Adsorption at oil-water interface

It is important to characterize the material adsorbed at the interface, as well as the conformation of the adsorbed proteins, because it determines the properties of the emulsions and gels. However, little is known about the conformational changes of proteins upon adsorption at oil-water interfaces. The main reason for this is the lack of experi-... 

Biopolymer incompatibility seems to provide phase-separated liquid and gel-like aqueous systems. In highly volume-occupied food systems aggregation, crystallisation and gelation of biopolymers and their adsorption at oil/water interfaces favour an increase in the free volume, which is accessible for other macromolecules. Denatura-tion of proteins during food processing decreases their solubility and co-solubility of proteins with one another and with polysaccharides and induces phase separation of the system. 

Murakami R, Takata Y, Ohta A, et al. Aggregate formation in oil and adsorption at oil/water interface thermodynamics and its application to the oleyl alcohol system. J Colloid Interface Sci 2004 270 262-269. 

In that most foodstuffs contain proteins, the formation of protein-surfactant complexes plays a direct role in 2. and an indirect role in 1. and 3., particularly with respect to their adsorption at oil-water interfaces and distribution between the phases. The surfactants used in foodstuffs are necessarily controlled, and permitted food emulsifiers are esters (or partial esters) formed from fatty acids (from animal and vegetable sources) with polyvalent alcohols such as glycerol, propylene glycerol, or sorbitol. 

The results of flic interfacial rheological studies on asphaltene adsorption at oil-water interfaces teach us a great deal about the behavior of asphaltenes and their role in emulsion stabili2ation. The conclusions drawn are based largely on the assumption that the rheological properties measured, namely flic elastic film modulus G are directly related to the surface excess concentration of asphaltenes. F. It is understood diat die elastic modulus actually depends on both the surface excess concenlration and the relative conformation (i.e., coimectivity) of the adsorbed asphaltenes. It is further understood that a minimum adsorbed level is required to observe a finite value of G and that the relationship between G and G is not linear. With these caveats in mind, we can conclude die following ... 

A stabilising effect in the presence of salt was also noted by Aronson and Petko [90]. Addition of various electrolytes was shown to lower the interfacial tension of the system. Thus, there was increased adsorption of emulsifier at oil/water interface and an increased resistance to coalescence. Salt addition also increased HIPE stability during freeze-thaw cycles. Film rupture, due to expansion of the water droplets on freezing, did not occur when aqueous solutions of various electrolytes were used. The salt reduced the rate of ice formation and caused a small amount of aqueous solution to remain unfrozen. The dispersed phase droplets could therefore deform gradually, allowing expansion of the oil films to avoid rupture [114]. 

Emulsions and foams are two other areas in which dynamic and equilibrium film properties play a considerable role. Emulsions are colloidal dispersions in which two immiscible liquids constitute the dispersed and continuous phases. Water is almost always one of the liquids, and amphipathic molecules are usually present as emulsifying agents, components that impart some degree of durability to the preparation. Although we have focused attention on the air-water surface in this chapter, amphipathic molecules behave similarly at oil-water interfaces as well. By their adsorption, such molecules lower the interfacial tension and increase the interfacial viscosity. Emulsifying agents may also be ionic compounds, in which case they impart a charge to the surface, which in turn establishes an ion atmosphere of counterions in the adjacent aqueous phase. These concepts affect the formation and stability of emulsions in various ways ... 

Low interfacial tension The adsorption of surfactant at oil-water interfaces causes a lowering of interfacial energy, thus facilitating... 

Nilsson, L., and Bergenstahl, B. (2006). Adsorption of hydrophobically modified stareh at oil-water interfaces during emulsification. Langmuir. 22, 8770-8776. 

The addition of Na2C03 in the brine can lead to an increase in surface charge in two ways (1) ionization of the organic acid at oil/water interface (2) adsorption of hydroxyl ions. Figure 12.11 shows that the -potential for oil in brine was -20 mv and oil in 0.15 wt.% NaaCOs solution in brine was -23 mv at 0% surfactant. The stabilized -potential of the surfactant-only system was about -33 mv. The -potential of the surfactant solution with 0.15 wt.% NaaCOs present stabilized at about -55 mv. 

K. Taiwo, H. Karbstein, and H. Schubert [/. Food Process Eng., 20, 1-16 (1997)] studied the influence of temperature on the kinetics of adsorption of a variety of food emulsifiers at oil-water interfaces. They used interfacial tension measurements to monitor the rate at which egg yolk present at 10 times its critical micelle concentration was transferred to a water-soybean oil interface. The rates of these processes are important in assessing the potential stability of oil-in-water emulsions of the type found in salad dressings and mayonnaise. The interfacial tension can be viewed as a property that reflects the contributions of the various species present at the interface, being an additive function of these contributions. Each individual contribution is proportional to the quantity of the material in question located at the interface between the oil and the water. The reaction of interest can be regarded as... 

