pubs.acs.org/Langmuir © 2010 American Chemical Society
Isomerism and Solubility of Benzene Mono- and Dicarboxylic Acid: Its Effect on Alumina Dispersions E-Jen Teh,†,‡ Yee-Kwong Leong,*,†,‡ and Yinong Liu‡ †
Centre for Strategic Nanofabrication and ‡School of Mechanical Engineering, The University of Western Australia, Crawley WA 6009, Australia Received November 17, 2009
Low molecular weight benzenedicarboxylic acid has a very well-defined molecular structure because of its rigid and planar backbone. Therefore, it is hypothesized to have high potential for highly directed bridging between surfaces. However, phthalic acid cannot participate in particle bridging because the two carboxylic acid groups on the benzene ring are located adjacent to each other which prevent the molecule from bridging between two surfaces. Yield stress measurements showed that isophthalic and terephthalic acid failed to cause significant rheological changes to alumina slurries within their solubility limit. However, upon increasing the concentration beyond the solubility limit, terephthalic acid increased the yield stress by a factor of 7 and isophthalic acid by a factor of 2 when compared to the same colloidal alumina system without additive. Benzoic acid, which has low solubility at low pH, also showed an increase in yield stress by a factor of 2 even though it lacks the second carboxylic group to link neighboring surfaces. These observations suggest that highly directed bridging is unlikely to operate when these acids are present in high concentration. Instead, the dominant mechanism is most likely attraction between the negatively charged precipitates and the positively charged alumina particles and/or capillary bridging.
Introduction Aluminum oxide (Al2O3) has many industrial and commercial applications. It is used extensively in paints,1 catalysts,2 refractories,3 ceramic components,4,5 and various colloidal processes.6-8 In order to control and ultimately achieve the desired rheological behaviors in alumina dispersions, a wide range of chemical additives can be added to the slurries at different stages of processing, depending on their functionalities. Polyelectrolytes such as poly(acrylic acid) (PAA) are commonly used in colloidal processing industries.9-11 At low molecular weight, PAA gave rise to a steric repulsive interaction which is gradually taken over by an attractive mechanism at high molecular weight.9,11 This attractive mechanism is attributed to bridging. When the surfaces of colloid particles are partially covered with chemical additives of a polymeric nature such as PAA,11,12 the adsorption of segments of the individual polymeric molecules onto the surface of more than one particle gives rise to a linking mechanism. This linking mechanism defined as bridging is often responsible in the formation of a network structure which usually flocculate and sediment rapidly. Partial surface coverage is critical for bridging as it ensures the “free” segments of the polymeric molecules to have ample bare surfaces *Corresponding author. E-mail:
[email protected]. (1) Morris, G. E.; Skinner, W. A.; Self, P. G.; Smart, R. S. C. Colloids Surf., A 1999, 155, 27–41. (2) Knozinger, H.; Ratnasamy, P. Catal. Rev.;Sci. Eng. 1978, 17, 31–70. (3) Ghassemi Kakroudi, M.; Huger, M.; Gault, C.; Chotard, T. J. Eur. Ceram. Soc. 2009, 29, 2211–2218. (4) Sigmund, W. M.; Bell, N. S.; Bergstrom, L. J. Am. Ceram. Soc. 2000, 83, 1557–1574. (5) Franks, G. V.; Gan, Y. J. Am. Ceram. Soc. 2007, 90, 3373–3388. (6) Johnson, S. B.; Franks, G. V.; Scales, P. J.; Boger, D. V.; Healy, T. W. Int. J. Miner. Process. 2000, 58, 267–304. (7) Lange, F. F. J. Am. Ceram. Soc. 1989, 72, 3–15. (8) Lewis, J. A. J. Am. Ceram. Soc. 2000, 83, 2341–2359. (9) Das, K. K.; Somasundaran, P. Colloids Surf., A 2003, 223, 17–25. (10) Cesarano, J.; Aksay, I. A. J. Am. Ceram. Soc. 1988, 71, 1062–1067. (11) Leong, Y. K.; Scales, P. J.; Healy, T. W.; Boger, D. V. Colloids Surf., A 1995, 95, 43–52. (12) Biggs, S. Langmuir 1995, 11, 156–162.
