萃取分离体系分子间弱相互作用的研究进展

黄焜，李晓佩，徐怡庄，刘会洲

(1中国科学院过程工程研究所，北京 100190；2北京大学化学与分子工程学院，北京 100871)



萃取分离体系分子间弱相互作用的研究进展

黄焜1，李晓佩1，徐怡庄2，刘会洲1

(1中国科学院过程工程研究所，北京 100190；2北京大学化学与分子工程学院，北京 100871)

摘要：萃取反应和传质大多发生在溶液相界面，而萃取体系的新相生成和相分离行为取决于溶液相内分子微观聚集结构的变化。研究萃取分离过程溶液相内及相界面发生的各种分子间弱相互作用及其随萃取反应条件的变化、加和与协同效应，是国际上萃取分离化学化工的前沿热点，对于深入认识萃取分离过程微观机理、调控分离选择性具有重要意义。本文从萃取分离体系的界面分子间相互作用出发，总结评述了国内外近年来利用各种实验手段表征液液萃取分离体系分子间弱相互作用的一些代表性工作和最新研究进展。

关键词：萃取；分离；界面；分子间相互作用；研究方法

2015-06-26收到初稿，2015-12-21收到修改稿。

联系人：黄焜，刘会洲。第一作者：黄焜（1972—），男，博士，研究员。

Received date: 2015-06-21.

引 言

液液萃取分离过程中，萃取剂分子与目标萃取物在溶液相内及相界面发生各种分子间弱相互作用，如离子-偶极、偶极-偶极静电相互作用、疏水相互作用、氢键、范德华力、π-π相互作用等。分离过程的效率和结果实质上往往取决于这些分子间弱相互作用的形式和变化。由于分子间弱相互作用的存在，受分子大小、极性、极化率、官能团取向、接受或给予电子能力差异的影响，不同目标物的萃取分离选择性和分离速率具有明显的差异。例如，溶剂萃取金属离子过程，有机萃取剂分子与不同金属离子的界面选择性识别配位或缔合行为与离子-偶极、偶极-偶极相互作用有关；胶团、反胶团萃取蛋白质过程，不同蛋白质分子的选择性溶解、增溶位点与超分子体系的氢键、疏水相互作用和静电相互作用等有关；而氢键对萃淋树脂、反相高效液相色谱分离的洗脱曲线、洗脱速率起着关键作用；在石油化工领域，烯烃和炔烃通过与亚铜盐、银盐或铂盐形成稳定的电荷转移配合物，从而实现二者的分离。另外，萃取分离体系通常伴随着相分离行为。外观均一的溶液体系往往形成许多微观的非均相聚集体和组装结构。这些微观聚集体的表面力和溶液结构变化与体系中的各种分子间弱相互作用有关，促使表观均一的溶液体系发生相分离行为，从而影响目标萃取物的相分配行为。因此，研究萃取分离体系中各种分子间弱相互作用及其随萃取反应条件的变化、加和与协同效应，对于深入认识萃取分离过程微观机理、调控分离选择性具有重要意义。

近年来，随着一些新兴萃取分离方法和分离体系的快速发展，关于萃取分离体系中的各种分子间弱相互作用研究越来越活跃，呈现出多层次、多角度的发展趋势。各种谱学分析方法、计算机分子模拟、量子化学计算方法以及热力学方法的发展为深入认识分子间弱相互作用对分离过程的影响机制提供了可能[1-9]。然而，目前对萃取分离体系分子间相互作用的实验研究还相对薄弱，特别是现有实验表征手段只能探测分子间相互作用力比较强同时分子间相互作用力类型比较简单的分离体系，对于分子间相互作用比较弱或比较复杂的分离体系，对其进行分子层次的探索仍然非常困难。本文综述了近年来关于各种实验手段表征液液萃取分离体系分子间弱相互作用的一些代表性工作，特别是关于液液萃取分离体系的界面分子间相互作用、新相生成和相分离行为以及二维相关光谱表征等方面的研究进展，以期为今后的研究工作提供参考。

