Release of Solubilizate from Micelle upon Core Freezing

Abstract

By combining NMR (yielding H chemical shift, spin relaxation, and self-diffusion data) and small-angle X-ray scattering experiments, we investigate the complex temperature dependence of the molecular and aggregate states in aqueous solutions of the surfactant [CH(CH)(OCHCH)OH], abbreviated as C18E20, and hexamethyldisiloxane, HMDSO. The latter molecule serves as a model for hydrophobic solubilizates. Previously, the pure micellar solution was demonstrated to exhibit core freezing at approximately 7-8 °C. At room temperature, we find that HMDSO solubilizes at a volume fraction of approximately 10% in the core of the C18E20 micelles, which consists of molten and thereby highly mobile alkyl chains. Upon lowering the temperature, core freezing is found, just like in pure micelles, but at a temperature shifted significantly to 3 °C. The frozen cores contain immobile alkyl chains and exhibit a higher density but are essentially devoid (volume fraction below 1%) of the solubilizate. The latter molecules are released, first gradually and then rather steeply, from the core in the temperature range that is roughly delimited by the two core freezing temperatures, one for pure micelles and one for micelles with solubilizates. The release behavior of systems with different initial HMDSO loading follows the same master curve. This feature is rationalized in terms of loading capacity being strongly temperature dependent: upon lowering the temperature, release commences once the loading capacity descends below the actual solubilizate content. The sharp release curves and the actual release mechanism with its molecular features shown in rich detail have some bearing on a diverse class of possible applications.

Strong Variation of Micelle–Unimer Coexistence as a Function of Core Chain Mobility

“…To evaluate the influence of core block T g on micelle–unimer coexistence, we extracted the micelle and unimer populations for the solutions of polymers 1a–g from fits to their signal decay curves, as shown in Figure C (also Figures S38–S42). The stark difference in unimer populations between the 100 and 0% n BA micelles suggests that the 0% n BA micelles exist in a “frozen core” state at the temperature of the NMR experiment (25 °C) . Interestingly, NMR diffusometry experiments before and after thermal annealing of several samples revealed no changes in the unimer percentage (Figures S43–S45), indicating that these results are thermally robust and reversible and are due to differences in chain mobility (or some system parameters closely correlated with mobility) and not due to the kinetic trapping of micelle chains.…”

“…The stark difference in unimer populations between the 100 and 0% nBA micelles suggests that the 0% nBA micelles exist in a "frozen core" state at the temperature of the NMR experiment (25 °C). 55 Interestingly, NMR diffusometry experiments before and after thermal annealing of several samples revealed no changes in the unimer percentage (Figures S43−S45), indicating that these results are thermally robust and reversible and are due to differences in chain mobility (or some system parameters closely correlated with mobility) and not due to the kinetic trapping of micelle chains. Additionally, NMR diffusometry experiments on 25% nBA micelles (polymer 1f) at concentrations ranging from 3 to 10 mg/mL revealed no changes in unimer percentages (Figure S46).…”

Tale of Two Tails: Molecular Exchange Kinetics of Telechelic Polymer Micelles

“…[109] The same group later exploited core freezing to trigger the release of a hydrophobic cargo from the micellar core. [110] In the context of the present thesis, C n -PEOx was employed to investigate the crystallization behavior of alkyl groups in micellar cores and how crystallization affects both micellar morphology and exchange kinetics as well as related phenomena.…”

Small-angle Scattering of X-rays and Neutrons as Structural Tools inNanoscience: Block Copolymer Micelles, Protein-detergent Complexes,Microgels and other Nanoparticles

“…There has been a large progress in analysis within the last decades and this has led to broad field of applications of the SAXS and SANS methods. In this lecture, the basic principles and theory will be described and some recent applications to block copolymer micelles [3], protein-detergent complexes [4] and microgels [5] will be presented. Figure 1.…”

“…Figure 1. Examples of structures: Block copolymer micelles [3], protein-detergent complexes [4] and hollow microgel [5].…”