(234b) The Adsorption and Reaction of a Titanate Coupling Reagent on the Surfaces of Different Nano-Particles in Supercritical Co2

Nano-particles have many special characteristics and are widely used in the fields of composite materials, biomaterials, sensors, etc., but they also have problems in that they are easily agglomerated because of the strong interactions between the particles. A good dispersion is the key in applications that make use of their unique characteristics. The surfaces of nano-particles can be modified to give better dispersibility, affinity, functionality, etc. A chemical modification of the particle surface can enhance the dispersibility of nano-particles in various continuous phases, change the surface activity of the nano-particles, and bring about new surface physical and chemical characteristics; and can be achieved through a reaction between the hydroxyl groups on the surface and modification reagents.

There are many methods of surface chemical modification, e.g. liquid methods, gaseous methods, mechano-chemical methods, etc. Each method has its own disadvantages. Liquid methods need the modification reagents to be dissolved in solvents, and there are the problems of solvent recovery, long operational procedures, high costs, and severe pollution. Gaseous methods usually operate at a high temperature in order to get the modification reagents to contact the particle surface in molecule form, and the reactors used are usually fluidized beds, fixed beds, agitating beds, etc. Gaseous methods are suitable for particles of micrometer size that do not easily agglomerate. It is difficult to achieve a uniform modification and to treat the exhaust gases that contain modification reagents. Mechano-chemical methods comprise mixing and grinding between the particles and modification reagents. The energy consumption in the process is high and there is the probable contamination by the grinding medium in the product.

Supercritical CO2 is a green solvent that has the characteristics of a high diffusion coefficient like a gas, a high solvating power like a liquid, low viscosity, low surface tension, and rapid osmosis into micro-porous materials. When supercritical CO2 is used as the solvent in a surface chemical modification process, the modification reagent can get into the voids of agglomerated particles so that the modification reagent can contact uniformly with the particle surface and react with hydroxyl groups on the surface. A more uniform modification of the particle surface can be achieved. Particle modification in supercritical CO2 does not lead to "caking" of the particles, and is especially suitable for nano-particles. The CO2 solvent can be separated quickly and outright from the particles by changing the temperature and pressure. There is little or no solvent waste. In our previous research, the organic modification of ultra-fine particles and nano-particles was performed using carbon dioxide as the solvent, and it was found that supercritical and liquid CO2 are green and effective solvents in the organic modification of inorganic particles.

The adsorption and reaction of the titanate coupling reagent, NDZ-201, (CH3)2CHOTi(OP(O)(OH)OP(O)(OC8H17)2)3 [isopropyl, tri(dioctylpyrophosphate) titanate] (for short, CA7), on the surfaces of seven different metal oxide particles: SiO2, Al2O3, ZrO2, Fe2O3, Fe3O4, TiO2 (anatase), and TiO2 (rutile) in supercritical CO2 was investigated. The modified particles were extracted with isopropanol for 72 hours, and then dried at 120 oC for 24 hours to remove CA7 physisorbed on the surfaces of nano-particles, in order to see if any CA7 has chemically reacted on the surface of the nano-particles. In our previous research, it was confirmed that an extraction time of 72 hours is enough to completely remove CA7 physisorbed on the nano-particle surfaces.

The spectra of all non-extracted modified samples show characteristic absorption peaks at 2961, 2933, 2874 and 2862 cm-1, which are the absorption peaks of the CH3- and CH2- groups of the coupling reagent CA7. This indicates that there are CA7 on the particle surfaces. The spectra of extracted modified SiO2, Al2O3, ZrO2, and TiO2 (A) particles still show the absorption peaks of the CH3- and CH2- groups, whereas the spectra of extracted modified Fe2O3, Fe3O4, TiO2 (R) particles do not show the absorption peaks anymore. This is interpreted by that physisorbed CA7 is removed from the surfaces of the particles by extraction, and that any remaining CA7 is evidence that these has chemically reacted and are bonded to the surface of the particles so that they are not removed by extraction. It can be inferred that CA7 on the various particle surfaces are in different forms. There are both physical adsorption and chemical reaction on the surfaces of SiO2, Al2O3, ZrO2, TiO2 (A) particles, whereas there is only physical adsorption on the surfaces of Fe2O3, Fe3O4, TiO2 (R) particles. Thus, the characteristics of the particle surfaces are different: some surfaces can react with CA7 and some cannot, and the reactivities between the particle surface and CA7 are different.

