Intermolecular forces

The molecules within a liquid crystal are attracted to one another through intermolecular forces. The intermolecular forces acting between the molecules are moderately strong. These forces are not strong enough to give the substance rigidity, so that this fluid will conform to the shape of its container. They are of sufficient strength to give a liquid crystal resistance to increase it surface area (surface tension) and resistance to flow (viscosity). Surface tension is caused by a balance between cohesive forces between the molecules within the liquid and adhesive forces between these molecules and other types of molecules at the liquid's interface, increasing with the strength of the attractive intermolecular forces. Viscosity also increases with the strength of the cohesive intermolecular forces. The width, length and arrangement of the liquid crystal's constituent molecules also affect viscosity, where large and long molecules can entangle each other. 

The discussion above is macroscopic, continuum description of forces acting within a liquid crystalline substance. At the discrete level, the intermolecular forces acting between individual molecules, can be subdivided 5 categories. These forces and their relative strength to that of a typical covalent or ionic chemical bond is listed below: 
 
 

Ion-dipole

Ions form within a liquid medium where electrolytic species are dissolved. In the presence of ions, a polar molecule will attempt to align itself in the direction of the ion. It will point the partially charged end that is opposite to that of the ion (i.e., partial positive end towards negative ion and partial negative end towards positive ion). The strength of this interaction depends on the dipole moment (µ) of the polar molecule, the charge of the ion (Q), the angle of alignment (), and the distance separating them (R). 

This interaction is especially important in solutions of ionic substances in a liquid solvent composed of polar molecules. An example of such a solution is salt in water. 
 
 

Ion-induced dipole

The electrostatic field of an ion can also polarize a neighboring neutral atom or molecule distorting the latter's electron distribution. The polarized species will be attracted to the ion. The strength of this interaction depends on the polarizability () of the atom or molecule, the charge of the ion (Q), and the distance between them (R). 

This interaction, which varies as 1/R⁴, is short-ranged. 
 
 

Dipole-dipole

Certain molecules develop permanent dipole moments either from uneven distribution of charges within their bonds or due to their shape. Neighboring polar molecules will attempt to align themselves to each other such that the ends of one molecule align toward the end of another molecule that has opposite partial charge to maximize the attractive interaction between them. In a liquid, the polar molecules are free to move with respect to one another, sometimes having attractive and at other time repulsive interactions. The net effect, averaged over time, the interactions are attractive. 

The strength of this interaction depends on the magnitude of the permanent dipole moments (_A, _B), the angles of alignment (_A, _B, _A, _B), and the distance (R) between the molecules.  This type of interaction weakens with increasing distance between the polar molecules and also weaken at high temperatures where random thermal energy cause bigger fluctuations in their alignment.  This is the strongest type of intermolecular interaction possible between two neutral molecules.  It is at best comparable to ordinary thermal energies and is much weaker than the energies of covalent bonds.
 
 

Dipole-induced dipole

The electrostatic field emanating from a polar molecule, though weak compared to an ion, can also polarize another atom or nonpolar molecule. 

The strength of this interaction depends on the magnitude of the dipole's permanent moment (_A), the polarizability of the other atom or molecule (_B), the angle of alignment (_A) between the dipole and the polarized species, and the distance (R) between them. Bigger atoms and molecules are more susceptible to the polarization of their electron distribution because these electrons are farther from their atomic nuclei.  Because of the 1/R⁶ dependence of this interaction is exceeding weak until the species come into contact with one another. 

Hydrogen-bonding

Hydrogen bonds are special dipole-dipole interactions between two molecules. In these situations a hydrogen atom attached to an electronegative atom, through a polar covalent bond of one molecule, aligns with the unshared lone electron pair of an electronegative atom on another neighboring molecule. The configuration of a hydrogen-bond is:

The atom X is covalently bonded to the hydrogen atom through a single bond.  The dashed line represent the interaction of X through the hydrogen to Y on another molecule.
 

The strength of these interactions varies from 4 Kilojoules (KJ) up to 25 KJ.  Although the strongest hydrogen-bonding interactions are only about 12% that of a covalent chemical bond, they are significantly stronger than the typical dipole-dipole and dispersion forces.  This interaction plays a significant role in the varies properties of water such as its high boiling point and its subtle blue tinge in color.  It is also responsible for holding our genetic material (DNA) together. 

London dispersion

This force is very weak but is present between all atoms and molecules regardless of their nature. This is due to random fluctuation of an atom's electronic distribution with time, producing an instantaneous, but transient dipole moment. This moment would induce nearby atoms or molecules to polarize and become attracted each other.  The behavior of this interaction is rather complex, a reasonable approximation is given by:

The strength of this interaction between species increases with their size. In general, the larger the molecule, the farther its electrons are away from their nuclei, and, consequently the greater its polarizability (_A, _B). The interaction also increases in strength with the frequency of the electronic fluctuation (_A, _B) and the distance between them (R). The interaction is significant only at contact distances, due to its 1/R⁶ dependence.