Binary-Phase Acetonitrile and Water Aerosols: Infrared Studies and

Article Cite This: ACS Earth Space Chem. XXXX, XXX, XXX−XXX

Binary-Phase Acetonitrile and Water Aerosols: Infrared Studies and Theoretical Simulation at Titan Atmosphere Conditions Rebecca Auchettl,† Mahmut Ruzi,† Dominique R. T. Appadoo,‡ Evan G. Robertson,† and Courtney Ennis*,† †

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Department of Chemistry and Physics, La Trobe Institute for Molecular Science, La Trobe University, Melbourne, Victoria 3086, Australia ‡ THz/Far-infrared Beamline, ANSTO Australian Synchrotron, 800 Blackburn Road, Clayton, Victoria 3148, Australia S Supporting Information *

ABSTRACT: Acetonitrile (CH3CN) and water (H2O) ice particles were generated within a collisional cooling cell coupled to the Australian Synchrotron light source. The evolution of the aerosols was tracked by infrared spectroscopy compiled over the 4000−50 cm−1 region. Gas pressure and temperature conditions were varied to replicate the lower altitudes of the Titan atmosphere allowing for comparison to far-infrared features detected by the Cassini−Huygens spacecraft. The experimental spectra show that CH3CN and H2O particles are microheterogeneous in composition and spherical in shape. CH3CN lattice bands display temperaturedependent shifts in frequency, implying that pure β-phase is present in the mixed particles. In addition, a red shift identified for the CN fundamental stretching mode indicates dipole−dipole and π-electron side-directed hydrogen bond coupling between segregated CH3CN and H2O phases exclusively at the grain interface. Discrete dipole approximation theory was implemented to evaluate various cluster architectures where segregated domains of pure CH3CN and H2O ices provided the best fit to experiment; confirming the infrared findings. Otherwise, simulations of competing architectures, such as core−shell and cubic shaped particles, did not provide convincing comparison to the aerosol spectra. We conclude that the far-infrared profiles for mixed CH3CN−H2O systems do not present as likely carriers for the unassigned 220 cm−1 “haystack” feature that has been identified in Titan’s lower atmosphere. KEYWORDS: planetary atmospheres, Titan, infrared spectroscopy, molecular astrochemistry, aerosol microphysics, lattice modes

1. INTRODUCTION Saturn’s largest moon Titan possesses a reducing atmosphere perhaps chemically analogous to that of the prebiotic Earth.1 Despite its distant location in the Solar system the Titan atmosphere harbors a rich chemical inventory of organic species, as shown by the NASA Voyager probes2−4 and the more recent Cassini spacecraft.5−7 These missions have confirmed that Titan is home to numerous nitrile species.8 The most abundant of these compounds, hydrogen cyanide (HCN), is formed via the UV photolysis and fast particle irradiation of Titan’s primary methane (CH4) and molecular nitrogen (N2) in the upper atmosphere.7,9,10 Further photochemistry involving HCN and hydrocarbon species generate abundances of secondary nitriles that include acetonitrile (CH3CN) and propionitrile (CH3CH2CN). It follows that secondary nitriles and hydrocarbons settle to lower altitudes where they sequentially condense, coalesce, and agglomerate to form fractal aerosols to micron dimensions.11,12 Accumulating as optically thick cloud layers, these icy particles precipitate to the surface where they deposit their chemical payload, which may include complex organics. © XXXX American Chemical Society

With over a decade in Saturn orbit, the Cassini mission heralded a new age of investigation into the chemical composition of Titan’s haze. The onboard composite infrared spectrometer (CIRS) detected several far-infrared (far-IR) bands attributed to cyanide-type molecules: including HCN (166 cm−1)13 and C4N2 (478 cm−1).14,15 However, there are a number of prominent far-IR bands that have remained unassigned, such as the broad haze feature centered at 220 cm−1.16 These signals have been observed to change intensity with the seasons, first observed in Titan’s northern winter (2007) but disappearing by mid-Spring (2016),17,18 suggesting the destruction of precursor by increased insolation. However, nitriles such as CH3CN are seen to resist photochemical destruction (Titan atmospheric lifetime of 7.7 years19) and accumulate at lower, polar altitudes. Here, nitriles are thought to integrate with aerosol material. Received: May 7, 2018 Revised: June 26, 2018 Accepted: June 27, 2018

