Liquid-vapor interface, surface tension

Specific surface energy or surface tension of a solid Specific free energy of an interface between UC and U vapor Specific energy or tension of a liquid-solid interface Surface tension of a solid in a foreign vapor Surface tension of a grain boundary... 

Harkins and Jura [20] describe a method to obtain the absolute surface area of a solid by the following method. Firstly, the powder is exposed to a high vapor pressure of water. Indeed it is best to expose it in a high-sensitivity calorimeter over a reservoir of water. The powder is then allowed to fall into the reservoir and the amount of heat produced is measured. By doing so, one eliminates the outer surface of the adsorbed film releasing the energy associated with the liquid-gas interface surface tension. Since the liquid-gas surface tension energy is known one may then calculate from the amount of heat released the area of the powder (or at least the outer surface area of the adsorbed film before immersion). 

Whereas the surface tensions refer to the liquid-vapor interface, interfacial tension is defined as surface free energy per unit area between two immiscible liquids. It has the same unit as the surface tension, and it results from... 

A case can be made for the usefulness of surface tension as a concept even in the case of a normal liquid-vapor interface. A discussion of this appears in papers by Brown [33] and Gurney [34]. The informal practice of using surface tension and surface free energy interchangeably will be followed in this text. 

The temperature distribution has a characteristic maximum within the liquid domain, which is located in the vicinity of the evaporation front. Such a maximum results from two opposite factors (1) heat transfer from the hot wall to the liquid, and (2) heat removal due to the liquid evaporation at the evaporation front. The pressure drops monotonically in both domains and there is a pressure jump at the evaporation front due to the surface tension and phase change effect on the liquid-vapor interface. 

At the interface the mass and thermal balance equations are valid. If one assumes that the liquid-vapor interface curvature is constant, accordingly (7)3-71)1111 = c/T men, Where Pq and Pl are the vapor and liquid pressure at the interface, a is the surface tension, and/ men is the meniscus radius. 

This measured concentration change is usually converted into a surface excess quantity, analogous to that usually calculable from surface tension data for adsorption at the liquid/vapor interface. In the case of... 

In nucleate boiling, bubbles are created by the expansion of entrapped gas or vapor at small cavities in the surface. The bubbles grow to a certain size, depending on the surface tension at the liquid-vapor interface and the temperature and pressure. Depending on the temperature excess, the bubbles may collapse on the surface, may expand and detach from the surface to be dissipated in the body of the liquid, or at sufficiently high temperatures may rise... 

With a liquid-vapor interface, Gibbs [36] has developed a thermodynamic treatment of the variation of surface tension with composition. This derivation comes from the book Physical Chemistry of Surfaces by Adamson [2, p. 340]. This derivation sets the stage for adsorption at the solid—liquid interface, which will be discussed next. 

For the liquid vapor interface, the surface tension is easUy measured as a function of the concentration as shown in Figure 9.10. The preceding equation can he used to determine the surface excess concentration of surfactant as a function of the sur ctant concentration if the sur ce tension of the solution as a fimction of surfactant concentration is known. For dilute aqueous solutions of organic substrates, the semi-empirical equation for the surface tension, y, of a solution of concentration C2,... 

Bubbles owe their existence to the surface-tension a at the liquid—vapor interface due to the attraction force on molecules at the interface toward the liquid phase. The surface teusion decreases with increasing temperature and becomes zero at the critical temperature. This explains why no bubbles are formed during boihng at supercritical pressures and temperatures. Surface tension has the unit N/m. 

Surface tension of liquid-vapor interface for vrater... 

T = surface tension of liquid-vapor interface, N/m Cfi = specific heat of the liquid, J/kg "C 7) = surface temperature of tile heater, °C = saturation temperature of die fluid, C C,/ — experimental constant that depends on surface-fluid combination Pri = Prandll number of the liquid II = experimental constant that depends on the fluid... 

The above techniques have been used in numerous calculations of solute free energy profiles. Wilson and Pohorille [52] and Benjamin[53] have determined the free energy profiles for small ions at the water liquid/vapor interface and compared the results to predictions of continuum electrostatic models. The transfer of small ions to the interface involves a monotonic increase in the free energy which is in qualitative agreement with the continuum model. This behavior is consistent with the increase in the surface tension of water with the increase in the concentration of a very dilute salt solution, and it represents the fact that small ions are repelled from the liquid/vapor interface. On the other hand, calculations of the free energy profile at the water liquid/vapor interface of hydrophobic molecules, such as phenol[54] and pentyl phenol[57] and even molecules such as ethanol [58], show that these molecules are attracted to the surface region and lower the surface tension of water. In addition, the adsorption free energy of solutes at liquid/liquid interfaces[59,60] and at water/metal interfaces[61-64] have been reported. 

The origin of the excess stress on liquid/vapor interfaces follows from the tendency of the liquid surface to contract. As a molecule inside a mass of liquid is under the effect of the forces of the surrounding molecules, while a molecule on the surface is only partly surrounded by other molecules, some work is necessary to bring molecules from the inside to the surface. This indicates that the force must be applied along the surface in order to increase the area of the surface. This force on the surface appears as excess stress (a difference between normal and transverse components of pressure tensor in the region of the interface) and defines the surface tension of the liquid. Excess stress a, for liquid/vapor interfaces, is always a positive quantity and is equal to the interfacial free energy. 

Liquid Mixtures Compositions at the liquid-vapor interface are not the same as in the bulk liquid, and so simple (bulk) composition-weighted averages of the pure-fluid values do not provide quantitative estimates of the surface tension at the vapor-liquid interface of a mixture. The behavior of aqueous mixtures is more difficult to correlate and estimate than that of nonpolar mixtures because small amounts of organic material can have a pronounced effect upon the surface concentrations and the resultant surface tension. These effects are usually modeled with thermodynamic methods that account for the activity coefficients. For example, a UNIFAC method [Suarez, J. T. C. Torres-Marchal, and P. Rasmussen, Chem. Eng. Set, 44 (1989) 782] is recommended and illustrated in PGL5. For nonaqueous systems the extension of the parachor method, used above for pure fluids, is a simple and reasonably effective method for estimating a for mixtures. 

Combined measurements of the surface tension, concentration gradient, and mass transfer coefficients on the liquid-vapor interface (P < Pm) and also measurements of the solution refractive index in the vapor or liquid phase (9,1 0). 

In other words, snrface tension may be considered to arise because of a degree of unsaturation of bonds that occurs when a molecnle resides at the surface and not in the bnlk. The term surface tension is used for solid-vapor or liquid-vapor interfaces. The term interfacial tension is more generally used for the interface between two liqnids, two solids, or a liqnid and a solid. 

In a recent study, a new model of fluids was described by using the generalized van der Waals theory. Actually, van der Waals over 100 years ago suggested that the structure and thermodynamic properties of simple fluids could be interpreted in terms of neatly separate contributions from intermolecular repulsions and attractions. A simple cubic equation of state was described for the estimation of the surface tension. The fluid was characterized by the Lennard-Jones (12-6) potential. In a recent study the dependence of surface tension of liquids on the curvature of the liquid-vapor interface has been described. ... 

We simply have to take into account that three interfacial tensions compete / snvt for the solid-vapor interface, f t for the liquid-vapor interface, and /j 1, for the solid-liquid interface. We have surface melting if /im - (/ini + /,ni) = A/ > 0. The effective free energy replacing eq. (263) is, for short range forces, being the melting temperature... 

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