Entropic force - Citizendia

In physics, an entropic force acting in a system is a macroscopic force whose properties are primarily determined not by the character of a particular underlying microscopic force (such as electromagnetism), but by the whole system's statistical tendency to increase its entropy. Physics (Greek Physis - φύσις in everyday terms is the Science of Matter and its motion. Macroscopic is commonly used to describe physical objects that are measurable and observable by the Naked eye. In Physics, a force is whatever can cause an object with Mass to Accelerate. Microscopic is a term used to describe objects smaller than those that can easily be seen by the naked Eye and which require a lens or Microscope to see In Thermodynamics (a branch of Physics) entropy, symbolized by S, is a measure of the unavailability of a system ’s Energy

A standard example of an entropic force is the elasticity of a freely-jointed polymer molecule: if the molecule is pulled into an extended configuration, the fact that more contracted, randomly coiled configurations are overwhelmingly more probable (i. A material is said to be elastic if it deforms under stress (e A polymer is a large Molecule ( Macromolecule) composed of repeating Structural units typically connected by Covalent Chemical bonds e. possess higher entropy) will result in the chain eventually returning (through diffusion) to such configurations. Diffusion is the net movement of particles (typically molecules from an area of high concentration to an area of low concentration by uncoordinated random movement To the macroscopic observer, the precise origin of the microscopic forces that drive the motion is irrelevant: The observer simply sees the polymer contract into a state of higher entropy, as if driven by an elastic force.

Entropic forces occur in the physics of gases and solutions, where they generate the osmotic pressure of a dilute solution, and in colloidal suspensions, where they are responsible for the crystallization of hard spheres. Osmotic pressure is the hydrostatic pressure produced by a difference in concentration between solutions on the two sides of a surface such as a semipermeable membrane A colloid is a type of mechanical Mixture where one substance is dispersed evenly throughout another Crystallization is the (natural or artificial process of formation of solid Crystals precipitating from a homogeneous --> identical Solution

Hydrophobic force

A very frequently cited example of entropic force is hydrophobic force. In Chemistry, hydrophobicity (from the combining form of water in Attic Greek hydro- and for fear phobos) refers to the physical property of Though hydrophobic force has an enthalpic component, about half of it originates from the entropy of the hydrogen bonded three dimensional network lattice of water molecules at room temperature. A hydrogen bond results from a Dipole-dipole force between an Electronegative atom and a Hydrogen atom bonded to Nitrogen, Oxygen Since each water molecule is capable of donating two hydrogen bonds through the two protons and accept two more hydrogen bonds through the two sp3 hybridized lone pairs, a water molecule, unlike Hydrogen fluoride (accept 3 but donate only 1) or ammonia (donate 3 but accept only 1) which mainly form linear chains, can form an extended three dimensional network lattices. Introduction of a non-hydrogen bonding surface disrupts this networks and the rest of the network immediately rearranges itself around that surface so as to minimize the number of disrupted hydrogen bonds. If the introduced surface had ionic or polar nature there would have been hydrogen molecules standing more or less orthogonal to this surface. But a non-hydrogen bonding surface forces the surrounding hydrogen bonds to be tangential and become locked in a clathrate like basket shape. Water molecules involved in this clathrate like basket envelope formation around the non-hydrogen bonding surface are entropically restrained. Thus any event that would minimize this type of surface would be entropically favoured. For example when two such particles comes very close they merge and the water trapped in between would be released from the trapped state and join the free bulk water molecules and lead to increase in entropy. That is the basis of the so called "attraction" between hydrophobic objects in solution. Though when two hydrophobic objects attract when they are very close, there may be a mild repulsion when they are about to come close. This mild repulsion may happen because the water molecules when trapped in between two hydrophobic surfaces coming from different direction are frustrated about whether to orient tangential to one surface or the other. However when the surfaces come further close this repulsion is more than compensated by the great increase in entropy when the water molecules are completely displaced of the clathrate-like sheath.

Hydrophobic force

See also