Temperature Switch: Why a Solution Reversely Changes Between Transparent @ 25 °C and Cloudy @ 45 °C

Abstract

The solubility of a substance determines how much of it will dissolve in a particular solvent at a given temperature. For most of the organic/inorganic compounds such as glucose and sodium chloride, their solubility in water increases with temperature. However, for polymers such as poly N-isopropylacrylamide and polyethylene oxide, they show lower critical solution temperature (LCST) behaviors and their solubility decreaseswhen temperature increases. A small molecule, 1,8-bis(2-(2-(2-methoxyethoxy)ethoxy)ethoxy)anthracene-9,10-dione) (AQ-1,8-3E-OCH3 ), shows an LCST behavior that its water solution is transparent at a temperature less than 45 °C, and cloudy at a temperature above 45 °C. More importantly, the whole process is reversible.[19] The aim of this study is to explain the LCST behavior of AQ-1,8-3E-OCH3 so that further application can be developed. After reviewing inorganic compounds, organic molecules, and polymers, The essay concluded that the hydrophilic tails (3E-OCH3 ) and the hydrophobic core (AQ) of AQ-1,8-3E-OCH3 endow its amphiphilic property to form micelles, thus being phase separated from water when it is heated. In the future, it is expected that AQ-1,8-3E-OCH3 can be applied for drug release. For example, drug molecules can be first surrounded in the AQ-1,8-3E-OCH3 micelles at a higher temperature, then get released to targeted cells at a lower temperature.