1000 2000 3000 4000 5000 6000
0.2
0.4
0.6
0.8
1.0
Su
rfa
ce
pre
ssu
re(m
N/m
)
Mma(A2)
NaCl 1000 ppm
NaCl 2000 ppm
NaCl 3000 ppm
Relaxation Experiment at different salt concentration
Napthenic Acid 1000 ppm in Toluene
Volume spread 20ul, Barrier speed 10mm/min
20 40 60 80 100
0.2
0.4
0.6
0.8
1.0
No
rma
lize
Su
rfa
ce
pre
ssu
re
Time(min)
pH 4
pH6
pH7
pH9
pH12
Relaxation Isotherm at different pH
20 40 60 80 100 120
0
5
10
15
20
Su
rfa
ce
pre
ssu
re(m
N/m
)
Mma(A2)
pH4
pH9
Slope comparision at different pH
Napthenic acid 1000 ppm in Toluene
Volume spread 20ul, Barrier speed - 10 mm/min
The measurements on LB were
done using the KSV NIMA
Langmuir Trough. The sample is
first dissolved in toluene and then
spread over the surface of water
& allowed toluene to evaporate.
Repeated compression-
decompression studies were then
carried out to get the excess
pressure vs area/molecule plots.
Monolayers of Napthenic Acids,
one of the surface active
components of crude oil, were
prepared using Langmuir Blodgett
technique.
Commercially available Poly-
Disperse Naphthenic acids (Acros
Chemicals) were used for Surface
Pressure-Mean Molecular Area
Isotherm experiments.
Effect of pH, nature and
concentration of electrolyte
concentrations on the film
properties were investigated.
It is found that Naphthenic acids
form stable film at air/water
interface. The pH of the bulk
phase and concentration of salt
has significant effect on the film
properties. The surface pressure
isotherms indicate the changes in
film behaviour as pH is varied
from 4, 9 and 12.
The surface pressure-area
isotherm deviates distinctly from
high to low mean molecular
area as the pH of sub
phase changes from 4 to 12.
The film on a bulk phase at pH 4
consists basically of protonated
naphthenic acid however when
the pH is higher deprotonation of
proton occurred (forming sodium
naphthenate) hence causing
solubilisation of sodium salt of
naphthenic acid into water
resulting in decline of surface
pressure.
Hysteresis pattern in
compression-relaxation cycles
were observed for almost
10 cycles.
Further, The interaction of
different metal salt with Napthenic
acid at different pH will be
evaluated by using surface
potential measurements.
Study of Langmuir Blodgett Monolayers of Napthenic Acids at
Air/Water Interface: Effect of pH and Salt on Film BehaviourManu Vashishtha1, Laksh Agarwal1, Abusaif Khan2, Rochish Thaokar1 and Vinay A. Juvekar1
1Department of Chemical Engineering, Indian Institute of Technology Bombay,
Powai, Mumbai – 400 076, India.2Department of Chemical Engineering, Institute of Chemical Technology, Mumbai
MATERIALS USED
RESULTS AND DISCUSSIONSABSTRACT
Napthenic Acid
METHOD
Effect of pH on Film Elasticity
Many molecules in the crude are acidic, which
ionize at the o/w interface, lower IFT and
stabilize the interface
•The acids are: simple alkyl carboxylic acids,
alkyl benzene carboxylic acids (benzoic
acids), naphthenic acids and metal salts,
fused aromatic ring acids, e.g. napthoic acids
•Above should be acidic stabilization
•Can form calcium soaps and stabilize
emulsions
• Tetra acids with carbon number 80 are the
ARN acids 1,2,3,4 butane tetra carboxylic
acid
Langmuir-Blodgett Trough
At pH 4 Carboxylic group remains protonated
At pH 9 Partial Deprotonation occurs
At pH 12 Complete Deprotonation occurs
Langmuir–Blodgett Trough is a laboratory
apparatus that is used to compress
monolayers of molecules on the surface of a
given sub phase (usually water) and
measures surface phenomena due to this
compression. It can also be used to deposit
single or multiple monolayers on a solid
substrate.
