Why the Surface Chemistry of TiO₂ Is Important

Titanium dioxide (TiO₂) is the most widely used white pigment in the coatings industry, primarily valued for its excellent opacity, brightness and durability. Surface polarity, wettability and adsorption capacity directly impact pigment dispersion, agglomeration tendencies and ultimately, the appearance and stability of the coating.

The surface characteristics of TiO₂ crucially dictate how dispersants, surfactants, resins and solvents interact with the pigment particles. Ignoring surface chemistry can lead to poor wetting and dispersion, increased viscosity and formulation instability—all of which compromise product performance. Conversely, a deeper understanding can significantly enhance product consistency, reduce formulation costs and accelerate product development.

Although traditional quality control (QC) methods emphasize particle size, rheology, brightness and tint strength, they do not adequately reflect these surface interactions, leaving formulators without vital information. Unfortunately, characterizing surface chemistry — a critical factor in any interaction between TiO₂ particles and other formulation components — is often neglected.

The Fundamentals of TiO₂ Surface Chemistry

The surface chemistry relates to the chemical characteristics and interactions occurring at the particle’s surface and their inherent surface properties (hydrophilic or hydrophobic nature). All dispersed particles spontaneously acquire a surface electrical charge when brought into contact with a polar medium (for example, water) via a variety of different charging mechanisms.

Crystal lattices are typically anisotropic; charge development occurs because of n and p defects within the crystal structure. This results in surface defects and, in the case of metal oxides such as TiO₂, a variety of amphoteric hydroxyl groups that can undergo reaction with either H⁺ or OH⁻. Hence, the magnitude of the surface charge for uncoated TiO₂ is dependent on the pH of the dispersion medium.

Further, titanium dioxide is manufactured synthetically by two processes, and the crystal structure contains lattice defects associated with ions such as Fe³⁺, Al³⁺ and Cr³⁺ from impurities in the initial crude raw ilmenite or rutile ores used, which can directly affect the surface chemistry.

Hence, while it is possible to manufacture TiO₂ with a fairly constant size and distribution uniformity, it is generally extremely difficult to ensure consistent surface chemistry. Thus, it should not be assumed that different lots or batches of the same material will necessarily have the same surface chemistry. Indeed, when comparing “similar” materials from different suppliers, it is more than likely that they will not have equivalent surface chemical behavior—which is why its measurement needs to be part of a rigorous QC protocol.

A widely accepted measurand for surface chemistry is the electrical charge of the particle surface. As a rule of thumb, a higher charge promotes better dispersibility when preparing a suspension; a zero charge implies instability and makes dispersibility problematic. However, determining the electrical charge (and potential) of surfaces is difficult.

Traditionally, the most widely used method for particulate dispersions is to measure what is termed the zeta potential (ZP). This parameter does not directly measure the surface charge but can correlate with it and certainly tracks changes brought about by changes in solution conditions, as shown in Figures 1 and 2.

FIGURE 1 | Schematic of the effect of pH on the particle charge of aqueous suspensions.  

Credit: Dr. David Fairhurst, Ralph J. Woerheide

Figure 1 illustrates the general features of the effect of solution pH on aqueous dispersions. The pH at which the zeta potential (and, potentially, the surface charge) is zero is termed the iso-electric point (IEP). Materials with an IEP pH 7 will have basic properties. Maximum dissociation/ionization of surface functional groups occurs at the extremes of pH where the value of the zeta potential plateaus.

FIGURE 2 | Typical zeta potential profiles.  

Credit: Dr. David Fairhurst, Ralph J. Woerheide

In Figure 2, the IEP for the specific (rutile) TiO₂ is at pH 6.8. Literature values for uncoated rutile range from pH 6.3 to pH 7.7, which speaks to the variability of the surface charge and the importance of QC for incoming batches.

