Monolayer of surfactant

Consider the case of an emulsion of 1 liter of oil in 1 liter of water having oil droplets of 0.6 /rm diameter. If the oil-water interface contains a close-packed monolayer of surfactant of 18 per molecule, calculate how many moles of surfactant are present. 

In a typical experiment, Israelachvili deposited monolayers of surfactants onto cleaved mica sheets, and evaluated the surface energies using the JKR equation. Fig. 11 contrasts results for mica coated with monolayers of (a) L-a-dipalmitoyl-phosphatidylethanolamine (DMPE) where j/a = = 27 mJ/m and (b) hexa-decyltrimethylammonium bromide (CTAB) where = 20 mJ/m and = 50 mJ/m. ... 

Tjandra et al. (1998) have proposed an interfacial reaction model for the kinetics of the reaction between 1-bromo octane and sodium phenoxide to give 1-phenoxyoctane in a nonionic microemulsion. In this model the microemulsion is assumed to consist of the aqueous phase and the interface is covered by a monolayer of surfactant molecules. It is thus possible to assess the interfacial area from the concentration of the surfactant in the microemulsion medium. 

The ITIES with an adsorbed monolayer of surfactant has been studied as a model system of the interface between microphases in a bicontinuous microemulsion [39]. This latter system has important applications in electrochemical synthesis and catalysis [88-92]. Quantitative measurements of the kinetics of electrochemical processes in microemulsions are difficult to perform directly, due to uncertainties in the area over which the organic and aqueous reactants contact. The SECM feedback mode allowed the rate of catalytic reduction of tra 5-l,2-dibromocyclohexane in benzonitrile by the Co(I) form of vitamin B12, generated electrochemically in an aqueous phase to be measured as a function of interfacial potential drop and adsorbed surfactants [39]. It was found that the reaction at the ITIES could not be interpreted as a simple second-order process. In the absence of surfactant at the ITIES the overall rate of the interfacial reaction was virtually independent of the potential drop across the interface and a similar rate constant was obtained when a cationic surfactant (didodecyldimethylammonium bromide) was adsorbed at the ITIES. In contrast a threefold decrease in the rate constant was observed when an anionic surfactant (dihexadecyl phosphate) was used. 

SECM has been extended successfully to investigate chemical processes at aqueous-air interfaces and Langmuir monolayers supported on them [19,34,93]. Initial work concentrated on molecular transfer processes across the aqueous-air interface, with and without a monolayer of surfactant. A submarine UME (described in section III) was utilized which could approach the air-water interface from below. The first study employed the DPSC mode to investigate the transfer kinetics of electrogenerated Br2, from aqueous... 

Enzymes are suspended in hydrated microemulsion surrounded by a monolayer of surfactant molecules dispersed in an apolar solvent [53-60,135] [Fig. 1(b)]. Micelles ( 2 nm sphere) are formed when lyophilized or aqueous preparation of enzymes are introduced with stirring or shaking into a solution of synthetic or natural surfactant in an organic solvent. 

Monolayers of surfactants at the air-water interface provide an unparalleled opportunity to study energy changes due to differences in orientation of the surfactant molecules as they are packed within various areas on the surface. The work reviewed here, from a small number of studies that have been widely scattered through the literature, shows that the surface properties of monolayers are quite sensitive to stereochemistry. 

Usually, activities of enzymes (hydrogenases included) are investigated in solutions with water as the solvent. However, enhancement of enzyme activity is sometimes described for non-aqueous or water-limiting surroundings, particular for hydrophobic (or oily) substrates. Ternary phase systems such as water-in-oil microemulsions are useful tools for investigations in this field. Microemulsions are prepared by dispersion of small amounts of water and surfactant in organic solvents. In these systems, small droplets of water (l-50nm in diameter) are surrounded by a monolayer of surfactant molecules (Fig. 9.15). The water pool inside the so-called reverse micelle represents a combination of properties of aqueous and non-aqueous environments. Enzymes entrapped inside reverse micelles depend in their catalytic activity on the size of the micelle, i.e. the water content of the system (at constant surfactant concentrations). 

Reverse micelles are well known to be spherical water in oil droplets stabilized by a monolayer of surfactant. The phase diagram of the surfactant sodium bis(2-ethylhexyl) sulfosuccinate, called Na(AOT), with water and isooctane shows a very large domain of water in oil droplets and often forms reverse micelles (3,23). The water pool diameter is related to the water content, w = [H20]/[ AOT], of the droplet by (23) D(nm) = 0.3w. From the existing domain of water in oil droplets in the phase diagram, the droplet diameters vary from 0.5 nm to 18 nm. Reverse micelles are dynamic (24-27) and attractive interactions between droplets take place. 

