Usually a surfactant contains one or more polar groups and a long aliphatic chain. The principle of surfactant treatment is the preferential adsorption of a polar group of a surfactant to the high-energy surface of a filler by electrostatic interaction. Ionic bonds can also be formed under certain circumstances. A typical example can be cited regarding the surface treatment of CaCO3 with stearic acid , which is necessary when producing nano-CaCO3. Stearic acid or its ammonium salt can be added directly during the precipitation step of CaCO3. Electron spectroscopy for chemical analysis (ESCA) studies carried out on the surface of CaCO3 covered with stearic acid showed that ionic bonds have been established between the surfactant molecules and the filler surface (Scheme 1). A further proof of the specific character of the above treatment is supplied by the fact that talc and silica absorb a significantly smaller amount of stearic acid at a unit surface than CaCO3. Evidently, the lack of ionic bonding formation results in a much lower amount of the proportionally bonded molecules. Nevertheless, even for the treated CaCO3, the particles still exist in the form of agglomerates because the interparticulate attraction is far beyond the obstructive effect resulting from the surfactant.
Papirer et al. related the absorbed amount of stearic acid to the equilibrium concentration of the solution by establishing adsorption isotherms . The quantity of stearic acid retained by the solid was determined either from the change in concentration of the supernatant solution or directly by analyzing the recovered solid. The physisorption isotherm of stearic acid on CaCO3 (50-100 nm) at 30 ° C was found to follow a type II behavior, corresponding to a multilayer adsorption. After extraction of the treated particles with hot toluene for 24 h, the chemisorption isotherm was found to be a Langmuir-type one, which confirms the formation of a monolayer of stearic acid on the surface of CaCO3. The monolayer (100% coverage) suggests a ratio of stearic acid of about 8% by weight. As the surface area of CaCO3, calculated on the assumption of a cross-sectional area of stearic acid of 0.21 nm2, is 35 m2/g (in excellent agreement with the value obtained from N2 adsorption), it was concluded that the stearic acid molecules are fixed perpendicular to the solid surface.
It should be noted that the shield made of C18 alkyl chains on the filler surface leads to a reduced surface energy. Contact angle measurements revealed a drastic decrease in surface energy with increasing degree of surface coverage by stearic acid. Before the treatment, CaCO3 possesses a high energy surface. Both the dispersive (yd) and the polar (yp) components of its surface energy are higher than 50 mJ/m2. When the particles were covered by a monolayer of stearic acid, the surface energy went down to 22 mJ/m2, coinciding with the value published for pure stearic acid.
Alkyl dihydrogenphosphate, containing functional groups such as olefine, chloro, methacryloxy, and mercapto, is another kind of effective surface modifier of calcium carbonate fillers [65, 66]. It was shown that alkyl dihydrogenphosphate molecules can react with CaCO3, resulting in a dibasic calcium salt of phosphate. The products are deposited on CaCO3 and provide the surface of CaCO3 with a certain hydrophobicity (Scheme 2), as indicated by its good dis-persibility in mineral oil . Relatively larger groups in the phosphates (like octyl dihydrogenphosphate, 3,7-dimethyl-6-octenyl dihydrogenphosphate, etc.) help to enhance the conversion of the agent through a lower hydrophilicity. Furthermore, a treatment with alkyl dihydrogenphosphates always results in an increase in the specific surface area of the treated particles. This can be ascribed to surface erosion and/or calcium salt formation.
Zhou and co-workers modified nanosized Y-TZP powders (10-20 nm, consisting of 97 mol% ZrO2 and 3 mol% Y2O3) with adipic acid and stearic acid, respectively (Fig. 3) . A covalent bond was established by the chemical reaction between the surface hydroxylic groups and the acid groups of the surfactants. Fourier transform infrared (FTIR) spectroscopy proved that a monolayer of aliphatic chains was coated on the Y-TZP particulates, since the peak representing free carboxyl was no longer detectable. As a result, the flowability of the powders was improved because of the change of their surface polarity.
HO C II
Figure 3. Formation of a monomolecular layer on Y-TZP particles. Reprinted with permission from , J. G. Zhou et al., J. Inorg. Mater. 11, 237 (1996). © 1996, Shanghai Institute of Ceramics, Chinese Academy of Sciences.
Yoshihara reported the application of hydrophilic dispersants of two-component structures (oxyethylene chains and acidic groups) for the preparation of a stable ultrafine particle dispersion (UFP) (chemical composition unknown, having hydrophilic and cationic nature, 120 nm in size) .
The results indicated that the interparticle attraction was reduced because of an adsorbed layer of dispersant on the surface of UFP by a hydrophilic interaction. On the principle of acid-base interaction, the oxyethylene chains might permeate the interface of the UFP agglomerates, and acidic groups contribute to hydrogen bonds and cationic hydroxyl groups on the UFP. Table 2 lists the effect of different dispersants on the average particle size of UFP. Obviously, the effect of the dispersants is not enough to separate the agglomerates completely. Furthermore, it was found that these dispersants could be easily desorbed from the UFP surfaces with the addition of small amounts of H2O to the dispersion, leading to an increase in the average particle size. That is, reagglomeration occurred with increasing H2O.
Table 2. Structure of dispersant and average particle size of UFP.
Reactive dispersant sizea (nm)
a Dispersal conditions: UFP, 10.0 g; dispersant, 1.0 g; diethylene glycol dimethyl ether, 16.5 g; paint shaker, 3 h.
Source: Reprinted with permission from , T. Yoshihara, Int. J. Adhes. Adhes. 19, 353 (1999). © 1999, Elsevier Science.
This dispersion system can be illustrated by the correlation among the three components in Figure 4. One of the major problems of the system is that the affinity of the dispersant for the UFP surface is lower than that of H2O.
To solve the problem, grafting of a polymer to surface hydroxyl groups of the UFP was conducted to inhibit H2O absorption and reagglomeration of the particles (Fig. 5). Figure 6 demonstrates the concept related to the application of grafting polymer to form hydrophobic layers that have an affinity for the dispersant. It can be expected to obtain a stable dispersion due to an improvement of interaction between the grafted UFP and the dispersant interface. The feasibility of the above proposed approach was subsequently demonstrated by an examination of the stability of the dispersion of poly-(dimethylsiloxane)-grafted UFP in diethylene glycol dimethyl ether. That is, a stable dispersion was kept even after 30 days in the case of polymer-grafted UFP, whereas the untreated UFP was completely precipitated after 1 week.
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