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Preparation of oil-in-water emulsions using polyamide 10 kDa hollow fiber membrane
Journal of Membrane Science 217 (2003) 173–180
Preparation of oil-in-water emulsions using polyamide 10 kDa hollow fiber membrane Lidietta Giorno∗ , Na Li1 , Enrico Drioli Institute on Membrane Technology, CNR-ITM, C/o University of Calabria, Via P. Bucci 17/C, 87030 Rende (CS), Italy Received 10 September 2002; received in revised form 5 March 2003; accepted 5 March 2003
Abstract In this work, asymmetric polyamide hollow fibre membranes having nominal molecular weight cut-off (NMWCO) of 10 kDa, internal diameter of 1.5 (±0.1) mm, and thickness of 0.4 (±0.05) mm have been used to prepare oil-in-water emulsions. Pure isooctane (or isooctane containing naproxen methyl ester) as oil dispersed phase, ultrapure water (or 50 mmol l−1 (50 mM) phosphate buffer pH 7.00) as continuous phase, and sodium dodecil sulphate (SDS) and polyvinyl alcohol (PVA) as emulsifier and stabiliser, respectively, were used. In order to achieve permeation of the apolar organic solvent, isooctane, through the hydrophilic ultrafiltration membrane it was necessary to pre-treat the membrane. The pre-treatment consisted in removing the polar internal liquid phase (water) and substituting it with the apolar solvent. Emulsion with narrow droplet size could be prepared, whose stability did not show any decrease for more than 20 days. Eighty-five percent of dispersed volume was contained in droplets with size of 1.87 (±0.58) m. © 2003 Elsevier Science B.V. All rights reserved. Keywords: Membrane emulsification; Polyamide membrane; Ultrafiltration membrane; Oil-in-water emulsion
1. Introduction Membrane emulsification is a technology that allows to obtain uniform emulsions at low energy input compared to the emulsion prepared using high-pressure homogenisers and rotor/stator systems [1,2]. Therefore, it is very useful for the preparation of emulsions containing labile compounds such as bioactive molecules sensitive to shear stress. In membrane emulsification, droplets are formed at the pore ∗ Corresponding author. Tel.: +39-0984-492040; fax: +39-0984-402103. E-mail addresses: [email protected] (L. Giorno), lina [email protected] (N. Li). 1 Present address: Department of Environmental and Chemical Engineering, Xi’an Jiao Tong University, Xi’an 710049, China.
mouth of a membrane by forcing the dispersed phase to permeate through the membrane and stripping the droplets from the pore into the continuous phase by action of the axial velocity [3,4]. The droplets size depends on the balance between the drag force on the droplets, interfacial tension forces, the buoyancy of the droplets, and the transmembrane pressure (TMP) [5]. Stability and density of emulsions prepared by membranes are very good, but productivity might represent a limiting aspect for large scale application. In other words, low permeation rate is in general associated with emulsions having narrow size and uniform distribution. As far as the authors know from the open literature, emulsification by membrane technology has
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been mainly applied to microporous inorganic membranes [5–9]. Microporous polymeric membranes (pore size 0.4 m) made of polypropylene have been recently used by Vladisavljevic et al. [10] to prepare water-in-oil emulsion. A linear correlation (y = mx) between membrane pore size (y) and droplet size (x) has been generally observed where the value of m may range from 2 to 10 depending on the properties of the organic, water and membrane [11]. The present work describes the preparation of oil-in-water emulsion by using ultrafiltration membranes (10 kDa nominal molecular weight cut-off (NMWCO)) made of polyamide. This represents the first attempt to use nanoporous hydrophilic polymeric membranes (pores diameter around 10 nm) to permeate apolar organic solvents for the preparation of emulsion. The emulsion was prepared by permeating the dispersed phase from shell-to-lumen or from lumen-to-shell. Emulsion preparation as a function of permeation rate and emulsifier concentration was investigated. The droplet size and size distribution, emulsion density and stability as a function of time, during preparation and afterwards (keeping it suspended by means of a magnetic stirrer at low rpm) were also studied.
2. Materials and methods Isooctane of reagent grade from Sigma–Aldrich was used as organic phase for the oil-in-water emulsion. Racemic mixture of naproxen methyl ester were obtained from commercial (S)-naproxen acid by racemization and esterification reactions (prepared by Prof. F. Trotta, University of Torino, Italy). Ultrapure water (bidistilled water, filtered with membrane of 0.2 m pore size), sodium dihydrogen phosphate anhydrous (NaH2 PO4 ) and disodium hydrogen phosphate anhydrous (Na2 HPO4 ) from Fluka (Germany) or Sigma (Italy), were used to prepare phosphate buffer solutions at pH = 7.00. Sodium dodecyl sulfate (SDS) and polyvinyl alcohol (PVA, molecular weight 22,000 and 97.5–99% of hydrolysis degree) from Fluka, were used as emulsifier and stabilizer, respectively. Isopropanol (IPA) of analyt-
ical grade from Fluka was used for the membrane pre-treatment. The membranes used in this study were polyamide membrane of 10 kDa NMWCO and 1.5 mm/2.2 mm inner/outer diameter (kindly provided by Forschungsinstitut Berghof, Germany) (named as PA 10 kDa in the following part). They are asymmetric with the dense selective layer on the lumen side and the sponge layer on the shell side. The lab-made membrane modules were prepared by assembling four capillary membranes inside a Pyrex glass cylinder of 1.2 cm i.d., 20 cm long. The internal/external membrane surface areas were 35.8 and 53.2 cm2 , respectively. Emulsions were made of isooctane as oil dispersed phase; ultrapure water or 50 mmol l−1 (50 mM) phosphate buffer pH 7.00 as continuous phase; and SDS and PVA as emulsifier and stabiliser, respectively. In some cases, the isooctane contained a racemic mixture of naproxen ester (the emulsion was applied to improve the efficiency of the enantioselective bioconversion of (S)-naproxen in a multiphase membrane reactor (Li et al., manuscript in preparation)). 2.1. Membrane pre-treatment In order to achieve permeation of the apolar organic solvent, isooctane, through the hydrophilic ultrafiltration membrane it was necessary to pre-treat the membrane. The pre-treatment consisted in removing the polar internal liquid phase (water) and substituting it with the apolar solvent. This was achieved by permeating through the membrane miscible solvents of gradually decreasing polarity, so that water was washed out and it was replaced with isooctane. The membrane was first washed with ultrapure water to remove the water-soluble residues in membrane and then the pure water permeability was tested to confirm the similar permeation property from membrane module to membrane module. Afterwards, the membrane was soaked in isopropanol for 60 min and then about 200 ml of isopropanol was filtrated through membrane under 0.03–0.06 MPa of TMP, to replace all water and water-insoluble residue in membrane matrix. The membrane was then soaked in a mixture of 50/50 (v/v) isopropanol/isooctane for 60 min. Finally, the membrane was soaked in pure isooctane overnight and then pure isooctane was filtrated.
