Surfactant transport processes

We now turn to a case in which the interface concentration is determined by the mass transfer process to and from the bulk fluids. We begin with the case in which the fastest of the surfactant transport processes is the adsorption desorption of surfactant between the exterior bulk fluid where it is assumed to be soluble and the interface, Bi 1. In particular, we assume that Bi (ca/V,Xjk)Pe, so that, according to (7-265),... 

In dilute solutions of surfactants adsorption processes are controlled by transport of the surfactant from the bulk solution towards the surface as a result of the concentration gradient formed in the diffusion layer the inherent rate of adsorption usually is rapid. For non-equilibrium adsorption the apparent (non-equilibrium) isotherm can be constructed for different time periods that are shifted with respect to the true adsorption isotherm in the direction of higher concentration (Cosovic, 1990) (see Fig. 4.10). 

As was stressed by Professor Ubbelohde, in the process of cell recognition not only the lateral diffusion of the binding sites has to be considered, but also the mechanical effects resulting from the local change of surface tension, inducing convection at the cell surface. It is well known, in the cell-to-cell contact inhibition of motion, in tissue culture, that a cell approaches another cell by touching it by means of microvilli and that this process can be affected when adding surfactants to the culture. Now the point is, What is the relative importance of both diffusion and convection Well, in binary surface films, it was observed that the transport process induced by two-dimensional convection is much more rapid than the two-dimensional diffusion. 

Weiss, J. 1999. Effect of Mass Transport Processes on Physicochemical Properties of Surfactant-Stabilized Emulsions. Department of Food Science, University of Massachusetts, Amherst. 280. 

One of the major obstacles to definitively examining the role of formulation components on enterocyte-based processes is the possible effect of excipient inclusion on other complicating factors. For example, the inclusion of lipids or surfactants in in vitro metabolic or transport screens runs the risk of affecting the thermodynamic activity of the drug in solution, thereby obscuring the role of metabolic and transport processes. Similarly, some surfactants and lipid-surfactant conjugates may cause transient increases in intestinal permeability as... 

Effect of Surfactants on Wetting, Penetration and Transport of Pesticides in Plants and Insects (16-17). Some of the factors which affect the wetting, penetration and transport processes are... 

We are often concerned with the dispersion of pollutants and other chemicals in the environment. Advection and mass flux are indiscriminate transport processes. In the water column of a lake, for example, these processes transport dissolved and particle-bound chemicals equally across the boundaries of the test volume. Settling of particles, in contrast, causes a downward flux of particle-bound chemicals while leaving dissolved chemicals in place. Similarly, surfactants or gases that join rising air bubbles are carried to the surface. These discriminate transport processes are very important in a variety of environmental situations ... 

In general, the surfactant is distributed along the interface by a combination of convection and diffusion, as well as transport to and from the interface from the bulk solvents. However, in many cases, the solubility of a surfactant in the two solvents is very low, and a good approximation is that the transport from the solvents is negligible. In this case, it is said that the surfactant is an insoluble surfactant, and the total quantity of surfactant on the interface is conserved. We have notpreviously derived a bulk-phase conservation equation to describe the transport of a solute in a solvent. Hence in this section we adopt the insoluble surfactant case, and follow Stone50 in deriving a surfactant transport equation that relates only to convection and diffusion processes on the interface. 

The shape of the capillary portion of the liquid-vapor interfacial area for sand (Fig. 1-1 lb) resembles simulation results of Reeves and Celia (1996) of interfacial areas in pore networks due to capillarity only. The discussion illustrates potential limitations in using cylindrical pore network models (Reeves Celia, 1996) especially for studies of volatile liquids and surfactants, and other multiphase transport processes where interfacial areas play a crucial role (Kim et al., 1997 Karkare Si. Fort, 1996). Furthermore, the overwhelming role of adsorbed liquid films casts doubts on several proposed constitutive relationships between capillary pressure (saturation) and interfacia] area (Skopp, 1985 Hassanizadeh Gray, 1993) most of which were based on assumed cylindrical capillary geometry in the absence of adsorption. 

