Hydrophobe
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In
chemistry Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science that covers the Chemical element, elements that make up matter to the chemical compound, compounds composed of atoms, molecules and i ...

chemistry
, hydrophobicity is the physical property of a
molecule File:Pentacene on Ni(111) STM.jpg, A scanning tunneling microscopy image of pentacene molecules, which consist of linear chains of five carbon rings. A molecule is an electrically neutral group of two or more atoms held together by chemical bo ...

molecule
that is seemingly
repelled
repelled
from a mass of
water Water (chemical formula H2O) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts ...

water
(known as a hydrophobe). (There is no repulsive force involved; it is an absence of attraction.) In contrast,
hydrophile A hydrophile is a molecule File:Pentacene on Ni(111) STM.jpg, A scanning tunneling microscopy image of pentacene molecules, which consist of linear chains of five carbon rings. A molecule is an electrically neutral group of two or more atoms ...

hydrophile
s are attracted to water. Hydrophobic molecules tend to be nonpolar and, thus, prefer other neutral molecules and nonpolar
solvent A solvent (from the Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, known as Latium. Through the power of the ...
s. Because water molecules are polar, hydrophobes do not dissolve well among them. Hydrophobic molecules in water often cluster together, forming
micelle Image:Micelle scheme-en.svg, 250px, Scheme of a micelle formed by phospholipids in an aqueous solution A micelle () or micella () (plural micelles or micellae, respectively) is an aggregate (or supramolecular assembly) of surfactant phospholipid ...

micelle
s. Water on hydrophobic surfaces will exhibit a high
contact angle The contact angle is the angle, conventionally measured through the liquid, where a liquid–vapor interface (chemistry), interface meets a solid surface. It quantifies the wetting, wettability of a solid surface by a liquid via the Young equation ...

contact angle
. Examples of hydrophobic
molecule File:Pentacene on Ni(111) STM.jpg, A scanning tunneling microscopy image of pentacene molecules, which consist of linear chains of five carbon rings. A molecule is an electrically neutral group of two or more atoms held together by chemical bo ...

molecule
s include the
alkane , the simplest alkane In organic chemistry, an alkane, or paraffin (a historical trivial name that also has other meanings), is an acyclic Acyclic may refer to: * In chemistry, a compound which is an open-chain compound, e.g. alkanes and acyclic ...
s,
oil An oil is any nonpolar chemical substance A chemical substance is a form of matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can b ...

oil
s,
fat In nutrition Nutrition is the biochemical and physiological process by which an organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual contiguous system that embodies ...

fat
s, and greasy substances in general. Hydrophobic materials are used for oil removal from water, the management of
oil spill An oil spill is the release of a liquid petroleum Petroleum (), also known as crude oil and oil, is a #Latent heat of vaporization, naturally occurring, yellowish-black liquid found in geological formations beneath the Earth, Earth's surfac ...
s, and chemical separation processes to remove non-polar substances from polar compounds. Hydrophobic is often used interchangeably with
lipophilicLipophilicity (from Greek language, Greek λίπος "fat" and :wikt:φίλος, φίλος "friendly"), refers to the ability of a chemical compound to dissolve in fats, oils, lipids, and non-polar solvents such as hexane or toluene. Such non-polar ...
, "fat-loving". However, the two terms are not synonymous. While hydrophobic substances are usually lipophilic, there are exceptions, such as the
silicones A silicone or polysiloxane is a polymer made up of siloxane (−R2Si−O−SiR2−, where R = organic group). They are typically colorless, oils or elastomer, rubber-like substances. Silicones are used in sealants, adhesives, lubricants, medic ...
and
fluorocarbon Fluorocarbons, sometimes referred to as perfluorocarbons or PFCs, are organofluorine compounds with the formula CxFy, i.e., they contain only carbon Carbon (from la, carbo "coal") is a chemical element Image:Simple Periodic Table Char ...
s. The term ''hydrophobe'' comes from the
Ancient Greek Ancient Greek includes the forms of the Greek language used in ancient Greece and the classical antiquity, ancient world from around 1500 BC to 300 BC. It is often roughly divided into the following periods: Mycenaean Greek (), Dark Ages () ...
ὑδρόφόβος (hýdrophóbos), "having a fear of water", constructed .Liddell, H.G. & Scott, R. (1940). ''A Greek-English Lexicon. revised and augmented throughout by Sir Henry Stuart Jones. with the assistance of. Roderick McKenzie.'' Oxford: Clarendon Press.


Chemical background

The hydrophobic interaction is mostly an
entropic
entropic
effect originating from the disruption of the highly dynamic
hydrogen bond A hydrogen bond (or H-bond) is a primarily Electrostatics, electrostatic force of attraction between a hydrogen Hydrogen is the chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of ...

hydrogen bond
s between molecules of liquid water by the nonpolar solute forming a
clathrate A clathrate is a chemical substance A chemical substance is a form of matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched ar ...
-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 Phase or phases may refer to: Science * State of matter, or phase, one of the distinct forms in which matter can exist *Phase (matter) In the physical sciences, a phase is a region of space (a thermodynamic system A thermodynamic system is a ...
separation.


