(PDF) Effect of Ionic Surfactants and Alcohols on the Morphology of

Jun 25, 2016 - Batch cooling and antisolvent crystallization of copper sulfate pentahydrate using surfactant additives is investigated to understand t...
0 downloads 0 Views 1MB Size
Download PDF
Article pubs.acs.org/IECR

Effect of Ionic Surfactants and Alcohols on the Morphology of CuSO4·5H2O Crystals: Combined Use of Factors and Significance of Threshold Surfactant Concentration Nitish Singh, Ribani Yeri, and Jayanta Chakraborty* Indian Institute of Technology, Kharagpur PIN-721302, India S Supporting Information *

ABSTRACT: Batch cooling and antisolvent crystallization of copper sulfate pentahydrate using surfactant additives is investigated to understand the effect of various factors on its morphology. It has been found that nonionic surfactants have a marginal effect on this ionic compound, whereas ionic surfactants are able to modify the morphology substantially. Alcoholic antisolvents are also found to be effective habit modifiers that produce elongated crystals. It has been shown that the effects of surfactants and antisolvents can be combined by using a surfactant−antisolvent pair. The effect of surfactant is observed after a certain threshold concentration. A detailed study of the critical micellar concentration (CMC) of sodium dodecyl sulfate in copper sulfate pentahydrate solution confirmed that this threshold concentration is close to the CMC of the surfactant in copper sulfate pentahydrate solution. Evidence suggests that a concentration higher than CMC is required to maintain a surfactant reservoir in the system, leading to a change in the crystal morphology. This insight may help to predict the minimum required surfactant concentration for habit modification for an unknown system.

1. INTRODUCTION The morphology or shape of crystals is an important property of crystalline solids. Fundamental properties like optical transparency,1 bioavailability of drugs2 and industrially relevant properties like bulk density3 and mechanical strength depend on the shape of the crystal. Manipulation of crystal size is usually achieved by manipulating the rate of nucleation and growth, whereas the growth morphology of a crystal can be manipulated by altering the relative growth rates of the crystal faces.4 The easiest way to manipulate the shape of a crystal is by changing the growth supersaturation.5 This method does not introduce any impurity, and the habit of the crystal can be changed by simply choosing the proper cooling profile.6,7 However, if the growth rates of different faces do not show different features at different supersaturations,8 this route is not effective. Varying the solvent and using certain additives are possible alternatives in such cases.7,9,10 A vast literature exists on the effects of additives on crystal morphology. A recent book11 and reviews12−15can be referred to for details. In general, surfactants are excellent additives that affect both the shape and the size of crystals. A large number of studies9,16−22have been carried out to elucidate these effects. The effects are often very complex. For example, in the presence of sodium dodecyl sulfate (SDS), CaCO3 forms very small (0.5 μm) prismatic crystals,16 whereas if polyvinylpyrrolidone (PVP) is present along with SDS, it forms flat sheets or even flower-like structures. Even the polymorph that is precipitated may change depending on the surfactant present. At very low concentration of SDS, vaterite is the dominant polymorph of CaCO3, whereas calcite is formed at higher concentration of SDS.18 Ionic surfactants are more frequently used than nonionic surfactants for altering crystal morphology © 2013 American Chemical Society

because of the strong ionic interaction between crystal sites and surfactant head groups. The concentration of surfactant used in crystallization experiments is a critical parameter.17,19 The effect of surfactant usually increases with increasing concentration. As surfactants form various self-assembled structures in solution, it is not clear whether the concentration effect is related to the appearance of such structures. The critical micellar concentration (CMC) of surfactants is reported in pure aqueous solution, and the CMC in a salt solution is usually not known. The effect of SDS micelle on crystallization of CaCO3 was studied by Bang et al.17 who showed that accelerated morphological transformation occurs in the presence of SDS micelles. Bujan et al.9 also reported the formation of different phases of calcium hydrogen phosphate above and below the CMC of the surfactant used. Konig et al.22 observed different crystallization kinetics above and below the CMC of SDS. In this work, we investigate the effect of surfactants on ionic copper sulfate pentahydrate crystals (CuSO4·5H2O, will be abbreviated as CS henceforth). Despite its industrial importance, very few studies6,7,23 have been conducted on the morphological development of CS crystals. Hence, a thorough investigation regarding the effect of all three types of surfactants and alcoholic antisolvents is first carried out. It is then shown that the effects of surfactants and antisolvents can be combined to achieve a combined effect. Next, it is shown that for SDS, the effect of surfactant is prominent only after a certain concentration. Measurement of CMC of SDS in CS solution confirms that this concentration is very close to the Received: Revised: Accepted: Published: 15041

