Adsorption fluid interfaces

Ordinary diffusion involves molecular mixing caused by the random motion of molecules. It is much more pronounced in gases and Hquids than in soHds. The effects of diffusion in fluids are also greatly affected by convection or turbulence. These phenomena are involved in mass-transfer processes, and therefore in separation processes (see Mass transfer Separation systems synthesis). In chemical engineering, the term diffusional unit operations normally refers to the separation processes in which mass is transferred from one phase to another, often across a fluid interface, and in which diffusion is considered to be the rate-controlling mechanism. Thus, the standard unit operations such as distillation (qv), drying (qv), and the sorption processes, as well as the less conventional separation processes, are usually classified under this heading (see Absorption Adsorption Adsorption, gas separation Adsorption, liquid separation). 

D. E. Sulhvan, M. M. Telo da Gama. Wetting transition and multilayer adsorption at fluid interfaces. In C. A. Croxton, ed. Fluid Interfaeial Phenomena. New York Wiley, 1986. 

In general, adsorption is a surface phenomenon, where gas or liquid is concentrated on the surface of solid particles or fluid interfaces. There are many adsorption systems. 

The rheological properties of a fluid interface may be characterized by four parameters surface shear viscosity and elasticity, and surface dilational viscosity and elasticity. When polymer monolayers are present at such interfaces, viscoelastic behavior has been observed (1,2), but theoretical progress has been slow. The adsorption of amphiphilic polymers at the interface in liquid emulsions stabilizes the particles mainly through osmotic pressure developed upon close approach. This has become known as steric stabilization (3,4.5). In this paper, the dynamic behavior of amphiphilic, hydrophobically modified hydroxyethyl celluloses (HM-HEC), was studied. In previous studies HM-HEC s were found to greatly reduce liquid/liquid interfacial tensions even at very low polymer concentrations, and were extremely effective emulsifiers for organic liquids in water (6). 

Information on the chemical potentials of components in a solution of biopolymers can serve as a guide to trends in surface activity of the biopolymers at fluid interfaces (air-water, oil-water). In the thermodynamic context we need look no further than the Gibbs adsorption equation,... 

Dickinson, E., Horne, D.S., Phipps, J.S., Richardson, R.M. (1993a). A neutron reflectivity study of the adsorption of p-casein at fluid interfaces. Langmuir, 9, 242-248. 

Miller, R., Fainerman, V.B., Makievski, A.V., Kragel, J., Grigoriev, D.O., Kazakov, V.N., Sinyachenko, O.V. (2000a). Dynamics of protein and mixed protein + surfactant adsorption layers at the water-fluid interface. Advances in Colloid and Interface Science, 86, 39-82. 

It is often useful to consider the adsorbent/fluid interface as comprising two regions. The region of the fluid phase (i.e., liquid or gas) forming part of the adsorbent/fluid interface may be called the adsorption space, while the portion of the adsorbent included in the interface is called the surface layer of the adsorbent. ... 

Dietrich, S., (1991), Fluid interfaces - wetting, critical adsorption, van der Waals tails, and the concept of the effective interface potential , in Taub, H., Torzo, G., Lauter, HJ. and Fain, S.C., (eds), Phase. Transitions in Surface Films 2, NATO Advanced Science Series, Physics, Vol. 267, 391-423. 

V. B. Fainerman, E. H. Lucassen-Reynders and R. Miller, Description of the adsorption behaviour of proteins at water/fluid interfaces in the framework of a two-dimensional solution model, Adv. Colloid Interface Sci. 106, 237-259 (2003). 

Having laid down the physico-chemical basis of interface and colloid science in Volume I, we can now make a start with the systematic treatment. Solid-gas and, in particular, solid-liquid interfaces are selected as the first topics. The argument for this choice is that such Interfaces do not change their area upon adsorption and/or charging, and are therefore the simplest group of model systems. In Volume III, dealing with fluid-fluid Interfaces, this restriction will be relaxed. 

