Bulk aqueous physical chemistry

On the basis of surface and bulk interaction with water. Small [85] classified bile acids as insoluble amphiphiles and bile salts as soluble amphiphiles. On account of the undissociated carboxylic acid group, the aqueous solubility of bile acids is limited [35] in contrast, many bile salts have high aqueous solubilities as monomers [33] and, in addition, their aqueous solubilities are greatly enhanced by the formation of micelles [5,6]. Because many bile salts are weak electrolytes, their ionization and solubility properties are more complicated than those of simple inorganic or organic electrolytes [5,35]. For example, the p/Tj, values of bile acids in water vary markedly as functions of bile salt concentration and, because micelles formed by the A (anionic) species can solubilize the HA (acid) species [5,35], the equilibrium precipitation pH values of bile acids also vary as functions of bile salt concentration. Finally, certain bile salts are characterized by insolubility at ambient temperatures [2,5,6,86,87], only becoming soluble as micelles at elevated temperatures (the critical micellar temperature) [6]. 

A simple method for estimating the pH-solubility relationship of bile acids and salts is to carry out aqueous acidometric titration of a bile salt in water with a stronger mineral acid [5,35], Once the molarities of bile salt and mineral acid are known, the titration curves provide a direct measurement of equivalence, equilibrium and metastable pH values, the pH at which precipitation of the HA species occurs (pHpp,), an estimate of the solubilities of the HA species in water (if the system is CMC) or in water plus micelles (if the system is CMC) and a calculation of the apparent pK (pATg). The methods, results and interpretation of such titration curves for the common bile salts, titrated with HCl, have been described in detail elsewhere [5,6]. 

The equilibrium pHpp, values and pK values for all 4 bile salts as functions of bile salt concentration (HjO, 37°C) are plotted in Fig. 8 (from ref. 35). For comparable bile salt concentrations, the HA species of UDC and GUDC precipitate at higher pH values than the HA species of CDC and GCDC, despite similar pA values. The physical-chemical reason why the pHpp, of the UDC and GUDC species 

By extrapolating the pA curves in Fig. 8 to infinite dilution, we obtain identical thermodynamic pK vsdues for CDC and UDC ( pK, 5.0) and for GCDC and 

Free bile salts Glycine conjugates Taurine conjugates 

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The actual recording of NMR spectra is not significantly different from one solvent to another except in the case of liquid crystal solvents, and in fact the bulk of NMR studies to date have been in organic solvents. However, most applications of NMR to physical chemistry have involved aqueous solutions. The following summary of basic NMR theory is an outline only, and the reader is referred to one of the fundamental texts for detail. 

F. Franks. Water A Comprehensive Treatise, Plenum Press, New York, 1972-1981. The seven volumes of this encyclopedic reference work that have appeared thus far contain discussions of all aspects of the chemistry and physics of liquid water and aqueous solutions. Of special interest are the chapters on bulk liquid water (Vol. 1), on water in electrolyte solutions (Vols. 3 and 6), and on clay-water systems, (Vol. 5). 

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