J. Phys. Chem. B 2008, 112, 1935-1939
1935
Be2+ Hydration in Concentrated Aqueous Solutions of BeCl2 Philip E. Mason,† Stuart Ansell,‡ George W. Neilson,§ and John W. Brady*,† Department of Food Science, Stocking Hall, Cornell UniVersity, Ithaca, New York 14853, ISIS Department, Rutherford Appleton Laboratory, Chilton, Didcot, OX11 0QX, U.K., and H.H.Wills Physics Laboratory, UniVersity of Bristol, BS8 1TL, U.K. ReceiVed: October 19, 2007
Neutron diffraction experiments were carried out on concentrated aqueous solutions of beryllium chloride at three concentrations: 1.5, 3, and 6 molal. By working with a specific (“null”) mixture of heavy water (D2O) and water (H2O), information on the local structure around Be2+ ions was extracted directly. For all three BeCl2 solutions, the results show that the Be2+ ion has a well-defined 4-fold coordination shell that is dominated by oxygen atoms. There is also a relatively small probability (10-15%) that there are direct contacts between Be2+ and Cl- at a distance of ∼2.2 Å. The oxygen atoms of the highly structured Be2+ first hydration shell are found to be situated at 2.6 Å apart, and form a pyramidal structure, in agreement with recent MD simulation results. The Cl- ions have approximately seven oxygen atoms (water molecules) in their hydration shells sited at 3.2 Å.
Introduction Be2+
The study of aqueous is of both medical and academic interest;1,2 it represents the archetypal small, highly charged cation, having the highest surface charge density of any divalent ion. From a medical viewpoint, beryllium is highly toxic and when ingested in the body causes severe damage to vital organs. It has technological uses as a material suitable in X-ray diffractometers and in fission nuclear reactors. The Be2+ ion can be expected to produce a high degree of covalency when combining with anions to form compounds.3 The reason for this arises mainly from the ion size of 0.31 Å4 and the high ionization enthalpy. Indeed, in solid BeCl2, the Be2+ ion has a maximum 4-fold coordination achieved by polymeric bridging. In aqueous solution, one might anticipate a similar type of structure around Be2+ with a very strong coordination shell of water molecules in relatively slow exchange with the bulk solution,5,6 having a rate constant of 10-2 s.7 Results from NMR spectroscopy8,9 (1H and 9Be) and potentiometry,10 which have mainly focused on the 1-4 beryllium atom aggregates formed by the addition of basic hydroxides to beryllium salts, suggest the presence of oligoberyllium clusters which are held together by bridging hydroxyl groups, an observation in agreement with recent quantum mechanical calculations.11 Similarly, other theoretical calculations12,13 have confirmed the 4-fold structure in solution. In all of these studies, the role of counterions in the structuring of these solutions is not addressed, although several studies have examined the coordination of the Be2+ ion to polydentate ligands.14 This report presents results of a recently developed neutron diffraction technique applied to aqueous solutions of BeCl2 in a water/heavy water (null) mixture with concentrations of H2O and D2O adjusted such that no correlations between the * Author to whom correspondence should be addressed. E-mail: jwb7@ cornell.edu. † Cornell University. ‡ Rutherford Appleton Laboratory. § University of Bristol.
hydrogen atoms and other atoms/ions appear in the scattering patterns.15 Although there have been previous X-ray diffraction structural studies of beryllium salt hydrates,16 the authors are unaware of any such studies in the liquid phase, a major problem being that, with only four electrons, correlations between Be2+ and other atoms (ions) in the solution are only weakly present in the X-ray scattering patterns even for highly concentrated solutions. Consequently, a significant degree of modeling would be required to analyze the data. By contrast, the results presented below provide information directly concerning the Be2+ local structure in concentrated aqueous solutions of BeCl2 at three concentrations, 1.5, 3, and 6 molal. Additionally, information on the hydration of the Cl- counterion is also obtained. Experimental Section Sample Preparation. Solutions of beryllium are acidic, due to the ionization of the water by the very polarizing Be2+ ion.11 Due to acute and permanent chronic poisoning risks of handling beryllium compounds, great care must be taken when dealing with this element. It is essential that handling of solid beryllium compounds be conducted in a fume cupboard to minimize the risk of exposure to airborne dust of the compound. The dissolution of BeCl2 is extremely exothermic (easily releasing enough energy to boil the water in the preparation of the three solutions used in this study); consequently, the dissolution of BeCl2 in water must be conducted with care and patience. Once in solution, the severe airborne poisoning risk is greatly reduced, and the solutions can be handled as a standard highly toxic solution. Here, BeCl2 solutions were prepared by the careful dissolution of BeCl2 (Aldrich 99% chloride, purified by sublimation) into null water (i.e., a water/heavy water mixture for which the average coherent neutron scattering length of hydrogen is effectively zero). Each solution was composed of BeCl2 in water (49.609 g of D2O (99.9% D) and 79.708 g of H2O) to yield solutions of mole ratios of 1.50, 3.00, or 6.00 mol of BeCl2 in 55.555 mol of water, hereafter referred to as 1.5, 3, and 6 molal for convenience. Because of the extremely exothermic dissolu-
10.1021/jp710180v CCC: $40.75 © 2008 American Chemical Society Published on Web 01/25/2008
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Figure 1. Normalized intermediate scattering functions I(Q) for (a) 1.5 molal BeCl2 in null water, (b) 3 molal BeCl2 in null water, (c) 6 molal BeCl2 in null water, and (d) null water.
