Adsorption layers state

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 comparison of W(C) dependence with Ao(C) isotherm gives a relation between formation of black spots and films, and the adsorption layer state. It has been shown [332] that the W(Q dependences for black spot and black films of a very small radius (25 pm) coincide. The comparison of the W(C) curve of CBF from NaDoS (see Fig. 3.78) with the surface tension isotherm of the same surfactant (see Fig. 3.77) indicates that black spots begin to form when the state of adsorption layers deviates from the ideal one (Henry s region in Aa(Q isotherm). The probability for observation of a black film steeply increases with the increase in surfactant concentration to about 10 5 mol dm 3 where the adsorption layer saturation is... 

For the solid-liquid system changes of the state of interface on formation of surfactant adsorption layers are of special importance with respect to application aspects. When a liquid is in contact with a solid and surfactant is added, the solid-liquid interface tension will be reduced by the formation of a new solid-liquid interface created by adsorption of surfactant. This influences the wetting as demonstrated by the change of the contact angle between the liquid and the solid surface. The equilibrium at the three-phase contact solid-liquid-air or oil is described by the Young equation ... 

It was shown earlier that the choice of a standard state can be based on the analysis of the adsorption equilibrium. Assuming that adsorption is a substitution process of a solvent molecule by the solute, that the adsorbate and solvent have the same size (n = 1), and that the adsorption layer can be treated as a separate phase, the equilibrium constant can be... 

Equation 1 has been used to derive various adsorption isotherms and equations of state for the adsorption layer. For example, since the chemical potentials of the components in the bulk and on the surface are balanced at equilibrium, Eq. 1 yields... 

Equation of state for the adsorption layer is described by Eq. 34, which 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]. 

Damaskin and Baturina [171] have studied unstable states during coumarin adsorption on mercury electrode. These instabilities were attributed to the nonequilibrium phase transitions in the adsorption layer, during which the orientation of coumarin molecules changed at the electrode surface. 

The appeareance of maxima on the adsorption isotherms and decrease in flotability can be explained by the hypothesis that in the presence of micelles no adsorption layer of the surfactant can be formed, the character of which corresponds to the equilibrium state only with monomers (sufficiently hydrophobic adsorption layer). Due to a heterogeneity of forces acting at the surfactant ion mineral interface it can be assumed that at concentrations S CMC some of the molecules will be bound much more firmly in a three-dimensional micelle than in... 

The Wicke and Eigenberger models are models for an ideal adsorption layer. They have been examined at the Institute of Catalysis, Siberian Branch of the U.S.S.R. Academy of Sciences [93-104,108,109] independently of Wicke and Eigenberger (the first publications refer to 1974). It was shown [93-96] that, for the detailed mechanisms of catalytic reactions either with the steps that are linear with respect to intermediates or with non-linear steps (but containing no interactions between various intermediates), the steady state of the reaction is unique and stable (autocatalytic steps are assumed to be absent). Thus the necessary condition for the multiplicity of steady states is the presence of steps for the interaction between various intermediates in the detailed reaction mechanism [93-100]. Special attention in these studies was paid to the adsorption mechanism of the general type permitting the multiplicity of steady states [102-104]... 

Combined measurement techniques were successfully applied in the study of surface forces in microscopic foam films such as study of longitudinal electrical condictivity, study of black films with X-rays forced rupture of films by a-particles irradiation, etc. They permit to find the relation between surface forces and parameters of film structure. It is important also surface forces measurements to be performed at controlled state of the adsorption layer. As far as surface forces act normally to film surface, it is interesting to understand the role of... 

Formation and stability studies of black foam films can be summarised as follows 1) surface forces in black foam films direct measurement of disjoining pressure isotherm DLVO- and non-DLVO-forces 2) thin foam film/black foam film transition establishing the conditions for the stability of both types of black films and CBF/NBF transition 3) formation of black foam films in relation to the state of the adsorption layers at the solution/air interface 4) stability of bilayer films (NBF) theory and experimental data. 

The region of jump-like changes in the dAsurface tension (see Fig. 3.77). This jump [364,365] is explained with a phase transition in the adsorption layer. Other authors have also noticed the flexion in Ao(C) isotherm and have considered it to be a transition from liquid-crystalline to gel state of the adsorption layer, e.g. in solutions of dodecylamine hydrochloride [374]. This transition can be found experimentally also from AV(C) dependence. As it is seen from Fig. 3.77 the minimum of AV coincides with the flexion point of Ao(lgC) isotherm. 

