Nanoparticle-Mediated, Light-Induced Phase Separations - Nano

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Nanoparticle-mediated, Light-induced Phase Separations Oara Neumann, Albert D. Neumann, Edgar Silva, Ciceron Ayala Orozco, Shu Tian, Peter Nordlander, and Naomi J. Halas Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.5b02804 • Publication Date (Web): 04 Nov 2015 Downloaded from http://pubs.acs.org on November 7, 2015

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Nanoparticle-mediated, Light-induced Phase Separations Oara Neumann*,§, Albert D. Neumann¶, Edgar Silva£, Ciceron Ayala-Orozco†,§, Shu Tian†,§, Peter Nordlander#,§, and Naomi J. Halas *,#,†,§ Department of Electrical and Computer Engineering, #Department of Physics and

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Astronomy, ¶Department of Civil and Environmental Engineering, £Department of Mechanical Engineering, †Department of Chemistry, §Laboratory for Nanophotonics and the Smalley-Curl Institute, Rice University, 6100 Main Street, Houston, Texas 77005

Corresponding Author: Naomi Halas Phone: (+) 1-(713)348-5612 Fax: (+) 1-(713)348-5686 E-mail: [email protected] RECEIVED DATE (to be automatically inserted after your manuscript is accepted if required according to the journal that you are submitting your paper to) 1

ACS Paragon Plus Environment

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Abstract Nanoparticles that both absorb and scatter light, when dispersed in a liquid, absorb optical energy and heat a reduced fluid volume due to the combination of multiple scattering and optical absorption. This can induce a localized liquid-vapor phase change within this reduced volume without the requirement of heating the entire fluid. For binary liquid mixtures, this process results in vaporization of the more volatile component of the mixture. When subsequently condensed, these two steps of vaporization and condensation constitute a distillation process mediated by nanoparticles and driven by optical illumination. Because it does not require the heating of a large volume of fluid, this process requires substantially less energy than traditional distillation using thermal sources. We investigated nanoparticle-mediated, light-induced distillation of ethanol-H2O and 1-propanol-H2O mixtures, using Au-SiO2 nanoshells as the absorber-scatterer nanoparticle and nanoparticle-resonant laser irradiation to drive the process. For ethanolH2O mixtures, the mole fraction of ethanol obtained in the light-induced process is substantially higher than that obtained by conventional thermal distillation, essentially removing the ethanol-H2O azeotrope that limits conventional distillation. In contrast, for 1-propanol-H2O mixtures, the distillate properties resulting from light-induced distillation were very similar to those obtained by thermal distillation. In the 1-propanol-H2O system, a nanoparticle-mediated, light induced liquid-liquid phase separation was also observed.

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Introduction Distillation is a process widely used to separate and purify liquids. In conventional distillation, a homogeneous mixture of liquids is heated to boiling, then its vapor is condensed; components of the mixture are separated based on their different boiling points. This is an energy intensive process, accounting for more than half of the total energy used industrially for chemical separations1,2 and is a determining factor in the cost of chemicals that require purification prior to commercial or industrial use. For example, in bioethanol production, distillation accounts for 70-85% of the total energy required for production.3,4 Reducing the energy requirements of distillation would have an immediate economic impact across this and many other industries, making the production of a large number of chemicals far more economically feasible. Recently we showed that illuminating a solution of light-absorbing and light-scattering nanoparticles dispersed in water, can result in steam generation without heating the fluid volume.5,6 This general effect has also been observed using porous, buoyant graphenic materials.7,8 For particle concentrations in the 109 to 1010 NP/mL range, light trapping caused by multiple scattering prior to absorption, concentrates the light absorption into a small volume of the liquid for efficient photothermal conversion.5 For top-down illumination, the energy is focused into a thin region just below the liquid-air interface enabling steam generation at high efficiencies.5 This effect has been demonstrated to be useful for powering a portable, standalone solar autoclave, and may have a range of promising applications,9 including the harnessing of solar power.10-13 We have also initially demonstrated that ethanol can be distilled from an H2O-ethanol mixture using

