Evaporation and Subsequent Adsorption of Alcohol Molecules at

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Evaporation and Subsequent Adsorption of Alcohol Molecules at Aqueous Droplet Surface Observed by Cavity-Enhanced Raman Spectroscopy Yasuhito Kihara, Hiroya Asami, and Jun-ya Kohno* Department of Chemistry, Faculty of Science, Gakushuin University, 1-5-1 Mejiro, Toshima-ku, Tokyo 171-8588, Japan ABSTRACT: Mass transfer toward and across liquid surfaces is important for the interpretation of various interfacial phenomena, such as evaporation, adsorption, and mass accommodation, which have been investigated by the use of various methods. These studies, however, have focused on only one of the mass-transfer processes occurring at the surface. We investigate the surface concentration of alcohol molecules at aqueous droplet surfaces on the several-millisecond time scale using cavityenhanced droplet Raman spectroscopy. A decrease and subsequent increase of the alcohol concentration are observed in a set of measurements, which arise from an evaporation and subsequent adsorption of the alcohol molecules at the surface. This facilitates an understanding of the surface kinetics of molecules at the liquid surfaces.

1. INTRODUCTION A wide range of natural and industrial processes include a mass transfer toward and across liquid surfaces, such as an adsorption of molecules from the bulk solution to the surface,1 evaporation from the solution to the gas phase,2−5 and mass accommodation from the gas phase.6 The adsorption process emerges from a dynamic surface tension, which originates from a delayed adsorption of surfactants to freshly formed surfaces. The dynamic surface tension must be taken into account to interpret various processes, such as coating, foaming,1 and detergent processes.7 The evaporation of molecules from the solution surface plays important roles in various processes, such as spray humidification8 and combustion of liquid fuels.9 The cloud formation in the atmosphere includes all the dynamic mass-transfer processes of atmospheric aerosols.10,11 The mass-transfer processes have been investigated extensively. Ward and Tordai analyzed the dynamic surface tension by diffusive adsorption of molecules from the subsurface of the solution to the surface.12 On the basis of their model, the adsorption and the desorption rate constants have been determined for various surfactants from the dynamic surface tension measured by an oscillating jet technique, a bubble pressure method, and so on.1,13−15 The dynamic surface tension has also been measured by employing small droplets. A damping oscillation behavior of the droplet gives the surface tension as well as viscosity of the droplets simultaneously.10,16 As has been demonstrated, however, the adsorption rates have been measured from the mechanical motions of the solutions of interest. Direct observation of the amount of adsorbed molecules is more desirable to facilitate the investigation of the surface dynamics. The evaporation process has been investigated so far by employing micrometer-sized droplets.2−5,17−22 The droplets are particularly suitable for the evaporation study, because (1) the size of the droplets is measurable with nanometer accuracy by © XXXX American Chemical Society

application of a cavity-enhanced spectroscopy and hence, the amount of evaporated species can be monitored in real time, (2) Raman scattered light or fluorescence produced by irradiation of a laser onto a droplet obtains its intensity by positive interference inside the droplet and can be observed with a high sensitivity. Moreover, the Raman scattered light or the fluorescence propagates in the vicinity of the droplet surface, which allows identification and quantification of the molecules at the droplet surface. Reid and co-workers measured the evaporation processes of alcohol molecules from the droplet surfaces and revealed that the evaporation proceeds in a diffusive way along with a surface cooling arising from the heat of evaporation.2−5 However, these studies on the mass transfer at the liquid surface focus on only one aspect of the dynamic processes of the surface molecules, such as the adsorption, evaporation, or mass accommodation, which, in principle, proceed simultaneously. All the kinetics have to be taken into account to totally understand the mass transfer of the molecules at the liquid surface. We have so far been studying the dynamics of molecules at the droplet surface; isolation of biomolecules from the droplet in a vacuum,23−27 and morphological and chemical dynamics induced by the collision of two droplets.28−31 A novel Raman spectroscopy method using a droplet has been developed by us to observe a contour of the cavity-enhanced Raman spectra.32 In the present report, we have measured the cavity-enhanced droplet Raman spectra of alcohol solutions on a severalmillisecond time scale from the generation of the droplet. A decrease and subsequent increase of the alcohol concentration at the droplet surface is observed in a set of measurements, Received: February 9, 2017 Revised: March 24, 2017 Published: April 11, 2017 A

