Control of α-Calcium Sulfate Hemihydrate Morphology Using Reverse

Article pubs.acs.org/Langmuir

Control of α‑Calcium Sulfate Hemihydrate Morphology Using Reverse Microemulsions Bao Kong,† Baohong Guan,*,† Matthew Z. Yates,*,‡ and Zhongbiao Wu† †

Department of Environmental Engineering, Zhejiang University, Hangzhou 310027, China Department of Chemical Engineering, University of Rochester, Rochester, New York 14627, United States

‡

S Supporting Information *

ABSTRACT: Alpha calcium sulfate hemihydrate (α-HH) is an important class of cementitious material and exhibits considerable morphology-dependent properties. In the reverse microemulsions of water/n-hexanol/cetyltrimethylammonium bromide (CTAB)/sodium dodecyl sulfonate (SDS), the morphology and aspect ratio of α-HH are successfully controlled by adjusting the mass ratio of CTAB/H2O and the concentration of SDS. As the ratio of CTAB/H2O is increased from 1.3 to 4.5, the crystal length decreases from 120 to 150 μm to 0.5−1.2 μm with the corresponding aspect ratio reduced sharply from 180 to 250 to 2−7. With increasing SDS concentration, the crystal morphology gradually changes from submicrometer-sized long column to rod, hexagonal plate, and even nanogranule. The preferential adsorption of CTAB on the side facets and SDS on the top facets contributes to the morphology control. This work presents a simple, versatile, highly efficient approach to controlling the morphology of α-HH on a large scale and will offer more opportunities for α-HH multiple applications.

1. INTRODUCTION Controlling the morphology of micrometer-sized and nanosized materials has attracted intensive interest in materials science because of morphology-dependent optical, electronic, magnetic, catalytic, and biomedical properties.1−4 Methods have been developed to prepare materials with a variety of morphologies, including wires, rods, tubes, disks, and hollow particles.5−9 One route to controlling particle morphology is through the confinement of the particle formation reaction in reverse microemulsion droplets.10−12 The confinement of reactants within microemulsion droplets can alter the nucleation process to change the particle size.11 When crystalline materials are nucleated within microemulsions, the preferential adsorption of surfactant onto certain facets of the growing crystals can influence the particle shape.13,14 When a species adsorbs onto a crystal facet, the free energy of that facet is lowered and the crystal growth on that facet is reduced.15 As a result, the crystal shape is altered by preferential adsorption to produce particles with an increased fraction of the surface covered by facets with the surface-adsorbed species.16−18 Crystal growth in reverse microemulsions is a complex process that involves the interplay of crystal growth kinetics and thermodynamics in the multiphase system.11,19 Controlling the crystal morphology is one of the most important issues.20,21 Herein, we report on the use of reverse microemulsions of water, n-hexanol, cetyltrimethylammonium bromide (CTAB), and sodium dodecyl sulfonate (SDS) to control the morphology of α-calcium sulfate hemihydrate (α-HH). α-HH is a very important class of cementitious materials. It has been widely applied in molding, special binder systems, and the © 2012 American Chemical Society

construction industry owing to its workability, fast setting time, and high strength.22 α-HH has also found use in a number of orthopedic applications, including bone cement, bone graft substitutes, and scaffolds for delivering growth factors for osseous regeneration.23−25 The performance of α-HH in various applications is closely associated with the crystal size and morphology. α-HH crystals with a spherical shape or a low aspect ratio are preferable for use in bone cement because they are easy to inject and have preferable mechanical properties.26 α-HH crystals with a high aspect ratio, such as whiskers and wires, can be used as a reinforcing agent in many polymer and ceramic composites that take advantage of the good thermal stability, chemical resistance, and compatibility of α-HH with these composites.27,28 The nanocrystalline form of α-HH has been used as a drug carrier,25,29 an important emerging application of α-HH. Morphological control of α-HH is therefore a prerequisite to obtaining the appropriate performance to meet the targeted application. A number of strategies have been employed to control the αHH morphology, including the manipulation of the pH, the electrolyte concentration,26 and the use of crystal growth modifiers.30 These techniques have produced α-HH rods,26 whiskers,27,28 and wires.31 However, the aspect ratio can be adjusted only within a narrow range, for example, 1.4−5.5 by changing the concentration of CaCl226 and 1.7−4.8 by the addition of potassium sodium tartrate and sodium citrate.30 Received: June 17, 2012 Revised: July 26, 2012 Published: July 27, 2012 14137

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Microemulsion synthesis was chosen in the present study to overcome the major challenge of effectively controlling the morphology over large ranges of size and aspect ratio.

