Interfacial adsorption layers

Rodin, V.V. and Izmailova, V.N. NMR method in the study of the interfacial adsorption layer of gelatin. Colloids Surfaces A, 106, 95,1996. 

The term structural-mechanical barrier was for the first time introduced by P. A. Rehbinder [2,46-48]. This is a strong factor of stabilization of colloidal systems related to the formation of interfacial adsorption layers of low and high molecular weight surfactants which lyophilize interfaces. The structure and mechanical properties of such adsorption layers are able to ensure very high stability of dispersion medium interlayers between dispersed particles. 

Ismailova, V.N., "Stmcture Formation and Rheological Properties of Proteins and Surface-Active Polymers of Interfacial Adsorption Layers" in Progress in Surface and Membrane Science 13(1979)... 

Abstract The effect of lecithin (natural surfactant) addition to gelatin on the surface rheological properties of the water/ heptane interfacial layer and emulsion films formed by these liquids was studied. It was found that the gelatin/ledthin mixtures form complexes in the aqueous phase. Self-assembly of these complexes leads to the formation of viscoelastic interfacial adsorption layers characterized by a yield stress and elastic modules that provide stability of the emulsion films and emulsion systems. The above mentioned parameters evolve in time, though the formation of equilibrium interfacial layers proceeds during several hours emulsion bilayer films require only several minutes. 

This work is devoted to the study of thermodynamic and rheological properties of interfacial adsorption layers of gelatin and natural surfactants (lecithin) in a wide range of mixing ratios formed at interfaces between water and hydrocarbons. 

In the first case, the interfacial adsorption layers are characterized by clearly expressed viscoelastic properties and plasticity reflected by the yield stress Fig. 5a presents the interfacial shear viscosity, t], yield stress xys and surface elastic modules Gj. One can see that the elastic modules and the viscosity for gelatin layers without a surfactant (Ciec = 0) are equal to 1.2 mN/m and 8.3 mN s/m, respectively. Upon addition of phosphoUpids at low lecithin-to-gelatin ratio these parameters do not change. However some threshold component ratio exists and the elastic modules and the yield stress sharply increase beyond this threshold. This is the evidence of the increase in the number of contacts between strucmre elements in a gel-like layer. 

In our earlier works the adsorption layer at the solid/ liquid interface was employed as a nanophase reactor for the generation of nanocrystalline metal particles and for their stabilization in the presence of the clay mineral [17]. The procedure consists of adsorbing the precursor ions of the nanocrystalline material in the interfacial adsorption layer of solid particles dispersed in the liquid phase and the synthesis is carried out in the adsorption layer by introducing the reducing agent. The nanoparticles can be grown attached to the surface, in well-controllable number and size between the silicate layers. 

Enthalpy of displacement isotherms were determined by the flow technique. The heat effects recorded on dodecylammonium and dodecyldiammonium ver-miculites are found to be endothermic in both cases, i.e., the measured heat exchange process results in heat extraction. Since the adsorption isotherms unambiguously indicate positive adsorption of n-butanol, the question arises as to why an exothermic exchange enthalpy is not recorded. In our opinion, the reason for this is the endothermic enthalpy of dilution [59-61,69], which overcompensates for the interlamellar adsorption of butanol. When, knowing the adsorption excesses, the enthalpy isotherm A21// = f Xy,) characteristic of the solid/Uquid interfacial adsorption layer can be calculated, it is indeed the exothermic adsorption enthalpy isotherm specific for the surfacial interaction that is obtained (Fig. 30). This measurement suggests that the interlamellar adsorption of n-butanol is thermodynamically preferred and is accompanied by the liberation of a very large amount of heat (A21// = 16.0-16.5 J/g). 

FIGURE 4.11 Schematic illustration of a torsion pendulum device for studying the rheological properties of the interfacial adsorption layer formed at an interface between polar and nonpolar liquids. (From Izmaylova, V.N. et al., Doklady AN SSSR, 206, 1150, 1972 Izmaylova, V.N. et al., Kolloidnyi Zh., 35, 860, 1973 Izmaylova, V.N. et al.. Surface Phenomena in Protein Systems, Khimiya, Moscow, Russia, 1988.)... 

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. 

The cohesion between the hydrophobic part of the interfacial adsorption layer and the adjacent nonpolar phase can be modeled nsing the cohesion between model hydrophobic snrfaces in the same liqnid. In snch a simnlation, the hydrophobic solid snrfaces represent the hydrophobic tails of the snrfactant molecnles. This approach allows one to overcome the difficnlties associated with the mutual solubility of the components (see Chapter 1). For the solid/liqnid/solid interface, the main parameter characterizing the interactions is the free energy of interaction, F (or Aoj), which can be established experimentally nsing Derjagnin s theorem, that is, p = %RF, where p is the cohesive force in a direct contact between two spherical particles immersed in a liqnid medinm. Snitable model systems include spherical molecularly smooth glass beads with a radius R 1-1.5 mm and hydrophobized surfaces of different natures, namely, HS and HL, immersed into the hydrocarbon and fluorocarbon liquids, HL and FL. Only dispersion forces are present in such systems, which makes the quantitative description of their interaction well defined and not complicated by the presence of various polar components. 

It is worth emphasizing here that while the structural-rheological properties (i.e., mechanical strength) of the interfacial adsorption layer play a determining role in the stability of the system toward coalescence, they alone may not be sufficient for complete stabilization. The prevention of coagulation also requires that the structural-mechanical barrier formed is lyophilic (hydrophilic) with respect to the surrounding polar liquid. The latter can be achieved by the introduction of common surfactants, for example, sodium dodecyl sulfate (SDS). 

Amelina, E. A., Kumacheva, E. E., Chalyh, A. E., and E. D. Shchukin. 1996. The structure of interfacial adsorption layers in systems of an aqueous solution of a block copolymer of ethylene oxide and propylene—Fluorocarbon. KoUoidnyi Zh. 58 437-441. 

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. 

Figure 3.1 Scheme illustrating the use of rotating suspension for studying the rheological characteristics of the interfacial adsorption layer between polar and nonpolar liquids [15],... 

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. 

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