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nanoparticles to absorb the focused sunlight, yielding fractions significantly richer in ethanol content than simple thermal distillation.6 Here we report an examination of nanoparticle-mediated, light-induced distillation of alcohols under controlled conditions, using plasmonic nanoparticles and resonant laser illumination. The distillate concentration was quantified as a function of the relative concentrations of the starting liquid mixture in an inert ambient environment, and in a closed-loop system. As the photothermally transducing nanoparticles, we here chose AuSiO2 nanoshells because of their highly regular size and shape, which enables us to quantify nanoparticle concentration and individual particle absorption and scattering cross sections to a high degree of accuracy.14,15 For ethanol-H2O mixtures, the wellknown azeotrope of these two liquids disappears entirely when distilled by this lightinduced process. In contrast, the light-induced distillation of 1-propanol-H2O mixtures shows only minor differences compared to the conventional distillation curve obtained using a thermal source. This dramatic difference in behavior of the two liquid mixtures, which differ primarily in the strength of their hydrogen bonding network,16-18 suggests that nanoparticle-mediated, light-induced vaporization differs from vaporization with a thermal source by disrupting the hydrogen bonding network of simple liquid mixtures. We performed distillation measurements in a 5 ml, well-insulated system under a nitrogen environment using a laser source that provided illumination at 808 nm with continuous 15W incident laser power.6 The laser wavelength corresponded to the peak resonant wavelength of the nanoshells, (Fig. S1, Supporting information) and light was focused onto the top surface of the liquid through an optical fiber. The vapor generated by laser illumination was condensed in a water-cooled column, and 50µL of each 4


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distillate fraction was collected (experimental schematic and photograph shown in Fig. S2, Supporting Information). The distillation samples were diluted (1/1000 in MQ water) and analyzed by gas chromatography (GC) on a Hewlett-Packard 5890 GC equipped with a glass column containing 80/120 Carbopack B-DA*/4% Carbowax 20M (Supelco, Bellefonte, PA) and a flame-ionization detector (Agilent Technologies). A 5 µL sample of the diluted distillate was injected into the GC unit, and the heating program was set to 250, 110, and 250°C for the injector, oven, and detector, respectively. Identification and quantification were performed using a calibration curve with ethanol standards prepared by diluting 200-proof ethanol (from molecular biology Sigma-Aldrich) with MQ water. The liquid-vapor phase diagram of ethanol and H2O was produced by nanoparticlemediated, light-induced distillation and compared to a standard equilibrium distillation curve at 1atm and 25°C (Fig 1a). The light-induced distillation is shown in red, and the distillation curve obtained with a standard thermal source is shown in blue. The dashed black line represents an equal concentration of alcohol in both liquid and the vapor states.

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Figure 1: (a) Liquid-vapor distillation diagram for nanoshell-ethanol-H2O mixtures (red dots) under laser exposure within a N2 environment, and ethanol-H2O mixtures (blue curve) using a conventional thermal source at atmospheric pressure. The black line represents an equal concentration of alcohol in both the liquid and the vapor state. (b) Time dependent mass loss for nanoshells mixed with various ethanol-H2O mixtures under 5W laser exposure (Inset: evaporation rate of ethanol as a function of ethanol mole fraction in the liquid mixture, showing linear behavior). (c) Time dependent temperature profile of nanoshells mixed with various ethanol-H2O mixtures under 5W laser exposure measured at two different locations: close to the surface and close to the bottom of the cuvette. Insert: schematic of the experimental geometry, showing the positions where the temperatures were measured (at the top and bottom of the solutions) under top laser illumination. (d) Vapor temperature as a function of ethanol mole fraction at 987 Torr

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(19.1psi) pressure under 15 W laser exposure (red dots) or electrical heating (blue triangles). The nanoshell concentration was 2.5x109 particles/mL for all measurements.