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Fig. 1. Schematic representation of the membrane emulsification system.
2.2. Equipment A schematic diagram of membrane emulsification system is shown in Fig. 1. An apparatus which includes two control panels fixed up with valves, flowmeters and pressure gauges to control separately the fluid dynamics conditions of the two circuits was used. A lab-made membrane module containing four capillary PA 10 kDa membrane was connected with this system. The dispersion phase (organic phase) was circulated in the lumen side (or in the shell side) of membrane module and under certain pressure permeated through membrane into continuous phase aqueous phase which was circulated in shell side (or lumen side) of membrane module where the emulsion was formed. A balance was used to measure the weight decrease of organic phase as a function of time from which it was possible to calculate the permeate rate and the percentage of the dispersed phase. The permeate flux was calculated as: Fd Jd = (1) ρd A
where Jd (m s−1 ) is the flux, Fd (g s−1 ) the mass flux of the dispersed phase, ρ (g cm−3 ) the density of the dispersed phase and A (m2 ) the membrane area. TMP, the pressure difference between the pressure of dispersion phase and the circulation pressure of continuous phase, is calculated according to the following equation: TMP = 21 (Pd, in + Pd, out ) − 21 (Pc, in + Pc, out )
(2)
where Pd, in and Pd, out are the pressure of dispersion phase at the inlet and outlet, respectively, and Pc, in and Pc, out are the pressure of continuous phase at the inlet and outlet, respectively. 2.3. Droplets’ measurement Droplets were observed by optical microscopy; the images were photographed by a camera and the pictures were then analysed by the “Scion Image” program. This program allows to automatically count and measure the droplets present in a selected area. An hemocytometer, with nine calibrated cells, was used to
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hold the emulsion samples [12]. A droplet of emulsion was positioned on the chamber and was covered with a cover glass. The chamber is constructed so that when the cover slip is in place, each of the nine squares has a volume of 0.1 mm3 or 1 × 10−4 cm3 . Since 1 cm3 is approximately 1 ml, the droplet concentration/ml can be determined as follows:
a membrane module was maintained in contact with isooctane for more than 3 months and allowed to obtain similar performance in terms of emulsion size and permeate flux.
Droplets/ml = (average count/square)
Droplet size can be controlled by membrane pore size. Katoh et al. [13] have prepared emulsion using Shirasu porous glass membrane [14] with different pore size in the range of 0.57–2.34 m and obtained a linear relationship (y = 5x) according to which the droplet size were five times bigger than the membrane pore size [13]. The organic dispersed phase was corn oil or kerosene and the continuous phase was deionised water or salt solution containing emulsifier (such as, sucrose esters, SE, polyglycerol esters, PGE, and SDS). In our experiments, pure isooctane or isooctane containing naproxen ester was used as dispersed phase and 50 mmol l−1 (50 mM) phosphate buffer with SDS and PVA 0.2 and 0.8 wt.%, respectively, was used as continuous phase. TMP from the dispersed phase (recycled along the shell circuit) was about 80 kPa, and continuous axial flow rate (recycled along the lumen circuit) was about 650 ml min−1 (1.5 m s−1 ). Permeation of the dispersed phase occurred then from shell-to-lumen. In Fig. 2, the behavior of droplets size prepared with 10 kDa cut-off with reference to the relationship reported by Katoh et al. [13] is shown. The droplet size
× (dilution factor) × 104 [chamber conversion factor] Each data is the result of three repeated measurements (different sampling from the same batch) and for each one at least 10 squares were counted. From these measurements, the mean droplet size and size distribution, the droplet density and stability were evaluated. 3. Results and discussion 3.1. Membrane performance Before the pre-treatment, isooctane could not permeate through the membrane even under 0.35 MPa of transmembrane pressure; after the pre-treatment, isooctane could easily permeate through the membrane even under 0.01–0.02 MPa transmembrane pressure. Operating in the opposite way, it was possible to remove the organic solvent and replace water. The efficiency of the treatment was evaluated by measuring pure water permeability of virgin membranes (110 (±10) l m−2 h−1 bar−1 , or 3.0 × 10−4 m3 m−2 s−1 MPa−1 ), of membrane containing the isooctane as internal liquid phase (no water could permeate until transmembrane pressure of 0.05 MPa), and after restoring water as internal liquid phase (permeability about 20% less compared to the virgin membrane). The isopropanol may swell the membrane, but when removing it using 50/50 IPA isooctane and then pure isooctane, the macroscopic membrane characteristics (shape, thickness, length) can be restored. In addition, no significant change of the membrane cross-section could be evidenced with scanning electron microscopy (SEM). On the other hand, membranes were very stable in contact with isooctane. In fact, after pre-treatment,
3.2. Droplet size, size distribution, and stability
Fig. 2. Relationship between membrane pore diameter (Dm) and particle diameter (Dp): comparison between microfiltration (literature data) and ultrafiltration (present work) results. The data shown in the figure refer to the use of 0.2% SDS and 0.8% PVA. Using 0.1% SDS and 0.4% PVA similar mean droplet size were obtained (about 1.9 m).
