Langmuir 1993,9,597400
597
Performance of Inhibitors in Calcium Fluoride Crystal Growth Inhibition Zahid Amjad Specialty Polymers and Chemical Division, Brecksville Research Center, The B.F. Goodrich Company, 9921 Brecksville Road, Brecksville, Ohio 44141 Received June 8, 1992. In Final Form: November 4, 1992
The influence of di- and tricarboxylicacids, poly(acry1ic acid), and glucose in supersaturated solutions of calcium fluoride was investigated under constant composition conditions. The rates of crystal growth measured in the presence of hydroxyl-containing di- and tricarboxylic acids at concentrations as low as 5.0 X 10-7 M were drastically reduced. At high inhibitor concentrations up to 10 X 1W M,although the rates were reduced,the crystallizationdid not stop. Kinetics analysis suggestedLangmuir-typeadsorption of inhibitorson the CaF2 surfacewith a relatively high affinity for the substrate in the concentration range investigated. Inhibitory effects of additives tested (citric (CAI, mdic (MA), succinic (SA),tartaric (TA), and fumaric (FA) acids, poly(acry1ic acid) (PAA),glucose (GL), etc.) tricarballylic (TCA), maleic (ME), are discuseed in tem of acid ionization constanta and the calcium-inhibitor complexationconstant. The order in tem of decreasing effectivenesson the rate of crystal growth of the various additives is as follows: citric acid > tricarballylic acid > malic acid > tartaric acid >> maleic acid > fumaric acid > glucose = control (no inhibitor). Introduction Crystallization inhibitors play an important role in several industrial processes where scaling has to be prevented. For example, calcium carbonate, calcium sulfate, magnesium hydroxide, calcium fluoride,etc. form scale in desalination, and sulfates of alkaline-earth-metal sulfates form scales in oil and gas recovery app1ications.l-4 Precipitation of mineral salts such as calcium carbonate has been reported to result in incrustation of clothes washed with hard waters6 The problemsassociatedwith the undesirableformation of mineral d e s have spurred research in the last decades toward inhibition or prevention of scales on equipment surfaces. Polyelectrolytes which have gained acceptance in industrial water treatment, desalination,and detergent and oil applications include polyacrylates, polymethacrylates, poly(maleic acids), acrylic and maleic acid based copolymers, sulfonated styrene-based copolymers, polyphoephates, and phosphonates. The effect of these polyelectrolyteson scale-formingminerals such as calcium carbonata,calcium sulfate,calciumphosphate,and barium sulfatehae been the subjectof numerous investigations.611 Results of these studies suggest that the influence of polyelectrolytee as crystal growth inhibitors may be explained by considering the adsorption of inhibitor molecules either generally on all crystal faces, reducing the rate of crystallization to zero, or on selective faces, leading to a change in morphology of the developing scale cryst.de. Polymeric type inhibitors, such as poly(acry1ic acid)-baaed copolymer,usually fall into the latter category. (1) Bochner, R.A,; Abdul-Rehman, A.; Nancoh, G.H. J. Chem. SOC. 1984,80,217. (2)Dubin, L.;Fulke, K. E. J. Cool. Tower Inst., in press. (3)Block,J.; Watson, B. Desalination 1976,19,359. (4)Amjad, Z.;Hooley, J. P.J. Colloid Interface Sci. 1986,111,496. (5) Nagarajan, M. K. J. Am. Oil Chem. SOC.1986,62,949. (6) Amjad, 2. Colloids Surf. 1990,48,95. (7)Weijner, M.P. C.; Van h m a l e n , G.M.Desalination 1985,54, 239. (8)Amjad, 2. Can. J. Chem. 1988,66,2188. (9) Amjad, 2.; Mador, W. F. Comion/85, National Aseociation of Corrosion Engineen Annual Meeting, Boston, MA,1985;Paper 357. (10)Imai,T.; Uchida, T.;Aho, S.;T " k i , T. Corrosion/l, National AnnualMeeting, St. Louis, MO,198% Amxiition of Corrmion E~~gineers Paper 423. (11)Zuhl, R.;Amjad, 2.; Masler, W. F. J. Cool. Tower Inst. 1987,8, 41.
