786 Overview Thermodynamic Assembly Micellization BCPs and lipids Thermodynamic Instability Creaming Sedimentation Aggregation Overview Unless thermodynamics dictate assembly all roads lead to separation ID: 935747
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Slide1
Assembly & In/stability
786
Slide2Overview
Thermodynamic
Assembly
Micellization
BCPs and lipidsThermodynamic InstabilityCreamingSedimentationAggregation
Slide3Overview
Unless thermodynamics dictate assembly,
all roads lead to separation!
Complex fluids are
metastableLS is a great tool to assess stability
Slide4Micellization
Slide5The
Ebers
papyrus (
Egypt, 1550 BC
) indicates the ancient Egyptians bathed regularly and combined animal and vegetable oils with alkaline salts
to create a soap-like substance.
2 main
i
ngredients!
Slide6How does soap work?
Rinse away…
econutssoap.com
Slide7Representative triglyceride found in a linseed oil, a
triester
(triglyceride) derived of
linoleic acid
, alpha-linolenic acid, and oleic acid.
Ester Group
+
NaOH
(lye)
Saponifcation
reaction = ester hydrolysis of salts
Slide8Representative triglyceride found in a linseed oil, a
triester
(triglyceride) derived of
linoleic acid
, alpha-linolenic acid, and oleic acid.
Ester Group
N
a+
N
a+
N
a+
Glycerin
Slide9Result: Soap
Start with liquid oils/fats like
glyceryl
tristearatteReact with lye or sodium hydroxide strong base Sodium stearate + glycerine (softener)Sodium stearate = soap = surfactant sodium
Slide10What differentiates surfactants?
Self-association leads to
reverse micelle formation
a
t critical micelle concentration
Increasing concentration
Measurable
via:
conductivity
, light scattering, surface tension, viscosity…
Slide11Dispersant
critical micelle concentration
by conductivity measurements
BA ~ 10 ppm
4F ~ 100 ppm
Slide12Dispersant micellization
by conductivity measurements
BA ~ 10 ppm
4F ~ 100 ppm
Slide13Surfactants below/above the
cmc
Low c
Nothing to scatter from
No size to measureHigh cNumber of scatterers growsMeasureable size
CMC can determine how surfactants interact in suspension: self-assembly may reduce or facilitate interaction with other components.
Slide14Stabilization by adsorption
Adsorption isotherms corroborate
LS particle characterization
2 dispersants:
cmc ~ 10 and ~ 100 ppm in heptaneEach stabilizes asphaltene colloids below
respective
cmc
Hashmi,
Firoozabadi
.
Soft Matter
7
, 8384 (2011).
Slide15Interparticle Interactions
Slide16In/Stability &
interparticle
interactions
physics.nyu.edu/~pc86
Universal
Repulsion Required
Brushes: short-ranged
Electrostatics: good candidate to explain stability
Slide17Van der Waals & Dispersion
Dipole-dipole and induced-dipole interactions
All material has some degree of
polarizability
UniversalAttractiveHamaker constant A F ~ - A / r 6NOTE: short range:
F
~
1
/
r
6
Slide18Inter-particle Potentials
Malescio
Nature Materials
2
, 501 (2003).
U
max
if repulsion is electrostatic,
DLVO theory describes
U
(
r
)
Induced dipole-dipole van
der
Waals
U
r
Irreversible aggregation
Slide19Electrostatics
Coulomb’s law
F
= kq
1q2 / r2Thin double layers in aqueous systems
Double layer indicates range of electrostatic interactions
-
electrostatic interactions (1/r
2
) are
longer-
ranged than van der Waals (1/
r
6
)
.
Slide20Size of the screening length
Electrostatic attraction is longer range in oil than in water
Double layer indicates range of electrostatic interactions
Aqueous suspensions
Thin double layers
Non-polar suspensions
Thick double layers
-
Slide21Balance
electrostatic force:
+
-
E
lectric
field
: diffusion with drift
Measurement Concept: Zeta Potential
Electrophoretic mobility:
Drift velocity:
Initial position
Diffusive trajectory
Final position
Against hydrodynamic force:
Related
to surface
charge:
Surface charge
“Phase
Analysis Light
Scattering”
Analogous to Doppler shift, but
with oscillating electric field
Slide22Electrophoretic
mobility & Zeta Potential
Hückel
Theory:
Balance electrostatic and hydrodynamic forces:
+
+
+
+
+
+
+
+
+
+
+
-
F
or
low ionic
strengths (
apolar
)
Smoluchowski
:
F
or high ionic strength (aqueous)
Slide23Effect of
Ionic strength
Ions “screen away” electrostatic repulsion, thus reducing zeta potential magnitude
Rajapaksa
TE, et. al. J. Biol. Chem. (2010)
Slide24Point of Zero Charge
Adjusting pH by adding acid/base can induce protonation/
deprotonation
of the particle surface, thus changing the surface charge
Slide25Sedimentation
Slide26Hard sphere systems: linear front
t = 0 min t = 60 min t = 95 min t = 150 min t = 210 min
1.57
µ
m
radius silica
spheres in water at
Φ
=0.05
Slide27Measured
v
~ 2 to
3x slower
than
expected
Φ
a
= 1.57
µ
m; silica particles in water
Hard sphere systems: linear front
Sedimentation Velocity:
µ
m/s
Silica zeta potential:
ζ
= -18.25 +/- 0.31 mV
Density difference
Gravity
Slide28Gelling systems: sudden collapse
Lietor-Santos et. al.
Langmuir.
26
(5), 3174 (2010).
Poon et. al.
Faraday Discuss.
112
, 143 (1999).
Slide29Unstable particles aggregate & settle
NP formation
Aggregation
FAST Sedimentation
FAST
Separation
Sedimentation speed ~ (aggregate size)
2
Slide30Stable
particles; no aggregation
NP formation
Aggregation
SLOW Sedimentation
SLOW Separation
Sedimentation speed ~ (aggregate size)
2
Slide31Growth & settling often simultaneous
Separation of time scales
Growth
S
edimentation
Time (hours)
Intensity (
kcps
)
Slide32Lab Tasks Day 2
3 surfactants
Prepare dilutions on log scale from stock
Run initial measurements
Prepare additional samples to ‘zoom in’ on cmcCNTs in suspensionAssess growth and settling over timeDepending on sonication/sample preparationSLS on CNTs and PS spheres