DOI: 10.1021/acs.jpcb.7b01277 J. Phys. Chem. B XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry B which is interpreted by an evaporative cooling of the droplet surface followed by an adsorption of alcohol molecules from the bulk solution to the surface.

CH/OH = =

2. EXPERIMENTAL SECTION A detailed description of the apparatus employed in this study has been reported previously.32 Here, we briefly describe the apparatus and the experimental procedures employed in the present study. Aqueous solutions of alcohols were produced in the atmosphere as droplets (∼80 μm in diameter), which were generated by a piezo-driven liquid-droplet nozzle (Microdrop Technologies GmbH, MD-K-130). A droplet was irradiated with the second harmonic of a Nd:YAG laser (Spectra-Physics, GCR-130) focused by a lens with a focal length of 100 mm down to a diameter of ∼0.4 mm at the droplet position. The diameter was measured from the burn pattern on photographic paper. The laser power was set to 6.0 mJ/pulse. Raman scattered light produced from the droplet was introduced to a Czerny-Turner spectrometer (Ritsu Oyo Kagaku, MC-10N) equipped with a CCD camera (Watec, WAT-902H2) through a couple of lenses and a color filter (HOYA, R59), which removed the Rayleigh scattered light. The Raman spectra were recorded by a shot-to-shot mode and averaged. The generation of droplets, laser firing, and the acquisition of the Raman spectrum were synchronized to TTL pulses produced by a delay generator (Stanford, DG645) with a repetition rate of 10 Hz. Employed samples were 8.5% v/v aqueous solutions of methanol, ethanol, and 2-propanol. The droplet nozzle was mounted on an XYZ stage. The XYZ stage had a large Z stroke (40 mm), so as to control the distance from the nozzle tip to the laser-irradiation position. We controlled the elapsed time from the droplet generation to the pulsed laser irradiation to the droplet (denoted as residence time in this report) by setting a time delay by the delay generator. Thus, we measured the time evolution of the droplet by Raman spectroscopy. The droplet position fluctuated by shot-to-shot because of a velocity jitter of the droplet and/or a small gas flow in the laboratory. The reproducibility of the CH/ OH ratio was confirmed through repeated experiments. However, the fluctuation disturbed the measurement at the longer residence time, because the positional fluctuation grows with the increase in the residence time.

⎛ dΓ Γ ⎞ Γ = k1Cs⎜1 − ⎟ − k2 dt Γ∞ ⎠ Γ∞ ⎝

ISRS(OH) = I0(OH)eGI0(OH)

(3)

(5)

∞

rate, on the other hand, is proportional to the amount of alcohol molecules at the surface, Γ . Γ∞

Eq 5 gives a solution where Γ exponentially approaches the kC Γ equilibrium value given by Γ = k C 1+s k . This solution does not ∞

1 s

2

explain the experimental results, where the amount of the alcohol molecules shows an initial decrease and subsequent increase with time. In the present analysis, we assume that the subsequent increase arises from (1) an exponential decrease of the evaporation rate constant, k2, with increase in the residence time, which results from a surface cooling and a backadsorption of alcohol molecules from the gas phase and (2) the adsorption of alcohol molecules from the bulk liquid, where the adsorption rate constant is independent of the residence time. These assumptions are examined in detail in Section 5.1. Hereafter, we denote the diffusive alcohol adsorption from the bulk liquid to the droplet surface as “adsorption”, which is distinguished from the “back-adsorption” representing the adsorption of alcohol molecules from the gas phase to the droplet. The above consideration is taken into account in the calculation as follows. We assume that the evaporation rate constant, k2, decreases exponentially with the residence time. Then, the surface excess of the alcohol molecules, Γ, follows the equation

where ISRS and I0 show the intensities of the stimulated and spontaneous Raman scattering at a wavelength, λ, respectively, and G is a gain parameter.33,34 Then, the stimulated Raman intensities of the CH and the OH bands are given as (2)

