Dynamics water-fluid interfaces

Miller, R., Fainerman, V.B., Makievski, A.V., Kragel, J., Grigoriev, D.O., Kazakov, V.N., Sinyachenko, O.V. (2000a). Dynamics of protein and mixed protein + surfactant adsorption layers at the water-fluid interface. Advances in Colloid and Interface Science, 86, 39-82. 

The rheological properties of a fluid interface may be characterized by four parameters surface shear viscosity and elasticity, and surface dilational viscosity and elasticity. When polymer monolayers are present at such interfaces, viscoelastic behavior has been observed (1,2), but theoretical progress has been slow. The adsorption of amphiphilic polymers at the interface in liquid emulsions stabilizes the particles mainly through osmotic pressure developed upon close approach. This has become known as steric stabilization (3,4.5). In this paper, the dynamic behavior of amphiphilic, hydrophobically modified hydroxyethyl celluloses (HM-HEC), was studied. In previous studies HM-HEC s were found to greatly reduce liquid/liquid interfacial tensions even at very low polymer concentrations, and were extremely effective emulsifiers for organic liquids in water (6). 

Regarding (1), the most academic approach, at the time of writing only embryonic attempts have been made. In sec. 2.2c the structure of water near Interfaces has been discussed and in sec. 3.9 the same has been done for charged surfaces. As to the dynamics, fig. 2.5 may be reconsidered. Although this figure does not specifically apply to water, for this liquid the dynamics may be similar. It may be inferred that residence times of fluid molecules are relatively... 

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

The following so-called dynamic capillary method was developed by Van Hunsel Joos (1987b) and complements the area of application with respect to other methods. This method allows measurements from 50 ms up about 1 s, similar to the inclined plate and growing drop techniques described above, and can be used at liquid/liquid and liquid/gas interfaces without modification. The principle of the experiment is schematically given in Fig. 5.23. Two fluids are contained in a tube of diameter R. The interface (or surface in case of studies at the water/air interface) is located in such a way that its interfacial tension can be measured by the capillary rise of the lower liquid in a narrow capillary c, which connects the both fluids. The height of the capillary rise h is determined via a cathetometer Cat. 

Although most polymers tend to accumulate at the fluid interface, reports involving the transfer of polymeric micelles (micellar shuttle) between two immiscible phases have been pubHshed. Poly(N-isopropylacrylamide) (PNIPAM), a thermally responsive polymer, is insoluble and can undergo a conformation change above its lower critical solution temperature of 32 ° C. The thermo reversible miceUization—demicellization process and micellar shuttle of PNIPAM-PEO diblock copolymer at a water-IL interface were investigated by dissipative particle dynamics (DPD) simulations (Soto-Figueroa et al, 2012). Simulation results confirm that the phase transfer behavior of polymeric micelles is controlled by the temperature effect that changes the diblock copolymer from hydrophilic to hydrophobic (as shown in Fig. 33). 

The absolute values of the interfacial tensions varied between different amphi-philes and solvents (Table 1). AOT, which is well known in the literature for the formation of microemulsions, showed the lowest surface tension at the interface of both solvents. The other nonionic snrfactants mentioned here. Span 80 and Brij 72 showed shghtly higher valnes. This was also observed for Lecithine, but this lipid precipitated partly during the spinning-drop measurements. Due to this phenomenon, it was not possible to measure accurate data for this emulsifying compound. The interfacial tension had also some influence on the mean size of the emulsion droplets and on the stability of the vesicles (Table 3). In addition to the stationary values of the surface tension, dynamic processes as the surfactant diffusion represented another important factor for the process of stimulated vesicle formation. If an aqueous droplet passed across the fluid interface it carried-over a thin layer of emulsifiers and thereby lowered the local surfactant concentration in the vicinity of the oil-water interface. In the short time span, before the next water droplet approached the interface, the surfactant films should entirely reform and this only occurred, if the surfactant diffusion was fast enough. 

In order to be realistic, environmental models must contain multiple phases, and are often referred to as multiphasic, or multimedia compartment models. Being so requires interfaces that separate the phases and media. These interfaces can be real or idealized (i.e., imaginary). Two-dimensional interface planes are assumed to exist between the air-water, water-sediment, and soil-air phases or media. In reality, chemical transport across the air-water interphase plane involves a true phase change. The watery interface plane at the water-bed sediment junction and airy interface plane at the soil-air junction are only separated by imaginary planer surfaces. Nevertheless, due to the dramatic changes that typically occur within the fluid and the associated media fluid dynamics on either interface side, different transport processes occur on opposite sides, typically. Therefore it is also practical to define an interfacial compartment for such imaginary and idealized interface situations. The interfacial compartment concept for multimedia, interphase chemical transport is based on the following ideas ... 