tion of BeCl2 in water (presumably mostly due to the hydration of the Be2+ ions), the solutions were allowed to thermally equilibrate for over 2 h before experimental measurements were made. Data Acquisition and Reduction. The difference methods of neutron diffraction with isotopic substitution (NDIS) are well established in the literature.17 For the present case, the procedures described in a recent paper, which established the method as applied to solutions of null water,14 were followed. Neutron scattering patterns were obtained for the null water mixture and the three BeCl2 solutions on the D20 diffractometer at the Institut Laue Langevin (ILL), Grenoble. Each data set was corrected for multiple scattering and absorption and subsequently normalized to a standard vanadium rod to give intermediary structure functions I(Q) (Figure 1). The I(Q)’s were then corrected for inelastic and incoherent scattering using the methods outlined in ref 14 to give structure factors (F(Q)) in units of barns str-1. The F(Q)’s for a solution of BeCl2 in null water (H2O) can be written as a linear sum of partial structure factors Sij(Q)
FBeCl2(Q) ) ASOO(Q) + BSClO(Q) + CSBeO(Q) + DSBeBe(Q) + ESBeCl(Q) + HSClCl(Q) (1) Fourier transformation of this equation gives the radial distribution function G(r)
G(r) ) AgOO(r) + BgClO(r) + CgBeO(r) + DgBeBe(r) + EgBeCl(r) + HgClCl(r) + J (2) where J ) -(A + B + C + D + E + H). The prefactors A-H are given by A ) cO2bO2, B ) 2cClcObClbO, C ) 2cBecObBebO, D ) cBe2bBe2, E ) 2cBecClbBebCl, and H ) cCl2bCl2, where cR is the concentration of species R, whose coherent neutron scattering length is bR. Subtraction of an equivalent function obtained for a sample of pure null water gives the function
∆F′BeCl2(Q) ) A′SO′O′(Q) + BSClO(Q) + CSBeO(Q) + DSBeBe(Q) + ESBeCl(Q) + HSClCl(Q) + J′ (3) where J′ ) -(A′ + B + C + D + E + H), and where the
TABLE 1: Prefactors for the Functions ∆F′BeCl2(Q) and ∆GBeCl2(r) in eqs 1 and 2; Units Are in millibarns str-1 concentrations (molal) prefactor (correlation)
1.5
3.0
6.0
A (OO + O′O′) A′ (O′O′) B (ClO) C (BeO) D (BeBe) E (BeCl) H (ClCl) total ) G
35.44 0.40 6.33 2.57 0.05 0.23 0.28 9.38
33.64 1.59 12.01 4.88 0.18 0.87 1.07 19.02
30.50 5.68 21.74 8.84 0.64 3.15 3.88 38.22
possibility of the presence of a strong correlation between the hydrated water molecules associated with the Be2+ ions is allowed for. The coefficient A′ is given by A′ ) cO′2bO2 and cO′ 2+ ion. It ) njO′ jO′ BecBe, where n Be is the hydration number of the Be should be noted that the value for A′ is undefined and can only be calculated after njO′ Be has been determined (see below). Furthermore, it is assumed that any correlations between hydrated oxygen atoms and other nonhydrated oxygen atoms are identical to those between nonhydrated water molecules. The values of these prefactors are given in Table 1. The Fourier transform of ∆F′(Q) yields the real space representation of the data defined as ∆GBeCl2(r), which can be written as
∆GBeCl2(r) ) A′gO′O′(r) + BgClO(r) + CgBeO(r) + DgBeBe(r) + EgBeCl(r) + HgClCl(r) + J′ (4) Note that in a previous paper the first term in eq 3 was ignored and it was pointed out that the correlations between hydrated oxygen atoms would be obscured by the Cl- ions in the case of nickel chloride. By contrast, and as is shown below, due to the fact that Be2+ ion is one of the smallest ions in nature, the oxygen correlations of its hydration shell are readily observed in ∆GBeCl2(r). However, in order to not unnecessarily complicate the analysis, the correlations between O′ and the O atoms of nonhydrated water molecules are assumed to be the same as those for O-O atom pairs. Determination of the coordination number njβR of species β around species R follows directly from integration of the
Be2+ Hydration in Aqueous Solutions of BeCl2
J. Phys. Chem. B, Vol. 112, No. 7, 2008 1937
Figure 2. The three ∆F′BeCl2(Q)’s in units of barns str-1 (eq 3) for the BeCl2 solutions: (a) 6 molal, displaced upward by 0.1 units; (b) 3 molal, displaced upward by 0.05 units; (c) 1.5 molal. Note that the lines through the three data sets are the edited back transform of the results for ∆GBeCl2(r) in Figure 3.
