An NMR technique to measure exchange kinetics at thermal equilibrium in dispersions of moderately soluble crystalline material is presented. By monitoring the exchange of molecules between pools in solid and dissolved form, one can characterize the surface specific exchange rate. Illustrative experiments were performed in a model system with β-type crystals of tripalmitin as the solid phase and tripalmitin, a fraction of it deuterated, dissolved in a medium-chain TG oil as the liquid phase. The concentration of deuterated tripalmitin in the solvent was followed by 2 H NMR after the crystals, which initially lack deuterated tripalmitin, were immersed in the liquid. The variation of the 2 H concentration in the solvent provided the surface specific exchange rate. No systematic errors, due to the slight difference in properties of the deuterated tripalmitin compared to hydrogenated tripalmitin, were observed. The methodology worked well between crystal concentrations of 2 and 4 wt%. Molecular or atomic crystallization or dissolution rates are relatively easy to measure in systems that are out of thermal equilibrium, e.g., by exhibiting a concentration or a chemical potential gradient. In contrast, exchange between different phases of the same molecular species under conditions of thermal equilibrium has drawn significantly less attention. This type of exchange can be imagined as molecular diffusion in and out over a phase boundary between two phases in equilibrium. In a suspension, this exchange is most pronounced at the outermost atomic/molecular layers of the solid particles involved, although there also may be an exchange between the inner and outer molecular layers. Hence, diffusion in and out of the solid phase predominantly represents solidification and dissolution of molecules on/from the solid-liquid interface, respectively.The exchange between solution and solid phase determines the rate of destabilization in systems with metastable compound solid phases. One example of such an exchange is TAG (fat) systems where several metastable and stable structures may exist, as exemplified by the cacao butter model recently reviewed by Sato et al.(1). Those authors identified several compound structures whose changing composition allows the formation of more stable forms. Hence, molecular exchange can strongly influence the kinetics of cacao butter and, in general, fat recrystallization.In general, exchange experiments performed at thermal equilibrium require marking the molecules by particular isotopes of the constituting atoms. Thereafter, the marked molecules or atoms can be followed by isotope-selective techniques such as radiochemical methods or NMR spectroscopy. If, as usual, one can assume that the marker isotope does not significantly alter the intermolecular interactions, such experiments can yield the exchange rates at thermal equilibrium. One methodologically related example (but not so concerning the nature of the exchange process) is NMR studies of hydrogen exchange rates in proteins (2): These provide vi...
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