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Manipulating the Size, the Morphology and the Polymorphism of

Acetaminophen using Supercritical Antisolvent (SAS)

Precipitation.

Matthias Rossmanna,b

,Andreas Braeuerb,c*

,Alfred Leipertzb,c

, and Eberhard Schlueckera

a Lehrstuhl für Prozessmaschinen und Anlagentechnik (iPAT), Friedrich-Alexander Universitaet Erlangen-

Nuernberg, Cauerstraße 4, 91058 Erlangen, Germany

b Erlangen Graduate School in Advanced Optical Technologies (SAOT), Friedrich-Alexander Universitaet

Erlangen-Nuernberg, Paul-Gordan-Straße 6, 91052 Erlangen, Germany

c Lehrstuhl für Technische Thermodynamik (LTT), Friedrich-Alexander Universitaet Erlangen-Nuernberg, Am

Weichselgarten 8, 91058 Erlangen, Germany

* Corresponding author.

E-mail address: andreas.braeuer@aot.uni-erlangen.de Fax: +49 9131 85 25851

Keywords: Supercritical Antisolvent Process, SAS, Particle, Nucleation, Micronization,

Precipitation, Crystallization, Polymorphism

Abstract

The supercritical antisolvent technology is used to crystallize Paracetamol particles.

Supercritical carbon dioxide (scCO2) is used as antisolvent. Ethanol, acetone and

mixtures of ethanol and acetone are used as solvents. The initial concentration of

paracetamol in the solution was varied between 1 and 5 wt %, the composition of the

ethanol/acetone solvent mixture between 50 and 90 wt % of ethanol and the

operation pressure between 10 and 16 MPa at a temperature of 313 K. The most

important finding is that the polymorph of paracetamol crystals can be adjusted

between monoclinic and orthorhombic by varying the content of ethanol in the

solution. The second important finding is that the occurrence of primary and

secondary crystal structures can be explained solely by the overall supersaturation

during the crystallization process. While X-ray diffraction was used to analyze the

polymorph of the particles, their morphology was analyzed using scanning electron

microscopy.

*ManuscriptClick here to view linked References

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1 Introduction

The usage of supercritical fluids for the production or micronization of pharmaceutical

compounds is investigated by an increasingly number of research groups because of

its numerous advantages [1-9]. The role of the supercritical fluid in the various

processes is highly diverse and ranges for example from solvent to dispersion

medium or antisolvent [5, 6, 8, 10-13]. The latter is the case in the supercritical

antisolvent process (SAS), where supercritical carbon dioxide (scCO2) is used as an

antisolvent. Due to the very gentle process route, especially thermo labile and special

compounds such as pharmaceuticals, polymers and biologically active proteins are in

the focus of research for SAS processing. The SAS process works on the basis of

antisolvent precipitation or crystallization. In this process, the solute that is to be

micronized is previously dissolved in a solvent and the resulting solution is then

injected into a supercritical fluid which acts as an antisolvent with respect to the

solute. The particles are generated due to the resulting supersaturation of the solute

which is compensated by nucleation and particle growth. By varying the process

parameters, the generation of tailor made particles with defined properties can be

achieved [1, 14, 15]. Conventional parameters that can be adjusted in the process

are pressure, temperature, solute concentration and the flow rate ratio of solution and

antisolvent. Some groups also reported on the influence of the type of solvent used

[9] respectively using mixtures of different solvents to adjust the solvent strength [16,

17].

The mechanisms that govern the single steps of particle generation in the SAS

process and therefore the properties of the resulting products are still in the focus of

recent research. Concerning the basic mechanisms, due to empirical investigations

and especially advanced optical in situ measurement techniques, a good

understanding exists [14, 15, 18-24]. Depending on the type of solute used, whether

it is an ideal or a non-ideal solute, different models can be applied to describe the

influence of the determining parameters[15].

For an ideal solute, which means that the solute is sparingly soluble in the

antisolvent, precipitation takes places. This results in amorphous particles, where the

size and the morphology of the generated particles can vary from spherical nano

particles to spherical micro or expanded micro particles.

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In the case of a non-ideal solute, which means that the solute is slightly soluble in the

antisolvent scCO2, crystallization occurs[17]. This is due to the fact, that the overall

level of supersaturation in the SAS process for these solutes is significantly lower

than for an ideal solute [15, 17]. In this case, the particle generation in the SAS

process can be described by the crystallization theory where the supersaturation is

the governing factor, and not by the time scale model. Therefore the manipulation of

the supersaturation by varying the corresponding parameters like for example vessel

pressure, solute concentration or temperature can be used to influence the properties

of the generated crystals.

The aim of this study is to further investigate the crystallization by the SAS process.

Considering the non-ideal SAS solute Paracetamol (acetaminophen, PCM) [17], the

influence of conventional SAS parameters like initial solute concentration and

pressure on the supersaturation and therefore the characteristics of the generated

PCM crystals are analyzed. Furthermore, the effect of the solvent on the

crystallization behaviour of PCM is examined in detail for the solvents ethanol,

acetone and mixtures of both. PCM itself is a well-known active pharmaceutical

ingredient and an important bioactive compound, which is used as an antipyretic and

analgesic drug. It is chosen as the example API, as it shows a polymorphic behavior

and exists in three known crystal forms, which are monoclinic (form I), orthorhombic

(form II) and an unstable not fully characterized form III [25]. Although it is an

established and worldwide used API, there is still ongoing research concerning PCM,

especially on its crystallization behaviour [26-28]. As it is known from other

crystallization techniques, the supersaturation and especially the solvent can play a

key role in controlling the desired polymorph [29, 30]. Therefore the conducted

experiments focus especially on the possibility of altering the polymorphic form of the

crystallized PCM via the variation of pressure, initial solute concentration and the

type of solvent used. A detailed investigation of the defined manipulation of the

polymorphic form of PCM via SAS-processing has, to the best knowledge of the

authors, not been conducted. Other authors have reported on different aspects of

PCM generation via SAS process, but not on a generation of different crystalline

modifications of PCM [31-35].

