1

Evaluation of currently available

methods for determining

log Kow values of surfactants

Geoff Hodges on behalf of the ERASM Hydrophobicity

of Surfactants Task Force (HoS TF).

SETAC Nantes, 22-26 May 2016

ERASM

Joint platform of the

European detergent and

surfactants producers.

Represented by A.I.S.E* and

CESIO**

Initiates and co-ordinates

joint industry activities for

improving the knowledge

about the Environmental and

Human Health risk

assessment of detergent-

based surfactants.

2

Environmental and Health Risk

Assessment and Management

(ERASM)

*Association Internationale de la Savonnerie,

de la Détergence et des Produits d’Entretien

**Comité Européen des Agents Surface et de

leurs Intermédiaires Organiques

Introduction

Requirement of log Kow for regulatory submissions such as REACh (not

accepted for surfactants under Harmonised Offshore Chemical Notification

Format (HOCNF)).

Experimental determination of octanol-water partition coefficients (log Kow) for

surfactants is challenging due to their amphiphilic nature (accumulating at

interfaces, phase separation, emulsion formation).

There is concern that existing methods for the experimental determination of log

Kow may give rise to technical difficulties and/or unreliable results.

Log Kow is used as an indicator for the tendency towards bioaccumulation (BCF).

Inaccurate determination can lead to incorrect assignment of bioaccumulation

potential.

Commercially available calculation methods have not been developed

specifically for surfactants.

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4

Experimental Determination of Kow

Partition Coefficient = Conc. of neutral species dissolved in partition solvent /

Conc. of neutral species dissolved in water

Distribution Coefficient = Conc. of all species dissolved in partition solvent /

Conc. of all species dissolved in water

log Kow = log [Boct]/ log [Bwater]

Water

octanol

Boct

Bwater

BH+oct

BH+water

1

1

11

11

1

1

5

Experimental Determination of Kow

log Kow = log [Boct]/ log [Bwater]

Consideration 1: Surfactants accumulate at

the interface of hydrophobic and hydrophilic

phases = measurement of log Kow is not well

defined and hence its measurement a

technical challenge

Consideration 2: Traditional methods (in

particular OECD 107 shake flask method)

tend to lead to emulsion formation.

Consideration 3: The majority of ionisable

surfactants (i.e. excluding non-ionics) have pKa

values which mean they are fully or largely

ionised under environmentally relevant

conditions:

anionics = very low (<5)

cationics = high (>9)

Water

octanol

Boct

Bwater

BH+oct

BH+water

1

1

1

1

1

1

1

1

6

Log Kow versus Log D

log Kow = log [Boct]/ log [Bwater]

log D is more relevant

Key factors to understand: pKa and pH

pKa can be predicted but predictions

can be variable

𝐥𝐨𝐠 𝐃𝐚𝐜𝐢𝐝𝐬 = 𝐥𝐨𝐠 𝐊𝐨𝐰 + 𝐥𝐨𝐠𝟏

𝟏+𝟏𝟎𝒑𝑯−𝐩𝐊𝐚to

correct for ionisation Water

octanol

Boct

Bwater

BH+oct

BH+water

1

1

1

1

1

1

1

1

Solubility: Liquid crystalline phases in

binary surfactant systems

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Idealised structures that may be encountered as the concentration

of surface active agent increases in a surface-active agent + water

system

Surfactants dissolve not only in

their mono-molecular form, but

they also may form different

types of soluble aggregates in

water

The maximum mono-molecular

solubility of a surfactant is

defined by as the critical micelle

concentration (CMC).

