Dispersive adhesion
Encyclopedia
Dispersive adhesion, also called adsorptive adhesion, is a mechanism for adhesion
which attributes attractive forces between two materials to intermolecular interactions between molecules of each material. This mechanism is widely viewed as the most important of the five mechanisms of adhesion due to its presence in every type of adhesive system and relative strength.
between two nonpolar molecules close together. The random nature of electron
orbit allows moments in which the charge distribution in a molecule is unevenly distributed, allowing an attraction
to another molecule with a temporary dipole
. A larger molecule allows for a larger dipole, and thus will have stronger dispersion forces.
differences between atoms in the molecule. This dipole causes a coulombic attraction between the two molecules.
atom
attached to a small, electronegative atom such as fluorine
, oxygen
or nitrogen
. This bond
is naturally polar, with the hydrogen atom gaining a slight positive charge and the other atom becoming slightly negative. Two molecules, or even two functional groups on one large molecule, may then be attracted to each other via Keesom forces.
of the molecules involved in the adhesive system
, the degree to which coatings wet
each other, and the surface roughness at the interface
.
in contact will have stronger intermolecular attractions and a larger adhesive strength due to the dispersive mechanism.
s can yield better wetting because they have a larger surface area. Thus, more intermolecular interactions at closer distances can arise, yielding stronger attractions and larger adhesive strength. Once the roughness becomes larger, on the order of 10 micrometres, the coating can no longer wet effectively, resulting in less contact area and a smaller adhesive strength.
Adhesion
Adhesion is any attraction process between dissimilar molecular species that can potentially bring them in close contact. By contrast, cohesion takes place between similar molecules....
which attributes attractive forces between two materials to intermolecular interactions between molecules of each material. This mechanism is widely viewed as the most important of the five mechanisms of adhesion due to its presence in every type of adhesive system and relative strength.
Source of dispersive adhesion attractions
The source of adhesive forces, according to the dispersive adhesion mechanism, is the weak interactions that occur between molecules close together. These interactions include London dispersion forces, Keesom forces, Debye forces and hydrogen bonds. Taken on an individual basis, these attractions are not very strong, but when summed over the bulk of a material, can become significant.London dispersion
London dispersion forces arise from instantaneous dipolesDipole
In physics, there are several kinds of dipoles:*An electric dipole is a separation of positive and negative charges. The simplest example of this is a pair of electric charges of equal magnitude but opposite sign, separated by some distance. A permanent electric dipole is called an electret.*A...
between two nonpolar molecules close together. The random nature of electron
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
orbit allows moments in which the charge distribution in a molecule is unevenly distributed, allowing an attraction
Attraction
In general, an attraction draws one object towards another one. The term may have the following specific meanings.* In physics, attraction may refer to gravity or to the electromagnetic force* Attractiveness...
to another molecule with a temporary dipole
Dipole
In physics, there are several kinds of dipoles:*An electric dipole is a separation of positive and negative charges. The simplest example of this is a pair of electric charges of equal magnitude but opposite sign, separated by some distance. A permanent electric dipole is called an electret.*A...
. A larger molecule allows for a larger dipole, and thus will have stronger dispersion forces.
Keesom
Keesom forces, also known as dipole-dipole interactions, result from two molecules that have a permanent dipole due to electronegativityElectronegativity
Electronegativity, symbol χ , is a chemical property that describes the tendency of an atom or a functional group to attract electrons towards itself. An atom's electronegativity is affected by both its atomic number and the distance that its valence electrons reside from the charged nucleus...
differences between atoms in the molecule. This dipole causes a coulombic attraction between the two molecules.
Debye
Debye forces, or dipole-induced dipole interactions, can also play a role in dispersive adhesion. These come about when a nonpolar molecule becomes temporarily polarized due to interactions with a nearby polar molecule. This “induced dipole” in the nonpolar molecule then is attracted to the permanent dipole, yielding a Debye attraction.Hydrogen Bonding
Sometimes grouped into the chemical mechanism of adhesion, hydrogen bonding can increase adhesive strength by the dispersive mechanism. Hydrogen bonding occurs between molecules with a hydrogenHydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
atom
Atom
The atom is a basic unit of matter that consists of a dense central nucleus surrounded by a cloud of negatively charged electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons...
attached to a small, electronegative atom such as fluorine
Fluorine
Fluorine is the chemical element with atomic number 9, represented by the symbol F. It is the lightest element of the halogen column of the periodic table and has a single stable isotope, fluorine-19. At standard pressure and temperature, fluorine is a pale yellow gas composed of diatomic...
, oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
or nitrogen
Nitrogen
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...
. This bond
Chemical bond
A chemical bond is an attraction between atoms that allows the formation of chemical substances that contain two or more atoms. The bond is caused by the electromagnetic force attraction between opposite charges, either between electrons and nuclei, or as the result of a dipole attraction...
is naturally polar, with the hydrogen atom gaining a slight positive charge and the other atom becoming slightly negative. Two molecules, or even two functional groups on one large molecule, may then be attracted to each other via Keesom forces.
Factors affecting adhesion strength
The strength of adhesion by the dispersive mechanism depends on a variety of factors including: chemical structureChemical structure
A chemical structure includes molecular geometry, electronic structure and crystal structure of molecules. Molecular geometry refers to the spatial arrangement of atoms in a molecule and the chemical bonds that hold the atoms together. Molecular geometry can range from the very simple, such as...
of the molecules involved in the adhesive system
System
System is a set of interacting or interdependent components forming an integrated whole....
, the degree to which coatings wet
Wetting
Wetting is the ability of a liquid to maintain contact with a solid surface, resulting from intermolecular interactions when the two are brought together. The degree of wetting is determined by a force balance between adhesive and cohesive forces.Wetting is important in the bonding or adherence of...
each other, and the surface roughness at the interface
Interface (chemistry)
An interface is a surface forming a common boundary among two different phases, such as an insoluble solid and a liquid, two immiscible liquids or a liquid and an insoluble gas. The importance of the interface depends on which type of system is being treated: the bigger the quotient area/volume,...
.
Chemical composition
The chemical structure of the materials involved in a given adhesive system plays a large role in the adhesion of the system as a whole because the structure determines the type and strength of the intermolecular interactions present. All things equal, larger molecules, which experience higher dispersion forces, will have a larger adhesive strength than smaller molecules of the same basic chemical fingerprint. Similarly, polar molecules will have Keesom and Debye forces not experienced by nonpolar molecules of similar size. Compounds which can hydrogen bond across the adhesive interface will have even greater adhesive strength.Wetting
Wetting is a measure of the thermodynamic compatibility of two surfaces. If the surfaces are well-matched, the surfaces will "desire" to interact with each other, minimizing the surface energy of both phases, and the surfaces will come into close contact. Because the intermolecular attractions strongly correlate with distance, the closer the interacting molecules are together, the stronger the attraction. Thus, two materials that wet well and have a large amount of surface areaSurface area
Surface area is the measure of how much exposed area a solid object has, expressed in square units. Mathematical description of the surface area is considerably more involved than the definition of arc length of a curve. For polyhedra the surface area is the sum of the areas of its faces...
in contact will have stronger intermolecular attractions and a larger adhesive strength due to the dispersive mechanism.
Roughness
Surface roughness can also affect the adhesive strength. Surfaces with roughness on the scale of 1-2 micrometreMicrometre
A micrometer , is by definition 1×10-6 of a meter .In plain English, it means one-millionth of a meter . Its unit symbol in the International System of Units is μm...
s can yield better wetting because they have a larger surface area. Thus, more intermolecular interactions at closer distances can arise, yielding stronger attractions and larger adhesive strength. Once the roughness becomes larger, on the order of 10 micrometres, the coating can no longer wet effectively, resulting in less contact area and a smaller adhesive strength.
Systems dominated by dispersive adhesion
All materials, even those not usually classified as adhesives, experience an attraction to other materials, due simply to dispersion forces. In many situations these attractions are trivial; however, dispersive adhesion plays a dominant role in various adhesive systems, especially when multiple forms of intermolecular attractions, as described above, are present. It has been shown by experimental methods that the dispersive mechanism of adhesion plays a large role in the overall adhesion in polymeric systems in particular.See also
- AdhesionAdhesionAdhesion is any attraction process between dissimilar molecular species that can potentially bring them in close contact. By contrast, cohesion takes place between similar molecules....
- Intermolecular forceIntermolecular forceIntermolecular forces are forces of attraction or repulsion which act between neighboring particles: atoms, molecules or ions. They are weak compared to the intramolecular forces, the forces which keep a molecule together...
- Van der Waals forces
- Hydrogen bonding