April 7, 2013 September 10, 2013 September 25, 2013 September 26, 2013 dx.doi.org/10.1021/ie4024403 | Ind. Eng. Chem. Res. 2013, 52, 15041−15048

Industrial & Engineering Chemistry Research

Article

experiments at lower temperature, the sonication was conducted by wrapping the vial with disposable rubber gloves containing ice. In some experiments, where CS/SDS may precipitate, the sonication period was reduced. The 2 h settling period was also omitted for such cases by using centrifugation. The duration of these steps were decided on the basis of the induction time. After filtration, absorbance of the dye was measured to quantify the amount solubilized in the micelles. 2.5. Quantitative Measurement of SDS Using Acridine Orange. Acridine orange test28 was used for determining the amount of SDS present in a solution. SDS forms a yellow colored complex with acridine orange, which can be extracted using toluene and quantified using UV−visible spectroscopy. Briefly, 100 μL of solution containing SDS was mixed with 100 μL of 1% acridine orange solution and then shaken for a minute. The complex was then extracted using 3 mL of toluene by shaking for 3 min. The toluene solution was analyzed using UV−vis spectroscopy against a calibration curve to determine the concentration of SDS. To quantify the amount of SDS present on crystal surface, acridine orange, glacial acetic acid, and toluene were added to a measured amount of dried CS powder and then stirred. The toluene turns yellow, indicating the formation of SDS−acridine orange complex. The mixture was filtered, and the filtrate was analyzed quantitatively for SDS. 2.6. Preparation of Microemulsion. Oil-in-water microemulsion was prepared by adding 40 μL of toluene to 15 mL of CS solution (with 0.5 mM SDS). The solution was stirred continuously at a speed of 200 rpm for 30 min. After being stirred, a transparent microemulsion formed. 2.7. Measurement of Size of Hydrated SDS Crystals. SDS was dissolved at a desired concentration in 0.9 M CS solution and kept in an ice bath. This solution was taken out periodically and visually observed under a strong light for any scattering. The observation period was very short so that no significant change in temperature occurred during observation. To supplement the visual observation, DLS (dynamic light scattering) measurements of samples were also taken. For DLS measurements, the solution was transferred to the measurement cell after the desired period. The cell was also equilibrated at the same temperature. The cell was then placed in the Zetasizer, which has accurate temperature control. The correlation curve was then obtained, and the particle size was measured using the built-in software.

CMC. This observation sets a lower limit on the concentration of SDS needed for habit modification. Such a limit, if it exists in general, can be used to predict the minimum surfactant concentration needed for effective habit modification.

2. EXPERIMENTAL SECTION 2.1. Materials. Copper sulfate pentahydrate (CuSO4· 5H2O), sodium dodecylsulfate (SDS), tween 80, polyethylene glycol 6000 (PEG 6000), toluene, and glacial acetic acid were purchased from Merck chemicals, India. Polyvinylpyrolidone (PVP) and cetyltrimethyl ammonium bromide (CTAB) were purchased from S. D. Fine Chemicals, India. Span 80 (sorbitanemonooleate), dioctyl sodium sulphosuccinate (AOT), sodium dodecyl benzene sulfonic acid (SDBS), acridine orange, and methyl orange (MO) were purchased from Loba Chemie, India. Sudan III dye was purchased from Sisco research laboratory. All chemicals were used as received. Double distilled water was used for all experiments. 2.2. Crystallization. Saturated solution of CS was prepared at 30 °C and filtered with 11 μm Whatman filter paper. The solution was then heated to 35 °C for about 10 min to dissolve any particle that might be present and finally cooled to 30 °C. Ten milliliters of this solution was taken in a 30 mL glass vial and immediately immersed in an ice bath for cooling crystallization. For testing of an additive, the additive was dissolved in a saturated CS solution at higher concentration and then diluted with fresh saturated CS solution to the desired concentration. Additives were used at low concentrations, and hence a significant change in solubility is not expected. To confirm this, the solubility of CS in SDS solution was tested quantitatively. It is found that over the range of SDS concentration used, the change in solubility of CS is below 2%, and hence such effects were not considered. The solution/ slurry was stirred continuously during the crystallization experiment using a Teflon-coated magnetic stirrer at a speed of 150 rpm to ensure good mixing. The temperature was maintained at 0 °C (±1 °C) throughout. The slurry was filtered with an 11 μm Whatman filter paper after 3 h. 2.3. Measurements and Characterization. Spectroscopic measurements were obtained using a Perkin-Elmer spectrophotometer (model: Lambda 35). SEM images were obtained using a JEOL JSM5800 scanning electron microscope unless otherwise mentioned. Surface tension of solutions was measured using a Du Nouy tensiometer (Test Master, Testing and Instruments, India). Precipitation of surfactant was tested by dynamic light scattering (DLS) using a Malvern zetasizer (nano series). FTIR spectra were obtained using a PerkinElmer (Spectrum Rx) spectrometer. 2.4. Measurement of CMC of SDS in CuSO4 Solution. CMC of SDS in a salt solution was measured using three wellknown techniques: dye micellization, surface tension, and dye solubilization. Methyl orange was used in dye micellization experiments. Details of these methods are available in the literature.24−27 For dye micellization experiments,25 the concentration of SDS was varied over a wide range, and the UV−visible spectra of samples were obtained. The peak wavelengths were plotted against concentrations of SDS. Such plots show a sharp change after a certain concentration of SDS. This concentration is interpreted as CMC. Sudan III was used for dye solubilization26 experiments. Excess amount of dye was added to the surfactant + CS solutions, and the mixture was sonicated for 30 min. The mixture was allowed to settle for 2 h before filtration. For

