Langmuir 1994,10, 1673-1675
1673
Nucleation of Crystals under Langmuir Monolayers: Kinetic and Morphological Data for the Nucleation of Ice R. J. Davey,' S. J. Maginn, R. B. Steventon, J. M. Ellery, and A. V. Murre11 Process Technology Department, ZENECA plc, P.O. Box 42, Hexagon House, Blackley, Manchester M9 3DA, U.K.
J. Booth, A. D. Godwin, and J. E. Rout Research and Technology Department, ICI Chemicals and Polymers, The Heath, Runcorn, Cheshire WA7 4QD, U.K. Received January 3,1994. In Final Form: April 26,1994" Over the past decade the use of Langmuir monolayers as ordered two-dimensionaltemplates for oriented crystallization of inorganic and organic substrates has been clearly demonstrated. Surprisingly no kinetic studies of nucleation in such systems have been reported, although it has been assumed that such templates reduce the activation energy for nucleation. Accordingly, this paper constitutes the first kinetic measurements to have been made and uses the nucleation of ice from water under an alcohol monolayer aa the system for study.
Introduction Interest in molecular recognition at interfaces' and particularly in those aspects related to biomineralization2 have led to a number of investigations in which Langmuir monolayers have been used as two-dimensional templates for crystallizationn from an aqueous supersaturated subphase. Thus, for example, Landau et al.394 were the first to demonstrate that for both organic (glycine) and inorganic (sodium chloride) materials it was possible to achieve oriented nucleation of crystals directed by the stereochemistry, geometry, and functionality of the monolayer. The extension of these studies to biologicallyrelated mineral phases such as calcium carbonate6 and barium sulfateshas illustrated the general validity of this approach and linked it to naturally occurring processes of mineralization. To date these studies have concentrated exclusively on the mechanism by which structural information is transferred from the two-dimensional monolayer to the three-dimensional crystal as inferred from a combinationof morphological and diffraction data.6 Two processes appear to operate, one in which the head groups of the monolayer mimic the molecular array of a particular surface of the crystallizing material4 and the second in which electrostatic interactions at the interface lead to ordered binding of particular i o n ~ . ~ > l In this contribution we wish to address the issue of nucleation under monolayers from a kinetic standpoint. This is the first time that the kinetics of this nucleation process have been studied and we believe that such data may provide further insights into the processes occurring. ClassicallyS the rate of nucleation is related to supersaturation, u, temperature, T, and the interfacial tension, y, of the crystal/fluid interface by an expression of the form Q
Abstract published in Advance ACS Abstracts, June 1, 1994.
N Ais Avagadro's number, u is the molar volume, and the preexponential factor, A, is related to the collision flux and sticking probability of crystallizing molecules. When nucleation takes place in the presence of a monolayer, this may be considered as a heterogeneous process for which eq 1 must be modified to take into account both the increased number density of nucleation sites and the extent of the structural match between monolayer and crystal. Formall? this amounts to an increase in the preexponential factor, A, and a modification of the exponential part by a parameter d which varies from 0 to 1 as the perfection of the structural match decreases. Clearly, if the match is perfect, then no energy barrier exists for the formation of a crystallfluid interface and hence the nucleation rate achieves its maximum value ( A ) at all supersaturations. As the structural match deteriorates, the value of the apparent interfacial tension will rise, eventually attaining that for homogeneous or heterogeneous nucleation in the absence of the monolayer. It follows that by measuring nucleation rates in the presence and absence of monolayers, eq 1may be used to estimate apparent interfacial tensions and hence provide an independent check on the structural thesis of monolayer nucleation. In this study we present preliminary data on the nucleationn of ice by monolayers of the amphiphilic alcohol C3&10H. We have measured the experimentally accessible induction time, t, and assumed it to be inversely proportional to the nucleation rate.8 The supersaturation in this system is related to the enthalpy of melting, AHf, the melting point, Tf,and the undercooling, AT, according to
(1) Addadi, L.;Berkovitch-Yellin,Z.; Weissbuch,I.; van Mil, J.; Shimon,
L. J. W.; Lahav, M.; Lieserowitz, L. Angew. Chem., Znt. Ed. Engl. 1985, 24, 466. (2) Mann, S.Nature 1988,332, 119-324. (3) Landau,E. M.;Grayer-Wolf, S.;Levanon, M.;Leiserovitz,L.;Lahav, M.; Sagiv, J. J . Mol. Cryst. Liq. Cryst. 1986, 134, 323. (4) Landau, E. M.; Popovitz-Biro, R.; Levanon, M.; Leiserowitz, L.; Lahav, M.; Sagiv, J. J. Am. Chem. SOC.1989,111, 1436-1439. (5) Mann, S.:Hevwood. B. R.; hiam, S.; Birchall, J. D. Nature 1988, 334,692-696. . (6) Heywood, B. R.; Mann, S.Langmuir 1992,8, 1492-1498. (7) Levsiller, F.; Jaquemain, D.; Lahav, M.; Lekrowitz, L.; Deutsch, M.; Kjaer, K.; Ale-Nielsen, J. Science 1991, 252, 1532-1535.
