In chemistry , hydrophobicity is the chemical property of a molecule that is seemingly repelled from a mass of water (called a hydrophobe ). In contrast, hydrophiles are attracted to water.
92-504: 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,
184-424: A suspension of rose-like V 2 O 5 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 V to V. The oxygen vacancies are met by water, and it
276-542: A charged object is not based on polarity. The deflection occurs because of electrically charged droplets in the stream, which the charged object induces. A stream of water can also be deflected in a uniform electrical field, which cannot exert force on polar molecules. Additionally, after a stream of water is grounded, it can no longer be deflected. Weak deflection is even possible for nonpolar liquids. Surface energy In surface science , surface energy (also interfacial free energy or surface free energy ) quantifies
368-402: A cube from the bulk of a material to the surface, energy is required. This energy cost is incorporated into the surface energy of the material, which is quantified by: where z σ and z β are coordination numbers corresponding to the surface and the bulk regions of the material, and are equal to 5 and 6, respectively; a 0 is the surface area of an individual molecule, and W AA
460-412: A difference of zero. A completely polar bond is more correctly called an ionic bond , and occurs when the difference between electronegativities is large enough that one atom actually takes an electron from the other. The terms "polar" and "nonpolar" are usually applied to covalent bonds , that is, bonds where the polarity is not complete. To determine the polarity of a covalent bond using numerical means,
552-399: A dipole moment because, by definition, D point groups have two or multiple C n axes. Since C 1 , C s ,C ∞h C n and C n v point groups do not have a centre of inversion, horizontal mirror planes or multiple C n axis, molecules in one of those point groups will have dipole moment. Contrary to popular misconception, the electrical deflection of a stream of water from
644-406: A drop of liquid on a solid substrate. If the surface energy of the substrate changes upon the addition of the drop, the substrate is said to be wetting . The spreading parameter can be used to mathematically determine this: where S is the spreading parameter, γ s the surface energy of the substrate, γ l the surface energy of the liquid, and γ s-l the interfacial energy between
736-402: A fear of water", constructed from Ancient Greek ὕδωρ (húdōr) 'water' and Ancient Greek φόβος (phóbos) '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, causing the water to form a clathrate -like structure around
828-464: A liquid medium. A wide variety of surface treatments have been previously used, including the adsorption on the surface of a molecule in the presence of polar groups, monolayers of polymers, and layers of inorganic oxides on the surface of organic pigments. New surfaces are constantly being created as larger pigment particles get broken down into smaller subparticles. These newly-formed surfaces consequently contribute to larger surface energies, whereby
920-566: 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
1012-537: A molar mass M = 18 and a boiling point of +100 °C, compared to nonpolar methane with M = 16 and a boiling point of –161 °C. Due to the polar nature of the water molecule itself, other polar molecules are generally able to dissolve in water. Most nonpolar molecules are water-insoluble ( hydrophobic ) at room temperature. Many nonpolar organic solvents , such as turpentine , are able to dissolve nonpolar substances. Polar compounds tend to have higher surface tension than nonpolar compounds. Polar liquids have
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#17328813344101104-423: A net dipole. The dipole moment of water depends on its state. In the gas phase the dipole moment is ≈ 1.86 debye (D), whereas liquid water (≈ 2.95 D) and ice (≈ 3.09 D) are higher due to differing hydrogen-bonded environments. Other examples include sugars (like sucrose ), which have many polar oxygen–hydrogen (−OH) groups and are overall highly polar. If the bond dipole moments of the molecule do not cancel,
1196-423: A positive charge (blue). The hydrogen fluoride , HF, molecule is polar by virtue of polar covalent bonds – in the covalent bond electrons are displaced toward the more electronegative fluorine atom. Ammonia , NH 3 , is a molecule whose three N−H bonds have only a slight polarity (toward the more electronegative nitrogen atom). The molecule has two lone electrons in an orbital that points towards
1288-528: A positively charged end. Polar molecules must contain one or more polar bonds due to a difference in electronegativity between the bonded atoms. Molecules containing polar bonds have no molecular polarity if the bond dipoles cancel each other out by symmetry. Polar molecules interact through dipole-dipole intermolecular forces and hydrogen bonds . Polarity underlies a number of physical properties including surface tension , solubility , and melting and boiling points. Not all atoms attract electrons with