2. EXPERIMENTAL SECTION 2.1. Materials. CTAB (≥99% purity), SDS (≥97% purity), and CaCl2 (≥96% purity) were all obtained from Sinopharm Chemical Reagent Co., Ltd., Shanghai, China. H2SO4 (95−98% purity) was purchased from Hangzhou Chemical Co., Ltd., Hangzhou, China. nHexanol (≥98% purity) was purchased from Shanghai Lingfeng Chemical Co., Ltd., Shanghai, China. 2.2. Synthesis of α-HH. For each synthesis, two separate microemulsions were first formed, with each having an aqueous phase, CTAB as the surfactant, and n-hexanol as the continuous oil phase. One microemulsion was formed with 0.1 mol kg−1 CaCl2 as the aqueous phase, and the other had 0.1 mol kg−1 H2SO4 as the aqueous phase. In both cases, the total volume of the aqueous phase was fixed at 10.0 mL. The CTAB surfactant was added in varying amounts to form microemulsions having a CTAB/H2O mass ratio ranging from 1.3 to 4.5. n-Hexanol was added to maintain a fixed CTAB/n-hexanol mass ratio of 0.3. When all components were added, the microemulsions became transparent after 30 min of stirring at room temperature. Only the microemulsion having the highest CTAB/H2O mass ratio of 4.5 was not transparent at room temperature but formed a transparent microemulsion after being stirred for 30 min at 60.0 °C. The transparent microemulsion containing CaCl2 was heated to 95.0 ± 0.5 °C in a 500 mL three-necked-flask reactor equipped with a glass condenser, a thermometer, and a Teflon impeller at a constant rate of 250 rpm. Then the microemulsion containing H2SO4 was added to the reactor, and the reactor was maintained at 95.0 °C for 1.0 h to form the calcium sulfate particles. The solid product was collected, washed with boiling deionized water four times, and rinsed with acetone before 2.0 h of drying at 60.0 °C. For reactions with mixed SDS and CTAB surfactants, the same procedure was followed except that the CTAB/H2O mass ratio was held constant at 2.5 while the CTAB/SDS mass ratio was fixed at 0.2, 0.3, and 0.4, respectively. 2.3. Characterization. The solid products were subjected to a powder X-ray diffraction (XRD) analyzer (D/Max-2550 pc, Rigaku Inc., Japan) with Cu Kα radiation at a scanning rate of 8°/min in the 2θ range from 5 to 80°. Thermogravimetry and differential scanning calorimetry (TG-DSC, STA-409PC, NET-ZSCH, Germany) were performed for further phase identification. Fourier transform infrared (FTIR) spectra were recorded on a spectrometer (IRAffinity-1, Shimadzu) with a resolution of 4 cm−1 over the frequency range of 400−4000 cm−1. The morphology and elemental analysis of the products were conducted via scanning electron microscopy (SEM) (Hitaches-570 Japan) equipped with an energy-dispersive spectrometer (EDS). Transmission electron microscopy (TEM) images and selected-area electron diffraction (SAED) patterns were obtained on a TEM at an acceleration voltage of 200 kV (FEI TECNAI G2 F20 STWIN, USA).

Figure 1. SEM images of α-HH synthesized at the mass ratio of CTAB/H2O: (a) 1.3, (b) 2.0, (c) 3.5, and (d) 4.5. (e, f) TEM and HRTEM images and SAED pattern of α-HH synthesized at a 3.5 CTAB/H2O mass ratio (CTAB/n-hexanol mass ratio = 0.30, reaction time = 1.0 h, and T = 95.0 °C).