Micellular solubility (up to phase

separation of lyotropic liquid

crystals) can be several orders of

magnitude higher than CMC

(therefore CMC underestimates

solubility)Ref: cohengroup.lassp.cornell.edu/content/droplet-breakup-

structured-fluids

8

Overview of use of current methods in

REACh submissions

Log Kow methodUse of log Kow methods for surfactants

(% of total surfactants)

2010 submissions 2013 submissions

Shake flask (OECD 107) 0 3

HPLC (OECD 117) 12 3

Slow-stir (OECD 123) 14 29

Computational (CMC + water solubility) 35 25

Computational (CMC alone) 11 11

Calculation methods – KOWWIN 20 29

Calculation methods – other 8 0

Total percentage 100 100

Aims of ERASM HoS TF

1. To advise on the suitability (or not) of existing methods for

determining the log Kow of surfactants

2. To carry out a programme to compare currently available

experimental and predicted methods for generating log Kow data for

different surfactant classes

3. To assess and recommend (where required) an alternative approach

or methodology for the determination of hydrophobicity of surfactants

that may not suffer from the drawbacks of established log Kow

methods

4. Improve bioaccumulation (BCF) assessments of compounds

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Compound structures

10

Neutral OO

OO

OHC8EO4

OO

OO

OHC12EO4

OO

OO

OO

OO

OHC12EO8

Anionic

OS

O

O

O Na+

C12SO4

Na+

OS

O

O

O

OO

OOC12EO4SO4

C11COOHO

O

Cationic

N+

ClC12TMAC

N+

ClC16TMAC

N+

ClC18BAC

References

N

N

N

Cl

NH

NH

OH

Cl

Cl

Cl

Cl

Cl

ATR PCPAmphoteric N+

OC12DMAO

N+

O O

C12DMB

C12APBNH

O

N+

O OH

OH-

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ERASM project - Hydrophobicity of surfactants

Log Kow calculation

KOWWIN, SPARC, COSMOtherm, ClogP, ACD, AlogP, Molinspiration, Hansch & Leo,+ atomic fragment methods in Chemdraw (Crippen/ Viswanadhan/ Broto)

Log Kow measurement

Slow stir (OECD 123),

Reduces emulsion formation; Operates below the critical micelle concentration (CMC); Has been applied to highly hydrophobic substances up to log Kow 8.2 (De Bruijn et al., 1989).

HPLC (OECD 117),

Adjustments made to the mobile phase to accommodate log Kow > 6 which may be expected for the more hydrophobic surfactants.

Computational method. (Kow = C n-octanol/C water)

pH Metric for ionic substances (underway).

Proton nuclear magnetic resonance (H-NMR) (underway)

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ERASM project - Hydrophobicity of surfactants

Log Kow calculation

KOWWIN, SPARC, COSMOtherm, ClogP, ACD, AlogP, Molinspiration, Hansch & Leo,+ atomic fragment methods in Chemdraw (Crippen/ Viswanadhan/ Broto)

Log Kow measurement

Slow stir (OECD 123),

Reduces emulsion formation; Operates below the critical micelle concentration (CMC); Has been applied to highly hydrophobic substances up to log Kow 8.2 (De Bruijn et al., 1989).

HPLC (OECD 117),

Adjustments made to the mobile phase to accommodate log Kow > 6 which may be expected for the more hydrophobic surfactants.

Computational method. (Kow = C n-octanol/C water)

pH Metric for ionic substances (underway).

Proton nuclear magnetic resonance (H-NMR) (underway)

Nonionics HPLC + Stir flask pH7

Anionics Stir flask +

Computational

pH7 +

pH2 and pH9 for carboxylate

Cationics Stir flask +

Computational

pH7

Amphoterics Stir flask +

Computational

pH7

Computational method (referred to in OECD 107)

Determines the solubility of the surfactant in n-octanol and water

separately to calculate log Kow

Surfactant solubility in n-octanol was determined by adapting the

OECD Test Guideline105.

The CMC was used as a surrogate for water solubility.

CMC studies were performed by two methods:

1. Surface tension of the solutions (OECD 115) (by Fraunhofer IME).

2. Using Solid Phase Micro Extraction* (SPME method by University of

Utrecht) (Haftka et al., 2014; 2016).

