Controlling Particle Size of a Poorly Water-Soluble Drug Using

Ind. Eng. Chem. Res. 2009, 48, 7581–7593

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Controlling Particle Size of a Poorly Water-Soluble Drug Using Ultrasound and Stabilizers in Antisolvent Precipitation Sameer V. Dalvi and Rajesh N. Dave* Otto H. York Department of Chemical, Biological and Pharmaceutical Engineering, New Jersey Institute of Technology, Newark, New Jersey 07102

The objective of this work was to develop a better understanding of a potentially scalable, liquid antisolvent (LAS) precipitation process, for the preparation of stable aqueous suspensions of ultrafine particles of poorly watersoluble active pharmaceutical ingredients (APIs). A novel combination of jets, ultrasound, polymers, and surfactants was used for the precipitation and stabilization of ultrafine particles of griseofulvin (GF). Use of ultrasound and high stream velocities enhances micromixing, whereas addition of polymers/surfactants inhibits/lowers the particle growth. A combination of ultrasound, high jet velocities, and stabilizers decreased the GF particle size to 1.04 µm ((0.46 µm) from 30.8 µm ((14.2 µm), when none of the treatments were used. A rational understanding was developed for predicting process performance and selecting suitable particle growth inhibitors/stabilizers. Favorable process conditions and combinations of polymer and surfactants were also identified experimentally for the precipitation of ultrafine particles of GF with a narrow particle size distribution (PSD). 1. Introduction One of the major challenges faced by pharmaceutical industries is achieving size and morphology control of highly potent, low-bioavailability drugs in large-scale manufacturing of active pharmaceutical ingredients (APIs) for oral solid dosage forms. The bioavailability of poorly water-soluble hydrophobic drugs [Biopharmaceutics Classification System (BCS) Class II] is limited by their solubility and dissolution rate.1,2 Many new drugs are poorly water-soluble,2 and their dissolution rates can be improved by decreasing particle size.3 Decreasing the size increases the surface area, which results in an increase in the rate of dissolution of these drugs in aqueous media such as body fluids.4,5 Although many techniques for size reduction are available, their applicability is limited because of poor control of particle size, morphology, and scalability in comparison to liquid antisolvent (LAS) precipitation,6-8 which is frequently scaled up through the impinging-jet technique. However, there is a need for systematic and detailed study of LAS precipitation with a focus on a basic understanding of the process, with the objective of achieving very small particle sizes. Precipitation of a solid solute is achieved in the LAS process through an increase in the molar volume of solution and, hence, a decrease in the solvent power for the solute by addition of a nonsolvent (antisolvent).9 Figure 1 schematically shows the steps involved in the precipitation process, namely, nucleation due to supersatuartion attained by mixing10,11 and simultaneous growth of nuclei by coagulation and condensation.6,12 Higher nucleation rates result in low or negligible growth and, hence, can potentially produce submicrometer particles. The stability of these particles in colloidal solution further depends on agglomeration or flocculation, driven by hydrophobic effects, electrostatic interactions, and weak van der Waals attractive forces. Deryagin-Landau-VerweyOverbeek (DLVO) theory effectively describes such interactions and suggests steric and electrostatic stabilization as a way to improve colloidal stability.10,13 Thus, to control the particle size and particle size distribution (PSD) and to improve the stability, it is necessary to increase the nucleation rate, inhibit the particle * To whom correspondence should be addressed. E-mail: dave@ njit.edu. Tel.: 973 596 5860. Fax: 973 642 7088.

growth, and control the agglomeration of particles by steric or electrostatic stabilization. Two main steps in the process of particle formation are the mixing of the solution and antisolvent streams to generate supersaturation and the precipitation of the particles, leading to two main time scales, the mixing time (τmix) and the precipitation or induction time (τprecipitation).6,14,15 τprecipitation is composed of τcondensation and τcoagulation.6 The dimensionless Damkohler number (Da) is the ratio of τmix to τprecipitation. Values of Da > 1 imply that the process is controlled by mixing, whereas values of Da < 1 imply that the precipitation step is the slowest and controls the process. Under conditions such that Da < 1, the mixing time is significantly shorter, mixing is uniform, and particle size is not affected by the mixing conditions. This produces rapid and uniform supersaturation in the solution. If attainment of supersaturation is slow and low in magnitude (as in the case of poor mixing or Da > 1), the metastable zone is crossed very slowly, and the process of particle growth dominates over the process of nucleation,12 resulting in large crystals. If the supersaturation attained is high and is realized over a very short period of time, then the metastable region is crossed quickly, the process of nucleation dominates the crystallization process,12 and a large number of nuclei are formed, resulting in the precipitation of ultrafine particles. Hence, it is necessary to decrease τmix and increase τprecipitation in order to keep Da < 1. Application of ultrasound and high stream velocities can be used to decrease the mixing time (τmix). The mixing time is mainly influenced by mesomixing and micromixing.16 Therefore, in this work, mixing time (τmix) is estimated as the sum of the characteristic times16 for micromixing (τmicro) and mesomixing (τmeso) τmix ) τmeso + τmicro τmeso )

