Polymeric surfactants adsorbed layer thickness

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. ... 

The inherently high colloid stability of nanoemulsions when using polymeric surfactants is due to their steric stabilization. The mechanism of steric stabilization was discussed above. As shown in Fig. 1.3 (a), the energy-distance curve shows a shallow attractive minimum at separation distance comparable to twice the adsorbed layer thickness 28. This minimum decreases in magnitude as the ratio between adsorbed layer thickness to droplet size increases. With nanoemulsions the ratio of adsorbed layer thickness to droplet radius (8/R) is relatively large (0.1 0.2) when compared with macroemulsions. This is schematically illustrated in Fig. 1.28 which shows the reduction in with increasing 8/R. 

The stabiHzation of organic pigment dispersions is achieved by use of polymeric anionic surfactants that provide strong adsorption on the polar surface of the pigment and hydroxyl groups for interaction with the aqueous phase. Non-polar intermediate sections of a polymeric anion add adsorbed layer thickness [3]. 

In addition to giving information about the shape and internal structure of colloidal aggregates, SANS studies can also be used profitably to determine the thickness and conformation of polymer layers adsorbed onto the surface of colloidal particles such as latex nanoparticles, and in some special cases, the surface of emulsion droplets. ° In such studies, the particles on which the polymer is adsorbed must generally be very accurately contrast matched to the solvent so as to allow information to be obtained only about the adsorbed layer. SANS studies have also been recently used in combination with differential scanning calorimetry and visual inspection of the solutions, to draw up a (simplified) partial phase diagram of the aggregation behavior of a polymeric surfactant in water.t ... 

In addition to ionic surfactants, nonionic surfactant molecules can also adsorb onto the particle surfaces to impart satisfactory stabihty to colloidal dispersions [20, 21]. Some very old examples include India ink and carbon black particles dispersed in the continuous aqueous phase containing a natural gum. This kind of colloidal stabilization mechanism (termed steric stabilization) was first illustrated experimentally by M. Faraday [31, 32]. Some representative polymeric materials (protective colloids) that are effective in preparing steri-cally stabilized aqueous colloidal dispersions are summarized in Table 2.7 [21]. A portion of an effective protective colloid must be hydrophobic enough to show a strong tendency to adsorb onto the hydrophobic particle surface. Furthermore, the adsorbed macromolecules must form a relatively thick hydrophilic layer surrounding the particle, which serves as a steric barrier to prevent the colloidal particles from flocculation. 

---