Pressure of an ideal gas The
internal energy of an
ideal gas depends only on its temperature, and not on the volume of its containing box, so it is not an
energy effect that tends to increase the volume of the box as gas
pressure does. This implies that the
pressure of an ideal gas has an entropic origin. What is the origin of such an entropic force? The most general answer is that the effect of thermal fluctuations tends to bring a
thermodynamic system toward a macroscopic state that corresponds to a maximum in the number of
microscopic states (or micro-states) that are compatible with this macroscopic state. In other words, thermal fluctuations tend to bring a system toward its macroscopic state of maximum
entropy. Neumann derived the entropic force for a particle undergoing three-dimensional Brownian motion using the
Boltzmann equation, denoting this force as a
diffusional driving force or
radial force. In the paper, three example systems are shown to exhibit such a force: •
electrostatic system of
molten salt, •
surface tension and, •
elasticity of rubber.
Polymers A standard example of an entropic force is the
elasticity of a freely jointed
polymer molecule. The entropic force by a freely jointed chain has a clear mechanical origin and can be computed using constrained
Lagrangian dynamics. With regards to biological polymers, there appears to be an intricate link between the entropic force and function. For example, disordered polypeptide segments in the context of the folded regions of the same polypeptide chain have been shown to generate an entropic force that has functional implications.
Hydrophobic force Another example of an entropic force is the
hydrophobic force. At room temperature, it partly originates from the loss of entropy by the 3D network of water molecules when they interact with molecules of
dissolved substance. Each water molecule is capable of • donating two
hydrogen bonds through the two protons, • accepting two more hydrogen bonds through the two
sp3-hybridized lone pairs. Therefore, water molecules can form an extended three-dimensional network. Introduction of a non-hydrogen-bonding surface disrupts this network. The water molecules rearrange themselves around the surface, so as to minimize the number of disrupted hydrogen bonds. This is in contrast to
hydrogen fluoride (which can accept 3 but donate only 1) or
ammonia (which can donate 3 but accept only 1), which mainly form linear chains. If the introduced surface had an ionic or polar nature, there would be water molecules standing upright on 1 (along the axis of an orbital for ionic bond) or 2 (along a resultant polarity axis) of the four sp3 orbitals. These orientations allow easy movement, i.e. degrees of freedom, and thus lowers entropy minimally. But a non-hydrogen-bonding surface with a moderate curvature forces the water molecule to sit tight on the surface, spreading 3 hydrogen bonds tangential to the surface, which then become locked in a
clathrate-like basket shape. Water molecules involved in this clathrate-like basket around the non-hydrogen-bonding surface are constrained in their orientation. Thus, any event that would minimize such a surface is entropically favored. For example, when two such hydrophobic particles come very close, the clathrate-like baskets surrounding them merge. This releases some of the water molecules into the bulk of the water, leading to an increase in entropy. Another related and counter-intuitive example of entropic force is
protein folding, which is a
spontaneous process and where
hydrophobic effect also plays a role. Structures of water-soluble proteins typically have a core in which hydrophobic
side chains are buried from water, which stabilizes the folded state. although formation of hydrogen bonds within the protein also stabilizes protein structure.
Colloids Entropic forces are important and widespread in the physics of
colloids, where they are responsible for the
depletion force, and the ordering of hard particles, such as the
crystallization of
hard spheres, the isotropic-
nematic transition in
liquid crystal phases of hard rods, and the ordering of hard polyhedra. Because of this, entropic forces can be an important driver of
self-assembly Entropic forces arise in colloidal systems due to the
osmotic pressure that comes from particle crowding. This was first discovered in, and is most intuitive for, colloid-polymer mixtures described by the
Asakura–Oosawa model. In this model, polymers are approximated as finite-sized spheres that can penetrate one another, but cannot penetrate the colloidal particles. The inability of the polymers to penetrate the colloids leads to a region around the colloids in which the polymer density is reduced. If the regions of reduced polymer density around two colloids overlap with one another, by means of the colloids approaching one another, the polymers in the system gain an additional free volume that is equal to the volume of the intersection of the reduced density regions. The additional free volume causes an increase in the entropy of the polymers, and drives them to form locally dense-packed aggregates. A similar effect occurs in sufficiently dense colloidal systems without polymers, where osmotic pressure also drives the local dense packing These effects are for anisotropic particles referred to as directional entropic forces.
Cytoskeleton Contractile forces in biological cells are typically driven by
molecular motors associated with the
cytoskeleton. However, a growing body of evidence shows that contractile forces may also be of entropic origin. The foundational example is the action of microtubule crosslinker Ase1, which localizes to
microtubule overlaps in the
mitotic spindle. Molecules of Ase1 are confined to the microtubule overlap, where they are free to diffuse one-dimensionally. Analogically to an ideal gas in a container, molecules of Ase1 generate pressure on the overlap ends. This pressure drives the overlap expansion, which results in the contractile sliding of the microtubules. An analogous example was found in the
actin cytoskeleton. Here, the actin-bundling protein
anillin drives actin contractility in
cytokinetic rings. == Controversial examples ==