224
The Journal of Physical Chemistry, Vol. 83, No. 2, 1979
(15) I. DraganiE, Z. DraganiE, Lj. PetkoviE, and A. NikoliE, J . Am. Chem. Soc., 95, 7193 (1973). (16) S. 0. Nielsen, 8.D. Michael, and E. J. Hart, J . Phys. Chem., 80, 2482 (1976).
A. T. Pudzianowski and R. N. Schwartz (17) E. Hayon, A. Treinin, and J. Wilf, J. Am. Chem. Soc., 94, 47 (1972). (18) P. Neta, V. Madhavan, H. Zemel, and R. W. Fessenden, J. Am. Chem. SOC.,99, 163 (1977). (19) J. Holcman and K. Sehested, J . Phys. Chem., 80, 1642 (1976).
Radiation Chemistry of Glassy Alkaline Ice Systems from a Structural Viewpoint Andrew T. Pudzianowski and Robert N. Schwartz” Department of Chemistry, University of Illinois at Chicago Circle, Chicago, Illinois 60680 (Received May 15, 1978; Revised Manuscript Received October 10, 1978)
Structural considerations have been extensively applied to critically examine the radiation chemistry of yirradiated 10 M NaOH aqueous glassy systems at 77 K. New mechanisms have been proposed for the production of 0- ions in such systems, based on theoretical and experimental results concerning the structural stabilization of these species by their immediate molecular environments. In addition, a model for the large-scale structure of NaOH aqueous glasses is proposed, based on available information on the nature of the trapped electrons in these systems and the spatial distribution of trapped electrons and 0- ions. This model involves very small clusters of hydrated Na’ and OH- ions, with an internal structure similar to that in concentrated NaOH crystdine hydrates such as NaOH.4H20, embedded in a matrix of ice-like “excess” water. The overall radiation chemistry of alkaline glasses is then considered in terms of these two different types of regions, and it is proposed that 0- ions are localized in the ion clusters, while trapped electrons reside in the water-rich regions.
(1) Introduction The problem of structural stabilization of species is a central one in radiation chemistry and is certainly of direct relevance to the understanding of mechanism. The authors’ aim has been the application of structural considerations to the radiation chemistry of the glassy systems formed by relatively concentrated ( 10 M) aqueous NaOH solutions which have been rapidly frozen to 77 K. (Many aspects of the relevant radiation chemistry have been comprehensively reviewed by Kevan.1$2When not otherwise specified, experimental results cited here are from these sources.) We believe our work to be the first in which a reasonably complete molecular picture of a particular system has been attempted. The principal paramagnetic products observed in NaOH glassy ices are trapped electrons le;) and stabilized 0- ions, formed in equal radiation chemical yields, along with trapped hydrogen atoms (H,) in -50-fold smaller yields. Note that, in contrast to y-irradiated pure ice, trapped OH radicals are not observed in irradiated 10 M NaOH glasses. We have drawn on experimental results3 for e; and on theoretical models for 0- stabilization which are consistent with the results of a preliminary electron spin echo (ESE) s t ~ d y The . ~ stabilization of reactant species is approached in terms of crystallographic results for the hydrates NaOH-4H20 and NaOH.7H20a6 Consideration of possible trapping sites for e- and 0has led us to independently propose a model for extended structure in these systems, which involves small clusters of ions, locally resembling concentrated crystalline hydrates, surrounded by excess water. Firm experimental evidence for just such a model in LiCl aqueous glasses has appeared very r e ~ e n t l y ,and ~ the results of a Raman spectroscopic study of 10 M NaOH, in both the liquid and glassy states, tend to support the model further.s In addition, the model is consistent with the results of a thermodynamic analysis of a number of glass-forming liquids,9 in which differently constituted solutions were N
0022-3654/79/2083-0224$0 1.0010
found to behave identically near the glassy phase transitions. Thus, it may well turn out that this type of model will prove to be generally useful for concentrated electrolyte solutions which form glasses and that its validity will also extend to the liquid state. (The latest approach regards the glassy phase as a minimum-energy configuration of the corresponding liquid?) For the specific radiation chemistry involved, the model’s essential feature is that it consists of two types of regions: small pockets in which the ions are localized, and the surrounding water-rich regions. In what follows, we show how the model and some of its features, such as the dimensions of the ion clusters, may be inferred from radiation chemical evidence. Sets of detailed mechanisms at the molecular level are proposed which are appropriate for both types of regions, and we go on to show how the structural model provides insight into various aspects of the observed radiation chemistry, e.g., the concentration dependence of radiation chemical yields. (2) S t r u c t u r a l Stabilization of Individual P r o d u c t a n d Reactant Species ( a ) e c . The work of Schlick et al.3 has provided a detailed picture of electron traps in y-irradiated NaOH glassy ices. The traps are, on the average, octahedral arrays of six water molecules, each of which is in a “hydrogen-bond” orientation with respect to the trapped electron, with the H-et- distance being 2.1 A for the nearest-neighbor hydrogens. The difference between e; yields in y-irradiated glasses and crystalline hydrates is both striking and important. At 77 K, the 10 M glass gives G = 1.9 f 0.1, while NaOH.3.5H20 (- 16 M) is characterized by a small yield of G = 0.05 f 0.02,1° from which it may be concluded that suitable traps are much less available in the hydrate. ( b ) 0-, OH-, and OH. Figure 1 shows models for the stabilization of the above species in an aqueous environment, as obtained by the present authors with the aid 0 1979 American Chemical Society
The Journal of Physical Chemistry, Vol. 83, No. 2, 1979
Radiation Chemistry of Glassy Alkaline I c e Systems H
dH
225
NaOH * 1120 -56M
yH
x x I ti
o,
A
(
B HsO’-O.
