基于熱重分析測定物質(zhì)的蒸氣壓

浙江大學(xué)學(xué)報(理學(xué)版第35卷第1期Journal of Zhejiang University(Science EditionVol 35 No. 12008年1月http://www.journalszju.educn/sciJan.2008基于熱重分析測定物質(zhì)的蒸氣壓鄭青,雷群芳,方文軍浙江大學(xué)化學(xué)系,浙江杭州310027)摘要:根據自由蒸發(fā)過(guò)程的 Langmuir關(guān)系,將熱重分析(TGA)用于正十六炕、正十七烷和正十八炕等3個(gè)正構烷烴的蒸氣壓測定,分別采用常數法和比較法計算蒸氣壓,其結果與文獻數據符合較好.結果表明,只要選擇合適的參考物質(zhì),TGA可以成功地應用于未知液體物質(zhì)蒸氣壓的測定,同時(shí),根據從TGA實(shí)驗得到的蒸發(fā)速率和蒸氣壓數據,計算了蒸發(fā)過(guò)程的表觀(guān)活化能和平均蒸發(fā)焓,發(fā)現活化能教值僅稍大于蒸發(fā)焓,證實(shí)TGA實(shí)驗中的蒸發(fā)過(guò)程接近非活化自由燕發(fā)過(guò)程,TGA是一種快速實(shí)用的蒸氣壓測量新方法,尤其適用于較低壓力范國的蒸氣壓測定,是傳統實(shí)驗方法的很好補充關(guān)鍵詞:熱重分析;正構烷烴;蒸氣壓;活化能;蒸發(fā)焓中圖分類(lèi)號:O642.4文獻標識碼:A文章編號:1008-9497(2008)01-059-04ZHENG Qing, LEI Qun-fang, FANG Wen-jun( Department of Chemistry, Zhejiang University, Hangzhou310027, China)Measurements of vapor pressures of alkanes by thermogravimetry. Journal of Zhejiang University( Science Edition)2008,35(1):59~62Abstract: Thermogravimetric analysis (TGA) was extended to the measurements on vapor pressures of three n-alkanes, hexadecane, heptadecane and octadecane, on the basis of the Langmuir equation for zero-order vaporizationprocess. The vapor pressures were calculated by two different methods, a constant-parameter one and a comparativeone, from the tGa data. These results are in good agreement with the reference data calculated from the antoine equation. It follows that TGa can be successfully extended to the vapor pressure measurements of some liquids proided an appropriate reference sample is used. Furthermore, the apparent energies of activation for the vaporizationprocess and the average enthalpies of vaporization are obtained through the vaporization rates and the vapor pressuredata. It was found that the values of the energy of activation are only slightly larger than those of the enthalpies of vapori-zation and it indicated that the vaporization process during the tga is nearly a nonr activated one. The experimental resultsshowed that the tga method is a rapid and practical one for constructing vapor pressure curves especially at low pressurerange. TGa can serve as a new remedy for the traditional methods of vapor pressure measurements.Key words: thermogravimetric analysis(TGA); alkane; vapor pressure; energy of activation; enthalpy of vaporization蒸氣壓數據常用于相平衡計算,由它還可以獲概括為動(dòng)態(tài)法和靜態(tài)法兩大類(lèi),應用中需考慮測定得蒸發(fā)焓、升華焓等重要熱力學(xué)函數;蒸氣壓也是評對象和測壓范圍來(lái)選擇-.