Surfactant surface excess concentration

The surface elasticity, therefore, decreases when either the thickness of the lamella or the bulk concentration of surfactant in it increases. Moreover, since T is proportional (equation 2.25) to (8y/81og C)T, the elasticity is very sensitive to change in the surface tension of the solution with change in the bulk phase concentration of the surfactant. Surface excess concentrations F for surfactants usually fall in the range 1-4 x 10-1° mol/cm2. Thus at 27°C (300 K), since R = 83 x 107 ergs moF1 deg-1,... 

The characterization of the adsorption of surfactant at a water-solid interface is more difficult than at the water-air interface for two reasons First, the interfacial tension cannot be measured directly and, second, its relation to the surfactant surface excess concentration is not always straightforward. 

We now have two terms, and an additional independent experiment is necessary to obtain the surfactant surface excess concentration. One way to obtain the surfactant surface excess concentration at a water-solid interface is by the depletion method. A known amount of a powder of known specific area solid is immersed in a known volume of an aqueous solution of the surfactant at a known concentration. After several hours of contact (required to reach adsorption equilibrium), the suspension is separated and the surfactant concentration is accurately determined. The difference between initial and final surfactant concentrations multiplied by the volume of liquid gives the amount of surfactant adsorbed on the solid. The surfactant surface excess concentration is obtained by dividing the amount of surfactant adsorbed by the surface of solid (specific area multiplied by weight of solid). The difficulty resides in the exact determination of the final surfactant concentration. In... 

It is dangerous to fully rely on Eq. (33) because, in the neighborhood of the critical micelle concentration, the surfactant activity (which is involved Si the equation) becomes significantly smaller than the concentration, due to associations taking place in the bulk of the liquid phase. The error made by assuming that the surfactant activity remains equal to the concentration results in underestimating the surfactant surface excess concentration. 

If the supply of surfactant to and from the interface is very fast compared to surface convection, then adsorption equilibrium is attained along the entire bubble. In this case the bubble achieves a constant surface tension, and the formal results of Bretherton apply, only now for a bubble with an equilibrium surface excess concentration of surfactant. The net mass-transfer rate of surfactant to the interface is controlled by the slower of the adsorption-desorption kinetics and the diffusion of surfactant from the bulk solution. The characteris-... 

It has been reported that the sonochemical reduction of Au(III) reduction in an aqueous solution is strongly affected by the types and concentration of organic additives. Nagata et al. reported that organic additives with an appropriate hydro-phobic property enhance the rate of Au(III) reduction. For example, alcohols, ketones, surfactants and water-soluble polymers act as accelerators for the reduction of Au(III) under ultrasonic irradiation [24]. Grieser and coworkers [25] also reported the effects of alcohol additives on the reduction of Au(III). They suggested that the rate of the sonochemical reduction of Au(III) is related to the Gibbs surface excess concentration of the alcohol additives. 

Let us consider the interface between two phases, say between a liquid and a vapour, where a solute (i) is dissolved in the liquid phase. The real concentration gradient of solute near the interface may look like Figure 3.1. When the solute increases in concentration near the surface (e.g. a surfactant) there must be a surface excess of solute nf, compared with the bulk value continued right up to the interface. We can define a surface excess concentration (in units of moles per unit area) as ... 

The surface excess concentration of surfactant corresponding to saturation of the surface or interface. Example one indicator of effectiveness is the maximum reduction in surface or interfacial tension achievable by a surfactant. This term has a different meaning from surfactant efficiency. 

The surface excess concentration (T), which is the surface concentration of surfactant, can be determined by the representative Gibbs adsorption equation. The T can be obtained from the slope of a plot shown in Figure 2.1 (y versus log[C] at constant temperature). 

When the two phases, gas and liquid, are in contact, component 1, the solvent, is present in large excess compared to component 2, the surfactant. In accordetnce with the Gibb s assumption, we choose a plane where the surface excess concentration of the solvent is equal to zero (Ff = 0) so that the changing surface tension is given only by the second term in the preceding equation. Because the chemical potential of the surfactant is given by... 

For the liquid vapor interface, the surface tension is easUy measured as a function of the concentration as shown in Figure 9.10. The preceding equation can he used to determine the surface excess concentration of surfactant as a function of the sur ctant concentration if the sur ce tension of the solution as a fimction of surfactant concentration is known. For dilute aqueous solutions of organic substrates, the semi-empirical equation for the surface tension, y, of a solution of concentration C2,... 

For surface-active solutes the surface excess concentration, p can be considered to be equal to the actual surface concentration without significant error. The concentration of surfactant at the interface may therefore be calculated from surface or interfacial tension data by use of the appropriate Gibbs equation. Thus, for dilute solutions of a nonionic surfactant, or for a 1 1 ionic surfactant in the presence of a... 