Uncoated TiO₂ is a hydrophilic material that is relatively easily wetted by water but only a coarse initial aqueous suspension can readily be prepared. This is because the IEP is at (roughly) neutral pH, and the natural state of the particles is one of aggregation/agglomeration. So, even if comminution is used to break particles apart, they will naturally re-aggregate over time unless a dispersing agent is used. However, a perfectly fine dispersion can be prepared at a solution pH either <3 or >9; the stability is, however, controlled totally electrostatically.

Pigment particles are often modified post-manufacture by being coated with layers of other chemicals (such as silica, alumina and organic coatings such as fatty acids) to reduce undesirable chemical activity (such as photoreactivity) and to improve dispersion in both aqueous and nonaqueous vehicles; obtaining a consistent surface modification is not without its challenges. These post-manufacture treatments are crucial for ensuring efficient pigment dispersion, optimal rheological behavior and long-term stability in coatings. The resultant modified surface chemistry of TiO₂ is strongly influenced by such treatments, as shown in Figure 3 and Table 1.

FIGURE 3 | Effect of surface modification on the particle charge of a rutile TiO₂.

Credit: Dr. David Fairhurst, Ralph J. Woerheide 

Figure 3 shows the zeta potential profiles and resultant IEPs for a pigmentary grade of rutile modified using alumina (R900) or a combination of silica and alumina (R960 and R931). The alumina and silica profiles (dotted curves) are for comparison.

TABLE 1 | Effect of surface modification on the IEP of a rutile TiO_2. 

Credit: Dr. David Fairhurst, Ralph J. Woerheide

The zeta potential plot and IEP (at pH 9.1) for R900 are very similar to that for pure alumina (IEP at pH 9.2), which suggests that the TiO₂ is well-coated.

However, a different picture emerges with the dual-coated materials. Often a silica coating is applied first followed by an overcoat of alumina. The R960 has a profile and IEP like that for an uncoated TiO₂, which suggests that there is considerable silica in combination with alumina at the surface. With the R931 there is an equal bulk amount of silica and alumina coating. However, the profile is shifted much closer to that for a pure silica, suggesting that the silica is dominant at the TiO₂ surface.

Hence, bulk percentages (elemental analysis) of the individual amounts of chemical “coating” (as provided in an MSDS) are not always a reliable indicator of the actual surface chemistry and how it will behave in solution.

Finally, the inability of an electrolyte to ionize in solvents of low dielectric has led to a mistaken belief that particles dispersed in a nonaqueous medium cannot acquire a charge. The charging mechanism is not the same as in aqueous dispersions; in nonaqueous systems it arises through the formation of ions in adsorbed films on the particle surface where acid-base (or electron donor-acceptor) interactions occur between the particle surface, the solvent and the dispersing agents. This is an extremely complex subject and beyond the scope of this article.

Examples of the Impact of Surface Chemistry

Several practical scenarios illustrate the impact of surface chemistry and why it matters:

• Dispersion stability: Two batches of TiO₂ with identical particle size but different surface treatments can exhibit significantly different dispersion behaviors. Formulators often encounter inconsistent batch performance, resulting in costly reformulations.
• Surfactant efficiency: Inefficient surfactant adsorption onto pigment surfaces because of incompatible surface chemistry can cause flocculation or settling, leading to storage stability problems.

• Raw material replacement: A coating formulation developed with a specific TiO₂ grade might fail when substituted with a seemingly identical alternative owing to subtle differences in surface chemistry, impacting consistency and product quality.

Working Principles of the MagnoMeter and its Utilization
 All solids and liquids possess a fundamental, intrinsic property called the relaxation time and it can be determined directly using a suitable NMR spectrometer. The relaxation time of particulate suspensions will, therefore, be intermediate between that for the specific solid and that for the dispersion liquid used. The actual value is a function of exactly how a liquid interacts with the particle surface (i.e., the wettability and resulting interfacial structure). 

