Hydrophobe

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165 degree water contact angle on a surface modified using plasma technology system surface chemistry. The contact angle is the red angle plus 90 degrees. 150 deg water contact angle.png
165 degree water contact angle on a surface modified using plasma technology system surface chemistry. The contact angle is the red angle plus 90 degrees.
Dew drop on a hydrophobic leaf surface Dew 2.jpg
Dew drop on a hydrophobic leaf surface
Cutting a water droplet using a superhydrophobic knife on superhydrophobic surfaces
Water drops on the hydrophobic surface of grass Drops I.jpg
Water drops on the hydrophobic surface of grass

In chemistry, hydrophobicity is the physical property of a molecule that is seemingly repelled from a mass of water (known as a hydrophobe). [1] In contrast, hydrophiles are attracted to water.

Contents

Hydrophobic molecules tend to be nonpolar and, thus, prefer other neutral molecules and nonpolar solvents. Because water molecules are polar, hydrophobes do not dissolve well among them. Hydrophobic molecules in water often cluster together, forming micelles. Water on hydrophobic surfaces will exhibit a high contact angle.

Examples of hydrophobic molecules include the alkanes, oils, fats, and greasy substances in general. Hydrophobic materials are used for oil removal from water, the management of oil spills, and chemical separation processes to remove non-polar substances from polar compounds. [2]

Hydrophobic is often used interchangeably with lipophilic, "fat-loving". However, the two terms are not synonymous. While hydrophobic substances are usually lipophilic, there are exceptions, such as the silicones and fluorocarbons.[ citation needed ]

The term hydrophobe comes from the Ancient Greek ὑδρόφοβος (hydróphobos), "having a fear of water", constructed from Ancient Greek ὕδωρ (húdōr) 'water',and Ancient Greek φόβος (phóbos) 'fear'. [3]

Chemical background

The hydrophobic interaction is mostly an entropic effect originating from the disruption of the highly dynamic hydrogen bonds between molecules of liquid water by the nonpolar solute, causing the water to form a clathrate-like structure around the non-polar molecules. This structure formed is more highly ordered than free water molecules due to the water molecules arranging themselves to interact as much as possible with themselves, and thus results in a higher entropic state which causes non-polar molecules to clump together to reduce the surface area exposed to water and decrease the entropy of the system. [4] [5] Thus, the two immiscible phases (hydrophilic vs. hydrophobic) will change so that their corresponding interfacial area will be minimal. This effect can be visualized in the phenomenon called phase separation.[ citation needed ]

Superhydrophobicity

A water drop on a lotus plant leaf DropConnectionAngel.jpg
A water drop on a lotus plant leaf

Superhydrophobic surfaces, such as the leaves of the lotus plant, are those that are extremely difficult to wet. The contact angles of a water droplet exceeds 150°. [6] This is referred to as the lotus effect, and is primarily a physical property related to interfacial tension, rather than a chemical property. [7]

Theory

In 1805, Thomas Young defined the contact angle θ by analyzing the forces acting on a fluid droplet resting on a solid surface surrounded by a gas. [8]

A liquid droplet rests on a solid surface and is surrounded by gas. The contact angle, thC, is the angle formed by a liquid at the three-phase boundary where the liquid, gas, and solid intersect. Contact angle.svg
A liquid droplet rests on a solid surface and is surrounded by gas. The contact angle, θC, is the angle formed by a liquid at the three-phase boundary where the liquid, gas, and solid intersect.
A droplet resting on a solid surface and surrounded by a gas forms a characteristic contact angle th. If the solid surface is rough, and the liquid is in intimate contact with the solid asperities, the droplet is in the Wenzel state. If the liquid rests on the tops of the asperities, it is in the Cassie-Baxter state. Contact angle microstates.svg
A droplet resting on a solid surface and surrounded by a gas forms a characteristic contact angle θ. If the solid surface is rough, and the liquid is in intimate contact with the solid asperities, the droplet is in the Wenzel state. If the liquid rests on the tops of the asperities, it is in the Cassie–Baxter state.

where

= Interfacial tension between the solid and gas
= Interfacial tension between the solid and liquid
= Interfacial tension between the liquid and gas

θ can be measured using a contact angle goniometer.

