Solvent isotope and density effects on the thermoresponsive behavior of amphiphilic materials

This study aims at providing additional insight and corroboration related to our previous work [Di Spirito et al., “A Pluronic block copolymer in H2O and D2O: The isotope effect on phase transition,” Phys. Fluids 36(12), 121712 (2024)] exploring the phase behavior of Pluronic F68 (Poloxamer 188) solutions in water (H2O) and heavy water (D2O). Such materials undergo thermal phase transitions, moving from a micellar liquid phase to a body-centered cubic (BCC) solid structure with increasing temperature. The study demonstrated that the substitution of water by heavy water as a solvent noticeably influences the system phase behavior when dealing with the same weight concentration. The liquid-to-solid phase transition in D2O occurs at temperatures 8-10 °C lower than in H2O, attributing this difference to the isotope effect. It examined samples with equal weight concentrations, not accounting for the effect of solvent density alone. To address this gap, the present work investigates various Pluronic F68 solutions prepared at both the same weight and volume concentrations in H2O and D2O. By analyzing how the transition temperature changes under different temperature ramp rates and polymer concentrations, we found that the gap of 8-10 °C difference observed at equal weight concentration reduces to 2-3 °C at identical volume concentration. This reduction isolates the influence of solvent density, linked directly to the isotope difference. Additionally, this study examines micellization, revealing that micelle formation and volume fraction are also significantly affected by the solvent isotope. Activation energy analysis further indicates that solvent isotope effects modulate the thermoresponsive behavior of Pluronic F68 solutions. Differential scanning calorimetry provided enthalpic signatures of the unimer-micelle and micelle-BCC transitions, revealing lower micellization enthalpies in D2O compared to H2O. These findings corroborate that phase properties of amphiphilic materials can be isotope-sensitive, underscoring the importance of considering isotope effects in rheological and scattering studies, where D2O and H2O are often treated as equivalent.