Preparation of suspensions

Abstract

A method for preparing a suspension of LDH particles comprising the steps of: preparing LDH precipitates by coprecipitation to form a mixture of LDH precipitates and solution; separating the LDH precipitates from the solution; washing the LDH precipitates to remove residual ions; mixing the LDH precipitates with water; and subjecting the mixture of LDH particles and water to a hydrothermal treatment step by heating to a temperature of from greater than 80 C. to 150 C. for a period of about 1 hour to about 144 hours to form a well dispersed suspension of LDH particles in water, wherein said LDH particles in suspension comprise platelets having a maximum particle dimension of up to 400 nm.

Claims

1. A method for preparing a suspension of LDH particles comprising the steps of: a) preparing LDH precipitates by coprecipitation to form a mixture of LDH precipitates and solution, wherein the LDH precipitates and the solution formed in step (a) are left in contact with each other for a period not exceeding 30 minutes; b) separating the LDH precipitates from the solution; c) washing the LDH precipitates to remove residual ions; d) mixing the LDH precipitates with water; and e) subjecting the mixture of LDH particles and water from step (d) to a hydrothermal treatment step by heating to a temperature of from greater than 80 C. to 150 C. for a period of about 1 hour to about 144 hours to form a dispersed suspension of non-aggregated LDH particles in water, wherein said LDH particles in suspension comprise platelets having a maximum particle dimension of up to 400 nm.

2. A method as claimed in claim 1 wherein step (e) comprises subjecting the mixture of LDH particles and water from step (d) to a hydrothermal treatment step by heating to a temperature of from greater than 80 C. to 150 C. for a period of about 1 hour to about 2 hours.

3. A method as claimed in claim 1 wherein the platelets exhibit a particle size distribution of no more than about 27%, based upon the peak value and the peak width at half maximum from the particle size distribution of an intensity average using photon correlation spectroscopy (PCS) measurement.

4. A method as claimed in claim 1 wherein the LDH precipitates and the solution formed in step (a) are left in contact with each other for a period not exceeding 20 minutes.

5. A method as claimed in claim 1 wherein the LDH precipitates and the solution formed in step (a) are left in contact with each other for a period not exceeding 10 minutes.

6. A method as claimed in claim 1 wherein the LDH precipitates and the solution formed in step (a) are left in contact with each other for a period not exceeding 5 minutes.

7. A method as claimed in claim 1 wherein the LDH precipitates and the solution formed in step (a) are left in contact with each other for a period not exceeding 1 minute.

8. A method as claimed in claim 1 wherein the LDH precipitates and solution formed in step (a) are stirred.

9. A method as claimed in claim 1 wherein the hydrothermal treating step is carried out while suppressing boiling.

10. A method as claimed in claim 1 wherein in step (a) a mixed metal ion solution and an alkaline solution are rapidly mixed together.

11. A method as claimed in claim 10 wherein the mixed metal ion solution and the alkaline solution are added together within a time period of less than 1 minute.

12. A method as claimed in claim 1 wherein precipitation in step (a) takes place at a temperature ranging from room temperature up to about 50 C.

13. A method as claimed in claim 1 wherein the LDH precipitates are washed one or more times with deionized water following separation from the solution.

14. A method as claimed in claim 1 wherein the mixture of water and LDH precipitates is held at the elevated temperature for a period of from 2-48 hours in step (e).

15. A method as claimed in claim 13 wherein the mixture of water and LDH precipitates is held at the elevated temperature for a period of from 1-24 hours.

16. A method as claimed in claim 1 wherein the largest dimension of the platelets in the suspension predominantly falls within the range of 20-400 nm.

17. A method as claimed in claim 1 wherein the largest dimension of the platelets in the suspension predominantly falls within the range of 40-300 nm.

18. A method as claimed in claim 1 wherein the suspension contains up to 10% w/w LDH platelets.

19. A method as claimed in claim 1 wherein the suspension contains up to 5% w/w LDH platelets.

20. A method as claimed in claim 1 wherein the suspension contains up to about 1% w/w LDH platelets.

21. A method as claimed in claim 1 wherein the suspension contains less than 1% w/w LDH particles.

22. A method as claimed in claim 1 further comprising the step of removing at least some of the water from the suspension to concentrate the particles to form a more concentrated suspension.

