The Types of Water in Cellulosic Fibers: DSC and NMR Spectroscopy

Outline Types of water Differential Scanning Calorimetry Nuclear Magnetic Resonance Spectroscopy Analytical determination of types of water Summary

Purpose Types of water are removed in different ways Water removal huge component of papermaking costs Trend towards use of non-wood fibers Has become increasingly important to understand how cellulosic materials behave in paper drying process Behavior should be known for wood fibers for effective comparisons Differential Scanning Calorimetry (DSC) and Nuclear Magnetic Resonance (NMR) spectroscopy If you know what kind of water is in the fiber and how much, it is easier to remove it efficiently Study the water using DSC and NMR DSC can evaluate the fractions of water determine concentration and drying rate NMR can determine location of fractions and drying rate

Types of water Free/Bulk water Freezing bound water Found in large pores Makes up inter-fiber free water in pores and intra-fiber water in lumen Removed by centrifugation, melting point similar to bulk water Freezing bound water Melting and freezing temperature depressed by about 2°C due to small size of micropores and presence of polymers like hemicellulose Loss causes most detrimental irreversible pore closure Hubbe, M. A. WPS 527 Coursepack. North Carolina State University. Heikkinen, S. et al. “NMR Imaging and Differential Scanning Calorimetry Study on Drying Pine, Birch, and Reed Pulps and Their Mixtures.” Journal of Applied Polymer Science. 2006. vol. 100. p. 937-945.

Types of water Non-freezing bound water “Water of hydration” associated with various surfaces Hydrogen-bonded to hydroxyl and carboxylic acid groups in micropores Amount calculated by subtracting total freezing water from the moisture ratio of the sample Hubbe, M. A. WPS 527 Coursepack. North Carolina State University. Heikkinen, S. et al. “NMR Imaging and Differential Scanning Calorimetry Study on Drying Pine, Birch, and Reed Pulps and Their Mixtures.” Journal of Applied Polymer Science. 2006. vol. 100. p. 937-945.

Differential Scanning Calorimetry (DSC) Most common analytical method for bulk surface measurements Measure difference in heat flow rate between a sample and an inert reference material as a function of time and temperature Two types Exothermic Endothermic Exothermic: Heat flows out of sample as result of cooling, crystallization, or oxidation Endothermic: Heat flows into sample due to heating or process like melting, evaporation, or a glass transition

DSC Temperature range about -100°C to 900°C Major applications Determination of drug purity, reaction time for enzyme degradation, degree of crystallization at a particular temperature Two modes: Heat flux Power compensated TA Instruments Q100 http://www.tainstruments.com/product.asp?n=1&id=16 Lucia, L. “DSC: A bulk analytical technique.” Lecture notes: WPS 595b Biomaterials Characterization. North Carolina State University. 9 February 2006.

Heat flux DSC Sample and reference heated or cooled by separate heating units Furnaces keep temperatures isothermal to one another throughout the test. Objective is to monitor electrical power used by heaters as temperatures are either increased or decreased linearly Power being sent to heaters is adjusted so that the same temperature is maintained for both sample and reference Difference in power to keep temperatures the same is used to generate curve Sample and reference positioned above their heaters Average temperature control circuit used to monitor the progress of the temperature control program Circuit insures that assigned temperature is actual average temperature of the sample and reference Skoog, D. A., Holler, F. J., & Nieman, T. A. (1998). Principles of Instrumental Analysis. 5th edition. Thomson Learning Inc. Bhadeshia, H. K. D. H. “Differential Scanning Calorimetry.” University of Cambridge, Materials Science & Metallurgy. 2002.

Power-compensation DSC Same heat energy transferred to sample and reference Transported heat carefully controlled Temperature difference is monitored Thermocouple below sample and reference platforms sends a measurement of the relative temperatures which are then converted to a differential heat flow between the two materials Skoog, D. A., Holler, F. J., & Nieman, T. A. (1998). Principles of Instrumental Analysis. 5th edition. Thomson Learning Inc. Bhadeshia, H. K. D. H. “Differential Scanning Calorimetry.” University of Cambridge, Materials Science & Metallurgy. 2002.