3. RESULTS AND DISCUSSION All of the crystallization experiments discussed in this work were carried out with identical concentration of copper sulfate in water, that is, the saturated solution at 30 °C. They were also subjected to identical supersaturation by cooling to the same temperature (0 °C) irrespective of the introduced additive. The crystals were allowed to grow until the supersaturation was exhausted. The crystals then were filtered and dried at 30 °C. Several trial runs had been conducted with and without surfactants to establish the time required for completion of the growth process. This was monitored in the following way: first the stirrer was stopped and the solution was allowed to settle for a minute, establishing a transparent solution at the top of the crystallizer. Next, a small amount (200 μL) of sample was drawn, the concentration of which was measured using absorption at 800 nm. Care was taken to avoid particles in the sample. The time versus concentration curve for crystallization experiments is shown in Figure 1. A large 15042

dx.doi.org/10.1021/ie4024403 | Ind. Eng. Chem. Res. 2013, 52, 15041−15048

Industrial & Engineering Chemistry Research

Article

explored. The nonionic surfactants failed to act as effective habit modifiers. Details of the surfactants used, the range of concentration explored, and micrographs of crystals produced are provided in the Supporting Information (Table S1 and Figure S1). In contrast, ionic surfactants have a significant effect on the morphology of CS crystals. Figure 2b shows the effect of SDS on the morphology of CS crystals. Crystals synthesized without any surfactant (Figure 2a) are prismatic and have smooth faces, but the crystals prepared with SDS (Figure 2b) are flake-like in visual appearance and formation of layers is observed in the SEM images. Another anionic surfactant, AOT, shows (Figure 2d) a different result; larger crystals are formed with sharp corners and minimal face roughening. This morphology is also different from that of the particles synthesized without any surfactant. Cationic surfactant CTAB (Figure 2c) also shows a prominent effect on morphology. However, it produces crystals with irregular shape. In this case, particles tend to form agglomerates during drying. 3.2. Production of Elongated Crystals Using Antisolvent. Usually, in industrial conditions, the most undesirable crystal shape is needle. It clogs the filter cloths, breaks into fines, and creates many other problems in downstream processes. Hence, knowledge about the formation of elongated crystals is important, and many additives are tested to identify the impurity responsible for formation of needle-like habits. It is known that alcohols have influence on the habit of crystals as additives.10,30 Hence, we tried a range of alcohols, methanol, ethanol, butanol, and isopropanol, as habit modifiers. However, in our case, alcohols do not show a substantial effect on the morphology of the CS crystals at low concentrations.

