Surfactants polymer adsorbed

The surface forces technique measures the force between molecules (eg. surfactants, polymers) adsorbed on mica sheets. In the case of large molecules such as polymers, the measurement is most sensitive to the regions closest to the solution and provides little direct information about the region adjacent to the surface. As it is a measurement between macroscopic surfaces, it is unable to provide information on microscopic chemical differences at the interface. Infrared spectroscopy could provide additional information about the quantity of adsorbed material on the mica surface, the identity and orientation of the adsorbed species, and possibly the nature of the surface linkage. 

Additional contact angles were measured at 0.001 M SDS concentration. The hematite disk was conditioned in the SDS solution before adding polymer to the solution. The contact angle was measured after adding sufficient quantities of polymer for concentrations of 1, 3, and 5 ppm. For both PAA and PAM, the contact angle remained constant and identical to the value obtained in the absence of polymer. No polymer was able to adsorb on the surfactant-coated disk. It can be concluded that whichever species, surfactant or polymer, adsorbed first was not... 

Because of their large interfacial area, emulsions are basically unstable. In order to produce a stable emulsion, a surfactant is mostly needed. The surfactants are adsorbed at the oil-water interface, forming a link between the two phases of different polarity. For this purpose, a wide variety of emulsifying agents is currently available. Polysaccharides such as arabic gum, tragacanth, Karaya gum, and different seaweed carbohydrate polymers have been employed. They, however, show considerable batch-to-batch variations and might support microbial growth. 

The analogous considerations are valid for polymer systems as well. Indeed, amphiphilic monomer units also tend to occupy interfacial areas of macromolecular associates as it is normal for low molecular weight surfactants to adsorb at polymer-poor solvent boundaries. And, if such interfacial groups of the polymer associate catalyze chemical transformation of a compound which tends to adsorb at the associate interfaces, this can result in unusual kinetics effects. Okhapkin et al. [18] studied the influence of temperature-induced aggregation on the catalytic activity of thermosensitive... 

Surfactants or polymers adsorbed on the particle surface are able to keep particles that far ap-part that the Van der Waals attraction cannot become effective. This phenomenon is called steric stabilisation. 

Two factors contribute to the stability of the gels prepared by the two-step concentrated emulsion. The repulsive forces between the charged surfactant molecules adsorbed on the surface of neighboring cells of the dispersed phase is one of them. The increased viscosity of the dispersed phase which contains the monomer constitutes the second factor, since the increased viscosity opposes the separation of the phases. The partial polymerization increases the viscosity of the dispersed phase, thus increasing the stability of the concentrated emulsion. Monomers that could not lead to gels in the one-step concentrated emulsion method were able to generate them when the two-step pathway was employed. Using this pathway, almost all monomers could be employed to prepare polymer materials. 

This section reports studies of the effect of strongly interacting adsorbates on the sorption of other adsorbates representing the same class (small cations, small anions, surfactants, polymers), e.g. sorption of copper is studied in absence and in the presence of other heavy metal cations at otherwise identical conditions (solid to liquid ratio, pH, equilibration time, etc.). A few examples of adsorption competition between anions or cations of inert electrolytes are also presented. This limitation does not imply that actual adsorption competition occurs only between adsorbates representing the same class. 

Most adsorption systems of practical importance contain strongly adsorbing species (multivalent cations and anions, surfactants, polymers). Systems without specific adsorption are difficult to realize even under laboratory conditions due to omnipresent strongly adsorbing impurities (cf. Chapter 3). On the other hand, the primary surface charging occurs also in more complex systems and it must be taken into account in modeling of specific adsorption. 

Fundamental investigation of the system at the molecular level. This requires investigations of the structure of the solid/liquid interface, namely the structure of the electrical double layer (for charge-stabiUsed suspensions), adsorption of surfactants, polymers and polyelectrolytes and conformation of the adsorbed layers (e.g., the adsorbed layer thickness). It is important to know how each of these parameters changes with the conditions, such as temperature, solvency of the medium for the adsorbed layers, and the effect of addition of electrolytes. 

The effect of the adsorbed surfactant-polymer complex on the rheology of the air-aqueous solution interface is easily detected by the talc particle test (Regis-mond, 1997). A small quantity of calcined talc powder is sprinkled on the surface of the aqueous solution in a 10-cm Petri dish. A gentle current of air is directed tangentially to the talc particles for 1-2 s and then removed. The observed movement is noted in the following categories fluid (F), viscous (V), gel (G) (= almost no flow), solid (S) (= no flow), and viscoelastic (VE) (= net movement, with some recovery upon removal of air current). 

The transition from non-adsorbing to adsorbing polymers can be achieved simply by changing the surfactant and thus increasing the attraction between polymer and surfactant. Hydrophilic polymers in w/o-droplet microemulsions lead to polymers incorporated in the droplets (Scheme 4.5). Attractive interactions lead to adsorption at the inside of the surfactant film. With increasing chain length confinement effects eventually occur (Scheme 4.6). In this case, the polymer is incorporated in more droplets and the droplets form clusters. Polymers adsorbing on the outside can also lead to droplet clusters. 

Time Effects. Surfactants that adsorb are often transported to the interface by diffusion. For most amphiphiles this is a fast process, the times needed ranging from a millisecond to a few minutes. For polymers, it can be much slower. For mixtures of surfactants, changes in surface composition and interfacial tension may take a long time. Several complications can arise, such as very slow adsorption of poorly soluble surfactants (e.g., phospholipids), or a greatly enhanced adsorption rate due to convection. In processes like foam formation, the interfacial tension at short time scales is of importance to obtain such values, one determines so-called dynamic surface tensions, i.e., values obtained at rapidly expanding surfaces. 

Anyway, bridging by adsorbed polymers will often depend on the history of the system. Consider a dispersion of small solid particles that tend to aggregate. To stabilize the suspension, an adsorbing polymer is added that can give a maximum surface excess Tpiateau of 6 mg m-2. If the specific particle surface area A is lm2 per ml and 3 mg of a suitable surfactant polymer is added per ml, a r value of at most 3 mg per m2 can result. This value is on the low side, but the particles are nevertheless stabilized apparently, polymer chains stick out far enough into the solvent to cause steric repulsion. Assume now that to 1 ml of the suspension 6 mg of polymer is added and that subsequently 1 ml of suspension without polymer is added. 

Surfactants in Aqueous Solution A very important component that is usually present in the lyophobic colloids is the surfactant. These molecules are amphiphilic, that is, a part of the molecule is much more polar than the other part. On the basis of the nature of the polar groups in the surfactant molecule, they are classified as ionic (anionic or cationic) and nonionic. When ionic-type surfactants are adsorbed onto polymer particles, they provide stabilization by electrostatic repulsion between them and when the nonionic type are adsorbed instead the mode of stabilization is by steric repulsion. Electrosteric stabilization is provided by polyelectrolyte chains that give place to both modes of repulsion electrostatic and steric. 

Cationic surfactants and polymers adsorb readily on silica due to electrostatic interaction since pH of most practical systems is above 2 and silica is negatively charged under these conditions. Anionic surfactants or polymers do not adsorb on silica at neutral pH due to the presence of similar charge on the solid and the adsorbate. However, anionic surfactants can adsorb on silica in the presence of multi-valent metal cations in the pH range in which metal ions hydrolyze to their first hydroxyl complex. This has been attributed to the chemisorption of the first hydroxyl species of metal ions on silica and modification of the quartz surface [3]. 

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