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Fig. 3. Droplet size distribution after about 5 days from the preparation.
have 1.87 (±0.58) m diameter. Eighty-five percent of the organic phase was present as droplets of 1.87 m (Fig. 3) with a coefficient of variation of 0.33. The equivalent molecular diameter for a 10 kDa molecule is less than 10 nm. Therefore, a significant deviation from the above mentioned relationship was observed. The reason for the deviation from the mentioned relationship might be in the assumption that the pores during emulsion preparation have the original dimensions. Swelling phenomena might affect the pore size of the membrane during emulsion preparation. In this case, on the basis of the particle dimension we can extrapolate the pore size from the figure, which results about 0.25 m. On the other hand, this pore size does not explain the permeability values obtained with membranes after restoring water as internal liquid phase. In fact, in this case (where the membranes have been eventually swollen), pure water permeability is 20% less compared to the virgin membrane (while with 0.25 m permeability should have been much higher). Further experiments with membranes having different pore size and characterisation of membranes by non-invasive microscopy technique (e.g. atomic force microscopy) will clarify the effective influence of membrane pre-treatment on polymeric membrane. The use of ceramic membranes will allow to confirm the relationship between pore and droplet size for nanoporous membrane. These preliminary results can be interpreted in various ways. For this reason, a proper microscopic theoretical treatment, on the basis of force balance
Fig. 4. Average droplet size as a function of time. Emulsion prepared by permeation from shell-to-lumen with no emulsifier and with 0.1% SDS and 0.4% PVA could not be followed longer than 1 and 24 h, respectively, due to coalescence and phase separation.
models, is necessary and is in progress. From other experimental tests and from the model rationalisation we will be able to better understand and verify our results. In particular, either the swelling effect than the linear relationship between pore size and droplet size. At the moment, from a macroscopic point of view, we can say that using polymeric membranes with NMWCO of 10 kDa and pre-treated with solvents as described, emulsions of about 1.87 m can be obtained. The emulsions are very stable. In fact, they have been kept in suspension by gently stirring with a magnetic stirrer (at about 1.047×10−1 rad s−1 (1 rpm)) for more than 20 days and no significant change in the average size and density was observed. In Fig. 4, the stability of the emulsion is illustrated in terms of average size as a function of time. 3.3. Emulsion using methylester of naproxen in isooctane as dispersed phase (permeation lumen-to-shell) The dispersion phase (organic phase) was recycled in the lumen side of membrane module and pressed through membrane pores into the continuous phase (aqueous phase). The continuous phase was recycled in the shell side of membrane module with 700–800 ml min−1 of flow rate (0.18 m s−1 ). The
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module was previously submitted to hydrodynamic compaction in order to reduce pore size of the shell side (that contained the sponge layer). The organic phase is 245 ml of isooctane solution containing 5 mM racemic naproxen methyl ester, and aqueous phase is 400 ml of 50 mM phosphate buffer solution pH = 7.0 containing 0.2 wt.% SDS and 0.8 wt.% PVA. This type of emulsion is useful for reaction purposes. After 32.67 h of running, 100 ml of organic phase permeated through membrane and dispersed in aqueous phase. The final organic content in emulsion was 20 vol.%. So the average dispersion flux of organic phase through membrane is 0.086 ml cm−2 h−1 . The emulsion was observed with optical microscopy. Fig. 5 is the photo of emulsion taken after 22 h of its preparation, magnified 10 times and diluted 200 times. It can been seen that the organic phase was mono-dispersed in the aqueous phase and the droplets are very fine and dense with uniform size distribution. The droplets size is estimated about 3.5 (±0.5) m with a concentration of 6 × 105 droplets/ml (in the diluted sample) and 1.2 × 108 droplets/ml in the original volume. During the preparation, the TMP was about 60–100 kPa. The analysis of emulsion property was followed for more than 1 month to investigate its stability. The emulsion kept good dispersion status of
organic phase in aqueous phase, and moreover, there was no appreciable change in the droplet size and size distribution of emulsion (see also Fig. 4). At higher permeation rate (using non-compacted module), the emulsion showed larger droplets size and wide size distribution. In fact, when the dispersion flux of organic phase through membrane was 5.30 (±1) ml cm−2 h−1 , droplets size ranged between 1 and 5 m. 3.4. Emulsion using pure isooctane as dispersed phase (permeation shell-to-lumen) Pure isooctane was used as dispersed phase; it was recirculated along the shell side and pressed through the membrane at a TMP of about 80 kPa. The continuous phase was recirculated along the lumen circuit at 650 (±50) ml min−1 (about 1.5 m s−1 ); it was made of pure water, or 50 mmol l−1 (50 mM) phosphate buffer containing SDS and PVA at different percentages, as indicated in the following. The use of pure water without emulsifier and stabiliser allowed to prepare fine emulsion but not stable. In fact, it was possible to observe droplets of about 2.5 m for about 1 h. During this period of time, the droplets first migrated over the water surface, then aggregated and coalesced.
