Surface Organic Modification of TiO 2 Powder and Relevant Characterization

Surface organic modification was conducted to TiO 2 with modifiers to improve the dispersity and comparability of pigment TiO 2 in application system by adjusting particle surface characteristics. Then, modification effects were characterized according to the changes in wetting contact angle and activation index of TiO 2 before and after modification. Moreover, the modification mechanisms of sodium stearate and sodium oleate were studied by analyzing the characteristics of TiO 2 surface functional groups inmodification system and effects ofmodifiers.The results showed that, after beingwet-processedwith sodium stearate and sodium oleate, TiO 2 could turn from surface hydrophilic to inductive hydrophobic with controllable degree. The wetting contact angle of modified TiO 2 increased from 7 to 125.6 and 121.3, respectively. The dispersity of TiO 2 in organic medium was stronger than that in inorganic medium. The modifiers formed absorption with chemical property on TiO 2 particle surface, so the inductive hydrophobicity of surface was stable.


Introduction
Titanium dioxide (TiO 2 ) is metallic oxide with excellent physical and chemical properties.Its particle in submicron scale (0.2-0.3 m) is the component of the best white pigment; nanometer TiO 2 (less than 100 nm) is excellent photocatalytic material.Therefore, TiO 2 draws general attention.With properties of high refractive index, no toxicity, strong stability, high brightness, high covering power, and so forth, TiO 2 has been widely used in fields of coating, plastic, papermaking, printing ink, chemical fiber, and so forth [1][2][3][4][5][6][7].As the photocatalyst, nanometer TiO 2 is applied in fields of water and air purification [8][9][10][11], breaking down procedures of organic pollutant degradation [12,13], medical waste treatment [14,15], and so forth.Actually, above properties and their roles are dependent on the control over the purity, crystal form, and size distribution of TiO 2 powder in synthesis and preparation.Moreover, the postprocessing is also important in optimizing the surface morphology and compatibility with application system.Therefore, the processing for TiO 2 powder is necessary when improving the application performance.
Micro-nano TiO 2 is poor in compatibility with high polymer material because of small grain size, high surface energy, and, especially, strong hydrophilia for surface hydroxylation.Actually, the bad dispersity and agglomeration of TiO 2 in organic system impede its pigment performance or photocatalytic performance.However, this problem can be solved by improving the dispersity of TiO 2 in application system.In the way of particle surface modification, the surface wettability can be adjusted and the morphology of surface functional groups be changed.Guosheng et al. conducted surface organic modification to nano-TiO 2 particle with silane coupling agent.The results show that TiO 2 turns from surface hydrophilic to surface hydrophobic [16].In addition, TiO 2 particle was also modified with sodium stearate and polyethylene glycol, lauryl sodium sulfate and triethanolamine, and sodium hexametaphosphate by Gang et al. [17], Xihai et al. [18], Hui [19], Ding et al. [20][21][22], and Weng et al. [23], respectively.All of these studies show that the dispersity of TiO 2 improves in corresponding medium.This work also indicates that the application effects of modifiers with carboxylic in structure are more ideal among above modifiers [24][25][26].Except for surface modifier, it is also important to make preassessment for TiO 2 modification effects.At present, the surface modification of powder is mainly evaluated by measurement with contact angle [27,28] and activation index [29].However, the modification effects and stability of changed surface wettability cannot be evaluated.Moreover, the modification mechanism cannot be reflected with the interaction between modifier and TiO 2 particle.This work conducted surface modification to TiO 2 particle in micrometer scale by wet-modification with modifiers, sodium stearate, and sodium oleate.Suspension was prepared with TiO 2 powder and water in the weight ratio of 1 : 1 and heated to and fixed at 65 ∘ C.Then, appropriate dosage of modifier was added to adjust pH value of modifier equal to or smaller than 7.The modified suspension was obtained after 30 minutes of heat preservation and constant stirring.After drying and dispersing the modified suspension, modified TiO 2 powder can be obtained.Then, the modification effects of TiO 2 were characterized according to the changes of wetting contact angle and activation index in different liquid mediums before and after modification.Finally, the modification mechanisms of TiO 2 were studied by testing the infrared spectrums and analyzing the morphology of surface functional groups before and after modification with sodium stearate and sodium oleate.

Experiments
2.1.Raw Materials and Reagent.TiO 2 used in tests was anatase-type titanium oxide powder manufactured by Henan Billions Chemicals Co., Ltd.In sample test, the particle size was median diameter ( 50 ) of 0.37 m; specific surface area was 6.125 m 2 /g −1 ; whiteness was 98.80%; covering power was 12.99 g/m 2 ; oil absorption was 24.40 g/100 g; and waterwetting contact angle was 7 ∘ .
The organic modifiers in test, sodium stearate and sodium oleate, were all chemically pure reagents provided by Beijing JingweiHuabo Co., Ltd.The reagents should be disposed as 5% water solution by heating.

