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VOJNOTEHNIČKI GLASNIK (MILITARY TECHNICAL COURIER), 2011, Vol. LIX, No. 4
EMULSIFICATION USING MICROPOROUS MEMBRANES
Vladisavljević T. Goran,
Chemical Engineering Department, Loughborough University, United Kingdom,
Vinča Institute of Nuclear Sciences, Belgrade, Serbia
FIELD: Chemical Technology
Abstract:
Membrane emulsification is a process of injecting a pure dispersed phase or pre-emulsion through a microporous membrane into the continuous phase. As a result of the immiscibility of the two phases, droplets of the dispersed phase are formed at the outlets of membrane pores. The droplets formed in the process are removed from the membrane surface by applying cross-flow or stirring of the continuous phase or using a dynamic (rotating or vibrating) membrane. The most commonly used membrane for emulsification is the Shirasu Porous Glass (SPG) membrane, fabricated through spi-nodal decomposition in a melt consisting of Japanese volcanic ash (Shira-su), boric acid and calcium carbonate. Microsieve membranes are increasingly popular as an alternative to highly tortuous glass and ceramic membranes. Microsieves are usually fabricated from nickel by photolithography and electroplating or they can be manufactured from silicon nitride via Reactive Ion Etching (RIE). An advantage of microsieves compared to the SPG membrane is in much higher transmembrane fluxes and higher tolerance to fouling by the emulsion ingredients due to the existence of short, straight-through pores. Unlike conventional emulsification devices such as high-pressure valve homogenisers and rotor-stator devices, membrane emulsification devices permit a precise control over the mean pore size over a wide range and during the process insignificant amount of energy is dissipated as heat. The drop size is primarily determined by the pore size, but it depends also on other parameters, such as membrane wettability, emulsion formulation, shear stress on the membrane surface, transmembrane pressure, etc. Key words: Membrane emulsification, microsieve, Shirasu Porous Glass (SPG) membrane.
Introduction
Conventional emulsification devices are high-pressure valve homoge-nisers, static mixers, and rotor-stator systems, such as colloid mills, stirred vessels, and toothed disc turbines [1]. In these devices, a preliminary
emulsified mixture, called pre-emulsion, is forced through a high-shear region near the rotor, or through valves, nozzles or baffles to promote mixing and turbulence and thereby to break up the large drops into smaller ones. This "top-down” approach does not allow a close control over the mean drop size and the drops are usually highly polydisperse. This is due to the exposure of the drops to variable shear and pressure fields, with very high shear rates near the rotor and negligible shear in "dead zones”. Over the past two decades new emulsification techniques have been developed for production of emulsions directly, without a pre-emulsification step, simply by injecting a dispersed phase through micro- channels, pores and nozzles. These "bottom-up” methods include membrane emulsification [2], ink-jet printing [3], microfluidic routes [4], and microchannel emulsification [5]. They can afford uniform drops of tuneable size, because the size of the drops is controlled by the size of the pores or channels, rather than by the size of the turbulent eddies or vortices which cannot be precisely controlled. This paper reviews the latest developments in formation of emulsions using microporous membranes.
Membrane emulsification (ME)
Membrane emulsification is a process that forms emulsion by injecting a pure dispersed phase or pre-emulsion through a microporous membrane into the continuous phase. Operating methods and systems for ME are described schematically in Fig. 1.
(a) Cross flow system
■=>L* * I* * -
• •
• •
■=>
(d) Rotating flat membrane
Stationar
cone
rotating
membrane
(b) Stirred cell - tube membrane
(c) Stirred cell - flat membrane
(e) Vibrating/rotating tube membrane
(f) Premix ME
Figure 1 - Typical operating methods for membrane emulsification (ME). Slika 1 - Tipični načini izvođenja membranskog emulgovanja (ME).
