Negative surface tension systems

Negative-surface-tension systems (a-) where surface tension drops as liquid flows down the column. 

Spray regime operation is desirable (321) for negative-surface-tension systems (i.e., where the mixture s surface tension decreases from the top tray toward the bottom tray). Froth regime operation is desirable for positive-surface-tension systems (surface tension increases from the top tray down), and when liquid entrainment needs to be minimized (321). In most commercial applications, vapor and liquid loading requirements override these desirability considerations and dictate the tray flow regime. For optimum tray performance, the tray layout must therefore accommodate the expected flow regime. 

Foaming is unlikely (107) in distillation columns where the bottom product has a lower surface tension than the top product ("negative-surface-tension systems ). Here the mass transfer counteracts the Marangoni effect. Foaming is also imlikely when two liquid phases are present throughout, because one of these phases will tend to act as a defoamer (107). 

Kakiuchi et al. [75] used the capacitance measurements to study the adsorption of dilauroylphosphatidylcholine at the ideally polarized water-nitrobenzene interface, as an alternative approach to the surface tension measurements for the same system [51]. In the potential range, where the aqueous phase had a negative potential with respect to the nitrobenzene phase, the interfacial capacity was found to decrease with the increasing phospholipid concentration in the organic solvent phase (Fig. 11). The saturated mono-layer in the liquid-expanded state was formed at the phospholipid concentration exceeding 20 /amol dm, with an area of 0.73 nm occupied by a single molecule. The adsorption was described by the Frumkin isotherm. 

To resolve the problem of negative /3 values obtained with the Frumkin theory, the improved Szyszkowski-Langmuir models which consider surfactant orientational states and aggregation at the interface have been considered [17]. For one-surfactant system with two orientational states at the interface, we have two balances, i.e., Ft = Fi + F2 and Ftco = Ficoi + F2C02, which can be used in conjunction with Eq. 24 to derive two important equations for determining the total surface excess and averaged molecular area required in the calculation of surface tension, i.e.,... 

The higher the negative deviation from ideality in monolayer formation, the lower the concentration required to attain a given surface tension below the CMC. The higher this deviation for micelle formation, the lower the CMC. Since the CMC is where the surface tension approximately levels out at near a minimum value, the minimum surface tension in such a system represents the relative enhancement of monolayer formation over micelle formation. This relative favorability of aggregate formation is often an important factor in many applications, as will be further discussed in this article. 

The results show that for a mixed solution, varies with the surface tension of solution. This is reasonable because the denseness of surface molecular packing is not the same at different 7 t the lower the O, the denser the packing and the greater the molecular interaction. This is most obvious in syetem of large negative j3,-values and less obvious or even vague in the case of weak interactions (such as in CyFNa C to SNa system). 

Groves (1978) provided an intuitive explanation based on a mechanical model in which water penetrates into the oil/surfactant system, forming liquid crystals but, more to the point, considerably expanding the interface. This is the reason why it is necessary to postulate that water is inconsiderable excess. The surface expands so that instead of a negative interfacial tension what we have is a positive surface pressure. At this point it is not unreasonable to visualize the surface expanding and stranding as postulated in the Gopal model. 

Solution The surface tension of liquid naphthalene (1) is greater than that of solid naphthalene (3). Therefore A312 is expected to be negative for all systems having 7 values greater than 7.,. This is the case for the first six compounds listed in Table 10.6. Therefore these substances are expected to display rejection by the solidification front. This is indeed observed for five of the six cases. The case of nylon-6,12, which deviates from the predicted behavior, is best understood by examining the product (73s - 711/2)(72/2 - y]12)- Values of this product for the various systems considered are listed in Table 10.6. The factor arising from the solid-liquid (3-1) naphthalene has the constant value -0.0186 for all cases, but differs when various solids are used as component 2. For nylon-6,12, the second factor becomes -0.0022, and the product of the two, 0.41 10 4 mJ m 2, is the smallest of all such products listed in the table. As the surface tension difference decreases, the sensitivity of the behavior to variations in d0 increases. ... 

This is the heat per unit area absorbed by the system during an isothermal increase in the surface. Since d y/dT is mostly negative the system usually takes up heat when the surface area is increased. Table 3.1 lists the surface tension, surface entropy, surface enthalpy, and internal surface energy of some liquids at 25°C. 

Rapid spreading is often observed when a liquid with low surface tension is introduced on a liquid with high surface tension. After a certain time in the course of mutual saturation of liquids A and B, the systems approach equilibrium and the positive initial spreading coefficient becomes 0 or negative. So, at se < 0, the excess of liquid B accumulates in a lens. The typical form of the lens is given in Fig. 3.117. At equilibrium this form has been studied in a number of works [e.g. 204]... 

The derivative of the surface tension with respect to temperature at the interface between condensed phases in binary systems can be either positive, or negative, or even change its sign when the temperature changes, which makes it different from the vapor-liquid interface in a one-component system. Within a certain approximation one may assume that in binary systems, as in single-component ones, the value r = -do/dT is the excess of entropy within the discontinuity surface. Consequently, for the interface between condensed phases, the excess of entropy can not only be positive (as it was with singlecomponent systems), but also negative. This situation is especially typical for the interface between two mutually saturated liquid solutions. 

In systems with the upper critical temperature, rcu, the surface tension decreases with increasing temperature, and consequently the excess of entropy within the surface layer is positive (Fig. III-l, a). In systems with the lower critical temperature, TCL, (Fig. III-l, b) the increase in interfacial tension is observed above the point at which system separates into two phases the value of tj is hence negative. The latter may serve as evidence for the existence of strong coorientation between molecules within the interfacial layer, which is due to the presence of directed chemical bonding, such as hydrogen bonding. 

The right-hand side of Eq. (10-60) is the entropy of a unit area of surface minus the entropy of the surface material content of liquid. (3y/5r),(.) is negative for liquid-vapor systems. The surface tension vanishes at the critical point. Equation (10-60) may be rewritten by use of the relation... 

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