Recent studies

The followmg summarizes the studies of electrokinetic barrier tests that have been done in the last 10 years or so. 

As mentioned earlier, Narasimhan and Sri Ranjan (2000) tested their barrier model with the data produced earlier by Yeung and Mitchell (1993). In Yeung s (1990) experiments, a hydraulic head of 500 cm was applied to a silty clay soil sample of 10-cm length, generating a hydraulic gradient of 50. An electric field of IV/cm applied for just 1 h per day successfully opposed the hydraulic flow. After 25 days, the sodium ions were still contained within the soil sample. For further details, the reader is referred to Chapter 30. 

Without the aid of an electric field, under the influence of the hydraulic gradient, the cadmium spread quite rapidly to the soil down-gradient, sections 1, 2, and 3 (Fig. 16.7). 

However, when a modest electric field of lOV was applied over 20 cm of soil (0.5V/cm), the cadmium migration was retarded and had only slightly migrated to section 3, the next section down-gradient of the contamination. At applied voltages 

A word of caution At very high pH (usually greater than 10), some heavy metals, such as Cd, Pb, and Cr, can form anions, and their solubility can increase with increasing pH. This would mean that under these extreme conditions in an electric field, they would travel back toward the anode, that is, back down-gradient. However, they would eventually find themselves in a lower pH environment and therefore become more insoluble again. In the cadmium experiments of Ruggeri above, the pH levels found were less than 10, and no evidence of reversal of cadmium was found. 

A large number of additional structures have been reported since these original six structures were described in 2000 (Table 1) [18, 19, 20, 21]. These more recent structures fall into three groups (a) structures of ligand-free ODCase, (b) structures of wild-type ODCase bound to inhibitors, and (c) structures of ODCase mutants. 

The following paragraphs list pertinent studies on the decomposition of halogen compounds which have been reported in the recent literature. 

Critical reviews on the mechanisms of the pyrolysis of halogen compounds in the gas-phase have been given by Maccoll and by Maccoll and Thomas . Commentary on these and related mechanisms has been given by Kwart and Waroblak Herndon and Manion , and Hartig et a/. . Holmes et have discussed the role of the protective carbonaceous films in these studies (see also ref. 824). 

There has been considerable interest in the elimination of hydrogen halides from halogenated hydrocarbon molecules and radicals which have been vibration-ally excited by chemical activation, by photochemical methods or by shock-tube techniques. Studies on fluorinated species have been reported by Trotman-Dickenson et a/.7 3.839-84i,846.908,909 by Pritchard et fl/.747,748.905.919.920  

Setser et a/, 6°. 63.836.939 b ve discussed similar studies on fluorinated, chlorinated and brominated species. The reactions of chlorinated species have also been reported by other workers .  

Papers on the pyrolytic decomposition of fluorinated hydrocarbons have included those on methyl fluoride , and ethyl fluoride , on polyfluorinated methanes and ethanes - - , on vinyl fluoride - , on per-fluoroethylene and on perfluoropropene . Isomerisation reactions of fluoro-olefins - - - have been discussed. Of related interest have been the reactions of fluorosilanes reported by Haszeldine et a/. 9J.806.8i4.8is ref. 778). 

X-ray analysis of the various samples that were produced indicated that the system ZnO-ZnClj-HjO includes four crystalline phases, two of which, ZnO and ZnClj. l HjO, are essentially the starting materials. Sorrell also found the 4 1 5 phase, reported by Droit, with an identical X-ray powder diffraction pattern to that reported by Nowacki Silverman (1961, 1962), and a 1 1 2 phase. Since neither the 1 1 2 nor the 4 1 5 phase lost or gained weight on exposure to air at about 50% relative humidity and 22 °C and no changes developed in the X-ray diffraction pattern following this exposure, he concluded that the previously reported 1 1 1 phase cannot be formulated from mixtures of ZnO and aqueous ZnCl,. 

The cementition reaction between zinc oxide powder and aqueous zinc chloride was found to be both rapid and extremely exothermic. Although at least four days equilibration was allowed before examining any of the cements in detail, Sorrell found evidence that reaction was complete within 20 to 30 minutes and occurred without observable development of intermediate phases. He also found that, as the concentration of reactants was increased, so reaction rate increased until, at sufficiently high concentrations, reaction occurred too quickly to allow proper mixing of the reactants. Preheating the zinc oxide at 900 °C for 16 hours was found to slow the reaction down, but only slightly. 