Q uDt

τmix ) km(ν/ε)1/2

(1) (2) (3)

where Q is the solution feed rate, u is the is the solution velocity, Dt is the turbulent diffisivity,16 km is a constant, ν is the kinematic viscosity, and ε is the energy dissipation rate. Use of high jet

10.1021/ie900248f CCC: $40.75  2009 American Chemical Society Published on Web 07/23/2009

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Ind. Eng. Chem. Res., Vol. 48, No. 16, 2009

Figure 1. Schematic of particle precipitation process.

velocities and ultrasound increases the energy dissipation rate (ε) and, hence, decreases the mixing time according to eqs 1-3. Ultrasound generates cycles of compression and rarefaction in the liquid medium, leading to the creation of tiny water-vapor bubbles in the solution as a result of cavitation.17-19 The bubbles ultimately collapse, and intense shock waves are generated and propagate through the liquid at velocities higher than the speed of sound. This imparts high velocities to the particles suspended in the solution,18,19 leading to the uniform micromixing of the solution and antisolvent. The sudden release of energy as a result of bubble explosion causes extremely rapid and localized temperature reductions in the solution, to generate rapid nucleation in the solution. The recent literature20-25 demonstrates the use of ultrasound, mainly for cooling crystallization, to reduce the particle size, size distribution, and agglomeration of the particles. It has been shown that the use of ultrasound decreases the induction time, reduces the width of the metastable zone, and induces nucleation at lower supersaturations. The precipitation time (τprecipitation) can be increased by using additives such as surfactants and polymers. According to the classical theory of homogeneous nucleation,10,11,26 the nucleation rate (J) is given by

[

J ) A exp -

B (ln S)2

]

(4)

with A ) N0υ B)

(

16πσ3Vs2 3k3T3

(5)

)

(6)

where S is the degree of supersaturation, N0 is the number of molecules of solute per unit volume, σ is the surface tension at the solid-liquid interface, k is the Boltzmann constant, Vs is the volume of a solute molecule, T is the temperature, and υ is the frequency of molecular transport to the solid-liquid interface. The molecular frequency of transport (υ) is given26 by the equation υ≈

kT 3πa03η

(7)

where η is the viscosity and a0 is the mean effective diameter of the diffusing species. Surfactants and polymers cause a decrease in the surface tension and an increase in the viscosity of a solution33,34 at any temperature. According to eqs 4 and 6, a decrease in surface tension increases the nucleation rate, and as a result, the particle size decreases.6,28,34 According to eq 7, an increase in the viscosity reduces the number of collisions and decreases the rate of mass transfer from the solution to the

