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Solid-State Hydrolysis of Calcium Tripolyphosphate Scales Y. Zhou and J. O. Carnali* Unilever Research, 45 River Road, Edgewater, New Jersey 07020 Received July 22, 1999. In Final Form: February 24, 2000 Precipitation of calcium tripolyphosphate (CaTPP) from strongly supersaturated solution is accompanied by tripolyphosphate hydrolysis if the bulk Ca/TPP mole ratio is in excess of about 2. Along with the tendency for hydrolysis, a change in the crystal habit from crystalline to amorphous is observed beyond this bulk ratio. The composition of the material changes as well from mainly Ca2NaP3O10 at a 2:1 Ca/TPP bulk ratio to Ca5(P3O10)2 at an 8:1 bulk ratio, though the water of hydration contents are similar. The crystallinity change is reflected in the IR and solid-state 31P NMR spectra, but, beyond these expected differences, the amorphous samples also appear to have a larger fraction of more mobile crystal water. Through a series of sequential experiments, it was identified that hydrolysis of these materials mainly occurs via a mechanism consisting of precipitation of CaTPP followed by hydrolysis of the precipitate. Bulk water does not play a critical role in the hydrolysis nor does surface adsorbed water, to the extent that it can clearly be differentiated from the crystal or bound water. Kinetic studies of the hydrolysis of the 8:1 material showed that its solid-state reaction rate (7.1 × 10-3 min-1 at 70 °C) is 2 orders of magnitude higher than that of NaTPP. The hydrolysis mechanism is found to lay midway between first order and diffusion controlled, leading us to suspect that loosely bound water within the crystal lattice constitutes the diffusing species. Although the facile hydrolysis at the stronger supersaturation can possibly be explained in terms of the higher Ca/TPP ratio in the precipitate leading to destabilization of the P-O-P bond, the observed trends seem to rather suggest the diffusion of crystal water within the strongly distorted crystal lattice as the dominant factor.
Background Chemical reactions in the solid phase are less wellstudied than in corresponding solutions. The same statement applies to even as simple a reaction as hydrolysis. The hydrolysis of a chemical species in solid form can conceivably occur via several routes, depending on the supply of water. Assuming an originally anhydrous species, water can arrive via the vapor phase in which case the amount of moisture sorbed and in turn the decomposition route depend on the water adsorption isotherm for the particular solid. An extreme case of “solid”state hydrolysis occurs when the solid is exposed to relative humidities above the so-called critical relative humidity (CRH), when so much water sorbs that a bulklike layer of water actually forms on the solid surface and solutionphase decomposition can then occur.1-9 Solid-state hydrolysis via surface reaction with adsorbed water is often modeled after the Avrami-Erofeyev theory for nucleated growth.10 According to this model, initiation of hydrolysis requires the presence of nucleation sites such as surface defects (cracks, asperities, edges, etc.), at which surface molecules can be activated and then react with * Author to whom correspondence should be addressed. Phone: (201) 840-2356. Fax: (201) 840-8299. E-mail joseph.carnali@ unilever.com. (1) Carstensen, J. T.; Attarchi, F.; Hou, X. J. Pharm. Sci. 1985, 74, 741-745. (2) Carstensen, J. T.; Attarchi, F. J. Pharm. Sci. 1988, 77, 318-321. (3) Yoshioka, S.; Ogata, H.; Shibazaki, T.; Ejima, A. Chem. Pharm. Bull. 1979, 27, 2363-2371. (4) Yoshioka, S.; Shibazaki, T.; Ejima, A. Chem. Pharm. Bull. 1982, 30, 3734-3741. (5) Carstensen, J. T. Solid Pharmaceutics: Mechanical Properties and Rate Phenomena; Academic Press: New York, 1980. (6) Teraoka, R.; Otsuka, M.; Matsuda, Y. J. Pharm. Sci. 1993, 82, 601-604. (7) Yoshioka, S.; Uchiyama, M. J. Pharm. Sci. 1986, 75, 92-96. (8) Yoshioka, S.; Uchiyama, M. J. Pharm. Sci. 1986, 75, 459-462. (9) Yoshioka, S.; Carstensen J. T. J. Pharm. Sci. 1990, 79, 799-801. (10) Okamura, M.; Hanano, M.; Awazu, S. Chem. Pharm. Bull. 1980, 28, 578-584.
adsorbed water molecules. The geometrical changes in the local lattice at the reaction site can in turn activate a neighboring surface molecule. Thus the reaction propagates, and a transformed area grows out from each nucleation site. Bulk, rather than surface, hydrolysis may occur if the water vapor can diffuse through the crystalline solid.11-13 An autocatalytic mechanism in this case is related to hydrolysis occurring along the diffusion path and hence varying the transport properties of the solid matrix.14 Also, crystal strain due to the discrepancy between the physical shape of the hydrolysis product and that of the starting material can create cracks which act as reaction centers, thus yielding a Avrami-type kinetics for bulk hydrolysis as well.10 Another potential route for hydrolysis involves solid hydrates, where the waters of hydration may provide the hydrolyzing species. Salts such as iron(II) chloride hexadydrate or iron(II) sulfate heptahydrate decompose upon heating.15 Potassium nitrilosulfonate dihydrate, K3N(SO3)3‚2H2O, also hydrolyzes 16 with the water of hydration acting to bridge formula units within the crystal lattice. The hydrolysis of sodium tripolyphosphate (Na5P3O10 ) NaTPP) to pyro- and orthophosphate is well-known in the detergent industry and is regularly accounted for in the design of industrial processes.17,18 In the dehydration of the solid sodium tripolyphosphate hexahydrate, a series (11) Vicens, J.; Decoret, C.; Royer, J.; Etter, M. C. Isr. J. Chem. 1985, 25, 306-311. (12) Etter, M. C.; Errede, L. A.; Vicens, J. Cryst. Struct. Commun. 1982, 11, 885-888. (13) Lamartine, R.; Vicens, J.; Perrin, R. Reaction Solids, Part B; Materials Science Monographs; Elsevier: Amsterdam, 1985; Vol. 28B, pp 721-724. (14) Pryde, C. A.; Hellman, M. Y. J. Appl. Polym. Sci. 1980, 25, 25732587. (15) Mahapatra, S.; Prasad, T. P. Thermochim. Acta 1988, 128, 305309. (16) Hall, J. R.; Johnson, R. A. Phosphorus Sulfur 1977, 3, 175-178. (17) Green, J. Ind. Eng. Chem. 1950 42, 1542-1546. (18) Patino, J. M. R. J. Am. Oil Chem. Soc. 1993, 70, 83-89.
