Penetration monolayers

The energetics and kinetics of film formation appear to be especially important when two or more solutes are present, since now the matter of monolayer penetration or complex formation enters the picture (see Section IV-7). Schul-man and co-workers [77, 78], in particular, noted that especially stable emulsions result when the adsorbed film of surfactant material forms strong penetration complexes with a species present in the oil phase. The stabilizing effect of such mixed films may lie in their slow desorption or elevated viscosity. The dynamic effects of surfactant transport have been investigated by Shah and coworkers [22] who show the correlation between micellar lifetime and droplet size. More stable micelles are unable to rapidly transport surfactant from the bulk to the surface, and hence they support emulsions containing larger droplets. 

For interpreting thesedata, and as a first step towards formulating a model for monolayer penetration, it is clearly desirable to calculate the amount of surfactant that has penetrated the monolayer. This has proved to be a difficult theoretical problem, but in recent years some limited solutions and a general solution have been found. In this paper we examine data for the penetration of cholesterol monolayers by hexadecy1-trimethyl-ammonium bromide (CTAB) (7) and compare the penetration or adsorption values calculated from the different treatments. 

Another type of interaction is the penetration of a surface-active constituent of the substrate into a spread monolayer. Penetration effects can be studied by injecting a solution of the surface-active material into the substrate immediately beneath the monolayer a) if there is no association between the injected material and the monolayer, tt and AK will both remain unaltered (b) if the injected material adsorbs on to the underside of the monolayer without actual penetration, AK will change appreciably but tt will alter very little (c) if the injected material penetrates into the monolayer (i.e. when there is association between both polar and non-polar parts of the injected and original monolayer materials), ir will change significantly and A V will assume an intermediate value between AV of the original monolayer and AV of a monolayer of injected material. Penetration is less likely to occur when the monolayer is tightly packed. 

UV/visible spectroscopy Orientational order ((S )). Measurement of adsorption and monolayer penetration. Restricted to monolayers containing molecules with chromophoric groups. 

Measurements of surface pressure and surface potential isotherms of monolayers penetrated by membrane extracts showed that the presence of the protein increased the compressibility of... 

MRJ Salton. Lytic agents, cell permeability and monolayer penetrability. J Gen Physiol 52 2275-2528, 1968. 

There are a few indirect methods for characterizing molecule-size physical recognition sites, such as examining the extent of monolayer penetration by probe molecules as a fiinction of their van der Waals radii and other chemical and physical properties (Scheme HI, Frame 4). We have used an electrochemical version of this approach, which assumes that the defect sites define an array of ultramicroelectrodes, to analyze our composite SAMs (Scheme IV). In these experiments, the shape of the cyclic voltammetric wave is correlated to the size and number density of sites through which the probe molecules can penetrate, as shown on the right side of Scheme IV (6). 

A difficulty in the physicochemical study of penetration is that the amount of soluble component present in the monolayer is not an easily accessible quantity. It may be measured directly, through the use of radioactive labeling (Section III-6) [263, 266], but the technique has so far been used only to a limited extent. 

Fig. IV 23. Penetration of cholesterol monolayers by CTAB (hexadecyl-trimethylam-monium bromide. [From D. M. Alexander, G. T. Barnes, M. A. McGregor, and K. Walker, Phenomena in Mixed Surfactant Systems, in J. F. Scamehom, ed., ACS Symposium Series 311, p. 133, 1986 (Ref. 269). Copyright 1986, American Chemical Society.]...

Fig. XIV-3. Steric effects in the penetration of sodium cetyl sulfate monolayers by cetyl alcohol and oleyl alcohol.

For ultrathin epitaxial films (less than "100 A), Grazingincidence X-ray Diffraction (GrXD) is the preferred method and has been used to characterize monolayer films. Here the incidence angle is small ("0.5°) and the X rays penetrate only "100-200 A into the specimen (see below). The exit angle of the diffracted X rays is also small and structural information is obtained about (hkl) planes perpendicular to the specimen sur e. Thus, GIXD complements those methods where structural information is obtained about planes parallel to the surface (e.g., Bra -Brentano and DCD). 

Figure 34 shows the critical load of all the samples. For the monolayer samples, Sample 1 has a higher critical load than Sample 2. The multilayers Samples 4, 5, and 6 have higher critical loads than monolayer Samples 1 and 2. Samples 5 and 6 have excellent scratch resistant properties. Only extremely small cracks are found in the scratch tracks of Samples 5 and 6. Therefore, there is no sudden change found in the force and penetration depth curves. Sample 7 has the lowest critical load, similar to the monolayer Sample 2. 

Salts of fatty acids are classic objects of LB technique. Being placed at the air/water interface, these molecules arrange themselves in such a way that its hydrophilic part (COOH) penetrates water due to its electrostatic interactions with water molecnles, which can be considered electric dipoles. The hydrophobic part (aliphatic chain) orients itself to air, because it cannot penetrate water for entropy reasons. Therefore, if a few molecnles of snch type were placed at the water surface, they would form a two-dimensional system at the air/water interface. A compression isotherm of the stearic acid monolayer is presented in Figure 1. This curve shows the dependence of surface pressure upon area per molecnle, obtained at constant temperature. Usually, this dependence is called a rr-A isotherm. 

The Langmuir-Blodged (LB) technique allows one to form a monolayer at the water surface and to transfer it to the surface of supports. Formation of the BR monolayer at the air/water interface, however, is not a trivial task, for it exists in the form of membrane fragments. These fragments are rather hydrophilic and can easily penetrate the subphase volume. In order to decrease the solubility, the subphase usually contains a concentrated salt solution. The efficiency of the film deposition by this approach (Sukhorukov et al. 1992) was already shown. Nevertheless, it does not allow one to orient the membrane fragments. Because the hydrophilic properties of the membrane sides are practically the same, fragments are randomly oriented in opposite ways at the air/water interface. Such a film cannot be useful for this work, because the proton pumping in the transferred film will be automatically compensated i.e., the net proton flux from one side of the film to the other side is balanced by a statistically equal flux in the opposite direction. 

Salt addition to the subphase has a strong influence on monolayer formation, too. The effect of salt was studied by spreading particles la on an aqueous KCl solution of different salt concentration, with the pH of the subphase always being 5. If no salt is present at pH 5, the particles simply disappear into the subphase, as discussed earlier. However, the presence of salt causes the metal ions to penetrate the particle shell and shield the ionic groups electrostatically. Consequently, the particles become less hydrophilic and monolayer formation is improved, as indicated by the larger value of Aq. As shown in Figure 6a, a KCl concentration of 10 moles is sufficient to cause formation of a stable particle layer even at pH 5. 

The penetration of ions from the subphase into the shell of spread particles is a general phenomenon and can be used to modify and functionalize the particle surface. For example, metal ions, such as Ba and Fe, or cationic polyelectrolytes, such as the polycation of polyallylamine, can be adsorbed at anionic particles, while anionic water-soluble dyes, such as phthalocyanine tetrasulfonic acid and 1.4-diketo-3.6-diphenylpyrrolo[3.4-c]pyrrole-4, 4 -disulfonic acid (DPPS) [157], can be adsorbed at cationic particles. However, since only a monolayer of the dye is adsorbed, a deep coloration of the particles is not obtained unless a dye with very high absorption coefficient is used [156],... 

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