Effect of thiocyanate anions on switching of poly (N-isopropylacrylamide) tethered to nanoparticle surface

Abstract

<sec>A recent experiment carried by Humphreys et al. (Humphreys B A, Wanless E J, Webber Grant B 2018 <i>J. Colloid Interface Sci</i>. <b>516</b> 153) shows that when poly (N-isopropylacrylamide) (PNIPAM) tethered to nanoparticle surface is immersed in potassium thiocyanate solution, the thiocyanate anions (SCN<sup>–</sup>) can increase the low critical solution temperature (LCST) of the PNIPAM below 500 mmol, though the LCST is reduced when at 1000 mmol. It is unclear why the SCN<sup>–</sup> increases the LCST at low concentration and reduces the LCST at high concentration. In this paper, using a molecular theory, we investigate the effect of SCN<sup>–</sup> on the switching and the structure of PNIPAM tethered to nanoparticle surface. In our model the PNIPAM-SCN<sup>–</sup> bonding (P—S bonds), electrostatic effects and their explicit coupling to the PNIPAM conformations are taken into consideration. We find that under the low SCN<sup>–</sup> concentration, as the SCN<sup>–</sup> concentration increases, the SCN<sup>–</sup> is associated with the PNIPAM chains through the PNIPAM—S bonds, and the PNIPAM segments become negatively charged, which makes electrostatic repulsion stronger and results in an increase in the LCST.</sec><sec>According to our model, the reduction of LCST at high SCN<sup>–</sup> concentration can be explained as follows: with the increase of SCN<sup>–</sup> concentration, more and more PNIPAM-SCN<sup>–</sup> bindings occur between SCN<sup>–</sup> and PNIPAM segments, which will lead the hydrophobicity of PNIPAM chains to increase. On the other hand, the P—S bonds have been filled at the high SCN<sup>–</sup> concentration, and the PNIPAM chains become more negatively charged. The increase of the SCN<sup>–</sup> is accompanied with an increase in the concentration of counterions (K<sup>+</sup>). The increase of counterion concentration will give rise to the counterion-mediated attractive interactions along the chains and electrostatic screening within the negatively charged PNIPAM, thus the LCST can be reduced when further increasing the SCN<sup>–</sup> concentration. The reduction of LCST can be attributed to the increased hydrophobicity of PNIPAM chains, or to the counterion-mediated attractive interaction along the chains and the screening of the electrostatic interactions. </sec><sec>By analyzing the distribution of PNIPAM segments near the critical temperature, we find that the distribution of volume fractions of the PNIPAM tethered to nanoparticle surface shows a maximum when the hydration of PNIPAM and PNIPAM-SCN<sup>–</sup> binding are stronger, which implies that a vertical phase separation may occur. Based on our theoretical model, a vertical phase separation and a two-step phase transition behaviors in the PNIPAM tethered to nanoparticle surface are predicted. We also analyze the height of the PNIPAM, which is a function of temperature at different SCN<sup>–</sup> concentrations, and then obtain the critical temperature of the two-step phase transition. The results show that the vertical phase separation and the two-step phase transition are promoted by competition between hydrophobicity, hydrophilicity and electrostatic effects due to the P—S bonds. Our theoretical results are consistent with the experimental observations, and provide a fundamental understanding of the effects of SCN<sup>–</sup> on the LCST of PNIPAM tethered to nanoparticle surface.</sec>