10.1021/la990977l CCC: $19.00 © 2000 American Chemical Society Published on Web 05/04/2000
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Figure 1. Heterogeneous phase boundary in the system CaCl2, Na5P3O10 at 55 °C, pH 10, and ionic strength 0.15 as NaCl. The size of the solid square symbols indicates the maximum variance in repetitive measurements. The portion of the diagram above the solid line exhibits a precipitated phase. Circle and diamond symbols are reproduced from the work of Quimby20 at 60 °C and of Diamond23 at 57 °C, respectively. The dotted line corresponds to a bulk 2:1 ratio of Ca to TPP.
of transition zones are set up perpendicular to the solid surface.19 Dehydration occurs in a series of steps spread through these zones from the surface to the hexahydrate core, with one step constituting hydrolysis. Then, in the latter stage of the dehydration, pyro- and orthophosphate recondense to yield Form II sodium tripolyphosphate. The mechanism of hydrolysis was considered in detail by Quimby.20 The hexahydrate can be isolated by repeated recrystallization from ethanol-water. However, the instability of the hexahydrate means that even the recrystallization process itself causes a measurable degree of hydrolysis. Hydrolysis is thought to initiate at the surface of the crystal at defects or points of strain21,22 and, once initiated, can be self-propagating since water is released which catalyzes further hydrolysis. Hydrolysis is postulated to occur via the mechanism
Na5P3O10‚6H2O S Na4P2O7 + NaH2PO4 + 5H2O (1) but the reaction product is usually richer in pyrophosphate, Na4P2O7, than the 1:1 mole ratio of pyrophosphate to orthophosphate expected from eq 1.21 Thus mechanism 2 20 has been proposed:
2Na5P3O10 + H2O S Na4P2O7 + 2 Na3HP2O7 (2) The calcium salt of tripolyphosphate is highly insoluble, and the boundary between the homogeneous and heterogeneous phase at 60 °C has been reported by Quimby20 and (partially) by Diamond23 as in Figure 1. The righthand boundary in this figure moves to higher NaTPP levels as the background level of sodium ion increases, implying that sodium ion is incorporated into the precipitate along this boundary. The heterogeneous phase region was studied semiquantitatively by Quimby.20 The dotted line in Figure 1 represents a bulk mole ratio of calcium to NaTPP of 2:1. This line divides the region into a portion characterized by a crystalline precipitate on the right (higher phosphate level) and one with an amorphous precipitate on the left (lower phosphate). Air-dried samples of these precipitates had water contents of 16-17 and 20-21% for crystalline and amorphous samples, respectively. Quimby found the latter to have a higher Ca/P (19) Prodan, E. A.; Lesnikovich, L. A. Thermochim. Acta 1992, 203, 269-288. (20) Quimby, O. T. J. Phys. Chem. 1954, 58, 603-618. (21) Griffith, E. J. Pure Appl. Chem. 1975, 44, 173-200. (22) Prodan, E. A.; Samuskevich, V. V.; Pytlev, S. I.; Galogaza, V. M.; Ambrazevich, N. M. Inorg. Mater. 1990, 26, 1285-1291. (23) Diamond, W. J. J. Phys. Chem. 1959, 63, 123-124.
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ratio, but Diamond and Grove24 found a Ca/P3O10 ratio of 2.5 throughout the heterogeneous region and proposed an empirical structure, Ca5(P3O10)2‚10H2O. Vance25 used the fact that the solution turbidity peaked at a Ca/P3O10 ratio of 2.5 to arrive at the same conclusion. The present study has focused on the solid-state hydrolysis of this calcium tripolyphosphate (CaTPP) precipitate. Aoki et al.26 studied the precipitation of TPP by Ca at pH 9 and noted that the precipitate contained the hydrolysis products (ortho- and pyrophosphate) to an increasing extent as the Ca/TPP bulk ratio increased. However, whether the hydrolysis occurred prior, during, or after the precipitation was not ascertained. The solidstate hydrolysis of NaTPP was studied by Zettlemoyer et al.27 and found to proceed with a first-order rate constant of 6 × 10-5 min-1 at 70 °C, so that appreciable hydrolysis could be detected after 8 days. An accelerated rate of hydrolysis of CaTPP might be expected since it is known that alkaline earth cations act as catalysts in solution to an extent which increases with the strength of the cation/ TPP binding.28 However, the acceleration previously reported is relatively weak, about a factor of 2 or 3, while rates as high as 7 × 10-3 min-1 will be reported below for precipitated CaTPP in excess Ca. Materials and Methods Materials. Sodium tripolyphosphate hexahydrate (Na5P3O10‚ 6H2O) was a gift of the FMC Corp. This sample was purified by repeated recrystallization from ethanol/water following the method proposed by Quimby20 and stored in a freezer. Periodic analysis by 31P NMR served to confirm that the material was maintained at a purity of 99% (anhydrous base). Calcium chloride dihydrate was from Fisher Scientific and all other chemicals employed were of reagent grade. Methods. Preparation of Solid Phases. Samples were prepared on a convenient size scale by combining together equal volumes of CaCl2‚2H2O solution and Na5P3O10‚6H2O solution. All solutions were first put through 0.22 µm filters to remove heterogeneous nucleation sites and were preequilibrated at the temperature of interest. The concentration of the respective solutions was adjusted to twice that desired in the final sample. In this way, transiently high supersaturation was avoidedsas evidenced by the fact that addition of CaCl2 solution to NaTPP solution gave the same results as did the reverse order of addition. Background ionic strength and pH buffering was provided by a pH 10.0 Borax buffer (0.0125 M borax, 0.0205 M NaOH, 0.15 M NaCl) used in all experiments. Comparison with carbonate buffer or with pH adjustment via NaOH, and with sodium acetate or sodium nitrate as the ionic strength source verified the absence of any specific effects in the present buffer system. Once mixed, the systems were stirred for a specified time at a specified temperature (usually 25 or 55 °C). The precipitate was then filtered off and collected on a 0.22 µm Millipore filter. To avoid contamination with CaCO3, some of these preparations were repeated in a drybox under N2. Elemental Analysis. For elemental analysis, the precipitate could be washed with a small quantity of distilled water, dried (see below), dissolved in dilute nitric acid (Trace Metal Grade), and then analyzed by inductively coupled plasma spectroscopy (Thermo Jarrell Ash AtomScan 25) for phosphorus and calcium. Phosphorus levels were also independently checked by a molybdenum blue method as described in Bassett et al.29 Briefly, (24) Diamond, W. J.; Grove, J. E. J. Phys. Chem. 1959, 63, 15281529. (25) Vance, R. F. J. Am. Oil Chem. Soc. 1969, 46, 639-641. (26) Aoki, S.; Imai, S.; Arai, Y. Gypsum Lime 1980, 169, 284-290. (27) Zettlemoyer, A. C.; Schneider, C. H.; Anderson, H. V.; Fuchs, R. J. J. Phys. Chem. 1957, 61, 991-994. (28) Thilo, E.; Wieker, W. J. Polym. Sci. 1961, 53, 55-59. (29) Bassett, J.; Denney, R. C.; Jeffery, G. H.; Mendham, J. Vogel’s Textbook of Quantitative Inorganic Analysis, 4th ed.; Longman Inc.: New York, 1988; p 756.
