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Adsorption of polyelectrolytes during crystallization of inorganic salts
JOURNAL OF COLLOID AND INTERFACE SCIENCE 21, 623--625 (1966)
ADSORPTION OF POLYELECTROLYTES DURING CRYSTALLIZATION OF INORGANIC SALTS Janet E. Crawford and B. R. Smith Department of Physical Chemistry, University of Sydney, N.S.W. Received March 29, t965 INTRODUCTION
La Met and his co-workers have made notable contributions to the field of nucleation (1), particularly in relation to the conditions necessary for the preparation of monodisperse sols. One problem in nucleation and crystal growth, of considerable interest in both industrial and biological situations, concerns the way in which certain polyelectrolytes inhibit or delay crystallization from supersaturated solutions of inorganic salts (2). With calcium sulfate, for example, as little as I p.p.m, of polyacrylic acid has an appreciable effect, the induction period before the visible onset of precipitation being lengthened, and the crystal growth rate reduced (3). The results indicated that the polyelectrolyte molecules adsorbed upon the crystal embryos (nuclei smaller than the critical size) preventing further growth; in due course, because of its thermodynamic instability, each embryo would redissolve, thus freeing the polyelectrolyte for interaction with other embryos. Williams and Ruehrwein (4) also described the mechanism of inhibition in terms of adsorption of the polyelectrolyte on the nuclei. They found that for the sodium salt of polymethacrylic acid, no difference could be detected in the polymer concentration in solution before and after precipitation of calcium carbonate. However, when the ammonia adduct of an isobutylene/ maleic anhydride copolymer was used as the polyetectrolyte impurity, adsorption was apparent. Determinations of the polyanion concentration were made by turbidimetric titration with a solution of polycation. In the course of more extensive studies of calcium sulfate and calcium oxalate (5) this question of polyelectrolyte adsorption was re-examined. Owing to difficulties with the Williams Reuhrwein analytical method, a new one was developed and used to follow the concentration of impurity during the course of the crystallization of calcium sulfate from supersaturated aqueous solution. 623
624
CRAWFORD
AND
SMITH
• ANALYTICAL ~V[ETHOD A known volume of the supernatant solution from the crystallization was spread as a monolayer at the air/water interface. B y measuring the surface pressure (v) at various surface areas (A) and referring to the known ~-A curve for the polyelectrolyte, several estimates of the amount of polyelectrolyte in the sample solution may be obtained and averaged. The presence of a varying concentration of calcium ions in the solution to be assayed was shown to affect t h e ~-A curve of the polyelectrolyte. This effect was minimized b y using as substrate 0.1 N calcium perchlorate. To ensure efficient spreading, the solutions were made 10 % with respect of n-propanol; the surface pressures caused b y the presence of n-propanol were subtracted from the total surface pressure to obtain the surface pressure of the polyelectrolyte. The determinations were performed in triplicate, and the results agreed t o w i t h i n -4-7 X l:0-6:monomole ( ± 4 % of the range measured). RESULTS The monolayer analytical method was used in an experiment designed to find out whether p01yacrylic acid (viscometric M.Wt. 20,000) adsorbed onto calcium sulfate crystals during their growth from a supersaturated solution.
°
"20
5.00
P
1 4.so C
]0
'\
4-00
3.50 ,
0
10
20
s
i~
o
~__~ q
30
Z,
0
50 days
FIG. 1. A plot of the: concentration of calcium sulfate (C) and of ~he concentration of polyacrylic acid (/~)•against time in days from the moment 0f mixing. The initial concentration of calcium sulfate was 50 raM.; the data are expressed in terms of the reciprocal of the measured resistance of the solution (1/R X 105 ohms-l). The initial concentration of the polyelectrolyte was 2 X 10-4 molar in •monomer units (14.4 p.p.m.).
