7422
Langmuir 2004, 20, 7422-7429
Effect of Varying the Oil Phase on the Behavior of pH-Responsive Latex-Based Emulsifiers: Demulsification versus Transitional Phase Inversion E. S. Read, S. Fujii, J. I. Amalvy,† D. P. Randall, and S. P. Armes* Department of Chemistry, School of Life Sciences, University of Sussex, Falmer, Brighton, BN1 9QJ, UK Received March 4, 2004. In Final Form: May 10, 2004 Sterically stabilized polystyrene latexes (previously described by Amalvy, J. I.; et al. Chem. Commun. 2003, 1826) were evaluated as pH-responsive particulate emulsifiers for the preparation of both oil-inwater and water-in-oil emulsions. The steric stabilizer was a well-defined AB diblock copolymer where A is poly(2-(dimethylamino)ethyl methacrylate) and B is poly(methyl methacrylate). Several parameters were varied during the emulsion preparation, including the polarity of the oil phase, the latex concentration, surface concentration of copolymer stabilizer, and solution pH. Nonpolar oils such as n-dodecane gave oil-in-water emulsions, and polar oils such as 1-undecanol produced water-in-oil emulsions. In both cases, these emulsions proved to be stimulus-responsive: demulsification occurred rapidly on adjusting the solution pH. Oils of intermediate polarity such as methyl myristate or cineole led to emulsions that underwent transitional inversion on adjusting the solution pH. All emulsions were polydisperse and typically ranged from 40 to 400 µm diameter, as judged by optical microscopy and Malvern Mastersizer measurements. Critical point drying of the emulsion droplets, followed by scanning electron microscopy studies, confirmed that the latex particles were adsorbed as a single monolayer at the oil/water interface, as anticipated.
Introduction The use of colloidal particles to prepare and stabilize emulsions (so-called “Pickering” emulsions) has been recognized for over a century.1,2 Most of this literature is concerned with various types of inorganic particles such as silica, barium sulfate, calcium carbonate, or carbon black. In principle, particulate emulsifiers offer a number of potential advantages over conventional surfactants.3 These include: (1) more robust, reproducible formulations; (2) reduced foaming problems; (3) lower toxicity profiles (e.g., reduced skin irritancy); and (4) lower cost. Given that the formulation of emulsions is important for a range of industrial sectors (pharmaceutics, agrochemicals, food science, home and personal care products, etc.), it seems likely that particulate emulsifiers will become increasingly widely used. Lumsden and Binks4 showed that the use of commercial hydrophobic silica sols as particulate emulsifiers enabled the preparation of submicrometer-sized water-in-oil emulsions (oil ) toluene). Transitional phase inversion occurred above a water volume fraction of around 0.7, leading to the formation of much larger toluene-in-water emulsions. A similar phase inversion was obtained at approximately the same volume fraction using hydrophilic silica sols, which initially formed toluene-in-water emulsions. This indicates that particle wettability is an important parameter for “Pickering” emulsions. The same group has reported the use of charge-stabilized polystyrene latex to produce water-in-oil emulsions.5a Under favorable circumstances, the particulate emulsifiers can sometimes be visualized at the surface of the oil droplets, but this † Member of research Career of CIC, Buenos Aires, Argentina, on leave from CIDEPINT (Centro de Investigacio´n y Desarrollo en Tecnologı´a de Pinturas), Av. 52, entre 121 y 122 s/n, (BI906AYB) La Plata, Buenos Aires, Argentina.
