Article pubs.acs.org/IECR
Manufacturing of Nanoemulsions Using Nanoporous Anodized Alumina Membranes: Experimental Investigation and Process Modeling Kah P. Lee and Davide Mattia* Department of Chemical Engineering, University of Bath, Claverton Down, Bath, BA2 7AY, United Kingdom S Supporting Information *
ABSTRACT: Tubular anodic alumina membranes, containing self-ordered and circular pores below 100 nm, were used to produce sunflower oil in water emulsions. Span 80 and Tween 20 were used as surfactants in the dispersed phase and continuous phase, respectively. Both dead-end and cross-flow processes were investigated. Membrane pore diameter and cross-flow velocity dominated the final droplet size. The droplet size yielded was consistently in the nanometer range and exhibited low polydispersity (1 μm.11 More recently, emulsions with droplet size in the nanometer range have attracted widespread interest due to their unique optical properties and long stability.12 Both properties could make these nanoemulsions interesting for the beverage and cosmetic industries whereby nutrients and drugs could be added without altering the appearance, color, and feel of existing products.11 Despite the complication of multiple upper particle size limits being proposed for nanoemulsions (500, 200, and 100 nm) in previous literature, a recently published review has rigorously and conveniently defined the average drop diameter of a nanoemulsion system to be less than 200 nm.13 So far, nanoemulsions are mostly obtained via ultrasonic and high-pressure valve homogenizer methods which, as discussed earlier, have a high energy consumption and tend to produce broad droplet size distributions.11 In some cases, nanoemulsions have been obtained coupling membrane emulsification with an external force field acting as stabilizer and, ultimately, determining the droplet size: Droplet sizes in the range 100−500 nm were achieved used SPG membranes in a dead-end mode in the presence of shear stress generated by intensive external agitation, which is energy consuming and difficult to scale up.8 Similarly, nanoemulsions by cross-flow membrane emulsification have been obtained using external agitation to stabilize the emulsion and determine the final drop size.14 A w/o/w emulsion with an average droplet size of ∼600 nm diameter was obtained via a two-pass cross-flow SPG membrane emulsification system.5 In contrast to the above methods, anodized alumina membranes (AAMs, Dp ∼ 200 nm, 13 mm diameter disk) have been used to prepare o/w nanoemulsions in a dead-end mode operation without the support of any external force field,15 with an average oil droplet size in the range 500−600 nm and narrow size distribution due to the uniform pore diameter and spacing of the membranes.16 While the size of these membranes makes them too small for practical application, the present authors have previously successfully fabricated tubular AAMs, which retain the regular pore structure while providing a scale for practical applications.17 In this article, nanoemulsions were produced using tubular nanoporous AAMs with pore sizes in the 20−100 nm range. These membranes have a very regular pore structure with circular pores and narrow size distributions.18 The use of the tubular form enables the use of a cross-flow emulsification setup, with the resulting o/w emulsions having narrow size distributions in the nanometer range (down to Dd < 120 nm). The results show that the force and torque balance used for micrometer scale droplets has to be modified for nanometerscale ones. In particular, much lower shear values and, hence, continuous phase flow rates, are needed to achieve droplet detachment and emulsions with a narrow size distribution.