Process of TiO 2 Organic
Modification.Surface modification for TiO 2 was conducted by water suspension wet surface modification.Suspension was prepared with TiO 2 powder and water in weight ratio of 1 : 1 (solid content 50%) and heated to constant temperature (65 ∘ C).Then, the modifier solution was added to TiO 2 water suspension by ratio to adjust the pH value equal to or less than 7. Modified suspension was obtained after thermal insulation and constant stirring for 30 minutes.TiO 2 powder can be obtained after drying and dispersing the modified suspension.

Performance and Structural Characterization
2.3.1.Wetting Contact Angle.Contact angle () refers to the included angle between gas-liquid surface and solid-liquid surface (including liquid) in the junction of three phases, gas, liquid, and solid.As the physical quantity quantitatively reflecting the wetting degree on solid surface, contact angle can represent the change in surface characteristics of TiO 2 before and after modification.Therefore, contact angle can be used to determine the modification effects.When measuring the angle, the sample to be tested should be pressed as the solid slice with smooth surface.Then, contact angle gauge (JC2000D, Shanghai powereach digital technology equipment Co. Ltd.) should be used to measure and take photo of wetting contact angle.

Activation Index.
Activation index refers to the percentage of powder floating on the water-gas interface accounting for the total volume after mixing the modified powder in water.Particles with surface hydrophobicity may float on the interface in the comprehensive action of adhesive force, gravity, buoyancy, water pressure, and so forth in moist periphery of three phases.The stronger the surface hydrophobicity of particle is, the greater the adhesive force in moist periphery and the larger the floating trend will be.Therefore, the floating powder can intuitively reflect the surface modification effects.
2 g modified sample was correctly weighed and placed in pear-shaped funnel with 200 mL water.After oscillation and stewing, the sedimentation at bottom in hierarchy was introduced to beaker for drying and weighing.The activation index of sample  could be calculated by following formula:

Infrared Spectroscopy.
Infrared spectroscopy analysis can be used to detect various keys in molecule, structure of functional group, and binding characteristics of different parts in composite materials.After compressing the sample to be tested and KBr, Spectrum 100 infrared spectrometric analyzer manufactured by Shanghai PerkinElmer Co., Ltd. was used for infrared spectrometry and analysis.The scanning scope of instrument was 4000 cm −1 -400 cm −1 , and the resolution ratio was 3 cm −1 .

Change of Modified TiO 2 Wetting Contact Angle with
Modifiers and Dosage.Figure 1 shows the influences of modifier dosage on the contact angle of TiO 2 wetted by water after TiO 2 particles were modified with sodium stearate and sodium oleate.Figure 2 shows the microscopic image of wetting contact angle.
According to Figure 1, the contact angle continuously enlarged with the increase of modifier dosage after TiO 2 is modified with sodium stearate and sodium oleate.The angles are 125.6 ∘ (modified with sodium stearate) and 121.3 ∘ (modified with sodium oleate), larger than 90 ∘ , critical between hydrophile and hydrophobicity, when modifier dosage is 1.5%.The contact angle of TiO 2 is 7 ∘ before modification.Obviously, TiO 2 surface has turned from strong hydrophilia to hydrophobicity after modification.However, the wetting contact angle on TiO 2 surface decreases in small range when the dosage further increases.The results show that both modifiers can change the wettability of TiO 2 surface.Moreover, the hydrophobicity formed in modification can be controlled with modifier dosage.In other words, the wettability on TiO 2 particle surface and its degree are controllable.Figure 2 intuitively reflects the changes in Figure 1.
In this work, TiO 2 was modified with sodium stearate and sodium oleate (in dosage of 1.5%), respectively.Figure 3 shows the contact angles of modified product and unmodified TiO 2 wetted by water, absolute ethyl alcohol, and kerosene.It can be seen that the contact angle of modified TiO 2 wetted by absolute ethyl alcohol is about 21 ∘ , while the angle wetted by water is larger than 120 ∘ (see Figure 1).This means that the hydrophilia of TiO 2 with alcohol is stronger than that with water because alcohol has double characters of organics and water.Therefore, modified TiO 2 with surface strong hydrophobicity has stronger wetting action than water.The wetting contact angle of modified TiO 2 in kerosene (organic nonpolar solvent) is slightly smaller than that in ethyl alcohol (less than 20 ∘ ).In other words, the hydrophilia of modified TiO 2 in kerosene is much stronger than that in alcohol.It means that the surface of modified TiO 2 particle has stronger organic nonpolarity.Moreover, the above results further specify that modifiers form the effective absorption on the surface of TiO 2 particle, so the particle turns from surface hydrophilia to hydrophobicity.Figure 3 also shows that the contact angle of unmodified TiO 2 wetted by alcohol and kerosene is about 20 ∘ larger than that wetted by water.In other words, the wetness degree of alcohol and kerosene is lower than water.This means that unmodified TiO 2 has strong surface hydrophilicity.
Before and after modification, wetting contact angle of TiO 2 particles changed in different mediums.This means that TiO 2 particle has extremely strong hydrophilia which may turn to strong hydrophobicity after organic modification.The wettability of modified TiO 2 particle in organic medium is stronger than that in inorganic medium, which is beneficial to the improvement of dispersity in organic medium.

