Degree of polymerization controls bound/free water balance and water-holding in multifunctional amphiphilic polymer esters

Moisture retention is a key determinant of sensory performance and stability in emulsified formulations, yet the molecular basis of water-holding in amphiphilic polymer esters used in cosmetics remains insufficiently elucidated. In this study, two compounds with different degrees of polymerization were synthesized from Polyglyceryl-2 Dipolyhydroxystearate (L-PD and S-PD), a water-in-oil emulsifier widely used in cosmetics. Hydrate formation revealed that L-PD exhibited a water-holding capability of 434%, compared to 152% for S-PD, with distinct hydrate structures. Based on the hypothesis that this difference in water-holding capability was due to differences in the degree of polymerization, the water-holding mechanism was investigated using both in vitro and in silico approaches. Experimental results showed that water in L-PD hydrates existed as both bound and free water, whereas S-PD hydrates contained predominantly free water. Simulations showed that differences in the degree of polymerization resulted in variations in the types of polymer configurations that could be adopted during water-holding. A higher degree of polymerization was found to favor polymer configurations with stronger affinity for water, promoting bound-water formation and enhancing the water-holding capability. This combined experimental and computational approach provides novel insights into the water-holding mechanism and is expected to aid the design of high-performance moisturizing materials for cosmetic applications.