When activity coefficients are used, y is plotted versus (log Ci + log/ ) to obtain T v The area per molecule at the interface provides information on the degree of packing and the orientation of the adsorbed surfactant molecule when compared with the dimensions of the molecule as obtained by use of molecular models. From the surface excess concentration, the area per molecule at the interface a, in square angstroms is calculated from the relation... 

The surface excess concentration ( surface concentration) at surface saturation Tm is a useful measure of the effectiveness of adsorption of the surfactant at the L/G or L/L interface, since it is the maximum value that adsorption can attain. The effectiveness of adsorption is an important factor in determining such properties of the surfactant as foaming, wetting, and emulsification, since tightly packed, coherent interfacial films have very different interfacial properties than loosely packed, noncoherent films. Table 2-2 lists values for the effectiveness of adsorption Tm, in mol/cm2, and the area per molecule at the interface at surface saturation asm, in square angstroms (which is inversely proportional to the effectiveness of adsorption) for a wide variety of anionic, cationic, nonionic, and zwitterionic surfactants at various interfaces. 

When the solid substrate is a nonpolar, low-energy surface, the contact angle can be used to determine the surface (excess) concentration of the surfactant at the solid-liquid interface rS . 

The slope of a plot of yM cos 0, the adhesion tension (Equation 6.3), versus yM consequently provides information on the surface (excess) concentrations of the surfactant at the three interfaces (Padday, 1967 Bargeman, 1973 Pyter, 1982). 

This expression implies, as expected, that E increases as the thickness of the bulk solution in the lamella or the bulk concentration of the surfactant in the lamella decreases. It also implies a very great dependence on the surface excess concentration of the surfactant, T, and indicates that if T is zero, there is no film elasticity. 

There is large body of data on the surface and interfacial tensions of aqueous surfactant solutions. This data show that the structure of the surfactant molecule has a pronounced effect on its ability to reduce these tensions. As the length of the alkyl or fluorinated alkyl chain increases, the CMC decreases and the surface excess concentration increases, causing a drop in the interfacial tension at a fixed surfactant concentration. At low surfactant concentrations the reduction in surface tension (or increase in surface pressure O = yo - y) is linear with the molar bulk solute concentration c (in the case of the dilute solution)... 

For most of the conventional amphiphiles it was demonstrated by Rosen [141] that at a surface pressure H = 20 mN/m the surface excess concentration reaches 84-100 % of its saturation value. Then, the (l/c)n=2o value can be related to the change in free energy of adsorption at infinite dilution AG , the saturation adsorption F and temperature T using the Langmuir and von Szyszkowski equations. The negative logarithm of the amphiphile concentration in the bulk phase required for a 20 mN/m reduction in the surface or interfacial tension can be used as a measure of the efficiency of the adsorbed surfactant ... 

Adsorption is an entropically driven process by which molecules diffuse preferentially from a bulk phase to an interface. Due to the affinity that a surfactant molecule encounters towards both polar and non-polar phases, thermodynamic stability (i.e. a minimum in free energy or maximum in entropy of the system) occurs when these surfactants are adsorbed at a polar/non-polar (e.g. oil/water or air/water) interface. The difference between solute concentration in the bulk and that at the interface is the surface excess concentration. The latter... 

It is well known that the air/solution interface of an amphiphile solution is well populated (Clint, 1992) by the adsorbed molecules. Accordingly, it has been shown that the concentration of the surfactant is always greater at the surface due to adsorption over and above the concentration of surfactant in the bulk. For calculation of Gibbs free energy changes, required different surface properties (e.g., the surface excess concentration, Tmax, minimum area per surfactant molecule at the air/water interface, Amin etc.). The surface excess concentration is an effective measure of the Gibbs adsorption at liquid/air interface, which was calculated by applying equation (Chattoraj Birdi, 1984)... 

The results shown in Figure 15 Indicate that relatively smaller amounts of alcohol are needed to keep surfactants dissolved in higher concentration solutions. This combined with the fact that surfactant surface excess is also lower at higher concentration may be the reason for a better performance of high surfactant concentration—low pore volume floods when compared to low surfactant concentration—large pore volume floods. 

Spectra were collected for a series alkylsulfonates of varying chain length in order to ascertain the effect of the alkyl length chain on alkyl chain order. The SF vibrational spectra of the three surfactants examined, DDS, UDS, and HS, are shown in Fig. l(d, e, and f). The peak intensities of the symmetric methyl and methylene vibrational modes were obtained from spectral fits, as done above. The surface excess concentration (Fi) of surfactant at the interface is obtained from the bulk aqueous concentration by way of the interfacial pressure isotherms. 

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