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*The SPME method prevents the problems with emulsions observed in octanol-water systems and

Measures only freely dissolved concentrations of surfactant (non micellar/ non aggregated)

Computational method (referred to in OECD 107)

Determines the solubility of the surfactant in n-octanol and water

separately to calculate log Kow

Surfactant solubility in n-octanol was determined by adapting the

OECD Test Guideline105.

The CMC was used as a surrogate for water solubility.

CMC studies were performed by two methods:

1. Surface tension of the solutions (OECD 115) (by Fraunhofer IME).

2. Using Solid Phase Micro Extraction* (SPME method by University of

Utrecht) (Haftka et al., 2014; 2016).

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*The SPME method prevents the problems with emulsions observed in octanol-water systems and

Measures only freely dissolved concentrations of surfactant (non micellar/ non aggregated)

Results from the two CMC

methods generally are in

good agreement

Comparison of methods

HPLC gives higher

values than slow stir.

This reflects increase in

hydrophobicity

Differences between two

HPLC runs in two

different labs

HPLC only applicable to

non-ionics (at present)

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(1:1 line shown for perfect comparison)

C8EO4

C12EO4C12EO8

C8EO4

C12EO8

0

1

2

3

4

5

6

7

-1 1 3 5 7

HP

LC

lo

g K

ow

Slow Stirring log Kow

Fraunhofer

Eadsforth etal

Comparison of methods

Overestimation of log D

likely due to:

• underestimation of

water solubility using

CMC

• some components are

fully soluble in n-

octanol (both methods)

No consistent trend in

comparison of data

between computational and

slow-stir methods

(c) Not method favoured

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C12EO8

C12AS

C12E4S

C12COONa

C12TMAC

C16TMAC

C18BAC

C12-16ADBC12AAPB

0

1

2

3

4

5

0 1 2 3 4 5

Co

mp

uta

tio

na

l lo

g D

Slow Stirring log D

(1:1 line shown for perfect comparison)

QSPR comparison with slow-stir data

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Non-ionic Anionic Cationic Amphoteric

1 2 3 4 5 6 7 8 9 10 11 12

Slow-stirring 2.66 4.58 4.00 1.92 1.47 3.18 0.74 1.89 1.32 3.72 2.60 4.57

KOWWIN 1.71 3.67 2.57 2.42 1.32 5.00 3.22 5.18 7.87 1.46 0.69 4.67 Some agreement

CLOGP 2.55 4.67 4.12 4.52 4.27 1.10 0.56 2.68 6.36 4.66 -2.30 6.00 Some agreement

Molinspiration 2.42 4.40 3.62 1.78 2.63 2.33 1.85 3.87 6.47 -1.62 0.65 3.27 Some agreement

H&L 2.77 4.93 4.53 1.60 1.28 1.10 2.14 4.30 3.69 Some agreement

ALOGP 2.27 4.10 3.57 2.17 3.73 3.10 3.94 5.77 8.26 1.95 1.00 4.50 Some agreement

ACD Labs 1.85 3.97 2.54 1.28 4.33 No apparent agreement

Crippen F. 2.02 3.68 3.06 Some agreement

Viswanadhan's F. 1.87 3.45 2.79 No apparent agreement

Broto 2.51 4.33 4.00 3.46 3.13 4.49 Some agreement

SPARC 3.50 5.5 6.45 No apparent agreement

> 1 difference

< 1 difference

< 0.5 difference

QSPR comparison with slow-stir data

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Non-ionic Anionic Cationic Amphoteric

1 2 3 4 5 6 7 8 9 10 11 12

Slow-stirring 2.66 4.58 4.00 1.92 1.47 3.18 0.74 1.89 1.32 3.72 2.60 4.57

KOWWIN 1.71 3.67 2.57 2.42 1.32 5.00 3.22 5.18 7.87 1.46 0.69 4.67 Some agreement