一般地靜態(tài)法多用于測價(jià)物質(zhì)穩定性的重要指標,比如先進(jìn)的航天航空燃定中、高壓蒸氣壓,測壓下限約為1kPa,等壓計法料的蒸氣壓就需要嚴格調控.通常,蒸氣壓數值范圍 ASTMD287986)是典型的靜態(tài)法;沸點(diǎn)計法則是寬,受物質(zhì)狀態(tài)、溫度等影響大,因此針對不同的研典型的動(dòng)態(tài)法,可用于相對低些的蒸氣壓測定,但最好究對象和具體要求,已經(jīng)形成了很多測定方法,如沸不低中國煤化工是為0.05Pa;努森扭點(diǎn)計法、等壓計法、流逸法、怒森隙透法等,總體上可矩隙CNMHG隙透法的測壓下限收稿日期:2006-12-18基金項目:國家自然科學(xué)基金資助項目(20573096);浙江省自然科學(xué)基金資助項目(Y404329)作鶚或據(132)::士明究生,主要從事物理化學(xué)研究浙江大學(xué)學(xué)報(理學(xué)版)第35卷分別為105、10-6、10-8Pa,是較好的測定低微蒸氣壓的方法.對于多組分體系來(lái)說(shuō),擬靜態(tài)法是一類(lèi)較好實(shí)驗部分的實(shí)驗方法,它是吸收了動(dòng)態(tài)法和靜態(tài)法的優(yōu)點(diǎn)而建立起來(lái)的,在流體相平衡研究和燃料開(kāi)發(fā)中應用較1.1試樣多47.近年來(lái)把熱重分析(TGA用于蒸氣壓測定得正十六烷、正十七烷和正十八烷(簡(jiǎn)寫(xiě)為C16到快速發(fā)展,熱重分析儀具有自動(dòng)化程度高操作方C2和C8)均為進(jìn)口試劑( Sigma-Aldrich,美國),純便測量速度快,用樣量少等優(yōu)點(diǎn)彌補了傳統動(dòng)態(tài)法度大于99%.文中計算需用的相關(guān)數據列于表1,其和靜態(tài)法的一些不足,因此受到關(guān)注.它在藥物、香料、中Tb為常壓沸點(diǎn),M為相對分子質(zhì)量,蒸氣壓染料等固體物質(zhì)的低蒸氣壓測定方面具有獨特優(yōu)p(10°Pa)與溫度T(K)的關(guān)系用 Antoine方程表勢1.本文則主要以具有較高沸點(diǎn)的正構烷烴為對示:logp=AA,B,C為 Antoine常數象,以探討熱重法在測定液體物質(zhì)蒸氣壓中的應用表1正十六、十七和十八炕的基本性質(zhì)Table 1 Fundamental properties of n-hexadecane, n-heptadecane and n-octadecaneAntoine constantsAlkatABTemp. range/K554.0226.444.173121845.672-117.054463.2~559.8575.0240.473.927301718.004-138.126436.6~576.0589.3254.494.332092068.963-111.927447.6~5901.2儀器與操作表2不同揮發(fā)率時(shí)的溫度Tv以及峰溫T和采用德國產(chǎn) NETZSCHSTA409PG/PC熱重分析終溫儀用經(jīng)過(guò)灼燒恒重的剛玉坩堝盛放試樣,其截面積為able2 Temperatures(Tv) at different volatile ratios2.83×10-5m2.試樣用量為10~11mg,用Metr十萬(wàn)(V), and peak(T )and final(T) temperatures分之一天平準確稱(chēng)量以空坩堝作參比,用高純氮氣為T(mén)emperature/℃載氣,流量為30mL·min-1,控制升溫速度為10℃186.1min-1,記錄溫度范圍為室溫至400℃,熱重(TG)曲線(xiàn)216.1228.6及其微分(DTG)數據由儀器軟件自動(dòng)給出V=0.4223.4236.5247.5V=0.5230.3242.8253.82結果與討論V=0.6235.3259.52.1 TG-DTG曲線(xiàn)243.7267.9圖1給出實(shí)驗測得的3種物質(zhì)的 TG-DTG曲線(xiàn)247.1259.5272.1246.5很明顯,在各自沸點(diǎn)溫度之前(530~550K),試樣已經(jīng)268.9258.6271.3284.1完全蒸發(fā),此后基線(xiàn)平直;DTG曲線(xiàn)只有一個(gè)銳利的峰,而且在峰值之前變化相對緩和些,峰值之后迅速變2蒸發(fā)過(guò)程的活化能化,說(shuō)明蒸發(fā)過(guò)程沒(méi)有發(fā)生裂解結焦等現象,并且符合零級蒸發(fā)動(dòng)力學(xué)特征表2列出了它們在不同揮發(fā)類(lèi)似于化學(xué)反應動(dòng)力學(xué)過(guò)程,蒸發(fā)過(guò)程也可用Arrhenius方程表示{101率V時(shí)的溫度Tv,以及峰溫T和終溫T其中,k為實(shí)驗溫度T時(shí)的蒸發(fā)速率常數kg·s-1·m-2,ko為指前因子,Ea為蒸發(fā)過(guò)程的表觀(guān)活化能小 kL. mol1R為氣體常數,J·mol-·K如中國煤化工率過(guò)程,則kp可以從CNMH(發(fā)表面積直接計算圖1 TG-DTG曲線(xiàn)TG-DTG curves for the n-alkanes其中,A為蒸發(fā)表面積,m為質(zhì)量,t為時(shí)間.