萃取分離體系分子間弱相互作用的研究進(jìn)展

黃焜，李曉佩，徐怡莊，劉會洲

(1中國科學(xué)院過程工程研究所，北京 100190；2北京大學(xué)化學(xué)與分子工程學(xué)院，北京 100871)

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萃取分離體系分子間弱相互作用的研究進(jìn)展

黃焜1，李曉佩1，徐怡莊2，劉會洲1

(1中國科學(xué)院過程工程研究所，北京 100190；2北京大學(xué)化學(xué)與分子工程學(xué)院，北京 100871)

摘要：萃取反應(yīng)和傳質(zhì)大多發(fā)生在溶液相界面，而萃取體系的新相生成和相分離行為取決于溶液相內(nèi)分子微觀聚集結(jié)構(gòu)的變化。研究萃取分離過程溶液相內(nèi)及相界面發(fā)生的各種分子間弱相互作用及其隨萃取反應(yīng)條件的變化、加和與協(xié)同效應(yīng)，是國際上萃取分離化學(xué)化工的前沿?zé)狳c，對于深入認(rèn)識萃取分離過程微觀機理、調(diào)控分離選擇性具有重要意義。本文從萃取分離體系的界面分子間相互作用出發(fā)，總結(jié)評述了國內(nèi)外近年來利用各種實驗手段表征液液萃取分離體系分子間弱相互作用的一些代表性工作和最新研究進(jìn)展。

關(guān)鍵詞：萃??；分離；界面；分子間相互作用；研究方法

2015-06-26收到初稿，2015-12-21收到修改稿。

聯(lián)系人：黃焜，劉會洲。第一作者：黃焜（1972—），男，博士，研究員。

Received date: 2015-06-21.

引 言

液液萃取分離過程中，萃取劑分子與目標(biāo)萃取物在溶液相內(nèi)及相界面發(fā)生各種分子間弱相互作用，如離子-偶極、偶極-偶極靜電相互作用、疏水相互作用、氫鍵、范德華力、π-π相互作用等。分離過程的效率和結(jié)果實質(zhì)上往往取決于這些分子間弱相互作用的形式和變化。由于分子間弱相互作用的存在，受分子大小、極性、極化率、官能團(tuán)取向、接受或給予電子能力差異的影響，不同目標(biāo)物的萃取分離選擇性和分離速率具有明顯的差異。例如，溶劑萃取金屬離子過程，有機萃取劑分子與不同金屬離子的界面選擇性識別配位或締合行為與離子-偶極、偶極-偶極相互作用有關(guān)；膠團(tuán)、反膠團(tuán)萃取蛋白質(zhì)過程，不同蛋白質(zhì)分子的選擇性溶解、增溶位點與超分子體系的氫鍵、疏水相互作用和靜電相互作用等有關(guān)；而氫鍵對萃淋樹脂、反相高效液相色譜分離的洗脫曲線、洗脫速率起著關(guān)鍵作用；在石油化工領(lǐng)域，烯烴和炔烴通過與亞銅鹽、銀鹽或鉑鹽形成穩(wěn)定的電荷轉(zhuǎn)移配合物，從而實現(xiàn)二者的分離。另外，萃取分離體系通常伴隨著相分離行為。外觀均一的溶液體系往往形成許多微觀的非均相聚集體和組裝結(jié)構(gòu)。這些微觀聚集體的表面力和溶液結(jié)構(gòu)變化與體系中的各種分子間弱相互作用有關(guān)，促使表觀均一的溶液體系發(fā)生相分離行為，從而影響目標(biāo)萃取物的相分配行為。因此，研究萃取分離體系中各種分子間弱相互作用及其隨萃取反應(yīng)條件的變化、加和與協(xié)同效應(yīng)，對于深入認(rèn)識萃取分離過程微觀機理、調(diào)控分離選擇性具有重要意義。

近年來，隨著一些新興萃取分離方法和分離體系的快速發(fā)展，關(guān)于萃取分離體系中的各種分子間弱相互作用研究越來越活躍，呈現(xiàn)出多層次、多角度的發(fā)展趨勢。各種譜學(xué)分析方法、計算機分子模擬、量子化學(xué)計算方法以及熱力學(xué)方法的發(fā)展為深入認(rèn)識分子間弱相互作用對分離過程的影響機制提供了可能[1-9]。然而，目前對萃取分離體系分子間相互作用的實驗研究還相對薄弱，特別是現(xiàn)有實驗表征手段只能探測分子間相互作用力比較強同時分子間相互作用力類型比較簡單的分離體系，對于分子間相互作用比較弱或比較復(fù)雜的分離體系，對其進(jìn)行分子層次的探索仍然非常困難。本文綜述了近年來關(guān)于各種實驗手段表征液液萃取分離體系分子間弱相互作用的一些代表性工作，特別是關(guān)于液液萃取分離體系的界面分子間相互作用、新相生成和相分離行為以及二維相關(guān)光譜表征等方面的研究進(jìn)展，以期為今后的研究工作提供參考。