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2 Experimental section

2.1 Materials

Paracetamol (acetaminophen, PCM) with a purity of ≥ 99 % was purchased from

VWR International and was delivered as a white free flowing powder. Before SAS

processing, PCM is crystalline (monoclinic form I). Ethanol (EtOH) with a purity of

99.9 % was purchased from Bundesmonopolverwaltung für Branntwein (Germany)

and acetone (Ac) with a purity of 99 % from Sigma Aldrich. The antisolvent carbon

dioxide (CO2) was supplied by Linde Company (Germany) with a purity of 99.9 %. All

chemicals were used as delivered.

2.2 SAS apparatus and methods

A schematic flow sheet of the optically accessible SAS plant is given in Fig. 1. The

solvent (S1) respectively the solution (S2) is conveyed through a nozzle into the high

pressure vessel using a syringe pump. The capillary nozzle, through which the liquid

feed is sprayed into the vessel, has an inner diameter of 150 µm.

The antisolvent CO2 is pressurized and brought to the vessel by a compressor (CP).

The antisolvent flow enters the vessel from the side, while the liquid feed is sprayed

from the top. The operating pressure is adjusted by a pneumatically driven back

pressure regulator valve (V) and the remaining flow is separated into liquid solvent

and gaseous antisolvent in a demister (D). Particles generated are either filtered

directly at the outlet of the vessel or in an external filter unit downstream. The

temperature of the whole plant is kept constant at 313 K by heating cartridges in the

vessel walls and heating tapes on all of the pipes. The plant is fully automated and

can be operated from an external control room.

The generation of particles using the supercritical antisolvent technology follows the

standard procedure [1, 36]. The vessel is continuously flushed with a certain flow rate

of CO2 and pure solvent (without solute) until steady conditions, such as the desired

overall molar fraction xCO2, the pressure, the temperature and all other operation

parameters, are adjusted. The liquid feed flow is then switched from solvent (no

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solute dissolved in it) to solution (solute dissolved in the solvent), which leads to the

generation of the solute particles due to antisolvent crystallization. The injection of

the solution is stopped, when the desired amount of solute is generated. Before the

particles can be analysed, the whole system has to be purged with pure antisolvent

for around two hours to remove any residual solvent from the vessel. After this drying

step, the vessel is stepwise depressurized and particle samples can be taken on

various positions within the vessel and on the filter frit.

2.3 Particle analysis

Particles generated were analysed using scanning electron microscopy (SEM) and

powder X-ray diffraction (XRD). To carry out the SEM measurements, samples of the

particles taken from the SAS plant were placed on a carbon film stuck on an

aluminium stub (Plano GmbH, Germany). To enhance the conductivity in order to

have a higher quality of the SEM images, the particles were then sputtered with a thin

film of gold palladium. Images of the samples were taken with an ULTRA 55 field

emission scanning electron microscope (SEM, Carl Zeiss AG, Germany).

To investigate into the crystalline modification of the generated PCM crystals and

their solid dispersions, crystallographic analysis was carried out by means of powder

diffraction. The measurements were carried out with a Phillips wide & small angle X-

ray diffractometer under nitrogen gas atmosphere and at ambient temperature with

diffraction angles from 5° to 40° with CuKα radiation at operation parameters of

40 mA and 45 KV. The XRD measurements of the powder samples were repeated

after a certain time to ensure that the generated polymorphic form of PCM is stable.

3 Results and discussion

3.1 Generation of paracetamol crystals from EtOH solutions

In Fig. 2, generated PCM crystals from ethanol solutions are given. In the left column

from image a) to c), the pressure was kept constant at 16 MPa and the PCM

concentration was increased from a) 1 wt% to b) 3 wt% and to c) 5 wt%. In the right

column from image d) to f), the concentration was kept constant at 1 wt% and the

pressure was decreased from d) 30 MPa to e) 16 MPa and f) 10 MPa. In all the

images given in Fig. 2, a primary and/or a secondary crystal structure can be found.

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The primary structure is flat and angular shaped and due to crystal growth and solid

bridging a secondary structure can be formed. This structure is rod- respectively

needle-like. The occurrence of the primary or secondary structure and the size of the

generated crystals can be explained by the level of supersaturation during the SAS

process.

In our previous work [15], we showed that Paracetamol is slightly soluble in CO2 and

therefore an overall lower level of supersaturation is reached during the SAS

crystallization. Thus the presence of the primary and secondary structures in Fig. 2

can be described using the crystallization theory [17]. This means that a higher

supersaturation leads to a high nucleation rate and small particles, whereas a lower

supersaturation leads to a lower rate of nucleation and a more distinct crystal growth

phase, resulting in larger particles (or even secondary structures).