Wenzel determined that when the liquid is in intimate contact with a microstructured surface, θ will change to θW*

where r is the ratio of the actual area to the projected area. [9] Wenzel's equation shows that microstructuring a surface amplifies the natural tendency of the surface. A hydrophobic surface (one that has an original contact angle greater than 90°) becomes more hydrophobic when microstructured – its new contact angle becomes greater than the original. However, a hydrophilic surface (one that has an original contact angle less than 90°) becomes more hydrophilic when microstructured – its new contact angle becomes less than the original. [10] Cassie and Baxter found that if the liquid is suspended on the tops of microstructures, θ will change to θCB*:

where φ is the area fraction of the solid that touches the liquid. [11] Liquid in the Cassie–Baxter state is more mobile than in the Wenzel state.[ citation needed ]

We can predict whether the Wenzel or Cassie–Baxter state should exist by calculating the new contact angle with both equations. By a minimization of free energy argument, the relation that predicted the smaller new contact angle is the state most likely to exist. Stated in mathematical terms, for the Cassie–Baxter state to exist, the following inequality must be true. [12]

A recent alternative criterion for the Cassie–Baxter state asserts that the Cassie–Baxter state exists when the following 2 criteria are met:1) Contact line forces overcome body forces of unsupported droplet weight and 2) The microstructures are tall enough to prevent the liquid that bridges microstructures from touching the base of the microstructures. [13]

A new criterion for the switch between Wenzel and Cassie-Baxter states has been developed recently based on surface roughness and surface energy. [14] The criterion focuses on the air-trapping capability under liquid droplets on rough surfaces, which could tell whether Wenzel's model or Cassie-Baxter's model should be used for certain combination of surface roughness and energy.[ citation needed ]

Contact angle is a measure of static hydrophobicity, and contact angle hysteresis and slide angle are dynamic measures. Contact angle hysteresis is a phenomenon that characterizes surface heterogeneity. [15] When a pipette injects a liquid onto a solid, the liquid will form some contact angle. As the pipette injects more liquid, the droplet will increase in volume, the contact angle will increase, but its three-phase boundary will remain stationary until it suddenly advances outward. The contact angle the droplet had immediately before advancing outward is termed the advancing contact angle. The receding contact angle is now measured by pumping the liquid back out of the droplet. The droplet will decrease in volume, the contact angle will decrease, but its three-phase boundary will remain stationary until it suddenly recedes inward. The contact angle the droplet had immediately before receding inward is termed the receding contact angle. The difference between advancing and receding contact angles is termed contact angle hysteresis and can be used to characterize surface heterogeneity, roughness, and mobility. [16] Surfaces that are not homogeneous will have domains that impede motion of the contact line. The slide angle is another dynamic measure of hydrophobicity and is measured by depositing a droplet on a surface and tilting the surface until the droplet begins to slide. In general, liquids in the Cassie–Baxter state exhibit lower slide angles and contact angle hysteresis than those in the Wenzel state.[ citation needed ]

Research and development

Water droplets roll down an inclined hydrophobic surface.
Water droplets on an artificial hydrophobic surface (left)

Dettre and Johnson discovered in 1964 that the superhydrophobic lotus effect phenomenon was related to rough hydrophobic surfaces, and they developed a theoretical model based on experiments with glass beads coated with paraffin or TFE telomer. The self-cleaning property of superhydrophobic micro-nanostructured surfaces was reported in 1977. [17] Perfluoroalkyl, perfluoropolyether, and RF plasma -formed superhydrophobic materials were developed, used for electrowetting and commercialized for bio-medical applications between 1986 and 1995. [18] [19] [20] [21] Other technology and applications have emerged since the mid-1990s. [22] A durable superhydrophobic hierarchical composition, applied in one or two steps, was disclosed in 2002 comprising nano-sized particles ≤ 100 nanometers overlaying a surface having micrometer-sized features or particles ≤ 100 micrometers. The larger particles were observed to protect the smaller particles from mechanical abrasion. [23]

