Aqueous-phase

The attempt to model vibrational spectra of species in aqueous solutions involves added complications compared to gas-phase spectra. First, one must account for the short-range solvation effects (i.e., H-bonding). Doing so requires that the model size be increased significantly to add enough water molecules to form at least one complete solvation sphere. In addition, a higher level of theory should be used to accurately account for the H-bonds. 

Solvation of solute vs. basis set -acetic acid and acetate. As a test of molecular orbital theory and the molecular cluster approximation to reproduce experimental vibrational frequencies, the molecules acetic acid and acetate were selected. The gas-phase frequencies of acetic acid were reproduced fairly well with molecular orbital theory. Consequently, discrepancies between observed and calculated frequencies for the aqueous species may be attributed to solvation effects. Acetate represents a bigger challenge because the charged species is likely to have stronger H-bonding associated with its solvation shell. 

A series of energy minimizations with an increasing number of H2O molecules in the solvation shell was performed for acetic acid with the HF/3-21G basis set (Kubicki 1999). (Note No continuum model was used in these calculations.) A comparison of measured and calculated frequencies is given in Table 3. From the results above, one can see that with the addition of eight water molecules, the calculated frequencies begin to 

Another method for predicting vibrational frequencies of species in aqueous solution 

For these reasons, the combination of spectroscopy and molecular modeling have become popular tools for studying mineral surface structures and reactions. Spectroscopic and microscopic techniques other than vibrational spectroscopy, such as EXAFS (Bargar et al. 1999), LEEDS (Henderson et al. 1998), and STM (Rosso et al. 1999), can provide more detail about surface structures and complexes and are discussed in other chapters of this volume. Vibrational spectroscopy is useful, however, for identifying proton speciation on mineral surfaces because infrared spectroscopy is 

The same phenomena of primary radical formation, distribution, and reactivity, as described in other chapters, apply to the radiolysis of the aqueous phase in food, but certain distinctive factors affect their fate. One factor is that, in the case of high dose treatment for sterilization, this phase will be frozen. Another factor is the high concentration of reactive solutes. Accordingly, the yields and reactions of Cs, H, and OH will be significantly modified, as already indicated in section 2.3. Their reactions with relevant salts, vitamins, and nucleic acids, either normally in foods or purposefully added to foods, are considered. 

Though reactions of primary radicals with sulfates and phosphates are unlikely, reactions of eg with endogenous or added nitrates or nitrites, which are used in the curing of meats, could lead to their reduction. The extent would be limited by competition for es by other solutes. Reduction of nitrate, already mentioned, is illustrative. Stoichiometrically, for every two moles of Cg reacting with one mole of NO3 , one mole of NCh would be formed. As explained, the extent of N02 formation even in frozen systems would depend significantly on the concentration of NO3 and on temperature. 

The nutritional importance aside, the radiolysis of ascorbic acid [8] and thiamin is of interest, because it illustrates certain distinctive kinetic processes. 

Ascorbic acid is highly reactive to all the primary water radicals, because of its carbonyl group and double bond. Reaction with ei or H reduces ascorbic acid to a ketyl radical, while reaction with OH oxidizes it to the relatively unreactive tricarbonyl radical ion [9]. Aside from a possible reaction with cytochrome-c (or ferrimyoglobin), the radical ion is most likely to undergo a complex disproportionation reaction that regenerates the ascorbic acid and produces dehydroascorbic acid, which has essentially the same vitamin activity. These reactions need to be considered, because ascorbic acid is added to foods to fortify them, to facilitate curing meats, and to enhance antioxidants. 

Reactions initiated by H and OH would lead to these and other radicals. H could react at the backbone C-H and at the carbonyl group, forming the peptide radical, but is more likely to add to aromatic or heterocyclic amino acid moieties, forming side chain radicals. OH could also react by abstraction or, more likely, by addition. The formation of addition radicals facilitates crosslinking through the side chains. Studies with polyphenylalanine peptides confirm such crosslinking and show how crosslinked products with and without hydroxyl groups can be produced [15, 16]. 

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A diffusion mechanism is also used in dialysis as a means of separating colloids from crystalloids. The rate of diffusion of molecules in gels is practically the same as in water, indicating the continuous nature of the aqueous phase. The diffusion of gases into a stream of vapour is of considerable importance in diffusion pumps. 

There is about 20% aqueous phase (milk or water) together with food additives, vitamins and colour. 

S = solid, A = air, W = water or aqueous phase, O = oily or organic water-insoluble phase.)... 

R. G. Laughlin, The Aqueous Phase Behavior of Surfactants, Academic, London, 1994. 

It is quite clear, first of all, that since emulsions present a large interfacial area, any reduction in interfacial tension must reduce the driving force toward coalescence and should promote stability. We have here, then, a simple thermodynamic basis for the role of emulsifying agents. Harkins [17] mentions, as an example, the case of the system paraffin oil-water. With pure liquids, the inter-facial tension was 41 dyn/cm, and this was reduced to 31 dyn/cm on making the aqueous phase 0.00 IM in oleic acid, under which conditions a reasonably stable emulsion could be formed. On neutralization by 0.001 M sodium hydroxide, the interfacial tension fell to 7.2 dyn/cm, and if also made O.OOIM in sodium chloride, it became less than 0.01 dyn/cm. With olive oil in place of the paraffin oil, the final interfacial tension was 0.002 dyn/cm. These last systems emulsified spontaneously—that is, on combining the oil and water phases, no agitation was needed for emulsification to occur. 

