What Is Shape Selective Catalysis By Zeolites
Shape selective catalysis is that type of catalysis which depends upon the pore structure of the catalyst and the size of the reactant and product molecules. Zeolite is a porous solid made up of silicon, aluminium and oxygen and a good shape-selective catalyst. It has cavities in its structure where ions or atoms or small molecules can reside.
Special Features Of Reactions In Thin Films And Multilayers
Interfacial reactions are of special importance for thin films because of the high ratio of interfacial area to volume. Multilayers, in parallel with their role in diffusion studies, permit the examination of interfacial reactions with great sensitivity because of their high density of interfaces. While facilitating structural studies of interfaces and reactions at them, multilayers are of particular interest in permitting kinetic studies of the reactions by techniques such as differential scanning calorimetry . It is important to note that the reactions in thin films are often different from those in bulk diffusion couples, a feature which has attracted much attention, e.g., in analyzing reactions at contacts in semiconductor devices ). The differences arise because in a thin film or multilayer the reaction can proceed to completion , while always having short diffusion distances. As a result, there is often only one product phase, rather than the full sequence of equilibrium intermediate phases seen in bulk reactions. Also, the single product phase may be metastable, as in solid state amorphization . Finally, the reaction may be explosive in that it accelerates without further heat input after being triggered or may proceed at a steady velocity .
J.-F. Li, J.-C. Dong, in, 2018
Measuring The Surface Energy With Contact Angle Measurements
The most common way to measure surface energy is through contact angle experiments. In this method, 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 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.
Read Also: How To Study Geography Maps
What Is Enzyme Catalysis
Many enzymes also act as catalysts for many reactions. As enzymes catalyze many reactions which occur in our body, plants and animals. So, they are known as biochemical catalysts and the phenomenon is called biochemical catalysis.
They perform best at optimum temperature and optimum pH
The activity of enzymes is increased in the presence of activators and coenzymes
Inhibitors can destroy or reduce the activity of enzymes
Surface Energy Modification Techniques
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 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.
Read Also: What Does Cognitive Psychology Focus On
Application Of Potential Energy Surfaces
A PES is a conceptual tool for aiding the analysis of molecular geometry and chemical reaction dynamics. Once the necessary points are evaluated on a PES, the points can be classified according to the first and second derivatives of the energy with respect to position, which respectively are the gradient and the curvature. Stationary points have physical meaning: energy minima correspond to physically stable chemical species and saddle points correspond to transition states, the highest energy point on the reaction coordinate . Three
- PES do not show kinetic energy, only potential energy.
- At T = 0 K , species will want to be at the lowest possible potential energy, .
- Between any two minima the lowest energy path will pass through a maximum at a saddle point, which we call that saddle point a transition-state structure.
The PES concept finds application in fields such as chemistry and physics, especially in the theoretical sub-branches of these subjects. It can be used to theoretically explore properties of structures composed of atoms, for example, finding the minimum energy shape of a molecule or computing the rates of a chemical reaction.
Surface Tension And Water
Surface tension in water might be good at performing tricks, such as being able to float a paper clip on its surface, but surface tension performs many more duties that are vitally important to the environment and people. Find out all about surface tension and water here.
Surface Tension: “The property of the surface of a liquid that allows it to resist an external force, due to the cohesive nature of its molecules.”
The between liquid molecules are responsible for the phenomenon known as surface tension. The molecules at the surface of a glass of water do not have other water molecules on all sides of them and consequently they cohere more strongly to those directly associated with them . It is not really true that a “skin” forms on the water surface the stronger cohesion between the water molecules as opposed to the attraction of the water molecules to the air makes it more difficult to move an object through the surface than to move it when it is completely submersed. .
Read Also: Does Angelina Jolie Have Biological Children
Surface And Chemical Properties
Particle surface energy changes drastically when the diameter of the particle is reduced below the 1 m nominal value . Wettability, measured by the surface contact angle, is significantly reduced when particle size is reduced below 10 nm. Specific surface area and other surface chemical properties of NPs change significantly when compared to microparticles and macroparticles. Nitridation, hydrogenation, and Grignard reactions on Mg NPs and Mg powders have been reported . Consistently, Mg NPs with an average size of 76 nm showed larger chemical reactivity when compared to 266 nm and 155 m particles . In the case of 155 m particles, there are no nitridation, hydrogenation, or Grignard reactions.
