Surfactants layer interaction

It is believed that solubilization is initiated when a biomolecule with charged surface groups approaches the bulk aqueous-lipophilic solvent interface, where the interactions cause the bulk interface s surfactant layer to bend, such that the protruding biomolecule becomes surrounded by the surfactant layer [105]. Ultimately, a filled w/o-ME forms, and partitions to the bulk organic phase [105]. 

R. Tilton, E. Blomberg, and P. Claesson, Effect of anionic surfactant on interactions between lysozyme layers adsorbed on mica. Langmuir 9, 2102-2108 (1993). 

Assuming that the internal droplets experience a hard-sphere-like repulsion when surfactant layers come in contact, an estimation of the van der Waals interactions can be obtained from the average length of the surfactant tails (1 3 nm) [20]. The coalescence frequency is therefore the unique free pa-... 

Apart from anomalous situations where surfactant interacts with the organic phase, the stability of HIPEs is linked to the interfacial tension of the system. Ruckenstein and coworkers [109] showed that the maximum volume of hydrocarbon which could be incorporated in an o/w HIPE increased with increasing surfactant concentration, presumably due to a concomitant decrease in the interfacial tension. Solans et al. [9] claimed that the interfacial tension between the aqueous phase and the liquid-crystalline surfactant layer in their highly... 

Liposomes were formed from 1,2-dipalmitoylphosphatidylcholine (DPPC) and cholesterol (Choi) and the effect of liposomal entrapment on pulmonary absorption of insulin was related to oligomerization of insulin (Liu et al. 1993). Instillation of both dimeric and hexameric insulin produced equivalent duration of hypoglycemic response. However, the initial response from the hexameric form was slightly slower than that from dimeric insulin, probably due to lower permeability across alveolar epithelium of the hexameric form caused by larger molecular size. The intratracheal administration of liposomal insulin enhanced pulmonary absorption and resulted in an absolute bioavailability of 30.3%. Nevertheless, a similar extent of absorption and hypoglycemic effects was obtained from a physical mixture of insulin and blank liposomes and from liposomal insulin. This suggests a specific interaction of the phospholipid with the surfactant layer or even with the alveolar membrane. 

A situation that commonly occurs with food foams and emulsions is that there is a mixture of protein and low-molecular-weight surfactant available for adsorption at the interface. The composition and structure of the developing adsorbed layer are therefore strongly influenced by dynamic aspects of the competitive adsorption between protein and surfactant. This competitive adsorption in turn is influenced by the nature of the interfacial protein-protein and protein-surfactant interactions. At the most basic level, what drives this competition is that the surfactant-surface interaction is stronger than the interaction of the surface with the protein (or protein-surfactant complex) (Dickinson, 1998 Goff, 1997 Rodriguez Patino et al., 2007 Miller et al., 2008 Kotsmar et al., 2009). 

In another method, Roberts and Tabor201 measured the electric double layer repulsion between a transparent rubber sphere and a plane glass surface separated by surfactant solution. As the surfaces were brought together, the double-layer interaction caused a distortion of the rubber surface which was monitored interferometrically. 

The electrostatic stabilization theory was developed for dilute colloidal systems and involves attractive van dcr Waals interactions and repulsive double layer interactions between two particles. They may lead to a potential barrier, an overall repulsion and/or to a minimum similar to that generated by steric stabilization. Johnson and Morrison [1] suggest that the stability in non-aqueous dispersions when the stabilizers are surfactant molecules, which arc relatively small, is due to scmi-stcric stabilization, hence to a smaller ran dcr Waals attraction between two particles caused by the adsorbed shell of surfactant molecules. The fact that such systems are quite stable suggests, however, that some repulsion is also prescni. In fact, it was demonstrated on the basis of electrophoretic measurements that a surface charge originates on solid particles suspended in aprotic liquids even in the absence of traces of... 

In this paper, a molecular thermodynamic approach is developed to predict the structural and compositional characteristics of microemulsions. The theory can be applied not only to oil-in-water and water-in-cil droplet-type microemulsions but also to bicontinuous microemulsions. This treatment constitutes an extension of our earlier approaches to micelles, mixed micelles, and solubilization but also takes into account the self-association of alcohol in the oil phase and the excluded-volume interactions among the droplets. Illustrative results are presented for an anionic surfactant (SDS) pentanol cyclohexane water NaCl system. Microstructur al features including the droplet radius, the thickness of the surfactant layer at the interface, the number of molecules of various species in a droplet, the size and composition dispersions of the droplets, and the distribution of the surfactant, oil, alcohol, and water molecules in the various microdomains are calculated. Further, the model allows the identification of the transition from a two-phase droplet-type microemulsion system to a three-phase microemulsion system involving a bicontinuous microemulsion. The persistence length of the bicontinuous microemulsion is also predicted by the model. Finally, the model permits the calculation of the interfacial tension between a microemulsion and the coexisting phase. 

Equations 1—8a can be simultaneously solved to obtain the surfactant density, the degree of dissociation, and the double layer force. The double layer interaction energy... 

