J. Phys. Chem. 1994,98, 13195-13197
13195
Neutron Diffraction Study of the Effect of a Polyelectrolyte on the Hydration of Nickel Ions? R. H. Tromp* and G. W. Neilson H. H. Willis Physics Laboratory, University of Bristol, Tyndall Avenue, Bristol BS8 ITL, U.K. M. C. Bellissent-Funel Laboratoire IRon Brillouin, CEN-Saclay, 91191 Gif sur Yvette, France Received: May 9, 1994; In Final Form: September 20, 1994@
The method of neutron diffraction and isotopic substitution was used to study the hydration of nickel (Ni2+) ions in a 2 m nickel poly(styrenesu1fonate) solution. The result shows that the polyion has a significant effect on the immediate Ni2+ hydration shell of six water molecules.
Introduction In aqueous solutions of polyelectrolytes (PES), the presence of large charged entities has a strong effect on the distribution and dynamics of the small counterions. This effect has been investigated by several experimental techniques. For example, when one compares PES and low molecular weight salt solutions of similar concentrations, self-diffusion measurements indicate slower diffusion of small ions in PE solution,’ osmotic pressure measurements show reduced activity of these ions,2 and the Nh4R line widths of counterions are often dramatically larger in PE s ~ l u t i o n On . ~ ~the ~ natural assumption that the small ionPE interaction is purely electrostatic in origin, the behavior of positive, monovalent ions in the presence of chainlike polyanions has been described with some success within the framework of the Poisson-Boltzmann-Smoluchowski (PBS) This method treats the polyion as a infinitely long, homogeneously charged cylinder and the solvent as a continuum. The small ions are represented by a continuous, smeared out charge distribution; Le., correlations due to the finite size of the small ions are neglected. For divalent ions, the PBS approach works much less well than for monovalent ions,8 presumably because divalent ions have higher interaction energies with each other and because they are more strongly hydrated, which casts doubts on the assumption of a continuous solvent. The experimental results reported here are part of a continuing effort to obtain a detailed picture of the way small ions interact with polyions and to identify the precise origin of the shortcomings of the PBS theory. The method of neutron diffraction and isotopic substitution was used to determine the local structure around the counterion a as defined in terms of the total radial distribution function Ga(r)(eq 1 below). Analysis of Ga(r)gives information on the content and range of the first and second coordination shells. Comparison between the Ga(r)’s of PES and low molecular weight salt solutions of similar molalities shows the closeness of approach of PES and the ion and the degree to which the PE affects the ionic hydration. To date, experiments have been carried out on lithium ions (Li+) in poly(acrylate) (PA) solutiong and on chloride ions (Cl-) in a solution of the polybase poly(ethy1enimine) (PEI).’O The hydration of the lithium ions turns out to be very weakly distorted by the
* Present address: Cavendish Laboratory, University of Cambridge, Madingley Road, Cambridge CB3 O H E , U.K. The neutron diffraction experiment was carried out in the Laboratoire Lion Brillouin (Laboratoire c o n ” CEA-CNRS) at Saclay, France. Abstract published in Advance ACS Abstrucrs, November 1, 1994. @
0022-365419412098-13195$04.50/0
PA. However, in the PEI solution, the environment of chloride ions at distances beyond the inner part of the first hydration shell was found to be strongly influenced by the polycation. To study the interaction between a PE and a divalent ion, a solution of nickel poly(styrenesu1fonate) (PSS, repeating unit CH2CH[C&S03-]) was chosen for the following reasons: (i) nickel isotopes have the most favorable neutron diffraction properties of all suitable divalent ions, (ii) PSS is a very widely studied compound and readily soluble with nickel ions as counterions, and (iii) nickel ions in aqueous solution are structurally isomorphous with magnesium ions.”
