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The Use of Analytical Centrifugation to Monitor Carbon black and Soot Dispersion
Stability
David Growney
30.11.16
1
Overview
(1) The importance of knowing the effective particle density for dispersant-adsorbed carbon black/soot particles for particle size
analysis via analytical centrifugation
(2) Is Carbon Black a Suitable Model Colloidal Substrate for Diesel Soot?
Growney, D. J.; Fowler, P. W.; Mykhaylyk, O. O.; Fielding, L. A.; Derry, M. J.; Aragrag, N.; Lamb, G.; Armes, S. P.; Langmuir 2015, 31, 8764-8773.
Growney, D. J.; Mykhaylyk, O. O.; Middlemiss, L.; Fielding, L. A.; Derry, M. J.; Aragrag, N.; Lamb, G.; Armes, S. P. Langmuir2015, 31, 10358-10369.
2
Background – Why soot dispersion stability?
3
Soot is a result of incomplete combustion - can stick to engine internals and irreversibly damage engine parts
Surfactants are incorporated into engine oil formulations to disperse soot so it can
be removed during vehicle service interval
BP were interested in determining which polymers give the best soot
dispersion performance
Background – Typical dispersant chemistry
4
Growney, D. J.; Mykhaylyk, O. O.; Armes, S. P.; Langmuir 2014, 30, 6047-6056
Diameter ~ 70 nm
400 nm
Relative pressure /P/P0
1/[
W((
P 0/P
)-1
)]
160
120
80
40
00.04 0.08 0.120 0.16 0.20 0.24 0.28 0.32
••
••
•BET surface area, As = 43 m2 g-1
Carbon black density, ρ = 1.89 g cm-3
(from helium pycnometry)
Use As = 6 / ρ.D
Carbon black primary grain size, D = 74 nm
A lower limit particle diameter?
Background - Why Carbon Black?Widely used as a model colloidal
substrate for genuine engine soot, due to expense of extraction ( ̴ £80k)
5
Background - How do linear diblock copolymers adsorb onto carbon black from n-alkanes?
Micelle adsorption
It has been shown that linear diblock copolymers adsorb onto carbon black as micelles rather than unimers
Growney, D. J.; Mykhaylyk, O. O.; Armes, S. P.; Langmuir 2014, 30, 6047-6056
Carbon Black
Carbon Black
6
0
0.5
1
1.5
2
2.5
3
3.5
4
0 1000 2000 3000 4000 5000 6000 7000 8000 9000
Γ = 3.5 mg m-2
Γ /
mg
/m2
Linear diblock concentration /ppm
Copolymer adsorption indirectly via UV supernatant assay (equilibrium micelles, n-heptane, 20oC)
Copolymer adsorption directly via TGA
5.49 %
7.18 %
8.75 %
1.35 %
0.60 %
6 wt. %
8 wt. %
10 wt. %
2 wt. %
Wei
ght
loss
/%
Temperature /˚C
Carbon blackis stable in N2
up to 600oC
Copolymer added
Copolymer adsorbed
Background - How much copolymer adsorption?
Growney, D. J.; Mykhaylyk, O. O.; Armes, S. P.; Langmuir 2014, 30, 6047-6056
7
(1) Calculating Effective density
8
=
Assume:
(i) density of copolymer shell is approximately that of solvent(ii) spherical particles
Growney, D. J.; Fowler, P. W.; Mykhaylyk, O. O.; Fielding, L. A.; Derry, M. J.; Aragrag, N.; Lamb, G.; Armes, S. P.; Langmuir 2015, 31, 8764-8773
(iii) Adsorbed layer thickness = star-like micelle radius
R = Radius of uncoated carbon black = 37 nmM 1 = Mass fraction of carbon black
ρ 1 = Density of carbon black = 1.89 g cm-3
x = Thickness of adsorbed PS-PEP copolymer = 22 nm
M 2 = Mass fraction of copolymer
ρ 2 = Density of copolymer/solvent shell = 0.75 g cm-3
Lascelles, S. F.; Armes, S. P.; J. Mater. Chem 1997, 7, 1339-1347
Considerations of relative volume, mass fractions for core and shell enables
calculation of effective particle density ρcomp
Effective particle density (ρcomp) for copolymer-coated carbon black particles is given by:
- Much lower than carbon black!
mcomp = mass of composite particle
Vcomp = volume of composite particle
Assume: x = star-like micelle radius
Calculate effective particle density of 1.03 g cm-3 for PS-PEP (28 mol% PS)
Calculating Effective density
Growney, D. J.; Armes, S. P. et al; Langmuir 2015, 31, 8764-8773
9
Effective density – Why is this important?
10
Growney, D. J.; Fowler, P. W.; Mykhaylyk, O. O.; Fielding, L. A.; Derry, M. J.; Aragrag, N.; Lamb, G.; Armes, S. P.; Langmuir 2015, 31, 8764-8773
Analytical centrifugation (LUMiSizer) relies on input of
sedimenting particle density for particle size analysis
ρ = 1.03 g cm-3ρ = 1.89 g cm-3
(c)(b)(a)
Carbon Black
20 mol% polystyrene content
35 mol% polystyrene content
28 mol% polystyrene content
What is the effect of micelle core thickness on carbon black stabilisation?
Altering diblock copolymer polystyrene content changes micelle core diameter and hence adsorbed layer thickness
94 nm micelles (DLS) 87 nm micelles (DLS) 84 nm micelles (DLS)
11
Does micelle size affect dispersion stability?
35 mol% PS content 28 mol% PS content 20 mol% PS content
Diblock copolymers with higher polystyrene contents show less sedimentation over a given time – higher stability 12
35 mol% PS content 28 mol% PS content 20 mol% PS content
Diblock copolymers with higher polystyrene contents form larger micelles and hence aid dispersion of carbon black particles – steric stabilisation
13
Does micelle size affect dispersion stability?
