Thermodynamics interfacial layer properties

Monte Carlo and Molecular Dynamics simulations of water near hydrophobic surfaces have yielded a wealth of information about the structure, thermodynamics and transport properties of interfacial water. In particular, they have demonstrated the presence of molecular layering and density oscillations which extend many Angstroms away from the surfaces. These oscillations have recently been verified experimentally. Ordered dipolar orientations and reduced dipole relaxation times are observed in most of the simulations, indicating that interfacial water is not a uniform dielectric continuum. Reduced dipole relaxation times near the surfaces indicate that interfacial water experiences hindered rotation. The majority of simulation results indicate that water near hydrophobic surfaces exhibits fewer hydrogen bonds than water near the midplane. 

Monolayers of micro- and nanoparticles at fluid/liquid interfaces can be described in a similar way as surfactants or polymers, easily studied via surface pressure/area isotherms. Such studies provide information on the properties of particles (dimensions, interfacial contact angles), the structure of interfacial layers, interactions between the particles as well as about relaxation processes within the layers. Such type of information is important for understanding how the particles stabilize (or destabilize) emulsions and foams. The performed analysis shows that for an adequate description of II-A dependencies for nanoparticle monolayers the significant difference in size of particles and solvent molecules has be taken into account. The corresponding equations can be obtained by using a thermodynamic model developed for two-dimensional solutions. The obtained equations provide a satisfactory agreement with experimental data of surface pressure isotherms in a wide range of particle sizes between 75 pm and 7.5 nm. Moreover, the model can predict the area per particle and per solvent molecule close to real values. Similar equations were applied also to protein monolayers at liquid interfaces. 

This is a rather more complex problem because the interfacial layer is not infinitesimally thin and some free energy is stored in this layer. There is a gradient of molecular density, composition, enthalpy, entropy, electrical potential (for charged molecules) and many other properties in this interfacial transition layer, as shown in Figure 3.1 a as a property-distance plot. Actually, Us is located in a layer of certain thickness, Ax, and some assumption must be made if we want to define Us in thermodynamical terms, because it is impossible to decide physically where phase a ends and phase ft begins. The thickness of this transition layer, Axp for any two immiscible phases is shown in Figure 3.2 a and is dependent on the molecular nature of phases a and /), and also on external factors such as temperature and pressure. It has been found experimentally that this interfacial layer is usually a few molecules in thickness for most non-electrolytes. 

According to Gibbs [1], one can view an interface as a layer of finite thickness within which the composition and thermodynamic characteristics are different from those in the bulk of phases in contact. This approach allows one to describe the properties of interfaces phenomenologically in terms of excesses of the thermodynamic functions in the interfacial layer in comparison with the bulk of individual phases. With this approach one does not need to introduce any model considerations regarding the molecular structure of the interfacial layer or utilize particular values of layer thickness. 

Thus, the important features of the structural-mechanical barrier are the rheological properties (See Chapter IX,1,3) of interfacial layers responsible for thermodynamic (elastic) and hydrodynamic (increased viscosity) effects during stabilization. The elasticity of interfacial layers is determined by forces of different nature. For dense adsorption layers this may indeed be the true elasticity typical for the solid phase and stipulated by high resistance of surfactant molecules towards deformation due to changes in interatomic distances and angles in hydrocarbon chains. In unsaturated (diffuse) layers such forces may be of an entropic nature, i.e., they may originate from the decrease in the number of possible conformations of macromolecules in the zone of contact or may be caused by an increase in osmotic pressure in this zone due to the overlap between adsorption layers (i.e., caused by a decrease in the concentration of dispersion medium in the zone of contact). 

An interdisciplinary team of leading experts from around the world discuss recent concepts in the physics and chemistry of various well-studied interfaces of rigid and deformable particles in homo- and hetero-aggregate dispersed systems, including emulsions, dispersoids, foams, fluosols, polymer membranes, and biocolloids. The contributors clearly elucidate the hydrodynamic, electrodynamic, and thermodynamic instabilities that occur at interfaces, as well as the rheological properties of interfacial layers responsible for droplets, particles, and droplet-particle-film structures in finely dispersed systems. The book examines structure and dynamics from various angles, such as relativistic and non-relativistic theories, molecular orbital methods, and transient state theories. 

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. 

