Conrad Peter, Ewing George E, Karlinsey Robert L, Sadtchenko Vlad
Department of Chemistry, Indiana University, Bloomington, Indiana 47405, USA.
J Chem Phys. 2005 Feb 8;122(6):064709. doi: 10.1063/1.1844393.
The mechanism of heterogeneous ice nucleation on inorganic substrates is not well understood despite work on AgI and other materials over the past 50 years. We have selected BaF(2) as a model substrate for study since its (111) surface makes a near perfect match with the lattice of the basal face of I(h) ice and would appear to be an ideal nucleating agent. Two series of experiments were undertaken. In one, nucleation of thin film water formed from deposition of vapor on BaF(2)(111) faces was explored with the finding that supercooling to -30 degrees C was required before freezing occurred. In the other series, nucleation of liquid water on submerged BaF(2) crystals was studied. Here supercooling to -15 degrees C was needed before ice formed. The reason why BaF(2) is such a poor nucleating agent contains clues to realistic mechanisms of heterogeneous nucleation. Our explanation of these results follows the model of Fletcher [J. Chem. Phys. 29, 572 (1958)] who showed that heterogeneous ice nucleating ability depends on how well ice wets a substrate. In this view, a smooth BaF(2)(111) face is poor at nucleation because ice only partially wets its surface. In an extension of Fletcher's model, our calculations, consistent with the experimental results demonstrate that the pitting of a submerged BaF(2) crystal dramatically improves its ice nucleating ability.
尽管在过去50年里对碘化银和其他材料进行了研究，但无机底物上异质冰核形成的机制仍未得到很好的理解。我们选择氟化钡作为研究的模型底物，因为其（111）面与I（h）型冰基面的晶格几乎完美匹配，似乎是一种理想的成核剂。进行了两个系列的实验。在其中一个实验中，研究了由蒸汽沉积在氟化钡（111）面上形成的薄膜水的成核情况，发现需要过冷到-30℃才会发生冻结。在另一个系列中，研究了浸没在水中的氟化钡晶体上液态水的成核情况。在这里，冰形成之前需要过冷到-15℃。氟化钡作为如此差的成核剂的原因包含了异质成核实际机制的线索。我们对这些结果的解释遵循了弗莱彻[《化学物理杂志》29, 572 (1958)]的模型，他表明异质冰核形成能力取决于冰对底物的润湿程度。从这个角度来看，光滑的氟化钡（111）面在成核方面很差，因为冰只是部分润湿其表面。在弗莱彻模型的扩展中，我们的计算与实验结果一致，表明浸没的氟化钡晶体的点蚀显著提高了其冰核形成能力。