This behavior is reflected in the SEM images given in Fig. 2, where the

supersaturation increases for both image columns from the top to the bottom due to

either an increase in concentration (left column) or a decrease of pressure (right

column). The primary structure of small separated crystals of PCM can be found in

the images c) and f). In these cases a high level of supersaturation occurs, due to a

high concentration of paracetamol in the initial solution and/or a low pressure during

the experiment. As paracetamol is slightly soluble in CO2, a higher pressure leads to

a higher solubility respectively to a lower supersaturation [15]. A higher initial PCM

concentration in the injected solution has the reverse effect [15], as the

supersaturation is defined by the ratio of the actual concentration to the saturation

concentration [20]. In the images a) and d), large crystals with the reported

secondary structure can be seen. No small single crystals are present at these

parameters, only large needle or rod like crystals (secondary structure) are

generated. This is due to a low level of supersaturation at either a low initial

concentration of paracetamol (a) or a high pressure (d) during the process. For the

parameters in the images b) and e) an intermediate level of supersaturation occurs.

In these SEM images, both a primary and a secondary structure can be seen. Single

crystals can still be identified, but they already have a more distinct growth phase and

start to grow together respectively start solid bridging. For the highest levels of

supersaturation achieved at the reported parameters, in the images c) and f)only

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small primary crystals are generated. In the images a) and b) also a higher

magnification of the SEM image is given, to highlight the shape of the crystallized

PCM particles. Irrespectively of the process conditions always the already described

flat angular crystalline structure can be found.

Next to the size and shape of the generated PCM crystals from the SEM images, the

crystalline modification respectively the polymorphic form was analyzed by carrying

out XRD measurements. In Fig. 3, example XRD spectra of the produced PCM

crystals and the corresponding SEM images are given. It has to be noted, that for

reasons of better comparability, the spectra have an arbitrary offset of 1000 counts to

each other. By comparing those spectra with the given spectra for the unprocessed

monoclinic form I paracetamol (given in Fig. 5), no change in the crystalline

modification can be found [37]. Paracetamol, irrespectively of the occurrence of

primary or secondary crystals respectively high or low concentration and pressure,

always crystallizes in the monoclinic form I, when generated from ethanol solutions.

Form I of paracetamol has a zig-zag pattern (flat angular shaped), which can be also

found in all the SEM images shown in Fig. 2.

As can be seen in Fig. 3, the angular position of the peaks is not shifted, which

means that the crystalline modification is the same for the three given experiments.

For the orthorhombic modification II, peaks in the XRD spectra would be observable

at around 10 degree (2 theta), at 22 degree and 35 degree and others would be

missing [25]. In Fig. 3, no significant intensity can be observed at those degrees

which also lead to the assumption that no modification mixture, even with a low

amount of form II, was generated.

Consequently, the generation of PCM crystals by SAS processing out of an ethanol

solution in the here analyzed range leads to the pure monoclinic form I. The main

determining factor for the final product (crystal size, occurrence of primary and/or

secondary structure) is the occurring supersaturation during the process, which is a

function of the chosen process parameters.

3.2 Generation of paracetamol crystals from acetone solutions

SEM images of produced PCM crystals from acetone solutions using the SAS

process are given in Fig. 4. For the left column from a) to c) the pressure is set to

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16 MPa in all the experiments and the PCM concentration was increased from a)

1 wt % to b) 3 wt % and c) 5 wt %, In the right column (d, e) the concentration is

constant at 3 wt % and the pressure was decreased from d) 16 MPa to e) 10 MPa.

As it was observable for the PCM particles crystallized from ethanol solutions

(compare Fig. 2), also for PCM particles crystallized from acetone solutions (compare

Fig. 4) a primary and a secondary structure can be found. The coherence between

the supersaturation during the process and the parameters pressure and

concentration is the same as discussed in the previous chapter. No experiments at

30 MPa and 1 wt % were conducted for acetone solutions

In Fig. 4, image a) the supersaturation has the lowest level for the examined values of

pressure and initial PCM concentration. In this case, the pressure is set to 16 MPa

and the initial PCM concentration was 1 wt %. The generated crystals show a

secondary structure and have a bigger size compared to the other SEM images.

Furthermore, no small primary crystalline structure can be found at these conditions.

In the images b) and d), where the pressure was set to 16 MPa and the concentration

is 3 wt %, a primary crystalline structure can still be clearly identified, and larger

crystals respectively a secondary structure can also be found. The supersaturation in

this case is higher than in the image a), due to a higher initial solute concentration. In

the images c) and e), the highest supersaturation was present during the

micronization due to either a high initial concentration (image c) or a low pressure

(image e). At those parameters only a primary crystalline structure of smaller crystals

is generated. The crystals already start to build solid bridges, but no distinct

secondary structure, like needles or rods, can be identified.

The enlarged zooms in the images b) and c) furthermore show that the morphology

of the crystals is different to the ones generated from ethanol solutions given in Fig. 2.

No distinct flat zigzag pattern can be found and the crystals show a more planar

cuboidal morphology. To further investigate into the exact crystalline structure of the

generated particles and not only their morphology, XRD measurements were also

conducted with the PCM powder generated from acetone solution.

