What is Syncrystallization? States of the pH Indicator Methyl Red in Crystals of Phthalic Acid

Abstract

The concept of syncrystallization was reinvestigated by focusing on phthalic acid (PA) grown with methyl red (MR). Crystals are alternately red and yellow in adjacent growth sectors. X-ray structures of MR and its cocrystals, revealing MR in the neutral, zwitterionic, and protonated states, as well as measurements of linear birefringence and linear dichroism of mixed crystals, were used to investigate mechanisms of PA coloring. These experiments were complemented by force field calculations of the lowest energy stable surfaces of expressed facets and energies of MR on and in crystals, as well as molecular orbital calculations of MR. Two MR species were detected in PA having distinct energies, polarizations, and face selectivities. Assignments of structures to these MRs, previously thought to be neutral and protonated, required a nuanced analysis of hydrogen bonds. The essential difference between yellow and red species is whether the MR carboxylic acid proton is inter- or intramolecularly hydrogen bound. Inferences about mixed crystal structure drawn from an examination of cocrystals of PA and MR are inconsistent with polarization spectroscopy signaling caution when using stoichiometric compounds as models of dilute solid solutions. Upon heating mixed crystals, linear dichroism diminishes and oriented, elongated pools of MR separate and pass through the bulk in directions perpendicular to the direction of elongation. These bâtonnets subsequently crystallize leaving macroscopic oriented crystals of a MR-rich phase within PA. No evidence was found for the simultaneous crystallization of MR and PA; however, the MR reorientation on heating as well as the separation and recrystallization of a MR-rich phase are distinct processes that could be embraced by the literal meaning of syncrystallization.

Large-area graphene-based nanofiltration membranes by shear alignment of discotic nematic liquid crystals of graphene oxide

“…Performances of the membranes as a function of membrane thickness were first evaluated by measuring the water permeability (using Reverse Osmosis water, known as RO water) and the retention for Methyl Red (an electroneutral probe molecule at pH∼5.5 (ref. 39 ); Fig. 4a,b ).…”

Evolution and Reversibility of Host/Guest Interactions with Temperature Changes in a Methyl Red@Palygorskite Polyfunctional Hybrid Nanocomposite

“…33, 34 These zwitterions are open shell systems, both similar to the trans conformer and reputed to be mostly responsible for absorption at room temperature. 77,78 Despite this, however, because of the approximation of the molecular mechanics methods adopted here, any optimization keeping in account the zwitterionic geometries ended in the dye molecule turning back to the corresponding trans conformer. Only one out of the four different conformersnamely, the trans form of methyl red was therefore used in the performed optimizations; such a constraint, however, is not expected to undermine the obtained outcomes, as this conformer represents the most reasonable approximation for the zwitterionic tautomers useable in these calculations.…”

Proton transferversusnontransfer in compounds of the diazo-dye precursor 4-(phenyldiazenyl)aniline (aniline yellow) with strong organic acids: the 5-sulfosalicylate and the dichroic benzenesulfonate salts, and the 1:2 adduct with 3,5-dinitrobenzoic acid

“…Also, with phosphoric acid (Halasz et al, 2007), the orange monohydrogen phosphate is an anilinium salt while the purple dihydrogen phosphate salt is diazenyl-group protonated. The dye methyl red {4-[(N,N 0dimethylanilino)diazenyl]benzene-2-carboxylic acid} also gives diazenyl-protonated salts and adducts with 2,5-dihydroxybenzoic acid (Benedict et al, 2006) and the matrix assisted laser disruption ionization (MALDI) host 2,6-dihydroxybenzoic acid (Cohen et al, 2007). It is of interest also that the azo dyes 4-[4-(N,N 0 -dimethylamino)phenyldiazenyl]benzenesulfonic acid (Burke et al, 2004) and 4-[4-(N,N 0 -diethylamino)phenyldiazenyl]benzenesulfonic acid (Burke et al, 2006) exist as sulfonate-diazenyl-group zwitterions.…”