Sorrell showed that the two discrete phases could be readily and reversibly interconverted. For example, the 1 1 2 phase was found to react 

The 4 1 5 phase was shown by thermogravimetric analysis to dissociate at about 160 °C to zinc oxide and the 1 1 2 phase, a process which was verified using X-ray diffraction (Sorrell, 1977). Once the 1 1 2 phase was formed it underwent characteristic dissociation at temperatures above 160 °C. 

The equilibrium relationships found by Sorrell (1977) were valid only for room temperature (22+2 °C) and, because samples were allowed to cure in sealed containers, for equilibrium water vapour pressures determined by the assembly of phases present. The phases which exist under such conditions were quite unequivocally found to be 4 1 5 and 1 1 2. However Sorrell pointed out that it is entirely possible that lower hydration states of either phase could be stable at higher temperatures or lower humidities. In particular the 4 1 4 phase (Feitknecht, 1933) may well be such a phase, particularly as one of the five waters of hydration is known to be held only loosely in the structure. Indeed, Sorrell reported that he observed a slight shoulder on the larger dehydration peak of the DTG curve of the 4 1 5 phase that might be assigned to the loss of this first water molecule. He did not, however, succeed in isolating or characterizing a 4 1 4 phase. 

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Figure V-8 illustrates that there can be a pH of zero potential interpreted as the point of zero charge at the shear plane this is called the isoelectric point (iep). Because of specific ion and Stem layer adsorption, the iep is not necessarily the point of zero surface charge (pzc) at the particle surface. An example of this occurs in a recent study of zircon (ZrSi04), where the pzc measured by titration of natural zircon is 5.9 0.1...

While much early work with dispersed electrochemical systems focused on silver halide sols [16], more recent studies by Rusling and co-workers and others exploited... 

The Washburn model is consistent with recent studies by Rye and co-workers of liquid flow in V-shaped grooves [49] however, the experiments are unable to distinguish between this and more sophisticated models. Equation XIII-8 is also used in studies of wicking. Wicking is the measurement of the rate of capillary rise in a porous medium to determine the average pore radius [50], surface area [51] or contact angle [52]. 

Foam rheology has been a challenging area of research of interest for the yield behavior and stick-slip flow behavior (see the review by Kraynik [229]). Recent studies by Durian and co-workers combine simulations [230] and a dynamic light scattering technique suited to turbid systems [231], diffusing wave spectroscopy (DWS), to characterize coarsening and shear-induced rearrangements in foams. The dynamics follow stick-slip behavior similar to that found in earthquake faults and friction (see Section XU-2D). 

More recently, studies employing STM have been able to address surface self-diffiision across a terrace [16, 17. 18 and 19], It is possible to image the same area on a surface as a fiinction of time, and watch the movement of individual atoms. These studies are limited only by the speed of the instrument. Note that the performance of STM instruments is constantly improving, and has now surpassed the 1 ps time resolution mark [20]. Not only has self-diflfiision of surface atoms been studied, but the diflfiision of vacancy defects on surfaces has also been observed with STM [18]. 

A recent study of the vibrational-to-vibrational (V-V) energy transfer between highly-excited oxygen molecules and ozone combines laser-flash photolysis and chemical activation with detection by time-resolved LIF [ ]. Partial laser-flash photolysis at 532 mn of pure ozone in the Chappuis band produces translationally-... 

More recently, studies of the hysteresis of these phase transitions have illuminated the importance of kinetic factors in solid-solid phase transitions [224]. The change between crystal stmctures does not occur at the same point when pressure is increasing, as when it is decreasing the difference between this up-stroke and down-stroke pressure... 

Early studies showed tliat tire rates of ET are limited by solvation rates for certain barrierless electron transfer reactions. However, more recent studies showed tliat electron-transfer rates can far exceed tire rates of diffusional solvation, which indicate critical roles for intramolecular (high frequency) vibrational mode couplings and inertial solvation. The interiDlay between inter- and intramolecular degrees of freedom is particularly significant in tire Marcus inverted regime [45] (figure C3.2.12)). 