increased to 2.0, the crystals were significantly shortened along the c-axis direction with the average length reduced to 20 μm and the average width slightly reduced to 200 nm (Figure 1b). Further increasing the CTAB/H2O ratio to 3.5 resulted in shorter rods of 1.0−3.0 μm in length and 200−300 nm in width, as shown in Figure 1c. The rods slightly decreased in length to 0.5−1.2 μm with a width of 150−400 nm when the CTAB/H2O ratio was increased to 4.5 (Figure 1d). The data show that the length of the c axis is strongly dependent on the CTAB/H2O ratio. However, the width of the crystal is much less dependent on the CTAB/H2O ratio. TEM, SAED, and high-resolution TEM (HRTEM) have been performed to investigate the morphology and structure of α-HH further. Figure 1e exhibits the α-HH crystal in the morphology of the long rod, and the inset SAED pattern shown in Figure 1f can be indexed to be the [11̅0] zone axis of α-HH. The indexes of the spots in the SAED pattern indicate that the α-HH crystal is single-crystalline and grows along the (001) direction. The fringe spacing of 0.603 nm in the HRTEM image (Figure 1f) corresponds to the (002) plane, further indicating the preferential growth along the (001) direction. The aspect ratio of α-HH, defined as the ratio of the length of the c axis to that of the a axis, is therefore adjustable over a wide range, as shown in Figure 2. Increasing the CTAB/H2O ratio from 1.3 to 4.5 causes the aspect ratio to drop from 180 to 250 to 2−7. Thus, the reverse microemulsion method offers a route to control the α-HH morphology over a much wider range than previously reported techniques by simply changing the mass ratio of CTAB/H2O. The addition of SDS was found to provide a means for additional control of the α-HH morphology. Figure 3 shows the crystals obtained with different SDS/CTAB mass ratios and

3. RESULTS AND DISCUSSION 3.1. Morphology Control of α-HH. In a reverse microemulsion, the growth of crystals depends strongly on the exchange of reactants, which is governed by the interactions between microemulsion droplets. These interactions can be modified by adjusting the size of the droplets, as determined by the ratio of surfactant to water.11,19 Calcium sulfate crystals were synthesized in reverse microemulsions with various mass ratios of CTAB/H2O whereas other experimental conditions were fixed. Both XRD and TG-DSC indicate that the crystals are pure α-HH phase (Supporting Information, Figures S1 and S2). Figure 1 shows the SEM images of the α-HH crystals. When the CTAB/H2O ratio was 1.3, the crystals were in the shape of whiskers with a length of up to 120 μm and a width of 200−700 nm (Figure 1a). When the CTAB/H2O ratio was 14138

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width to 300−400 nm (Figure 3b), showing a rodlike shape. Thus, a low concentration of SDS triggered a dramatic decrease in the aspect ratio. Further increases in the SDS/CTAB ratio to 0.3 resulted in crystals having a hexagonal plate morphology with an average width of 500 nm (Figure 3c). The plate morphology was a result of the aspect ratio being further decreased to about 0.5. Increasing the SDS/CTAB ratio to 0.4 resulted in α-HH crystals in the form of spherical nanogranules of about 50 nm in diameter (Figure 3d). These results show that the α-HH morphology can be changed from submicrometer-sized long column to rod, hexagonal plate, and then nanogranule as the SDS/CTAB ratio is increased. On the basis of the experimental results above, a schematic illustration of the α-HH morphological evolution in reverse microemulsions is shown in Figure 4. When SDS is absent from the microemulsions, α-HH prefers 1D growth and the aspect ratio mainly depends on the mass ratio of CTAB/H2O. When SDS is added, α-HH shows diverse morphologies, from rodshapes, plate-shapes, and spherically shapes with increasing mass ratio of SDS/CTAB. The reverse microemulsion technique therefore offers much more flexible control of the morphology over a wide particle size range than in previous studies and may allow for new applications of α-HH that exploit morphology-dependent properties. 3.2. Growth Mechanism of α-HH. Both the amount of cationic and anionic surfactant and the ratio of cationic to anionic surfactant play key roles in controlling the morphology of α-HH in the microemulsion synthesis. We believe that strong interactions of the surfactants with the α-HH crystal surface are responsible for the observed morphological changes. The surface interactions are closely associated with the structure of α-HH and the nature of the surfactants. As shown in Figure 5, the crystal lattice of α-HH consists of repeating, ionically bonded Ca and SO4 atoms in chains of −Ca−SO4−Ca−SO4− in which each S atom is covalently bonded to four O atoms that form a tetrahedral corner.32,33 The chains' structure may explain the fact that α-HH normally crystallizes in the 1D shape. These chains are hexagonally arranged and form a framework parallel to the c axis with continuous channels with a diameter of about 4.5 Å, where one water molecule is attached to every two calcium sulfate molecules.34 The structure presents a denser distribution of SO42− ions on the side facets of {110} and {100} and a denser distribution of Ca2+ ions on the top facets of {111}, with

Figure 2. Aspect ratio of α-HH as a function of the mass ratio of CTAB/H2O (CTAB/n-hexanol mass ratio = 0.30, reaction time = 1.0 h, and T = 95.0 °C).

