Hydrophobic silica is a form of silicon dioxide (commonly known as silica) that has hydrophobic groups chemically bonded to the surface. The hydrophobic groups are normally alkyl or polydimethylsiloxane chains. Hydrophobic silica can be processed in different ways; such as fumed silica, precipitated silica, and aerosol assisted self assembly, all existing in the form of nanoparticles.
Structure
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Hydrophobic silica has an orthorhombic crystal structure (its space group name is Pmna under the bipyramidal point group).[1] Orthorhombic structures are the product of stretching a cubic lattice along two of its orthogonal pairs, resulting in a rectangular prism shaped crystal structure.
Properties
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Hydrophobic silica displays water resistant properties because of its nanostructure and chemical properties. When applied to a surface of a material, the nanoparticles adhere to the host material and prevent liquids from permeating the rough texture. The water only comes into contact with the tips of the nanoparticles coating the outside of the material. Due to lack of attraction, the water is repelled from the hydrophobic silica.[3]
Processing
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Initially, silica is hydrophilic due to the presence of the silanol (Si-OH) groups on the surface of the particle. These silanol groups can chemically react with various reagents to render the silica hydrophobic. There are many different methods of processing silica to become hydrophobic, mainly by adding hydrocarbon groups.
Hydrophobic fumed silica
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Fumed silica can react with chlorosilanes in a fluidized bed reactor at 400 °C[4]
Hydrophobic precipitated silica
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Precipitated silica can be hydrophobized with e.g. alkylchlorosilanes or trimethylsilanol in the precipitated solution. The hydrophobised silica is filtered, washed, dried, and tempered to 300&#;400 °C to finish the reaction.[5]
Hydrophobic plasma polymer coated silica
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Silica particles can become hydrophobic through plasma polymerization. In this process, plasma polymerized 1,7-octadiene (ppOD) (related to the diene hydrocarbons) is used to deposit polymer films onto the silica particles. The ppOD films are deposited through the use of radio frequencies, along with a reactor containing a rotating chamber. Using low specific energy plasma conditions, the ppOD films chemically render the silica particles hydrophobic.[6]
By using the ppOD films, the hydrophilic polar Si-OH groups in the polymer itself are concealed by non-polar CxHy hydrocarbon groups, so when it's applied as a film to the silica particles, they become hydrophobic as well.[7]
Aerosol assisted self-assembly
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The goal of this process is to quickly and continuously create nanostructured particles deriving from a colloid precursor containing a solvent and silica particles. Aerosol assisted self assembly is a one step process with a high production rate. The process takes a few seconds in terms of reaction time, and there's no requirement for heating and chemically treating the particles after development.
The first part of the process is to create the colloid precursor which consists of the silica nanoparticles and the solvent. The initial silica nanoparticles are in an amorphous crystalline phase and the solvent is composed of trimethylsilyl chloride (TMCS) and ethyl alcohol. To synthesize hydrophobic nanostructured silica using this method, the colloid precursor containing the solvent and silica particles is sprayed by an aerosol generator. The droplets are then transported by a carrier gas to a furnace where they are heated. Upon entry into the furnace, the ethyl alcohol evaporates from the colloid precursor, allowing self-assembly to occur between the silica particles and the surface treating agent, TMCS.
The results of this process causes the silica particles to group together to combine into spherical nanostructured particles. By grouping these silica nanoparticles into a nanostructured particle, a certain percentage of porosity develops within the nanostructure related to the amount of TMCS concentration. Increasing the amount of TMCS concentration reduces the specific surface area of the silica nanostructured particles. The exhibited hydrophobicity is a result of the chemical reaction occurring between the silica particles and the TMCS. When the original SiO2-OH groups are replaced with hydrolytically stable Si(CH3) groups, this hydrophobicity occurs due to the prevention of the silica particles from interacting with water.[8]
Applications
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Hydrophobic silica is used to solve technical problems in a number of products including, but not limited to, paints, inks, adhesives, plastics, coatings, toners, defoamers, silicone rubber, sealants, cosmetics, food additives, polyester resins, cable gels, and greases. It's often manufactured as both single and multiphase composites in order to enhance properties such as dispersion, stability behavior, resistance to water, and functionality. Hydrophobic silica has been added to aerogels to increase their ability to adsorb organic pollutants without disrupting the network structure.[9]
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Treated aggregated fumed silica
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Hydrophobic silica can be used to treat other surfaces to become hydrophobic, this is due to the morphology of the silica particles once they adhere to their host. The silica particles then alter the surface of its host material resulting in a hydrophobic surface.
Aggregated fumed silica can be applied to large surfaces to render them hydrophobic. Micro and nanoscale structures, resembling ball and block like forms, are attributed to the hydrophobic characteristics. Due to the change in the original surface's texture, the roughness of the surface causes its hydrophobicity to increase. This is because when water comes into contact with the rough surface, it only touches the tips of the rough texture and doesn't permeate any deeper through the rest of the air occupied structure. The water can't spread through the surface, thus yielding hydrophobic properties.[3]
Additional applications
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References
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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.
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.
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 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.
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.
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,&#;
Three primary factors govern the selection between hydrophilic and hydrophobic 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.
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.
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.
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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