Fig. 5. Example of emulsion prepared using isooctane containing naproxen ester as dispersed phase, permeated from lumen-to-shell (SDS 0.2%, PVA 0.8%, diluted 200 times).
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Fig. 6. Example of emulsion prepared using pure isooctane as dispersed phase, permeated from shell-to-lumen (SDS 0.1%, PVA 0.4%, no dilution).
Experiments with 50 mmol l−1 (50 mM) phosphate buffer and two concentration of SDS and PVA were carried out. In particular, SDS 0.1 wt.%–PVA 0.4 wt.%, and SDS 0.2 wt.%–PVA 0.8 wt.% were used. The results showed that the presence of emulsifier and stabiliser influenced the droplet size and size distribution, the density and the stability. The amount of emulsifier and stabiliser influenced the density and stability. In other words, when SDS and PVA were present in the continuous phase, smaller droplets (1.9 (±0.5) m) were prepared compared to when pure water was used. Changing the amount of SDS and PVA there was no significant change in the size and size distribution, while emulsion density and stability were influenced. For example, when lower SDS and PVA concentrations were used (0.1 and 0.4 wt.%, respectively), density was much lower and emulsion lasted for no more than 24 h. The mean droplet size as a function of time for the different concentrations of emulsifier and stabiliser are reported in Fig. 4. Fig. 6
shows a microscopic picture of this type of emulsion, for which no dilution has been practised.
4. Conclusions In the present study, oil/water emulsions were prepared by membrane emulsification technology using polyamide ultrafiltration membrane with 10 kDa NMWCO pre-treated with organic solvents. The pre-treatment of membrane was of key importance to allow permeation of the oily phase. The stability of membrane was very good, showing no decrease in performance for more than 3 months of contact with organic solvent. The emulsion was a dispersion of isooctane containing racemic naproxen ester in a continuous aqueous phase containing SDS and PVA as emulsifier and stabilizer. When permeation was carried out from shell-to-lumen, the organic droplets size were 1.87 (±0.58) m. During a period of 20–30
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days of observation with microscopy, the emulsions maintained good stability. When permeation was carried out from lumen-to-shell through compacted membranes, the organic droplets were larger (3.5 (±0.5) m) due to the larger pore size of the external surface, but stability was good as well. Emulsions were also prepared using pure isooctane as dispersed phase and similar results were observed. The membrane compaction, dispersion flux of organic phase, presence and concentration of emulsifier and stabiliser affected the droplets size, size distribution, emulsion density, and stability. This study, showed that polymeric ultrafiltration membranes can be successfully applied to prepare very stable emulsion with narrow size distribution.
[4]
[5]
[6] [7]
[8]
[9]
Acknowledgements This work was carried out with the support of the Italian National Research Council (Consiglio Nazionale delle Ricerche, CNR). References [1] H. Karbstein, H. Schubert, Development in the continuous mechanical production of oil-in-water macro-emulsions, Chem. Eng. Process 34 (1995) 205–211. [2] J. Klier, C.J. Tucker, T.H. Kalantar, D.P. Green, Properties and applications of microemulsions, Adv. Mater. 12 (23) (2000) 1751–1757. [3] V. Schröder, H. Schubert, Influence of emulsifier and pore size on membrane emulsification, in: Food Emulsions and
[10]
[11]
[12] [13]
[14]
Foams, The Royal Society of Chemistry, Cambridge, 1999, pp. 70–80. H. Yuyama, T. Watanabe, G-H. Ma, M. Nagai, Preparation and analysis of uniform emulsion droplets using SPG membrane emulsification technique, Colloids Surf. A: Physicochem. Eng. Aspects 168 (2000) 159–174. V. Schröder, O. Behrend, H. Schubert, Effect of dynamic interfacial tension on the emulsification process using microporous ceramic membranes, J. Colloid Interface Sci. 202 (1998) 334–340. S.M. Joscelyne, G. Tragardh, Membrane emulsification—a literature review, J. Membr. Sci. 169 (2000) 107–117. K. Kandori, Applications of microporous glass membranes: membrane emulsification, in: A.G. Gaonkar (Ed.), Food Processing: Recent Developments, Elsevier, Amsterdam, 1995, pp. 113–142. T. Nakaschima, M. Shimizu, Preparation of monodispersed O/W emulsion by porous-glass membrane, Kagaku Kogaku Ronbunshu 19 (1993) 984–990. Y. Mine, M. Shimizu, T. Nakashima, Application of sizecontrolled microporous glass membranes for designing simple and multiple emulsions, Recent Res. Dev. Agric. Food Chem. 3 (1999) 131–137. G.T. Vladisavljevic, S. Tesch, H. Shubert, Preparation of water-in-oil emulsions using microporous polypropylene hollow fibers: influence of some operating parameters on droplet size distribution, Chem. Eng. Process. 41 (2002) 231– 238. M. Rayner, G. Tragardh, Membrane emulsification modelling: how can we get from characterization to design? Desalination 145 (2002) 165–172. H.J. Philips, J.E. Terryberry, Counting actively metabolising tissue cultured cells, Exp. Cell. Res. 13 (1957) 341–347. R. Katoh, Y. Asano, A. Furuya, K. Sotoyama, M. Tomita, Preparation of food emulsions using a membrane emulsification system, J. Membr. Sci. 113 (1996) 131–135. T. Nakashima, M. Shimizu, Porous glass from calcium alumino boro-silicate glass, Ceramics 21 (1986) 408–415.