Solid-State Hydrolysis of CaTPP Scales TPP is hydrolyzed to orthophosphate by heating in H2SO4. The resulting orthophosphate is complexed with molybdate ion and then reduced with hydrazinium sulfate to give a blue complex whose concentration was detected spectrophotometrically at 820 nm. The P/Ca ratio for some of the precipitates was confirmed by EDX analysis using a Kevex Sigma 2 Energy Dispersive system operated at accelerating voltages between 12 and 14 kV. FT-IR Analysis. Further identification of the solid phase was made via Fourier transform infrared spectroscopy (Bio-Rad FTS60A, operating in the range 700-4000 cm-1, 8 cm-1 resolution). The dried precipitate was ground and mixed at a level of about 0.5% with powdered potassium bromide. Phosphate Speciation. 31P NMR was used to measure the relative levels of tripolyphosphate, pyrophosphate, and orthophosphate. Samples were dissolved in 0.1 N alkaline (pH 10) ethylenediaminetetraacetic acid (EDTA) and spectra were collected on a Bruker DMX 500 MHz spectrometer at a frequency of 202.4 M Hz using a 5 mm broad band probe (it was separately verified that no further hydrolysis of the tripolyphosphate occurred during this determination in alkaline EDTA30). A 5% D2O solution was added as a lock solvent, and shifts are reported relative to 85% H3PO4. Typical acquisition parameters were 7000 Hz spectral width, 50° pulse width, and a 60 s relaxation delay, to ensure maximized detection of the observable magnetization.31 Morphological Studies. The morphologies of the precipitated solids were studied using an Electro Scan Model E-3 environmental scanning electron microscope (ESEM) and by X-ray powder diffraction using a Rigaku Rotaflex (employing Cu KR radiation and SC-30 scintillation counting). Water Content of the Hydrate. The degree of hydration of the precipitate was determined via 1H NMR. The Ca/TPP precipitate was prepared as above, and free water removed by storing over Drierite in a desiccator overnight at 4 °C. A portion of this precipitate was usually further dried by storing under vacuum over P2O5, again at 4 °C, for several more hours. Small quantities of the solid, ranging from 10 to about 40 mg, were then dissolved in 1 mL of 100 mol % D2O, which was 1 M in DCl. As a proton standard, the deuterated solvent also contained 10 mg/mL of H8-1,4-dioxane. When fully dissolved, a portion of each solution was transferred to clean, dry 5 mm NMR tubes and then sealed. The above operations with the deuterated solvents were all performed in a drybox. 1H NMR was then performed on the samples using a Bruker DMX-500 spectrometer with a 5 mm 1H triple axes inverse (TXI) probe. Four transients were accumulated with an 8 s relaxation delay between pulses and a spectral width of 6000 Hz. The peak from the Ca/TPP precipitate (water of hydration) was referenced against the dioxane internal standard in order to calculate the water content of the precipitate. The water content of the P2O5/vacuum treated portion of the sample was typically only slightly lower than that of the Drierite/ desiccator treated portion. As an example, for the 8:1 sample, the water content was found to be 23.0 versus 24.1 ( 0.5%, respectively. Nevertheless, the water levels determined are sensitive to the kinetics of the drying procedure employed. The true level of hydration, in equilibrium with a realistic relative humidity, could only be determined by a longer study which was precluded by the instability of the precipitate. Heterogeneous Phase Boundary. The heterogeneous phase boundary for the calcium/TPP system was re-determined turbidometrically using a fiber optic probe connected to a Brinkmann colorimeter, Model PC 800. Points on the right branch of the phase boundary in Figure 1 were determined by titrating 100 mL of a NaTPP solution with 0.25 mL aliquots of a M CaCl2 solution. The transmittance was monitored over the 15 min between increments, and the heterogeneous phase boundary was assumed to have been crossed when this transmittance fell by g5%. In practice, precipitation was rapid and voluminous along this branch so that the phase boundary was easily detected visually. Points on the left branch were determined by titrating 100 mL of a CaCl2 solution with 0.5-1 mL aliquots of NaTPP solution. Precipitation was much slower on this boundary with the heterogeneous phase, and incremental additions were spaced (30) Kura, G.; Tsukuda, T. Polyhedron 1993, 12, 865-870. (31) Gard, D. R.; Burquin, J. C.; Gard, J. K. Anal. Chem. 1992, 64, 557-561.
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Figure 2. Calcium distribution in a bulk 2 mM Ca2+ solution with varying Ca/TPP bulk ratios after 1 h at 55 °C: square symbols, milligrams of Ca found in the resulting precipitate; circle symbols, free Ca2+ in solution; diamond symbols, milligrams of Ca2+ assumed complexed with TPP (obtained by mass balance). The total calcium per sample is 8 mg. 1-3 h apart (in light of hydrolysis, as shown below, this side of the boundary should be considered unreliable). Measurement of Hydrolysis Rates in Solid Phases. The precipitate was dried overnight at 4 °C, as detailed above, and then split into 30 mg aliquots. Each aliquot was loaded into a 2 × 2 in. square of Parafilm which was then folded up tightly. At time t ) 0, an aliquot was immersed into a silicon oil bath maintained at the elevated temperature of interest. After a predetermined period of 5-90 min, the aliquot was removed and quickly immersed in a second silicon oil bath maintained in a freezer at -15 °C to stop any reaction. The solid was then dissolved in a small amount of 0.1 M alkaline (pH 10) EDTA, preventing further hydrolysis,30 and then analyzed by 31P NMR for the degree of TPP conversion to pyro- and orthophosphate.
Results Descriptive Details. Heterogeneous Phase Boundary. The phase boundary determined at pH 10 and 0.15 M ionic strength at 55 °C is compared to that found by Quimby20 and by Diamond23 in Figure 1. The boundary is found to be in good agreement, reproducing the minimum and the homogeneous phase region at low- and high-NaTPP levels. Precipitate Analysis. A clue to the identity of the Ca/ TPP precipitate comes from a coarse analysis of the precipitate which forms when a 2 mM Ca 2+ solution is combined with NaTPP to give varying Ca/NaTPP bulk ratios. After 1 h at 55 °C, the calcium content in the collected precipitate was analyzed, the calcium present as free Ca2+ in the mother liquor determined with a calcium sensitive electrode and the calcium present as a soluble complex with TPP estimated by mass balance. The results, shown in Figure 2, parallel the findings of Aoki et al.26 and of Diamond.23 At low Ca/NaTPP, the calcium is fully complexed with TPP. However, at Ca/ TPP levels of around 2, no more calcium can be complexed and the excess calcium leads to a precipitate whose composition would appear to be around 2:1 Ca/TPP. Further increase in the calcium level results in the appearance of free Ca2+ in solution, as the binding/ precipitation capacity of the TPP is exceeded. Analysis of the precipitate near the peak levels shown in Figure 2 shows a P/Ca ratio of 1.45 ( 0.05, which would match the bulk Ca/TPP ratio on the abscissa. Speciation of the Precipitated Phase. The composition of the precipitated phase was examined in a preliminary fashion by fixing the bulk Ca2+ level at 2, 4, or 8 mM and varying the NaTPP level to traverse the heterogeneous phase region. The resulting precipitates were analyzed by 31P NMR for tripolyphosphate content and the interesting results shown in Figure 3 obtained. At 2 mM Ca2+, along the bottom of the heterogeneous phase region in
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Zhou and Carnali Table 1. Percent of Phosphate as TPP in the Wet Cake and in the Precipitate from the Mother Liquor (Indicated as WC and ML, Respectively) and Percent of Original TPP Precipitated, at 4 and 8 mM Ca, at the Times Indicated at 4 mM Ca time (min)
Figure 3. Tripolyphosphate fraction of phosphate in the precipitate resulting from Ca2+ solution [bulk levels ) 8 mM (square symbols), 4 mM (circle symbols), 2 mM (diamond symbols)] at varying Ca/TPP ratios at 55 °C after 1 h. The arrow emphasizes the critical nature of the 2:1 Ca/TPP ratio. Compositions underlined by star symbols are further discussed in the text.
