2406
Langmuir 1991, 7, 2405-2408
Inhibition of Calcium Fluoride Crystal Growth by Polyelectrolytes Zahid Amjad The B. F. Goodrich Company, Specialty Polymers and Chemicals Division, Brecksville Research Center, 9921 Brecksville Road, Brecksville, Ohio 44141 Received December 3,1990. In Final Form: March 27,1991 The kinetics of calcium fluoride crystal growth in the presence of polyelectrolytes of 37 "C has been investigated. The polyelectrolytes studied include the following: poly(acry1ic acid) (PAA), poly(acry1aacid) (P-SA), and poly(diallyldimethy1ammide) (P-Am), poly(2-acrylamido-2-methylpropanesulfonic monium chloride) (P-DADMAC). It has been found that addition of low levels of PAA of low molecular weight to the calcium fluoride supersaturated solutions has a striking inhibitory effect upon the rate of crystal growth of calcium fluoride. The effect is interpreted in terms of adsorption, following the Langmuir isotherm of PAA ions at the active growth sites. The kinetic data in the presence of PAA of varying molecular weight (800-240OOO) suggest optimum effectivenesswith an 2000 molecular weight. The order, in terms of decreasing effectiveness on the rate of crystal growth, of various polymers of similar molecular weight studied is PAA >> P-SA > P-Am == P-DADMAC = control (no polymer).
-
Introduction The influence of inhibitors on the crystallization of alkaline-earth metal fluorides has been reported by several investigators. Shyu and Nancollas,' using a seeded growth technique, reported that trace amounts of phosphate ion stabilized supersaturated calcium fluoride solutions and lengthened the induction period before the onset of crystallization. The duration of the induction period was found to be greatly influenced by the concentration of phosphate ion. Results of previous studies have shown that 1-hydroxyethylidine-1,l-diphosphonicacid is an effective crystal growth inhibitor when added to CaF212 S~FZ BaS04,' , ~ and calcium phosphate systems,M while in the case of the more soluble CaSO~2Hz0,8~ this inhibitor appeared to have only a slight influence on the growth kinetics. Results of a recently reported study on the inhibition of CaF2 crystallization by phytic acid, 2-phosphonobutane1,2,4-tricarboxylic acid, and benzene polycarboxylicacid,lO reveal that these inhibitors, when present at low concentration, markedly influence the crystallization kinetics. Similar results were observed for these inhibitors in the case of calcium carbonate,11-13calcium phosphate dihydrate,"~'~and hydroxyapatite.loJ7 A surface adsorption mechanism has been proposed to account for the effect of a variety of inhibitors on the crystallization of sparingly soluble sa1ts.2*4*6J4-17 (1) Shyu, L. J., Nancollas, G.H. Croat. Chem. Acta 1980,53, 281. (2) Shyu, L. J. Ph.D.Thesis, StateUniversityof New York at Buffalo, New York, 1982. (3) Bochner,R. A.; Abdul-Rehman,A.; Nancollas,G.H. J. Chem. SOC. 1984,80, 217. (4) Rizkallas, E. N. J. Chem. SOC., Faraday Trans. 2 1983, 79,1857. (5) Amjad, Z. Langmuir 1987,3, 1063. (6) Varsanik, R. G.Mater. Perform. 1976,14, 18. (7) Francis, M. D. Calif. Tiesue Res. 1969, 3, 151. (8) Liu, S. T.; Nancollas, G. H. J. Colloid Interface Sci. 1973,3,422. (9) Amjad, Z. Desalinatfon 198454,263. (10) Amjad, Z. Langmurr, in press. (11) Amjad, Z. Langmuir 1987, 3, 224. (12) Ashcraft,R. W. Corrosion/86,National Aeeociation of Corrosion Engineers Annual Meeting, Boston, MA, 1985. (13) Dubin, L.; Fulke, K. E. J. Cooling Tower Inst. 1982, 3, 1. (14) Amjad, Z. Can. J . Chem. 1988,66, 2181. (15) Amjad, Z. J. Collord Interface Sci. 1986, 117, 98. (16) Kouteoukos, P. G.;Amjad, Z.; Nancollas,G. H., J. Colloidlnterface Sci. 1981, 83, 599. (17) Amjad, Z. In Adsorption and Surface Chemistry of Hydroxyapatite; Misra, D. N., Ed.;Plenum Press: New York, 1984.
Polyelectrolytes such as poly(acry1ic acid), poly(methacrylic acid), and poly(maleic acid) are used quite extensively in industrial water treatment, geothermal oil production, and desalination, combined with phosphates and/ or phosphonates to avoid the precipitation of scale-forming salts on the equipment surfaces. The inhibitory effect of poly(acrylic acids) on the precipitation of calcium oxalate,'s CaHPO1.2H20,lg CaCOs,20and CaS0,*2H2021*nhas been the subject of much research in the past several decades because of its relevance in both industrial and biological systems. It has been noted that these polyeletrolytes, when present a t low concentration, markedly influence the crystallization kinetics of calcium carbonate, calcium sulfates, calcium phosphates, and calcium oxalate. A surface adsorption mechanism has been proposed to account for the effect of polyelectrolytes on the crystallization of sparingly soluble salts. In the present work we have investigated the effect of poly(acry1ic acid) concentration and molecular weight on the rate of crystal growth of calcium fluoride at sustained supersaturation. In addition, kinetic studies have also been performed in the presence of polyelectrolytes containing non-carboxyl functional groups. The structures and molecular weights of these polyelectrolytes are summarized in Table I.
