Adsorption at fluid interfaces

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. 

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.)... 

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. 

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). 

Leimard-Jones, J.E., in "Fowler s Statistical Mechanics", Cambridge, 1936 Levich, V.G., Physico-Chemical Hydrodynamics, Prentice-Hall, Englewood Cliffs, N.Y., 1962 Li, D., Gaydos, J. and Neumaim, A.W., Langmuir, 5(1989)1133 Lucassen-Reynders, E.H., Progr. Surface Membrane Sic., 3(1976)253 Lucassen-Reynders, E.H., Adsorption at Fluid Interfaces, in "Surfactant Science Series", Marcel Dekker, New York, 11(1981)... 

The adsorption at fluid interface can be described by at least three consecutive or competitive processes (1) diffusion of the molecule from the bulk to an interface and attachment to this interface (2) penetration of new molecules into the adsorbed layer (3) molecular rearrangement of the adsorbed molecules (this process is very important in the case of proteins). For the last two processes energy... 

Proteins, on adsorption at fluid interfaces, undergo a change from their globular configuration in solution to an extended chain structure. This has often been referred to as surface denaturation. 

The discussion above introduced some basic concepts related to the properties of fluid-fluid, and particularly liquid-vapor interfaces. The practical effects of surface tension lowering were not addressed because they generally appear in the context of phenomena such as emulsification, foaming, wetting, and detergency, to be discussed later. For further details on the subject of surface tension lowering and surfactant adsorption at fluid interfaces, the reader is referred to the works cited in the Bibliography. 

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. 

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. 

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... 

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

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). 

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. 

The adsorption of proteins at fluid interfaces is a key step in the stabilization of numerous food and non-food foams and emulsions.1 Our general goal is to relate the amino acid sequence of proteins to their surface properties, e. g. to the equation of state or other structural and thermodynamic properties. To improve this understanding, the effect of guanidine hydrochloride (Gu HC1) on /1-casein adsorption is evaluated in the framework of the block-copolymer model for the adsorption of this protein. At first the main features of the model are presented, and then the effect of Gu HC1 is interpreted using the previously introduced concepts. 

In the linear case, non-equilibrium properties of adsorption layers at fluid interfaces can be quantitatively described by the interfacial thermodynamic modulus (Defay, Prigogine Sanfeld 1977),... 

This chapter describes the origins of surface activity of proteins and some basic characteristics of their adsorption at fluid/fluid and solid/liquid interfaces, which have great importance in various colloidal systems. Understanding the surface activity of proteins would establish the need for various chemical and physicochemical alterations of proteins, which can influence their surface activity. Some of these modifications are presented briefly in this chapter, and in more detail in the following chapters. 

Adsorption kinetics and isotherms. According to the Gibbs equation for ideal systems, the adsorption of classical surface-active molecules at fluid interfaces decreases the surface tension ... 

Adsorption layers of the same kind as at fluid interfaces are also formed at low-energy solid -water surfaces, as it was established on PE, polystyrene, paraffin, carbon black, and other related materials. The classical Langmuir or Frumkin adsorption isotherm is often applicable to describe this behaviour. Studies on surfactant adsorption at various solid surfaces have been summarised in a great number of reviews [2, 7, 8, 54, 98, 101, 111, 121, 126, 141, 144, 145, 177, 186, 190, 194-198]. The adsorption at the solid/liquid interfaces is governed by a number of factors ... 

This implies that a smaller molecule will expel a larger one from the surface when their total concentration is increased at constant Ci/C2. Thus, a two-dimensional solution treatment which expresses the chemical potentials of the surface layer components by means of Eq. (2.7) enables us to derive equations of state and adsorption isotherms at fluid interfaces depending on the system considered (ideal or non-ideal surface layer, single surfactant or mixture of... 

Theoretical models have reached a state that allows a quantitative description of the equilibrium state by thermodynamic models, the adsorption kinetics of surfactants at fluid interfaces, the transfer across interfaces and the response to transient or harmonic perturbations. As result adsorption mechanisms, exchange of matter mechanisms and the dilational rheology are obtained. For some selected surfactant systems, the characteristic parameters obtained on the various levels coincide very well so that a comprehensive understanding was reached. 

Miller, R. and Kretzschmar, G., Adsorption kinetics of surfactants at fluid interfaces, Adv. Colloid Interf. Set, 37, 97-121, 1991. 

Amphiphilic molecules supplied to a (aqueous) solution adsorb at available interfaces. For that reason, amphiphilic molecules are also called surface active agents or, for short, surfactants. The decrease in interfacial tension y resulting from adsorption is, for a reversible process, given by the Gibbs equation (Equation 3.88) or Equation 7.2. Adsorbed layers of surfactants at fluid interfaces are extensively discussed in Chapter 7, and Chapter 14 deals with generic theoretical aspects of the adsorption process. 

Most practical systems, for example, essentially all biological fluids, are multi protein systems. The various proteins compete with each other (and with other surface-active components) for adsorption at any interface present. As a rule, the interface will at first become covered by the molecules that have the highest rate of arrival (that is, the smaller ones that have the highest diffusion coefficient and the ones that occur most abundantly in the solution). At later stages, the initially adsorbed molecules may be displaced in favor of other molecules that have a higher affinity for the surface. 

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