Linearization of eq 1then means that a plot of ln(1lt) vs l/a2 should yield a straight line whose slope gives the apparent interfacial tension. The choice of the ice system (8) Garside, J.; Sohnel, 0. Precipitation; Butterworth-Heinemam Ltd.: Oxford, 1992. van der Leeden, M. C.; Kashiev, D.; van Rosmalen, G. M. J. Cryst. Growth 1993, 130, 221-232.
0743-7463/94/2410-1673$04.50/0 0 1994 American Chemical Society
Letters
1674 Langmuir, Vol. 10, No.6,1994
A n
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L Control
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-12
-14
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Temperature (C)
Figure 1. Induction times as a function of temperature.
0
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200
300
400
500
(c1" Figure 2. Linearation of eq 1: 0 , control; A, monolayer.
4 t
was made largely for experimental reasons. Firstly it has been extensively studied previouslygJOand the catalytic effect of the aliphatic series of both odd and even chain length alcohols demonstrated. Secondly, the monolayer experiments can be performed on small sample volumes without the use of a Langmuir trough. We chose C&10H in this initial study because, giving a freezing point of about -7.5 "C, it allowed a reasonable range of undercoolings in which to perform isothermal experiments. It was also availablea t the high purity necessary. In addition to these kinetic data, some frozen drops were examined using cry0 scanning electron microscopy in order to provide related morphological data.
I
1
I
1
Experimental Section Measurement of induction times was carried out on a O.2-cma sample of distilled water held in a small thermostated stainless steel cell which was connected to a chiller bath. Its temperature could be controlled (10.1"C)at any present value in the range 0to -15 "C. Monolayers were introduced onto the water surface from chloroform solution and the cell was cooled to a predetermined constant temperature which was monitored with time using a thermocouple. The induction times were then estimated (k5 min) measured from the point a t which the set temperature was reached to that a t which freezing occurred as judged by the appearance of the freezing exotherm in the temperature profile. To examinethe microstructural differencesbetween water frozen in the absence and presence of catalytic monolayers, we used the cell described above in which water droplets (5pL) were frozen on a glass cover slip. Some frozen droplets were fractured with a razor blade to expose their internal structure. Samples were then cooled to liquid nitrogen temperatures, gold coated, and examined by a Hitachi S650 SEM fitted with a cryostage.
d
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*.
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.
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Figure 3. Scanning electron micrographs of frozen drops: (a) control; (b) with monolayer.
Results and Discussion At temperatures above about -5 "Cand, particularly in the absence of alcoholmonolayers, the measured induction times showed a high degree of scatter. Thus, for example, control experiments at -4 "C froze in times ranging from 400 to 900 min. This reflects the stochastic nature of the freezing process and for this reason each data point was (9) Gavish, M.; Popovitz-Biro, R.; Lahav, M.; Leiserowitz, L. Science repeated 8 times and averages were taken. Hence, Figure 1990,250,973-975. 1 shows the induction times as a function of freezing (10) Mann,S.;Archibald,D.D.;Didymue,J.M.;Douglas,T.;Heywood, temperature both with and without the nucleating monoB. R.; Meldrum, F. C.; Reeves, J. J. Science 1993,261,1286-1292.
