PREPARATION AND CHARACTERISATION OF PERFLUORODECALIN IN WATER PICKERING EMULSION

Transcription

1 PREPARATION AND CHARACTERISATION OF PERFLUORODECALIN IN WATER PICKERING EMULSION L.S. Chin, M.Lim, R.Amal 1 1 ARC Centre of Excellence for Functional Nanomaterial, School of Chemical Engineering, The University of New South Wales, Sydney NSW 2052, Australia address: r.amal@unsw.edu.au ABSTRACT Pickering emulsion is emulsion stabilised by solid particles between two liquid phases. The preparation of perfluorodecalin (PFD) in water Pickering emulsion using functionalised magnetite iron oxide nanoparticles (MION) as stabilisers is reported herein for the first time. The effects of (1) coating MION with polyethyleneimine (PEI) and polymethacrylic acid (PMAA) and (2) applying shear, on the size of the Pickering emulsion formed were investigated. The functionalised MION suspended in water were then mixed with PFD solution in the ratio of 2:1 and homogenised for 3 min. The properties of PEI-MION and PMAA-MION were characterised with dynamic light scattering (DLS), transmission electron microscopy (TEM) and zeta potential measurements. The size of the emulsion was determined using an optical microscope. The size of Pickering emulsion formed with PEI-MION is larger (255 µm) compared to that with PMAA-MION (35 µm) regardless of the molecular weight of the polymers indicating that Pickering emulsion formation is mainly affected by MION surface structure. Homogenising the Pickering emulsion is essential to form more uniform sizes, however, the size is independent of the homogenising speed and time.

2 INTRODUCTION There are many applications of emulsion in various fields such as pharmaceuticals, food, cosmetics, and petroleum industries. The emulsions are mixture of two immiscible liquids whereby one of the liquid is dispersed in another in the form of droplets. Generally, surfactants are employed in providing the stability of these emulsion droplets against Ostwald ripening, a mechanism whereby the emulsion droplets coalesce and induce phase separation. Specifically, these surfactants adsorbed in between the two immiscible liquids, providing a barrier against the coalescence among the droplets themselves. Pickering emulsion, on the other hand, is emulsion utilising solid particles as emulsifiers rather than molecular type surfactants. The difference between the solid stabilised emulsion as compared to the molecular type surfactants is their relative higher stability (of up to a few months) and its higher emulsification fraction. The formation of Pickering emulsion is based on the wetting property of the particles. Basically, the three phase contact angle of the particles has to be around 90 for the particles to adsorb in between the liquid-liquid interface. When the contact angle is less than 90, the emulsion formed will be oil in water emulsion and vice versa. It was reported that the energy required to remove these particles from the liquid-liquid interface is 10 7 k B T, whereby k B is Boltzmann s constant and T is the room temperature (Melle et al., 2005). The morphology and the property of the surface particles dictate the size of the emulsion formation. Kaiser etal. (Kaiser et al., 2009) had reported that increasing the polystyrene molecular weight coated on the magnetic iron oxide particles reduces the size of the Pickering emulsion formed. Madivala etal.(madivala et al., 2009) observed change in emulsified fraction (change in Pickering emulsion s size) upon changing the aspect ratio of the hematite particles. By coating polystyrene on iron oxide nanoparticles, the Pickering emulsion remained stable while carbonyl iron particles without any coating exhibits complete destabilisation upon magnetic retraction (Kaiser et al., 2009, Melle et al., 2005). Similar stabilisation was observed when hybrid poly (n-isopropylacrylamide) (NIPAM) cross-linked magnetic nanoparticles system was used as stabilisers (Brugger and Richtering, 2007). On the other hand, it was observed in classical emulsion (whereby molecular type surfactants are used as emulsifier) that the size of the emulsion reduced upon increasing the surfactants weight concentration with respect to the oil phase or using strong shearing force. It has also been shown in the Pickering emulsion that upon increasing the concentration of silica particles, the size reduced until a saturation point is reached (Frelichowska et al., 2010). Shearing of the iron oxide Pickering emulsion for longer period produced emulsion of smaller sizes (Kaiser et al., 2009). The same trend is also observed for some polymeric particles (Thareja and Velankar, 2008, Thareja et al., 2010). Herein, we report for the first time the preparation of stable perfluorodecalin in water magnetic iron oxide Pickering emulsion using branched chain polyethyleneimine (PEI) and polymethacrylic acid (PMAA) functionalised iron oxide nanoparticles (MION) of approximately 50 nm. Different from conventional hydrocarbon oil, perfluorodecalin has higher density than water which is the continuous phase in the Pickering emulsion formed. In this study, the effect of polymer coating on MION, shearing speed and shearing rate in Pickering emulsion formation were investigated. 1