At the most fundamental level, monolayers of surfactants at an air-liquid interface serve as model systems to examine condensed matter phenomena. As we see briefly in Section 7.4, a rich variety of phases and structures occurs in such films, and phenomena such as nucleation, dendritic growth, and crystallization can be studied by a number of methods. Moreover, monolayers and bilayers of lipids can be used to model biological membranes and to produce vesicles and liposomes for potential applications in artificial blood substitutes and drug delivery systems (see, for example, Vignette 1.3 on liposomes in Chapter 1). 

E. Ruckenstein Phase transitions in insoluble monolayers of surfactants, Encyclopedia of Surface and Colloid Science (2007) Taylor Francis. 

Ruckenstein, E. Nonconventional first order transitions in insoluble monolayers of surfactants. Colloids and Surfaces A 2001,183, 423. 

An optical three-layer model has proved superior to a one-layer model for the interpretation of the ellipsometric data. The refractive indices of the film and surface layers are determined and it is found that the index for the surface is higher than that for the film core. A Lorenz-Lorentz type treatment of NBF reveals that there are approximately seven water molecules per molecule of surfactants in both NaDoS and NaDoBS films. The optical data obtained by the three-layer model for NBF from NaDoS indicate that the thickness of the aqueous core is zero while that of the adsorption monolayers of surfactant molecules with refractive index 1.365 is 1.8 nm, i.e. the thickness of NBF is 3.6 nm. 

The foam bilayer is the main model system used to obtain experimental results for the stability of bilayers. The proof that the studied foam films are bilayers is based on the experimentally measured h(Cei) dependences and I"I(/i) isotherms. In both cases films with the same thickness are obtained, which corresponds to that of bilayers and does not change with further increase in Cei or IT (e.g. Figs. 3.44, 3.57, 3.62). This leads to the conclusion that the NB foam films do not contain a free aqueous core between its two monolayer of surfactant molecules. A similar conclusion is drawn from the investigatigations of NB foam films by infrared spectra [320,417] and by measuring longitudinal electric conductivity of CB and NB foam films [328,333,418]. 

Micelles form when a suitable amphiphile [e.g., sodium bis(2-ethyl-hexyl)sulfosuccinate (AOT)], is introduced into a hydrocarbon solvent (e.g isooctane). Reverse micelles containing water form when water is taken up by an isooctane—AOT solution. At water contents exceeding what is needed to saturate the polar head groups forming the micelle wall, the system can properly be termed a water-in-oil microemulsion, in which water droplets stabilized by a monolayer of surfactant are dispersed in an organic solvent. For convenience, the terms reverse micelle and microemulsion are sometimes considered equivalent. There is a considerable literature on the properties of proteins, particularly enzyme activity, in reverse micelles (see Luisi and Steinmann-Hofmann, 1987, and references cited therein). 

Microemulsions. The structure of microemulsion systems has been reviewed (22). Both bicontinuous and droplet-type structures, among others, can occur in microemulsions. The droplet-type structure is conceptually more simple and is an extension of the emulsion structure that occurs at relatively high values of IFT. In this case, very small thermodynamically stable droplets occur, typically smaller than 10 nm (7). Each droplet is separated from the continuous phase by a monolayer of surfactant. Bicontinuous microemulsions are those in which oil and water layers in the microemulsion may be only a few molecules thick, separated by a monolayer of surfactant. Each layer may extend over a macroscopic distance, with many layers making up the microemulsion. 

Table 3.2. Relations between the characteristic functions of Langmuir mono-layers. (For flat interfaces, the (a) equations apply to a monocomponent monolayer of surfactant s in the presence of low M solutes, whereas the (b) equations apply to the same in their absence.)...

The adsorption of surfactants onto ODS is more straightforward and well understood. Here, the formation of the monolayer of the surfactant on the ODS surface proceeds through the adsorption of the surfactant through its alkyl chain with the polar head group pointing into the bulk aqueous phase. Such an adsorption only results in a monolayer coverage, and the chromatographic behavior of these adsorbed monolayers can be discerned with established principles. The absorbed monolayers of surfactants on reversed-phase silica have been characterized by several spectroscopic methods [2] and will not be discussed here. 

Stability), the area per molecule is determined by the cross-sectional area of the sulphate group, which is in the region of 0.4 mn With nonionic surfactants consisting of an alkyl chain and a poly(ethylene oxide) (PEO) head group, adsorption onto a hydrophobic surface is determined by the hydrophobic interaction between the alkyl chain and the hydrophobic surface. For the vertical orientation of a monolayer of surfactant molecules, the area per molecule will depend on the size of the PEO chain, which is in turn directly related to the number of ethylene oxide (EO) units in the chain. If the area per molecule is smaller than that predicted from the size of the PEO chain, the surfactant molecules may associate on the surface to form bilayers and hemimicelles (as discussed in detail in Chapter 5). This information can be related directly to the stability of the suspension. 

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