Observation of a Brine Layer on an Ice Surface with an Environmental Scanning Electron Microscope at Higher Pressures and Temperatures Ja ́ n Krausko, †,‡ Jir ̌ í Runs ̌ tuk, § Vile ́ m Nedě la, § Petr Kla ́ n, †,‡ and Dominik Heger* ,†,‡ † Department of Chemistry, Faculty of Science, Masaryk University, Kamenice 5, 62500 Brno, Czech Republic ‡ RECETOX, Faculty of Science, Masaryk University, Kamenice 5, 62500 Brno, Czech Republic § Institute of Scientific Instruments of the ASCR, v.v.i., Kra ́ lovopolska ́ 147, 61264 Brno, Czech Republic * S Supporting Information ABSTRACT: Observation of a uranyl-salt brine layer on an ice surface using backscattered electron detection and ice surface morphology using secondary-electron detection under equilibrium conditions was facilitated using an environmental scanning electron microscope (ESEM) at temperatures above 250 K and pressures of hundreds of Pa. The micrographs of a brine layer over ice grains prepared by either slow or shock freezing provided a complementary picture of the contaminated ice grain boundaries. Fluorescence spectroscopy of the uranyl ions in the brine layer confirmed that the species exists predominately in the solvated state under experimental conditions of ESEM. 1. INTRODUCTION Natural ice and snow accumulate and concentrate significant amounts of impurities that can be stored or chemically transformed, and eventually released to the environment. 1-4 The impurities are rejected from the freezing solution to the ice grain boundaries, free ice surfaces, or liquid/brine inclusions. 1 Information about compartmentation and phase speciation of chemical impurities in ice is thus essential for the assessment of their fate. The location of impurities and their interactions with the water molecules of ice, still not sufficiently clarified, must be studied at lower temperatures because thawing erases the information. Various techniques such as absorption, 5-7 fluorescence, 8 or X-ray photoelectron 9 spectroscopies have been utilized on solid ice samples. Microscopy is one of the few direct methods for observing ice impurities and evaluating their location and compartmentation. 10-12 Low-temperature scanning electron microscopy (LTSEM) 13 has already been used for the observation of natural 14-16 and laboratory-generated ice surfaces, grain boundaries, 17 vitrified sulfuric acid, 18 as well as solid impurities at the grain boundaries or dust particles/coagulated solutes 14 on the grain surfaces, for example. 19,20 LTSEM requires low pressure in the specimen chamber (usually below 10 -4 Pa) which causes potentially undesirable ice sublimation, called (thermal) etching, revealing impurities that would otherwise remain buried under the ice surface. When the impurities keep their location while the surrounding ice sublimes, a 3D morphology of the ice boundaries is revealed. Various shapes of coagulated impurities, such as grain boundary filaments formed from triple junction tubes (also called veins), 19 thin ridges, 14 cobweb-like structures, 21 nodules, sheets, as well as stems and leaves, 20 have been observed. A site-specific energy-dispersive spec- trometer for X-ray microanalysis built into the LTSEM, 21 and a confocal optical microscopy coupled with Raman spectrosco- py, 12 were used to determine unambiguously that the observed features are concentrated impurities excluded from the solutions. The environmental scanning electron microscope (ESEM) has been constructed for observation of various wet non- conductive samples at high pressures. 22 Maintaining a water vapor pressure on the order of 10 3 Pa or more obviates the need for conductive surface coating, and allows the sample to be preserved essentially in its natural form. 23 As a result, the sample surface structure is highly hydrated, thus the technique is suitable for the investigation of dynamic processes, such as sample solidification or dissolution. Only a few studies have examined noncontaminated ice at high pressures using the ESEM. 24 Leu and co-workers examined ice particle sizes in thin ice films; 25,26 Zimmermann and co-workers observed ice nucleation on various solid supports, such as silver iodide, kaolinite, or montmorillonite; 27 and Varanasi and co-workers observed frost formation on hydrophobic surfaces coated, for example, with a thin layer of (trideca fl uoro-1,1,2,2- tetrahydrooctyl)trichlorosilane at 260 K. 28 Neshyba and co- workers showed well-resolved surfaces of hexagonal ice crystals possessing strands which are characteristic for growing and ablating facets. 29 Pedersen and co-workers observed ice crystals growing into contact. 30 In most of the previous studies of ice, SEM has been used at pressures below 0.1 Pa and temperatures below 180 K. Received: January 30, 2014 Revised: April 23, 2014 Published: April 24, 2014 Article pubs.acs.org/Langmuir © 2014 American Chemical Society 5441 dx.doi.org/10.1021/la500334e | Langmuir 2014, 30, 5441-5447