Surfactant adsorption densities determination

Fig. 48 Surfactant aggregation numbers determined at various adsorption densities (average number at each adsorption density shown along the adsorption isotherm)...

The adsorption density of surfactants was determined by calculating the difference in the surfactant concentration before and after the adsorption of a mineral, as described earlier (5). 

The adsorption densities ( r ) on minerals (C< CMC) of the salt type are in some cases higher because of precipitation of the ionic surfactant with multivalent cations in the bulk phase. Measurements were carried out to determine the fraction f the precipitated surfactant by divalent cations Ca and Ba " leading to a decrease in its equilibirum concentration. They showed a shift of the adsorption maximum towards lower values of r, even after a correction of the adsorption density due to the precipitation. On the other hand, a direct co-adsorption of the precipitated surfactant on a mineral surface cannot be excluded. 

The value of Qst can be determined from the slope of the plot (log c)r against T. Measurements in the system N-dodecylammonium acetate-quartz showed that -AG increased with increasing temperature over the whole concentration range whereas the adsorption density against temperature showed a minimum over the greatest part of the concentration range. In the range of equilibrium surfactant concentrations of 1 - 9 x 10"4 mol 1" the values of - AG 12.55 to 17.58 kJ/mol were found for temperatures between 5 and 45 °C. 

A direct determination of the adsorption density of surfactants as dependent on Na+, Ca+, and Fe3+ concentrations was performed by Dobias179). His results are shown in Fig. 17. To interpret the results of adsorption measurement, he used the Stem-Graham equation (Eq. 51). Under the assumption that Na+, Cl-, Br- and cetyltrimethylam-monium ion (the surfactant used) are not the PDI, he modified the equation to get the form... 

As mentioned above, in order to fully characterize polymeric surfactant adsorption, three parameters must be determined (i) the adsorbed amount F (mgm or mol m ) as a function of the equihbrium concentration that is, the adsorption isotherm (ii) the fraction of segments in direct contact with the surface p (the number of segments in trains relative to the total number of segments) and (iii) the segment density distribution p(z) or the hydrodynamic adsorbed layer thickness 5. ... 

Region IV and the plateau in it correspond to the maximum surface coverage as determined by micelle formation in the bulk or monolayer coverage, whichever is attained at the lowest surfactant concentration further increase in surfactant concentration does not alter the adsorption density. A schematic representation of adsorption by lateral interactions is given in Figure 7.8. 

The thermodynamic description of the interfacial state of liquid systems is the basis for the development of relationships between the adsorption density T at a liquid interface, the surface tension and the surfactant bulk concentrations. Beside this the interfacial tension is an intrinsic parameter which determines the shape of a curved interface as well as different other types of capillary phenomena. 

The interplay between these various factors is complex and often requires experimental measurement under as realistic conditions as possible to appropriately determine the impact of surfactant on wettability. It is the migration to, and the adsorption of, the surfactant at the fluid and solid interfaces along with the orientation and density of the adsorbed surfactant molecules that modifies the fluid-surface interfacial tension/ wettability. Surfactant adsorption at an interface is a necessary, but not a sufficient condition for wettability alteration. Although details of adsorption will be covered in Chapter 4, this section includes a brief treatise on it with the other known variables that can affect wettability modification with surfactants. 

The best fit between experimental results and theory is achieved when both the change in hydrostatic pressure along the height of the forming bubble at the moment of its detachment from the capillary orifice and the expansion of bubble during its rising are taken into account. Surface tension and density of foaming solution (see Eq. (1.9)) determine the size of bubbles when they are formed slowly. The surfactant kind and concentration affect both the rate of formation of adsorption layers at bubble surface and the stability of foam obtained. 

Therefore, the effect of the monolayer is brought down to additional resistance of the equivalent by thickness aqueous layer h. It was shown that the permeability of the adsorption layer depends on surface tension (packing density) and size of the diffusing gas molecules [482], For many surfactants h is within the range of 7 to 12 nm. This means that the permeability of thick films is determined by the rate of molecular diffusion, while for black films (h 10 nm) Eq. (3.147) is valid and their permeability is determined by the properties of the surfactant monolayers. Electrolytes do not affect significantly the permeability of monolayers. It was considered that gas diffusion through the monolayer occurred as a result of creation of microscopic vacancies between the surfactant molecules. This model was called model of energy barrier. However, later this model proved unsatisfactory. [483]... 

Other parameters related to the adsorption of surfactants on the film surface can also be used as characteristics of the surfactant. They determine the charge density and, respectively, the potential of the diffuse electric layer [Pg.531]

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