Surfactant Adsorption on Metal Oxide Pigments

1
Surfactant Adsorption on Metal Oxide Pigments

• Jeff Harwell
• Institute for Applied Surfactant Science
• University of Oklahoma

2
Outline

• I. Overview. The 4 Region Surfactant Adsorption
  Isotherm
• II. Critical Background Information
• A. Micelle Formation and Shape Factors
• B. Surfactant Phase Behavior
• C. Metal Oxide Surface Charge
• III. Implications for Dispersion Stability

3
Overview

• The 4 Region Surfactant Adsorption Isotherm
• (Generic Isotherm)

4
The 4 Region Adsorption Isotherm
5
Region 1. Henrys Law
?s H Cs

• Isotherm slope 1 on log-log plot.
• Monomers adsorb independently.
• Adsorption potential not a function of adsorption
  level.
• Proportional to surface charge density.
• In excess of electrostatic potential alone.

6
Region 2. Admicelle Formation

• Isotherm slope gt 1
• Micelle-like aggregates form on patches of the
  surface.
• Concentration called the Critical Admicelle
  Concentration (CAC).
• CAC varies with surface charge and electrolyte
  but is always below the CMC

7
Region 3. Admicelle Competition

• Slope lt 1
• Surface coverage is still patchy.
• Aggregates compete for sites, so isotherm slope
  decreases.
• May occur well below complete surface coverage

8
Region 4. Micelle Competition

• Slope 0
• First micelle fixes chemical potential
  additional surfactant forms new micelles.
• Isotherm plateaus for monoisomeric surfactants,
  decreases for mixtures.

9
Generic Isotherm Shape Holds for Ionic Surfactants
3-?-C9 isomer of ABS on kaolinite
cac
cmc
10
and for Nonionic Surfactants
Nonylphenol poly- ethoxylate on fumed silica
Note nonionics show very low adsorption on
aluminas
11
Hydrogen bonding important for nonionic adsorption
12
Critical Background Information

• Micelles, micelle shapes,
• surfactant phase behavior, and oxide surface
  charge

13
Critical Micelle Concentration (CMC)

• Monomer concentration plateaus at CMC.
• Micelle concentration increases above CMC.
• Surfactant adsorption plateaus at the
  CMC--whether at air/water, solid/solution, or
  liquid/liquid interface.
• This initiates Region 4 of the isotherm.

14
CMC decreases with NaCl and Tail Carbon Number
CMC
CMC
NaCl
Tail Carbon Number
15
CAC Decreases as the CMC Decreases
Effect of NaCl largely cancels out above CMC
Effect of NaCl can be large BELOW the CMC
Isotherm B is at higher NaCl than Isotherm A
Shift in CAC
16
Micelle Shapes Vary
17
Monomers modeled as cones
18
Monomers modeled as cones
19
Monomers modeled as inverted cones
20
Micelle Shape vs Packing Factor
? lt 1/3
? 1
? gt 1
21
Many variables effect packing factor
1/3 lt ? lt 1/2
? V/a0Lc
a0, area of head group, varies with electrolyte
for ionics or for mixed micelles
(anionic/nonionic, etc.) or addition of hydrotrope
NaCl or surfactant
V, volume of tail, varies with branching of
hydrophobe
Oil or hydro- trope
Lc, length of tail, varies with addition of
nonpolar oils (Hoffman effect--rod-to-sphere
? lt 1/3
22
Admicelle shape?
The shape of the admicelles may vary from
spherical, to rod-like, to lamellar, depending on
many factors, including the effective
dimensionless packing factor, ?.
Preponderance of studies indicates that admicelle
always forms with a head group layer toward the
solution, but shapes vary.
23
Surfactant phase behavior

• Surfactants form liquid crystals at higher
  concentrations
• Liquid crystals increase viscosity and thus
  increase dispersion stability
• Effect is INDEPENDENT of adsorption on oxide

24
Nonionic Surfactants

• Phase behavior shows same trends with as ionics
  with concentration
• Opposite temperature dependence from ionics for
  water solubility

25
Transition Sequence
Viscosity
increases
decreases
increases
decreases
26
Liquid Crystal Is made of Micelles
Here ? is an effective shape factor
hexagonal
lamellar
1/3 lt ? lt 1/2
? 1
27
Oxide Surface Charge
Oxide Surface Charge varies with pH
Net surface charge is zero at the Point of Zero
Charge, PZC
28
Surface Charge Affects Adsorption for Ionics
Pxs adsorption on alumina vs pH
29
Plateau adsorption vs pH
SDS plateau adsorption on alumina vs. pH
Substantial adsorption occurs on like- charged
surface!
pzc
30
Oxide PZC Table
For pH gt pzc surface carries a negative charge
and cationics adsorb strongly
For pH lt pzc surface carries a positive charge
and anionic surfactants adsorb strongly.
31
Application to Dispersions

• Strong concentration dependence and existence of
  optimum stability region

32
Electrostatic stabilization

• Stable suspension due to
• Brownian or colloidal stabilization
• Particle/particle net interaction is repulsive
• Particles are small enough not to settle with
  gravity
• Low solid/solution ratio

33
Repulsion of large particles
Because particles repel but are too large to stay
suspended by Brownian forces, they settle to form
clay that become dilatant on aging.
34
Chain aggregation
Post-It-Note stickiness results in high volume
sediment easily redisperse with shear form in
Region 2 of adsorption isotherm.
35
Region 1 Adsorption Effect
Small reduction in surface charge reduces
particle/particle repulsion but has slight effect
on dispersion
36
Region 2 Adsorption Effect
Admicelle bridge causes slight stickiness
between particles inducing reversible aggregate
formation
37
Region 3 Adsorption Effect
Admicelle coverage in Region 3 leads to
steric/electrostatic repulsion between
particles--except for nonionic near cloud
point--and formation of clay.
38
Suspension Stability Varies along Isotherm
G
Adsorption
Stability
Suspension stability shows a maximum
with surfactant coverage
39
Implication for Dispersions

• The same surfactant that has no effect in Region
  1
• Induces weak aggregation leading to greater
  suspension stability in Region 2 but
• Re-disperses the particles in Region 3--leading
  to clay formation then
• Shows no concentration dependence in Region 4!
• To understand effects we must know if
  equilibration is at, below, or above the CMC.
• Comparing many surfactants at the same weight
  percent in solution is essentially a random
  search!

40
Conclusions and Summary

• Surfactant adsorption on oxides is dominated by
  surface aggregate formation
• Surface surfactant aggregates, called admicelles,
  are very micelle-like (NaCl, C-number,
  solubilization, etc.)
• Oxide surface charge--and therefore adsorption of
  ionic surfactants--is strongly pH dependent
• Effect on dispersions depends strongly on
  fractional surface coverage by admicelles
• CMC is a critical parameter for design