moduli, according to the following equations [28]:

*+G*=A0∂π∥∂A

*-G*=A0∂π⊥∂A

where π|| and π⊥ refer to the surface pressure determined with the sensor positioned parallel or perpendicular to the barriers, respectively (see Fig. 2.1). In turn, G* and E* can be expressed as:

*ω=G′ω+iG″ω=G′ω+iωηsω

*ω=E′ω+iE″ω=E′ω+iωηdω

Both parameters (G* and E*) are complex numbers with a real (elastic response) and an imaginary (viscous response) part, as is clear in Eqs. (2.3) and (2.4). The imaginary parts arise from the fact that the compression speed is finite, so there may be friction resisting the compression flow, and the resistance is characterized by the compression (dilatational) viscosity, ηd. On the other hand, the shear elastic viscosity, ηs, is the ratio between the shear stress and the rate of shear. In order to obtain each component of G*and E*, a sinusoidal perturbation in the film area may be performed with a frequency ω [25]. An instantaneous response is characteristic of an elastic material, whereas a retarded response indicates viscoelasticity.

For most lipids, however, shear can be neglected and the elastic compressibility can be directly obtained by the elastic compressibility modulus ɛ = − MMA (∂π/∂MMA). For lipid molecules with high intermolecular cohesion, strong lipid–lipid attractions are present within molecules that form the monolayer, and the resulting film has a high ɛ value, in the order of 102 mN/m (these are named “liquid-condensed”) or higher (named “solid”) [18]. In contrast, when lipids with low intermolecular interactions are spread at the air–water interface, softer films are formed (compressibility modulus from 101 to 102 mN/m), named “liquid-expanded” monolayers [18]. For intermediate interactions, phase transitions induced by compression can be observed. The phase state can also be modulated by temperature like any 3D-phase state: an increase in temperature decreases the surface pressure of the phase transition from an expanded to a denser phase state.

The phase transitions in pure lipid monolayers are first order, since two phases can be detected in the monolayer using different techniques. According to the Gibbs phase rule when applied to 2D systems by Crisp [1], lateral pressure should remain constant during the whole transition of monolayers composed of a pure lipid, and therefore, the compressibility modulus should be zero at equilibrium. However, during the phase transition of pure lipids, the isotherm generally shows a nonzero but low slope (see, e.g., Fig. 2.2A, gray line). Observation of this region of the isotherm (normally called “plateau”) has been reported frequently and studied from different points of view. The simplest explanation given for the lack of constancy of surface pressure during the phase transition is related to the presence of impurities...