(1) Ramsden, W. Proc. R. Soc. 1903, 72, 156. (2) Pickering, S. U. J. Chem. Soc. 1907, 91, 2001. (3) Binks, B. P. Curr. Opin. Colloid Interface Sci. 2002, 7, 21. (4) Binks, B. P.; Lumsdon, S. O. Langmuir 2000, 16, 2539.
usually requires freeze-fracture techniques in combination with field emission scanning electron microscopy.5b,c Recently, Weitz and co-workers reported the use of micrometer-sized polystyrene latex particles to prepare water-in-oil emulsions.6 It was demonstrated that thermal annealing of the adsorbed monolayer of latex particles enabled the permeability of the so-called “colloidosomes” to be controlled. This advance augurs well for a number of applications. For example, living cells can be encapsulated within such colloidosomes, which are then protected from the external environment. In 1996, we reported the synthesis of a series of welldefined, low polydispersity 2-(dimethylamino)ethyl methacrylate-block-methyl methacrylate [PDMA-PMMA] diblock copolymers.7 Subsequently, these copolymers were used as steric stabilizers for the synthesis of micrometersized, sterically stabilized polystyrene latex via alcoholic dispersion polymerization,8 with the hydrophobic PMMA block being adsorbed onto the latex surface and the hydrophilic PDMA block acting as a steric stabilizer layer. Submicrometer-sized latexes of the same type can be readily prepared, either by adding a small amount of water to the dispersion polymerization formulation or by carrying out the latex synthesis under aqueous emulsion conditions.9 Recently, we demonstrated that such sterically stabilized latexes can be exploited as stimulus-responsive particulate emulsifiers for the stabilization of oil-in-water emulsions.9,10 For nonpolar oils such as n-hexadecane or (5) (a) Binks, B. P.; Lumsdon, S. O. Langmuir 2001, 17, 4540. (b) Binks, B. P.; Kirkland, M. Phys. Chem. Chem. Phys. 2002, 4, 3727. (c) Binks, B. P. Adv. Mater. 2002, 14, 1824. (6) Dinsmore, A. D.; Hsu, M. F.; Nikolaides, M. G.; Marquez, M.; Bausch, A. R.; Weitz, D. A. Science 2002, 298, 1006. (7) Baines, F. L.; Billingham, N. C.; Armes, S. P. Macromolecules 1996, 29, 3416. (8) Baines, F. L.; Dioniso, S.; Billingham, N. C.; Armes, S. P. Macromolecules 1996, 29, 3096. (9) Amalvy, J. I.; Unali, G.-F.; Li, Y. T.; Granger-Bevan, S.; Armes, S. P.; Rodrigues, J. A.; Whitby, C. P.; Binks, B. P. Langmuir 2004, 20, 4345.
10.1021/la049431b CCC: $27.50 © 2004 American Chemical Society Published on Web 07/29/2004
Behavior of pH-Responsive Latex-Based Emulsifiers
Langmuir, Vol. 20, No. 18, 2004 7423
Table 1. Summary of the Physicochemical Properties of the Five Oils Evaluated in This Study
a
oil type
chemical structure
density (g cm-3)
boiling point (°C)
Γopa
n-dodecane isopropyl myristate methyl myristate cineole 1-undecanol
CH3(CH2)10CH3 CH3(CH2)12CO2CH(CH3)2 CH3(CH2)12CO2CH3 C10H18O CH3(CH2)10OH
0.750 0.850 0.855 0.912 0.830
216 192 323 176 146
0 3.1 4.3 9.0 20.0
Polarities were calculated from experimental values of the oil-air and oil-water surface tensions.11
n-dodecane, the latex particles are adsorbed at the oil/ water interface at around pH 8 and stable oil-in-water emulsions are formed. This in itself is unusual: the use of (charge-stabilized) latexes as emulsifiers typically leads to the formation of water-in-oil emulsions.5a,6 We attribute this to the presence of the steric stabilizer, because in its neutral form the PDMA block of the stabilizer has some affinity for the oil phase. Moreover, adding sufficient acid to the aqueous continuous phase leads to spontaneous desorption of the pH-responsive latex particles from the surface of the oil droplets, allowing such emulsions to be broken relatively easily. In the present work, we have examined the effect of varying the nature of the oil phase on the performance of these new pH-responsive latex emulsifiers. Recent studies of silica sols by Binks and Clint11 suggest that interesting differences should