Figure 1. Temporal evolution of concentration of CS in mother liquor during batch cooling crystallization.

number of crystals are formed within the first 5−10 min, and the majority of the supersaturation is exhausted in 2 h, but the concentration of CS in the growth solution reaches the reported saturation concentration29 in 3 h. Therefore, the slurry was filtered after 3 h in all experiments of cooling crystallization in this work. 3.1. Effect of Various Surfactants on Morphology of CS. Although the effect of surfactant on crystal morphology is well discussed in the literature, the specific system is not explored systematically. Hence, we briefly discuss the effect of various surfactants on the morphology of CS. All three classes of surfactants, cationic, anionic, and nonionic, have been

Figure 2. Effect of ionic surfactants on morphology of CS crystals. (a) Without surfactant, (b) 0.5 mM SDS, (c) 4.06 mM CTAB, (d) 37.5 mM AOT. 15043

dx.doi.org/10.1021/ie4024403 | Ind. Eng. Chem. Res. 2013, 52, 15041−15048

Industrial & Engineering Chemistry Research

Article

but for AOT a much higher concentration (∼37 mM) is needed. For SDS, the crystals synthesized with various concentrations are shown in Figure 5. With lower concentration of SDS (0.03 mM), the habit remains effectively unchanged (Figure 5a), and the effect becomes very prominent at higher concentration (about 1.2 mM; Figure 5d). The lowest concentration of SDS at which the effect could be perceived is around 0.5 mM. Giulietti et al.7 also reported a similar observation on the effect of SDS on CS crystals. However, for the cooling profile followed in their work, the steps are hardly discernible, whereas a prominent effect is observed in the current work at comparable (even lower) concentrations of surfactant. This is probably due to the difference in cooling profiles. The key question is whether this is purely a concentration effect or due to the appearance of self-assembled structures. The first surfactant structures that form in a surfactant solution are spherical micelles. However, the CMC of SDS in CS solution is not reported, and in the next section we show how this minimum required concentration of SDS for habit modification can be related to the CMC of SDS in CS solution. 3.5. Determination of CMC. Several methods are available for measurement of CMC. The two most convenient methods are the surface tension and the conductivity method. The conductivity method could not be used for the present case because of the presence of salt at high concentration. The modulation of conductivity by SDS is much smaller than the measured conductivity of the solution, and hence the signal-tonoise ratio is not favorable. On the other hand, the well-known surface tension method could be used for CMC determination even in the presence of a salt.31 Two more methods, dye solubilization and dye micellization as detailed in the Experimental Section, were also chosen to supplement the results obtained by surface tension measurements. Dye solubilization method has previously been used32 for CMC determination in salt solution. Use of dye micellization technique in the presence of salt is not known to us. The CMC obtained by this method for a salt-free water−surfactant system is found to be very accurate, and hence we apply this method for salt solution as well. The measured CMC values using this method show a good agreement with the values found by the two other methods. These values are used as supplementary to the other experiments.

A large amount of an alcohol (1−5 mL) was added to 10 mL of saturated CS solution at 30 °C to initiate antisolvent crystallization. In this case, elongated crystals are formed. A larger amount of an alcohol produces very small elongated crystals. The most prominent effect is observed for ethanol where elongated crystals with aspect ratio 10 are formed with the longest dimension around 200 μm. For other alcohols, the effect is less prominent. These crystals are shown in Figure 3.

Figure 3. Elongated crystals of CS formed by antisolvent crystallization using ethanol.

3.3. Combinations of Factors To Achieve a Combined Effect. It seems natural to try to combine various factors to see if combined effects can be achieved, for example, if SDS could be used as an additive in antisolvent crystallization to form elongated particles with flaky nature. This is indeed found to be the case, and the particles are shown in Figure 4a. A similar effect is observed for AOT as well. The particles synthesized via antisolvent crystallization using AOT as additive are shown in Figure 4b. It is possible because the two factors could work in synergy. 3.4. Effect of SDS Concentration on Morphology. The effect of surfactant is not observed until a certain concentration is reached. For example, the flaky particles appear if the SDS concentration is above 0.5 mM. It is clear from Figure 2 that this concentration is also specific to a surfactant. For SDS, the effect is observed around a threshold concentration of 0.5 mM,

Figure 4. CS crystal synthesized by antisolvent crystallization. Ethanol is used as antisolvent in the presence of (a) 6.3 mM SDS and (b) 37.5 mM AOT. 15044

dx.doi.org/10.1021/ie4024403 | Ind. Eng. Chem. Res. 2013, 52, 15041−15048

Industrial & Engineering Chemistry Research

Article

Figure 5. Effect of concentration of SDS on morphology of CS crystals. (a) 0.03 mM SDS, (b) 0.06 mM SDS, (c) 0.7 mM SDS, (d) 1.2 mM SDS.

