Solubility measurement and molecular simulation of unsolvated and solvated estrogen receptor agonist (R)-equol in binary solvents (alcohols + n-heptane) from 273.15 K to 333.15 K

In this work, the solubility of unsolvated crystalline (R)-equol in six (alcohols +  n -heptane) binary solvents and that of solvated crystalline (R)-equol in methanol along with ethanol were firstly systematically measured and analyzed. The experimental results revealed that the solubility of unsolvated crystalline (R)-equol was positively correlated with temperature and mass fraction of alcohol. And the solubility order was 2-propanol > 1-propanol > 1-butanol > 1-pentanol > 3-methyl-1-butanol > 2-methyl-1-propanol >  n -heptane. The transformation temperatures between unsolvated and solvated forms were acquired at 316.28 K and 310.50 K, respectively. Meanwhile, the experimental solubility data of unsolvated crystalline (R)-equol were correlated using modified Apelblat, λh , quartic polynomial and CNIBS/R-K equations, in which the modified Apelblat equation achieved the best regression performance. Besides, the dissolution of unsolvated crystalline (R)-equol was endothermic. Furthermore, solvent parameters, solvation free energy and molecular electrostatic potential surface obtained through molecular simulation were used to explain the experimental phenomena, and found the polarity of solvents, van der Waals and hydrogen bonding interactions made different contributions to various dissolution processes.