Surfactants adsorption kinetics

Miller R, Fainerman VB, Aksenenko EV, Makievski AV, Kraegel J, Liggieri L, Ravera F, Wuestneck R, and Loglio G (2000) "Surfactant Adsorption Kinetics and Exchange of Matter for Surfactant Molecules with Changing Orientation within the Adsorption Layer" in Emulsion, Foams, and Thin Films, Mittal and Kumar Editors, Ch. 18, Marcel Dekker, pp. 313-327 Miller R, Fainerman VB, Makievski AV, Leser M, Michel M and Aksenenko EV (2004) Determination of Protein Adsorption by Comparative Drop and Bubble Profile Analysis Tensiometry. Colloids Surfaces B 36 123-126 Neumann AW and Spelt JK Eds., Applied Surface Thermodynamics, Surfactant Science Series, Vol. 63, Marcel Dekker Inc., New York, 1996 Noskov B and Logho G (1998) Dynamic surface elasticity of surfactant solutions. Colloids Surfaces A 143 167-183... 

For soluble surfactant adsorption layers the vertical mass transfer occurs under two different conditions, after the formation of a fresh surface of a surfactant solution and during periodic or aperiodic changes of the surface area. From the thermodynamic point of view the "surface phase" is an open system. The theoretical and practical aspects of this issues have been outlined in many classical papers, published by Milner (1907), Doss (1939), Addison (1944, 1945), Ward Tordai (1946), Hansen (1960, 1961), Lange (1965). New technique for measuring the time dependence of surface tension and a lot of theoretical work on surfactant adsorption kinetics under modem aspects have recently been published by Kretzschmar Miller (1991), Loglio et al. (1991), Fainerman (1992), Joos Van Uffelen (1993), MacLeod Radke (1993), Miller et al. (1994). This topic will be discussed intensively in Chapters 4 and 5. The relevance of normal mass exchange as a surface relaxation process is discussed in Chapter 6. 

The indices "diff and "kin" refer to the diffusional and transfer steps, respectively. Recently, theoretical models of surfactant adsorption kinetics were developed mainly to take into account specific experimental conditions or surfactant properties, such as molecular charge (Dukhin et al. 1983, 1991, Borwankar Wasan 1986, Chang Franses, 1992, Miller et al. 1994a), micelle formation (Rakita Fainerman 1989, Dushkin Ivanov 1991, Fainerman 1992, Serrien et al 1992) or other specific effects (Lin et al. 1991, Ravera et al. 1993, 1994, Jiang et al. 1993). Some of these models will be discussed in sections or chapters below. 

As an experimental prerequisite for studies at liquid/liquid interfaces, the two liquids have to be mutually saturated. For example, even solvents like alkanes are remarkably soluble in water and a transfer of solvent molecules across the interface would influence surfactant adsorption kinetics. The table in Appendix 5E summarises the mutual solubility of some solvents with water. 

The studies of adsorption layers at the water/alkane interface give excess to the distribution coefficient of a surfactant, which is a parameter of particular relevance for many applications. Theoretical models and experimental measurements of surfactant adsorption kinetics at and transfer across the water/oil interface will be presented. The chapter will be concluded by investigations on mixed surfactant systems comprising experiments on competitive adsorption of two surfactants as well as penetration processes of a soluble surfactant into the monolayer of a second insoluble compound. In particular these penetration kinetics experiment can be used to visualise separation processes of the components in an interfacial layer. 

Other theoretical models of surfactant adsorption kinetics take into account specific experimental conditions or surfactant properties surfactant charge [27, 28, 29, 30, 31, 32, 33], micelle formation [34, 35, 36, 37, 38] or other specific effects [39, 40, 41, 12, 13, 42, 43]. These other effects however will not further discussed here, as most of the surfactants studied in literature turned out to follow a diffusion mechanism. Thus, below we will give more details mainly on models based on a diffusion mechanism. 

Thermodynamics of Surfactant Adsorption Kinetics of Surfactant Adsorption Dynamic Surface Tension of Solutions Drop and Bubble Shape Experiments Adsorption Behaviour of Mixed Systems... 

