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Langmuir 2006, 22, 2876-2883
Structuring of Macroions Confined between Like-Charged Surfaces Aysen Tulpar, Paul R. Van Tassel, and John Y. Walz* Department of Chemical Engineering, Yale UniVersity, New HaVen, Connecticut 06520-8286 ReceiVed NoVember 11, 2005. In Final Form: January 4, 2006 We investigate the structuring of charged spherical nanoparticles and micelles (i.e., “macroions”) between two surfaces as a function of bulk macroion concentration. Structuring is deduced from measured force profiles between a silica particle and a silica plate in the presence of an aqueous macroion (Ludox silica nanoparticle or sodium dodecyl sulfate micelle) solution, obtained with an atomic force microscope. We observe oscillatory force profiles that decay with separation. We find that the wavelength of the force profiles scales with the bulk number density as F-1/3, rather than with the effective macroion size. Only at very high silica nanoparticle concentration (above 10 vol %) in a low ionic strength solution does the wavelength become smaller than that predicted by the simple F-1/3 scaling; however, the original scaling is recovered upon the addition of a small amount of electrolyte. A comparison between the measured wavelength and the predicted spacing between the macroions in the bulk shows that the two variables differ in both magnitude and bulk density scaling. This finding suggests that confined macroions are more ordered than those in the bulk and the nature of this ordering is maintained over a relatively wide range of bulk concentration.
Introduction The ordering of charged nanoparticles or micelles, generically referred to here as “macroions”, next to a similarly charged surface has been demonstrated both experimentally1 and theoretically.2 The ordering of macroions confined between two charged surfaces has also been studied, although to a lesser degree and primarily via theoretical predictions.3,4 In this manuscript, we investigate the ordering of nanometer-sized particles and micelles in the gap region between a micron-sized charged sphere and a macroscopic charged plate. The study is performed by measuring the interaction force profile between the particle and plate using an atomic force microscope (AFM). Specifically, the ordering of the nanoparticles and micelles leads to an oscillatory force profile with a very specific wavelength (see Figure 1).5 The two parameters that are of interest to us in this study are the wavelength of the oscillations, λ, which is related to the spacing between the ordered layers in the gap region, and the range over which this ordering is observed. The first observation of an oscillatory force profile dates back to 1981, when Horn and Israelachvili6 measured the force between two mica sheets in octamethylcyclotetrasiloxane with the surface forces apparatus. These authors found that the wavelength of the * To whom correspondence should be addressed. Address: Virginia Polytechnic and State University, Department of Chemical Engineering (0211), Blacksburg, VA 24060. (1) (a) Pouligny, B.; Aastuen, D. J. W.; Clark, N. A. Phys. ReV. A. 1991, 44, 6616-6625. (b) van Winkle, D. H.; Murray, C. A. J. Chem. Phys. 1988, 89, 3885-3891. (c) Murray, C. A.; Sprenger, W. O.; Wenk, R. A. J. Phys.: Condens. Matter 1990, 2, SA385-SA388. (2) (a) Gonzalez-Mozuelos, P.; Medina-Noyola, M. J. Chem. Phys. 1991, 94, 1480-1486. (b) Gonzalez-Mozuelos, P.; Medina-Noyola, M. J. Chem. Phys. 1990, 93, 2109-2115. (c) Yi, J.-H.; Kim, S.-C. J. Chem. Phys. 1997, 107, 81478151. (d) Gonzalez-Mozuelos, P.; Alejandre, J. J. Chem. Phys. 1996, 105, 59495955. (3) (a) Hsu, J.-P.; Tseng, M.-T.; Tseng, S. Chem. Phys. 1999, 242, 69-79. (b) Henderson, D.; Trokhymchuk, A.; Nikolov, A.; Wasan, D. T. Ind. Eng. Chem. Res. 2005, 44, 1175-1180. (c) Gonzalez-Mozuelos, P. J. Chem. Phys. 1993, 98, 5747-5755. (d) Gonzalez-Mozuelos, P.; Alejandre, J.; Medina-Noyola, M. J. Chem. Phys. 1991, 95, 8337-8345. (e) Kim, Y.-W.; Kim, S.-C.; Suh, S.-H. J. Chem. Phys. 1999, 110, 1230-1234. (f) Choudhury, N.; Ghosh, S. K. J. Chem. Phys. 1996, 104, 9563-9568. (g) Trokhymchuk, A.; Henderson, D.; Nikolov, A.; Wasan, D. T. Langmuir 2004, 20, 7036-7044. (4) Murray, C. A.; Grier, D. G. Annu. ReV. Phys. Chem. 1996, 47, 421-462. (5) Israelachvili, J. Intermolecular and Surface Forces; Academic Press: San Diego, CA, 1992; Chapter 13. (6) Horn, R. G.; Israelachvili, J. N. J. Chem. Phys. 1981, 75, 1400-1411.