1380-438: A result of polar bonds due to differences in electronegativity as described above, or as a result of an asymmetric arrangement of nonpolar covalent bonds and non-bonding pairs of electrons known as a full molecular orbital . While the molecules can be described as "polar covalent", "nonpolar covalent", or "ionic", this is often a relative term, with one molecule simply being more polar or more nonpolar than another. However,
1472-583: 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
1564-406: A surface freshly prepared in vacuum . Surfaces often change their form away from the simple " cleaved bond " model just implied above. They are found to be highly dynamic regions, which readily rearrange or react , so that energy is often reduced by such processes as passivation or adsorption . The most common way to measure surface energy is through contact angle experiments. In this method,
1656-673: 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
1748-582: 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. The development of hydrophobic passive daytime radiative cooling (PDRC) surfaces, whose effectiveness at solar reflectance and thermal emittance
1840-528: A surface. As such the Gibbs free energy of the system is minimized when the surface is curved. The Kelvin equation is based on thermodynamic principles and is used to describe changes in vapor pressure caused by liquids with curved surfaces. The cause for this change in vapor pressure is the Laplace pressure. The vapor pressure of a drop is higher than that of a planar surface because the increased Laplace pressure causes
1932-417: A symmetrical molecule such as bromine , Br 2 , has zero dipole moment, while near the other extreme, gas phase potassium bromide , KBr, which is highly ionic, has a dipole moment of 10.41 D. For polyatomic molecules, there is more than one bond. The total molecular dipole moment may be approximated as the vector sum of the individual bond dipole moments. Often bond dipoles are obtained by
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#17328813344102024-416: A tendency to rise against gravity in a small diameter tube. Polar liquids have a tendency to be more viscous than nonpolar liquids. For example, nonpolar hexane is much less viscous than polar water. However, molecule size is a much stronger factor on viscosity than polarity, where compounds with larger molecules are more viscous than compounds with smaller molecules. Thus, water (small polar molecules)
2116-446: A two equal vectors that oppose each other will cancel out. Any molecule with a centre of inversion ("i") or a horizontal mirror plane ("σ h ") will not possess dipole moments. Likewise, a molecule with more than one C n axis of rotation will not possess a dipole moment because dipole moments cannot lie in more than one dimension . As a consequence of that constraint, all molecules with dihedral symmetry (D n ) will not have
2208-420: Is OWRK, which requires the use of two probe liquids and gives out as a result the total surface energy as well as divides it into polar and dispersive components. Contact angle method is the standard surface energy measurement method due to its simplicity, applicability to a wide range of surfaces and quickness. The measurement can be fully automated and is standardized. In general, as surface energy increases,
2300-470: Is a dipole across the whole ozone molecule. A molecule may be nonpolar either when there is an equal sharing of electrons between the two atoms of a diatomic molecule or because of the symmetrical arrangement of polar bonds in a more complex molecule. For example, boron trifluoride (BF 3 ) has a trigonal planar arrangement of three polar bonds at 120°. This results in no overall dipole in the molecule. Carbon dioxide (CO 2 ) has two polar C=O bonds, but
2392-490: 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
2484-431: Is a vector, parallel to the bond axis, pointing from minus to plus, as is conventional for electric dipole moment vectors. Chemists often draw the vector pointing from plus to minus. This vector can be physically interpreted as the movement undergone by electrons when the two atoms are placed a distance d apart and allowed to interact, the electrons will move from their free state positions to be localised more around
2576-421: Is an essential requirement for pigment dispersions; for wetting to be effective, the surface tension of the pigment's vehicle must be lower than the surface free energy of the pigment. This allows the vehicle to penetrate into the interstices of the pigment aggregates, thus ensuring complete wetting. Finally, the particles are subjected to a repulsive force in order to keep them separated from one another and lowers
2668-468: 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
2760-426: Is calculated by multiplying the amount of charge separated and the distance between the charges. These dipoles within molecules can interact with dipoles in other molecules, creating dipole-dipole intermolecular forces . Bonds can fall between one of two extremes – completely nonpolar or completely polar. A completely nonpolar bond occurs when the electronegativities are identical and therefore possess