1 萃取体系的界面传质与分子间相互作用

通常地，传统意义上的油水两相有机溶剂萃取大多是发生在两个互不相溶的液相界面上的反应和传质。与界面分子/离子的结构、构象、取向、聚集行为相关的各种分子间弱相互作用的方向性和选择性决定了界面萃取传质的效率和反应历程。澳大利亚墨尔本大学的Stevens等[10]曾详细评述了各种表征液/液分离体系界面分子间弱相互作用的光谱实验手段。他们指出，虽然光谱方法探测界面分子信息具有独特优势，但是，由于背景噪声和体相分子的干扰，界面光谱信号较弱，很难给出界面分子相互作用如何影响分离过程的直接实验证据。

常见的研究界面分子相互作用的方法大多采用Langmuir膜。先在气液界面制备Langmuir膜，然后把Langmuir膜转移到特殊材质的基底上，用振动光谱测量基底上的Langmuir膜，以此来了解界面分子的信息[11-21]。然而，这种方法不能原位地反映界面分子的相互作用，得到的界面信息也大打折扣。另外，由于Langmuir膜与基底存在相互作用，可能还会对检测结果造成干扰。为解决此问题，原位界面光谱测量技术逐渐引起了人们的重视。然而，早先的原位界面光谱测量仅限于有紫外-可见吸收或有荧光的物质[22]，这使得原位界面光谱测量只能应用在极少数情况下。有限的界面测量方法使得大多数研究者转向用分子模拟的方法去探究界面分子行为[23-30]。

近年来，关于原位界面光谱的研究发展迅速。代表性的工作主要有两大类：红外反射吸收光谱（IRRAS）和界面和频振动光谱（SFG）。红外反射吸收光谱（IRRAS）最早由Dluhy等[22, 31-38]提出。一般地，IRRAS测量可以分别得到偏振IRRAS和非偏振IRRAS两种信号。偏振IRRAS分辨率高，可以反映界面分子的结构和取向；相对而言，非偏振IRRAS分辨率低，不能反映这些信息。然而，偏振IRRAS谱图的信噪比较差[39]，在实际测量中，一般通过增加扫描次数来提高谱图的信噪比，然而即使扫描次数增加到1024次，谱图质量依然不高。为此，Dluhy等[39-41]把二维相关谱引入到非偏振IRRAS谱图的分析中。通过二维相关分析，非偏振IRRAS谱图也可以用来反映界面分子的结构和取向的信息，并且比一维偏振IRRAS更加灵敏。Dluhy 等[39-40]在研究蒸馏水表面的二棕榈酰磷脂酰胆碱（DPPC）单分子膜随膜压变化的相转化过程中发现：在低膜压时，偏振IRRAS结果表明，DPPC的亚甲基反对称伸缩振动峰（vasCH2）包含两个谱峰，分别对应DPPC分子的无序构象和有序构象。而随着膜压的增加，vasCH2的谱峰仅呈现一个单峰。他们认为，在高膜压下，DPPC分子构象更加有序。通过非偏振IRRAS得不到上述信息。然而二维相关非偏振IRRAS谱图表明：在低膜压下，vasCH2的谱峰可用两个高度重叠的谱峰描述，表明在低膜压时DPPC分子的无序构象和有序构象共存；在高膜压时，vasCH2的谱峰为一个单峰，并且随着膜压的继续增加，峰位发生微弱的移动，表明有序构象DPPC分子的堆积。此外，Dluhy等还结合偏振IRRAS、βν相关方法和kν相关方法确定了四环素萃取过程中，四环素和磷脂单分子层的作用位点。研究结果表明，在低膜压下，四环素和磷脂分子的作用位点为四环素的环A和磷脂分子的头基；在高膜压下，四环素和磷脂分子的作用位点变为四环素的环B和磷脂分子的头基。同时，Dluhy等结合偏振IRRAS和βν相关方法对二组分磷脂单分子层的甲基和亚甲基随膜压变化的相对速率以及嵌入磷脂单分子层中的蛋白质分子随膜压变化的构象转变进行了研究。

