Wetting adsorption effects

Since the effect of a surfactant on an interfacial phenomenon is a function of the concentration of surfactant at the interface, we can define the effectiveness of a surfactant in adsorbing at an interface as the maximum concentration that the surfactant can attain at that interface, i.e., the surface concentration of surfactant at surface saturation. The effectiveness of adsorption is related to the interfacial area occupied by the surfactant molecule the smaller the effective cross-sectional area of the surfactant at the interface, the greater its effectiveness of adsorption. Effectiveness of adsorption, therefore, depends on the structural groupings in the surfactant molecule and its orientation at the interface. Another parameter characterizing the performance of surfactants, important in high-speed interfacial phenomena such as wetting and spreading, is the rate of adsorption of the surfactant at the relevant interface(s). This will be discussed in Section IV of Chapter 5. 

Figure 33 depicts a prednisolone calibration curve in dry ACN. The nonlinearity of this profile is obvious. Because there is absolutely no indication of adsorption effects, we hypothesize that there is a following second order chemical reaction. Hydrocortisone gives similar behavior in dry aprotic solvents. Both of these steroids are characterized by an aliphatic alcohol groip which is not associated with the other three ideally behaving steroids. This circumstantial evidence suggests that the anion radical electrode reaction product is being protonated by reactant diffusing toward the electrode. The likelihood that this is the process occurring is further enhanced by the fact that addition of sufficient amounts of a weak proton donor (e.g., H2O) leads to a linear calibration curve with lower sensitivity, as shown in Figure 34. The latter figure illustrates calibration curves for dry and wet ACN and DMF, as well as aqueous base.

In the presence of salt it was assumed that electrical double layer repulsion is suppressed and the action of the SDS/NaCl solutions was simply to enhance wetting. In a parallel study with isopropanol /water mixtures 5 we found that there exists a linear relationship between wjj and yl where Wq = 0 when the liquid surface tension is equal to the critical wetting tension yc. The value of Yc in these alcohol/water mixtures was ca 22 mNm i which is lower than Ys PET because of the well known adsorption effect 20 and hence the complete inhibition of adhesion due to wetting occurs when yc YS Similar adsorption effects have been observed for SDS solutions 2 and this is reflected by the value of yc oi ca lOmNm determined in this work from contact angle measurements on Mylar film. The measured Wq (= 0.95 yN) in water gives the fallowing expression for as a function of yl assuming that a linear interpolation can be made between YL YC 10 Nm l and yl Ywater 72.2 mNm"l. 

The foregoing is an equilibrium analysis, yet some transient effects are probably important to film resilience. Rayleigh [182] noted that surface freshly formed by some insult to the film would have a greater than equilibrium surface tension (note Fig. 11-15). A recent analysis [222] of the effect of surface elasticity on foam stability relates the nonequilibrium surfactant surface coverage to the foam retention time or time for a bubble to pass through a wet foam. The adsorption process is important in a new means of obtaining a foam by supplying vapor phase surfactants [223]. 

The mechanisms that affect heat transfer in single-phase and two-phase aqueous surfactant solutions is a conjugate problem involving the heater and liquid properties (viscosity, thermal conductivity, heat capacity, surface tension). Besides the effects of heater geometry, its surface characteristics, and wall heat flux level, the bulk concentration of surfactant and its chemistry (ionic nature and molecular weight), surface wetting, surfactant adsorption and desorption, and foaming should be considered. 

Hysteresis is observed not only in the sorption isotherms but also in calorimetric measurements of heat of wetting at different moisture contents, and it is thus a combined entropy and enthalpy phenomenon. A reliable explanation for this effect is not currently available, but there is speculation that it is due to the stresses which are induced as the cellulose swells. Since the swelling of cellulose is not completely reversible, mechanical recovery is incomplete and hysteresis will therefore be present both in the internal stress-strain curve of the sample, and also in the water adsorption isotherm. 

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