ADSORPTION OF POLYELECTROLYTES
625
A 50-mM. solution of calcium sulfate was prepared by mixing equal volumes of 100 mM. calcium perchlorate and 100 m~I sodium sulfate. Sufficient polyacrylic acid was added to the calcium perchlorate solution to give a final polyeleetrolyte concentration of 2 X 10-4 monomolar (about 14.4 p.p.m.). The solution was kept in a thermostat at 30.0°C. and stirred vigorously with a magnetic stirrer. At this temperature the saturation solubility of calcium sulfate is 15.4 raM. The kinetics of crystallization were followed by measuring the resistance of the solution. Samples were removed periodically for polyelectrolyte analysis. These results are shown in Fig. 1. DISCUSSION
The results of Fig. 1 indicate the way in which the polyelectrolyte is adsorbed during the process of calcium sulfate precipitation. Under the same conditions, but in the absence of any added impurity, t h e solutloi~ would have an induction period of about 15 minutes, and crystallizat~bn would be 90 % completed in about 100 minutes. Figurefl shows that in the presence of polyelectrolyte, the induction period h a s been markedly lengthened and the rate of precipitation decreased. ~: The change in polymer concentration closely paralleled the decrease the concentration of calcium sulfate in solution. I t islp0Stulated that the initial polymer concentration was such that very few growth sites re~ained free of adsorbed polyelectrolyte, and crystallization wasprevented. As the polymer concentration decreased by adsorption, an increasing number of nuclei would be able to grow without interference. As a resUlt,'there would be an increasing number of sites on which the polyelect~oIyte ic'buld adsorb. The crystals produced in the presence of polyelectrolyte were much smaller and more numerous than those formed in its absence. : REFERENCES 1. 2. 3. 4. 5.
See LAMER, V, K., Ind. Eng. Chem. 44, 1270 (1952). K~ATOKWL,S.,et al., J. Colloid Sci. 19, 373 (1964). McCARTNEY,E. m., AND ALEXANDER,A. E., J. Colloid Sci. 13; 383 (1958): WILLL~MS,F. V., .~UD RUEHRW~IN,'R. A., J. A m . Chem. Soc. 79, 4898 (1957). ~: C~AWFORD,J. E., B. Sc. (Hens.) thesis, University of Sydney, I964.
ADSORPTION OF POLYELECTROLYTES DURING CRYSTALLIZATION OF INORGANIC SALTS Janet E. Crawford and B. R. Smith Department of Physical Chemistry, University of Sydney, N.S.W. Received March 29, t965 INTRODUCTION
La Met and his co-workers have made notable contributions to the field of nucleation (1), particularly in relation to the conditions necessary for the preparation of monodisperse sols. One problem in nucleation and crystal growth, of considerable interest in both industrial and biological situations, concerns the way in which certain polyelectrolytes inhibit or delay crystallization from supersaturated solutions of inorganic salts (2). With calcium sulfate, for example, as little as I p.p.m, of polyacrylic acid has an appreciable effect, the induction period before the visible onset of precipitation being lengthened, and the crystal growth rate reduced (3). The results indicated that the polyelectrolyte molecules adsorbed upon the crystal embryos (nuclei smaller than the critical size) preventing further growth; in due course, because of its thermodynamic instability, each embryo would redissolve, thus freeing the polyelectrolyte for interaction with other embryos. Williams and Ruehrwein (4) also described the mechanism of inhibition in terms of adsorption of the polyelectrolyte on the nuclei. They found that for the sodium salt of polymethacrylic acid, no difference could be detected in the polymer concentration in solution before and after precipitation of calcium carbonate. However, when the ammonia adduct of an isobutylene/ maleic anhydride copolymer was used as the polyetectrolyte impurity, adsorption was apparent. Determinations of the polyanion concentration were made by turbidimetric titration with a solution of polycation. In the course of more extensive studies of calcium sulfate and calcium oxalate (5) this question of polyelectrolyte adsorption was re-examined. Owing to difficulties with the Williams Reuhrwein analytical method, a new one was developed and used to follow the concentration of impurity during the course of the crystallization of calcium sulfate from supersaturated aqueous solution. 623
624
CRAWFORD
AND
SMITH
• ANALYTICAL ~V[ETHOD A known volume of the supernatant solution from the crystallization was spread as a monolayer at the air/water interface. B y measuring the surface pressure (v) at various surface areas (A) and referring to the known ~-A curve for the polyelectrolyte, several estimates of the amount of polyelectrolyte in the sample solution may be obtained and averaged. The presence of a varying concentration of calcium ions in the solution to be assayed was shown to affect t h e ~-A curve of the polyelectrolyte. This effect was minimized b y using as substrate 0.1 N calcium perchlorate. To ensure efficient spreading, the solutions were made 10 % with respect of n-propanol; the surface pressures caused b y the presence of n-propanol were subtracted from the total surface pressure to obtain the surface pressure of the polyelectrolyte. The determinations were performed in triplicate, and the results agreed t o w i t h i n -4-7 X l:0-6:monomole ( ± 4 % of the range measured). RESULTS The monolayer analytical method was used in an experiment designed to find out whether p01yacrylic acid (viscometric M.Wt. 20,000) adsorbed onto calcium sulfate crystals during their growth from a supersaturated solution.
