Effective charge of PNIPAM microgels determined by conductivity measurements

Understanding the effective charge of colloidal particles is crucial to control the stability and performance of a colloidal system. In particular, the electrostatic interaction potential between the particles determines the stability and thermodynamic properties of electrostatically charged colloidal dispersions. In the case of thermosensitive microgels, which are solvent and ions permeable particles, it is expected that the microgel's size and the number of ions within it are affected by temperature changes. This temperature-dependent microgel-solvent ion interchange regulates the microgel net charge and, consequently, the interaction potential. Here, a straightforward experimental method based on conductivity measurements is presented to determine the temperature dependence of the effective net charge of poly-N-isopropylacrylamide, PNIPAM, microgels under different salinity conditions. Our results show that the net charge of the microgel decreases with increasing temperature. Specifically, microgels reduce their net charge by around 40%, when the temperature is changed from 25 to 40 degrees C. This scenario could be explained by the entrance of counterions into the microgel after its collapse to partially neutralize the increase of the electrostatic repulsion due to the closer proximity among the charged groups present in the polymer particle.