2γ cos θ rp
(3)
Fγ = 2πγrp
(4)
Fb =
4 3 πrd (ρ − ρd )g 3
Fi = ρd πrp2Vd 2
(5) (6)
where Fd, Fγ, Fb, and Fi are the forces caused by drag, IFT, buoyancy, and inertia, respectively; kx is the wall correction factor (typically 1.7 for spherical particles);20 ρ is the density of the continuous phase; Vc is the average velocity of the continuous phase; rd is the radius of the droplet; τw is the wall shear stress; ρd is the density of the dispersed phase; g is the gravity acceleration; and Vd is the average velocity of the dispersed phase within the pore. Using an algebraic torque balance along the droplet contact line located around the membrane pore border, the droplet size can be estimated from the following equation:1,21 (Fd + Fb)h = Fγrp
(7)
where h is the height of the droplet. As a result of low dispersed phase velocity within the membrane pore, the inertial force is small compared to the three other forces and is therefore neglected from eq 7. If the drag force is very high, the buoyancy force can also be neglected. According to Peng and Williams,1 the droplet shape is significantly deformed at high shear rate and is bent toward the membrane surface. Equation 7, therefore, can be simplified to Fd = Fγ
(8)
For the detailed derivation of eqs 3−8, see Peng and Williams.1 Substituting eqs 3 and 4 into eq 8 yields an equation to predict the droplet size:19,22 Dd =
4Dpγ 6k xτw
(9)
3. EXPERIMENTAL SECTION 3.1. Membrane Fabrication and Characterization. Tubular AAMs were prepared by anodizing the inner wall of aluminum alloy tubes, based on the detailed procedures described in a previous publication.17 Briefly, A1050 grade aluminum alloy tubes (>99.5% Al, Haynes Tube, 100 mm (length) × 6.35 mm (o.d.) × 0.3 mm (thickness)) were used as the starting material. After several pretreatment processes, i.e. annealing, cleaning, and electropolishing, a one-step potentiostatic anodization was performed. The inner surface of the aluminum alloy tube is anodized by recirculating the electrolyte using a peristaltic pump. Only symmetric AAMs were prepared in this study, and the anodization parameters were chosen
2. THEORY For membrane with cylindrical pores, as those in AAMs, the critical pressure required to force the dispersed phase through the pores and into the continuous phase can be estimated based on the capillary pressure:1 Pc =
Fd = 3πfk xρVc 2rd 2 = 10.205πrd 2τw
(2) 14867
dx.doi.org/10.1021/ie401960n | Ind. Eng. Chem. Res. 2013, 52, 14866−14874
Industrial & Engineering Chemistry Research
Article
Figure 1. Schematics of (a) dead-end and (b) cross-flow membrane emulsification apparatus.
pulsation. Then the dispersed phase within the inner bore of the membrane would permeate through the membrane and form droplets in the continuous phase. The experiment was conducted for 1 h. In the cross-flow configuration (Figure 1b), the membrane was mounted in a minimodule such that a continuous phase could flow in the membrane bore, while the dispersed phase was pressurized through the membrane from the outer side. The tubular AAMs were individually housed in a minimodule made of a 13 mm inner diameter acrylic tube by using epoxy adhesive (Araldite) (Figure 2). The inner bore of the tubular
based on the specific conditions within the well-established selfassembly regime.23 In particular, 20 V anodization was performed using 0.5 M sulfuric acid at 0 ± 1 °C for 12 h whereas 40 and 50 V anodizations were performed using 0.3 M oxalic acid at 12 ± 2 °C for at least 10 h. After anodization, residual aluminum removal and pore opening were performed on the middle 50-mm-long section, to create an active membrane area with opened through pores. The resulting membrane is ready for use. The membrane pore structure was characterized by use of a field emission scanning electron microscope (FESEM, JEOL 6301F). Standard image analysis processing using ImageJ was applied to determine the mean pore diameter and porosity.18 Membrane thickness was measured by analysis of FESEM micrographs of the membrane cross sections by taking the average of three readings per membrane. 3.2. Emulsion Formulation and Interfacial Tension Measurement. Due to the highly hydrophilic nature of AAMs, only oil in water (o/w) emulsions were investigated. Sunflower oil (SFO) for domestic use (Co-operative, U.K.) and Milli-Q water were used as the dispersed phase and continuous phase, respectively. In a previous study, 4 v/wt % Span 80 (Sigma) and 4 v/wt % Tween 20 (Sigma) used as surfactants in the oil and aqueous phase, respectively, showed good and stable droplet formation in the micrometer range.3 In the current study, the AAMs used have a much smaller pore size and, therefore, the IFT should be minimized to lower the critical pressure for droplet formation. Different compositions (0, 1, 2, 4, 6, 10 v/wt %) of Span 80 (Sigma) in SFO were prepared. Tween 20 (Sigma) was added into Milli-Q water in the same range of composition. The IFT between each pair of oil/ aqueous solutions was measured using a goniometer (Dataphysics OCA20), based on the pendant drop method. 3.3. Dead-End and Cross-Flow Membrane Emulsification (DE-ME and CF-ME). As shown schematically in Figure 1a, for DE-ME, the inner bore of the membrane was filled with the oil/dispersed phase. Vertically, the top of the membrane was connected to compressed air (up to 400 kPa) via a regulator to control the transmembrane pressure. A pressure transducer (Swagelok, industrial standard, ±5 kPa) was connected to monitor the pressure settings. Then the membrane was positioned to be totally submerged into a 10 mL borosilicate glass test tube containing the continuous aqueous phase. The pressure was slowly increased to the set point, i.e., 50 or 80 kPa, to avoid membrane failure by sudden pressure
Figure 2. Picture of a tubular AAM housed in an acrylic tube filled with dispersed oil phase. Membrane active length and diameter are 50 mm and 6.5 mm, respectively.