Several studies have been reported pertaining to the influenceof di- and tricarboxylicacids on the crystalgrowth of scale-formingsalts. Meyer and Sellinger12in their study on the effect of citrate ion on calcium phosphate phase transitions have shown that citrate ion appears to have only a minimal effect on the rate of amorphous-crystalline transformation but a small, but measurable, stabilizing effect on the amorphous phase at higher concentrations ( 1 2 X lo4 M). Nancollaset al.13investigated the influence of tricarboxylic acids on calcium phosphate precipitation usingthe seeded growth method. Comparisonof the effect of citric acid, isocitric acid, and tricarballylic acid suggest that the hydroxyl group in the molecular backbone plays a key role in the effectiveness of these tricarboxylate ions as inhibitors. Brecevic et al.,14 using spontaneous precipitation, arrived at similar conclusions after studying the effectsof di- and tricarboxylicacids on the precipitation of amorphous calcium phosphate and dicalcium phosphate dihydrate. In earlier papers1"17 it was reported that the crystal growth of calcium fluoride was markedly affected by a trace amount of inhibitors such as polyphosphates, phosphonates, phytic acid, and poly(acry1ic acid) added to the calcium fluoride supersaturated solution. The marked inhibitory activity by these additiveswas explained by a Langmuir adsorption isotherm. In this paper we present results on the effect of di- and tricarboxylicacids, poly(acrylic acid), and glucose on the crystal growth of calcium fluoride at sustained supersaturation. Experimental Section All experiments were performed at 37.0 f 0.1 OC in a thermostated double-walled,water-jacketed Pyrex vessel, volume totaling 250 mL. Reagent grade chemicals and carbon dioxide free distilled water were used in the preparation of the solutions. The preparation of Supersaturated solutions of calcium fluoride was done as described in detail elsewhere.'B Calcium fluoride
(12)Meyer, J. L.;Selinger,A. H.Miner. Electrolyte Metab. 1980,9, 207. (13)Nancollas. G.H.:Tomon. M. B. Faraday Discups. Chem. SOC. 1976,61,2976. . (14)Breceivic,L.;Sendijarevic;Furedi-Milhofer,H.Colloids Surf. 1984, 11.55. (15) Shyu, L. J.; Nancolla~,G. H. Croat. Chem. Acta 1980,59,281. (16)Amjad, 2. Langmuir 1991,7,600. (17)Amjad, 2. Langmuir lQ91,7 , 2405.
Q 1993 American Chemical Society
698 Langmuir, Vol. 9, No.2, 1993
seed crystale were prepared and characterized as described previously.16 The specific surface area of the seed crystals as determined by the Brunauer-Emmett-Teller (BET) method was found to be 3.3 m2 gl. In a typical crystal growth experiment, a metastable supersaturated solution was prepared by mixing calcium chloride and sodium fluoride solution such that the final calcium fluoride concentration would be 0.600 mM. Inhibitors were added after the addition of NaF, but before the addition of calcium chloride, asa dilute solutionin water. The calcium fluoride supersaturated solutions were stirred continuously (-350 rpm) while nitrogen gas, presaturated with water at 37 "C, was bubbled through the solution to exclude carbon dioxide. Satisfactory stability of the supersaturated solution was verified by a constant millivolt reading for at least 30 min. Following the addition of seed crystale to the metastable supersaturated solution,the onset of growth resulted in a lowering of fluoride concentration, which was restored to its preset value by the simultaneous addition of titrant solutions from mechanically coupled 10-mL burets mounted on a modified pH stat (Model600series,BrinkmannInstrumenta, Westbury,NY),using a fluoride combination electrode (Orion Model 900900). The titrant solutions in the two burets consisted of calcium chloride and sodium fluoride. These solutions were prepared such that the additional volume contained the same ratio of calciumchloride to sodium fluoride as the supersaturated solution. They were added at such a rate so as to replace the calcium fluoride which precipitated. In experiments in which the effect of inhibitor was investigated, the inhibitor was introduced both in the working solution and in the titrant so as to avoid dilution due to the addition of titrants during the course of the crystallization reaction. The rates of crystallization (mol of CaF2 crystallized/ min) were calculated from the rates of addition of titrants and corrected for surface area changes.16 Constancy of calcium concentration during the experiments was verified by periodically withdrawing the aliquots and analyzing the filtered (0.22-ccm Millipore filter) samples for calcium as described previously.l8
Results and Discussion The structures of inhibitors and the experimental conditions emploved in this study are summarized in Tables I and 11,respectively. Typical plots of the amount of the calcium fluoride crystallized as a function of time are shown in Figure 1. The rates of crystallization R in Table I1 were calculated from the slopes of linear plots such as those in Figure 1. It can be seen (Table 11, experiments 2,6, and 6) that R,normalized for the initial surface area of each inoculating seed, is constant, confirmingthat crystallizationtakes place on the seed crystals. Crystallization experiments made in the presence of inhibitors are summarized in Table 11. The marked reduction in the rate of crystallization in the presence of tricarballylic acid, TCA, is illustrated by the rate plots in Figure 1. In these experiments,TCA was introduced into the calcium fluoride supersaturated solution prior to the inoculation with seed crystals. It can be seen that TCA at a concentration of 2.0 X lW7 M (experiment 7, Table 11)exhibits no significant inhibitory effect on the crystal growth of CaF2. However, increasing the TCA concentration to 5.0 X lW7 M resulted in 20 % reduction in the rate of crystallization (experiment 9, Table 11). The observed rate increase in the latter stages of the crystallization (Figure 1) in the presence of TCA may be interpreted in terms of the depletion of the solution TCA by adsorption on the surface of growing CaF2 crystals. It is interesting tonote that duringseeded growth of calcium oxalatemonohydrate in the presence of poly(acrylicacid), PAA, Nancollas and SheehanlB reported that crystal growth of CaC20~H20is preceded by an initial slow reaction after which crystallization of CaC204*H20takes
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(18)Sheehan, M.E.;Nancoh, G. H.Inuest. Urol. 1980,17,446.