growing solid-liquid interface as a result of diffusion. This increases the precipitation time (τprecipitation). The adsorption of polymers and surfactants at the growing solid interface reduces the interfacial surface energy6,28,34 and inhibits particle growth.10,27-32 In this work, griseofulvin (GF), an antifungal agent, was chosen as a poorly water-soluble, highly hydrophobic model drug having a solubility of about 12 ppm.36 Micronization of GF has been attempted by many researchers,36-54 mainly using various supercritical fluid techniques (see Table 1), and often, long bipyramidal needle-shaped particles with sizes up to several micrometers have been precipitated. The LAS process has also been used to precipitate GF particles, with chloroform as the solvent and hexane and HFE-7100 as antisolvents. However, the resulting particle size ranged from 40-100 µm, with large needlelike and platelike particles.37 The LAS process has also been studied by other researchers for the precipitation of pharmaceuticals other than GF.6,8,9,15,58-61 In these studies, the particles precipitated with sizes mostly ranging from a few micrometers to millimeters. It has been also shown that nanoparticles (up to 300 nm) are immediately precipitated by mixing the antisolvent and solution streams but that their size grows to over 10 µm within 30 min, suggesting that significant Ostwald ripening and agglomeration of the particles occur, thereby increasing the particle size.6 Therefore, the main objective of this work was to precipitate ultrafine particles of GF with a narrow particle size distribution. GF was precipitated from its organic solution in acetone using water as the antisolvent. A novel combination of ultrasound and stabilizers was used for the control of particle size by LAS precipitation. A reduction in τmix was achieved by applying higher mixing energies through ultrasound and high stream velocities. An increase in τprecipitation was achieved through the use of polymer, surfactant, and their interaction at the growing solid-liquid interface. The effects of various process parameters such as ultrasound power, jet velocity, antisolvent temperature, and concentration and type of stabilizers on the particle size and particle size distribution were studied in order to identify favorable process conditions for the precipitation of ultrafine particles of GF with a narrow PSD. 2. Experimental Section 2.1. Materials. GF was a gift received from Johnson & Johnson, Langhorne, PA. Acetone (99.8% pure), Tween 80 (derived from a nonanimal source), sodium dodecyl sulfate (SDS; >99% ACS reagent), poly(vinylpyrrolidone) (PVP; molecular weight 360000), and quaternized hydroxyethylcellulose ethoxylate (Polymer JR 400) were purchased from SigmaAldrich (St. Louis, MO). Hydroxypropyl methyl cellulose (HPMC) (4000 cPs, FCC) was purchased from Spectrum Chemicals & Laboratory Products (New Brunswick, NJ). All


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morphology

ref

Table 1. List of Literature Reports Available on the Micronization of Griseofulvin process rapid expansion of supercritical solutions (RESS)

solvent 1. supercritical trifluoromethane

200-300 nm

2. supercritical CO2

800-3000 nm

3. supercritical trifluoromethane

5. supercritical CO2

13.4-35.4 µm L 1.0-1.3 µm D 0.9-1.4 µm around 1 µm (primary size of 200 nm) 0.8-1.2 µm

1. acetone

40 µm-20 mm

2. tetrahydrofuran 3. dichloromethane 4. chloroform

2-50 µm few millimeters 106.7 ( 42.6 µm L 1.27 ( 0.41 µm W 10-100 µm

4. supercritical trifluoromethane

supercritical antisolvent (SAS)

precipitation with compressed antisolvent (PCA)

dichloromethane

aerosol solvent extraction system

acetone, ethanol, dimethylformamide (DMF) acetone as solvent

gas antisolvent (GAS) supercritical fluid expansion depressurization (SFED)

particle size

gaseous CO2 as antisolvent acetone, ethanol, and SC CO2

59 menthol used as solid cosolvent

62

long needles

65

quasispherical spongy-like

71

cubic, near spherical tetragonal bipyramidal needles long needles long needles long needles

with acetone as cosolvent -

72

63 64 58

61

10 µm to many millimeters

long needles

supercritical CO2 as antisolvent poly(sebacic anhydride) as growth inhibitor; supercritical CO2 as antisolvent -

around 100 µm

bipyramidal

-

67

1.5 µm

spherical and agglomerated

-

66

of these chemicals were used without further purification. Deionized Millipore water was used as an antisolvent. 2.2. Apparatus and Experimental Procedure. An organic solution of GF in acetone (5-50 mg/mL) was added to water (50-500 mL) containing surfactants and polymers, maintained at a constant temperature (1-60 °C), through a stainless steel nozzle (0.01-0.03-in. i.d.) at a flow rate of 1-100 mL/min using a high-performance liquid chromatography (HPLC) pump (LabAlliance, State College, PA). The vessel volume varied between 100 and 600 mL depending on the amount of antisolvent used. However, for most experiments, a 250-mL vessel was used (except when the effect of the antisolvent-tosolution ratio on the particle size was studied). The ultrasound horn was immersed in antisolvent at an immersion depth (below the antisolvent level) of 1.5 in. The tip (1/2-5/32-in. i.d.) of the ultrasound horn (Omni Ruptor 250 from Omni International, Marietta, GA) was directed over a nozzle such that the flow of the solution was dispersed instantaneously into antisolvent by the vibrating (0-125 W operating at a frequency of 20 kHz) surface of the tip. A schematic of the experimental apparatus is shown in Figure 2. The appearance time (tapp) of the particles

Figure 2. Schematic of experimental apparatus.

