4.1. pH and Zeta Potential Measurements

The pH measurements were performed at 15 °C in a thermostatic bath using the ELMETRON CP-401 pH-meter (ELMETRON Sp. J., Zabrze, Poland), equipped with a glass electrode for viscous liquids ERH-12-6.

The Zeta potential was measured by analyzing the electrophoretic mobility of the molecule in an electric field using the ZetaSizer Nano ZS device (Malvern Instruments Ltd., Malvern, UK). A capillary measuring cell of the DTS1060 type was used. Measurements were carried out under both isothermal and non-isothermal conditions; all tests were repeated 3 to 5 times.

In the case of isothermal measurements, the measuring cell was filled with the medium at a storage temperature of 5 °C and placed in the measuring chamber of the device. The tests were carried out at a constant temperature, close to the phase transition temperature, which was 40 °C for chitosan solutions containing disodium or magnesium glycerophosphate, and 20 °C for solutions containing calcium glycerophosphate, respectively.

The tests under non-isothermal conditions were carried out in the temperature range from 5 °C to 50 °C. The temperature change interval was 2 °C, and the heating rate was 1 deg/min.