1 萃取體系的界面?zhèn)髻|(zhì)與分子間相互作用

通常地，傳統(tǒng)意義上的油水兩相有機溶劑萃取大多是發(fā)生在兩個互不相溶的液相界面上的反應(yīng)和傳質(zhì)。與界面分子/離子的結(jié)構(gòu)、構(gòu)象、取向、聚集行為相關(guān)的各種分子間弱相互作用的方向性和選擇性決定了界面萃取傳質(zhì)的效率和反應(yīng)歷程。澳大利亞墨爾本大學(xué)的Stevens等[10]曾詳細(xì)評述了各種表征液/液分離體系界面分子間弱相互作用的光譜實驗手段。他們指出，雖然光譜方法探測界面分子信息具有獨特優(yōu)勢，但是，由于背景噪聲和體相分子的干擾，界面光譜信號較弱，很難給出界面分子相互作用如何影響分離過程的直接實驗證據(jù)。

常見的研究界面分子相互作用的方法大多采用Langmuir膜。先在氣液界面制備Langmuir膜，然后把Langmuir膜轉(zhuǎn)移到特殊材質(zhì)的基底上，用振動光譜測量基底上的Langmuir膜，以此來了解界面分子的信息[11-21]。然而，這種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膜與基底存在相互作用，可能還會對檢測結(jié)果造成干擾。為解決此問題，原位界面光譜測量技術(shù)逐漸引起了人們的重視。然而，早先的原位界面光譜測量僅限于有紫外-可見吸收或有熒光的物質(zhì)[22]，這使得原位界面光譜測量只能應(yīng)用在極少數(shù)情況下。有限的界面測量方法使得大多數(shù)研究者轉(zhuǎn)向用分子模擬的方法去探究界面分子行為[23-30]。

近年來，關(guān)于原位界面光譜的研究發(fā)展迅速。代表性的工作主要有兩大類：紅外反射吸收光譜（IRRAS）和界面和頻振動光譜（SFG）。紅外反射吸收光譜（IRRAS）最早由Dluhy等[22, 31-38]提出。一般地，IRRAS測量可以分別得到偏振IRRAS和非偏振IRRAS兩種信號。偏振IRRAS分辨率高，可以反映界面分子的結(jié)構(gòu)和取向；相對而言，非偏振IRRAS分辨率低，不能反映這些信息。然而，偏振IRRAS譜圖的信噪比較差[39]，在實際測量中，一般通過增加掃描次數(shù)來提高譜圖的信噪比，然而即使掃描次數(shù)增加到1024次，譜圖質(zhì)量依然不高。為此，Dluhy等[39-41]把二維相關(guān)譜引入到非偏振IRRAS譜圖的分析中。通過二維相關(guān)分析，非偏振IRRAS譜圖也可以用來反映界面分子的結(jié)構(gòu)和取向的信息，并且比一維偏振IRRAS更加靈敏。Dluhy 等[39-40]在研究蒸餾水表面的二棕櫚酰磷脂酰膽堿（DPPC）單分子膜隨膜壓變化的相轉(zhuǎn)化過程中發(fā)現(xiàn)：在低膜壓時，偏振IRRAS結(jié)果表明，DPPC的亞甲基反對稱伸縮振動峰（vasCH2）包含兩個譜峰，分別對應(yīng)DPPC分子的無序構(gòu)象和有序構(gòu)象。而隨著膜壓的增加，vasCH2的譜峰僅呈現(xiàn)一個單峰。他們認(rèn)為，在高膜壓下，DPPC分子構(gòu)象更加有序。通過非偏振IRRAS得不到上述信息。然而二維相關(guān)非偏振IRRAS譜圖表明：在低膜壓下，vasCH2的譜峰可用兩個高度重疊的譜峰描述，表明在低膜壓時DPPC分子的無序構(gòu)象和有序構(gòu)象共存；在高膜壓時，vasCH2的譜峰為一個單峰，并且隨著膜壓的繼續(xù)增加，峰位發(fā)生微弱的移動，表明有序構(gòu)象DPPC分子的堆積。此外，Dluhy等還結(jié)合偏振IRRAS、βν相關(guān)方法和kν相關(guān)方法確定了四環(huán)素萃取過程中，四環(huán)素和磷脂單分子層的作用位點。研究結(jié)果表明，在低膜壓下，四環(huán)素和磷脂分子的作用位點為四環(huán)素的環(huán)A和磷脂分子的頭基；在高膜壓下，四環(huán)素和磷脂分子的作用位點變?yōu)樗沫h(huán)素的環(huán)B和磷脂分子的頭基。同時，Dluhy等結(jié)合偏振IRRAS和βν相關(guān)方法對二組分磷脂單分子層的甲基和亞甲基隨膜壓變化的相對速率以及嵌入磷脂單分子層中的蛋白質(zhì)分子隨膜壓變化的構(gòu)象轉(zhuǎn)變進(jìn)行了研究。

