Colloidal Calcium Hydroxide in Organic Medium: Synthesis and Analysis

JOURNAL OF COLLOID AND INTERFACE SCIENCE ARTICLE NO.

189, 151–157 (1997)

CS974799

Colloidal Calcium Hydroxide in Organic Medium: Synthesis and Analysis BRUNO DELFORT, 1 MAURICE BORN, AGNE`S CHIVE´,

AND

LOI¨C BARRE´

Institut Franc¸ais du Pe´trole, 1-4 av de Bois-Pre´au, Rueil Malmaison, France Received November 14, 1996; accepted January 27, 1997

A colloidal dispersion of calcium hydroxide in a hydrocarbonated medium was synthesized by the hydrolysis of calcium hydride in the presence of a surfactant under specific experimental conditions. The hydrolysis of calcium hydride is assumed to take place in a water in oil microemulsion. The resulting product can be defined as a distribution in hydrocarbon of inorganic particles made of crystallized calcium hydroxide surrounded by surfactant in a reverse type micelle association. Morphological characterization of the colloidal dispersion has been investigated using small angle X-ray scattering, transmission electron microscopy, and X-ray diffractometry. The inorganic particles appear as disks with diameters ranging from 120 to 300 A˚ and a thickness of 30 A˚. q 1997 Academic Press Key Words: colloidal calcium hydroxide; synthesis; small angle X-ray scattering; X-ray diffractometry; transmission electron microscopy.

vestigated by different techniques, such as small angle Xray scattering (SAXS) or small angle neutron scattering (SANS) and transmission electron microscopy (TEM) (12– 17). A core shell model whereby salt spheres are surrounded by a stabilizing surfactant shell has been proposed. In a previous study, we reported the synthesis of colloidal calcium thiophosphate by an original process (18). In this article, we introduce a new colloidal dispersion in which the inorganic particles are made of calcium hydroxide and the surfactant is a calcium alkylarylsulfonate. We will begin by presenting the synthesis of such a compound, and then we will analyze it from both chemical and morphological standpoints. EXPERIMENTAL

Materials INTRODUCTION

Alkaline and earth alkaline inorganic derivatives as colloidal species in hydrocarbon media have attracted considerable interest especially in the field of additives for lubricants (1, 2). Most of the studies concern colloidal calcium or magnesium carbonate (3, 4), calcium borate (5, 6), and sodium or potassium carbonate (7). They consist of the colloidal dispersion of inorganic cores surrounded by a surfactant according to a reverse micelle-type association. These colloidal compounds are generally prepared by achieving the synthesis of the inorganic chemical species at the same time as the micellization in a one step process. For example, colloidal calcium carbonate is prepared by the reaction of calcium hydroxide with carbon dioxide under specific conditions, in the presence of a surfactant such as calcium alkyl sulfonate or calcium alkylphenate (8, 9). The reaction mechanism of some of these compounds has been investigated (10) and colloidal parameters such as shapes and sizes of the particles have been determined (11). The structure of these different compounds has been in1

To whom correspondence should be addressed.

The surfactant is an industrial C-24 monoalkylarylsufonic acid containing 30% by weight nonactive diluting oil (Paramins), with an equivalent average molecular weight of 700. Its calcium salt derivative was prepared by neutralization with calcium hydroxide according to the conventional procedure. Calcium hydride was used as received (Merck). Inorganic diluting oil was a 130 Neutral Solvent grade (Total). Synthesis Calcium hydride (60 g, 1.38 mol) was dispersed in a solution of 90.4 g (0.126 mol) of calcium alkylarylsulfonate in 70.3 g of mineral oil, 350 cc of toluene, and 15 cc of methanol. The mixture was maintained at a temperature of between 5 and 107C, then a solution of 44.9 g (2.49 mol) of water in 155 cc of tetrahydrofuran was slowly added for 5 h while stirring vigorously. After a few additional hours of reaction, the mixture was filtered and the volatile solvents were removed under vacuum evaporation to yield 209 g of a brown viscous and homogeneous product. Analysis The infrared spectrum was recorded with a Digilab FTS40 spectrometer through two KBr plates.

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0021-9797/97 $25.00 Copyright q 1997 by Academic Press All rights of reproduction in any form reserved.