Figure 1, the precipitate was essentially composed on 100% tripolyphosphate. At 8 mM Ca2+, the highest bulk calcium level examined, the precipitate was composed of TPP for bulk Ca/NaTPP mole ratios up to 2. For levels in excess of 2, however, the presence of hydrolysis products was detected. Expressed in terms of the mole percentage of phosphate present as TPP, the TPP content fell to a plateau of roughly 50%. At an intermediate Ca2+ level of 4 mM, the onset of hydrolysis again occurred at bulk Ca/NaTPP ratios over 2 but plateaued at roughly 70% TPP. Though these results are quantitatively affected by the temperature (55 °C) and the time period (60 min) over which the precipitation is allowed to occur, they show a qualitative trend which can be illustrated using Figure 1. The phosphate in the precipitate occurring at low bulk Ca2+ levels or between the right-hand side heterogeneous phase boundary and the dotted 2:1 Ca/TPP line is in the form of TPP. However, at high bulk Ca2+ levels and well in from the right-hand phase boundary, a large fraction of the phosphate is hydrolyzed to ortho- and pyrophosphate. Evidence of Solid-State Hydrolysis. The normal course of precipitation from the mother liquor was interrupted as follows. A system at 4 mM calcium, 4:1 Ca/TPP, and 55 °C was filtered 2 min after initial mixing and the precipitate collected left to sit, in the wet cake state, at 55 °C for the remainder of a 2 h period. Small samples of solid were taken for determination of phosphate speciation at 10, 30, 60, and 120 min. In parallel runs, precipitate in the mother liquor was collected at 10, 30, 60, or 120 min and analyzed in the same way. It was thus possible to compare the extent of hydrolysis occurring in the wet cake with that occurring in the mother liquor. Note that in the latter case, the amount of precipitate was also increasing for a time beyond the initial 2 min sampling. Results in Table 1 show the mole percentage of phosphate as TPP in the solid for both the wet cake and the precipitate collected from the mother liquor at the times indicated as well as the total percentage of TPP which has precipitated at the indicated time. Over 50% of the original TPP in the system precipitates within 2 min. The extent of hydrolysis in the wet cake and in the freshly collected precipitate parallel one another closely over the entire period of observation. Clearly, an important component of the hydrolysis reaction begins with precipitation of Ca/TPP which subsequently hydrolyzes in the “solid state”seither in the bulk solid or at the surface (see below). Phosphate hydrolysis in any occluded water must be negligible due to the insolubility of CaTPP. A similar experiment but at 8 mM calcium, 8:1 Ca/TPP, gave analogous results (Table 1). In this case, the wet cake was removed at 10 s, by
0.17 2 10 30 60 120
%TPP WC
at 8 mM Ca
%TPP ML
%TPP ppt
90
53 75 87 90 94
%TPP WC
% TPP ML
%TPP ppt
91 85 78 70 62
76 68 63
85 78 70 51
74 62 54 44
94 98 99
Figure 4. Incremental percentage of phosphate as TPP dissolved from the preformed 4:1 Ca/TTP precipitate. The material is stirred in 0.1 N alkaline EDTA solution, which is exchanged at the time corresponding to each data point for fresh solution. The spent solution is analyzed for the total phosphate and for the TPP dissolved in the incremental time period. The solid is fully dissolved by the last data point.
which time over 80% of the TPP had already precipitated. This time, hydrolysis in the wet cake was seen to lag slightly behind that in the mother liquor, but the above conclusion is clearly still applicable. In experiments not shown in Table 1, it was found that the wet cake isolated at the beginning of the hydrolysis run underwent hydrolysis at a faster rate than a wet cake isolated from the middle of the run. Thus the higher degree of supersaturation occurring initially seemed to affect the hydrolysis within a run just as it affects overall hydrolysis for an entire run (see below). Mechanism of Hydrolysis. The speciation of the 4:1 precipitate from fresh mother liquor was found to not be affected by the presence of partially hydrolyzed precipitate isolated from a prior precipitation run. Thus hydrolysis, whether in solution or in the solid phase, was shown to not be catalyzed in any way by the presence of priorly hydrolyzed solidseliminating this form of heterogeneous catalysis as a mechanism. Further insight into the hydrolysis mechanism comes from a coarse analysis of the 4:1 precipitate species, performed by sequentially dissolving the solid particles in alkaline, 0.1 N EDTA under constant stirring. The solid was periodically (every 10-100 s) filtered off from the EDTA solution, the filtrant analyzed for phosphate speciation, and the filtrate resuspended in fresh solution for another dissolution period. The percentage of tripolyphosphate in the incrementally dissolved precipitate was shown (as in Figure 4) to remain constant from the start until the completion of dissolution. Thus the precipitate particles had undergone a consistent level of hydrolysis from the surface to the core. This result would suggest that solid-state hydrolysis is not surface initiated to a significant extent. Quantitative Details. Of the prior work reported on the Ca/TPP precipitate, only the work of Aoki et al.26 acknowledges hydrolysis of TPP as being significant. In
Solid-State Hydrolysis of CaTPP Scales
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Table 2. Elemental Analysis of the Precipitate Phasesa boron calcium phosphorus sodium water P/Ca
2:1
4:1
8:1
0.05 ( 0.02 19 ( 0.5 21 ( 0.5 3.5 ( 1 16 ( 0.5 1.43 ( 0.07
0.3 21 ( 0.5 20 ( 0.5 1.0 ( 0.5 24 ( 3 1.22 ( 0.05
1.2 ( 0.2 21 ( 0.5 17 ( 0.5 0.5 ( 0.5 24 ( 1 1.00 ( 0.05
a Results presented as percent by weight of solid except P/Ca atomic ratio.
