Changes of surface tension

Equation 9 states that the surface excess of solute, F, is proportional to the concentration of solute, C, multipHed by the rate of change of surface tension, with respect to solute concentration, d /dC. The concentration of a surfactant ia a G—L iaterface can be calculated from the linear segment of a plot of surface tension versus concentration and similarly for the concentration ia an L—L iaterface from a plot of iaterfacial teasioa. la typical appHcatioas, the approximate form of the Gibbs equatioa was employed to calculate the area occupied by a series of sulfosucciaic ester molecules at the air—water iaterface (8) and the energies of adsorption at the air-water iaterface for a series of commercial aonionic surfactants (9). 

The change of surface tension with temperature in the case of states far removed from the critical point is found experimentally to be linear (Erankenlieim (4) 1836) ... 

Factors influencing jet breakup may include (a) flow rates, velocities and turbulence of liquid jet and co-flowing gas, (b) nozzle design features, (c) physical properties and thermodynamic states of both liquid and gas, (d) transverse gas flow,[239] (e) dynamic change of surface tension, 1151[2401 (f) swirlj241 242 (g) vaporization and gas compressibility,[243] (h) shock waves,[244] etc. 

When the polymerization has proceeded to such an extent that all of the monomer droplets have vanished, which occurs after 60-80% conversion, all of the residual monomer is located in the latex particles. The monomer concentration in the particles now declines as polymerization proceeds further, i.e., in this final period the reaction is first order. At the end of the polymerization, the emulsion consists of polymer particles with a size distribution between 50 and 150 pm, which is larger than the original micelles, but smaller than the original monomer droplets. The changes of surface tension and overall rate of polymerization with conversion are schematically shown in Fig. 2.2. 

In the case of the interfacial tension of two pure liquids we have had to deal with the superficial system in equilibrium with a two phase two component system of three dimensions. If we add to this system a third component the problem becomes still more complicated. The simplest case is that in which the added substance is soluble in one phase and completely insoluble in the other, the original liquids being themselves mutually insoluble. The change of interfacial tension should then run parallel to the change of surface tension of the liquid in which the third component dissolves. 

If the original liquids are mutually soluble and the third component is soluble in only one of them, the mutual solubility will be diminished by its addition—according to Nernst s law, at low concentrations. The rise or fall of interfacial tension will thus depend on two superimposed effects, the change of surface tension of the better solvent owing to addition of the solute, and that in each of the two liquids due to diminished concentration of the other. The latter effect tends to increase the tension while the former may work in either direction. 

As was stressed by Professor Ubbelohde, in the process of cell recognition not only the lateral diffusion of the binding sites has to be considered, but also the mechanical effects resulting from the local change of surface tension, inducing convection at the cell surface. It is well known, in the cell-to-cell contact inhibition of motion, in tissue culture, that a cell approaches another cell by touching it by means of microvilli and that this process can be affected when adding surfactants to the culture. Now the point is, What is the relative importance of both diffusion and convection Well, in binary surface films, it was observed that the transport process induced by two-dimensional convection is much more rapid than the two-dimensional diffusion. 

Niebergall (Nl), 1963 Deals with the effects of changes of surface tension during heat and mass transfer in wetted-wall equipment. 

It is well known that the surface tension of water decreases when a detergent is added. Detergents are strongly enriched at the surface, which lowers the surface tension. This change of surface tension upon adsorption of substances to the interface, is described by the Gibbs adsorption isotherm. 

Very great precautions must be taken against accidental contamination of the surface, which cannot be renewed except by removing the bubble or drop, and replacing it by a new one. The method has recently proved useful with metals, and in studying the slow changes of surface tension which take place in solutions of some colloidal electrolytes. 

On a liquid-gas interface, the partial pressure of the adsorbed gas is substituted in Equation 1.59. On the solid-gas and solid-liquid interfaces, only the excess surface concentration can be measured directly, and not the surface tension. The Gibbs adsorption isotherm is suitable for the calculation of the change of surface tension. 

