Effects of temperature and anaesthetic molecules on the aggregation of block copolymers
An increase in temperature induces allregation of Pluronics. The dehydration of POP segments enhances the hydrophobic interaction and drives the formation of aggregates. The POP/POE mass ratio and relative molar mass of the Pluronics are factors that affect the onset of aggregation. Pluronics appear to aggregate in the order of clusters (without hydrophobic core), larger aggregates and well defined micelles as the temperature is increased. The length of the POE segments was found to be important in determining the temperature dependence of the size of aggregates. The core of the micelles contains some water and the amount decreases with increasing hydrophobicity of the Pluronics. An addition of small amounts (several mM) of the anesthetics induces the aggregation of Pluronics at 25°C. The addition of 1 mM of anesthetic is estimated to be equivalent to ca. a 1-2°C increase in temperature. It is deduced that, whereas an increase in temperature removes water of hydration from around the POP segments, the anesthetic molecules tend to replace water of hydration around these segments. The hydrogen bonding (between acidic hydrogen of anesthetic molecules and oxygen of POP segments) and the hydrophobic Interaction (between anesthetic molecules and POP segments) we considered to be the chemical and physical driving force, respectively, for the aggregation. The anesthetic molecules are found to distribute in both the hydrophobic core and hydrophilic corona but with a preference for the interfacial region either between the hydrophobic and hydrophilic segments or. between the hydrophilic segment and bulk water. This study of triblock copolymer aggregates as a solvation model for membranes has shown that inhalation anesthetics may affect the hydration water, the so called "biological water", of a membrane. It is proposed that the dehydration effect shown by the anesthetics could play an important role in the mechanism of anesthesia.