The smooth silkiness we equate with moisturised hair, is actually from hair cuticle layers sealed in a lipid complex including fatty acids, ceramics, glycolipids and cholesterol. One important fatty acid within the cuticle layer is called 18-methyl eicosanoic acid (18-MEA). 18-MEA is bound to the epicuticle which is the thin outer layer of the hair cuticles. This is lipid rich area is also referred to as the external protective f-layer (fatty layer) of hair.
Too much moisture makes your hair fibre swell and weakens it. 18-MEA also controls how much water leaves the hair. As long as this hydrophobic layer remains present, less moisture is lost from the hair.