Adsorption onto Hydrophobic Surfaces

In comparison, the adsorption of the AOT was found to persist to lower surfactant concentrations than was the case for the nonionic surfactant Ci2E4. 

Fragneto et al. [66] made a more complete study of the adsorption of Ci2E4 onto an OTS hydrophobic surface. Two different isotopically labelled OTS surfaces (h-OTS, d-OTS) were used to establish the structure of the OTS layer in more detail. Subsequent measurements using differently labelled Ci2E4 and solvent established that the adsorbed amount and structure of the adsorbed layer was similar to that obtained at the air-water interface. Furthermore, they were able to quantify the extent of penetration of the alkyl chains into the OTS layer. 

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Kronberg et al. have examined the thermodynamics of surfactant micellization and adsorption onto hydrophobic surfaces [47]. They considered two main contributions for the Gibbs free energy of transfer from the aqueous solution to the micelle or the surface ... 

The following protocol for passive adsorption is based on methods reported for use with hydrophobic polymeric particles, such as polystyrene latex beads or copolymers of the same. Other polymer particle types also may be used in this process, provided they have the necessary hydrophobic character to promote adsorption. For particular proteins, conditions may need to be optimized to take into consideration maximal protein stability and activity after adsorption. Some proteins may undergo extensive denaturation after immobilization onto hydrophobic surfaces therefore, covalent methods of coupling onto more hydrophilic particle surfaces may be a better choice for maintaining native protein structure and long-term stability. 

Figure 15.12 Detergent molecules can be used to solubilize carbon nanotubes by adsorption onto the surface through hydrophobic interactions and create half-micelle structures with the hydrophilic head groups facing outward into the aqueous environment.

Entropy-related adsorption, known as hydrophobic sorption, involves the partitioning of nonpolar organics from a polar aqueous phase onto hydrophobic surfaces, where they are retained by dispersion forces. The major feature of hydrophobic sorption is the weak interaction between the solute and the solvent. The entropy change is due largely to the destruction of the cavity occupied by the solute in the solvent and the destruction of the structured water shell surrounding the solvated organic. 

Entropy-related adsorption, denoted hydrophobic sorption (or solvophobic interaction) is the partitioning of nonpolar organics out of the polar aqueous phase onto hydrophobic surfaces. Fig. 5.6 shows a schematic model of forces that contribute to the sorption of hydrophobic organics, relevant to the subsurface environment. 

Balazs and Lewandowski (1990) have performed simulations of the adsorption of triblock copolymers onto a planar surface, and examined the conformations of the adsorbed chains. Monte Carlo simulations were performed of the motion of hydrophilic-hydrophobic chains on a cubic lattice. These simulations revealed a complex structure in the interfacial region due to the self-assembly of chains, driven by the solvent-incompatible block, reducing adsorption onto the surface. The influence on the surface coverage of length of the hydrophilic segement, polymer concentration, interaction energy between hydrophilic block and the... 

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... 

Polyoxypropylene or Polyoxyethylene Units Polyoxypropylene units increase the hydrophobic nature of the surfactant, its adsorption onto polar surfaces, and its solubility in organic solvents. Polyoxyethylene units decrease the hydrophobic character of the surfactant. 

The interaction of nanoparticles with the proteins is governed from the same type of interactions described for carbon nanotubes. Since NPs carry charges, they can electrostatically adsorb biomolecules with different charges, which depend on the pH that the immobilization takes place and the pi of the protein [3,191]. Moreover, hydrophobic interactions, hydrogen bonds and non-specific absorption can play a role for enzyme non-covalent adsorption onto the surface of nanoparticles. 

The relative extent of protein adsorption onto polymer surfaces is influenced by the surface tensions of the substrate material, of the suspending liquid and of the proteins themselves. For one and the same substrate material the extent of protein adsorption depends on the relative hydrophobic ty of the proteins. For the situation where YlV > YPV more hydrophobic proteins will adsorb to the largest extent. 

This chapter reviews results for the adsorption of ionic surfactants onto hydro-phobic surfaces. Even with this limitation there are many possible combinations. Both anionic and cationic surfactants are considered. The hydrophobic surfaces may be either uncharged or positively or negatively charged. Thus, the electrostatic interactions between the adsorbing surface and the surfactant may be either nonexistent, attractive, or repulsive. Finally, the adsorbing materials may be either a single species or a combination of surfactants. This review is not a comprehensive overview of all of the published material on this topic. Rather it presents representative results concerning the adsorption of ionic surfactants onto hydrophobic surfaces. 

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... 

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