Heterogeneous Ice Nucleation Rate Coefficient of Water Droplets Coated by a Nonadecanol Monolayer

Abstract

The heterogeneous ice nucleation rate coefficient (j het) of water droplets coated with a monolayer of 1-nonadecanol was determined from multiple freezing/melting cycles. Freezing was monitored optically with a microscope for droplet radii between 31 and 48 μm and with a differential scanning calorimeter for radii between 320 and 1100 μm. The combination of these two techniques allows the surface area of the 1-nonadecanol nucleating agent to be varied by more than a factor of 1000, showing that j het increases only by ∼5 orders of magnitude over a temperature range of 18 K. This is roughly 5 times less than the change in the ice nucleation rate coefficient for homogeneous ice freezing at around 238 K or for heterogeneous ice freezing in the presence of a solid ice nucleus, such as Al2O3. This temperature dependence of j het can be reconciled with the framework of classical nucleation theory, when assuming a reduced compatibility of the alcohol monolayer with the ice embryo as the temperature decreases. We attribute this finding to an enhanced ability of the alcohol monolayer to adapt to the ice structure close to the ice melting point due to larger thermal density fluctuations in the monolayer, which in turn makes the monolayer serve as a better ice nucleus.

Refreeze experiments with water droplets containing different types of ice nuclei interpreted by classical nucleation theory

“…Subcritical ice clusters may be produced at a high rate but are prevented from reaching a critical size by the confinement in the pores. Marcolli (2014) showed that the CNT-based parameterization by Zobrist et al (2007) is able to describe the observed melting point depression in pores as a function of temperature and should therefore be well suited to estimate critical embryo sizes. The critical embryo volume for homogeneous ice nucleation predicted by this parameterization is 109 nm 3 at 254 K and increases to 1441 nm 3 at 265 K. We therefore consider the energy barrier and critical embryo size predicted by CNT as a quantity with a physical basis.…”

“…Several parameterizations of j hom (T ) have been proposed in the literature. Three different parameterizations are considered in this study to evaluate the measured data, namely the parameterization provided by Zobrist et al (2007), hereafter referred to as Z07, the parameterization given by Pruppacher and Klett (1997), P&K97, and the parameterization from Ickes et al (2015), Ick15. Differences between these parameterizations are discussed by Ickes et al (2015) and concern mainly the treatment of F diff (T ), σ (T ), and n s .…”

“…In the present study, we perform refreeze experiments similar to those of Vali (2008) and in order to characterize and compare the properties of single nucleation sites. We fitted the freezing rates from the refreeze experiments using three different CNT-based parameterizations from Pruppacher and Klett (1997), Zobrist et al (2007), and Ickes et al (2015) under the assumption that ice nucleation occurs at a single site of critical size, namely the most effective one in the sample.…”

“…Birch pollen washing water containing macromolecules with 100-300 kDa show similar freezing temperatures to the whole pollen grains (Pummer et al, 2012). Zobrist et al (2007) performed refreeze experiments with water droplets coated by a nonadecanol monolayer, which arranges itself in a 2-D crystalline lattice on the water surface. The structural match of this 2-D crystal with the ice lattice has been considered as a key reason for the good ice nucleation ability of long-chain alcohol monolayers (Popovitz-Biro et al, 1994;Majewski et al, 1995;Knopf and Forrester, 2011).…”

“…The structural match of this 2-D crystal with the ice lattice has been considered as a key reason for the good ice nucleation ability of long-chain alcohol monolayers (Popovitz-Biro et al, 1994;Majewski et al, 1995;Knopf and Forrester, 2011). The refreeze experiments by Zobrist et al (2007) are reevaluated here assuming that instead of the whole monolayer only an active site in it is responsible for ice nucleation.…”