J. Phys. Chem. 1980, 84, 1464-1468
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Nucleation of Ice by Sorption on Monodisperse Silver Iodide Particles in the 20-1200 Diameter Size Range
A
J. Rosinski," G. Cooper,+ National Center for Atmospheric Research, Boulder, Colorado 80307
and T. C. Kerrlgan Drexel University, Depaflment of Mathematical Sciences, Philadelphia, Pennsylvania 19 104 (Received September 20, 1979) Publication costs assisted by the National Center for Atmospheric Research
Two techniques have been employed to show that silver iodide particles smaller than 100 A in diameter nucleate ice by sorption. The first technique involves specially prepared membrane filters on which monodisperse silver iodide particles in the 20 to 1200 A diameter size range nucleate ice at temperatures between -18 and -10 "C. The predominant mode of ice nucleation is sorption at below 3% supersaturation with respect to water. The second technique involves the Bigg-Warner expansion chamber in which free aerosol particles in this size range nucleate ice at -100% supersaturationwith respect to liquid water. Nucleation practically ceased for particles smaller than 30 A in diameter, however.
Introduction Experimental evidence shows that ice nucleation on the surface of a silver iodide particle takes place through adsorption of water molecules at specific sites. The existence of these sites indicates that the surface of a silver iodide particle is not energetically homogeneous but contains a random distribution of minute sites of different free energies of interaction with water molecules. Adsorption of water molecules at one of these sites is by itself insufficient for ice nucleation, however. In order to produce an ice embryo, the orientation of adsorbed water molecules must also result in an icelike structure. This icelike orientation of water molecules is governed by the configuration of dipole moments. In fact, Rosinski has suggested (in Langer et al.I), that studies of ice-forming nuclei should consider the spatial distribution of constituent link dipole moments rather than crystalline similarity. This point of view unifies the studies of ice nucleation on organic and inorganic surfaces. Extensive studies of adsorption sites and dipole orientations of water molcules on silver iodide crystal surfaces have been undertaken by Hale (Kiefer and Hale,2 Hale et al.s4). Molecular water and ice clusters have been investigated by Plummer and Hale (Hale and Plummer+6 The energetically Plummer and Hale: Plummer et al.8~~). nonuniform distribution of adsorption sites and the specific orientation of adsorbed water molecules at each site give each potential ice-nucleating site a different probability of becoming active. This probability increases with decreasing temperature and increasing concentration of water molecules in the vicinity of the site. In addition, both experimental studies (Anderson and Hallett,lo Garvey,l' Rosinski et Rosinski and Nagamoto13) and theoretical studies (Fukuta and Paik,14Kiefer and H a l e 9 show that an effective ice-nucleating site consists of an -10 A diameter patch surrounding an Ag+ ion. 'Current address: Solar Energy Research Institute, Golden, Colo. 80401.
*This research was performed as part of the National Hail Research Experiment managed by the National Center for Atmospheric Research and sponsored by the Weather Modification Program, Research Applications Directorate, National Science Foundation. 0022-3654/80/2084-1464$01 .OO/O
The principal objective of this work is to show that small silver iodide particles (down to 20 A in diameter) are capable of adsorbing and organizing water molecules into ice embryos. The investigation is based on two experiments designed to detect silver iodide particles active as iceforming nuclei by sorption. The first makes use of a new membrane filter technique, and the second makes use of the Bigg-Warner expansion chamber. Both require the production of monodisperse silver iodide particles. Finally, we note that the solubility of angstrom-sized silver iodide particles in water excludes the possibility that the particles under investigation nucleated ice by contact, condensation followed by freezing, or freezing (Mathews et ale1%
