Surface transport mechanisms in molecular glasses probed by the exposure of nano-particles.

For a glass-forming liquid, the mechanism by which its surface contour evolves can change from bulk viscous flow at high temperatures to surface diffusion at low temperatures. We show that this mechanistic change can be conveniently detected by the exposure of nano-particles native in the material. Despite its high chemical purity, the often-studied molecular glass indomethacin contains low-concentration particles approximately 100 nm in size and 0.3% in volume fraction. Similar particles are present in polystyrene, another often-used model. In the surface-diffusion regime, particles are gradually exposed in regions vacated by host molecules, for example, the peak of a surface grating and the depletion zone near a surface crystal. In the viscous-flow regime, particle exposure is not observed. The surface contour around an exposed particle widens over time in a self-similar manner as 3 (Bt)1/4, where B is a surface mobility constant and the same constant obtained by surface grating decay. This work suggests that in a binary system composed of slow- and fast-diffusing molecules, slow-diffusing molecules can be stranded in surface regions vacated by fast-diffusing molecules, effectively leading to phase separation.

Coordination polymer flexibility leads to polymorphism and enables a crystalline solid-vapour reaction: a multi-technique mechanistic study.

Despite an absence of conventional porosity, the 1D coordination polymer [Ag4 (O2 C(CF2 )2 CF3 )4 (TMP)3 ] (1; TMP=tetramethylpyrazine) can absorb small alcohols from the vapour phase, which insert into AgO bonds to yield coordination polymers [Ag4 (O2 C(CF2 )2 CF3 )4 (TMP)3 (ROH)2 ] (1-ROH; R=Me, Et, iPr). The reactions are reversible single-crystal-to-single-crystal transformations. Vapour-solid equilibria have been examined by gas-phase IR spectroscopy (K=5.68(9)×10(-5) (MeOH), 9.5(3)×10(-6) (EtOH), 6.14(5)×10(-5) (iPrOH) at 295 K, 1 bar). Thermal analyses (TGA, DSC) have enabled quantitative comparison of two-step reactions 1-ROH→1→2, in which 2 is the 2D coordination polymer [Ag4 (O2 C(CF2 )2 CF3 )4 (TMP)2 ] formed by loss of TMP ligands exclusively from singly-bridging sites. Four polymorphic forms of 1 (1-A(LT) , 1-A(HT) , 1-B(LT) and 1-B(HT) ; HT=high temperature, LT=low temperature) have been identified crystallographically. In situ powder X-ray diffraction (PXRD) studies of the 1-ROH→1→2 transformations indicate the role of the HT polymorphs in these reactions. The structural relationship between polymorphs, involving changes in conformation of perfluoroalkyl chains and a change in orientation of entire polymers (A versus B forms), suggests a mechanism for the observed reactions and a pathway for guest transport within the fluorous layers. Consistent with this pathway, optical microscopy and AFM studies on single crystals of 1-MeOH/1-A(HT) show that cracks parallel to the layers of interdigitated perfluoroalkyl chains develop during the MeOH release/uptake process.

Fast surface crystallization of molecular glasses: creation of depletion zones by surface diffusion and crystallization flux.

Molecular glasses can grow crystals much faster at the free surface than in the interior. A property of this process is the creation of depressed grooves or depletion zones around the crystals on the initially flat amorphous surface. With scanning electron microscopy and atomic force microscopy, we studied this phenomenon in indomethacin, which crystallizes in two polymorphs (α and γ) of different morphologies. The observed depletion zones are well reproduced by the known coefficients of surface diffusion and the velocities of crystal growth. At the slow-growing flanks of needle-like α IMC crystals, depletion zones widen and deepen over time according to the expected kinetics for surface diffusion responding to a crystallization flux. Before fast-advancing growth fronts, depletion zones have less time to develop; their steady-state dimensions agree with the same model revised for a moving phase boundary. These results support the view that surface diffusion enables fast surface crystal growth on molecular glasses. Our finding helps understand crystal growth in thin films in which the formation of deep depletion zones can cause dewetting and alter growth kinetics.

Fast Surface Crystal Growth on Molecular Glasses and Its Termination by the Onset of Fluidity.