(4)

where k1, Γ, and Γ∞ refer to the rate constant of adsorption, the surface excess at time t and that at a saturation adsorption, respectively.13 Cs is the subsurface concentration, which is assumed to be identical to the bulk concentration (1.9 mol dm−3) in the present calculation. The first and second terms on the right side of the equation represent the rates of the adsorption and evaporation of alcohol molecules at the solution surface, where the rate constants are denoted as k1 and k2, respectively. The adsorption rate is proportional to a subsurface concentration, Cs, of the alcohol molecules as well as the Γ number of vacant sites at the surface, 1 − Γ . The evaporation

(1)

ISRS(CH) = I0(CH)eGI0(CH)

⎛ I (CH) ⎞⎞ I0(CH) ⎛ − 1⎟⎟⎟ exp⎜⎜GI0(OH)⎜ 0 I0(OH) ⎝ ⎝ I0(OH) ⎠⎠

The parameter I0(CH)/I0(OH) is linearly dependent on the concentration of the alcohol molecules at the droplet surface. Therefore, we can derive the surface concentration and its temporal profile from the analysis of experimentally obtained CH/OH values using eq 4 (see Section 5.2). 3.2. Model Calculation of Surface Concentration. We performed a model calculation of the surface concentration of the alcohol molecules based on the Langmuir adsorption kinetics, which is given as

3. THEORETICAL CALCULATION 3.1. Intensity of Stimulated Raman Scattering. The Raman spectra observed in this study result from a cavity enhancement in the droplet, as well as enhancement by a stimulated Raman effect. The stimulated Raman scattering gains its intensity as ISRS(λ) = I0(λ)eGI0(λ)

ISRS(CH) ISRS(OH)

⎛ dΓ Γ ⎞ Γ = k1Cs⎜1 − ⎟ − k 2e−κt dt Γ∞ ⎠ Γ∞ ⎝

(6)

The surface excess, Γ/Γ∞, is obtained as a function of the residence time by a numerical integration of eq 6 with a time

The stimulated Raman intensity ratio, CH/OH, obtained in the experiment is then calculated as B

DOI: 10.1021/acs.jpcb.7b01277 J. Phys. Chem. B XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry B step of 0.1 ms by using k1Cs/Γ∞, k2/Γ∞, and κ as the parameters. The variable Γ/Γ∞ is considered to be proportional to the spontaneous Raman intensity ratio, I0(CH)/I0(OH), because (1) I0(CH) shows the amount of CH bonds which corresponds to the alcohol concentration at the surface, and (2) division by I0(OH) calibrates the experimental fluctuation because the large excess of water molecules in the solutions gives I0(OH) regardless of the alcohol concentration. Then, we obtain I0(CH) Γ =a I0(OH) Γ∞

(7)

by setting the proportionality constant to be a. Substituting eq 7 into eq 4 and defining GI = GI0(OH), we obtain CH/OH = a

⎛ ⎛ Γ ⎞⎞ Γ exp⎜⎜GI ⎜a − 1⎟⎟⎟ Γ∞ ⎝ ⎝ Γ∞ ⎠⎠

(8)

Eqs 6 and 8 yield the calculated values of CH/OH as a function of time, where the input parameters are k1Cs/Γ∞, k2/ Γ∞, κ, a, and GI.

Figure 2. CH/OH ratios as a function of the residence time. The solutions were 8.5% v/v aqueous solutions of (a) methanol, (b) ethanol, and (c) 2-propanol. Red curves represent fitted results to the theoretical calculation.