Experimental Section The experimental program is divided into three parts: the generation of monodisperse silver iodide aerosol particles of known size, the development of a membrane filter technique to detect silver iodide particles active as iceforming nuclei, and the application of the Bigg-Warner chamber to detect silver iodide particles active as iceforming nuclei. 1. Generation of Monodisperse Aerosol Particles. Polydispersed AgI aerosol particles were produced thermally in a generator (Figure 1) consisting of a quartz chamber (vol 300 cm3)with two tubes penetrating into its interior. The chamber was placed inside a temperature-controlled oven. Within the chamber AgI liquid was brought to equilibrium with its vapor. A continuous, particle-free carrier gas flow (-8 cm3/s) displaced some of the vapor, which was then mixed with particle-free gas at a higher flow rate (50-100 cm3/s). Polydisperse AgI aerosol particles were thus produced by homogeneous nucleation. To produce monodisperse aerosol particles the polydisperse aerosol particles were first brought to a charge equilibrium within a =Kr charge equilibrator. On leaving the equilibrator, although most particles were electrostatically neutral, the particles in the size range of interest were predominantly unit-charged according to Boltzmann's law. The electrostatic mobility of these particles is therefore a monotonic function of diameter. Monodisperse aerosol particles were produced by electrostatic fraction-
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@ 1980 American Chemical Society
Nucleation of Ice by Sorption on Monodisperse AgI
The Journal of Physical Chemistry, Vol. 84, No. 12, 1980
To Electrostatic
Classifier Agl Aerosol
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generated in a high-stability atomizer (Cooperlg). 2. The Membrane Filter Technique. The membrane filter technique for the detection of silver iodide particles active as ice-forming nuclei consisted of the following. Monodisperse silver iodide aerosol particlea and monodisperse sodium chloride aerosol particles were deposited on a membrane filter (Millipore Corp. Bedford, Mass. 01730). The filter was then exposed in a dynamic chamber (Langer and Rodgers20)to a continuous stream of particle-free air saturated with respect to ice at some predetermined temperature higher than the temperature of the chamber. The ice crystals which formed on the filter were photographed and counted. Filters used in this experiment had nominal pore sizes of 0.45,0.22, and 0.1 pm in diameter. The sodium chloride particles were 900 A in diameter and had particle area density of 1.2 X lo7 particles/cm2. The initial supersaturation with respect to water at the filter surface during exposure was 3% in all experiments. Vaseline (0.5 cm3/ filter) was used to assure thermal contact between the filter and the chamber's cold stage. Preparation of the filters was carried out under a particle-free hood. The background counts were 0 to 2 ice crystals per sampling area on each filter. Two sequences of particle deposition were investigated. In the first case NaCl particles were deposited first and AgI particles were deposited second. In the second case this order was reversed. Because these particles were deposited soon after being generated, almost all particles carried one electron per particle at the time of deposition. Losses of AgI particles during transport between the electrometer-particledetector and the filter were estimated to be 86,63, and 47% for 20, 30, and 40 A diameter particles (Davies21). 3. The Bigg- Warner Technique. The Bigg-Warner technique for the detection of silver iodide particles active as ice-forming nuclei consisted of the following. All air in this experiment passed through activated charcoal, through a dryer, and through an absolute filter. A compressed air line was used to sweep out and pressurize the expansion chamber. Positive air pressure was maintained within the chamber in order to prevent room air from entering. The walls of the chamber were wetted with glycerine in order to prevent ice crystals from forming there. A tray with sugar solution was placed at the bottom of the chamber. The chamber was flushed with particle-free humidified air. Air containing sample particles was passed through the chamber for up to 3 min. The chamber was pressurized to 10 in. Hg at -9 "C and then after 1 min allowed to expand adiabatically. The final temperature ranged between -16 and -17 "C. Ice crystals which were grown in the sugar solution were counted by an observer. Background ice crystal numbers were normally below 10. Ice crystal counts higher than 50 were regarded as due to the formation of ice on the AgI nuclei. As in the membrane filter experiment, almost all AgI particles used in this experiment carried one electron per particle at the time of nucleation. The concentration of AgI particles used in this experiment was between 14 and 42/cm3. Each experiment was repeated at least 10 times. Calculations showed that coagulation in the chamber is negligible. Finally, the entire series of experiments was repeated after several months in order to eliminate error due to contamination or malfunction of the aerosol classifier.
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Flgure 2.
MONODISPERSED AEROSOL
Schematic flow diagrams.
