J. Phys. Chem. lQ83, 87, 4699-4703
significant charge transfer from the radical. This is confirmed by the large scalar contribution to the 7Litransverse relaxation rate from 143 coupling. The observed 7Li contact shifts are 1order of magnitude higher in CH3CN than in THF, which confirms a weaker interaction of Li+ with CH3CN. The 'Li contact shifts in CH,CN decrease as the temperature increases, because a contact shift in the Li+-radical complex is proportional to the inverse absolute temperature (Curie law dependence) and because the mole fraction of the complex decreases with the temperature increase (AHf = -3.3 f 0.8 kJ mol-'). In THF the mole fraction of the Li+-radical complex increases with the temperature increase (AHf = 4.6 f 0.2 kJ mol-'); in this
4699
case the experimental contact shifts increase as temperature increases. The combination of data of this study enables one to place the nitroxide radical between CH3CN and THF in its ability to solvate the Li+ cation in the types of systems addressed in this study. Acknowledgment. We gratefully acknowledge use of the Colorado State University Regional NMR Center, funded by National Science Foundation Grant No. CHE78-18581. Registry No. Lithium-7,13982-05-3;oxygen-17,13968-48-4; chlorine-35, 13981-72-1;acetonitrile, 75-05-8;nitromethane, 7552-5; tetrahydrofuran, 109-99-9;lithium, 7439-93-2.
Crystal Growth in Aqueous Solution at Elevated Temperatures. Barlum Sulfate Growth Kinetics 0. L. Gardner and 0. H. Nancollas' chemistry Department, State University of New York at Buffalo, Buffalo, New York 14214 (Recelved:January 3, 1983)
The solubility and seeded crystallization of barium sulfate (barite) has been studied at 105-150 "C in aqueous sodium chloride solutions at ionic strengths of 0.1-1.0 M. The rate of growth was proportional to the square of the supersaturation, in agreement with the results previously established at ambient temperatures. The formation of barium sulfate crystals is controlled by a surface reaction and is independent of fluid dynamics at the solid-liquid interface. Over the temperature range 25-125 "C, the apparent activation energy for growth is 33.5 f 4.0 kJ mol-'. Added strontium ions, at concentrations similar to those found in natural brine solutions, decreased the reaction rate and lowered the apparent solubility of the grown crystalline phase. The significant decrease in the measured growth rate at 125 "C in the presence of phosphonates indicates that these molecules are effective barium sulfate scale inhibitors in petroleum and geothermal brines.
Introduction Barite along with calcium carbonate, silica, and calcium sulfate are the principal minerals found in scale deposits. Barium sulfate formation as scale has been a serious problem in oil and gas wells as well as in higher temperature geothermal With the exception of solubility determinations, little is known regarding barite formation at temperatures above about 100 "C. The kinetics of crystal growth of barium sulfate has been extensively studied at ambient temperatures and pressures. Nucleation studies have shown the kinetics to be homogeneous at supersaturations above a critical value while at lower concentrations the mechanism appeared to be heterogeneous since the particle number was constant and dependent only on the number of foreign particles in sol ~ t i o n . " ~Data on nucleation and growth, available pri~~
~
(1)Vetter, 0.J. G. J.Pet. Technol. 1975,1515. ( 2 ) Gevecker, J. R. Proc. Annu. Southwest. Pet. Short Course 1976, 23, 117. (3)Mitchell, D.M.; Grist, D. M.; Boyle, M. J., presented at the 1979 Society of Petroleum Engineers International Symposium on Oilfield and Geothermal Chemistry, Houston, TX, Jan 1979,185. (4) Kharaka, Y. K.; Lico, M. S.; Carothers, W. W. J. Pet. Technol. 1 - -9-m -,.-x-g- .. (5)Walton, A. G.; Hlabse, T. Talanta 1963,10,601. (6)Nielsen, A. E. Acta Chem. Scand. 1958,12,11. (7)Cobbett, W. G.; French, C. M. Discuss. Faraday SOC.1954,No. 18, 113. 0022-3654/83/2087-4699$0 1.50/0