°
"20
5.00
P
1 4.so C
]0
'\
4-00
3.50 ,
0
10
20
s
i~
o
~__~ q
30
Z,
0
50 days
FIG. 1. A plot of the: concentration of calcium sulfate (C) and of ~he concentration of polyacrylic acid (/~)•against time in days from the moment 0f mixing. The initial concentration of calcium sulfate was 50 raM.; the data are expressed in terms of the reciprocal of the measured resistance of the solution (1/R X 105 ohms-l). The initial concentration of the polyelectrolyte was 2 X 10-4 molar in •monomer units (14.4 p.p.m.).
ADSORPTION OF POLYELECTROLYTES
625
A 50-mM. solution of calcium sulfate was prepared by mixing equal volumes of 100 mM. calcium perchlorate and 100 m~I sodium sulfate. Sufficient polyacrylic acid was added to the calcium perchlorate solution to give a final polyeleetrolyte concentration of 2 X 10-4 monomolar (about 14.4 p.p.m.). The solution was kept in a thermostat at 30.0°C. and stirred vigorously with a magnetic stirrer. At this temperature the saturation solubility of calcium sulfate is 15.4 raM. The kinetics of crystallization were followed by measuring the resistance of the solution. Samples were removed periodically for polyelectrolyte analysis. These results are shown in Fig. 1. DISCUSSION
The results of Fig. 1 indicate the way in which the polyelectrolyte is adsorbed during the process of calcium sulfate precipitation. Under the same conditions, but in the absence of any added impurity, t h e solutloi~ would have an induction period of about 15 minutes, and crystallizat~bn would be 90 % completed in about 100 minutes. Figurefl shows that in the presence of polyelectrolyte, the induction period h a s been markedly lengthened and the rate of precipitation decreased. ~: The change in polymer concentration closely paralleled the decrease the concentration of calcium sulfate in solution. I t islp0Stulated that the initial polymer concentration was such that very few growth sites re~ained free of adsorbed polyelectrolyte, and crystallization wasprevented. As the polymer concentration decreased by adsorption, an increasing number of nuclei would be able to grow without interference. As a resUlt,'there would be an increasing number of sites on which the polyelect~oIyte ic'buld adsorb. The crystals produced in the presence of polyelectrolyte were much smaller and more numerous than those formed in its absence. : REFERENCES 1. 2. 3. 4. 5.
See LAMER, V, K., Ind. Eng. Chem. 44, 1270 (1952). K~ATOKWL,S.,et al., J. Colloid Sci. 19, 373 (1964). McCARTNEY,E. m., AND ALEXANDER,A. E., J. Colloid Sci. 13; 383 (1958): WILLL~MS,F. V., .~UD RUEHRW~IN,'R. A., J. A m . Chem. Soc. 79, 4898 (1957). ~: C~AWFORD,J. E., B. Sc. (Hens.) thesis, University of Sydney, I964.