In recent research, superhydrophobicity has been reported by allowing alkylketene dimer (AKD) to solidify into a nanostructured fractal surface. [24] Many papers have since presented fabrication methods for producing superhydrophobic surfaces including particle deposition, [25] sol-gel techniques, [26] plasma treatments, [27] vapor deposition, [25] and casting techniques. [28] Current opportunity for research impact lies mainly in fundamental research and practical manufacturing. [29] Debates have recently emerged concerning the applicability of the Wenzel and Cassie–Baxter models. In an experiment designed to challenge the surface energy perspective of the Wenzel and Cassie–Baxter model and promote a contact line perspective, water drops were placed on a smooth hydrophobic spot in a rough hydrophobic field, a rough hydrophobic spot in a smooth hydrophobic field, and a hydrophilic spot in a hydrophobic field. [30] Experiments showed that the surface chemistry and geometry at the contact line affected the contact angle and contact angle hysteresis, but the surface area inside the contact line had no effect. An argument that increased jaggedness in the contact line enhances droplet mobility has also been proposed. [31]

Many hydrophobic materials found in nature rely on Cassie's law and are biphasic on the submicrometer level with one component air. The lotus effect is based on this principle. Inspired by it, many functional superhydrophobic surfaces have been prepared. [32]

An example of a bionic or biomimetic superhydrophobic material in nanotechnology is nanopin film.[ citation needed ]

One study presents a vanadium pentoxide surface that switches reversibly between superhydrophobicity and superhydrophilicity under the influence of UV radiation. [33] According to the study, any surface can be modified to this effect by application of a suspension of rose-like V2O5 particles, for instance with an inkjet printer. Once again hydrophobicity is induced by interlaminar air pockets (separated by 2.1 nm distances). The UV effect is also explained. UV light creates electron-hole pairs, with the holes reacting with lattice oxygen, creating surface oxygen vacancies, while the electrons reduce V5+ to V3+. The oxygen vacancies are met by water, and it is this water absorbency by the vanadium surface that makes it hydrophilic. By extended storage in the dark, water is replaced by oxygen and hydrophilicity is once again lost.[ citation needed ]

A significant majority of hydrophobic surfaces have their hydrophobic properties imparted by structural or chemical modification of a surface of a bulk material, through either coatings or surface treatments. That is to say, the presence of molecular species (usually organic) or structural features results in high contact angles of water. In recent years, rare earth oxides have been shown to possess intrinsic hydrophobicity. [34] The intrinsic hydrophobicity of rare earth oxides depends on surface orientation and oxygen vacancy levels, and is naturally more robust than coatings or surface treatments, having potential applications in condensers and catalysts that can operate at high temperatures or corrosive environments. [35]

Applications and potential applications

Hydrophobic concrete has been produced since the mid-20th century.[ citation needed ]

Active recent research on superhydrophobic materials might eventually lead to more industrial applications.[ citation needed ]

A simple routine of coating cotton fabric with silica [36] or titania [37] particles by sol-gel technique has been reported, which protects the fabric from UV light and makes it superhydrophobic.

An efficient routine has been reported for making polyethylene superhydrophobic and thus self-cleaning. [38] 99% of dirt on such a surface is easily washed away.

Patterned superhydrophobic surfaces also have promise for lab-on-a-chip microfluidic devices and can drastically improve surface-based bioanalysis. [39]

In pharmaceuticals, hydrophobicity of pharmaceutical blends affects important quality attributes of final products, such as drug dissolution and hardness. [40] Methods have been developed to measure the hydrophobicity of pharmaceutical materials. [41] [42]

The development of hydrophobic passive daytime radiative cooling (PDRC) surfaces, whose effectiveness at solar reflectance and thermal emittance is predicated on their cleanliness, has improved the "self-cleaning" of these surfaces. Scalable and sustainable hydrophobic PDRCs that avoid VOCs have further been developed. [43]

See also

Related Research Articles

<span class="mw-page-title-main">Surface energy</span> Excess energy at the surface of a material relative to its interior

In surface science, surface energy quantifies the disruption of intermolecular bonds that occurs when a surface is created. In solid-state physics, surfaces must be intrinsically less energetically favorable than the bulk of the material, otherwise there would be a driving force for surfaces to be created, removing the bulk of the material. The surface energy may therefore be defined as the excess energy at the surface of a material compared to the bulk, or it is the work required to build an area of a particular surface. Another way to view the surface energy is to relate it to the work required to cut a bulk sample, creating two surfaces. There is "excess energy" as a result of the now-incomplete, unrealized bonding between the two created surfaces.