Changes in Activation Index of Modified TiO 2 .
Activation index is an important index evaluating modification.The stronger the hydrophobicity TiO 2 particles are, the larger the activation index will be.Figure 4 shows the changes in activation index of TiO 2 powder modified by sodium stearate and sodium oleate.According to the figure, the activation index of modified TiO 2 greatly increases with the dosages of sodium stearate and sodium oleate increasing from 0% to 1.5% and from 0% to 1%, respectively.Moreover, the activation index reached the maximum, 98.6% and 89.7%, when the dosages of sodium stearate and sodium oleate were 1.5% and 1%, respectively.However, with the further increase of sodium stearate and sodium oleate, the activation index of TiO 2 may slightly decrease or keep stable.This is because the surface modification of TiO 2 is incomplete if the modifier dosage is little, so the activation index is small.With the increase of modifier dosage, however, the unimolecular coverage is realized with modifier on the surface of TiO 2 to make the modifier dosage saturated.At this time, the activation index is the maximum.If the modifier dosage further increases, multilayer coats will form on the surface of TiO 2 particles to make some types of group outward and reduce the hydrophobicity of TiO 2 .Therefore, the activation index will decrease.The change of activation index shows that after modifying the sodium stearate and sodium oleate of TiO 2 , the hydrophilia on surface will turn to strong hydrophobic effects.This is obviously the interaction between TiO 2 particle and modifier molecule, as well as the coating on the surface of TiO 2 [30,31].
This means that TiO 2 surface turns from hydrophilia to strong hydrophobicity after being modified with sodium stearate and sodium oleate.This is obviously the result of TiO 2 particle and modifier molecule covering TiO 2 surface.Above results are consistent with the analysis on contact angle test.

IR Analysis on TiO 2 Modified with Sodium Stearate and
Sodium Oleate.The mechanism of TiO 2 surface showing strong hydrophobicity can be further understood and hydrophobicity stability be determined by studying the properties of interaction between modifiers, sodium stearate and sodium oleate and TiO 2 surface.Therefore, the infrared spectroscopies were measured for sodium stearate and sodium oleate, as well as TiO 2 modified by these two modifiers.Figures 5 and 6 show the measurement results.
Figure 5 shows that there are symmetrical and dissymmetrical characteristic absorption peaks of -CH 2 -at 2,917 cm −1 and 2,849 cm −1 in the infrared spectroscopy of sodium stearate (see Figure 5(a)).Moreover, there is characteristic absorption peak of -CH 3 at 2,956 cm −1 and stretching absorption peaks of carbonyl (C=O) and carboxyl (COO-) at 1,557 cm −1 and 1,470 cm −1 .All of these reflect the structural characteristics of sodium stearate.Compared with raw material of sodium stearate and TiO 2 , there are absorption peaks at 2,996 cm −1 and 2,885 cm −1 in the infrared spectroscopy of modified TiO 2 .This is obviously the characteristics of -CH 2and -CH 3 , showing that sodium stearate has attached to TiO 2 surface.
According to the comparison, the absorption peak of modified TiO 2 at 1,504 cm −1 resulted from the displacement of absorption peaks at 1557 cm −1 and 1470 cm −1 in the infrared spectrum of sodium stearate.It means that the chemical environment of C=O and -COO − had significant change after sodium stearate reacted with TiO 2 .This may result from the chemical action between functional group of sodium stearate and surface functional group of TiO 2 (the hydroxide radical produced in Ti or Ti hydrolysis).Therefore, it is believed that the attachment of sodium stearate to TiO 2 particle surface has chemical properties.Sodium stearate is closely and firmly combined with TiO 2 particle, so the changes in surface characteristics of TiO 2 for modification are stable.
As shown in Figure 6, the infrared spectral features of TiO 2 modified with sodium oleate are similar to those of TiO 2 modified with sodium stearate.In the infrared spectroscopy of modified TiO 2 , there are characteristic absorption peaks representing methylene and methyl at 2,852 cm −1 and 2,922 cm −1 .This means that sodium oleate has attached to TiO 2 surface.The absorption peak at 1,465 cm −1 indicates that chemical action occurred between -COO − and TiO 2 in sodium oleate.Therefore, sodium oleate is also closely combined with TiO 2 particle for stable modification of TiO 2 .