ation of these polydisperse aerosol particles (Liu and Piu,I7 Langer et a1.18). A schematic diagram of the flow systems is given in Figure 2. When the generator was operating near the melting point of AgI (552 "C), particles from 20 to 100 A in diameter were produced abundantly. As the temperature was increased, the size distribution of aerosol particles shifted toward larger sizes (up to -2000 A in diameter). The size of monodispersed KgI particles down to 250 A in diameter was confirmed by means of an electron microscope. The silver iodide particles used in these experiments were nearly chemically pure. Even the surface of a pure AgI particle, however, may have lattice defects (the surface of thermally generated AgI is deficient in iodine atoms) sufficient to modify adsorption of the first water molecules. As a further complication, oxygen may have entered the lattice since these experiments were performed using air as the carrier gas. Monodisperse NaCl aerosol particles required in the membrane filter experiment were also produced by electrostatic classification of polydisperse aerosol particles
Results Little ice-nucleating activity was evident in membrane filter experiments in which NaCl particles were deposited first and AgI particles were deposited second (Figures 3
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I Sodium Chlorid
Porticle Area Density
Flgure 3. Fraction of different-sized Agl aerosol particles detected on a 0.22 pm membrane filter: NNI d = 750 A. f = 1.3 X NN2 d
= 5 0 0 A . f = 1.0 X l o ? NN3 d = 3 5 0 A, f = 2 . 0 X NN4 d = 2 5 0 A. f = 3.9 X IO-'. Area density (1.6 to 4.1) X 10'/cm-2; temp -16 O C .
and 4). In this case, detection of AgI particles depended on AgI particle sue and NaCl particle area density. It did not depend on the area density of AgI particles (3.9 X 10' to 4.2 X lo' particles/cm2) used in the experiments. In contrast, a surprising difference in nucleating activity appeared in membrane filter experiments in which the sequence of deposition was reversed (Figure 5). The fractions of different-sized AgI particles detected on filters were plotted vs. the numbers of AgI particles deposited on filters (Figure 6) for the AgI first-NaCI second sequence of deposition. The fraction decreased with increasing number of AgI particles for the three filter types (0.45,0.22,0.1 pm diameter nominal pore size). The fraction of particles detected on 0.45 pm filters was nearly two orders of magnitude lower than that on 0.22 and 0.1 pm filters. Because of this difference in sensitivity, all other membrane filter experiments were performed using 0.22 pm filters. A series of membrane filter experiments was then performed to investigate the dependence of AgI particle icenucleating activity on temperature and particle size. The results are given in Figure 7 for an AgI particle area density of 104/cm2. Note that the data presented in Figure 7 are raw in the sense that the tube losses mentioned in the Experimental Section have not been calculated for a filter holder. A series of experiments was also performed in the Bigg-Warner expansion chamber in which AgI aerosol particles were momentarily exposed to water vapor supersaturations of -100% with respect to liquid water. The fractions of AgI particles nucleating ice for 40.35, and 30 A were 0.2,O.l to 0.01, and -0,001, respectively. AgI particles practically ceased to nucleate ice for sizes below 30 A in diameter.
Discussion To account for the apparent lack of nucleating activity associated with the NaCl first-AgI second sequence of particle deposition, first recall that each NaCl particle and
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The Journal of phvsical Cfmmktty, Vol. 84, No. 12. 1980
Nucleation of Ice by Sorption on Monodisperse Agl
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Flgure 5. Detection of 200 A diameter Agl particles for two sequences of particle deposition: A. NaCl particles first. Agl particles second; 6, Agl particles first. NaCl particles second. Temp -16 'C: area density of AgI particles. cm-': A, 3.8 X 10' and 6. 1.0 X lo'. 100 .
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Flgure 6. Fraction fol different-sized Agl aerosol particles active as Ice-fwming nuclei on 0.1, 0.22. and 0.45 Frn membrane filters: temp -18 OC.
gives the AgI particles time to nucleate ice by sorption. In order to account for the diminished ice-nucleating activity associated with the 0.45 Fm filter data (Figure 6), note that the 0.45 pm filter data lie below the f = 0.1 level. This suggests that a large number of AgI particles were deposited below the filter face rather than on the knobs and so did not experience supersaturation a t the time of filter development. Further, 900 A diameter NaCl particles were small enough to be deposited deeper in the large filter pores. Some of the particles did not therefore experience supersaturation at the time of filter development. As a result, fewer droplets were produced and so a higher supersaturation tended to he sustained at the filter face. Some combination of the loss of AgI particles to the filter pores and a sustained high supersaturation at the filter face may result in the data presented in Figure 6.
Finally, we offer some remarks concerning the data presented in Figure 7. The high activity of particles -30 A in diameter is due either to a favorable configuration of ions in the structure of AgI particles of this size or to protection from wetting and dissolution by the filter surface. The first explanation is more plausible. There is no explanation offered for the trough between 50 and 80 A or the peak between 100 and 500 A. Some variation in deposition pattern of particles in this size range may account for these variations.