The reported CMC of SDS in pure water is around 8 mM, and CMC decreases substantially in salt solution.27 In our case, the initial concentration of salt was 1.5 M, which becomes a thermodynamically nonequilibrium state when cooled to 0 °C. However, the time required for precipitation to start (induction time) is much more than the time required to achieve thermal equilibrium and a single surface tension measurement. Hence, we measured the CMC of the salt solution at both of the concentrations (1.5 and 0.92 M, respectively) at 30 and 0 °C. Dye micellization requires a longer measurement period and hence could not be used for 1.5 M CS solution at 0 °C. Dye solubilization protocol could be used for both cases because the hydrophobic dye solubilizes very quickly in the CS−SDS−SDS micelle system. The results for CMC measurements in CS solutions of two different concentrations at 30 °C using dye micellization technique are shown in Figure 6. It can be seen that a sharp change in peak wavelength is observed after the SDS concentration crosses 0.1 mM for 0.92 M CS at 30 °C. A similar transition is observed at a lower SDS concentration (0.01 mM) for a more concentrated (1.5 M) CS solution. Figure 7 shows the plots of surface tension against SDS concentration for 1.5 M CS measured at 0 °C. In this case, the CMC is found to be about 0.1 mM. Figure 8 shows the results for dye solubilization experiments conducted at two different temperatures for 0.92 M CS solution. It can be seen that the peak absorbance of dye rapidly increases after a certain concentration, which is interpreted as CMC. The CMC in 0.92 M CS solution is 0.3 mM at 30 °C and 0.7 mM at 0 °C. The former value is in good agreement with the value found by the dye solubilization method. The values of CMC measured using various techniques are summarized in Table 1. The initial concentration of salt in the

Figure 6. Determination of CMC of SDS in CS solution using dye micellization: peak wavelength versus SDS concentration for two different concentrations of CS at 30 °C. Inflection point indicates CMC.

system before precipitation starts was 1.5 M, and it can be seen from Table 1 (column 4, rows 2, 3, and 5) that if the SDS concentration is above ∼0.1 mM, micelles are present in the solution before cooling. Hence, 0.5 mM SDS solution (the critical concentration required for morphology change) will certainly have micelles in 1.5 M CS solution at 30 °C, and a large amount of SDS will be stored as micelles. When this solution is cooled to 0 °C, the CMC remains less than 0.5 mM, and a large portion of surfactants remains in micelles. As the crystallization proceeds, the CS concentration reduces, and according to Table 1, column 3, the CMC value starts to increase gradually toward 0.7 mM. During this period, some of 15045

dx.doi.org/10.1021/ie4024403 | Ind. Eng. Chem. Res. 2013, 52, 15041−15048

Industrial & Engineering Chemistry Research

Article

periodically and observed under a strong light. Visual observation of scattering or slight turbidity should indicate onset of precipitation in such cases. It was observed that, although SDS does precipitate, the kinetics of this process is much slower than the kinetics of precipitation of ionic CS. Any visual evidence of precipitation of SDS is observed only after 40−50 min, while by this time about 75% of CS would precipitate for a 1.5 M CS solution (Figure 1), and the particle shape is also determined by this time. Table 2 shows the induction time versus SDS concentration for this Table 2. Induction Time for SDS Precipitation in CS Solution versus SDS Concentration at 0 °C in 0.9 M CS Solution Figure 7. Determination of CMC of SDS in CS solution using surface tension: surface tension versus SDS concentration for 1.5 M CS solution at 0 °C.

Table 1. CMC of SDS in CS Solution of Two Different Concentrations at Two Different Temperaturesa CMC (mM) temp (°C)

0.92 M

1.5 M

dye solubilization

0 30 30 0 30

0.7 0.3 0.1 0.7 0.3

0.1 0.03 0.01 0.1 0.1

dye micellization surface tension a

induction time (min)