Functional Group Morphology of TiO 2 Particle Surface in Water Medium
. TiO 2 is one of the most typical oxides of surface hydroxylation, while the surface hydroxylation of TiO 2 as the semiconductor material is complex with various hydroxyl groups.Perrin made quantitative analysis on hydration reaction of titanium ion (Ti 4+ ) in water medium.The hydration reactions of Ti 4+ in different levels and constants are as follows [32]:    [Ti (OH) 3+ ] (7) Therefore, the relationship between components, Ti 4+ , Ti(OH) 3+ , Ti(OH) 2 2+ , Ti(OH) 3 + , and Ti(OH) 4 , and pH value (− log[H + ]) is shown in Figure 7 [32].
Figure 7 shows that with the increase of pH value in system, Ti 4+ tends to generate polyhydroxy.When pH < 1, Ti(OH) 2 2+ is taken as the principal; when pH = 2, Ti(OH) 3 + is taken as the principal; when pH > 4, Ti 4+ will turn to Ti(OH) 4 component.The result also shows that even when pH = 0∼1, the concentration of OH − in system is extremely small.If the external action of hydrolysis is weak, no Ti 4+ and Ti(OH) 3+ component is represented, meaning that the capability of Ti 4+ is strong in generating various hydroxylates.It is obvious because the constants (pk 1 ∼pk 4 ) of Ti 4+ hydrolysis reaction in different levels keep in very high value (13.06∼14.15).
The hydroxylation characteristic of unsaturated Ti on TiO 2 surface can be deducted according to the hydrolysis behavior of Ti 4+ .In other words, Ti is covered by a large amount of hydroxyl, so surface functional groups of TiO 2 are mainly OH-group as shown in Figure 8 [33].This is similar with the molecule model of TiO 2 with surface attached with water proposed by Huaxiang and Xuxu [33].

Dissociation Morphology of Sodium Stearate and Sodium
Oleate in Water Solution.The molecular formula of sodium stearate is CH 3 (CH 2 ) 16 COONa, while that of sodium oleate is CH 3 (CH 2 ) 7 CH=CH(CH 2 ) 7 COONa (R-COONa for short).Both of them are composed of long chain of polar hydrophilic carboxyl and nonpolar hydrophobic alkane.They can dissolve in water solution, and then the dissolved components are rehydrated.The reaction formula is as follows: Dianzuo and Yuehua [34] studied the relationship between the concentration of R-COO and R-COOH in sodium oleate solution and pH value (see Figure 9) [34].The figure shows that the main morphology of R-COONa is R-COOH in the modification condition of TiO 2 , and only few R-COONa have coexistence of R-COOH and R-COO − in this work.Therefore, it is believed that the action groups of TiO 2 modified with sodium stearate and sodium oleate are mainly R-COOH, followed by R-COO − .

Model of Action between Modifier and TiO 2 Surface.
The modification mechanism of sodium stearate and sodium oleate can be obtained according to above analysis on surface functional groups of TiO 2 particle and the dissociation hydrocarbon chain distribute externally on particle surface.Therefore, TiO 2 has strong hydrophobicity.OH − on TiO 2 surface is covered by R-COOH and R-COO − attachment, so the absorption peaks reflect hydroxyl in infrared spectroscopy of TiO 2 after modification.
The reaction model of modifier on TiO 2 particle surface in wet process is established as according to above result analysis (See Figure 10).

Conclusions
Surface modification was conducted to TiO 2 particle with sodium stearate and sodium oleate.TiO 2 particle surface turns from hydrophilia to hydrophobicity, while the hydrophobicity degree can be controlled by changing modifier dosage.
In aqueous medium, wetting contact angle of TiO 2 particle greatly enlarges after modifying sodium stearate and sodium oleate.The increase of activation index shows that wettability in water medium becomes poor.The wetting contact angle of modified TiO 2 particle in absolute ethyl alcohol and kerosene is significantly lower than that in water medium, showing the good compatibility of organic matrix.TiO 2 infrared spectroscopic analysis before and after modification shows that the modifier forms effective attachment on TiO 2 particle surface.With such attachment, inductive hydrophobicity forms on TiO 2 particle surface.It is relatively stable because attachment of modifier has certain chemical properties.The action model of TiO 2 modified with sodium stearate and sodium oleate was established based on the analysis on the functional groups of TiO 2 particle surface and the dissociation morphology of modifiers.

Figure 1 :
Figure 1: The wetting contact angle of modified TiO 2 in water.

Figure 2 : 2 Figure 3 :Figure 4 :
Figure 2: Microscopic image of wet contact angle of TiO 2 .(a) Wet contact angle of TiO 2 , (b) wet contact angle of modified TiO 2 with sodium stearate, and (c) wet contact angle of modified TiO 2 with sodium oleate.