The most powerful teclmique for studying VER in polyatomic molecules is the IR-Raman method. Initial IR-Raman studies of a few systems appeared more than 20 years ago [16], but recently the teclmique has taken on new life with newer ultrafast lasers such as Ti sapphire [39]. With more sensitive IR-Raman systems based on these lasers, it has become possible to monitor VER by probing virtually every vibration of a polyatomic molecule, as illustrated by recent studies of chlorofonn [40], acetonitrile [41, 42] (see example C3.5.6.6 below) and nitromethane [39, 43]. 

The case of W = 3 is somewhat more complicated because the corresponding orthogonal matrix is expressed in terms of three angles, namely, Yi2>Ti3i and 723 [36,84,85]. This case was recently studied by us in detail [85] and here we briefly repeat the main points. 

A similar approach is followed in a recent study of the Lewis-acid catalysis of a Michael addition in acetonitrile. See Fukuzumi, S. Okamoto, T. Yasui, K Suenobu, T. Itoh, S. Otera, J. Chem. Lett. 1997, 667. 

Density functional theory calculations have shown promise in recent studies. Gradient-corrected or hybrid functionals must be used. Usually, it is necessary to employ a moderately large basis set with polarization and diffuse functions along with these functionals. 

A more quantitative approach to the influence of the thiazole ring on the reactivity of a lateral functional chain was made in a recent study by Noyce and Fike (383), already discussed in Section 10.4. The first-order rates of solvolysis for three isomeric 1-thiazolylethyl chlorides were determined in 80% ethanol. The order of relative reactivity observed. 

A recent study instigated by Kolosova (521) and taken up and proved by Chanon and Metzger (597) increased the yield of this reaction to 70% in the case of chloroacetaldehyde and to 50% in the case of di-bromoether, whereas other authors have been either unable to obtain the product in this way or in very low yield (11%) (316). 

Humphlett and Lamon (522) have recently studied the intermediary compounds of this reaction and have shown with the help of infrared and ultraviolet spectroscopy that 176 was not present in the reaction mixture (Scheme 90) instead, a compound containing an hydroxyl radical and not a carbonyl function was present (Scheme 91). 

Striking confirmation of the conclusion that the BET area derived from a Type IV isotherm is indeed equal to the specific surface is afforded by a recent study of a mesoporous silica, Gasil I, undertaken by Havard and Wilson. This material, having been extensively characterized, had already been adopted as a standard adsorbent for surface area determination (cf. Section 2.12). The nitrogen isotherm was of Type IV with a well defined hysteresis loop, which closed at a point below saturation (cf. F, in Fig. 3.1). The BET area calculated from it was 290 5 0 9 m g , in excellent agreement with the value 291 m g obtained from the slope of the initial region of the plot (based on silica TK800 as reference cf. p. 93). 

A vast amount of research has been undertaken on adsorption phenomena and the nature of solid surfaces over the fifteen years since the first edition was published, but for the most part this work has resulted in the refinement of existing theoretical principles and experimental procedures rather than in the formulation of entirely new concepts. In spite of the acknowledged weakness of its theoretical foundations, the Brunauer-Emmett-Teller (BET) method still remains the most widely used procedure for the determination of surface area similarly, methods based on the Kelvin equation are still generally applied for the computation of mesopore size distribution from gas adsorption data. However, the more recent studies, especially those carried out on well defined surfaces, have led to a clearer understanding of the scope and limitations of these methods furthermore, the growing awareness of the importance of molecular sieve carbons and zeolites has generated considerable interest in the properties of microporous solids and the mechanism of micropore filling. 

Although its use as a transparent electrode in diodes has made it one of the most useful of the conductive polymers, the intractable nature of the material made the stmcture of polyaniline difficult to determine. Recent studies of polyphenyleneamineimines have conclusively shown that the stmcture of PANI is an exclusively para-linked system (21). 

Apphcations of ultrasound to electrochemistry have also seen substantial recent progress. Beneficial effects of ultrasound on electroplating and on organic synthetic apphcations of organic electrochemistry (71) have been known for quite some time. More recent studies have focused on the underlying physical theory of enhanced mass transport near electrode surfaces (72,73). Another important appHcation for sonoelectrochemistry has been developed by J. Reisse and co-workers for the electroreductive synthesis of submicrometer powders of transition metals (74). 

More recently, studies of wine and beer have initiated techniques of statistically vaUd sensory analysis. Scientific studies involving wine continue in these areas, building on past discoveries. Natural phenols as desirable dietary components and monitors of storage and aging reactions are currently active fields. Viticultural research, as well as enological, continues to improve grapes and the wines made from them (11). 

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