界面和频光谱实际上是一种二阶非线性光谱，包括和频振动光谱（SFG）和二次谐波（SHG）两种。由于其光谱信号的界面选律，表征界面分子结构信息的变化具有优异的选择性和灵敏性。通过对和频偏振信号进行拟合，可以对界面分子的取向、构象堆积、界面环境、分子间相互作用等信息进行剖析。Diat、Martin-Gassin等[42-43]曾采用界面和频光谱（SHG/SFG）研究了水/十二烷界面酰胺萃取硝酸过程，证实了分子动态模拟的推论。他们认为，水-油两相界面萃取剂分子的聚集、取向行为导致其在界面运动受限，从而对萃取剂分子的界面反应活性造成影响。平躺在液-液界面的酰胺分子与在界面有一定取向聚集排列的酰胺分子对硝酸分子萃取反应速率的影响是不同的。他们的研究发现，SHG和频振动谱峰的偏振信号与酰胺分子的界面取向排列、自组装聚集行为相关，其峰位的相对变化及峰强的时间相关起伏波动对应硝酸分子萃取速率的实时变化。他们的研究[44-45]还关注到金属离子的萃取过程也存在类似现象。2011年在智利举办的第19届国际溶剂萃取会议上，以Diat为代表的这方面最新研究进展[46]引起了与会国际同行广泛关注。2014年，在德国维尔茨堡举办的第20届国际溶剂萃取会议上，Diat等[47]又进一步提出萃取剂分子界面聚集、取向行为影响金属离子界面传质动力学及分离选择性的机理模型。除此之外，Teramae等[48]发现碱金属阳离子的萃取选择性顺序与萃取剂分子界面取向络合导致的和频光谱偏振信号选择性增强有关。Frey等[49]采用SHG手段直接证明了TBP萃取速率受制于界面分子聚集、取向导致的分子间相互作用变化。王鸿飞、郭源等[50-53]则采用SFG详细研究了液-液界面水分子取向、聚集及其对各种无机离子界面吸附选择性的影响，提出了界面分子-离子相互作用变化的时间相关函数。笔者等[54]采用红外反射光谱结合和频光谱研究了三辛基氧膦（TOPO）在油水两相界面与水溶液中的稀土离子的界面相互作用。实验结果表明，水溶液中与稀土离子共存的盐离子种类和盐浓度对TOPO-稀土离子的界面相互作用影响较大。不同的无机盐阴阳离子在油水两相界面的正/负竞争吸附导致稀土离子的界面萃取行为发生变化。在含高浓度Mg2+的水溶液中，TOPO萃取La3+的萃取率大大下降，而在含高浓度SCN-的水溶液中，TOPO萃取La3+的萃取率明显增加。这是因为，离子半径小、带电量高的Mg2+会促使界面TOPO分子发生去水化效应，TOPO分子碳链在界面有序堆积，分子间的相互作用增强，不利于TOPO与水溶液中的La3+接触；而离子极化率高、体积大的SCN-有利于界面TOPO分子的水化，可明显增强TOPO与水溶液中的La3+接触。该研究从分子水平解释了水溶液中与目标离子共存的其他阴阳离子在界面的竞争吸附行为对有机膦萃取剂萃取效率的影响，有助于理解萃取过程的界面微观机理。

2 萃取体系的相分离行为与介质无机盐离子作用机制

液液萃取过程通常伴随互不相溶两液相的相分离行为，分散相在连续相内不断聚并，最终形成两层液相共存体系。目标被萃物在两层液相的分配比以及其分配速率与萃取体系的成相行为密切相关。近年来，随着一些新兴萃取体系和萃取方法的快速发展，如微乳相萃取、三液相萃取、双水相萃取、离子液体萃取、凝胶萃取等，人们越来越关注这些萃取体系的相分离行为或新相生成对目标物的相分配行为影响。研究萃取分离体系分子间各种弱相互作用，如氢键、范德华力、疏水相互作用、C—H…O弱相互作用等对萃取体系成相行为的影响已成为萃取分离化学的前沿热点。人们的研究发现，这些萃取体系的萃取分离性能大多与溶液相内分子有序组装、聚集行为等溶液结构的变化密切相关，受介质盐离子种类及浓度的影响，萃取体系伴随新相的生成以及相分离行为，目标物在萃取体系的相分配行为也将发生明显变化[55-58]。因此，研究盐离子在这些萃取分离体系中的作用机理是深入认识萃取体系相分离行为微观机制的必要前提。但是，在实际分离体系中，由于可能存在萃取剂分子与目标萃取物、介质无机盐阴离子与阳离子、水分子与目标萃取物、水分子与水分子之间的众多复杂相互作用，至今尚不清楚溶液介质中的无机盐离子如何影响萃取体系的相分离行为以及其对目标被萃物相传质行为的作用机理，也无法给出直接的实验证据，从微观分子层次深入认识萃取分离的历程和机理。