In this chapter, we have attempted to review and summarize the state of knowledge of the mechanisms of stabilization of W/0 emulsions by asphaltenes. This has necessarily taken us into flic realm of asphaltene chemistry, physical chemistry, adsorption at interfaces, film structure, and film rheology. The self-assembly of asphaltenes at oil-water interfaces into a physically crosslinked, mechanically strong. 

LIGHT SCATTERING STUDY OF ADSORPTION OF SURFACTANT MOLECULES AT OIL-WATER INTERFACE... 

The adsorption of surfactant molecules at oil-water interfaces has attracted much interest, in relation to the oil recovery techniques (1). The systems containing oil, water and emulsifier molecules form generally two phases an aqueous phase containing sometimes solubilized oil in the form of small droplets surrounded by emulsifier molecules and an oil phase which also can contain solubilized water. When the amount of emulsifier is large enough, the system can form only one phase, i.e. all the water (or oil) can be solubilized in the oil (or water). The system is again a dispersion of very small droplets of water (or oil), surrounded by emulsifier molecules, in a continuous medium containing the oil (or water). Such dispersions are currently called microemulsions. The droplet size is usually of the order of lOOA (2). 

Redox reactions at the interface between immiscible liquids fall into two classes. The first class includes spontaneous processes that occur in the absence of external electromagnetic fields. This type of redox transformation has been investigated in bioenergetics [2], model membrane systems [20] and at oil/water interfaces [1]. Redox reactions in the second class occur at the interface between immiscible electrolytes when external electrical fields are applied to the interface, and under these conditions interfacial charge transfer reactions take place at controlled interfacial potentials [11, 35, 36]. Such electrochemical interfacial reactions are usually multi-stage processes that proceed through five stages (i) diffusion of reactants to the interface (ii) adsorption of reactants onto the interface (iii) electrochemical reaction at the interface (iv) desorption of products from the interface (v) diffusion of products from the interface. 

Goual, L. Firoozabadi, A. (2002). Measuring Asphaltenes and Resins, and Dipwle Moment in Petroleum Fluids, AiChE Journal, Vol.48, No.ll, pp. 2646-2663 Goual, L. Firoozabadi, A. (2004). Effect of Resins and DBSA on Asphaltene Precipitation from Petroleum Fluids. AiChE Journal, Vol.50, No.2, pp. 470-479 Goual, L. Horvath-Szabo, G. Masliyah, J.H. Xu, Z. (2005). Adsorption of Bituminous Components at Oil/Water Interfaces Investigated by Quartz Crystal Microbalance Implications to the Stability of Water-in-OU Emulsions. Langmuir, Vol.21, No.l8, pp. 8278-8289... 

The performance of demulsifiers can be predicted by the relationship between the film pressure of the demulsifier and the normalized area and the solvent properties of the demulsifier [1632]. The surfactant activity of the demulsifier is dependent on the bulk phase behavior of the chemical when dispersed in the crude oil emulsions. This behavior can be monitored by determining the demulsifier pressure-area isotherms for adsorption at the crude oil-water interface. 

Replacement of gas by the nonpolar, e.g., hydrocarbon phase (or oil phase) is used to modify the interactions between molecules in a spread film of investigated long-chain substances [6,15,17,18]. The nonpolar solvent-water interface possesses the advantage over that between gas and water, that the cohesion (i.e., interactions between adsorbed molecules due to dipole and van der Waals forces) is negligible. Thus, at the oil-water interfaces behavior of adsorbates is much closer to ideal, but quantitative interpretation may be uncertain, in particular for the higher chains which are predominantly dissolved in the oil phase to an unknown activity. Adsorption of dipolar substances at the w/a and w/o interfaces changes surface tension and modifies the surface potential of water [15] ... 

Studies of the adsorption of surface active electrolytes at the oil-water interface provide a convenient method for testing electrical double layer theory and for determining the state of water and ions in the neighborhood of an interface. The change in the surface amount of the large ions modifies the surface charge density. For instance, the surface ionic area of 100 per ion corresponds to 16, /rC/cm. The measurement of the concentration dependence of the changes of surface potential were also applied to find the critical concentration of formation of the micellar solution [18]. 

Thermodynamics of adsorption at liquid interfaces has been well established [22-24]. Of particular interest in view of biochemical and pharmaceutical applications is the adsorption of ionic substances, as many of biologically active compounds are ionic under the physiological conditions. For studying the adsorption of ionic components at the liquid-liquid interface, the polarized liquid-liquid interface is advantageous in that the adsorption of ionic components can be examined by strictly controlling the electrical state of the interface, which is in contrast to the adsorption studies at the air-water or nonpolar oil-water interfaces [25]. 

No data are as yet available on the effect of length of tail on the emulsion size. As has already been noted (p. 46) fatty acids with short hydrocarbon tails are very water soluble and the sodium soaps soluble to a greater degree, interfacial adsorption is consequently small. Thus the concentration required to produce a saturated film at the oil-water interface will be correspondingly greater. This necessitates a high sodium ion concentration in the... 

It may be noted that similar lyotropic series for the anions have been observed at other interfaces in addition to the air-water interface. Beutner s series (see p. 249) for the oil-water interface adsorption is... 

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