23. A method as claimed in claim 1 wherein step (d) comprises mixing the LDH precipitates with water in order to disperse the LDH precipitates in the water and to break up any loose aggregates of LDH precipitates into platelets.

24. A method for forming LDH particles comprising forming a suspension as claimed in claim 1 and separating the LDH particles from the suspension.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows a schematic diagram of a typical individual particle of a layered double hydroxide;

(2) FIG. 2 shows a schematic diagram demonstrating aggregation of LDH particles;

(3) FIG. 3 shows TEM images of LDH particles from a suspension in accordance with an embodiment of the present invention and from an ethanol suspension of LDH prepared by a conventional method;

(4) FIG. 4 shows the particle size distribution of Mg.sub.2AlCl-LDH samples: (X) stirred for 10 minutes at room temperature, and then dispersed with ultrasonic treatment for 20 minutes, with two peaks at 320 nm and 2300 nm; (Y) aged at 50 C. for 18 hours, and dispersed with ultrasonic treatment for 20 minutes, with two broad peaks at 220 nm and 955 nm; (Z) as-prepared with the current method, with one sharp peak at 114 nm;

(5) FIG. 5 shows the dispersion of Mg.sub.2AlCl-LDH aggregates with heating duration during the hydrothermal treatment at 100 C.;

(6) FIG. 6 shows the relationship between primary particle size of the LDH particles and duration of the hydrothermal treatment step, for hydrothermal treatment steps utilizing a temperature of 100 C.;

(7) FIG. 7 shows the particle size distribution of LDH samples produced using a hydrothermal heating step having a temperature of 100 C. and heating time of from 1 hour to 2 hours;

(8) FIG. 8 shows the particle size distribution of LDH samples produced using a hydrothermal heating step having a temperature of 150 C. and heating time of from 1 hour to 2 hours; and

(9) FIG. 9 shows the particle size distribution of LDH samples produced using a hydrothermal heating step having a temperature of 80 C. and heating time of from 8 hour to 16 hours.

DETAILED DESCRIPTION OF THE DRAWINGS

(10) It will be appreciated that the drawings attached to this specification have been provided for the purposes of illustrating preferred embodiments of the present invention.

(11) FIG. 1 shows a schematic diagram of a typical LDH particle. The typical LDH structure has metal hydroxide sheets that carry a net positive charge due to limited substitution of trivalent cations for divalent cations. As the structure includes metal hydroxide sheets, separate sheets tend to form layers one upon another, with anions or other negatively charged material being positioned between the sheets in order to balance the charge. Where anions are present between the sheets, they are often referred to interlayer anions. It has been found that interlayer anions may be exchanged or substituted.

(12) LDH particles exhibit preferable growth along the a and b axis (as shown in FIG. 1) to form hexagonal platy sheets having an aspect ratio typically of 5-10. As can be seen from FIG. 1, the LDH particle shown schematically in FIG. 1 comprises layers 12, 14, 16, 18, 20 of hydroxide sheets. Each layer represents one positively charged brucite-like hydroxide layer (formula: M.sup.II.sub.nM.sup.III(OH).sub.2(n+1).sup.+) and between two layers are anions and water molecules (X.sup..yH.sub.2O).sub.z, Although FIG. 1 shows five layers, it will be appreciated that the LDH particles may contain a greater or lesser number of brucite-like hydroxide layers. In general, there may be 5-150 such brucite-like hydroxide layers in crystalline LDH particles. Such LDH particles typically have a particle size ranging from about 30-1000 nm in the largest particle dimension, with a thickness typically from about 5-100 nm.

(13) FIG. 2 shows a schematic diagram of aggregation of individual LDH nanoparticles. It has previously been found that LDH particles tend to aggregate together when placed in suspension to form larger particles, typically having a particle size in the range of 1-10 microns. For example, the aggregated particle 22 shown schematically in FIG. 2 comprises an aggregation of a plurality of individual particles, some of which are numbered as 24, 26, 28. (Each of these individual nanoparticles is typically composed of a number of brucite-like hydroxide layers, as shown in FIG. 1.) It has been postulated that aggregation of individual LDH particles to form an aggregated particle occurs due to the following:

(14) 1) the sheets of hydroxides on the top and bottom surfaces of the individual particles are typically positively charged and also typically have anions or other negatively charged material associated therewith to balance the charge. It is possible that the surface anions or surface negatively charged material may be shared between the overlapping part of adjacent particles;

(15) 2) the individual LDH particles have some defects so that they might share some top points and edges of lattice cells; and

(16) 3) amorphus or very small particles may act as a glue between the individual platy particles.