In electrocapillarity experiments the change of surface tension with changing surface potential is measured. With mercury such measurements were already done in the nineteenth century [147], Mercury has the advantage that the surface tension can easily be measured, since it is a liquid. Such measurements yielded important information about the behavior of electrified surfaces. In particular, the relation between surface charge and surface potential or the adsorption of ions to metal surfaces was investigated by electrocapillary experiments. 

A variation in the time of flow when the walls of the tube are covered with an immiscible liquid, such as oleic acid, is really due to changes of surface tension, which are iniportant with some types of viscometer. It is possible, however, that some slip may occur with some lubricating oils, due to the regular reflexion of large molecules from the walls, as contrasted with the diffuse scattering of smaller molecules by irregularities of molecular dimensions in the walls. ... 

In some cases (dyes, etc.) the adsorption of solute in the surface may be so great that a solid film may be formed. The surface tensions of solutions containing hybrid ions may be greater or less than that of water.3 The change of surface tension of a solution with time was found by McBain, Ford, and Mills to be very slow, many days being needed to attain the niaximum effect,... 

The black spots on soap films, which are not more than 10 to 20 molecules thick, can remain for weeks in equilibrium with the thicker, coloured parts of the film,4 and hence it is assumed that they have the same vapour pressure as the normal liquid, and that Thomson s formula can be applied for a radius of curvature of 200 x 10 cm. or less. Bakker<5 gave reasons for supposing that the surface tension is independent of the radius of curvature of the capillary layer, although he recognised that in very thin films it has abnormal values, and he calculated that the maximum ascent of a liquid occurs in a tube of 2 5 m[jL radius. Woodland and Mack found no change of surface tension in a tube of 6 7 [I radius. 

Change of surface tension with surfactant concentration - the critical micelle concentration... 

Figure 3. Example of surface tensio-elastogram9 giving change of surface tension and dilational modulus with time (schematic). A start of periodic surface deformation. Abscissa time after sweeping of surface (minutes). Ordinate ...

Change of surface tension of an aqueous 0.2 mol/1 Triethylbenzyl ammonium chloride solution in dependence of the number of purification cycles j (according to Lunkenheimer Miller 1988)... 

One of the most important equations in surface thermodynamics is that which links changes of surface tension to adsorption processes, The derivation of this equation is given by Appendix III. 

The last boundary condition on the velocity comes from the balance between the change in surface tension due to temperature variations along the surface with the tractive force induced at the free surface. Here, as in Section 10.5, the rate of change of surface tension is taken to be linear with temperature... 

Fig. 4,44 Surface tension response for a Triton X-100 solution at I TO mol/I a) change of surface tension with time b) same as in (a) but plotted over the area change A(t)/A during oscillation according to...

The change of surface tension coefficient along the interface gives rise to a tangential force directed from the smaller values of 2 to the larger values ... 

Fig. 17.9 Thermo-capillary motion of liquid in a shallow rectangular tray (o) and the change of surface tension coefficient along the tray (/ ).

The second reason for the anomalous change of surface tension of a solid polymer containing surfactant lies in the structural and conformational conversions of the polymer itself under the influence of the surfactant. Such factors as the increase of the polymer surface tension when surfactant is added cannot be explained by the surfactant adsorption on the polymer surface only (see Fig. 2.13). Later we will consider this in detail. As was noted above, if the rate of aggregation of the surfactant molecules is higher than or equal to the rate of polymerization, the system surface tension alters during polymerization in the same way as in the coiu-se of the equilibrium process. At a high rate of polymerization, the formation of micelles of the maximum possible size can be hindered by the rapid increase of the system viscosity. In this case, when an IS substance is applied the split into two phases is not observed and the system appears to be more oversaturated by surfactant than in the first case. 

FIGURE 6.14 Change of surface tension with movement of the oscillating barrier at a given distance from the starting position of the barrier. Solid line purely elastic behavior dotted line viscoelastic behavior. The eccentricity and tilt of the ellipse vary with distance from the barrier unless multiple reflections render the surface deformation uniform. From... 

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