Langmuir 2004, 20, 6977-6980
6977
Spontaneous Formation of Mesostructures in Colloidal Monolayers Trapped at the Air-Water Interface: A Simple Explanation J. C. Ferna´ndez-Toledano, A. Moncho-Jorda´, F. Martı´nez-Lo´pez, and R. Hidalgo-A Ä lvarez* Biocolloid and Fluid Physics Group, Department of Applied Physics, Faculty of Science, University of Granada, Granada E-18071, Spain Received February 12, 2004. In Final Form: June 25, 2004 The spontaneous formation of loosely bound ordered aggregates, foam, voids, chains, striations, and loops (see Figure 1a), called mesostructures hereafter, has been observed in colloidal monolayers trapped at the air-water interface.1-6 The distance between particles in these mesostructures is of the order of the particle radius (micrometers), implying that the colloidal interaction potential has a minimum at such distances, which could induce the phase separation of colloidal monolayers in dense and dilute regions. This is at odds with the accepted theory (Derjaguin-Landau-Verwey-Overbeek (DLVO)) of colloidal interactions,7,8 which predicts a secondary minimum at distances of nanometers between pairs of interacting particles. Moreover, the introduction of capillary, hydrophobic, and dipolar interactions9 between particles in an extended DLVO theory is not able to explain the spontaneous formation of mesostructures either. Recently, a great deal of effort has focused on understanding the mechanism behind the phenomenon of long-range attraction between colloidal particles confined in interfaces.1,2,10-11 In particular, this attraction has been employed to explain the spontaneous formation of mesostructures. Here, we show that the appearance of our mesostructures is due to the contamination of colloidal monolayers by silicone oil (poly(dimethylsiloxane)), which arises from the coating of the needles and syringes used to deposit and spread the particle solution at the air-water interface. The difference in the interfacial tension of water and silicone oil accounts for the formation of the experimentally observed mesostructures.
We can distinguish two different sets of experimental studies on the behavior of colloidal monolayers depending on the morphology of the structures formed during the spreading of colloidal particles on a determined liquid interface. In the first set, we have aggregates with fractal character-scale invariance, whose cluster structure is selfsimilar in a statistical sense and can be featured through a fractal dimension.12,13 The formation of fractal aggregates can be explained using short-range interactions (usually, with ranges of the order of nanometers). In the second set, it has been reported that in some cases colloidal monolayers can form mesostructures spontaneously. Colloidal mesostructures require an attractive contribution to the interparticle potential that is longer than the van der Waals force and stronger than the conventional * Corresponding author. Current address: Departamento de Fı´sica Aplicada, Facultad de Ciencias, Universidad de Granada, E-18071, Spain. Fax: +34- 958 243 214. E-mail:
[email protected]. (1) Ruiz-Garcı´a, J.; Ga´mez-Corrales, R.; Ivlev, B. I. Physica A 1997, 236, 97. (2) Ghezzi, F.; Earnshaw, J. C. J. Phys.: Condens. Matter 1997, 9, L517. (3) Ruiz-Garcı´a, J.; Ivlev, B. I. Mol. Phys. 1998, 95, 371. (4) Ruiz-Garcı´a, J.; Ga´mez-Corrales, R.; Ivlev, B. I. Phys. Rev. E 1998, 58, 660. (5) Sear, R. P.; Chung, S.-W.; Markovich, G.; Gelbart, W. M.; Heath, J. R. Phys. Rev. E 1999, 59, R6255. (6) Ghezzi, F.; Earnshaw, J. C.; Finnis, M.; McCluney, M. J. Colloid Interface Sci. 2001, 238, 433. (7) Derjaguin, B. V.; Landau, L. Acta Physicochim. URSS 1941, 14, 633. (8) Verwey, E. J.; Overbeek, J. Th. G. Theory of the Stability of Lyophobic Colloids; Elsevier: Amsterdam, The Netherlands, 1948. (9) Martı´nez-Lo´pez, F.; Cabrerizo-Vı´lchez, M. A.; Hidalgo-A Ä lvarez, R. J. Colloid Interface Sci. 2000, 232, 303. (10) Ivlev, B. I. J. Phys.: Condens. Matter 2002, 14, 4829. (11) Nikolaides, M. G.; Bausch, A. R.; Hsu, M. F.; Dinsmore, A. D.; Brenner, M. P.; Gay, C.; Weitz, D. A. Nature 2002, 420, 299. (12) Robinson, D. J.; Earnshaw, J. C. Phys. Rev. E 1992, 46, 2045. (13) Stankiewicz, J.; Cabrerizo-Vı´lchez, M. A.; Hidalgo-A Ä lvarez, R. Phys. Rev. E 1993, 47, 2663.