particular correlation identified in gRβ(r) over its range r1 < r (Å) < r2, according to the equation njβR ) Fcβ∫rr12 dr 4πr2gRβ(r), where F is the total number density of the solution and has a value of order 0.1 Å-3. Results and Discussion The results for the ∆F′BeCl2(Q)’s are shown in Figure 2 and provide information on the statistical accuracy of the data; on the basis of previous experiments with this technique, it is clear that structural results can be obtained from these data. As with all neutron diffraction experiments which exploit the use of isotopic substitution, the results for the ∆G(r)’s for the three solutions are straightforward to analyze; once correlations have been clearly identified as being caused by specific atom-atom interactions, coordination numbers and interatomic distances can be obtained directly. In the present case, the only drawback is the absence of information on correlations between the hydrogen atoms of the water molecules and other atoms and ions in the solution. The first point to note is that the ∆G(r) for each concentration (Figure 3) has essentially the same shape, which scales with ion concentration. Inspection of ∆G(r) shows the presence of three relatively strong peaks centered at 1.6, 2.6, and 3.2 Å, and further structure out to around 6 Å. The peak at 1.6 Å is consistent with the Be-O nearest neighbor correlation. Integration of this peak over the range 1.2-2 Å gives a coordination number of 3.8(0.2), which is in excellent agreement with that inferred from NMR studies of solutions,8,9 and reflects the 4-fold coordination found in the solid state. More substantially, the sharpness of the correlation, with a half-width of 0.4 Å, which is comparable to that of the strongly hydrating Ni2+ cation,18 confirms the high degree of covalency of Be2+ even in solution and, while there is some resolution broadening due to the limited range in Q of the experimental data, confirms the idea that the hydrated Be2+ ion can, like Ni2+, be considered as a pseudo-
molecular entity for the purposes of simulation methods.19 This result is also consistent with those from computer simulation studies.11-13 The peak centered at 2.6 Å, with a shoulder at 2.2 Å, is less straightforward to interpret, as it almost certainly contains more than one correlation. The results from recent MD simulations of hydrated Be2+ ions can be used to understand the origins of the peak itself.11,12 On the basis of the fact that the four hydrated oxygen atoms of Be2+ are situated at 1.6 Å, it must also be the case that a strong correlation between these oxygen atoms must exist at a distance distinct from that found in bulk water. Indeed, a relatively simple calculation based on the ab initio MD results of Asthagiri and Pratt,11 which give a beryllium-oxygen separation rBeO of 1.6 Å and an O′BeO′ bond angle of 109°, shows that the distance between the O′ atoms of the Be2+ hydration shell is 2.6 Å. Using this analysis as a guide, one can then analyze further the peak centered at 2.6 Å. Integration of the peak gives the expected value of 3 for the O′-O′ correlation of the hydrated oxygen atoms. Additionally, the peak can accommodate a small amount of Cl- density (at the 10-15% level), most probably sited at around 2.2 Å, where a shoulder appears in ∆G(r). Integration of a Gaussian peak centered at this position gives a coordination number of 0.5. The peak at 3.2 Å is clearly due to correlations between Clions and the oxygen atoms of the water molecules. The result is consistent with other studies of Cl- in solution, and integration over the correlation gives a hydration number for Cl- of ∼7(1). The reason for the larger error in this case is that the extraction of this function assumes that there is no contamination from remnant correlations between oxygen atoms themselves, where it is assumed that the water coordination is unaffected by the addition of salt or from the expected “second hydration shell” of beryllium (at ∼3.8 Å).12,20-22 This is probably the case in LiCl solutions below 3 molal23 but may not necessarily be the case here with the highly polarizing Be2+ ion. Nevertheless,
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Figure 3. The three ∆GBeCl2(r) in units of barns str-1 (eq 4) for the BeCl2 solutions: (a) 6 molal, displaced upward by 0.1 units; (b) 3 molal, displaced upward by 0.05 units; (c) 1.5 molal. The dashed lines were used to edit out all of the nonphysical data below the first peak centered at 1.6 Å. These lines together with their continuations were then Fourier transformed to give the lines through the experimental data shown in Figure 2.