界面和頻光譜實際上是一種二階非線性光譜，包括和頻振動光譜（SFG）和二次諧波（SHG）兩種。由于其光譜信號的界面選律，表征界面分子結(jié)構(gòu)信息的變化具有優(yōu)異的選擇性和靈敏性。通過對和頻偏振信號進(jìn)行擬合，可以對界面分子的取向、構(gòu)象堆積、界面環(huán)境、分子間相互作用等信息進(jìn)行剖析。Diat、Martin-Gassin等[42-43]曾采用界面和頻光譜（SHG/SFG）研究了水/十二烷界面酰胺萃取硝酸過程，證實了分子動態(tài)模擬的推論。他們認(rèn)為，水-油兩相界面萃取劑分子的聚集、取向行為導(dǎo)致其在界面運動受限，從而對萃取劑分子的界面反應(yīng)活性造成影響。平躺在液-液界面的酰胺分子與在界面有一定取向聚集排列的酰胺分子對硝酸分子萃取反應(yīng)速率的影響是不同的。他們的研究發(fā)現(xiàn)，SHG和頻振動譜峰的偏振信號與酰胺分子的界面取向排列、自組裝聚集行為相關(guān)，其峰位的相對變化及峰強的時間相關(guān)起伏波動對應(yīng)硝酸分子萃取速率的實時變化。他們的研究[44-45]還關(guān)注到金屬離子的萃取過程也存在類似現(xiàn)象。2011年在智利舉辦的第19屆國際溶劑萃取會議上，以Diat為代表的這方面最新研究進(jìn)展[46]引起了與會國際同行廣泛關(guān)注。2014年，在德國維爾茨堡舉辦的第20屆國際溶劑萃取會議上，Diat等[47]又進(jìn)一步提出萃取劑分子界面聚集、取向行為影響金屬離子界面?zhèn)髻|(zhì)動力學(xué)及分離選擇性的機理模型。除此之外，Teramae等[48]發(fā)現(xiàn)堿金屬陽離子的萃取選擇性順序與萃取劑分子界面取向絡(luò)合導(dǎo)致的和頻光譜偏振信號選擇性增強有關(guān)。Frey等[49]采用SHG手段直接證明了TBP萃取速率受制于界面分子聚集、取向?qū)е碌姆肿娱g相互作用變化。王鴻飛、郭源等[50-53]則采用SFG詳細(xì)研究了液-液界面水分子取向、聚集及其對各種無機離子界面吸附選擇性的影響，提出了界面分子-離子相互作用變化的時間相關(guān)函數(shù)。筆者等[54]采用紅外反射光譜結(jié)合和頻光譜研究了三辛基氧膦（TOPO）在油水兩相界面與水溶液中的稀土離子的界面相互作用。實驗結(jié)果表明，水溶液中與稀土離子共存的鹽離子種類和鹽濃度對TOPO-稀土離子的界面相互作用影響較大。不同的無機鹽陰陽離子在油水兩相界面的正/負(fù)競爭吸附導(dǎo)致稀土離子的界面萃取行為發(fā)生變化。在含高濃度Mg2+的水溶液中，TOPO萃取La3+的萃取率大大下降，而在含高濃度SCN-的水溶液中，TOPO萃取La3+的萃取率明顯增加。這是因為，離子半徑小、帶電量高的Mg2+會促使界面TOPO分子發(fā)生去水化效應(yīng)，TOPO分子碳鏈在界面有序堆積，分子間的相互作用增強，不利于TOPO與水溶液中的La3+接觸；而離子極化率高、體積大的SCN-有利于界面TOPO分子的水化，可明顯增強TOPO與水溶液中的La3+接觸。該研究從分子水平解釋了水溶液中與目標(biāo)離子共存的其他陰陽離子在界面的競爭吸附行為對有機膦萃取劑萃取效率的影響，有助于理解萃取過程的界面微觀機理。

2 萃取體系的相分離行為與介質(zhì)無機鹽離子作用機制

液液萃取過程通常伴隨互不相溶兩液相的相分離行為，分散相在連續(xù)相內(nèi)不斷聚并，最終形成兩層液相共存體系。目標(biāo)被萃物在兩層液相的分配比以及其分配速率與萃取體系的成相行為密切相關(guān)。近年來，隨著一些新興萃取體系和萃取方法的快速發(fā)展，如微乳相萃取、三液相萃取、雙水相萃取、離子液體萃取、凝膠萃取等，人們越來越關(guān)注這些萃取體系的相分離行為或新相生成對目標(biāo)物的相分配行為影響。研究萃取分離體系分子間各種弱相互作用，如氫鍵、范德華力、疏水相互作用、C—H…O弱相互作用等對萃取體系成相行為的影響已成為萃取分離化學(xué)的前沿?zé)狳c。人們的研究發(fā)現(xiàn)，這些萃取體系的萃取分離性能大多與溶液相內(nèi)分子有序組裝、聚集行為等溶液結(jié)構(gòu)的變化密切相關(guān)，受介質(zhì)鹽離子種類及濃度的影響，萃取體系伴隨新相的生成以及相分離行為，目標(biāo)物在萃取體系的相分配行為也將發(fā)生明顯變化[55-58]。因此，研究鹽離子在這些萃取分離體系中的作用機理是深入認(rèn)識萃取體系相分離行為微觀機制的必要前提。但是，在實際分離體系中，由于可能存在萃取劑分子與目標(biāo)萃取物、介質(zhì)無機鹽陰離子與陽離子、水分子與目標(biāo)萃取物、水分子與水分子之間的眾多復(fù)雜相互作用，至今尚不清楚溶液介質(zhì)中的無機鹽離子如何影響萃取體系的相分離行為以及其對目標(biāo)被萃物相傳質(zhì)行為的作用機理，也無法給出直接的實驗證據(jù)，從微觀分子層次深入認(rèn)識萃取分離的歷程和機理。