this case, it is understood that the hydrolysis observed occurred via a solution phase mechanism. While it is probable that some solution phase hydrolysis does occur in these systems, the strong dependence of the extent of hydrolysis on the bulk Ca2+ level led us to more closely investigate the extent of solid-phase hydrolysis. Three precipitates at the upper left, center, and lower righthand corners of Figure 3 (marked with star symbols) were chosen for further study, with bulk Ca2+ levels of 2, 4, and 8 mM and initial Ca/TPP ratios of 2:1, 4:1, and 8:1, respectively. The resulting precipitates from these solutions will thus be referred to as 2:1, 4:1, and 8:1, respectively. To isolate the fraction of hydrolysis occurring via a solid-phase route, it was convenient to first obtain the precipitates in as hydrolysis-free a state as possible. For the 2:1 sample, the precipitate formed in the usual way was found to be >97% TPP. The 4:1 and 8:1 precipitates, however, were hydrolyzed to a large degree after 1 h at 55 °C. Thus the procedure was modified by forming the precipitate at room temperature. The degree of supersaturation was reduced in this way as was indicated for the 8:1 by the induction time which increased from 0 s at 55 °C to about 5 s at room temperature. That of the 4:1 at 25 °C was several hours. For comparison, the induction time of the 2:1 system at 55 °C was about 5 min. The room temperature precipitation route resulted in isolated precipitates of >97% TPP content. It is acknowledged that the room temperature precipitate may differ, in ways other than in the extent of hydrolysis, from the 55 °C analogue. However, in the 8:1 case, it will be shown below that its propensity toward hydrolysis is still large with respect to the 2:1 species. Elemental Analysis of the Precipitated Phases (Table 2). (a) The 8:1 Precipitate. Elemental analysis of this precipitate indicated an atomic P/Ca ratio of 1.0 ( 0.05 as opposed to the expected 1.2 for Ca5(P3O10)2. High levels of boron were also found, in excess of that expected from adsorption.32 Assuming, then, that the boron was present as the contaminate calcium borate, a mass balance could be made by accounting for the high level of calcium in the precipitate. A consistent 24% crystal water, corresponding to the formula Ca5(P3O10)2‚14H2O, was determined. As noted above, the drying regime employed does not guarantee that all of the water determined is water of hydration, as opposed to surface water. The role of boron as an impurity was verified by preparation of the 8:1 precipitate in unbuffered solution, the pH being adjusted and kept constant via addition of dilute NaOH. A material with a P/Ca ratio of 1.17 ( 0.05 was obtained, as expected. Precipitates obtained by both the buffered and unbuffered routes hydrolyzed at identical rates. (b) The 4:1 Precipitate. Elemental analysis of this precipitate indicated an atomic P/Ca ratio close to the expected 1.2 of Ca5(P3O10)2. The water content of the 4:1 precipitate was found to be the most sensitive of the three precipitates to the drying conditions and time employed. (32) Irani, R. R.; Callis, C. F. J. Phys. Chem. 1960, 64, 1398-1407.
Figure 5. Infrared spectra of the CaTPP precipitate obtained as described in the text: (a, top) 8:1 material; (b, bottom) 2:1 material.
The 24% level observed (corresponding to 14 mol of bound water) differs from the 10 mol found by Rakotomavo et al.33 (c) The 2:1 Precipitate. Elemental analysis of this precipitate showed an atomic P/Ca ratio of approximately 1.43 ( 0.07, well above the expected 1.2. The high level of sodium ion found suggested that the dominant chemical species was Ca2NaP3O10 as had been proposed by Quimby20 for precipitates from the calcium poor region near the heterogeneous phase boundary. The 16% water level reported corresponds to 4 mol of bound water. The overall formula Ca2NaP3O10‚4H2O is the same as that reported by Rakotomavo et al.33 Physical Analysis of the Precipitated Phases. (a) IR Spectroscopy. The IR spectra of the 8:1 and 2:1 precipitates are shown in Figure 5a,b, respectively. The 8:1 spectrum is characterized by sharp lines at 915 and 995 cm-1, a broad line at 1135 cm-1, a peak at 1650 cm-1, and a broad peak at 3350 cm-1. Similarly, the 2:1 sample has the same sharp lines at about 915 and 995 cm-1, a 1130 cm-1 peak now resolved into a triplet, a sharp peak at 1258 cm-1 (previously a shoulder), the sharp peak at 1650 cm-1, and a broad, but less symmetrical, peak at 3450 cm-1. A weak doublet appears in both Figure 5a,b at 1430 and 1500 cm-1, which has previously been identified33-35 as arising from coprecipitated CaCO3. This doublet grew in intensity if the special precautions to exclude CO32- applied in Figure 5a,b were not taken. The assignment of the peaks in the spectra can be made with reference to the prior work.26,33,36 The fine structure in the 2:1 precipitate at around 1130 cm-1 is characteristic of the asymmetric stretching vibrations of PO3. Aoki et al.26 also observed its collapse into a broad band at highCa/TPP levels. The sharp lines at 915 and 995 cm-1 are assigned for both samples (Figure 5a,b) to the asym(33) Rakotomavo, S.; Meullemeestre, J.; Vierling, F.; Schwing, J. P. Analusis 1988, 16, 227-236. (34) Li, J.; Liao, H.; Sjo¨stro¨m, M. Biomaterials 1997, 18, 743-747. (35) SÄ lo´sarczyk, A.; Paluszkiewicz, C.; Gawlicki, M.; Paszkiewicz, Z. Ceram. Int. 1997, 23, 297-304. (36) Prodan, E. A.; Ol’shevskaya, O. P. Inorg. Mater. 1979, 15, 15711576.
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metric stretch of POP. The sharp line at 1258 cm-1 in the 2:1 sample is assigned, according to Prodan and Ol’shevskaya,36 to the asymmetric stretching vibration of PO2. This peak becomes a broad shoulder in the 8:1 sample. The peak at 1650 cm-1 belongs to the H-O-H (δ(OH)) bending mode of water of hydration (crystal water).37 The broad peak at 3300-3400 cm-1 does not appear to be the O-H vibration from protonated TPP (i.e. HP3O10), as that peak is much sharper as shown, for example, by the hydroxyl adsorption in hydroxyapatite.34 The broad peak is present in the starting material (Na5P3O10‚6H2O) and is likewise assigned to the symmetric and antisymmetric stretch of waters of hydration (ν(O-H)), in agreement with Slo´sarczyk et al.35 From the above absence of the P(dO)O-H hydroxyl stretching vibration and the missing P-(OH) stretching frequency at 850-870 cm-1,37 it is concluded that both the 8:1 and the 2:1 precipitates at pH 10 are fully deprotonated. The nonexistence of Ca4H2(P3O10)2 in the precipitate at this pH was also observed by Rakotomavo et al.33 The 4:1 precipitate (not shown) had an IR spectrum very similar to that of the 2:1 one. (b) Precipitate Morphology. The XRD spectra were taken of precipitates at a fixed bulk NaTPP level (1 mM) as a function of the bulk calcium level. In general, the crystalline character of the precipitate decreased with the bulk calcium level. At 2 mM Ca, the precipitate was basically crystalline, with relatively sharp lines at 29.2, 30.5, and 31.2° 2θ. At Ca/TPP levels above 2.0, the spectra took on a progressively more amorphous character. At 8 mM Ca, the sample was entirely amorphous. Previous work33 at Ca/NaTPP levels in excess of 2.5 reported a similar lack of crystallinity and attributed this finding to the rapid rate of crystal growth with subsequent small and imperfect crystals. Aoki et al.,26 at pH 9.0, assigned XRD peaks occurring at 28.8, 30.6, and 31.5° 2θ to Ca4H2(P3O10)2, but this assignment seems speculative in light of their IR data which showed no sign of the P(dO)O-H hydroxyl band. These authors also observed that the spectra became ill defined for Ca/NaTPP g 2 due to the poorly defined crystals which result. One additional qualitative point regarding our XRD spectra is their lack of any lines assignable to calcium carbonate.35 Morphological analysis by ESEM largely confirmed the XRD findings. The 8:1 sample was found to be amorphous (or perhaps microcrystalline) down to a size range of 500 nm. The 2:1 sample is crystalline, the primary particles consisting of agglomerated platelets of plate dimension on the order of a few micrometers. The surface area of the crystalline material was noticeably lower that than of the 8:1 material. Extent of Solid-State Hydrolysis. The extent of hydrolysis occurring in the solid state is expressed for various conditions in Figure 6. Since the descriptive results do not suggest a surface mediated reaction, the fraction of remaining TPP (X/X0) is plotted loge/linear versus the reaction time, anticipating simple linear hydrolysis kinetics. As expected in eqs 1 and 2, the reaction products of TPP hydrolysis are found to be richer in pyrophosphate than in orthophosphate. This finding holds for the hydrolysis occurring during preparation of the precipitate as well as for that occurring during the temperature treatment in the dry state. In both cases, the pyro to ortho mole ratio is roughly 1.5, independent of temperature. Similar ratios were reported by Prodan and Ol’shevskaya.36 Effect of Water Content. (a) In Figure 6 results for a hydrolysis run with the 8:1 precipitate, dried as described (37) Brecˇevic˘ , Lj.; Fu¨redi-Mihofer, H. Calcif. Tissue Res. 1972, 10, 82-90.