Figure 3. SEM images of α-HH synthesized at SDS/CTAB mass ratios of (a) 0, (b) 0.2, (c) 0.3, and (d) 0.4 (CTAB/H2O mass ratio = 2.5, CTAB/n-hexanol mass ratio = 0.30, reaction time = 1.0 h, and T = 95.0 °C).

a fixed mass ratio of 2.5 CTAB/H2O. The XRD analysis confirms the precipitation of the pure α-HH phase as shown in Figure S3 in the Supporting Information. Without SDS in the microemulsions, α-HH crystals had a long columnar shape with a length of 5−20 μm and a width of 200−300 nm (Figure 3a). The corresponding aspect ratios were 15−60 as revealed in Figure 2. When the SDS/CTAB ratio was 0.2, the crystals decreased in length to 2.5−5.0 μm and slightly increased in

Figure 4. Schematic illustration of the α-HH morphological evolution in a CTAB-stabilized reverse microemulsion with or without SDS. 14139

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That is, the supply of Ca2+ and SO42− ions to the growing crystal surface becomes more constrained. The growth is then restrained along the c axis, and the aspect ratio is decreased (Figure 6b). The kinetic and thermodynamic factors work in conjunction with one another because the crystal surface tends to be covered by slow-growing and low-energy facets. Unlike CTAB, SDS is an anionic surfactant that adsorbs more strongly on the positively charged {111} facets of α-HH. Adsorbed SDS molecules blocked the crystal growth along the c axis, decreasing the aspect ratio of α-HH from 15 to 60 to 5−15 (Figure 6c) and 0.5 (Figure 6d). The complex morphological changes observed with added SDS indicate that the surfactant influences particle nucleation as well as growth. At the highest concentration of SDS investigated, the particle size is significantly reduced, suggesting that SDS is effective at promoting the formation of greater numbers of nuclei. To confirm the interactions between the surfactants and crystal surfaces of α-HH, the Fourier transform infrared (FTIR) spectra of α-HH were recorded as shown in Figure 7. The

Figure 5. Structure of α-HH along the c-axis direction.

parallel channels partially filled with water molecules.32,34 The crystals also have electrostatic charge imparted by the dipole on water molecules resulting from the preferred orientation.35 Therefore, the {110} and {100} facets of the α-HH crystal are negatively charged and are expected to interact strongly with CTAB. The {111} facets are positively charged, resulting in the strong adsorption of SDS. The preferential adsorption of CTAB molecules to the {110} and {100} facets of α-HH will lower the surface free energy of those facets relative to that of the {111} facets. The crystals will then tend to grow to minimize the surface area covered by the {111} facet, resulting in rod-shaped crystals as illustrated in Figure 6.36−38 Factors other than the surface free energy are

Figure 7. FTIR spectra of α-HH synthesized at CTAB/H2O mass ratios of (a) 1.3, (b) 2.0, (c) 3.5, and (d) 4.5 (CTAB/n-hexanol mass ratio = 0.30, reaction time = 1.0 h, and T = 95.0 °C).

bands at 3550 and 3606 cm−1 can be assigned to O−H stretching, the band at 1008 cm−1 can be assigned to ν1 SO42− stretching, the bands at 1096, 1115, and 1154 cm−1 can be assigned to ν3 SO42− stretching, and the bands at 601 and 660 cm−1 can be assigned to ν4 SO4−2 stretching. The characteristic absorption peaks of HH further verify that all of the samples are composed of HH. Two bands are observed in the region of 2800−3000 cm−1 and are assigned to the asymmetric (2928 cm−1) and symmetric (2848 cm−1) stretching vibrations of CH2, which indicates that CTAB is present on the surfaces of the crystals. The band at 1154 cm−1 corresponding to ν3 SO42− stretching increases in width with an increase in the CTAB/ water ratio, and it exhibits a large red shift in comparison to that of analytical-grade reagent α-HH, where the band appears at 1195 cm−1.40 These changes suggest that CTAB interacts strongly with the SO42− groups of α-HH. The presence of CTAB on the surfaces of α-HH was further confirmed by the energy-dispersive spectrometry (EDS) analysis (Supporting Information, Figure S4). These results suggest that CTAB is strongly adsorbed on the rod-shaped α-HH crystals. Figure 8 shows the FTIR spectra of α-HH synthesized with different SDS concentrations. α-HH synthesized without SDS displays characteristic peaks of α-HH (Figure 8a). The stretching vibration bands of CH2 groups (2800−3000 cm−1) appear rather intensively for all samples, indicating that SDS