be obtained as the polarity of the oil is increased. Five representative oils were chosen, and their physical properties are summarized in Table 1. Experimental Section Materials. Each of the oils listed in Table 1 was purchased from Aldrich and was used as received. The PDMA-PMMA diblock copolymer was prepared by group transfer polymerization according to the general protocol described by Baines et al.7 Its block composition was confirmed to be approximately 80 mol % by 1H NMR spectroscopy. Gel permeation chromatography analysis indicated an Mn of 38 400 and a Mw/Mn of 1.14 versus poly(methyl methacrylate) standards. Latex Syntheses. The PDMA-PMMA diblock copolymer was used to prepare sterically stabilized polystyrene latex via dispersion polymerization in a 7:3 methanol/water mixture using AIBN initiator as described previously.9 Purification was achieved using ultrafiltration with periodic replacement of the serum: surface tensiometry was used to confirm that the final latex was not contaminated with nonadsorbed block copolymer stabilizer. 1H NMR analysis of the resulting latex dissolved in CDCl 3 indicated a PDMA-PMMA stabilizer content of approximately 7 ( 1 wt %. Scanning electron microscopy studies of the latex particles confirmed their spherical morphology and indicated a particle diameter range of 100-250 nm. This is consistent with both dynamic light scattering and disk centrifuge photosedimentometry studies. 1-Pyrenylmethyl methacrylate was prepared by the reaction of 1-pyrenemethanol (1.0 g) with excess methacryloyl chloride (0.494 g) in dry THF (25 g) for 5 h at 0 °C in the presence of triethylamine (0.87 g) and hydroquinone inhibitor (1 mg). The resulting reaction was filtered to remove the amine salt, and 10 wt % Na2CO3 aqueous solution was added to neutralize the excess methacryloyl chloride. The 1-pyrenylmethyl methacrylate was extracted with dichloromethane and dried with anhydrous Na2SO4. This solution was filtered, and the dichloromethane was removed under vacuum. 1H NMR spectroscopy confirmed the compound to be 1-pyrenylmethyl methacrylate (see Supporting Information), and the yield (based on 1-pyrenemethanol) was calculated to be almost 100%. Pyrene-labeled PS latex was synthesized under the same conditions as the conventional PS latex described above using 1-pyrenylmethyl methacrylate as a comonomer (1.0 wt % based on styrene). The PDMA-PMMA (10) Amalvy, J. I.; Armes, S. P.; Binks, B. P.; Rodrigues, J. A.; Unali, G.-F. Chem. Commun. 2003, 1826. (11) Binks, B. P.; Clint, J. H. Langmuir 2002, 18, 1270.
stabilizer content of the pyrene-labeled PS latex was 6 ( 1 wt % as judged by 1H NMR studies of the dissolved latex in CDCl3. Scanning electron microscopy was used to investigate the morphology and size distribution of the pyrene-labeled PS latex.
Emulsion Synthesis Stock solutions of the latexes at the desired solids content were prepared by dilution. The solution pH of 5 mL aliquots of the diluted latexes was adjusted as required by adding a few drops of concentrated HCl or NaOH solution, using a pH meter to monitor the solution pH. The latex dispersions were then homogenized at 20 °C with 5 mL of each of the five oils for 2 min using an IKA Ultra-Turrax T-18 homogenizer with a 10 mm dispersing tool operating at 12 000 rpm. Emulsion stabilities after standing for 24 h at room temperature (20-25 °C) were assessed by visual observation. In some cases, a more accurate emulsion stability assessment was obtained using graduated vessels by monitoring the movement of the oil-emulsion and emulsion-water interfaces with time. Emulsion Characterization Conductivity Measurements. The conductivity of the emulsions immediately after preparation was measured using a digital conductivity meter (Hanna model Primo 5). The emulsions were classified according to their conductivities. A high conductivity indicated an oil-inwater emulsion, and a low (immeasurable) conductivity (
