1Department of Chemical and Pharmaceutical Engineering, Faculty of Chemistry and Pharmacy,

Sofia University, Bulgaria

Efficient control of the bulk and surface rheological properties by using C8-C18

fatty acids as co-surfactants

Efficient control of the bulk and surface rheological properties by using C8-C18

fatty acids as co-surfactants

Zlatina Mitrinova,1* Zhulieta Popova,1 Slavka Tcholakova,1 Nikolai Denkov,1 Bivash R. Dasgupta2 and K.P.

Ananthapadmanabhan2

2UNILEVER R&D, Trumbull, USA

Content

1.Introduction.

2.Aim of the study

3.Materials and methods.

4.Experimental results.

5.Conclusions.

Introduction

Surfactant adsorption and surface modulus

A0, 0, 0 A0 +A0

< 0

> 0

A0

> 0

< 0

A0-A0 > 0

< 0

0 sinA t A A t

0 sint t

Surface modulus

*

lnG

A

Surface propertiesGolemanov et al., Langmuir 24, 2008

Oscillating drop

1 22 2D ST LSG G G

A/A0, %

0.0 0.5 1.0 1.5 2.0 2.5 3.0

GL

S, m

N/m

100

101

102

103

SLES+CAPB

SLES+CAPB+MAc

SLES+CAPB+LAc

Golemanov et al., Langmuir 24, 2008

Significant effect of FAc on SLES+CAPB surface rheological properties

Surface condensed layer formation

Ca

10-5 10-4 10-3 10-2

10-2

10-1

1 mPa.s, 290 m4 mPa.s, 150 m4 mPa.s, 200 m10 mPa.s,110 m

V

nV Ca

Value of n <=> Viscous friction mechanism

V

0 – yield stress k – consistency n – power law index

n < 1 (concentrated foams)

Foam rheology

Denkov et al., 2008; Tcholakova et al., 2008; Denkov et al., 2009.

VV R

RCa

0( ) nk

Capillary number, Ca

10-7 10-6 10-5 10-4 10-3 10-2

Dim

ensi

on

less

vis

cou

s st

ress

10-3

10-2

10-1

100

SoapSLES+Betaine+LAc+MAc+PAc

SLES+Betaine+Akypo+LADA+NaCIT

n=0.2

n=0.5

Inside film friction Surface dissipation

Denkov et al., 2008; Tcholakova et al., 2008

0.5VF Ca 0.2VS LSG Ca

A0A0 A0 + A

Mechanism of energy dissipation in sheared foam

Time, s

0 20 40 60 80 100 120 140

Sh

ear

stre

ss, P

a

0

10

20

30

40

50

BS+LAc

BS 1100 s

G 3 mN/m

G 100 mN/m

1~

nR

R32 ≈ 700 m

Bubble break-up during inside foam friction

Golemanov et al.,, 2008 BS+LAc, R32 ≈ 150 m

BS, R32 ≈ 300 m

Much smaller bubbles are formedin presence of long-chain FAc as co-surfactant

Aim of the study

To clarify the effect of fatty acid on foam and bulk

rheological properties

Studied factors:

Fatty acid chain length Fatty acid concentration pH

Materials

Concentrated solution:

(300 mM = 10 wt %)

SLES + CAPB = 2:1 + FAc

Sodium laureth sulfate-SLES-1EO

Cocoamidopropyl betaine-CAPB

Fatty acids (FAc):C8Ac - C18Ac

Diluted solution:

(15 mM = 0.5 wt %)

SLES + CAPB = 2:1 + FAc

Basic solution (BS):

(300 mM = 10 wt %)

SLES + CAPB

Methods for systems characterization

Oscillating drop

1 22 2D ST LSG G G

Rheometer

Tensiometer

P. Garrett et al., 1993

(Mitrinova et al. Langmuir, 2013)

FAc concentration, mM

0 50 100 150 200 250

0,

Pa.

s

10-3

10-2

10-1

100

101

102

300 mM BS+ C8Ac

+ C10Ac

+ C12Ac

+ C14Ac

Formation of wormlike micelles in presence of C8Ac and C10Ac,while longer FAc lead to formation of cylindrical micelles.

Elongated micelles

Entangled wormlike micelles

C8Ac-C10Ac

C12Ac-C18Ac

Effect of fatty acids on solution rheology

Shear rate, s-1

10-1 100 101 102

Ap

par

ent

visc

osi

ty, P

a.s

10-3

10-2

10-1

100

101

102

10 wt % BS only

10 wt % BS + 0.1 wt % C8Ac

+ 0.3 wt % C8Ac

+ 0.5 wt % C8Ac

+ 0.7 wt % C8Ac

10 wt % BS + 1 wt % C8Ac

+ 1.5 wt % C8Ac

Frequency of oscillation, s-1

10-1 100 101

G',

G",

Pa

10-1

100

101

102

103

10 wt % BS + 1 wt % C8Ac

10 wt % BS + 1 wt % C10Ac

Full: G'Empty: G"