Preparation of oil-in-water emulsions using polyamide 10 kDa hollow fiber membrane Lidietta Giorno∗ , Na Li1 , Enrico Drioli Institute on Membrane Technology, CNR-ITM, C/o University of Calabria, Via P. Bucci 17/C, 87030 Rende (CS), Italy Received 10 September 2002; received in revised form 5 March 2003; accepted 5 March 2003
Abstract In this work, asymmetric polyamide hollow fibre membranes having nominal molecular weight cut-off (NMWCO) of 10 kDa, internal diameter of 1.5 (±0.1) mm, and thickness of 0.4 (±0.05) mm have been used to prepare oil-in-water emulsions. Pure isooctane (or isooctane containing naproxen methyl ester) as oil dispersed phase, ultrapure water (or 50 mmol l−1 (50 mM) phosphate buffer pH 7.00) as continuous phase, and sodium dodecil sulphate (SDS) and polyvinyl alcohol (PVA) as emulsifier and stabiliser, respectively, were used. In order to achieve permeation of the apolar organic solvent, isooctane, through the hydrophilic ultrafiltration membrane it was necessary to pre-treat the membrane. The pre-treatment consisted in removing the polar internal liquid phase (water) and substituting it with the apolar solvent. Emulsion with narrow droplet size could be prepared, whose stability did not show any decrease for more than 20 days. Eighty-five percent of dispersed volume was contained in droplets with size of 1.87 (±0.58) m. © 2003 Elsevier Science B.V. All rights reserved. Keywords: Membrane emulsification; Polyamide membrane; Ultrafiltration membrane; Oil-in-water emulsion
1. Introduction Membrane emulsification is a technology that allows to obtain uniform emulsions at low energy input compared to the emulsion prepared using high-pressure homogenisers and rotor/stator systems [1,2]. Therefore, it is very useful for the preparation of emulsions containing labile compounds such as bioactive molecules sensitive to shear stress. In membrane emulsification, droplets are formed at the pore ∗ Corresponding author. Tel.: +39-0984-492040; fax: +39-0984-402103. E-mail addresses: [email protected] (L. Giorno), lina [email protected] (N. Li). 1 Present address: Department of Environmental and Chemical Engineering, Xi’an Jiao Tong University, Xi’an 710049, China.
mouth of a membrane by forcing the dispersed phase to permeate through the membrane and stripping the droplets from the pore into the continuous phase by action of the axial velocity [3,4]. The droplets size depends on the balance between the drag force on the droplets, interfacial tension forces, the buoyancy of the droplets, and the transmembrane pressure (TMP) [5]. Stability and density of emulsions prepared by membranes are very good, but productivity might represent a limiting aspect for large scale application. In other words, low permeation rate is in general associated with emulsions having narrow size and uniform distribution. As far as the authors know from the open literature, emulsification by membrane technology has
0376-7388/03/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0376-7388(03)00126-1
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been mainly applied to microporous inorganic membranes [5–9]. Microporous polymeric membranes (pore size 0.4 m) made of polypropylene have been recently used by Vladisavljevic et al. [10] to prepare water-in-oil emulsion. A linear correlation (y = mx) between membrane pore size (y) and droplet size (x) has been generally observed where the value of m may range from 2 to 10 depending on the properties of the organic, water and membrane [11]. The present work describes the preparation of oil-in-water emulsion by using ultrafiltration membranes (10 kDa nominal molecular weight cut-off (NMWCO)) made of polyamide. This represents the first attempt to use nanoporous hydrophilic polymeric membranes (pores diameter around 10 nm) to permeate apolar organic solvents for the preparation of emulsion. The emulsion was prepared by permeating the dispersed phase from shell-to-lumen or from lumen-to-shell. Emulsion preparation as a function of permeation rate and emulsifier concentration was investigated. The droplet size and size distribution, emulsion density and stability as a function of time, during preparation and afterwards (keeping it suspended by means of a magnetic stirrer at low rpm) were also studied.
2. Materials and methods Isooctane of reagent grade from Sigma–Aldrich was used as organic phase for the oil-in-water emulsion. Racemic mixture of naproxen methyl ester were obtained from commercial (S)-naproxen acid by racemization and esterification reactions (prepared by Prof. F. Trotta, University of Torino, Italy). Ultrapure water (bidistilled water, filtered with membrane of 0.2 m pore size), sodium dihydrogen phosphate anhydrous (NaH2 PO4 ) and disodium hydrogen phosphate anhydrous (Na2 HPO4 ) from Fluka (Germany) or Sigma (Italy), were used to prepare phosphate buffer solutions at pH = 7.00. Sodium dodecyl sulfate (SDS) and polyvinyl alcohol (PVA, molecular weight 22,000 and 97.5–99% of hydrolysis degree) from Fluka, were used as emulsifier and stabilizer, respectively. Isopropanol (IPA) of analyt-
ical grade from Fluka was used for the membrane pre-treatment. The membranes used in this study were polyamide membrane of 10 kDa NMWCO and 1.5 mm/2.2 mm inner/outer diameter (kindly provided by Forschungsinstitut Berghof, Germany) (named as PA 10 kDa in the following part). They are asymmetric with the dense selective layer on the lumen side and the sponge layer on the shell side. The lab-made membrane modules were prepared by assembling four capillary membranes inside a Pyrex glass cylinder of 1.2 cm i.d., 20 cm long. The internal/external membrane surface areas were 35.8 and 53.2 cm2 , respectively. Emulsions were made of isooctane as oil dispersed phase; ultrapure water or 50 mmol l−1 (50 mM) phosphate buffer pH 7.00 as continuous phase; and SDS and PVA as emulsifier and stabiliser, respectively. In some cases, the isooctane contained a racemic mixture of naproxen ester (the emulsion was applied to improve the efficiency of the enantioselective bioconversion of (S)-naproxen in a multiphase membrane reactor (Li et al., manuscript in preparation)). 2.1. Membrane pre-treatment In order to achieve permeation of the apolar organic solvent, isooctane, through the hydrophilic ultrafiltration membrane it was necessary to pre-treat the membrane. The pre-treatment consisted in removing the polar internal liquid phase (water) and substituting it with the apolar solvent. This was achieved by permeating through the membrane miscible solvents of gradually decreasing polarity, so that water was washed out and it was replaced with isooctane. The membrane was first washed with ultrapure water to remove the water-soluble residues in membrane and then the pure water permeability was tested to confirm the similar permeation property from membrane module to membrane module. Afterwards, the membrane was soaked in isopropanol for 60 min and then about 200 ml of isopropanol was filtrated through membrane under 0.03–0.06 MPa of TMP, to replace all water and water-insoluble residue in membrane matrix. The membrane was then soaked in a mixture of 50/50 (v/v) isopropanol/isooctane for 60 min. Finally, the membrane was soaked in pure isooctane overnight and then pure isooctane was filtrated.