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Figure 6. First-order kinetic plot of fractional level of tripolyphosphate versus time for 8:1 precipitate: in wet cake (dotted line) or dried as described in the text (circle symbols), both at 55 °C. Temperature variation of the fractional level of tripolyphosphate versus time is also shown for the dried precipitate at 45 °C (square symbols), 55 °C (circle symbols), and 70 °C (diamond symbols). The size of the symbols is representative of the uncertainty in the average values. Rate constants are derived from the fitted lines shown.
above, are compared with a run made with freshly precipitated, wet material (30% solids). The rate of the hydrolysis reaction at 55 °C (negative slope of the loge X/X0 vs t plot) is clearly not greatly affected by the large difference in water content between the two samples. If surface adsorbed water does play a mechanistic role in hydrolysis, then the hydrolysis rate would be expected to increase with the level of surface water, at least at low levels. Although the level of surface water in the isolated 8:1 precipitate is not well-quantified, it is certainly reduced considerably by first the desiccation and then the vacuum treatment over P2O5. The fact that the hydrolysis rate is independent of this drying treatment suggests that bulk and surface adsorbed water (as opposed to crystal or water of hydration) is not critical to hydrolysis. (b) Effect of the Bulk Ca2+/TPP Ratio. Hydrolysis runs with 8:1 and 4:1 precipitates were compared at 55 °C. Recall that in the mother liquor at 55 °C, the 4:1 precipitate did hydrolyze, though to a lesser extent than the 8:1 species. With the material prepared at room temperature, however, hydrolysis of the solid-state 4:1 species essentially did not occur over a 60 min time scale, reinforcing the notion that these solid-state experiments do not exactly parallel the hydrolysis process in the mother liquors though the 8:1 precipitate behaves the same in both instances. (c) Effect of Reaction Temperature. Solid-state hydrolysis of the 8:1 precipitate was followed at 45, 55, and 70 °C, and the results obtained are plotted assuming first-order kinetics in Figure 6. The hydrolysis rate constants (k) obtained from the slopes are increasing functions of temperature: 45 °C, 6.7 × 10-4 min -1; 55 °C, 2.4 × 10-3 min -1; 70 °C, 7.1 × 10-3 min -1. From this temperature variation of k, an Arrhenius plot was assembled assuming the relationship
k ) k0 exp (-∆G/RT)
(3)
The estimated activation energy of hydrolysis is thus determined as ∆G ) 85 kJ mol-1 (20.3 kcal mol-1). This value can be compared with various previous measurements in the literature, made in solution and in the solid state. Solution measurements include the 20.5 kcal mol-1 determined by Gill and Riaz38 for the hydrolysis of longchain polyphosphates to trimetaphosphate and orthophosphate, the value of 30 kcal mol-1 for the hydrolysis of 1% tetramethylammonium tripolyphosphate in tetra(38) Gill, J. B.; Riaz, S. A. J. Chem. Soc. A 1969, 183-187.
Solid-State Hydrolysis of CaTPP Scales
methylammonium bromide solution at pH 10,39 25.8 kcal mol-1 for hydrolysis of 1% NaTPP in 0.65 N NaBr,40 or 22.8 kcal mol-1 for 1% NaTPP with no background electrolyte. In the solid state, previous measurements include the 21-28 kcal mol-1 obtained36 on mixed MIMII2 P3O10‚xH2O salts at 100% relative humidity and the 41.5 kcal mol-1 measured by Zettlemoyer et al.27 on NaTPP over the temperature range 85-105 °C. The range of typical rate constants, k, can be compared with some solution values for NaTPP, for example 10-3 day-1 as measured by Williard et al.41 at pH 7 and 25 °C, 4.9 × 10-3 h-1 as measured by Zinder et al.42 at pH 7 and 60 °C, and 8.7 × 10-3 min-1 observed by Kura and Tsukuda30 at 90 °C. Again in the solid state,27 6 × 10-4 min-1 was measured for Na5TPP at 85 °C. Corrected for the temperature difference, the present solid-state rate constant for the 8:1 material at 70 °C is 2 orders of magnitude higher that that for NaTPP. Discussion Context. The catalytic effect of cations on the alkaline hydrolysis of polyphosphates in solution has been attributed by Kura43,44 to the formation of an inner sphere complex (ion pair) between the cation and phosphate. The small crystal radius of cations such as Li+ favors formation of the complex, which then reduces the electron density around the phosphorus, rendering the phosphorus atom more susceptible to nucleophilic attack by OH- or H2O. Sodium also shows this effect.40 Lanthanide cations (which are often used as models for calcium in complexation studies) also form inner coordination sphere complexes with TPP in solution. In a multinuclear NMR study, Nieuwenhuizen et al.45 found evidence for monodentate coordination of the β (central) PO2 group and for one of the R,γ PO3 groups, while the other R,γ PO3 group was coordinated in a bidentate fashion. Rapid interconversion of the R,γ PO3 groups between mono- and bidentate coordination occurs in solution. Such complexation again catalyzes solution hydrolysis of phosphates, and again P-O bond activation is thought to be due to reduction in the charge density upon forming the complex. A further catalytic effect is felt when one of the coordinating water molecules deprotonates.46 The resulting coordinated OHis a more effective nucleophile than its noncoordinated counterpart. Internal nucleophilic attack by a coordinated hydroxide ion was also shown by Norman and Cornelius47 to greatly catalyze the hydrolysis of co-coordinated tripolyphosphate. In this case, a cobalt complex functioned as a template to position phosphate and nucleophile favorably to one another. The pH dependence of the catalysis demonstrated that the aquo ligand deprotonated prior to the nucleophilic attack. A similar finding was made by Haight et al.48 in which one cobalt(III) complex (39) Van Wazer, J. R.; Griffith, E. J.; McCullough, J. F. J. Am. Chem. Soc. 1952, 74, 4977-4978. (40) Van Wazer, J. R.; Griffith, E. J.; McCullough, J. F. J. Am. Chem. Soc. 1955, 77, 287-291. (41) Williard, J. W.; Sullivan, J. M.; Kim, Y. K. J. Chem. Eng. Data 1984, 29, 290-293. (42) Zinder, B.; Hertz, J.; Oswald, H. R. Water Res. 1984, 18, 509512. (43) Kura, G. Bull. Chem. Soc. Jpn. 1987, 60, 2857-2860. (44) Kura, G. J. Chromatogr. 1988, 447, 91-101. (45) Nieuwenhuizen, M. S.; Peters, J. A.; Sinnema, A.; Kieboom, A. P. G.; van Bekkum, H. J. Am. Chem. Soc. 1985, 107, 12-16. (46) Huskens, J.; Kennedy, A. D.; van Bekkum, H.; Peters, J. A. J. Am. Chem. Soc. 1995, 117, 375-382. (47) Norman, P. R.; Cornelius, R. D. J. Am. Chem. Soc. 1982, 104, 2356-2361. (48) Haight, G. P., Jr.; Hambley, T. W.; Hendry, P.; Lawrance, G. A.; Sargeson, A. M. J. Chem. Soc., Chem Commun. 1985, 488-491.
