Evaporation is a type of vaporization that occurs on the surface of a liquid as it changes into the gas phase. The surrounding gas must not be saturated with the evaporating substance; when the molecules of the liquid collide, they transfer energy to each other based on how they collide with each other. When a molecule near the surface absorbs enough energy to overcome the vapor pressure, it will escape and enter the surrounding air as a gas; when evaporation occurs, the energy removed from the vaporized liquid will reduce the temperature of the liquid, resulting in evaporative cooling. On average, only a fraction of the molecules in a liquid have enough heat energy to escape from the liquid; the evaporation will continue until an equilibrium is reached when the evaporation of the liquid is equal to its condensation. In an enclosed environment, a liquid will evaporate. Evaporation is an essential part of the water cycle; the sun drives evaporation of water from oceans, moisture in the soil, other sources of water.
In hydrology and transpiration are collectively termed evapotranspiration. Evaporation of water occurs when the surface of the liquid is exposed, allowing molecules to escape and form water vapor. With sufficient energy, the liquid will turn into vapor. For molecules of a liquid to evaporate, they must be located near the surface, they have to be moving in the proper direction, have sufficient kinetic energy to overcome liquid-phase intermolecular forces; when only a small proportion of the molecules meet these criteria, the rate of evaporation is low. Since the kinetic energy of a molecule is proportional to its temperature, evaporation proceeds more at higher temperatures; as the faster-moving molecules escape, the remaining molecules have lower average kinetic energy, the temperature of the liquid decreases. This phenomenon is called evaporative cooling; this is. Evaporation tends to proceed more with higher flow rates between the gaseous and liquid phase and in liquids with higher vapor pressure.
For example, laundry on a clothes line will dry more on a windy day than on a still day. Three key parts to evaporation are heat, atmospheric pressure, air movement. On a molecular level, there is no strict boundary between the vapor state. Instead, there is a Knudsen layer; because this layer is only a few molecules thick, at a macroscopic scale a clear phase transition interface cannot be seen. Liquids that do not evaporate visibly at a given temperature in a given gas have molecules that do not tend to transfer energy to each other in a pattern sufficient to give a molecule the heat energy necessary to turn into vapor. However, these liquids are evaporating, it is just that the process is much slower and thus less visible. If evaporation takes place in an enclosed area, the escaping molecules accumulate as a vapor above the liquid. Many of the molecules return to the liquid, with returning molecules becoming more frequent as the density and pressure of the vapor increases; when the process of escape and return reaches an equilibrium, the vapor is said to be "saturated", no further change in either vapor pressure and density or liquid temperature will occur.
For a system consisting of vapor and liquid of a pure substance, this equilibrium state is directly related to the vapor pressure of the substance, as given by the Clausius–Clapeyron relation: ln = − Δ H v a p R where P1, P2 are the vapor pressures at temperatures T1, T2 ΔHvap is the enthalpy of vaporization, R is the universal gas constant. The rate of evaporation in an open system is related to the vapor pressure found in a closed system. If a liquid is heated, when the vapor pressure reaches the ambient pressure the liquid will boil; the ability for a molecule of a liquid to evaporate is based on the amount of kinetic energy an individual particle may possess. At lower temperatures, individual molecules of a liquid can evaporate if they have more than the minimum amount of kinetic energy required for vaporization. Note: Air used here is a common example. Concentration of the substance evaporating in the air If the air has a high concentration of the substance evaporating the given substance will evaporate more slowly.
