Solution phase models Aqueous solutions

We now have seven candidates, and the phase rule allows seven phases to be stable together. We shall state as a working hypothesis that these seven phases—(1) aqueous solution, (2) quartz, (3) kaolinite, (4) hydromica (illite) (5) chlorite, (6) montmorillonite, and (7) phillip-site—are the stable assemblage in the intermediate model, and we shall test this hypothesis against various evidence. Some of these phases may prove unstable and be replaced by some other phase—e.g., phillipsite by another zeolite or a feldspar. Chlorite might also be replaced by some of the magnesium silicates described by Arrhenius (3). 

The model of icebergs around nonpolar solute molecules in aqueous solution is clearly not a very realistic one. However, if solutions of hydrocarbons (or noble gases) are cooled, then the solid phase that sometimes separates out consists of a so-called gas hydrate (clathrate), in which water provides a particular kind of hydrogen-bonded framework containing cages that are occupied by the nonpolar solute molecules. Obviously, such gas hydrates (clathrates) represent more realistic models for the phenomenon of hydrophobic hydration [176]. 

The relative stabilities of the tautomers of dicoumarol, which can potentially exist in six tautomeric forms, and its derivatives were studied by the PM3 method (04JST(678)55). The calculations have been carried out both in the gas phase and in a continuum model of water, on isolated tautomers of 10 systems obtained by single proton transfer. In the most cases, the predominance of the a,/-benzopyran tautomeric structure 154a was predicted in the gas phase. In aqueous solution, the calculations imply significant qualitative differences in the relative stabilities of the tautomers and predict the predominance of the tetra-keto structure 154b, even though the a,a -benzopyran structure is the one with the largest dipole moment. 

The efficiency of solution-phase (two aqueous phase) enzymatic reaction in microreactor was demonstrated by laccase-catalyzed l-DOPA oxidation in an oxygen-saturated water solution, and analyzed in a Y-shaped microreactor at different residence times (Figure 10.24) [142]. Up to 87% conversions of l-DOPA were achieved at residence times below 2 min. A two-dimensional mathematical model composed of convection, diffusion, and enzyme reaction terms was developed. Enzyme kinetics was described with the double substrate Michaelis-Menten equation, where kinetic parameters from previously performed batch experiments were used. Model simulations, obtained by a nonequidistant finite differences numerical solution of a complex equation system, were proved and verified in a set of experiments performed in a microreactor. Based on the developed model, further microreactor design and process optimization are feasible. 

In Chapter 3, we introduced the concept of the surfactant and discussed how molecules with hydrophobic and hydrophilic sections will self-assemble into lyotropic phases in aqueous solution. The stability of a specific phase depends on molecular shape and concentration in the solvent. In the discussions that follow, we focus on the phase behavior of a simplified model membrane containing only lipids, even though the real cell membrane consists of a complex mbcture of many different varieties of lipids, sterols, and membrane proteins. Figure 6.3 shows molecular structures for some common lipid molecules found in the cell membrane. For a detailed description of the cell membrane, I recommend The Structure of Biological Membranes as further reading. ... 

Figure 8 Koopmans energies (bold lines) and corrected vertical energies (doited lines) of CH3SH, CH3S"(H20)n (n=l-4) in gas phase and aqueous solution calculated at the ROHF/6-31G basis set. The results in aqueous phase are obtained through the use of the SCRF model and the Born charge term. The calculated vertical values were scaled to experiment [164]. Reproduced with permission from ref. [164].

Molecular modelling calculations have been used to study the interactions between D-talopyranose, D-talofuranose and Pb " and ions in the gas phase. In aqueous solutions Pb ions form carbohydrate complexes with both forms of the sugar whereas Hg " ions do not. The calculations implied that the reverse ought to be true. ... 

As with resoles, we can use a three-phase model to discuss formation of a novolac. Whereas the resole is activated through the phenol, activation in novolacs occurs with protonation of the aldehyde as depicted in Scheme 12. The reader will note that the starting material for the methylolation has been depicted in hydrated form. The equilibrium level of dissolved formaldehyde gas in a 50% aqueous solution is on the order of one part in 10,000. Thus, the hydrated form is prevalent. Whereas protonation of the hydrate would be expected to promote dehydration, we do not mean to imply that the dehydrated cation is the primary reacting species, though it seems possible. 

The interactions between water and aqueous solutions and another phase have been modeled in various ways. The most simple models consist of an aqueous system in contact with a hard or soft wall described by... 

Phospholipids e.g. form spontaneously multilamellar concentric bilayer vesicles73 > if they are suspended e.g. by a mixer in an excess of aqueous solution. In the multilamellar vesicles lipid bilayers are separated by layers of the aqueous medium 74-78) which are involved in stabilizing the liposomes. By sonification they are dispersed to unilamellar liposomes with an outer diameter of 250-300 A and an internal one of 150-200 A. Therefore the aqueous phase within the liposome is separated by a bimolecular lipid layer with a thickness of 50 A. Liposomes are used as models for biological membranes and as drug carriers. 

Yamaoka T, Tamura T, Seto Y et al (2003) Mechanism for the phase transition of a genetically engineered elastin model peptide (vpgig)(40) in aqueous solution. Biomacromolecules 4 1680-1685... 

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