For our purposes, adsorption from solution is of more direct relevance than gas adsorption. Most, if not all, topics in the five volumes of FICS Involve one or more elements of it. In the present chapter, the basic elements will be introduced, restricting ourselves to low molecular weight, uncharged adsorbates and solid surfaces. Adsorption of charged species leads to the formation of electrical double layers, which will be treated in chapter 3. Adsorption at fluld/fluid Interfaces follows in Volume III. Adsorption of macromolecules will be Introduced in chapter 5. Between monomers, short oligomers, longer oligomers and polymers there is no sharp transition in the present chapter we shall go as far as non-ionic surfactants, but omit most of the association and micelle formation features, which will be addressed in a later Volume. There will be some emphasis on aqueous systems. 

The first boundary condition is equivalent to the well-known Levich approach (ca=1 for according to which, it is supposed that the concentration values vary only within a very thin concentration layer while it is supposed to keep its bulk value elsewhere [9], Eq. (3b) has been proposed by Coutelieris et al. [8] in order to ensure the continuity of the concentration upon the outer boundary of the cell for any Peclet number. Furthermore, eq. (3c) and (3d) express the axial symmetry that has been assumed for the problem. The boundary condition (3e) can be considered as a significant improvement of Levich approach, where instantaneous adsorption on the solid-fluid interface cA(ri=Tia,ff)=0) is also assumed for any angular position 0. In particular, eq. (3e) describes a typical adsorption, order reaction and desorption mechanism for the component A upon the solid surface [12,16] where ks is the rate of the heterogeneous reaction upon the surface and the concentration of component A upon the solid surface, c,is, is calculated by solving the non linear equation... 

The use of an evanescent wave to excite fluorophores selectively near a solid-fluid interface is the basis of the technique total internal reflection fluorescence (TIRF). It can be used to study theadsorption kinetics of fluorophores onto a solid surface, and for the determination of orientational order and dynamics in adsorption layers and Langmuir-Blodgett films. TIRF microscopy (TIRFM) may be combined with FRAP ind FCS measurements to yield information about surface diffusion rates and the formation of surface aggregates. 

G. Kretzschmar and R. Miller. Dynamic Properties of Adsorption Layers of Amphiphilic Substances at Fluid Interfaces, Adv. Colloid Interface Set 36 (1991) 65. See also R. Miller, G. Kretzschmar, Ibid 37 (1991) 97. 

E. H. Lucassen-Reynders, Adsorption of Surfactant Monolayers at Gas/Liquid and Liquid/Liquid Interfaces, in Progr. Surface Membrane ScL, D.A. Cadenhead, J.F. Danielli, Eds., 10 (1976) 253, and Adsorption at Fluid Interfaces, chapter 1 in Anionic Surfactants Physical Chemistry and Surfactant Action, E.H. Lucassen-Reynders, Ed., Marcel Dekker (1981). (Two reviews by the same author she emphasizes adsorption thermodynamics and 2D equations of state.)... 

The strongly amphipathic nature of proteins, resulting from their mixture of polar and nonpolar side chains, causes them to be concentrated at interfaces. As a result of their great stability in the adsorbed state, it is possible to study them at fluid interfaces by the classical techniques of insoluble monolayers. A review of early work along these lines in this series (Bull, 1947) serves as an excellent introduction to the subject. The effect of adsorption on the biological activity of proteins was treated by Rothen (1947) in the same volume. Further... 

Adsorption may be followed at fluid/fluid interfaces by measuring changes in interfacial pressure, potential, or viscosity, using spread monolayers for calibration purposes. The most accurate method for measuring rates of adsorption is by the rate of increase of interfacial area at constant interfacial pressure. If (1/A) (dA/dt) is the fractional rate of increase of interfacial area expressed in sec-1 and n is the interfacial concentration in g cm-2 found from measurements on spread monolayers, then the rate of adsorption dn/dt in g cm-2 sec-1 is given by... 