大量的研究表明，無機鹽離子行為遵循Hofmeister離子序列[59-63]，即存在SO42->OH->F-> Cl->Br->NO3->I->SCN->ClO4-的陰離子順序和Ba2+>Ca2+>Mg2+>Rb+>Li+>Cs+>Na+>K+>NH4+的陽離子順序。但是，長期以來，關(guān)于Hofmeister離子如何調(diào)控水溶液中的溶質(zhì)分子聚集組裝行為以及如何影響萃取分離體系相分離行為的作用機制一直未達(dá)成共識[64-75]，其中一個主要的分歧是：無機鹽離子是否與溶質(zhì)分子發(fā)生直接的相互作用。一些研究者持“間接作用機理”觀點，將Hofmeister離子分為親水離子（kosmotropic離子）和疏水離子（chaotropic離子）兩組，認(rèn)為Hofmeister離子通過與水的相互作用，可間接影響溶質(zhì)分子在水溶液中的溶解和聚集分相行為；而另一些研究者則持“直接作用機理”觀點，認(rèn)為Hofmeister離子與溶質(zhì)分子存在直接的相互作用。Florin等[76]認(rèn)為，相對于水溶性聚合物分子水化層內(nèi)的水分子，位于kosmotropic離子周圍的水分子具有更低的能量。因此，當(dāng)水溶性聚合物分子和kosmotropic離子相互靠近時，kosmotropic離子會“奪取”聚合物分子水化層中的水分子，使得聚合物分子在水中的溶解度降低，即產(chǎn)生“鹽析效應(yīng)”，進(jìn)而原本呈均一相的聚合物水溶液會發(fā)生相分離行為，形成兩層互不相溶的液相。與kosmotropic離子相反，chaotropic離子周圍的水分子具有較高的能量，chaotropic離子可促進(jìn)聚合物分子的水化，增加聚合物分子在水中的溶解度，即表現(xiàn)為“鹽溶效應(yīng)”，這將抑制聚合物水溶液的相分離行為。Collins等[77]認(rèn)為，無機鹽離子與水溶液中的聚合物溶質(zhì)分子的界面水化層相互作用，可間接調(diào)控聚合物分子在水溶液中的溶解和聚集組裝行為。當(dāng)存在kosmotropic離子時，聚合物分子的界面水化層會被kosmotropic離子破壞，減小聚合物分子界面的水化效應(yīng)，出現(xiàn)鹽析現(xiàn)象。而chaotropic離子因極化率高和體積大等不易與水結(jié)合，因此，chaotropic離子的加入會促進(jìn)聚合物溶質(zhì)分子的界面水化相互作用，出現(xiàn)鹽溶現(xiàn)象。Schott等[78-90]詳細(xì)研究了無機鹽對聚乙二醇等非離子型表面活性劑水溶液的濁點、臨界膠團(tuán)濃度和膠團(tuán)穩(wěn)定性的影響。Nucci等[91]基于雙態(tài)氫鍵模型，對不同溫度下30種無機鹽水溶液的羥基伸縮振動峰的分析結(jié)果得出：無機鹽離子的加入改變了溶質(zhì)分子周圍的水氫鍵網(wǎng)絡(luò)結(jié)構(gòu)，其對氫鍵結(jié)構(gòu)的影響遵循Hofmeister離子序列。但是，隨后的研究又對上述研究者的觀點提出了質(zhì)疑。Bakker等[92-93]基于飛秒二色泵浦探測光譜研究了水分子的取向遷移時間相關(guān)行為。結(jié)果指出，水的氫鍵結(jié)構(gòu)并沒有因為無機鹽離子的加入而發(fā)生改變。Pielak等[94]提出，如果間接作用機理合理，那么本體水應(yīng)該存在兩種結(jié)構(gòu)體：其一為水分子堆積稀疏，排列有序；其二為水分子堆積密積，排列無序。chaotropic離子會破壞本體水的氫鍵結(jié)構(gòu)，而kosmotropic離子有利于本體水氫鍵結(jié)構(gòu)的增強。然而，實驗結(jié)果表明(?c-p/?p)T的符號與離子種類沒有直接關(guān)系。Thormann等[67]認(rèn)為如果間接作用機理合理，kosmotropic離子引起水分子氫鍵結(jié)構(gòu)變化時所對應(yīng)的熵變(ΔSstr)應(yīng)該呈負(fù)值，而chaotropic離子對應(yīng)的ΔSstr為正值。然而，計算結(jié)果表明，ΔSstr的正負(fù)與離子種類也沒有直接關(guān)系。