marily from experiments involving the heterogeneous generation of barite crystals, are useful for understanding the dependence of crystal size and morphology on important solution parameters including supersaturation, lattice ion molar ratio, pH, and the effects of added metal ions and growth inhibitors. In spontaneous precipitation experiments, following heterogeneous nucleation, the initial rate of growth of barite is proportional to the first power of the solution supersaturation. At later stages the effective order of the reaction is 2-4 at high and low levels of supersaturation, respectively. The linear dependence on concentration suggests that dfiusion of ions to the growing crystal surface may be important at high supersaturation while, at conditions close to equilibrium, surface reactions are rate limiting.g Kinetic data at 25 "C on the growth of barium sulfate seed crystals indicate an initial secondary nucleation process followed by a surface-controlled reaction in which the rate is proportional to the square of the supersaturation (S)l0-l2 expressed as (TBa - TOBe). Tbe and ToBaare the (8) Liteanu, C.; Lingrew, H. Reu. Anal. Chem. 1976,3,108.
(9)Nielsen, A. E. In "Industrial Crystallization 78";de Jong, E. J., Jancic, S. J. Eds.; North-Holland Publishing, Co.: New York, 1979;p 159. (10)Nancollas, G. H.; Purdie, N. Tram. Faraday SOC.1963,59,735. (11)Nancollas, G. H.; Liu, S. T. SOC.Pet. Eng. J. 1975,509. (12)Liu, S.T.;Nancollas, G. H.; Gasiecki, E. A. J. Cryst. Growth 1976, 33,11.
0 1983 American Chemical Society
4700
The Journal of Physical Chemistry, Vol. 87,No. 23, 1983
Gardner and Nancollas
TABLE I: Crystal G r o w t h of Barium Sulfate' expt no. 9 5 4 7 20 a
104~,,, 1.690 1.780 1.758 3.764 1.466
M
0
ionic strength, M
0.786 0.884 0.831 0.587 0.466
0.20 0.20 0.20 1.0 0.20
Equivalent barium a n d sulfate concentrations.
10-~k,,b
temp, "C
seed concn, m g L-'
min-' m - z
105 125 125 125 150
37.1 42.7 92.7 44.8 26.4
6.33 7.69 7.29 5.29 3.42
L2mol-'
R a t e constant ( e q 1).
molar concentrations of barium sulfate at time t and at equilibrium, respectively. At high relative supersaturation, u (+/ToBa = 2.6-4.3), a two-dimensional surface nucleation growth model has been proposed to explain the seeded-growth re~u1ts.l~It is interesting to note that dissolution of barite, in contrast to many other systems, follows a surface rate-limiting mechanism with a rate proportional to the square of the solution undersaturation.11J4 A number of added metal ions and organic inhibitor molecules have been found to influence both the growth rate and the apparent solubility of barite in aqueous solution. Metal ions present in brine such as Sr, Cu, and Pb are known to incorporate into the growing barite phase and, in certain cases, from a series of continuous solid solution^.'^-'^ Scale inhibitors are effective at ambient temperatures,18J9and there has been considerable recent interest in studying the role of mineral precipitation and inhibition in the formation of scale deposits a t the high temperatures of geothermal brines.20v21The Los Alamos Scientific Laboratory has been involved in extensive studies since 1971 involving computer modeling of geothermal systems22as well as conducting field experiments on the potential for geothermal energy development from hot, dry rock locations.23 In the present work, the mineralization of barium sulfate has been studied by using a seeded-growthmethod at 105-150 "C both in pure solution and in the presence of added metal ions and growth inhibitors.
