In chemistry , hydrophobicity is the physical property of a molecule that appears to be repelled from a mass of water (known as a hydrophobe ). [1] (Strictly speaking, there is no repulsive force involved; it lacks attraction.) In contrast, hydrophiles are attracted to water. Hydrophobic molecules are non-polar and, thus, prefer other neutral molecules and non-polar solvents . Since water molecules are polar, hydrophobes do not dissolve well in them . Hydrophobic molecules in water often join together to form micelles . Water on hydrophobic surfaces will exhibit a higher contact angle .
Examples of hydrophobic molecules include alkanes , oils , fats , and greasy substances in general . Hydrophobic materials are used for chemical separation processes for oil removal from water, oil spill management, and removal of non-polar substances from polar compounds. The term hydrophobe comes from Ancient Greek , “having a horror of water”, derived from βος (hýdrophóbos) Ancient Greek (húdōr) ‘water’, and Ancient Greek βος (phobos) ‘fear’.
Chemical background
The hydrophobic interaction is mostly an entropic effect resulting from the dissolution of highly dynamic hydrogen bonds between liquid water molecules by non-polar solutes forming a clathrate -like structure around non-polar molecules . This structure is in a higher order than that of free water molecules because the water molecules arrange themselves to interact as closely as possible, and thus results in a higher entropic state that is non-polar. The molecules clump together to reduce the exposed surface area . water and to reduce the entropy of the system. [4] [5]Thus, the two immiscible phases (hydrophilic versus hydrophobic) would change, minimizing their respective interfacial area. This effect can be observed in a phenomenon called phase separation.
Superhydrophobicity
Superhydrophobic surfaces, such as the leaves of the lotus plant, are those that are extremely difficult to wet. The contact angle of water droplet is greater than 150°. [6] This is known as the lotus effect, and is primarily a physical property related to interfacial tension rather than a chemical property.
Theory
In 1805 , Thomas Young defined the contact angle by analyzing the forces acting on a droplet of liquid resting on a solid surface surrounded by gas.
{\displaystyle \gamma _{\text{SG}}\ =\gamma _{\text{SL}}+\gamma _{\text{LG}}\cos \theta \,}Where from
\gamma _{{\text{SG}}}~~= intermolecular~~ tension~~ between ~~solid~~ and ~~gas\gamma _{{\text{SL}}}~~= interfacial~~ tension~~ between~~ solid ~~and~~ liquid\gamma _ {{\text {LG}}} ~~= intermolecular~~ tension~~ between~~ liquid~~ and ~~gasCan be measured with a contact angle goniometer .
Wenzel determined that when the liquid is in close contact with a finely structured surface, will change to W* .
{\displaystyle \cos \theta _{W}*=r\cos \theta \,}where r is the ratio of the actual area to the projected area. [8] Wenzel’s equation shows that the microstructure of the surface enhances the natural tendency of the surface. A hydrophobic surface (whose original contact angle is greater than 90°) becomes more hydrophobic when microstructured – its new contact angle exceeds the original. However, a hydrophilic surface (whose original contact angle is less than 90°) becomes more hydrophilic when microstructured – its new contact angle becomes less than the original. [9] Cassie and Baxter found that if the liquid is suspended at the peaks of the microstructures, will change to Cb * :
{\displaystyle \cos \theta _{\text{CB}}*=\varphi (\cos \theta +1)-1\,}where is the area fraction of the solid that touches the liquid . [10] The liquid in the Cassie-Baxter state is more dynamic than in the Wenzel state. [ citation needed ]
We can predict whether a Wenzel or Cassie-Baxter state should exist by computing the new contact angle with both equations. By reducing the free energy argument, the relation that predicts the smaller new contact angle is the state most likely to exist. Stated mathematically, for the Cassie–Baxter state to exist, the following inequality must be true.