capillary interaction.2 Bowen and Sharif14 have proposed that these forces between confined colloidal particles can be obtained from direct solutions of the nonlinear PoissonBoltzmann equation (PBE) for two like-charged spheres. On the other hand, Sader and Chan15 have rigorously proved that within the Poisson-Boltzmann theory the interaction between identical colloidal particles is never attractive, irrespective of whether the particles are isolated or confined. These authors establish a necessary condition for the existence of attractive interactions, which indicates the possibility that an osmotically driven process is behind the observed attractive interactions. In two-dimensional systems, capillary attraction,16 intrinsic to interfacial phenomena, depending on the particle size can play an important role in their stability. These capillary forces are due to the meniscus around a partly immersed particle. In this respect, Stamou et al.17 have proposed a mechanism for attraction, which is based on nonuniform wetting causing an irregular shape of the particle meniscus. For two particles in contact and optimal orientations, one obtains the capillary energy Ec ) -(3/4)πγhc2, where γ is the interfacial tension and hc is the undulation amplitude of the contact line. Thus, the effect of the contact-line undulations will be significant (greater than the thermal energy (kBT)) when their amplitude is of the order of nanometers or larger. However, the predictions of the above-mentioned equation have not been verified experimentally yet. Very recently, Nikolaides et al.11 presented quantitative measurements of the attractive interactions between (14) Bowen, W. R.; Sharif, A. O. Nature 1998, 393, 663. (15) Sader, J. E.; Chan, D. Y. C. Langmuir 2000, 16, 324. (16) Kralchevsky, P. A.; Denkov, N. D. Curr. Opin. Colloid Interface Sci. 2001, 6, 383. (17) Stamou, D.; Duschl, C.; Johannsmann, D. Phys. Rev. E 2000, 62, 5263.
10.1021/la0496237 CCC: $27.50 © 2004 American Chemical Society Published on Web 07/14/2004
6978
Langmuir, Vol. 20, No. 17, 2004
Letters
colloidal particles at the oil-water interface and showed that the attraction can be explained by capillary forces that arise from a distortion of the interface shape due to electrostatic stresses caused by the particles’ dipolar field. Megens and Aizenberg18 have argued that this effect cannot account for the observed attraction, on the fundamental grounds that it is inconsistent with force balance. As concluded from the different theoretical approaches used to account for the formation of mesostructures, the physical origin of this long-range attraction remains unclear. In fact, the existence of long-range attractive forces between colloidal particles confined in interfaces is one of the great current controversies of colloid science. In this context, we decided to study experimentally the behavior of colloidal monolayers formed by polystyrene microspheres spread at the air-water interface. These colloidal particles were synthesized from styrene by freeemulsifier polymerization19 with potassium persulfate (K2S2O8) as an initiator and potassium bicarbonate (KHCO3) as a buffer. They were cleaned by steam stripping and then with serum replacement and mixed-bed ionexchange resin. The average particle diameter and polydispersity index were 600 ( 25 nm and 1.004, respectively. The surface charge density of -0.053 ( 0.005 C/m2 was determined by conductometric titration. The experiments at the air-water interface were carried out using a small cylindrical cell of Teflon (area, 2.01 cm2; height, 1.0 cm). The colloidal microspheres were spread at the interface using a syringe and methanol as a spreading agent. This is the usual procedure employed to spread colloidal particles at the air-water interface. All the colloidal dispersions were sonicated for 5 min to ensure a good initial monodispersity. The interface has to be as planar as possible to prevent the particle migration because of gravity. After methanol evaporation, a thin glass plate covers the cell to prevent contamination of the monolayer and the convective