2852-425: Is less viscous than hexadecane (large nonpolar molecules). A polar molecule has a net dipole as a result of the opposing charges (i.e. having partial positive and partial negative charges) from polar bonds arranged asymmetrically. Water (H 2 O) is an example of a polar molecule since it has a slight positive charge on one side and a slight negative charge on the other. The dipoles do not cancel out, resulting in
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2944-403: Is no overall dipole in the molecule. The diatomic oxygen molecule (O 2 ) does not have polarity in the covalent bond because of equal electronegativity, hence there is no polarity in the molecule. Large molecules that have one end with polar groups attached and another end with nonpolar groups are described as amphiphiles or amphiphilic molecules. They are good surfactants and can aid in
3036-402: Is only strictly true for amorphous solids ( glass ) and liquids, isotropy is a good approximation for many other materials. In particular, if the sample is polygranular (most metals) or made by powder sintering (most ceramics) this is a good approximation. In the case of single-crystal materials, such as natural gemstones , anisotropy in the surface energy leads to faceting . The shape of
3128-425: Is possible to determine the pairwise intermolecular energy. Incorporating this value into the surface energy equation allows for the surface energy to be estimated. The following equation can be used as a reasonable estimate for surface energy: The presence of an interface influences generally all thermodynamic parameters of a system. There are two models that are commonly used to demonstrate interfacial phenomena:
3220-410: 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. Chemical polarity#Nonpolar molecules 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
3312-409: 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
3404-438: Is the molar volume of the liquid, R is the universal gas constant , T is temperature (in kelvin ), and R 1 and R 2 are the principal radii of curvature of the surface. Pigments offer great potential in modifying the application properties of a coating. Due to their fine particle size and inherently high surface energy, they often require a surface treatment in order to enhance their ease of dispersion in
3496-401: Is the interfacial energy between the solid and gas phases, γ s-l the interfacial energy between the substrate and the liquid, γ l-g is the interfacial energy between the liquid and gas phases, and θ is the contact angle between the solid–liquid and the liquid–gas interface. The energy of the bulk component of a solid substrate is determined by the types of interactions that hold
3588-471: Is the pairwise intermolecular energy. Surface area can be determined by squaring the cube root of the volume of the molecule: Here, M̄ corresponds to the molar mass of the molecule, ρ corresponds to the density, and N A is the Avogadro constant . In order to determine the pairwise intermolecular energy, all intermolecular forces in the material must be broken. This allows thorough investigation of
3680-404: 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,
3772-425: Is to break down aggregates and form stable dispersions of optimally sized pigment particles. This process generally involves three distinct stages: wetting, deaggregation, and stabilization. A surface that is easy to wet is desirable when formulating a coating that requires good adhesion and appearance. This also minimizes the risks of surface tension related defects, such as crawling, cratering, and orange peel . This
Hydrophobe - Misplaced Pages Continue
3864-402: Is uneven – since the central atom has to share electrons with two other atoms, but each of the outer atoms has to share electrons with only one other atom, the central atom is more deprived of electrons than the others (the central atom has a formal charge of +1, while the outer atoms each have a formal charge of − 1 ⁄ 2 ). Since the molecule has a bent geometry, the result
3956-534: The Gibbs ideal interface model and the Guggenheim model. In order to demonstrate the thermodynamics of an interfacial system using the Gibbs model, the system can be divided into three parts: two immiscible liquids with volumes V α and V β and an infinitesimally thin boundary layer known as the Gibbs dividing plane ( σ ) separating these two volumes. The total volume of the system is: All extensive quantities of
4048-444: The conversion factor of 10 statcoulomb being 0.208 units of elementary charge, so 1.0 debye results from an electron and a proton separated by 0.208 Å. A useful conversion factor is 1 D = 3.335 64 × 10 C m. For diatomic molecules there is only one (single or multiple) bond so the bond dipole moment is the molecular dipole moment, with typical values in the range of 0 to 11 D. At one extreme,
4140-415: 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
4232-603: The atoms, as electrons will be drawn closer to the atom with the higher electronegativity. Because electrons have a negative charge, the unequal sharing of electrons within a bond leads to the formation of an electric dipole : a separation of positive and negative electric charge. Because the amount of charge separated in such dipoles is usually smaller than a fundamental charge , they are called partial charges , denoted as δ+ ( delta plus) and δ− (delta minus). These symbols were introduced by Sir Christopher Ingold and Edith Hilda (Usherwood) Ingold in 1926. The bond dipole moment