agglomerated chains of spherical particles spherical

comments

bipyramidal, diamond shape

60

73

was measured by visual observation as the time required for the suspension to begin turning opaque. The solution was insonated for 10 min. The aqueous suspension of GF particles (0.2-3 mg/mL) thus obtained was then analyzed for particle size and particle size distribution by laser scattering (Beckman Coulter LS Coulter 230). A field-emission scanning electron microscope (LEO 1530 VP FE-SEM) was used to observe the surface morphology of the precipitated GF particles. The aqueous suspensions were subsequently freeze-dried using a lyophilizer (Labconco-FreeZone Plus 2.5 L Benchtop Cascade Freeze-Dry System). X-ray diffraction (XRD) [Phillips X’pert materials research diffractometer (MRD)] and differential scanning calorimetry (DSC) (FP84 Mettler-Toledo KK) were used to compare the physical characteristics of GF before and after processing. 3. Results and Discussions 3.1. Initial Screening of Effects of Process Parameters on Particle Size, Distribution, and Morphology. Preliminary experiments were performed to screen the effects that ultrasound and stabilizers can have on the particle size and particle size distribution. The particle size decreases and the size distribution narrows with the use of ultrasound and surfactant, as reported in Table 2 and Figure 3. The mean particle size was around 30.8 µm ((14.2 µm) for EXP 1 (Table 2), where only magnetic stirring (τmix ) 8413 ms) was used to mix the antisolvent with the solution stream. The particles had large bipyramidal needlelike structures (Figure 3, EXP 1), indicating one-dimensional particle growth. However, with the use of Tween 80 as a surfactant (EXP 2), the particle size decreased to 13.8 µm ((10 µm). Addition of surfactant decreases the surface tension at the solid-liquid interface and thereby increases the nucleation rate,

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Table 2. Details of Preliminary Experiments on the Precipitation of GF from 10 mg/mL Acetone Solution Added at 1 mL/min through a 0.03-in.-i.d. Nozzle to 200 mL of Water at 20 °C (S ≈ 41.5) EXP

velocitya (cm/s)

temperatureb (°C)

probe diameter (mm)

power (W)

surfactant (wt %)

appearance time (s)

τmix (ms)

mean size (µm)

std dev (µm)

size distribution (µm) D10/D50/D90

1 2 3 4

3.66 3.66 3.66 3.66

23 22 20 20

3.97 3.97

magnetic stirring magnetic stirring 125d 125d

0.013c 0.013c

540 620 334 -

8413 8413 494 494

30.8 13.8 9.1 4.6

14.2 10.0 4.2 1.6

6.6/18.7/61.1 4.8/11.0/26.0 4.0/8.6/15.0 2.0/4.6/6.6

a

Velocity of organic solution. b Temperature of antisolvent. c Tween 80 as surfactant. d Diameter of a probe ) 5/32 in.

Figure 3. Preliminary experiments (EXP 1, no ultrasound, no Tween 80; EXP 3, with ultrasound, no Tween 80; EXP4, with ultrasound, with Tween 80) on the precipitation of GF from its acetone solution (10 mg/mL) added through a 0.03-in.-i.d. nozzle to 200 mL of water at 20 °C.

which results in a finer particle size. Further, the adsorption of surfactant on the particle surface prevents growth and agglomeration of the particles, so that the PSD becomes narrower (Table 1, EXP 2). Use of ultrasound (Table 1, EXP 3) further reduced the particle size to 9.1 µm ((4.23 µm). In this case, enhanced mixing (as evident from decreased mixing time of 494 ms) due to the cavitation produced by ultrasound generated rapid and uniform supersaturation and resulted in the precipitation of finer particles. The combined effect of ultrasound and Tween 80 (Table 1, EXP 4) yielded particles with a mean size of 4.6 µm ((1.6 µm). The combination of ultrasound and surfactant makes it possible to attain a more uniform supersaturation at a lower surface tension and thus promotes a high nucleation rate and reduced particle growth. Also, as is evident from Figure 3, the use of both ultrasound and surfactant controls the one-dimensional growth of the particles. In light of this screening study, all further experiments were conducted with the use of ultrasound to enhance micromixing and surfactants/ polymers as additives to inhibit the growth of particles, in order

to examine the possibility of manipulating the particle size by fine-tuning the process parameters. 3.2. Effect of Solution Flow Rate. Table 3 presents the effect of the solution flow rate on the size of the GF particles. It can be seen that an increase in the organic solution flow rate resulted in a decrease in particle size up to a flow rate of 20 mL/min, after which any further change in the particle size was marginal. Increasing the flow rate increases the jet velocity and, hence, the Reynolds number (Re), which increases the extent of mixing of solution with antisolvent through an increase in shear forces. As shown in Table 3, τmix decreased from 497 ms for a flow rate of 1 mL/min to 0.8 ms for a flow rate of 100 mL/min. This resulted in a rapid and more uniform supersaturation and increased the nucleation rate. Also, the appearance time (tapp) of the particles decreased from 360 s for a flow rate of 1 mL/min to 4 s for a flow rate of 100 mL/min. Hence, the particle size decreased, and the particle size distribution became narrower. All further experiments were therefore performed