The preceding discussion allows one to address the problem of the phase state of sixrface and interfacial layers of pol3rmers in composites. Despite the non-uniformity, they can be characterized by their intrinsic dimensions, thermodynamic fimctions (entropy, enthalpy, specific volume), and the distinctions of mean local properties from the properties of the polymer in the brdk. In a number of instances these distinctions may be similar to the difference in the properties between amorphous and crystalhne regions in semicrystalhne polymers. The redistribution of fractions of different molecular mass in a smface layer, taking account of hmited thermodynamic immiscibility of polymer homo-logues, provides a basis to consider the transition layer as an independent phase. However, whether the surface and interfacial layers can be considered as an independent phase in the thermod5mamical meaning or not is a very important question. 

In addition to the thermodynamic characteristics of the adsorption equilibrium the dynamic dilational visco-elasticity of the surfactant interfacial layers is a very important quantity [5]. This Ireqnency dependent property of a liquid interface is a significant quantity in the stabilization of foams and emulsions. Of course, in many practical situations mixtures of surfactants with particles, polymers or proteins are used, however, these rather complex systems are not the subject of this work. 

In Chapter 4, we considered the thermodynamic properties of homogeneous phases with consistent properties through the entire phase. It should be obvious that this is not the case at the interface between two different phases of a system. The area of contact between two phases where the molecules of both phases are interacting is called the interfacial layer, which is usually considered to be a few molecules thick in a neutral species. If we are considering the bulk properties of the phase, the effect of this region on the properties of the phase can be considered to be vanishingly small. 

The characteristic effect of surfactants is their ability to adsorb onto surfaces and to modify the surface properties. Both at gas/liquid and at liquid/liquid interfaces, this leads to a reduction of the surface tension and the interfacial tension, respectively. Generally, nonionic surfactants have a lower surface tension than ionic surfactants for the same alkyl chain length and concentration. The reason for this is the repulsive interaction of ionic surfactants within the charged adsorption layer which leads to a lower surface coverage than for the non-ionic surfactants. In detergent formulations, this repulsive interaction can be reduced by the presence of electrolytes which compress the electrical double layer and therefore increase the adsorption density of the anionic surfactants. Beyond a certain concentration, termed the critical micelle concentration (cmc), the formation of thermodynamically stable micellar aggregates can be observed in the bulk phase. These micelles are thermodynamically stable and in equilibrium with the monomers in the solution. They are characteristic of the ability of surfactants to solubilise hydrophobic substances. 

In the first part of this century, electrochemical research was mainly devoted to the mercury electrode in an aqueous electrolyte solution. A mercury electrode has a number of advantageous properties for electrochemical research its surface can be kept clean, it has a large overpotential for hydrogen evolution and both the interfacial tension and capacitance can be measured. In his famous review [1], D. C. Grahame made the firm statement that Nearly everything one desires to know about the electrical double layer is ascertainable with mercury surfaces if it is ascertainable at all. At that time, electrochemistry was a self-contained field with a natural basis in thermodynamics and chemical kinetics. Meanwhile, the development of quantum mechanics led to considerable progress in solid-state physics and, later, to the understanding of electrostatic and electrodynamic phenomena at metal and semiconductor interfaces. 

In assessing the meaning of the parameters y and 3j/dT, one eventually asks the question What is the interface and how thick is it The interface certainly has the thickness of the layer of molecules at the termination of the liquid phase but the disruption of normal liquid structure may extend somewhat further into the bulk. Since the thickness of the interface is not known, it is difficult to give a molecular interpretation of the thermodynamic properties of this region. However, effective thermodynamic conventions have been developed for discussing interfacial properties. These are outlined in detail in the following section. 

In the linear case, non-equilibrium properties of adsorption layers at fluid interfaces can be quantitatively described by the interfacial thermodynamic modulus (Defay, Prigogine Sanfeld 1977),... 

A great deal of our knowledge about the double layer comes from measurements of macroscopic, equilibrium properties, such as interfacial capacitance and surface tension. In general, we are interested in the way in which these properties change with potential and with the activities of various species in the electrolyte. The next section will deal with experimental aspects in some detail. For the moment, we will concentrate on the theory that we use to suggest and interpret experiments. Since our concern now is with macroscopic, equilibrium properties, we can expect a thermodynamic treatment to describe the system rigorously without a postulated model. This is an important aspect, because it implies that we can obtain data that any successful structural model must rationalize. 

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