With conventional distillation, an ethanol-H2O mixture forms an azeotrope (i.e., equivalent alcohol composition in the vapor and liquid phase) at 0.85 mole fraction ethanol, at standard temperature and pressure. Nanoparticle-mediated, light-induced distillation of ethanol-H2O shows a very different behavior relative to conventional distillation. With this process we observe a preferential vaporization of ethanol, even at the highest mole fractions of ethanol in the starting liquid mixture. No azeotrope is observed under these conditions, but rather, an enrichment of the more volatile component of the liquid mixture in the distillate (Fig. 1a). The absence of an azeotrope was further verified by mass loss measurements (Fig. 1b), which showed a continuous increase in the evaporation rate as a function of ethanol mole fraction of the liquid mixture. Extracting the slope from the linear region of each of the mass loss curves yields the vaporization rate as a function of liquid mole fraction of ethanol (Fig. 1b, insert). Here we observe a constant, monotonic increase in vaporization rate with mole fraction, also indicating the absence of an azeotrope. The temperature in the fluid mixture was recorded simultaneously with mass loss measurements (due to vaporization) by positioning two infrared thermocouples at the top and the bottom of the solution. These temperatures were recorded as a function of time during laser illumination (Fig. 1C). The temperature at the surface of the nanoshell-H2Oethanol mixture solutions was 75°C, close to the boiling point of ethanol (78.4°C); while the temperature at the bottom of the solution showed no significant temperature increase. The thermal gradient of the liquid mixture established during resonant laser illumination 7


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confirms that the bulk of the solution remained well below the boiling point while vaporization was proceeding, a characteristic property of this nanoparticle-mediated vaporization regime. We also performed pressure and temperature measurements of nanoparticle-mediated, light-induced distillation of the ethanol-H2O liquid mixtures in a closed system, allowing us to compare the vapor properties of ethanol-H2O mixtures at elevated pressures. We compared the laser irradiation results with those obtained using a conventional thermal source. A pressure sensor (PV350 pressure vacuum module adapter for a Fluke milivoltmeter, ECO-Tronic Pressure Transmitters) and a thermocouple (OMEGA Engineering, Inc.) were mounted onto the experimental vessel to monitor the vapor properties. A vacuum jacket was included to prevent heat loss (Figure S3, Supplemental Information shows the closed loop setup). A plot of temperature versus mole fraction of ethanol at a fixed pressure at 987 Torr (19.1psi) was measured in a closed-loop system using nanoparticles and optical illumination (red dots), and compared with that obtained using no nanoparticles and a conventional thermal source (blue triangles) (Fig. 1d). For vaporization of the liquid mixture using a conventional thermal source, a minimum in the temperature-composition diagram can be observed at an ethanol mole fraction of ~0.85, indicating the presence of an azeotrope for this liquid mixture. However, for the nanoparticle-mediated laser-induced vaporization, no such minimum is observed. (Vaporization with a thermal source was also conducted for ethanol-H2O mixtures with and without nanoshells present as a control; however, no difference in the vaporization due to the presence of nanoshells alone was observable.) The vapor reaches a higher temperature for ethanol-H2O mixtures undergoing nanoparticle-mediated, laser-induced 8


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vaporization (92.5°C) than for conventional heating (60.4°C) at this elevated pressure. In the case of pure ethanol, conventional heating produces vapor at 34.5°C while for nanoparticle-mediated laser-induced heating the ethanol vapor temperature reaches 40°C under the same conditions. The boiling behavior of ethanol-H2O mixtures is strongly affected by the hydrogen bonding network of the liquid mixture.19,20 Previous studies have shown that ethanol and H2O molecules do not mix randomly, but instead form clusters due to hydrogen bonding,21-28 where the cluster size depends on the ethanol-H2O molecular ratio. For liquid mixtures with a high H2O content, the H2O molecules form strong hydrogenbonded shells around the ethyl group of the ethanol molecule and induce a large decrease in vaporization enthalpy.27,29 In ethanol-rich solutions, only one hydrogen is involved in network bonding; consequently, the solutions are thought to contain mainly linear chains or cyclic clusters of ethanol molecules.28,29 To examine whether it is feasible that nanoparticle-based local heating may be disrupting the hydrogen bonding network of the liquid mixture, we determined the enthalpy of vaporization for pure H2O and ethanol with dispersed nanoshells using the Clausius-Clapeyron equation (Supporting Information, Figure S4). The vaporization enthalpy obtained was 42.88KJ/mole for nanoshells in H2O and 39.54KJ/mole for nanoshells in ethanol,

both values obtained slightly reduced

relative to those reported for the pristine liquid (43.99KJ/mole for water and 42.3KJ/mole for ethanol).24,30 This difference in the heat of vaporization with and without nanoparticles suggests that the presence of nanoshells in the liquid may perturb the hydrogen bonding network of the pure liquid in each case.