A. HYDRATED 0’ ION 1.2.40
0-H H’
H
H
i i
i(270 i]
____,&L.H-o 1 a l’
COMPLEX
~ 2 . 5 0 (2 81 i i b-2 20 ( 2 . 4 8 i i
\H
H/
o.-H---$L--H-o /H’
~
;
a
\
H
/H
i o\H
‘-I
C HYDRATED OH’ ION
$
1-2 40 ( 2 70 b-2 60 A (2 82 A )
C ‘ I
o\H D HYDRATED OH RADICAL
1 Hi
a-2 50 (2 81 b - 2 4 0 A 12 70 A)
Flgure ‘1. INDO-optimized structural models for stabilized species in an aqueous environment. The distances denoted a and b are interoxygen.
of open- and closed-shell INDO calculation^.^ The distances given are for direct optimization of geometry and, in parentheses, for results obtained after application of a scaling factor to the directly optimized distances. The models shown were all found to yield greater stabilization energies than their tetrahedrally coordinated counterparts. It should be kept in mind that they represent “gas-phase” situations, energetically ideal structures that might not be attainable in a low-temperature solid. (However, see below.) For 0, the experimental ESE results5 are not as detailed as one might hope. While they do indicate that the 0- ion in irradiated alkaline ice is stabilized by a firfit shell of water molecules, no information is given as to the number and location of nearest-neighbor hydrogens. The presence of a metal cation 3.87 A, on the average, from the 0- is also indicated. For both 0- and OH-, the octahedral coordination and scaled distances in Figure 1compare very favorably with the first coordination shell for the anions in lKF.4H20, NaOH.4Hz0, and N a 0 H ~ 7 H ~ 0However, .~ our !structural models involve only the first shell, while it turns out that the nature of the second coordination shell in of great importance to the consideration of mechanism. Our earlier theoretical study also included hydrated OH radical, since this species has long been thought to be an intermediate in the production of 0- in aqueous alkaline glasses. (3) Structural Considerations Based on NLIOH Crystalline Hydrates T o discuss the second coordination shells of reactant species in alkaline glass irradiation, we have used NaOH hydrates as models. Figure 2 shows the detailed first coordination shell for the OH- ion in NaOH$tH20and NaOH.4H206 and in NaOH.7HZ0.l1 The second coordination shells are indicated in parentheses, next to each first-shell species. The approximate concentrations of the corresponding solutions are also indicated. In the tetra- and heptahydrate, the first-shell waters are tetrahedrally coordinated. The extended structure is therefore a compromise between the coordination characteristic of ions and the tetrahedral {coordinationpreferred by water. Note that each ionic species is separated from
NaOH * 7H20-8M
Figure 2. Details of first and second coordination shells of each OHin NaOH.H,O, NaOt-1.4H2O, and NaOH-7H2O. Indicated distances are interoxygen, in A, and the nearest neighbors of each first-shell species are given in parentlheses.
all other ions by at least one bridging water molecule. This point plays a prominent role in the detailed mechanisms to follow. In the highly concentrated monohydrate, each ionic species has other ions in its first coordination shell, and the water molecules are all six-coordinated. Here the high concentration of‘ ions forces a complete breakdown of the tetrahedral water structure, and the coordination of all species is “ionic”.