大量的研究表明，无机盐离子行为遵循Hofmeister离子序列[59-63]，即存在SO42->OH->F-> Cl->Br->NO3->I->SCN->ClO4-的阴离子顺序和Ba2+>Ca2+>Mg2+>Rb+>Li+>Cs+>Na+>K+>NH4+的阳离子顺序。但是，长期以来，关于Hofmeister离子如何调控水溶液中的溶质分子聚集组装行为以及如何影响萃取分离体系相分离行为的作用机制一直未达成共识[64-75]，其中一个主要的分歧是：无机盐离子是否与溶质分子发生直接的相互作用。一些研究者持“间接作用机理”观点，将Hofmeister离子分为亲水离子（kosmotropic离子）和疏水离子（chaotropic离子）两组，认为Hofmeister离子通过与水的相互作用，可间接影响溶质分子在水溶液中的溶解和聚集分相行为；而另一些研究者则持“直接作用机理”观点，认为Hofmeister离子与溶质分子存在直接的相互作用。Florin等[76]认为，相对于水溶性聚合物分子水化层内的水分子，位于kosmotropic离子周围的水分子具有更低的能量。因此，当水溶性聚合物分子和kosmotropic离子相互靠近时，kosmotropic离子会“夺取”聚合物分子水化层中的水分子，使得聚合物分子在水中的溶解度降低，即产生“盐析效应”，进而原本呈均一相的聚合物水溶液会发生相分离行为，形成两层互不相溶的液相。与kosmotropic离子相反，chaotropic离子周围的水分子具有较高的能量，chaotropic离子可促进聚合物分子的水化，增加聚合物分子在水中的溶解度，即表现为“盐溶效应”，这将抑制聚合物水溶液的相分离行为。Collins等[77]认为，无机盐离子与水溶液中的聚合物溶质分子的界面水化层相互作用，可间接调控聚合物分子在水溶液中的溶解和聚集组装行为。当存在kosmotropic离子时，聚合物分子的界面水化层会被kosmotropic离子破坏，减小聚合物分子界面的水化效应，出现盐析现象。而chaotropic离子因极化率高和体积大等不易与水结合，因此，chaotropic离子的加入会促进聚合物溶质分子的界面水化相互作用，出现盐溶现象。Schott等[78-90]详细研究了无机盐对聚乙二醇等非离子型表面活性剂水溶液的浊点、临界胶团浓度和胶团稳定性的影响。Nucci等[91]基于双态氢键模型，对不同温度下30种无机盐水溶液的羟基伸缩振动峰的分析结果得出：无机盐离子的加入改变了溶质分子周围的水氢键网络结构，其对氢键结构的影响遵循Hofmeister离子序列。但是，随后的研究又对上述研究者的观点提出了质疑。Bakker等[92-93]基于飞秒二色泵浦探测光谱研究了水分子的取向迁移时间相关行为。结果指出，水的氢键结构并没有因为无机盐离子的加入而发生改变。Pielak等[94]提出，如果间接作用机理合理，那么本体水应该存在两种结构体：其一为水分子堆积稀疏，排列有序；其二为水分子堆积密积，排列无序。chaotropic离子会破坏本体水的氢键结构，而kosmotropic离子有利于本体水氢键结构的增强。然而，实验结果表明(∂c-p/∂p)T的符号与离子种类没有直接关系。Thormann等[67]认为如果间接作用机理合理，kosmotropic离子引起水分子氢键结构变化时所对应的熵变(ΔSstr)应该呈负值，而chaotropic离子对应的ΔSstr为正值。然而，计算结果表明，ΔSstr的正负与离子种类也没有直接关系。