Amjad Table I. List of Dicarboxylic Acidr, Tricarboxylic Acidr, and Poly(carboxylic ncidr) carboxylic acid structure mol wt abbrev source fumaric acid CHCOOH 116.07 FA Aldrich II HW&H
maleic acid
CHCOOH
116.07
ME
Aldrich
succinic acid
CHCOOH CH2COOH
118.09
SA
Aldrich
134.09
MA
Aldrich
150.09
TA
Aldrich
176.12
TCA
Aldrich
192.13
CA
Fisher
PAA
BFGoodrich
GL
Fisher
II
I
CHzCOOH
malic acid
HOCHCOOH
I
CHpCOOH
tartaric acid
OH
I I HCCOOH I HCCOOH
OH
tricarballylic acid
H2-H
I
HCCOOH
I
HzCCOOH
citric acid
H W O H
I
HOCCOOH
I
HgCOOH
poly(acry1ic acid)
(CH2CHh
I
2100
COOH
Dglucose
CHO
I
180
HCOH
I
HOCH
I
HCOH
I
HCOH
I
CHPH
place at a rate almost independent of the concentration of the PAA. Similar observationshave ale0 been made in the constant composition studies involving the crystal growth of C&04-2H2019 and CaHPO4-2HzOm in the presence of polyelectrolytes. The influence of di- and tricarboxylicacids containing zero, one, and two hydroxyl groups (viz., citric (CA), tricarballylic (TCA), tartaric (TA),malic (MA), succinic (SA), maleic (ME), and fumaric (FA) acids) was ale0 studied by a series of experiments summarized in Table 11. Figure 2 illustrates the plots of newly grown CaF2 on CaF2 seed crystals as a function of time in the presence of 5.0 X 1V M concentration of each inhibitor present initially in the supersaturated solution. As can be seen among the tricarboxylic acids, the acid containing the hydroxylgroup (e.g., citric acid) is a more effectiveinhibitor than the non-hydroxyl-containingacid, TCA. Regarding the performanceof dicarboxylicacids, the acid containing a monohydroxy1 group (e.g., MA) is a more effective inhibitor than the acid containing the dihydroxyl group (e.g.,TA). It isnoteworthythatundersimilarexperimemtal conditions dicarboxylic acids (e.g., FA, ME, SA) which are devoid of hydroxyl group(s) exhibit poor performance in inhibiting CaF2 crystallization from euperaaturated solution. It is interestingtonote that low molecular weight poly(acrylic acid) which does not contain any hydroxyl groupsshowsthe beet performanceas a CaFz crystalgrowth inhibitor. On the basis of the kinetic data (Table II), the order in terms of decreasing effectiveneea for all the inhibitors studied is as follows: PAA > CA > TCA > MA > TA > SA > FA ME = control (no inhibitors).
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(19)Amjad, 2.Can. J. Chem. 1988,66, 1529. (20) Amjad, 2.Langmuir 1989,5,1222.