1.5 0.75 0.45 0.325 0.165 0.04

40 60 80 80 80 120

system where the CS concentration is fixed at 0.9 M. It can be noted that for the threshold SDS concentration (0.5 mM), the induction time is 80 min. The induction time is also dependent on concentration. Hence, as SDS is consumed by the CS particles, and the concentration of SDS decreases, induction time becomes larger. To supplement the visual observations, DLS studies were also conducted. It has been found that visual observation using a strong light is accurate enough to estimate the induction time. DLS (dynamic light scattering) measurements reveal that before the induction time, nanosized aggregates are present, which are probably micelles. The particle size suddenly increases to micrometer within a very short period around the observed induction time. It may be noted that even though SDS starts to precipitate after the induction period, the crystals remain very small for 3−4 h and hence stay suspended in the system. They settle at the bottom only after overnight storage at 0 °C. This phenomenon is in contrast with precipitation of SDS in pure water by cooling the solution below the Kraft point. In that case, larger particles settle at the bottom of the vial within 10 min. 3.7. Mechanism of Surfactant Interaction. It can be concluded from the foregoing discussions that the surfactants remain in the solution during the period of crystallization of CS and micelles are present in the system. Now there can be two possible ways by which the micelles can interact with the crystal surface: first, the micelles may attach with the crystal faces as colloidal particles along with monomers and thereby modify the surface and in turn modify the growth. Micelles may also disintegrate and form a layer of surfactant on the crystal face after adsorption. The other possibility, as discussed previously, is that the micelles may act as a reservoir of surfactants. Micelles acting as a surfactant reservoir are known in the field of polymerization. In emulsion polymerization, hydrophobic monomer is solubilized in water as swollen micelles along with surfactant monomers and “inactive” micelles (i.e., micelles without monomer). The polymerization starts in the swollen micelles, and the swollen micelles absorb monomers from the solution and grow. As the surface area increases due to growth, extra surfactant is needed to cover that surface. However,

Figure 8. Determination of CMC of SDS in CS solution using dye solubilization. This plot shows the CMC for 0.92 M CS solution at 30 and 0 °C.

method

concn of SDS (mM)

Three different methods have been used to determine CMC.

the micelles will disintegrate because of the change in CMC. Some micelles will also disintegrate and supply the surfactants required with a newly generated crystal surface. Because of the presence of micelles, the concentration of surfactant in the solution remains high during the crystallization period. 3.6. Precipitation of SDS. It may be possible that SDS also precipitates from the solution along with CS at 0 °C. To test whether simultaneous precipitation of surfactant occurs, SDS was dissolved in 0.9 M CS solution at various concentrations and then kept in an ice bath. CS does not precipitate at this concentration, and hence any observed precipitation is due to SDS. The CS + SDS solution was taken out from the ice bath 15046

dx.doi.org/10.1021/ie4024403 | Ind. Eng. Chem. Res. 2013, 52, 15041−15048

Industrial & Engineering Chemistry Research

Article

and the micelles provide the pool for surfactants required for morphology change. Hence, the CMC of a surfactant in a salt solution can be an important parameter for habit modification, provided such a feature exists in general.

dissolved surfactants quickly get consumed, and the extra surfactants are supplied by disintegration of the inactive micelles.33 The same mechanism could be invoked to explain the need for above CMC concentration in the current case. To test whether micelle disintegration occurred during CS precipitation, we conducted the following experiment: A small amount of toluene was added to the CS + SDS solution to convert the micelles into microemulsion drops just prior to the crystallization experiments. The oil to surfactant ratio was kept high. The microemulsion in CS solution formed a stable and transparent single phase. After a significant amount of precipitation (after about 40 min), oil droplets were seen to form at the top of the aqueous phase, indicating phase separation. Because surfactants do not precipitate until 80 min, a large amount of surfactant must have been consumed during crystallization, and micelles have been disintegrated. To quantify the amount of surfactant consumed during crystallization, the concentration of SDS in the mother liquor was monitored using acridine orange test as detailed in the experimental section. It was found that when 1.2 mM SDS was used, the concentration of SDS in mother liquor reduced to 0.6 mM after 30 min. This amount of SDS could not be provided by dissolving the monomers as the CMC for this case is about 0.1 mM (Table 1). Hence, a surfactant pool maintained by micelles is required to bring about the morphology change. The surfactants present on crystal surface was also extracted using a mixture (toluene + glacial acetic acid + acridine orange) and quantified using acridine orange test. It is found that the major amount (80%) of the added surfactant is with the crystals, while only about 15% remains in the lean filtrate after the crystallization. To quantify the amount of SDS present as final impurity with the crystals after this washing step, the crystals were tested using FTIR and acridine orange test. For acridine orange test, the washed crystals were redissolved in water, and the standard acridine orange test was repeated on this solution. The FTIR spectrum for the habit-modified crystals was found to be similar to that for the pure crystals precipitated without surfactant (see Figures S2 and S3 of the Supporting Information). However, more sensitive acridine orange test reveals that about 7% of the surfactant added remains with the crystal even after the washing step. Yet the mole fraction of SDS that remains with the crystal is of the order of 10−4, which is negligible for all practical purposes.