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Table 3. Effect of Organic Solution Flow Rate on Particle Size and Distribution of GF Particles Precipitated from 10 mg/mL Acetone Solution Added through a 0.03-in.-i.d. Nozzle to 200 mL of Water (Containing Tween 80 above its CMC) at 20 °C with 125 W Sonication through a 5 /32-in. Probe (S ≈ 41.5) organic solution flow rate (mL/min)

velocity (cm/s)

Re

particle size (µm)

std dev (µm)

appearance time (s)

τmix (ms)


1 10 20 30 40 60 70 90 100

3.65 36.48 72.95 109.43 145.91 218.86 255.34 328.29 364.77

45.8 457.5 914.9 1372.4 1829.9 2744.8 3202.3 4117.2 4574.7

4.57 3.60 2.77 2.77 2.65 2.54 2.32 2.51 2.35

1.61 2.58 1.17 1.14 1.17 0.99 0.85 0.99 0.91

360 180 90 60 35 17 10 6 4

497 5.7 2 1 0.8 0.8 0.8 0.8 0.8

2.0/4.6/6.6 1.6/2.93/5.9 1.5/2.3/4.6 1.6/2.4/4.6 1.5/2.2/4.5 1.5/2.2/4.1 1.5/2.1/3.5 1.5/2.2/4.0 1.4/2.0/3.9

Table 4. Effect of Antisolvent-to-Solution Ratio on Particle Size and Distribution of GF Particles Precipitated from 10 mg/mL Acetone Solution Added through a 0.03-in.-i.d. Nozzle to 200 mL of Water (Containing Tween 80 above its CMC) at 100 mL/min with 125 W Sonication through a 5/32-in. Probe antisolvent-to-solution ratio

solvent content (wt %)

solubility of GF (mg/L)

S

particle size (µm)

std dev (µm)

appearance time (s)

τmix (ms)


5 10 20 30 40 50

13.7 7.3 3.8 2.6 1.9 1.6

23.3 14.8 11.5 10.5 10 9.7

71.5 61.2 41.5 30.8 24.4 20.2

7.10 5.53 3.28 3.33 3.67 4.04

5.20 3.15 1.58 1.65 1.87 2.10

4 7 13 28 45 65

0.6 0.6 0.8 0.9 1.0 1.0

2.0/5.6/14.4 1.9/4.9/10.2 1.6/2.9/7.9 1.8/2.8/8.0 1.5/2.6/8.1 1.7/3.6/7.0

Table 5. Effect of Antisolvent Temperature on Particle Size and Distribution of GF Particles Precipitated from 10 mg/mL Acetone Solution Added through a 0.03-in.-i.d. Nozzle to 200 mL of Water (Containing Tween 80 above its CMC) at 100 mL/min with 125 W Sonication through a 5/32-in. Probe antisolvent temperature (°C)

solubility of GF (mg/L)

S

particle size (µm)

std dev (µm)

appearance time (s)

τmix (ms)


60 50 40 30 25 20 10 2 1.5 1

171 83.9 39.3 17.5 11.5 7.4 2.9 1.3 1.3 1.2

1.9 3.8 8 17.8 26.9 41.5 102.9 223.1 234.5 246.6

7.65 6.02 4.08 3.17 2.93 2.88 2.61 2.59 2.43 2.23

4.81 4.38 2.08 1.49 1.30 1.20 1.13 1.10 1.06 1.11

8 8 8 8 8 14 28 32 33 34

0.5 0.6 0.6 0.7 0.7 0.9 1.0 1.0 1.0 1.0

2.8/6.5/14.0 2.3/4.9/10.8 1.8/3.6/7.1 1.6/2.8/5.4 1.5/2.6/4.9 1.5/2.6/4.7 1.4/2.3/4.2 1.5/2.2/4.0 1.4/2.0/4.1 1.3/1.9/4.0

