Interfacial parameters, importance

Key Concepts of Interfacial Properties in Food Chemistry CASE STUDY LIPID OXIDATION OF EMULSIONS The case of lipid oxidation in an emulsified system is a perfect example to illustrate the importance of interfacial properties in food chemistry. The goal of this case study is not to completely describe the very complex mechanisms of lipid oxidation in emulsions. Indeed, many investigators over the past years have focused on this research area. Instead, the key interfacial parameters that influence lipid oxidation in emulsions are emphasized. 

The overall interfacial area for the whole reactor can be determined by chemical techniques. These techniques, however, must be used with restrictions. For example, chemical methods are difficult to use for fast-coalescing systems, since the presence of a chemical compound may reduce coalescence rates. Furthermore, in fast-coalescing systems, the specific area may depend strongly on the position in the reactor, which complicates the interpretation of an average value obtained with chemical methods. Indeed, both physical and chemical techniques should be used together to describe the phenomena that occur within gas-liquid reactors. While chemical methods allow the determination of the much-needed average interfacial area, information on the variations of the interfacial parameters, such as aL and dsv, within the reactor, which is important for scale-up, cannot be obtained by this method. 

The most important achievement of this research has been the finding that the addition of some surfactants to the binary liquid-liquid systems investigated can induce or increase interfacial convection that enhances greatly the initial mass transfer rates in comparison with values predicted by Pick s law. This shows that surfactants can be used as a means to manipulate the interfacial region to obtain variations in interfacial parameters, which induce interfacial convection and produce an... 

Pressure drop and liquid holdup are very important parameters, indispensable for the design of trickle-bed reactor (6, 7, 13, 16, 17, 19, 23, 27, 29, 30, 32, 42). Their values influence directly the interfacial parameters between the fluid phases and between liquid and solid pliases too. Many workers have proposed different correlations for predicting the two-phase pressure drop in co-current downward flow through packed beds (6, 17, 19, 23,... 

The mixing of the two phases proved to be an important parameter for the reaction rate. An increased efficiency of the mixing resulted in an increase in the reaction rate but did not change the dimer selectivity. Elsewhere, batch laboratory experiments showed that no reaction occurred in the organic phase. This could indicate the possibility of the participation of an interfacial reaction. 

The aspect of sample preparation and characterization is usually hidden in the smallprint of articles and many details are often not mentioned at all. It is, however, a very crucial point, especially with surface and interface investigations since there might be many unknown parameters with respect to surface contaminations, surface conformations, built-in stresses, lateral sample inhomogeneities, roughness, interfacial contact etc. This is in particular important when surfaces and interfaces are investigated on a molecular scale where those effects may be quite pronounced. Thus special care has to be taken to prepare well defined and artifact free specimens, which is of course not always simple to check. Many of these points are areas of... 

The investigations of interfacial phenomena of immiscible electrolyte solutions are very important from the theoretical point of view. They provide convenient approaches to the determination of various physciochemical parameters, such as transfer and solvation energy of ions, partition and diffusion coefficients, as well as interfacial potentials [1-7,12-17]. Of course, it should be remembered that at equilibrium, either in the presence or absence of an electrolyte, the solvents forming the discussed system are saturated in each other. Therefore, these two phases, in a sense, constitute two mixed solvents. 

It was shown later that a mass transfer rate sufficiently high to measure the rate constant of potassium transfer [reaction (10a)] under steady-state conditions can be obtained using nanometer-sized pipettes (r < 250 nm) [8a]. Assuming uniform accessibility of the ITIES, the standard rate constant (k°) and transfer coefficient (a) were found by fitting the experimental data to Eq. (7) (Fig. 8). (Alternatively, the kinetic parameters of the interfacial reaction can be evaluated by the three-point method, i.e., the half-wave potential, iii/2, and two quartile potentials, and ii3/4 [8a,27].) A number of voltam-mograms obtained at 5-250 nm pipettes yielded similar values of kinetic parameters, = 1.3 0.6 cm/s, and a = 0.4 0.1. Importantly, no apparent correlation was found between the measured rate constant and the pipette size. The mass transfer coefficient for a 10 nm-radius pipette is > 10 cm/s (assuming D = 10 cm /s). Thus the upper limit for the determinable heterogeneous rate constant is at least 50 cm/s. 