<span class="mw-page-title-main">Adhesion</span> Molecular property

Adhesion is the tendency of dissimilar particles or surfaces to cling to one another.

<span class="mw-page-title-main">Hydrophobic effect</span> Aggregation of non-polar molecules in aqueous solutions

The hydrophobic effect is the observed tendency of nonpolar substances to aggregate in an aqueous solution and to be excluded by water. The word hydrophobic literally means "water-fearing", and it describes the segregation of water and nonpolar substances, which maximizes the entropy of water and minimizes the area of contact between water and nonpolar molecules. In terms of thermodynamics, the hydrophobic effect is the free energy change of water surrounding a solute. A positive free energy change of the surrounding solvent indicates hydrophobicity, whereas a negative free energy change implies hydrophilicity.

<span class="mw-page-title-main">Wetting</span> Ability of a liquid to maintain contact with a solid surface

Wetting is the ability of a liquid to maintain contact with a solid surface, resulting from intermolecular interactions when the two are brought together. This happens in presence of a gaseous phase or another liquid phase not miscible with the first one. The degree of wetting (wettability) is determined by a force balance between adhesive and cohesive forces. There are two types of wetting: non-reactive wetting and reactive wetting.

<span class="mw-page-title-main">Contact angle</span> The angle between a liquid–vapor interface and a solid surface

The contact angle is the angle between a liquid surface and a solid surface where they meet. More specifically, it is the angle between the surface tangent on the liquid–vapor interface and the tangent on the solid–liquid interface at their intersection. It quantifies the wettability of a solid surface by a liquid via the Young equation.

<span class="mw-page-title-main">Lotus effect</span> Self-cleaning properties

The lotus effect refers to self-cleaning properties that are a result of ultrahydrophobicity as exhibited by the leaves of Nelumbo, the lotus flower. Dirt particles are picked up by water droplets due to the micro- and nanoscopic architecture on the surface, which minimizes the droplet's adhesion to that surface. Ultrahydrophobicity and self-cleaning properties are also found in other plants, such as Tropaeolum (nasturtium), Opuntia, Alchemilla, cane, and also on the wings of certain insects.

<span class="mw-page-title-main">Cassie's law</span>

Cassie's law, or the Cassie equation, describes the effective contact angle θc for a liquid on a chemically heterogeneous surface, i.e. the surface of a composite material consisting of different chemistries, that is non uniform throughout. Contact angles are important as they quantify a surface's wettability, the nature of solid-fluid intermolecular interactions. Cassie's law is reserved for when a liquid completely covers both smooth and rough heterogeneous surfaces.

<span class="mw-page-title-main">Ultrahydrophobicity</span> Material property of extreme resistance to wetting

In chemistry and materials science, ultrahydrophobic surfaces are highly hydrophobic, i.e., extremely difficult to wet. The contact angles of a water droplet on an ultrahydrophobic material exceed 150°. This is also referred to as the lotus effect, after the superhydrophobic leaves of the lotus plant. A droplet striking these kinds of surfaces can fully rebound like an elastic ball. Interactions of bouncing drops can be further reduced using special superhydrophobic surfaces that promote symmetry breaking, pancake bouncing or waterbowl bouncing.

<span class="mw-page-title-main">Coffee ring effect</span> Capillary flow effect

In physics, a "coffee ring" is a pattern left by a puddle of particle-laden liquid after it evaporates. The phenomenon is named for the characteristic ring-like deposit along the perimeter of a spill of coffee. It is also commonly seen after spilling red wine. The mechanism behind the formation of these and similar rings is known as the coffee ring effect or in some instances, the coffee stain effect, or simply ring stain.

Self-cleaning glass is a specific type of glass with a surface that keeps itself free of dirt and grime.

<span class="mw-page-title-main">Water-in-water emulsion</span>

Water-in-water (W/W) emulsion is a system that consists of droplets of water-solvated molecules in another continuous aqueous solution; both the droplet and continuous phases contain different molecules that are entirely water-soluble. As such, when two entirely aqueous solutions containing different water-soluble molecules are mixed, water droplets containing predominantly one component are dispersed in water solution containing another component. Recently, such a water-in-water emulsion was demonstrated to exist and be stable from coalescence by the separation of different types of non-amphiphilic, but water-soluble molecular interactions. These molecular interactions include hydrogen bonding, pi stacking, and salt bridging. This w/w emulsion was generated when the different water-solvated molecular functional groups get segregated in an aqueous mixture consisting of polymer and liquid crystal molecules.