Concentration of other substances in the air If the air is saturated with other substances, it can have a lower capacity for the substance evaporating. Flow rate of air This is in part related to the concentration points above. If "fresh" air is moving over the substance all the time the concentration of the substance in the air is less to go up with time, thus encouraging faster evaporation; this is the result of the boundary layer at the evaporation surface decreasing with flow velocity, decreasing the diffusion distance in the stagnant layer. The amount of minerals
Thermolysin is a thermostable neutral metalloproteinase enzyme produced by the Gram-positive bacteria Bacillus thermoproteolyticus. It requires one zinc ion for enzyme activity and four calcium ions for structural stability. Thermolysin catalyzes the hydrolysis of peptide bonds containing hydrophobic amino acids; however thermolysin is widely used for peptide bond formation through the reverse reaction of hydrolysis. Thermolysin is the most stable member of a family of metalloproteinases produced by various Bacillus species; these enzymes are termed'neutral' proteinases or thermolysin -like proteinases. Like all bacterial extracellular proteases thermolysin is first synthesised by the bacterium as a pre-proenzyme. Thermolysin is synthesized as a pre-proenzyme consisting of a signal peptide 28 amino acids long, a pro-peptide 204 amino acids long and the mature enzyme itself 316 amino acids in length; the signal peptide acts as a signal for translocation of pre-prothermolysin to the bacterial cytoplasmic membrane.
In the periplasm pre-prothermolysin is processed into prothermolysin by a signal peptidase. The prosequence acts as a molecular chaperone and leads to autocleavage of the peptide bond linking pro and mature sequences; the mature protein is secreted into the extracellular medium. Thermolysin has a molecular weight of 34,600 Da, its overall structure consists of two spherical domains with a deep cleft running across the middle of the molecule separating the two domains. The secondary structure of each domain is quite different, the N-terminal domain consists of beta pleated sheet, while the C-terminal domain is alpha helical in structure; these two domains are connected by a central alpha helix, spanning amino acids 137-151. In contrast to many proteins that undergo conformational changes upon heating and denaturation, thermolysin does not undergo any major conformational changes until at least 70 °C; the thermal stability of members of the TLP family is measured in terms of a T50 temperature. At this temperature incubation for 30 minutes reduces the enzymes activity by half.
Thermolysin has a T50 value of 86.9 °C, making it the most thermo stable member of the TLP family. Studies on the contribution of calcium to thermolysin stability have shown that upon thermal inactivation a single calcium ion is released from the molecule. Preventing this calcium from binding to the molecule by mutation of its binding site, reduced thermolysin stability by 7 °C. However, while calcium binding makes a significant contribution to stabilising thermolysin, more crucial to stability is a small cluster of N-terminal domain amino acids located at the proteins surface. In particular a phenylalanine at amino acid position 63 and a proline at amino acid position 69 contribute to thermolysin stability. Changing these amino acids to threonine and alanine in a less stable thermolysin-like proteinase produced by Bacillus stearothermophillus, results in individual reductions in stability of 7 °C and 6.3 °C and when combined a reduction in stability of 12.3 °C. In the synthesis of aspartame, less bitter-tasting byproduct is produced when the reaction is catalyzed by thermolysin.
Determining protein stability in cell lysate using the fast parallel proteolysis assay. The MEROPS online database for peptidases and their inhibitors: M04.001 Thermolysin at the US National Library of Medicine Medical Subject Headings
In organic chemistry, an alkane, or paraffin, is an acyclic saturated hydrocarbon. In other words, an alkane consists of hydrogen and carbon atoms arranged in a tree structure in which all the carbon–carbon bonds are single. Alkanes have the general chemical formula CnH2n+2; the alkanes range in complexity from the simplest case of methane, where n = 1, to arbitrarily large and complex molecules, like pentacontane or 6-ethyl-2-methyl-5- octane, an isomer of tetradecane. IUPAC defines alkanes as "acyclic branched or unbranched hydrocarbons having the general formula CnH2n+2, therefore consisting of hydrogen atoms and saturated carbon atoms". However, some sources use the term to denote any saturated hydrocarbon, including those that are either monocyclic or polycyclic, despite their having a different general formula. In an alkane, each carbon atom is sp3-hybridized with 4 sigma bonds, each hydrogen atom is joined to one of the carbon atoms; the longest series of linked carbon atoms in a molecule is known as its carbon skeleton or carbon backbone.