We recall that in the regime BC the rate constants of the fast and slow micellar processes, and ks, do not affect the surfactant adsorption kinetics, and cannot be determined from the fit of the data. In principle, it is possible to observe the kinetic regime AB (and to determine kj with faster methods or with slower surfactants. 

In general, the surfactant adsorption is a consequence of two stages the first stage is the diffusion of surfactant from the bulk solution to the subsurfac e the second stage is the transfer of the surfactant molecules from the subsurface to the surface. The following important cases of surfactant adsorption kinetics can be specified ... 

The aim of this chapter is to compare and contrast adsorption kinetics of model cationic surfactants at air-water and solid-liquid interfaces, so as to draw general conclusions and identify dominant processes. Recently, strides have been made in understanding surfactant adsorption kinetics, and in this area development and application of new surface selective techniques has been key. Methods of relevance in this chapter are neutron reflectivity (NR), ellipsometry, and optical reflec-tometry (OR). These techniques are based on scattering and/or interference of neutron radiation or polarized laser light, and hence the principal advantages are that they directly probe surface layer structures and adsorption densities. In the text the terms surface excess, adsorbed amount, and surface density are used interchangeably to express two-dimensional concentrations, either at air-water or solid-liquid surfaces. The main surfactants considered are the family of n-alkyltrimethylam-monium bromides C ,TAB, of alkyl chain carbon number m. 

The time required for surface tension reduction depends on diffusion processes involved in surfactant adsorption. Kinetic models for surfactant adsorption divide the adsorption process into two steps [67]. The first step is the transport of the surfactant to the subsurface, driven by a concentration gradient or hydrody-... 

Surface tension methods measure either static or dynamic surface tension. Static methods measure surface tension at equilibrium, if sufficient time is allowed for the measurement, and characterize the system. Dynamic surface tension methods provide information on adsorption kinetics of surfactants at the air-liquid interface or at a liquid-liquid interface. Dynamic surface tension can be measured in a timescale ranging from a few milliseconds to several minutes [315]. However, a demarkation line between static and dynamic methods is not very sharp because surfactant adsorption kinetics can also affect the results obtained by static methods. It has been argued [316] that in many industrial processes, sufficient time is not available for the surfactant molecules to attain equilibrium. In such situations, dynamic surface tension, dependent on the rate of interface formation, is more meaningful than the equilibrium surface tension. For example, peaked alcohol ethoxylates, because they are more water soluble, do not lower surface tension under static conditions as much as the conventional alcohol ethoxylates. Under dynamic conditions, however, peaked ethoxylates are equally or more effective than conventional ethoxylates in lowering surface tension [317]. 

Two important parameters, a and pf arise which depend on the equilibrium and kinetic properties of the surfactant. First, a measures the fractional change in equilibrium surface tension with a fractional change in surfactant adsorption ... 

When p approaches infinity, Equation 7 reveals that equals zero, which corresponds to infinitely fast sorption kinetics and to an equilibrium surfactant distribution. In this case Equation 6 becomes that of Bretherton for a constant-tension bubble. Equation 6 also reduces to Bretherton s case when a approaches zero. However, a - 0 means that the surface tension does not change its value with changes in surfactant adsorption, which is not highly likely. Typical values for a with aqueous surfactants near the critical micelle concentration are around unity (2JL) ... 

Brunauer classification, 1 591 and column performance, 1 604—606 gas adsorption, 1 622-623, 626-629 of nonionic surfactants, 24 142—143 predicting, 24 139-140 Adsorption kinetics... 

Jin F, Balasubramaniam R, Stebe KJ (2004) Surfactant adsorption to spherical particles the intrinsic length-scale governing the shift from diffusion to kinetic-controlled mass transfer. J Adhes 80 773-796... 

Kotsmar, Cs., Pradines, V., Alahverdjieva, V.S., Aksenenko, E.V., Fainerman, V.B., Kovalchuk, V.E, Kragel, J., Leser, M.E., Noskov, B.A., Miller, R. (2009). Thermodynamics, adsorption kinetics and rheology of mixed protein-surfactant interfacial layers. Ach cmces in Colloid and Interface Science, 150, 41-54. 