(17) It has been found that the aggregation of the particles causes settling of the LDH particles from suspension.

(18) The inventors have found that the method in accordance with the first aspect of the present invention produces a suspension in which the LDH particles are dispersed and exhibit little tendency to aggregate. Accordingly, suspensions in accordance with the present invention tend to exhibit little or no segregation or settling for extended periods of time, for example, for at least one month and, in some instances, up to 6 months. Without wishing to be bound by theory, the inventors have postulated that the hydrothermal treatment step of the method of the present invention has the following effects:

(19) a) The higher temperatures present during the hydrothermal treating step may provide extra kinetic energy for the individual LDH nanoparticles to undergo stronger Brownian motion to collide with aggregates and break aggregates into individual nanoparticles. Once these nanoparticles are separated individually, they are kept apart from one another due to the electrostatic repulsion between these nanoparticles, because these nanoparticles overall carry positive charges (zeta potential is 30-50 mV);

(20) b) in the hydrothermal treatment step, the LDH particles become more perfect because the higher temperatures in the hydrothermal treating step allow the M.sup.II ions and M.sup.III ions in the hydroxide sheets to move around to a more desired distribution. Thus, the positive charge in each hydroxide sheet becomes more evenly distributed, thus reducing the sharing of surface anions by adjacent LDH particles;

(21) c) some of the smaller amorphus particles probably dissolve during the hydrothermal treatment step to promote the growth of LDH crystallites.

(22) Each of the above postulated mechanisms would act to reduce aggregation of the individual LDH particles.

(23) The inventors have also found that there may also be benefits obtained by causing the precipitation of the LDH particles to occur very quickly. When precipitation processes take place, two processes act to form the particles, namely nucleation and growth. In nucleation, very small particles nucleate from the solution once appropriate precipitating conditions have been reached. Nucleation of particles typically takes place on surface defects in the vessel in which precipitation is occurring or on impurities in the solution, such as dust particles in the solution. Seed particles may also be used. Further precipitation can then occur by virtue of growth, in which precipitating solids are deposited on the nucleated particles to increase the particle size of the particles. In preferred embodiments of the present invention, precipitation occurs by virtue of very rapid mixing of the mixed metal ion solution with an alkaline solution. For example, the mixed metal ion solution and an alkaline solution may be fully mixed together in less than a minute, more preferably less than 20 seconds, even more preferably less than 5 seconds, most preferably with the mixed metal ion solution and alkaline solution being mixed together in less than 2 seconds. This rapid mixing causes a high rate of nucleation of small particles.

(24) It will also be appreciated that vigorous stirring during mixing of the mixed metal ions solution and the alkaline solution is likely to assist in preparing LDH particles having a narrow particle size distribution. It is believed that vigorous stirring may help to evenly spread the metal ions in the alkaline solution, promote the precipitation of LDH particle to occur homogeneously, and lead to a uniform LDH particle size distribution. The uniformity in LDH particles at this stage is advantageous to the stability of the suspension and uniformity of LDH particle size after the hydrothermal treatment.

(25) It is further preferred that the mixture of precipitated particles and solution obtained from mixing the mixed metal ion solution and alkaline solution remains together for not more than 30 minutes after initial mixing. Without wishing to be bound by theory, the inventors have postulated that an aging phenomenon occurs when the precipitated particles and the solution (which is still at a slightly alkaline pH for MgAl-LDH, for example) remain together. This aging phenomenon causes redistribution of lattice ions in the hydroxide, growth of the particles and aggregation of the particles. The present inventors have further postulated that minimising the length of time of contact between the solution and the precipitated LDH particles minimises this aging phenomenon and thereby minimises growth and aggregation of the LDH particles. It is believed that this further enhances the beneficial effects of the hydrothermal treatment step.

(26) The inventors have postulated that the rapid mixing of the salt solution and the alkaline solution within a very short time period provides an equal opportunity for each metal ion to precipitate at the same time and minimises the time for the nucleates to grow, thus resulting in relatively uniform primary LDH crystallites, which assist in obtaining monodispersed LDH particles after the hydrothermal treatment.