The chemical reaction mechanism of titanate coupling reagents on inorganic surfaces is the following:

MOH + R'O-Ti (Y-R-X-O)3 --- MO-Ti(Y-R-X-O)3 + R'OH (1)

For CA7, equation (1) can be expressed as:

MOH + (CH3)2CHOTi(OP(O)(OH)OP(O)(OC8H17)2)3--- MO-Ti(OP(O)(OH)OP(O)(OC8H17)2)3 + (CH3)2CHOH (2)

There are one isopropoxy and three organic long-chains in the structural formula of CA7. The isopropoxy group can react with protons on the particle surfaces to release isopropanol and the remaining group forms a chemical bond on the particle surface. The protons are from physically adsorbed water and the hydroxyl groups on the particle surfaces.

Under the same conditions, the reaction of CA7 is different on different particle surfaces. It depends on the surface characteristics of the particles. As can be seen from equations (1) and (2), protons on particle surfaces are necessary for the reaction to occur.

The OH groups of TiO2 (anatase) are amphoteric and those of TiO2 (rutile) are basic. This shows that some OH groups of TiO2 (anatase) can provide protons, thus, CA7 can react with the surface of TiO2 (A) particles. But there are no protons on the surface of TiO2 (rutile), thus, there is no reaction of CA7 with the surface of TiO2 (R) particles.

Since SiO2, ZrO2 and Al2O3 particles are also amphoteric, it is deduced that they have some OH groups that can provide protons, similar to TiO2 (anatase), that can react with CA7. Fe3O4 and Fe2O3 particles are basic, similar to TiO2 (rutile), and there are no protons on the surface of Fe3O4 and Fe2O3 particles that can react with CA7.

Therefore, it is concluded that some OH groups of amphoteric (or acidic) particles can provide protons and the titanate coupling reagent can react with these OH groups. On these particles, there are physisorption and chemical reaction on the surfaces. On the other hand, the OH groups of basic particles do not provide protons and the titanate coupling reagent cannot react with these OH groups. On these particle surface, there is only physisorption on the surfaces. The acid-base properties of the OH groups on the particle surfaces is the key that determines whether the surface chemical reaction of the titanate coupling reagent occurs. For basic particles, a chemical reaction of titanate coupling reagents on these particle surfaces can be induced by pre-treating to give surface acidity, such as by pre-coating with SiO2 and Al2O3 films.

The quantity of CA7 on the particle surfaces can be estimated from the weights of samples at 800 oC. The total quantity of CA7 adsorbed on the particle surfaces (mol/m2) are in the order: TiO2 (rutile) ~ Fe3O4 > Fe2O3 ~ SiO2 ~ Al2O3 > ZrO2 > TiO2 (anatase). On the other hand, the quantity of CA7 reacted on particle surfaces (mol/m2) is in the order: Al2O3 > TiO2 (anatase) ~ZrO2 > SiO2 >> TiO2 (rutile) = Fe3O4 = Fe2O3. Since the reaction environments are the same, including the same solvent (supercritical CO2), the same concentration of CA7, the (almost) same volume of particles, the same reaction time (one hour), the same extraction solvent (isopropanol) and time (72 hours), etc., a particle surface reactivity with CA7 can be defined as the molar number of CA7 per unit area. In this study, the surface reactivity of particles with CA7 has the order: Al2O3 > TiO2 (anatase) ~ZrO2 > SiO2 >> TiO2 (rutile) = Fe3O4 = Fe2O3, which is the same as the order of the quantity of CA7 reacted on the particle surfaces (mol/m2).

References

(1) Poliakoff, M.; George, M. W.; Howdle, S. M.; Bagratashvili, V. N.; Han, B. X.; Yan, H. K. Chin. J. Chem. 1999, 17(3), 212.

(2) Leitner, W. Accounts Chem. Res. 2002, 35(9), 746.

(3) Wang, Z. H.; Dong, J. H.; Xu, N. P.; Shi, J. AICHE J. 1997, 43(9), 2359.

(4) Subra, P.; Jestin, P. Powder Technol. 1999, 103(1), 2.

(5) Cooper, A. I. Adv. Mater. 2003, 15(13), 1049.

(6) Tripp, C. P.; Combes, J. R. Langmuir 1998, 14 (26), 7350.

(7) Combes, J. R.; White, L. D.; Tripp, C. P. Langmuir 1999, 15(22), 7870.

(8) Jia, X. Q.; McCarthy, T. J. Langmuir 2002, 18(3), 683.

(9) Wang, Z. W.; Wang, T. J.; Wang, Z. W.; Jin, Y. Powder Technol. 2004, 139(2), 148.

(10) Wang, Z. W.; Wang, T. J.; Wang, Z. W.; Jin, Y. J. Supercrit. Fluids. 2005, in press.

(11)Primet, M.; Pichat, P.; Mathieu, M. V. J. Phys. Chem. 1971, 75(9), 1221.