Article pubs.acs.org/Langmuir
Water Vapor Uptake of Ultrathin Films of Biologically Derived Nanocrystals: Quantitative Assessment with Quartz Crystal Microbalance and Spectroscopic Ellipsometry Elina Niinivaara,† Marco Faustini,‡ Tekla Tammelin,§ and Eero Kontturi*,†,∥ †
Department of Forest Products Technology, School of Chemical Technology, Aalto University, 02150 Espoo, Finland Sorbonne Universités, UPMC Univ Paris 06, CNRS, Collège de France, UMR 7574, Chimie de la Matière Condensée de Paris, F-75005, Paris, France § High Performance Fibre Products, VTT Technical Research Center of Finland, Espoo, Finland ∥ Polymer and Composites Engineering (PaCE) Group, Department of Chemical Engineering, Imperial College London, London SW7 2AZ, U.K. ‡
S Supporting Information *
ABSTRACT: Despite the relevance of water interactions, explicit analysis of vapor adsorption on biologically derived surfaces is often difficult. Here, a system was introduced to study the vapor uptake on a native polysaccharide surface; namely, cellulose nanocrystal (CNC) ultrathin films were examined with a quartz crystal microbalance with dissipation monitoring (QCM-D) and spectroscopic ellipsometry (SE). A significant mass uptake of water vapor by the CNC films was detected using the QCM-D upon increasing relative humidity. In addition, thickness changes proportional to changes in relative humidity were detected using SE. Quantitative analysis of the results attained indicated that in preference to being soaked by water at the point of hydration each individual CNC in the film became enveloped by a 1 nm thick layer of adsorbed water vapor, resulting in the detected thickness response.
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vapor sorption properties of isotropic, homogeneous films of cellulose nanocrystals (CNCs): rigid nanoscaled rods, derived by acid hydrolysis from the plant matrix.14 CNCs provide a suitable platform to study the interfacial interactions of water vapor and solid surfaces due to the fact that not only are they known to be impregnable by water,15 their extensive specific surface area provides a vast number of adsorption sites. The water vapor uptake of the CNC films was assessed by quartz crystal microbalance with dissipation monitoring (QCM-D) and spectroscopic ellipsometry (SE). The combination of these techniques and materials provide a unique template to study vapor adsorption on a biologically derived surface: water is taken up exclusively on the surface of CNCs; the thickness of the surface layer can be monitored by thickness changes within the whole film by SE while QCM-D accurately provides the mass increase due to accumulated vapor. Although the combination of SE and QCM-D has previously been utilized in adsorption studies,16−18 no entries exist where water vapor adsorption on plant-based materials has been explicitly studied with this combination.
INTRODUCTION Since water vapor accounts for 2−3% of Earth’s atmosphere, vast amounts of research have been carried out to understand and utilize the intricacies of water vapor sorption; examples of such include corrosion research1 and sensor materials,2 among others. While the fundamentals of vapor adsorption have been probed extensively on various inorganic materials like mica,3 silicon dioxide,4 and graphite,5 research on biological materials has focused on interactions with liquid water because of its practical relevance: for example, the structural role of water in protein conformation6 or in turgor pressure in plants.7 Vapor uptake can, however, be just as important in many biological phenomena. In addition, the predominant trend in materials technology is to substitute synthetic materials with natural ones like cellulose, the principal structural component of all plants. Examples of such include packaging and barrier materials,8 diagnostics,9 and pharmaceuticals,10 not to mention the extensive literature on applications as niche materials, like anticoagulants,11 super-strong composites,12 or chiral templates.13 Vapor adsorption is also relevant in such applications as biological materials are more sensitive toward humidity than synthetic materials. The challenge with analyzing the vapor adsorption of biological materials is that most techniques designed for fundamental research are only suitable for flat, homogeneous, and ideally single crystal surfaces. Here, we investigated the © XXXX American Chemical Society
Received: May 13, 2015 Revised: October 9, 2015
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DOI: 10.1021/acs.langmuir.5b01763 Langmuir XXXX, XXX, XXX−XXX
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prior to the mass change determinations. Before the water vapor adsorption measurements, the CNC thin films were allowed to stabilize inside the humidity module at 11% RH for approximately 18 h by passing a saturated solution of LiCl through the module at a rate of 100 μL min−1.23 Finally, the water vapor adsorption experiments, with six incrementally increasing values of % RH, were carried out using five different saturated salt solutions and Milli-Q water (Table S1). Each salt solution was passed through the QHM 401 humidity module at a rate of 0.1 mL min−1 for 30 min at 23 °C. When Milli-Q water is passed through the humidity module, the resulting rapid increase in % RH causes possible condensation effects inside the measurement chamber. As a result, any changes in frequency (Δf) and dissipation (ΔD) (see below) detected during this step of the experiment are unreliable. The highest possible % RH achievable with the QCM-D setup used was 97% RH (K2SO4 saturated salt solution)as such, it was assumed that as complete a hydration as possible occurred at this % RH. The QCM-D device detects changes in frequency (Δf) and dissipation (ΔD) using several overtones simultaneously (QTools Software); however, for the sake of comparability, mass change analyses due to water vapor adsorption were performed on experimental values taken from the third overtone (15 MHz). In addition, frequency and dissipation change data from all of the overtones were compared to gain more information on the viscoelastic properties of the CNC thin films when exposed to different humidity levels. The change in dissipation energy (ΔD = D − D0) is defined as eq 2:
EXPERIMENTAL SECTION
Materials. Ashless Whatman filter paper was obtained from Whatman, GmbH, Dassel, Germany. Sulfuric acid (H2SO4), sodium chloride (solid) (NaCl), and 0.1 M sodium hydroxide (NaOH) were purchased from Sigma-Aldrich Finland Oy, Helsinki, Finland. Lithium chloride (solid) (LiCl), magnesium chloride hexahydrate (solid) (MgCl2·6H2O), magnesium nitrate hexahydrate (solid) (Mg(NO3)2· 6H2O), and potassium sulfate (solid) (K2SO4) were purchased from VWR Chemicals (VWR International, Helsinki, Finland). Ethanol (Aa grade 99.5% w/v) was purchased from the Altia Corporation (Rajamäki, Finland). Water was purified in a Milli-Q system (Millipore Corporation, resistivity 18.2 MΩ cm). CNC thin films were prepared on AT-cut silicon dioxide QCM-D sensors purchased from Q-Sense, AB, Gothenburg, Sweden, with a fundamental resonance frequency, f 0, of 5 MHz and a sensitivity constant, C, of approximately 0.177 mg m−2 Hz−1. CNC Preparation. CNCs were prepared from cotton-based Whatman 1 filter paper using an established procedure with a 64 wt % (45 °C, 45 min) sulfuric acid hydrolysis.14,19 10 g of finely ground Whatman filter paper was hydrolyzed in 175 mL of 64% w/w sulfuric acid at 45 °C for 45 min. Hydrolysis was quenched by the addition of 1800 mL of Milli-Q water. The resulting suspension was then centrifuged to remove the excess water, after which the CNCs were dialyzed using Milli-Q water until their conductivity was