The number of carbon atoms may be considered as the size of the alkane. One group of the higher alkanes are waxes, solids at standard ambient temperature and pressure, for which the number of carbon atoms in the carbon backbone is greater than about 17. With their repeated –CH2 units, the alkanes constitute a homologous series of organic compounds in which the members differ in molecular mass by multiples of 14.03 u. Alkanes are not reactive and have little biological activity, they can be viewed as molecular trees upon which can be hung the more active/reactive functional groups of biological molecules. The alkanes have two main commercial sources: natural gas. An alkyl group abbreviated with the symbol R, is a functional group that, like an alkane, consists of single-bonded carbon and hydrogen atoms connected acyclically—for example, a methyl or ethyl group. Saturated hydrocarbons are hydrocarbons having only single covalent bonds between their carbons, they can be: linear wherein the carbon atoms are joined in a snake-like structure branched wherein the carbon backbone splits off in one or more directions cyclic wherein the carbon backbone is linked so as to form a loop.
According to the definition by IUPAC, the former two are alkanes, whereas the third group is called cycloalkanes. Saturated hydrocarbons can combine any of the linear and branching structures. Alkanes are the acyclic ones, corresponding to k = 0. Alkanes with more than three carbon atoms can be arranged in various different ways, forming structural isomers; the simplest isomer of an alkane is the one in which the carbon atoms are arranged in a single chain with no branches. This isomer is sometimes called the n-isomer; however the chain of carbon atoms may be branched at one or more points. The number of possible isomers increases with the number of carbon atoms. For example, for acyclic alkanes: C1: methane only C2: ethane only C3: propane only C4: 2 isomers: n-butane and isobutane C5: 3 isomers: pentane and neopentane C6: 5 isomers: hexane, 2-methylpentane, 3-methylpentane, 2,2-dimethylbutane, 2,3-dimethylbutane C12: 355 isomers C32: 27,711,253,769 isomers C60: 22,158,734,535,770,411,074,184 isomers, many of which are not stable.
Branched alkanes can be chiral. For example, 3-methylhexane and its higher homologues are chiral due to their stereogenic center at carbon atom number 3. In addition to the alkane isomers, the chain of carbon atoms may form one or more loops; such compounds are called cycloalkanes. Stereoisomers and cyclic compounds are excluded; the IUPAC nomenclature for alkanes is based on identifying hydrocarbon chains. Unbranched, saturated hydrocarbon chains are named systematically with a Greek numerical prefix denoting the number of carbons and the suffix "-ane". In 1866, August Wilhelm von Hofmann suggested systematizing nomenclature by using the whole sequence of vowels a, e, i, o and u to create suffixes -ane, -ene, -ine, -one, -une, for the hydrocarbons CnH2n+2, CnH2n, CnH2n−2, CnH2n−4, CnH2n−6. Now, the first three name hydrocarbons with single and triple bonds, it is impossible to find compounds with more than one IUPAC name. This is because shorter chains attached to longer chains are prefixes and the convention includes brackets.
Numbers in the name, referring to which carbon a group is attached to, should be as low as possible so that 1- is implied and omitted from names of organic compounds with only one side-group. Symmetric compounds will have two ways of arriving at the same name. Straight-chain alkanes are sometimes indicated by the prefix "n -". Although this is not necessary, the usage is still common in cases where there is an important difference in properties between the straight-chain and branched-chain isomers, e.g. n-hexane or 2- or 3-met
Silicones known as polysiloxanes, are polymers that include any synthetic compound made up of repeating units of siloxane, a chain of alternating silicon atoms and oxygen atoms, combined with carbon and sometimes other elements. They are heat-resistant and either liquid or rubber-like, are used in sealants, lubricants, cooking utensils, thermal and electrical insulation; some common forms include silicone oil, silicone grease, silicone rubber, silicone resin, silicone caulk. More called polymerized siloxanes or polysiloxanes, silicones consist of an inorganic silicon-oxygen backbone chain with organic side groups attached to the silicon atoms; these silicon atoms are tetravalent. So, silicones are polymers constructed from inorganic-organic monomers. Silicones have in general the chemical formula n, where R is an organic group such as an alkyl or phenyl group. In some cases, organic side groups can be used to link two or more of these -Si-O- backbones together. By varying the -Si-O- chain lengths, side groups, crosslinking, silicones can be synthesized with a wide variety of properties and compositions.