Figure 1. Schematic example of a structural force profile.
measured force corresponded to the known diameter of the solvent molecules. A similar observation was made by Pashley and Israelachvili7 in pure water. Richetti and Ke´kicheff8 observed oscillatory forces in aqueous solutions of cetyltrimethylammonium bromide, a cationic surfactant, above its critical micelle concentration (cmc). This latter work is perhaps the first observation of a structural force in a charged system. More recently, Piech and Walz9 performed force measurements between silica surfaces in the presence of both silica nanoparticles and a negatively charged polyelectrolyte using an AFM. These authors found that the wavelength of the oscillatory forces, λ, could be related to the bulk number density of nanoparticles, F, via the simple expression
λ ) F-1/3
(1)
suggesting a simple space-filling behavior. (For the polyelectrolyte solutions, this relationship was found to be valid in the dilute (7) Israelachvili, J. N.; Pashley, R. M. Nature 1983, 306, 249-250. (8) Richetti, P.; Kekicheff, P. Phys. ReV. Lett. 1992, 68, 1951-1954. (9) Piech, M.; Walz, J. Y. J. Phys. Chem. B 2004, 108, 9177-9188.
10.1021/la0530485 CCC: $33.50 © 2006 American Chemical Society Published on Web 02/17/2006
Structuring of Macroions between Two Surfaces
Langmuir, Vol. 22, No. 6, 2006 2877
polymer regime; in the semidilute regime, the scaling exponent was minus one-half.) It should be mentioned that, with the nanoparticle system, the highest concentration of silica nanoparticles used in these experiments was 4.5 vol %. McNamee et al.10 also performed force measurements in silica nanoparticle suspensions, as well as in sodium dodecyl sulfate (SDS) micellar solutions, using an AFM. These authors observed oscillatory forces at concentrations above 30 vol % for the silica system and above 30 cmc for the SDS system. This lower concentration limit was presumably controlled by the relatively stiff cantilever used in the experiments (spring constant of 0.68 N/m). On the basis of their results, McNamee et al. concluded that the wavelength of the force profiles was controlled by the effective size of the particles or micelles, Deff, defined as
λ ≈ Deff ) 2(R + κ-1)
(2)
where R is the radius of the particle or the micelle and κ-1 is the Debye length of the bulk solution. The principal goal of the present work is to investigate the wavelength of oscillatory force profiles, as they occur in the presence of a macroion solution. Specifically, we seek to determine whether this wavelength is controlled by the bulk number density, as suggested by the experiments of Piech and Walz,9 or whether the effective size of the macroions is an important factor, as observed by McNamee et al.10. We perform force measurements between a 5 µm silica sphere and a flat silica surface in silica nanoparticle suspensions with a concentration range of 2.5-11 vol %, and in SDS micellar solutions with a concentration range of 2-60 cmc. Because of the very flexible cantilever used (spring constant of 0.006-0.01 N/m), we are able to detect oscillatory force profiles over a much wider range than what was possible with previous studies. We also compare the measured wavelength with the spacing between macroions in the bulk, as predicted through a statistical mechanics approach. Knowledge of the nature of these force oscillations will aid in understanding the ordering of macroions under confinement, as well as the effects of added macroions on colloidal stability. Experimental Section Materials. Water was purified with a NANOPure (Barnstead Thermolyne Corp., Dubuque, IA) that was equipped with a 0.22 µm filter. KNO3 (Aldrich, Milwaukee, WI) was recrystallized twice from HPLC-grade ethanol (Aldrich, Milwaukee, WI) and purified water (90:10). Standardized solutions of 1.0095 N KOH (Aldrich, Milwaukee, WI) and 0.987 N HNO3 (Aldrich, Milwaukee, WI) were used as received to adjust the pH of the experimental solutions. The flat substrate used in the AFM measurements consisted of polished fused silica flats (Melles Griot, Irvine, CA) with a diameter of 12.5 mm and a thickness of 3 mm. The flats had a root-mean-square (rms) roughness of