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involves all three phosphate groups (R,β,γ) of TPP. A second cobalt species containing a coordinated hydroxide ion binds at the R phosphate, thus neutralizing the charge at the phosphorus center and encouraging intramolecular nucleophilic attack. Hydrolysis of adenosine triphosphate analogues is also accelerated by the binding of cations. Binding of single divalent cations to the central (β) phosphate of the triphosphate was proposed by Cooperman49 to negate the negative charge and activate the site to attack by hydroxide ion. Binding by a second cation shifts the first cation into coordination with both the R and β phosphate groups. The incoming cation then coordinates with the more basic, terminal γ phosphate, again labilizing it toward nucleophilic attack by OH- or H2O.50 Tetas and Lowenstein51 proposed that the original metal binding would occur at R,β or γ,β for bidentate complexes and at the γ phosphate for monodentate complexes because of the higher charge density on the terminal phosphate. Again, hydroxo metal (M) complexes, e.g. M(ATP)(OH), were postulated,50 and intramolecular attack by the metal bound OH- group at the γ - phosphorus was proposed as part of the metal catalyzed hydrolysis. In the solid state, it is reasonable to assume that the counterions to TPP are directly coordinated to the tripolyphosphate oxygens, though coordination through waters of hydration may well also occur. Evidence for this coordination in the case of monovalent cations such as Li and Na is available from solid-state NMR52 and X-ray diffraction.53 ESCA studies of the calcium tripolyphosphate precipitate have provided evidence of possible tridentate coordination of metal cations via two terminal oxygens and the oxygen from the bridging phosphorus.54 In a study by Prodan and Ol’shevskya,36 the solid-state hydrolysis of the mixed monovalent/divalent salts MIM2II(P3O10)‚xH2O was found to be more facile than that of the respective MI3MII(P3O10)‚yH2O salts. The former salts also crystallize with more difficulty, and their infrared spectra indicate an enhanced association of the MII cation with the β phosphate in the tripolyphosphate chain, labilizing that phosphorus to nucleophilic attack. It was hypothesized that π-bond delocalization in tripolyphosphate stabilizes this anion and that divalent cations such as Co2+, Ni2+, and especially Cu2+ decrease this delocalization and so the stability. Differentiation of 2:1 and 8:1 Precipitates. The 2:1 and 8:1 precipitates differ in several ways which may also help to explain their differing tendency toward hydrolysis. The higher degree of supersaturation found in the 8:1 wash liquor leads to more rapid and imperfect crystallization, a higher surface area, and the inclusion of impurities from the buffer employed. The 2:1 precipitates in a more crystalline morphology but has a different composition, closer to NaCa2P3O10 than to the Ca5(P3O10)2 of the 8:1 material. Many of the differences observed between the 8:1 and 2:1 precipitates (spectroscopic and otherwise) can partially be explained by this differing degree of crystallinity between them.55 The fine structure (49) Cooperman, B. S. Biochemistry 1969, 8, 5005-5010. (50) Sigel, H.; Amsler, P. E. J. Am. Chem. Soc. 1976, 98, 7390-7400. (51) Tetas, M.; Lowenstein, J. M. Biochemistry 1963, 2, 350-357. (52) Ramasamy, R.; Mota de Freitas, D.; Geraldes, F. G. C.; Peters, J. A. Inorg. Chem. 1991, 30, 3188-3191. (53) Mootz, D.; Altenburg, H. Acta Crystallogr. 1969, B25, 10771089. (54) Rizkalla, E. N.; Antonious, M. S.; Anis, S. S. Inorgan. Chim. Acta 1985, 96, 171-178. (55) Stutman, J. M.; Termine, J. D.; Posner, A. S. Trans. N. Y. Acad. Sci. 1965, 27, 669-675.
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in the IR peak at 1130 cm-1 is one such example,56 as are the solid-state 31P NMR spectra which are sharp for the 2:1 but broad for the 8:1 precipitate (not shown).57 The relative water content of the precipitates has not been completely characterized. The thermally liable nature of the precipitates, especially the 8:1 species, has prevented the unequivocal differentiation between bound and surface water. From the results at hand, it appears that the bound water levels after the overnight drying at 4 °C are higher for the 8:1 than for the 2:1 species. However, the 4:1 and 8:1 precipitates have about the same water contents. Thus, the water level of the precipitate does not, alone, appear to be the dominant factor in explaining the differing propensity of the precipitates to undergo solidstate hydrolysis. Another difference which seems significant in light of the varying susceptibility toward hydrolysis between the 2:1 and 8:1 precipitates is the ν(O-H) portion of the water IR spectrum shown in Figure 5. The water peak is broad for both samples but is more symmetrical in the case of the 8:1 precipitate, with the suggestion of a shoulder at around 3250 cm-1, while being distinctly skewed toward high wavenumber for the 2:1 precipitate. Assuming that there are two or more kinds of crystal waters with different locations in the local lattice, both the 2:1 and 8:1 precipitates have in common a portion of their crystal water which has an adsorption centered at around 3450 cm-1. However, the 8:1 precipitate has a larger portion of its crystal water adsorbing in the vicinity of 3250 cm-1. This structurally different water adsorbing at a lower wavenumber could be bridging or nonbonding water molecules. Grodzicki and Piszczek58 observed that higher hydrates often have a portion of their crystal water uninvolved with the metal ion coordination sphere, but rather acting to bridge water, which is so coordinated, with the respective anion. This water may play a role in the hydrolysis via some enhanced mobility or it might also function by happening to sit in a critical position with respect to the TPP moleculesso that it is well-suited to take part in the hydrolysis reaction. The above difference in crystal habit between the 2:1 and 8:1 precipitates may directly account for the subordinance and dominance, respectively, of this type of crystal water. Direct evidence for there being at least two types of crystal water in the 8:1 material was sought by preparing the respective deuterates and watching the D2O exchange back to H2O. 2:1 and 8:1 precipitates were made on small scales using 100% D2O solvent, dehydrated borax buffer, and anhydrous CaCl2 as one of the solutes. The only significant source of H2O was the sodium tripolyphosphate hexahydrate. The precipitates were dried as above, only under a N2 blanket, and were exposed to air only briefly before being examined by IR spectroscopy. A D2O (deuterate) peak is visible in both samples, most notably as the ν(O-D) stretch at about 2500 cm-1. Despite the precautions taken, considerable back-exchange with H2O is already found, as shown in Figure 7a (8:1) and Figure 7b (2:1). A full 30 min exposure to ambient air was sufficient to fully exchange H2O for D2O. Examination of the 2:1 deuterate shows that the ν(O-D) stretch has a comparable peak shape and width to that of the exchanged hydrate (ν(O-H) stretch)ssignifying that there is either only one type of crystal water or, if there is more than one, that they exchange at comparable rates. The 8:1 deuterate, (56) Figlerowicz, M.; Utzig, E.; Alejska, M.; Bratek-Wiewio’rowska, M. D.; Wiewio’rowski, M. J. Mol. Struct. 1997, 416, 197-208. (57) Rodrigues, A.; Lebugle, A. Colloids Surf. A 1998, 145, 191204. (58) Grodzicki, A.; Piszczek, P. Pol. J. Chem. 1996, 70, 620-631
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Figure 7. ν(O-H) portion of the infrared spectrum of the CaTPP precipitates prepared as the deuterates but in which partial exchange back to the respective hydrates has occurred: (a, top) 8:1 material; (b, bottom) 2:1 material.
