J. Chem. SOC., Faraday Trans. I, 1989, 85(6), 1365-1372 The Structure of Concentrated Aqueous Ammonium Nitrate Solutions P. A. M. Walker, D. G. Lawrence and George W. Neilson* H. H. Wills Physics Laboratory, University of Bristol, Bristol BS8 ITL J. Cooper I.C.I. Ltd, D.R.G., Ardeer Site, Stevenston, Ayrshire KA20 3LN Neutron diffraction studies have been carried out on heavy water solutions of ammonium nitrate at two salt concentrations, 18 and 12 mol/kg-'. The first-order difference method of isotopic substitution was applied to both ND: and NO;, thus enabling the determination of detailed structural information regarding the hydration of these ions. Both ions show relatively weak interactions with water. At the highest concentration, ca. 18 mol kg-', there is evidence which demonstrates a direct interaction between the ND,' and NO; ions.Ammonium nitrate, AN, solutions are of considerable technical and scientific interest. They are essential components in fertilisers, and non-military explosives where they are fabricated in the form of emulsions or as ternary mixtures of AN/alkaline-earth-metal nitrate/aqueous solution. The scientific interest in these systems stems from the complex phase diagram of AN, which has at least five phases at atmospheric pressure.'*2 Furthermore, mixtures of AN and certain metal nitrate salts at certain concentrations can form glasses in a reversible way.3 Despite the extensive studies of AN solutions very little is known of their micro- structural properties. One reason for this is that X-ray diffraction methods applied to aqueous solutions of AN are unable to resolve the local ionic structure because both NH,+ and NO; are only weakly observed compared with the solvent water molecules.In fact, several authors conclude that, based on X-ray diffraction results alone, NH,+ is indistinguishable from a water m01ecule~~~ and it is a matter of debate whether the nitrate ion can be considered as being hydrated at this condition being almost totally dependent on the type of cation pre~ent.~ One objective of the study described below is to establish the validity of such claims. The main aim of our work was to determine the ionic structure of AN solutions as a function of concentration. This was done by application of the first-order isotopic difference method of neutron diffraction* to nitrogen atoms (15N and ") in both the ammonium ions and the nitrate ions.The results of this study enabled us to focus on the local hydration of these two types of ions and draw inferences regarding their relative hydrating strengths. Experimental and Data Analysis Neutron diffraction experiments were carried out on a series of AN-heavy-water solutions (table 1). Each sample was prepared under glove-box conditions by standard procedures. The ratio of heavy water to water was determined from infrared spectrometry and the isotopic content of samples was measured using a mass spectrometer. 13651366 Structure of Concentrated Aqueous ND,NO, Table 1. molality molecular ratio, numberodensity 15N abundance sample label /mol kg-' R (D,O:AN) (Yo) solution 1 (10 mol kg-') "ND,"O, I 17.88 2.8 0.10 - 15ND,NN0,11 17.97 2.8 0.10 95.1 "D,15N0, I11 17.99 2.8 0.10 99.0 "ND,"NO, IV 11.99 4.2 0.1 99.8 'SND,"NO, V 11.99 4.2 0.1 99.8 solution 2 (12 rnol kg-') '5ND,'5N0,VI 11.99 4.2 0.1 - 0 .0 8 ) 1 - '2 0.04. !i e Y n 0.00. Q -0.04 I 1 I 0 4 8 12 16 k /A- Fig. 1. First-order difference function A,,(k) for ND; ion in 18 mol kg-' ND,NO, in heavy water. Crosses represent raw data; full curve back Fourier transform of G,,(r) shown in fig. 3. 0.08 - IL. 0.04 v1 EJ d" e Y h 0.00 -0.04 I I 1 4 8 12 16 klA-' Fig. 2. First-order difference function AN2(k) for NO; ion in 18 mol kg-l ND,NO, in heavy water. Crosses represent raw data; full curve back Fourier transform of GN,(r) shown in fig. 4. The diffraction data were gathered on the D4B diffractometer at the ILL.All experiments were carried out in vacuo under ambient conditions. The data were corrected for absorption, multiple and incoherent scattering effects, and put on an absolute scale of barns sr-l nucleus-' by reference to a vanadium standard.lO The structure factors, F(k), calculated in this way were used to calculate first-order difference functions A,(k) (fig. 1 and 2), and their Fourier transformations G,(r) [fig. 3 and 5 (later)] from which structural information concerning the hydration of both NDZ and ND; is inferred.P. A . M. Walker, D. Lawrence, G. W. Neilson and J . Cooper I367 I I I I 1 0 1 2 3 4 5 6 r / A Fig. 3. The ammonium ion total radial distribution function GN,(r), for 18 mol kg-' ammonium nitrate in heavy water.0.10' If we use the following notation for ammonium nitrate in heavy water: N,,,D,N,,,O, - D,O then : F(k) = C i bi[Soo(k) - 11 + CD 6i[S,D(k) - I] + 2 ~ 0 C , 60 6D[SoD(k)- 13 + 2cNl co 60 6Nl[SoN,(k) - 11 + 2cKl CD 6, 6 N l [ S D N l ( k ) - 11 + 2c0 'N, '0 6N,[SON2(k) - '1 + 2cLl 'X2 'D 6N2[SDNp(k) - '1 + +'if 6i2[SN2Ny(k) - '1. 