Bare surface tension

For short chain crystals such as obtained for n-Alkanes, the eqnilibrinm melting temperature To must be replaced be maximum equilibrium melting temperature corresponding to the finite thickness of the crystals for m = Nch- This takes into account a certain melting point depression due to the bare surface tension of the top and bottom snrface. 

I note that the bare surface tension, a/o, has an empirical meaning only, although it must be still related to the localization of the end-points of the loop. In fact, being constant, [Pg.37]

In (8), Eq = V(JD is the applied electric field across the slit before reduction due to water polarization E E = 0 EJe )). Equations (6) and (7) suggest an expression for apparent contact angle cos0g cosOf + EoDEoIAj. Here, Wei is associated with volume rather than with the surface layer alone. Equation (6) also presumes that bare surface tensions, jab ia, b = s,, v) remain unaffected by the field. While the latter is usually true for the solid-vapor term, the alignment of water molecules in the field can modify molecular interactions at the surface and hence further affect ysi and yi, an effect confirmed by simulations (see Sect. 4.2). 

From Eqs. (43) and (44), the bare surface tension can be expressed in units of the bulk correlation length... 

Mercury forms alloys, called amalgams, with other metals such as gold, silver, zinc, and cadmium. It is not soluble in water, but will dissolve in nitric acid. It has a high electric conductivity, making it useful in the electronics industry. However, unlike most other metals, it is a poor conductor of heat. Because of its high surface tension, it does not wet the surfaces that it touches. This characteristic also accounts for its breakup into tiny droplets when poured over a surface. If spilled, it should not be collected with bare hands, but with a thin piece of cardboard to scoop it up. 

We measure n versus P, solve the integral, and get the difference between the surface tension of the bare solid surface and that of the surface-vapor interface 7s — 7sv-... 

If the air in the tube is evacuated, leaving only the vapour of the liquid, there is usually a slight, barely measurable, increase in the surface tension.4... 

It is a matter of course that the different surfactant coverages are also reflected in the corresponding surface tensions y of the latexes (see Fig. 4b). An increase of the surface tension with increasing diameter is observed. The miniemulsions based on polystyrene particles exceeding 100 nm have a surface tension of close to the one of pure water (72 mN nr1)- This is due to the fact that the bare particle surface is so large that adsorption equilibrium ensures a very low surfactant solution concentration. Smaller particles with their higher sur-... 

A limit of adsorption experiments is that only changes of the surface tension upon adsorption of a substance can be measured. In many practical applications this is not a severe limitation because Ksv- One example is alumina. For y-AbO. McHale et al. measured a value of 1.67 J/m for ys - ysv [871- This is close to the surface tension of bare K-AI2O3, so that 1.67 J/m % ys-... 

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. 

A similar smface tension treatment can be made for the interface between metal oxides and metals. Native oxides typically have lower surface free energies than the bare metal, in turn driving the surface oxidation of most metals. However it is not true that all oxides have lower surface tensions than all metals or semiconductors. For the case of growth of a metal oxide film on a dissimilar metal, or a metal on a supporting metal oxide substrate, the initial phases of growth are determined by the respective surface tensions. These are described by the Young-Dupre equation... 

After the creation of a fresh surface (or an almost bare surface) the adsorption increases until equilibrium has been reached. Liquid flow lateral to the surface creates concentration and surface tension gradients which induce additional surface flow. This effect is called the Marangoni-Gibbs effect. 

An additional benefit of the enhanced coverage and minimal defect density can be seen in Figure 6. Alkaline conditions allow dissolution of Si02, such that even at neutral pH 5% of bare MCM-41 is dissolved after a period of 4 hours. Presumably this is driven by the elevated free energy inherent to high surface area materials. (By analogy, surface tension drives coalescence of small droplets in, say, an aerosol into larger droplets.)... 

Available Experimental Techniques. Examination of Equation 5a shows that in order to evaluate the surface tension of the bare solid, 03, it is necessary to have independent knowledge of three quantities, 7T3., ( i3,and a.3 (i = 1,2). Only the first two of these quantities are experimentally accessible bythe conventional techniques of surface chemistry. 

Upon the establishment of the value of the surface tension for the bare solid, the assumption of negligible film pressures for the other two liquids may be relaxed. Equation 19 is used to obtain values of the solid-liquid tension, 033, and Equation 16b can then be solved for the film pressure, n 2 ... 

Solid-Liquid Interfacial Tensions. For such systems as are of interest in the present discussion, the solid-liquid interfacial tensions, like the surface tensions of the bare solids, cannot be evaluated by the usual techniques of surface chemistry. In the case of the solid-liquid tensions, however, there remains an avenue of approach which has been, as yet, rather ignored by workers in this field the study of crystallization rate phenomena. 

Referring to the form in which Yoimg s equation was finally expressed (Equation 16b), it is seen that with the information at hand it is possible to obtain estimates of the surface tensions of the bare solids. The discussion is restricted to three of the solid surfaces for which data were given in Table I. The data for the glassy fluorocarbon surface have been fully treated by the method of interaction parameters, as given in Table II. The liquids of interest for the three surfaces are those for which solid-liquid interfacial tension estimates are given in Table IV, and, in addition, water. 

Complications such as these extend also to the case of polytetra-fluoroethylene. The large difference in estimated solid-vacuum tensions between this polymer and polyethylene is not imexpected, since a proportionately large difference exists for the liquid surface tensions of hydrocarbons and fluorocarbons having five to eight carbon atoms [58]. The underlying cause of this difference is, however, more obscure. The inter molecular forces for fluorocarbons apparently have features wuich lead to anomalous behavior, at least from the point of view of solubility parameter theory [59]. Thus, theoretical calculations of the surface tension for the bare solid in the case of polytetrafluoroethylene would face a number of difficulties not encountered with paraffin crystals. 

Surface tensions for the bare solids are estimated to be polytetrafluoroethylene, 34 paraffin, 45 and polyethylene, 55 dynes per cm. 

In the spray application of thin coatings it is important that the droplets spread quickly to cover bare islands on the surface the results of this study point the way to formulations in which a high spreading rate is assured by the choice of solvents of suitable volatility and surface tension. Similar considerations apply to the spraying of disinfectants and insecticides. 

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