{\displaystyle \cos \theta <{\frac {\varphi -1}{r-\varphi }}}A recent alternative criterion for the Cassie–Baxter state claims that the Cassie–Baxter state exists when the following 2 criteria are met: 1) the contact line forces overcome the body forces of the unsupported droplet weight and 2) the microstructures. are long enough to contain the liquid which prevents the microstructure from touching the base of the microstructure. [12]
A new criterion for the switch between Wenzel and Cassie–Baxter states has recently been developed based on surface roughness and surface energy. [13] The criterion focuses on the air-trapping ability under liquid droplets on rough surfaces, which may suggest that Wenzel’s model or Cassie-Baxter’s model should be used to account for some combination of surface roughness and energy. [ citation needed ]
The contact angle is a measure of static hydrophobicity, and the contact angle hysteresis and slide angle are dynamic measures. Contact angle hysteresis is a phenomenon characterized by surface asymmetry. [14]When a pipette injects a liquid onto a solid, the liquid will make some contact angle. As the pipette injects more liquid, the droplet will increase in volume, increasing the contact angle, but its three-phase boundary will remain constant until it suddenly moves outward. The contact angle of the droplet just before moving outwards is called the forward contact angle. The receding contact angle is now measured by pumping the liquid back out of the droplet. The droplet volume will decrease, the contact angle will decrease, but its three-phase limit will remain constant until it abruptly withdraws inward. The contact angle that was just before the droplet inward decrease is called the periodic contact angle. The difference between increasing and decreasing contact angles is called contact angle hysteresis and can be used to characterize surface asymmetry, roughness, and mobility. [15]Surfaces that are not homogeneous will have domains that impede the movement of the contact line. 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 angle and contact angle hysteresis than in the Wenzel state. [ citation needed ]
Research and development
Deitre and Johnson found 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 telomeres. The self-cleaning property of superhydrophobic micro-nano-structured surfaces was reported in 1977 [16] Perfluoroalkyl, perfluoropolyether, and RF plasma-formed superhydrophobic materials were developed, used for electrowetting and commercialized for biomedical applications between 1986 and 1995. [17] [18] [19] [20] Other techniques and applications have emerged since the mid-1990s. [21]A durable superhydrophobic hierarchical structure implemented in one or two steps was disclosed in 2002 consisting of nano-sized particles 100 nanometers overlaying a surface containing micrometre-sized features or particles 100 µm. Larger particles were sawed to protect smaller particles from mechanical friction. [22]
In recent research, superhydrophobicity has been reported by allowing alkylketene dimers (AKDs) to solidify into a nanostructured fractal surface. [23] Since then several papers have presented fabrication methods for the fabrication of superhydrophobic surfaces, including particle deposition, [24] sol-gel techniques, [25] plasma treatment, [26] vapor deposition, [24] and casting techniques. . [27] The current opportunity for research impact mainly lies in fundamental research and practical formulation. [28]There have been recent debates regarding 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 models and promote the contact line perspective, water droplets were placed in a rough hydrophobic region on a smooth hydrophobic spot, a smooth A thicker hydrophobic spot in a hydrophobic region, and a hydrophilic spot in a hydrophobic region. [29] Experiments showed that 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 increasing the jaggedness in the contact line, increasing the mobility of the droplets, has also been proposed. [30]
Many hydrophobic substances found in nature depend on Cassie’s law and are bivalent at the submicrometer level with one component air. The lotus effect is based on this principle. Inspired by this, several functionalized superhydrophobic surfaces have been prepared. [31]
An example of a superhydrophobic material in bionic or biomimetic nanotechnology is the nanopin film.
One study presents a vanadium pentoxide surface that switches reversibly between superhydrophobicity and superhydrophilicity under the influence of UV irradiation. [32] According to the study, any surface can be modified for this effect, for example with an inkjet printer, by application of a suspension of rose-like V 2 O 5 particles. Once again the hydrophobicity is induced by the interlaminar air pockets (separated by a 2.1 nm distance). The UV effect is also explained. UV light creates electron-hole pairs, with the holes reacting with the lattice oxygen, creating surface oxygen vacancies, while the electrons convert V 5+ to V 3+ .to reduce. The oxygen vacancies are filled by water, and it is water absorption by the vanadium surface that makes it hydrophilic. By extended storage in the dark, the water is replaced by oxygen and the hydrophilicity is once again lost.
A significant majority of hydrophobic surfaces have their hydrophobic properties that are conferred by structural or chemical modification of the surface of the bulk material, either through coatings or surface treatments. That is to say, water has higher contact angles as a result of the presence of molecular species (usually biological) or structural features. In recent years, rare earth oxides have been shown to possess intrinsic hydrophobicity. [33] The intrinsic hydrophobicity of rare earth oxides depends on surface orientation and oxygen vacancy levels, [34] and is inherently stronger than that of coatings or surface treatments, with potential applications in condensers and catalysts. which can operate in high temperature or corrosive environment.
Applications and potential applications
Hydrophobic concrete has been produced since the middle of the 20th century.
Active recent research on superhydrophobic materials may eventually lead to more industrial applications.
A simple routine is described by the sol-gel technique of coating the cotton fabric with silica [35] or titania [36] particles, which protect the fabric from UV light and make it superhydrophobic.
An efficient routine has been reported to make polyethylene superhydrophobic and thus self-cleaning. [37] 99% of the dirt on such a surface is easily washed off.
Patterned superhydrophobic surfaces also hold promise for lab-on-a-chip microfluidic devices and could significantly improve surface-based bioanalysis. [38]
In pharmaceuticals, the hydrophobicity of pharmaceutical blends affects important quality characteristics of the final products, such as drug dissolution and hardness. [39] Methods have been developed to measure the hydrophobicity of pharmaceutical materials.