however, shows a distinctly narrower ν(O-D) stretch than ν(O-H) and has a shape reminiscent of the 2:1 material, missing the shoulder seen in the 8:1 hydrate. It is tempting to ascribe the missing shoulder to a second type of crystal water species in the 8:1 material, which is so mobile that it exchanges too rapidly to be observed as the deuterate. The shapes of these ν(O-D) stretches do not change in either the 8:1 or 2:1 cases as the exchange goes on to completion. A related piece of evidence for there being distinctly different species of bound water comes from some preliminary solid-state 1H NMR relaxation studies (Bruker DMX 400 spectrometer, 4 kHz spinning rate). The water line is reasonably broad for both the 2:1 and the 8:1 precipitates but the spin-echoes from a Carr-PurcellMeiboom-Gill T2 pulse sequence show a possibly significant difference. At a delay time of 1.94 ms between pulses, Figure 8 shows that the 8:1 precipitate (8a) apparently contains two nonequivalent water species. Although separate relaxation times for these two species could not be determined, it was qualitatively observed that the species responsible for the shoulder in Figure 8a relaxed more quickly while the species responsible for the downfield peak relaxed on a comparable time scale to that of the 2:1 sample. The more rapid relaxation is in keeping with the suspected higher mobility of this structurally different water in the 8:1 precipitate. Analysis of Hydrolysis Kinetics. The fair degree of adherence to first-order kinetics shown in Figure 6 is in agreement with the kinetics of NaTPP hydrolysis in solution17,39,40 and in the solid phase.27 A more rigorous test is to plot the natural logarithm of [-loge(X/X0)] versus that of time, i.e., to assume a variable order rate law of the form loge(X/X0) ) -ktn, where a slope of unity would then be expected. At the three temperatures studied, the plots (shown in Figure 9) are again linear but have a slope of 0.7 ( 0.05. This result would suggest a more complex mechanism, perhaps a mixture of first-order kinetics (n ) 1) and a diffusion controlled reaction (n ) 0.5).36 Generalized diffusion controlled mechanisms arise when a reactant must diffuse through a layer of reaction product or inert media to reach co-reactant. One scenario possibly applicable to the present case is that the diffusing species
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Figure 10. Diagrammatic representation of the nucleophilic attack by water at the phosphorus center.
Figure 8. Residual solid-state 1H NMR magnetization following a T2 pulse sequence. The delay time shown is 1.94 ms for: (a, top) the 8:1 precipitate and (b, bottom) the 2:1 precipitate.
Figure 9. Variable-order kinetic plot of the data from Figure 6 (symbols have the same meanings as those assigned in Figure 6).
is a more mobile type of crystal water within the precipitated solid. This reactant could diffuse through the solid, perhaps via defects, to reach hydrolyzable TPP. Conceivably, not all TPP sites in the solid may be equally reactive and such unreactive zones and previously hydrolyzed TPP would constitute the diffusion barriers. Conjecture. In analyzing the facile hydrolysis of the 8:1 versus the 2:1 precipitate, several factors have been discussed as being pertinent. The difference in supersaturation conditions under which the precipitation occurred should be considered the most crucial. It is also noteworthy that the 4:1 precipitate was intermediate between the 8:1 and the 2:1 materials in terms of the level of crystallinity but had a composition (Ca/P ratio and water
content) almost identical to that of the 8:1 precipitate. Yet its IR spectra and tendency toward hydrolysis mirrored the 2:1 precipitate. The high reactivity of the 8:1 precipitate can arise in a number of ways, assuming that it occurs by solid-state hydrolysis via bound crystal water. In passing, we acknowledge making the assumption that the attacking nucleophile is water and not coordinated hydroxide anion (OH-). The absence of any OH- peaks in the IR spectrum and the fact that the existence of an OH- ligand is not required for electroneutrality leads to this assumption. Further, we seek to attribute the reactivity difference to a difference in the local structure within the solid and not with the acknowledged difference in the surface morphology. The activation of a P-O-P bond in the 8:1 species to hydrolysis could occur due to the larger number of Ca2+ cations bound per phosphate backbone in this solid. From the change in the PO2 stretch in the IR, we conjecture, following Prodan and Ol’shevskaya,36 that the electron density around this central phosphorus has been distorted by a more asymmetric association of a Ca2+ ion with this phosphatesperhaps via association of a Ca2+ ion with a single TPP chain. This type of coordination would lower the electron density around the central P atom, encouraging nucleophilic attack by water, as sketched in Figure 10. Such a conjecture is supported by the NMR studies of Ramasamy et al.52 who noted that lanthanide(III) ions bind symmetrically, at low Ln/TPP ratio, with two TPP entities in the first coordination sphere of Ln. Since Ln(III) is sometimes said to complex ligands similarly to Ca(II), it seems reasonable that higher multivalent cation/TPP ratios would give rise to a less symmetric association. Calcium could contribute to the facile hydrolysis of the 8:1 precipitate in a second way. The dual coordination of a water of hydration and the phosphate to a single calcium cation could serve to optimally orient the activated phosphorus center to the water nucleophile (i.e. templating action). Yet a third factor to consider is the difference found between the water of hydration in the two species. IR and solid-state 1H NMR seem to distinguish a more mobile water component associated with the 8:1 species. The role of water mobility may come into play to the extent that the hydrolysis reaction has some diffusion component. Since such a component has been demonstrated experimentally, it is postulated that there is a loosely coordinated water species present in the 8:1 matrix, presumably as a consequence of the less regular nature of its lattice. This nucleophilic species might be thought to diffuse through the lattice to activated phosphorus sites. All three explanations follow from the higher degree of supersaturation present when the 8:1 precipitation occurs, and the current data do not allow selecting any one factor as dominating. However, consideration of the intermediate 4:1 precipitate may slightly favor the water mobility explanation since this material has the same calcium and water levels as does the 8:1 material yet does not show the mobile water species in the IR nor the instability
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toward hydrolysissfeatures which can perhaps be linked to the lower degree of supersaturation at which this material precipitates. Acknowledgment. The authors wish to thank Dr. Gordon Welch, of Unilever Research, Port Sunlight, for
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valuable discussions which lead to the initiation of this work. Dr. Leon Van Gorkom and Ms. Sibel Alkan, of Unilever Research, U.S., are also thanked for their help with the NMR measurements. LA990977L