Example 1

(27) The following procedure was used to prepare a suspension in accordance with the present invention: 1) Prepare 10 mL salt solution containing 0.3 M MgCl.sub.2, 0.1 M AlCl.sub.3. (Solution A); 2) Prepare 40 mL 0.15 M NaOH solution (Solution B); 3) Add solution A into solution B within 2 seconds to precipitate under vigorous stirring; 4) Stir at room temperature for 10 min; 5) Separate via centrifugation; 6) Wash two times with deionized water via centrifugation; 7) Manually disperse the LDH slurry in 40 mL water and place into an autoclave; 8) Hydrothermally treat the suspension in the autoclave at 100 C. for 8 hrs; 9) Cool down the autoclave to room temperature and store the suspension.
This suspension contained about 0.4% w/w Mg.sub.2Al(OH).sub.6Cl.H.sub.2O, with the product yield being about 60%.

(28) FIG. 3 shows a photomicrograph of a suspension made in accordance with the above procedure, which shows well dispersed, hexagonal shaped particles. FIG. 3 also shows a photomicrograph of an LDH suspension prepared by a conventional method and then dispersed in ethanol. This shows much less well dispersed particles that still exhibit apparent aggregation.

Example 2

(29) The following procedure was used to prepare a suspension in accordance with the present invention: 1) Prepare 10 mL salt solution containing 0.2 M Co(NO.sub.3).sub.2, 0.1 M Al(NO.sub.3).sub.3. (Solution A); 2) Prepare 40 mL alkaline solution 0.15 M NaOH and 0.013 M Na.sub.2CO.sub.3 (Solution B); 3) Add solution A into solution B within 2 seconds to precipitate under vigorous stirring; 4) Stir at room temperature for 30 min; 5) Separate via centrifugation (pH10); 6) Wash two times with deionzed water via centrifugation; 7) Manually disperse the LDH slurry in 40 mL water and place into an autoclave; 8) Hydrothermally treat the suspension in the autoclave at 100 C. for 8 hrs; 9) Cool down the autoclave to room temperature and store the suspension.
This suspension contains about 0.5% w/w Co.sub.2Al(OH).sub.6(CO.sub.3).sub.0.5.H.sub.2O, the product yield is about 60%.

Example 3

(30) The following procedure was used to prepare a suspension in accordance with the present invention: 1) Prepare 10 mL salt solution containing 0.3 M Mg(NO.sub.3).sub.2 and 0.1 M Al(NO.sub.3).sub.3 (Solution A); 2) Prepare 40 mL 0.15 M NaOH (Solution B); 3) Add 10 mL solution A into 40 mL solution B to precipitate under vigorous stirring; 4) Stir at room temperature for 10 min; 5) Separate via centrifugation; 6) Wash two times with deionized water via centrifugation; 7) Disperse the slurry in 40 mL water and place into an autoclave (45 mL); 8) Heat the autoclave at 100 C. for 16 hrs. 9) Cool down the autoclave to room temperature and store the suspension.
This suspension contains about 0.4% w/w Mg.sub.2Al(OH).sub.6NO.sub.3.H.sub.2O, the product yield is about 60%.

Example 4

(31) The following procedure was used to prepare a suspension in accordance with the present invention: 1) Prepare 10 mL salt solution containing 0.3 M MgCl.sub.2 and 0.1 M AlCl.sub.3 (Solution A); 2) Prepare 40 mL 0.15 M NaOH and 0.013 M Na.sub.2CO.sub.3 (Solution B); 3) Add 10 mL solution A into 40 mL solution B to precipitate under vigorous stirring; 4) Stir at room temperature for 10 min; 5) Separate via centrifugation; 6) Wash two times with deionized water via centrifugation; 7) Disperse the slurry in 40 mL water and place into an autoclave (45 mL); 8) Heat the autoclave at 100 C. for 16 hrs. 9) Cool down the autoclave to room temperature and store the suspension.
This suspension contains about 0.45% w/w Mg.sub.3Al(OH).sub.8(CO.sub.3).sub.0.5.H.sub.2O, the product yield is about 60%.