cross
flow
Vladisavljević, G., Emulsification using microporous membranes, pp. 62-78
VOJNOTEHNIČKI GLASNIK (MILITARY TECHNICAL COURIER), 2011, Vol. LIX, No. 4
Direct Membrane Emulsification (ME)
In direct ME shown in Figs. 1(a)-(e), droplets are produced in situ by injecting a pure liquid (the dispersed phase) through the membrane into a second immiscible liquid (the continuous phase) [2]. Hydrophobic and hydrophilic membranes are needed to produce water-in-oil (W/O) and oil-in-water (O/W) emulsions, respectively. At low production rates, droplets can be formed in the absence of any shear, solely by the action of interfacial tension [6, 7]. At small interpore distances "push-off” force as a result of droplet-droplet interactions on the membrane surface may assist in droplet detachment [8]. In order to obtain uniform droplets at higher throughputs, a shear stress is generated on the membrane surface. The size of the drops formed under shear stress on the membrane surface is a result of interplay between interfacial tension force, viscous force in the pores due to flow of the dispersed phase and drag force arising from a movement of the continuous phase relative to the drop surface. Shear stress on the membrane surface is usually generated using a cross-flow pump (Fig. 1 a) [1] or a stirrer (Figs. 1b and 1c) [9]. Cross-flow ME systems usually employ a tubular glass, ceramic or metallic membrane. The continuous phase flows inside the membrane tube, whereas the pressurised dispersed phase permeates across the membrane wall radially, from the shell side to the tube side [10]. To further increase the throughput, the continuous phase can be introduced into the membrane tube radially, thereby forming spiral streamlines in the axial direction that exert a strong centrifugal force on the inner surface of the membrane ("swirl flow”) [11]. Cross-flow systems are easy to scale up and offer a constant shear rate along the membrane surface. On the other hand, stirring systems are easier to operate and a batch volume can be very small, just 10 mL, which can be advantageous for some medical applications, e.g. preparation of emulsions loaded with anticancer drugs [12].
The shear stress can also be generated by a dynamic membrane shown in Figs. 1(d) and (e), in which case the droplet detachment from the membrane surface is facilitated by rotating [13-14] or vibrating [15-16] the membrane within the otherwise static continuous phase. In a dynamic membrane system, the shear on the membrane surface is decoupled from the cross-flow velocity. Cross flow of the continuous phase is only used to carry droplets away from the module and not to provide a surface shear. The shear is controlled by the rotational speed or the frequency and amplitude of the membrane. Thus, very low cross-flow velocities can be used and recirculation of the product stream is not necessary, which is useful to prevent secondary break-up of large droplets. Kosvintsev et al. [17] modified a commercial Weissenberg ‘plate and cone’ rheometer by replacing a normally impervious plate underneath the cone with the
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membrane and permitting the injection of the dispersed phase from underneath the membrane under constant shear-stress condition (Fig. 1d). It has been shown that the simple paddle-stirred cell (Fig. 1c) with a nonuniform shear profile on the membrane surface provides the same degree of drop size uniformity as the modified Weissenberg rheometer with a constant shear-stress operation [17]. Because of that, the modified We-issenberg rheometer was abandoned as a drop generation device and a simple paddle-stirred cell has become increasingly popular as a viable alternative to cross-flow ME systems. Membrane vibration through piezoactuation has been first applied by Zhu and Barrow [15] to provide extra control over droplet detachment in cross-flow ME. Holdich et al. [16] have introduced a vibrating tubular membrane system to control drop generation without any cross flow.
Premix Membrane Emulsification (ME)
In premix ME shown in Fig. 1(f), fine droplets are produced by passing a pre-emulsion through the membrane [18] or the porous bed of uniform particles [19]. If the membrane surface is wetted by the dispersed phase of the original emulsion, a phase inversion can occur during the process, leading to the formation of a W/O emulsion from an O/W premix or vice versa [20]. Another important factor for the operation of the process is the pressure difference across the membrane. If the pressure difference is lower than the capillary pressure in a pore, the droplet cannot penetrate the pore, which leads to demulsification [21] rather than homogenization of the pre-emulsion. In order to achieve additional droplet size reduction and improve droplet size uniformity, the emulsion can repeatedly be passed through the same membrane, usually up to 5 times [22-25]. Repeated membrane homogenisation was originally developed for homogenisation of large multilamellar and unilamellar lipid vesicles using track-etch polycarbonate membranes [26]. An overview of the droplet break-up mechanisms and governing process parameters in premix ME has been provided by Nazir et al. [27].