fluxes produced by the air motion. The dynamic processes at the colloidal monolayer were monitored using a phase contrast microscope with a chargecoupled device (CCD) camera attached to it with 1280 × 1024 pixels. Possible vibrations were avoided by placing the experimental setup formed by the cell, microscope, and camera on an antivibratory table. The magnification of the objective was fixed so that the pixel size was close to the particle diameter. A frame grabber acquired the images. More details on the experimental setup are given elsewhere.20 Figure 1 shows two pictures of two different runs of a typical experiment performed under identical experimental conditions at the stationary state, which is reached after 30 min. The liquid subphase is in both cases water (water from a Millipore Milli-Q system) without an additional electrolyte. The microspheres are spread on the air-water interface using a syringe (1 mL Plastipak) and a needle (Microlance 3). We observed that the morphology of the colloidal monolayers at the air-water interface depends on the number of times that the same needle and syringe are used. At the beginning, we always observed stable monolayers (see Figure 1b). However, after a certain number of times of using the same syringe and needle, we observed the spontaneous formation of mesostructures (see Figure 1a), and this phenomenon was more evident with the repeated use of the same syringe and needle in the deposition of drops (containing poly-
styrene microspheres, water, and methanol) on the airwater interface. Once the cause of the spontaneous formation of mesostructures was experimentally confirmed to be due to the repeated use of the same syringe and needle, we decided to contact with the manufacture of both materials. The manufacture of the syringes and needles used in this study confirms that both constituents are made with an internal coating only of silicone oil.21 Also, we observed that the surface tension of a water drop changes from 73 to 62 mJ/m2 as a result of the contact with a needle for 1 s. These surface tension values were experimentally obtained using the axisymmetric drop shape analysis (ADSA).23 The surface tension of the water-air interface remained constant when it was put in contact with a needle, which had been previously washed with toluene. We never observed the formation of mesostructures using glass syringes and the washed needle. It should be noted that the silicone oil is partially miscible in methanol,22 and this alcohol is very often used as a spreading agent in the formation of colloidal monolayers trapped at the air-water interface. It is feasible that some silicon oil molecules (even drops!) remained at the interface after evaporation of the methanol. The pattern formations of stable colloidal monolayers or
(18) Megens, M.; Aizenberg, J. Nature 2003, 424, 1014. (19) Kotera, A.; Furusawa, K.; Takeda, Y. Kolloid Z. Z. Polym. 1970, 239, 677. (20) Moncho-Jorda´, A.; Martı´nez-Lo´pez, F.; Hidalgo-A Ä lvarez, R. J. Colloid Interface Sci. 2002, 249, 405.
(21) Becton Dickinson, S. A. www.bd.com. (22) Tikhomiroff, C.; Allais, S.; Klvana, M.; Hisiger, S.; Jolicoeur, M. Biotechnol. Prog. 2002, 18, 1003. (23) Rotenberg, Y.; Boruvka, L.; Neumann, A. W. J. Colloid Interface Sci. 1983, 93, 169.
Figure 1. Different morphologies of colloidal monolayers made of 0.6-µm particles: (a) mesostructures; (b) the second stable colloidal monolayer. Image dimension: (a) 593 × 474 µm2; (b) 127 × 191 µm2. The spreading volume was in all cases 5 µL.
Letters
Figure 2. Spontaneous formation of mesostructures in colloidal monolayers contaminated with different superficial densities of silicone oil on the air-water interface: (A) 1.27 × 10-3, (B) 2.55 × 10-3, (C) 3.81 × 10-3, (D) 5.08 × 10-3 µg/cm2. Image dimension: 593 × 474 µm2. Methanol was always used as a spreading agent.
Langmuir, Vol. 20, No. 17, 2004 6979
Figure 4. Spontaneous formation of mesostructures in a colloidal monolayer contaminated with 5.08 × 10-3 µg/cm2 of superficial density of silicone oil on the air-water interface. Image dimension: 82 × 108 µm2.