4324-427: The bulk phases. The concentration of molecules present at the interface can be defined as: where c iα and c iβ represent the concentration of substance i in bulk phase α and β , respectively. It is beneficial to define a new term interfacial excess Γ i which allows us to describe the number of molecules per unit area: Surface energy comes into play in wetting phenomena. To examine this, consider
4416-405: The contact angle decreases because more of the liquid is being "grabbed" by the surface. Conversely, as surface energy decreases, the contact angle increases, because the surface doesn't want to interact with the liquid. The surface energy of a liquid may be measured by stretching a liquid membrane (which increases the surface area and hence the surface energy). In that case, in order to increase
4508-440: The contact angle of the surface is measured with several liquids, usually water and diiodomethane . Based on the contact angle results and knowing the surface tension of the liquids, the surface energy can be calculated. In practice, this analysis is done automatically by a contact angle meter. There are several different models for calculating the surface energy based on the contact angle readings. The most commonly used method
4600-457: The crystal (assuming equilibrium growth conditions) is related to the surface energy by the Wulff construction . The surface energy of the facets can thus be found to within a scaling constant by measuring the relative sizes of the facets. In the deformation of solids, surface energy can be treated as the "energy required to create one unit of surface area", and is a function of the difference between
4692-465: The difference between the electronegativity of the atoms is used. Bond polarity is typically divided into three groups that are loosely based on the difference in electronegativity between the two bonded atoms. According to the Pauling scale : Pauling based this classification scheme on the partial ionic character of a bond, which is an approximate function of the difference in electronegativity between
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#17328813344104784-453: 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 (that is, the atoms on the surface must have more energy than the atoms in the bulk), otherwise there would be a driving force for surfaces to be created, removing the bulk of the material by sublimation . The surface energy may therefore be defined as
4876-406: 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
4968-441: 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. Cutting a solid body into pieces disrupts its bonds and increases
5060-541: The following expression: where For a slab, we have two surfaces and they are of the same type, which is reflected by the number 2 in the denominator. To guarantee this, we need to create the slab carefully to make sure that the upper and lower surfaces are of the same type. Strength of adhesive contacts is determined by the work of adhesion which is also called relative surface energy of two contacting bodies. The relative surface energy can be determined by detaching of bodies of well defined shape made of one material from
5152-483: The following properties are typical of such molecules. When comparing a polar and nonpolar molecule with similar molar masses, the polar molecule in general has a higher boiling point, because the dipole–dipole interaction between polar molecules results in stronger intermolecular attractions. One common form of polar interaction is the hydrogen bond , which is also known as the H-bond. For example, water forms H-bonds and has
5244-408: The forces acting on a fluid droplet resting on a solid surface surrounded by a gas. where 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
5336-489: The formation of stable emulsions, or blends, of water and fats. Surfactants reduce the interfacial tension between oil and water by adsorbing at the liquid–liquid interface. Determining the point group is a useful way to predict polarity of a molecule. In general, a molecule will not possess dipole moment if the individual bond dipole moments of the molecule cancel each other out. This is because dipole moments are euclidean vector quantities with magnitude and direction, and
5428-472: The fourth apex of an approximately regular tetrahedron, as predicted by the VSEPR theory . This orbital is not participating in covalent bonding; it is electron-rich, which results in a powerful dipole across the whole ammonia molecule. In ozone (O 3 ) molecules, the two O−O bonds are nonpolar (there is no electronegativity difference between atoms of the same element). However, the distribution of other electrons
5520-435: The geometry of CO 2 is linear so that the two bond dipole moments cancel and there is no net molecular dipole moment; the molecule is nonpolar. Examples of household nonpolar compounds include fats, oil, and petrol/gasoline. In the methane molecule (CH 4 ) the four C−H bonds are arranged tetrahedrally around the carbon atom. Each bond has polarity (though not very strong). The bonds are arranged symmetrically so there
