WANG Na, PANG Shu-feng, ZHANG Yun-hong

Institute of Chemical Physics, School of Chemistry, Beijing Institute of Technology, Beijing 100081, China

Study on the Water Transfer of Magnesium Acetate Aerosols Led by the Rapid and Slow Change of Relative Humidity

WANG Na, PANG Shu-feng*, ZHANG Yun-hong

Institute of Chemical Physics, School of Chemistry, Beijing Institute of Technology, Beijing 100081, China

A combination of vacuum FTIR spectrometer (Vertex 80v, Bruker, German) and novel relative humidity (RH) adjusting equipment,which provides the pressure by pure water vapor, is used to study the hygroscopicity of magnesium acetate (Mg(CH3COO)2) aerosols.The RH can change not only rapidly but also slowly by the RH adjusting equipment.Because the RH is decided by the pure vapor, the real-time RH can be gained by calculating the integrated intensity of a feature band of vapor in an IR spectrum.Such the synchronism between FTIR spectrum and RH canbe ensured.The high-quality spectra of aerosols are obtained and the water peak and feature peaks of Mg(CH3COO)2are analyzed during the slow and rapid RH changing process.The result shows that the areas of acetate ions and water decreases continuously at constant high RHs.After a slow cycle of RH (1.05×104minutes), the water area decreases from 1.5 to 1.1, which means that the water content decreases after a cycle of RH.This phenomenon is reported at first up to date.The detailed analysis suggests that the hydrolysis of Mg(CH3COO)2at high RH produces acetic acid, which was put out from the aerosols owing to the decrease of the pressure around the aerosols droplets.Furthermore, the dynamic hygroscopicity of Mg(CH3COO)2aerosols is studied by changing RH as a pulse mode.It reveals that there is only water transfer hysteresis and no water loss after a pulse (10 seconds) when the RH is above 70%.Compared to slow process, it can be concluded that the hydrolysis reaction rate is slower than that of a pulse RH.The water transfer limited on rapid process should rise from some species on the surface of aerosols.

Magnesium acetate aerosol; Water transfer process; Evaporation

Hygroscopicity of the aerosol, which is defined as the water uptake or loss with the change of relative humidity (RH), is a key factor to affect the size of the aerosols and the Earth’s radiative forcing[1].The most reports about hygroscopicity are the physical and chemical properties of aerosols under equilibrium state, which is the thermodynamic[2-6].While the other important problem is the velocity of mass transfer and the immediate species accompanying mass transfer, which is the kinetic[7-9].Usually, some observations in dynamic process cannot be found in the thermal stable state, which is important in predicting the physical and chemical properties of aerosols in real environment[10-13].

In this communication, the pressure of the sample cell is provided by a pure vapor, which was put to or from the surface of aerosols leading to the rise or decrease of the RH.The RH can change not only rapidly but also slowly.As the pressure of vapor changes, the FTIR spectra are collected synchronously by the rapid scan measurement mode.The RH values can be acquired by calculating the integrated area of a feature band of vapor in an IR spectrum.Figure 1 describes the RH adjusting system and the representative rates of RH change.By this pressure equipment combined to rapid scan FTIR technique, the rapid-change of RH can reach 19.24% per 0.36 second by a pulse mode [show in Figure 1(c)], while for slow change of RH, the velocity of RH change is only 0.7% per 274.5 second [shown in Figure 1(b)].By the rapid scan mode, a spectrum can be acquired every 0.1 second.

Fig.1 (a) Scheme of the RH adjusting system and the changing trend of RH as a function of experimental time in a slow process(b) and rapid process (c)

At first, the hygroscopicity of magnesium acetate aerosols is studied by a slow change of RH.The RH and water area as a function of time have been shown in Figure 2(a).It can be found that the water area is 1.5 at the first 80% RH, and decreases to 1.1 at the end 80% RH after a cycle of RH.It means that some water drop after 1.05×104seconds.Specially, the water area decreases at the terrace of RH above 60%.So the quantitative water and acetate ions are calculated by the integrated areas of their feature bands as the RH keeps constant.Figure 2(b) gives the calculated results at 90%, 80% and 70% RH, respectively.It shows that acetate ions and water content decreases continuously at constant RH.It is particularly obvious at 90% and 80%RH.This phenomenon is observed at first up to date.In order to confirm the result, we performed the other experiment, in which the RH was adjusted by the nitrogen/vapor mixed gas.However, the water content keeps constant at the same RH.The previous work[14-15]has reported that magnesium acetate aerosol entered the gel state below 60% RH.Above 70% RH, the aerosols are liquid state.In usual, water transfer keeps synchronous with the RH in the liquid state.The present continuous water-loss at high RH suggested the hydrolysis of Mg(CH3COO)2droplets.Mg(CH3COO)2is a salt composed of a weak acid and a weak base.So it is in equilibrium with hydrated cations and dissociated acetate/acetic acid in aqueous solution.Because the decrease in RH was achieved by vapor off the droplets in present experiment, the acetic acid can be blown away by the vacuum pump.The hydrolysis reaction can cost water, which leads to decrease of water continuously.