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Elemental analysis of calcium and sulfur was performed by X-ray fluorescence with a Philips PW 1480 spectrometer. Potentiometric analysis was performed using a Tacussel TT automatic titrator according to the ASTM 2896 method. Dialysis of colloidal dispersions was performed in n-heptane through a rubber membrane according to a well-known procedure (19). Dialysis time ranged from 8 to 16 h. Thermogravimetric analysis was performed with Mettler TG50 apparatus at a heating rate of 57C/min. Transmission electron microscopy (TEM) measurements were performed with a JEOL 120 CX apparatus. A drop of a very dilute suspension of colloidal calcium thiophosphates in xylene was placed on a copper grid covered with carbon film. Electron micrographs were taken of the particles retained on the film after evaporation of the solvent. Small-angle X-ray scattering (SAXS) measurements were performed with a Cu rotating anode (Rigaku) providing an ˚ , a curved coated X-ray beam with a wavelength of 1.54 A gold mirror combined with a Ni filter, and a one-dimensional position-sensitive proportional counter (Elphyse). The detector had a resolution of 150 mm (FWHM). The X-ray generator was operated at 1 kW (40 kV 1 25 mA). The range of scattering angles corresponded to wave vectors be˚ 01 for a sample-to-detector distance tween 0.01 and 0.22 A of 80 cm. For the experiments, the colloidal suspension was diluted to approximately 1 wt% of calcium hydroxide in xylene and was placed in a 1.85-mm-diameter sealed glass capillary. After normalization in terms of thickness, transmission, and measuring time, the solvent (xylene) signal was subtracted from the sample signal. Experimental data were converted to an absolute scale and desmeared following Lake’s method (20). X-ray diffractometry measurements were performed on a u-2u Philips diffractometer. The incident wavelength was ˚ , and the detector moved step by step ( D2u CoKa Å 1.789 A Å 0.057 ) between 27 and 827 2u. The measuring time for each step was 5 s. RESULTS AND DISCUSSION

In this system, the synthesis of calcium hydroxide and its micellization occur simultaneously in a one-step process. The synthesis involves the hydrolysis of calcium hydride, which leads to a colloidal species in the presence of a surfactant. This takes place in a multiphase system, where a solid reactant such as calcium hydride is dispersed in a mixture of apolar solvents containing a surfactant. This is illustrated in Fig. 1. The hydrolysis reaction is controlled by the introduction of water and it is very fast and exothermic. In our experiments the surfactant is calcium alkylarylsulfonate, but alkylarylsulfonic acid could also be used, producing its calcium salt according to in situ neutralization, since the conversion of calcium hydride to calcium hydroxide has started. The precise mechanistic details of such a synthesis have

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FIG. 1. Synthesis of colloidal calcium hydroxyde.

not yet been fully investigated, but it can be suggested that this process initially involves the addition of water to the system, leading to a microemulsion water in oil, followed by the adsorption of these reverse micelles onto the solid calcium hydride particles allowing hydrolysis to take place inside the micelles. In our experiments, calcium hydride was used in excess and was removed by filtration at the end of the reaction. Some additional experiments using calcium oxide instead of calcium hydride have been performed yielding smaller amounts of colloidal calcium hydroxide. Optimization is in progress and the results will be communicated. The synthesized colloidal calcium hydroxide will be characterized from a chemical point of view first and then from a more physical one, including morphological characteristics. Chemical Structure The chemical composition of the synthesized product was determined using different techniques. This is summarized in Fig. 2 and discussed below. The hydrolysis of calcium hydride produces exclusively calcium hydroxide. This was confirmed by the infrared spectrum as shown in Fig. 3 with typical sharp nOH signals at 3642 cm01 . Calcium hydroxide is intrinsically insoluble in hydrocarbonated media. Therefore, since the medium remains optically homogeneous, it can be suggested that calcium hydroxide is incorporated in apolar hydrocarbonated solvents as reverse micelles surrounded by a layer of surfactant. The potentiometric titration of calcium hydroxide is found to be 7.18 basic meq per gram. This corresponds to a content of 26.6% by weight of calcium hydroxide in the crude product (i.e., colloidal dispersion in oil), and therefore to 14.4%

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FIG. 2. Chemical composition of colloidal calcium hydroxide.

by weight of calcium. The total calcium content in the crude colloidal product is found to be 16.3% and includes the calcium from the surfactant counter ion which is consequently estimated at 1.9%. In this experiment, the synthesis was performed in a diluting oil, with one part originating from the nonactive part of the surfactant representing 30% by weight and an additional

part being added during the synthesis in order to obtain a liquid-state final product. Therefore, the analysis of the crude product depends on this dilution. The exact content of both the diluting oil and the colloidal fraction—i.e., inorganic particles plus surfactant—was determined using a dialysis technique through a rubber membrane in refluxing n-heptane. This allows the larger molecules such as colloidal spe-

FIG. 3. Infrared spectra of colloidal calcium hydroxide (colloidal fraction).

