**3. Organic modification of TiO2**

Modifying TiO2 by organic agents is realized by coating them with organic substances such as surfactants, coupling agents and polymers. It can improve the affinity of TiO2 particles with organic matrices, resulting in better dispersion of TiO2. Thus, the pigmentary properties of TiO2, such as tint-reducing power, hiding power, and whiteness, are shown.

In the modification, there are two mechanisms: physical adsorption of organic agents on the surface of TiO2 and chemical bonding between TiO2 and organic agents [30]. The principle of physical adsorption is that the hydrophilic group of the organic coating agent is adsorbed on the surface of TiO2 particles. In contrast, the oleophilic group is outwardly affinity to the surrounding polymer matrix. Therefore, the polymer chains can penetrate the TiO2 aggregates and separate the TiO2 particles, finally improving the dispersion of TiO2. For chemical bonding, the hydroxyl groups on the surface of TiO2 particles act as active sites, which will react with organic coating agents and form covalent bonds. As a result, the TiO2 particles change from hydrophilic to hydrophobic. Several kinds of organic agents can be used for the surface modification of TiO2, including surfactants, coupling agents and polymers.

### **3.1 Surfactants**

Surfactants can be divided into cationic, anionic, and nonionic surfactants. One can use surfactants singly or together to modify TiO2 particles to evaluate the performance of TiO2. Li et al. [31] chose anionic sodium dodecyl sulfate (SDS) and nonionic nonylphenol ethoxylate (NPEO, Tergitol NP-9) to study the effect of surfactants on the behaviors of TiO2 in aqueous solution. The results showed that both surfactants could be adsorbed onto the surface of nano-TiO2 but that only SDS can significantly decrease the zeta potential of TiO2. Both surfactants reduced the aggregation of TiO2 and retarded the aggregate sedimentation at surfactant concentrations ≥0.015% (w:v). In addition, SDS exerted a more substantial reductive effect than NP-9.

Wei et al. [32] used different surfactants, such as cetyltrimethylammonium bromide (CTAB), sodium dodecylbenzene sulfonate (SDBS), and diethanolamine (DEA), to modify TiO2 particles. The crystal type of TiO2 has no noticeable change with the addition of different surfactants, but the morphology, size, and dispersion of the TiO2 particles have changed to some extent. Among the three surfactants, CTAB is beneficial in reducing TiO2 particle size and improving TiO2 dispersion and agglomeration. And this CTAB-coated TiO2 had the greatest photostability in methyl orange degradation.

Wittmar et al. [33] prepared modified TiO2 particles by adding a cationic imidazolium salt solution. It was found that an increase in the alkyl chain length was beneficial, leading to a narrowing of the particle size distribution and a decrease of the agglomerate size in dispersion. The smallest average nanoparticle sizes in dispersion were around 30 nm.

Zhang et al. [34] discussed the influence of different surfactants on the thermal stability, weather fastness, and pigmentary properties of TiO2 particles. The results are collected in **Table 3**. Compared with neopentyl glycol (NPG),



**Table 3.** *Routine index of TiO2 [34].*

polyethylene glycol (PEG) and trimethylolethane (TME), trimethylolpropane (TMP) can bring the highest whiteness (97.64 of *L*) to TiO2 particles only in the content of 0.3 wt%.