4. RESULTS Figure 1 shows cavity-enhanced Raman spectra of 8.5% v/v aqueous solution of ethanol, where the residence time was set

methanol, ethanol, and 2-propanol. The CH/OH ratios are obtained from the average of 100 single-shot Raman spectra. The CH/OH ratios decrease to zero at 2.4−4 ms, but then increase above 6 ms with an increase of the residence time. The initial CH/OH values increase in the order of methanol, ethanol, and 2-propanol, which indicates that the CH/OH ratio relates to the concentration of the CH bonds in the solutions. The time dependences of the CH/OH ratio, however, were almost the same for the solutions. The decrease and subsequent increase of the CH/OH ratio were likely to correspond to the evaporation and the adsorption of the alcohol molecules from the bulk to the surface of the droplet.

5. DISCUSSION 5.1. Evaporation and Subsequent Adsorption of Alcohol Molecules to Surface. The cavity-enhanced Raman scattering originates from the surface of the droplet, because the enhancement results from an interference of the Raman scattered light propagating beneath the droplet surface by total reflection.32,35 Then, the Raman intensity ratio, CH/ OH, represents the amount of CH bonds at the droplet surface, where the evaporation and the adsorption of the alcohol molecules occur. The CH/OH ratio decreases to zero at a residence time of 2.4−4 ms, and increases again at 6−9 ms. This result indicates that the amount of alcohol molecules decreases and increases at the residence times of 2.4−4 and 6− 9 ms, respectively. The decrease and increase of the alcohol molecules originate from an evaporation of the alcohol molecules to the gas phase and the adsorption of the alcohol molecules from the bulk solution inside the droplet. The dynamics of the alcohol molecules at the droplet surface is given in Figure 3. The dynamics were considered as follows: (1) the nascent droplet is homogeneous for both the bulk and the surface, (2) alcohol molecules evaporate from the droplet surface, (3) the surface concentration, as well as temperature, decreases, (4) the alcohol molecules are adsorbed from the bulk to the surface, and (5) the surface concentration of the alcohol increases again. Here, we ignore heating of the droplet due to the surface energy of the nascent prolate droplet, which is

Figure 1. Cavity-enhanced Raman spectra of an 8.5% v/v aqueous solution of ethanol. The residence time was set to (a) 3, (b) 6, and (c) 8 ms.

to 3, 6, and 8 ms. The spectra were the average of 100 singleshot spectra. Two Raman bands in the spectra at 2986 and 3470 cm−1 were assignable to the CH and OH stretching vibrational modes, respectively. The band at 3280 cm−1 has been reported to be an artificial background,35 and was excluded from analysis in the present study. The CH and OH bands originated from ethanol and water molecules in the solution, respectively, because the cavity-enhanced Raman spectra of ethanol and pure water consist of only CH and OH bands, respectively.29 From these spectra, we calculated the ratio of the CH and OH band intensities (CH/OH ratio) as a measure of the ethanol concentration at the surface of the droplet. To evaluate the dynamics of the ethanol molecules at the droplet surface, we measured the CH/OH ratio as a function of the residence time of the droplet. Figure 2 shows the CH/OH ratios as a function of the residence time of the droplet composed of aqueous solutions of C

DOI: 10.1021/acs.jpcb.7b01277 J. Phys. Chem. B XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry B

parameters as the variables, we performed a least-squares fitting to the experimental results. The fitting results are shown in Figure 2 as red curves, which reproduce the experiments reasonably well. Figure 4 shows the calculated Γ/Γ∞ in comparison with the calculated and experimental CH/OH values as a function of the Figure 3. Schematic drawing of the dynamics of the alcohol molecules at the droplet surface, which consists of 5 stages: (1) the nascent droplet is homogeneous for both the bulk and the surface, (2) alcohol molecules evaporate from the droplet surface, (3) the surface concentration, as well as temperature, decreases, (4) the alcohol molecules are adsorbed from the bulk to the surface, and (5) the surface concentration of the alcohol increases again.