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DOI: 10.1021/acsearthspacechem.8b00059 ACS Earth Space Chem. XXXX, XXX, XXX−XXX

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ACS Earth and Space Chemistry In surveying potential molecular carriers for the “haystack” features, previous studies by our workgroup have focused on the IR signatures of pure CH3CN and CH3CH2CN aerosols under simulated Titan conditions.20 Having ruled out these nitriles as candidate pure ice carriers, in this article we turn our attention to binary-phase CH3CN and water (H2O) aerosols. Titan’s atmosphere is strongly reducing and mostly devoid of oxygen-bearing gases, with carbon monoxide being the most enriched at molar ratio x(CO) = 5 × 10−5.21 Although mostly bound to the surface (94 K), solid H2O ice is continuously supplied from the icy environments surrounding Titan orbit, such as the H2O ejected from the surface vents of Enceladus.22 Although, to this point crystalline H2O has not been detected by its prominent far-IR lattice bands at 230 and 160 cm−1.23 Formed in situ, gas-phase CH3CN has been confirmed in the Titan stratosphere by ground based observations,24−26 albeit at abundances insufficient for homogeneous nucleation (x(CH3CN) = 2.0 × 10−9).25 However, like most nonvolatile species entering Titan’s lower stratosphere (100−50 km), the relative humidities for H2O and CH3CN reach 100% to trigger their condensation. Additionally, these “cold trap” conditions could allow CH3CN to cocrystallize with other nitriles and compatible polar species, or condense on the surface of descending particles containing H2O ice. Atmospheric aerosols alter the radiation balance of an atmosphere by the scattering and absorption of solar radiation as well as by acting as cloud condensation nuclei, in turn affecting interconnected physicochemical properties of the atmosphere. Previous work by our group and affiliates27−30 have shown that temperature and pressure conditions influence the nucleation kinetics and morphology of laboratory aerosols. The particle morphology (phase, size, shape, and architecture) all dictate aerosol scattering and absorption cross sections. Here, the particle size distribution has a major influence as aerosols less than 100 nm diameter (dp) have negligible contribution toward the scattering of solar radiation. This is opposed to aerosols of the dp = 100−2500 nm regime that scatter UV photons through to IR.31 Studies at the Australian Synchrotron have also demonstrated that changes in ice morphology influence both aerosol formation rates and their final size distribution.20,23 It has also been shown that the farIR features of amorphous phases formed by rapid condensation can split and narrow upon warming. Finally, extinction spectra have been modeled by Mie theory to track particle size distributions over aerosol lifetimes.29,32,33 Several studies have employed specialized gas cells to mimic terrestrial and planetary atmospheres, where production of pure-phase aerosols are simulated by collisional cooling in a cold bath gas (e.g., CH3CN/CH3CH2CN,20 H2O/D2O,23,29,30 HCl34). In replicating the Titan stratospheric conditions (T = 90−150 K and P = 10−1−102 mbar) between 50 and 250 kms,2 we can constrain the chemical composition, particle size distribution, and phase of aerosols observed in Titan’s lower atmosphere. In this work, we present the far-IR spectra of binary-phase CH3CN and H2O particles as possible contributors to Titan’s aerosol body. Both pure-phase CH3CN and H2O ices show prominent far-IR lattice features near 220 and 100 cm−1 with profiles that could be altered by strong intermolecular coupling between the species in the condensedphase (note polar CH3CN and H2O molecules are highly miscible in the liquid phase35). This work also explores the morphology of binary-phase CH3CN−H2O aerosols under replicated Titan conditions by employing a variety of

experimental deposition methods. Results are presented across four sections. First, in Section 3.1, the IR spectra for pure H2O and pure CH3CN, aerosols are presented. Second, in Section 3.2, we investigate the binary aerosol spectra frequency shifts and the interactions at the grain interface. Third, in Section 3.3, we confirm the formation of the π-electron side-directed coupling via investigation of the dangling OH feature. Finally, in Section 3.4, we examine the temporal evolution and condensation kinetics of the binary-phase aerosol populations through IR analysis.

