Composition of sodium glycocholate micellar solutions

Composition and existence range of aggregates formed by sodium glycocholate in aqueous solutions were studied. Electromotive force measurements provided hydrogen, sodium, and glycocholate ion free concentrations. Lead(II) glycocholate solubility measurements yielded the free concentration of glycocholate ions, as well. Experimental data obtained at 25 °C and at three different concentrations of N(CH3)4Cl, used as a constant ionic medium, can be explained by assuming the presence of aggregates with different compositions, depending on reagent and ionic medium concentrations. The distribution of the species found at the same concentration of ionic medium and close to neutrality remains constant. As expected, the size of the aggregates increases by increasing ionic medium and reagent concentrations. A dimer is the prevailing species at low concentrations, and all of the found species have even anion aggregation numbers. In solutions with the highest concentration and ionic strength, aggregate multiples of eight are present at a high percentage. These results agree with structural investigations that proposed as building blocks of sodium glycocholate micellar aggregates a dimer and an octamer. A strong analogy with the composition of taurococholate aqueous solutions is observed. The comparison with deoxycholate, glycodeoxycholate, and taurodeoxycholate shows wide differences.