Identifying thermal phase transitions of lignin–solvent mixtures using electrochemical impedance spectroscopy
Identifying thermal phase transitions of lignin–solvent mixtures using electrochemical impedance spectroscopy
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使用电化学阻抗谱识别木质素与溶剂混合物的热相变
DOI：
10.1039/c5gc02342d
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发表时间：
2016
期刊：
影响因子：
9.8
通讯作者：
Roberts, Mark E.
中科院分区：
文献类型：
--
作者：
Klett, Adam S.;Gamble, Jordan A.;Thies, Mark C.;Roberts, Mark E.
木质素在可再生生物聚合物中是独一无二的，因为它具有显著的芳香性，使其在从涂料到碳纤维的广泛应用中具有潜在的吸引力。最近的研究表明，热醋酸(ACoH)-水混合物可用于从硫酸盐木质素中回收相对分子质量可控的超净木素。这一发现的一个主要特征是存在一个液-液平衡区，一个相富含提纯的木质素，另一个相富含溶剂。虽然目视方法可以用来确定固体木质素在ACoH-水混合物存在下熔融形成LLE的温度，但由于溶剂不透明，只有在较低的ACoH浓度下才能看到相变。为此，发展了一种电化学阻抗谱(EIS)技术来测量针叶木硫酸盐木素在ACoH-水混合物中的相变温度。在电化学电池中，双层充电的电阻(即极化电阻Rp)与电解液中自由离子的浓度和迁移率有关，这两者都受到存在的相的影响。当木质素-ACoH-水混合物通过相变加热时，Rp是一个强烈的温度函数，Rp的最大值对应于目测得到的相变温度。当体系受热时，醋酸盐离子与固体木质素结合，形成液化的富含木质素的相。这种缔合增加了系统的整体阻抗，因为流动的醋酸根离子从溶剂相中剥离，因此不能再吸附在极化电极表面。一旦新的富含木质素的相完全形成，则在Rp中出现最大值，并且木素聚合物不可能与ACoH进一步结合。除亚常温外，相变温度与溶剂组成密切相关，从70/30ACoH/水时的18℃线性增加到10/90wt%ACoH/水时的97℃。
Lignin is unique among renewable biopolymers in having significant aromatic character, making it potentially attractive for a wide range of uses from coatings to carbon fibers. Recent research has shown that hot acetic acid (AcOH)–water mixtures can be used to recover “ultraclean” lignins of controlled molecular weight from Kraft lignins. A key feature of this discovery is the existence of a region of liquid–liquid equilibrium (LLE), with one phase being rich in the purified lignin and the other rich in solvent. Although visual methods can be used to determine the temperature at which solid lignin melts in the presence of AcOH–water mixtures to form LLE, the phase transition can be seen only at lower AcOH concentrations due to solvent opacity. Thus, an electrochemical impedance spectroscopy (EIS) technique was developed for measuring the phase-transition temperature of a softwood Kraft lignin in AcOH–water mixtures. In electrochemical cells, the resistance to double-layer charging (i.e., polarization resistance Rp) is related to the concentration and mobility of free ions in the electrolyte, both of which are affected by the phases present. When the lignin–AcOH–water mixture was heated through the phase transition, RP was found to be a strong function of temperature, with the maximum in RP corresponding to the transition temperature obtained from visual observation. As the system is heated, acetate ions associate with the solid lignin, forming a liquefied, lignin-rich phase. This association increases the overall impedance of the system, as mobile acetate ions are stripped from the solvent phase and thus are no longer available to adsorb on the polarizing electrode surfaces. The maximum in RP occurs once the new lignin-rich phase has completely formed, and no further association of the lignin polymer with AcOH is possible. Except at sub-ambient temperatures, the phase-transition temperature was a strong function of solvent composition, increasing linearly from 18 °C at 70/30 AcOH/water to 97 °C at 10/90 wt% AcOH/water.
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