2. EXPERIMENTAL SECTION 2.1. Setup. All experiments were performed in an Enclosive Flow Cooling (EFC) cell installed at the THz/Far-IR beamline at the Australian Synchrotron. This setup has been previously described,23 and only experimental details exclusive to the present work are discussed below. The setup has been prepared to generate binary-phase CH3CN−H2O aerosols under conditions simulating the Titan atmosphere. A qualitative and quantitative approach consistent with that reported by Signorell and co-workers28,36−38 was adopted herein, where IR spectra (including far-IR region) were collected to track the formation and depletion of the binaryphase CH3CN−H2O aerosol population. The EFC cell was filled with a N2 gas pressure of 5, 20, or 100 mbar, aligning with Titan pressures between ca. 50 and 150 km altitude.39 Temperatures were then set between 95 and 130 K corresponding to Titan’s stratospheric profile; maintained by a liquid-N2 bath and heating elements positioned along the external walls of the cell and monitored by k-type thermocouples. Here, a cell temperature of 100 K coincides with the temperature of the Titan nitrile haze layer.40 A 5 to 10 min rest period between experiments ensured temperatures and gas convection had settled before sample injection. Background IR spectra were recorded at this point. Acetonitrile (Sigma-Aldrich, 99.8%) was treated to freeze− pump−thaw cycles to remove dissolved volatiles, while H2O was collected from a Milli-Q source to a purity of 18.2 MΩ cm with each sample stored in separate volumes. A vapor pressure equivalent of each gas was collected in dry N2 (10−20 psi) before injection to the EFC cell via individual solenoid valves. Binary CH3CN−H2O aerosols were formed under three injection schemes (i) simultaneous injection (CH3CN and H2O), (ii) sequential injection with CH3CN first deposited (CH3CN → H2O), and (iii) sequential injection with H2O first deposited (H2O → CH3CN). Simultaneous injection of CH3CN and H2O (with no delay between pulses) aimed to form mixed CH3CN−H2O aerosols (similar to premixed D2O/ H2O aerosols generated by Wong et al.30). The sequential injection studies were expected to generate core−shell architectures, with the delay between injection pulses set at 0.5 s. The volume mixing ratio (VMR) of CH3CN in the predominantly N2 (98%) Titan atmosphere (denoted as x(CH3CN)) has been measured at around 2 × 10−9 by Marten et al.25 The VMR of H2O has been difficult to constrain, with over an order-of-magnitude variation between measurements obtained by Cassini CIRS (1.4 × 10−10) and Herschel PACS instruments (0.023 × 10−10).41−43 We therefore expect the enriched experimental mixing ratios for CH3CN and H2O diluted in N2 to be more comparable to concentrations observed in Titan’s lower stratosphere, where number densities are thought sufficiently high for saturation and subsequent nucleation of aerosol particles. B


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ACS Earth and Space Chemistry White-type, multipass optics within the EFC cell were set to the shortest 2.4 m path length (four passes) allowing maximum light transmittance through the cell. Coupled to a Bruker IFS125HR spectrometer, the mid-IR experiments (spanning the 4000−600 cm−1 region) used the internal Globar source, a potassium bromide (KBr) beamsplitter and windows, and a mercury cadmium telluride midband (MCTm) detector. For the far-IR measurements (600−50 cm−1), the synchrotron edge-radiation source, a 6 μm Mylar multilayer beamsplitter, polyethylene windows, and a silicon bolometer were employed. The collection procedure commenced with injection of the samples, where a small delay in timing ( 11 nm) having been previously formed at temperatures above 90 K using the same EFC setup.23 Therefore, we suggest that the H2O ice is most likely the Ic phase. Note this is also the

