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On Synthesizing Solid Polyelectrolyte Microspheres from Evaporating Liquid Marbles Prasad S. Bhosale and Mahesh V. Panchagnula* Department of Mechanical Engineering, Tennessee Technological University, Cookeville, Tennessee 38505 Received February 5, 2010. Revised Manuscript Received March 24, 2010 We report a simple evaporation process using liquid marbles as precursors to produce high sphericity, precisely diameter controlled polyelectrolyte microspheres. We use poly(diallyldimethylammonium chloride) (PDDA) as the test polyelectrolyte for this experimental study. We present measurements of the rate of mass loss during evaporation to demonstrate evidence of two limiting physical processes. At short times, the rate of mass loss is well described by the “D2 law” regime, which is vapor diffusion limited. At long times, the rate of water diffusion inside the nearly solid polyelectrolyte microsphere becomes the rate-limiting step. The transition between these two limiting processes is accompanied by changes in the physical morphology inside the microsphere. We compare the estimated values of the water diffusion coefficients with the values reported in the literature to demonstrate good agreement.
Introduction Liquid marbles are formed when a small volume of liquid is encapsulated in a particulate matrix. Such marbles have been shown to be effective agents to transport liquids.1-6 Previous evaporation studies have shown that the rate of mass loss from a liquid marble is less than that from a sessile drop of the same liquid in a similar atmosphere.2,7-10 As a result, it can be concluded that liquid marbles are also effective storage devices. In the literature, liquid marbles have been studied while employing liquids of homogeneous chemical composition—water, glycerin, ionic liquids—in preparing the marbles and characterizing the rate of mass loss. It would be interesting to investigate the possibility of first creating liquid marbles from a liquid of nonhomogenous chemical composition, such as a solution. This would further generalize the applicability of liquid marbles as transport and storage agents. In addition, during the evaporation process, it is anticipated that the solute concentration in the marble would vary over a wide range of values (from an extremely dilute limit up to its solubility limit). It would be interesting to study the underlying physical processes as they are relevant to several multiphase applications.11 We choose polyelectrolyte solutions for this study as they offer a rich set of possibilities for biochemical applications. Polyelectrolyte compound storage and the ability to sequester molecules
Methods and Materials
*To whom correspondence should be addressed. (1) Aussillous, P.; Quere, D. Nature 2001, 411(6840), 924–927. (2) Bhosale, P. S.; Panchagnula, M. V.; Stretz, H. A. Appl. Phys. Lett. 2008, 93, 034109. (3) Bormashenko, E.; Bormashenko, Y.; Musin, A. J. Colloid Interface Sci. 2009, 333(1), 419–421. (4) McHale, G.; Herbertson, D. L.; Elliott, S. J.; Shirtcliffe, N. J.; Newton, M. I. Langmuir 2007, 23(2), 918–924. (5) Forny, L.; Pezron, I.; Saleh, K.; Guigon, P.; Komunjer, L. Powder Technol. 2007, 171, 15–24. (6) Gao, L. C.; McCarthy, T. J. Langmuir 2007, 23(21), 10445–10447. (7) Tosun, A.; Erbil, H. Y. Appl. Surf. Sci. 2009, 256(5), 1278–1283. (8) Dandan, M.; Erbil, H. Y. Langmuir 2009, 25(14), 8362–8367. (9) McHale, G.; Shirtcliffe, N. J.; Newton, M. I.; Pyatt, F. B.; Doerr, S. H. Appl. Phys. Lett. 2007, 90, 5. (10) Shih, Y. P.; Coughanowr, D. R. AIChE J. 1968, 14(3), 502–504. (11) Hu, B.; Matar, O. K.; Hewitt, G. F.; Angeli, P. Chem. Eng. Sci. 2006, 61, 4994–4997. (12) Stein, E. W.; Volodkin, D. V.; SMcShane, M. J.; Sukhorukov, G. B. Biomacromolecules 2006, 7, 710–719.
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within such storage devices are relevant to protein-based sensors.12 Further, synthesis of polyelectrolyte microspheres has also attracted much interest for applications ranging from drug delivery to chemical storage. For example, they can form the template for controlled release applications. Toward this end, a process which can be used to produce precisely controlled diameter microspheres is necessary.13 We herein demonstrate that by using precursor liquid marbles of the polyelectrolyte solution and allowing the solvent to evaporate provides such a procedure. We show that this process produces microspheres that are not only of a controlled diameter but also of a high sphericity. We choose a relatively commonly employed polyelectrolyte— poly(diallyldimethylammonium chloride) (PDDA)—for the study, but also note that the procedure is general enough that it can be employed over a wider range of chemicals. We demonstrate two physical phenomena manifested during this evaporation process. First, we demonstrate from measurements of the rate of mass loss the possibility of the formation of a near solid or gel-like phase during the evaporation process. Second, the evaporation process on liquid marbles presents an interesting case study in phase inversion. Phase inversion, a process where the dispersed and continuous phases undergo an inversion in roles, has only been experimentally studied in a limited set of physical systems.11,14
Poly(diallyldimethylammonium chloride) (PDDA) of molecular weight between 400 000 and 500 000 obtained from SigmaAldrich was used as the polyelectrolyte in this experimental study. Liquid marbles were prepared by placing a drop of 23 wt % PDDA precursor solution in a Petri dish containing Cab-O-Sil TS-530 brand hydrophobic fumed silica nanoparticulate material. The concentration by weight of the PDDA solution was ascertained independently by weighing the solids content after complete evaporation of the solvent. The precursor liquid marble volume was controlled using a micropipet. By gently rolling the drop of PDDA solution in the particulate material without (13) Toprak, M. S.; McKenna, B. J.; Waite, J. H.; Stucky, G. D. Chem. Mater. 2007, 19(17), 4263–4269. (14) Binks, B. P.; Murakami, R. Nat. Mater. 2006, 5(11), 865–869.
