Non-equilibrium interfacial

In contrast to the situation for pure liquids, for which data are sccirce, there is an abundance of information about non-equilibrium interfacial tensions for solutions, particularly for solutions of surface-active substances. One reason is that the latter effect is more mundane and therefore easier to observe. Moreover, it also has practical importance in many industrial processes. Explanations are readily offered, although on closer inspection interpretation of the observed relaxation is not always straightforward. 

Methods for measuring non-equilibrium Interfacial tensions of (surfactant) solutions can be divided into two groups. 

Figure 1.32. Non-equilibrium interfacial tension at the oil-water interface system water + hexane, containing palmitic acid, of which the concentration c is indicated. The drawn curves relate to a model interpretation involving diffusion. (Redrawn from J. van Hunsel, G. Bleys and P. Joos, J. Colloid Interface Set 114 (1986) 432.)...

Brenner ([5], pp. 428-432, and pp. 436-438), Middleman [21] (pp. 39-42) and Edwards et al. ([13], pp. 48-52) address the basic nature of macro-scale interfacial force balances at an arbitrary curved fluid in the state of hydrostatic equilibrium (a state that serves as a standard from which non-equilibrium interfacial transport processes depart). 

There are various direct measurements of micellar solutions giving access to the dynamics rate constants - mainly based on disturbance of the equilibrium state by imposing various types of perturbations, such as stop flow, ultrasound, temperature and pressure jump [14,15[. This aspect is also not further elaborated here we focus instead on the impact of micellar kinetics on interfacial properties, to demonstrate that tensiometry and dilational rheology are suitable methods to probe the impact of micellar dynamics. The first work on this subject was published by Lucassen already in 1975 [16[ and he showed that the presence of micelles in the bulk have a measurable impact on the adsorption kinetics, and hence on the dilational elasticity, when measured by a longitudinal wave damping technique. Subsequent work demonstrated the effect of micellar dynamics on non-equilibrium interfacial properties [17-29]. The physical idea of the impact of micellar dynamics on the dynamic properties of interfacial layers can be easily understood from the scheme given in Figure 13.1. 

By virtue of their simple stnicture, some properties of continuum models can be solved analytically in a mean field approxunation. The phase behaviour interfacial properties and the wetting properties have been explored. The effect of fluctuations is hrvestigated in Monte Carlo simulations as well as non-equilibrium phenomena (e.g., phase separation kinetics). Extensions of this one-order-parameter model are described in the review by Gompper and Schick [76]. A very interesting feature of tiiese models is that effective quantities of the interface—like the interfacial tension and the bending moduli—can be expressed as a fiinctional of the order parameter profiles across an interface [78]. These quantities can then be used as input for an even more coarse-grained description. 

Salvador [100] introduced a non-equilibrium thermodynamic approach taking entropy into account, which is not present in the conventional Gerischer model, formulating a dependence between the charge transfer mechanism at a semiconductor-electrolyte interface under illumination and the physical properties thermodynamically defining the irreversible photoelectrochemical system properties. The force of the resulting photoelectrochemical reactions are described in terms of photocurrent intensity, photoelectochemical activity, and interfacial charge transfer... 

It is possible to find a range in which the electrode potential is changed and no steady state net current flows. An electrode is called ideally polarized when no charge flows accross the interface, regardless of the interfacial potential gradient. In real systems, this situation is observed only in a restricted potential range, either because electronic aceptors or donors in the electrolyte (redox systems) are absent or, even in their presence, when the electrode kinetics are far too slow in that potential range. This represents a non-equilibrium situation since the electrochemical potential of electrons is different in both phases. 

This chapter describes recent work in our laboratories examining density modification of DNAPLs through a combination of batch non-equilibrium rate measurements and DNAPL displacement experiments in 2D aquifer cells. The objective of this work was to evaluate the applicability of nonionic surfactants as a delivery mechanism for introducing hydrophobic alcohols to convert the DNAPL to an LNAPL prior to mobilizing the NAPL. Three different nonionic surfactants were examined in combination with n-butanol and a range of DNAPLs. Overall, it was found that different surfactants can produce dramatically different rates of alcohol partitioning and density modification. However, for some systems interfacial tension reduction was found to be a problem, leading to unwanted downward... 