(4) Radiation Chemical Evidence for the Cluster Model The NaOH glassy ices studied in radiation chemistr:y are most frequently prepared from 10 M solutions. If maximal ordering were present in the solid, it would exhibit a regular structure similar to that of NaOH.7H20. However, maximal ordering in such systems leads to very low yields of e; relative to the corresponding glassy ices, so the greater disorder in the latter must be responsible for increased yields. The detailed nature of electron traps in these sysitems implies the availability of large numbers of water molecules not incorporateid into ion hydration shells. In fact, the structures of NaOH.4HZ0 and NaOH.7HZ0 (Figure 2) preclude the existence of regions in the crystal where this condition is met. Such “water-rich’’ regions must therefore be characteristic of the glassy state, from which it follows that complementary “ion-rich’’ regions must also be present. We picture the glass as consisting of clusters of hydrated ions, internally more concentrated than the overall molar concentration of‘the bulk solution, embedded in a matrix composed of the excess water molecules, Le., those not involved in ion Iiydration. We propose that the internal structure of such ion clusters would resemble small regions in a crystalline hydrate, whose overall concentration is greater than 10 M. We therefore assume that NaOH.4H20 will serve as a good model. In the radiation chemistry of aqueous systems, the initial step in the molecular sequence leading to observed products is most probably the ionization of a water molecule, to produce a water cation and a mobile electron (ern-): H20H 2 0 ++ ern(1)
226
The Journal of Physical Chemistry, Vol. 83, No. 2, 1979
A. T. Pudzianowski and R. N. Schwartz
The fate of e,- will either be capture by some chemical species or stabilization in a physical trap to produce et-. As we have already seen, 10 M NaOH glasses a t 77 K provide an ample supply of physical traps. It has been possible to estimate the distance traveled by the mobile electron prior to trapping. On the basis of scavenger studies, this is between 20 and 40 A, which agrees with the average e,--O- distance of 30-35 A as determined by electron paramagnetic resonance (EPR) methods.12 This may be indicative of how far a mobile electron must go before it encounters a water-rich region, assuming the initial ionization took place in an ion cluster. This assumption will be made when we consider the production mechanism for 0- in alkaline glasses. We can assume that reaction 1 involves a water molecule in the first shell of a hydroxide ion. In an ion cluster, such a water molecule would probably bridge two OH- ions, as in the NaOH-4Hz0structure (Figure 2). Following (l), the next set of reactions is
-
+ OHOH + OH--
HzO+
+ H20 0- + H20 OH
(2)
(3)
These would be acid-base reactions involving proton transfer between hydrogen-bonded nearest neighbors, and the water molecule ionized in (1)would ultimately become the 0- ion, with the two water molecules produced in (2) and (3) augmenting its stabilization. The et-0- distance of 30-35 8, would then serve as a rough indication of the size of the ion cluster in which 0- is produced. Further evidence is available in the form of average 0-0- distances, obtained by ESE techniques, on the order of 70 .&.I3 This is roughly twice the average e;-0- distance and fits in neatly with the cluster model if we assume the clusters are so small that, on the average, only one 0- per ion cluster is produced in an ionization path. The model helps explain why the reaction sequence (1)-(3), otherwise expected to occur in an irradiated NaOH crystalline hydrate, does not lead to large yields of e t and 0- in the maximally ordered solid. If em-of reaction 1 has no physical traps available close to the ionization site, rapid capture by a water cation might take place resulting in no net reaction. The very low yield of et- observed in NaOH.3.5H20 might be indicative of the relative contribution of OH- vacancies to the overall distribution of trapping sites for ern-,since such vacancies would seem to be the only recourse in a crystalline hydrate. On the basis of these considerations, we suggest the following rough model for the extended structure of alkaline glassy ices. We consider the ion clusters to be roughly spherical, with radii of perhaps 20 .& since em-must travel a t least that far before becoming trapped. This limits the total number of hydrated ions per cluster to something like 10-20. The model is pictured in Figure 3a.