ion concentration was determined by ion exchange on a strong acid resin (Dowex 50). Seed crystals of barium sulfate were prepared at low supersaturation by the M sulfuric acid and barium dropwise addition of 3 X chloride solutions into 1 L of distilled water, stirred, and maintained at 80 "C over a period of 24 h. The crystals were filtered with a Millipore filter (0.22-pm filters) which was throughly prewashed to remove any surfactants prior to use. Scanning electron microscopic analysis (IS1model 11) of the barite crystals showed regular rhomobohedrons of diameter 4-12 pm. The mean particle size (Particle Data Inc. particle counter computerized ELZONE system Model I11 LTSCD/ADC with 76-pm orifice tube) was 9.5 pm and the specific surface area (Quantasorb 11, Quantachrome, Greenvale, NY) 0.45 m2 g-l. The crystals were aged a t -25 "C in a saturated solution of barium sulfate for 2 weeks and were then dried over P205. Strontium solutions were prepared from reagent-grade strontium chloride and were analyzed by EDTA titrations. Nitrilotrimethylenephosphonate (NTMP) was obtained from Monsanto Chemicals (Dequest 2001); the pH of the growth solutions containing NTMP was adjusted to 6.5 with sodium hydroxide. The stainless steel autoclave used in the early stages of this study was similar to that described p r e v i ~ u s l y . It ~~,~~ incorporated a Teflon-coated l-L vessel and internal wetted surfaces were coated (3-6-pm thickness) with either poly(tetrafluoroethy1ene) (PTFE) or fluoroethylpropylene (FEP). However, after three to four experiments (125-150 "C) blistering and subsequent cracking exposed the underlying steel surfaces to the aqueous phase. Subsequently, Experimental Section a The autoclave vessels have been described p r e v i ~ u s l y . ~ ~ ~ ~2-L titanium autoclave (Parr Instrument Co., Model 2501) was used. It incorporated slurry and filtered liquid Stock solutions used for seed preparation and for cryssampling tubes together with a seed addition tube enabling tallization experiments were prepared from reagent-grade seed crystals to be introduced by nitrogen pressurization.26 barium nitrate and sodium sulfate using triply distilled A Teflon lid and liner provided additional protection water. Analysis for barium ions was carried out by using against metal corrosion at temperatures > 225 "C. TitaEDTA titrations (metal phthalein indicator). The sodium nium sampling tubes were covered with heat-shrunk Teflon tubing and the remaining internal titanium parts were also covered with Teflon tape. Filtration through the (13)Van Rosmalen, G. M.; van Leeden, M. C.; Gouman, J. K h t . Tech. liquid sampling tube was carried out by using either an 1980,15,1213. (14)Bovington, C.H.; Jones, A. L. Tram. Faraday SOC.1970,66,764. extrafine fritted glass filter or, at higher temperatures, a (15)Cowan. J. C.:Weintritt. D. J. 'Water-Formed Scale Deuosits": Teflon cylinder and membrane (ZITEX, extrafine; ChemGulf Publishing Co.:' Houston, TX 1976. plast Inc.). The autoclave pressure was that of water vapor (16)Leach, 0. L. Econ. Geol. 1980,75,1168. (17)Gordon, L.; Reimer, C. C.; Burit, P. B. Anal. Chem. 1954,26,842. at the stated temperatures. (18)Liu, S.T.; Nancollas, G. H. J.Colloid Interface Sci. 1975,52,582. In the crystal growth experiments supersaturated solu(19)Leung, W. H.;Nancollas, G. H. J. Cryst. Growth 1978,44, 163. tions were prepared by the addition of calculated volumes (20)Feber, R. C.; Holley, C. E., Jr. 'Proceedings of Workshop on Scale of barium chloride and sodium sulfate solutions to 850 mL Control in Geothermal Energy Extraction Systems, Los Alamos, NM, Oct 1977,7664-C". of the medium solution of known concentration. Following (21)Feber, R. C.,presented at the Workshop on Scale Control in temperature equilibration (2-4 h) the dry barium sulfate Geothermal Systems, Los Alamos, NM, April 1980. seed crystals (30-100 mg) were introduced into the su(22)Merson, T. J. Oct. 1977, "Proceedings of Workshop on Scale Control in Geothermal Systems, Los Alamos, NM, Oct., 1977",L.A.persaturated solution through the seed addition tube. 7664-C,p 66. During the reaction, 5-mL filtered samples were withdrawn (23)Pettitt, R. A. In "Proceedings of Conference on Water for Energy at known times and analyzed for barium. A wash sample Development"; Pacific Grove, CA, Dec 1976;Karodi, G. M., Krizek, R. M., Eds.; 1978;p 151. (24)Nancollas, G. H.; Reddy, M. M.; Tsai, F. J.Phys. E 1972,5,1186. (25)Nancollas, G.H.; Reddy, M. M.; Tsai, F. J. Cryst. Growth 1973, 20,125.