The MagnoMeter uses low-field Nuclear Magnetic Resonance (LF-NMR) relaxometry, an innovative yet established technique, to determine the relaxation time of suspensions and slurries; the basic measurement of relaxation time is illustrated in Figure 4. In principle, any type of particle - of any size or shape - can be studied. However, the dispersing liquid must contain at least one NMR active nucleus such as the hydrogen atom (proton). So, for example, measurements can be made in hexane but not carbon tetrachloride. A very useful benefit, particularly in non-aqueous media, when investigating the wettability of surfaces is that mixtures of liquids can be studied. The solids concentration can be very high indeed - the limit is simply dictated by whether the sample can be transferred into an NMR tube

Measurements with a MagnoMeter can specifically detect changes in relaxation times of molecules interacting with particle surfaces, providing insight into how surface chemistry influences pigment dispersibility and formulation stability.

In summary, the MagnoMeter methodology: 

• Measures the fundamental relaxation time of virtually and type of liquid (water, solvent or oil) bound (adsorbed) to the surface of TiO₂ particles.
• Detects subtle differences in molecular dynamics at the TiO₂ particle-liquid interface.
• Identifies changes through surface treatments or adsorption of surfactants, polyelectrolytes and polymers. 

Importantly, this approach is non-invasive/non-destructive, rapid, requires minimal sample preparation, and provides reproducible quantitative data that complements existing QC methods. 

NMR Relaxation Time and the Zeta Potential 

The zeta potential (ZP), denoted by the Greek letter zeta (ζ), is an empirical parameter that is related non-linearly to the surface potential and, thus, it is not a direct measure of the surface charge. ZP measurements are useful primarily for systems in which particles are electrostatically charged. They are of very limited value in systems in which the particle surface groups are not dissociated or ionized, or for particles that are completely sterically stabilized by, for example, polymers. There are two commercially available techniques to measure ZP - Electrophoretic Light Scattering (ELS) and Phase Analysis Light Scattering (PALS). Both require that samples be highly diluted. 

In contrast, the relaxation time is directly related to the number and type of surface groups and the surface lyophobic-lyophilic balance, which, in turn, determines the particle surface charge in relation to the properties of the bulk fluid used. The technique works with industrially relevant solids concentrations. Importantly, no dilution is necessary, which permits analysis of samples as they are intended to be used.

Further, dispersing TiO₂ often utilizes inter-particle repulsion mechanisms such as steric forces (from adsorption of polymeric materials), which would not be detected by ZP measurements. Advantageously, this will be detected by the MagnoMeter. Thus, such measurements offer intelligence that a ZP measurement cannot. 

Practical Applications of the MagnoMeter in Coatings. 

The ability to characterize surface chemistry rapidly and precisely has several practical advantages:

• Raw material screening: Quickly distinguishes variations between different grades or batches of TiO₂, enabling formulators to predict formulation behavior before costly and time-consuming stability tests.

• Evaluation of dispersants and surfactants: Precisely quantifies adsorption efficiency of dispersants and surfactants on TiO₂, optimizing dispersant selection and dosage, reducing material costs and development time. 

• Assessing equivalence of raw material replacements: Provides immediate comparative data on surface interactions, confirming whether an alternative TiO₂ source will perform equivalently in existing formulations.  

• Stability prediction: Early identification of potential formulation instability related to pigment dispersion and particle interaction, significantly shortening product development cycles and improving long-term product reliability. 

Conclusion
Neglecting the surface chemistry of TiO₂ can have significant consequences, yet it remains misunderstood and overlooked because of limitations of traditional measurement techniques. The MagnoMeter addresses this critical gap, offering a fast, reliable, and scientifically robust method to evaluate TiO₂ surface chemistry directly. By adopting the MagnoMeter, coatings formulators can achieve unprecedented insight into pigment performance, streamline development processes, and ensure more consistent and stable coatings formulations.

At the simplest level LF-NMR relaxation information helps maintain a more consistent product, enhancing end-use value and profitability.  At the more complex level relaxation information can reduce in-process modifications, and reworking, and so make products more competitive.

Appendix: Explanation of the Measurement Principle

FIGURE 4 | Working principle of low-field NMR relaxation measurements.

Credit: Dr. David Fairhurst, Ralph J. Woerheide