Aqueous stability

Various carboxylic acid salts have also been reported. Gallardo35 produced the maleate salt of neomycin which, it was claimed, improved the aqueous stability of the antibiotic. A practically tasteless compound, the citrate salt,has been described by Szyszka3 . Neomycin mandelate has been claimed to be particularly useful in the treatment of urogenital infections3 while the di-hydroxy-dinaphthylmethane-dicarboxylate4(3 and the pamoate salts43,67,68 have a low intestinal absorption and are thus effective treatments for intestinal infections. 

Swart et al have demonstrated the stability of neomycin hydrochloride in aqueous solution over the pH range 2-9293. Further investigations by Simone and Popino298 confirmed the aqueous stability at 23°C but at 45°C potency losses of up to 94% were noted over a period of 2 years with solutions in the pH range 4-8. 

When the ion-pair partitioning is indicated in the quadrant diagram (below) it becomes obvious that a circle of equilibria is present. Knowing the octanol pKa, the log P and the aqueous pKa should allow one to calculate the partition coefficient of the ion pair. From these equilibria one can write that the difference in log P between the acid and its salt is the same as the difference between the pKa s (Equation 9). The closer the pKa s, the more lipid soluble the ion pair will be, relative to the acid. Internal hydrogen bonding or chelation that stabilizes an ion pair will affect the octanol stability more than the aqueous stability, where it is less needed, and so will decrease the delta pKa. Chelation should therefore favor biolipid solubility of ion pairs. Ultimate examples are available in some ionophores. This is one of the properties of some of the herbicides I pointed out earlier. 

These references have extensive listings of inorganic complexes and their aqueous stabilities. Special note If the complexing agent is one which protonates, the concentrations of the non-protonated agent (as a function of pH) must be calculated. These values are to be used in the construction of the E-pH diagram. 

Hydride surface termination has the capability for ideal surface passivation, with each hydrogen atom bonding to a single surface-dangling bond. On silicon, hydride termination has been well researched and shown to provide many advantages, including aqueous stability and limited air stability [13]. The hydride-terminated surface is also of interest as it can be used as a precursor for wet chemical reactions. 

Nordstrom, D.K. and Barber, L.B. (2005) Aqueous stability of gadolinium in surface waters receiving sewage treatment plant effluent, Boulder Creek,... 

Its formation may well arise from the reaction of water with (C5H5)2Ti-containing species, the water being produced in the methanation reaction. The study underscores the necessity of having a catalyst compatible with the products of the catalyzed reaction. In the case of CO reduction by H2, this implies at least modest aqueous stability. 

Fleisher, D., et al. 1986. Oral absorption of 21-corticosteroid esters A function of aqueous stability and intestinal enzyme activity and distribution. J Pharm Sci 75 934. 

Jarvinen, K., et al. 1994. The effect of modified beta-cyclodextrin, SBE4-beta-CD, on the aqueous stability and ocular absorption of pilocarpine. Curr Eye Res 13 897. 

The 2MgO - B203 - H20 borate is known as ascharite. It can be prepared from the Mg0-B203-H20 system at temperatures of 150-450°C. The presence of magnesium chloride at a concentration of 25% extends its aqueous stability range down to 83°C. Ascharite is almost insoluble in water, and a solubility of 0.0085% B203 at 25°C has been reported (456). Its dehydration has been studied (123, 165). 

Table 4.1 Aqueous stability constants (log ) for lanthanide (3+) and other ions...

The preparation, characterization, aqueous stability, and photophysical properties of NIR emitting lanthanide complexes with tetradentate chelating ligands 36 and 37 were described by Raymond and coworkers [61, 62]. In aqueous solution, the chelating ligand 36 or 37 forms stable complexes with Ln(III) ions, and sensitized NIR lanthanide luminescence was detected for the complexes with Pr(III), Nd(III), Ho(III), or Yb(III) ions. For [Ln(36)2] complexes, the luminescence decay curves were biexponential due to partial hydrolysis of the complexes or alternately the presence of a slowly exchanging equilibrium mixture with a hydrated form of the complexes. For [Ho(37)2] , the NIR band due to Fs -> I transition of the Ho(III)... 

Non-polar supports like polystyrene/divinylbenzene copolymers or carbon are also used as column materials. Alumina is polar and acidic while TiOi, and zirconia are much more neutral. They all have good aqueous stability compared to silica. Normal phase chromatography is restricted to the separation of stereochemical isomers, diastereomers, low molecular weight aromatic compounds and functionalized long chain aliphatic compounds. 

Hirayama, F. Kurihara, M. Uekama, K. Improving the aqueous stability of prostaglandin E2 and prostaglandin A2 by inclusion complexation with methylated-P-cyclodextrins. Chem. Pharm. Bull. 1984, 32 (10), 4237-4240. 

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