(754d) Populations and Size Distributions of Solute-Rich Mesoscale Structures in Aqueous Amino Acid Solutions and Their Role in Crystal Nucleation
Aqueous solutions of well soluble molecules such as small amino acids are usually assumed to be essentially homogenous systems with some degree of local structuring due to specific interactions on the sub-nanometre scale (e.g. molecular clusters, hydration shells) usually not exceeding several solute molecules. However, recent theoretical and experimental studies have indicated the presence of much larger molecular assemblies or mesospecies in solutions of small organic and inorganic moleculesas well as proteins1.
We investigated both supersaturated and undersaturated aqueous solutions of two simple amino acids (glycine and DL-alanine) using Dynamic Light Scattering (DLS), Brownian Microscopy/Nanoparticles Tracking Analysis (NTA) and Cryogenic Transmission Electron Microscopy (Cryo-TEM). Colloidal scale mesospecies (nanodroplets), were previously reported in supersaturated solutions of these amino acids and were implicated as intermediates species on a non-classical crystallization pathway2, 3.
Surprisingly, we found that these mesostructures were also present in undersaturated solutions at solute concentrations well below the solid-liquid equilibrium (saturation) concentration at a given temperature, with mean sizes ranging from 100 to 300 nm and size distributions widening towards larger sizes with increasing solute concentrations. While DL-alanine mesostructures appeared to be solid-like and could be filtered out by 100 nm syringe PTFE filters, glycine mesostructures were comfortably passing through these filters and are thought to be liquid-like.
Using experimentally measured size distributions of these mesostructures from NTA, SAXS intensity data were fitted using various structural models and an excellent agreement was obtained using a mass fractal model with the fractal dimension df=2.7 suggesting fairly compact clusters with highly irregular interfaces. This is not surprising, since it is expected that the surface tension between the mesostructures and surrounding bulk solution would be very low as they only differ in glycine concentration. The result may be an undulating interfacial topology similar to that found in complex surfactant or block-copolymer systems.
We note that the mesospecies are not a separate phase and the system is better described as a thermodynamically stable mesostructured liquid containing solute-rich domains dispersed within bulk solute solution. Hence, at a given temperature, if excess crystalline solid is present, the complete mesostructured liquid will be in equilibrium with the solid and solute molecules will be stably distributed between solute-rich mesospecies and the surrounding bulk solution. The local concentration of glycine in mesostructures is higher than that in the surrounding bulk solution so when the system is supersaturated nucleation is more likely to occur in them. However, if the nuclei produced are smaller than the critical nucleus size, they will redissolve when they are exposed to the surrounding bulk solution.
1. P. G. Vekilov, Crystal Growth & Design, 2004, 4, 671-685.
2. Y. Ma, H. Cölfen and M. Antonietti, The Journal of Physical Chemistry B, 2006, 110, 10822-10828.
3. A. Jawor-Baczynska, J. Sefcik and B. D. Moore, Crystal Growth & Design, 2013, 13, 470-478.