Example 5

(32) The following procedure was used to prepare a suspension in accordance with the present invention: 1) Prepare 10 mL salt solution containing 0.3 M MgCl.sub.2, 0.04 M FeCl.sub.3 and 0.06 M AlCl.sub.3 (Solution A); 2) Prepare 40 mL 0.15 M NaOH (Solution B); 3) Add 10 mL solution A into 40 mL solution B to precipitate under vigorous stirring; 4) Stir at room temperature for 10 min; 5) Separate via centrifugation; 6) Wash two times with deionized water via centrifugation; 7) Disperse the slurry in 40 mL water and place into an autoclave (45 mL); 8) Heat the autoclave at 100 C. for 16 hrs. 9) Cool down the autoclave to room temperature and store the suspension.
This suspension contains about 0.4% w/w Mg.sub.2Fe.sub.0.04Al.sub.0.06(OH).sub.2Cl.H.sub.2O, the product yield is about 60%.

Example 6

(33) The following procedure was used to prepare a suspension in accordance with the present invention: 1) Prepare 10 mL salt solution containing 0.3 M MgCl.sub.2 and 0.1 M AlCl.sub.3 (Solution A); 2) Prepare 40 mL 0.15 M NaOH (Solution B); 3) Add 10 mL solution A into 40 mL solution B to precipitate under vigorous stirring; 4) Stir at room temperature for 10 min; 5) Separate via centrifugation; 6) LDH slurry is exchanged with Na.sub.2SO.sub.4 (40 mL 0.05 M) for 30 min; 7) Separation and then wash 1 time; 8) Disperse the slurry in 40 mL water and place into an autoclave (45 mL); 9) Heat the autoclave at 100 C. for 16 hrs. 10) Cool down the autoclave to room temperature and store the suspension.
This suspension contains about 0.4% w/w Mg.sub.2Al(OH).sub.6(SO.sub.4).sub.0.5H.sub.2O, the product yield is about 60%.

(34) FIG. 4 shows a graphical representation of the particle size distribution of suspensions of LDH particles prepared in accordance with the present invention (line Z in FIG. 4), the particle size distribution of freshly prepared LDH precipitates that have been subjected to ultrasonication for 20 minutes (i.e. no hydrothermal treatment, Line X in FIG. 4) and the particle size distribution of fresh LDH precipitates that have been subjected to ultrasonication for 20 minutes and aged overnight at 50 C. (line Y in FIG. 4). The particle size distribution was measured using photon correlation spectroscopy (PCS).

(35) The Mg.sub.2AlCl-LDH suspension obtained after hydrothermal treatment at 100 C. for 16 hours has a narrow particle size distribution with an equivalent hydrodynamic diameter of 114 nm, with all particles inclusively within 45-250 nm. However, the suspensions of the same LDH material made conventionally without a hydrothermal treatment and dispersed with the assistance of ultrasonication in water have a much wider particle size distribution and larger diameters (200-3000 nm). In particular, the suspension of freshly precipitated Mg.sub.2AlCl-LDH, after ultrasonication for 20 min, consists of a bimodal particle size distribution, with diameters at 320 nm and 2300 nm, respectively. After aging at 50 C. overnight, the aggregates decrease in size to 220-955 nm. This means that the aggregates can be only partially segregated after aging and/or ultrasonication. However, the inventor's experiments have demonstrated that these partially segregated particles can be also segregated further into individual nanoparticles by the process of the present invention. Visually, the well-dispersed suspension looks very transparent while the conventional ones are turbid. These evidences suggest that the aggregates are completely segregated and well dispersed into much smaller particles after the hydrothermal treatment at 100 C. for 16 hours.

(36) FIG. 5 shows a graphical representation of particle size distribution for LDH suspensions subjected to hydrothermal treatments of differing duration and a hydrothermal treatment temperature of 100 C. As can be seen from FIG. 5, after hydrothermal treatment at 100 C. for a suitable period of time (4, 8, 16 or 48 hours), the LDH particles in as-made Mg.sub.2AlCl-LDH suspensions become very uniform, with a narrow particle size distribution. However, the 2-hour or 144-hour treatment leads to extra distribution bands with much bigger particles (FIG. 5), indicating the presence of some degree of aggregation. It seems that 2-hour heat treatment at 100 C. does not disperse all aggregates into individual LDH crystallites. Compared with the freshly prepared LDH suspension (FIG. 4, curve X), 2-hour treated LDH suspension contains much smaller particles (69 vs. 320 nm), which further suggests that the freshly prepared LDH aggregates are only partially segregated with ultrasonication. However, there are two groups of bigger particles with size of 800 nm and over 4 m, respectively. Subsequently, these two groups of particles disappeared when the treatment time is extended to 4 hours, resulting in one narrow particle size distribution band (89 nm) in the LDH suspension. This suggests that 2-hour treatment at 100 C. is not sufficient to fully disperse the aggregates while the 4-hour treatment completes the segregation of LDH particles. As the treatment time increases further, the LDH particle size distribution band is shifted to the big particle size side, indicating the continuous growth of LDH particles with time. However, if the hydrothermal treatment is continued to 144 hours at 100 C., the LDH particle size distribution band becomes quite broad and the LDH crystallite size is hundreds of nanometers. As a consequence, a weak and broad band is seen around 5 m, presumably due to the re-aggregation of these bigger LDH crystallites.

