Interfacial film, complexed

Citrus oils readily form oxygenated products that are likely to congregate at oil/water interfaces and thereby cause a detectable change in IFT. The aldehydic components of citrus oil could react with the amine groups of the gelatin molecules present in the aqueous phases formed by complex coacervation and thereby affect IFT. In addition to chemical reactions, physical changes can occur at an interface and alter IFT. A visible interfacial film can form simply due to interfacial interactions that alter the interfacial solubility of one or more components. No chemical reactions need occur. An example is the formation of a visible interfacial film when 5 wt. per cent aqueous gum arabic solutions are placed in contact with benzene (3). Interfacial films or precipitates can also form when chemical reactions occur and yield products that congregate at interfaces. 

A discontinuous precipitate often was observed at citrus oil/ aqueous phase interfaces kept at 50 C for prolonged periods. The precipitate particles congregate on the water side of the citrus oil/aqueous phase interface and disperse into the aqueous phase upon agitation. If the aqueous phase is distilled water, or a supernatant phase, the precipitate particles cause the aqueous phase to become noticeably cloudy. Precipitate particles or interfacial films were not detected at citrus oil/complex coacervate phase interfaces. However, such interfaces normally were not kept for prolonged periods, because their IFT values rapidly decayed to a value too low to measure. 

Some Properties of Interfacial Films of Colloidal Electrolyte-Protein Complexes... 

Small concentrations of di- and trivalent ions do, however, produce very low interfacial tensions with the simple paraffin chain salts l and if substances such as cholesterol, which form complexes with the paraffin chain ions, are present in the oil, again very low tensions are possible. There is little doubt that either the polyvalent ion, or the other molecule of the complex, binds the paraffin chain ions together laterally in the interfacial film, so that more ions crowd into it for equal concentration of paraffin chain ion in the water, with a lower tension as a consequence of the greater crowding. 

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... 

The formation and stabilization of 0/W emulsions prepared with mixed emulsifier systems has been extensively investigated. However, the mechanisms proposed differ greatly. One of the primary hypotheses attributes the enhanced stability to the formation of a molecular "complex" or layer at the oil/water interface (8-11). The mixture of emulsifier types increases the packing density of the adsorbed interfacial film. Several investigators have shown that more closely packed complexes produce more stable emulsions (9,12-14). Friberg, et al. (15-17) have attributed the enhanced stability of mixed emulsifier emulsions to the formation of liquid crystals at the oil/water interface, which reduce the van der Waals attractive forces. 

A full analysis of interactions in multiple emulsions would obviously have to take account of forces of repulsion. The systems are too complex to (Fig. 7) allow any reasonable estimate of repulsive forces at this stage, although simplified models are being developed to allow an approach along this route. One complication resides in the possible lamellar nature of interfacial films or liquid crystalline structures, discussed earlier. 

Emulsion stability is determined by the strength of the interfacial film and the way the adsorbed molecules in it are packed. If the adsorbed molecules in the film are closely packed, and it has some strength and viscoelasticity, it is difficult for the emulsified liquid droplets to break the film. In other words, coalescence is difficult. The emulsion is therefore stable. The molecular structure and the properties of the emulsifiers in the film affect the film s properties. The molecules in the film are more closely packed if the emulsifier has straight chains rather than branched chains. The film strength is increased if mixed emulsifiers are used rather than a single one. The reasons are that (1) the molecules in the film are closely packed, (2) mixed liquid crystals are formed between droplets, and (3) molecular complexes are formed in the interface by emnlsifier compositions. For example, an oil-soluble surfactant mixed with a water-solnble snrfactant works very well to stabilize emulsions (Kang, 2001). 

Nonsurface-active PS as X strongly increase the surface pressure and the elastic character of the interfacial film. This imexpected behavior would arise from incompatibility with the aqueous bulk phase and possible complexation onto the adsorbed protein. 

Nearly a decade later, Oldroyd [1953, 1955] proposed a constitutive model similar to that of Frohlich and Sack, vahd at small deformations. The model considered low concentration of monodispersed drops of one Newtonian hquid in another. The interfacial tension and the viscoelastic properties of the interfacial film were incorporated by means of convected derivatives. The model provided the following relation for the complex modulus ... 