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175
Fig. 1. Schematic representation of the membrane emulsification system.
2.2. Equipment A schematic diagram of membrane emulsification system is shown in Fig. 1. An apparatus which includes two control panels fixed up with valves, flowmeters and pressure gauges to control separately the fluid dynamics conditions of the two circuits was used. A lab-made membrane module containing four capillary PA 10 kDa membrane was connected with this system. The dispersion phase (organic phase) was circulated in the lumen side (or in the shell side) of membrane module and under certain pressure permeated through membrane into continuous phase aqueous phase which was circulated in shell side (or lumen side) of membrane module where the emulsion was formed. A balance was used to measure the weight decrease of organic phase as a function of time from which it was possible to calculate the permeate rate and the percentage of the dispersed phase. The permeate flux was calculated as: Fd Jd = (1) ρd A
where Jd (m s−1 ) is the flux, Fd (g s−1 ) the mass flux of the dispersed phase, ρ (g cm−3 ) the density of the dispersed phase and A (m2 ) the membrane area. TMP, the pressure difference between the pressure of dispersion phase and the circulation pressure of continuous phase, is calculated according to the following equation: TMP = 21 (Pd, in + Pd, out ) − 21 (Pc, in + Pc, out )
(2)
where Pd, in and Pd, out are the pressure of dispersion phase at the inlet and outlet, respectively, and Pc, in and Pc, out are the pressure of continuous phase at the inlet and outlet, respectively. 2.3. Droplets’ measurement Droplets were observed by optical microscopy; the images were photographed by a camera and the pictures were then analysed by the “Scion Image” program. This program allows to automatically count and measure the droplets present in a selected area. An hemocytometer, with nine calibrated cells, was used to
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hold the emulsion samples [12]. A droplet of emulsion was positioned on the chamber and was covered with a cover glass. The chamber is constructed so that when the cover slip is in place, each of the nine squares has a volume of 0.1 mm3 or 1 × 10−4 cm3 . Since 1 cm3 is approximately 1 ml, the droplet concentration/ml can be determined as follows:
a membrane module was maintained in contact with isooctane for more than 3 months and allowed to obtain similar performance in terms of emulsion size and permeate flux.
Droplets/ml = (average count/square)
Droplet size can be controlled by membrane pore size. Katoh et al. [13] have prepared emulsion using Shirasu porous glass membrane [14] with different pore size in the range of 0.57–2.34 m and obtained a linear relationship (y = 5x) according to which the droplet size were five times bigger than the membrane pore size [13]. The organic dispersed phase was corn oil or kerosene and the continuous phase was deionised water or salt solution containing emulsifier (such as, sucrose esters, SE, polyglycerol esters, PGE, and SDS). In our experiments, pure isooctane or isooctane containing naproxen ester was used as dispersed phase and 50 mmol l−1 (50 mM) phosphate buffer with SDS and PVA 0.2 and 0.8 wt.%, respectively, was used as continuous phase. TMP from the dispersed phase (recycled along the shell circuit) was about 80 kPa, and continuous axial flow rate (recycled along the lumen circuit) was about 650 ml min−1 (1.5 m s−1 ). Permeation of the dispersed phase occurred then from shell-to-lumen. In Fig. 2, the behavior of droplets size prepared with 10 kDa cut-off with reference to the relationship reported by Katoh et al. [13] is shown. The droplet size
× (dilution factor) × 104 [chamber conversion factor] Each data is the result of three repeated measurements (different sampling from the same batch) and for each one at least 10 squares were counted. From these measurements, the mean droplet size and size distribution, the droplet density and stability were evaluated. 3. Results and discussion 3.1. Membrane performance Before the pre-treatment, isooctane could not permeate through the membrane even under 0.35 MPa of transmembrane pressure; after the pre-treatment, isooctane could easily permeate through the membrane even under 0.01–0.02 MPa transmembrane pressure. Operating in the opposite way, it was possible to remove the organic solvent and replace water. The efficiency of the treatment was evaluated by measuring pure water permeability of virgin membranes (110 (±10) l m−2 h−1 bar−1 , or 3.0 × 10−4 m3 m−2 s−1 MPa−1 ), of membrane containing the isooctane as internal liquid phase (no water could permeate until transmembrane pressure of 0.05 MPa), and after restoring water as internal liquid phase (permeability about 20% less compared to the virgin membrane). The isopropanol may swell the membrane, but when removing it using 50/50 IPA isooctane and then pure isooctane, the macroscopic membrane characteristics (shape, thickness, length) can be restored. In addition, no significant change of the membrane cross-section could be evidenced with scanning electron microscopy (SEM). On the other hand, membranes were very stable in contact with isooctane. In fact, after pre-treatment,
3.2. Droplet size, size distribution, and stability
Fig. 2. Relationship between membrane pore diameter (Dm) and particle diameter (Dp): comparison between microfiltration (literature data) and ultrafiltration (present work) results. The data shown in the figure refer to the use of 0.2% SDS and 0.8% PVA. Using 0.1% SDS and 0.4% PVA similar mean droplet size were obtained (about 1.9 m).
