Van der Waals force

With the van der Waals forces (van der Waals interaction), named after the Dutch physicist Johannes van der Waals Diderik ( 1837-1923 ), refers to the relatively weak non-covalent interactions between atoms or molecules whose interaction energy of about the sixth power of the distance decrease. Thus, the van der Waals forces can be split as a cause of Van der Waals bonding in modern terms into three components:

• Keesom interaction between two dipoles (dipole -dipole forces),
• Debye interaction between a dipole and a polarizable molecule (dipole -induced dipole forces)
• Londonsche dispersion forces ( London forces ) between two polarizable molecules ( induced -dipole induced -dipole forces). London forces are often referred to as Van der Waals force in the narrow sense.

All Van -der- Waals forces, in comparison to atomic bonding and ionic bonding weak forces, the dispersion is generally the dominant interaction of the three components is. For example, take the van der Waals forces of HCl to HI, although the dipole moment is decreased. The van der Waals forces are the attractive interaction term in the Lennard -Jones potential.

Cause

The van der Waals force generally occurs between nonpolar ( uncharged ) nanoparticles ( noble gas atoms, molecules ) and leads to a weak attraction of this nanoparticles.

Electrons in nanoparticles (atomic ) can only move within certain limits, which leads to an ever-changing charge distribution in the microparticles. Once the center of gravity of the positive charges is spatially separated from the centroid of the negative charges, one can speak of a dipole, because there are two ( di -, the Greek δύο "two" borrowed ) electric poles. Individual non-polar molecules can, however, only be described as temporary dipoles, because their polarity is dependent on the electron distribution, and it changes constantly. ( In polar molecules, however, the dipole is permanent due to the electronegativity of the atoms and the spatial structure, so they are called permanent dipoles or dipoles in the strict sense. )

Come now two nonpolar molecules to each other long enough (ie, at low particle velocity ) near, then go for an electrostatic interaction with each other.

If the particles A as neighbors B shows a marked negatively charged side, the electrons are repelled neighbor B (from the side facing ). Thus, the dipoles align to each other. Such a charge transfer by an electric field is called induction. That is, the negative pole of a temporary dipole influiert vis -à-vis the neighboring molecule a positive pole. Thus, from a particle B " influenzierter " dipole. In the literature, this is " induced dipole " (Latin inducere: (back ) Insert ) called.

Between the original temporary dipole and the induced dipole, it is the van der Waals attraction. From now on, the dipoles interact with each other, their electron shift is synchronized.

Come, therefore, occur two atoms or molecules close enough so one of the following situations.

• Two temporary dipoles meet: the particles attract each other.
• A temporary dipole in a particle hits without dipole moment: The dipole induced in the non- dipole a rectified dipole moment, which also results in a return force of attraction between two particles.

Van-der- Waals binding energy: 0.5-5 kJ / mol (corresponding to 5-50 MeV / molecule)

Quantum mechanical view

In the above description, however, the electrons are treated as classical particles and does not take into account the findings of quantum mechanics. In the quantum mechanical model of the atom, the electron is described by a stationary wave function whose squared modulus remains the same at a certain point in the atom. This initially suggests the idea that the electron behaving like a classical extended charge distribution, with a charge density that is given by the product of electron charge and the absolute square of the wave function:

Thus, the spontaneous emergence of temporary dipoles the charge distribution would be immutable, and hence not possible. Since, typically, is axially symmetric around the nucleus would be the dipole moment, such as a noble gas atom is always zero.

On closer inspection of the quantum mechanical charge density operator

Where the position operator of the electron is, but this proves to be a fallacy. An electron does not behave like an extended charge distribution, but as a point charge, whose whereabouts are undefined. For the expectation value of the charge density, although results actually

However, it is not an eigenvalue of the charge density operator. The charge density has a certain amount of imprecision that leads just that with a certain probability not the focus of the electronic charge distribution in the nucleus and thus a dipole moment arises. In this way, the picture of quantum mechanics to understand the Van -der- Waals forces.

Van-der- Waals binding

Since the said dipole moments are small, the resultant electrical attraction is weak and has only a very small range. Thus, the van der Waals bond can ever come about, two atoms or molecules must come close. This approach is all the more "difficult" ( statistically unlikely ), the more kinetic energy the molecules, ie the higher the temperature is. With increasing temperature, the thermal motion predominates over the Van der Waals bond. This often represents the transition from the liquid to the gaseous state

There are solids that are held together solely by van der Waals bonding. The occurring only at low temperatures noble gas crystals are an example of this.

Clearly, the influence of the van der Waals forces on the example of alkanes understand. Here is the boiling point ( the melting point is not, as at this point other influences to come) initially increases with increasing molar mass. Isomers at the boiling point rises with an increasing or decreasing chain spherical shape, since the ball for a given volume, the smallest surface. This phenomenon can be found, for example, the following isomers (all C5H12 ) 2,2- dimethylpropane (bp 9.5 ° C), 2-methyl butane (boiling point 28 ° C) and n-pentane (boiling point 36.1 ° C).

Geckos use the van der Waals forces to stick without glue or suction cups on surfaces. The bottoms of her feet are full of tiny hairs. Each hair can only transfer a small force by the high number reaches the sum of the forces still advocates that the animal can run upside down on ceilings. This is also possible to smooth appearing surfaces such as glass. The sum of the van der Waals forces of gecko is about 40 N.