with a solution flow rate of 100 mL/min to maintain uniform mixing conditions. 3.3. Effect of Antisolvent-to-Solution Ratio. Table 4 presents the effects of the antisolvent-to-solution ratio on the particle size and size distribution. Particle size decreased initially and then experienced a slight minimum at an antisolvent-tosolution ratio of about 20. As reported in Table 4, with an increase in ratio from 5 to 50, the supersaturation ratio changed from 71.5 to 20.2, which actually suggests that the particle size should be lower at lower ratios. However, at a low ratio, the solvent content of the solution is very high, as shown in Table 4. Therefore, the particle growth rate due to Ostwald ripening is very high because of the increased solubility of GF56 (as reported in Table 4) in the mixture of antisolvent and solvent. Hence, the particle size is greater at lower ratios. At higher ratios, the solvent content is low, and hence, the extent of growth by Ostwald ripening is low. However, the supersaturation ratio decreases with increasing ratio (Table 4), and hence, the particle size increases. Also, an increase in ratio decreases the quality of mixing because of the decrease in specific ultrasound energy added to the solution. This can be verified from the increase in τmix of particles with increasing antisolvent-to-solution ratio. Therefore, all further experiments were conducted at an antisolvent-to-solution ratio of 20 where particle size was observed to experience a minimum. 3.4. Effect of Antisolvent Temperature. Table 5 shows that a decrease in antisolvent temperature decreases the particle size

and narrows the size distribution. Reduction in antisolvent temperature decreases the equilibrium solubility of GF in solution56 and hence increases the supersaturation. As shown in Table 5, a change in antisolvent temperature from 60 to 1 °C changed the supersaturation ratio (S) from 1.9 to 246.6. According to eq 4, as the supersaturation (S) increases, the nucleation rate increases, and hence, the particle size decreases. Also, the decrease in antisolvent temperature from 60 to 1 °C increased the viscosity of water from 0.4665 to 1.728 cP.55 This increased the mixing time and the appearance time of the particles (as shown in Table 5), indicating a decrease in molecular transport rate (according to eq 7) and a reduction in particle collision frequency, which resulted in a decrease in particle growth by preventing coagulation and agglomeration. It also reduced the extent of Ostwald ripening because of a decrease in equilibrium solubility (as shown in Table 5) with decreasing temperature.6 These multiple effects reduced the particle size (as shown in Table 5) and controlled the growth of the particles as the antisolvent temperature decreased. All further experiments were thus performed using antisolvent at a temperature of 1 °C. 3.5. Effect of Ultrasound Energy and Ultrasound Probe Characteristics. Figure 4 shows the variation in particle size with ultrasound power and specific energy intensity (watts per kilogram per square meter of probe cross section) for three different sonicator probe diameters. It can be seen that the mean particle size could be decreased dramatically from 23.49 to 4.87

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Ind. Eng. Chem. Res., Vol. 48, No. 16, 2009 Table 6. Effect of Initial GF Concentration in Acetone on Size and Distribution of Particles Precipitated by Introducing Organic Solution through a 0.03-in.-i.d. Nozzle at 100 mL/min in 200 mL of Water (Containing Tween 80 above its CMC) Maintained at 1 °C and Sonication through a 1/2-in. Probe at an Operating Power of 75 W (τmix ≈ 1 ms)

Figure 4. Effect of ultrasound power on the size of GF particles precipitated from 10 mg/mL acetone solution added to water (containing Tween 80 above its CMC) at 1 °C through a 0.03-in.-i.d. nozzle at 100 mL/min.

Figure 5. Effect of initial GF concentration in acetone solution on the size of GF particles precipitated by introducing organic solution in water (containing Tween 80 above its CMC) at 1 °C through a 0.03-in.-i.d. nozzle at 100 mL/min and sonication through a 1/2-in. probe operating at 75 W.

µm with the use of an ultrasound power of 25 W. However, a further increase in ultrasound energy only improved the particle size slightly. A slight minimum was obtained at an operating power of about 75 W. An increase in particle size with increasing ultrasound energy could be due to agglomeration of particles as interparticle collisions increase as a result of higher velocities imparted to particles by cavitation events caused in the solution by ultrasound.18,19 It was also observed that a wider probe (1/2-in. i.d.) performed better than a probe with a smaller diameter (5/32-in. i.d.) (see the inset in Figure 4). It appears that an increase in diameter of the probe increases its capacity to process a large volume of liquid, as similar or smaller particle sizes are obtained for a wide probe with a lower specific energy intensity as compared to a smaller probe. Further, it has been shown that the precipitation rate is proportional to the horn diameter,21,23 which also suggests that a large probe diameter can result in a lower particle size. The effects of the depth of immersion (distance of the tip from the top of the liquid level) of the sonicator probe on the particle size and PSD were also studied, but it was found that the immersion depth did not have a significant impact for the rather small range available in the experimental apparatus. Nonetheless, at an immersion depth of 1.5 in., the appearance time was the lowest (indicating more mixing), and hence, all further experiments were carried out at that depth using a probe of 1/2-in. i.d. operating at 75 W. 3.6. Effect of Initial GF Concentration in Acetone Solution. Figure 5 and Table 6 show that the particle size initially decreased with increasing GF concentration, exhibited a mini-