kinetically favored phase of H2O56 as well as the form that is observed to possess the higher nucleation rate.32 The SI (Figures S-1−S-3) contains simulated IR spectra retrieved from H2O Ic aerosol DDA calculations at 100 K, with a particle sizes ranging from 10 to 1000 nm. The OH stretching band profile for the theoretical H2O Ic particles with dp = 20−30 nm closely matches both the profile of the experiment spectra and our estimation of the H2O particle size from the integrated band intensities of the dOH band (see Table S-3). Together, this supports the assignment Ic H2O spherical aerosols of 20−30 nm for the pure H2O experiments. Figure 2b depicts the far-IR spectra for pure CH3CN aerosols at 95 and 130 K. The lowest intramolecular (ν8 CCN bend) mode can be identified at 392 cm−1 showing some evidence of peak splitting typical of molecular crystals (Table S-2). The β-phase CH3CN crystalline solid can then be verified by observing the translational (νLT) and libration (νLXY) lattice modes, located at 125 and 90 cm−1 respectively.57 Although α-CH3CN is the most thermodynamically stable phase under our EFC cell conditions, rapid condensation generates β-phase CH3CN crystals as seen previously for supercooled aerosol20 and thin film58 experiments. Similarly, high pressure Raman studies of CH3CN−H2O systems by Chen et al.59 showed that the α-CH3CN is not identified, rather formation of the β-phase solid. In our previous pure CH3CN aerosol studies,20 the scattering contribution toward IR extinction was measured to give an indication of the particle size distribution. In this publication, we used Mie scattering to calculate the size of the aerosols produced. We found the CH3CN particle size to be dp = 65−120 nm under conditions comparable to the present experiments. To confirm this size regime, we used DDA to investigate amorphous and crystalline CH3CN aerosol spectral profiles for different dp and temperatures (assuming a spherical shape), but found that in the dp range 10 to 1000 nm there is D


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Figure 3. Experimental mid-IR (3800−2600 cm−1) spectra for (a) pure H2O, (b) pure CH3CN, (c) 1:3 CH3CN−H2O summation, and (d) H2Oinjected first, (e) CH3CN injected first, and (f) simultaneously deposited CH3CN−H2O aerosols. Spectra recorded at 90 (black) and 130 K (red). The dOH, linear, and side-directed binding regions are included as inset for H2O-bearing architectures with normalization to the OH stretch feature.

but rather as (i) individual homogeneous aerosol particles or as (ii) pure and segregated crystalline phases (grains) in the same particle, where the only interaction between the species occurs at the boundary interface. See Figure S-4 for a schematic of these forms. The most pronounced disparity between band prof iles of the pure-phase and codeposited studies concern the OH[3(W)] feature. In the pure H2O spectra (Figure 3a), the band has a width of 200 cm−1 at 130 K, which increases to 220 and 230 cm−1 for the two sequential depositions (Figure 3d,e). There is no significant width increase observed for the simultaneous deposition in Figure 3f. This broadening can be attributed to the difference in H2O particle size between experiments. Previous work by Medcraft et al.29 showed that smaller, more amorphous H2O particles result in broader OH stretch features. At the other end of the size scale, DDA calculations predict a particle size dependence for the relative contributions of the [4(SH)], [3(W)], and [2(SL)] components toward the OH stretching band (Figure S-2) that manifests at larger sizes (dp ≫ 100 nm) and results in band shapes different from those observed here. The symmetric OH[3(W)] band profile of the experimental aerosol, when compared to the DDA results, provides further evidence for smaller H2O particles of the order of dp = 20−30 nm. Spectra showing the mid-IR ν2 CN stretching band located at 2250 cm−1 for β-CH3CN and its H2O binary aerosols is provided in Figure 4 (deconvoluted) and Figure S-5 (raw data). Most evident in the 130 K x(CH3CN) = 0.8 binary ice shown in Figure 4, a pronounced satellite peak is observed toward the red edge at 2245 cm−1. This peak likely indicates that a portion of the CH3CN component interacts with H2O via side-directed π-type hydrogen bonding, as a similar satellite peak is computed for the (H2O)2−(CH3CN)2 cluster.61 Other studies have assigned the CH3CN···HOH complex by way of a red-shifted CN satellite band arising from strong dipole−

