Surface tension reduction dynamic

IV. DYNAMIC SURFACE TENSION REDUCTION IV.A. Dynamic Regions... 

In high-speed spray cleaning, a critical factor is the dynamic surface tension reduction of the surfactant solution (Chapter 5, Section IV), rather than its... 

The efficiency of dynamic surface tension reductions within homoiogous series of surfactants in aqueous geiatin soiution... 

From the discussion of dynamic surface tension (Chapter 5, Section IV), the maximum rate of reduction of surface tension occurs when t = t, the time required for the surface tension to reach half of the value between that of the solvent, y , and the meso-equilibrium surface tension value, ym. From equation 5.3, it has been shown (Rosen, 1991) that... 

Less attention has been paid to reducing spray drift by modification of the dynamic surface tension of the spray dispersion. This is not surprising, in that a reduction in the dynamic surface tension may adversely affect the retention of the spray droplets on the leaf surfaces. Furthermore, the extensive use of tank mixes may bring about an unintended reduction of the dynamic surface tension via the contribution of the surface-active agents in the partner products. 

Nevertheless, a reduction in spray retention brought about by an increase in the dynamic surface tension may be offset by increasing the elasticity of the spray droplet (Hart, and Young, 1987 Wirth cl al., 1991). Therefore, an approach that involves the use of polymers to both increase the viscosity and avoid the use of surface-active agents is conceivable. 

Fig. 5 Efficiency of reducing dynamic surface tension at 0.1 s surface age in terms of the relative reduction in surface tension at 10" M concentration as a function of CAC ...

All three class types of surfactant studied exhibit a common behaviour in their dynamic surface tension properties in aqueous gelatin solution all show a maximum efficiency in DST reduction at a surface age of 0.1 s as their homologous series are ascended. 

There would appear then to be only limited evidence that oils which exhibit antifoam effects, when present as emulsified bulk phase, can also produce antifoam effects when present only as solubilizates in aqueous micellar solutions of surfactants. In many instances, alternative explanations for supposed observations of the latter are possible, which do invoke the presence of the oils as bulk phase. However some of the observations described here are difficult to dismiss. Of particular interest in this context are the findings of Koczo et al. [15], Lobo et al. [21], and Binks et al. [16] concerning the effect of solubilized alkanes on the foam stability of aqueous micellar solutions of various surfactants. Attempts to explain such effects by recourse to dynamic surface tension behavior after the manner of Ross and Haak [11] would appear to be unconvincing (see reference [22]). It is, however, possible that it may concern the effect of the solubilized oil on the relevant disjoining pressure isotherm. Wasan and coworkers [15,21] have suggested that the phenomenon is a consequence of the effect of solubilization of alkanes on intermicellar interactions. Lobo et al. [21] find that the instability of the foams formed from certain ethoxyl-ated alcohols in the presence of solubilized alkanes depends on the magnitude of the micellar second virial coefficient describing those interactions. Reduction of the... 

There is also a difference in dynamic surface properties between methyl ester ethoxylates and alcohol ethoxylates. As shown in Fig. 12 for pure 7-mol homologs, the methyl ester ethoxylate maintains a lower surface tension than its alcohol ethoxylate counterpart as measurements become more dynamic (bubble rate of bubble tensiometer is increased). This suggests that methyl ester ethoxylate is more effective in lowering surface tension (can achieve the same surface tension reduction with a lower surfactant concentration at the interface) and/or it diffuses through aqueous solution at a faster rate. 

Equation (46), one form of the Gibbs equation, is an important result because it supplies the connection between the surface excess of solute and the surface tension of an interface. For systems in which y can be determined, this measurement provides a method for evaluating the surface excess. It might be noted that the finite time required to establish equilibrium adsorption is why dynamic methods (e.g., drop detachment) are not favored for the determination of 7 for solutions. At solid interfaces, 7 is not directly measurable however, if the amount of adsorbed material can be determined, this may be related to the reduction of surface free energy through Equation (46). To understand and apply this equation, therefore, it is imperative that the significance of r2 be appreciated. 

Freshly prepared macroemulsions change their properties with time. The time scale can vary from seconds (then it might not even be appropriate to talk about an emulsion) to many years. To understand the evolution of emulsions we have to take different effects into account. First, any reduction of the surface tension reduces the driving force of coalescence and stabilizes emulsions. Second, repulsive interfacial film and interdroplet forces can prevent droplet coalescence and delay demulsification. Here, all those forces discussed in Section 6.5.3 are relevant. Third, dynamic effects such as the diffusion of surfactants into and out of the interface can have a drastic effect. 

The combined evidence of all three class types of surfactant suggest a strong degree of common behaviour in their dynamics in aqueous gelatin solution they all exhibit a maximum efficiency in DST reduction as their homologous series are ascended. Efficiency can be defined in several ways. Here, efficiency is represented in two different ways, both of which are designed to be independent of absolute values of surface tension lowering. The results for the three classes can then be combined and compared. 

Although a rapid adsorption of proteins is necessary to facilitate the reduction in surface tension, it is not a rate-limiting step under dynamic flow conditions. The rate of conformational rearrangement/reorientation of proteins at the interface is a rate-limiting step in reducing the interfacial tension. Results from the adsorption behavior of denatured and reduced proteins indicate that the effectiveness of a protein film in reducing the interfacial energy is dependent... 

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