The results of the XRD measurements are given in Fig. 5. The spectra given have an

arbitrary offset of 200 counts to enhance a better comparability. The spectra a), b),

and c) are from generated powders at different process conditions whereas spectrum

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d) represents the diffraction pattern of the unprocessed (monoclinic) PCM. The

spectrum a) origins from PCM powder generated at 10 MPa and 1 wt %. The

parameters for b) are 16 MPa, 1 wt % and for c) 10 MPa and 3 wt %.

The X-ray diffractometry spectra of the powder generated from acetone solutions all

show the same behavior with respect to the d-spacing position of the peaks. The

height of the peaks, which represent the crystallinity, does however vary with the

process conditions. The influence of the pressure on the crystallinity can be seen by

comparing spectrum a) with spectrum b). In the experimental run for spectrum a) the

pressure was set to 10 MPa and for b) it was set to 16 MPa, at otherwise similar

conditions. The d spacing position of the peaks does not change with the increased

pressure, but the overall counts for all intensities are lower for a higher pressure.

The grey bars at different angles (d spacing) in Fig. 5, represent the previously

mentioned characteristic angles where peaks occur for the orthorhombic phase II for

PCM [37]. By looking at the highlighted areas, it can be seen that all the conducted

experiments with acetone solutions lead to PCM with an orthorhombic modification

phase II. From literature [37] it is also known, that also other d spacing angles could

be used to identify the corresponding modifications of PCM. For the differentiation

between monoclinic and orthorhombic modification of PCM, only the highlighted grey

areas are discussed in more detail in this work, although also other positions, could

be used. The lack of respective signal intensities[25] in the XRD spectra related to

modification I indicates that no crystalline mixture of both forms, with a significant

amount of form I, was generated from PCM/acetone solutions.

As it is the case for ethanol solutions, also PCM crystals can be successfully

generated from acetone solutions. The size and the structure, primary and/or

secondary structure, of the crystals can also be adjusted by varying the investigated

parameters pressure and concentration The coherence between the level of

supersaturation and the crystal properties also follow classical crystallization theory,

as it was described for ethanol. The primary crystalline structure, which in the case of

PCM crystallization from acetone solutions has a more planar cuboidal morphology

can be related to the orthorhombic modification form II. XRD investigation shows, that

using acetone as the solvent, instead of ethanol, paracetamol crystals with

orthorhombic modification II are generated. This modification does not alter during

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storage, as the produced powder samples were measured by X-ray diffractometry

repeatedly during several months after storage at ambient conditions.

3.3 Generation of paracetamol crystals from mixtures of ethanol

and acetone

Following the previously shown possibility of altering the polymorphic form of PCM by

changing the organic solvent, the usage of mixtures of different ratios of ethanol and

acetone must also lead to a change in the crystalline form at otherwise constant

conditions. To confirm the influence of the solvent mixture on the PCM crystals

generated, experiments with different ratios of ethanol to acetone were conducted.

The solvent mixtures used were 50/50, 70/30, 80/20 and 90/10 in weight ratio of

ethanol to acetone.

In Fig. 6 SEM images of the generated paracetamol crystals at different process

conditions and ethanol/acetone ratios are given. The images a) show crystals

generated from a 50/50 ethanol/acetone mixture at a pressure of 10 MPa and a

solute concentration of 1 wt %. These images only differ in their magnification to give

a better overview of the uniform size distribution of the generated crystals and also to

allow a detailed consideration of the single crystals. Only primary crystals can be

found at these conditions, which resemble the orthorhombic morphology of the ones

generated from pure acetone solutions. In the images b) also one experimental result

with different magnifications is given. In this case, the solvent ratio is 50/50, the

pressure is 10 MPa and the concentration was set to 3 wt %. Also here, only

uniformly sized primary crystals can be found. The crystals are slightly smaller

compared to the experiment at 1 wt % (images a)) due to the higher supersaturation

on account of an increased initial PCM concentration. The effect of the different initial

concentrations is not as dominant as it was observed for using either acetone or

ethanol as solvents. The morphology of the generated crystals for both experiments

is a planar cuboidal-orthorhombic structure, similar to the PCM particles generated

from PCM/acetone solutions.

In the SEM images c) and d) in Fig. 6, PCM crystals generated from solvent mixtures

with an ethanol to acetone weight ratio of 70/30 (c) and 80/20 (d) are shown. In both

images, the pressure was 10 MPa and the concentration was 3 wt %, so only the

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solvent ratio was varied. The images show only primary crystal structures with no

secondary structure formed. The average size of the crystals is not much influenced

by varying the solvent composition but the morphologies are different. By comparing

the images b), c) and d) the influence of the different solvent ratio at otherwise

constant conditions can be seen. By increasing the amount of ethanol in the solvent

composition, the morphology changes from compact orthorhombic crystals towards

more flake like crystals with an increased amount of fragments.

To have a further insight into the structure respectively the polymorphic modification

of the generated PCM crystals from ethanol/acetone mixtures, X-ray diffractometry

analysis was also carried out. The spectra of the investigated powders are given in

Fig. 7. The spectra a) and b) represent the experiments given in Fig. 6 in the images

a) and b). They vary only in the initial solute concentration, which was increased from

1 wt % (XRD Spectrum a)) to 3 wt % (Spectrum b)). The other parameters were kept

constant at 10 MPa and a solvent composition of 50/50 ethanol to acetone. No

significant influence of the varied concentration can be found in the two

corresponding spectra. The d-spacing position of the peaks does not change, only a

slightly higher intensity of the peaks occurs for the experiment with the lower

concentration. This can be attributed to the larger size of the crystals because of the

lower level of supersaturation caused by a lower initial PCM concentration.

