![Dew 2](https://upload.wikimedia.org/wikipedia/commons/8/8c/Dew_2.jpg)
In
chemistry, hydrophobicity is the physical property of a
molecule
A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and bioche ...
that is seemingly
repelled from a mass of
water
Water (chemical formula ) is an Inorganic compound, inorganic, transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living ...
(known as a hydrophobe). In contrast,
hydrophile
A hydrophile is a molecule or other molecular entity that is attracted to water molecules and tends to be dissolved by water.Liddell, H.G. & Scott, R. (1940). ''A Greek-English Lexicon'' Oxford: Clarendon Press.
In contrast, hydrophobes are no ...
s are attracted to water.
Hydrophobic molecules tend to be
nonpolar
In chemistry, polarity is a separation of electric charge leading to a molecule or its chemical groups having an electric dipole moment, with a negatively charged end and a positively charged end.
Polar molecules must contain one or more polar ...
and, thus, prefer other neutral molecules and nonpolar
solvent
A solvent (s) (from the Latin '' solvō'', "loosen, untie, solve") is a substance that dissolves a solute, resulting in a solution. A solvent is usually a liquid but can also be a solid, a gas, or a supercritical fluid. Water is a solvent for ...
s. Because water molecules are polar, hydrophobes do not
dissolve well among them. Hydrophobic molecules in water often cluster together, forming
micelle
A micelle () or micella () (plural micelles or micellae, respectively) is an aggregate (or supramolecular assembly) of surfactant amphipathic lipid molecules dispersed in a liquid, forming a colloidal suspension (also known as associated coll ...
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 meets a solid surface. It quantifies the wettability of a solid surface by a liquid via the Young equation. A given system of solid, liq ...
.
Examples of hydrophobic
molecule
A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and bioche ...
s include the
alkanes,
oil
An oil is any nonpolar chemical substance that is composed primarily of hydrocarbons and is hydrophobic (does not mix with water) & lipophilic (mixes with other oils). Oils are usually flammable and surface active. Most oils are unsaturated ...
s,
fat
In nutrition, biology, and chemistry, fat usually means any ester of fatty acids, or a mixture of such compounds, most commonly those that occur in living beings or in food.
The term often refers specifically to triglycerides (triple est ...
s, 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
Lipophilicity (from Greek λίπος "fat" and 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 solvents are themselves lipo ...
, "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 rubber-like substances. Silicones are used in sealants, adhesives, lubricants, medicine, cookin ...
and
fluorocarbons.
The term ''hydrophobe'' comes from the
Ancient Greek
Ancient Greek includes the forms of the Greek language used in ancient Greece and the ancient world from around 1500 BC to 300 BC. It is often roughly divided into the following periods: Mycenaean Greek (), Dark Ages (), the Archaic p ...
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
Entropy is a scientific concept, as well as a measurable physical property, that is most commonly associated with a state of disorder, randomness, or uncertainty. The term and the concept are used in diverse fields, from classical thermodynam ...
effect originating from the disruption of the highly dynamic
hydrogen bonds between molecules of liquid water by the nonpolar solute forming a
clathrate
A clathrate is a chemical substance consisting of a lattice that traps or contains molecules. The word ''clathrate'' is derived from the Latin (), meaning ‘with bars, latticed’. Most clathrate compounds are polymeric and completely envelop t ...
-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.
Superhydrophobicity
![DropConnectionAngel](https://upload.wikimedia.org/wikipedia/commons/f/f7/DropConnectionAngel.jpg)
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 meets a solid surface. It quantifies the wettability of a solid surface by a liquid via the Young equation. A given system of solid, liq ...
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 architec ...
, and is primarily a physical property related to
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.
![Contact angle](https://upload.wikimedia.org/wikipedia/commons/e/e4/Contact_angle.svg)
:
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 ''θ''
W*
:
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*:
:
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.
Research and development
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 architec ...
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 Electrowetting is the modification of the wetting properties of a surface (which is typically hydrophobic) with an applied electric field.
History
The electrowetting of mercury and other liquids on variably charged surfaces was probably first ex ...
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
Dimer may refer to:
* Dimer (chemistry), a chemical structure formed from two similar sub-units
** Protein dimer, a protein quaternary structure
** d-dimer
* Dimer model, an item in statistical mechanics, based on ''domino tiling''
* Julius 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 Cassie's law, or the Cassie equation, describes the effective contact angle Theta, 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. ...
and are
biphasic
Biphasic, meaning having two phases, may refer to:
* Phase (matter)
In the physical sciences, a phase is a region of space (a thermodynamic system), throughout which all physical properties of a material are essentially uniform. Examples ...
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 to the study and design of engineering systems and modern technology.
The word ''bionic'', coined by Jack E. Steele in August 1 ...
or
biomimetic
Biomimetics or biomimicry is the emulation of the models, systems, and elements of nature for the purpose of solving complex human problems. The terms "biomimetics" and "biomimicry" are derived from grc, βίος (''bios''), life, and μίμησ ...
superhydrophobic material in
nanotechnology is
nanopin film Nanopin film is an experimental material in nanotechnology developed in 2005 with unusual superhydrophobic properties . A droplet of water makes contact with the surface of this film and forms an almost perfect sphere with a contact angle of 178 °. ...
.
One study presents a
vanadium pentoxide
Vanadium(V) oxide (''vanadia'') is the inorganic compound with the formula V2 O5. Commonly known as vanadium pentoxide, it is a brown/yellow solid, although when freshly precipitated from aqueous solution, its colour is deep orange. Because o ...
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 suspende ...
of rose-like V
2O
5 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 inexpensi ...
. 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 V
5+ to V
3+. 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 attracted to water molecules and tends to be dissolved by water.Liddell, H.G. & Scott, R. (1940). ''A Greek-English Lexicon'' Oxford: Clarendon Press.
In contrast, hydrophobes are no ...
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
Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is ...
or
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
Polyethylene or polythene (abbreviated PE; IUPAC name polyethene or poly(methylene)) is the most commonly produced plastic. It is a polymer, primarily used for packaging ( plastic bags, plastic films, geomembranes and containers including b ...
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
In materials science, hardness (antonym: softness) is a measure of the resistance to localized plastic deformation induced by either mechanical indentation or abrasion. In general, different materials differ in their hardness; for example hard ...
. Methods have been developed to measure the hydrophobicity of pharmaceutical materials.
The development of hydrophobic
passive daytime radiative cooling
Passive daytime radiative cooling (PDRC) is a renewable cooling method proposed as a solution to global warming of enhancing terrestrial heat flow to outer space through the installation of thermally-emissive surfaces on Earth that require zer ...
(PDRC) surfaces, whose effectiveness at
solar reflectance
Albedo (; ) is the measure of the diffuse reflection of solar radiation out of the total solar radiation and measured on a scale from 0, corresponding to a black body that absorbs all incident radiation, to 1, corresponding to a body that refle ...
and
thermal emittance Thermal emittance or thermal emissivity (\varepsilon) is the ratio of the radiant emittance of heat of a specific object or surface to that of a standard black body. Emissivity and emittivity are both dimensionless quantities given in the range of ...
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.
See also
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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