The investigation of the chemical modification of dextran to determine the importance of various reaction parameters that may eventually allow the controlled synthesis of dextran-modified materials has began. The initial parameter chosen was reactant molar ratio, since this reaction variable has previously been found to greatly influence other interfacial condensations. Phase transfer catalysts, PTC s, have been successfully employed in the synthesis of various metal-containing polyethers and polyamines (for instance 26). Thus, the effect of various PTC s was also studied as a function of reactant molar ratio. 

Regime-IV flow patterns are of pragmatic interest when interphase heat and mass transfer are of key importance because the existence of the discrete phase generates a large interfacial area per unit tube volume. Evaluation of the interfacial area is made difficult because the bubbles or drops of the discrete phase are usually not of uniform size or shape. By assuming a characteristic size and shape for the drops or bubbles, the interfacial area and the other parameters can be estimated with reasonable accuracy for many situations. 

Our goal is to develop a property-performance relationship for different types of demulsifiers. The important interfacial properties governing water-in-oil emulsion stability are shear viscosity, dynamic tension and dilational elasticity. We have studied the relative importance of these parameters in demulsification. In this paper, some of the results of our study are presented. In particular, we have found that to be effective, a demulsifier must lower the dynamic interfacial tension gradient and its ability to do so depends on the rate of unclustering of the ethylene oxide groups at the oil-water interface. 

In the macrocomposite model it is assumed that the load transfer between the rod and the matrix is brought about by shear stresses in the matrix-fibre interface [35]. When the interfacial shear stress exceeds a critical value r0, the rod debonds from the matrix and the composite fails under tension. The important parameters in this model are the aspect ratio of the rod, the ratio between the shear modulus of the matrix and the tensile modulus of the rod, the volume fraction of rods, and the critical shear stress. As the chains are assumed to have an infinite tensile strength, the tensile fracture of the fibres is not caused by the breaking of the chains, but only by exceeding a critical shear stress. Furthermore, it should be realised that the theory is approximate, because the stress transfer across the chain ends and the stress concentrations are neglected. These effects will be unimportant for an aspect ratio of the rod Lld> 10 [35]. 

One approach to the study of solubility is to evaluate the time dependence of the solubilization process, such as is conducted in the dissolution testing of dosage forms [70], In this work, the amount of drug substance that becomes dissolved per unit time under standard conditions is followed. Within the accepted model for pharmaceutical dissolution, the rate-limiting step is the transport of solute away from the interfacial layer at the dissolving solid into the bulk solution. To measure the intrinsic dissolution rate of a drug, the compound is normally compressed into a special die to a condition of zero porosity. The system is immersed into the solvent reservoir, and the concentration monitored as a function of time. Use of this procedure yields a dissolution rate parameter that is intrinsic to the compound under study and that is considered an important parameter in the preformulation process. A critical evaluation of the intrinsic dissolution methodology and interpretation is available [71]. 

The phenomena at the liquid/liquid interface are of outstanding importance for the removal of oily soil from the surface. The interfacial tension is one of the decisive parameters in the rolling-up process. This parameter vary considerably, de-... 

It is important to point out thatEq. (44) is a nonquadratic form of the rate equation and gives rise to linear Tafel plots the slopes of these plots depend strongly on the interfacial field parameter p (see Fig. 16). The... 

This parameter helps distinguishing the relative importance of interfacial kinetics and bulk transport. For LpEM < Tpem water transport through the PEM is dominated by interfacial water exchange, whereas for LpEM > bulk permeation of water prevails. The data obtained in Monroe et al. yield Lpem -100-300 im. This indicates that the interfacial vaporization resistance exceeds the resistance due to bulk transport in the membrane when the membrane thickness is LpEM < 100 im. 

On the other hand, the merits of such insights are obvious. It would become possible to evaluate the relative importance of surface and bulk mechanisms of PT. The transition from high to low proton mobility upon dehydration could be related to molecular parameters that are variable in chemical synthesis. It could become feasible to determine conditions for which high rates of interfacial PT could be attained with a minimal amount of hghtly bound water. As an outcome of great practical value, this understanding could direct the design of membranes that operate well at minimal hydration and T > 100°C. 

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