In the XRD spectra c), d) and e) in Fig. 7, the pressure is constant at 10 MPa and the

concentration was set to 3 wt %. From the bottom to the top (c) to e)), the solvent

composition was changed from 70/30 (c) to 80/20 (d) and to 90/10 (e) weight ratio of

ethanol to acetone. The measured spectra show, that increasing the amount of

ethanol in the solvent mixture, a transition from the orthorhombic modification II

towards the monoclinic modification I occurs. At a mixture of 70 wt % EtOH to 30%

acetone, still orthorhombic features can be detected at the determining highlighted

angles, represented by the grey bars. The intensity of those peaks is lower than for

example for the 50/50 EtOH/acetone mixture or the pure acetone XRD samples. As

other peaks that belong to the monoclinic XRD fingerprint start to occur slightly, it has

to be assumed that at this solvent composition already a mixture of monoclinic and

orthorhombic modifications is generated.

A further increase of the quantity of ethanol in the solvent blend towards a ratio of

80/20 and 90/10 wt % of EtOH to acetone, finally leads to a pure monoclinic

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modification I of the generated paracetamol crystals. Those two solvent mixtures are

represented by the spectra d) (80/20) and e) (90/10) and can be compared with

spectrum f), which represents the unprocessed paracetamol in the pure monoclinic

form I. This means that referred only to the crystalline modifications of PCM, mixtures

of EtOH and acetone result in the generation of PCM in the orthorhombic

modification form II until the content of EtOH does not exceed 70 wt %. The

coherence between the supersaturation and the previously discussed PCM crystal

size and structure (primary and/or secondary) is nevertheless still valid also for the

solvent mixtures processed.

4 Conclusion

Different strategies can be applied to adjust the properties of PCM via the SAS

process. As PCM is a so called non-ideal solute, which means it is slightly soluble in

scCO2, the crystallization theory can be applied to describe the behaviour of PCM.

The supersaturation can be varied by the parameters concentration and pressure. An

increase of concentration and/or a decrease of pressure lead to a higher level of

occurring supersaturation and vice versa. Depending on the level of supersaturation,

the PCM crystals show a primary and/or a secondary structure. Low levels of

supersaturation results in bigger crystals that show a distinct secondary structure due

to a prolonged crystal growth phase. An increase of supersaturation by varying one

of the two discussed parameters leads to a decrease of the formed secondary

structure and an even higher supersaturation results in smaller single crystals that

show predominantly a primary structure. This behavior was found for both the

solvents used. Neither the initial PCM concentration nor the pressure variation

enabled the altering of the polymorphic form of PCM. This was found to be

achievable by changing the solvent. Using ethanol as the organic solvent, always the

monoclinic polymorphic form I of PCM was generated irrespectively of the pressure

and the concentration. Changing the organic solvent in the initial solution to acetone

resulted in a different polymorph of PCM. Crystallizing PCM via the SAS process with

acetone as solvent eventuates in the orthorhombic form II. Also a mixture of both

solvents can favour either form I or form II. It was found that already 30 % of acetone

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in the initial mixture is sufficient to change the polymorphic form from monoclinic, as it

is the case for pure EtOH, to orthorhombic.

Generating PCM in an amorphous state was neither able by varying the process

conditions nor the type of solvent. This follows the previously proposed criterion of

ideal and non-ideal solutes, that a switch between precipitation and crystallization is

not possible by just changing the process parameters. If an API, which belongs to the

non-ideal solutes, should be required in a precipitated amorphous form, the authors

propose a co-precipitation with another solute to produce an amorphous solid

dispersion.

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4.1 Acknowledgments:

The authors gratefully acknowledge financial support for parts of this work by the

German Research Foundation (DFG) which additionally funds the Erlangen Graduate

School in Advanced Optical Technologies (SAOT) in the framework of the German

Excellence Initiative to Promote Science and Research at German Universities.

Furthermore, we would like to acknowledge the Institute of Particle Technology of the

Friedrich-Alexander Universitaet Erlangen-Nuernberg (FAU) for providing us access

to the SEM and the Department of Chemistry and Pharmacy for providing us access

to the XRD.

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5 References [1] M. Perrut, Supercritical fluids applications in the pharmaceutical industry, S.T.P. Pharma

Sciences, 13 (2003) 83-91.

[2] P.B. Deshpande, G.A. Kumar, A.R. Kumar, G.V. Shavi, A. Karthik, M.S. Reddy, N.

Udupa, Supercritical fluid technology: Concepts and pharmaceutical applications, PDA

Journal of Pharmaceutical Science and Technology, 65 (2011) 333-344.

[3] H.S. Tan, S. Borsadia, Particle formation using supercritical fluids: Pharmaceutical

applications, Expert Opinion on Therapeutic Patents, 11 (2001) 861-872.

[4] E. Badens, V. Majerik, G. Horváth, L. Szokonya, N. Bosc, E. Teillaud, G. Charbit,

Comparison of solid dispersions produced by supercritical antisolvent and spray-freezing

technologies, International Journal of Pharmaceutics, 377 (2009) 25-34.

[5] J. Fages, H. Lochard, J.-J. Letourneau, M. Sauceau, E. Rodier, Particle generation for

pharmaceutical applications using supercritical fluid technology, Powder Technology, 141

(2004) 219-226.

