Adsorption isotherm ionic surfactants

The deviations from the Szyszkowski-Langmuir adsorption theory have led to the proposal of a munber of models for the equihbrium adsorption of surfactants at the gas-Uquid interface. The aim of this paper is to critically analyze the theories and assess their applicabihty to the adsorption of both ionic and nonionic surfactants at the gas-hquid interface. The thermodynamic approach of Butler [14] and the Lucassen-Reynders dividing surface [15] will be used to describe the adsorption layer state and adsorption isotherm as a function of partial molecular area for adsorbed nonionic surfactants. The traditional approach with the Gibbs dividing surface and Gibbs adsorption isotherm, and the Gouy-Chapman electrical double layer electrostatics will be used to describe the adsorption of ionic surfactants and ionic-nonionic surfactant mixtures. The fimdamental modeling of the adsorption processes and the molecular interactions in the adsorption layers will be developed to predict the parameters of the proposed models and improve the adsorption models for ionic surfactants. Finally, experimental data for surface tension will be used to validate the proposed adsorption models. 

The adsorption of ionic surfactants creates an adsorption layer of surfactant ions, a Stern layer of counterions and a diffusive layer distributed by the electric field of the charged surface. Every layer has its own contribution to surface tension. For example, the adsorption of dodecyl sulfate (DS") ions from the sodium dodecyl sulfate solution is described by the modified Frumkin isotherm as... 

In general, the adsorption of ionic surfactants follows the Langmuir isotherm, as discussed in Section 4.1. The adsorption of the surfactants onto the solid surfaces is dependent on the orientation and the packing efficiency of the solid surfaces. The onset of the adsorption plateau may occur at the critical micelle concentration (c.m.c.) of the surfactant in water, as shown in Figure 4.28. If the adsorption isotherm... 

The adsorption of ionic surfactants on hydrophobic surfaces may be represented by the Stern-Langmuir isotherm [17]. Consider a substrate containing sites... 

As this subject was covered in detail in Chapter 5, only a summary will be provided at this point. Surfactant adsorption is usually reversible, and hence thermodynamics can be applied for deriving the adsorption isotherm. Eor example, the adsorption of ionic surfactants onto hydrophobic surfaces may be represented by the Stern-Langmuir isotherm [13]. Consider a substrate containing sites (molm ) on which F molm of surfactant ions are adsorbed. The surface coverage 0 is (F/NJ and the fraction of uncovered surface is (1 — 0). The Stern-Langmuir... 

Fig. III-8. A model S-shaped four-region isotherm for the adsorption of ionic surfactant on an oppositely charged surface (a) The structure of the adsorbed surfactant layers corresponding to different regions of the adsorption isotherm (b)...

The result of these investigations leaves the following view of adsorption of ionic surfactants onto hydrophobic surfaces. Measured adsorption isotherms (Langmuir type) and comparisons between particle sizes as measured by soap titration and... 

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. 

Surfactant adsorption theories are based on different physical and geometrical models of the adsorbed layer, resulting in a variety of surface equations of state or equivalently in several different adsorption isotherms. The usual approach in the theoretical description of the adsorption of ionic surfactants is the generalization of an adsorption isotherm (or equation of state) of nonionic surfactants by incorporating the electrostatic contribution in the adsorption free energy [4, 5, 6, 7, 8]. The validity of the ionic models derived is usually tested by applying the models for the description of the surface... 

Adsorption of ionic surfactants at the solid/solution interface is of vital importance in determining the stability of suspension concentrates. As discussed in Chapter 5, the adsorption of ionic surfactants on solid surfaces can be measured directly by equilibrating a known amount of solid (with known surface area) with surfactant solutions of various concentrations. After reaching equilibrium, the solid particles are removed (for example by centrifugation) and the concentration of surfactant in the supernatant liquid is determined analytically. From the difference between the initial and flnal surfactant concentrations (Ci and C2 respectively) the number of moles of surfactant adsorbed, F, per unit area of solid is determined and the results may be fitted to a Langmuir isotherm. 

Results on the adsorption of ionic surfactants on pesticides are scarce. However, Tadros [81] obtained some results on the adsorption of NaDBS and CTABr on a fungicide, namely ethirimol. For NaDBS, the shape of the isotherm was of a Langmuir type, giving an area/DBS at saturation of 0.14 nm. The adsorption of CTA showed a two-step isotherm with areas/CTA of 0.27 and 0.14 nm. These results suggest full saturation of the surface with surfactant ions that are vertically oriented. 

Tin literature there are many studies on the adsorption of ionic surfactants at aqueous solution/alkane interface, such as [11, 26, 28-31]. A direct comparison of the adsorption behaviour of CnTAB with alkyl chain lengths 10, 12, 14 and 16 at the water/air and water/hexane interfaces was presented in [8] and it was shown that the FIC model described the experimental data for both interfaces quite well. The new model proposed in [6], leading to the set of Eqs. (7), (9) and (10), has shown to be superior over the FIC model as it allows to assume that the oil molecules provide not only a hydrophobic environment for the adsorbing surfactant molecules but the adsorb themselves at the interface. The equilibrium interfacial tension isotherms presented in Fig. 3 for four CnTABs (n = 10, 12, 14 and 16) adsorbed at the aqueous phosphate buffer solution/hexane interfaces allow to demonstrate the feasibility of the given physical picture of a co-adsorption of surfactant and alkane molecules. 

Typical adsorption isotherms are shown in Figs. 16 and 17. Despite the large experimental scatter, a steep increase in adsorption can be seen at low concentrations, followed by a plateau at concentrations exceeding the CMC. Similar behavior has been observed before with model surfactants [49-54] and has also been predicted by modem theories of adsorption [54]. According to Fig. 16, adsorption increases modestly with salinity provided that the calcium ion concentration remains low. The calcium influence, shown in Fig. 17, cannot be explained by ionic strength effects alone but may be due to calcium-kaolinite interactions. 

The description of a mixed adsorption layer of ionic and nonionic surfactants requires the appropriate adsorption isotherms. For example, the Frumkin isotherm gives... 

A new analysis of the adsorption layer of ionic surfactants with new adsorption isotherms and equations of states was made in [42]. The effect of mono and bivalent anions on the adsorption of cethyltrimethyl ammonium salts was recently examined in [43]. 

A well-studied and often-cited adsorption isotherm of an ionic surfactant is that of sodium dodecylsulfate at the alumina-water interface at pH 6.5 and 0.1 M NaCl. At pH 6.5 and 0.1 M NaCl the S-F isotherm [23]... 

For ionic surfactants another effect often dominates and usually salt tends to stabilize emulsions. Reason without salt the distance between surfactants in the interface is large because the molecules electrostatically repel each other. This prevents a high surface excess. The addition of salt reduces this lateral repulsion and more surfactant molecules can adsorb at the interface. Then, according to the Gibbs adsorption isotherm Eq. (3.52), the surface tension is reduced and the emulsion is stabilized. 

The second objective is to verify experimentally the predicted relationship between polymer polarity and surfactant adsorption by studying the adsorption of a non ionic surfactant that shows a saturation type isotherm behavior on vinyl acrylic latexes of varying polarity. 

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