在實(shí)驗1鄭青,等:基于熱重分析測定物質(zhì)的蒸氣壓61溫度范圍內,如果E可視為常數,由式(1)得Im,p和ν的關(guān)系示于圖3.可見(jiàn),p和y確實(shí)存在良好的對1/T呈直線(xiàn)關(guān)系.TGA實(shí)驗得到的相應結果示直線(xiàn)關(guān)系,關(guān)聯(lián)實(shí)驗數據,可得常數k=3.8653×于圖2,于是從直線(xiàn)斜率可計算得到蒸發(fā)過(guò)程的表105Pa·kgs·mol.5·s1·m2·K05,進(jìn)一步計算觀(guān)活化能,其數值列于表3得蒸發(fā)系數a=1.87×10.需要指出的是,實(shí)驗條件下的a數值遠小于1,這主要是因為實(shí)驗在載氣存在下的流動(dòng)體系中進(jìn)行,與真空狀態(tài)相差很遠,文獻12中也有類(lèi)似的結果報道1.8192.02.1222.3242.5262.71000/T/K圖2蒸發(fā)過(guò)程的Ink,與1/T的關(guān)系Fig. 2 Plots of Ink a versus 1/t during the0.06008vaporization processw/kg:mor∵s:m:K"表3從TGA數據關(guān)聯(lián)得到的蒸發(fā)動(dòng)力學(xué)參數和圖3蒸氣壓p和變量ν的關(guān)系蒸發(fā)焓3 Relationship between vapor pressure p and variable vTable 3 Correlation results for the parameters of(2)比較法:根據式(4),可以得到vaporization kinetics and the enthalpy of vaporization(dm/dt)from TGA dataPsm T"PRef, T-(dm/dt).TMS.(5)n如mw:my8:-可見(jiàn),只要比較兩者在相同溫度下的蒸發(fā)速率,便可式中,下標Sam表示未知試樣,Ref表示參考物質(zhì)Alkane range/K pointsC16397~5205010.08769.725.53964.4從參考物質(zhì)的蒸氣壓計算出未知物質(zhì)的蒸氣壓C1409~532509.90170.825.66566C1402~542579.63771.625,76768.9由上述兩種方法得到的3種物質(zhì)的蒸氣壓實(shí)驗值,及其與文獻7數據計算值的比較示于圖4.可2.3蒸氣壓和蒸發(fā)焓見(jiàn),兩者符合較好,說(shuō)明TGA可以用于測定液體物對于自由蒸發(fā)過(guò)程,蒸發(fā)速率可以用 Langmuir質(zhì)的蒸氣壓以后將進(jìn)一步擴充體系范圍方程[8-10表示:400001 dMAat=aL2Rr」(3)△ Comparative式中,a是蒸發(fā)系數,在真空條件下,數值為112.重排式(3),可以得到20000(2R)I(T1 d=ky,(4)其中,k=(2πR)1/a是與溫度和物質(zhì)無(wú)關(guān)的常數,y=[(-dm/dt)](T/M)2則取決于溫度和物質(zhì)因此,只要在相同條件下對參考物質(zhì)和未知物質(zhì)作熱重分析,就可以從參考物質(zhì)的蒸氣壓求出未知物溜定的蒸壓值與文獻值的比較質(zhì)的蒸氣壓根據具體的計算過(guò)程,則有常數法和比中國煤化工ssures from TGA較法等處理方法11,這里分別用這兩種方法計算,CNMHGdata以作比較另外,蒸氣壓p隨溫度T的變化可用 Clausius(1)常數法:由參考物質(zhì)的p和v求出各實(shí)驗溫 Clapeyron方程表示度下的k,然后求算術(shù)平均作為常數k;或者以p對Inp(6)作真秀分數級嘉求出常數,本文討論的3種物質(zhì)的浙江大學(xué)學(xué)報(理學(xué)版)第35卷式中,△H是摩爾蒸發(fā)焓.假定△H灬與溫度無(wú)關(guān),定[.石油化工,1997,26(9):626-631積分式(6),有:NIE Li, LEI Qun-fang, ZONG Han-xing, et alIn p=Inz-△H、Measurement of the saturated vapor pressure of multcomponent systems [J]. 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