[6] J. Kluge, G. Muhrer, M. Mazzotti, High pressure homogenization of pharmaceutical

solids, The Journal of Supercritical Fluids, 66 (2012) 380-388.

[7] R.T.Y. Lim, W.K. Ng, R.B.H. Tan, Amorphization of pharmaceutical compound by co-

precipitation using supercritical anti-solvent (SAS) process (Part I), Journal of Supercritical

Fluids, 53 (2010) 179-184.

[8] A. Tabernero, E.M. Martín del Valle, M.A. Galán, Supercritical fluids for pharmaceutical

particle engineering: Methods, basic fundamentals and modelling, Chemical Engineering and

Processing: Process Intensification, 60 (2012) 9-25.

[9] P. Girotra, S.K. Singh, K. Nagpal, Supercritical fluid technology: A promising approach in

pharmaceutical research, Pharmaceutical Development and Technology, 18 (2013) 22-38.

[10] B. Subramaniam, R.A. Rajewski, K. Snavely, Pharmaceutical processing with

supercritical carbon dioxide, Journal of Pharmaceutical Sciences, 86 (1997) 885-890.

[11] G. Muhrer, U. Meier, F. Fusaro, S. Albano, M. Mazzotti, Use of compressed gas

precipitation to enhance the dissolution behavior of a poorly water-soluble drug: Generation

of drug microparticles and drug-polymer solid dispersions, International Journal of

Pharmaceutics, 308 (2006) 69-83.

[12] K. Moribe, Y. Tozuka, K. Yamamoto, Supercritical carbon dioxide processing of active

pharmaceutical ingredients for polymorphic control and for complex formation, Advanced

Drug Delivery Reviews, 60 (2008) 328-338.

[13] P.-C. Lin, C.-S. Su, M. Tang, Y.-P. Chen, Micronization of ethosuximide using the rapid

expansion of supercritical solution (RESS) process, The Journal of Supercritical Fluids, 72

(2012) 84-89.

[14] E. Reverchon, E. Torino, S. Dowy, A. Braeuer, A. Leipertz, Interactions of phase

equilibria, jet fluid dynamics and mass transfer during supercritical antisolvent micronization,

Chemical Engineering Journal, 156 (2010) 446-458.

[15] M. Rossmann, A. Braeuer, S. Dowy, T.G. Gallinger, A. Leipertz, E. Schluecker, Solute

solubility as criterion for the appearance of amorphous particle precipitation or crystallization

in the supercritical antisolvent (SAS) process, Journal of Supercritical Fluids, 66 (2012) 350-

358.

[16] A. Gokhale, B. Khusid, R.N. Dave, R. Pfeffer, Effect of solvent strength and operating

pressure on the formation of submicrometer polymer particles in supercritical microjets, The

Journal of Supercritical Fluids, 43 (2007) 341-356.

[17] S. Varona, J. Fernández, M. Rossmann, A. Braeuer, Solubility of paracetamol and

polyvinylpyrrolidone in mixtures of carbon dioxide, ethanol, and acetone at elevated

pressures, Journal of Chemical and Engineering Data, 58 (2013) 1054-1061.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

16

[18] A. Braeuer, S. Dowy, E. Torino, M. Rossmann, S.K. Luther, E. Schluecker, A. Leipertz,

E. Reverchon, Analysis of the supercritical antisolvent mechanisms governing particles

precipitation and morphology by in situ laser scattering techniques, Chemical Engineering

Journal, 173 (2011) 258-266.

[19] I. De Marco, O. Knauer, F. Cice, A. Braeuer, E. Reverchon, Interactions of phase

equilibria, jet fluid dynamics and mass transfer during supercritical antisolvent micronization:

The influence of solvents, Chemical Engineering Journal, 203 (2012) 71-80.

[20] S. Dowy, E. Torino, S.K. Luther, M. Rossmann, A. Braeuer, Imaging the supersaturation

in high-pressure systems for particle generation, Chemical Engineering Journal, 168 (2011)

896-902.

[21] O. Boutin, T. Petit-Gas, E. Badens, Powder Micronization using a CO2 supercritical

antisolvent type process: Comparison of different introduction devices, Industrial and

Engineering Chemistry Research, 48 (2009) 5671-5678.

[22] A. Montes, A. Tenorio, M.D. Gordillo, C. Pereyra, E.J. Martínez de la Ossa, Screening

design of experiment applied to supercritical antisolvent precipitation of amoxicillin:

Exploring new miscible conditions, The Journal of Supercritical Fluids, 51 (2010) 399-403.

[23] Y. Wang, Y. Wang, J. Yang, R. Pfeffer, R. Dave, B. Michniak, The application of a

supercritical antisolvent process for sustained drug delivery, Powder Technology, 164 (2006)

94-102.

[24] A. Braeuer, R. Adami, S. Dowy, M. Rossmann, A. Leipertz, Observation of liquid

solution volume expansion during particle precipitation in the supercritical CO 2 antisolvent

process, Journal of Supercritical Fluids, 56 (2011) 121-124.

[25] S. Gurunath, S. Pradeep Kumar, N.K. Basavaraj, P.A. Patil, Amorphous solid dispersion

method for improving oral bioavailability of poorly water-soluble drugs, Journal of Pharmacy

Research, 6 (2013) 476-480.