Experimental Section Poly(styrenesu1fonate) (PSS) of 70 000 g/mol was purchased from Aldrich in the sodium form. NaPSS dissolved in water was converted into the H+form in a ion-exchange column, the concentration of which was obtained by titration. In this way, also, a rough estimate of the degree of sulfonation was obtained and found to be not significantly different from 100%. Isotopic samples of nickel carbonate (62NiC03 and 5*NiC03) were prepared by dissolving nickel powder in concentrated nitric acid, which was boiled to dryness and heated until only nickel oxide was left. The nickel oxide was dissolved in moderately concentrated hydrochloric acid, which was dried to solid NiC12 until no weight change occurred after further heating. After adding a small excess of sodium carbonate, the nickel carbonate precipitate was filtered off and extensively flushed with water to remove Na+ and C1-. The carbonate content was obtained by titration, and a measure of the purity was obtained by heating a known amount of the nickel carbonate until the weight remained constant. The weight of the nickel oxide which was obtained was in accordance with the initial amount of nickel carbonate. Nickel carbonate was added to an HPSS solution by an amount which neutralizes 80% of the charges of PSS. Consequently, 20% of the polyelectrolyte charges in the experimental sample were neutralized by D+ (initially H+). The PSS was not fully neutralized by Ni2+ to improve solubility. D20 solutions of NiPSS were obtained by cyclic freeze drying and dissolving in D20. Finally, NiPSS solutions were made up of 1.8 m Ni2+,of which the density was found to be 1.30(2) glmL and, consequently, a atomic number density was found to be 0.07(1) atom k3. The neutron diffraction experiments were carried out on the spectrometer 7C2 situated at the hot source of the Orphie reactor at the Laboratoire Lion Brillouin, Saclay, France. The neutron 0 1994 American Chemical Society
13196 J. Phys. Chem., Vol. 98, No. 50, 1994
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Figure 2. G&), Fourier transform of AN,(Q), for a 2 m D20 solution of NiCl2 (dashed) and a 1.8 m D20 solution of NPSS (full line). To
make the first coordination sphere correlation peaks directly comparable, the data sets were multiplied by the atomic number density and divided by the atomic fraction of nickel and the difference in the scattering length of the nickel isotopes, and the sums of the scattering factors (eq 1) were added.
where
e is the atomic number density.
Results and Discussion Figure 1 shows the AN~(Q) of 1.8 m NiPSS and 2 m NiC12 in D20.15 The most significant difference in AN~(Q) between the solutions is observed below Q 1 A-1. For NiPSS, there is a clear increase in the intensity with decreasing value of Q. This can be explained qualitatively by a long-range ordering of nickel ions along the PSS chain. A similar effect has been observed in other wide-angle neutron diffraction experiments on polyelectrolyte solutions?JO Figure 2 shows the corresponding G&)'s, the sum of the scattering coefficients (eq 1) being added. The average level of GI(r) when r approaches zero should be minus the sum of these scattering coefficients. To meet this condition in the case of NiPSS, &i(r) had to be multiplied by 0.8. The origin of this mismatch is not quite certain; it might be due to an
inaccurate knowledge of the exact atomic composition of the NiPSS solution, which would affect the atomic number density and the atomic fractions, the latter being incorporated in the scattering coefficients. This adjustment of the normalization on the basis of the level of &i(r) at r near zero will introduce a rather large experimental error in the coordination numbers. However, relevant for the subsequent analysis is the fact that the ratio of coordination numbers will be affected much less and the shape of &i(r) not at all. Comparison of &i(r) in NiPSS and NiClz solutions shows that the positions of the peaks corresponding to the correlations between Ni2+ and the atoms of the hydration water molecules turn out to be indistinguishable. Moreover, the shapes of the correlations are almost the same. For NiPSS, the area under the NiO peak, centered at 2.05 A, corresponds to 6( 1) 0 atoms. Both the peak positions and the number of 0 atoms are within experimental error, in agreement with the 6-fold coordination of a nickel ion, as was found many times before in low molecular