Cotton is more absorbent than nylon mainly due to the difference in their molecular structures and properties.

1. Natural Fiber vs Synthetic Fiber: Cotton is a natural fiber derived from the seed hairs of cotton plants, while nylon is a synthetic polymer created in labs. Natural fibers tend to have greater absorbency because they are composed of materials that occur in living organisms and are designed to interact with water.
2. Porous Structure: Cotton fibers have a porous, cellulose-based structure that allows for capillary action, which draws moisture into the fabric. The numerous tiny gaps and spaces within the cotton fibers provide ample surface area for water molecules to adhere to and be absorbed.
3. Hydrophilic Nature: Cotton fibers are hydrophilic, meaning they attract and readily bond with water molecules. This property makes cotton fabrics excellent at absorbing and holding moisture.

On the contrary, nylon is a hydrophobic material, which repels water rather than absorbs it. Nylon fibers are smooth and uniform, lacking the natural pores and irregularities that allow cotton to soak up water efficiently. Instead, water beads up on the surface of nylon, preventing absorption.

In practical terms, this difference in absorbency is why cotton is preferred for items like towels, bedding, and clothing that need to wick away sweat or dry quickly after becoming wet. Meanwhile, nylon is often used for applications requiring water resistance, such as rain gear or outdoor equipment.