3 EXPERIMENTAL Materials Iron (II) sulphate heptahydrate (FeSO 4.7H 2 O) and sodium hydroxide (NaOH) were obtained from Ajax Finechem (Sydney, Australia). Potassium nitrate (KNO 3 ) was obtained from May and Baker (England). Perfluorodecalin (PFD, C 10 F 18, 95%) was obtained from Alfa Aesar. Polyethyleneimine (PEI, branched, M w ~ 25 kda) was obtained from Sigma-Aldrich (Sydney Australia). Polymethacrylic acid (PMAA, M w ~ 17 kda and 3 kda) was synthesised and received from Ms. Hilda Wiogo, a PhD student in our lab. All chemicals above were used as received without further purification. Synthesis of 50 nm MION g of FeSO 4.7H 2 O was dissolved in 320 ml of distilled water followed by addition of 2.0 M KNO 3 (40 ml) and 1.0 M NaOH (40 ml) in deoxygenated condition. The dark green floc was heated to 90 C for 2 h. After the heating, the dark iron oxide nanoparticle was cooled to room temperature and separated using a neodymium magnet. The nanoparticle was rinsed 5 times with deoxygenated, distilled water and redispersed in 200 ml of distilled water. Functionalisation of MION with PEI and PMAA MION prepared (100 ml) was pre-sonicated in a 250 ml beaker for 4 min at 20 % amplitude using an ultrasonic probe (Sonicator 3000, Misonix, Inc) followed by addition of g of PEI pre-dissolved in 100 ml distilled water. The resulting mixture was sonicated again at the same condition followed by constant stirring at 400 rpm for 24 h. After stirring, the resulting PEI coated MION (PEI-MION) was separated using a neodymium magnet and rinsed a few times with distilled water. The particles were then redispersed in 20 ml of distilled water to yield a concentration of 15.9 g/l. In another set of experiment, similar procedure was repeated using mixture of MION (5 ml) and PMAA (5 ml, 2.88 g/l). The final PMAA coated MION (PMAA-MION) had concentration of 3.05 g/l. + PEI or PMAA 50 nm Bare MION PEI-MION or PMAA-MION Fig. 1: Schematic diagram of functionalisation of bare MION with PEI or PMAA. 2

4 Preparation of PFD in Water Magnetic Pickering Emulsion Typically, for PEI-MION, 100 µl of PEI-MION (15.9 g/l) was diluted with 1.9 ml of distilled water in a vial. The MION solution was either homogenised for 1 min at shearing rate of 9500 rpm using a homogeniser (IKA T10 basic) (Fig. 2a) or without homogenisation (Fig. 2b). PFD (0.5 ml) was added into the mixture followed by additional homogenisation at shear rate of rpm. Similar procedure was repeated with using PMAA-MION (500 µl, 3.05 g/l). The size of the PMAA polymer used has M w ~ 17 kda otherwise mentioned. ON ON Add PFD a) OFF ON Add PFD b) Fig. 2: Schematic diagram of PFD in water magnetic Pickering emulsion preparation. Characterisation The size, stability and zeta potential analysis of MION were determined using Brookhaven Zetaplus particle sizer. Transmission emission microscopy (TEM) sample was prepared by dropping one drop of diluted PEI or PMAA-MION onto the carbon grid. The size of the as prepared emulsion was monitored under optical microscope by dispersing into a water bath contained in a glass petri dish using 5x (for PEI-MION) and 10 x (for PMAA-MION) magnification.. To determine whether the emulsion formed was PFD in water or water in PFD, analysis was performed by diluting the sample with PFD followed by strong agitation using hand shake and observed under optical microscope. If the size of the emulsion increases, PFD in water emulsion resulted and vice versa. The emulsions measured in our experiment are all appeared to be PFD in water emulsion. The emulsion s size of at least 400 droplets was determined using image processing software, ImageJ. The ph of the samples was determined using the ph meter attached to the Zetaplus sizer. 3