Superhydrophobicity

Superhydrophobic surfaces, such as the leaves of the lotus plant, are those that are extremely difficult to wet. The
contact angle The contact angle is the angle, conventionally measured through the liquid, where a liquid–vapor interface (chemistry), interface meets a solid surface. It quantifies the wetting, wettability of a solid surface by a liquid via the Young equation ...

contact angle
s of a water droplet exceeds 150°. This is referred to as the
lotus effect 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 architectu ...
, and is primarily a physical property related to
interfacial tension Surface tension is the tendency of liquid surfaces to shrink into the minimum surface area possible. Surface tension is what allows heavier than water i.e., denser than water objects such as razor blades, insects (e.g. Gerridae, water striders ...

interfacial tension
, rather than a chemical property.


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. :\gamma_\text\ =\gamma_\text+\gamma_\text\cos\theta \, where :\gamma_\text\ =
Interfacial tension Surface tension is the tendency of liquid surfaces to shrink into the minimum surface area possible. Surface tension is what allows heavier than water i.e., denser than water objects such as razor blades, insects (e.g. Gerridae, water striders ...

Interfacial tension
between the solid and gas :\gamma_\text\ = Interfacial tension between the solid and liquid :\gamma_\text\ = 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 ''θ''W* :\cos\theta_W* = r \cos\theta \, where ''r'' is the ratio of the actual area to the projected area. 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. Cassie and Baxter found that if the liquid is suspended on the tops of microstructures, ''θ'' will change to ''θ''CB*: :\cos\theta_\text* = \varphi(\cos\theta + 1) - 1 \, 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. :\cos\theta<\frac 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.


Research and development

Water droplets on an artificial hydrophobic surface (left) Dettre and Johnson discovered in 1964 that the superhydrophobic
lotus effect 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 architectu ...
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
nanostructured
surfaces was reported in 1977. Perfluoroalkyl, perfluoropolyether, and RF plasma -formed superhydrophobic materials were developed, used for
electrowettingElectrowetting is the modification of the wetting 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) ...

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 lawCassie's law, or the Cassie equation, describes the effective contact angle The contact angle is the angle In Euclidean geometry, an angle is the figure formed by two Ray (geometry), rays, called the ''sides'' of the angle, sharing a common endp ...
and are
biphasic Biphasic, meaning having two phases, may refer to: * Phase (matter), in the physical sciences, a biphasic system, e.g. one involving liquid water and steam * Biphasic sleep, a nap or siesta in addition to the usual sleep episode at night * Phase (p ...
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 Bionics or biologically inspired engineering is the application of biological methods and systems found in nature Nature, in the broadest sense, is the natural, physical, material world or universe The universe ( la, universus) is a ...
or
biomimetic Biomimetics or biomimicry is the emulation of the models, systems, and elements of nature for the purpose of solving complex human Humans (''Homo sapiens'') are the most populous and widespread species of primates, characterized by bipedality ...
superhydrophobic material in
nanotechnology Nanotechnology, also shortened to nanotech, is the use of matter on an atomic, molecular, and Supramolecular complex, supramolecular scale for industrial purposes. The earliest, widespread description of nanotechnology referred to the particul ...

nanotechnology
is
nanopin filmNanopin film is an experimental material in nanotechnology developed in 2005 with unusual hydrophobic, superhydrophobic properties . A droplet of water makes contact with the surface of this film and forms an almost perfect sphere with a contact angl ...
. One study presents a
vanadium pentoxide Vanadium(V) oxide (''vanadia'') is the inorganic compound In chemistry Chemistry is the scientific discipline involved with Chemical element, elements and chemical compound, compounds composed of atoms, molecules and ions: their composition ...

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 Suspension or suspended may refer to: Science and engineering * Suspension (topology), in mathematics * Suspension (dynamical systems), in mathematics * Suspension of a ring, in mathematics * Suspension (chemistry), small solid particles suspended ...
of rose-like V2O5 particles, for instance with an
inkjet printer Inkjet printing is a type of computer printing that recreates a digital image by propelling droplets of ink onto paper and plastic substrates. Inkjet printers were the most commonly used type of printer in 2008, and range from small inexpensiv ...
. 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 A hydrophile is a molecule or other molecular entity that is intermolecular force, attracted to water molecules and tends to be dissolution (chemistry), dissolved by water.Liddell, H.G. & Scott, R. (1940). ''A Greek-English Lexicon'' Oxford: Claren ...
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.


Applications and potential applications

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 silica or titanium dioxide, titania particles by sol–gel process, 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 Dissolution testing, drug dissolution and Tablet hardness testing, hardness. Methods have been developed to measure the hydrophobicity of pharmaceutical materials.


See also

* * * * * * * * * * * *


References


External links


What are superhydrophobic surfaces?

The difference between Hydrophobic and Hydrophilic
{{authority control Chemical properties Intermolecular forces Surface science Articles containing video clips