They can vary in consistency from liquid to gel to rubber to hard plastic. The most common siloxane is a silicone oil; the second largest group of silicone materials is based on silicone resins, which are formed by branched and cage-like oligosiloxanes. F. S. Kipping coined the word silicone in 1901 to describe polydiphenylsiloxane by analogy of its formula, Ph2SiO, with the formula of the ketone benzophenone, Ph2CO. Kipping was well aware that polydiphenylsiloxane is polymeric whereas benzophenone is monomeric and noted that Ph2SiO and Ph2CO had different chemistry; the discovery of the structural differences between Kipping's molecules and the ketones means that silicone is no longer the correct term and that the term siloxanes is correct according to the nomenclature of modern chemistry. Silicone is confused with silicon, but they are distinct substances. Silicon is a chemical element, a hard dark-grey semiconducting metalloid which in its crystalline form is used to make integrated circuits and solar cells.
Silicones are compounds that contain silicon, hydrogen and other kinds of atoms as well, have different physical and chemical properties. Compounds containing silicon-oxygen double bonds, now called silanones but which could deserve the name "silicone", have long been identified as intermediates in gas-phase processes such as chemical vapor deposition in microelectronics production, in the formation of ceramics by combustion; however they have a strong tendency to polymerize into siloxanes. The first stable silanone was obtained in 2014 by others. Most common are materials based on polydimethylsiloxane, derived by hydrolysis of dimethyldichlorosilane; this dichloride reacts with water as follows: n Si2Cl2 + n H2O → n + 2n HClThe polymerization produces linear chains capped with Si-Cl or Si-OH groups. Under different conditions the polymer is a cyclic, not a chain. For consumer applications such as caulks silyl acetates are used instead of silyl chlorides; the hydrolysis of the acetates produce the less dangerous acetic acid as the reaction product of a much slower curing process.
This chemistry is used in many consumer applications, such as adhesives. Branches or cross-links in the polymer chain can be introduced by using organosilicone precursors with fewer alkyl groups, such as methyltrichlorosilane and methyltrimethoxysilane. Ideally, each molecule of such a compound becomes a branch point; this process can be used to produce hard silicone resins. Precursors with three methyl groups can be used to limit molecular weight, since each such molecule has only one reactive site and so forms the end of a siloxane chain; when silicone is burned in air or oxygen, it forms solid silica as a white powder and various gases. The dispersed powder is sometimes called silica fume. Silicones exhibit many useful characteristics, including: Low thermal conductivity Low chemical reactivity Low toxicity Thermal stability; the ability to repel water and form watertight seals. Does not stick to many substrates, but adheres well to others, e.g. glass. Does not support microbiological growth.