Letters layer. A number of features are apparent. Firstly the monolayer enhances the rate of nucleation; induction times are reduced at equivalent freezingtemperatures. Secondly a t high temperatures the technique becomes unreliable due to the high scatter discussed above. Thirdly, at -8 "C the induction time in the presence of the alcohol is reduced to near zero. This agrees well with the nucleation temperature for this alcohol measured by Gavish et al.g Figure 2 shows the same data plotted according to eq 1 above with the best straight lines as shown. From the gradients of these plots, the interfacial tensions were calculated to be 3.8 and 3.7 mJ/m2 in the absence and presence of the alcohol, respectively. Given that the true ice/water interfacial tension is around 22 mJ/m2, it is concluded that nucleation in these experiments is heterogeneous. Following the arguements above, these data suggest, surprisingly and in contradiction with the model suggested recently by Mann et al.,lothat the presence of the monolayer does not reduce the free energy for forming acritical nucleus. Further, since the critical radius is given by rc = 2yVIRTa (3) it follows that the monolayer does not reduce the critical nucleus size. At first sight this conclusion is not consistent with the structural model of Gavish et aL9 or the experimental data of Majewski et al.12 which point to an epitaxial relationship between the basal surface of hexagonal ice and the oxygen array in the alcohol monolayer. According to our results the catalytic effect is related to the preexponential factor in the nucleation expression and hence is due to the increased number of hetero sites rather than their superior structural compatibility with ice. Indeed the datasuggest that the structuralmatch between ice and the monolayer is no better than with other heteronuclei: only the monolayer is active at high tem(11) Majeweki, J.; Popovitz-Biro,R.;Kjner, K.; Als-Nieleen, J.; Lahav, M.; Lieeerowitz, L. Submitted for publication in J. Am. Chem.SOC. (12) Majewski, J.; Margulie, L.; Jacquemain, D.; Leveiller, F.; B o b , C.; h a d , T.;Tamon, Y.; Lahav, M.; Laierowotz, L. Science 1993,261, 899-902.
Langmuir, Vol. 10, No. 6, 1994 1675 peratures, other nuclei types are evidently inactive until lower temperatures are reached. By use of eq 3 together with our estimates of the interfacial tension, the critical nucleus size can be calculated to lie in the range 30-15 A as the freezing temperature falls from -2 to -5 O C . These values are in close agreement with the value of 20 A estimated from grazing angle X-ray diffraction data by Majewski et al." for the crystallite size of ice frozen under C31H630H.We thus conclude that in the nucleation of ice by alcohol monolayers the structural match between the hydroxyl array and the basal plane of hexagonal ice is essential for the catalysis of the nucleation process since it provides active, heterogeneous sites at temperatures close to 0 "C. This mechanistic insight is a powerful demonstration of the role of kinetic data in aiding our understanding of these processes. Examples of the microstructure of frozen drops are shown in Figure 3. Without a monolayer the drop surfaces were of a grainy texture (Figure 3a), each grain being 30-60 pm across and roughly hexagonal. Sectioning revealed each drop to be integrally frozen but showed little structural detail. In contrast the presence of the monolayer (Figure 3b) created a 5-pm skin on the drop comprising many smaller crystallites. This is presumably a layer of ice that first nucleates in contact with the monolayer and its appearance is consistent with visual observations which confirm the initiation of freezing at the drop surface. The formation of this layer and its tendency to crack and flake away from the rest of the solidified drop during sectioning suggest that the freezing process as catalyzed by the monolayer is discontinuous. Crystalsnucleated under the monolayer do not, apparently, grow to become part of the bulk drop. Their growth is limited to the surface region, perhaps because as shown elsewhere11J2 they nucleate with their slowest growing basal surfaces directed into the remaining liquid. Subsequent secondary nucleation in the bulk drop would not impose such a morphological constraint on growth and hence complete solidification would take place more rapidly yielding the observed interface between the two solid regions.
