Adsorption from surface tension

From measurements of surface tension to compute the extent of adsorption of phenol in aqueous solutions of concentrations 0.3, 0.2 and 0.05 mole 1 .  

The surface tension y of aqueous solutions of phenol of concentration c has been measured at 20 °C by Goard and Rideal (J. Chem. Soc, 1925, 127, 1672) and their results are given in table 1. 

For such dilute solutions the adsorption F per unit area is given by 

We accordingly plot y against log c and evaluate F from the slope. 

The simple formula here used is valid only in dilute solutions. In concentrated solutions the concentration must be replaced by the activity and the precise signihcance of F has to be watched (compare problem 138). 

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The value of 9 can be estimated on purely theoretical grounds from estimates of the adsorption of surfactant which, in turn, can be estimated from the Gibbs adsorption equation relating adsorption to surface-tension lowering. 

Thus, in principle, we could determine the adsorption excess of one of the components from surface tension measurements, if we could vary ii independently of l2. But the latter appears not to be possible, because the chemical potentials are dependent on the concentration of each component. However, for dilute solutions the change in p for the solvent is negligible compared with that of the solute. Hence, the change for the solvent can be ignored and we obtain the simple result that... 

Ule may calculate the surface adsorptions and surface mole fractions of and CyF from surface tension-concen-... 

A second way of classifying the material is on the basis of the experimental methods involved. For mobile interfaces, surface tension is easily measured. For these it is easiest to examine the surface tension-adsorption relationship starting with surface tension data. When insoluble surface films are involved, we shall see how the difference in y between a clean surface and one with an adsorbed film may be measured directly. For solid surfaces, surface tension is not readily available from experiments. In this case adsorption may be measurable directly, and the relationship between adsorption and surface tension may be examined from the reverse perspective. 

This measured concentration change is usually converted into a surface excess quantity, analogous to that usually calculable from surface tension data for adsorption at the liquid/vapor interface. In the case of... 

The Gibbs adsorption equation enables the extent of adsorption at a liquid surface to be estimated from surface tension data. 

Surfactants accumulate at interfaces, a process described as adsorption. The simplest interfaces are the air/water (A/W) and oil/water (O/W). The surfactant molecule orients itself at the interface, with the hydrophobic portion orienting towards the hydrophobic phase (air or oil) and the hydrophilic portion orienting at the hydrophihc phase (water) this is shown schematically in Figure 10.14. As a result of adsorption, the surface tension of water is reduced from its value of 72 mN m before adsorption to 30-40mN m, while the interfacial tension for the O/W system decreases from a value of 50 mN m (for an alkane oil) before adsorption to a value of 1-lOmN m , depending on the nature of the surfactant. 

To compare the directly measured adsorption of phenol in aqueous solution with that calculated from surface tension. 

It should be noted first that the Frumkin model is the most general one with respect to its application to surfactants of different nature. In spite of the fact that, e.g., for oxyethylated nonionic or ionic surfactants this model is essentially biased, in the majority of practical cases it can be recommended irrespectively of the nature of the surfactant. In the Frumkin model, three parameters are necessary to describe the adsorption and surface tension isotherm. Leaving aside the molar area co which can be estimated from the molecular geometry [16, 84], we concentrate on the results which follow from our development for the parameters a and b for surfactant molecules with linear hydrocarbon chain. Figure 3.59 illustrates the dependence of the Frumkin constant a on the molar area co of various surfactants at n<- = 10. Note that for ionic surfactants the co values are equal to the doubled values of co, from corresponding tables. 

The surface excess obtained by the second-harmonic generation in the concentration range below the CMC, however, changes with concentration in contradiction to the usual interpretation of surface tension data. Moreover, the absolute values of the adsorption determined by two experimental methods differ by one order of magnitude. These discrepancies were explained by means of the concept of a depth-dependent distribution of surfactant molecules [66]. Different distributions can lead to identical adsorption values. The surface excess determined by the second-harmonic generation can be attributed only to the very top layer, whereas the values obtained from surface tension techniques are apparently more sensitive to the near-surface layer. 

If the rate of formation of the interface is much faster the rate of adsorption of the surfactant, the surface tension of the spray solution will not be far from that of pure water. Alternatively, if the rate of surfactant adsorption is faster than the rate of formation of the fresh interface, the surfactant will lower the dynamic surface tension and hence smaller droplets are produced. With liquid jets, an important factor may be considered that enhances surfactant adsorption (24). Addition of surfactants reduces the surface velocity (which is in general lower than the mean velocity of flow of the jet) below that obtained with pure water. This results from surface tension gradients which enhances adsorption (the molecules will move to the areas with high surface tension). 

It is most impressive to find how theoretical knowledge has led to some fascinating developments in the technology. The purpose of this handbook is also to further this development. The molecular description of liquid surfaces has been obtained from surface tension and adsorption studies. The emulsion (microemulsion) formation and stability are described by the interfacial film structures. The surfaces of solids are characterized by contact angle and adsorption studies. The ultimate in interfaces is an extensive description of chemical physics of colloid systems and interfaces. Contact angle and adhesion is described at a very fundamental level. The thermodynamics of... 