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To examine nanoparticle-mediated, light-induced distillation in another liquid mixture, we investigated the distillation of H2O-1-propanol, a different azeotropic H2O-alcohol mixture with a weaker hydrogen bonding network (Fig. 2). We studied nanoparticlemediated, light-induced vaporization of 1-propanol-H2O mixtures between 0 and 0.9 mole fraction of 1-propanol, using a constant concentration of nanoshells. Under the same conditions, we obtained the liquid-vapor equilibrium diagram for 1-propanol-H2O mixtures with nanoshells dispersed in solution (Fig. 2a). In contrast to the H2O-ethanol case, the 1-propanol content obtained by nanoparticle-mediated, light-induced distillation follows the conventional thermal distillation curve very closely. We also performed mass loss measurements (Fig. 2b) and determined the rate of vaporization (Fig. 2b, inset) for the liquid mixture. In this case, the maximum rate of vaporization was obtained at the mole fraction corresponding to the azeotrope; the azeotrope composition is shifted slightly compared to the thermal case but is still present. The temperatures at the surface of the solution and at the bottom of the container were recorded during mass loss measurements, showing a similar temperature gradient to that shown in the H2O-ethanol system in Figure 1d (Fig. 2c). However, in this case while vaporization was occurring, the surface temperature of the 1-propanol-H2O solution was measured to be nominally 80°C, much lower than the 1-propanol boiling point (97.4°C). No temperature increase was observed at the bottom of the flask.

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Figure 2: (a) Liquid-vapor distillation diagram for nanoshell 1-propanol-H2O mixtures (red dots) under laser exposure and standard equilibrium distillation curve at 1atm and 25°C (blue curve). The black line represents an equal concentration of alcohol in both the liquid and the vapor state. (b) Time dependent mass loss for nanoshell mixed with various 1-propanol-H2O mixtures under 5W CW power laser illumination (Inset: evaporation rate of 1-propanol vs. NS-1-propanol-H2O mole fraction showing a distinct minimum at 0.7 mole fraction of 1-propanol). (c) Time dependent temperature profile of nanoshell mixed with various 1-propanol-H2O mixtures under 5W CW laser exposure measured at two different locations: close to the surface and close to the bottom of the cuvette. Insert: schematic of the experimental geometry showing where the temperatures were measured (top and bottom of the solutions) under top-down laser illumination. (d) Vapor temperature as a function of 1-propanol mole fraction at 987 Torr pressure under

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15 W laser exposure (red dots) or electrical heating (blue triangles). The nanoshell concentration was 3.5x109particles/mL for all measurements.

The temperature-composition phase diagram of the 1-propanol-H2O mixture obtained by heating with a conventional thermal source (Fig. 2d, blue triangles) shows the boiling points of the coexisting phases and the 1-propanol azeotrope, which corresponds to 0.45 1-propanol mole fraction at 987 Torr. Under laser irradiation (Fig. 2d, red dots) the temperature versus 1-propanol mole fraction diagram shows an azeotrope at the same concentration obtained for the thermal process. The vapor generated from the 1-propanolH2O mixtures reaches a higher temperature under laser irradiation compared to the conventional thermal process, similar behavior to what was observed for the ethanol-H2O mixture. For 1-propanol mole fractions of 0.5-0.9, we observe a nanoparticle and light-induced phase separation into two distinct liquid layers: the upper with a higher propanol concentration than the lower phase. While nanoshells dispersed in H2O can be stably dispersed for several months (Fig. 3a), when nanoshells are dispersed in a 1-propanolH2O mixture at room temperature and then exposed to laser illumination for approximately 3-5 minutes, a phase separation appears and the nanoshells selectively transfer into the upper, propanol rich liquid phase (Fig. 3b). When the light illuminating the nanoshells is stopped, the particles gradually re-disperse in the bulk liquid mixture. (The color change observed in these images is an optical effect, due to a combination of increased nanoshell concentration and enhanced scattering upon transfer into the higher refractive index phase.) The composition of the two observed liquid phases was quantified by gas chromatography (GC) (Fig. 3c). The phase separation process that occurs when the 1-propanol-H2O mixture system is at 0.49-0.9 mole fraction is controlled by several factors, including intermolecular interactions, light-driven nanoparticle dynamics, photo-induced pressure gradients, 12