Figure D3.5.6 Adsorption kinetics of a small molecule surfactant. Surface tension of polyoxyethylene (10) lauryl ether (Brij) at the air-water interface decreases as time of adsorption increases. Brij concentration is 0.1 g/liter, as measured by the drop volume technique and the maximum bubble pressure method (UNITD3.6).

It has recently been shown that the inverted setup is better suited to measurement of the adsorption kinetics of protein samples at, e.g., the oil/water interface since it prevents reservoir depletion. Reservoir depletion can occur if the concentration of surfactant in the solvent phase is low. If the protein is present in the drop-forming phase, then the concentration within the drop itself may decrease during the adsorption process. This in turn would affect the measured rate of adsorption. In this case, it is preferable to form an inverted oil droplet in the protein solvent. 

An almost overwhelmingly large number of different techniques for measuring dynamic and static interfacial tension at liquid interfaces is available. Since many of the commercially available instruments are fairly expensive to purchase (see Internet Resources), the appropriate selection of a suitable technique for the desired application is essential. Dukhin et al. (1995) provides a comprehensive overview of currently available measurement methods (also see Table D3.6.1). An important aspect to consider is the time range over which the adsorption kinetics of surface-active substances can be measured (Fig. D3.6.5). For applications in which small surfactant molecules are primarily used, the maximum bubble pressure (MBP) method is ideally suited, since it is the only... 

Surface properties of proteins in general, 296-298 (table) purification methods based on, 272 Surface tension and interfacial properties, 609-628. see also Interfaces Surfactants, see also Interfacial tension definition and adsorption kinetics of, 617-618, 639... 

The adsorption and desorption kinetics of surfactants, such as food emulsifiers, can be measured by the stress relaxation method [4]. In this, a "clean" interface, devoid of surfactants, is first formed by rapidly expanding a new drop to the desired size and, then, this size is maintained and the capillary pressure is monitored. Figure 2 shows experimental relaxation data for a dodecane/ aq. Brij 58 surfactant solution interface, at a concentration below the CMC. An initial rapid relaxation process is followed by a slower relaxation prior to achieving the equilibrium IFT. Initially, the IFT is high, - close to the IFT between the pure solvents. Then, the tension decreases because surfactants diffuse to the interface and adsorb, eventually reaching the equilibrium value. The data provide key information about the diffusion and adsorption kinetics of the surfactants, such as emulsifiers or proteins. 

The study of adsorption kinetics of a surfactant on the mineral surface can help to clarify the adsorption mechanism in a number of cases. In the literature we found few communications of this kind though the adsorption kinetics has an important role in flotation. Somasundaran et al.133,134 found that the adsorption of Na dodecylsulfonate on alumina and of K oleate on hematite at pH 8.0 is relatively fast (the adsorption equilibrium is reached within a few minutes) as expected for physical adsorption of minerals with PDI H+ and OH". However, the system K oleate-hematite exhibits a markedly different type of kinetics at pH 4.8 where the equilibrium is not reached even after several hours of adsorption. Similarly, the effect of temperature on adsorption density varies. The adsorption density of K oleate at pH 8 and 25 °C is greater than at 75 °C whereas the opposite is true at pH 4.8. Evidently the adsorption of oleic acid on hematite involves a mechanism that is different from that of oleate or acid soaps. 

While microemulsions are thermodynamically stable, and the stability of emulsions has a kinetic origin, in both cases the adsorption of the dispersant upon the interface of the globules is responsible for stability. For this reason it appears natural to attempt to explain the above equality between the two inversion temperatures on the basis of surfactant adsorption. In addition, both the micro and macro-emulsions obey in many cases the Bancroft rule [8,9], which indicates that the phase in which a larger amount of dispersant is present becomes the continuous phase there are, however, some violations of this rule which will be discussed later in the paper. 

Investigations of the adsorption kinetics showed that a surfactant adsorption layer is formed. In the beginning, the adsorption ran very fast. After one minute, already 40% of the equilibrium amount was adsorbed. Then the adsorption became slower until after 10 to 30 min the adsorption equilibrium is reached. The fast adsorption gives... 

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