For surfactants, the hydrophobic free energy of transfer of the lipophilic hydrocarbon tail from water to oil provides the driving force for aggregation. But the hydrophilic head-groups prefer an aqueous environment and an interface between the polar region and the lipophilic domains results. With hydrocarbon tails, like alkanes, there are a large number of accessible tail conformations, so that the hydrophobic region is usually fluid-like aroimd room temperature. The interface can be a dynamic one of a well-defined... 

Mass transfer in the feed and strip solutions is limited by the extent of concentration polarization. On the feed side of the membrane, concentration polarization refers to an increase in the concentration of solutes at and near the feed-membrane interface because of evaporation of water into the membrane pores (Fig. 1). The resulting solute concentration gradient between the membrane-feed interface, where the concentration is greatest, and the bulk solution induces diffusive transport of rejected solutes back through the concentration polarization boundary layer into the bulk stream. Bulk solution is simultaneously transported to the membrane wall by convection. When equilibrium has been established under a given set of operating conditions (stream flow rate, temperature, fluid dynamics imposed by membrane module design), the rate of back diffusion is equal to the rate at which the solutes are carried to the membrane surface by convective flow. ... 

Stuart s studies of the structure of the liquid-vapor interfaces of metals and alloys can also be related to his previous research. He developed the first theory of transport in dense simple fluids that explicitly recognizes, and accounts for, the different dynamics associated with short-range repulsion and longer-ranged attraction. He has contributed to the theory of the three-molecule distribution function in a liquid and the theory of melting, and he developed the Random Network Model of water and the first consistent... 

In recent years a great many studies have reported on the dynamic systems where a drop of liquid is placed on a smooth solid surface. ° The system liquid drop-solid is a very important system in everyday life, for example, rain drops on tree leaves or other surfaces. It is also significant in all kinds of systems where a spray of fluid is involved, such as in sprays or combustion engines. The dynamics of liquid drop evaporation rate is of much interest in many phenomena. The liquid-solid interface can be considered as follows. Real solid surfaces are, of course, made up of molecules not essentially different in their nature from the molecules of the fluid. The interaction between a molecule of the fluid and a molecule of the boundary wall can be regarded as follows. The molecules in the solid state are not as mobile as those of the fluid. It is therefore permissible for most purposes to regard the molecules in the solid state as stationary. However, complexity arises in those liquid-solid systems where a layer of fluid might be adsorbed on the solid surface, such as in the case of water-glass. 

The oil-water interface is one of the most important systems. The liqnid-liquid interface constitutes a phase separation where two different molecules meet. We can directly measure the magnitude of the surface tension, with rather high precision. It wonld thns seem that much useful information can be obtained if we could measure a dynamic parameter of the interface, such as the freezing phenomenon. It is widely known that liqnids can be cooled below their freezing temperature without solidification (snpercooled fluid) and that they can be heated above their boiling temperature without vaporization (snperheated liquid). 

In many flotation systems, the electrical nature of the mineral/water interface controls the adsorption of collectors. The flotation behavior of insoluble oxide minerals, for example, is best understood in terms of electrical double-layer phenomena. A very useful tool for the study of these phenomena in mineral/water systems is the measurement of electrokinetic potential, which results from the interrelation between mechanical fluid dynamic forces and interfacial potentials. Two methods most commonly used in flotation chemistry research for evaluation of the electrokinetic potential are electrophoresis and streaming potential. 

Surfactants. Some compounds, like short-chain fatty acids, are amphiphilic or amphipathic that is, they have one part that has an affinity for the nonpolar media (the nonpolar hydrocarbon chain), and one part that has an affinity for polar media, that is, water (the polar group). The most energetically favorable orientation for these molecules is at surfaces or interfaces so that each part of the molecule can reside in the fluid for which it has the greatest affinity (Figure 4). These molecules that form oriented monolayers at interfaces show surface activity and are termed surfactants. As there will be a balance between adsorption and desorption (due to thermal motions), the interfacial condition requires some time to establish. Because of this time requirement, surface activity should be considered a dynamic phenomenon. This condition can be seen by measuring surface tension versus time for a freshly formed surface. 

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