GF concentration (mg/mL)

S

particle size (µm)

std dev (µm)

appearance time (s)


5 10 15 25 40

20.8 41.5 62.3 83.0 128.3

4.06 2.63 2.66 2.91 3.54

2.16 1.06 1.47 1.45 1.74

105 141 128 90 40

1.7/3.6/7.2 1.5/2.3/4.4 1.2/2.2/4.8 1.4/2.5/5.1 1.5/3.3/6.0

mum, and again increased with further increase in concentration. An increase in the concentration increased the supersaturation obtained after mixing (as shown in Table 6) and, hence, resulted in an increase in the nucleation rate according to eq 4, thus decreasing the particle size. On the other hand, an increase in concentration also increased the growth rate, as the latter is directly proportional to the supersaturation attained or the masstransfer gradient available for particle growth.10,11,62 The appearance time also showed a maximum with increasing GF concentration (shown in Figure 5). A decrease in appearance time at higher GF concentrations suggests an increase in the frequency of collisions among particles and an increase in agglomeration. Thus, at lower concentrations, the low nucleation rate increased the particle size, whereas at higher concentrations, particle growth and agglomeration increased the particle size. Consequently, an optimal concentration was found at which the minimum particle size could be obtained. Hence, all further experiments were carried out at an initial GF concentration of 10 mg/mL. 3.7. Effect of Surfactant Concentration. In this work, a nonionic surfactant (Tween 80) and an anionic surfactant (SDS) were used as stabilizers. Figure 6 shows the effects on the size of the GF particles of the concentrations [in terms of both weight percentage and critical micelle concentration (CMC)] of these surfactants when added to antisolvent phase. It can be observed that the particle size initially decreased with an increase in surfactant concentration, passed through a minimum, and then increased again. The adsorption of surfactant at the solid-liquid interface decreased the interfacial surface tension and increased the rate of nucleation (according to eq 4 and 6), resulting in an initial reduction of the particle size. Also, adsorption made the particles of GF, which is a highly hydrophobic molecule, less hydrophobic and decreased particle growth by reducing the hydrophobic interactions and coagulation. However, an increase in surfactant concentration above the CMC caused micelle formation in the solution, which left the particles unprotected.57 This increased coagulation and agglomeration, thereby increasing the particle size, as shown in Figure 6b. Figure 6a shows that the concentration of Tween 80 required to control particle growth is significantly lower than that of SDS. This can be explained by considering the difference in the molecular sizes of Tween 80 and SDS. The Tween 80 molecule has a relatively large hydrophobic chain and a large hydrophilic headgroup as compared to the SDS molecule. The relatively large hydrophobic chain occupies more space at the solid-liquid interface, and the bulkier hydrophilic headgroup occupies more volume in the aqueous phase than for SDS. Thus, the excludedvolume effect of a Tween 80 molecule is greater than that of a SDS molecule. This offers more steric hindrance and increases the energy barrier to prevent aggregation.63 Thus, the Tween 80 concentration required for stabilization is lower than the corresponding concentration of SDS. A similar observation of


Figure 6. Effects of type and concentration of surfactant on the particle size of GF particles precipitated from 10 mg/mL acetone solution added at 100 mL/min through a 0.01-in.-i.d. nozzle to water at 1 °C with sonication through a 1/2-in. probe operating at 75 W: (a) based on weight percent, (b) based on CMC.

Figure 7. Effects of polymer concentration and type on the particle size of GF particles precipitated from 10 mg/mL acetone solution added at 100 mL/min through a 0.01-in.-i.d. nozzle to water at 1 °C with sonication through a 1/2-in. probe operating at 75 W.