estimated to be ∼3 × 10−4 K for the prolate droplet with an aspect ratio of 3. The recurrence of the alcohol molecules at the droplet surface is explained below. The evaporation of the alcohol molecules from the droplet surface is followed by (1) cooling of the outermost surface region5 of the droplet by the heat of evaporation, (2) the back-adsorption of the evaporated alcohol molecules from the gas phase, and (3) the adsorption of alcohol molecules from the bulk liquid of the droplet caused by the concentration gradient at the droplet surface built by the evaporation. The cooling and the back-adsorption from gas phase decrease the evaporation rate of the alcohol molecules from the droplet surface. The back-adsorption rate is estimated to be ∼3% of the evaporation from a mass-accommodation coefficient of ethanol at room temperature.36 On the other hand, the rate of the alcohol adsorption from the bulk liquid is likely to be independent of the surface cooling, because the evaporation and cooling proceed not in the bulk droplet2 but on the outermost layer of the droplet surface.5,37 Here, we observe the droplet surface in the depth of ∼1 μm by the cavity enhancement condition of the droplet, which exceeds the depth of the concentration gradient built by the evaporation.5 5.2. Analysis Based on Model Calculation. We confirmed the discussion of Section 5.1 in a model calculation. In the calculation, we assumed that the evaporation rate decreases exponentially with increase in the residence time. This assumption can be supported by literature data given by Reid and co-workers, who reported temporal evolution of ethanol concentration at the surface of an aqueous droplet under a reduced pressure, 7−100 kPa.5 When the adsorption of ethanol to the surface is ignored, eq 6 becomes

dΓ Γ = −k 2e−κt dt Γ∞

Figure 4. Calculated Γ/Γ∞ (blue curve) in comparison with the calculated (red curve) and experimental (blue plots) CH/OH values as a function of the residence time. Panel b is the enlarged view of Panel a. The pink curves represent the results calculated by changing by ±2% of the parameter, a (see text).

residence time for the ethanol aqueous droplet. Panel b is an enlargement of panel a. The result indicates that the surface concentration of alcohol decreases by ∼66% of the initial value at the residence time of 5.6 ms and returns to 80% at 10 ms. The errors of the fitting parameters are evaluated by calculation of the sum of squares deviation with changing one of the parameters around the fitted value, where the other parameters are set to the fitted values. We set the errors of the parameters when the parameter differentiates 10% of the sum of the squares deviation from the fitted value. Pink curves in Figure 4 show the results calculated by changing the parameter, a, where the a values differs by ±2% of the original value. Obtained parameters by the least-squares fitting and their errors are summarized in Table 1. Bleys and Joo reported the values of k1 and Γ∞ of propanol as 3 × 10−6 m s−1 and 6 × 10−6 mol m−2, respectively, for propanol. By use of these values, the obtained parameter for 2-propanol by our calculation, k1Cs/Γ∞ = 49 s−1, gives Cs as 0.098 mol dm−3, which is ∼5% of the bulk concentration. Ward and Tordai reported that the initial

(9)

which is integrated as ln

k Γ = 2 (e−κt − 1) Γ0 Γ∞κ

(10)

Table 1. Obtained Parameters by the Least-Square Fitting and Their Errors

This equation reproduces the result under pressure of 100 kPa in the literature5 when k2/Γ∞ and κ were set to 150 and 500 s−1, respectively. Then, the time evolution of the Raman intensity ratio, CH/ OH, is calculated from eqs 6 and 8 by taking k1Cs/Γ∞, k2/Γ∞, κ, a, and GI as the input parameters. Among them, the stimulated Raman gain factor, GI, is set to 16.4, which is reported by Reid and co-workers.34 Taking the other four

