Microscopic Heterogeneities

It is now believed from studies on the natural photosynthetic systems that microenvironments for the photoinduced ET reaction play an important role in the suppression of the back ET [1-3]. As such reaction environments, molecular assembly systems such as micelles [4], liposomes [5], microemulsions [6-8] and colloids [9] have been extensively investigated. In them, the presence of microscopically heterogeneous phases and interfacial electrostatic potential is the key to the ET rate control. 

Although these results need to be understood - and confirmed in other similar systems - the authors claim that they are in qualitative agreement with a model of droplet formation and chain localization resulting from the existence of microscopic heterogeneities within the spinodal boundaries of the phase diagram [256]. 

Texture and structure are not explicitly defined terms. Structure is usually applied to macroscopic heterogeneity due to macroscopic movement such as flow structure or foliated structure, whereas texture usually refers to microscopic heterogeneity, such as holocrystalline texture, porphyritic texture, and lamellar texture due to exsolution. 

Dispersed System Disperse Phase Dispersing Agents and Dispersion Medium. Dispersed system is an apparently homogeneous, system which consists of a microscopically heterogeneous mixture of two or more finely divided phases, eg liquid-liquid (emulsions such as milk) liquid-solid (suspension, such as clay in water) gas-liquid (aerosols, such as fogs, clouds, mists) and gas-solid (such as smoke, dusts)... 

The resulting fluid consisting of a microscopically heterogenous mixture of two phases both,of which. are liquid (or semi-liquid) at ordinary temperatures is called emulsion. For... 

The determination of mechanistic rate laws for soil chemical processes is very difficult since microscopic heterogeneity is pronounced in soils and even for most soil constituents such as clay minerals, humic substances, and oxides. Heterogeneity can be enhanced due to different particle sizes, types of surface sites, etc. As will be discussed more completely in Chapter 3, the determination of mechanistic rate laws is also complicated by the type of kinetic methodology one uses. With some methods used by soil and environmental scientists, transport-controlled reactions are occurring and thus mechanistic rate laws cannot be determined. 

In common with conventional surfactants, Inisurfs and Transurfs, Surfmers form micelles in aqueous solutions above the CMC. The organized monomer aggregates of colloidal dimension are microscopically heterogeneous and may affect polymerization kinetics and polymer structure and properties. 

Oil and water do not mix, but on addition of a suitable surfaetant a microemulsion can be formed depending on the relative concentrations of the three components. Microemulsions (i.e. surfactant/water/oil mixtures) can also be used as reaction media see references [859-862] for reviews. Microemulsions are isotropic and optically clear, thermodynamically stable, macroscopically homogeneous, but microscopically heterogeneous dispersions of oil-in-water (O/W) or water-in-oil (W/O), where oil is usually a hydrocarbon. The name microemulsion, introduced by Schulman et al. in 1959 [863], derives from the fact that oil droplets in O/W systems or water droplets in W/O systems are very small (ca. 10... 100 nm nanodroplets). Unlike conventional emulsions, microemulsion domains fluctuate in size and shape with spontaneous coalescence and breakup. The oil/water interface is covered with surfactant molecules and this area can amount to as much as 10 m per litre ( ) of microemulsion. 

H-imaging NMR using the protons of adsorbed hydrocarbon molecules (DMP) has been used to determine the distribution of coke in the pellet. The microscopic heterogeneity observed by Xe NMR has been seen also, at the macroscopic scale, by imaging NMR. The study of the dififiision during the adsorption of DMP reveals competing processes. 

The main difference between solid-state reactions and those in solution is that of freedom of molecular motion (1-3) due to restriction of mobility of reactants in solids. Another important feature is the heterogeneous progress of reactions (3,4) frequently observed in solid states due to the microscopically heterogeneous states of aggregation or free volume distribution of the reaction media. In the case of poly(methyl methacrylate) (PMMA), which is an organic glass and is usually regarded as an inert matrix for photophysical and photochemical processes, a marked deviation from... 

Previous work has shown that gels formed from cross-linked polyacrylamide (6), alginate (6), hydrolyzed starch-polyacrylonitrile (7), and a cross-linked poly(acrylic acid) (8), consist of discontinuous structures with microregions of extremely low viscosity. Such microscopic heterogeneity has been attributed to permanent or diffusing fluctuations within the gel (i) or to a structure of closely packed, swollen microgels in a continuous water phase (2, 3). 

For microscopically heterogeneous surfaces, in the limit, polyatomic solids, three types of behavior can be distinguished. The liquid boundary may preferentially rest on one or the other type of surface, so that the contact angle approximates the minimum or the maximum available local values. In fact, the postulation of this type of behavior has been a basis for an explanation of hysteresis [29]. A second possibility is that there are locally varying angles, but on so small a scale as quickly to average to a single observable one. In a formal way one may then write... 

A hypothesis is developed to explain the formation mechanism of the ROS ring by the polymer rheology in high pH caustic media influenced by the microscopic heterogeneity of porous medium. 

---