Langmuir, Vol. 9, No.2, 1993 699
Calcium Fluoride Crystal Growth Inhibition Table 11. Crystallisation of CtaF'; on CoFa Seed Crystals in the Prewnce of Additives'
expt 2 3 5 6 7 9 10
concn (108 M)
inhibitor
rate x 108 (mol min-1 mZ) 68.2 62.7
'
68.lb 64.3c 63.1 51.7 41.1 40.2 26.9 17.2 6.9 50.2 43.0 34.1 20.2 12.8 3.5 46.5 36.1 28.4 38.1 32.6 17.7 60.1 56.4 63.3 56.9 42.5 64.6 54.1 TCA > MA > TA. The effects of tricarboxylic acids (viz., CA, TCA), benzenehexacarboxylicacid (BHCA),trimesic acid (TMA), and poly(acry1ic acid) (PAA) (mol wt 5O00) as hydroxyapatite (HAP)crystal growth inhibitors have been studied
Amjad
600 Langmuir, Vol. 9, No. 2, 1993
by the constant composition technique.21 Results of this study indicate that the effectiveness of various inhibitors at a fired weight concentration of 5.0 X lo* M is in the order PAA >> MA > CA > TCA. It is of interest to note that the results obtained in the present study on CaF2 crystal growth show a similar trend in inhibitor effectiveness as observed in the case of HAP.21 Nancollas et al.,l3 using the pH stat technique, studied the precipitation of calcium phosphate in the presence of 1-(hydr0xyethylidene)-1,l-diphosphonicacid (HEDP), CA, TCA, and pyrophosphate (PYP)and concluded that the two most effective inhibitors were HEDP and PYP, whereas CA and TCA showed moderate to poor effectiveness as inhibitors for calcium phosphate. This is in agreement with the results obtained in the present study for CA and TCA and also with earlier results on CaFz crystal growth in the presence of HEDP, PYP, and TCA which showed HEDP to be the most effective inhibitor followed by PYP and TCA. The affinity constants reported for HEDP, PYP, and TCA were 23416.8, and 4.5 X 105, respectively.le It is interesting to note that in the case of HAP seeded growthat constant supersaturation,glucosehas been found to be a better inhibitor than CA.22 This is in contrast to the results obtained in the present study, where CA exhibited a superior inhibitory performance compared to glucoee (Table11,experiments20,41,and42). The affinity constants reported for CA and GL in the case of the HAP system were 1.2 X lo4 and 10.2 X lo4, respectively, comparedto 10 X 105 for CA obtained in the present study. Thus, the marked differences in the degree of inhibition of GL and CA may be due to the difference in their specific adsorptions and orientations of the molecules on the surface of HAP and CaF2 crystala Brecevic et al.14 investigated the influence of di- and tricarboxylic acids on the precipitation of amorphous calcium phosphate (ACP) and dicalcium phosphate dihydrate (DCPD)and on the morphologyof DCPD crystals grown in the presence of inhibitors. In ACP systems, it was shown that malic, succinic, maleic, and fumaric acid anions had no significanteffecton the metastabilityperiod while the effectivenessof other carboxylicacids decreased in the order citric > tricarballylic > tartaric. The effectivenessof these inhibitors was explained in terms of the Cacomplexcharge and the number of functionalgroups (-COOH, -OH). Regarding DCPD crystal modification (21) Amjad, Z. Langmuir 1987, 3, 1063. (22) D a b , E.;Koutaoukos, P.G. J . Chem. SOC.,Faraday Trans. 1 1989,85, 2466.
by carboxylic acids, succinic, fumaric, and maleic acids did not affect the shape of DCPD crystals. Among the carboxylicacidswhich did show some inhibitoryinfluence, the ranking in terms of decreasing effectivenesswas malic > citric > tricarballylic > tartaric. The experimentalresults discused in this paper on CaF2 crystal growth inhibition by a variety of inhibitors suggest that these inhibitors can be classified into the following two categories: (a) those such as unsaturated dicarboxylic acids (fumaric, maleic) and glucose, which have no pronounced effect on the rate of crystal growth; (b) those such as mono- and dihydroxycarboxylic acids and poly(carboxylic acids) (malic, citric, succinic, tartaric, tricarballylic, poly(acry1icacid)), exhibiting a significanteffect at all examined concentrations. The differences in the ability of the class b inhibitors (viz., PAA, CA, TA, TCA, etc.) to retard or inhibit crystallization of CaF2 could be rationalized in terms of their abilities to form a complex with the calcium ion. Table I11 summarizes the comparative data on the acid ionization constant, Ki,calciuminhibitor complexation constant, KML,and the affmity constant for a variety of inhibitors. It can be seen that, on the basis of affinity constant values, the order of effectiveness for the di- and tricarboxylic acids is CA > TCA > MA > TA, whereas the values of the complexation constants follow the order CA > TA > MA > TCA. On the basis of data presented in Table 111, it seems evident that it is not simply the ability of these carboxylates to complex Ca2+,but also the affiity of their adsorption to the crystal surface that influences their ability to inhibit crystal growth. The data summarizedin Table XI1 suggest that the order of crystal growth inhibition observed with the various carboxylates does not follow the calciuminhibitor complexation constant (KMI,)values. On the basis of the kinetic data (Table 11), it appears that the most effective inhibitors for CaF2 are those which have high affiiities for the crystalsurface. Apparently the larger number of functional groups of the negatively charged species increases the polar attraction between the adsorbate and the positive sites of the crystdsolution interface. This is consistent with the results of studies by Amjad on the performance of BHCA, CA, and TCA for CaC03, CaHP04 2H20, and Ca~(P04)30Hsystems.21923924The relatively good inhibitory activity shown by BHCA compared to CA and TCA has been attributed to the relatively highly charged BHCA ion. (23) Amjad, Z. Langmuir 1987,3, 224. (24) Amjad, Z. J. Colloid Interface Sci. 1987, 117, 98.