Choice of membrane for ME
The Shirasu Porous Glass (SPG) membrane is the earliest and most common membrane for ME, manufactured from the Na2O-CaO-MgO-Al2O3-B2O3-SiO2 mother glass through phase separation by spinodal decomposition [1, 28-29]. The mother glass is prepared by mixing and melting Shirasu, calcium carbonate, and boric acid. Shirasu is a Japanese volcanic ash deposit from the southern Kyushu containing about 77%
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Vladisavljević, G., Emulsification using microporous membranes, pp. 62-78
VOJNOTEHNIČKI GLASNIK (MILITARY TECHNICAL COURIER), 2011, Vol. LIX, No. 4
SiO2 and 10-15% Al2O3 in addition to minor amounts of other inorganic oxides. After the mother glass is formed into tubes, it is treated at 650-750 °C for the period ranging from several hours to several tens of hours. This heat treatment causes the phase separation of the homogeneous glass melt into the acid-insoluble Al2O3-SiO2 phase and the acid-soluble Na2O-CaO-MgO-B2O3 phase, as shown in Fig. 2. The phase separated glass is then immersed into a hydrochloric acid solution to dissolve the Na2O-CaO-MgO-B2O3 phase, which results in the formation of a porous Al2O3-SiO2 skeleton with a typical porosity of 50-60%. If the volume percent of the acid soluble phase is outside the above range, separation may take place by the nucleation and growth mechanism leading to the formation of discrete spherical particles of one phase surrounded by a continuous matrix of the other.
Figure 2 - Fabrication of the Shirasu Porous Glass (SPG) membrane.
Slika 2 - Dobijanje membrane od Shirasu poroznog stakla (SPG)
The SPG membrane has a uniform internal structure with no voids or cracks [30] and is commercially available with a wide range of mean pore sizes (0.050-20 ^m) [6]. The surface of the SPG membrane can be rendered hydrophobic by a chemical modification with organosilane compounds [31] or by a physical coating process with silicone resin [32]. The SPG can
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also be made with thermoresponsive hydrophilic-hydrophobic surface properties via the layer-by-layer deposition of silica nanoparticles and grafting poly(N-isopropylacrylamide) (PNIPAM) brushes on the particle surfaces [33]. A hydraulic resistance of the SPG membrane is relatively high due to the high wall thickness of around 100 p,m, but it can be reduced by making the membrane structure asymmetric [34]. The SPG has a higher water tolerance than the VYCOR® porous glass due to a higher Al2O3 content (10-15% as compared to 0.5% of the Vycor glass). The chemical durability of the SPG against alkaline solutions is relatively poor, but can be improved by incorporating ZrO2 into the glass skeleton [35]. The alkali resistance of the SPG membrane containing 3 mol% ZrO2 in the mother glass is about 3.5 times higher than that of the conventional SPG membrane [35].
Figure 3 - Uniform droplets formed in direct ME using the SPG membrane Slika 3 - Uniformne kapi dobijene direktnim ME primenom SPG membrane
The formation of uniform oil droplets on the surface of the PG membrane is shown in Fig. 3. In direct ME, the mean droplet size is 3-4 times larger than the mean pore size of the SPG membrane and a relative span of droplet size distribution is in the range of 0.25-0.45 [36]. A disadvantage of direct ME using the SPG membrane is that the transmembrane flux should be maintained at relatively low levels (10-100 Lm"2h-1) to avoid transition from dripping to a continuous outflow regime. Uniformly sized droplets can only be formed in the dripping regime [37-38].
In the past 10 years, microsieve membranes are increasingly being used in ME to obtain high transmembrane fluxes at low transmembrane
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Vladisavljević, G., Emulsification using microporous membranes, pp. 62-78
VOJNOTEHNIČKI GLASNIK (MILITARY TECHNICAL COURIER), 2011, Vol. LIX, No. 4
pressures [39]. Microsieves are microfiltration membranes manufactured by semiconductor fabrication methods [40] Typical microsieves used for ME are Aquamarijn™ sieves fabricated in silicon nitride by reactive ion etching [41-43], nickel microsieves formed by electroplating (Stork Veco BV, NL and Micropore Technologies Ltd, UK [44-45, 7]) and stainless steel microsieves with laser drilled pores (Disperse Technologies Ltd, UK [46]). Because microsieves are ultra-thin foils with straight-through pores and a very low internal pore area, they are less prone to fouling by emulsion ingredients than highly tortuous SPG membranes. It is especially an important issue in pre-mix ME, because a whole emulsion, rather than a pure dispersed phase, is injected through the membrane.