no particle size dependence on the CH3CN band profiles across both the mid- and far-IR regions. 3.2. Binary Aerosol Frequency Shifts: Interactions at the Grain Boundary. Summation spectra derived from the 1:3 superposition of CH3CN and H2O profiles are shown in Figure 2c across the far-IR region and Figure 3c for the OHstretching region. These spectra represent an unphysical system devoid of intermolecular interactions between the codeposited CH3CN and H2O molecules. Therefore, through subtraction, we use summation spectra to identify spectral features that arise due to intermolecular interactions in codeposited systems. Shifts in frequency or alteration of band profile may imply an interface, if not complete mixing, between two species in the bulk ice. The mid-IR spectra of sequentially deposited (Figure 3d) H2O-first and (3e) CH3CN-first binary-phase CH3CN−H2O aerosols are compared to the summation spectra. Here, a mixing ratio of 1:3 (or x(CH 3 CN) = 0.3) is assumed for all CH 3 CN−H2 O codeposited experiments. This mixing ratio is derived by integration of the ν2 CN stretch peak for CH3CN and the OH stretch peak for H2O and applying literature absorption coefficients (A-values) obtained from Dello Russo and Khanna58 and Gerakines et al.60 for the respective bands and obtain number densities for each component. Shifts in band position (both +ve:, blue-shift; −ve, red-shift) arising from the codeposition of CH3CN and H2O vary with cell temperature and vapor deposition order. However, the frequency shifts follow no discernible trend and are generally less than ±11 cm−1 in magnitude. We note the largest shift is recorded for the H2O [4(SH)] band at 130 K, which displays δν = +9, + 3, and +11 cm−1 shifts from the pure ice position for each of the three codeposition experiments (Table S-4). These shifts correspond to the alterations of the pure H2O hydrogen bonding network. This infers that CH3CN and H2O are not condensed as a mixed amorphous or cocrystalline solid, E


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that are bound by hydrogen bonding at the grain interface.63 However, note that most intensity is still observed at the ν2 position at 2250 cm−1, indicating that CH3CN predominantly remains in the pure β-phase. Figure 4d also presents binary aerosols with a lower CH3CN mole fraction of x(CH3CN) = 0.3, confirming a significant decrease of the satellite band contribution to the overall feature as the concentration of the H2O increases. This supports the findings of Takamuku et al.63 3.3. Dangling OH Feature: Formation of Linear and Side-Directed CH3CN−H2O Complexes. The Figure 3a inset highlights the region associated with the dOH stretch feature, identified at 3691 cm−1 in the pure H2O experiment. This feature originates from surface hydroxyl groups oriented away from the bulk ice (i.e., those groups not involved in hydrogen bonding), which are therefore free to oscillate at a higher frequency than the bulk ice.64 Figure 3b shows the CH3CN mid-IR absorption bands that overlap with the rededge of the H2O stretching feature. Here, peaks for the ν5 CH3 asymmetric and the ν1 CH3 symmetric stretching modes are observed at 3000 and 2940 cm−1, respectively, as well as the weak ν2 + ν4 combination band at 3162 cm−1. However, the CH3CN peaks are dominated by the profile of the composite OH stretch feature (Figure 3c). Binary-phase experiments (Figure 3d−f) show that two new features are identified in the vicinity of the dOH peak; red-shifted −34 cm−1 (3657 cm−1) and −74 cm−1 (3618 cm−1) for all binary experiments. Redshifts of 10−70 cm−1 are associated with weakly adsorbed species such as N2, O2, CO, O3, and C2H4.65 Previous studies66,67 have shown that a shift of ca. −74 cm−1 magnitude can be attributed to a linear CH3CN···HOH complex (see

Figure 4. Experimental mid-IR spectra (2280−2220 cm−1) for the CN stretch region of (a) pure CH3CN, (b) x(CH3CN) = 0.8 mole fraction at 4 cm−1 resolution, (c) x(CH3CN) = 0.8 mole fraction at 1 cm−1 resolution, and (d) x(CH3CN) = 0.3 mole fraction of CH3CNfirst aerosols. The experimental curve is in black, the curve fit of the main band is in blue, and the curve fit of the satellite peak is in red.

dipole interactions (Figure S-6e).62,63 The appearance of the CN satellite peak supports our preferred architecture consisting of microheterogeneous CH3CN and H2O clusters