Gurau等[95]采用和頻光譜研究了溶液中不同陰離子對溶液表面十八胺單分子膜的聚集取向行為。基于十八胺分子的甲基對稱伸縮振動(vs(CH3))與亞甲基對稱伸縮振動(vs(CH2))比值的變化與Hofmeister離子序列順序相同這一實驗現(xiàn)象，作者認(rèn)為Hofmeister離子序列可能與無機鹽陰離子-十八胺分子之間存在直接相互作用。Chen等[96]采用SFG光譜研究了水溶液中的聚（N-異丙基丙烯酰胺）（PNIPAM）分子與不同陰離子的相互作用。結(jié)果表明，在含有不同無機陰離子的PNIPAM水溶液表面，位于3200 cm-1處水分子的伸縮振動峰隨著Hofmeister離子序列呈現(xiàn)出規(guī)律性變化。他們認(rèn)為，盡管PNIPAM本身不帶電，但是陰離子，尤其是體積大、極化率高、易去水化的chaotropic離子，可與PNIPAM分子發(fā)生直接相互作用，吸附在PNIPAM溶液表面，使得PNIPAM溶液表面帶電。在靜電場下，表面層的水分子排列更加有序[97]，使得3200 cm-1處的和頻信號增加。此外，由于PNIPAM本身不帶電，吸附在PNIPAM分子上的陰離子并不會改變PNIPAM在溶液表面的取向和濃度，因此3200 cm-1處的和頻信號隨Hofmeister離子序列呈規(guī)律性變化。與PNIPAM不同，十八胺分子會與溶液中的H+結(jié)合而帶電，因此陰離子與十八胺的相互作用還涉及靜電作用。在靜電作用的影響下，十八胺在溶液表面的排布密度、取向以及電荷數(shù)量等都會發(fā)生改變，這些因素都會對表面水分子的取向和堆積結(jié)構(gòu)產(chǎn)生影響。因此，在十八胺體系中，3200 cm-1處和頻信號的變化并不符合Hofmeister序列。Zhang等研究了PEO-PPO共聚物[98]、PNIPAM[99-100]以及類彈性蛋白多肽V5-120和V5A2G3-12[101]溶液的相行為及其受無機陰離子的影響。結(jié)果表明，陰離子對含有這幾種溶質(zhì)分子的水溶液相行為的調(diào)控都遵循Hofmeister序列。Gibb 等[102]研究了kosmotropic和chaotropic陰離子與溶液中兩親分子的相互作用。結(jié)合核磁波譜和等溫量熱掃描的實驗結(jié)果，他們認(rèn)為chaotropic陰離子的去水化效應(yīng)使得其能與溶液中兩親分子官能基團(tuán)發(fā)生直接的作用；而kosmotropic陰離子因親水性強，不易去水化，與兩親分子不存在直接相互作用。Finney等[103]基于中子散射實驗結(jié)果，提出叔丁醇水溶液的鹽析分相可能是通過“陰離子橋”實現(xiàn)的。當(dāng)沒有鹽存在時，叔丁醇分子的極性基團(tuán)暴露在水溶液中，而非極性基團(tuán)發(fā)生相互作用抱團(tuán)在一起，這時叔丁醇能與水以任意比例互溶。而當(dāng)叔丁醇水溶液中加入無機鹽后，無機鹽離子通過與叔丁醇分子的極性基團(tuán)發(fā)生直接相互作用，形成“鹽橋效應(yīng)”，把叔丁醇分子連接起來，使得叔丁醇分子的非極性基團(tuán)暴露出來，從而產(chǎn)生鹽析效應(yīng)。

無機鹽的陰離子對萃取分離體系的相分離行為影響較大。然而，陽離子的作用也不能忽略。Tasaki[104]對水溶液中鉀離子和聚乙二醇間的相互作用進(jìn)行了分子動力學(xué)模擬。研究結(jié)果表明，在水溶液中，聚乙二醇分子鏈呈螺旋狀，位于螺旋鏈內(nèi)側(cè)的鉀離子與聚乙二醇存在直接相互作用。Florin[105]測定了聚乙二醇對81Br、23Na、7Li、133Cs 和35Cl 5種核的弛豫時間影響。結(jié)果表明，這5種核的弛豫時間隨著聚乙二醇濃度的增加都不斷減小。作者認(rèn)為，聚乙二醇造成的陰離子不對稱性水化是陰離子弛豫時間減小的主要原因。而陽離子的弛豫時間減小是因為聚乙二醇和陽離子發(fā)生了直接相互作用。筆者等[106]采用二階導(dǎo)紅外光譜和Raman光譜研究了聚合物雙水相萃取體系中的聚合物分子與無機鹽陰離子、陽離子的相互作用。結(jié)果表明，在30% EOPO+Na2CO3和30% EOPO+K2CO3體系（均為質(zhì)量分?jǐn)?shù)）中，隨著體系中鹽濃度的增加，EOPO分子的EO鏈段v(C－O)紅外譜峰先紅移然后藍(lán)移。在30% EOPO+KSCN和30% EOPO+NaSCN體系中，隨著體系中鹽濃度的增加，EOPO分子的PEO鏈段v(C－O)紅外譜峰只發(fā)生紅移。Raman曼光譜表征進(jìn)一步指出，隨著體系中碳酸鉀濃度的增加，EOPO分子的PEO鏈段v(CH2)先藍(lán)移后紅移，而v(C－O)譜峰先紅移后藍(lán)移。在硫氰酸鈉溶液中加入EOPO后，鈉核的弛豫時間大幅度減弱。這些實驗結(jié)果均證實了鉀離子、鈉離子與EOPO之間確實存在直接相互作用。該研究為認(rèn)識陽離子與水溶性聚合物分子間的相互作用提供了分子水平的證據(jù)。

