Humectants are hygroscopic substances used to retain moisture in many personal care products, particularly in phospholipid-based skin creams and haircare products. Here we studied the effect of humectants on the thermotropic behavior and nanostructure of fully hydrated 1, 2-dipalmitoyl-sn-glycero-3-phosphocholine, DPPC, from 25 to 50 °C. Three humectants have been investigated: betaine, sarcosine and acetamide monoethanolamine, AMEA. Small angle X-ray scattering, SAXS, differential scanning calorimetry, DSC, and isothermal calorimetry, ITC, have been applied. The general trend is that all humectants induce an increase in both the pre-transition temperature (gel to ripple phase) and the main transition temperature (ripple to fluid phase). At lower concentrations of humectant (0.05-0.4 M), however, we observed a lowering in the pre-transition temperature. Deduced electron density profiles of the gel phase of DPPC display for all three humectants similar effects: the membrane thickness marginally increases with increasing concentration of humectant, and concomitantly the interstitial water layer decreases significantly. This suggests that the membrane becomes more rigid upon addition of humectants leading to a reduced inter-membrane repulsion (reduction of the Helfrich undulation force). Furthermore, DSC results reveal strong enthalpy variations compared to DPPC in pure water. Complementing ITC measurements on the solution/dissolution enthalpy of the humectants confirmed that changes in the lipid/water partition coefficient across the main transition are responsible for an overall increase of the enthalpy in the presence of AMEA and leads to a decrease with sarcosine. Betaine instead has no great influence on the main transition enthalpy. In conclusion, by combining SAXS, DSC and ITC (i) nanostructural changes in the DPPC bilayer as well as (ii) different lipid/water partitioning behavior of the humectants can be unraveled.
CORE (COnnecting REpositories)
CORE (COnnecting REpositories)