How to Choose Between Hydrophilic and Hydrophobic ...

In the realm of nanomaterials, fumed silica emerges as a versatile substance with properties that can be tailored for various applications. One significant distinction lies in its hydrophilic and hydrophobic attributes. Understanding the factors influencing the selection between these two forms of fumed silica is paramount for optimizing product performance. Let&#;s delve into the world of fumed silica and explore the key considerations that shape this choice.

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The Genesis of Fumed Silica

Fumed silica comes into being through a high-temperature hydrolysis process involving silicon halides in a hydrogen-oxygen flame. The resultant nanoscale amorphous powder exhibits unique characteristics that find utility across diverse industries.

What Are The Materials And Processes Used To Manufacture Fumed Silica?

Fumed silica, or pyrogenic silica, colloidal silicon dioxide, is an amorphous white inorganic powder that has high specific surface area, nano-scale primary particle size and a relatively high (among silica&#;

Hydrophilic Fumed Silica: Properties and Applications

Hydrophilic fumed silica is characterized by surface hydroxyl groups that readily interact with water molecules, endowing it with water-attracting qualities. This type of fumed silica smoothly wets surfaces and disperses within water-based systems. It&#;s particularly suited for non-polar or low-polarity products.

Applications of hydrophilic fumed silica extend to non-polar systems such as methyl siloxanes, polyvinyl chloride (PVC), and various hydrocarbons. It also finds a place in low-polarity systems like MS Polymers and STP-E, contributing to stability and dispersion.

Hydrophobic Fumed Silica: A Shift in Properties

By treating hydrophilic fumed silica with modifying agents, its surface hydroxyl groups can be either reacted or masked, leading to hydrophobicity. Hydrophobic fumed silica repels water, making it ideal for medium to high-polarity systems.

In contrast to its hydrophilic counterpart, hydrophobic fumed silica suits systems like polyurethanes, polyols, epoxy resins, and other polar environments. Its reduced polarity aligns with higher-polarity product systems.

Why Fumed Silica Need To Be Treated To Be Hydrophobic?

Fumed silica is available in both hydrophilic and hydrophobic forms. However, its hydrophilic nature makes it unsuitable for certain applications. Untreated fumed silica has many silanol groups on its surface,&#;

Factors Influencing the Choice

Three primary factors govern the selection between hydrophilic and hydrophobic fumed silica:

1. Inherent Properties of Fumed Silica

The polarity of the product system determines the compatibility of fumed silica. Hydrophilic fumed silica aligns with non-polar and low-polarity systems, whereas hydrophobic fumed silica is suited for medium to high-polarity environments. As the polarity of the system increases, the polarity of the fumed silica should decrease.

2. Application System and Scenario

The nature of the application system is a key determinant. Hydrophilic fumed silica finds favor in non-polar systems such as PVC, while hydrophobic fumed silica is fitting for epoxy resins and high-polarity environments. Matching the fumed silica polarity to the application system is crucial for optimal performance.

3. Cost Considerations

Cost plays a significant role in the selection process. Hydrophilic fumed silica is generally more cost-effective than its hydrophobic counterpart. This aspect is vital for applications where cost sensitivity is a primary concern.

conclusion

In conclusion, the choice between hydrophilic and hydrophobic fumed silica rests on a careful analysis of the product&#;s nature, the application system, cost considerations, and the desired dispersion method. Hydrophilic fumed silica is best suited for products requiring interaction with water molecules, while hydrophobic fumed silica serves well in scenarios where water repellency or specific processing methods are required. Ultimately, the decision should be aligned with the product&#;s requirements and the overall cost-effectiveness of the chosen fumed silica variant.

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Hydrophobic colloidal silicon dioxide is a synthetic amorphous silica derivative in which the surface of the fumed silica particle has been modified by the addition of dimethyl silyl groups. The surface modification is achieved via a controlled chemical process that involves the attachment of dimethyl silyl groups, rendering the silica less wettable. It is approved for use in pharmaceutical products as an excipient and is supplied as a light, fine, white or almost white amorphous fluffy powder.

Hydrophobic colloidal silica occurs as a light, fine, white or almost white amorphous powder, not wettable by water.

Silicon Dioxide: Definitions and Types

Silicon and oxygen are the two most abundant elements in the earth&#;s crust. In nature, silicon almost always exists in combination with oxygen, either as free silica (SiO2), in conjunction with other elements (for instance, in silicates, which are the main minerals in rocks and soil), or as combined silica (SiO3). The different silicon compounds have substantially different chemical properties, applications, and hazards.

Free silica (Silicon dioxide) is a hard, low-reactivity, colourless substance that occurs naturally in rocks and minerals or can also be industrially produced in the form of synthetic amorphous silica. All forms of Silica, whether natural, synthetic, crystalline, cryptocrystalline or amorphous, are assigned a single CAS Registry number [-86-9]. For convenience, the different forms of Silica can be divided into three main groups as shown in the chart below:

Chemical Structure of Silica

At the fundamental level of Silica&#;s chemical properties is the Silica tetrahedron (SiO4), which consists of a central Silicon cation covalently bonded to four oxygen atoms, arranged in the shape of a tetrahedron. Silica tetrahedra may be linked and arranged in a variety of ways, from simple to complex three-dimensional frameworks. Crystalline forms of Silica exhibit a highly ordered crystal lattice, determined by the ordered arrangement of the Silica tetrahedra. Amorphous forms, on the other hand, have random, disordered lattices. The orientation of the bonds is random, and there is no long-range periodicity.