(26)Chieng, P. C. Ph.D. Thesis, State University of New York at Buffalo, Buffalo, NY, 1982.
The Journal of Physical Chemistry, Vol. 87,No. 23, 1983
Barium Sulfate Growth Kinetics
4701
TABLE 11: Barium Sulfate Thermodynamic Solubility Products, pK,, (Molal Units) temp, "C ref 29 30 31 32 33 27
18 10.06
25
35
50
60
65
75
80
95
9.63 9.98 9.67 9.96 9.70 9.66 9.62 9.59 9.96 9.83 9.71 9.49 9.44 9.72 9.70 9.42
125 150 175
100
200
225
250
275
300
9.59 9.60 9.73 9.94 10.25 10.53 10.80 11.12 11.44 9.58 9.45 9.55 9.70 9.90 10.15 10.45 10.90 11.45 12.15 9.22 9.34 9.67 10.34 11.05 9.50 9.40 9.47 9.60 9.83
9.68
11 34 35 36 37 38 39
10.03 10.06 9.97 9.87 9.99 9.77
(105) 9.24 9.30 9.39
this work
8.0 T
n 1.6
/
61
0
TIME 0
20
40
$0"
in)
Flgure 1. Barium sulfate growth at 125 "C in 0.2 M sodium chloride 5. solutions. Experiments: (0)4 and (0)
(approximately 5 mL) was removed prior to removal of sample for analysis. Typical experiments were made over 24-72-h periods. A sample of the crystal slurry was withdrawn prior to the conclusion of each experiment and filtered, and the dried solid was examined in the scanning electron microscope.
Results Crystal growth experiments are summarized in Table I; growth curves of total barium concentration vs. time are shown in Figure 1. Solubility values were determined from growth runs that were allowed to proceed to equilibrium over a period of 3-70 h. The experimentally determined thermodynamic solubility products at 105 (5.71 X 10-lom2), 125 (4.955 X 1W0 m2),and 150 (4.088 X 1@l0m2)"C agreed with those found by Strube12' and the calculated values reported by H e l g e s ~ nsummarized ~~ in Table 11. The (27) Strubel, G. N.; Jahrbuch, F. Mineralogie Monulshefte 1967,223. (28) Helgeson, H. C. Am. J. Sci. 1969,116, 321. (29) Blount, C. W. Amer. Mineral. 1977,62,942. (30) Melcher, A. C. J. Am. Chem. SOC.1910,32,50. (31) Templeton, C. C. J. Chem. Eng. Data 1960, 5 , 514. (32) Malinen, S. D.; Uchameyshvilli, N. Ye.; Khitarov, N. I. Geokhimiya 1969,927. (33) Helgeson, H. C. Am. J. Sci. 1969, 267, 729. (34) Kohbrausch, F. Z . Phys. Chem. 1908,64, 129. (35) Latimer, W. M.; Hicks, J. F. G., Schutz, P. W. J. Chem. Phys. 1933, 1, 620.