These results confirm that asphaltene film properties are heavily influenced by the nature of the oil phase and the asphaltene concentration. The complex molecular structures and aggregation propensity of asphaltenes are the main effects which influence the interfacial properties of these components. Small concentrations of asphaltenes, present in a highly aromatic oil phase, represent conditions which makes it possible to dissolve asphaltenes as small association structures. Diffusion of the asphaltene molecules from the bulk toward the interface requires time in order to rearrange into structured interfacial films. Hence, the IFPYV and the interfacial viscosity increases markedly after aging, meaning increased strength and elasticity of the interfacial film. 

In another view. Void and Void (214) suggest that holes are formed in the interfacial film and this allows the droplets to merge. Ivanov and Dimitrov (234) indicated that holes are due to surfactant depletion at the interface. However, extensive studies conducted to understand the mechanism of destabilization of the thin/thick films formed between two droplets, or between droplet and bulk phase, indicate that the process is much more complex and may involve more than one mechanism. These are not all fully understood as yet for crude oil and bitumen systems. 

The surface of O/W emulsions, which are stabilized by surface active salts of polyelectrolytes, usually carries a negative charge. O/W emulsions are formed if the interfacial film consists of a molecular complex with an oil-soluble component and an ionizable water-soluble component (25). The simple oriented-wedge theory of... 

Sr, and Zn) promote W/O emulsions which flocculate some of the flocculated emulsions are stable against coalescence, others are not. The stabilization of coalescence by some oleates of polyvalent metals was found to result from the formation of a thick film of complex hydrolysis products. This observation confirms the findings of Wasan et al. (17) that coalescence rates could be inversely related to interfacial viscosities or thickness of the interfacial film (0.7-0.3 ym). 

Proteins and proteoglycans are important stabilisers of emulsions and foams in many food and non-food applications [3]. The interactions between protein molecules, either when adsorbed at interfaces or when present in the bulk, are very complex and are connected with changes in the conformation of the folded polypeptide chains. These changes have a wide range of characteristic time-scales [4]. When these particular features of proteins are superimposed on the above picture of a deformed interfacial film it is seen that the task of understanding the mechanism of action of proteins as surfactants is a daunting task. 

Despite this donble complexity, it is possible to use enzymes in surfactant solutions to get a deeper insight in their structure and dynamics. In this chapter, we showed that enzymes can yield useful information on surfactant hydration, interfacial film rigidity, and partitioning of cosurfactants. Enzymes are also useful as an independent check for pH in reverse microemulsions. We could also show that for some of the properties of microemulsions such as cosurfactant partition coefficients, semi-qnantitative results can be obtained. 

Studies on the form of interaction between natural surfactants species present in oil and interfacial film can assist in developing more efficient methods of separation. From this point of view, asphaltenes can be studied by seeking a greater understanding of this complex fraction in terms of structure and composition, as a species that can contribute to the stabilization of these emulsions. 

Figure 8.16 Hypothetical structure of a molecular complexed interfacial film at a propellant water interface. From Sanders [90]. The oriented liquid crystal nature of molecular complexes with their attendant layers of oriented water molecules suggests that the interfacial region around an emulsified propellant droplet can be viewed as consisting of alternating shells of oriented water and molecular complex molecules. The propellant interface would consist of a monolayer of adsorbed molecular complex molecules with the polar heads oriented towards an adjacent hydration layer. The hydration layer of water molecules in turn would be surrounded with a bimolecular shell of complex molecules with the polar heads on one side of the shell oriented towards the inner hydration layer and the polar heads on the other side oriented towards an outer hydration shell. This configuration of alternating layers of oriented water and bimolecular complex molecules would extend into the bulk phase with diminishing orientation until it disappeared. ...

The main factors which affect the viscosity of emulsions are listed in Table 8.6. The properties of the disperse phase, the continuous phase and the emulsifying agent or agents all influence the emulsion viscosity. Each factor does not act independently and the interpretation of emulsion viscosity data is complicated by this fact and the fact that particles can deform under shear depending on the nature of the interfacial film. As we have also discussed, emulsions are complex systems, often highly structured, and at phase boundaries or on the point of inversion are very sensitive to small perturbations in the system. We will deal here first with mobile emulsions and then consider briefly the semi-solid state. 

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