AAM was connected to the 225 mL stainless steel syringe. The gap between the acrylic tube and AAM surface was filled with the dispersed oil phase, which was connected to filtered compressed air via a regulator. The continuous aqueous phase was dispensed by pulseless flow driven by a syringe pump (Nexus 4000). The syringe pump was started to make sure the inner bore of a tubular AAM was completely filled with the continuous aqueous phase. Gradually increasing to the pressure set point, i.e., 50 or 80 kPa, the dispersed oil phase would start to permeate through the membrane, forming droplets that were 14868
dx.doi.org/10.1021/ie401960n | Ind. Eng. Chem. Res. 2013, 52, 14866−14874
Industrial & Engineering Chemistry Research
Article
measurements, i.e., to avoid the occurrence of multiple scattering. Three measurements were repeated for each sample tested. There are a variety of systems to define the droplet size polydispersity index (PDI).24 In this study, the grade of dispersity is used to describe the polydispersity:24
detaching from the membrane surface. The applied pressure was limited to the range between the critical pressure (as estimated in Table 1) and the membrane burst pressure, which Table 1. Operating Parameters Investigated for Membrane Emulsificationa AAM anodization voltage (Dp) 20 40 50 50
V V V V
(25 (50 (60 (60
± ± ± ±
2 3 5 5
nm) nm) nm) nm)
Pc (±2 kPa)
PA (±5 kPa)
60 30 25 25
80 80 50 80
PDI =
dv ,84 − dv ,16 2dv ,50
(10)
a
The mode of operation for all experiments was dead-end mode or cross-flow mode (Vc = 0.01, 0.02, 0.04, 0.10 m s−1).
where dv,i is the volume-average diameter below which i% of the particles lie. An emulsion can be then classified as monodisperse if PDI < 0.14, quasi-monodisperse if 0.14 < PDI < 0.41, and polydisperse if PDI > 0.41.
is approximately 1 bar for a low voltage AAM (i.e., 20 V).17 Multiple sets of experiments were performed. As summarized in Table 1, the main parameters investigated include membrane pore diameter, applied pressure, and cross-flow velocity. Due to the fixed capacity of the syringe and hence the continuous phase, the emulsification time is variable (ranging from 20 to 60 min) for different cross-flow velocities. The first 50 mL of resulting emulsion samples was purged in each experiment. Subsequently, three batches of 50 mL samples were collected, in order to verify that steady state had been attained. 3.4. Dynamic Light Scattering (DLS) Analysis. The size and distribution of the droplets in the resulting o/w emulsions were analyzed using the dynamic light scattering technique (Zetasizer Nano-ZS, Malvern Instruments Ltd.). The collected emulsion samples were significantly diluted for reliable
4. RESULTS AND DISCUSSION 4.1. Membrane Morphology. Figure 3a,b shows the surface morphology of the fabricated AAMs. The AAMs were fabricated at the optimum conditions for self-ordered structure. As expected, the membrane pore is circular in shape and the pore diameter is proportional to the anodization voltage. The pore arrangement is not perfectly hexagonal, when compared to other flat AAMs,16 due to the presence of alloying elements in the starting material. The pore diameters for 20, 40, and 50 V membranes are approximately 25 ± 2, 50 ± 3, and 60 ± 5 nm, respectively. In agreement with previous findings,18 the pore diameter is linearly proportional to the anodization voltage. SEM micrographs of the membrane cross section show that the membranes consist of straight and symmetric channels. For comparison, the micrographs of other popular membranes used
Figure 3. SEM micrographs showing the surface morphology of (a) 20 and (b) 50 V AAMs and (c) cross section of a 50 V AAM. For comparison with other membranes used for emulsification, SEM micrographs of (d) SPG6 and (e) PCTE25 membranes are included. 14869
dx.doi.org/10.1021/ie401960n | Ind. Eng. Chem. Res. 2013, 52, 14866−14874
Industrial & Engineering Chemistry Research
Article
formulation is based on using 4 v/wt % Span 80 in SFO as the dispersed phase and 4 v/wt % water as the continuous phase. 4.3. Emulsion Characterization. The DLS analysis of three different batches of samples collected at different time intervals in the same experiment was used to evaluate that the experiments had achieved steady state when samples were collected. Both the average droplet diameter and the statistical mode obtained for different batches are within a 10% error span. See the Supporting Information for typical droplet size distribution curves (Figure S1 in the Supporting Information) and tabulated data obtained from DLS analysis for each sample prepared based on different membrane emulsification process parameters (Table S1 in the Supporting Information). In all cases, only a single peak was obtained from the size distribution curve. Specifically, the proportionality constant that correlates the pore diameter and average droplet diameter is calculated based on eq 1. 