Effect of molecular size and hydrogen bonding on three surface-facilitated processes in molecular glasses: Surface diffusion, surface crystal growth, and formation of stable glasses by vapor deposition

Abstract

Recent work has shown that diffusion and crystal growth can be much faster on the surface of molecular glasses than in the interior and that the enhancement effect varies with molecular size and intermolecular hydrogen bonds (HBs). In a related phenomenon, some molecules form highly stable glasses when vapor-deposited, while others (notably those forming extensive HBs) do not. Here we examine all available data on these phenomena for quantitative structure-property relations. For the systems that form no HBs, the surface diffusion coefficient D s decreases with increasing molecular size d (d = Ω 1/3 , where Ω is the molecular volume); when evaluated at the glass transition temperature T g , D s decreases ∼5 orders of magnitude for 1 nm of increase in d. Assuming that center-of-mass diffusion is limited by the deepest part of the molecule in the surface-mobility gradient, these data indicate a mobility gradient in reasonable agreement with the Elastically Collective Nonlinear Langevin Equation theory prediction for polystyrene as disjointed Kuhn monomers. For systems of similar d, the D s value decreases with the extent of intermolecular HB, x (HB), defined as the fraction of vaporization enthalpy due to HB. For both groups together (hydrogenbonded and otherwise), the D s data collapse when plotted against d/[1 − x(HB)]; this argues that the HB effect on D s can be described as a narrowing of the surface mobility layer by a factor [1 − x(HB)] relative to the van der Waals systems. Essentially the same picture holds for the surface crystal growth rate u s. The kinetic stability of a vapor-deposited glass decreases with x(HB) but is not better organized by the combined variable d/[1 − x(HB)]. These results indicate that surface crystal growth depends strongly on surface diffusion, whereas the formation of stable glasses by vapor deposition may depend on other factors.

Ultrastable glasses: new perspectives for an old problem

“…In general, measured D s values on organic glasses range from 10 -11 to 10 -16 m 2 /s at T g , 4 to 8 orders of magnitude larger than the bulk counterpart, as shown in Fig. 8 This eightfold enhancement of surface diffusion with respect to bulk diffusion [131] allows to circumvent the restrictions to molecular rearrangement imposed by the slow bulk dynamics. Indeed, at the surface layer, molecules exhibit diffusion levels that allow them to reach stable packing arrangements in timescales comparable to the residence time of these molecules on the surface.…”

“…Enhanced molecular diffusion at the surface is responsible for fast surface crystal growth in many organic glasses as crystal growth rate is nearly proportional to the surface diffusion coefficient, D s [130]. One way to measure surface diffusion is by the embossing of surface gratings onto glassy surfaces and subsequent measurement by diffraction of the decay of the grating pattern [89,131]. Surface diffusion can also be accessed by measuring the evolution of the response when a probe is placed on the surface, inducing the formation of a meniscus due to surface-mediated flow.…”

“…The relevance of surface diffusion as an indicator of the formation of stable glasses has been analyzed in detail in several reports involving organic molecules with different degrees of hydrogen bonding (HB) and/or variations of molecular size [131]. HB reduces surface diffusion due to their robustness at surface layers, which makes the barrier for diffusion approximately the same on the surface as in the bulk.…”

Recent Advances in the Application of Characterization Techniques for Studying Physical Stability of Amorphous Pharmaceutical Solids

“…Recent studies showed that surface diffusion of molecular glasses can be strongly affected by several factors, including strength of molecular interaction [47,48], molecular size [49], and the addition of polymer [43]. With the increase in the strength of molecular interaction and molecular size, surface diffusion of amorphous solids exhibits a tendency to slow down [47][48][49][50][51]. For instance, Chen et al found that the surface diffusion of polyalcohol glasses showing extensive hydrogen bonding is much slower than that of the molecular glasses of comparable size but with no or limited hydrogen bonds [48].…”

Direct Measurement of Lateral Molecular Diffusivity on the Surface of Supersaturated Amorphous Solid Dispersions by Atomic Force Microscopy

“…The existence and extent of intermolecular hydrogen bonds have been shown to play an important role for the molecular mobility on amorphous surfaces as well as for the physical stabilization of ASDs. 32,33 Because of the ability of IMC to act as both a hydrogen bond acceptor (carboxylic acid and amid) and donor (carboxylic acid), the formation of hydrogen bonds between IMC molecules as well as between IMC and the acceptor groups of SP (tertiary amide and carboxylic ester) was suspected (see Figure 3 for chemical structures).…”