Kinetic between immiscible phases

This description of the dynamics of solute equilibrium is oversimplified, but is sufficiently accurate for the reader to understand the basic principles of solute distribution between two phases. For a more detailed explanation of dynamic equilibrium between immiscible phases the reader is referred to the kinetic theory of gases and liquids. 

This overview sets out the rationale for, and potential benefits of, studies of the kinetics of partitioning of compounds between immiscible phases. 

A detailed analysis of this behavior, as well as its analogy to the mercury-KF solution system, can be found in several papers [1-3,8,14]. The ions of both electrolytes, existing in the system of Scheme 13, are practically present only in one of the phases, respectively. This allows them to function as supporting electrolytes in both solvents. Hence, the above system is necessary to study electrical double layer structure, zero-charge potentials and the kinetics of ion and electron reactions at interface between immiscible electrolyte solutions. 

The theoretical results described have implications for the design of experimental approaches for the study of transfer processes across the interface between two immiscible phases. The current response in SECMIT is clearly sensitive to the relative diffusion coefficients and concentrations of a solute in the two phases and the kinetics of interfacial transfer over a wide range of values of these parameters. 

Goldel et al. [77,78] investigated the kinetics of transfer of CNT between immiscible co-continuous phases during melt mixing. They demonstrated that if complete transfer of MWCNTs from San to PC takes 5 min in a discontinuous micro-compounder, it takes only 60-90 s in a continuous twin-screw extruder. The pre-mix of CNTs in one of the two polymer phases can be a determining factor for the final dispersion of CNTs in the blend. Moreover, the final morphology may be inconsistent with the thermodynamic predictions. Indeed, the first polymer that is in contact with the CNTs is thus adsorbed in a partially irreversible way as described by Baudouin et al. [79,80] and this adsorption can be responsible for the final blend morphology. 

Goldel A, Kasaliwal GR, Potschke P, Heinrich G. The kinetics of CNT transfer between immiscible blend phases during melt mixing. Polymer 2012 53 411-21. 

Some of the earliest attempts to address the difficulties associated with making kinetic measurements at immiscible liquid-liquid interfaces were made by Lewis [16,17] using the stirred cell design illustrated in Fig. 2. The Lewis cell employs direct contact between the two immiscible liquids, and reaction rates are evaluated by measuring concentration changes in the bulk of one of the two phases, usually by a batch extraction technique. The rate of change of concentration, dc/dt, is related to the interfacial reaction flux, 7, by... 

Potential differences at the interface between two immiscible electrolyte solutions (ITIES) are typical Galvani potential differences and cannot be measured directly. However, their existence follows from the properties of the electrical double layer at the ITIES (Section 4.5.3) and from the kinetics of charge transfer across the ITIES (Section 5.3.2). By means of potential differences at the ITIES or at the aqueous electrolyte-solid electrolyte phase boundary (Eq. 3.1.23), the phenomena occurring at the membranes of ion-selective electrodes (Section 6.3) can be explained. 

It follows from study of the kinetics of transfer of ions across the phase boundary between two immiscible electrolyte solutions (see chapter 9) that ion-exchanger ions, where the ion is as nearly as possible symmetrically surrounded by hydrophobic groups on all sides, are especially suitable. Amphiphilic (amphipathic) substances, in whose molecules the hydrophobic part is separated from the hydrophilic part, are less suitable because they have a tendency to become adsorbed on the membrane/water phase boundary, thus retarding ion transfer across this boundary. 

Figure 4. Charge-transfer processes at the liquid-liquid interface, (a) Probing ET at the liquid-liquid interface with the SECM. The kinetics of ET between two redox couples confined to different immiscible liquid phases can be measured with the SECM operating in the conventional feedback mode. Electroneutrality is maintained by transfer of the common ion (shown as an anion) across the interface (IT). Adapted with permission from Ref. [38]. Copyright 1995, American Chemical Society, (b) Schematic diagram of facilitated ion transfer reaction studied by SECM.

The use of copolymers as surfactants is widespread in macromolecular chemistry in order to compatibilize immiscible blends. These additives are sometimes named surfactants , interfacial agents or more usually compatibi-lizers . Their effect on improving different properties is observed interfacial tension and domain size decrease, while there is an increase in adhesion between the two phases and a post-mixing morphology stabilization (coalescence prevention). The aim of the addition of such copolymers is to obtain thermodynamically stable blends, but the influence of kinetic parameters has to be kept in mind as long as they have to be mastered to reach the equilibrium. Introducing a copolymer can be achieved either by addition of a pre-synthesized copolymer or by in-situ surfactant synthesis via a fitted re-... 

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