Figure 5. Temporal far-IR spectra (500−50 cm−1) for (a) H2O-injected first, (b) CH3CN injected first, and (c) simultaneously deposited CH3CN−H2O aerosols at 100 K (black) and 130 K (red) at 4 cm−1 resolution. F


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ACS Earth and Space Chemistry Figure S-6b), while a ca. −34 cm−1 shift is likely derived from side-directed π-type CH3CN interactions (see Figure S-6c). This observation reaffirms that a portion of the H2O component interacts with CH3CN via hydrogen bonding in all binary-phase systems. The binding energies of the linear σtype and side-directed π-type CH3CN−(H2O)1 complexes are predicted to be similar, e.g., 22.2 and 19.1 kJ mol−1 at MP2/ aug-cc-pVDZ level, respectively.68 This indicates that both types of interactions are feasible, through at the surface of CH3CN particles, the linear-type interaction site may be less sterically accessible. Hujo et al. showed that the dangling OH band is enhanced by a factor of 2 to 3 with N2 adsorbed on a H2O surface.69 In principle, the size of water particles in the ACN mixtures could be extracted from the summed intensities of the perturbed dangling OH bands, but this would require specific oscillator strengths associated with absorbed CH3CN to be determined, which is beyond the scope of this study. 3.4. Temporal Evolution: CH3CN and H2O Condensation Kinetics. For the segregated CH3CN−H2O cluster model favored by our previous observations, it is expected that the IR band profiles would follow the nucleation and growth dynamics of the individual components over time. This hypothesis is based on the different freezing points of the two components, where the more polar H2O species will condense to form solid particles over a shorter time scale compared to CH3CN. Therefore, we next consider temporal IR studies for validation of the segregated cluster model by inspecting both fundamental and lattice band profiles for the pure-phase and codeposited binary particles. Over the observed lifetime of the aerosol population within the EFC cell (∼120 s), no new absorption features or significant shifts in fundamental peak position were detected in the temporal mid-IR spectra (Figure S-7). However, a continual decrease in absorption intensity over time coincides with the aerosol population being depleted by diffusion (and sticking) to the cold walls of the cell. The decay of IR band intensity is more evident at higher temperature (130 K) due to a higher diffusion rates under uniform EFC cell pressure. Figure 5 depicts the far-IR spectra for binary-phase aerosols at 100 and 130 K, formed by our three deposition methods. With a higher freezing temperature, the νTO H2O feature rapidly reaches a maximum absorbance within one second following injection for all deposition and temperature schemes. The H2O lattice bands appear unaffected by cell temperature, producing similar band profiles at both 100 and 130 K. In comparison, there is generally a time delay of 1−2 s before CH3CN reaches its absorption maximum. Also note that CH3CN ice, particularly in the (5b) CH3CN → H2O and (5c) CH3CN and H2O experiments, displays a more prominent νLXY band at 130 K; approaching the intensity of the H2O νTO band. This could indicate a delayed organization of CH3CN to the β-phase solid at 100 K, as preformed H2O particles disrupt the condensation of crystalline CH3CN through strong dipole coupling. However, at 130 K, the CH3CN aerosols more rapidly form the pure phase solid. The order of sequential CH3CN and H2O deposition (5a and 5b) has little bearing on the relative intensities of the two components due to the similar microstructures expected for the segregated clusters. In both sets of spectra, H2O is the dominant carrier by way of its 230 cm−1 νTO band, while the νLXY feature for CH3CN appears much weaker (and also significantly blended with the weaker νLA lattice mode of H2O).