Langmuir 2011, 27(1), 49–58
on other particles to “anchor” themselves onto during the random Brownian collision. Low molecular weight (LMW) charged molecules can also exhibit particle bridging, known as highly directed bridging.13 This is where electrostatic attraction between charged functional groups on the LMW molecules and the protonated oxide surface group on the colloids form molecular links between particles which results in the formation of “bridges” in the network structure and thus causes an increase in the interparticle strength.14 As LMW molecules are usually small, it is possible to select molecules with a well-defined structure such as backbone rigidity, positions of functional groups, and also limited conformational structures.13 As the study of bridging by polyelectrolytes such as PAA is made complicated by the fact that it is difficult to ensure definitively the number of polymer segments per molecule that are adsorbed on each surface and those that take part in the actual bridging mechanism,13 LMW charged molecules serve as a model system to further understand the bridging flocculation in complex macromolecules system. Of late, there is a growing number of studies revealing the molecular attributes necessary to enhance highly directed bridging by LMW molecules, specifically LMW carboxylic acid.13,15,16 Leong15 studied the effect of molecular configuration of adsorbed cis- and trans-1,2-ethylenedicarboxylic acids on the rheology of alumina and found that while its cis counterpart weakens the flocculated structure of the R-Al2O3 dispersion by forming a steric layer on the particles, trans-1,2-ethylenedicarboxylic acid enhances the strength of the flocculated structure of the R-Al2O3 dispersion. This was attributed to highly directed bridging. However, the various molecular factors that control how molecules with charged functional groups participate in (13) (14) (15) (16)
Leong, Y. K. Phys. Chem. Chem. Phys. 2007, 9, 5608–5618. Leong, Y. K. J. Chem. Soc., Faraday Trans. 1997, 93, 105–109. Leong, Y. K. Langmuir 2002, 18, 2448–2449. Leong, Y. K. Colloids Surf., A 2008, 325, 127–131.
Published on Web 12/08/2010
DOI: 10.1021/la1045783
49
Article
Teh et al.
highly directed bridging is not fully explored, but preliminary work by Leong13,15,16 shows that molecular structure plays a major role in determining the bridging quality to maximize the strength of the particle-particle bond and the network. Cannon and Warner17 studied titanium dioxide (TiO2) dispersions in the presence of benzenecarboxylic acids and found that with increasing weight percent of additive, benzene-1,4-dicarboxylic acid (terephthalic acid) showed the highest increase in viscosity compared to benzene-1,3-dicarboxylic acid (isophthalic acid) and benzene-1,2-dicarboxylic acid (phthalic acid) while benzoic acid showed negligible changes. They reasoned that the difference in the molecular structure of the adsorbed acids is the cause of the different rheological behavior of the TiO2 dispersions. They deduced that terephthalic acid displayed the strongest “highly directed bridging” behavior due to its second carboxylic acid group being positioned at an angle of 180° from each other, therefore rendering it the most available for interparticle interactions. Benzoic acid, lacking the second carboxylic acid, failed to link between two surfaces, resulting in negligible changes in viscosity. However, in this study, the pH at each acid concentration was not clearly specified. The pH dependence of TiO2 colloidal dispersions in the presence of benzenecarboxylic acids was investigated by Johnson et al.18 They found that with increasing pH past the second pKa, terephthalic acid increased the viscosity dramatically, and this was again attributed to highly directed bridging. Johnson et al.18 also used infrared spectroscopy measurements to determine the various adsorption modes of benzenecarboxylic acid used in their study. However, in order to relate the adsorption modes of these acids to their rheological behavior, they must remain soluble at the conditions employed. Up to date, the effect of solubility of adsorbed molecules on the interparticle interaction has not been widely studied. This is partly because the additive molecules studied so far have high solubility9,11,15 or remain soluble within the experimental conditions employed.19 Nevertheless, not all additive molecules have high solubility. In the event of precipitation of the additives, i.e., molecules coming out of the solution, the interparticle interaction between colloidal surfaces is not fully known. Thus, this current study attempts to fill this knowledge gap in addition to exploring the effect of functional groups, isomer, and concentration of LMW benzenecarboxylic acids when adsorbed on alumina/water interface.
All dispersions used in the zeta potential and rheology studies were prepared via ultrasonification using a Branson digital Sonifier (20 kHz, 600 W). The zeta potential of the alumina suspensions was measured via electroacoustic method using a ZetaProbe instrument (Colloidal Dynamics, USA). The alumina slurries for zeta potential measurements had an initial solid loading of 1.48 vol % (5.2 wt % solid). All slurries for rheology measurements were prepared in batches of ∼100 g. An appropriate amount of additives was dissolved with purified water (conductivity