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FIG. 4. Thermogravimetric behavior of colloidal calcium hydroxide. Determination of surfactant and inorganic particle content.

cies which remain inside the membrane to be separated, while the smaller molecules such as the diluting oil dialyze through the pores of the membrane. From this analysis, the colloidal fraction is found to be 54.4% by weight of the crude product. Elemental analysis of the two colloidal and noncolloidal fractions indicates that all the calcium—i.e., all the calcium hydroxide—is located in the nondialyzing colloidal fraction. Another important parameter is the average ratio between the particle inorganic cores and the surfactant. This was determined using thermogravimetric analysis and is shown in Fig. 4. A sample of the isolated colloidal fraction was analyzed and exhibited a weight loss corresponding to the surfactant hydrocarbonated chain degradation in the 350 to 5007C range. This allows us to determine the weight fraction of the organic surfactant part and that of the inorganic calcium hydroxide including the weight of the calcium surfactant counter ions. The resulting calcium hydroxide will be further converted to calcium oxide in the 670 to 7807C range through the loss of a molecule of water. With 50% in weight of inorganic species and 50% of surfactant chain, the average calcium hydroxide inorganic particle to surfactant ratio in the colloidal fraction is found to be about 1. Physical Structure The previous determinations are rather statistical and do not take account of shape, size, and distribution of the colloidal species. To access this information, other investigations have been performed. X-ray diffractometry. The X-ray diffractometry spectrum of the dialysed product (Fig. 5) exhibits Bragg peaks

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which can be ascribed to Portlandite Ca(OH)2 (JCPDS, 4˚; c Å 733; hexagonal structure of Brucite type; a Å 3.59 A ˚ 4.91 A ). However, the 00l reflections, which correspond to spacing between planes perpendicular to the c axis, are very broad compared to the hk0 reflections. This indicates the high anisotropy of the crystallite. Crystallite sizes have been measured in different directions, parallel (001 reflection) or perpendicular (100 and 110 reflections) to the c axis, using the Sherrer formula (21) T( ⊥hkl) Å

Kl , ( D2u )cos u

where D2u is the width at half of the maximum hkl reflection corrected by instrumental broadening. The calculation gives ˚ in the ab direction and a value of a value of 180 { 20 A ˚ 30 A in the c direction. Small angle X-ray scattering. The scattering intensity per unit volume, I(q), from diluted particles in a solvent can be expressed in the form (22) I(q) Å A 2e ne£ ( r1 0 r2 ) 2 F(q)S(q), where q is the scattering vector which has a magnitude of 4p sin u / l, Ae is the amplitude scattered by one electron, i.e., A 2e Å 7.9 1 10 026 cm2 , ( r1 0 r2 ) 2 is the contrast term expressing the electronic density difference between particles and solvent, £ is the volume of the particle, F(q) is the form factor, and S(q) is the structure factor. The sample dilution minimizes the possibility of particle

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FIG. 5. Comparison of the X-ray diffractometry diagram of the dialyzed fraction with the Portlandite (sticks) Ca(OH)2: JCPDS 4-733; indexation of the peaks are reported on the graph.

interaction and we consider that S(q) is close to 1 in the experimental q range. The electronic densities of the constituents of the system are given in Table 1. From these values, it is clear that the alkyl aryl group of the surfactant does not contribute to the scattered intensity. The sample will be considered a twophase system: micellar cores (including Ca(OH)2, the counterion, and the sulfonate group) and the solvent composed of xylene, diluting oil, and the alkyl aryl group. The log(I(q)) versus log(q) plot is shown in Fig. 6. At large q values, we observe a q 04 dependence which is the Porod limit. At small q values we do not observe a flattening of the curve which would indicate the beginning of the Gui˚ 01 q range, the intensity nier domain. In the 10 02 –10 01 A 02.1 follows a q dependence. This behavior is characteristic of flat objects and is in accordance with WAXS results. In

this case the form factor for flat disks of radius R and of thickness 2h can be expressed by (22)

F

F(q) Å n

S

2 (qh) 2 exp 0 (qR) 2 3

DG

for 1/R õ q õ 1/h.

TABLE 1 Electronic Densities of the Different Species Electronic density ˚ 3) (e0/A

Constituents

a

Ca(OH)2 Ca// SO30

0.692 0.467a 1.08a

Alkyl aryl group Oil Xylene

0.266a 0.266a 0.284a FIG. 6. SAXS spectrum of the diluted product.

From reference (10).

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morphologies can be observed: individual and circular ob˚ , and more jects with diameters in the range 120–300 A ˚ and large contrasted rods with small dimensions of 30–60 A ˚ . The rods are located dimensions in the range 120–300 A into large aggregates. These two morphologies can be interpreted either as two kinds of objects (rods and spheres), or as thin disks seen on the edge and on the face. The second explanation seems correct with respect to SAXS and Xray diffractometry measurements. Moreover, the dimensions correspond fairly well with those measured by X-ray. CONCLUSION

FIG. 7. Ln (q 2 I(q) versus plot of the SAXS spectrum.