Gurau等[95]采用和频光谱研究了溶液中不同阴离子对溶液表面十八胺单分子膜的聚集取向行为。基于十八胺分子的甲基对称伸缩振动(vs(CH3))与亚甲基对称伸缩振动(vs(CH2))比值的变化与Hofmeister离子序列顺序相同这一实验现象，作者认为Hofmeister离子序列可能与无机盐阴离子-十八胺分子之间存在直接相互作用。Chen等[96]采用SFG光谱研究了水溶液中的聚（N-异丙基丙烯酰胺）（PNIPAM）分子与不同阴离子的相互作用。结果表明，在含有不同无机阴离子的PNIPAM水溶液表面，位于3200 cm-1处水分子的伸缩振动峰随着Hofmeister离子序列呈现出规律性变化。他们认为，尽管PNIPAM本身不带电，但是阴离子，尤其是体积大、极化率高、易去水化的chaotropic离子，可与PNIPAM分子发生直接相互作用，吸附在PNIPAM溶液表面，使得PNIPAM溶液表面带电。在静电场下，表面层的水分子排列更加有序[97]，使得3200 cm-1处的和频信号增加。此外，由于PNIPAM本身不带电，吸附在PNIPAM分子上的阴离子并不会改变PNIPAM在溶液表面的取向和浓度，因此3200 cm-1处的和频信号随Hofmeister离子序列呈规律性变化。与PNIPAM不同，十八胺分子会与溶液中的H+结合而带电，因此阴离子与十八胺的相互作用还涉及静电作用。在静电作用的影响下，十八胺在溶液表面的排布密度、取向以及电荷数量等都会发生改变，这些因素都会对表面水分子的取向和堆积结构产生影响。因此，在十八胺体系中，3200 cm-1处和频信号的变化并不符合Hofmeister序列。Zhang等研究了PEO-PPO共聚物[98]、PNIPAM[99-100]以及类弹性蛋白多肽V5-120和V5A2G3-12[101]溶液的相行为及其受无机阴离子的影响。结果表明，阴离子对含有这几种溶质分子的水溶液相行为的调控都遵循Hofmeister序列。Gibb 等[102]研究了kosmotropic和chaotropic阴离子与溶液中两亲分子的相互作用。结合核磁波谱和等温量热扫描的实验结果，他们认为chaotropic阴离子的去水化效应使得其能与溶液中两亲分子官能基团发生直接的作用；而kosmotropic阴离子因亲水性强，不易去水化，与两亲分子不存在直接相互作用。Finney等[103]基于中子散射实验结果，提出叔丁醇水溶液的盐析分相可能是通过“阴离子桥”实现的。当没有盐存在时，叔丁醇分子的极性基团暴露在水溶液中，而非极性基团发生相互作用抱团在一起，这时叔丁醇能与水以任意比例互溶。而当叔丁醇水溶液中加入无机盐后，无机盐离子通过与叔丁醇分子的极性基团发生直接相互作用，形成“盐桥效应”，把叔丁醇分子连接起来，使得叔丁醇分子的非极性基团暴露出来，从而产生盐析效应。

无机盐的阴离子对萃取分离体系的相分离行为影响较大。然而，阳离子的作用也不能忽略。Tasaki[104]对水溶液中钾离子和聚乙二醇间的相互作用进行了分子动力学模拟。研究结果表明，在水溶液中，聚乙二醇分子链呈螺旋状，位于螺旋链内侧的钾离子与聚乙二醇存在直接相互作用。Florin[105]测定了聚乙二醇对81Br、23Na、7Li、133Cs 和35Cl 5种核的弛豫时间影响。结果表明，这5种核的弛豫时间随着聚乙二醇浓度的增加都不断减小。作者认为，聚乙二醇造成的阴离子不对称性水化是阴离子弛豫时间减小的主要原因。而阳离子的弛豫时间减小是因为聚乙二醇和阳离子发生了直接相互作用。笔者等[106]采用二阶导红外光谱和Raman光谱研究了聚合物双水相萃取体系中的聚合物分子与无机盐阴离子、阳离子的相互作用。结果表明，在30% EOPO+Na2CO3和30% EOPO+K2CO3体系（均为质量分数）中，随着体系中盐浓度的增加，EOPO分子的EO链段v(C－O)红外谱峰先红移然后蓝移。在30% EOPO+KSCN和30% EOPO+NaSCN体系中，随着体系中盐浓度的增加，EOPO分子的PEO链段v(C－O)红外谱峰只发生红移。Raman曼光谱表征进一步指出，随着体系中碳酸钾浓度的增加，EOPO分子的PEO链段v(CH2)先蓝移后红移，而v(C－O)谱峰先红移后蓝移。在硫氰酸钠溶液中加入EOPO后，钠核的弛豫时间大幅度减弱。这些实验结果均证实了钾离子、钠离子与EOPO之间确实存在直接相互作用。该研究为认识阳离子与水溶性聚合物分子间的相互作用提供了分子水平的证据。