The LIGA process (German acronym for LIthographie, Galvanofor-mung, Abformung, which means Lithography, Electroplating, and Molding) was originally developed in the early 1980s at the Karlsruhe Research Center (Forschungszentrum Karlsruhe). As shown in Fig. 4a, the UV-LIGA process consists of three main steps: UV lithography (UVL), membrane nickel electroforming and membrane release. The UVL starts with spin coating of positive photoresist on a flat conducting metal substrate. If the substrate is not electrically conductive, it should be coated with a thin layer of conductive material, such as gold. The photoresist is exposed to UV light through a mask that determines the geometry of the pores. The photoresist is then developed in a developer to remove the exposed parts of the photoresist, rinsed in deionised water and dried. The sieve is then electroformed in a nickel electrolytic plating bath to deposit a nickel film in the voids left by the removed photoresist. The nickel foil will be perforated at the regions on the substrate masked by the photoresist. Starting from a particular size of the photoresist areas on the substrate, the final perforation diameter can be adjusted as small as desired by nickel overgrowth over the masked areas in the electroplating operation. The membrane is released from the substrate by submerging in water and applying ultrasound. A pure nickel membrane is inherently hydrophobic, but can be made hydrophilic by a treatment with a wetting agent. Nickel microsieves can be produced with a variety of different pore shapes, including slotted pores [47].
The process used for fabrication of Aquamarijn® microsieves is described in Fig. 4b. It consists of four main steps: chemical vapour deposition (CVD) of a silicon nitride film on a silicon substrate, photolithography, reactive ion etching (RIE) of the silicon nitride film, and anisotropic wet etching of the silicon substrate. First, a layer of silicon nitride is deposited on the surface of a single crystal silicon substrate by CVD. This layer is formed by a reaction between dichlorosilane (SiH2Cl2) and ammonia (NH3) at elevated temperatures and low pressures. A positive photoresist is then applied to the surface of the silicon nitride film and exposed to the
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UV light through a mask. After the exposure, the photoresist is developed in a dilute NaOH solution to remove the exposed photoresist and form the photoresist mask having a pattern of openings. The silicon nitride exposed in these openings is etched by CHF3/O2 reactive ion etching (RIE) to form a pattern of pores corresponding in locations to the openings in the photoresist film. After dry etching of silicon nitride, the silicon underneath the membrane is etched by anisotropic wet etching using a KOH solution. Finally, the microsieve is treated with air plasma to grow a hydrophilic silicon dioxide layer on the membrane surface.
(a) Nickel microsieve
(b) Silicon nitride microsieve
Conductor
CVD of Si3N4
—-------Si3N4
Si
Photoresist
deposition
Positive
photoresist
Photoresist
deposition
Positive
photoresist
UV Light
UV Light
Mask
Mask
Photo-
lithography
Photo-
lithography
Developing
Developing
Nickel
electroforming
Microsieve
release
П П П n —
О О
Nickel
membrane
Wetting agent deposition
Wetting
О Q О Q------------agent
1 m m m f
RIE of Si3N4
Removing the 1 photoresist
л
Wet etching of __Si3N4
the Si substrate 3 a 1=1 1=1 c membrane
Oxidation of Si3N4 surface
SiO2
Figure 4 - Fabrication of microsieve membranes.
Slika 4 - Dobijanje membrana od mikrosita.
Precision pores suitable for ME can also be made by pulsed laser drilling of stainless steel [14, 46]. A focused laser beam can melt and vaporise a small area of the material to produce micropores. Pulsed lasers need to be used to prevent heating up of the surroundings of the intended pore.
Vladisavljević, G., Emulsification using microporous membranes, pp. 62-78
VOJNOTEHNIČKI GLASNIK (MILITARY TECHNICAL COURIER), 2011, Vol. LIX, No. 4
Governing operating parameters in ME
The effect of operating parameters on the droplet size in direct ME has been the subject of several reviews [48-53]. The major factors affecting the droplet size are microstructure and wetting properties of the membrane (pore size distribution, pore geometry, pore spacing and tortuosity), but other parameters also play an important role, such as transmembrane flux, shear stress on the membrane surface, viscosities of the continuous and dispersed phase, choice of surfactants, emulsion formulation, etc. The droplet size in ME has been predicted analytically, using several force balance and torque balance models [54-58], and computationally, using the Computational Fluid Dynamics (CFD) simulations [54, 59-60] and the Surface Evolver software [61-62].