6kl[SN1 Nl(k)- 'I2 + 2cN1 'N, 'XI 6N2[SN1 N2(k> - '1 c, is the atomic concentration of atom ' a ' whose neutron-scattering length is 6,, and Sclp is the partial structure factors of the atom pair ap*. The Fourier transformation of SZp(k)- 1 is [g,&r) - 11 = [Sclp(k) - 13 k sin kr dk 2n ' s pr where p is the total number density of the solution. A first-order difference ANi(k) can be formed from the F(k)s of two isotopically distinct but chemically identical solutions which have had the nitrogen atoms of their ammonium ions (or nitrate ions) changed from NN to I5N.In particular A,i(k) = W ) - m k ) = AISNio(k>- Il+B[S,p(k)- lI+C[SNiNj(& ll+DISNiNi(k>- 11 where B = 2cD C N t bD A6N2, D = cKt(6iz - b;z) and A6Nz = (6Nz - bX). It is straightforward to show that the Fourier transformation of ANi(k) can be written as: where E = -(A+B+C+D). GNz(r) = AgNtO(r) + BgiYl D('> + 'gN, N,('> + DgNt N,(') +1368 Structure of Concentrated Aqueous ND,NO, 0.01 - 0.00 L E e z -0.01 h v 'U -0.02 Fig. 4. As fig. 3, but the ordinate scale expanded. Full curve, GN,(r) for ND: in 18 mol kg-l ammonium nitrate heavy water solution; dashed curve, G,,(r) for ND,f in 12 mol kg-' ammonium nitrate heavy-water solution.Fig. 5. The nitrate ion total radial distribution function GN,(r), for 18 mol kg-l ammonium nitrate in heavy water. The truncation of the Fourier sum in k-space produces spurious ripples in the trans- form at short distances. Before the first intra-molecular peak, we have, as is usual, set G,(r) = E, as r + 0. Clearly it would be wrong to do so after this point, though it can be seen from the fact that the values appear to drop below the limit, E, that the low r oscillation still produces a small distortion. We have chosen to present the unadjusted data (fig. 3-6). However, techniques of back transformation and truncation were used to identify real features from those produced artificially. The uncertainties involved were taken into account when the errors in table 3 were calculated. Structural analysis follows from the calculated GN(r)s where peak positions can be determined directly and coordination numbers from integration over ranges in r .For example the number of atoms of type x in the range rl < r ,< r2 around N, say is given by: nGl = pc, 1: 4nr2g",,(r) dr.P . A . M. Walker, D. Lawrence, G. W. Neilson and J . Cooper 0.10 0.00 - L $ e h -0.01 t u " -0.02 2 3 5 1369 Fig. 6. As fig. 5, but the ordinate scale expanded. Full curve, GN2(r) for NO; in 18 rnol kg-' ammonium nitrate heavy water solution; dashed curve, GN2(r) for NO; in 12 rnol kg-' ammonium nitrate heavy-water solution. Table 2. Coefficients of G(r) in barns/lO-, sample differences A B C D E solution 1 (18 mol kg-' AN/D,O) "ND4NN0,-'5ND,NN0, 6.148 1 1.61 8 1.70 1 1.459 - 20.926 NND4NN0,-"ND4'5N0, 6.395 12.102 1.768 1.524 - 21.789 solution 2 (12 rnol kg-' AN/D,O) "ND4NN0,-'5ND4NN0, 5.15 10.20 1.15 0.18 - 16.68 '5ND,NN0,-'5ND,'5ND, 5.15 10.20 0.80 0.18 - Table 3.Intramolecular parameters of ND,' and NO; ND; NO; concentration/ rnol kg-' rN,/A Ar1I2/A n rNo/A AF/A n 18 1.04 (2) 0.31 (3) 4.04 (10) 1.23 (2) 0.28 (4) 3.05 (10) 12 1.02 (2) 0.29 (5) 3.9 (1) 1.23 (2) 0.28 (4) 3.0 (1) 7.8 (NaNO,) 1.23 (2) 0.36 (5) 3.0 (3) - 5 (ND4C1) 1.05 (2) 0.29 (3) 4.0 (1) - - - - - Table 2 shows that the G(r)s are dominated by N-0 and N-D correlations. Consequently, the interactions between the ions themselves can only be inferred from the first-order difference functions. However, as we shall see below it is possible to obtain information about the possibility of direct ND; NO, contacts by comparing results as a function of concentration.1370 Structure of Concentrated Aqueous ND,NO, Results and Discussion The first-order difference functions A,(k) and the total radial distribution functions GNi(r) for ND,+ and NO, are shown in fig.1-6. The results are in broad agreement with earlier studies of both these ions in other aqueous solutions, where we found the G(r)s to be relatively featureless