(37) The above results suggest that, for the shorter hydrothermal treatment times in the present invention, temperatures in the upper part of the treatment temperature range should be used, whilst at the longer hydrothermal treatment times, temperatures in the lower part of the range should be used.

(38) Regardless of the aggregates, the primary Mg.sub.2AlCl-LDH particle size, i.e. the peak value from PCS curves in FIG. 5 constantly increase from 70 to 300 nm with increasing the hydrothermal treatment time from 2 to 144 hours, as clearly shown in FIG. 6 (bold curve). The relationship is almost linear, indicating a rough growth rate of 1.5 nm per hour. It is therefore concluded that the Mg.sub.2AlCl-LDH particle size can be tailored in the range of 70-300 nm by adjusting hydrothermal treatment time at 100 C.

(39) In the case of Mg.sub.2AlCO.sub.3-LDHs (i.e. CO.sub.3 being the interlayer anion), the relationship between the treatment time and the LDH particle size is quite similar. As shown in FIG. 6 (thin curve), the hydrodynamic diameter of LDH-OO.sub.3 particles almost linearly increase from 45 to 118 nm with the time from 4 to 72 hours in the logarithmic scale. Very interestingly, the average particle size of LDH-OO.sub.3 is smaller roughly by 40-50 nm than that of LDH-Cl at every identical time point, suggesting a similar growth pattern but different growth preference in these two systems.

(40) The effect of hydrothermal treatment temperature was also investigated. Table 1 shows the particle size of Mg.sub.2AlCl-LDH under the various hydrothermal treatments as specified in Table 1.

(41) TABLE-US-00001 TABLE 1 Mg.sub.2Al-LDH-Cl particle size (nm) under different conditions. time 80 C. 100 C. 125 C. 150 C. 2 hr 69 15 89 19 94 20 (465 110) 4 hr .sup.99 36 .sup.a 89 16 111 22 118 18 .sup.(2500 660) .sup.b (3300 540) 8 hr 87 19 101 19 124 30 160 30 (4600 420) 16 hr 92 18 114 19 162 46 194 34 (4200 500) (4300 520) 48 hr 159 29 144 hr 284 91 (4200 500) Note: .sup.a The peak value and the peak width at half maximum of small particles are from the particle size distribution on intensity average. .sup.b The data in the parenthesis are corresponding to big size particles that are not dispersed or re-aggregated.

(42) The treatment temperature seems to more strongly influence LDH particle size than treatment time. As given in Table 1 for Mg.sub.2AlCl-LDHs, an increase in temperature by 10 C. can lead to an increase in the hydrodynamic diameter by 10-15 nm on average after hydrothermal treatment for 8 or 16 hours, or the primary particle size is doubled when temperature increases from 80 to 150 C. for treatment time of 8 or 16 hours. This shows the quick growth of LDH crystallites at higher temperatures. In contrast, if treatment time is short, such as 2 or 4 hours, the particle growth seems to be much slower (3-5 nm per 10 C.). This comparison suggests that in the beginning of hydrothermal treatment, the major event is segregation of LDH aggregates to individual LDH crystallites in the aqueous suspension. Subsequently LDH crystallites grow continuously with time.

(43) Similarly, incomplete dispersion is observed at lower temperatures for short treatment duration while re-aggregation takes place at higher temperatures even at short treatment time (see Table 1). For example, the treatment at 80 C. for 4 hours does not disperse all the aggregates while the treatment at 150 C. for 4 hours is long enough to produce bigger LDH crystallites to re-form the micrometer-scaled aggregates.

(44) A number of LDH suspensions were prepared in accordance with the present invention. Table 2 summarises the conditions under which the suspensions were prepared and gives the average particle size and the measured zeta potentials.