IO
20
30
40
50
Figure 2. Plot of the integrated form of eq 1. Experiments: (0) 4 and (0) 5.
solubility at 125 "C in 1.0 M sodium chloride (2.19 x M) is about 5% higher than that r e p ~ r t e d . ~From ' the measured solubility values at 150 "C for solutions of ionic strength 0.14.2 M, the concentration of the BaSO, ion pair was negligible. The rate of barium sulfate growth, R , can be described by eq 1, where k , is the growth rate constant and s is a
-R = dTB,/dt = kpS"
(1)
function of the available surface area for crystal growth. Analysis of the growth curves in Figure 1 gave a value of n = 2, as can be seen from the linear plots of the integrated form of eq 1 shown in Figure 2. Values of the rate constant, k,, were independent of the surface area, s, as is seen from the results of experiments 4 and 5 in Table I. In contrast to the results at lower temperatures, at 150 "C in solutions with initial relative supersaturation, u = 0.64 [I = 0.2 M (NaCl)] and 1.56[1 = 0.1 M (NaCl)], seeded growth is preceded by the spontaneous nucleation of barium sulfate. However, following seed addition, analysis of the subsequent growth curve also indicated a secondorder dependence on supersaturation. The morphology of the crystals grown at 125 "C indicated a gradual change from the initial regular rhombic habit to slightly larger (36) Cphen, E.; Blekkingh, J. J. A. Z . Phys. Chem. 1940, 186, 257. (37) SillBn, L. G.; Martell, A. E. Spec. Pub2.-Chem. SOC.1964, No. 17. (38) Rosseinsky, D. R. Trans. Faraday SOC.1968, 54, 116. (39) Selivanova, N. M.; Kapustinkskii, A. F. Zh. Fiz. Khim. 1953,27, 565.
4702
The Journal of Physical Chemistry, Vol. 87,No. 23, 1983
Gardner and Nancollas
TABLE 111: Crystal Growth of Barium Sulfate at 1 2 5 "C in 0.2 M NaCl and the Influence of Strontium Ion
' Rate
expt no.
1 0 4 ~ (initial), M
4 5 14 15
1.758 1.780 1.742 1.666
~
after 6 0 h
barite seed concn, mg L"
10-4kg: L2 mol-' min-' m-'
0.975 0.950 0.700 0.713
92.7 42.7 36.6 44.0
7.29 7.69 8.58 8.38
1 0 4 ~ 0 BMa ,
~ 1 0 5 M~ ~ ~ after ~ 5h
0.00 0.00 1.00 10.0
0.953 0.960 1.020 1.007
constant (eq 1 ) .
TABLE IV: Crystal Growth of Barium Sulfate and the Influence of NTMP 10-3kg,4
Lz mol-
--
expt
104T,,, M
lO'[NTMP], M
T, "C
5 17 16 A0 A3 R3
1.779 1.631 1.804 0.197 0.197 0.196
0
125 125 125 25 25 25
Rate constant (eq 1).
2.0 10.1
0 1.9 11.2
U
min-' m - z
0.88 0.70 0.88 0.88 0.88
76.9 15.8 2.2 2.3 0.4 0.04
0.86
k,'/kgb
1 0.20 0.03 1
0.18 0.02
k g ' , rate constant for NTMP experiments; experiments AO, A 3 , R3; ionic strength - 1 . 2 x
M (ref 19, Table I).