4. CONCLUSION The trace Titan gases CH3CN and H2O are both expected to play some role in aerosol formation in Titan’s atmosphere where they may cocrystallize as binary-phase ice at lower altitudes. Despite no direct observations, the photoproduct CH3CN would be expected to condense at similar altitudes to its nitrile relatives and polar species such as H2O. This work presents the first far- and mid-IR spectra of these CH3CN and H2O binary-phase aerosols formed under temperature and pressure conditions that replicate Titan’s stratosphere. Particles generated in the EFC cell were determined to be (i) spherical, (ii) crystalline, and (iii) segregated as microheterogeneous H2O and CH3CN grains, with evidence for side-directed π-type CH3CN···HOH hydrogen bonding at the grain interface. These physical attributes were determined by examination of frequency shifts and band profiles indicative of predominantly unmixed CH3CN and H2O ice particles with intermolecular interaction between grain boundaries. We show that the antiparallel dipole−dipole configuration of β-CH3CN (see Figure S-6) is perturbed at the interface when the CH3CN··· HOH configuration is established; inducing significant shifts in the lattice band positions from the pure-phase counterparts. In addition, the ν2 CN stretching band develops a satellite peak at 2240 cm−1 that is assigned to the side-directed π-type coupling within the CH3CN···HOH network. By applying DDA theory, we investigated several possible aerosol architectures and configurations for the mixed CH3CN−H2O ices. Comparison between DDA modeled spectra and the experimental data confirm we do not form either (i) spherical particles of the pure-phase components, (ii) cubic shaped particles, or (iii) layered core−shell architectures. Instead, (iv) microheterogeneous particles formed by contact freezing return the most convincing simulated spectra to validate the inferences from IR band analysis performed in previous sections. We conclude that the investigations of binary CH3CN and H2O aerosols have given new insight into the phase, size distribution, and internal structure of these particles. This information may find relevance for improved models of Titan’s haze composition and optical properties, as well as providing future IR surveys with aerosol signatures for minor ices in planetary (Titan and trans-Neptunian object) atmospheres. Further, it is expected that co-condensates aerosols, formed from the more abundant atmospheric species such as HCN, C2H2, and C2H6, populate Titan’s lower atmosphere. Having now validated our far-IR aerosol procedure on the CH3CN− H2O system, we hope to apply an aligned methodology to study more Titan relevant co-condensate systems involving the aforementioned species in future work. In addition, we have elucidated the dependence of far-IR lattice features on temperature, data that could be adopted by future Titan explorations as a probe for atmospheric conditions. Importantly, this work rules out CH3CN−H2O binary aerosols as a carrier for the unassigned 220 and 100 cm−1 “haystack” features due to our experimental far-IR spectra providing a poor semblance to Cassini CIRS spectra. G


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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsearthspacechem.8b00059. Description of discrete dipole approximation. Discrete dipole approximation far-IR spectra for H2O Ic spherical aerosols. Discrete dipole approximation mid-IR spectra for H2O Ic spherical aerosols. Discrete dipole approximation mid-IR spectra for H2O Ic cubic aerosols. Schematic of generalized aerosol shapes and architectures. Experimental mid-IR spectra (2280−2220 cm−1) at for the CN stretch region. Structures for CH3CN and H2O complexes. Temporal mid-IR spectra (1550− 1350 cm−1) at 100 and 130 K. Mid- and far-IR vibrational bands for H2O in pure and binary phase. Mid- and far-IR vibrational bands for CH3CN in pure and binary phase. H2O particle size calculations from pure spectra. Peak frequency shifts from the pure-phase absorption at 100 and 130 K. Peak frequency shifts for three deposition techniques at 100 mbar. Temporal farIR band positions for pure and binary aerosols at 130 K (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Evan G. Robertson: 0000-0003-4346-4457 Courtney Ennis: 0000-0003-1774-8982 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS R.A. was supported by an Australian Government Research Training Program (RTP) Scholarship. R.A. thanks the Australian Institute of Nuclear Science and Engineering (AINSE) for their kind financial support under the AINSE Postgraduate Research Award (PGRA) scheme to enable this work. R.A. thanks the Royal Australian Chemical Institute Physical Chemistry division for their financial support and their invitation to present this work at the 2017 RACI Congress in Melbourne. C.E. thanks the Australian Research Council for a Discovery Early Career Research Award (DE150100301) and the La Trobe Institute for Molecular Sciences for organizational support. This research was undertaken at the THz/Far-Infrared beamline at the Australian Synchrotron, ANSTO. This project was completed with the assistance of resources from the National Computational Infrastructure (NCI), which is supported by the Australian Government.

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REFERENCES

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