Low-temperature thermal properties of the aqueous solutions of simple aminoalcohols: Finding of peculiarities in 1-amino-2-propanol solutions

• We found peculiar behaviors of Cp and Tg in 1-amino-2-propanol aqueous solutions.
• Our findings indicate the presence of a phase separation on a scale of 1 nm or so.
• The separation is attributed to NH2 and OH groups to take gauche conformations.

Low-temperature heat capacity (Cp) and glass-transition temperature (Tg) values of the aqueous solutions of three simple aminoalcohols (hereafter abbreviated as AA); i.e., 2-aminoethanol (2AE), 3-amino-1-propanol (3A1P) and 1-amino-2-propanol (1A2P), were examined by differential scanning calorimetry (DSC). Both the 2AE and 3A1P aqueous solutions revealed gradual decrease of Cp values on cooling in an AA molar-fraction (xAA) in the range of 0.2–0.7, while displaying no crystallization. On the other hand, the 1A2P aqueous solutions revealed a Cp hump at the temperature slightly above Tg on both cooling and heating curves when the xAA approaches 0.19. The Tg values of the 1A2P solutions simultaneously displayed a maximum as a function of the xAA, which suggests formation of an ordered aggregate structure with development of a certain hydrogen-bond network. We propose, as the origin of the anomalies, formation of a molecular-aggregate structure of nanometer-sized regions of two kinds; one region is composed of the hydrophilic groups of AA molecules and water molecules and the other of the hydrophobic groups of AA molecules.

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