■

ASSOCIATED CONTENT

S Supporting Information *

Details of surfactant studied and range of concentration explored, SEM micrographs of CS crystals synthesized with nonionic surfactants, and FTIR spectra of CS powder with and without surfactant. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*Tel.: +91 3222 283950. Fax: +91 3222 282250. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS We gratefully acknowledge research funding from the Department of Chemical Engineering, IIT Kharagpur. We thank Prof. D. Ray from the Department of Chemistry, IIT Kharagpur, for helpful discussion on FTIR results. We thank Ms. G. Amulya who reproduced some of the results. We also thank the reviewers whose comments and suggestions helped us to improve the manuscript substantially.

■

REFERENCES

(1) Huang, L.; Liao, Q.; Shi, Q.; Fu, H.; Ma, J.; Yao, J. Rubrene micro-crystals from solution routes: their crystallography, morphology and optical properties. J. Mater. Chem. 2010, 20, 159. (2) Blagden, N.; DeMatas, M.; Gavan, P. T.; York, P. Crystal engineering of active pharmaceutical ingredients to improve solubility and dissolution rates. Adv. Drug Delivery Rev. 2007, 59, 617. (3) Sirola, I.; Luksic, J.; Simuni, B.; Kujundzic, N. Effect of crystal size and shape on bulk density of pharmaceutical powders. J. Cryst. Growth 1997, 181, 403. (4) Zhang, Y.; Sizemore, J. P.; Doherty, M. F. Shape evolution of 3dimensional faceted crystals. AIChE J. 2006, 52, 1906. (5) Yang, G.; Kubota, N.; Sha, Z.; Louhi-Kultanen, M.; Wang, J. Crystal shape control by manipulating supersaturation in batch cooling crystallization. Cryst. Growth Des. 2006, 6, 2799. (6) Giulietti, M.; Seckler, M. M.; Derenzo, S.; Valarelli, J. V. Changes in copper sulfate crystal habit during cooling crystallization. J. Cryst. Growth 1996, 166, 1089. (7) Giulietti, M.; Seckler, M. M.; Derenzo, S.; Schiavon, L. H.; Valarelli, J. V.; Nývlt, J. Effect of selected parameters on crystallization of copper sulphate pentahydrate. Cryst. Res. Technol. 1999, 34, 959. (8) Boerrigter, S. X. M.; Cuppen, H. M.; Ristic, R. I.; Sherwood, J. N.; Bennema, P.; Meekes, H. Explanation for the supersaturationdependent morphology of monoclinic paracetamol. Cryst. Growth Des. 2002, 2, 357. (9) Bujan, M.; Sikiric, M.; Filipovic-Vincekovic, N.; Vdovic, N.; Garti, N.; Furedi-Milhofer, H. Effect of anionic surfactants on crystal growth of calcium hydrogen phosphate dehydrate. Langmuir 2001, 17, 6461. (10) Yan, G.; Wang, L.; Huang, J. The crystallization behavior of calcium carbonate in ethanol/water solution containing mixed nonionic/anionic surfactants. Powder Technol. 2009, 192, 58. (11) Sangwal, K. Additives and Crystallization Processes: From Fundamentals to Applications; Wiley: New York, 2007. (12) Canselier, J. P. The effects of surfactants on crystallization phenomena. J. Dispersion Sci. Technol. 1993, 14, 625.

4. CONCLUSIONS Ionic surfactants are effective habit modifiers for copper sulfate pentahydrate crystals, and nonionic surfactants do not affect the shape of the crystal significantly. The effect is also specific to the surfactant used. SDS produces flake-like crystals, whereas AOT produces prismatic crystals of different morphology and CTAB produces crystals of irregular shape. Antisolvent crystallization using ethanol produces elongated crystals. It has been shown that the effects of surfactant and antisolvents can be combined to achieve a combined effect. Elongated flakes are produced if SDS is used in antisolvent crystallization. The effect of surfactant is observable only after a certain concentration. Although this concentration is much less than the CMC of the surfactant in water, it is shown to be close to the CMC of the surfactant in aqueous CuSO4·5H2O solution. The kinetic study of surfactant concentration in the crystallizer indicates that the amount of surfactant required to bring about the morphology change is more than the monomer solubility, 15047

