Surfactant mediated charging and electrostatic particle interaction in nonpolar dispersions

Transcription

1 Surfactant mediated charging and electrostatic particle interaction in nonpolar dispersions 85 th ACS Colloid and Surface Science Symposium Montreal, June 21, 2011 Qiong Guo, Crystal C. Clemmons, Carlos E. Espinosa, and Sven H. Behrens Georgia Institute of Technology School of Chemical & Biomolecular Engineering

2 The problem of introducing ions in a nonpolar liquid 2a Electrostatic self-energy of a monovalent ion: 2 UB e 8 0a U B B kt 2a Bjerrum length: 2 B e 4 0kT (water: 0.7 nm, hexane: 29 nm) Thermal energy scale Ion size Probability of charging exceedingly low in oils BUT: surfactant additives ( charge control agents ) can dramatically raise the electric conductivity promote particle charging prevent explosion hazards due to flow electrification used in electrophoretic displays (Kindle etc.)

3 Case of ionic surfactants CMC AOT (in hexadecane) S. K. Sainis, J.W. Merrill, E.R. Dufresne, Langmuir 24, (2008).

4 Case of ionic surfactants Disproportionation: - (in hexadecane) S. K. Sainis, J.W. Merrill, E.R. Dufresne, Langmuir 24, (2008).

5 Case of ionic surfactants Dissociation: S. K. Sainis, J.W. Merrill, E.R. Dufresne, Langmuir 24, (2008).

6 Case of ionic surfactants CMC I. II. Transition III. S. K. Sainis, J.W. Merrill, E.R. Dufresne, Langmuir 24, (2008).

7 Particle charging with ionic surfactants Electrostatic surface potential of PMMA microparticles in AOT/dodecane Significant surface charging only for C > CMC. Kemp, R., Sanchez, R., Mutch, K. J., Bartlett, P., Langmuir 26, 6967 (2010).

8 Hypothesized mechanism for particles charging asymmetric adsorption of charged micelles 1 dissociation of individually adsorbed surfactant molecules dissociation of surface groups due to acid-base interaction with the surfactant Micelles as charge acceptors + 1 M. F. Hsu, E. R. Dufresne, D. A. Weitz, Langmuir 21, 4881 (2005); G. S. Roberts, R. Sanchez, R. Kemp, T. Wood, P. Bartlett, Langmuir 24, 6530 (2008). 2 R. Kemp, R. Sanchez, K. J. Mutch, P. Bartlett, Langmuir 26 (10), (2010). 3 S. Poovarodom and J. C. Berg, J. Colloid Interface Sci. 346 (2), (2010).

9 Nonionic surfactants A. S. Dukhin; P. J. Goetz, J. Electroanal. Chem. 588, 44 (2006).

10 Nonionic surfactants Span 85: mixture of sorbitan trioleate and tetra-oleate, HLB ~ 1.8 Span 85 A. S. Dukhin; P. J. Goetz, J. Electroanal. Chem. 588, 44 (2006).

11 Span 85 in hexane Spherical micelles above a CMC of ~10 mm (DLS diameter and interfacial tension) CMC Linear conductivity increase in hexane above and below the CMC. Q. Guo, V. Singh, SHB, Langmuir 26, 3203 (2010).

12 Deliberate contamination with ionizable impurities Adding large amounts of the most likely ionic impurity does not increase conductivity! Does this mean, ionizable impurities play no role? Q. Guo, V. Singh, SHB, Langmuir 26, 3203 (2010).

13 Hypothesized charging mechanism 2 C < CMC Ionizable impurity 2 C > CMC Two steps: 1) inclusion of impurity in surfactant micelle or pre-micellar complex 2) charge disproportionation rate limiting step!

14 Zeta potential of supended PMMA sulfate particles m 0.11 m C. E. Espinosa, Q. Guo, V. Singh, SHB, Langmuir 26, (2010).

15 Zeta potential of supended PMMA sulfate particles Particle charge before and after solvent replacement: repeated dilution & centrifugation Aqueous dispersion Particles in alcohol Particles in hexane/span 85 C. E. Espinosa, Q. Guo, V. Singh, SHB, Langmuir 26, (2010).

