Abstract
Colloidal Unimolecular Polymer (CUP) particles are spheroidal, 3–9 nm with charged groups on the surface and a hydrophobic core, which offer a larger surface water fraction to improve the analysis of its characteristics. Differential scanning calorimetry (DSC) was performed to determine the characteristics of surface water. These properties include the amount of surface water, the layer thickness, density, specific heat of the surface water above and below the freezing point of water, melting point depression of free water, effect of charge density and particle size. The charge density on the CUP surface was varied as well as the molecular weight which controls the particle diameter. The surface water is proportional to the weight fraction of CUP <20%. Analogous to recrystallization the CUP particles were trapped in the ice when rapidly cooled but slow cooling excluded the CUP, causing inter-molecular counterion condensation and less surface water. The density of surface water was calculated to be 1.023 g/mL to 1.056 g/mL depending on the surface charge density. The thickness of surface water increased with surface charge density. The specific heat of surface water was found to be 3.04 to 3.07 J/g·K at 253.15 K and 3.07 to 3.09 J/g·K at 293.15 K. The average area occupied by carboxylate and ester groups on the CUP surface were determined.
Highlights
The freezing of water has been an extensively studied thermodynamic property
This work discussed the thermal properties of Colloidal Unimolecular Polymer (CUP) with different molecular weight, monomer ratio and CUP surface charge density, based on the heat of fusion, specific heat and melting point depression aspects
Rapid cooling of CUP solutions will result in larger amount of surface water due to more rapid ice crystal growth and less time for CUP particles to migrate and undergo
Summary
The freezing of water has been an extensively studied thermodynamic property. The freeze thaw of sperm, eggs, cryogenic preservation, crop freezing, food preservation by freezing, paint freeze thaw stability, road and walkway ice and aircraft icing are just a few examples of where ice formation is a critical issue. Surface associated water plays an important role in most of these examples. The study of surface water has been difficult due to the low ratio of surface to free water present in a system. Nano particles can offer a significantly enhanced window into surface water due to the high surface water to particle weight ratio. Colloidal Unimolecular Polymer (CUP) particles are a new class of spheroidal nanoscale polymer particles (3–9 nm) with charged hydrophilic groups on the surface and a hydrophobic backbone [1]
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