Experimental Section All experimenta were conducted at 37 k 0.1 O C in a 2WmL double-walled glass cell fitted with Teflon lid. Rsagent grade chemicals were used throughout and stock solutions were prepared with doubly distilled water. Calcium concentrations were determined by ion exchanging the metal ion for hydrogen ion on a Dowex-50ion-exchange resin and titrating the liberated acid with a standard.lO Fluoride solutions were prepared in polyethylene bottles and concentrations were determined by using a fluoride combination electrode(Orion Mode1960900). Calcium fluoride seed crystals were prepared and characterized as described previously.10 The specific surface area of the seed crystals as determined by the BET method was found to be 3.3 (18) Sheehan, M. E.; Nancoh, G.H. Invest. Urol. 1980,17,446. (19) Amjad, Z. Longmuir 1989,5,1222. (20) Libutti, B. L.; Knudsen, J. G.;Mueller, R. W. Corroeion 84
National hociation of Corrosion Engineere Annual Meeting, I k Orleans, LA, 1984; Paper 84. (21) Flesher, P.; Streatfield,;Peame, A. 5.;Hydes, 0. D. Proc. Int. Symp. Fresh Water Sea 3rd 1970,1,493. (22) Amjad, Z.; Masler, W. F. Corroeion/8S, National Aseocition of Corrosion Engineere Annual Meeting, Boeton, MA, 1985; Paper 367. 0 1991 American Chemical Society
Amjad
2406 Langmuir,Vol. 7, No. 10, 1991 Table I. Structures and Molecular Weights of Polyelectrolytes polyelectrolyte acronym structure MW 800 poly(acry1icacid) poly(acry1icacid)
2100
poly(acry1icacid)
6000
poly(acry1ic acid)
2oooo
poly(acry1ic acid)
50000
poly(acry1ic acid)
24oooO
poly(acry1amide)
6000
poly(diallyldimethylammonium chloride)
6Ooo
poly(2-acrylamido- P-SA 2-methylpropanesulfonic acid)
(CHzCNn
I
6000
O= CNHC(CHJ,CH$O,H
m2 g-1, The polyelectrolytes used in this study were selected from a range of commercial and experimental materials (The B. F. Goodrich Co.). Polyelectrolyteconcentrations expressed herein are on a dry polymer basis. In a typical crystal growth experiment, a metastable supersatured solution was prepared by mixing equimolar volumes of calcium chloride and sodium fluoride such that final calcium fluoride concentration would be 0.500 X 10-9 M. Polyelectrolytes were added after the addition of NaF, but before the addition of calcium chloride, as dilute solutions in water. The calcium fluoridesupersaturated solutionswere stirred continuously(-320 rpm) while nitrogen gas, presaturated with water at 37 "C, was bubbled through the solution to exclude carbon dioxide. The crystallization experiments were initiated by innoculation of metastable supersaturated solution withwell-characterized CaFz seed crystals. The crystal growth reaction was monitored by the addition of titrant solutions from mechanically coupled automatic burets mounted on a modified pH stat (Model 600 series, Brinkmann Instruments, Westbury, NY) using a fluoride combination electrode. The titrant solutions in burets consisted of calcium chloride and sodium fluoride. In the investigation of the crystal growth in the presence of inhibitor, in order to avoid dilution the appropriate amount of inhibitor was added in one of the two burets. The molar concentration ratio of the titrant corresponded to the stoichiometry of CaF2. Sodium chloride was added to calcium fluoride supersaturated solutions in order to maintain the ionic strength to within &1% . During experiments, aliquots were withdrawn from time to time, filtered (0.22wm filter, Mililpore Corp.), and analyzed for calcium as described previously.1° The rates of crystallization were determined from rates of addition of titrants and corrected for surface area changes.10
Results and Discussion The experimental conditions and the kinetic results obtained are summarized in Table 11. Figure 1illustrates the plots of CaF2grown asa function of time in the presence of varying concentrations of PAA-C (MW 6000). As can be seen in Figure 1, the crystallization rates of CaFz are highly sensitive to the concentration of polymer C in solution. Interestingly, a polymer concentration as low as 0.04 ppm significantly reduces the crystal growth rate of CaFz. Figure 1 further indicates that, at 0.10 ppm
Table 11. Kinetic Data for CaFi Crystal Growth in the Presence of Polymers. concn, rate x I@, expt polyelectrolyte ppm mol min-1 m-2 64.6 10 61.3 12 62.5 13 14 58.7b 18 PAA-A 0.01 63.3 17 PAA-A 0.10 37.3 0.10 13.6 16 PAA-A 19 PAA-B 0.10 24.8 22 PAA-B 0.25 7.5 23 PAA-c 0.01 60.6 24 PAA-c 0.04 48.3 26 PAA-C 0.10 30.9 25 PAA-C 0.15 26.8 0.15 25.5 29 PAA-c 0.25 9.2 27 PAA-c 30 PAA-c 0.50 7.6 31 PAA-c 1.50
