Thermodynamics water-fluid interfaces

Using surface and interfacial tension data for some members of the homologous series of cationic surfactants we want to demonstrate to suitability of the thermodynamic approach of competitive adsorption for the formation of adsorption layers at different water/fluid interfaces, including those to alkane vapor and liquid alkane. We will restrict ourselves here to hexane as the oil or vapor phase. The particular effects of the alkane chain length have been discussed for example in [9]. For oils different from alkanes less systematic data exist, however, a specific impact of the molecular structure can be expected and the molecular characteristics might be rather different from those we obtained for alkanes. 

Information on the chemical potentials of components in a solution of biopolymers can serve as a guide to trends in surface activity of the biopolymers at fluid interfaces (air-water, oil-water). In the thermodynamic context we need look no further than the Gibbs adsorption equation,... 

Proteins and Lipids Can Alter the Thermodynamic and Dynamic Characteristics of Water at Fluid Interfaces... 

Fluid interfaces are those systems in which the two phases forming the interface are mobile. Thus, they include liquid liquid and liquid gas interfaces. The example chosen here to illustrate the application of thermodynamics to these interfaces is a closed system containing both gas and liquid phases consisting of two components, propanol and water. The liquid phase is designated p and the gas phase a. Now imagine a microprobe moving up through p to the interface (see fig. 8.4). The concentration of the alcohol is constant in this phase and equal to In the gas phase, its concentration is much lower. Near the interface the alcohol concentration rises before it falls to the lower gas phase value c%. This observation is... 

As shown in Figure 3.9, the L2 phase is able to solubilize a very large amount of a hydrocarbon such as decane or hexadecane. In fact, a composition containing up to 75% decane and water/surfactant/cosurfactant proportions corresponding to the L2 phase is still clear, fluid and isotropic, forms spontaneously, and is thermodynamically stable. The structure of this microemulsion can be (to some extent) regarded as a dispersion of tiny water droplets (reverse micelles) in a continuous phase of the hydrocarbon. The surfactant and cosurfactant are mainly located at the water/oil interface. This type of system is often referred to as a w/o microemulsion. 

In short, one of the most significant findings in the fields of electrolytes is the view that ions may adopt a non-monotonous concentration profile at the air-water interface. Please note that this is not in contradiction to thermodynamic analysis of the surface tension isotherms. Thermodynamics does not predict a distinct concentration profile instead, it measures an integral quantity from the fluid to the gas phase. There are many... 

Mixtures of aqueous electrolytes, hydrocarbons, and amphiphilic compounds have been the subjects of extensive research, especially those systems forming amorphous isotropic solutions, called microemulsions. Several books and papers have treated this subject [1-5]. The term microemulsion was first introduced by Hoar and Schulman [5]. Microemulsions are thermodynamically stable, isotropic, transparent colloidal solutions of low viscosity, consisting of three components a surfactant (amphiphile), a polar solvent (usually water), and a nonpolar solvent (oil) [1-7]. The surfactant monomers in these fluids reside at oil water interface and effectively lower the interfacial-free energy, resulting in the formation of optically clear, thermodynamically stable formulations. The innate formation of colloidal particles is typically up to nanometer scale globular droplets each... 

The decisive trick to link water uptake in the PEM with external thermodynamic conditions is the introduction of region II in Figure 2.19. The semipermeable and fixed mesh at the interface between regions II and III lifts the condition of mechanical equilibrium at this interface. The liquid pressure is uniform across the interface, while the total fluid pressure undergoes a discontinuous transition, that is, P jj iir... 

It is evident that the optimal interface between the water and amphiphilic molecules is the surface area of the hydrophilic part. The shapes in which the molecules arrange themselves depend partly on the optimal siuface area, as well as partly on the fluid volume of the hydrocarbon chains and the maximum length at which they can still be considered fluid. Although many structures can fit the geometry, one is usually best from a thermodynamic perspective. Large structures create too much order, while small structures cause the surface area to be larger than optimal, so a medimn-sized structure usually wins out. 

At the physical interface between the living and nonliving, cell membranes are essentially composed of phospholipids, which globally exhibit a structure with a polar hydrophilic end and a nonpolar hydrophobic end. The assembly constitutes a kind of bi-dimensional liquid, sometimes called a fluid mosaic model. The hydrophilic heads are immersed in the aqueous medium (water) on each side of the double layer, and the hydrophobic tails congregate inside the membrane. The thermodynamic stability of such a liquid is maximal for a thickness of 40 A (4nm), and the mechanical properties of the membranes result from the combination of the lipids with stabilizing elements, which maintain the cohesion of the assembly. The chemical study of these stabilizing elements allows a clear distinction between the cell membranes of the three fundamental types of organism. 

---