In chemistry, hydrophobicity is the physical property of a molecule (known as a hydrophobe) that is seemingly repelled from a mass of water.(Strictly speaking, there is no repulsive force involved; it is an absence of attraction.) In contrast, hydrophiles are attracted to water.
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.
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.
The term hydrophobe comes from the Ancient Greek ὑδρόφόβος (hýdrophóbos), "having a horror of water", constructed from Ancient Greek ὕδωρ (húdōr), meaning 'water',and Ancient Greek φόβος (phóbos), meaning 'fear'.
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 forming 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.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.
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°.This is referred to as the lotus effect, and is primarily a physical property related to interfacial tension, rather than a chemical property.
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.
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. 90°) becomes more hydrophilic when microstructured – its new contact angle becomes less than the original. Cassie and Baxter found that if the liquid is suspended on the tops of microstructures, θ will change to θCB*: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
where φ is the area fraction of the solid that touches the liquid.Liquid in the Cassie–Baxter state is more mobile than in the Wenzel state.
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.
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.
A new criterion for the switch between Wenzel and Cassie-Baxter states has been developed recently based on surface roughness and surface energy.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.
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.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. 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.
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.Perfluoroalkyl, perfluoropolyether, and RF plasma -formed superhydrophobic materials were developed, used for electrowetting and commercialized for bio-medical applications between 1986 and 1995. Other technology and applications have emerged since the mid 1990s. 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.
In recent research, superhydrophobicity has been reported by allowing alkylketene dimer (AKD) to solidify into a nanostructured fractal surface.Many papers have since presented fabrication methods for producing superhydrophobic surfaces including particle deposition, sol-gel techniques, plasma treatments, vapor deposition, and casting techniques. Current opportunity for research impact lies mainly in fundamental research and practical manufacturing. 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. 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.
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.
An example of a bionic or biomimetic superhydrophobic material in nanotechnology is nanopin film.
One study presents a vanadium pentoxide surface that switches reversibly between superhydrophobicity and superhydrophilicity under the influence of UV radiation.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.
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.. 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.
Hydrophobic concrete has been produced since the mid-20th century.
Active recent research on superhydrophobic materials might eventually lead to more industrial applications.
A simple routine of coating cotton fabric with silicaor titania 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.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.
In pharmaceuticals, hydrophobicity of pharmaceutical blends affects important quality attributes of final products, such as drug dissolution and hardness.Methods have been developed to measure the hydrophobicity of pharmaceutical materials.
Adhesion is the tendency of dissimilar particles or surfaces to cling to one another. The forces that cause adhesion and cohesion can be divided into several types. The intermolecular forces responsible for the function of various kinds of stickers and sticky tape fall into the categories of chemical adhesion, dispersive adhesion, and diffusive adhesion. In addition to the cumulative magnitudes of these intermolecular forces, there are also certain emergent mechanical effects.
An artificial membrane, or synthetic membrane, is a synthetically created membrane which is usually intended for separation purposes in laboratory or in industry. Synthetic membranes have been successfully used for small and large-scale industrial processes since the middle of twentieth century. A wide variety of synthetic membranes is known. They can be produced from organic materials such as polymers and liquids, as well as inorganic materials. The most of commercially utilized synthetic membranes in separation industry are made of polymeric structures. They can be classified based on their surface chemistry, bulk structure, morphology, and production method. The chemical and physical properties of synthetic membranes and separated particles as well as a choice of driving force define a particular membrane separation process. The most commonly used driving forces of a membrane process in industry are pressure and concentration gradients. The respective membrane process is therefore known as filtration. Synthetic membranes utilized in a separation process can be of different geometry and of respective flow configuration. They can also be categorized based on their application and separation regime. The best known synthetic membrane separation processes include water purification, reverse osmosis, dehydrogenation of natural gas, removal of cell particles by microfiltration and ultrafiltration, removal of microorganisms from dairy products, and Dialysis.
Wetting is the ability of a liquid to maintain contact with a solid surface, resulting from intermolecular interactions when the two are brought together. The degree of wetting (wettability) is determined by a force balance between adhesive and cohesive forces. Wetting deals with the three phases of materials: gas, liquid, and solid. It is now a center of attention in nanotechnology and nanoscience studies due to the advent of many nanomaterials in the past two decades.
A Langmuir–Blodgett trough is a laboratory apparatus that is used to compress monolayers of molecules on the surface of a given subphase and measures surface phenomena due to this compression. It can also be used to deposit single or multiple monolayers on a solid substrate.
The contact angle is the angle, conventionally measured through the liquid, where a liquid–vapor interface meets a solid surface. It quantifies the wettability of a solid surface by a liquid via the Young equation. A given system of solid, liquid, and vapor at a given temperature and pressure has a unique equilibrium contact angle. However, in practice a dynamic phenomenon of contact angle hysteresis is often observed, ranging from the advancing (maximal) contact angle to the receding (minimal) contact angle. The equilibrium contact is within those values, and can be calculated from them. The equilibrium contact angle reflects the relative strength of the liquid, solid, and vapourmolecular interaction.
The lotus effect refers to self-cleaning properties that are a result of ultrahydrophobicity as exhibited by the leaves of Nelumbo or "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.
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 surfaces 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.
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, or pancake.
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.
A Pickering emulsion is an emulsion that is stabilized by solid particles which adsorb onto the interface between the two phases. This type of emulsion was named after S.U. Pickering, who described the phenomenon in 1907, although the effect was first recognized by Walter Ramsden in 1903.
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.
Adsorption is the adhesion of ions or molecules onto the surface of another phase. Adsorption may occur via physisorption and chemisorption. Ions and molecules can adsorb to many types of surfaces including polymer surfaces. A polymer is a large molecule composed of repeating subunits bound together by covalent bonds. The adsorption of ions and molecules to polymer surfaces plays a role in many applications including: biomedical, structural, and coatings.
A hydrophobic coating is a thin surface layer that repels water. It is made from hydrophobic (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.
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.
An ideal solid surface is flat, rigid, perfectly smooth, and chemically homogeneous, and has zero contact angle hysteresis. Zero hysteresis implies the advancing and receding contact angles are equal.
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, and 3) Photocatalytic.
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.