Although extraction of lipids from membranes can be induced in atomic force apparatus (Leckband et al., 1994) and biomembrane force probe (Evans et al., 1991) experiments, spontaneous dissociation of a lipid from a membrane occurs very rarely because it involves an energy barrier of about 20 kcal/mol (Cevc and Marsh, 1987). However, lipids are known to be extracted from membranes by various enzymes. One such enzyme is phospholipase A2 (PLA2), which complexes with membrane surfaces, destabilizes a phospholipid, extracts it from the membrane, and catalyzes the hydrolysis reaction of the srir2-acyl chain of the lipid, producing lysophospholipids and fatty acids (Slotboom et al., 1982 Dennis, 1983 Jain et al., 1995). SMD simulations were employed to investigate the extraction of a lipid molecule from a DLPE monolayer by human synovial PLA2 (see Eig. 6b), and to compare this process to the extraction of a lipid from a lipid monolayer into the aqueous phase (Stepaniants et al., 1997). 

Saturating the aqueous phase with sodium chloride. 

Primary alcohols, hexyl and higher, do not dissolve appreciably the aqueous phase remains clear. 

Petroleum ether is preferable to diethyl ether because it removes very little acetic acid from the aqueous phase. 

Extensive discussions have focused on the conformation of the alkyl chains in the interior ". It has been has demonstrated that the alkyl chains of micellised surfactant are not fully extended. Starting from the headgroup, the first two or three carbon-carbon bonds are usually trans, whereas gauche conformations are likely to be encountered near the centre of tlie chain ". As a result, the methyl termini of the surfactant molecules can be located near the surface of the micelle, and have even been suggested to be able to protrude into the aqueous phase "". They are definitely not all gathered in the centre of tire micelle as is often suggested in pictorial representations. NMR studies have indicated that the hydrocarbon chains in a micelle are highly mobile, comparable to the mobility of a liquid alkane ... 

Solubilisation is usually treated in terms of the pseudophase model, in which the bulk aqueous phase is regarded as one phase and tire micellar pseudophase as another. This allows the affinity of the solubilisate for the micelle to be quantified by a partition coefficient P. Different definitions of P can be found in the literature, differing in their description of the micellar phase. Frequently P is... 

The kinetic data are essentially always treated using the pseudophase model, regarding the micellar solution as consisting of two separate phases. The simplest case of micellar catalysis applies to unimolecTilar reactions where the catalytic effect depends on the efficiency of bindirg of the reactant to the micelle (quantified by the partition coefficient, P) and the rate constant of the reaction in the micellar pseudophase (k ) and in the aqueous phase (k ). Menger and Portnoy have developed a model, treating micelles as enzyme-like particles, that allows the evaluation of all three parameters from the dependence of the observed rate constant on the concentration of surfactant". ... 

Herein k js is the observed pseudo-first-order rate constant. In the presence of micelles, analogous treatment of the experimental data will only provide an apparent second-order rate constant, which is a weighed average of the second-order rate constants in the micellar pseudophase and in the aqueous phase (Equation 5.2). 

Herein Pa and Pb are the micelle - water partition coefficients of A and B, respectively, defined as ratios of the concentrations in the micellar and aqueous phase [S] is the concentration of surfactant V. ai,s is fhe molar volume of the micellised surfactant and k and k , are the second-order rate constants for the reaction in the micellar pseudophase and in the aqueous phase, respectively. The appearance of the molar volume of the surfactant in this equation is somewhat alarming. It is difficult to identify the volume of the micellar pseudophase that can be regarded as the potential reaction volume. Moreover, the reactants are often not homogeneously distributed throughout the micelle and... 

Herein [5.2]i is the total number of moles of 5.2 present in the reaction mixture, divided by the total reaction volume V is the observed pseudo-first-order rate constant Vmrji,s is an estimate of the molar volume of micellised surfactant S 1 and k , are the second-order rate constants in the aqueous phase and in the micellar pseudophase, respectively (see Figure 5.2) V is the volume of the aqueous phase and Psj is the partition coefficient of 5.2 over the micellar pseudophase and water, expressed as a ratio of concentrations. From the dependence of [5.2]j/lq,fe on the concentration of surfactant, Pj... 

In all surfactant solutions 5.2 can be expected to prefer the nonpolar micellar environment over the aqueous phase. Consequently, those surfactant/dienophile combinations where the dienophile resides primarily in the aqueous phase show inhibition. This is the case for 5.If and S.lg in C12E7 solution and for S.lg in CTAB solution. On the other hand, when diene, dienophile and copper ion simultaneously bind to the micelle, as is the case for Cu(DS)2 solutions with all three dienophiles, efficient micellar catalysis is observed. An intermediate situation exists for 5.1c in CTAB or C12E7 solutions and particularly for 5.If in CTAB solution. Now the dienophile binds to the micelle and is slid elded from the copper ions that apparently prefer the aqueous phase. Tliis results in an overall retardation, despite the possible locally increased concentration of 5.2 in the micelle. 

Assuming complete binding of the dienophile to the micelle and making use of the pseudophase model, an expression can be derived relating the observed pseudo-first-order rate constant koi . to the concentration of surfactant, [S]. Assumirg a negligible contribution of the reaction in the aqueous phase to the overall rate, the second-order rate constant in the micellar pseudophase lq is given by ... 

When the dienophile does not bind to the micelle, reaction will take place exclusively in the aqueous phase so that the second-order rate constant of the reaction in the this phase (k,) is directly related to the ratio of the observed pseudo-first-order rate constant and the concentration of diene that is left in this phase. 

It requires a certain flexibility of mind to accept the proposal of using the same THF as extraction solvent in some cases. Ue discovered this possibility, when we tried to remove this solvent from carboxylic acids in a water-pump, vacuum it appeared difficult to remove the last traces of this solvent, even when heating at 70-80°C in a vacuum of 10-15 mmHg was applied. It seemed that there is some weak complexation. This led us to the idea of using THF for the extraction of carboxylic acids from the aqueous phase (after saturation with... 

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