Fluorinated interfacial adsorption layers

Rheological Properties of Interfacial Adsorption Layers in Fluorinated Systems... 

The rheological behavior of interfacial adsorption layers formed between the nonpolar phase (fluorinated or nonfluorinated) and the aqueous fluorinated or regular nonionic surfactant solution can be studied using the torsion pendulum instrument shown in Figure 4.11. The results of such studies for the adsorption layer of Pluronic F-68 formed at the interface between its aqueous solution and three different nonpolar phases are illustrated in Figure 4.31, which shows the shear stress, t, as a function of the time of deformation, t. The shear stress was applied to the entire thickness of the adsorption layer, and the time, t, was proportional to the shear deformation at a constant angular velocity, Q = 0.084 rad/s. 

Several independent experimental methods were applied that allowed comparison of the properties of these systems [ 18-20]. We present only the principal results of the three foDowing approaches (i) rheological studies of interfacial adsorption layers (lAL) by the rotating suspension method (ii) observation of the compression of two nonpolar droplets in the surfactant aqueous solution, with measurement of the force needed for their coalescence and (iii) evaluation of the free energy of interaction between nonpolar groups of lAL and various nonpolar liquids by measuring the contact rupture force between two methylated (or fluorinated) smooth solid particles in a given liquid. 

The stabilizing effect of interfacial adsorption layers (lAL) formed by ordinary hydrocarbon surfactants (HS) and by the fluorinated (FS) ones has been studied at the boundaries of their aqueous solutions and hydrocarbon (HL) or fluorocarbon (FL) nonpolar liquid phase. 

The adsorption of fluorinated surfactants at the electrode-solution boundary is of considerable practical interest for the application in electrochemical systems [60-64) (see Chapter 8). The electrochemical behavior of Zonyl FSN (nonionic), Zonyl FSD (cationic), Zonyl FSA (anionic), Fluorad FC-99 (anionic), and Fluorad FC-135 (cationic) at Hg and Pt electrodes has been investigated by using cyclic voltammetry and interfacial differential capacitance measurements. When the electrode is relatively hydrophobic, such as Hg, and the surface charge density is relatively low, the fluorinated surfactants, as well as hydrocarbon surfactants, are adsorbed with their hydrophobic segments oriented toward the electrode. The interaction of fluorinated surfactants with the Hg electrode is weaker and the adsorbed layer is less compact than those of hydrocarbon surfactants. When the electrode is more hydrophilic, such as Pt, or the surface charge density is high, the surfactants adsorb with their hydrophilic end group toward the electrode surface. 

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