Monolayers, insoluble experimental study

The penetration of soluble proteins into spread, insoluble lipid monolayers was first studied by Schulman and Rideal. Their injection technique allows measurement of the surface pressure increase of the insoluble penetrated monolayer after the soluble species is coadsorbed. The interpretation of this result cannot lead to a quantitative interpretation of the penetration unless the degree of penetration is known. This value can be determined if the soluble species is radio-activly labeled. An experimental and theoretical study of the change in monolayer surface pressure and composition during penetration by a soluble component has shown that information on the mechanism of this process can be obtained. 

As with the water/oil interface, amphiphiles can specifically adsorb to the interface between water and air. One limiting case is complete adsorption resulting in the so-called insoluble monolayer. Experimental studies of insoluble monolayers at the water/air interface are routinely carried out using a film balance developed by Langmuir [1] over 70 years ago with this, the average area per molecule can be varied and the resulting change in surface... 

It was found centuries later that some lipid-like substances (almost insoluble in water) formed self-assembly monolayers (SAMs) on the surface of water (Gaines, 1966 Adamson and Gast, 1997 Birdi, 1989, 1999, 2002 Chattoraj and Birdi, 1984). A few decades ago, experiments showed that monomolecular films of lipids could be studied by using rather simple experimental methods (Figure 4.1). 

Experimental techniques for studying insoluble monolayers Surface pressure... 

For insoluble monolayers of cholesterol and dipalmitoyl choline the relaxation at pressures below the collapse point were studied by Joos et al. ), using oscillatory and stress relaxation techniques. They found experimental evidence (and presented theory) for a double-exponential decay, representing two consecutive processes. The longer r s are 0(10 s) and 0(10 s) for cholesterol and the lipid, respectively, so these relaxations are relatively slow and may therefore be overlooked, especicJly in automated apparatus. No molecular mechanism was proposed the two r s did not exhibit a clear relationship with the surface pressure at which the experiments were carried out. 

Reaction at liquid surfaces covers a wide field. However, special experimental methods have been developed mainly for the study of insoluble or partly soluble monolayers at the liquid-air interface. Adsorption and reaction of monolayers at the liquid-air interface and liquid-liquid interfaces can be studied by similar techniques. It is therefore convenient to treat these together. Comprehensive information about liquid interfaces is contained in publications by Adamson, Davies , Alexander , Davies and Rideal and Gaines . ... 

Insoluble monolayers on an aqueous substrate have been investigated by means of the capillary wave method for many years. Lucassen and Hansen (1966) in their pioneering work neglected the surface viscosity and considered only pure elastic films. Subsequent studies showed that the surface elasticity of real surface films is a complex quantity, and both the equilibrium surface properties and the kinetic coefficients of relaxation processes in the films influence the characteristics of surface waves. However, it has been discovered recently that the real situation is even more complicated and the macroscopic structure of surface films influences the dependency of the damping coefficient of capillary waves on the area per molecule (Miyano and Tamada 1992, 1993, Noskov and Zubkova 1995, Noskov et al. 1997, Chou and Nelson 1994, Chou et al. 1995, Noskov 1991, 1998, Huhnerfuss et al. this issue). Some peculiarities of the experimental data can be explained, if one takes into account the capillary wave scattering by two-dimensional particles (Noskov et al. 1997). 

Surface self-diffusion is the two-dimensional analogue of the Brownian motion of molecules in a liquid bulk. Measurements of self-diffusion have to be performed in complete absence of any Marangoni flow caused by surface tension differences. Such experimental conditions are best established in an insoluble monolayer where one part consists of unlabelled and the other of radio-tracer labelled molecules. The movement of molecules within the surface monolayer can be now observed by using a Geiger-Miiller counter. There are possible effects of liquid convective flow in the sublayer which was discussed for example by Vollhardt et al. (1980a). With e special designed apparatus Vollhardt et al. (1980b) studied the self-difihision of different palmitic and stearic acid and stearyl alcohol and obtained self-diffusion coefficients between l-i-4 lO cm /s. 

The studies of adsorption layers at the water/alkane interface give excess to the distribution coefficient of a surfactant, which is a parameter of particular relevance for many applications. Theoretical models and experimental measurements of surfactant adsorption kinetics at and transfer across the water/oil interface will be presented. The chapter will be concluded by investigations on mixed surfactant systems comprising experiments on competitive adsorption of two surfactants as well as penetration processes of a soluble surfactant into the monolayer of a second insoluble compound. In particular these penetration kinetics experiment can be used to visualise separation processes of the components in an interfacial layer. 

The pendant drop experiments are a very new experimental technique to study penetration systems. The insoluble monolayer is spread onto the drop surface carefully by using a microsyringe [221]. The exchange of the drop bulk phase can be easily performed by using a coaxial double-capillary as shown in Fig. 4.20. 

Electrochemistry of LB films of fullerenes has been widely studied and remains the subject of much research effort from both theoretical and experimental approaches. Bard etal. have studied basic electrochemistry of Ceo fullerene LB films on an electrode in acetonitrile solutions [23]. The study indicated that reduction of the fullerene films could form insoluble films with incorporated electrolyte cations or lead to dissolution. The study on Cgo LB films has become a focus of considerable interest however, it is difficult to fabricate high-quality LB films of pure Cgo due to its intrinsic hydropho-bicity. Kajiyama et al. applied a multistep creep method as an LB technique for constructing a fairly homogeneous Ceo monolayer, which is regularly packed in a hexagonal array [44]. Kunitake etal. developed the electrochemical replacement method to form epitaxial adlayers of fullerenes on Au(lll) surfaces [45]. The wet process method consists of the transfer of Langmuir films of fullerene onto iodine-modified Au(lll) surfaces at an air-water interface followed by the electrochemical removal and replacement of iodine adlayers with fullerene adlayers in solution. The fullerene adlayers prepared by this method showed excellent quality and uniformity. A visuahzing... 

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