5612-456: The interactions that occur for single molecules. During sublimation of a substance, intermolecular forces between molecules are broken, resulting in a change in the material from solid to gas. For this reason, considering the enthalpy of sublimation can be useful in determining the pairwise intermolecular energy. Enthalpy of sublimation can be calculated by the following equation: Using empirically tabulated values for enthalpy of sublimation, it
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#17328813344105704-447: The likelihood of flocculation . Dispersions may become stable through two different phenomena: charge repulsion and steric or entropic repulsion. In charge repulsion, particles that possess the same like electrostatic charges repel each other. Alternatively, steric or entropic repulsion is a phenomenon used to describe the repelling effect when adsorbed layers of material (such as polymer molecules swollen with solvent) are present on
5796-483: 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
5888-522: The liquid. The most commonly used surface modification protocols are plasma activation , wet chemical treatment, including grafting, and thin-film coating. Surface energy mimicking is a technique that enables merging the device manufacturing and surface modifications, including patterning, into a single processing step using a single device material. Many techniques can be used to enhance wetting. Surface treatments, such as corona treatment , plasma treatment and acid etching , can be used to increase
5980-509: 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 ὑδρόφοβος ( hydróphobos ), "having
6072-453: 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 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
6164-410: The molecule is polar. For example, the water molecule (H 2 O) contains two polar O−H bonds in a bent (nonlinear) geometry. The bond dipole moments do not cancel, so that the molecule forms a molecular dipole with its negative pole at the oxygen and its positive pole midway between the two hydrogen atoms. In the figure each bond joins the central O atom with a negative charge (red) to an H atom with
6256-412: The molecules to evaporate more easily. Conversely, in liquids surrounding a bubble, the pressure with respect to the inner part of the bubble is reduced, thus making it more difficult for molecules to evaporate. The Kelvin equation can be stated as: where P 0 is the vapor pressure of the curved surface, P 0 is the vapor pressure of the flat surface, γ is the surface tension , V m
6348-471: The more electronegative atom. The SI unit for electric dipole moment is the coulomb–meter. This is too large to be practical on the molecular scale. Bond dipole moments are commonly measured in debyes , represented by the symbol D, which is obtained by measuring the charge δ {\displaystyle \delta } in units of 10 statcoulomb and the distance d in Angstroms . Based on
6440-400: 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
6532-542: 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
6624-411: 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
6716-426: 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 chemical property related to interfacial tension , rather than a chemical property. In 1805, Thomas Young defined the contact angle θ by analyzing
6808-551: 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
6900-453: The resulting particles often become cemented together into aggregates. Because particles dispersed in liquid media are in constant thermal or Brownian motion , they exhibit a strong affinity for other pigment particles nearby as they move through the medium and collide. This natural attraction is largely attributed to the powerful short-range van der Waals forces , as an effect of their surface energies. The chief purpose of pigment dispersion
6992-484: The reverse process: a known total dipole of a molecule can be decomposed into bond dipoles. This is done to transfer bond dipole moments to molecules that have the same bonds, but for which the total dipole moment is not yet known. The vector sum of the transferred bond dipoles gives an estimate for the total (unknown) dipole of the molecule. A molecule is composed of one or more chemical bonds between molecular orbitals of different atoms. A molecule may be polar either as
7084-434: The rod: Also, since the volume ( V ) of the rod remains constant, the variation ( δV ) of the volume is zero, that is, Therefore, the surface energy density can be expressed as The surface energy density of the solid can be computed by measuring P , r , and l at equilibrium. This method is valid only if the solid is isotropic , meaning the surface energy is the same for all crystallographic orientations. While this
7176-409: The same force. The amount of "pull" an atom exerts on its electrons is called its electronegativity . Atoms with high electronegativities – such as fluorine , oxygen , and nitrogen – exert a greater pull on electrons than atoms with lower electronegativities such as alkali metals and alkaline earth metals . In a bond, this leads to unequal sharing of electrons between