5 RESULTS AND DISCUSSION Synthesis of 50 nm MION and Functionalisation of MION with PEI and PMAA The first step of preparation of the Pickering emulsion was to synthesised the solid stabilisers, MION. This facile synthesis of 50 nm MION was based on modified Sugimoto s (Sugimoto and Matijevic, 1980) method (Fig. 3). In brief, the synthesis is based on the oxidation of Fe 2+ ions in the presence of KNO 3 to produce Fe(OH) 2 gels followed by crystallisation of iron oxide nanoparticles from this hydroxide gels upon ph and temperature adjustment. From particle sizing measurement, the particles are unstable and stability can be achieved by incorporating polymers such as PEI or PMAA. Fig. 3: TEM image of synthesized 50 nm MION. The scale bar on the left is 200 nm while the scale bar on the right is 20 nm. Functionalisation of MION with PEI and PMAA were performed using the method similar to that described in Goon etal. (Goon et al., 2009). In this method, PEI-MION was prepared at the ratio of 6 to 1 for PEI to MION concentration followed by high speed stirring for 1 day at room temperature. The resulted aggregated size was found to be 180 nm and stable for at least 16 h similar to the observation from Goon etal. (Goon et al., 2009) without heating. With PMAA-MION, only 1 to 1 ratio of PMAA to MION was used, however yield similar result as PEI-MION with relatively smaller size of 160 nm. There is IEP shift for both functionalised MION as can be seen from Fig. 4b, the PEI-MION shows an increase in isoelectric point (IEP) to approximately 11 indicating that the charge on the particle is positive at ph ~ 7.0, while PMAA shows a decrease in IEP (~ 4) indicating the negatively charge nature of the particles at ph ~ 7.0 when comparing to bare MION which has IEP of approximately 6. 4

6 200 a) 180 Hydrodynamic diameter (nm) Zeta Potential (mv) b) Time (hr) Fig. 4: a) Size and stability of the PEI-MION and PMAA-MION. b) Zeta potential analysis comparing PMAA-MION, PEI-MION and bare MION. Effects of Functionalisation on Emulsion Size ph 17 kda PMAA-MION Bare MION 25 kda PEI-MION Previously, the preparation of PFD in water Pickering emulsion was attempted using bare MION however these emulsion droplets grew rapidly and very unstable. It was found that functionalising the bare MION with polymer resulted in stable Pickering emulsion formed. The preparation of PFD in water requires such that PEI-MION or PMAA-MION is initially dispersed in water phase using homogeniser. This step of homogenising the solution is to break up the aggregates of MION before the addition of PFD. It was shown that without this pre-homogenising step, the emulsion formed after the PFD addition is relatively polydisperse (Fig. 5). 5

7 a) b) Fig. 5: PEI-MION stabilised Pickering emulsion formed (a) without pre-homogenisation and (b) with pre-homogenisation of MION solution. The scale bar is 500 µm. The preparation of the Pickering emulsion using PEI-MION was compared to PMAA- MION. It was observed that emulsion prepared using 25kDa PEI-MION has average size of about 255 µm while the size of 17kDa PMAA-MION has the average size of approximately 35 µm from ImageJ analysis (Tab. 1, Fig. 6 (b) and (f)). It was observed from the optical microscope that there are two distinct emulsion size of PMAA-MION stabilised Pickering emulsion (Fig 6 (f)). This suggested that the size distribution could be bimodal however, the smaller size emulsion (35 µm) is very significant such that no trace of the larger emulsion size was visible using ImageJ analysis. In order to confirm the result, ph measurement was performed. While PEI is basic and PMAA is acidic, there is a concern about whether the polymers will induce change in ph during the emulsion preparation. However, the ph of these two emulsions was measured to be 6.5 indicating that ph changes did not occur and alter the change in size of emulsion as ph alteration had shown to induce emulsion size change (Brugger and Richtering, 2007, Brugger et al., 2008). Further analysis investigates whether the polymer size affects the change in the emulsion size was also being conducted using PMAA-MION as the model. Two different PMAA polymers were compared, 3 kda and 17 kda (Tab. 1). It was observed that 3 kda PMAA-MION also shows similar size of 35 µm regardless of the polymer size even though the MION particle itself is unstable and aggregated over 1 µm when observed using particles sizer. This suggested that regardless of the aggregated size of the functionalised MION, the surface of the MION plays important role in controlling the size of the emulsion. Notice also there are small particles and droplets among the main emulsion droplets of PEI-MION batch, however cannot be resolved with the current method of analysis. These droplets could be either the aggregation of the MION particles or the smaller emulsion drops (Fig. 6). Further analysis required in order to differentiate these from the majority of the emulsion droplets. 6

8 a) b) c) d) e) f) g) Fig. 6: Emulsion size with respect to condition change. a-e are emulsions with PEI- MION as stabilisers while f and g are emulsion with PMAA-MION as stabilisers. a-c corresponds to change in the shearing rate from rpm to rpm. b, d and e refers to change in shearing time from 3 min to 9 min. b and f compares 25 kda PEI- MION and 17 kda PMAA-MION respectively as stabilisers. f and g compares 17 kda PMAA and 3 kda PMAA functionalised MION respectively. The scale bars from a-e are 500 µm while f and g are 200 µm. Emulsion Tab. 1: Size of Pickering emulsion with respect to conditions applied. Polymers Coated Shearing (rpm) Rate Shearing Time (min) a) 25 kda PEI b) 25 kda PEI c) 25 kda PEI d) 25 kda PEI e) 25 kda PEI f) 17 kda PMAA g) 3 kda PMAA Average (µm) Size 7