The diffusing capacity of the lung for carbon monoxide (CO) is a measure of the ability of the alveolar capillary membrane to transfer or conduct gases from the alveoli to the blood. This transport process is entirely a passive one brought about by diffusion. As described previously in Section 2.2, the barriers for diffusion consist of surfactant, alveolar epithelium, interstitital fluid, capillary endothelium, plasma, and the red blood cell membrane. 

The adsorption kinetics of surfactant molecules to a liquid interface is controlled by transport processes in the bulk and the transfer of molecules from a solution state into an adsorbed state or vice versa. In this paragraph qualitative and quantitative models are discussed. 

Dynamic adsorption layers differ from equilibrium layers not only by the existence of an angular dependence but also by the difference in the adsorbed amount averaged over the bubble surface (Sadhal Johnson, 1983). Usually, in foam flotation, the surfactant yield is calculated under the assumption of equilibrium adsorption at the surface of buoyant bubbles. The theory of dynamic adsorption layers lead to substantial changes in the notion of surfactant flotation. Thus, the mechanism of transport at the bubble-solution interface has a substantial effect on the transport process at the surfactant solution-foam boundary. 

The results of the preceding section allow us now to move on to describe the surfactant transport from the depth of the bulk phase to the interface or in the opposite direction. If any adsorption barriers are absent, this process determines the adsorption and desorption rates. The main step in the solution of this problem consists in the formulation of the surfactant diffusion equations for micellar solutions. The problem of surfactant diffusion to the interface was considered and solved for the first time by Lucassen for small perturbations [94]. He used the simplified model (5.146) where micelles were assumed to be monodisperse and the micellisation process was regarded as consisting of one step. Later Miller solved numerically the problem of adsorption on a fresh liquid surface using the same assumptions [146], Joos and van Hunsel applied also the same model to the interpretation of dynamic surface tension of... 

Buzza et al. (105) have presented a qualitative discussion of the various dissipative mechanisms that may be involved in the small-strain linear response to oscillatory shear. These include viscous flow in the films. Plateau borders, and dispersed-phase droplets (in the case of emulsions) the intrinsic viscosity of the surfactant monolayers, and diffusion resistance. Marangoni-type and marginal regeneration mechanisms were considered for surfactant transport. They predict that the zero-shear viscosity is usually dominated by the intrinsic dilatational viscosity of the surfactant mono-layers. As in most other studies, the discussion is limited to small-strain oscillations, and the rapid events associated with T1 processes in steady shear are not considered, even though these may be extremely important. 

The emulsification process is so dynamic and complex that an accurate model and theoretical treatment is almost impossible. With certain limitations its is possible to obtain order-of-magnitude estimates of such steps as droplet formation rate and surfactant transport and adsorption rates. However, the work involved is seldom worth the trouble in practice. Flocculation and coagulation rates during preparation are difficult to analyze because of the dynamics of the process and the turbidity of the flow involved. Collision rate theory... 

Experiments demonstrate that intrafacial tension varies with both interfacial composition and temperature. At equilibrium, both these quantities are uniform along an interface and hence so is inteifacial traision. If transport processes (e.g., diffusion of surfactant to a newly created interface) cause interfadal tension to be time dependrait while remaining uniform at each time, changes in intrafacial shape may occur. Such processes may be followed by, for instance, monitoring the dimensions of a sessile or pendant drop as a function of time. 

Okuda, I., McBride, J.F., Gleyzer, S.N., and Miller, C.T., 1996. Physicochemical transport processes affecting the removal of residual DNAPL by nonionic surfactant solutions. Environ. Sci. Technol. 30 1852-1860. 

The increase of the length of sand packs from 1.1 to 4 feet and the decrease of displacement velocity from 2.8 to 1.0 ft/ day during tertiary flooding did not change the oil recovery appreciably. The surfactant recovery, interfacial tension between effluent oil and brine as well as the pressure drop history remained approximately the same indicating that the transport process in porous media was nearly identical for these two cases. 

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