k1Cs/Γ∞/s−1 k2/Γ∞/s−1 κ/102 s−1 a D

MeOH

EtOH

2-PrOH

85 ± 10 139 ± 7 1.5 ± 0.2 1.01 ± 0.02

43 ± 10 113 ± 9 2.9 ± 0.3 1.08 ± 0.02

49 ± 8 121 ± 4 2.2 ± 0.2 1.14 ± 0.01

DOI: 10.1021/acs.jpcb.7b01277 J. Phys. Chem. B XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry B subsurface concentration has a small value,12 which supports the present calculation result. The parameters, k2/Γ∞ and κ, represent the initial evaporation rate of the alcohol molecules and its decrease rate, respectively. As described before, the parameters, k2/Γ∞ and κ, are determined to be 150 and 500 s−1, respectively, for the aqueous ethanol droplet under a 100 kPa pressure by our calculation based on the literature results.5 The obtained results of k2/Γ∞ in our analysis are in good agreement with the literature value, 150 s−1. On the other hand, obtained κ values are less than the literature value, 500 s−1. These results show that the initial evaporation rate is almost the same regardless of the kind of alcohol molecules and of the ambient pressure, whereas the temporal decrease of the evaporation rate is less under atmospheric pressure than that under the reduced pressure. This is probably because the back-adsorption of the evaporated alcohol molecules is more significant in the atmospheric pressure than in the reduced pressure. In addition, methanol has a larger evaporation ate than ethanol and 2propanol, which is to do with the larger vapor pressure of methanol. The a value shows the spontaneous Raman intensity ratio of CH and OH bonds with respect to the surface excess, Γ/Γ∞ (see eq 7). The a values are almost the same for all the alcohol molecules. Since the alcohol concentration is small, the Raman scattering intensity of the OH band is considered to be common to all the alcohol solutions. On the other hand, methanol has less Raman cross section than the others,38 which indicates that methanol has less value of Γ/Γ∞. The less Γ/Γ∞ value for methanol is conceivable because the solubility of methanol is more than the others.

