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Langmuir 2008, 24, 12858-12866
Viscoelastic and Shear Viscosity Studies of Colloidal Silica Particles Dispersed in Monoethylene Glycol (MEG), Diethylene Glycol (DEG), and Dodecane Stabilized by Dodecyl Hexaethylene Glycol Monoether (C12E6) Justice M. Thwala,*,† Jim W. Goodwin,‡ and Paul D. Mills§ UniVersity of Swaziland, PriVate Bag 4, Kwaluseni M201, Swaziland, Southern Africa, Department of Physical and Colloid Chemistry, Bristol UniVersity, BS8 1TS Bristol, England, and ICI Films, P.O. Box 90 Wilton Centre, Middlesbrough, CleVeland TS6 8JE, England ReceiVed August 16, 2008. ReVised Manuscript ReceiVed September 11, 2008 Silica dispersions stabilized by a nonionic surfactant, dodecyl hexaethylene glycol monoether (C12E6), were studied using rheological measurements. The viscosity-shear rate flow behavior of silica in monoethylene glycol (MEG) is shear thinning at low shear rates, leading to a Newtonian plateau at high shear rates for all dispersions studied. All rheological properties showed an increase above a critical surfactant concentration. The dispersions were stable at low levels of C12E6 concentrations because of electrostatic repulsions as deduced from the zeta potentials of silica that were on the order of about -30 to -65 mV in monoethylene glycol (MEG). Instability on further addition of C12E6 to the silica particles, a phenomenon normally obtained with high-molecular-weight polymers, was observed in MEG. Viscoelatic measurements of silica in monoethylene glycol at various surfactant concentrations showed a predominantly viscous response at low frequency and a predominantly elastic response at high frequencies, indicative of weak flocculation. Instability is explained in terms of hydrophobic and bridging interactions. Restabilization observed at high surfactant concentration was due to the steric repulsion of ethoxy groups of micellar aggregates adsorbed on silica particles. The study also revealed that the presence of trace water introduced charge repulsion that moderated rheological measurements in glycol media and introduced the charge reversal of silica particles in dodecane.
1. Introduction It is well known that concentrated dispersions are encountered in many industrial applications, such as paints, dyestuffs, and pharmaceutical and pesticide formulations.1 A number of dispersants have always been used in industry, but the role and mechanism of their stabilization has remained subtle. Particles (fillers) are also added to these dispersions to improve the handling properties of the final product. The production processes, however, is interrupted by the production of side reactants such as water, methanol, or acids resulting in poor final products. Studies in these systems are not popular for two reasons, namely, the low dielectric constants and the high viscosities make them difficult to study using conventional methods. Steric stabilization in these systems may be obtained by using polymers that are either physically or chemically bound to the surfaces of the particles. When the sterically stabilized particles approach each other, strong repulsion occurs as a result of the steric interaction between the adsorbed layers.2 Such interactions are easily reflected in the flow characteristics (rheology) of these dispersions. In previous studies, the rheological investigation of a number of systems utilized rheological parameters such as the Bingham yield value, σB, plastic viscosity, ηpl, complex modulus, * Author to whom correspondence should be addressed. Tel: +2686036616. Fax: 2685285276. E-mail:
[email protected]. † University of Swaziland. ‡ Bristol University. § ICI Films.
(1) Hughes, D. F. K.; Robb, I. D. Langmuir 1999, 15, 8795–8799. (2) Tadros, Th. F.; Taylor, P.; Bognolo, G. Langmuir 1995, 11, 4678–4684. (3) Nestor, J.; Obiols-Rabasa, M.; Esquena, J. C.; Solans, B.; Levecke; Booten, K.; Tadros, Th. F. J. Colloid Interface Sci. 2008, 319, 152–159. (4) Chaari, K.; Bouaziz, J.; Bouzouita, K. J. Colloid Interface Sci. 2005, 285, 469–475. (5) Goodwin, J. W.; Smith, W. Faraday Discuss. Chem. Soc. 1974, 57, 126. (6) Nasu, A.; Otsubo, Y. J. Colloid Interface Sci. 2007, 310, 617–623.