4.3.1. Effect of Membrane Pore Size. Membrane pore diameter is an influential factor for the resulting emulsions’ droplet size. To visualize the effect of the membrane pore diameter, the data from Table S1 in the Supporting Information was extracted to construct the plots in Figure 5. As can be seen, the droplet size increases with membrane pore diameter. However, this correlation is nonlinear, as evidenced by the varied proportionality constants obtained for different membranes or cross-flow velocities. This is in contrast to linear correlations obtained for membranes with pore sizes in the micrometer range, including SPG27 and PCTE membranes.25 A nonlinear behavior was also observed in a study using commercial alumina membranes (Membraflow) in the nanometer range (100 and 800 nm).28 The proportionality constants are smaller for membranes with larger pore diameters, as shown in Figure 5b. However, the proportionality constants were smaller for the AAMs with 50 nm pore diameter compared to AAMs with 60 nm pore diameter when the shear rate was low (Vc = 0.01 m s−1) or in a dead-end configuration. A reasonable explanation is that, when the shear rate is low, the membrane morphology has an even higher influence on droplet formation. It is well-known that AAMs formed at 40 V in oxalic acid provide the optimum pore structure and arrangement.18
for emulsification are included, namely SPG and PCTE membranes. While SPG membranes have been described to have a uniform pore diameter, their pore shape is actually rather irregular.6 As for PCTE membranes, despite the uniform pore size and circular pore shape, the dispersed phase flow is low due to the low porosity (3−5%).25 4.2. Interfacial Tension Measurement. Figure 4 illustrates the IFT measured for different SFO/water solutions
Figure 4. IFT between SFO and water with varying concentration of surfactants in each phase. The data points shown are the average of three measurements with less than 10% error by standard deviation.
with varying surfactant concentration. When no surfactant was used, the IFT between pure SFO and water obtained was 24.37 mN m−1. This is close to the literature value of approximately 25 mN m−1.26 As expected, the IFT decreases with increasing dosage of either Span 80 or Tween 20 in SFO and water, respectively. In good agreement with previous studies,3 the IFT between 4 v/wt % Span 80 in SFO and 4 v/wt % water is very low, 0.47 mN m−1. Further increase in surfactant dosage has minimal effect on reducing the IFT. Therefore, the emulsion
Figure 5. Dependence of (a) obtained average droplet size and (b) proportionality constant on membrane pore diameter. 14870
dx.doi.org/10.1021/ie401960n | Ind. Eng. Chem. Res. 2013, 52, 14866−14874
Industrial & Engineering Chemistry Research
Article
This enables 40 V AAMs to have a lower proportionality constant than the 50 V AAMs. Despite the nonlinearity, the proportionality constants obtained are well-aligned with the lower end of the range of 3−10 reported in the literature for membranes with regular and circular pore structures.1 In some cases, the presence of higher shear by cross-flow velocity has reduced the proportionality constants to just below 3. This significant achievement for nanometer-scale droplet formation can be attributed to the highly ordered pore structure and the predominant circular pore shape of the AAMs compared to other less-organized membranes, such as the SPG ones shown in Figure 3d. PCTE membranes also have highly monodispersed pore diameters with circular pore geometry (Figure 3e), and very low proportionality constants (1 μm), the gap between the model and experiment is even larger for nanoemulsion systems. When using eq 9 to estimate the droplet size based on the system in the current work, the resulting discrepancy increases dramatically to 50−180 times. Even the droplet size obtained from the dead-end setup (6 μm). In a previous investigation, dead-end emulsification (no shear) using a AAM of 220 nm average pore diameter yielded an average drop diameter of approximately 600 nm.15 Based on eq 9, a substantial shear rate of 144 Pa would be needed to generate an equivalent result. It is clear that the model behind eq 9 does not work for nanoemulsion systems. Equation 9 was developed based on the assumption of significant shape deformation during droplet formation as a result of high shear stress.1 The height of the droplet is approximated as equivalent to the membrane pore radius as a result of such deformation, as can be seen in the simplification step from eq 7 to eq 8. However, in the current study, this assumption might not hold, due to the low shear values used (
