Solution surface tension

Because this is less than 1 mN/m, extremely low oil-surfactant solution surface tensions are necessary. 

The difference between hydrophobicities of the hydroxyethyl (HE) grouping and the hydroxypropyl (HP) unit is evident in the relative aqueous solution surface tensions(7,8) of the two cellulosic polymers in these comparative references the M.S. of the products is not equal. The dramatic influence of the more hydrophobic HP groupings on surface pressures is illustrated in Figure 2. 

Surfactants are compounds that exhibit surface activity, or more generally, interfacial activity, and migrate to the interface when placed in solution. This migration results in lowering the solution surface tension (interfacial tension) as compared to the surface tension of the pure solvent. Thermodynamically, adsorption of a surfactant is deLned by the Gibbs adsorption equation ... 

For liquids of low viscosity, a useful measurement technique is the tensiometer, schematically represented in Fig. 2.56. Here, the surface tension is related to the force it takes to pull a platinum ring from a solution. Surface tension for selected polymers are listed in Table 2.12 [71 ], for some solvents in Table 2.13 [58] and between polymer-polymer systems in Table 2.14 [71],... 

With type 1 solutes, surface tension in aqueous solution mildly increases with concentration. Because activities generally increase with concentration, from Eq. (50), these solutes have a negative surface excess concentration (i.e., they are depleted in the surface layer). Inorganic electrolytes show this behavior. In the bulk solution, these ions are stabilized by interacting with the extended ionic environment of the solution. In the surface layer, this environment is limited in extent in one direction. 

A simple and largely applied method for foam formation is dispersion of gas through porous plates (filters) placed at the lower parts of foam generation apparatus [5-10], This method is employed in flotation, in gas adsorption and dust collection in set-up with turbulent gas emulsion, and in the equipment for foam separation. The dispersity of a foam thus obtained depends on filter pore size or capillary diameter, hydrophility of the material used in the dispersion device construction, physicochemical properties of the foaming solution (surface tension, viscosity, surfactant concentration, etc.) and conditions of the dispersion process. 

Equalising Eqs. (1.32) and (1.33) and accounting for Eq. (1.34), an expression is derived which indicates that excess (capillary) pressure in bubbles is determined by both specific foam surface area and solution surface tension but does not depend on the shape of films and bubbles... 

Table IX Aqueous-Solution Surface Tension (ff soin) of Fluorosilicone Surfactants...

Generalizations from the aqueous-solution surface tensions in Table IX are risky, because values are as dependent on the hydrophilic-lipophilic balance (HLB) as on the intrinsic surface activity of the hydrophobe. The data in Table IX are consistent with earlier observations that longer per-fluorinated groups are most effective in producing low surface tensions (in this case CF3(CF2)5-) and that a terminal CF2H- is detrimental. 

Surfactant surface activity is most completely presented in the form of the Gibbs adsorption isotherm, the plot of solution surface tension versus the logarithm of surfactant concentration. For many pure surfactants, the critical micelle concentration (CMC) defines the limit above which surface tension does not change with concentration, because at this stage, the surface is saturated with surfactant molecules. The CMC is a measure of surfactant efficiency, and the surface tension at or above the CMC (the low-surface-tension plateau) is an index of surfactant effectiveness (Table XIII). A surfactant concentration of 1% was chosen where possible from these various dissimilar studies to ensure a surface tension value above the CMC. Surfactants with hydrophobes based on methylsiloxanes can achieve a low surface tension plateau for aqueous solutions of —21-22 mN/m. There is ample confirmation of this fact in the literature (86, 87). 

Attenuated total reflection infrared critical micelle concentration electron spectroscopy for chemical analysis hydrophilic-lipophilic balance poly(chlorotrifluoroethylene) poly(dimethylsiloxane) poly(tetrafluoroethylene) poly(trifluoropropylmethylsiloxane) glass transition temperature critical surface tension of wetting Owens-Wendt solid surface tension surface tension of aqueous solution surface tension of liquid... 

By contrast, Gibbs monolayers are continually at equilibrium with the subphase which, in this case, may be called a solution. Surface tension and concentration are related through Gibbs law, so that the surface pressure can be related to the solution concentration. By analogy with [II. 1.1.6 and 7J we have for a single adsorbed component in an ideal solution... 

The prevalent explanation for isopropanol s role in this kind of dampening system is that its surface tension, about 29 dynes/cm, sufficiently lowers the aqueous fountain solution surface tension to allow wetting of the inked form-roller by that solution, as illustrated in Figure 1. That is, the fountain solution wets and spreads onto, and is carried by the ink film on the form-roller to the printing plate, as a relatively thin, uniform film. 

The Gibbs adsorption isotherm shows the dependence of the extent of adsorption of an adsorbent on its bulk concentration or pressure. However, we also need to know the state of the adsorbate at the surface. These are interrelated because the extent of material adsorb-tion on a surface depends on the state of the surface. The behavior of the molecules in the surface film is expressed by a surface equation of state which relates the spreading pressure, n, which is the difference between the solvent and solution surface tensions, %= % - y to the surface concentration of the adsorbent. This equation is concerned with the lateral motions and interactions of the molecules present in an adsorbed film. In general, the surface equation of state is a two-dimensional analogue of the three-dimensional equation of state of fluids, and since this is related to monomolecular films, it will be described in Sections 5.5 and 5.6. It should be remembered that on liquid surfaces, usually monolayers form, but with adsorption on solid surfaces, usually multilayers form (see Section 8.3). 

This particle that does not naturally float can be made to float. Suppose that a surfactant is added to the water so that the solution surface tension becomes y = 40 mN/m and the contact angle becomes 0=140°. In this case the net force on the particle becomes +1.22 X 10 2 — 1.37 X 10-5 mN = +1.22 X 10 2 mN, the particle now floats. 

The dissolution of other compounds in water alters its surface tension. Experimental values of the variation of water solution surface tension with the solution concentration are tabulated in the Handbook of Physics and Chemistry. For salts like NaCl and (NH4)2S04, the dependence of the solution surface tension, aw, on the solution molarity is practically linear over the range of atmospheric interest... 

Figure 14 shows the knee-shaped curve of solution surface tension vs. concentration characteristic of aqueous surface-active agents the steeper the curve, the more efficient the wetting agent. It is generally assumed that the bend of the curve coincides with the critical micelle concentration (c.m.c.) of the respective compound in the aqueous medium. Since the discontinuities in the slopes of the individual curves of Figure 14 occur in the region of the c.m.c. values reported by various... 

The Marangoni effect A surface-active solute tends to concentrate at the liquid surface. When liquid drains from a film surrounding a bubble, it thins a part of the film. In the thin part, surface eirea increases (Fig. 14.9a). The additional surface is supplied by liquid from the bulk, which is leaner in the surface-active solute. Surface tension at the thinned surface therefore rises. This causes a surface flow from the nonthinned (low-surface-tension) surface to the thinned (high-surface-tension) surface, which counteracts film drainage and restores the film. 

---