3 二维相关光谱探测萃取分离体系的分子间弱相互作用

利用二维相关谱分析技术研究分子间弱相互作用最先起源于核磁领域。相比一维谱图，由于增加了一个维度，提高了谱图的分辨率，在一维核磁谱图上无法分辨的重叠峰或者被掩盖的小峰，在二维谱上可以通过交叉峰的信息进行有效分辨。另外，二维核磁谱图中的交叉峰还能提供自旋核之间相互作用的信息。因此，二维核磁技术在解析复杂化合物的结构（如蛋白质三维空间结构的解析）、研究分子内和分子间相互作用等方面占据非常重要的地位[107-116]。Noda等[117-122]首先将二维核磁相关谱分析方法引入红外、Raman光谱，通过对样品施加一定外部扰动，如电、热、磁、声、化学或机械力等，分子的电偶极跃迁会发生相应变化。这种基于外部扰动的宏观弛豫速率较振动弛豫速率慢得多，借助于传统的红外、Raman等光谱分析手段就可以对宏观弛豫过程进行检测。在扰动过程中，通过测量多张一维谱图，然后对这些谱图进行二维相关分析即可得到二维相关谱。二维相关光谱的提出，实现了红外、Raman等光谱技术的二维化，在探测体系分子间弱相互作用领域的应用越来越广泛[123-128]。

应用二维相关光谱的交叉峰，可以探测萃取分离体系由于分子取向、作用位点、分子构象、聚集行为等变化带来的离子-偶极、偶极-偶极、氢键等分子间弱相互作用的变化，为从分子水平认识分离过程微观机制、优化分离选择性、调控分离行为提供了可能。近年来，这方面的研究在诸如分子印迹分离技术、超分子自组装分离技术等新兴分离方法的发展过程中起到了至关重要的作用。根据二维相关谱的性质，二维相关谱可分为同步相关谱和异步相关谱。其中，异步相关谱在辨别萃取分离体系分子间弱相互作用方面具有更大的优势。然而，迄今为止，二维相关光谱在探测更为复杂的分离体系分子间弱相互作用、研究萃取分离过程界面分子间相互作用方面仍然存在一定问题[129-140]。

Thomas等[141]基于Noda等提出的广义二维相关谱，发展了移动窗口二维相关谱分析技术。在移动窗口二维相关谱中，一个维度为光谱变量坐标，另一个维度为扰动变量坐标，利用移动窗口二维相关谱，可以观察分离体系分子间相互作用导致的交叉峰随外部扰动变化的实时信息[142]。在此基础上，Ozaki等[143-146]通过数学变换转置动态光谱矩阵以及动态光谱矩阵在二维相关谱计算公式中的位置，得到了一种新的二维相关谱分析方法，称为样品-样品相关。为了便于区分，他们把Noda提出的二维相关分析称为波长-波长相关，而他们提出的样品-样品相关可以更好地反映样品特征的变化（如浓度扰动带来的分子间相互作用变化），与波长-波长相关互为补充。此外，Ozaki等[147]还提出了混二维相关谱。混二维相关谱即对两种不同扰动下的动态光谱进行相关分析得到二维相关谱。作者对3种扰动情况进行了考虑：（1）两扰动相互独立，互不相干；（2）两扰动有一定的关联性；（3）两扰动相同，但是扰动施加的体系不同。利用混二维相关谱，可以直接对两体系或不同扰动下的两组光谱的关联性进行研究。根据Thomas等提出的移动窗口二维相关谱的概念，Ozaki等[148]又提出扰动相关移动窗口二维相关谱，即直接对扰动变量和光谱变量进行二维相关分析，与移动窗口二维相关谱相同，该方法也可以显示出体系随扰动变化的信息，但其计算过程比移动窗口二维相关谱更为简单。

二维相关分析其实是一种光谱数据的数学分析方法。从这个意义上来讲，二维相关分析与主成分分析相类似，属于一种化学计量分析方法。起先，研究者分别基于二维相关分析和主成分分析对光谱数据进行解释。在这些研究中，二维相关分析和主成分分析的地位是平行的[146,149-152]。之后Jung等[153-156]把二维相关分析与主成分分析结合起来，利用主成分分析或奇异值分解对一维光谱数据进行了降噪处理，然后对降噪的光谱数据进行二维相关分析，得到了高信噪比的二维相关谱。

