Surface tension method definition

It was made clear in Chapter II that the surface tension is a definite and accurately measurable property of the interface between two liquid phases. Moreover, its value is very rapidly established in pure substances of ordinary viscosity dynamic methods indicate that a normal surface tension is established within a millisecond and probably sooner [1], In this chapter it is thus appropriate to discuss the thermodynamic basis for surface tension and to develop equations for the surface tension of single- and multiple-component systems. We begin with thermodynamics and structure of single-component interfaces and expand our discussion to solutions in Sections III-4 and III-5. 

A solid, by definition, is a portion of matter that is rigid and resists stress. Although the surface of a solid must, in principle, be characterized by surface free energy, it is evident that the usual methods of capillarity are not very useful since they depend on measurements of equilibrium surface properties given by Laplace s equation (Eq. II-7). Since a solid deforms in an elastic manner, its shape will be determined more by its past history than by surface tension forces. 

Surface properties of proteins in general, 296-298 (table) purification methods based on, 272 Surface tension and interfacial properties, 609-628. see also Interfaces Surfactants, see also Interfacial tension definition and adsorption kinetics of, 617-618, 639... 

The total porosity of solids can be estimated by simple pycnometric methods using the differences in surface tension between gases and mercury. By definition, the porosity Pr(%) is given by ... 

Tnteractions at surfaces have long been at the center of interest in the study of surfactant monolayers and have been thought to influence both static and dynamic surface properties considerably (1,2). Although the theoretical interpretation and even the definition of surface interactions may be controversial, the experimental method has not been in doubt. Invariably, the equilibrium surface pressure vs. molar area relationship has been used as a criterion for assessing interactions in mono-layers since interactions, no matter what their precise definition, must appear in the measurable quantity of surface tension (y) or surface pressure (7r = y° — y) at a given surface concentration (r) or molar... 

Eq. (2.18) is the exact definition of the experimental relationship for the determination of surface tension by measuring the corresponding pressure differences and radii of curvature. This relationship is the basis of many experimental surface and interfacial tension methods measuring for example the volume of detaching drops (Section 5.2), the pressure inside bubbles (Section 5.3) or drops (Section 5.5), and the shape of sessile or pendent drops (Section 5.4). 

Using the above mentioned straightforward method the distribution coefficient of a surfactant can be determined by measuring the surface tension of the aqueous solution after equilibration with a definite volume of the oil phase. The following Fig. 4.40 demonstrates that the distribution coefficient K is a function of temperature T, in particular for non-ionic surfactants... 

We recall the lUPAC definition The mechanic properties of the interfacial layer between two fluids, including the equilibrium shape of the surface, may be calculated by applying the standard mathematical techniques of mechanics to the forces associated with the surface of tension. The resulting equations— which comprise the subject of capillarity— form the basis of experimental methods of measuring surface tension. lUPAC Manual of Symbols and Terminology for Physicochemical Quantities and Units, App. II, Part I,... 

One of the most popular and simple methods for making a surface tension measurement is the Wilhelmy plate or rod method. Here, either a flat plate (held vertically) or a simple rod is dipped into the fluid and then retracted slowly. The plate is designed to be very hydrophilic, so the water in the trough will wet the plate and be pulled upward as it is retracted. In this method, calculation of the surface tension is easy as the method is analogous to our previous definition of surface tension by pulling out a film from the bulk (Section 3.3). 

It must be stressed that the CMC value obtained from (4.6) is a function of the contribution factors Oi, 02, and /3j. In other words, the CMC depends on the solution properties employed in the determination and therefore differs with the method used. For this reason, measured CMC values define a narrow concentration range. The CMC values obtained from the solution properties mainly due to a monomeric surfactant contribution are found to be less than those due to a surfactant micelle contribution, as can be seen in Fig. 4.6. In this case, random errors are taken into account for the CMC determination methods. For example, the CMC value obtained from surface tension measurements is less than that obtained from turbidity. In the literature, however, CMCs have often been presented as definite concentra-tions, especially since the appearance of a separation model for micelliz-... 

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