Monolayers of micro- and nanoparticles at fluid/liquid interfaces can be described in a similar way as surfactants or polymers, easily studied via surface pressure/area isotherms. Such studies provide information on the properties of particles (dimensions, interfacial contact angles), the structure of interfacial layers, interactions between the particles as well as about relaxation processes within the layers. Such type of information is important for understanding how the particles stabilize (or destabilize) emulsions and foams. The performed analysis shows that for an adequate description of II-A dependencies for nanoparticle monolayers the significant difference in size of particles and solvent molecules has be taken into account. The corresponding equations can be obtained by using a thermodynamic model developed for two-dimensional solutions. The obtained equations provide a satisfactory agreement with experimental data of surface pressure isotherms in a wide range of particle sizes between 75 pm and 7.5 nm. Moreover, the model can predict the area per particle and per solvent molecule close to real values. Similar equations were applied also to protein monolayers at liquid interfaces. 

It is known that the three film types are thermodynamically unstable. Long-living films can be obtained when suitable surfactants are employed, creating an energy barrier to film thinning, due either to the repulsion of the diffuse electric layers or the steric interaction of the adsorption surfactant layers. Since the minimum surfactant concentration that provides stable... 

For separating and purifying proteins, a forward extraction operation facilitates the selective transfer of a target protein from an aqueous solution containing some kinds of proteins into an organic phase, and the extracted proteins are quantitatively recovered into a fresh aqueous solution by the subsequent back extraction. The transfer selectivity is based on the interaction between surfactants, which form reverse micelles, and the protein surface. On the other hand, the quantitative recovery of an objective protein from reverse micelles is accomplished by severing proteins from the enclosure with a surfactant layer. 

The basic pH brings about an electrostatic repulsion between the protein surface and anionic surfactants. On the other hand, the high salt concentration avoids the electrostatic adsorption of the surfactants. Both factors modify the electrostatic interaction between the proteins solubilized in reverse micelles and the surfactant layer. Indeed, however, in the back extraction process, the proteins solubilized in the droplets cannot suffer such a modification because of the little degree of coalescence of the reverse micelles containing proteins with a bulk aqueous solution or vacant reverse micelles. This is one of the reasons why the back extraction process is much slower than the forward extraction process. 

The DLVO theory does not explain either the stability of water-in-oil emulsions or the stability of oil-in-water emulsions stabilized by adsorbed non-ionic surfactants and polymers where the electrical contributions are often of secondary importance. In these, steric and hydrational forces, which arise from the loss of entropy when adsorbed polymer layers or hydrated chains of non-ionic polyether surfactant intermingle on close approach of two similar droplets, are more important (Fig. 4B). In emulsions stabilized by polyether surfactants, these interactions assume importance at very close distances of approach and are influenced markedly by temperature and degree of hydration of the polyoxyethylene chains. With block copolymers of the ethylene oxide-propylene oxide... 

Satisfactory results are obtained with compounds of different nature. Of particular interest is the case of basic compounds, such as phenethylamines and 3-blockers, which experience large efficiency enhancements in SDS systems.This makes the use of special columns less necessary. The surfactant layer adsorbed on the column prevents the interaction of basic compounds with free silanol groups, which accounts for the low efficiencies observed with conventional columns in aqueous-organic RPLC. For acidic compounds such as sulfonamides, the efficiencies are comparable with both MLC and conventional RPLC, but for low polar compounds such as steroids, efficiencies are comparably poorer in MLC. However, in this technique, low polar steroids are eluted at sufficiently short retention times via a small amount of... 

In sections 7.3.1-7.3.4 we have considered only relatively simple dilute emulsions. Many pharmaceutical preparations, lotions or creams are, in fact, complex semisolid or stmc-tured systems which contain excess emulsifier over that required to form a stabilising mono-layer at the oil/water interface. The excess surfactant can interact with other components either at the droplet interface or in the bulk (continuous) phase to produce complex semisolid multiphase systems. Theories derived to explain the stability of dilute colloidal systems cannot be applied directly. In many cases the formation of stable interfacial films at the oil/water interface cannot be considered to play the dominant role in maintaining... 

Double Layer Interactions and Interfacial Charge. Schulman et al (42) have proposed that the phase continuity can be controlled readily by interfacial charge. If the concentration of the counterions for the ionic surfactant is higher and the diffuse electrical double layer at the interface is compressed, water-in-oil microemulsions are formed. If the concentration of the counterions is sufficiently decreased to produce a charge at the oil-water interface, the system presumably inverts to an oil-in-water type microemulsion. It was also proposed that for the droplets of spherical shape, the resulting microemulsions are isotropic and exhibit Newtonian flow behavior with one diffused band in X-ray diffraction pattern. Moreover, for droplets of cylindrical shape, the resulting microemulsions are optically anisotropic and non-Newtonian flow behavior with two di-fused bands in X-ray diffraction (9). The concept of molecular interactions at the oil-water interface for the formation of microemulsions was further extended by Prince (49). Prince (50) also discussed the differences in solubilization in micellar and microemulsion systems. 

Figure 21 An enzyme entrapped in the water pool of a reverse micelle may interact with a water-insoluble substrate. The surfactant layer separating the aqueous from the hydrocarbon phase is in fast movement (very much like in regular micelles) and regions of the enzyme may be exposed to the organic solvent...

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