Transport across liquid surfactant membranes

Actinides, in nitric acid waste, 182 Advancing-front model, ELMs, 18,68 Alkali metal cations transport across bulk liquid membranes, 89-92 transport across liquid surfactant membranes, 93-95 transport across polymer-supported liquid, 95-96... 

An idealized schematic diagram of alkali metal cation transport across a liquid surfactant (emulsion) membrane by an lonizable crown ether is shown in Figure 7. Thus a metal cation is transported from an external aqueous source phase across the liquid surfactant membrane which forms the outer surface of the emulsion droplet into an interior aqueous receiving phase. Metal ion transport is driven by a pH gradient and back transport of protons from the internal to the external aqueous solution according to the mechanism illustrated earlier in Figure 1. In this system, transport is rapid due to the thin organic membrane. 

Figure 8. Influence of Polyether Ring Size Upon Competitive Transport of Alkali Metal Cations Across a Liquid Surfactant Membrane by (a) (b) 2, and (c) 3. (Adapted with permission...

The concept of emulsion liquid membranes (ELM) was first proposed by Li in 1968 [1]. Since their inception in the late 1960s they have been referred to as surfactant liquid membranes, double emulsion membranes or ELM. Regardless of the terminology used, the workings of such systems are as follows they consist of an emulsion formed by an organic solvent and water, which can be stabilized by the addition of surfactant. This emulsion is then contacted with a continuous phase containing the desired solute, stirred to yield globules, and transported across the extremely thin membrane layer that separates internal phase droplets... 

In the present paper, we examine the influence of structural variation within series of crown ether carboxylic acid and crown ether phosphonic acid monoalkyl ester carriers upon the selectivity and efficiency of alkali metal transport across three types of liquid organic membranes. Structural variations within the carriers include the polyether ring size, the lipophilic group attachment site and the basicity of ethereal oxygens. The three membrane types are bulk liquid membranes, liquid surfactant (emulsion) membranes and polymer-supported liquid membranes. 

Figure 7. Schematic Diagram of Proton-coupled Transport Across a Liquid Surfactant (Emulsion) Membrane.

Fluctuations of interfaces are directly relevant to a number of interfacial phenomena. One example, ion transfer across a liquid-liquid interface, will be discussed in Section 6.1. Another example is the behavior of monolayers of surfactants on water surfaces. Surface fluctuations are also fundamental to several processes in water-membrane systems, such as unassisted ion transport across lipid bilayers and the hydration forces acting between two membranes. Here, however, the problem is more complicated because not only capillary waves but also bending motions of the whole bilayer have to be taken into account. Furthermore, the concept of the surface tension is less clear in this case. This topic is discussed in Molecular Dynamics Studies of Lipid Bilayers. 

The biological efficiency of an insoluble pesticide will be significantly affected by the presence of micelles, due in part to improved solution of the a.i. In addition, surfactant micelles influence the rate of dissolution of a solute as well as its membrane permeability. The driving force for transportation across a membrane may involve dissolution, diffusion, or convection in bulk liquid and crossing of the membrane. Diffusion in bulk liquid obeys Pick s law ... 

Emulsion liquid membranes (ELMs), first invented by Li [2], are made by forming a surfactant-stabilized emulsion between two immiscible phases. The solute (i.e, the chemical species that is desired) is selectively transported from a feed phase across a thin liquid film... 

Apart from the above effect on dissolution rate, surfactant micelles also affect the membrane permeability of the solute [8j. Solubilization can, under certain circumstances, help the transport of an insoluble chemical across a membrane. The driving force for transporting the substance through an aqueous system is always the difference in its cdiemical potential (or to a first approximation the difference in its relative saturation) between the starting point and its destination. The principal steps involved are dissolution, diffusion or convection in bulk liquid and crossing of a membrane. As mentioned above, solubilization will enhance the diffusion rate by affecting transport away from the boundary layer adjacent to the crystal [8j. However, to enhance transport the solution should remain saturated, i.e. excess solid particles must be present since an unsaturated solution has a lower activity. 

Emulsion Liquid Membranes. The emulsion liquid membrane (ELM) was developed by U and Cussler in the late 1960 s and early 1970 s (65-66), A water-in-oil emulsion is formed by an organic solvent and water, often containing acid. The emulsion can be stabilized by the addition of a surfactant. This emulsion is then stirred into an aqueous source solution, and transport occurs across the extremely thin membrane bubbles (Figure 8b). When extraction is complete the emulsion is collected and broken to obtain the concentrated target substance. 

---