尽管二维相关谱具有更高的谱图分辨率，然而对于复杂的体系，若其中多个组分的一维谱峰在二维相关谱中产生多个交叉峰，且这些交叉峰彼此之间相互交叠、干扰，仍然很难对二维相关谱进行有效分析。针对此问题，2010年，Noda[157-160]提出了投影二维相关分析法。该方法可以大大简化二维相关谱分析。此外，Noda等[161]还提出从光谱数据的奇异值分解对光谱数据进行降噪处理，用于提取异步相关谱的内核矩阵，其矩阵容量比常规异步相关谱矩阵小得多，但更方便分析。

基于Noda等的工作，Dluhy等[140]提出“model-dependent”二维相关分析。“modeldependent”二维相关分析与传统二维相关分析（亦称为“model-free”二维相关谱）的区别在于：传统二维相关分析为动态光谱之间的相关，而“model-dependent”二维相关分析建立的是动态光谱与一个数学函数之间的相关性。相对于“model-free”二维相关谱，“model-dependent”二维相关谱更具有“开放性”，因为在“model-dependent”二维相关谱中，数学函数的选择并没有具体的规定，函数可以为任意形式。因此，通过改变函数，可以创造不同类的“modeldependent”二维相关谱。例如，Dluhy等[132-134, 137-138]最先提出的bn相关分析和kn相关分析法，已用于分析抗生素萃取分离过程的机理，对抗生素与磷脂单分子层的作用位点、磷脂单分子层的甲基和亚甲基随膜压变化的相对速率、以及嵌入在磷脂单分子层中的蛋白质随膜压变化的构象转变等进行了研究。

二维相关谱的实际应用过程中，可能会存在干扰峰，即二维相关谱交叉峰的出现可能与分子间相互作用无关，而是源于其他的相关性。这些干扰峰的存在，使得二维相关谱仍然无法成为反映分子间相互作用的可靠工具。针对此问题，徐怡庄等[162-163]以浓度作为外部扰动，通过选择合适的相互作用两物质的初始浓度序列，提出了正交浓度设计方法（OSD），该方法成功消除了同步相关谱中的干扰峰。之后，他们进一步发展了OSD方法，提出了双正交浓度设计方法（DOSD）[164]、异步正交浓度设计方法（AOSD）[165]、双异步正交浓度设计方法（DAOSD）等系列方法[166]，不仅消除了同步相关谱中的干扰峰，异步相关谱中的干扰峰也可被去除。基于上述方法，他们最终将二维相关谱中的交叉峰与体系分子间相互作用建立起了对应关系，使其成为反映分子间相互作用的可靠工具。

另外，实际分离体系中，由于噪声的干扰，或当相互作用的两物质谱峰相互重叠，或仅一个物质有特征吸收峰时，异步相关谱也可能会给出错误的结果，这使得异步相关谱在一些特定体系的应用过程中仍然无法准确反映分子间弱相互作用。例如，协萃体系中不同萃取剂分子间的偶极-偶极相互作用，即使使用二阶导数光谱，也不能得到相关分子间弱相互作用的信息。而采用通常的异步相关谱或其改进（如AOSD谱），由于噪声的影响，AOSD谱中会出现多余的交叉峰或者本该出现的交叉峰被噪声淹没，极易造成谱图的错误解读，得出与事实不相符的结论[132, 167-168]。针对此问题，笔者等提出，通过对一维动态光谱的谱图进行排序，增加异步相关谱的信噪比[169]，并在此基础上发展了一种基于改良参比光谱的异步相关谱（称为ASMR谱）[170]，以期提高谱图的信噪比。相比文献报道的以平均光谱为参比光谱的方法[171-172]，当以浓度变化作为外部扰动时，采用异步相关谱检测分子间弱相互作用时，参比光谱的选择并不是任意的，必须确保在新参比下异步相关谱中的干扰峰可以被去除掉。ASMR谱可以写成额外异步相关谱和AOSD谱的加和，改变相互作用两物质的浓度序列排序时，ASMR谱峰的绝对值强度可以随着浓度序列的排序变化而变化。模拟实验和实际体系实验证实了额外异步相关谱的强度远大于AOSD谱的强度，因此ASMR谱的强度远大于AOSD谱，同时ASMR谱的信噪比也远远高于AOSD谱。将ASMR谱用于分析协萃体系不同萃取剂分子间的相互促进或抑制作用、生物大分子（如溶菌酶）萃取过程与聚氧乙烯-聚氧丙烯共聚物EOPO分子间的相互作用以及稀土萃取过程稀土离子与萃取剂酯基间的偶极-静电相互作用。结果表明，在一维谱图和二阶导谱图上无法观测到的相互作用信息，在ASMR谱上有明显的体现。根据ASMR谱交叉峰的峰型，可以判断由于分子间相互作用导致的特征峰变化，这为从谱峰的微细变化认识分子间弱相互作用对萃取分离微观过程的影响创造了有利条件。另外，针对萃取分离体系中相互作用的两物质仅一种物质含有特征吸收峰的情况，由于ASMR谱无法反映分子间相互作用导致的峰强变化，可能对体系分子间是否存在相互作用的判定得出错误的结论。笔者[173]发展了一种带有辅助交叉峰的ASMR谱（称为ASAP方法）。通过在研究体系中引入一种虚拟物质构造异步相关谱的辅助交叉峰，成功解决了上述问题，并以苯甲酰胺萃取Tb3+体系的偶极-静电相互作用为研究对象，验证了ASAP方法的可靠性。结果表明，辅助交叉峰确实可以反映苯甲酰胺萃取Tb3+体系分子间相互作用导致的特征峰峰宽、峰位以及峰强的变化，而常规的红外光谱除了苯甲酰胺浓度变化引起的谱图变化外，观测不到任何与分子间相互作用相关的谱图变化信息。此外，当相互作用的两物质的特征峰发生严重重叠时，ASAP方法还可用于辨别复杂的交叉峰。辅助交叉峰的引入有助于分辨分子间相互作用导致的不同物质特征峰的差异性变化，较ASMR谱和常规的AOSD谱具有明显优势，为正确判定分离体系由于分子结构、构象变化引起的分离行为变化提供了可靠依据。

