Van der Waals forces are intermolecular forces. They include attraction and repulsion between atoms and molecules. This force is fundamental in diverse fields which include condensed matter physics, polymer science, nanotechnology, supra-molecular chemistry, biological structure, and surface science.
In this article, we look into Van der Waals forces their types, the equation of the Van der Waals force, and their applications.
What are Van der Waals Forces?
Van der Waals forces are a set of intermolecular forces that arise from the interaction between molecules. Unlike stronger covalent and ionic bonds, which involve the sharing or transfer of electrons between atoms, van der Waals forces are weaker and result from transient electrical charges that occur even in neutral molecules.
The strength of these forces ranges from 0.4 KJ.mol-1 to 4 KJ.mol-1. The force is extremely weak when the distance between the atoms is bigger than 0.6 nm. These forces are named after Dutch physicist Johannes Diderik van der Waals, who first described them in the late 19th century.
Definition of Van der Waals Force of Attraction
Van der Waals forces are weak intermolecular attractions between atoms or molecules, arising from temporary or permanent dipoles. These include London dispersion forces, dipole-dipole interactions, and induction forces, which are crucial for the physical properties of various substances.
Who is Johannes Diderik Van der Waals?
Johannes Diderik Van der Waals was a Dutch physicist. He formulated the Van der Waals equation of state. The Van der Waals force is named after him. It is a distance-dependent interaction between atoms or molecules. The force plays a fundamental role in various fields including surface science, matter physics, polymer science, and more.
Types of Van der Waals Forces
These are three types of Van der Waals forces, which are:
- Keeson Interaction [Permanent Dipole-Permanent Dipole (Hydrogen bond)]
- Debye Force [Permanent Dipole-Induced Dipole]
- London Dispersion Force [Instentanous Dipole-Induced Dipole]
Let’s discuss these forces in detail as follows:
Keeson Interaction
The Keeson interaction is a type of intermolecular force that arises from the electrostatic attraction between permanent dipoles in polar molecules. This force is named after Willem Hendrik Keeson. The energy of a Keeson interaction depends on the inverse sixth power of the distance between the molecules. In contrast, the interaction energy of two spatially fixed dipoles depends on the inverse third power of the distance.
Key Features of the Keeson Interaction
Some of the key features of keeson interaction are:
- Permanent Dipoles: The interaction occurs only among molecules with permanent dipole moments, i.e., two polar molecules.
- Weak Interaction: Keeson interactions are very weak van der Waals interactions and do not occur in aqueous solutions.
- Orientation Effect: The electrostatic interactions align the permanent dipoles of the polar molecules, causing an orientation effect.
- Dipole-Dipole Interactions: The dipoles of the polar molecule induce the formation of opposite terminals in the non-polar molecules, leading to an orientation effect.
Some example of Keeson interaction are HCl, H2O, CO2, SO2, and NH3 etc
Check: What is Acetone?
Debye Forces
This force is named after Peter J. W. Debye. It is an attractive interaction between the permanent dipoles of polar molecules and the dipoles they may induce in similar molecules. It is caused by the interaction of permanent dipoles with dipoles induced by them in electron clouds. This interaction is always attractive and does not vanish with distance. This force is one of the essential parts of force fields frequently used in molecular mechanics.
Key Features of Debye Force
Some of the key features of Debye Force are:
- Interaction: Debye forces result from the interaction between the permanent dipoles of polar molecules and the dipoles induced by them in electron clouds
- Strength: Debye forces are stronger than the London dispersion force (instantaneous dipole-induced dipole) but weaker than the Keeson orientation effect
- Attractive nature: Debye forces are always attractive and do not vanish at higher temperatures
Some example of Debye forces are a polar molecule like hydrogen bromide (HBr) and a non-polar or symmetrical molecule like argon (Ar).
Check: Argon Gas Formula
London Dispersion Forces
Some of the key features of London Dispersion Forces are:
London dispersion forces are present between any two molecules, even polar ones. These forces occur when the molecules are almost touching. This force is sometimes called an induced dipole-induced dipole attraction. This force is a temporary attractive force. It results when the electrons in two adjacent atoms or molecules occupy positions that make the atoms form an instantaneous dipole. They are the weakest intermolecular forces. They are responsible for any compound’s liquid, solid, and solution states.
Key Features of London Dispersion Forces
- Temporary Dipole: An atom or molecule can develop a temporary (instantaneous) dipole when its electrons are not symmetrically distributed with respect to the nucleus.
- Induced Dipole: A second atom or molecule can be distorted by the appearance of the dipole in the first atom or molecule, leading to an electrostatic attraction between them.
- Weak Intermolecular Force: London dispersion forces are weaker than dipole-dipole and hydrogen bonding forces.
