1. 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
1Department of Chemical and Pharmaceutical Engineering, Faculty of Chemistry and Pharmacy, Sofia University, Bulgaria
2UNILEVER R&D, Trumbull, USA

2. Content
Introduction.
Aim of the study
Materials and methods.
Experimental results.
Conclusions.

3. Introduction

4. Surfactant adsorption and surface modulus
A0 +A0
 < 0
 > 0 A0
 > 0
 < 0
A0-A0
 > 0
 < 0
Surface modulus

5. Surface properties Oscillating drop
Golemanov et al., Langmuir 24, 2008
Golemanov et al., Langmuir 24, 2008
Oscillating drop
SLES+CAPB+MAc
SLES+CAPB+LAc
SLES+CAPB
Significant effect of FAc on SLES+CAPB surface rheological properties Surface condensed layer formation

6. n < 1 (concentrated foams)
Foam rheology V
n=0.2
n=0.5
0 – yield stress k – consistency n – power law index
n < 1 (concentrated foams)
Value of n <=> Viscous friction mechanism
Denkov et al., 2008; Tcholakova et al., 2008; Denkov et al., 2009.

7. Mechanism of energy dissipation in sheared foam
A0 + A
Inside film friction
Surface dissipation
Denkov et al., 2008; Tcholakova et al., 2008 7

8. Bubble break-up during inside foam friction
BS+LAc, R32 ≈ 150 m
BS, R32 ≈ 300 m
Golemanov et al.,, 2008
R32 ≈ 700 m
BS+LAc BS
G  3 mN/m
G  100 mN/m
Much smaller bubbles are formed
in presence of long-chain FAc as co-surfactant

9. 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

10. Materials Fatty acids (FAc): Sodium laureth sulfate-SLES-1EO
C8Ac - C18Ac
Cocoamidopropyl betaine-CAPB
Concentrated solution:
(300 mM = 10 wt %)
SLES + CAPB = 2:1 + FAc
Basic solution (BS):
(300 mM = 10 wt %)
SLES + CAPB
Diluted solution:
(15 mM = 0.5 wt %)
SLES + CAPB = 2:1 + FAc

11. Methods for systems characterization
Oscillating drop
Tensiometer
Rheometer
P. Garrett et al., 1993

12. Effect of fatty acids on solution rheology
Elongated micelles
Entangled wormlike micelles
C8Ac-C10Ac
C12Ac-C18Ac
Formation of wormlike micelles in presence of C8Ac and C10Ac,
while longer FAc lead to formation of cylindrical micelles.
(Mitrinova et al. Langmuir, 2013)

13. Oscillatory measurements
2 % deformation
1/tR
1/tR
tR relaxation time of network restructuring: s for C8Ac and 0.7 s for C10Ac

14. Combined relaxation time
Cole-Cole plot
reptation relaxation
reaction model
trep – reptation time;
tbr – breaking time;
Combined relaxation time
Kern et al., 1992
Systems:
TR, sec
tbr, sec
trep, sec
BS+ C8Ac
0.5
0.6
0.4
BS+ C10Ac
0.14
1.4
0.01
– stress relaxation modulus

15. Effect of FAc on surface rheological properties
(Mitrinova et al. Langmuir, 2013)
Oscillating drop
C8Ac and C10Ac are unable to increase surface modulus of BS C12Ac-C18Ac increase surface modulus above 100 mN/m

16. Effect of FAc on foam rheology
(Mitrinova et al. Langmuir, 2013)
Foams with GD > 100 mN/m exhibit much higher foam friction, compared to solutions with lower surface modulus

17. 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
(Mitrinova et al. Langmuir, 2013)

18. Equilibrium surface tension of diluted solutions
Significant decrease of equilibrium surface tension for all FAc
Most effective - C16 and C14Ac, followed by C12Ac
(Mitrinova et al. Langmuir, 2013)

19. Effect of pH on surface modulus and foam friction
(Mitrinova et al. Colloid and surfaces A, 2013)
15 mM BS
0.88 mM C14Ac
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

20. Effect of pH on BS+C14Ac surafce tension
(Mitrinova et al. Colloid and surfaces A, 2013)
σEQ is low at pH between 3 and σEQ incerase at high pH and reaches values for BS at pH=11
At natural pH ≈ 6 молекулите FAc molecules are protonated
At high pH significant molar fraction of FAc is ionized

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

22. Thank you for your attention!