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Fig. 3. Droplet size distribution after about 5 days from the preparation.
have 1.87 (±0.58) m diameter. Eighty-five percent of the organic phase was present as droplets of 1.87 m (Fig. 3) with a coefficient of variation of 0.33. The equivalent molecular diameter for a 10 kDa molecule is less than 10 nm. Therefore, a significant deviation from the above mentioned relationship was observed. The reason for the deviation from the mentioned relationship might be in the assumption that the pores during emulsion preparation have the original dimensions. Swelling phenomena might affect the pore size of the membrane during emulsion preparation. In this case, on the basis of the particle dimension we can extrapolate the pore size from the figure, which results about 0.25 m. On the other hand, this pore size does not explain the permeability values obtained with membranes after restoring water as internal liquid phase. In fact, in this case (where the membranes have been eventually swollen), pure water permeability is 20% less compared to the virgin membrane (while with 0.25 m permeability should have been much higher). Further experiments with membranes having different pore size and characterisation of membranes by non-invasive microscopy technique (e.g. atomic force microscopy) will clarify the effective influence of membrane pre-treatment on polymeric membrane. The use of ceramic membranes will allow to confirm the relationship between pore and droplet size for nanoporous membrane. These preliminary results can be interpreted in various ways. For this reason, a proper microscopic theoretical treatment, on the basis of force balance
Fig. 4. Average droplet size as a function of time. Emulsion prepared by permeation from shell-to-lumen with no emulsifier and with 0.1% SDS and 0.4% PVA could not be followed longer than 1 and 24 h, respectively, due to coalescence and phase separation.
models, is necessary and is in progress. From other experimental tests and from the model rationalisation we will be able to better understand and verify our results. In particular, either the swelling effect than the linear relationship between pore size and droplet size. At the moment, from a macroscopic point of view, we can say that using polymeric membranes with NMWCO of 10 kDa and pre-treated with solvents as described, emulsions of about 1.87 m can be obtained. The emulsions are very stable. In fact, they have been kept in suspension by gently stirring with a magnetic stirrer (at about 1.047×10−1 rad s−1 (1 rpm)) for more than 20 days and no significant change in the average size and density was observed. In Fig. 4, the stability of the emulsion is illustrated in terms of average size as a function of time. 3.3. Emulsion using methylester of naproxen in isooctane as dispersed phase (permeation lumen-to-shell) The dispersion phase (organic phase) was recycled in the lumen side of membrane module and pressed through membrane pores into the continuous phase (aqueous phase). The continuous phase was recycled in the shell side of membrane module with 700–800 ml min−1 of flow rate (0.18 m s−1 ). The
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module was previously submitted to hydrodynamic compaction in order to reduce pore size of the shell side (that contained the sponge layer). The organic phase is 245 ml of isooctane solution containing 5 mM racemic naproxen methyl ester, and aqueous phase is 400 ml of 50 mM phosphate buffer solution pH = 7.0 containing 0.2 wt.% SDS and 0.8 wt.% PVA. This type of emulsion is useful for reaction purposes. After 32.67 h of running, 100 ml of organic phase permeated through membrane and dispersed in aqueous phase. The final organic content in emulsion was 20 vol.%. So the average dispersion flux of organic phase through membrane is 0.086 ml cm−2 h−1 . The emulsion was observed with optical microscopy. Fig. 5 is the photo of emulsion taken after 22 h of its preparation, magnified 10 times and diluted 200 times. It can been seen that the organic phase was mono-dispersed in the aqueous phase and the droplets are very fine and dense with uniform size distribution. The droplets size is estimated about 3.5 (±0.5) m with a concentration of 6 × 105 droplets/ml (in the diluted sample) and 1.2 × 108 droplets/ml in the original volume. During the preparation, the TMP was about 60–100 kPa. The analysis of emulsion property was followed for more than 1 month to investigate its stability. The emulsion kept good dispersion status of
organic phase in aqueous phase, and moreover, there was no appreciable change in the droplet size and size distribution of emulsion (see also Fig. 4). At higher permeation rate (using non-compacted module), the emulsion showed larger droplets size and wide size distribution. In fact, when the dispersion flux of organic phase through membrane was 5.30 (±1) ml cm−2 h−1 , droplets size ranged between 1 and 5 m. 3.4. Emulsion using pure isooctane as dispersed phase (permeation shell-to-lumen) Pure isooctane was used as dispersed phase; it was recirculated along the shell side and pressed through the membrane at a TMP of about 80 kPa. The continuous phase was recirculated along the lumen circuit at 650 (±50) ml min−1 (about 1.5 m s−1 ); it was made of pure water, or 50 mmol l−1 (50 mM) phosphate buffer containing SDS and PVA at different percentages, as indicated in the following. The use of pure water without emulsifier and stabiliser allowed to prepare fine emulsion but not stable. In fact, it was possible to observe droplets of about 2.5 m for about 1 h. During this period of time, the droplets first migrated over the water surface, then aggregated and coalesced.
