Ionic surfactants, total surface

When sur-f actants adsorb on metal oxide sur-f aces (e.g., minerals), at low concentrations, the adsorbate molecules are widely dispersed enough that no signi-ficant interactions between adsorbed sur-f actants occurs. Above a certain critical concentration, dense sur-factant aggregates form on the surface (72). These are called admicelles. For ionic surfactants, the admicelles are bilayered structures (72). Above the CMC, the total adsorption of surfactant can increase or decrease slowly. 

An interest represents the comparison of (po with AV-potential, i.e. the total potential difference at the solution/air interface. Fig. 3.24 plots q>o(C) and AV(Q dependences for a solution of non-ionic surfactant. The measurement of AV-potential is performed by the method of vibrating plate over the solution surface [203]. (po and AV change simultaneously and reach a maximum value at the same surfactant concentration. Surely, their absolute values are different, as expected from the following equation [204]... 

Fig. 4 The total potential energy of interaction Vt as a function of distance of surface separation H for two similar oh droplets in an oil-in-water emulsion. (A) Electrostatic stabilization by a monolayer of ionic surfactant. (B) Steric stabilization by a monolayer of non-ionic surfactant. V van der Waals attractive force Vr electrostatic repulsive force Vs steric repulsive force.

Here, d is the micelle hydrodynamic diameter (usually measured by dynamic light scattering) as before, CMC stands for the critical micellization concentration, C, is the total concentration of ionic surfactant 4 is the ionic strength due to added inorganic electrolyte (if any), and is the degree of ionization of the micelle surface ionizable groups (non-neutralized by bound counterions). 

An estimate of the total desorption flow from the surface of a strongly retarded region in the neighbourhood of the rear pole of the bubble is derived as follows. When electrostatic retardation of adsorption-desorption kinetics does not exists, the results of Chapter 8 [Eq. (8.145)] can be applied. For ionic surfactant, the equation for surface tension variation somewhat differs from that for non-ionic surfactant. With regard to these differences, the following estimate of desorption flow results. 

Donetsk, Ukraine) Fainerman obtained Ay 0.4 mN/m, and from the see of Azow Ay =0.1 mN/m. Parallel to these measurements the content of ionic surfactants was determined by another method and found to be smaller than the surfactant concentration calculated from the surface tension. This is reasonable because the ionic surfactants are only part of the impurities. If we compare the total content of organic compounds determined by the well known oxidation method it usually exceeds by 10 times the content determined from the surface tension by means of Gibbs equation. 

During the process of adsorption of surfactant ions at a liquid-fluid interface the surface electric potential and charge density increase with time. This leads to the formation of an electric double layer inside the solution. The charged surface repels the new-coming surfactant molecules (Fig. 4.10), which results in an apparent deceleration of the adsorption process. On the other hand, the existence of the electric double layer (DEL in agreement with the nomination given in [2]) changes the amount of adsorbed surfaetant ions needed to reach equilibrium. This decreases the rate of adsorption so that the total rate is a counterbalance of various influences and it cannot be estimated a priori if a deceleration or an acceleration of the equilibration of an adsorption layer results. The most recent analysis of the different relaxation processes inherent in the adsorption process of ionic surfactants has been performed by Danov et al. [33]. In this work the inclusion of counterions into the Stem layer was performed for the first time. 

A possible interpretation of the shape of the surface tension isotherm at the CMC was given by Rusanov and Fainerman in the framework of a quasichemical approach to micellisation [62]. The general idea is as follows the total surfactant concentration is related to the concentrations of micelles and monomers by the mass balance condition (5.18) and the mass action law in form of Eq. (5.23). From these conditions, one of two quantities can be expressed as a function of the other. For a single non-ionic surfactant this gives (see also Eq. (5.40))... 

It is well-established now that the concentration of surfactant ions in micellar solutions changes when the total surfactant concentration c is increased. This leads to changes in the adsorption value and, consequently, to changes in the surface tension. These alterations, however, are small, even for ionic surfactants. For relatively dilute solutions, i.e. c< 10 CMC, as a first approximation one can consider that the monomer concentration ci is constant (ci CMC). Actually, for c > CMC surface tension changes are usually low and in the range of accuracy of conventional methods. This fact evidences an approximate constancy of the adsorption. 

Hoeft [44] also studied the cooperative and competitive adsorption of ionic surfactant mixtures onto hydrophobic surfaces. When shorter alkyl chain surfactants (sodium octyl sulfonate and sodium decyl sulfonate) are adsorbed, the decyl will displace the octyl surfactant. For mixtures of sodium dodecyi sulfonate and sodium octyl sulfonate, however, there appears to be an association between the surfactant molecules leading to enhanced adsorption of the sodium dodecyi sulfonate with no depletion of the octyl sulfonate adsorption. This is shown in Fig, 2, where the lines indicate the expected adsorption determined using a two-component Langmuir adsorption isotherm with the adsorption parameters determined analyzing the data from adsorption of each species individually. Also shown in Fig. 2 is the concentration of surface-active materials in the aqueous phase at equilibrium. In each of these experiments the total molar concentration and amount of surfactant solution added to the latex was a constant, as was the amount of latex. Thus a lower value for the bulk concentration corresponds to greater adsorption. 

Below the CMC, the surfactant mixing in monolayers composed of similarly structured surfactants approximately obeys ideal solution theory. This means that the total surfactant concentration required to attain a specified surface tension for a mixture is intermediate between those concentrations for the pure surfactants involved. For mixtures of ionic/nonionic or anionic/cationic surfactants, below the CMC, the surfactant mixing in the monolayer exhibits negative deviation from ideality (i.e., the surfactant concentration required to attain a specified surface tension is less than that predicted from ideal solution theory). The same guidelines already discussed to select surfactant mixtures which have low monomer concentrations when micelles are present would also apply to the selection of surfactants which would reduce surface tension below the CMC. 

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