Surface-tension value experiments

For ionic solids interacting with Coulomb pair potentials, similar calculations can be carried out. However, this is a rather complex matter because Coulomb, van der Waals attraction and Pauli repulsion should all be taken into account. In addition, there are uncertainties in the choice of suitable pair-potential equation (many inter-atomic potential equations, including Lennard-Iones were tried), and the calculated Gf results are highly dependent on the particular choice of pair-potential model. As an example, Gf = 212m) m 2 was calculated theoretically for the NaCl (100) crystal, which is near to the experimental value of Gf = 190 m) m 2 from extrapolation of the molten salt surface tension values, but far away from Gf = 300 mj m 2, which was found from crystal cleavage experiments. 

Similarly to the oxyethylated alcohols, the adsorption behaviour of Tritons agrees better with the two-state model. It is seen from the analysis of the experimental results reported in [62] that only in the high concentration range (dashed line in Fig. 3.37) the results are consistent with those obtained for the Tritons with other degrees of oxethylation, cf Tables 3.14 and 3.15. Higher surface tension values exhibited by Triton X-165 for the concentration below 5-10 mol/1 can be possibly ascribed to the lack of adsorption equilibrium in the experiments [62]. Similar results were obtained in [62] also for Triton X-100 (not shown in Fig. 3.37). We do not believe that the data listed in Table 3.15 are sufficient to make unambiguous... 

Each separate experimental determination of the surface-tension value took hours. It had to be done at night, when the lab was unoccupied, because the jets were easily disturbed by vibration. Slow work, but Bohr also dawdled. The academy had allowed two years. Toward the end of that time Christian Bohr realized his son was procrastinating to the point where he might not finish his paper before the deadUne. The experiments had no end, Bohr told Rosenfeld some years later on a bicycle ride in the country I always noticed new details that I thought I had first to understand. At... 

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. 

The film pressure is defined as the difference between the surface tension of the pure fluid and that of the film-covered surface. While any method of surface tension measurement can be used, most of the methods of capillarity are, for one reason or another, ill-suited for work with film-covered surfaces with the principal exceptions of the Wilhelmy slide method (Section II-6) and the pendant drop experiment (Section II-7). Both approaches work very well with fluid films and are capable of measuring low values of pressure with similar precision of 0.01 dyn/cm. In addition, the film balance, considerably updated since Langmuir s design (see Section III-7) is a popular approach to measurement of V. 

Since capillary tubing is involved in osmotic experiments, there are several points pertaining to this feature that should be noted. First, tubes that are carefully matched in diameter should be used so that no correction for surface tension effects need be considered. Next it should be appreciated that an equilibrium osmotic pressure can develop in a capillary tube with a minimum flow of solvent, and therefore the measured value of II applies to the solution as prepared. The pressure, of course, is independent of the cross-sectional area of the liquid column, but if too much solvent transfer were involved, then the effects of dilution would also have to be considered. Now let us examine the practical units that are used to express the concentration of solutions in these experiments. 

Changes in the shape of the absorption spectrum correspond very well with micelle formation. The ratio of absorbance at 550 nm to that at 500 nm(both are absorptions of merocyanine) is constant below the CMC whereas the value increases continuously with concentration above CMC. This indicates that the merocyanine is a sensitive probe to detect micelle formation. During the photoirradiation experiment shown in Figure 2, the ratio of absorbance started to increase at the A /Aq value where the surface tension showed a sudden drop. 

The quoted authors (D9) collected data on bubble volumes in water, aqueous glycerol, and petroleum ether. They have used Eq. (6) for verifying bubble volumes obtained for flow rates up to 3 cm3/sec. They find that theory and experiment agree excellently only in the flow range of 1.5 to 3.0 cm3/sec and not below 1.5 cm3/sec. This discrepancy has been qualitatively explained by them on the basis of surface tension effects, but there is no quantitative explanation. Although the equation has not been verified from 3 to 15 cm3/sec, the authors feel that it would be applicable. Beyond 20 cm3/sec, the experimental values have been compared with those obtained by using Eqs. (8)—(9) and considerable deviation has been observed. 

There is a corresponding paucity of experimental determinations of the surface tension of solids, probably because no direct experimental method has been developed. A review of the work on the surface tension of solid metals has been given by Shaler 27). These values were obtained, in most cases, near the melting point of the metals and thermodynamic equilibrium was achieved. These experiments are thus quite different from those where the nonequilibrium state persists, with incomplete relief of surface stress. As this review is mainly concerned with high surface area adsorbents in a state of considerable surface stress in vacuo at least), the above results with metals will not concern us further. 

Magnesium oxide crystals about 500 A. in diameter were prepared in vacuo by Nicolson 26). Lattice determinations by X-rays showed that the parameter of these small crystals was smaller than that of large crystals. The surface tension obtained from these experiments (- -3,020 dynes/cm.) was 46% of the theoretical value. Similar experiments were carried out with sodium chloride crystals made in vacuo (size about 2000 A), and the agreement between experiment and theory was better, the observed surface tension (- -390 dynes/cm.) being 70% of that calculated. 

The magnitude of ycr for Teflon of 18 mN/m thus suggests that -CF2- groups exhibit this low surface tension. The value of ycr for -CH2-CH3- alkyl chains gave a higher value of 22 mN/m than for Teflon. Indeed, from experience, one also finds that Teflon is a better water-repellant surface than any other material. The magnitudes of ycr for different surfaces are seen to provide much useful information (Table 5.4). 

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