CLOGP 2.55 4.67 4.12 4.52 4.27 1.10 0.56 2.68 6.36 4.66 -2.30 6.00 Some agreement

Molinspiration 2.42 4.40 3.62 1.78 2.63 2.33 1.85 3.87 6.47 -1.62 0.65 3.27 Some agreement

H&L 2.77 4.93 4.53 1.60 1.28 1.10 2.14 4.30 3.69 Some agreement

ALOGP 2.27 4.10 3.57 2.17 3.73 3.10 3.94 5.77 8.26 1.95 1.00 4.50 Some agreement

ACD Labs 1.85 3.97 2.54 1.28 4.33 No apparent agreement

Crippen F. 2.02 3.68 3.06 Some agreement

Viswanadhan's F. 1.87 3.45 2.79 No apparent agreement

Broto 2.51 4.33 4.00 3.46 3.13 4.49 Some agreement

SPARC 3.50 5.5 6.45 No apparent agreement

> 1 difference

< 1 difference

< 0.5 difference

• Wide variation in logKow values depending on choice of QSPR.

• No one QSPR is preferred for all classes (a mixed approach is needed).

• There are some regulatory objections since surfactants are not in the

training set.

• Nonionics and anionics are generally better predicted than cationics or

amphoterics.

Summary: experimental

1. Correlations between log Kow/D data from experimental or model predictions are

generally variable, although non-ionic log Kow values were more consistent.

2. The slow-stirring method is the most broadly applicable for measuring all the test

compounds. Main limitation is that there are difficulties with recovery and

sensitivity of the analytical method for water phase (some repeat studies

underway)

3. HPLC method is currently only appropriate for non-ionics and needs to be

validated for the other surfactant classes.

4. pKa is important for ionisable surfactants. If the pKa is around the environmental

pH (5-9), log Kow/D should be measured two both sets of conditions under which

the surfactant is fully neutral and fully ionised.

5. If the pKa is <5 or >9 then testing at pH 7 is recommended to represent relevant

environmental conditions.19

Summary: Predictive methods

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Category Recommended methods Comments

Non-ionic

Several methods compare well to the slow stir data (ClogP,

Molinspiration, L&H, AlogP and Broto atomic fragment

approach).Use Weight of Evidence

Anionic

KOWWIN, Molinspiration, Hansch and Leo with modifications

and AlogP show some agreement with slow stir data. Other

approaches are unable to calculate or are less predictive

Do not include counter-ion in

SMILES

CationicAll approaches provide variable predictions of slow stir

values

Include [N+] and counter-ion

in SMILES

AmphotericAll approaches provide variable predictions of slow stir

values.

Include [N+] and counter-ion

in SMILES. Subtract value

of counter-ion after

calculation

Summary : Next steps

The Task Force is currently progressing studies to investigate

alternative methods for determining hydrophobicity which may

overcome some of the problems associated with current logKow

methods. These are:

• Liposome water partitioning

• Immobilised Artificial Membranes (IAM)

• Kfibre-water (using SPME fibres)

• pHmetric method (alternative logKow method)

• H-NMR method (alternative logKow method)

• Cosmotherm (to determine Klip-water)

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Charles Eadsforth (Shell)

Günter Oetter (BASF)

Ping Sun (Procter & Gamble)

Bart Bossuyt (Huntsman)

Dennis Miller (Clariant Produkte)

Judie Adnett (Dow Chemical Company)

Joachim Venzmer (Evonik Industries AG)

Marc Geurts (Akzo Nobel BV)

Marie-Hélène Enrici (Solvay Novecare)

Alain Bouvy (CEFIC)

Diederik Schowanek (Procter & Gamble)

Eleanor Michie (Shell)

Jayne Roberts (Unilever)

Josef Mueller and Matthias Kotthoff (Fraunhofer)

Joris Haftka and Joop Hermens (Utrecht University)

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Acknowledgements

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Thank you