The ln(q 2 I(q)) versus q 2 plot (Fig. 7) shows linear be˚ 01 . This havior in the small q domain up to qmin Å 10 02 A ˚ . The means that disk radius is larger than 1/qmin Å 100 A thickness 2h of the disks can be estimated from the slope ˚ , in good agreement of this curve and we find a value of 32 A with the WAXS value. Transmission electron microscopy. The transmission electron micrograph of a direct deposit of the diluted micellar dispersion onto the carbon film is shown in Fig. 8. Two

This synthesis leads to a new stable colloidal product, containing up to 27% by weight calcium hydroxide in a hydrocarbon medium. The synthesis of colloidal calcium hydroxide involves the hydrolysis under specific conditions of calcium hydride in organic solvents, in the presence of a surfactant. It may be suggested that the reaction could take place in a water in oil microemulsion. The colloidal species has been described from chemical and physical standpoints. This compound could be defined as colloidal calcium hydroxide in a reverse micelle association. The inorganic core is made of crystallized calcium hydroxide, with the structure of Portandlite. These cores are found to be shaped like thin disks, with ˚ and a thickness of diameters ranging from 120 to 300 A ˚. 30 A These sizes were measured with good agreement using three different techniques, namely SAXS, X-ray diffractometry, and TEM. This agreement is illustrated in Table 2.

FIG. 8. Transmission electron micrograph of colloidal calcium hydroxide particles.

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TABLE 2 Comparison of Dimensions of Micellar Cores Measured by SAXS, X-Ray, Diffractometry, and TEM

SAXS X-ray diffractometry TEM

Diameter ˚) (A

Thickness ˚) (A

ú200 180 120–300

32 30 §30

Further experiments will include investigations concerning the micellization mechanism and the influence of experimental synthesis conditions on chemical and colloidal structure. ACKNOWLEDGMENTS The authors thank Phillipe Dascotte for microscopy experiments, Isabelle Cle´menc¸on and Bernadette Rebours for WAXS, and Didier Espinat for helpful discussions.

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5. Inoue, K., J. Soc. Trib. Lub. Eng. 49, 263 (1993). 6. Inoue, K., and Nose, Y., ‘‘Method for Producing Alkaline Earth Metal Borate Dispersions,’’ U.S. Patent No. 4 683 126, 1987. 7. Hunt, M. W., ‘‘Overbased Alkali Metal Sulfonates,’’ U.S. Patent No. 4 867 891, 1989. 8. Whittle, J., ‘‘Method of Preparing Overbased Calcium Sulfonates,’’ U.S. Patent No. 4 427 559, 1984. 9. Belle, C., Beraud, C., Faure, D., Gallo, R., Hornaert, P., Martin, J. M., and Rey, C., J. Chim. Phys. 87, 93 (1990). 10. Roman, J. P., Hoornaert, P., Faure, D., Bivier, C., Jaquet, F., and Martin, J. M., J. Colloid Interface Sci. 144(2), 324 (1991). 11. Mansot, J. L., Hallouis, M., and Martin, J. M., Colloids Surf. A: Physicochem. Eng. Aspects 71, 123 (1993). 12. O’Sullivan, T. P., Vickers, M. E., and Heenan, R. K., J. Appl. Crystallogr. 24, 732 (1991). 13. Markovic, I., Ottewill, R. H., Cebula, D. J., Field, I., and Marsh, J. F., Colloid Polym. Sci. 262(8), 648 (1984). 14. Markovic, I., and Ottewill, R. H., Colloid Polym. Sci. 264(1), 65 (1986). 15. Markovic, I., and Ottewill, R. H., Colloid Polym. Sci. 264(5), 454 (1986). 16. Ottewill, R. H., Sinagra, E., McDonald, I. P., Marsh, J. F., and Heenan, R. K., Colloid Polym. Sci. 270(6), 602 (1992). 17. Giasson, S., Espinat, D., Palermo, T., Ober, R., Pessah, M., and Morizur, M. F., J. Colloid Interface Sci. 153(2), 335 (1992). 18. Delfort, B., Chive´, A., and Barre´, L., J. Colloid Interface Sci. 186, 300–306 (1997). 19. Altgrelt, K. H., and Gouv, T. H., in ‘‘Chromatography in Petroleum Analysis,’’ Vol. 11, pp. 417–419. Dekker, New York, 1979. 20. Lake, J. A., Acta Crystallogr. 23, 191 (1967). 21. Klugg, H. P., and Alexander, L. E., ‘‘X-Ray Diffraction Procedures for Polycrystalline and Amorphous Materials,’’ Wiley Interscience, New York, 1974. 22. Guinier, A., and Fournet, G., ‘‘Small Angle Scattering of X-Rays.’’ Wiley, New York, 1955.

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