Organic glasses can grow crystals much faster on the free surface than in the interior, a phenomenon important for fabricating stable amorphous materials. This surface process differs from and is faster than the glass-to-crystal (GC) growth mode existing in the bulk of molecular glasses. We report that similar to GC growth, surface crystal growth terminates if glasses are heated to gain fluidity. In their steady growth below the glass transition temperature Tg, surface crystals rise above the amorphous surface while spreading laterally and are surrounded by depressed grooves. Above Tg, the growth becomes slower, sometimes unstable. This damage is stronger on segregated needles (α indomethacin, nifedipine, and o-terphenyl) than on crystals growing in compact domains (γ indomethacin). This effect arises because the onset of liquid flow causes the wetting and embedding of upward-growing surface crystals. Segregated needles are at greater risk because their slow-growing flanks appear stationary relative to liquid flow at a low temperature. The disruption of surface crystal growth by fluidity supports the view that the process occurs by surface diffusion, not viscous flow. Compared to the bulk GC mode, surface crystal growth is disrupted less abruptly by fluidity. Nevertheless, to the extent that fluidity damages them, both processes are solid-state phenomena terminated in the liquid state.

Termination of Solid-State Crystal Growth in Molecular Glasses by Fluidity.

Fast crystal growth can abruptly emerge as molecular liquids are cooled to become glasses, a phenomenon presently unknown for other materials. This glass-to-crystal (GC) mode can cause crystallization rates orders of magnitude faster than those predicted by standard models. While GC growth is known for 12 systems, its features vary greatly with growth rates spanning a factor of 10(4). We report that the general condition for GC growth to exist is that liquid diffusion be slow relative to crystal growth according to D/u < 7 pm. This condition holds for all liquids exhibiting GC growth and suggests that the phenomenon is a solid-state process terminated by fluidity. GC growth must solidify several molecular layers before rearrangement by diffusion. We propose that GC growth propagates by a nonequilibrium crystal/liquid interface 3 nm wide that solidifies by local mobility. These results explain the prevalence of GC growth among organic liquids and guide its discovery in other materials.

Anticounterfeit protection of pharmaceutical products with spatial mapping of X-ray-detectable barcodes and logos.

Counterfeit pharmaceutical products are a global threat to public health, and they undermine the credibility and the financial success of the producers of genuine products. The escalating circulation of counterfeit drugs demands new anticounterfeit measures that permit rapid screening, are nondestructive, and cannot be circumvented easily. Herein we describe a micro-X-ray diffraction (µ-XRD) protocol for this purpose capable of reading barcodes and logos fabricated on various substrates using soft-lithography stamping of compounds that can be read by X-ray diffraction but are invisible to the naked eye or optical microscopy. This method is demonstrated with barcodes and logos of compounds, approved by the Food and Drug Administration, printed on flat substrates as well as commercial aspirin and ibuprofen tablets. The µ-XRD protocol is nondestructive, automated, and user-friendly and can be used to certify the authenticity of drug tablets by mapping hidden patterns printed under the tablet coating and on packages.

Chemical double mutant cycles for the quantification of cooperativity in H-bonded complexes.

Chemical double mutant cycles have been used in conjunction with new H-bonding motifs for the quantification of chelate cooperativity in multiply H-bonded complexes. The double mutant cycle approach specifically deals with the effects of substituents, secondary interactions, and allosteric cooperativity on the free energy contributions from individual H-bond sites and allows dissection of the free energy contribution due to chelate cooperativity associated with the formation of intramolecular noncovalent interactions. Two different doubly H-bonded motifs were investigated in carbon tetrachloride, chloroform, 1,1,2,2-tetrachloroethane, and cyclohexane, and the results were similar in all cases, with effective molarities of 3-33 M for formation of intramolecular H-bonds. This corresponds to a free energy penalty of 3-9 kJ mol(-1) for formation of a bimolecular complex in solution, which is consistent with previous estimates of 6 kJ mol(-1). This result can be used in conjunction with the H-bond parameters, alpha and beta, to make a reasonable estimate of the stability constant for formation of a multiply H-bonded complex between two perfectly complementary partners, or to place an upper limit on the stability constant expected for a less complementary system.