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viscosity, microconvection processes, and

Brownian motion. Theoretical

and

experimental studies show that 1-propanol and H2O molecules form mixtures with specific characteristic hydration compositions to minimizing their surface contact with H2O. Several model clusters for 1-propanol surrounded by H2O molecules have been reported for 1-propanol-H2O mixtures, including centered nonpolar alkyl groups and fractal clusters. All sizes and types of clusters have an impact on the boiling-evaporation process.17,24,31-37 Under laser illumination, the increase in kinetic energy perturbs the electrostatic field that stabilizes the hydrophilic nanoshells leading to a change in the type and size of the 1propanol-H2O clusters surrounding the nanoparticles.38,39 This perturbation allows the nanoparticles to come in closer proximity to one another until the attractive forces, such as induced dipole interaction (i.e. van der Waals force including London dispersion), eventually causing the particles to agglomerate.28,38,40-46 Therefore, the nanoshells are trapped in the denser organic phase rich in 1-propanol and form flocculants in the glass vessel under top-down laser illumination. When the light illuminating the nanoshells is stopped, these particles gradually re-disperse in the bulk liquid mixture. This indicates that the nanoshell-induced local heating process significantly shifts the chemical and hydrodynamic equilibrium, particularly in the presence of light. The dual role of nanoshells as localized light absorbers and high locally charged particles in alcohol-H2O mixtures requires additional study to clearly understand and control light-driven NS-1propanol-H2O phase separation. We attempted to performed light induced phase separation process using nanoshell-2propanol-H2O mixtures and the same illumination conditions, but a phase separation with this system was not observed. T. Takamuku, et al. investigated the structure of propanolwater mixtures by large-angle X-ray scattering and reported that even though 1-propanol and 2-propanol are not significantly different chemically, the enthalpy (-380Jxmol-1) of 1-propanol-H2O mixture is much larger than that for 2-propanol-H2O mixture (650Jxmol-1) at the mole fraction reflecting the minimum of the enthalpy of mixing. They suggested that the 2-propanol-H2O mixture solution hydrate differently compared with the 1-propanol-H2O mixture solution due to the different shape of the propyl group in each case. The hydrophobic group is more flexible in the case of 1-propanol compared to 13


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2-propanol. Therefore it may be possible that the 1-propanol disrupts more of the structure of H2O molecule network than the 2-propanol molecules.38,47 In general, the enthalpic gain of H-bonding among the polar fraction of alcohol molecules overcomes the enthalpic loss due to the water intermolecular structure disruption induced by the nonpolar groups. Conversely, in hydrophobic-H2O systems with larger alkyl groups, the enthalpic losses are due to increases in the size of the hydrophobic groups. Therefore, the larger the hydrophobic group the more the tetrahedral-like structure of water is perturbed by increasing alcohol concentration. By engineering the surface of the nanoparticles to control the hydrophobicity of the particle, understanding the intermolecular cluster formation of liquid mixtures, and controlling the local induced heating process we may be able to more generally extend this effect to other important liquid separation processes.

Figure 3: (a) Photograph of nanoshells suspended in 0.57 mole fraction 1-propanol-H2O solution showing homogeneity of the solution prior to laser irradiation and (b) phase separation of the solution occurring after 5 minutes under 15 W CW laser illumination. (c) The liquid mole fraction content of the two separate phases induced by laser illumination of 0.57 mole fraction nanoshell 1-propanol-H2O mixture. The black dashed 14


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line represents the 0.57 1-propanol mole fraction. (d) Normal Raman (black) of 1propanol and SERS spectra of Au nanoshells withdrawn from 1-propanol-H2O liquid mixture (blue) before and (red) after laser-induced phase separation. (e) TEM image of nanoshells after suspension in 1-propanol-H2O mixed prior to, and (f) following laserinduced phase separation (Inset: cryo-TEM image, error bars 50 nm).