3 二維相關(guān)光譜探測萃取分離體系的分子間弱相互作用

利用二維相關(guān)譜分析技術(shù)研究分子間弱相互作用最先起源于核磁領(lǐng)域。相比一維譜圖，由于增加了一個維度，提高了譜圖的分辨率，在一維核磁譜圖上無法分辨的重疊峰或者被掩蓋的小峰，在二維譜上可以通過交叉峰的信息進(jìn)行有效分辨。另外，二維核磁譜圖中的交叉峰還能提供自旋核之間相互作用的信息。因此，二維核磁技術(shù)在解析復(fù)雜化合物的結(jié)構(gòu)（如蛋白質(zhì)三維空間結(jié)構(gòu)的解析）、研究分子內(nèi)和分子間相互作用等方面占據(jù)非常重要的地位[107-116]。Noda等[117-122]首先將二維核磁相關(guān)譜分析方法引入紅外、Raman光譜，通過對樣品施加一定外部擾動，如電、熱、磁、聲、化學(xué)或機械力等，分子的電偶極躍遷會發(fā)生相應(yīng)變化。這種基于外部擾動的宏觀弛豫速率較振動弛豫速率慢得多，借助于傳統(tǒng)的紅外、Raman等光譜分析手段就可以對宏觀弛豫過程進(jìn)行檢測。在擾動過程中，通過測量多張一維譜圖，然后對這些譜圖進(jìn)行二維相關(guān)分析即可得到二維相關(guān)譜。二維相關(guān)光譜的提出，實現(xiàn)了紅外、Raman等光譜技術(shù)的二維化，在探測體系分子間弱相互作用領(lǐng)域的應(yīng)用越來越廣泛[123-128]。

應(yīng)用二維相關(guān)光譜的交叉峰，可以探測萃取分離體系由于分子取向、作用位點、分子構(gòu)象、聚集行為等變化帶來的離子-偶極、偶極-偶極、氫鍵等分子間弱相互作用的變化，為從分子水平認(rèn)識分離過程微觀機制、優(yōu)化分離選擇性、調(diào)控分離行為提供了可能。近年來，這方面的研究在諸如分子印跡分離技術(shù)、超分子自組裝分離技術(shù)等新興分離方法的發(fā)展過程中起到了至關(guān)重要的作用。根據(jù)二維相關(guān)譜的性質(zhì)，二維相關(guān)譜可分為同步相關(guān)譜和異步相關(guān)譜。其中，異步相關(guān)譜在辨別萃取分離體系分子間弱相互作用方面具有更大的優(yōu)勢。然而，迄今為止，二維相關(guān)光譜在探測更為復(fù)雜的分離體系分子間弱相互作用、研究萃取分離過程界面分子間相互作用方面仍然存在一定問題[129-140]。

Thomas等[141]基于Noda等提出的廣義二維相關(guān)譜，發(fā)展了移動窗口二維相關(guān)譜分析技術(shù)。在移動窗口二維相關(guān)譜中，一個維度為光譜變量坐標(biāo)，另一個維度為擾動變量坐標(biāo)，利用移動窗口二維相關(guān)譜，可以觀察分離體系分子間相互作用導(dǎo)致的交叉峰隨外部擾動變化的實時信息[142]。在此基礎(chǔ)上，Ozaki等[143-146]通過數(shù)學(xué)變換轉(zhuǎn)置動態(tài)光譜矩陣以及動態(tài)光譜矩陣在二維相關(guān)譜計算公式中的位置，得到了一種新的二維相關(guān)譜分析方法，稱為樣品-樣品相關(guān)。為了便于區(qū)分，他們把Noda提出的二維相關(guān)分析稱為波長-波長相關(guān)，而他們提出的樣品-樣品相關(guān)可以更好地反映樣品特征的變化（如濃度擾動帶來的分子間相互作用變化），與波長-波長相關(guān)互為補充。此外，Ozaki等[147]還提出了混二維相關(guān)譜?；於S相關(guān)譜即對兩種不同擾動下的動態(tài)光譜進(jìn)行相關(guān)分析得到二維相關(guān)譜。作者對3種擾動情況進(jìn)行了考慮：（1）兩擾動相互獨立，互不相干；（2）兩擾動有一定的關(guān)聯(lián)性；（3）兩擾動相同，但是擾動施加的體系不同。利用混二維相關(guān)譜，可以直接對兩體系或不同擾動下的兩組光譜的關(guān)聯(lián)性進(jìn)行研究。根據(jù)Thomas等提出的移動窗口二維相關(guān)譜的概念，Ozaki等[148]又提出擾動相關(guān)移動窗口二維相關(guān)譜，即直接對擾動變量和光譜變量進(jìn)行二維相關(guān)分析，與移動窗口二維相關(guān)譜相同，該方法也可以顯示出體系隨擾動變化的信息，但其計算過程比移動窗口二維相關(guān)譜更為簡單。