To differentiate between the different silica analogues, new CAS Registry numbers have been assigned. These are shown in the chart below:

Synthetic Amorphous Silicon Dioxide

Synthetic amorphous silicon dioxides are further divided into three main types, namely:

• Pyrogenic or fumed silicon dioxide
• Precipitated silicon dioxide, and
• Surface-modified silicon dioxide

Even though they share the same chemical structure and synthetic origin, Synthetic silicas exhibit different properties, as briefly reviewed below:

1). Pyrogenic (Fumed) Silicon Dioxide

Pyrogenic silicon dioxide is produced using a high-temperature process in which silicon tetrachloride is vaporised in an oxygen-hydrogen atmosphere according to the following chemical reaction:

SiCl4 + 2H2 + O2 -> SiO2 + 4HCl

The raw materials used in the silica production process are all inorganic and very pure. As a result, the synthesis produces only hydrochloric acid (which is easily removed) and silica in very high states of purity (typically > 99.9%). Silicon dioxide produced pyrogenically exists in the form of chain-like, branched aggregates, giving rise to a fluffy, light powder. (The term &#;fume&#; alludes to the method of manufacture, which involves the use of a flame.) Varying processing conditions allows the production of silica products with different specific surface areas, typically between 50 and 400 m2/g.

The pyrogenic method for producing Silica was invented in by Harry Klopfer, a scientist at Degussa (now part of Evonik AG). This method is what is still used by Evonik (for the production of pyrogenic silica marketed under the AEROSIL® brand name) and Cabot Corporation (for the Cab-O-sil® fumed silica brand). Note that these silica grades can be used in their native (unprocessed) state. They can also be further processed (for example, spray drying, granulation, or surface chemical modification) to turn them into other technical silica grades.

2). Precipitated Silicon Dioxide

Precipitated silicon dioxide is silica produced in an aqueous solution at temperatures >60 °C. In this process, sodium silicate (waterglass) undergoes controlled neutralisation with either concentrated sulphuric acid or hydrochloric acid. The Silica precipitates out as a slurry of (hydrated) silica, which is washed and filtered to remove by-products. It is then dried in hot air and milled or passed through a classifier.

Precipitated silicon dioxide has been known since the mid 17th century. It was not until the s that its practical uses and industrial production were fully established. Currently, Precipitated silica is produced in volumes that are up to x10 greater than for Pyrogen silica. The method and conditions have been fine-tuned and now permit the production of many types of synthetic silica that are structurally and characteristically different, including exhibiting internal pore volume/specific surface area, larger particle sizes, and water content.

3). Surface-Modified Silicon Dioxide

The Silica grades described thus far are available for use in their native or unmodified state. These materials have freely accessible silanol groups (Si-OH) on the surfaces of Silica particles, rendering them hydrophilic. Frequently, it is desirable to have hydrophobic silica, i.e a product that repels water.  Hydrophobicity can be achieved through a post-synthesis step in which the silanol groups are reacted with organic groups. The added organic groups are tightly bound to the surface (via covalent bonds) and are only broken via thermal decomposition.

Pharmaceutical-approved hydrophobic silica is produced by reacting hydrophilic silicon dioxide with dimethylchlorosilane immediately after the production of Silica particles in the hydrogen flame chamber. This process is also conducted at high temperatures and allows dimethysilyl groups to be bound irreversibly onto the surface of the silica via siloxane bonds. This produces a material that, while appearing identical to the precursor Silica, is very hydrophobic, repels water and does not absorb moisture from the environment.

Colloidal Silica, Silica Colloids (Sol-Gels) and Fumed Colloidal Silica

&#;Colloidal&#; is used in reference to both pyrogenic and precipitated silica. It may be confused with Silica colloids, which are also obtained via the wet chemical route. Note that the International Union of Pure and Applied Chemistry (IUPAC) defines colloids as systems (dispersions) in which particles of colloidal size (1 nm&#; nm) of any nature (solid, liquid, or gas) are dispersed in a continuous phase of different composition or state.

Thus, in the strictest sense, the term &#;colloidal silica&#; applies to stable dispersions (or sols) consisting of discrete particles of amorphous silica having sizes of between 5 and 100 nm. These colloidal silicas are commercially available in the form of sols or dried powders (e.g., xerogels, dry precipitates, aerogels, or calcinated coarcervates). In a broad sense, however, many other forms of silica (other than wet or dry silica sols above) are colloidal on the grounds that they are composed of particles in a colloidal state of subdivision (1- nm). Moreover, the silica particles and aggregates are self-supporting and stabilised dispersions of silica particles in a continuous air phase and are unaffected by gravitational forces. Finally, fumed silica is commonly referred to as &#;colloidal&#; because the silica powders are made by condensing a silica precursor from a vapour phase. In this sense, fumed silica particles are dispersed in a gas during its production process.

Physical Form

The SiO2 molecules in synthetic silicon dioxide do not exist in isolation. While the primary structure is the tetrahedron, consisting of one silicon atom bonded to four oxygen atoms, tetrahedrons arrange into networks. During the synthesis process, minute droplets of SiO2 initially aggregate into so-called nuclides, which combine stochastically into even larger particles, facilitated by weak physical interactions such as van der Waal&#;s forces.

Another important property of synthetic silica is its specific surface area. Fumed silicon dioxide, in particular, has only one surface, which is external and little or non-existent internal pore volume. Precipitated silica, on the other hand, is mesoporous and exhibits an internal surface. Generally, the higher the specific surface area, the greater the degree of agglomeration. It is these aggregates that partly contribute to the unique functionalities of amorphous silicon dioxide.