Phospholipids assemble spontaneously into bilayers driven by the attraction of their "tail" portions to each other and of their "head" portions to each other. the tail regions are

Tails of the phospholipids in the phospholipid bilayer are hydrophobic,  consisting of fatty acid chains.  
Phospholipids are composed of two fatty acids, glycerol and phosphate groups, forming an amphipathic molecule. Amphipathic molecules are built from hydrophilic (water-loving) and hydrophobic (not attracted to water) molecules. The phospholipids in the plasma membrane are arranged in two layers, called a phospholipid bilayer. Hydrophobic molecules (fatty acids) are on the interior of the membrane, whereas the water-loving heads (phosphate head) point outwards, toward either the cytoplasm or the fluid that surrounds the cell. The head group and fatty acids are attached to a glycerol unit.

The answer to this will be from your lab notes/procedure. it would be best to ask another student who participated in the lab as we do not know what was done, what enzyme or if the temperature is celsius or fahrenheit. if i were to guess though i'd say the rate stopped due to enzyme denturing at high temperature