The second and third terms on the right hands side of Eq. 9.8 remain constant for a given electrode-electrolyte system, and hence the electrode potential is a linear function of the interfacial potential A MIS of the electrode. This definition of the electrode potential holds valid for all electronic and ionic electrodes, whether the electrode reaction is in equilibrium or non-equilibrium. The potential defined by Eq. 9.8 is called the absolute electrode potential. 

It can be considered from the scheme that one has to distinguish between the foam kinetics, i.e. the rate of generation of foam under well defined conditions (air input and mechanical treatment) and the stability and lifetime of a foam once generated. The foam kinetics is also sometimes termed foamability in the literature. These quantities can be related to interfacial parameters such as dynamic surface tension, i.e. the non-equilibrium surface tension of a newly generated surface, interfacial rheology, dynamic surface elasticity and interfacial potential. In the case of the presence of oily droplets (e.g. an antifoam, a... 

Non-equilibrium liquid films formed in the process of spreading have been considered in some early works, especially in the test of the theory of interfacial tension and the rule of Antonov [204], A review on the rule of Antonov and its interpretation on the basis of isotherms of disjoining pressure in wetting films is presented in [532]. However, these works do not deal with precise measurement of film thickness and the studies confined only the kinetics of spreading and lens formation. 

The following five chapters deal with problems associated with solid phases, in some cases involving surface and interfacial problems. In Chapter 14, Steele presents a review of physical adsorption investigated by MD techniques. Jiang and Belak describe in Chapter 15 the simulated behavior of thin films confined between walls under the effect of shear. Chapter 16 contains a review by Benjamin of the MD equilibrium and non-equilibrium simulations applied to the study of chemical reactions at interfaces. Chapter 17 by Alper and Politzer presents simulations of solid copper, and methodological differences of these simulations compared to those in the liquid phase are presented. In Chapter 18 Gelten, van Santen, and Jansen discuss the application of a dynamic Monte Carlo method for the treatment of chemical reactions on surfaces with emphasis on catalysis problems. Khakhar in... 

At equilibrium the thermodynamical and mechanical interpretations should be equivalent. In sec. 1.2.3, in connection with fig. 1.2.1, this equivalence was addressed and found to be achieved provided the extension of the area is done reversibly, that is, at low Deborcih number (De 1). Only under this condition has the interface enough time to come to equilibrium, that is, to achieve full relaxation of all adsorption equilibria. The values of y obtained in this way are the equilibrium values y (eq.), i.e. those tabulated in reference books. When we want to distinguish these from the non-equilibrium, or dynamic, interfacial tensions, we call them the static Interfaclal tension. Under non-equilibrium conditions the mechanical interpretation remains valid, but the thermodynamical one becomes ill-defined. 

In this section we address the measurement of interfacial tensions that are time dependent because the interface is not at equilibrium. Sometimes such tensions are called dynamic surface tensions but we prefer non-equilibrium surface tensions. Their measurement will be discussed in this section, particularly against the background of the techniques described so far. Most of the interpretation (in terms of surface rearrangements, transport to and from interfaces, etc.) and additional monolayer techniques (wave damping, for instance) will be deferred to chapters 3 and 4. 

The results of spinning drop experiments with the equilibrated oil (styrene) and aqueous (1 1 SLS/LA) phases are shown in Figure 5. No "tails" were formed on droplets of the pre-equilibrated styrene when injected into the capillary tube containing the pre-equilibrated aqueous phase. Thus, the formation of an interfacial layer in the spinning drop tensiometer is a non-equilibrium affect. 

Static, equilibration studies indicated that a molecular association forms at the styrene/water interface in the presence of mixed emulsifiers. Interfacial layers were also observed in spinning drop experiments between various oil phases and aqueous mixed emulsifier solutions. The formation of these interfacial layers as a function of time was found to be a non-equilibrium effect that depended primarily on the chemical structure of the oil phase. Oil phase water solubility had little effect. 

There are two types of interfacial potential differences equilibrium and non-equilibrium potentials. (From now on we will use potential as shorthand for potential difference . Potentials of individual phases cannot be measured, but some potential differences can be.) The equilibrium potentials can again be subdivided into two categories electron transfer and ion transfer potentials. The metal/metal ion potentials can be considered as... 

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