G*, storage modulus, G′, loss modulus, G′′, and high-frequency rigidity modulus, G∞.3-10 This work was therefore undertaken to study the dispersion properties of silica in monoethylene glycol (MEG), diethylene glycol (DEG), and dodecane. Monoethylene glycol and diethylene glycol are intermediate-polarity solvents with dielectric constants, εr, within the range of 30-40; dodecane is a low-polarity organic solvent whose dielectric constants, εr, is approximately 2.11 Most studies of steric stabilization utilize long-chain polymer formation as a means of stabilization12-16 whereas in the present study we used a short-chain nonionic surfactant, dodecyl hexaethylene glycol monoether,C12E6. Dodecyl hexaethylene glycol monoether, C12E6, being a glycol, is normally used during the production of polyethylene terephthalate polymer products. It was therefore found to be suitable for the purpose of this study. Rheology and electrophoresis were then used to study stability using flow and electrokinetic properties of silica particles in glycols, namely, MEG and DEG and low-polarity media typified by dodecane. Results presented in this study may lead to applications in plastic and paint manufacturing. The data described in this article will lead to an understanding of some of the salient features of (7) Seyssiecq, I.; Marrot, B.; Djerroud, D; Roche, N. Chem. Eng. J. 2008, 142, 40–47. (8) Mikulasek, P.; Wakeman, R. J.; Marchant, J. Q. Chem. Eng. J. 1997, 67, 97–102. (9) Starck, P.; Mosse, W. K. J.; Nicholas, N. J.; Spiniello, M.; Tyrell, J.; Nelson, A.; Qiaco, G. G.; Ducker, W. A. Langmuir 2007, 23, 7587–7593. (10) Liang, W.; Bognolo, G.; Tadros, Th. F. Langmuir 2000, 16, 1306–1310. (11) Lide, D. R., Ed.; CRC Handbook of Chemistry and Physics, 84th ed.; CRC Press LLC: Boca Raton, FL, 2003; pp 6-155. (12) Kogan, M.; Dibble, C. J.; Rogers, R. E.; Solomon, M. J. J. Colloid Interface Sci. 2008, 318, 252–263. (13) Otsubo, Y. Langmuir 1994, 10, 1018–1022. (14) Zingg, A.; Winnefeld, F.; Holzer, L.; Pakusch, J.; Becker, S.; Gauckler, L. J. Colloid Interface Sci. 2008, 323, 301–312. (15) Otsubo, Y. Langmuir 1999, 15, 1960. (16) Liang, W.; Bognolo, G.; Tadros, Th. F. Langmuir 1995, 11, 2899–2904.
10.1021/la8026754 CCC: $40.75 2008 American Chemical Society Published on Web 10/14/2008
Studies on Dispersed Colloidal Silica Particles
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dispersion stability in nonaqueous systems. The study also shows the effect of the addition of small quantities of soluble species of varying acid/base behavior such as water and terephthalic acid on the zeta potential and rheological properties of silica particles.
2. Experimental Section 2.1. Materials. The solvents used were analar-grade monoethylene glycol, analar-grade diethylene glycol, BDH-grade acetone, analar-grade ethanol, BDH-grade dodecane, and purite water (conductivity of 3 × 10-6 Ω-1 m-1). The nonionic surfactant dodecyl hexaethylenedodecyl glycol monoether, C12E6, was obtained from Nikko Chemicals Co. Ltd., Tokyo, Japan. 2.2. Preparation of Silica Particles. Silica particles were prepared using the method by Stober et al.,17 followed by several cycles of centrifugation at speeds of G′) until the crossover points (until G′ > G′′). At low volume fractions (φ < 0.3), the loss modulus, G′′, is greater than the elastic modulus, G′. The dispersion is predominantly viscous. At high volume fraction (φ > 0.3), the loss modulus, G′′, is less than the elastic modulus, G′. The dispersion is elastic. The exact volume fractions at which the dispersion changes from viscous to elastic are clearly shown by plotting G′′/G′ () tan δ) at various volume fractions, as shown in Figure 11b. The crossover point for MEG occurs
(24)
where K is the proportionality constant. The value of m was found to be 26 for MEG. The gel point is the point where an interconnecting network of particles from clusters of particles occurs. The law scaling fit for the oscillation-volume fraction data for MEG was found to be of the form
G′ ) 2.3 × 107φ26; R2 ) 0.99
(25)
The constant of proportionality is a function of particleparticle interactions. According to the Smulochowski theory of gelation,17 the values of m being an integer means that the gel is not a uniform network of particles but a nonuniform continuous network of particles with a fractal structure. Figure 12a,b shows the variation of G′ and G′′ (at ω ) 10 Hz) with volume fraction for silica particles in monoethylene glycol (MEG). The scaling relationship obtained for dodecane is