Figure 3. Rough schematic of the proposed model for extended structure in NaOH aqueous glasses. The hatched portions denote the water matrix. Spatial distribution and equality of yield are implied only See text for additional comments. for 0-and e.;
these reactions, the dots indicate that the water molecule initially undergoing ionization bridges two OH- ions through hydrogen bonds, as in the NaOH-4H20crystal structure. Since this water molecule was originally coordinated to two additional species not shown in the reactions, the resulting 0- has a roughly tetrahedral stabilization shell of four surrounding species, if complete rigidity of the matrix is maintained during the reaction. However, depending on the amount of energy being dissipated into the local matrix a t each point along the ionization path, some rearrangement may be possible. This would lead to an inhomogeneous distribution of stabilization sites for 0-. The implied inhomogeneity of 0- sites has been suggested previously, based on the results of an EPR study of y-irradiated NaOH glassy ice enriched in 170.14 In addition, the cluster concept is consistent with the presence of metal cations near the 0-,at the average distance noted earlier, since some sites may include Na+ as a nearest neighbor and others may not. We consider the sequence (4)-(6) to be the most likely mechanism for 0- production in alkaline glasses. Unless the water ionized in the first step bridges two OH- ions, it seems unlikely that an 0- would be formed. Consider, for example, the following set of reactions, in which the water molecule is coordinated to only one OH-: OH-.-H20-.H20 -OH--H20+-.H20
+ ern-
(7)
The next step in such a sequence would be
( 5 ) Detailed Mechanisms
Since the proposed model involves two types of regions, two different sets of reactions must be presented to explain the overall radiation chemistry. ( a ) Ion Clusters. To suggest the structural aspect of 0formation, we rewrite reactions (1)-(3) as in eq 4-6. In (HzO)50H-...HzO...OH-(HzO), (Hz0)50H-...H20+...0H-(H20)5 + e,- (4)
OH---H,O+..-H20
+
H2O...OH*.*H20
(8)
The result is a stabilized OH radical, which is not observed in concentrated y-irradiated alkaline glasses. The acidbase dissociation of OH radical in aqueous solution is governed by a pK value of 11.7,15so it is highly unlikely that the products of reaction 8 would spontaneously form 0- and H30+. It is of interest to note that the 4 G 0 for OH radical (HZO)jOH-...H~O+...OH-(Hzo)s dissociation and the difference between the INDO total energies4 for the geometry-optimized species OH(H,O), (Hz0)5HzO...OH...OH-(HzO)j (5) and O-(H,0)5H30+are comparable.16 (Hz0)5HzO...OH...OH-(Hzo), In the production of 0-,there is one further mechanism (H20)5HZO...O-...H20(~20)5 (6) to consider, which may be viewed as a minor contributor
-
-
Radiation Chemistry of Glassy Alkaline Ice Systems
The Journal of Physical Chemistry, Vol. 83, No. 2, 1979 227
both on energetic and statistical grounds. This involves the direct ionization of OH- to produce OH radical, possibly in an excited state which we denote as OH*. The sequence of rcactions, with structural aspects implied, is
(Hz0)5OH-...IIzO...OM-(HzO)5 -ww-+ (H20)50H*...Hz0...0H-(HzO)5-t- em- (9)
(H20)50H*...HzO...OH-(Hz0)5 +
-
(Hz0)50-...H30+...OH-( HZO), (10)
(H20)5O-...H3Of...0H-(H2o), (HzO),0--H20.-H20 (HzO)5 (11) The 10- production mechanisms suggested above, especially the main sequence (4)-(6), are much more detailed than the macroscopic mechanisms accepted previously,’i2 and differ from those initially discussed in terms of structure by the present author^.^ ( b ) Water-Rich Regions. The radiation chemistry here is that of pure water ice, but modifications must be introduced to take into account the proposed influx of ernfrom adjacent ion clusters. The essential sequence of reactions in pure water radiation chemistry is HzOHzO+
-
+ ernOH + H30+
HzOf
+ HzO
(12)
(13) In y-irradiated pure water ices, the products of (13) may be stabilized, and trapped OH radicals are observeld, as well as e; and H,. Thus, we expect all these species to be formed in the water-rich regions of y-irradiated alkaline glasses but, as already mentioned, OH is not observed and the yields of Ht are low. A possible explanation is to postulate a cascade of emcoming into these regions during irradiation. In addition, ionizations such as (12) within these regions alslo lead to ern-,so at any given moment there must be more electrons present than chemical products, Le., OH and H30+. T o account for the disappearance of O H from these regions, we propose the scavenging reaction
OH,,
+ e--
OH,,;
(14)
where e- denotes both em-and e;. The OH radical is an efficient electron scavenger, as is evident from its electron affinity of 1.83 eV.l7 In view of the equal yields of 0- and e; observed in these systems, this scavenging reaction also helps account for the lack of an excess of e;. Reaction 15 may also account for the disappearance of OH,, + H H20 (15)
-
OH. This reaction may be connected with the following sequence, involving scavenging of electrons by H30+:18
- +
H30aqf+ eH30aq
HzO
H30,,
Haq
(16) (17)
Thus, (‘14) and (15) may account for the disappearance of OH from water-rich regions, while (16) and (17) provide a mechanism for Ht formation in alkaline glasses.
