Gravity-assisted distillation on a chip: Fabrication, characterization, and applications
Graphical abstract
Introduction
The distillation process is widely used in industrial processes and laboratories for sample pre-treatment. The conventional distillation apparatus is usually composed of four units, namely, heating source, distilling flask, condenser, and receiving flask. As disadvantages, this technique presents chemical-consuming and laborious assays that undermine parameters such as precision, simplicity, cost, and green chemistry compatibility. In this regard, the macro-to-microscale conversion is a powerful alternative. The safety represents other advantage of the microsystems on their macroscale analogues since some species can explode under the distillation conditions [[1], [2], [3]]. Despite the relevance of the distillation and the consequent gains with the miniaturization, little attention has been paid to develop microscale distillers. The major challenges for distillation process miniaturization are phase separation and vapour driving, integration of the functional steps in a single device, complexity and cost of microfabrication, and separation efficiency. In microchannels, surface forces such as viscosity, wall friction, and surface tension take over from the gravity [[1], [2], [3], [4], [5], [6], [7], [8]]. Accordingly, alternatives to gravitational forces are needed to provide the phase separation and controllably drive and collect the vapour within a microscale channel.
The alternatives to gravity for microdistillations include the use of inert carrier gas or vacuum pump in membrane distillation [6,7,9]. In this case, the vapour is separated from sample by microporous and hydrophobic membranes and the temperature gradient is vertically reached by placing a microfluidic chip on a heating source and flowing cooling water above the sample channel. As hurdles, the separation efficiency is poor because of the concentration polarization in the membrane across the liquid phase side and further steps for gas-liquid separation are required when carrier gas is employed. Another type of microfluidic distillation, called zero-gravity [[1], [2], [3], [4], [5]], performs the phase separation in a horizontal microchannel that is either heated or cooled at its ends to attain a temperature gradient. Micropillars around the distillation channel separate the liquid from vapour by capillary force. The micropillars establish fractional distillations with improved separation efficiency by increasing the number of vaporization-condensation equilibria (plates). However, the use of these micropillars requires a pre-treatment step, namely, the sample should be previously degassed to get rid of any entrapped air [[2], [3], [4]]. Moreover, both the aforesaid microchips involve a complex, high-cost, and time-consuming fabrication. The zero-gravity microdevices are obtained by standard photolithography, dry or wet chemical etching, and thermal or anodic bonding procedure that needs long time and high temperatures. With regard to the membrane distillation, the fabrication is further laborious since the membrane surface is thermally uniform, thus requiring the generation of a vertical temperature gradient. Such fabrication relies on photolithography with multiple and subsequent steps of film deposition, UV exposure, resist development, and adhesive bonding in UV-curable resins [7,9,10]. Additionally, the membrane and zero-gravity microdistillers presented non-integrated functional components making it impossible to construct lab-on-a-chip platforms, i.e., systems with multifunctional units incorporated into a single device.
Considering the relevance and downsides of the microscale-based distillation processes, we describe in this paper for the first time a totally integrated microdistiller in agreement with the apparatus of a conventional flash distillation. The chip was composed of a single piece of polydimethylsiloxane (PDMS) and its fabrication was rapid, simple, and cost-effective by avoiding the use of cleanroom facilities and bonding step. Such technique was based on sequential steps of polymerization and scaffold removal (PSR) [[11], [12], [13]], which granted the simple incorporation of the components of flash distillation (heating resistor, distillation flask, condenser, and distillate collector) into a single chip. This integration contributed for automation and operation simplicity of the system that was termed distillation-on-a-chip (DOC). Gravity-assisted separations were attained through the 3D printing of a distillation chamber with somewhat large size (approximately 900 μL). In this regard, the gravity overcame the surface forces, thus driving the distillation. The 3D printing technology has generated a new revolution in the fabrication of microfluidic chips by creating 3D structures in a simple and single-step way [[14], [15], [16]]. While the phases were separated through the gravity, the distillate was driven by capillary force and vapour pressure in a microchannel. As proof of concept applications, the DOC was successfully used in i) desalination for sample pre-treatment and ii) determination of ethanol in alcoholic beverages. Desalinations at harsh salinity conditions (NaCl 600.0 mmol L−1) were obtained with high throughput and salt removal efficiency (roughly 99%). Finally, the concentrations of ethanol obtained in real samples using the microdistiller were in agreement with those results recorded by gas chromatography.
Section snippets
Chemicals
Sodium chloride (NaCl), acetone, 1-butanol, and ethylene glycol were supplied from Labsynth (São Paulo, Brazil), whereas ethanol was supplied from Merck (Darmstadt, Germany). Sylgard 184 silicone was purchased from Dow Corning (Midland, MI). Deionized water (Milli-Q, Millipore Corp., Bedford, MA) was obtained with resistivity no less than 18 MΩ cm. Alcoholic beverage samples were acquired from a supermarket in Campinas, Brazil, which included whiskey, vodka, rum, and cachaça (typical Brazilian
Microchannel
The fabrication of the microdistiller basically required a laboratory oven and a low cost 3D printer. In fact, PSR is a simple method to fabricate microscale devices by avoiding the time-consuming, complex, and high cost steps of bonding and channel engraving [[11], [12], [13]] that are traditionally used to construct the chips applied in distillation [[1], [2], [3], [4], [5], [6], [7], [8], [9]]. SEM-FEG image of the cross-section of the collection channel is shown in Fig. 2(a) inset. The
Conclusions
A microscale distiller in agreement with the apparatus of conventional flash distillation and with ability to drive gravity-assisted distillations was, to our knowledge, described for the first time. The gravitational forces took over from the surface forces because of the somewhat large dimensions of the distillation chamber that was indirectly obtained by 3D printing. Vertical temperature gradient was achieved in the same piece of PDMS device due to the low thermal conductivity of this
Acknowledgements
We would like to thank Electron Microscopy Laboratory from LNNano for the SEM-FEG images. Rafael Defavari from CNPEM is also thanked for taking the photos.
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