weight salt solutions. However, integrating the NiD peak at 2.63 8, results in only 9(2) D atoms,significantly less than 12, twice the number of oxygen atoms under the first peak, found in NiCl2 solution. This indicates that the first coordination sphere of the nickel ions in NiCl2 solution is significantly affected by replacing chloride ions with PSS polyanions and is no longer fully composed of D20. At distances beyond the first hydration shell (i.e., '3 A), the shape and intensity of GNi(r) differ considerably. The feature of the second hydration zone, which extends from about 4 to 6 A, is appreciably displaced to lower r values by about 0.2 A in the case of the NiPSS solution. Additionally, the intensity in this range, which for the case of NiC12 is mainly due to NiO and NiD correlations, is for NiPPS reduced by about 40% in the normalized &i(r). These observations are tentatively explained by oxygen atoms of SO3- groups of the PSS chain penetrating the first hydration sphere of the nickel ions. In such a configuration, the number of D atoms contributing to the NiD correlation peak is reduced. On grounds of the number 9(2) D atoms in the first hydration sphere, it can be concluded that a considerable fraction of Ni ions is affected in this way. A sulfur atom and the two other oxygen atoms of the SO3- group will be at distances from the nickel ion where the second hydration zone is found in the case of NiCb altering the shape and intensity of &,(I) in this region. There is probably a further effect on the second hydra-
Polyelectrolyte on the Hydration of Nickel Ions tion zone from the association of SO3- groups with the outer regions of intact Ni hydration complexes. Hydration of SO3groups is expected to be weak, being similar to the hydration of S042-,16and will not restrict contacts between Ni2+hydration complexes and the polyanion. A similar observation was made in a solution of nickel adenosine triphosphate (NiATP). l7 A reduced peak amplitude at 2.7 8, and reduced intensity between 3 and 3.5 8, was found, which was also interpreted as oxygen atoms of phosphate groups penetrating into the first hydration s here of nickel ions. In the case of ATP, a feature at 3.2 was interpreted as the correlation between the nickel ion and the other atoms of the associated phosphate group. Most consistent with the observed feature was a configuration of a phosphate group coordinated with the nickel ion by two of its oxygen atoms. A similar feature is not observed here. This suggests that for the case of the sulfonate group, only one oxygen is involved in the direct association of the nickel ion with the anion, leaving more dynamic freedom to the sulfonate group, causing the correlation peaks to be less well resolved or, as in the resolved or, as in the present case, unobservable. In order to explain the differences between the ionic interactions in solutions of NiPSS and NiC12, both highly soluble in water, it should be noted that in a solution of NiPSS, chargebearing oxygen atoms are arranged on the PSS chain with a regular inter charge spacing between 3 and 6 8,. These charges probably exert a cooperative attractive influence on the divalent nickel ions and cause a more intimate Ni2+-anion contact than in the case of NiCL. The difference in the shapes of the electron shells of chloride ions and oxyanionic SO3- groups might result in a different Ni2+-anion interaction, too. However, in concentrated low molecular weight salt solutions of Nioxyanion salts, no penetration of the Ni2+ hydration sphere appears to take place1*J9 or only to a very weak extent.20q21 Therefore, the observed effect of PSS chains on the Ni2+ coordination sphere is proposed to be typical for anions having several charged groups positioned close to each other. The intimate contact between the divalent Ni ions and the polyanion is in contrast with the situation found in a solution of LiPAA, where nearly no influence of the polyanion on the Li+ hydration could be detected. The difference between the results for divalent and monovalent metal ions is consistent with the general observation that the presence of polyions has a much stronger effect on the activity and dynamics of divalent cations than on those of monovalent cations. Though this is obviously caused by different electrostatic interactions, these neutron diffraction experiments represent the first direct observations of this effect in terms of atomic structure.