二維相關(guān)分析其實是一種光譜數(shù)據(jù)的數(shù)學(xué)分析方法。從這個意義上來講，二維相關(guān)分析與主成分分析相類似，屬于一種化學(xué)計量分析方法。起先，研究者分別基于二維相關(guān)分析和主成分分析對光譜數(shù)據(jù)進(jìn)行解釋。在這些研究中，二維相關(guān)分析和主成分分析的地位是平行的[146,149-152]。之后Jung等[153-156]把二維相關(guān)分析與主成分分析結(jié)合起來，利用主成分分析或奇異值分解對一維光譜數(shù)據(jù)進(jìn)行了降噪處理，然后對降噪的光譜數(shù)據(jù)進(jìn)行二維相關(guān)分析，得到了高信噪比的二維相關(guān)譜。

盡管二維相關(guān)譜具有更高的譜圖分辨率，然而對于復(fù)雜的體系，若其中多個組分的一維譜峰在二維相關(guān)譜中產(chǎn)生多個交叉峰，且這些交叉峰彼此之間相互交疊、干擾，仍然很難對二維相關(guān)譜進(jìn)行有效分析。針對此問題，2010年，Noda[157-160]提出了投影二維相關(guān)分析法。該方法可以大大簡化二維相關(guān)譜分析。此外，Noda等[161]還提出從光譜數(shù)據(jù)的奇異值分解對光譜數(shù)據(jù)進(jìn)行降噪處理，用于提取異步相關(guān)譜的內(nèi)核矩陣，其矩陣容量比常規(guī)異步相關(guān)譜矩陣小得多，但更方便分析。

基于Noda等的工作，Dluhy等[140]提出“model-dependent”二維相關(guān)分析?！癿odeldependent”二維相關(guān)分析與傳統(tǒng)二維相關(guān)分析（亦稱為“model-free”二維相關(guān)譜）的區(qū)別在于：傳統(tǒng)二維相關(guān)分析為動態(tài)光譜之間的相關(guān)，而“model-dependent”二維相關(guān)分析建立的是動態(tài)光譜與一個數(shù)學(xué)函數(shù)之間的相關(guān)性。相對于“model-free”二維相關(guān)譜，“model-dependent”二維相關(guān)譜更具有“開放性”，因為在“model-dependent”二維相關(guān)譜中，數(shù)學(xué)函數(shù)的選擇并沒有具體的規(guī)定，函數(shù)可以為任意形式。因此，通過改變函數(shù)，可以創(chuàng)造不同類的“modeldependent”二維相關(guān)譜。例如，Dluhy等[132-134, 137-138]最先提出的bn相關(guān)分析和kn相關(guān)分析法，已用于分析抗生素萃取分離過程的機理，對抗生素與磷脂單分子層的作用位點、磷脂單分子層的甲基和亞甲基隨膜壓變化的相對速率、以及嵌入在磷脂單分子層中的蛋白質(zhì)隨膜壓變化的構(gòu)象轉(zhuǎn)變等進(jìn)行了研究。

二維相關(guān)譜的實際應(yīng)用過程中，可能會存在干擾峰，即二維相關(guān)譜交叉峰的出現(xiàn)可能與分子間相互作用無關(guān)，而是源于其他的相關(guān)性。這些干擾峰的存在，使得二維相關(guān)譜仍然無法成為反映分子間相互作用的可靠工具。針對此問題，徐怡莊等[162-163]以濃度作為外部擾動，通過選擇合適的相互作用兩物質(zhì)的初始濃度序列，提出了正交濃度設(shè)計方法（OSD），該方法成功消除了同步相關(guān)譜中的干擾峰。之后，他們進(jìn)一步發(fā)展了OSD方法，提出了雙正交濃度設(shè)計方法（DOSD）[164]、異步正交濃度設(shè)計方法（AOSD）[165]、雙異步正交濃度設(shè)計方法（DAOSD）等系列方法[166]，不僅消除了同步相關(guān)譜中的干擾峰，異步相關(guān)譜中的干擾峰也可被去除?；谏鲜龇椒ǎ麄冏罱K將二維相關(guān)譜中的交叉峰與體系分子間相互作用建立起了對應(yīng)關(guān)系，使其成為反映分子間相互作用的可靠工具。