Fig. 5. Example of emulsion prepared using isooctane containing naproxen ester as dispersed phase, permeated from lumen-to-shell (SDS 0.2%, PVA 0.8%, diluted 200 times).
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Fig. 6. Example of emulsion prepared using pure isooctane as dispersed phase, permeated from shell-to-lumen (SDS 0.1%, PVA 0.4%, no dilution).
Experiments with 50 mmol l−1 (50 mM) phosphate buffer and two concentration of SDS and PVA were carried out. In particular, SDS 0.1 wt.%–PVA 0.4 wt.%, and SDS 0.2 wt.%–PVA 0.8 wt.% were used. The results showed that the presence of emulsifier and stabiliser influenced the droplet size and size distribution, the density and the stability. The amount of emulsifier and stabiliser influenced the density and stability. In other words, when SDS and PVA were present in the continuous phase, smaller droplets (1.9 (±0.5) m) were prepared compared to when pure water was used. Changing the amount of SDS and PVA there was no significant change in the size and size distribution, while emulsion density and stability were influenced. For example, when lower SDS and PVA concentrations were used (0.1 and 0.4 wt.%, respectively), density was much lower and emulsion lasted for no more than 24 h. The mean droplet size as a function of time for the different concentrations of emulsifier and stabiliser are reported in Fig. 4. Fig. 6
shows a microscopic picture of this type of emulsion, for which no dilution has been practised.
4. Conclusions In the present study, oil/water emulsions were prepared by membrane emulsification technology using polyamide ultrafiltration membrane with 10 kDa NMWCO pre-treated with organic solvents. The pre-treatment of membrane was of key importance to allow permeation of the oily phase. The stability of membrane was very good, showing no decrease in performance for more than 3 months of contact with organic solvent. The emulsion was a dispersion of isooctane containing racemic naproxen ester in a continuous aqueous phase containing SDS and PVA as emulsifier and stabilizer. When permeation was carried out from shell-to-lumen, the organic droplets size were 1.87 (±0.58) m. During a period of 20–30
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days of observation with microscopy, the emulsions maintained good stability. When permeation was carried out from lumen-to-shell through compacted membranes, the organic droplets were larger (3.5 (±0.5) m) due to the larger pore size of the external surface, but stability was good as well. Emulsions were also prepared using pure isooctane as dispersed phase and similar results were observed. The membrane compaction, dispersion flux of organic phase, presence and concentration of emulsifier and stabiliser affected the droplets size, size distribution, emulsion density, and stability. This study, showed that polymeric ultrafiltration membranes can be successfully applied to prepare very stable emulsion with narrow size distribution.
[4]
[5]
[6] [7]
[8]
[9]
Acknowledgements This work was carried out with the support of the Italian National Research Council (Consiglio Nazionale delle Ricerche, CNR). References [1] H. Karbstein, H. Schubert, Development in the continuous mechanical production of oil-in-water macro-emulsions, Chem. Eng. Process 34 (1995) 205–211. [2] J. Klier, C.J. Tucker, T.H. Kalantar, D.P. Green, Properties and applications of microemulsions, Adv. Mater. 12 (23) (2000) 1751–1757. [3] V. Schröder, H. Schubert, Influence of emulsifier and pore size on membrane emulsification, in: Food Emulsions and
[10]
[11]
[12] [13]
[14]
Foams, The Royal Society of Chemistry, Cambridge, 1999, pp. 70–80. H. Yuyama, T. Watanabe, G-H. Ma, M. Nagai, Preparation and analysis of uniform emulsion droplets using SPG membrane emulsification technique, Colloids Surf. A: Physicochem. Eng. Aspects 168 (2000) 159–174. V. Schröder, O. Behrend, H. Schubert, Effect of dynamic interfacial tension on the emulsification process using microporous ceramic membranes, J. Colloid Interface Sci. 202 (1998) 334–340. S.M. Joscelyne, G. Tragardh, Membrane emulsification—a literature review, J. Membr. Sci. 169 (2000) 107–117. K. Kandori, Applications of microporous glass membranes: membrane emulsification, in: A.G. Gaonkar (Ed.), Food Processing: Recent Developments, Elsevier, Amsterdam, 1995, pp. 113–142. T. Nakaschima, M. Shimizu, Preparation of monodispersed O/W emulsion by porous-glass membrane, Kagaku Kogaku Ronbunshu 19 (1993) 984–990. Y. Mine, M. Shimizu, T. Nakashima, Application of sizecontrolled microporous glass membranes for designing simple and multiple emulsions, Recent Res. Dev. Agric. Food Chem. 3 (1999) 131–137. G.T. Vladisavljevic, S. Tesch, H. Shubert, Preparation of water-in-oil emulsions using microporous polypropylene hollow fibers: influence of some operating parameters on droplet size distribution, Chem. Eng. Process. 41 (2002) 231– 238. M. Rayner, G. Tragardh, Membrane emulsification modelling: how can we get from characterization to design? Desalination 145 (2002) 165–172. H.J. Philips, J.E. Terryberry, Counting actively metabolising tissue cultured cells, Exp. Cell. Res. 13 (1957) 341–347. R. Katoh, Y. Asano, A. Furuya, K. Sotoyama, M. Tomita, Preparation of food emulsions using a membrane emulsification system, J. Membr. Sci. 113 (1996) 131–135. T. Nakashima, M. Shimizu, Porous glass from calcium alumino boro-silicate glass, Ceramics 21 (1986) 408–415.