except for the presence of the strong intra-molecular peak. ND,f Coordination (fig. 3, 4) In boLh solutions the ND,+ structure exhibits the same strong intra-molecular peak at ca. 1.04 A (table 3) in good agreement with that found in our previous studies1'* l2 and in a diffraction study of the p 0 ~ d e r .l ~ Integration of this peak gives values of about 4 and gives credence to our data normalisation procedures. The rest of the G(r) is, as expected, relatively featureless. However, at the highest concentration 18 mol kg-' where there are effectively less Jhan three D,O molecules to each ND,NO, molecule, a small peak centred at ca. 2.15 A is observed (fig. 3 and 4). If this peak is assumed to contain 0 atoms a coordination number of 1 is obtained. There are two possible explanations for this observation. It could arise from an oxygen a$om of a near-neighbour water molecule whose D atoms are sited under the peak at 3 A, or it could arise from an oxygen atom belonging to an NO; anion. Because it is not present at the lower concentration we feel that the latter explanation is more probable and our result provides the first experimental evidence of ion pairing in aqueous ammonium nitrate solutions.One possible configuration for this interaction is sbown in fig. 7. As we will show below, the nitrate-deuterium distance is estimated to be 3 A, a result consistent with the G,(r)$or the nitrate ion shown in fig. 6, where the closest distance of approach would be 2.3 A. The longer-range structure of both solutions is broadly similar, and tbeir analysis can be carried out by integrating the two broad peaks centred at 3.0 and 3.4 A. Although our interpretation is no+ unique y e suggest the configuration for the 18 mol kg-l solution is one in which the peak at 3.0 A contains one N and about 6( 0.1) 0 atoms which belong to nearest-neighboyr water molecules.The peak centred at 3.4 A when integrated over the range 3.2-4.1 A can readily accommodate the two remaining 0 atoms of the NO, ion, the 12 D atoms of the nearest-neighbour hydration shell and the 5 or 4.5 other oxygen or deuterium atoms, respectively. It is not possible to be more precise because of the complexity of the solution. Interpretation of GN(r) for the 12 mol kg-l AN solution is also subject to the same uncerJainties as those of the 18 rnol kg-l solution. The results do not exhibit a peak at 2.15 A. We note that in the 12 mol kg-: solution no hydration structure is observed within a distance of approach of ca. 2.4 A. The structure beyond this distance is moved only slightly closer at the hjgher concentration.However, there is a relative increase in the size of the peak at 3.2 A and this increase we ascribe to an oxygen atom belonging to an NO, ion. The rest of the peak can accommodate 6 or 7 other oxygen atqms forming a near-neighbour weakly defined hydration shell. The peak centred at 3.4 A is similar to that for the 18 mol kg-l case and there is sufficient area under it to contain 14 D atoms, and one N atom, two 0 atoms and a remaining four other atoms. The general features of the NDI coordination are similar to those found in our earlier study of 5 mol kg-l ammonium chloride. However, there is one significant diffcrence in that, whereas the ND,+ in ammonium chloride shows no resolved peak at 2.9 A, one is clearly evident in the AN study described here.This result agrees well with a recent molecular dynamics study by Walker and Allen.'* Furthermore, these results are in better agreement with an earlier molecular dynamics study by Heinzinger and Szasz15 who found at infinite dilution a relatively strong correlation between NDZ and 10 water molecules. The present study at higher concentrations gives coordination numbersP . A . M. Walker, D. Lawrence, G. W. Neilson and J . Cooper 1371 D 1.04A D D 2.15 A - - - - - 0 1.23A 0 1.23A A L 0 Fig. 7. A two-dimensional projection of a possible ND,+ ... NO; configuration consistent with experimental results (fig. 4 and 6). Clearly, from the distances between the atoms, as shown, the ND; and NO, ions cannot be coplanar. appreciably less than this, and it is clearly of interest to determine the extent of concentration and counter-ion effects on ND,+ hydration.NO; Coordination (fig. 5, 6) In contrast to the ND: coordination, there is no obvious dissimilarity between the G(r)s for the two solutions. In both cases there appears to be no short-range structure. However, there is 9 small but appreciable difference in