dx.doi.org/10.1021/ie4024403 | Ind. Eng. Chem. Res. 2013, 52, 15041−15048

Industrial & Engineering Chemistry Research

Article

(13) Mann, S. The chemistry of form. Angew. Chem., Int. Ed. 2000, 39, 3392. (14) Schmidt, C.; Ulrich, J. Morphology prediction of crystals grown in the presence of impurities and solvents − an evaluation of the state of the art. J. Cryst. Growth 2012, 353, 168. (15) Füredi-Milhofer, H.; Sarig, S. Interactions between polyelectrolytes and sparingly soluble salts. Prog. Cryst. Growth Charact. Mater. 1996, 32, 45. (16) Wei, H.; Shen, Q.; Zhao, Y.; Wang, D.; Xu, D. Crystallization habit of calcium carbonate in the presence of sodium dodecyl sulfate and/or polypyrrolidone. J. Cryst. Growth 2004, 260, 511. (17) Bang, J.; Song, K.; Lee, M. G.; Jeon, C. W.; Jang, Y. N. Effect of critical micelle concentration of sodium dodecyl sulfate dissolved in calcium and carbonate source solutions on characteristics of calcium carbonate crystals. Mater. Trans. 2010, 51, 1486. (18) Wei, H.; Shen, Q.; Zhao, Y.; Zhou, Y.; Wang, D.; Xu, D. On the crystallization of calcium carbonate modulated by anionic surfactants. J. Cryst. Growth 2005, 279, 439. (19) Wei, X.; Yang, J.; Li, Z.; Su, Y.; Wang, D. Comparison investigation of the effects of ionic surfactants on the crystallization behavior of calcium oxalate: From cationic to anionic surfactant. Colloids Surf., A 2012, 401, 107. (20) Rodríguez-hornedo, N.; Murphy, D. Surfactant-facilitated crystallization of dihydrate carbamazepine during dissolution of anhydrous polymorph. J. Pharm. Sci. 2004, 93, 449. (21) Mahmoud, M. H. H.; Rashad, M. M.; Ibrahim, I. A.; Abdel-Aal, E. A. Crystal modification of calcium sulphate dihydrate in the presence of some surface-active agents. J. Colloid Interface Sci. 2004, 270, 99. (22) Kö n ig, A.; Emons, H. H. Crystallization kinetics of MgSO4.7H2O in presence of tenside. Cryst. Res. Technol. 1988, 23, 319. (23) Lu, F.; Wang, H. S.; Zhao, X.; Ozaki, Y. Nucleation and growth of CuSO4.5H2O crystals on the liquid state stearic acid Langmuir− Blodgett film. J. Cryst. Growth 2008, 310, 4652. (24) Patist, A.; Bhagwat, S. S.; Penfield, K. W.; Aikens, P.; Shah, D. O. On the measurement of critical micelle concentrations of pure and technical-grade non-ionic surfactants. J. Surfactants Deterg. 2000, 3, 53. (25) Khamis, M.; Bulos, B.; Jumean, F.; Manassra, A.; Dakiky, M. Azo dyes interactions with surfactants. Determination of the critical micelle concentration from acid−base equilibrium. Dyes Pigm. 2005, 66, 179. (26) McElhanon, J. R.; Zifer, T.; Kline, S. R.; Wheeler, D. R.; Loy, D. A.; Jamison, G. M.; Long, T. M.; Rahimian, K.; Simmons, B. A. Thermally cleavable surfactants based on Furan-Maleimide Diels-Alder adducts. Langmuir 2005, 21, 3259. (27) Hunter, R. J. Foundations of Colloid Science, 2nd ed.; Oxford University Press: New York, 2001; p 459. (28) Sokoloff, R. L.; Frigon, R. P. Rapid spectrophotometric assay of dodecyl sulfate using acridine orange. Anal. Biochem. 1981, 118, 138. (29) Green, D. W.; Perry, R. H. Perry’s Chemical Engineers’ Handbook, 8th ed.; McGraw Hill: New York, 2008. (30) Barata, P. A.; Serrano, M. L. Salting-out precipitation of potassium dihydrogenphosphate (KDP): IV. Characterisation of the final product. J. Cryst. Growth 1998, 194, 109. (31) Gurkov, T. D.; Dimitrova, D. T.; Marinova, K. G.; Bilke-Crause, C.; Gerber, C.; Ivanov, I. B. Ionic surfactants on fluid interfaces: determination of the adsorption; role of the salt and the type of the hydrophobic phase. Colloids Surf., A 2005, 261, 29. (32) Williams, R. J.; Phillips, J. N.; Mysels, K. J. The critical micelle concentration of sodium laurylsulfate at 25 °C. Trans. Faraday Soc. 1955, 51, 728. (33) Odian, G. G. Principles of Polymerization, 4th ed.; John Wiley & Sons, Inc.: New York, 2007; p 350.

15048

dx.doi.org/10.1021/ie4024403 | Ind. Eng. Chem. Res. 2013, 52, 15041−15048