(45) TABLE-US-00002 TABLE 2 Summary of Mg.sub.2Al-LDH Particle size and Zeta potential Particle size Potential Compounds .sup.a Conditions (nm) (mV) Mg.sub.2AlCl-LDH 100 C., 4 h, 0.4%.sup.b 89, 95, 97, 51.9 87, 92.sup.c 100 C., 8 h, 0.4% 101 48.3 100 C., 16 h, 0.4% 114 47.0 100 C., 48 h, 0.4% 159, 159, 49.1 171, 160 100 C., 144 h, 0.4% 284 47.9 100 C., 16 h, 1.0% 101 53.0 100 C., 16 h, 2.0% 116 51.3 100 C., 16 h, 3.0% 115 45.0 100 C., 16 h, 4.0% 120 50.4 100 C., 72 h, 1.0% 155 80 C., 8 h, 1.0% 85 38.5 80 C., 16 h, 1.0% 89 36.9 Mg.sub.2AlCO.sub.3-LDH 100 C., 4 h, 0.4% 46, 43 39.5, 48.5 100 C., 8 h, 0.4% 58 37.2 100 C., 16 h, 0.4% 75, 71, 68 42.0, 48.9 100 C., 72 h, 0.4% 118 48.5 Mg.sub.2AlNO.sub.3-LDH 100 C., 16 h, 0.4% 112, 117, 119 Mg.sub.2AlSO.sub.4-LDH 100 C., 16 h, 0.4% 142 28.4, 28.1 Mg.sub.3AlCl-LDH 100 C., 16 h, 0.4% 106 43.8, 45.7 Mg.sub.3AlCO.sub.3-LDH 100 C., 16 h, 0.4% 143 33.6 Ni.sub.2AlCO.sub.3-LDH 100 C., 16 h, 0.5% 41, 43 40.6, 41.2 Co.sub.2AlCl-LDH 100 C., 16 h, 0.25% 127 37.2, 38.1 Co.sub.0.5Mg.sub.1.5AlCl- 100 C., 16 h, 0.2% 125 43.1, 42.8 LDH Mg.sub.2Al.sub.0.7Fe.sup.III.sub.0.3Cl- 100 C., 16 h, 0.4% 123 LDH Mg.sub.2Al.sub.0.6Fe.sup.III.sub.0.4Cl- 100 C., 16 h, 0.4% 110 37.7, 39.0 LDH Mg.sub.2Al.sub.0.5Fe.sup.III.sub.0.5Cl- 100 C., 16 h, 0.4% 90 LDH Notes for Table 2: .sup.a The composition of compound is nominal. .sup.bThis indicates that the suspension was hydrothermally treated at 100 C. for 4 hours, with nominal LDH weight percentage of 0.4% in the suspension. .sup.cMultiple value was obtained from different repeated suspensions

Example 7

(46) A further sample of LDH particles were prepared using the same procedure as given in Example 1, except that the hydrothermal treatment step in step (8) of example 1 was conducted at 100 C. for 1 hour, 1.5 hours and 2 hours. Step (7) of example 1, which involved manually dispersing the LDH slurry in 40 mls water and placing the slurry into an autoclave, consisted of hand shaking the water and LDH particles.

(47) The particle size distributions of the suspensions obtained in these experimental runs are shown in FIG. 7. The average particle size and the particle size distribution, determined from the peak value and the peak width at half maximum from the particle size distribution of an intensity average using photon correlation spectroscopy (PCS) measurement, are shown in Table 3.

(48) TABLE-US-00003 TABLE 3 Sample Preparation Peak size Conditions (nm) 100 C., 1 h 79 15 100 C., 1.5 h 77 15 100 C., 2 h 91 17

Example 8

(49) A further batch of experimental runs were conducted using the procedure outlined in example 7, except that the hydrothermal treatment step in step (8) of example 1 was conducted at 150 C. for 1 hour, 1.5 hours and 2 hours. Step (7) of example 1, which involved manually dispersing the LDH slurry in 40 mls water and placing the slurry into an autoclave, consisted of hand shaking the water and LDH particles.

(50) The particle size distributions of the suspension obtained in these experimental runs are shown in FIG. 8. The average particle size and the particle size distribution, determined from the peak value and the peak width at half maximum from the particle size distribution of an intensity average using photon correlation spectroscopy (PCS) measurement, are shown in table 4.