the shape of the G(r)s in the region around 2.8 A where a shoulder appears in the results at high coacentration and a peak is visible at lower concentration. If the region 2.14 < r < 3.0 A is assigned to N-D nearest-neighbour correlations, integration over this range gives coordination numbers of 3.3 at 12 mol kg-l and ca. 5 at 18 mol kg-l. This latter result is consistent with the above discussion concerning direct cation-anion contact.It is also possible that the remaining three deuterium atoms belong to water molecules weakly coordinated to the NO; ion in a configuration suggested originally by Caminiti et al. (ref. 16, fig. 2a) and subsequently identified by us17 in an earlier stgdy of NO; in 7.8 mol kg-' NaNO,. In contrast to our earlier paper,17 a peak at 2.05 A, which had been assigned to the D above the NO; plane, is not observed in this study. It will be interesting to see whether this observation is a result of the stronger Na+ cation influencing the NO, coordination. Experiments are in hand to investigate the effect of strong cations such as Ni2+ on the NO; hydration by studying AN/Ni(NO,), heavy water solutions using the difference methods described in this paper." The recent computer simulation by Walker and Allen14 of NO; hydration shows fair agreement with the above results.However, more realistic potentials are being developed in order to obtain a better fit to the experimental results. Conclusions and Future Prospects The first-order difference method has been applied to aqueous solutions of ammonium nitrate. The results show that at the highest concentration there is strong evidence for direct cation-anion contacts. Furthermore, as expected, the general form of the cation and anion coordination is relatively weak compared with ions such as Li'or Na+ on the one hand and Cl- on the other. There is a clear need for a more definitive picture before we can resolve the detailed ionic structure.One possible means is by elimination of the hydrogen correlations as could be undertaken using a 'null' water mixture,lg i.e. a mixed solution of water and heavy water with concentrations chosen such that correlations between nitrogen and hydrogen atoms are not present in either ANl(k) and ANz(k) or in their Fourier trans- forms, GN,(r) and GNP(r).1372 study AN solutions at even higher concentrations. Structure of Concentrated Aqueous ND,NO, The use of temperature is also expected to help in our analysis, and will enable us to The authors thank Professor J. E. Enderby for his helpful suggestions during the course of these investigations. We are also grateful to Mr P. Gullidge for assistance with the sample preparations, and Dr P. Chieux for his help with the diffraction experiments.The financial support of S.E.R.C. and I.C.I. is greatly appreciated. References 1 P. W. Bridgman, Proc. Am. Acad. Arts Sci., 1915, 51, 5999. 2 S. B. Hendricks, E. Posnjak and F. C. Kracek, J. Am. Chem. SOC., 1932, 54, 2766. 3 I. V. Vasilokova, M. P. Snarev and T. A. Znchenco, Servia Fiziki Khimiya (University of Leningrad), 4 A. H. Narten, J. Phys. Chem., 1970, 74, 765. 5 P. M. Vollmar, J. Chem. Phys., 1963, 39, 2236. 6 R. Caminiti, G. Licheri, G. Piccaluga and G. Pinna, J. Chem. Phys., 1977, 19, 371. 7 R. Caminiti, G. Licheri, G. Piccaluga and G. Pinna, J. Chem. Phys., 1978, 68, 1967. 8 A. K. Soper, G. W. Neilson, J. E. Enderby and R. A. Howe, J. Phys. Chem., 1977, 10, 1793. 9 J. E. Enderby and P. Gullidge, in Methods of Experimental Physics, ed. K. Skold and D. L. Price 10 J. E. Enderby and G. W. Neilson, Water, a Comprehensive Treatise, ed. F. Franks (Plenum Press, New I I N. A. Hewish and G. W. Neilson, Chem. Phys. Lett., 198 1, 84, 425. 12 S. Cummings, J. de Physique Colloq., 1984, C7, 131. 13 B. W. Lucas, M. Ahtec and A. W. Hewat, Acta Cryst., 1979, B35, 1038; Handbook of Chemical Physics 14 P. A. M. Walker and M. P. Allen, Molec. Simulations, 1989, 2, 307. 15 Gy. I. Szasz and K. Heinzinger, Nuturforsch, 1979, A340, 840. 16 R. Caminiti, G. Licheri, G. Paschina, G. Piccaluga and G. Pinna, J. Chem. Phys., 1980, 72, 4522. 17 G. W. Neilson and J. E. Enderby, J. Phys. C: Solid State Phys., 1982, 15, 2347. 18 D. G. Lawrence, unpublished report, University of Bristol, 1986. 19 D. H. Powell, G. W. Neilson and J. E. Enderby, J. Phys. Condensed Matter, submitted. 1971, 1, 74. (Academic Press, 1987) vol. 23B, p. 471. York, 1979) vol. 6, chap. 1. Edu. 1985 (C.R.C. Press Inc., Florida, 1985). Paper 81022081; Received 2nd June, 1988