- Cumulative Effect: Although the London dispersion force between individual atoms and molecules is relatively weak, the effect is cumulative over the volume of materials, making it stronger in bulk solids and liquids.
Some examples of London dispersion forces are n-pentane and neopentane.
Check: Propane Formula
Formula for van der waals force is given as:
(P+ n²a/V²)(V−nb)=nRT
- P is pressure measured,
- V is volume of gas in moles (n),
- a is constant related to the strength of the attraction between molecules,
- b is constant related to the volume of gas molecules,
- R is gas constant, and
- T is temperature.
The van der Waals equation accounts for the volume of gas molecules and the intermolecular forces of attraction. It is used to calculate the behavior of real gases.
Characteristics of Van der Waals Forces
- Van der Waals forces are significantly weaker compared to covalent and ionic bonds.
- These forces are composed of multiple minor interactions that are additive.
- Unlike other bond types, Van der Waals forces do not exhibit saturation.
- These forces lack directional characteristics.
- Temperature changes generally do not affect Van der Waals forces, except in the case of dipole-dipole interactions.
- Effective at short ranges, the strength of Van der Waals forces increases as the distance between atoms or molecules decreases.
Factor Affecting Van der Waals Force
There are several factors that affects the strength of van der Waals forces:
Larger molecules have more electrons, leading to stronger van der Waals forces due to increased polarizability. This makes it easier for larger molecules to induce temporary dipoles in neighboring molecules.
Molecules with a linear shape can come closer together and have more surface area in contact than spherical molecules, leading to stronger van der Waals forces. Branched molecules have weaker van der Waals forces compared to their linear counterparts because of the reduced effective contact area.
Polarizability refers to how easily the electron cloud around a molecule can be distorted. Molecules with more easily distortable electron clouds have stronger van der Waals forces. Large atoms or molecules with loosely held outer electrons are more polarizable.
Permanent Dipole Interactions: While van der Waals forces include interactions between non-polar molecules, they also involve dipole-dipole interactions where present. Molecules with a permanent dipole moment can align such that their positive and negative ends attract, enhancing the overall intermolecular force.
Contact Surface Area: The amount of surface area that molecules have in contact with each other can influence the strength of van der Waals forces. Greater surface area allows for more opportunities for these forces to act between molecules.
Increasing temperature can weaken van der Waals forces because higher kinetic energy allows molecules to overcome these weak attractions more easily. Conversely, lowering the temperature can enhance the effectiveness of van der Waals forces.
Applications of Van der Waals forces
Some applications of Van der Waals forces include:
- Gecko Adhesion: Geckos can climb smooth surfaces and hang from ceilings. This is due to the exploitation of Van der Waals forces. These forces allow them to adhere to surfaces with only weak attraction.
- Intermolecular Interactions: Van der Waals forces play a significant role in intermolecular interactions between molecules and atoms in gases, organic liquids, and solids, contributing to the properties of these substances.
- Van der Waals Equation: The Van der Waals equation calculates the behavior of non-ideal (real) gases, considering the intermolecular forces of attraction, and helps determine actual values for such gases.
- Viscosity: Van der Waals forces can affect the viscosity of a substance by increasing the attraction between molecules, making it more difficult for them to move past each other.
Read More,
Van der Waals Force – FAQs
What are Van der Waals Forces?
Van der Waals forces are weak intermolecular forces that arise from either the temporary or permanent dipoles in molecules. These forces include London dispersion forces, dipole-dipole interactions, and ion-dipole forces, with London dispersion being the weakest.
How do Van der Waals Forces work?
These forces work through the attraction and repulsion between molecules, primarily influenced by the electronic fluctuations that create temporary dipoles within nonpolar molecules or between permanent dipoles in polar molecules.
What role do Van der Waals Forces play in nature?
In nature, Van der Waals forces are crucial for various biological processes, helping proteins and nucleic acids maintain their structures. They also contribute to the unique properties of water, like high surface tension.
What do you mean by Van der Waals force?
Van der Waals forces are attractive forces between molecules. They occur due to temporary electron distribution shifts, creating positive and negative charges.
What are the Different Types of Van der Waals Force?
Van der Waals forces include three types. These are London dispersion forces, dipole-dipole interactions, and hydrogen bonding forces
What is the Van der Waals Equation?
The equation of Van der Waals force is
(P+ n²a/V²)(V−nb)=nRT
Why is Van der Waals Force Called London Force?
Van der Waals interection between instantaneous dipole and induced dipole are often refered to as London Forces as this is named after German-American physicist Fritz London who contributed his work in the same field.
Why Van der Waals Force is Weakest?
Van der Waals forces are the weakest among intermolecular forces because they are based on relatively subtle interactions between temporary fluctuations in electron distribution within molecules.
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