Multicomponent crystals have received significant attention in recent years, given their considerable potential to develop new products or improve the properties of known ones. Many studies have examined the synthesis and structure of multicomponent (mostly binary) crystals. The thermodynamic aspects related to the driving force for their formation and stability are, however, still relatively unexplored. This work describes a structure–energetics study of binary crystals consisting of glycine (Gly) and fumaric acid (FA) or maleic acid (MA), two regioisomers with different proton transfer abilities. Single-crystal X-ray diffraction experiments showed that the mechanochemically synthesized materials corresponded to a new FA:Gly2 cocrystal and a MA:Gly salt that had been previously prepared by crystallization from solution. A packing analysis further suggested that the stoichiometry difference (1:2 vs 1:1) is possibly related to differences in hydrogen bonding ability between FA and MA. Calorimetric and solubility measurements indicated that: (i) The two binary crystals are stable relative to decomposition into their precursors under ambient conditions (298.15 K; 1 bar), since the process is endergonic (Δr G m° > 0) in both cases. (ii) The stability is of enthalpic nature because Δr H m° > 0 and Δr H m° > |TΔr S m°|. (iii) The formation of a salt, rather than a cocrystal, does not bring any notable stability advantage, since the obtained Δr G m° for MA:Gly exceeds that found for FA:Gly2 by ∼1.0 kJ·mol–1 only. (iv) The similarity appears to be originated by an enthalpy–entropy compensation effect: the Δr H m° contribution (reflecting lattice enthalpy differences between the binary crystals and their components) is approximately twice as large for MA:Gly than for FA:Gly2, but the difference is balanced, to a considerable extent, by a larger and opposing TΔr S m° contribution. (v) The number of classical hydrogen bonds established by a FA or MA molecule with Gly does not seem to be a reliable predictor of stability, as six of these bonds are present in FA:Gly2 and only four in MA:Gly. (vi) Finally, the results here obtained show that the formation of a binary crystal is not necessarily advantageous in terms of solubility enhancement. Indeed, while the solubility of FA from FA:Gly2 was found to be ∼4 times larger than that of pure FA, no analogous benefit was observed for the MA:Gly salt, which exhibited a ∼3 times lower MA solubility when compared with pure MA.

A new concept is presented for green analytical applications based on coupling on-line high-intensity focused ultrasound (HIFU) with a sequential injection/flow injection analysis (SIA/FIA) system. The potential of the SIA/HIFU/FIA scheme is demonstrated by taking mercury as a model analyte. Using inorganic mercury, methylmercury, phenylmercury, and diphenylmercury, the usefulness of the proposed methodology for the determination of inorganic and total mercury in waters and urine was demonstrated. The procedure requires low sample volumes and reagents and can be further applied to all chemical reactions involving HIFU. The inherent advantages of SIA, FIA, and HIFU applications in terms of high throughput, automation, low reagent consumption, and green chemistry are accomplished together for the first time in the present work.

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The influence of temperature (T) and ionic strength (I m ) on the stoichiometric (molality scale) acidity constants of nicotinic acid in aqueous solution was investigated by potentiometry (H þ -glass electrode). The background salt used was potassium chloride, and the temperature and ionic strength ranges covered were 283.15 K < T < 318.15 K and 0.05 mol 3 kg À1 < I m < 0.52 mol 3 kg À1 , respectively. Acidity constants at zero ionic strength were derived by means of a DebyeÀH€ uckel type formalism, and their temperature dependence was obtained through a van't Hoff analysis. This led to pK a1 = 2.19 ( 0.06 and pK a2 = 4.86 ( 0.03 at 298.15 K and to the corresponding standard molar enthalpies and entropies of proton dissociation in the temperature range of the experiments, Δ r H m,l °= (4.5 ( 3.5) kJ 3 mol À1 , Δ r S m,l °= À(26.8 ( 11.8) J 3 K À1 3 mol À1 , Δ r H m,2 °= (12.5 ( 2.1) kJ 3 mol À1 , and Δ r S m,2 °= À(51.2 ( 7.0) J 3 K À1 3 mol À1 . These values were compared with previously reported data.