9 Effects of Shearing Rate and Period Kaiser etal. (Kaiser et al., 2009) observed that the size of the emulsion reduced upon sonication for a period of time. In our experiment, instead of sonication, homogenisation of the emulsion at different shearing rate and time was investigated using PEI-MION as model. Particularly, the homogenising speed was varied from rpm to rpm for shearing rate measurement. With the shearing time measurement, the shearing rate was fixed at rpm while changing the homogenising time from 3 min to 9 min (Tab. 1). It was observed that regardless of the change in shearing rate or period, the size of the emulsion which indicating that shearing rate and period did not introduce emulsion deformation. CONCLUSION Functionalisation of MION with PMAA resulted in smaller size emulsion of 35 µm while with PEI yield larger emulsion of 255 µm regardless of the polymers molecular weight. This suggested that surface modification of MION can significantly affected size of the emulsion produced. Although homogenising is important to form uniform Pickering emulsion, the emulsion size is independent of shearing rate and shearing time. ACKNOLEDGEMENT The author would like to thank Ms. Hilda Wiogo for her chemicals and Australian Research Council for the funding. REFERENCES BRUGGER, B. & RICHTERING, W MAGNETIC, THERMOSENSITIVE MICROGELS AS STIMULI-RESPONSIVE EMULSIFIERS ALLOWING FOR REMOTE CONTROL OF SEPARABILITY AND STABILITY OF OIL IN WATER-EMULSIONS. ADVANCED MATERIALS, 19, BRUGGER, B., ROSEN, B. A. & RICHTERING, W MICROGELS AS STIMULI-RESPONSIVE STABILIZERS FOR EMULSIONS. LANGMUIR, 24, FRELICHOWSKA, J., BOLZINGER, M. A. & CHEVALIER, Y EFFECTS OF SOLID PARTICLE CONTENT ON PROPERTIES OF O/W PICKERING EMULSIONS. JOURNAL OF COLLOID AND INTERFACE SCIENCE, 351, GOON, I. Y., LAI, L. M. H., LIM, M., MUNROE, P., GOODING, J. J. & AMAL, R FABRICATION AND DISPERSION OF GOLD-SHELL-PROTECTED 8

10 MAGNETITE NANOPARTICLES: SYSTEMATIC CONTROL USING POLYETHYLENEIMINE. CHEMISTRY OF MATERIALS, 21, KAISER, A., LIU, T., RICHTERING, W. & SCHMIDT, A. M MAGNETIC CAPSULES AND PICKERING EMULSIONS STABILIZED BY CORE SHELL PARTICLES. LANGMUIR, 25, MADIVALA, B., VANDEBRIL, S., FRANSAER, J. & VERMANT, J EXPLOITING PARTICLE SHAPE IN SOLID STABILIZED EMULSIONS. SOFT MATTER, 5, MELLE, S., LASK, M. & FULLER, G. G PICKERING EMULSIONS WITH CONTROLLABLE STABILITY. LANGMUIR, 21, SUGIMOTO, T. & MATIJEVIC, E FORMATION OF UNIFORM SPHERICAL MAGNETITE PARTICLES BY CRYSTALLIZATION FROM FERROUS HYDROXIDE GELS. JOURNAL OF COLLOID AND INTERFACE SCIENCE, 74, THAREJA, P., MORITZ, K. & VELANKAR, S. S INTERFACIALLY ACTIVE PARTICLES IN DROPLET/MATRIX BLENDS OF MODEL IMMISCIBLE HOMOPOLYMERS: PARTICLES CAN INCREASE OR DECREASE DROP SIZE. RHEOLOGICA ACTA, 49, THAREJA, P. & VELANKAR, S. S. YEAR. EFFECT OF PARTICLES ON RHEOLOGY AND MORPHOLOGY OF IMMISCIBLE PI/PDMS POLYMER BLENDS. IN, BRIEF BIOGRAPHY OF PRESENTER Lip Son Chin, graduated with Bachelor of Science specialised in Nanotechnology at UNSW in 2009 is currently pursuing his PhD in Chemical Engineering, working with Professor Rose Amal and Dr. May Lim. The title of his project is magnetic emulsion for islet cell harvesting and preservation. 9