the results for the Cl- hydration are in good agreement with that found experimentally in a variety of other solutions.24 The above result for Be2+ hydration fits in nicely with other results for the alkaline earth group of ions, many of which have already been investigated by the methods of neutron and X-ray diffraction.25-29 There is a clear trend toward a significant increase in coordination number as one progresses from Be2+ (4) to Mg2+ (6), which is isomorphic with Ni2+, to Ca2+ (6-9, depending on concentration), to Sr2+ (8). There is also evidence for a general weakening of the coordination as defined in terms of the correlations between the ion itself and the nearest neighbor water molecules. It is of further interest to ask whether the results can help in understanding the relative solubilities of beryllium salts in water, where one finds that BeF2 is infinitely soluble, while BeCl2, BeBr2, BeI2, and Be(NO3)2 are progressively less so. In light of the above results, it appears that the reason for this sequence, at least in part, rests with the unique hydration structure of the Be2+ ion. It first should be noted that the form of the overall shape of ∆G(r) scales with concentration. Second, there is a well-defined hydration shell for Be2+ with a relatively high probability (10-15%) that contacts can arise between Be2+ and the Cl- counterion.30,31 (It will be shown in a subsequent, more general publication that this observation is also consistent with MD simulations.) Finally, it should be noted that the Be-Cl potential is sufficiently strong to counter both the buildup of subsequent hydration shells and any entropic loss due to localization of the Cl- ions; only at very high dilution might one expect to find free Be2+ ions in solution. The Be2+-O correlation exhibits a sharpness similar to that for other strongly coordinating cations such as Li+, Ni2+, Cr3+, etc., where the isolated metal ion can localize significantly more water at a greater cost entropically. Given that the F- anion is significantly smaller than Cl-, it seems highly likely that the Be2+-F- potential in aqueous solutions of beryllium fluoride will be even stronger and the solubility consequently much
greater, as is indeed the case. This situation contrasts with that for aquated beryllium salts containing larger anions such as Brand I-, the solubilities of which are appreciably smaller. Conclusions The neutron scattering results presented above provide the first detailed structural information about the Be2+ ion in concentrated aqueous solutions and show that, as might be expected, Be2+ has a coordination number of 4. The overall hydration structure is tetrahedral. The results present a challenge for computer simulations designed to determine the extent to which the quantum chemistry of a strongly polarizing ion such as Be2+ ion is crucial in defining its structural properties in solution. Additionally, the evidence for the association between Be2+ and Cl- raises interesting questions regarding the degree to which ion pairing occurs for other beryllium salts such as BeSO4 in concentrated solutions; such a study is currently being undertaken and will include the use of molecular dynamic simulations to help resolve any observed differences in structure between solutions of BeCl2 and BeSO4. Acknowledgment. The authors thank Gabriel Cuello and the D20 staff of the Institut Laue Langevin for their help with the neutron scattering experiments. This project was supported by grant GM63018 from the National Institutes of Health. References and Notes (1) Alderighi, L.; Gans, P.; Midollini, S.; Vacca, A. AdV. Inorg. Chem. 2000, 50, 109-172. (2) Fontenot, A. P.; Newman, L. S.; Kotzin, B. L. Clin. Immunol. 2001, 100, 4-14. (3) Cotton, F. A.; Wilkinson, G. Inorganic Chemistry; John Wiley & Sons Inc.: New York, 1980; pp 274-278. (4) Yamaguchi, T.; Ohtaki, H.; Spohr, E.; Palinkas, G.; Heinzinger, K.; Probst, M. M. Z. Naturforsch. 1986, 41A, 1175-1185.
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