Higher surface area accounts mostly for the differences in catalytic properties. However, improved catalysis properties with peculiar properties have been reported with transition metal NPs . Thermal stability is a significant issue with small metal NPs. Indeed, with high surface energy, these small NPs tend to agglomerate, forming aggregation/agglomeration NPs or even microparticles and macroparticles.
Surface Free Energy Arises From Molecular Interactions
Term surface free energy describes the excess energy that the surface has compared to the bulk of the material. At the bulk, the molecules have similar molecules on their sides and are pulled equally to all directions.
This causes a zero-net force on the molecule. However, at the air solid interface, the molecules have similar neighboring molecules only on their side and below. There is very little interaction with the molecules in air which causes excess energy at the solid interface. The magnitude of the surface free energy depends on the interaction between the molecules. In the case of metals, the surface free energy is high due to strong metallic bonds between the metal atoms. In polymers, the molecular forces are typically much weaker and thus the surface free energy of the polymers is low.
The most commonly used surface free energy theory, OWRK, divides the surface into two components: polar and dispersive. The dispersive interactions include van der Waals forces such as London dispersive force, Debye inductive force, and Keesom orientational force. Polar interactions are hydrogen bonding and dipole-dipole interactions. The polar interactions can be further divided into acid and base components.
You May Like: What Is Agriculture In Geography
What Is Emulsification
The process of formation of emulsions is called emulsification. In this process, one immiscible liquid is dispersed in another immiscible liquid. Thus, we can say to emulsifying two immiscible liquids is called emulsification. For example, in oil-water cutting fluid emulsion used for metalworking is formed by emulsifying oil in the water medium.
How Is It Measured
A high surface energy is vital for wetting, which is the first step in getting an intimate covering of a coating, adhesive or ink on to a surface. Surface treatments are one of the common methods for increasing surface energy. There are numerous types of treatments including , Plasma and Corona treatments, UV, Ozone and Flame treatments.
A safe, readily available and cheap solvent is water but with a surface tension of 72mN/m, which is very high, it prefers to keep to itself and not react with the world. To overcome the liquid being a sphere where it is energetically stable, the solid it is going on to must have a higher surface energy than the surface tension of the liquid.
Using our example of a water containing liquid, such as printing ink, the water based ink will have a high surface tension and prefer to be a sphere. Unless the solid has a high surface tension, it will not attract the ink toward it and the liquid will sit like water on a waxed car bonnet. With a high surface energy, the solid will pull the liquid closer, leading to increased wetting, flow and chance of interaction and bonding compared to a low surface energy.
If we measure both, it gives us a picture of the molecular world so we can predict the way a material will react, and can be used as a quality tool to confirm your process will work.
Don’t Miss: What Is Mechanical Weathering In Geography
What Is Surface Energy
Have you ever tried drowning an ant? Doesnt work, does it? They seem to be bodies sliding across the waters surface till they safely reach solid ground. But that doesnt make sense because their breathing mechanism is kind of located in the underbelly!
They arent body sliding. They are just walking across just like how they would do on land except with a different manoeuvring technique. They are able to do this because of the phenomenon known as surface tension. Because of cohesive forces across liquid molecules, surface tension causes the existence of a thin film across the surface. Surface energy is the work done per unit area to produce this new surface. And if the insect is light enough, to not damage the layer, it can use the thin layer as support to walk across.
Surface Modified Pigments For Coatings
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 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 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.
Recommended Reading: What Is Delta In Geography
How Can Surface Tension Be Measured
It’s hard to imagine being able to pull apart the surface of a liquid bubble. But there are a couple of ways to measure this force fairly easily. One of them is to measure the force required to pull a thin wire ring out of a container of liquid. Do this very slowly and the force required to just detach the ring from the surface divided by the circumference of the ring , gives the surface tension. . Another measurement is based on the pressure necessary to create and detach a bubble from the end of a needle immersed in the liquid here we are measuring the force necessary to create a liquid surface. More information on these techniques is readily available.