7268-468: The solid creeps and even though the surface area changes, the volume remains approximately constant. If γ is the surface energy density of a cylindrical rod of radius r and length l at high temperature and a constant uniaxial tension P , then at equilibrium, the variation of the total Helmholtz free energy vanishes and we have where F is the Helmholtz free energy and A is the surface area of
7360-431: The substrate and the liquid. If S < 0 , the liquid partially wets the substrate. If S > 0 , the liquid completely wets the substrate. A way to experimentally determine wetting is to look at the contact angle ( θ ), which is the angle connecting the solid–liquid interface and the liquid–gas interface (as in the figure). The Young equation relates the contact angle to interfacial energy: where γ s-g
7452-403: The substrate made from the second material. For example, the relative surface energy of the interface " acrylic glass – gelatin " is equal to 0.03 N/m. Experimental setup for measuring relative surface energy and its function can be seen in the video. To estimate the surface energy of a pure, uniform material, an individual region of the material can be modeled as a cube. In order to move
7544-431: The substrate together. High-energy substrates are held together by bonds , while low-energy substrates are held together by forces . Covalent , ionic , and metallic bonds are much stronger than forces such as van der Waals and hydrogen bonding . High-energy substrates are more easily wetted than low-energy substrates. In addition, more complete wetting will occur if the substrate has a much higher surface energy than
7636-449: The surface area of a mass of liquid by an amount, δA , a quantity of work , γ δA , is needed (where γ is the surface energy density of the liquid). However, such a method cannot be used to measure the surface energy of a solid because stretching of a solid membrane induces elastic energy in the bulk in addition to increasing the surface energy. The surface energy of a solid is usually measured at high temperatures. At such temperatures
7728-404: The surface area, and therefore increases surface energy. If the cutting is done reversibly , then conservation of energy means that the energy consumed by the cutting process will be equal to the energy inherent in the two new surfaces created. The unit surface energy of a material would therefore be half of its energy of cohesion , all other things being equal; in practice, this is true only for
7820-436: 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
7912-399: The surface energy of the substrate. Additives can also be added to the liquid to decrease its surface tension. This technique is employed often in paint formulations to ensure that they will be evenly spread on a surface. As a result of the surface tension inherent to liquids, curved surfaces are formed in order to minimize the area. This phenomenon arises from the energetic cost of forming
8004-414: The surface of the pigment particles in dispersion. Only certain portions (anchors) of the polymer molecules are adsorbed, with their corresponding loops and tails extending out into the solution. As the particles approach each other their adsorbed layers become crowded; this provides an effective steric barrier that prevents flocculation . This crowding effect is accompanied by a decrease in entropy, whereby
8096-402: The system can be written as a sum of three components: bulk phase α , bulk phase β , and the interface σ . Some examples include internal energy U , the number of molecules of the i th substance n i , and the entropy S . While these quantities can vary between each component, the sum within the system remains constant. At the interface, these values may deviate from those present within
8188-482: The total energies of the system before and after the deformation: Calculation of surface energy from first principles (for example, density functional theory ) is an alternative approach to measurement. Surface energy is estimated from the following variables: width of the d-band, the number of valence d-electrons , and the coordination number of atoms at the surface and in the bulk of the solid. In density functional theory , surface energy can be calculated from
8280-440: The two bonded atoms. He estimated that a difference of 1.7 corresponds to 50% ionic character, so that a greater difference corresponds to a bond which is predominantly ionic. As a quantum-mechanical description, Pauling proposed that the wave function for a polar molecule AB is a linear combination of wave functions for covalent and ionic molecules: ψ = aψ(A:B) + bψ(A B ). The amount of covalent and ionic character depends on
8372-415: The values of the squared coefficients a and b . The bond dipole moment uses the idea of electric dipole moment to measure the polarity of a chemical bond within a molecule . It occurs whenever there is a separation of positive and negative charges. The bond dipole μ is given by: The bond dipole is modeled as δ — δ with a distance d between the partial charges δ and δ . It
8464-451: 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
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