[26] T. Delmas, U.V. Shah, M.M. Roberts, D.R. Williams, J.Y.Y. Heng, Crystallisation of the

orthorhombic form of acetaminophen: Combined effect of surface topography and chemistry,

Powder Technology, 236 (2013) 24-29.

[27] P.J. Frawley, N.A. Mitchell, C.T. Ó'Ciardhá, K.W. Hutton, The effects of

supersaturation, temperature, agitation and seed surface area on the secondary nucleation of

paracetamol in ethanol solutions, Chemical Engineering Science, 75 (2012) 183-197.

[28] N.A. Mitchell, C.T. Óciardhá, P.J. Frawley, Estimation of the growth kinetics for the

cooling crystallisation of paracetamol and ethanol solutions, Journal of Crystal Growth, 328

(2011) 39-49.

[29] C. Sudha, K. Srinivasan, Supersaturation dependent nucleation control and separation of

mono, ortho and unstable polymorphs of paracetamol by swift cooling crystallization

technique, CrystEngComm, 15 (2013) 1914-1921.

[30] I. Pasquali, R. Bettini, F. Giordano, Supercritical fluid technologies: An innovative

approach for manipulating the solid-state of pharmaceuticals, Advanced Drug Delivery

Reviews, 60 (2008) 399-410.

[31] F. Fusaro, M. Mazzotti, G. Muhrer, Gas Antisolvent Recrystallization of Paracetamol

from Acetone Using Compressed Carbon Dioxide as Antisolvent, Cryst. Growth Des., 4

(2004) 881-889.

[32] B. Shekunov, J. Baldyga, P. York, Particle formation by mixing with supercritical

antisolvent at high Reynolds numbers, Chemical Engineering Science, 56 (2001) 2421-2433.

[33] B.Y. Shekunov, M. Hanna, P. York, Crystallization process in turbulent supercritical

flows, Journal of Crystal Growth, 198-199 (1999) 1345-1351.

[34] A. Weber, L.V. Yelash, T. Kraska, Effect of the phase behaviour of the solvent–

antisolvent systems on the gas–antisolvent-crystallisation of paracetamol, The Journal of

Supercritical Fluids, 33 (2005) 107-113.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

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[35] V.J. Pereira, R.L. Matos, S.G. Cardoso, R.O. Soares, G.L. Santana, G.M.N. Costa,

S.A.B. Vieira de Melo, A new approach to select solvents and operating conditions for

supercritical antisolvent precipitation processes by using solubility parameter and group

contribution methods, The Journal of Supercritical Fluids, 81 (2013) 128-146.

[36] E. Reverchon, Supercritical antisolvent precipitation of micro- and nano-particles,

Journal of Supercritical Fluids, The, 15 (1999) 1-21.

[37] G.L. Perlovich, T.V. Volkova, A. Bauer-Brandl, Polymorphism of paracetamol :

RRRelative stability of the monoclinic and orthorhombic phase revisited by sublimation and

solution calorimetry, Journal of Thermal Analysis and Calorimetry, 89 (2007) 767-774.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

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Figure captions:

Fig. 1: Schematic of the SAS plant. (S1: solvent, S2: solution, AS: antisolvent CO2, S:

solvent; SP: syringe pump; HPV: optically accessible high pressure vessel; V: back pressure

regulating valve; D: demister)

Fig. 2: SEM images of PCM crystals generated from ethanol solutions at 313 K and

xCO2 = 0.98. For the left column from a) to c) the pressure was 16 MPa in all the experiments

and the PCM concentration was increased from a) 1 wt% to b) 3 wt% and c) 5 wt%, In the

right column (d), e), f)) the concentration is constant at 1 wt% and the pressure was decreased

from d) 30 MPa to e) 16 MPa and f) 10 MPa.

Fig. 3: XRD spectra of the generated PCM crystals from ethanol solutions. a) 10 MPa,

5 wt %; b) 16 MPa, 1 wt%; c) 30 MPa, 5 wt %.

Fig. 4: SEM images of PCM crystals generated from acetone solutions at 313 K and

xCO2 = 0.98. For the left column from a) to c) the pressure was 16 MPa in all the experiments

and the PCM concentration was increased from a) 1 wt % to b) 3 wt % and c) 5 wt %. In the

right column (d), e)) the concentration is constant at 3 wt % and the pressure was decreased

from d) 16 MPa to e) 10 MPa. For 30 MPa, no experiments from acetone solutions were

conducted.

Fig. 5: Exemplary XRD spectra of the generated PCM particles crystallized from acetone

solutions. a) 10 MPa, 1 wt %; b)16 MPa, 3 wt %; c) 10 MPa, 3 wt %; d) reference spectra of

monoclinic PCM (as delivered)).