The quantities determined from surface tension measurements at 25°C are summarized in Table 11.1. The data are compared with those previously reported for TX-lOO in water [21]. In addition to CMC and surface tension, we are able to determine the area per molecule, a, at the air/solution interface when it is satmated with surfactant molecules. It is calculated from a linear fit to the data below the CMC by using the Langmuir-Szyszkowski adsorption equation. We can notice that both ILs display very high CMC valnes. Indeed, there is a factor of 1000 between these valnes and the CMC measured in water. 

Not only the equilibrium sruface tension but also the kinetic properties of a surfactant adsorption monolayer play an important role in various phenomena related to the stability of foams and emulsions [5,30], rising of bubbles and flotation [31]. Indeed, many processes are accompanied by disturbances (expansion, compression) of the adsorption monolayer or by formation of new surface of the solution. The surfactant solution has the property to damp the disturbances by diffusion of the surfactant from the bulk to the interface, or vice versa. The main subject of this section is the theory of adsorption and surface tension under such dynamic conditions. 

Usually Ax measurements are coupled with measurements of the surface excess T of adsorbate which causes the change of X T data are derived from surface tension measurements by means of Gibb s adsorption equation. The derivative 9(A)09r, or the integral slope AX/AF, then gives a measure of the average surface potential change per molecule of adsorbate in the... 

A typical plot of y versus Inc is shown in Fig. 4.16. We will now discuss three regimes of behaviour, indicated in this figure, and relate the variation of surface tension to the adsorbed structure at the water surface, and to micellization in the bulk. Surface adsorption and micelle formation are clearly correlated, since the CMC is most often determined from surface tension measurements. It might be supposed that the observed concentration independence of y above the CMC arises because the surface excess is saturated due to formation of a complete monolayer at the CMC. According to this picture, above the CMC, surfactant molecules are unable to adsorb at the air-water interface, and so form micelles in bulk. However, we shall see that this interpretation is an oversimplification of the subtle physical chemistry involved. 

Downer et al. [29] attempted to remove divalent metal ions by an addition of EDTA in amounts sufficient to chelate the contaminants, but below the concentration at which EDTA affects the surface tension. However, adsorption isotherms derived from surface tension with a Gibbs prefactor of 2 did not agree with those obtained from neutron reflection data. A better agreement was found when using a prefactor of 1.7, consistent with about 30% dissociation of counterions. 

Table 1. Cmc and molecular area (S) of C Em in bmimBF4, water and formamide at 25 °C. [a] Estimated through a fit to the Szyszowski-Langmuir adsorption equation, [b] Deduced from surface tension measurements, [c] From (Rosen et al., 1982 van Os et al., 1993). [d] From (Berthod et al., 2001). [e] From (Couper et al., 1975). [f] From (McDonald, 1967 Jonstromer et al., 1990), some cmc measurements were performed at 21 °C.

Since the drop volume method involves creation of surface, it is frequently used as a dynamic technique to study adsorption processes occurring over intervals of seconds to minutes. A commercial instrument delivers computer-controlled drops over intervals from 0.5 sec to several hours [38, 39]. Accurate determination of the surface tension is limited to drop times of a second or greater due to hydrodynamic instabilities on the liquid bridge between the detaching and residing drops [40],... 

A modification of the foregoing procedure is to suspend the plate so that it is partly immersed and to determine from the dry and immersed weights the meniscus weight. The procedure is especially useful in the study of surface adsorption or of monolayers, where a change in surface tension is to be measured. This application is discussed in some detail by Gaines [57]. Equation 11-28 also applies to a wire or fiber [58]. 

A recent design of the maximum bubble pressure instrument for measurement of dynamic surface tension allows resolution in the millisecond time frame [119, 120]. This was accomplished by increasing the system volume relative to that of the bubble and by using electric and acoustic sensors to track the bubble formation frequency. Miller and co-workers also assessed the hydrodynamic effects arising at short bubble formation times with experiments on very viscous liquids [121]. They proposed a correction procedure to improve reliability at short times. This technique is applicable to the study of surfactant and polymer adsorption from solution [101, 120]. 

We have considered the surface tension behavior of several types of systems, and now it is desirable to discuss in slightly more detail the very important case of aqueous mixtures. If the surface tensions of the separate pure liquids differ appreciably, as in the case of alcohol-water mixtures, then the addition of small amounts of the second component generally results in a marked decrease in surface tension from that of the pure water. The case of ethanol and water is shown in Fig. III-9c. As seen in Section III-5, this effect may be accounted for in terms of selective adsorption of the alcohol at the interface. Dilute aqueous solutions of organic substances can be treated with a semiempirical equation attributed to von Szyszkowski [89,90]... 

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