4 结 语

研究和表征萃取分离体系中的各种分子间弱相互作用，对于深入认识萃取分离过程微观机理、调控萃取分离选择性具有重要意义。近年来，关于萃取分离体系分子间弱相互作用的分子模拟、量化计算和热力学分析报道较多，但从实验表征出发，特别是利用各种谱学手段探测分子间弱相互作用的研究还相对薄弱。

由于实际萃取分离体系的复杂性，目前采用各种谱学实验手段研究和表征萃取反应、传质过程发生在溶液相界面的分子间相互作用、研究与萃取体系新相生成和相分离行为相关的溶液相内分子微观聚集结构变化等刚刚起步，二维相关谱技术表征萃取体系分子间弱相互作用时，针对的研究对象也比较简单。已报道的这些实验工作进一步与分子模拟、量化计算、热力学分析等手段相结合是将来的发展趋势。

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Foundation item: supported by the National Basic Research Program of China(2012CBA01203, 2013CB632602) and the National Natural Science Foundation of China (51574213, 51074150).

Research progress on intermolecular weak interaction in extraction and separation system

HUANG Kun1, LI Xiaopei1, XU Yizhuang2, LIU Huizhou1
(1Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China;2College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China)

Abstract:The extraction reaction and separation in most cases occur on liquid-liquid interface, and generally, the formation of a new phase and phase-separation kinetics in extraction systems depend on step-by-step evolution of molecular aggregates in solution microstructure. Therefore, the research on various intermolecular interaction appeared in solutions or on the interface of contacting liquid phases during extraction and separation process, and exploration about the influence from the change, interrelationship and synergistic effect of those intermolecular interactions are becoming one of the research focuses and frontier around the world in current separation sciences and chemical engineering. It is very important towards understanding the microscopic mechanism in the separation process, and also how to control the separation selectivity. In present paper, recent research progress and some typical works about the intermolecular interaction on the interfaces of liquid phases, and various experimental techniques and methods employed to describe the intermolecular weak interaction in extraction and separation systems are reviewed.

Key words:extraction; separation; interface; intermolecular interaction; research method

Corresponding author:HUANG Kun, khuang@ipe.ac.cn; LIU Huizhou, hzliu@ipe.ac.cn

基金项目：国家重点基础研究发展计划项目（2012CBA01203，2013CB632602）；国家自然科学基金项目（51574213，51074150）。

中图分类号：TQ 028

文献标志码：A

文章编号：0438—1157（2016）01—0152—13

DOI:10.11949/j.issn.0438-1157.20150999