G′ ) 11.24 × 104φ7; R2 ) 0.92
(26)
The value of m was found to be 7 for dodecane, and the crossover point is 0.57 for dodecane. From these data, it can be
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seen that the tendency of the silica particles to gel is much greater in dodecane than in MEG as deduced from the values of the integer m. 3.5.2. Viscoelasticity as a Function of Surfactant Concentration. Figure 13a,b) shows the variation of G*, G′, and G′′ versus ω (Hz) for a volume fraction of 0.46. The experiment represented by Figure 13a was run in the absence of the C12E6 surfactant. The experiment represented by Figure 13b was run in the presence of the C12E6 surfactant at a concentration of 3.6%. The results were obtained in the linear viscoelastic region, 30-150 mγ, for both surfactant concentrations. It can be seen that G*, G′′, and G′ increase with frequency, whereas η′ decreases as expected (note the logarithmic scale of ω). This is true for all of the C12E6 concentrations studied, except of course the fact that all viscoelastic parameters increase with increasing C12E6 above the critical flocculation C12E6 concentrations (1.3% v/v for φ ) 0.35 and 0.6% for φ ) 0.46) reflecting an increase in flocculation. It is well known that the viscoelastic function (G′′, G′) of ordinary flocculated suspensions shows a plateau at low frequencies.15The plateau is explained in terms of relaxation processes due to the network structure of the particles. However, this plateau region was not observed for all suspensions irrespective of surfactant concentration. The lack of the plateau in the frequency-dependent plots indicates the relatively weak interactions between particles with short relaxations times due to the short chains and low molecular weight of the surfactant used.
4. Conclusions Studies have been conducted on the rheological behavior of silica particles in glycol as a function of volume fraction and surfactant, C12E6, concentration. The following conclusions can be drawn from the ongoing study: • The viscosity-shear rate flow behavior of silica in monoethylene glycol is shear thinning at low shear rates, leading to a Newtonian plateau at high shear rates for all dispersions studied.
Thwala et al.
• Dispersions are stable at low C12E6 concentrations with predominantly viscous flow. The silica dispersions, however, flocculate weakly at high C12E6 concentrations. The stability of silica particles in glycols is determined by electric effects at low C12E6 concentrations. The increase in the ηr, σβ, and G∞ rheological parameters with C12E6 concentration indicates that at the onset of flocculation the dispersions show viscoelasticity. On the basis of rheology and zeta potential instability at moderate C12E6 concentration, is suggested to be due to bridging and hydrophobic interactions. Restabilization at high C12E6 concentration is due to steric repulsions of ethoxy groups of the adsorbed micellar aggregates. • Viscoelatic measurements of silica in monoethylene glycol at various surfactant concentrations showed a predominantly viscous response at low frequency and a predominantly elastic response at high frequencies indicative of weak flocculation. • The critical flocculation concentration, φfloc, varied with the volume fraction of the dispersions in MEG and depends on the free energy of flocculation. • The results shows that the rheology (G∞) of concentrated sterically stabilized silica particles in nonaqueous systems with added water is moderated by the total interaction potential, VT, and is greatly affected by the repulsive term, VR. Acknowledgment. We extend our thanks to Dr. Jim Goodwin’s group for the assistance they gave us in the running of the Bohlin constant stress viscometer and the shear wave rheometer for the rheological measurements. We also thank technicians John Dimery and Les for their help in the production of the transmission electron micrographs and in the running of the Penkem and Matec for the electrophoretic studies. Lastly, we extend our thanks to Imperical Chemical Industries Plc (ICI, Wilton, U.K.) for providing the samples, supervision, and financing (to P.D.M.) throughout this work. LA8026754
