Salt hydrate pairs

Zacharis, E., Omar, I.C., Partridge, J. and Robb, D.A. (1997) Selection of salt hydrate pairs for use in water control in enzyme catalysis in organic solvents. Biotechnol. 

Berberich lA, Kaar IL, Russell Al (2003) Use of salt hydrate pairs to control water activity for enzyme catalysis in ionic liquids. Biotechnol Prog 19 1029-1032... 

Salt Hydrate Pairs Used to Effectively Control Water Activity, a, in Nonaqueous Enzymology... 

Nomenclature Na2HP04 12/7 refers to the salt hydrate pair (e.g., Na2HP04 12 H20/Na2HP04 7 H2O). All salt hydrates are commercially available unless indicated otherwise. 

All of this describes just the thermodynamically favored directions of water transfer, for ideal crystalline solids. Many salt hydrate pairs seem to behave approximately ideally. However, if water activity is to be controlled close to the transition value, the rates of water release and uptake must be sufficient. Different salt pairs have very different rates of water exchange. It is difficult to give quantitative values, because the rates will depend on the size and shape of the crystals in each of the salt hydrate forms. This will depend on how they have been crystallized and handled subsequently. For example, cycling between hydrate forms, with gain and loss of water, will usually lead to a reduction in crystal size, and hence more rapid water exchange in future cycles. 

The equilibrium water activity achieved depends on the choice of salt hydrate pair used and the temperature. In most cases the temperature dependence is higher than for saturated salt solutions. There is also a maximum temperature at which the higher hydrate will melt to give a liquid phase, so above this the biocatalyst will probably be seriously affected. Table 8-4 gives water activity values for some pairs that can be recommended for biocatalysis, together with an indication of the rates of transfer, and the maximum temperature. A compilation from the literature1241 gives information on temperature dependence, and notes some other hydrate pairs whose use has not been (fully) tested. 

There have been some cases of confusion in the control of water activity between saturated salt solutions (see above) and salt hydrate pairs. These can both be useful methods, but the principles and recommended applications are quite different. Avoid phrases like control of water activity using salts , which do not make it clear which method is being used. 

Berberich et al. used salt hydrate pairs to control water activity in [BMIM][PF6]. The results were in good agreement with that obtained for water activity control using saturated salt solutions. The advantage of pre-equilibration is that the contact of the enzyme with the used salt and thus enzyme deactivation can be avoided. On the other hand it is only applicable for initial rate measurements. This disadvantage can be overcome by controlling water activity with salt hydrate pairs. Berberich et al. measured initial rate - water activities for the transesterification reaction of methyl methacrylate with 2-ethylhexanol in either hexane or [BMIM][PF6]. Both reaction systems gave similar profiles [72],... 

Table 2. Water Activities of Some Salt Hydrate Pairs at 25°C from Hailing (39)...

Divalent or higher-valent cations and, in particular, transition metal cations, are likely to be covalently solvated by solvents that are strong electron pair donors (have large solvatochromic P values). This solvation often persists in crystals, so that the salt that is in equilibrium with the saturated solution in such solvents may not be the anhydrous salt (nor the salt hydrate). Equation (2.56) omits any consideration of the solvent of crystallization and pertains to the solventless (anhydrous) salt. For a salt hydrated by n water molecules in the crystal, the activity of water raised to the nth power must multiply the right-hand side of Eq. (2.56) for it to remain valid. A similar consideration applies for salts crystallizing with other kinds of solvent molecules, the activity of the solvent in the saturated solution replacing that of water. Such situations must be... 

An alternative method is based on the fact that salt hydrates containing different numbers of water molecules are interconverted at fixed water activities. The first salt hydrate used was Na COj 10 H2O. This is converted to Na2C03 7 H2O at a water activity of 0.74 at 24°C. The salt hydrates act as a buffer of the water activity. As long as both salt hydrates are present the water activity remains at 0.74. The salt hydrates can be added directly to the organic reaction mixture. The pair of salt hydrates should be chosen to give a water activity suitable for the enzymatic conversion (Zacharis et al., 1997). 

Water Activity Control Using Pairs of Salt Hydrates... 

One simple and convenient method is the addition directly to the reaction mixture of suitable pairs of solid salt hydrates. A given salt hydrate will give up its water at a characteristic water activity, transforming to a lower hydrate or an anhydrous form. If the pair are placed in a system of water activity below their characteristic transition value, the (higher) hydrate will tend to give up water to the rest of the system. Water release will continue until the whole system reaches the transition water activity (or... 

Conversion Saltpetre.—To illustrate the use of Janecke s method of representation we may consider briefly the equilibria formed by the reciprocal salt-pairs (Na,K)—(NOgjCl) and water, which are of importance for the manufacture of conversion saltpetre, and which have been studied by Uyeda and by Reinders/ In this system no compounds or salt hydrates are formed. 

The water activity of pre-equilibrated substrates and enzymes may change when they are put in contact with the SCF for example, SCCO2 may take up water from the enzyme. Furthermore, if water is consumed or produced in the reaction, the water activity of the reaction system will change. Putting salt hydrates in contact with the reaction mixture should buffer the water activity. Suitable pairs of solid salt hydrates may be used to control the water activity more accurately. Examples of salt hydrates and saturated solutions are given in Table 4.9-5. 

Total solvent-phase salt hydration includes the hydration numbers of the dissociated cations and anions together with ion pairs and assumes that is independent of water activity. 

To put to test the above suggestion, we examined nanoporous carbonaceous sorbent D4609 in combination with rather concentrated mixtures of NaCl and HCl or NaOH taken in equal proportions. In the first salt/acid pair the species that determine the rates of movement of solute fronts are hydrated Na cation and CC anion with ionic radii of 3.58 and 3.32 A, respectively. The difference in their sizes is not too large, but obviously sufficient for a successfid separation. As illustrated by Fig. 12.6, the fronts of NaCl and HCl diverge by one-third of the total bed volume (A = 0.33) in both the forward and reverse experiments. Here, HCl behaves as a clearly... 

Table 3.4 Water activity (aw) of saturated salt solutions and pairs of salt hydrates...

Na salt Ion-pairing reagent used in hplc. SI. hygroscopic cryst. (EtOH). Forms a quarter-hydrate and a hemihydrate. 

The two strands, in which opposing bases are held together by hydrogen bonds, wind around a central axis in the form of a double helix. Double-stranded DNA exists in at least six forms (A-E and Z). The B form is usually found under physiologic conditions (low salt, high degree of hydration). A single turn of B-DNA about the axis of the molecule contains ten base pairs. The distance spanned by one turn of B-DNA is 3.4 nm. The width (helical diameter) of the double helix in B-DNA is 2 nm. 

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