Conclusions It has been shown that silver iodide aerosol particles down to 20 A in diameter nucleate ice by sorption in the temperature range from -18 to -10 OC. Ice nucleation proceeds from 20 A on a substrate, where 30 A diameter AgI particles nucleate ice more readily than larger particles
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heterogeneous ice nucleation. Adsomtion and migration of water molecules over an AgI surface lead to a moiolayer over the AgI particle. At this time, water molecules in the surrounding air see only an H 2 0 cluster whose surface molecules have icelike orientations. Since sgch a cluster is far more stable than a pure water cluster of the same size, the probability that it will nucleate ice before falling apart is far greater than for its pure water counterpart. Acknowledgment. J. R. thanks Drs. B. Hale and P. Plummer, Department of Physics and the Graduate Center for Cloud Physics Research, University of Missouri-Rolla, Rolla, Missouri, for valuable discussions of this problem, The continuing interest and support of Drs. A. M. Kahan and B. A. Silverman from the Atmospheric Water Resources Management Division, Bureau of Reclamation, Denver, Colorado, are gratefully acknowledged. All filters used in the experiments were processed by Miss J. Baird, University of Colorado, Boulder, Colorado.
References and Notes (1) G. !-anger, J. Rosinski, and S. Bemsen, J. Atmos. Sci., 20, 557 (1963). (2) J. Kiefer and B. N. Hale, “The water monomer on the basal planes
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(A)
Figure 7. Fraction f of different sized AgI aerosol particles as a function of ice-nucleation temperature.
under our experimental conditions. Evidently the surfaces of these small particles differ from the surfaces of larger particles, where nucleating sites may be required for the production of ice. Ice nucleation proceeds from 30 8, for free AgI aerosol particles under our experimental conditions. The ability of AgI particles -20 8, in diameter to nucleate ice suggests a bridge between homogeneous and
of ice and AgI; an effectlve pair potential study”, Proceedings, 9th International Conference on Atmospheric Aerosols, Condensation and Ice Nuclei, Galway, Ireland, 1977. (3) B. N. Hale, J. Kiefer, S. Terrazas, and R. C. Ward, J . Phys. Chem., in press. (4) 8. N. Hale, J. Kiefer, and C. A. Ward, J. Phys. Chem., in press. ( 5 ) B. N. Hale and P. L. M. Piummer, J. Atmos. Sci., 31, 1615 (1974). (6) B. N. Hale and P. L. M. Plummer, J. Chem. Phys., 81,4012 (1974). (7) P. L. M. Plummer and B. N. Hale, “Ice nucleation mechanisms: applications of a molecular model”, Proceedings, 8th International Conference on Nucleation, Leningrad, U.S.S.R., 1973,p 158. (8) P. L. M. Plummer, B. N. Hale, J. Kiefer, and E. M. Stein, Colloid Interface Sci., 11, 45 (1976). (9) P. L. M. Plummer, J Glaciology, 21, 565 (1978). (10) B. J. Anderson and J. Hallett, J. Atmos. Sci., 33, 822 (1976). (11) D. M. Garvey, J. Appl. Meteor., 30, 165 (1973). (12) J. Roslnski, G. Langer, C. T. Nagamoto, C. W. Thomas, J. A. Young, and N. W. Wogman, J. Appl. Meteor., 12, 1303 (1973). (13)J. Rosinski and C. T. Nagamoto, J. Appl. Meteor., 13, 778 (1974). (14) N. Fukuta and Y. Paik, J . Appl. Phys., 44, 1092 (1973). (15) J. Kiefer and B. N. Hale, J . Chem. Phys., 87, 3206 (1977). (16) L. A. Mathews, 0.W. Reed, P. St.-Amand, and R. J. Stirton, J. Appl. Meteor., 11, 913 (1972). (17) B. Y. H. Liu and D. Y. H. Pui, J . Aerosol Sci., 5 , 465 (1974). (18)G. Langer, G. Cooper, C. T. Nagamoto, and J. Rosinski, J. Appl. Meteor., 17, 1039 (1978). (19) G. Cooper, Am Ind. Hyg. Assoc. J., 40,734 (1979). (20) G. Langer and J. Rodgers, J. Appl. Meteor., 14, 560 (1975). (21) C. N. Davies, “Aerosol Science”; Academic Press: London, 1966, p 393. (22) B. Binek and S. Przyborowski, Staub Rinehart. Luff, 25, 533 (1965).
