A Comparative Study on Chiral and Racemic 12-Hydroxyoctadecanoic Acids in the Solutions and Aggregation States: Does the Racemic Form Really Form a Gel?

Chiral and racemic forms of the title compound (12HOAs) were reinvestigated. Gels formed separately from d- and l-12HOAs became a solution containing crystallites on maintaining contact, confirming that Dl-12HOA forms no gel, contrary to several reports published over more than ten years, which stated that Dl-12HOA formed a gel (for example, P. Terech et al., Langmuir1994, 10, 3406). The powder X-ray diffraction pattern of d-12HOA was different from that of Dl-12HOA and the simulated patterns of two crystal structures of Dl-12HOA. 13C NMR spectroscopy for solutions of C6D6 showed significant difference in the concentration dependence of the low-field shifts at the carboxyl C due to hydrogen-bond formation. Analysis based on multiple equilibria among monomer, dimer, and tetramer was successfully applied to the behavior of d-12HOA, but not to that of Dl-12HOA. For Dl-12HOA, a modified model including hexamer was applied. It is concluded that association of dimers to tetramers and higher aggregates is more favorable in the Dl-12HOA solution than in the d-12HOA solution and that d- and l-12HOA molecules recognize each other even in a dilute solution (lower than 2 mmol L−1) of Dl-12HOA. Different NMR spectra were also observed in the solid state.