Growney Lubrizol - National Physical Laboratory

The Use of Analytical Centrifugation to Monitor Carbon black and Soot Dispersion

Stability

David Growney

([email protected])

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.

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Background – Why soot dispersion stability?

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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

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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


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?


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(1) Calculating Effective density

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=

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

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Effective density – Why is this important?

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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



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)

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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

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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.

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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…


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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

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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


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Is Regal 250R a suitable model for diesel Soot?PS-PEP diblock copolymer dispersant


TEM shows well dispersed primary particles of carbon black/soot

Carbon Black Soot

TEM

OM

TEM

OM


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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


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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


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Carbon Black Soot


TEM shows relatively well dispersed primary particles of carbon black/soot

TEM

OM

TEM

OM


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Carbon Black Soot


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

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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

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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

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