Fig. 6: SEM images of PCM crystals generated from ethanol/acetone mixture solutions at

313 K and xCO2 = 0.98. In the images a), the conditions were p = 10 MPa, c = 1 wt %,

EtOH/Ac = 50/50. In the right images b) p = 10 MPa, c = 3 wt %, EtOH/Ac = 50/50. The top

and the bottom images of a) and b) differ only in their magnification. In the bottommost row

in image c) the conditions were p = 10 MPa, c =3 wt %, ratio of EtOH/Ac = 70/30 and in

image d) p = 10 MPa, c = 3 wt %, EtOH/Ac = 80/20

Fig. 7: Exemplary XRD images of the generated PCM crystals from ethanol/acetone mixture

solutions and reference spectra of monoclinic PCM. a) 10 MPa, 1 wt %

EtOH/acetone = 50/50; b) 10 MPa, 3 wt %, EtOH/acetone = 50/50; c) 10 MPa, 3 wt %,

EtOH/acetone = 70/30, d)10 MPa, 3 wt %, EtOH/acetone = 80/20, e) 10 MPa, 3 wt %,

EtOH/acetone = 90/10, f) reference spectra of unprocessed monoclinic PCM

Fig. 1: Schematic of the SAS plant. (S1: solvent, S2: solution, AS: antisolvent CO2, S: solvent; SP: syringe pump; HPV: optically accessible high pressure vessel; V: back pressure regulating valve; D: demister)

Figure(s)

Fig. 2: SEM images of PCM crystals generated from ethanol solutions at 313 K and xCO2 = 0.98. For the left column from a) to c) the pressure was 16 MPa in all the experiments and the PCM concentration was increased from a) 1 wt% to b) 3 wt% and c) 5 wt%, In the right column (d), e), f)) the concentration is constant at 1 wt% and the pressure was decreased from d) 30 MPa to e) 16 MPa and f) 10 MPa

Sup

ersatu

ration

100µm

200µm

100µm

d)

e)

f)

zoom

zoom

30MPa, 1 wt.%

16MPa, 1 wt.%

10MPa, 1 wt.%

c = con

st. ; pre

ssure

de

crease

200µm

100µm

100µm

a)

b)

c)

16MPa, 1 wt.%

16MPa, 3 wt.%

16MPa, 5 wt.%

p = co

nst. ; co

nce

ntratio

n in

crease

Fig. 3: XRD spectra of the generated PCM crystals from ethanol solutions. a) 10 MPa, 5 wt %; b) 16 MPa, 1 wt%; c) 30 MPa, 5 wt %.

0

500

1000

1500

2000

2500

3000

5 10 15 20 25 30 35 40

Co

un

ts

2θ

a) 10MPa; 5wt.%,

ethanol

b) 16MPa; 1wt.%,

ethanol

c) 30MPa; 5wt.%,

ethanol

Fig. 4: SEM images of PCM crystals generated from acetone solutions at 313 K and xCO2 = 0.98. For the left column from a) to c) the pressure was 16 MPa in all the experiments and the PCM concentration was increased from a) 1 wt % to b) 3 wt % and c) 5 wt %. In the right column (d), e)) the concentration is constant at 3 wt % and the pressure was decreased from d) 16 MPa to e) 10 MPa. For 30 MPa, no experiments from acetone solutions were conducted

20µm

20µm

d)

e)

20µm

20µm

20µm

a)

b)

c)

zoom

Sup

ersatu

ration

16MPa, 1 wt.%

16MPa, 3 wt.%

16MPa, 5 wt.%

16MPa, 3 wt.%

10MPa, 3 wt.%

p = co

nst. ; co

nce

ntratio

n in

crease

c = con

st. ; pre

ssure

de

crease

0

500

1000

1500

2000

2500

5 10 15 20 25 30 35 40

Co

un

ts

2θ

DB11 DB12 DB13 PCM unprocessed

a) 10MPa; 1wt.%,

acetone

d) unprocessed

monoclinic PCM

b) 16MPa; 3wt.%,

acetone

c) 10MPa; 3wt.%,

acetone

Fig. 5: Exemplary XRD spectra of the generated PCM particles crystallized from acetone solutions. a) 10 MPa, 1 wt %; b)16 MPa, 3 wt %; c) 10 MPa, 3 wt %; d) reference spectra of monoclinic PCM (as delivered)).

Fig. 6: SEM images of PCM crystals generated from ethanol / acetone (50/50) solutions at 313K and xCO2=0.98. For the left row from a) and b) the pressure was 10 MPa and the PCM concentration was 1 wt%. In the right row , c) and d), the pressure was also 10 MPa and the concentration was increased to 3 wt%

20µm 100µm

10µm 10µm

10µm

c)

10µm

d)

10MPa, 3 wt.% EtOH/Ac = 80/20

10MPa, 3 wt.% EtOH/Ac = 70/30

20µm

a) b)

20µm

10MPa, 1 wt.%

EtOH/Ac 50/50

10MPa, 3 wt.%

EtOH/Ac 50/50

0

500

1000

1500

2000

2500

5 10 15 20 25 30 35 40

Co

un

ts

2θ

a) 10MPa; 1wt.%,

EtOH/Ac=50/50

f) unprocessed

monoclinic PCM

b) 10MPa; 3wt.%,

EtOH/Ac=50/50

c) 10MPa; 3wt.%,

EtOH/Ac=70/30

d) 10MPa; 3wt.%,

EtOH/Ac=80/20

e) 10MPa; 3wt.%,

EtOH/Ac=90/10

Fig. 7: Exemplary XRD images of the generated PCM crystals from ethanol/acetone mixture solutions and reference spectra of monoclinic PCM. a) 10 MPa, 1 wt % EtOH/acetone = 50/50; b) 10 MPa, 3 wt %, EtOH/acetone = 50/50; c) 10 MPa, 3 wt %, EtOH/acetone = 70/30, d)10 MPa, 3 wt %, EtOH/acetone = 80/20, e) 10 MPa, 3 wt %, EtOH/acetone = 90/10, f) reference spectra of unprocessed monoclinic PCM