The beginning of the curves, indicating black film formation, is probably related to the state of the adsorption layers at the surfactant solution/air interface. However, the infinite stability of black films is not a function only of the adsorption layers. To find the reasons for... 

As already mentioned (see Chapter 3), at the instant of foam formation the films and borders are in non-equilibrium state. The films thin mainly due to the capillary pressure, while the borders thin due to gravity or a pressure drop (when the foam is dried by the Foam Pressure Drop Technique [21-23]). The surfactant adsorption layers decrease the flow rate through the borders and films and the process of thinning becomes similar to the flow in thin gaps with solid surfaces. As indicated in Sections 3.2.1 and 5.3 the degree of retardation of the flow depends on the surfactant type and concentration as well as on the film type. A complete immobility at the film and border surfaces usually is not reached. 

For NBFs (bilayer films) there is a correlation between the concentration Cm and the state of the adsorption layer Cm is close to the concentration of saturation of the adsorption layer. On the other hand, Cm is directly related to the stability of the bilayer films. Therefore, it can be concluded that Cm is identical with the critical surfactant concentration of film rupture, corresponding to a certain arbitrary time, for instance, at film lifetime xcr = 1 s. Hence, Cm of bilayer films is a function of the film lifetime. According to Eq. (3.129)... 

Such a relationship between Cm and the state of the adsorption layer is also found for CBFs (see Chapter 3). However, no quantitative link between Cm and the stability of these films has been found. As far as in the formation of black spots the adsorption layers at film surface plays a significant role, the clarification of the decrease in surface tension Act with the surfactant concentration is also important. Being a characteristic of surfactants Cm is in agreement with the commonly used quantity Act but is also related to the film properties. It is also important that Cm is in correlation with foam stability and, thus, is a more precise, suitable and physically better grounded characteristic. Such a correlation has been found for aqueous emulsion films [69,70]. 

In a series of papers [109,110,112,113] this catalytic action has been explained in the context of the mechanism of micellar catalysis and has been attributed to the reactants concentrating in the foam adsorption layers to a local change in the pH to the effect of reactant molecules orientation in the adsorption layer and to the surface charge of the transition state (intermediate complex). Compared to micellar catalysis the higher efficiency of film catalysis in a foam has been attributed to the structural features of the surface layers in the foam (the type of adsorption films) that facilitate formation of the reaction transition states of the reaction. However, no special studies that would have unambiguously confirmed these assumptions were undertaken. 

Summary Solid state NMR studies of molecular motions and network structure in poly(dimethylsiloxane) (PDMS) filled with hydrophilic and hydrophobic Aerosil are reviewed and compared with the results provided by other methods. It is shown that two microphases with significantly different local chain mobility are observed in filled PDMS above the glass transition, namely immobilized chain units adsorbed at the filler surface and mobile chain units outside this adsorption layer. The thickness of the adsorption layer is in the range of one to two diameters of the monomer unit ( 1 nm). Chain units in the adsorption layer are not rigidly linked to the surface of Aerosil. The chain motion in the adsorption layer depends significantly on temperature and on type of the filler surface. With increasing temperature, both the fiaction of less mobile adsorbed chain units and the lifetime of the chain units in the adsorbed state decrease. The lifetime of chain units in the adsorbed state approaches zero at approximately 200 K and 500 K for PDMS chains at the surface of hydrophobic and hydrophilic Aerosil, respectively. 

Summarizing this section it can be stated that the adsorption bonds in filled PDMS have a dynamic origin. With increasing temperature, the frequency of adsorption-desorption processes in the adsorption layer increases and the adsorption-desorption equilibrium shifts to the chain desorption. At room temperature, the lifetime for the dimethylsiloxane chain units in the adsorption state is very short chain units adhere to the filler surface only for tens of microseconds. 

Fig. 13. Schematic view of adsorbed chain fragments at different t peratures PDMS chains are completely desorbed at about 200 K and 500 K for hydrophobic and hydrophilic Aerosil, respectively less mobile chain units in the adsorbed state are shown by filled circles the dotted line corresponds to the estimated border for the adsorption layer...

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