另外，實際分離體系中，由于噪聲的干擾，或當(dāng)相互作用的兩物質(zhì)譜峰相互重疊，或僅一個物質(zhì)有特征吸收峰時，異步相關(guān)譜也可能會給出錯誤的結(jié)果，這使得異步相關(guān)譜在一些特定體系的應(yīng)用過程中仍然無法準(zhǔn)確反映分子間弱相互作用。例如，協(xié)萃體系中不同萃取劑分子間的偶極-偶極相互作用，即使使用二階導(dǎo)數(shù)光譜，也不能得到相關(guān)分子間弱相互作用的信息。而采用通常的異步相關(guān)譜或其改進(jìn)（如AOSD譜），由于噪聲的影響，AOSD譜中會出現(xiàn)多余的交叉峰或者本該出現(xiàn)的交叉峰被噪聲淹沒，極易造成譜圖的錯誤解讀，得出與事實不相符的結(jié)論[132, 167-168]。針對此問題，筆者等提出，通過對一維動態(tài)光譜的譜圖進(jìn)行排序，增加異步相關(guān)譜的信噪比[169]，并在此基礎(chǔ)上發(fā)展了一種基于改良參比光譜的異步相關(guān)譜（稱為ASMR譜）[170]，以期提高譜圖的信噪比。相比文獻(xiàn)報道的以平均光譜為參比光譜的方法[171-172]，當(dāng)以濃度變化作為外部擾動時，采用異步相關(guān)譜檢測分子間弱相互作用時，參比光譜的選擇并不是任意的，必須確保在新參比下異步相關(guān)譜中的干擾峰可以被去除掉。ASMR譜可以寫成額外異步相關(guān)譜和AOSD譜的加和，改變相互作用兩物質(zhì)的濃度序列排序時，ASMR譜峰的絕對值強度可以隨著濃度序列的排序變化而變化。模擬實驗和實際體系實驗證實了額外異步相關(guān)譜的強度遠(yuǎn)大于AOSD譜的強度，因此ASMR譜的強度遠(yuǎn)大于AOSD譜，同時ASMR譜的信噪比也遠(yuǎn)遠(yuǎn)高于AOSD譜。將ASMR譜用于分析協(xié)萃體系不同萃取劑分子間的相互促進(jìn)或抑制作用、生物大分子（如溶菌酶）萃取過程與聚氧乙烯-聚氧丙烯共聚物EOPO分子間的相互作用以及稀土萃取過程稀土離子與萃取劑酯基間的偶極-靜電相互作用。結(jié)果表明，在一維譜圖和二階導(dǎo)譜圖上無法觀測到的相互作用信息，在ASMR譜上有明顯的體現(xiàn)。根據(jù)ASMR譜交叉峰的峰型，可以判斷由于分子間相互作用導(dǎo)致的特征峰變化，這為從譜峰的微細(xì)變化認(rèn)識分子間弱相互作用對萃取分離微觀過程的影響創(chuàng)造了有利條件。另外，針對萃取分離體系中相互作用的兩物質(zhì)僅一種物質(zhì)含有特征吸收峰的情況，由于ASMR譜無法反映分子間相互作用導(dǎo)致的峰強變化，可能對體系分子間是否存在相互作用的判定得出錯誤的結(jié)論。筆者[173]發(fā)展了一種帶有輔助交叉峰的ASMR譜（稱為ASAP方法）。通過在研究體系中引入一種虛擬物質(zhì)構(gòu)造異步相關(guān)譜的輔助交叉峰，成功解決了上述問題，并以苯甲酰胺萃取Tb3+體系的偶極-靜電相互作用為研究對象，驗證了ASAP方法的可靠性。結(jié)果表明，輔助交叉峰確實可以反映苯甲酰胺萃取Tb3+體系分子間相互作用導(dǎo)致的特征峰峰寬、峰位以及峰強的變化，而常規(guī)的紅外光譜除了苯甲酰胺濃度變化引起的譜圖變化外，觀測不到任何與分子間相互作用相關(guān)的譜圖變化信息。此外，當(dāng)相互作用的兩物質(zhì)的特征峰發(fā)生嚴(yán)重重疊時，ASAP方法還可用于辨別復(fù)雜的交叉峰。輔助交叉峰的引入有助于分辨分子間相互作用導(dǎo)致的不同物質(zhì)特征峰的差異性變化，較ASMR譜和常規(guī)的AOSD譜具有明顯優(yōu)勢，為正確判定分離體系由于分子結(jié)構(gòu)、構(gòu)象變化引起的分離行為變化提供了可靠依據(jù)。

4 結(jié) 語

研究和表征萃取分離體系中的各種分子間弱相互作用，對于深入認(rèn)識萃取分離過程微觀機理、調(diào)控萃取分離選擇性具有重要意義。近年來，關(guān)于萃取分離體系分子間弱相互作用的分子模擬、量化計算和熱力學(xué)分析報道較多，但從實驗表征出發(fā)，特別是利用各種譜學(xué)手段探測分子間弱相互作用的研究還相對薄弱。

由于實際萃取分離體系的復(fù)雜性，目前采用各種譜學(xué)實驗手段研究和表征萃取反應(yīng)、傳質(zhì)過程發(fā)生在溶液相界面的分子間相互作用、研究與萃取體系新相生成和相分離行為相關(guān)的溶液相內(nèi)分子微觀聚集結(jié)構(gòu)變化等剛剛起步，二維相關(guān)譜技術(shù)表征萃取體系分子間弱相互作用時，針對的研究對象也比較簡單。已報道的這些實驗工作進(jìn)一步與分子模擬、量化計算、熱力學(xué)分析等手段相結(jié)合是將來的發(fā)展趨勢。

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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

基金項目：國家重點基礎(chǔ)研究發(fā)展計劃項目（2012CBA01203，2013CB632602）；國家自然科學(xué)基金項目（51574213，51074150）。

中圖分類號：TQ 028

文獻(xiàn)標(biāo)志碼：A

文章編號：0438—1157（2016）01—0152—13

DOI:10.11949/j.issn.0438-1157.20150999