Surface-enhanced Raman Spectroscopy (SERS) of nanoshell aggregates removed directly from the liquid mixture before and after laser-induced phase separation (after separation we sample only the less dense phase into which the nanoshells are transferred) allows us to probe the resulting changes in the 1-propanol concentration. Quartz and silicon substrates (Addison Engineering, Inc.) were cleaned by immersing in “piranha solution” (H2SO4:H2O2 = 3:1) for 1 h, followed by several rinsing with deionized water (18.3 MΩ, Millipore). Films of nanoshell aggregates supported on quartz and silicon were formed by drop-casting nanoshells suspended in the H2O-1-propanol mixture before and after phase separation onto each substrate. SERS spectra were obtained using a Renishaw inVia micro-Raman spectrometer with a 785 nm excitation laser at 0.15mW power. Backscattered light was collected using a 50X objective lens (Leica, Germany) with a 10 second integration time. In Figure 3d, the SERS spectra of H2O-1-propanol mixtures (0.57 mole fraction 1-propanol) before laser illumination (blue), of the lowerdensity, nanoshell-enriched phase after laser illumination (red), and the normal Raman spectrum of a solution of pure 1-propanol (black), are shown. Peak assignments are presented in Table S1. The principal Raman peak of 1-propanol at 864 cm-1, induced by the in-plane skeletal modes ν(CCC), ν(CCO), is easily seen in all spectra.48,49 In addition, a spectral feature characteristic of 1-propanol is also observed at 1454.6 cm-1, associated 15


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with the asymmetric deformation mode of the methyl group combined with the scissoring modes of the methylene group. This substantial increase in the two characteristic 1propanol peaks following phase separation, which correlates with the increased concentration of 1-propanol in the less dense separated phase, also suggests that the 1propanol molecules are more plentiful at the nanoshell surface in this phase than prior to phase separation. This locally enriched 1-propanol concentration is also corroborated by TEM images (Fig. 3e, f), and a slight reduction in zeta potential of the less dense separated phase relative to the parent solution prior to separation (Fig. S6). Cryo-TEM images performed on nanoshells from the 0.57 mole fraction 1-propanol-H2O mixture, obtained before (Fig. 3e) and after (Fig. 3f) laser-induced phase separation, indicate that a micelle-like assembly surrounds the nanoparticles. This locally enriched 1-propanol concentration may play a role in the laser-induced phase separation process. Clearly further studies are warranted to examine this novel phase-separation phenomenon in greater detail, to understand the specific contributions of nanoparticle surface chemistry, as well as physical parameters such as local temperature at the nanoparticle surface, that control this process. In conclusion, we have shown that combining light-absorbing nanoparticles and resonant illumination may give rise to altogether new ways to separate liquids. We have shown that nanoparticle-mediated, laser-induced distillation of liquid mixtures can result in distillation properties quite distinct from conventional thermal distillation in the case of ethanol-H2O mixtures, yet yield results remarkably similar to the conventional process for the liquid mixture of 1-propanol-H2O, with its weaker hydrogen-bonding network. We have also observed a light-induced phase separation in the 1-propanol-H2O liquid 16


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mixture over a range of mole fractions, where the nanoparticles transfer into one of the two separated phases. Both these processes clearly indicate that the combination of lightabsorbing nanoparticles and resonant illumination allows us to alter physicochemical processes, foundational to our understanding of liquids, and ultimately to a multitude of industrial processes, in surprising new ways. Supporting Information available. This material is available free of charge via the Internet at http://pubs.acs.org The authors declare no competing financial interest. Acknowledgments: We gratefully acknowledge the Robert A. Welch Foundation (C1220 and C-1222); AFOSR MURI-FA9550-15-1-0022, and the Bill and Melinda Gates Foundation for financial support, Dr. Zongmin Xiu, Julius Müller, and Christyn A. Thibodeaux for help with the GC/HPLC, Dr. Sandra W. Bishnoi, Dr. Jared Day, Dr. Surbhi Lal, Dr. Christopher DeSantis, Greg Bond, and Dr. Alexander S. Urban, for helpful discussions.

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Nanoparticle-Mediated, Light-Induced Phase Separations - Nano

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