Critical Droplet Theory Explains the Glass Formability of Aqueous Solutions

Abstract

When pure water is cooled at $10 6 K=s, it forms an amorphous solid (glass) instead of the more familiar crystalline phase. The presence of solutes can reduce this required (or ''critical'') cooling rate by orders of magnitude. Here, we present critical cooling rates for a variety of solutes as a function of concentration and a theoretical framework for understanding these rates. For all solutes tested, the critical cooling rate is an exponential function of concentration. The exponential's characteristic concentration for each solute correlates with the solute's Stokes radius. A modification of critical droplet theory relates the characteristic concentration to the solute radius and the critical nucleation radius of ice in pure water. This simple theory of ice nucleation and glass formability in aqueous solutions has consequences for general glass-forming systems, and in cryobiology, cloud physics, and climate modeling. DOI: 10.1103/PhysRevLett.110.015703 PACS numbers: 64.60.QÀ, 05.40.Àa, 64.70.dg Ice nucleation and growth are of major interest in fields ranging from cryobiology to atmospheric physics. Ice is a key issue in cryopreservation of cells and tissues [1] and in cryocooling of samples for macromolecular crystallography [2,3], where solutes like salts, sugars, alcohols, and polyols can have dramatic effects on ice formation. In atmospheric physics, models of cloud formation are sensitive to the nature of the critical nucleus of ice crystals [4] with implications for climate models [5]. Supercooled water is an interesting system in its own right [6], and the formation of crystalline phases from supercooled solutions is an active area of study [7].Previous experiments have focused on properties such as the melting and glass transition temperatures, and models to explain the data have largely been phenomenological. Similar models have been applied to explain glass formability in a wide variety ofnonaqueous systems. Here, we report measurements of the minimum cooling rates [or ''critical cooling rates'' (CCRs)] required to prevent ice formation in aqueous solutions during cooling to $100 K or below. We show that a surprisingly simple statistical modification to classical thermodynamic nucleation theory provides an excellent account of these data.We studied eight different solutes (see Fig. 1), including a salt (sodium chloride), simple alcohols (methanol, ethanol), sugars (dextrose, trehalose), polyols (glycerol, ethylene glycol), and polyethylene glycol 200 (PEG 200). All are compact and highly soluble and can have large effects on critical cooling rates required for vitrification.The effects of these solutes on ice nucleation were evaluated by measuring the critical cooling rate above which no ice was observed. Below the critical rate, a sample turns opaque on cooling, indicating the formation of polycrystalline ice. As the rate increases, a transition to transparent samples is observed. This optical transition corresponds to a transition in the x-ray diffraction patterns obtained from the cooled sam...

Efficient cryoprotection of macromolecular crystals using vapor diffusion of volatile alcohols

“…However, cryogenic cooling itself can damage the crystal and compromise diffraction quality, often due to ice formation (Haas and Rossmann, 1970; Juers and Matthews, 2001; Juers and Matthews, 2004; Kriminski et al, 2002; Low et al, 1966). Cooling-induced damage is typically reduced by cooling faster and/or adding cryoprotective agents such that the system cools through the freezing point of water to the glass transition before ice can form (Chinte et al, 2005; Shah et al, 2011; Warkentin et al, 2013). The use of pressure to prevent the formation of ice I during cooling has also been successfully applied to several systems (Burkhardt et al, 2012; Kim et al, 2005; Thomanek et al, 1973).…”

“…Recent experiments showed that both methanol and ethanol require lower concentrations (w/v) than traditional cryoprotective agents (e.g. glycerol and ethylene glycol) to prevent ice formation in small volumes of plunge-cooled solution (Warkentin et al, 2013). Despite their effectiveness, volatile alcohols have seen little use for cryoprotection in macromolecular crystallography, due in part to the difficulty of working with their high vapor pressures.…”

Quenching device for electrolytic aqueous solutions

“…There is a claim by [9] that concentrated aqueous solutions, including those of sodium chloride, have been amorphized by cooling rates as low as 60 K s −1 . However, amorphization was not determined by diffraction, but by visual inspection of the transparency of the cooled droplet, by the method described in [10].…”

Microfluidic cryofixation for correlative microscopy

“…15,16 Furthermore, positions close to the bottom of the channel (10 μm or less) result in cooling rates of the order of 10 5 to 10 6°C s −1 , corresponding to the critical cooling rate for pure water. 17,18 Therefore, under ideal conditions, the ultra-rapid microfluidic cooling technique presented here has the potential to greatly reduce or eliminate the need for cryoprotectants. Interestingly, at the same distance from the heater, a channel four times taller (h = 60 μm) results in a rate Fig.…”

“…Yet, these rates should still be high enough to allow for vitrification at low cryoprotectant concentrations. 18,19 In a first proof-of-principle we used budding yeast as a model system. Yeast cells were grown and resuspended in PM medium containing 800 mM sorbitol as described in the ESI.…”