Distribution between trains and loops in molecules adsorbed at solid/Iiquid interfaces is also possible and has been shown to occur for flexible polymers. There are some indications that protein molecules at solid/liquid interfaces do not always undergo the drastic conformational changes that occur at fluid/fluid interfaces. At a solid/liquid interface, an adsorbing molecule cannot penetrate the solid phase. Furthermore, adsorption may be confined to sites and thus be localized. Using infrared difference spectroscopy, Morrissey and Stromberg (1974) found a bound fraction (number of carbonyl surface... 

It would appear that, at fluid/fluid interfaces, proteins give adsorption isotherms for which interfacial pressure is a linear function of the logarithm of the bulk concentration over appreciable ranges as has been found for simpler compounds. What has not been satisfactorily explained is the reason for the very low values of A. Joos has inter-... 

Adsorption Data for Proteins at Fluid/Fluid Interfaces... 

Apart from those mentioned, other general features of protein adsorption at solid/liquid interfaces are as follows (1) Adsorption is sensitive to pH, as it is for fluid/fluid interfaces, a maximum usually being observed near the isoelectric point of the protein (Dillman and Miller, 1973 Norde, 1976). (2) Greater adsorption occurs at hydrophobic interfaces than at hydrophilic ones (MacRitchie, 1972 Brash and... 

The kinetics of competitive adsorption of the food components at fluid interfaces... 

The adsorption of emulsifiers at fluid-fluid interfaces is considered to play an important role in the formation and stabilization of food dispersions (Dickinson, 1982). During the formation of a dispersed system, the emulsifier must be adsorbed at the interface to prevent the recoalescence of the initially formed dispersed particles (bubbles or droplets). In addition, during... 

The d)mamics of adsorption of emulsifiers at fluid interfaces have been determined by tensiometry and surface rheology (Figure 14.3) that is, from the time dependence of surface pressure and surface dilatational modulus (E). We found that tt and E increase with time (9), which should be associated with emulsifier adsorption (Patino and Nino, 1999 Nino et al., 2003 Carrera et al., 2005). 

Solutes are one of the major components of foods, and they have significant effects on their adsorption at fluid interfaces. In addition, the study of the effects of ethanol and/or sucrose on protein adsorption at fluid interfaces is of practical importance in the manufacture of food dispersions. The presence of ethanol in the bulk phase apparently introduces an energy barrier for the protein diffusion towards the interface. This could be attributable to competition with previously adsorbed ethanol molecules for the penetration of the protein into the interface. However, if ethanol causes denaturation and/or aggregation of the protein in the bulk phase, the diffusion of the protein towards the interface could be diminished. The causes of the higher rate of protein diffusion from aqueous solutions of sucrose, in comparison with that observed for water, must be different in aqueous ethanol solutions. Since protein molecules are preferentially hydrated in the presence of sucrose, it is possible that sucrose limits protein unfolding in the bulk phase and reduces protein-protein interactions in the bulk phase and at the interface. Both of these phenomena may increase the rate of protein diffusion towards the interface. Clearly, the kinetics of adsorption of proteins at interfaces are highly complex, especially in the presence of typical food solutes such as ethanol and sucrose in the aqueous phase. 

In summary, tensiometry and surface rheology give complementary information about emulsifier adsorption and interactions at fluid interfaces as a function of emulsifier concentration, aqueous phase composition, and the scale of adsorption time. 

An increase in diffusion rates occurs as a consequence of increasing protein bulk concentration (Patino et al., 1999 Home and Patino, 2003 Baeza et al., 2004a). Excluded volume effects can have an effect similar to increasing protein concentration because of the increased thermod)mamic activity of the protein in the bulk solution — that would perform as a more concentrated one (Carp et al., 1999) — and can lead to an enhancement of protein adsorption at fluid interfaces (Tsapkina et al., 1992). 

The adsorption isotherms in Table 5.2 can be applied to both fluid and solid interfaces. The surface tension isotherms in Table 5.2, which relate a and Fj, are usually apphed to fluid interfaces, although they could also be used for sohd-liquid interfaces if a is identified with the Gibbs superficial tension. (The latter is defined as the force per unit length that opposes every increase of the wet area without any deformation of the sohd.)... 

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