What is the hydrophobic effect?

The hydrophobic effect is the phenomenon that is observed in nonpolar substances when they come into contact with water. In an aqueous solution, hydrophobic molecules tend to aggregate excluding water molecules, something that can be easily seen in a mixture of water and oil; the drops of oil are added until two phases are separated, one aqueous and one oily.

The hydrophobic effect is key in many biological processes. For example, it participates in the stabilization of cell membranes, vesicle formation, protein folding or DNA stabilization. In this sense, the hydrophobic effect is essential for terrestrial life. It also has numerous applications; for example in material and fabric coatings water repellents.

Entropy and hydrophobic effect

Water molecules interact with each other and with polar substances by Hydrogen bonds and dipole-dipole interactions, something that does not occur in hydrophobic substances. When hydrophobic substances are present in an aqueous medium, they tend to bind together in a way that maximizes hydrogen bonding between water molecules.

Although it is not fully understood what it is due to or why it occurs, it seems that the main cause is the result of entropic effect. Entropy is a state function, that is, a characteristic that describes the equilibrium state of a thermodynamic system. Entropy is usually assimilated with degree of disorder of matter of the system and, in general, entropy tends to increase in natural and spontaneous processes (2nd Law of Thermodynamics).

Hydrogen bonds form a dynamic, three-dimensional network of water molecules. When a nonpolar molecule is introduced, it cannot interact with the water molecules and the network is interrupted. The water molecules in contact with the hydrophobic molecule reorient the hydrogen bonds tangentially, forming a kind of capsule around the hydrophobic molecule, the so-called solvation layer.

The water molecules in the solvation shell have the restricted movementlose translational and rotational entropy, that is, have lower entropy and this makes the process unfavorable from a thermodynamic point of view.

If instead of one hydrophobic molecule we introduce two, a solvation shell can form around each one, further decreasing the entropy. But if they join together they can share the solvation shell; in this way the surface of the solvation shell will be smaller and the entropy will be greater than when they are separated, that is why the aggregation process is thermodynamically favorable and occurs spontaneously.

In addition, when hydrophobic molecules are added the free energy of the system decreases (ΔG < 0), which also favors the hydrophobic effect:

Free energy

Where:

  • G is the free energy
  • H is the enthalpy
  • T is the temperature
  • yes is the entropy

Using calorimetry, the entropy and enthalpy of the process can be measured. Keeping the temperature constant, it has been found that the hydrophobic effect is driven by entropy and the effect on the mobility of water molecules.

As the temperature increases, the mobility of the molecules increases and the entropy reduction in the solvation shell is less, so the hydrophobic effect decreases. At high temperatures the miscibility between water and hydrophobic molecules increases.

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