(294e) Understanding the Role of Polymers on the Nucleating Behavior of Water in Dilute Supercooled Solutions Using Molecular Dynamics Simulations

The ice recrystallization inhibition (IRI) phenomenon is of utmost importance in the field of medicine (e.g., cryopreservation of organs, drug delivery), agriculture (e.g., crop damage), and industrial applications (e.g., jet aviation fuel, de-icing of turbine blades) [1-3]. Hence, understanding the nucleation behavior of water in dilute polymeric solutions is important for the development of suitable artificial IRI agents. Although Poly (Vinyl Alcohol) (PVA) is found to be one of the most potent biomimetic IRI agents, the molecular understanding of the nucleation behavior of water in the presence of such polymers is still lacking.

Here, in this work, we use molecular dynamics to elucidate the role of the concentration, degree of supercooling, degree of polymerization, and amphiphilicity of PVA and PVA-like polymers on the homogeneous nucleation of water in dilute polymeric solutions using the seeding method, based on the Classical Nucleation Theory (CNT) [1,4,5]. The seeding simulations are used to determine the critical cluster size (N_c), which has an equal probability to either grow or melt. The predicted N_c is then used to evaluate the interfacial free energy, the nucleation free energy barrier and then finally the nucleation rate of the supercooled water, based on the CNT formalism.

In line with the literature [1,2], our results indicate that the homogeneous nucleation rate of water in dilute supercooled polymeric solutions decreases with an increase in the solute concentration. The increase in the chemical potential difference and the interfacial free energy between ice and the supercooled solution, with an increase in the solute concentration, led to an increase in the nucleation free energy barrier, which finally decreased the nucleation rate of water. The nucleation rate also decreased with a decrease in the degree of supercooling due to the reduced propensity of the water molecules to nucleate at higher temperatures. The effect of the chain length and amphiphilicity of the polymers on the inhibition of ice nucleation is dominant at a higher degree of supercooling, whereas, at lower supercooling, the inhibition is mainly due to an increase in the solute concentration.

Our simulation results provide preliminary molecular insights into the factors affecting the nucleating tendency of water in presence of amphiphilic polymers, which may play a pivotal role in designing artificial IRI agents, for use in cryobiology, drug delivery and tissue engineering applications.

Figure 1: (a) The initial configuration for seeding simulations with Ih seed (red), PVA (black), and supercooled liquid water (cyan), (b) the time evolution of the number of ice molecules in the largest seed in the simulation box. The image represents the critical seed size of (N_c=2818) molecules for 2 wt% PVA 4-mer solution at 252.5 K, and c) the nucleation rate of water in supercooled solution at 240 K. The solid and empty symbols represent the data for solutions with 4-mer and 8-mer polymer molecules, respectively.

References

[1] Mochizuki K. et. Al., J. Am. Chem. Soc. 139, 17003−17006 (2017)

[2] Naullage P.M. and Mollinero V., J. Am. Chem. Soc. 142, 4356−4366 (2020)

[3] Naullage P.M. et. al., J. Phys. Chem. 121, 26949−26957 (2017)

[4] Espinosa J. R. et. al., J. Chem. Phys. 141, 18C529 (2014)

[5] Goswami A. et. al., J. Chem. Phys. 153, 094502 (2020)