Tween 80 being a better stabilizer than SDS was made for the stabilization of ascorbyl palmitate nanocrystals.64 However, when compared in terms of CMC, as can be seen in Figure 6b, SDS performs better as a growth inhibitor than Tween 80 below the CMC. The CMC of Tween 80 is 58.7 µM, and that of SDS is 7.2 mM in water at 1 °C.66 Tween 80 is a nonionic surfactant, whereas SDS is an anionic surfactant. Hence, adsorption of SDS on the particle surface (at concentrations below the CMC) makes the particle surface negatively charged. This increases the energy barrier by increasing repulsion among the particles67 and, hence, prevents particle growth and agglomeration. 3.8. Effect of Polymer Concentration. Figure 7 and Table 7 show that the use of polymers such as HPMC, PVP, and

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Polymer JR 400 decreases the particle size of GF. Addition of polymers lowers the surface tension28,34,69 and increases the nucleation rate. Moreover, adsorption of polymer molecules at the solid-liquid interfaces reduces the interfacial surface energy of newly formed hydrophobic surfaces and offers steric or electrostatic stabilization,27,28,70 thus preventing growth and agglomeration. Also, use of a polymer increases the appearance time of particles. An increase in appearance time upon addition of polymer suggests a decrease in transport rates due to an increase in the viscosity of the solution.32,68 As can be seen from Figure 7, the particle size goes through a minimum as the polymer concentration increases in the solution. Similar observations have been made by others.70,71 Increase in polymer concentration beyond critical flocculation concentration (CFC) initiates depletion or bridging flocculation of the particles in a suspension.28,71,72,74 Increasing the osmotic pressure in the solution with the polymer concentration causes an attraction between colloidal particles. Also, the use of ultrasound imparts high velocities to the particles,18,19 causing increased collisions among the particles. This leads to an unbalanced osmotic pressure and causes the depletion of the adsorbed polymer layer for weakly adsorbing28 or nonadsorbing74 polymers, or it can cause bridging flocculation in case of strongly adsorbing polymers.72 SEM images (Figure 8) show the particles of GF bridged together when precipitated in the presence of HPMC, thus confirming this phenomenon. Three polymers, namely, HPMC, PVP, and Polymer JR 400, were used in this work. Because HPMC and PVP are neutral polymers, they mainly contribute to the steric hindrance. Polymer JR 400, being a cationic polymer, contributes to the electrostatic forces in the solution. Among the three polymers used, HPMC was the most effective, and Polymer JR 400 was the least effective. This difference can be attributed to the aqueous solubilities of these polymers and, consequently, to the quantity of adsorbed polymers, which is inversely proportional to the solubility of the polymers in the liquid phase (according to Lundelius’ rule).80 Charged polymers such as Polymer JR 400 have high solubilities in water as compared to neutral polymers such as HPMC,81 which is the least water-soluble polymer among the three investigated here. This implies that Polymer JR 400 should be the least effective in controlling the particle size. Also, the addition of stabilizers can change the aqueous solubility of drug molecules, which, in turn, might affect the particle size.71 It has been shown that HPMC does not change the aqueous solubilities of APIs such as ibuprofen.82 Therefore, it is expected that HPMC will not affect the aqueous solubility of griseofulvin (8.6 mg/L at 20 °C), which is less soluble in water than ibuprofen (21 mg/L at 20 °C). Nevertheless, it is possible that polymers such as Polymer JR 400 that are highly soluble in water as compared to HPMC might increase the solubility of drug molecules in water. This increase in aqueous solubility decreases the supersaturation attained and, hence, results in higher particle sizes as compared to polymers with low water solubilities such as HPMC. A comparison of HPMC and PVP shows that HPMC is more effective. This difference in behavior can be attributed to two factors; surface tension and suspension viscosity. It has been reported69 that the reduction in surface tension upon use of HPMC is greater than that upon use of PVP (the surface tension of a 4% aqueous HPMC solution is 46.1 mN/m, and that of 4% aqueous PVP is 62.1 mN/m, as compared to 72.1 mN/m for pure water69), which indicates that a higher nucleation rate can be obtained with the use HPMC. Also, the appearance times of GF particles with the use of HPMC are higher than those

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Table 7. Effects of Concentrations of HPMC, PVP, and Polymer JR 400 Dissolved in Antisolvent on Size and Distribution of Particles Precipitated by Adding 10 mg/mL Acetone Solution of GF to 200 mL of Water Maintained at 1 °C through a 0.01-in.-i.d. Nozzle at 100 mL/min with Sonication through a 1/2-in. Probe at an Operating Power of 75 W HPMC

PVP

Polymer JR 400

concentration appearance size distribution percentage concentration appearance size distribution percentage concentration appearance size distribution percentage (wt %) time (s) (µm) D10/D50/D90

Controlling Particle Size of a Poorly Water-Soluble Drug Using

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