Nuclear Magnetic Resonance (NMR) Spectroscopy Determination of molecular structure for organic and inorganic compound Based on measurement of adsorption of electro-magnetic radiation in the radio frequency range 60 to 800 MHz Concept that certain atomic nuclei have magnetic and spin moments 1H, 13C, 19F, and 31P

NMR Spin and charge of the nuclei cause behavior similar to bar magnets Energy levels split causing nuclei to possess one of two magnetic forces Each nucleus is able to switch between energy states through the absorption of a photon.

NMR Placing nuclei in strong magnetic field, can monitor energy transition when photon is absorbed The excess of lower energy spin is essential for monitoring of the energy adsorption later When nuclei then subjected to radio waves some absorb radiation and are raised to higher energy Difference in energy between low and high energy states provides signal Signal sensitivity is directly proportional to magnetic field When no magnetic field is present, sample contain an identical number of nuclei for each spin state. They align with the magnetic field because it is a lower energy state Once subjected to radio waves, some are then aligned against magnetic field As new machines are developed, the magnetic fields increase in strength and sensitivity increases Skoog, D. A., Holler, F. J., & Nieman, T. A. (1998). Principles of Instrumental Analysis. 5th edition. Thomson Learning Inc. Argyropoulos, D.S. “NMR Spectroscopy.” Lecture notes: WPS 595b Biomaterials Characterization. North Carolina State University. 24 January 2006.

NMR Distinguish between nuclei of different elements because of chemical shifts in peaks Fourier transform Two-dimensional Two different pulse frequencies bombard sample Pulsed Radiofrequency (RF) radiation 90-degrees to magnetic field causes nuclei to jump into higher-energy alignment Pulse simultaneously excites nuclei in all local environments Nuclei re-emit RF radiation and create interference pattern known as a free-induction decay (FID) Chemical shifts due to electron environment surrounding nuclei Frequencies are extracted from the FID by a Fourier transform of the time-based data. http://www.chemistry.adelaide.edu.au/external/soc-rel/content/ftnmr.htm

Analytical determination DSC: Detect different types of water using procedures analogous to paper drying Temperature fluctuations help determine which type of water is present and in what quantity NMR: Observe effects of drying because protons in each type of water give off specific signal Proton-NMR well-suited for study of water/cellulose interactions, relaxation times, and for distribution of moisture within paper sheet

Ogiwara et al- NMR Level of bound water strongly dependent on type and condition of fibers Temperature-dependence of experimental readings Amount of detectable bound water was more accurate as experimental temperature decreased This boundary temperature is specific for each cellulose sample and a given water content and corresponded well with the glass transition temperature of the system Below the Tc value, water molecules trying to force their way into cellulose actually destroy the cellulose-cellulose hydrogen bonds and form new water-cellulose hydrogen bonds. The water in that region is therefore without the ability to move. This causes the proton relaxation time in NMR to become longer than that for free water. Above the boundary temperature however, the cellulose-cellulose bonds are easily broken to form water-cellulose ones and the water therefore has a large freedom of motion. Ogiwara, Y., Kubota, H., & Hayashi, S. “Temperature Dependency of Bound Water of Cellulose Studied by a High-Resolution NMR Spectrometer.” Journal of Applied Polymer Science. 1970. vol. 14 p.303-309.

Ogiwara et al- NMR Determined boundary temperature, Tc, where water molecules become bound to cellulose The lower the water content became, the greater the changes in Tc Similar DSC studies agreed that glass transition temperature was lower for moist cellulose than for an air-dried sample These results showed that Tc represents the glass transition temperature for a given compound

Heikkinen- DSC and NMR NMR Drying setup for NMR imaging probe used to measure water contents of pulps gravimetrically as a function of drying time Drying rates used during the imaging process Water content decreased rapidly to an inflection point around 61-68% water by weight Decreased again to a point just below 37-45% weight Remaining moisture tightly bound to fibers and was hard to remove Narrowest lines and highest intensities matched with highest water content Increases in line widths with time correspond to decreases in water content and matched two inflection points Water distribution measured using two-dimensional NMR imaging before and after drying There will be graphs from this paper

Heikkinen- DSC and NMR DSC Used to determine fraction of each type of water and rate at which it disappeared from the sample Explain method briefly Graphs There will be graphs and further explanation

Summary Three types of water Analyze using Each treated differently to maximize efficiency Analyze using DSC NMR Combination Movement towards utilization of non-wood fibers for papermaking Will elaborate on these more