(2) Carbon Black versus Soot
Growney, D. J.; Mykhaylyk, O. O.; Middlemiss, L.; Fielding, L. A.; Derry, M. J.; Aragrag, N.; Lamb, G.; Armes, S. P. Langmuir 2015, 31, 10358-10369.
14
Already widely used as a model colloidal substrate for genuine engine soot
Can we use carbon black as a model colloidal substrate?
Need to determine whether a particular carbon black (Regal 250R, Cabot) can be used as a soot mimic
Numerous techniques have been used to characterise the two…
Growney, D. J.; Mykhaylyk, O. O.; Middlemiss, L.; Fielding, L. A.; Derry, M. J.; Aragrag, N.; Lamb, G.; Armes, S. P. Langmuir 2015, 31, 10358-10369.
15
Carbon Black versus Soot – XPS Analysis
XPS can be used to assess differences in sulphur content between Regal 250 R carbon black and diesel soot 17
Carbon Black versus Soot – XPS Analysis
XPS can be used to assess differences in surface chemistry via oxygen content for Regal 250 R carbon black and diesel soot 18
18
Carbon Black versus Soot – PS-PEP Adsorption Isotherms
UV spectroscopy can be used to determine adsorbed amount via supernatant depletion assay
Very similar equilibrium adsorbed amounts for carbon black versus soot
Copolymer adsorption indirectly via UV supernatant assay (equilibrium micelles, n-dodecane, 20oC)
Optical microscopy shows very similar results for both dispersions (micron scale)
Carbon Black Soot
OMOM
Growney, D. J.; Mykhaylyk, O. O.; Middlemiss, L.; Fielding, L. A.; Derry, M. J.; Aragrag, N.; Lamb, G.; Armes, S. P. Langmuir 2015, 31, 10358-10369.
19
Is Regal 250R a suitable model for diesel Soot?PS-PEP diblock copolymer dispersant
Optical microscopy shows very similar results for both dispersions (micron scale)
TEM shows well dispersed primary particles of carbon black/soot
Carbon Black Soot
TEM
OM
TEM
OM
Growney, D. J.; Mykhaylyk, O. O.; Middlemiss, L.; Fielding, L. A.; Derry, M. J.; Aragrag, N.; Lamb, G.; Armes, S. P. Langmuir 2015, 31, 10358-10369.
20
Is Regal 250R a suitable model for diesel Soot?PS-PEP diblock copolymer dispersant
Optical microscopy shows very similar results for both dispersions (micron scale)
Is Regal 250R a suitable model for diesel Soot?PS-PEP diblock copolymer dispersant
TEM shows well dispersed primary particles of carbon black/soot
Analytical centrifugation gives extremely similar particle size distributions
Carbon Black Soot
TEM
OM
TEM
OMLUMiSizerLUMiSizer
Growney, D. J.; Mykhaylyk, O. O.; Middlemiss, L.; Fielding, L. A.; Derry, M. J.; Aragrag, N.; Lamb, G.; Armes, S. P. Langmuir 2015, 31, 10358-10369.
21
Growney, D. J.; Mykhaylyk, O. O.; Middlemiss, L.; Fielding, L. A.; Derry, M. J.; Aragrag, N.; Lamb, G.; Armes, S. P. Langmuir 2015, 31, 10358-10369.
22
Is Regal 250R a suitable model for diesel Soot?Dispersant olefin copolymer
Carbon Black Soot
Optical microscopy shows larger agglomerates with soot versus carbon black
OMOM
Growney, D. J.; Mykhaylyk, O. O.; Middlemiss, L.; Fielding, L. A.; Derry, M. J.; Aragrag, N.; Lamb, G.; Armes, S. P. Langmuir 2015, 31, 10358-10369.
23
Is Regal 250R a suitable model for diesel Soot?Dispersant olefin copolymer
Carbon Black Soot
Optical microscopy shows larger agglomerates with soot versus carbon black
TEM shows relatively well dispersed primary particles of carbon black/soot
TEM
OM
TEM
OM
Growney, D. J.; Mykhaylyk, O. O.; Middlemiss, L.; Fielding, L. A.; Derry, M. J.; Aragrag, N.; Lamb, G.; Armes, S. P. Langmuir 2015, 31, 10358-10369.
24
Is Regal 250R a suitable model for diesel Soot?Dispersant olefin copolymer
Carbon Black Soot
Optical microscopy shows larger agglomerates with soot versus carbon black
TEM shows relatively well dispersed primary particles of carbon black/soot
Analytical centrifugation gives extremely different particle size distributions
TEM
OM
TEM
OM LUMiSizerLUMiSizer
Conclusions (1)
It is important to understand soot dispersion stability - can stick to engine internals and irreversibly damage engine parts
Diblock copolymers adsorb as micelles onto carbon black/soot –adsorbed amount can be understood via UV spectroscopy/TGA
Diblock copolymer micelle core thickness affects dispersion stability – larger core, thicker adsorbed layer, greater stabilisation effect
25
Sedimenting particle density is a very important parameter for calculating diameter via analytical centrifugation – it is essential to calculate effective particle density for
polymer-adsorbed carbon black/soot prior to particle size analysis
Conclusions (2)
Carbon black is widely used as a model colloidal substrate for genuine diesel soot
Optical microscopy, TEM and analytical centrifugation can be used to compare and contrast colloidal dispersions of carbon black versus soot
26
XPS can help to characterise the surface chemistries, whilst UV spectroscopy can give us
an understanding of differences in adsorbed amount with carbon black versus soot
Acknowledgements
Prof. Steve Armes FRS
BP (Castrol)
University Of Sheffield
Armes Group
27