Figure 6. Schematic representation of α-HH growth in the CTABstabilized reverse microemulsion with or without SDS.

also at play because a surface free energy argument does not explain the strong dependence of the crystal aspect ratio on the ratio of CTAB/H2O used in the microemulsion. In all cases, the α-HH crystals are much larger than the microemulsion droplets.39 Although the initial nucleation of α-HH occurs within the microemulsion droplets, the growth of large crystals requires the microemulsion droplets to transport reagents to the surface of the growing crystal. The decrease in the aspect ratio of α-HH with an increase in the mass ratio of CTAB/H2O (Figure 2) can be explained by the interactions between the microemulsion droplets and the surfaces of the growing crystals. At a low CTAB/H2O ratio (e.g., 1.3), the microemulsion has a relatively high fraction of water that allows droplets to interact more freely with each other and with the crystal surface (Figure 6a). However, at a high CTAB/H2O ratio (e.g., 4.5), both the side facets and top facets of the crystals tend to adsorb CTAB molecules. The strong adsorption of the surfactants also blocks reactive sites to slow down the rate of addition of new material to the crystal facet. 14140

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for ionic crystals by the selective adsorption of surfactants on the faces.

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ASSOCIATED CONTENT

S Supporting Information *

XRD patterns, TG-DSC curves of the crystals, and EDS analysis of the side facets of the crystals. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*(B.G.) Tel: +86 571 88982026. Fax: +86 571 88273687. Email: [email protected]. (M.Z.Y.) Tel: 1-585-273-2335. Fax: 1-585-273-1348. E-mail: [email protected].

Figure 8. FTIR spectra of α-HH synthesized at SDS/CTAB mass ratios of (a) 0, (b) 0.2, (c) 0.3, and (d) 0.4 (CTAB/H2O mass ratio = 2.5, CTAB/n-hexanol mass ratio = 0.30, reaction time = 1.0 h, and T = 95.0 °C).

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge project 21176219 supported by the NSFC and project 2009AA064002 supported by the Hi-Tech R& D Program of China. We thank J. Wu for his important suggestion regarding the mechanism investigation and L. Wang for his contribution to the crystallography description.

and/or CTAB is present. The bands at 1096 and 1154 cm−1 exhibit gradually enhanced peaks, and the band at 1115 cm−1 becomes masked with an increase in the concentration of SDS (Figure 8b−d). It is the mutual interference between the SO42− stretching bands of α-HH and SO3− in SDS that leads to these changes (1170 and 1065 cm−1 are the characteristic bands of SO3− stretching).41 Also, a significant variation was observed in the two O−H stretching bands at 3550 and 3606 cm−1. Both bands gradually become masked with increasing SDS concentration, indicating a strong interaction between SDS molecules and the H2O molecules of α-HH. The crystal structure of α-HH shows that H2O molecules exist in the channels formed by the −Ca2+−SO42−− chains along the c axis.32,34 The interaction of SDS with the oriented dipoles of H2O can enhance SDS adsorption on the {111} facets. This interaction would then restrain the crystal growth along the caxis direction and thus result in a reduction in the aspect ratio. Therefore, it can be concluded that CTAB and SDS exert their effects on the morphology of α-HH by preferential adsorption onto the different facets: the interaction between CTAB and the {110} and {100} facets results in a rodlike shape, and the strong binding of SDS to the {111} facets leads to a decrease in the aspect ratio of the rods, eventually leading to a morphological transition to platelike shape and then spherical shape as the SDS concentration is increased.

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REFERENCES

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4. CONCLUSIONS The morphology of α-HH was successfully controlled over a large range of size using reverse microemulsions of water, nhexanol, CTAB, and SDS. By adjusting the mass ratio of CTAB/H2O, the aspect ratio of rodlike α-HH crystals can be continuously tuned from 2 to 7 to 180−250. SDS added to the microemulsion suppresses c-axis growth to produce morphological transitions from rods to hexagonal plates and eventually to nanogranules as the SDS concentration is increased. Morphology control is ascribed to the strong preferential adsorption of SDS to the {111} crystal facets and CTAB to the {110} and {100} facets by electrostatic attraction. This study highlights the development of α-HH crystal growth and offers an approach to synthesizing α-HH with abundant morphologies and sizes for its multiple applications. This simple, efficient method may be extended to address the morphology control 14141

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dx.doi.org/10.1021/la302459z | Langmuir 2012, 28, 14137−14142