Resistance to oxygen and ultraviolet light. This property has led to widespread use of silicones in the construction industry and the automotive industry. Electrical insulation properties; because silicone can be formulated to be electrically insulative or conductive, it is suitable for a wide range of electrical applications. High gas permeability: at room temperature, the permeability of silicone rubber for such gases as oxygen is 400 times that of butyl rubber, making silicone useful for medical applications in which increased aeration is desired. Conversely, silicone rubbers can not be used. Silicone can be developed into rubber sheeting, where it has other properties, such as being FDA compliant; this extends the uses of silicone sheeting to industries that demand hygiene, for example and beverage and pharmaceutical. Silicones are used in many products. Ullmann's Encyclopedia of Industrial Chemistry lists the following major categories of application: Electrical, elec
The viscosity of a fluid is a measure of its resistance to deformation at a given rate. For liquids, it corresponds to the informal concept of "thickness": for example, syrup has a higher viscosity than water. Viscosity can be conceptualized as quantifying the frictional force that arises between adjacent layers of fluid that are in relative motion. For instance, when a fluid is forced through a tube, it flows more near the tube's axis than near its walls. In such a case, experiments show; this is because a force is required to overcome the friction between the layers of the fluid which are in relative motion: the strength of this force is proportional to the viscosity. A fluid that has no resistance to shear stress is known as an inviscid fluid. Zero viscosity is observed only at low temperatures in superfluids. Otherwise, the second law of thermodynamics requires all fluids to have positive viscosity. A fluid with a high viscosity, such as pitch, may appear to be a solid; the word "viscosity" is derived from the Latin "viscum", meaning mistletoe and a viscous glue made from mistletoe berries.
In materials science and engineering, one is interested in understanding the forces, or stresses, involved in the deformation of a material. For instance, if the material were a simple spring, the answer would be given by Hooke's law, which says that the force experienced by a spring is proportional to the distance displaced from equilibrium. Stresses which can be attributed to the deformation of a material from some rest state are called elastic stresses. In other materials, stresses are present which can be attributed to the rate of change of the deformation over time; these are called. For instance, in a fluid such as water the stresses which arise from shearing the fluid do not depend on the distance the fluid has been sheared. Viscosity is the material property which relates the viscous stresses in a material to the rate of change of a deformation. Although it applies to general flows, it is easy to visualize and define in a simple shearing flow, such as a planar Couette flow. In the Couette flow, a fluid is trapped between two infinitely large plates, one fixed and one in parallel motion at constant speed u.
If the speed of the top plate is low enough in steady state the fluid particles move parallel to it, their speed varies from 0 at the bottom to u at the top. Each layer of fluid moves faster than the one just below it, friction between them gives rise to a force resisting their relative motion. In particular, the fluid applies on the top plate a force in the direction opposite to its motion, an equal but opposite force on the bottom plate. An external force is therefore required in order to keep the top plate moving at constant speed. In many fluids, the flow velocity is observed to vary linearly from zero at the bottom to u at the top. Moreover, the magnitude F of the force acting on the top plate is found to be proportional to the speed u and the area A of each plate, inversely proportional to their separation y: F = μ A u y; the proportionality factor μ is the viscosity of the fluid, with units of Pa ⋅ s. The ratio u / y is called the rate of shear deformation or shear velocity, is the derivative of the fluid speed in the direction perpendicular to the plates.
If the velocity does not vary linearly with y the appropriate generalization is τ = μ ∂ u ∂ y, where τ = F / A, ∂ u / ∂ y is the local shear velocity. This expression is referred to as Newton's law of viscosity. In shearing flows with planar symmetry, it is what defines μ, it is a special case of the general definition of viscosity, which can be expressed in coordinate-free form. Use of the Greek letter mu for the viscosity is common among mechanical and chemical engineers, as well as physicists. However, the Greek letter eta is used by chemists and the IUPAC; the viscosity μ is sometimes referred to as the shear viscosity. However, at least one author discourages the use of this terminology, noting that μ can appear in nonshearing flows in addition to shearing flows. In general terms, the viscous stresses in a fluid are defined as those resulting from the relative velocity of different fluid particles; as such, the viscous stresses. If the velocity gradients are small to a first approximation the v
Chemical Reviews is peer-reviewed scientific journal published twice per month by the American Chemical Society. It publishes review articles on all aspects of chemistry, it was established in 1924 by William Albert Noyes. As of 1 January 2015 the editor-in-chief is Sharon Hammes-Schiffer; the journal is abstracted and indexed in Chemical Abstracts Service, CAB International, EBSCOhost, ProQuest, PubMed and the Science Citation Index. According to the Journal Citation Reports, the journal has a 2017 impact factor of 52.613. Official website