Polymeric surfactants adsorption isotherms

The adsorption of polymeric surfactants is more complex, since in this case the process is irreversible and produces a high-affinity isotherm with a steep rise in the adsorption value at low polymer concentrations (in this region most of the molecules are completely adsorbed). Subsequently, the adsorbed amount remains virtually constant, giving a plateau value that depends on the molecular weight, temperature and solvency of the medium for the chains (this topic was discussed in detail in Chapter 6). 

A plot of r versus C2 gives the adsorption isotherm. Two types of isotherms can be distinguished a Langmuir type for reversible adsorption of surfactants (Fig. 4.10) and a high affinity isotherm (Fig. 4.11) for irreversible adsorption of polymeric surfactants. 

A characteristic increase of 90 after the horizontal section is apparently more pronounced when the potassium oxalate K2C2O4 is used as the electron donor instead of the sulfide ions (Fig. 2.25). A qualitative similarity of the adsorption isotherms and the MO concentration dependence on the initial quantum yield indicates that the adsorbed dye molecules take part in the reaction. Note that all kinetic curves attain the same value of the stationary quantum yield ratio depends on the nature of polymeric surfactant used for stabilization of CdS colloid. With PAA, this ratio equals ca. 0.5, and 0.6 with PVS. 

A consideration of the adsorption kinetics is very important m an estimation of the effectiveness of surfactants under the dynamic conditions of emulsion polymerization. In a stalagmometric study of dynamic and static adsorption of emulsifiers of various structure at the air-water interface, it was established that adsorption values of micelle-forming sui c-tants differ significantly in the period of drop formation (Nikitina et ai, 1961). This was explained by the considerable period needed for establishment of adsorption equilibrium connected with the kinetics of adsorption layer formation. The authors concluded tirat for usual concentrations of surfactant solutions the period of estaUishm t of adsorption equilibrium can be taken as equal to 2 min. Figure 2 shows the adsorption isotherms of... 

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. ... 

Examples of the Adsorption Isotherms of Nonionic Polymeric Surfactants... 

In both cases a plateau adsorption value F is reached at a given value of C2. In general, the value of F is reached at a lower C2 for polymeric surfactant adsorption when compared to small molecules. The high-affinity isotherm obtained with polymeric surfactants implies that the first added molecules are virtually... 

This is attributed to the failure of the large molecules to enter the pores of the solid. More complex isotherm shapes are encountered as in the case of the adsorption of alkyl surfactants on silica and alumina. For example, the adsorption isotherm of sodium dode-cylsulfonate on alumina consists of four regions depending on the dominant adsorption mechanism. Adsorption of polymeric reagents on minerals typically results in a pseudo-Langmuirian type isotherm as shown in Fig. 4.7 for the adsorption of polyacrylamide on Na-kaolinite (Hollander et al., 1981). 

Adsorption of ionic, nonionic and polymeric surfactant on the agrochemical solid gives valuable information on the magnitude and strength of the interaction between the molecules and the substrate as well as the orientation of the molecules. The latter is important in determining colloid stability. Adsorption isotherms are fairly simple to determine, but require careful experimental techniques. A representative sample of the solid with known surface area A per unit mass must be available. The surface area is usually determined using gas adsorption. N2 is usually used as the adsorbate, but for materials with relativdy low surface area, such as those encountered with most agrochemical solids, it is preferable to use Kr as the adsorbate. The surface area is obtained from the amount of gas adsorbed at various relative pressures by application of the BET equation [96]. However, the surface area determined by gas adsorption may not represent the true surface area of the solid in suspension (the so-called wet surface). In this case it is preferable to use dye adsorption to measure the surface area [99]. 

The high affinity isotherm obtained with poiymeric surfactants implies that the first added moiecuies are virtuaiiy compieteiy adsorbed and such a process is irreversible. The irreversibility of adsorption is checked by carrying out a desorption experiment. The suspension at the plateau value is centrifuged and the supernatant liquid is replaced by pure carrier medium. After redispersion, the suspension is centrifuged again and the concentration of the polymeric surfactant in the supernatant liquid is analytically determined. For lack of desorption, this concentration will be very small indicating that the polymer remains on the particle surface. 

This section will deal with the above interfacial aspects starting with the equilibrium aspects of surfactant adsorption at the air/water and oil/water interfaces. Due to the equilibrium aspects of adsorption (rate of adsorption is equal to the rate of desorption) one can apply the second law of thermodynamics as analyzed by Gibbs (see below). This is followed by a section on dynamic aspects of surfactant adsorption, particularly the concept of dynamic surface tension and the techniques that can be applied in its measurement. The adsorption of surfactants both on hydrophobic surfaces (which represent the case of most agrochemical solids) as well as on hydrophilic surfaces (such as oxides) will be analyzed using the Langmuir adsorption isotherms. The structure of surfactant layers on solid surfaces will be described. The subject of polymeric surfactant adsorption will be dealt with separately due to its complex nature, namely irreversibility of adsorption and conformation of the polymer at the solid/liquid interface. 

Another characteristic is the surface activity how much surfactant is needed, or more precisely, how large should the surfactant activity be, to reach a certain value of r (or of y) The differences between a soap and a polymer are illustrated in Figure 2.8. It is seen that the polymer is far more surface active the value of me needed to obtain the same F is smaller by about 2 orders of magnitude for the polymer the difference expressed in mass concentration is even more, by about 4 orders of magnitude. Soaps and pol3miers also differ in the applicability of eqn. (2.35). For a soap, it nearly always appears to hold, also in an emulsion. For a polymer, equilibrium is often not obtained, at least in the timescales considered see e.g. ref 1 for explanations. This is borne out by the two curves for /3-casein in Figure 2.8 the one obtained by emulsification does not agree at all with the adsorption isotherm obtained by unhindered adsorption on a macroscopic interface. For a soap, the value of F can be calculated from the adsorption isotherm when an emulsion is made from known quantities of materials and the specific surface area is known. This is not so for a polymeric surfactant, where F has to be determined separately. 

In view of such applications, the adsorption of a grafted (rake-type) polymeric siloxane surfactant containing 48% PDMS, 39% PEO, and 13% PPO on carbon black particles dispersed in mixtures of water with polar organic solvents has been investigated [58]. The adsorption was foimd to obey the Langmuir isotherm below the critical micelle concentration and a sharp increase in the adsorbed amount was observed at higher surfactant concentrations. DLS and SANS data indicate that the structure of the adsorbed layer is similar to that of micelles. 

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