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Conclusions The hydration of nickel ions (Ni2+) in aqueous solutions of NiC12 and nickel poly(styrenesu1fonate) (NiPSS) of 2 and 1.8 m, respectively, has been studied with neutron diffraction
J. Phys. Chem., Vol. 98, No. 50, 1994 13197 combined with isotopic substitution. The presence of PSS polyanions does not affect the 6-fold coordination of Ni2+ by oxygen atoms and leaves the hydration of Ni2+ largely intact. A detailed investigation of the radial distribution function, however, suggests a significant degree of association of Ni2+ hydration complexes with the polyelectrolyte chain: there is evidence for direct contacts between Ni2+ and oxygen atoms of the sulfonate groups. A degree of association for the case of Ni2+ is in contrast with conclusions on Li+ in a polyelectrolyte solution. This difference corroborates the general observation that the behavior of monovalent ions is less affected by polyelectrolytes than divalent ions. For the case of NiPSS, the Q-space data shown an increase with decreasing Q at low Q values, which is not observed in NiC12 solution. A similar observation has been made several times in polyelectrolyte solutions and is, therefore, associated with correlations between counterions and the macromolecular chains over distances much larger than the size of a nickel hydration complex.
Acknowledgment. We thank Pauline Elks for assistance with the sample preparations and Stuart Ansell for help with the neutron diffraction experiment. We also acknowledge financial support from the SERC. References and Notes (1) Huizenga, J. R.; Grieger, P. F.; Wall, F. T. J. Am. Chem. SOC.1950, 72, 2636. (2) E.g.: Alexandrowicz, 2.J . Polym. Sci. 1959, 40, 91. (3) van der Klink, J. J.; Zuidenveg, L. H.; Leyte, J. C. J. Chem. Phys. 1974, 60, 2391. (4) Reddy, M. R.; Rossky, P. J.; Murthy, C. S. J.Phys. Chem. 1987, 91, 4923. ( 5 ) Jackson, J. L.; Coriell, S. R. J . Chem. Phys. 1963, 38, 959. (6) RymdBn, R.; Stilbs, P. J . Phys. Chem. 1985, 89, 2425. (7) Mills, P.; Anderson, C. F.; Record, M. T. J . Phys. Chem. 1985, 89, 3984. (8) Nilsson. L. G.: Nordenskiold. L.: Stilbs. P.: Braunlin. W. H. J. Phvs. Chem.' 1985, 89, 3385. (9) van der Maarel. J. R. C.; Powell. D. H.: Jawahier, A. K.; LevteZuidenveg, L. H.; Neilson, G. W.; Bellissent-Funel, M. C. J . Chem. Piys. 1989, 90, 6709. (10) Bieze, T. W. N.; Tromp, R. H.; van Strien, M. H. J. M.; van der Maarel, J. R. C.; Neilson, G. W.; Leyte, J. C.; Bellissent-Funel, M. C. Accepted for publication in J . Phys. Chem. (11) Skipper, N. T.; Neilson, G. W.; Cummings, S. C. J . Phys. Condensed Matter 1989, I , 3489. (12) Blech, I. A.; Averbach, B. L. Phys. Rev. A 1965, 137, 1113. (13) Paalman, H. H.; Ping, C. J. J . Appl. Phys. 1962, 33, 2635. (14) Enderby, J. E.; Neilson, G. W. Rep. Prog. Phys. 1981, 44, 593. (15) Powell, D. H. Ph.D. Thesis, University of Bristol, 1989. (16) Caminiti, R. Chem. Phys. Let?. 1982, 88, 103. (17) Gullidge, P. M. N.; Neilson, G. W. Chem. Phys. Lett. 1990, 165, 457. (18) Lichen, G.; Pinna, G.; Navarra, G; Vlaic, G. Z. Naturforsch. Teil A. 1983, 38, 559. (19) Newson, J. P.; Neilson, G. W.; Enderby, J. E.; Sandstrom, M. Chem. Phys. Lett. 1981, 82, 399. (20) Caminiti, R. J . Chem. Phys. 1982, 77, 5682. (21) Lichen, G.; Paschina, G.; Piccaluga, G.; Pinna, G. J . Chem. Phys. 1984, 81, 6059.