Surfactant type
The role of surfactants in ME is to rapidly adsorb to the newly formed oil-water interface to facilitate the droplet detachment and stabilise the forming droplet by reducing the interfacial tension. The effect of dynamics of adsorption of surfactants at the liquid-liquid interface on a droplet size has been studied by several researchers [10, 61, 63]. As a rule, the faster the emulsifier molecules adsorb to the newly formed interface, the smaller the droplet size of the resultant emulsion becomes. Surfactant molecules should not adsorb to the membrane surface, since otherwise the membrane can become fouled by the surfactant molecules or the dispersed phase can spread on the membrane surface. The effect of the emulsifier charge on droplet formation has been investigated by Nakashima et al. [64] for the SPG membrane and by Kobayashi et al. [65] for silicon microsieves. It has been found that surfactant molecules must not have the charge opposite to that of the membrane surface. An untreated SPG membrane has a negative surface potential of -15 to -35 mV in an aqueous solution at pH 2-8, due to dissociation of acidic silanol groups (Si-OH ^ SiO- + H+). Hence, for this case, the use of cationic emulsifiers such as alkyl-substituted quaternary ammonium salts must be avoided. The same conclusion is valid for hydrophilic silicon microsieves, which also contain silanol groups on the surface. Zwitterionic surfactants are also unsuitable, even when they carry a net negative charge. For example, an SPG membrane can become fouled by lecithin at pH 3 due to electrostatic interactions between positively charged groups (e.g. -N(CH3)3+ or -NH3+) on the lecithin molecules and negatively charged silanol groups on the SPG membrane, although at pH 3 the net charge of lecithin molecules is negative [66]. Consequently, positively charged surfactants cannot be used in conjunction with SPG mem-
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branes and silicon microsieves. One solution to this limitation is to prepare emulsion first using nonionic or anionic surfactant and then to displace this surfactant by the surfactant with positively charged groups using the surfactant-displacement method [67].
Transmembrane pressure and wall shear stress
The minimum transmembrane pressure for driving the dispersed phase through the pores is known as the capillary pressure Pcap, and is given by the Laplace equation:
P _ 4ycos6
cap _ ~~d
(1)
where у is the equilibrium interfacial tension at the liquid-liquid interface, в is the contact angle at the interfacial line between the two liquid phases and the membrane surface, and dp is the mean pore diameter of the membrane. The capillary pressure in premix ME is given by [68]:
P _
cap
y[2 + 2a6 / V2a6 -1 x arccos(\ / a3) - 4a2 ]
a + V a
■\la2 -1
(2)
where a = d1/dp and d1 is the mean droplet size in the pre-emulsion. If d1/dp » 1, e.g. for very large droplets, the capillary pressure is given by Eq. (1). In premix ME, the optimum transmembrane pressure is typically 10-50 times greater than the capillary pressure [23]. In direct ME using the SPG membrane, the transmembrane pressure is up to 8Pcap and the wall shear stress is 2-40 Pa [37]. The higher the shear stress on the membrane surface, the higher the maximum pressure that can be applied to obtain uniform droplets. The shear stress on the membrane surface in a cross-flow system with a tubular membrane is [69]:
T_ (di /4№Pfr / L) (3)
where d) is the inner diameter of the membrane tube and Apfr is the pressure drop for overcoming friction resistance in the membrane tube over a length L. In the case of laminar flow inside the membrane tube (Ret < 2300), Eq. (3) is simplified to: t= 8ricv/dj, where vt is the mean velocity of the continuous phase inside the membrane tube and п is the viscosity of the continuous phase. In the case of a turbulent flow, Eq. (3) has the form:
T_KPcv2t/8) (4)
Vladisavljević, G., Emulsification using microporous membranes, pp. 62-78
VOJNOTEHNIČKI GLASNIK (MILITARY TECHNICAL COURIER), 2011, Vol. LIX, No. 4
where pc is the density of the continuous phase and Я is the friction factor, which is at 2,500 < Ret < 100,000 given by the Blasius equation:
Я = 0.3164 Re -025 (5)
In ME using a stirred cell with a simple paddle stirrer, the shear stress on the membrane surface varies with the radial distance r from the stirrer axis, according to the equations [70]:
For r < rtrans t = 0.825цсог — 8 (6)
For r > rtrans Т = 0.825цc®rtrans f— ^ r 06 8 (7)
where 8 is the boundary layer thickness, о is the angular velocity of the stirrer, and rtrans is the transitional radius, i.e. the radial distance at which the shear stress is greatest:
=123 7 [057+035 717
, 0.036
nu
0.116
Re „
1000 +1.43 Re c
(8)
where D is the stirrer diameter, C is the cell diameter, b is the blade height, nb is the number of impeller blades, and Rec is the Reynolds number given by copcD2/(2nnc).
