Surface tension direct measurement

Ms and Msl are the internal surface energies per unit area for the bare and the immersed surface, respectively. Hence from measuring the heat of immersion one can obtain information about the internal surface energy, but it is not possible to measure the surface tension directly. 

Equilibrium. From a thermodynamic point of view, interfacial tension is an equilibrium parameter. When enlarging an interface at a high velocity, equilibrium distribution and orientation of the molecules in the interface cannot be directly attained, and in order to measure y, the rate of change in interfacial area should be slow and reversible. Nevertheless, when enlarging a liquid surface at conditions that do not allow the establishment of equilibrium, a force can be measured, hence a surface or interfacial tension can be derived, which differs from the equilibrium value. It may be a transient value, but it is also possible that a constant surface tension is measured it then concerns a steady state. In other words, from a mechanical point of view, interfacial tension need not be an equilibrium value. 

A correction factor has to be applied to the measured surface tension according to a method of Harkins and Jordan [44], which has been justified theoretically by Freud and Freud [45]. Table 5 [45] shows the details of measurements for four liquids Tl is the surface tension directly obtained from the force and /l is the corrected value. The correction factor (F) is a function of R V and RJr where R is the radius... 

A Kriiss K6 tensiometer with a platinum du Noiiy ring was used during the surface tension measurements, and the experiments were performed at a temperature of 20 °C. Concentrated surfactant solutions were prepared, and the pH was adjusted with sodium hydroxide or hydrochloric acid. The samples were prepared by dilution with Milli-Q water, buffered to the appropriate pH. The sample volumes were approximately 13 ml and the surface area of the samples were ca 15.5 cm. The surface tension was measured directly after pouring the liquid into the sample vessel. The surface tension value for each sample was multiplied by the appropriate correction factor, according to Harkins and Jordan. [7] The cmc was found at the break point in the surface tension versus concentration plot. 

Equilibrium will occur when the presence of a surface-active agent does not lower Y any further. This concentration is typically at the critical micelle concentration [47]. Modification of fhe confacf angle (and fherefore emulsion sfability) can be achieved by a modification of the aqueous, oil, or solid phase so as to alter Yow, Yos, or Ysw- As described later, this can be achieved with the use of surfactants. The interfacial tension and surface tension are measurements of droplet deformation. Neither Ysw nor Yso can be directly measured, because the solid cannot be deformed. To solve Young s equation and to determine the solid/water and solid/oil interfacial surface tensions, the equation-of-state approach for interfacial surface tensions is required [48] ... 

On liquid mercury, one can obtain the data necessary to use Eq. (9.11) in one of two ways by measuring the surface tension directly, or by measuring the double-layer capacitance as a function of potential and integrating it twice. 

The variation of the integral capacity with E is illustrated in Fig. V-12, as determined both by surface tension and by direct capacitance measurements the agreement confrrms the general correctness of the thermodynamic relationships. The differential capacity C shows a general decrease as E is made more negative but may include maxima and minima the case of nonelectrolytes is mentioned in the next subsection. 

A direct measurement of surface tension is sometimes possible from the work of cleaving a crystal. Mica, in particular, has such a well-defined cleavage plane that it can be split into large sheets of fractional millimeter thickness. Orowan... 

The Wilhelmy hanging plate method (13) has been used for many years to measure interfacial and surface tensions, but with the advent of computer data collection and computer control of dynamic test conditions, its utility has been greatly increased. The dynamic version of the Wilhelmy plate device, in which the liquid phases are in motion relative to a solid phase, has been used in several surface chemistry studies not directly related to the oil industry (14- 16). Fleureau and Dupeyrat (17) have used this technique to study the effects of an electric field on the formation of surfactants at oil/water/rock interfaces. The work presented here is concerned with reservoir wettability. 

The adsorption of organic molecules offers a rich phenomenology. A large number of studies have been performed on mercury electrodes, where the surface tension can be measured directly, and the surface charge and the capacity obtained by differentiation. We will not attempt to survey the literature, but consider a simple example the adsorption of aliphatic compounds. 

The traditional electrochemical techniques are based on the measurement of current and potential, and, in the case of liquid electrodes, of the surface tension. While such measurements can be very precise, they give no direct information on the microscopic structure of the electrochemical interface. In this chapter we treat several methods which can provide such information. None of them is endemic to electrochemistry they are mostly skillful adaptations of techniques developed in other branches of physics and chemistry. 

In place of concentration of reactant or product any physical property, which is directly related with concentration, such as viscosity, surface tension, refractive index, absorbance etc. can be measured for the determination of the rate of reaction. 

There is also the possibility of having surface tension affected directly by the presence of an electrostatic field. To some extent this will be a matter of definition since the outward pressure due to a surface charge could be defined as an apparent effect on surface tension. Hurd, Schmid, and Snavely (H15) measured the surface tension of water and water solutions when fields up to 0.7 V/micron were applied across the air-solution interface. The results showed a reduction in surface tension of less than 1 %. These data must not be considered conclusive, however, because insufficient details are reported to permit assessment of the exact nature of the electrostatic field applied or of the validity of a number of corrections that had to be applied but were reported to be very large and difficult to apply. 

The Gibbs adsorption theory (Birdi, 1989,1999, 2002, 2008 Defay et al., 1966 Chattoraj and Birdi, 1984) considers the surface of liquids to be monolayer. The surface tension of water decreases appreciably on the addition of very small quantities of soaps and detergents. The Gibbs adsorption theory relates the change in surface tension to the change in soap concentration. The experiments that analyze the spread monolayers are also based on one molecular layer. The latter data indeed conclusively verifies the Gibbs assumption (as described later). Detergents (soaps, etc.) and other similar kind of molecules are found to exhibit self-assembly characteristics. The subject related to self-assembly monolayer (SAM) structures will be treated extensively (Birdi, 1999). However, no procedure exists that can provide information by direct measurement. The composition of the surface of a solution with two components or more would require additional comments. 

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