Surface tension is not the same as viscosity, although sometimes people confuse the two. Viscosity is a measure of the resistance to flow or, more precisely, how hard it is to move liquid molecules past each other. It’s related to the friction between molecules. Viscosity is why it takes muscle to push your hand through water it’s the friction you feel as the molecules slide past each other. Its fairly easy to measure viscosity. One common measure of viscosity is the time it takes a ball bearing of a certain size to travel a fixed distance when dropped into a liquid. Again, surface tension and viscosity reflect very different properties. Water has high surface tension but low viscosity. Mineral oil has high viscosity but low surface tension.
We Know Surface Energy Per Unit Area=mechanical Energy + Heat How Can We Prove By Thermodynamics That #h=
needed to increase a surface area #A#
Where the surface tension is #=gamma#
Hence at constant temperature and pressure, surface tension
equals Gibbs free energy per surface area:
is Gibbs free energy and #A#
From this it is easy to understand why decreasing the surface area
of a mass of liquid is always spontaneous ##
not coupled to any other energy changes. It follows that in order to
increase surface area, a certain amount of energy must be added.
Gibbs free energy is defined by the equation #G = H TS#
Read Also: What Is Density In Geography
Which Model Should I Use
- Non-polar surfaces with low surface energies: Zisman
- Moderately polar surfaces: Fowkes or OWRK
- Polar surfaces : Oss-Good
- To find polar and dispersive interactions of liquids: Reference surface & Fowkes/OWRK
There are also additional surface energy models that are not discussed here, such as:
- The Wu model:4 Good for materials with low surface energy up to 40 mN/m
- The Schultz model:11 Used for high energy surfaces like bare metals
- The Neumann equation of state:12 Approximates surface energy with only a single liquid by ignoring the impact of different interaction components
How Surface Tension Works
At the interface between a liquid and the atmosphere , the liquid molecules are more attracted to each other than they are to the air molecules. In other words, the force of cohesion is greater than the force of adhesion. Because they two forces are not in balance, the surface may be considered to be under tension, like if it was enclosed by an elastic membrane (hence the term “surface tension”. The net effect of cohesion versus adhesion is that there is an inward force at the surface layer. This is because the top layer of a molecule is not surrounded by liquid on all sides.
Water has an especially high surface tension because water molecules are attracted to each other by their polarity and able to engage in hydrogen bonding.
Read Also: Are You In Love Psychology Test
Mathematical Definition And Computation
The geometry of a set of atoms can be described by a vector, r, whose elements represent the atom positions. The vector \ could be the set of the Cartesian coordinates of the atoms, or could also be a set of inter-atomic distances and angles. Given \, the energy as a function of the positions, \\), is the value of \\) for all values of \ of interest. Using the landscape analogy from the introduction, \\) gives the height on the “energy landscape” so that the concept of a potential energy surface arises. An example is the PES for water molecule ) that show the energy minimum corresponding to optimized molecular structure for water- O-H bond length of 0.0958 nm and H-O-H bond angle of 104.5°
Figure \: PES for water molecule: Shows the energy minimum corresponding to optimized molecular structure for water- O-H bond length of 0.0958nm and H-O-H bond angle of 104.5°. of Wikipedia .
The Dimensionality of a Potential Energy Surface
To define an atoms location in 3-dimensional space requires three coordinates , \,and \ or \, \ and \ in Cartesian and Spherical coordinates) or degrees of freedom. However, a reaction and hence the corresponding PESs do not depend of the absolute position of the reaction, only the relative positions . Hence both translation and rotation of the entire system can be removed . So the dimensionality of a PES is
What Are Lyophilic Sols
Lyophilic means liquid loving. Those sols in which dispersed phase and dispersion medium have strong attraction between them are called lyophilic sols. For example, a colloidal solution is formed by dissolving starch in water. In this colloidal solution dispersion medium is water and the dispersed phase is starch. This sol can be prepared by heating water at 100 and dissolving starch in it. It is a stable sol and cannot be separated easily due to the strong attraction between the dispersed phase and dispersion medium. Egg albumin sol is another example of lyophilic sol.
You May Like: How To Get The Mode In Math