Conclusions
Microengineering "bottom-up” strategies for emulsion formation offer a great potential in manufacturing droplets of a controlled size in a gentle manner. Membrane emulsification involves injecting a pure dispersed phase or pre-emulsion through a microfiltration membrane into the continuous phase. In order to ensure droplet detachment from the membrane surface, a shear stress is generated on the membrane surface by agitating or recycling the continuous phase in the static membrane system or vibrating/rotating the membrane in an otherwise static continuous phase. In principle, any microporous membrane can be used, but the majority of investigations have been done using the Shirasu Porous Glass (SPG) membrane and metal microsieves. The advantage of using microsieve membranes is in much higher transmembrane fluxes that can be achieved and their straight pores with a low internal surface area that do not easily become fouled by the emulsion constituents.
Acknowledgment
The research work of G.T. Vladisavljević in membrane emulsification was sponsored by the Humboldt Foundation, The Fulbright Commission, the Japan Society of for the Promotion of Science (JSPS) (Invitation and Postdoctoral Fellowship) and the Royal Society. The support of these funding organisations is greatly appreciated.
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Vladisavljević, G., Emulsification using microporous membranes, pp. 62-78
VOJNOTEHNIČKI GLASNIK (MILITARY TECHNICAL COURIER), 2011, Vol. LIX, No. 4
EMULGOVANJE PRIMENOM MIKROPOROZNIH MEMBRANA
OBLAST: hemijske tehnologije
Sažetak:
Pri membranskom emulgovanju se čista disperzna faza ili gruba emulzija propuštaju kroz mikroporoznu membranu u kontinualnu fazu pri čemu se usled nemešljivosti dve faze, na površini membrane formi-raju kapi disperzne faze. Ove kapi se odnose sa površine membrane recirkulacijom kontinualne faze iznad membrane u unakrsnom toku, mešanjem kontinualne faze ili primenom dinamičke membrane, koja može biti rotirajuća ili vibraciona. Za emulgovanje se najčešće koristi membrana od mikroporoznog stakla proizvedena procesom spinodalne dekompozicije u rastopljenoj smeši Japanskog vulkanskog pepela pod nazivom “Shirasu”, krečnjaka i boroksida. U poslednje vreme se sve vi-še koriste mikrosita od nikla proizvedena primenom fotolitografije i gal-vanskog niklovanja i mikrosita od silicijum nitrida, proizvedena reaktiv-nim jonskim nagrizanjem. Prednost mikrosita u odnosu na staklene membrane je u tome što omogućavaju veće transmembranske flukse-ve i što se manje prljaju zbog pravih kratkih pora. U odnosu na klasič-no emulgovanje u emulgatorima visokog pritiska i rotor-stator uređaji-ma, pri membranskom emulgovanju se može precizno kontrolisati srednja veličina kapi u širokom opsegu a zanemarljivo mala količina energije se rasipa u obliku toplote. Veličina kapi je prvenstveno defini-sana veličinom pora, ali zavisi i od kvašljivosti membrane, sastava emulzije, napona smicanja na površini membrane, transmembranskog pritiska, itd.
Ključne reči: membransko emulgovanje, mikrosito, porozna staklena Shirasu membrana.
Datum prijema članka: 25. 09 2011
Datum dostavljanja ispravki rukopisa: 29.09.2011.
Datum konačnog prihvatanja članka za objavljivanje: 30. 09. 2011.
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