A Ramsden emulsion, sometimes named Pickering emulsion, is an emulsion that is stabilized by solid particles which adsorb onto the interface between the water and oil phases. Typically, the emulsions are either water-in-oil or oil-in-water emulsions, but other more complex systems such as water-in-water, oil-in-oil, water-in-oil-in-water, and oil-in-water-in-oil also do exist. Pickering emulsions were named after S.U. Pickering, who described the phenomenon in 1907, although the effect was first recognized by Walter Ramsden in 1903.

<span class="mw-page-title-main">Janus particles</span> Type of nanoparticle or microparticle

Janus particles are special types of nanoparticles or microparticles whose surfaces have two or more distinct physical properties. This unique surface of Janus particles allows two different types of chemistry to occur on the same particle. The simplest case of a Janus particle is achieved by dividing the particle into two distinct parts, each of them either made of a different material, or bearing different functional groups. For example, a Janus particle may have one half of its surface composed of hydrophilic groups and the other half hydrophobic groups, the particles might have two surfaces of different color, fluorescence, or magnetic properties. This gives these particles unique properties related to their asymmetric structure and/or functionalization.

A wetting transition may occur during the process of wetting of a solid surface with a liquid. The transition corresponds to a certain change in contact angle, the macroscopic parameter characterizing wetting. Various contact angles can co-exist on the same solid substrate. Wetting transitions may occur in a different way depending on whether the surface is flat or rough.

<span class="mw-page-title-main">Superhydrophobic coating</span> Water-repellant coating

A superhydrophobic coating is a thin surface layer that repels water. It is made from superhydrophobic (ultrahydrophobicity) materials. Droplets hitting this kind of coating can fully rebound. Generally speaking, superhydrophobic coatings are made from composite materials where one component provides the roughness and the other provides low surface energy.

The surface chemistry of paper is responsible for many important paper properties, such as gloss, waterproofing, and printability. Many components are used in the paper-making process that affect the surface.

Icephobicity is the ability of a solid surface to repel ice or prevent ice formation due to a certain topographical structure of the surface. The word “icephobic” was used for the first time at least in 1950; however, the progress in micropatterned surfaces resulted in growing interest towards icephobicity since the 2000s.

<span class="mw-page-title-main">Edward Bormashenko</span>

Edward Bormashenko is a professor of Materials Science and the Head of the Laboratory of Interface Science of the Ariel University in Israel. He was born in 1962 in Kharkiv, Ukraine and lives in Israel since 1997. He studied in the V. N. Karazin Kharkiv National University. His research is in the polymer science and surface science. He accomplished his PhD in Moscow Institute of Plastics in 1990.

Self-cleaning surfaces are a class of materials with the inherent ability to remove any debris or bacteria from their surfaces in a variety of ways. The self-cleaning functionality of these surfaces are commonly inspired by natural phenomena observed in lotus leaves, gecko feet, and water striders to name a few. The majority of self-cleaning surfaces can be placed into three categories:

  1. superhydrophobic
  2. superhydrophilic
  3. photocatalytic.
<span class="mw-page-title-main">Liquid marbles</span>

Liquid marbles are non-stick droplets wrapped by micro- or nano-metrically scaled hydrophobic, colloidal particles ; representing a platform for a diversity of chemical and biological applications. Liquid marbles are also found naturally; aphids convert honeydew droplets into marbles. A variety of non-organic and organic liquids may be converted into liquid marbles. Liquid marbles demonstrate elastic properties and do not coalesce when bounced or pressed lightly. Liquid marbles demonstrate a potential as micro-reactors, micro-containers for growing micro-organisms and cells, micro-fluidics devices, and have even been used in unconventional computing. Liquid marbles remain stable on solid and liquid surfaces. Statics and dynamics of rolling and bouncing of liquid marbles were reported. Liquid marbles coated with poly-disperse and mono-disperse particles have been reported. Liquid marbles are not hermetically coated by solid particles but connected to the gaseous phase. Kinetics of the evaporation of liquid marbles has been investigated.

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