1.Xinjiang Laboratory of Phase Transitions and Microstructures of Condensed Matter Physics, Yili Normal University, Yining 835000, China 2.Laboratory of Micro-Nano Electro Biosensors and Bionic Devices, Yili Normal University, Yining 835000, China 3.Center of Urology, the Xinjiang Uygur Autonomous Region People’s Hospital, Urumqi 830000, China
Fund Project:Project supported by the Joint Funds of Xinjiang Natural Science Foundation, China (Grant No. 2019D01C333) and the National Natural Science Foundation of China (Grant No. 21764015)
Received Date:06 May 2019
Accepted Date:02 July 2019
Available Online:01 November 2019
Published Online:05 November 2019
Abstract:A recent experiment carried by Humphreys et al. (Humphreys B A, Wanless E J, Webber Grant B 2018 J. Colloid Interface Sci. 516 153) shows that when poly (N-isopropylacrylamide) (PNIPAM) tethered to nanoparticle surface is immersed in potassium thiocyanate solution, the thiocyanate anions (SCN–) 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– 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– on the switching and the structure of PNIPAM tethered to nanoparticle surface. In our model the PNIPAM-SCN– 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– concentration, as the SCN– concentration increases, the SCN– 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.According to our model, the reduction of LCST at high SCN– concentration can be explained as follows: with the increase of SCN– concentration, more and more PNIPAM-SCN– bindings occur between SCN– 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– concentration, and the PNIPAM chains become more negatively charged. The increase of the SCN– is accompanied with an increase in the concentration of counterions (K+). 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– 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. 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– 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– 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– on the LCST of PNIPAM tethered to nanoparticle surface. Keywords:poly (N-isopropylacrylamide) tethered to nanoparticle surface/ switching/ effect of thiocyanate anions
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2.分子场理论模型考虑浸没在硫氰酸钾(KSCN)水溶液中接枝在纳米粒子表面的PNIPAM球面刷系统(图1), 球形纳米粒子的半径R = 30 nm, 纳米粒子的球心定为坐标原点, 建立球坐标系, 沿半径方向标记为r轴, 纳米粒子表面接枝${N_{\rm{p}}}$个PNIPAM链, 每个PNIPAM分子有N个单体, 每个PNIPAM分子单体体积${v_{\rm{p}}} = \;0.16\;{\rm{n}}{{\rm{m}}^3}$. 单位面积接枝的PNIPAM分子数, 即接枝密度定义为$\sigma = {N_{\rm{p}}}/4{\text{π}}{R^2}$. 阴离子(SCN–)、阳离子(K+)和水分子的体积近似相等, 可取值为${v_i}\; = \;0.03\;{\rm{n}}{{\rm{m}}^3}$ (i = –, +, w), 假定各种分子不均匀分布仅在径向(r方向)上. 需要说明的是, 由于理论计算的限制, 理论研究PNIPAM球面刷体系采取的PNIPAM分子的单体数目N = 50, 这个值小于实验观测样品PNIPAM分子的分子链长(N > 100)[15], 但这对于研究SCN–影响PNIPAM球面刷构象转变的机理, 预言新的相结构本质特性应该是足够的, 实验已经证实, PNIPAM刷构象转变的LCST与PNIPAM的分子量不相关[23-25]. 图 1 接枝在纳米粒子表面的PNIPAM球面刷系统(其中SCN–通过P—S键与PNIPAM结合) Figure1. Schematic representation of the PNIPAM tethered to nanoparticle surface. Bonding between PNIPAM and SCN– by formation of P—S bonds.