(4) Hopkins, R. J.; Howle, C. R.; Reid, J. P. Measuring Temperature Gradients in Evaporating Multicomponent Alcohol/water Droplets. Phys. Chem. Chem. Phys. 2006, 8, 2879−2888. (5) Homer, C. J.; Jiang, X.; Ward, T. L.; Brinker, C. J.; Reid, J. P. Measurements and Simulations of the near-Surface Composition of Evaporating Ethanol-Water Droplets. Phys. Chem. Chem. Phys. 2009, 11, 7780−7791. (6) Davidovits, P.; Kolb, C. E.; Williams, L. R.; Jayne, J. T.; Worsnop, D. R. Update 1 of: Mass Accommodation and Chemical Reactions at Gas−Liquid Interfaces. Chem. Rev. 2011, 111, PR76−PR109. (7) Carter, D. L.; Draper, M. C.; Peterson, R. N.; Shah, D. O. Importance of Dynamic Surface Tension to the Residual Water Content of Fabrics. Langmuir 2005, 21, 10106−10111. (8) Yule, A. J.; Al-Suleimani, Y. On Droplet Formation from Capillary Waves on a Vibrating Surface. Proc. R. Soc. London, Ser. A 2000, 456 (1997), 1069−1085. (9) Lavieille, P.; Lemoine, F.; Lavergne, G.; Virepinte, J. F.; Lebouché, M. Temperature Measurements on Droplets in Monodisperse Stream Using Laser-Induced Fluorescence. Exp. Fluids 2000, 29, 429−437. (10) Bzdek, B. R.; Power, R. M.; Simpson, S. H.; Reid, J. P.; Royall, C. P. Precise, Contactless Measurements of the Surface Tension of Picolitre Aerosol Droplets. Chem. Sci. 2016, 7, 274−285. (11) Nozière, B.; Baduel, C.; Jaffrezo, J.-L. The Dynamic Surface Tension of Atmospheric Aerosol Surfactants Reveals New Aspects of Cloud Activation. Nat. Commun. 2014, 5, 3335. (12) Ward, A. F. H.; Tordai, L. Time-Dependence of Boundary Tensions of Solutions I. The Role of Diffusion in Time-Effects. J. Chem. Phys. 1946, 14, 453−461. (13) Bleys, G.; Joos, P. Adsorption Kinetics of Bolaform Surfactants at the Air/water Interface. J. Phys. Chem. 1985, 89, 1027−1032. (14) Joos, P.; Bleys, G.; Petre, G. Adsorption Kinetics of Nonanediol and Nonane Dicarbonic Acid at the Air-Water Interface. J. Chem. Phys. 1982, 79, 387−393. (15) Prpich, A. M.; Elias Biswas, M.; Chen, P. Adsorption Kinetics of Aqueous N-Alcohols: A New Kinetic Equation for Surfactant Transfer. J. Phys. Chem. C 2008, 112, 2522−2528. (16) Yamada, T.; Sakai, K. Observation of Collision and Oscillation of Microdroplets with Extremely Large Shear Deformation. Phys. Fluids 2012, 24, 022103. (17) Duffey, K. C.; Shih, O.; Wong, N. L.; Drisdell, W. S.; Saykally, R. J.; Cohen, R. C. Evaporation Kinetics of Aqueous Acetic Acid Droplets: Effects of Soluble Organic Aerosol Components on the Mechanism of Water Evaporation. Phys. Chem. Chem. Phys. 2013, 15, 11634−11639. (18) Mitchem, L.; Buajarern, J.; Hopkins, R. J.; Ward, A. D.; Gilham, R. J. J.; Johnston, R. L.; Reid, J. P. Spectroscopy of Growing and Evaporating Water Droplets: Exploring the Variation in Equilibrium Droplet Size with Relative Humidity. J. Phys. Chem. A 2006, 110, 8116−8125. (19) Newbold, F. R.; Amundson, N. R. A Model for Evaporation of a Multicomponent Droplet. AIChE J. 1973, 19, 22−30. (20) Popp, J.; Lankers, M.; Schaschek, K.; Kiefer, W.; Hodges, J. T. Observation of Sudden Temperature Jumps in Optically Levitated Microdroplets due to Morphology-Dependent Input Resonances. Appl. Opt. 1995, 34, 2380−2386. (21) Heinisch, C.; Wills, J. B.; Reid, J. P.; Tschudi, T.; Tropea, C. Temperature Measurement of Single Evaporating Water Droplets in a Nitrogen Flow Using Spontaneous Raman Scattering. Phys. Chem. Chem. Phys. 2009, 11, 9720−9728. (22) Schaschek, K.; Popp, J.; Kiefer, W. Morphology Dependent Resonances in Raman Spectra of Optically Levitated Microparticles: Determination of Radius and Evaporation Rate of Single Glycerol/ Water Droplets by Means of Internal Mode Assignment. Berichte der Bunsengesellschaft für Phys. Chemie 1993, 97, 1007−1011. (23) Kohno, J.-Y.; Toyama, N.; Kondow, T. Ion Formation to the Gas Phase by Laser Ablation on a Droplet Beam. Chem. Phys. Lett. 2006, 420, 146−150.

6. CONCLUSIONS We report the evaporation and subsequent adsorption of alcohol molecules at the surface of aqueous droplets in a set of experiments with a cavity-enhanced droplet Raman spectroscopy. Model analysis of the results gives a temporal profile of the surface concentration of the alcohol molecules. This provides a general method to observe a total measurement of the mass-transfer processes across the liquid surface, which facilitates the analysis of atmospheric aerosols.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +81-3-3986-0221; Fax: +81-3-5992-1029; E-mail: jun-ya. [email protected]. ORCID

Jun-ya Kohno: 0000-0001-6239-9289 Notes

The authors declare no competing financial interest.

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REFERENCES

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DOI: 10.1021/acs.jpcb.7b01277 J. Phys. Chem. B XXXX, XXX, XXX−XXX