(6) Discussion Figure 3b summarizes the distribution of species after irradiation, as inferred from the considerations of the previous section. It should be pointed out that reactions 4-6 imply that irradiated clusters are left with ai? overall positive charge, since two OH- ions have been consumed and only one negative product, 0-, remains in the cluster. Since the figure does not explicitly show this, it should be kept in mind. In addition, OH- vacancies may be minor
contributors to the distribution of e; sites, and such traps would necessarily be located in ion clusters. This is idso not shown in the figure, which is intended as a summary of what we consider the major results to be. We now consider the implications of the cluster model and the mechanisms in alkaline glass radiation chemistry. The pronounced concentration dependence of product yields is discussed first. NaOH solutions yield glasses only when the OH- concentration exceeds 5 M.’ Below this value, a rapidly frozen solution forms an opaque ice, usually described as polycrystalline. In very dilute ices, y irradiation produces OH in increasing yield up to an OH- concentration of 0.5 M, and no 0- is observed. Above this value, the OH yiield decreases and 0- begins to appear. This behavior is consistent with the production of OH by reactions 2 and 13 and with the idea that 0- will not be formed unless the HzO ionized in reaction 1bridges two OH ions. At low OHconcentrations this condition is less likely to be met, (and hence the O H of reaction 2 will not be consumed. The detailed reactions (4)-(8) show this clearly. The yield of 0- then increases linearly with OH- concentration, finally reaching a plateau a t around 10 M. The e; yield also increases linearly with OH- concentration, reaching a plateau a t 8-10 M. The actual plateau yield, G = 1.9, is very much larger than that in pure water ice, which is about The ice formed by rapid freezing is presumably polycrystalline; however, when irradiated water vapor is condensed on a cold surface to form amorphous ice, the e; yield is significantly larger.2 The amorphous ice apparently has many more physical traps than the more orldered polycrystalline ice, paralleling the situation in NaOH glasses and crystalline hydrates. Thus, we might infer that the water-rich regions in alkalline glasses resemble amorphous ice in their structural aspects. In the cluster model, we would expect clusters to become larger with increasing overall concentration, with some corresponding decrease in the amount of “excess” water as more H 2 0 is incorporated into ion hydration. This picture would acciount for the observed plateaus: in a large cluster, 0- would probably be formed only near the periphery, since the interior of such a cluster would present the same problem as in a crystalline hydrate, and the e,of reaction 4 would have to travel over a distance greater than 20-40 A before becoming trapped. If e,- cannot escape the ionization site, the formation of 0- may be blocked. This last idea is supported by a detailed scavenger study of y-irradiated 10 M NaOH glass at 77 K.19 When NaN03 was added, no tr,sce of e; remained a t a nitrate concentration of 0.1 M, and the 0- yield increased linearly up to 3 M, at which point G(O-) was roughly twice that observed in the absence of nitrate. This increase was interpreted as being due to tlhe inhibition of e- reactions with HzO+, OH, and 0-. In the first two cases, this would block the formation of 0- Zimbrick and Bowmanlg present a mechanism for 0-formation which is the same as our reactions 1-3, except for their inclusion of the reaction HzOf + H 2 0 H30+ OH This represents a case in which the ionized HzO does not bridge two OH- ions, as in (7) and (8),and we have alre,ady commented that this would be an unlikely reaction. We now consider the possibility that the existence of water-rich regions may be directly verified by experiment. Since metal cations are known to be close to 0- ions in alkaline glasses, a complementary demonstration that metal ions are not in close proximity to e; would conclusively support ithe notion that water-rich regions are the
-
+
228
The Journal of Physical Chemistry, Vol. 83, No. 2, 1979
primary trapping sites for this species. Earlier work20 indicated that no ESE modulation from 23Na nuclei is observed from e;, while 0- in the same samples shows such modulation. However, the results of more recent ESE studies21have shown that there is, in fact, a small modulation effect from Na+ on the e< signal, and a larger effect has been seen in CsOH glasses. It has not yet been possible to obtain et--cation distances, and the mechanism for modulation from the metal ions is not understood well. It is possible to infer rough e