(51) TABLE-US-00004 TABLE 4 Sample Preparation Peak size Conditions (nm) 150 C., 1 h 121 30 150 C., 1.5 h 124 27 150 C., 2 h 139 29

(52) The results obtained in example 8 demonstrate that it is possible to perform the method of the present invention at a temperature of 150 C. whilst still obtaining a narrow particle size distribution (having no secondary peak in the distribution curve at larger particle sizes). In some instances where the process of the present invention is operated at the higher limits of the temperature ranges described herein, it may be desirable to carefully disperse the precipitates before the hydrothermal treatment. This assists in ensuring that the final particle size distribution does not exhibit secondary peaks at particle sizes above 400 nm.

Example 9

(53) The following procedure was used to prepare a suspension in accordance with the present invention: 1) Prepare 10 mL salt solution containing 0.6 M MgCl.sub.2, 0.2 M AlCl.sub.3. (Solution A); 2) Prepare 40 mL 0.30 M NaOH solution (Solution B); 3) Add solution A into solution B within 2 seconds to precipitate under vigorous stirring; 4) Stir at room temperature for 10 min; 5) Separate via centrifugation; 6) Wash two times with deionized water via centrifugation; 7) Manually disperse the LDH slurry in 30 mL water and place into an autoclave; 8) Hydrothermally treat the suspension in the autoclave at 80 C. for 8 hr; 9) Cool down the autoclave to room temperature and store the suspension.
This suspension contained about 1.0% w/w Mg.sub.2Al(OH).sub.6Cl.H.sub.2O, with the product yield being about 60%.

(54) The particle size distributions of the suspensions obtained in this run together with an example that was heated at 80 C. for 8 and 16 hr are shown in FIG. 9. The average particle size and the zeta potential are shown in Table 5.

(55) TABLE-US-00005 TABLE 5 Sample Preparation Peak size Zeta Potential Conditions (nm) (mV) 80 C., 8 h 85 20 38.5 80 C., 16 h 89 19 36.9

(56) The present invention provides a suspension of LDH particles in water and a method for preparing such a suspension. In some embodiments, the suspension is stable for extended periods of time and will typically not exhibit any separation or segregation for up to a month from formation. Suitably, such stable suspensions will not exhibit separation or segregation for a period of up to 6 months from formation. The suspension includes LDH particles having a narrow particle size distribution, with the largest particle dimension of the particles predominantly falling within the range of 20-400 nm, more suitably 40-300 nm. The suspension may contain up to 10% w/w LDH particles, although the suspensions more preferably contain 1% w/w or less LDH particles. Unlike Liu et al, which shakes and heats the suspension at atmospheric pressure, the present invention utilises a hydrothermal heating step. The present invention also uses higher heating temperatures and, in some embodiments, different conditions in the co-precipitation step, when compared to Liu et al. In particular, Liu et al is specific in directing the reader to age the precipitated particles and alkaline solution formed in the precipitation step for 1 hour. In contrast, the present inventors have found that the precipitated particles and the solution remaining after precipitation should desirably remain in contact with each other for not longer than 30 minutes.

(57) The suspensions in accordance with the present invention may be used in biomedical applications, for example, to prepare bio-inorganic hybrid composites as described in European patent application no. 987328, the entire contents of which are herein incorporated by cross reference. The suspension may be also used to manufacture polymer/clay nanocomposites for membrane separation, biomedical materials and other uses. The suspensions may also be useful as a component in the manufacture of polymers, with the LDH nanoparticles acting as a filler in the polymer.

(58) The current invention can be used to make, for example LDH where the interlayer anion is chloride, nitrate, sulfate, and carbonate. Where the suspension is to be used in an application that requires exchange of the interlayer anion, LDH-carbonate is not preferred, but it is a good candidate to make polymer-LDH nanocomposite as well as polymer fillers, retardants etc. In some cases, LDH-CO.sub.3 can be exchanged with CF, NO.sub.3.sup. and SO.sub.4.sup.2 in a lightly acidic solution.

(59) If it is desired to make LDH that is free of carbonate as the interlayer anion, it is preferred to conduct the co-precipitation step in an inert atmosphere, such as in a nitrogen atmosphere, because air contains carbon dioxide that may be absorbed by the solutions, leading to carbonate ions going into the interlayer space, or in a carbon dioxide or carbonate free environment.

(60) Those skilled in the art will appreciate that the present invention may be susceptible to variations and modifications other than those specifically described. It is to be understood that the present invention encompasses all such variations and modifications that fall within its spirit and scope.