Properties of cell walls prepared using supercritical fluids
Paul Callaghan (VUW), Robert Franich, Stefan J. Hill, and Roger Newman (Scion)
Presentation overview:
•Wood cell wallspolymerswater
•Fibre-saturation point
•Supercritical CO2
•Water extraction
•Wood material properties
•Summary
Structure of wood cell wall
H2O ?
Fibre-saturation point
Stamm et al, 1935-1971: cell water as gas, liquid, ‘solid solution’ phases
Cells from green wood –full lumens & fully-water-swollencell walls – xylem sap
FSP –empty lumens & fully-water-swollencell walls
-a chemical phenomenon at molecular/supramolecular scales
•FSP- independent of scale –log/fibres•Discrete water binding sites - OH•Exothermic wood cell wall hydration – a molecular chemical interaction
Partial specific volume of water at %mc below FSP
Stamm & Seborg, 1935; Stamm, 1967
‘Compression’ of water molecules at binding sites
200 bar
1000 bar
Wood material dynamics
Wood at variable moisture content – dimensional, conformational & MoE change
Water an integral part of the cell wall supramolecular cellulose-hemicellulose-lignin nanocomposite dynamics-
- ‘Velcro’ mechanics
Where is water located and how structured?
Difficulty in preparing wood specimens for study of FSP &material dynamics – heating, solvents, azeotropes, critical-point, high-pressure
Supercritical CO2 water extraction
Theory: Physical-chemical interaction of cell water and carbon dioxide according to Henry’s Law, the Le Chatelier Principle and the Phase Rule.
Variables:P, T, phase (gas, liquid, supercritical)
[CO2] P, 1/T
CO2 + H2O H2CO3 H+ + HCO3- H+ + CO3
2-
F = C – P + 2
The reactant’s properties
80.36 ε0 at 20 °C
40.68
108.9
1.85
Rel static permittivity
Dipole moment D
Enthalpy evap kJ/mol
Entropy evap J/(mol·K)
1.60 ε0 at 0 °C, 50 B
0
15.33 at –57.5°C
70.8
Supercritical Fluids
Phase-changedriving chemicalchange
Supercritical CO2 wood dewatering
Fibre SaturationPoint
Comparison of oven-dried (105C) and
SCCO2 dewatered radiata pine wood
Dewatering & air-drying
0
50
100
150
200
250
1 3 5 7 9 11 13 15 17 19 21 23 25
y = -33.837x + 233.2
y = -1.4143x + 36.781
FSP=28%
Dewatering & drying sequence
Wo
od
sp
ecim
en m
c (%
)
SCCO2 chemi-mechanical dewatering
Diffusion-evaporation drying
Practical specimen preparation
Wo
od
sp
ecim
en m
c (%
)
Preparation time (min)
0
50
100
150
200
250
0 2 4 6 8 10 12
5 SCCO2 – gaseous CO2 sequences
1H NMR imaging of SCCO2 dewatering
Cycle
105% 96% 94% 91% 91% mean0 168% 216% 221% 231% 223% 159%
192% 199% 207% 218% 208%165% 173% 167% 172% 123%116% 117% 117% 123% 144%
107% 146% 151% 161% 165% mean1 103% 101% 99% 91% 89% 142%
155% 167% 165% 162% 157%185% 200% 207% 209% 199%105% 116% 113% 103% 94%
91% 96% 94% 86% 83% mean2 143% 151% 157% 152% 138% 113%
82% 86% 103% 122% 84%161% 111% 75% 73% 89%124% 142% 141% 131% 105%
44% 43% 43% 43% 48% mean3 69% 72% 75% 81% 70% 59%
72% 81% 91% 95% 76%46% 50% 52% 54% 48%43% 42% 43% 44% 49%
45% 38% 36% 40% 46% mean4 50% 46% 37% 36% 38% 43%
44% 53% 56% 40% 38%39% 39% 47% 54% 48%45% 38% 38% 40% 46%
39% 39% 39% 39% 41% mean5 43% 41% 40% 40% 42% 40%
41% 45% 43% 40% 41%37% 37% 36% 37% 37%39% 39% 38% 38% 40%
SCCO2 dewatering
Preparation of woodspecimen with uniform mc distributionapproaching FSP
Re-wetting?
Further drying?
15968
402
Solid State 13C NMR Spectrum
O
HO
HO
OH
OH
OH
C1
C2
C3
C4
C5
C6
Pinus radiata wood at 12% mc
180 160 140 120 100 80 60 40 20 ppm
acetyl acetyl
methoxylignin
C1c
C1h
C4i/sC6i/s
C3,5
C2
Cellulose LigninHemicellulose
T1(H) Nutation-Diffusion NMR
Lignin
Cellulose
Hemicellulose
Lignin
T1
(H)
(ms)
Water in cell wall at FSP
NMR evidence of a water layer between the cellulose aggregate -phase and the matrix
in the supramolecular nanocomposite of green wood.
Stamm’s ‘solid-solution’ ?Cellulose
Hemicellulose
Lignin
Water layer
SCCO2 dewatering process - summary
Reaction of lumen water with SCCO2
with reversal of chemistry in gas phase
Bound water bond strength higher than enthalpy of SCCO2 reaction – no change
Wood material derived approaching FSP from green
Material suitable for wood-water dynamics & cell wall studies – Green to FSP & below
Acknowledgements
Hank Kroese, Suzanne Gallagher,
Bernard Dawson & Meeta Patel
FRST – ‘Wood Products for the Future’
Contract C04X0205