Published on Web 04/08/2010
DOI: 10.1021/la100551r
10745
Article
Bhosale and Panchagnula
Figure 1a shows a photograph of a set of PDDA solid microspheres. These microspheres were obtained from a controlled evaporation process where the ambient temperature and relative humidity were respectively maintained at 60 °C and 0% (by the use of dry nitrogen). Figure 1b depicts an optical micrograph of one ∼50 μm diameter solid PDDA microsphere obtained from the same process. This image of the microsphere also shows the nanoparticulate layer that was used to form the liquid marble. As mentioned before, the procedure for synthesizing microspheres provides the ability to control both the diameter and the sphericity because the polyelectrolyte mass and its surface area are both parameters relevant to controlled release applications.15 The solid microsphere diameter is determined by the polyelectrolyte concentration in the precursor solution and the initial volume of the liquid marble. For example, using a 1 wt % polyelectrolyte solution and by initiating the process with a precursor liquid marble of volume 0.5 ( 0.01 μL, the final polyelectrolyte microsphere diameter is expected to be controlled in the range 200 ( 0.5 μm. Thus, the current approach can produce precisely controlled diameter microspheres. The sphericity is controlled by the underlying physical processes during evaporation. It has been shown that the effective surface tension of a liquid marble made from hydrophobic
particles is on the same order of magnitude as that of the liquid alone.16,17 As a result, the capillary length (a length scale that characterizes the competition between gravity and surface tension forces)18 is also comparable. In addition, as the evaporation process proceeds, the effect of gravity in relation to surface tension forces is further minimized, thereby increasing the tendency to remain spherical. There are two possible physical effects due to which the solid microsphere could deviate from a spherical shape. First, due to gravity, the shape of the liquid marble just prior to solidification itself could deviate from a spherical shape. Rienstra19 has presented a perturbation solution that describes the shape of a drop under the action of gravity. From this perturbation solution, it can be estimated that the maximum deviation in diameter for a typical solid microsphere (∼200 μm diameter) will be less than 0.3%. Second, the liquid marble has a contact zone with the solid substrate that could result in a small flat circular zone on the solidified microsphere. From the theoretical analysis presented by Aussillous and Quere,20 it can be estimated that for the same microsphere this contact zone will be less than 1 μm in diameter. Hence, it can be concluded that the physical processes attempt to retain sphericity to a high degree. The previous conclusion can further be substantiated by the images presented in Figure 2. Figures 2a-d depict a time sequence of environmental scanning electron microscope (ESEM) images obtained during the evaporation process of a PDDA solution marble. Figures 2e-h present images of a pure water liquid marble obtained from an evaporation process under similar conditions. For comparison, the two images in each vertical column were obtained after the same elapsed time since the start of the evaporation process and therefore lend themselves to comparison. As can be observed from the sequence of images in Figures 2a-d, the PDDA marble remains spherical during the entire evaporation process. In addition, the size of the marble in Figures 2a-c decreases gradually. However, the size of the PDDA marble in Figures 2c,d is approximately constant, indicating that a solid microspherical core has been established. The nanoparticulate material that encapsulates the PDDA solution also appears to coarsen in texture and thicken with the passage of time. In comparison, the images in Figures 2e-h obtained from an evaporating water marble show that the marble does not retain its sphericity. In fact, a clear buckling of the free surface is observed in Figures 2g,h due to the compressive stresses on the thickening nanoparticulate layer. In a related study, Bhosale et al.2 were able to correlate the buckling point and the related departure from sphericity to increased evaporation rate. The final set of data presented in Figure 3 was obtained on an evaporating PDDA marble using a thermogravimetric analyzer (TGA). Data from three separate TGA experiments are reported in this figure wherein the PDDA solution marbles were allowed to evaporate in an isothermal atmosphere maintained at 25, 40, and 60 °C and at nearly 0% RH. During the course of each experiment, the water in the marble preferentially diffuses out and is reflected as a net decrease in the overall mass measured by the TGA. It is well-known that a spherical liquid drop in a quiescent isothermal atmosphere evaporates following the D2 law (where D is the diameter of the drop) which implies that the surface area of the drop decreases linearly with time.21 This behavior has also been observed empirically in other liquid marble evaporation
(15) Park, W.; Na, K. Colloids Surf., B 2009, 72(2), 193–200. (16) Bormashenko, E.; Pogreb, R.; Whyman, G.; Musin, A.; Bormashenko, Y.; Barkay, Z. Langmuir 2009, 25(4), 1893–1896. (17) Bormashenko, E.; Pogreb, R.; Whyman, G.; Musin, A. Colloids Surf., A 2009, 351(1-3), 78–82.
(18) de Gennes, P. G.; Brochard-Wyart, F.; Quere, D. Capillarity and Wetting Phenomena - Drops, Bubbles, Pearls, Waves; Paris, 2002. (19) Rienstra, S. W. J. Eng. Math. 1990, 24, 193–202. (20) Aussillous, P.; Quere, D. Proc. R. Soc. London A 2006, 462(2067), 973–999. (21) Turns, S. R. An Introduction to Combustion; Wiley & Sons: New York, 2002.
Figure 1. (a) Solid PDDA microspheres. (b) Optical micrograph of an ∼50 μm diameter solid PDDA marble showing the nanoparticulate sheath still intact. allowing it to come in direct contact with the glass, the liquid is entirely encapsulated by the particulate material resulting in a liquid marble. TS-530 is morphologically consistent of long chain (∼5 μm) silica nanoparticles (diameter