Figure 3. Spontaneous formation of mesostructures in a colloidal monolayer contaminated with 50.8 × 10-3 µg/cm2 of superficial density of silicone oil on the air-water interface. Each image dimension: 105 × 137 µm2.
mesostructures were reproducible for six different experiments. The surface particle density of the colloidal monolayers was between 0.6 and 0.8%. The hypothetical presence of some oily film (say from dissolved styrene oligomers) as a possible cause for the mesostructure formation was already pointed out by Kralchevsky and Denkov.16 In our case, we observed that
Figure 5. Spontaneous formation of mesostructures in a colloidal monolayer contaminated with 5.08 × 10-3 µg/cm2 of superficial density of silicone oil on the air-water interface. Each image dimension: 112 × 143 µm2.
the cleaning of latex particles by steam stripping (where residual monomer is removed) does not affect the spontaneous formation of mesostructures. Therefore, we exclude in our case that some oily film from dissolved
6980
Langmuir, Vol. 20, No. 17, 2004
styrene oligomers is generated on the surface of the aqueous subphase when spreading the suspension of latex particles in methanol. To check the effect of silicone oil on the spontaneous formation of mesostructures, we performed some experiments contaminating the colloidal particles with silicone oil (AR 200 from Fluka; density, 1.042 g cm-3; refractive index, 1.450) dissolved in methanol. Figure 2 shows the mesostructures obtained by this procedure for different concentrations of silicone oil. The pattern formation of these structures depends on the number of molecules of silicone oil present at the air-water interface. In addition, the molecular weight of the silicone oil might affect the geometric pattern of mesostructures. This is likely to explain the great number of geometric patterns found by different authors.1-6 The surface tension of the silicone oil-air interface is 35 mJ m-2, whereas it is 73 mJ m-2 for the water-air interface. The very small amounts of silicone oil change the surface tension of the air-liquid interface and lead to the formation of a bi-dimensional emulsion with hydrophobic (silicone oil droplets) and hydrophilic patches (water surface) with different interfacial tensions. When the polystyrene microspheres (hydrophobic in character) are spread on this nonuniform interface, most of them tend to locate on the hydrophobic silicon patches and form mesostructures. Basically, in our case, the mesostructures are the result of the decoration with microspheres of a bi-dimensional emulsion.24 This means that the mean particle distance between the colloidal particles depends on the manner in which they are accommodated inside of the hydrophilic and hydrophobic patches at the air-liquid interface. This explains the appearance of circular clusters in mesostructures very often. We have observed that the most irregular mesostructures (loops) are formed when the silicone oil concentration at the air-water interface is great (see (24) Binks, B. P.; Whitby, C. P. Langmuir, in press.
Letters
Figure 3), whereas the circular clusters are always present in any concentration range of the contaminating agent (see Figures 4 and 5). The pattern formation of these mesostructures is quite similar to that obtained previously by other authors.1-6 A more detailed description of the formation mechanism of mesostructures at an air-water interface contaminated by silicone oil is outside the scope of this letter. Further understanding and experimental work is needed to develop a theoretical model that is able to predict the appearance of any kind of mesostructures. More figures and videos of the formation of mesostructures are given at http://caos.ugr.es. In short, in our case, the mechanism behind the phenomenon of long-range attraction between colloidal particles25 that are confined at the air-water interface and form mesostructures is not related to any nonaccepted physical interaction but to the formation of an oily film due to the contamination caused by the silicone oil. This contamination arises from the coating of the needles and syringes used to deposit and spread the particle solution at the air-water interface. Our results support the suggestion made by Kralchevsky and Denkov16 on the effect of an oily film in the apparently spontaneous formation of mesostructures on the air-water interface. Acknowledgment. The authors acknowledge the financial support from “Ministerio de Ciencia y Tecnologı´a, Plan Nacional de Investigacio´n (I+D+i), MAT2003- 08356C04-01”. Supporting Information Available: Mesostructures formed by silicon oil at the air-water interface. The silicon superficial density is 3.81 × 10-3 µg/cm2. This material is available free of charge via the Internet at http://pubs.acs.org. LA0496237 (25) Belloni, L. J. Phys.: Condens. Matter 2000, 12, R549.