Oscillatory measurements

1/tR 1/tR

tR relaxation time of network restructuring: 5 s for C8Ac and 0.7 s for C10Ac

1R

R

t

2 % deformation

– stress relaxation modulus

1 2/ 1.23 8br rept t forC Ac

1 2/ 10.2 10br rept t forC Ac

Systems: TR, sec tbr, sec trep, sec

BS+ C8Ac 0.5 0.6 0.4

BS+ C10Ac 0.14 1.4 0.01

G'/Gosc

0.0 0.5 1.0 1.5 2.0 2.5 3.0

G''/

Go

sc

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

10 wt % BS 1 wt % C10Ac10 wt % BS

1 wt % C8Ac

0.38

1.234.61

10.2

Cole-Cole plot

Kern et al., 1992

0R

e

TG

1/2( . )R br repT t t

Combined relaxation timereptation relaxation

reaction model

trep – reptation time;

tbr – breaking time;

15 mM BS1.1 mM FAc

C8Ac C10Ac C12Ac C14Ac C16Ac C18Ac

Su

rfac

e m

od

ulu

s, m

N/m

1

10

100

1000

BS only15 mM BS3.5 mM FAc

C8Ac and C10Ac are unable to increase surface modulus of BS C12Ac-C18Ac increase surface modulus above 100 mN/m

Effect of FAc on surface rheological properties

Oscillating drop

1 22 2D ST LSG G G

(Mitrinova et al. Langmuir, 2013)

Capillary number, Ca10-6 10-5 10-4 10-3

Dim

ensi

on

less

Vis

cou

s S

tres

s

10-2

10-1

BS only

Empty: + 1 mM FAcFull: + 3.5 mM FAc

15 mM BS

BS+C8AcBS+C10Ac

BS+C18Ac

BS+C14AcBS+C12Ac

BS+C16Ac

Foams with GD > 100 mN/m exhibit much higher foam friction, compared to solutions with lower surface modulus

Effect of FAc on foam rheology

(Mitrinova et al. Langmuir, 2013)

Effect of FAc chain length on mean bubble radius

C8Ac and C10Ac do not change R32 for BS system C12Ac-C18Ac lead to

much smaller bubbles due to the high GD

Mea

n b

ub

ble

siz

e,

m

120

140

160

180

200

220

240

260

15 mM BS only

15 mM BS1.1 mM FAc

15 mM BS3.5 mM FAc

(Mitrinova et al. Langmuir, 2013)

Equilibrium surface tension of diluted solutions

(Mitrinova et al. Langmuir, 2013)

15 mM BS only

FAc concentration, mM

10-2 10-1 100 101

Su

frac

e te

nsi

on

, mN

/m

20

22

24

26

28

30

+ C8Ac

+ C10Ac

+ C14Ac

+ C12Ac

+ C16Ac

+ C18Ac

Significant decrease of equilibrium surface tension for all FAcMost effective - C16 and C14Ac, followed by C12Ac

pH

2 4 6 8 10 12

Su

rfac

e m

od

ulu

s, m

N/m

101

102

103

= 0.2 Hz

At pH between 3 and 8 surface modulus are high (above 100 mN/m) and there is significant foam friction

At pH = 11 GD reaches values for BS and foam friction decrease

Effect of pH on surface modulus and foam friction

15 mM BS 0.88 mM C14Ac

15 mM BS

2.2 mM C14Ac

Capillary number, Ca

10-6 10-5 10-4 10-3

Dim

ensi

on

less

Vis

cou

s S

tres

s

10-2

10-1

pH=8

BS (no C14Ac)

pH=11pH=10

pH=6

pH=9

pH=3

(Mitrinova et al. Colloid and surfaces A, 2013)

σEQ is low at pH between 3 and 8 σEQ incerase at high pH and reaches values for BS at pH=11

Effect of pH on BS+C14Ac surafce tension

pH

2 4 6 8 10 12

Su

rfac

e te

nsi

on

, mN

/m

20

22

24

26

28

30

32

34

15 mM BS

15 mM BS0.88 mM C14Ac

15 mM BS+ 2.2 mM C14Ac

At natural pH ≈ 6 молекулите FAc molecules are protonatedAt high pH significant molar fraction of FAc is ionized

(Mitrinova et al. Colloid and surfaces A, 2013)

In presence of C8Ac and C10Ac :Apparent viscosity of concentrated solutions significantly

increase due to the formation of entangled wormlike micelles;

Foam rheological properties are not affected;

In presence of C14Ac, C16Ac, C18Ac:Apparent viscosity of concentrated solutions is slightly

higher than those for BS system due to the presence of cylindrical micelles;

Surface modulus of diluted solution significantly increase;Foam viscous friction is higher due to additional dissipation

in adsorption layer.

Conclusions

In presence of C12Ac: In presence of C12Ac intermediate properties are observed.

Thank you for your attention!