pubs.acs.org/Langmuir © 2009 American Chemical Society
Temperature-Dependent Colloidal Stability of Hydrophobic Nanoparticles Caused by Surfactant Adsorption/Desorption and Depletion Flocculation Thomas Dederichs, Martin M€oller, and Oliver Weichold* DWI an der RWTH Aachen e.V. und Institut f€ ur Technische und Makromolekulare Chemie der RWTH Aachen, Pauwelsstr. 8, D-52056 Aachen, Germany Received April 6, 2009. Revised Manuscript Received June 2, 2009 Nanoparticles coated with self-assembled dodecyltrimethylammonium bromide shells are shown to undergo colloidal destabilization at higher temperatures. This is caused by two different mechanisms depending on the surfactant concentration. Up to a surfactant concentration of 55 mM, the surfactant micelles dissolve before the breakdown of the dispersion. In this case, the breakdown is triggered by desorption of surfactant molecules from the particle surface causing flocculation via hydrophobic interactions. Since the surfactant concentration influences the adsorptiondesorption equilibrium, the breakdown temperature increases with increasing surfactant concentration from approximately 100 to 160 °C. Beyond 55 mM, surfactant micelles are still present when the dispersion breaks down and destabilization is caused by high temperature depletion flocculation. Since higher surfactant concentrations result in a larger number of micelles in solution, the breakdown temperature for concentrations above 55 mM decreases with increasing surfactant concentration.
Introduction Core-shell nanoparticles are clearly structured assemblies with distinct concentric nanometer-sized domains of different chemistry. These arrangements open up the possibility for new, tailormade properties, and thus, core-shell nanoparticles have seen increasing interest over the past decade.1-3 Besides protection of the core, alteration of the particle charge or polarity, and enhancement of surface reactivity, interest in such structures arises from the fact that the core-shell architecture is intimately connected with functionality. Especially organic shells are indispensable parts of stimuli-responsive, so-called “smart” materials. The stimuli can be light,4 heat,5-7 and/or properties of the dispersing medium such as pH,8,9 solvent polarity,10 or ionic strength.11 This renders these particles potentially interesting for biomedical and industrial applications.12 Often, the response to a stimulus causes a change in the supramolecular structure of the shell, which can then lead to a change in the surface polarity of the assembly. Consequently, when applied to dispersions, such stimuli directly affect the colloidal stability.4,6-9 A sudden stability alteration results in a change of the sample‘s appearance from transparent to turbid, and the sample can act like an optical switch. *Corresponding author. Telephone: þ49 241 8023307. Fax: þ49 241 8023301. E-mail:
[email protected].
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Among the core-shell nanoparticles, those with self-assembled surfactant mono- and bilayer shells are of particular interest, due to the increased dispersion stability of such particles in aqueous media.13-18 The mono- or bilayer shells can be assembled either during synthesis of the particles or in a subsequent modification step. Yet, the properties and the stability of these self-assembled surfactant structures on particle surfaces have not been fully explored. For a particular nanoparticle coated with a surfactant mono- or bilayer, the room temperature stability of the selfassembled shell depends on the surfactant concentration19-25 and low concentrations lead to colloidal instability. On the other hand, the stability of self-assembled structures, for example, micelles, depends among other factors on the temperature. In particular, the critical micelle concentration (cmc) of ionic surfactants increases significantly with temperature.26-28 The question thus arises, how the thermal behavior of ionic surfactants translates to nanoparticle dispersions stabilized by them. For hydrophobic silica particles coated with nonionic surfactants, a loss of the dispersion stability in aqueous media is (13) Shen, L.; Laibinis, P. E.; Hatton, T. A. Langmuir 1999, 15, 447–453. (14) Nikoobakht, B.; El-Sayed, M. A. Langmuir 2001, 17, 6368–6374. (15) Jana, N. R.; Gearheart, L.; Murphy, C. J. Langmuir 2001, 17, 6782–6786. (16) Pileni, M.-P. Nat. Mater. 2003, 2, 145–150. (17) Swami, A.; Kumar, A.; Sastry, M. Langmuir 2003, 19, 1168–1172. (18) Fan, H.; Yang, K.; Boye, D. M.; Sigmon, T.; Malloy, K. J.; Xu, H.; Lopez, G. P. Science 2004, 304, 567–571. (19) Bremmell, K. E.; Jameson, G. J.; Biggs, S. Colloids Surf., A 1999, 146, 75– 87. (20) Koopal, L. K.; Goloub, T.; de Keizer, A.; Sidorova, M. P. Colloids Surf., A 1999, 151, 15–25. (21) Adler, J. J.; Singh, P. K.; Patist, A.; Rabinovich, Y. I.; Shah, D. O.; Moudgil, B. M. Langmuir 2000, 16, 7255–7262. (22) Weichold, O.; Dederichs, T.; M€oller, M. J. Colloid Interface Sci. 2007, 306, 300–306. (23) Krishnakumar, S.; Somasundaran, P. Colloids Surf., A 1996, 117, 227–233. (24) Kawasaki, H.; Ban, K.; Maeda, H. J. Phys. Chem. B 2004, 108, 16746– 16752. (25) Dederichs, T.; M€oller, M.; Weichold, O. Langmuir 2009, 25, 2007–2012. (26) Goddard, E. D.; Benson, G. C. Can. J. Chem. 1957, 35, 986–991. (27) Evans, D. F.; Wightman, P. J. J. Colloid Interface Sci. 1982, 86, 515–524. (28) Evans, D. F.; Allen, M.; Ninham, B. W.; Fouda, A. J. Solution Chem. 1984, 13, 87–101.