crystals having rounded and less distinct edges and surfaces. Growth layers were evident on the surfaces that were initially smooth and featureless. In the field, strontium concentrations of (0.1-5) X M are typically found in brine solutions that are supersaturated with respect to barite. The influence of added S P on barium sulfate crystallization at 125 "C in solutions of 0.2 M NaCl is shown in Table I11 and Figure 3. The effective value of n = 2 in eq 1 and the equilibration times are similar to those for growth in the absence of strontium. For experiments 14 and 15, where strontium chloride was added to the supersaturated solution before the introduction of barite seed crystals, the concentration of barium ion after an initial 4-5-h period was about 5% higher than the value for the experiments (4 and 5) in the absence of added Sr2+. This suggests that strontium ions were incorporated into the growing crystal surface by substituting for barium lattice ions. Growth data in the presence of strontium over extended times, plotted in Figure 4, show the striking influence of the added cation. In the absence of strontium, the concentration of barium ion decreased rapidly immediately upon the introduction of seed crystals and reached a steady equilibrium value within 2-4 h. In the presence of strontium ion, however, the initial rapid decrease in barium concentration was followed by a much slower reduction over a time period of 50-60 h. The growth rate of this (Ba,Sr)S04solid phase was thus considerably slower than that of pure barite. The final equilibrated phase had an effective solubility which was approximately 30% lower than that for barium sulfate (Table 111). An analysis of the kinetic data using the integrated form of the rate equation (eq 1) indicated that, during the initial rapid growth of barium sulfate seed, the rate was again proportional to the square of the supersaturation. The rate constants, k,, calculated for the growth experiments in the presence of added Sr2+(Table 111)are close to those found for growth in pure barium sulfate solutions. The shape of the growth curve (Figure 3), the initial and final equilibration periods (Figures 3 and 4; Table 111), and the calculated rate constant (Table 111) are virtually independent of the presence of strontium ion in the concentration range 10-5-10-4 M. Scanning microscopic analysis of the grown (Ba,Sr)S04 crystals indicated an almost complete change from the regular rhombic barite to irregular aggregates of 5-10 pm in length, primarily com-
1'21 I .o
I
I
I
I
I
I
0
IO
20
30
40
50
Figure 3. Seeded gowth of barium sulfate in the presence of strontium 4. ion. Experiments: (0)15, (A)5, and (0)
1.2
0.6I
I
T I ME( hrs)
I
I
The Journal of Physical Chemistry, Vol. 87, No. 23, 1983 4703
Barlum Sulfate Growth Kinetics
Thx IO4 M
i
1.0
TI ME (mid
0
1
I
IO
20
I
30
I
40
' hrkh 40
Flgure 5. Influence of NTMP on the seeded growth of barium sulfate 17, and (0)16. at 125 "C. Experlments: (A)5 (0)
of seed crystals are shown in Figure 5. After an initial rate close to that of the uninhibited crystal growth, the crystallization process was markedly reduced at an inhibitor concentration as low as 2.0 X lo-' M. At this inhibitor level (experiment 17) the rate of growth was essentially zero after 20 h while the relative supersaturation, u, still remained at a value of 0.14 (initial u = 0.70). At higher inhibitor concentrations (experiment 16; where [NTMP] = lo4 M), the rate was reduced to zero after only 3 h of reaction, with u remaining at 0.51. The morphology of the barite crystals grown in the presence of NTMP appeared to be similar to the seed but with a slightly greater curvature of the surfaces and edges than was found for the 125 "C crystals grown in the absence of NTMP. The growth kinetics of barium sulfate in the presence of NTMP was found to follow eq 1. Plots of the integrated form of this equation confirmed the proportionality of the rate to the square of the solution supersaturation. The slopes of these integrated rate plots gave the k, and k,' values in the absence and the presence of inhibitor, respectively, in Table IV.