16 Zeta potential of supended PMMA sulfate particles Particle charge before and after solvent replacement: repeated dilution & centrifugation Aqueous dispersion Particles in alcohol Particles in hexane/span 85 Particle charge in hexane NOT due to surface headgroups!! C. E. Espinosa, Q. Guo, V. Singh, SHB, Langmuir 26, (2010).

17 ELECTROPHORETIC MOBILITY / (10-8 m 2 /Vs) ELECTROPHORETIC MOBILITY / (10-8 m 2 /Vs) ZETA POTENTIAL / mv Charging ZETA POTENTIAL / mv Electrophoretic mobility depends on field strength! Zero field mobility and zeta potential m PMMA mm 1 mm 2mM 5 mm 10 mm 20 mm 30 mm 50 mm FIELD STRENGTH / (kv/m) CMC CONCENTRATION C SPAN 85 / mm 0.52 m Dia m Dia C. E. Espinosa, Q. Guo, V. Singh, SHB, Langmuir 26, (2010).

18 ELECTROPHORETIC MOBILITY / (10-8 m 2 /Vs) ELECTROPHORETIC MOBILITY / (10-8 m 2 /Vs) ZETA POTENTIAL / mv Charging ZETA POTENTIAL / mv Electrophoretic mobility depends on field strength! Zero field mobility and zeta potential m PMMA mm 1 mm 2mM 5 mm 10 mm 20 mm 30 mm 50 mm FIELD STRENGTH / (kv/m) CMC CONCENTRATION C SPAN 85 / mm 0.52 m Dia m Dia Particle charging appears not to require micelles! C. E. Espinosa, Q. Guo, V. Singh, SHB, Langmuir 26, (2010).

19 Interaction measurements PMMA coated glass coverslips 1 m PMMA particles microscope find 2-d particle locations* Pair interaction energy from radial distribution function and 2-d Ornstein- Zernicke integral equation with hypernetted chain closure ** *J.C. Crocker and D.G. Grier, J. Colloid Interface Sci. 179, 298 (1996) **SHB, D.G. Grier, Phy. Rev. E, 88, (2001)

20 Insensitivity to plate separation

21 Interaction measurements PMMA coated glass coverslips 1 m PMMA particles microscope find 2-d particle locations* Pair interaction energy from radial distribution function and 2-d Ornstein- Zernicke integral equation with hypernetted chain closure ** *J.C. Crocker and D.G. Grier, J. Colloid Interface Sci. 179, 298 (1996) **SHB, D.G. Grier, Phy. Rev. E, 88, (2001)

22 ) Screened Coulomb form confirmed by logarithmic plot ) exp( 2 / 1 ) exp( 4 ) ( ) ( * r d d r e Z r u Screened Coulomb Potential fits well above and below CMC

23 Screened Coulomb form confirmed by logarithmic plot

24 ) Screened Coulomb form confirmed by logarithmic plot ) exp( 2 / 1 ) exp( 4 ) ( ) ( * r d d r e Z r u Screened Coulomb Potential fits well above and below CMC

25 Ion size from conductivity and particle interaction???

26 Ion size from conductivity and particle interaction d H 2 2 e 2 12 B = size of small ion responsible for conductivity and screening Compare to DLS result of (2.8 ± 0.2) nm for micelle size

27 Ion size from conductivity and particle interaction d H 2 2 e 2 12 B = size of small ion responsible for conductivity and screening Compare to DLS result of (2.8 ± 0.2) nm for micelle size consistent with small pre-micellar complex as ionic species below the CMC.

28 Conclusions Nonionic surfactants can promote charging in nonpolar liquids Differences to charging by ionic surfactants: - linear conductivity increase - surface charging - screened Coulomb interaction even below the CMC! Mechanism of surface charging not yet understood: - micelles not needed - ionic surfactant group not needed - particle surface head group possibly irrelevant - possibly relevant: (Lewis) acid-base interaction between PMMA and surfactant

29 Acknowledgements Virendra Singh Qiong Guo Carlos Espinosa Adriana San Miguel Sven Behrens The Camille & Henry Dreyfus Foundation