Published on Web 07/02/2009
DOI: 10.1021/la901216g
10501
Article
Dederichs et al.
observed upon increasing temperature. This is a result of the lower critical solution temperature of the ethylene oxide block, which becomes insoluble at higher temperatures and results in sedimentation of the particles.6 However, the self-assembled surfactant shell remains intact. We have recently reported on the dispersion stability of hydrophobic boehmite nanoparticles with ionic surfactant shells.25 In aqueous solution at room temperature, two mechanisms were found leading to colloidal instability: hydrophobic flocculation at low surfactant concentrations and depletion flocculation at high concentrations. Herein, we use this particle-surfactant system to study colloidal stabilities at high temperature. We present a conceptually new approach to affect the colloidal stability using thermoresponsive core-shell nanoparticles. Our concept is based on the thermal disintegration of the selfassembled, ionic surfactant shell, which is held together by weak hydrophobic interactions. It is shown that besides thermal disintegration of the surfactant shell depletion effects are also found at high temperature.
Materials and Methods Materials. Disperal OS225 is a hydrophobic, nanocrystalline boehmite with branched alkylbenzenesulfonic acid molecules (length of alkyl chain: 9-12) on the surface of the inorganic particle and a specific surface area of 256 m2/g (BET after 3 h at 600 C).29 This nanocrystalline boehmite can be easily dispersed in aqueous cationic surfactant solution. Dynamic light scattering shows at the cmc of a dodecyltrimethylammonium bromide solution a monomodal particle size distribution with an average particle size of 50 nm.25 Dodecyltrimethylammonium bromide (C12TAB, g98%) was purchased from AppliChem and used as received. Methods. The dry particles were dispersed at a particle concentration of 1.0 g/L in previously prepared aqueous dodecyltrimethylammonium bromide solutions (with total surfactant concentrations of cC12TAB) by stirring overnight at room temperature. Bluish opalescent to whitish opaque dispersions were obtained. In order to test the colloidal stability at elevated temperatures, 10 mL of the sample was heated with a rate of 1 K/min in a sealed, thermocontrolled stainless steel vessel while stirring. In order to measure the transmission through the sample, a white superbright LED and a light-intensity sensor (TSL250R, Texas Advanced Optoelectronic Solutions Inc.) were used. One thermoelement type K was attached to the body of the stainless steel vessel in order to control the heating process. A second thermoelement was in direct contact with the sample solution to record the temperature of the dispersion and the transmission simultaneously. Additionally, the optical appearance of the samples, such as the presence of a precipitate, was checked visually during the experiments. Dynamic light scattering was recorded on a Malvern Zetasizer Nano ZS instrument. The particle-size distribution reflects the scattering intensity. Critical micelle concentrations in aqueous solution were determined by measuring the electrical conductivity using a Mettler Toledo “Sevenmulti” conductometer. The conductivity is plotted against the surfactant concentration and fitted with two straight lines whose intersection point is assigned as the cmc.30
Results and Discussion The hydrophobic boehmite nanoparticles are easily dispersed in C12TAB solutions, since the surfactant molecules adsorb tailon on the particle surface. This results in the formation of a (29) Product information data sheet, Sasol GmbH Germany. (30) Nesmerak, K.; Nemcova, I. Anal. Lett. 2006, 39, 1023–1040.
10502 DOI: 10.1021/la901216g
charged, self-assembled shell (ideally a surfactant monolayer) and provides the required dispersion stability in aqueous media at room temperature. Transmission Measurements at High Temperature. The colloidal stability of the prepared dispersions during heating was monitored by turbidity measurements. The measured absolute transmission values depend on a number of factors and, thus, vary slightly from one experiment to the next. In order to compare the temperature-dependent behavior of dispersions, a normalized transmission was calculated as In = I(T)/Imax, with Imax being the highest transmission value in the particular measurement, and plotted against the temperature (Figure 1). Figure 1A shows a typical curve together with the characteristic temperatures that can be extracted from it. Tonset and Tendset can be calculated from all experimental curves, while Tbend occurs only in a certain concentration interval. All these temperatures are determined as the intersection of straight lines fitted by linear regression to the transmission values before and after the transmission decay. All samples under investigation show a decrease in transmission to In