Discussion The initial growth surges that were observed in ambient-temperature growth studies of barium sulfateloJ1are absent at the higher temperatures of the present study. These rapid initial growth rates appear to be due to nucleation at surface defects and their absence may be attributed to the increased growth rates at higher temperatures which compete favorably with any concurrent nucleation processes. Results reported here have demonstrated that the kinetic rate equation (eq 1) found at 25 "C also satisfactorily describes the growth of barite over the temperature range 105-150 "C. The form of the rate equation and the constancy of the calculated activation energy for growth (25-125 "C; 33.5 f 4.0 kJ mol-l) suggest the same surface-controlled crystallization process over a wide range of temperature. Diffusion of ions to the growth surface therfore does not contribute significantly to the seeded growth at these relatively low supersaturations. At high supersaturations, nucleation and flux rates to the scaling surfaces may become important. The formation and solubility of scale-forming minerals in geothermal brine may be markedly affected by the presence of other metal ions which can coprecipitate. Thus, the majority of natural barite samples contain varying amounts of strontium ion and it has been sug-
gested that barium and strontium sulfate form a continuous series of solid solutions. Homogeneous precipitation studied7 have shown that for initial molar Ba/Sr ratios of 1.3-2700, Sr appears to be heterogeneously distributed throughout the solid phase. Strontium concentration levels in brines are usually larger (10-5-10-3 M) than those of barium and thus the influence of Sr2+on the kinetics of barium sulfate as well as the formation of (Ba,Sr)S04 phases may be of considerable importance. The present results indicate the transformation of BaS0, into a (Ba,Sr)S04structure having a significantly lower apparent solubility than pure barite. The amount of strontium incorporated into the barite seed crystal (approximately 5%) is similar to that found in homogeneous precipitation studies.17 The equilibration of the mixed sulfate phase is considerably slower than that found for pure barium sulfate, indicating the importance of a solid-state rather than a surface-solution limiting reaction. Epitaxial studies of strontium sulfate growth on prismatic barite crystals at much higher strontium solution concentrations have indicated the formation of microcrystalline SrSO, at etch pits and step^.^!^^ This is a rapid process occurring over an initial 10-min period and is followed (5-60 h) by a much slower process during which the SrS0, supersaturation is reduced. The first stage corresponds to nucleation of SrS04 microcrystals at growth steps, and the second involves the subsequent growth and thickening of the newly formed crystals. In the present study morphological changes occurring during the reactions preceding final equilibration appeared to confirm a slow transformation of the original barite structure. The influence of typical scale inhibitors in the geothermal temperature range is of particular interest. These compounds can modify the kinetics of both nucleation and growth of scale and, at ambient temperatures, various phosphonic acids, including NTMP, have been found to inhibit the growth of barium ~u1fate.l~ Barium sulfate growth at 125 "C was found to be significantly reduced at NTMP concentrations of 10-'-104 M. Indeed, the growth reaction could be entirely suppressed at high levels of supersaturation with inhibitor levels in this range. Examination of the calculated rate constant ratios, k,'/k, (Table IV), shows that the values at 125 "C are close to those at 25 "C, indicating a similar inhibition mechanism over the whole temperature range. Previous studies of NTMP inhibition at 25 "C have been interpreted by using a Langmuir adsorption in terms of the blocking of active growth sites by inhibitor mole~ules.'~J~ Metal phosphonate coordination has also been suggested as an explanation for mineral growth i n h i b i t i ~ n . ~ ~ ~ ~ ~ Acknowledgment. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this work, and to the Department of Energy for a grant through Los Alamos Scientific Laboratory (LASL). We thank Dr. Roy C. Feber (LASL) for his contributions and comments throughout the period of this study. Registry No. Sr, 7440-24-6; NTMP, 6419-19-8; barium sulfate, 1721-43-1.
(40) Melikhov, I. V.;Rudm, V. N. SOC.Phys. Crystallogr. 1978,23,462. (41) Melikhov, I. V.;Komarov, V. F.; Podoinitsyn, V. A.; Erokhin, V. I.; Nozdran, G. I. SOC.Phys. Crystallogr. 1979, 24,317. (42) van Rosmalen, G.M.; van der Leeden, M. C.; Gouman J. Krist. Tech. 1980, 15, 1213. (43) van Rosmalen, G. M.; van der Leeden, M. C.; Gouman J. Krist. Tech. 1980, 15, 1269.
