However, this time the entropy term, which is magnified by the large temperature, will outweigh the enthalpy term, and consequently the repair of the membrane will not be spontaneous, hence it has lost its elasticity. When the membrane is more fluid, there are less Van der Waals interactions between constituent non-polar tails and in addition less interactions between polar heads. This means there will be less bonds disrupted by the damage of the cell membrane. This makes the enthalpy term smaller and it can no longer outweigh the entropy term. Another factor that affects the elasticity of the membrane is the ratio of saturated to unsaturated hydrocarbon tails.
Saturated hydrocarbons have only single bonds and consequently are straight chains.
Unsaturated hydrocarbons have one or more double bonds which create kinks in the side chain so that they are not straight. If the phospholipids are saturated and therefore straight, they can pack close together and form numerous Van der Waals bonds. Unsaturated hydrocarbons are not straight and thus are not able to pack together as tightly. Consequently, they form less strong and close Van der Waals bonds between other hydrophobic tails' and we are back to the situation of the membrane being in a more fluid or random arrangement again, as compared to completely saturated hydrocarbons. Very short hydrocarbon chains result in a similar effect.