Fig.2 (a) The RH and water area in the Mg(CH3COO)2aerosols with the time (b) The water and acetate content change at the RH of 90%, 80% and 70% terraces

In order to understand the water-loss in detail at high RHs, another experiment has been performed, where the RH changes very quickly by three pulses, shown in Figure 3(a).Because the magnesium acetate aerosols are liquid above 70% RH, water transfer should be synchronous with RH.However, it can be found the hysteresis in humidification than dehumidification process.While water content keeps constant at the highest RH after three pulsed RH cycles.The results show that there are some species on the surface of magnesium acetate aerosols produced in a pulse RH cycle, which limit the water transfer in a short time[16].When the RH is raised, the species can dissolve, so the water content can reach the original value after several pulsed RHs.In addition, it also can be demonstrated that the time scale of hydrolysis reaction is longer than that spent by a pulse RH cycle (10 seconds).

As a conclusion, the hygroscopicity of magnesium acetate aerosols has been studied with the various RH.The results show that the hydrolysis reaction of magnesium acetate takes place and acetic acid evaporates owing to the subpressure of sample cell in a slow RH change process.Because the time of hydrolysis reaction is longer than a pulse (10 seconds), water-loss cannot be found after a pulse-RH change process.And the short-timed species are formed on the surface of magnesium acetate droplets which led to the hysteresis of water-uptake than water-loss in the pulsed rise of RH.

Fig.3 (a) The changing trends of relative humidity and relevant water content along with experimental time and (b) the water content of pulse 1(black), pulse 2(red), and pulse 3(blue) as a function of RH of magnesium acetate

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*通訊聯(lián)系人

O657.3

A

利用不同的濕度調(diào)節(jié)方法研究醋酸鎂氣溶膠中水的傳質(zhì)過程

王 娜，龐樹峰*，張韞宏

北京理工大學(xué)化學(xué)學(xué)院，化學(xué)物理研究所，北京 100081

利用一種自制的濕度調(diào)節(jié)設(shè)備與真空紅外光譜儀相結(jié)合, 提出了一種研究醋酸鎂氣溶膠的吸濕性和傳質(zhì)動(dòng)力學(xué)的新方法。濕度調(diào)節(jié)裝置通過改變純水汽壓力來調(diào)節(jié)樣品室的相對(duì)濕度，這樣可以實(shí)現(xiàn)濕度以不同的速度發(fā)生變化。與紅外光譜手段相結(jié)合，濕度緩慢變化，讓氣溶膠時(shí)刻處于準(zhǔn)穩(wěn)態(tài)過程，可以研究氣溶膠在熱力學(xué)穩(wěn)態(tài)的吸濕性質(zhì)。濕度脈沖式改變，可以研究氣溶膠的動(dòng)態(tài)吸濕性質(zhì)以及傳質(zhì)動(dòng)力學(xué)過程。紅外光譜的掃描方式隨著濕度的改變速度進(jìn)行調(diào)節(jié)。由于實(shí)驗(yàn)中的相對(duì)濕度由純水氣提供，因此通過對(duì)紅外方光譜中純水氣的特征吸收峰面積的定量計(jì)算獲得與光譜同步的相對(duì)濕度值。研究發(fā)現(xiàn)，當(dāng)濕度穩(wěn)定在一個(gè)高濕度時(shí)，醋酸根和液態(tài)水的峰面積在持續(xù)下降。并且首次發(fā)現(xiàn)經(jīng)過一個(gè)準(zhǔn)穩(wěn)態(tài)的濕度循環(huán)后(1.05×104s)，在80%RH條件下，水峰面積由1.5降低至1.1。通過改變實(shí)驗(yàn)方案并對(duì)其結(jié)果進(jìn)行對(duì)比可知，在高濕度條件下，醋酸鎂發(fā)生水解，生成的醋酸由于樣品室內(nèi)的負(fù)壓而揮發(fā)，因此引起氣溶膠含水量下降。采用脈沖發(fā)式快速改變樣品室的濕度，一個(gè)濕度脈沖循環(huán)需要10 s。計(jì)算脈沖循環(huán)過程中氣溶膠的含水量發(fā)現(xiàn)，在濕度高于70%RH時(shí)，氣溶膠含水量沒有減少。但是加濕過程的水傳輸比去濕過程的水傳輸快。這些現(xiàn)象說明醋酸鎂的水解速率小于濕度的脈沖改變速率。在濕度快速變化的過程中，氣溶膠液滴表面難溶物的形成引起水傳輸發(fā)生受阻現(xiàn)象。

醋酸鎂氣溶膠； 水傳質(zhì)動(dòng)力學(xué)； 揮發(fā)

2015-02-04，

2015-06-11)

Foundation item：Supported by the NSFC (41175119, 21373026, 21473009)

10.3964/j.issn.1000-0593(2016)05-1581-04

Received：2015-02-04; accepted：2015-06-11

Biography：WANG Na, (1989—), female, graduate student of Beijing Institute of Technology e-mail: wangna.jz@163.com *Corresponding author e-mail: sfpang@bit.edu.cn