Typical LB measurements gives us information
about the following:
•Packing(Mean molecular Area)
•Elasticity(Steepness of the curve)
•Hysteresis(Molecular Interaction)
•Relaxation(Stability of the films)
•Poly-Disperse Naphthenic Acids
(Acros Chemicals)
•Toulene (Sigma)
•Sodium Hydroxide (Merck)
•Chloroform (Merck)
•De-Ionized Water (Mili Q)
•Buffer Capsules (Merck)
20 40 60 80 100 120
5
10
15
20
Su
rafc
e p
ressu
re(m
N/m
)
Mma(A2)
Cycle 1
Cycle 2
Cycle 3
Cycle 4
Cycle 5
Typical Langmuir Blodgett Experiment
20 40 60 80 100 120
0
10
20
Su
rfa
ce
pre
ssu
re(m
N/m
)
Mma(A2)
pH4
pH9
pH12
Effect of pH
Napthenic Acid 1000ppm in Toluene
Volume spread 20ul, barrier speed 10mm/min
pH 4
No Dissociation
pH 9
Partial Dissociation
pH 12
Complete Dissociation
20 40 60 80 100 120
0
10
20
Su
rfa
ce
pre
ssu
re(m
N/m
)
Mma(A2)
pH4
pH9
pH12
Hysteresis study
Napthenic Acid 1000ppm in Toluene
Volume spread 20ul, barrier speed 10mm/min
Slope = 0.346Slope = 0.96
Slow Hysteresis at pH 4 - Entanglement
Fast Hysteresis at pH 9 - Charge Repulsion
No Hysteresis at pH 12 - Completely Soluble
20 40 60 80 100 120
2
4
6
8
10
12
14
16
18
20
Su
rfa
ce
pre
ssu
re(m
N/m
)
Mma(A2)
NaCl 1000 ppm
NaCl 2000 ppm
NaCl 3000 ppm
Salt Effect at pH 6
Napthenic acid 1000 ppm in Toluene
Volume spread 20ul, Barrier Speed 10mm/min
NO Effect since No Ionization at Interface
Effect of pH
Hysteresis Isotherm
Effect of Salt
Study of Relaxation Curves
Study suggests that pH has influence on the
film properties of Napthenic Acid at Air /Water
Interface. At pH 9 partial deprotonation
causes formation of negatively charged
carboxylate ions, which produce more steeper
rise in Surface Pressure Isotherm at pH 4.
Higher value of slope at pH 9 indicates the
formation of rigid films due to charge
interactions.
This study indicates that pH has influence on
molecular interaction among Napthenic Acid
molecules at Air/Water Interface.
At pH 4 molecules are unionized and show
large Hysteresis pattern because of slow
relaxation of Napthenic Acid molecules held
together by Vander-Wall/Hydrophobic
interactions.
However partial deprotonation of Napthenic
Acid molecules at pH 9 causes repulsion
between negatively charged carboxylate ions
resulting in faster rate relaxation and less
area Hysteresis.
At pH 12 Napthenic Acid molecules are
completely deprotonated making them soluble
in water resulting in decrease in Surface
Pressure and hence no Hysteresis is
observed.
Effect of NaCl concentration on the surface
properties of Napthenic Acid were studied
results in all three concentration range found
to be similar.
References
Future Scope
Conclusion
• Surface morphology of Napthenic Acid film
using Brewster Angle Microscopy(BAM)
• Effect of CaCl2 concentration(Monovalent
cation vs Divalent cation)
• Visco-Elastic properties(G’ & G’’) of
Napthenic Acid films using Oscillation
Barrier Technique at Air/Water Interface
• Dipping Experiments of Napthenic Acid
monolayer for contact angle measurement
• Atomic Force Microscopy of deposited
Napthenic Acid films
• Alkalinity of the aqueous phase has
significant influence on the film properties
of surface active molecule(Napthenic
Acid) present in crude oil
• Low pH (pH 4) causes viscous behavior in
the films
• Moderate alkalinity (pH 9) induces
elasticity in the films
• High alkalinity (pH 12) cause complete
solubalization of Napthenic Acid in the
form of Metal Napthenates.
• Fastest Relaxation at pH 9 indicates the
rearrangement of molecules at interface
• Song Gao, Kevin Moran, Zhenghe Xu and
Jacob Masliyah J. Phys. Chem. B,
2010, 114 (23), pp 7710–7718.
• Ramesh Varadaraj and Cornelius Brons
Energy Fuels, 2007, 21 (1), pp 199–204.
• Øystein Brandal , Ann‐Mari D. Hanneseth
, Pål V. Hemmingsen , Johan Sjöblom ,
Sunghwan Kim , Ryan P. Rodgers & Alan
G. Marshall Journal of Dispersion Science
and Technology, 27:295–305, 2006.
This study indicates that pH has influence on
stability of the films of Napthenic Acid at
Air/Water Interface.
At pH 9 faster relaxation of Napthenic Acid
molecules is observed due to re-orientation
of molecules at Air/Water Interface
High concentrations of salt (NaCl 3000 ppm)
allow Napthenic Acid molecules to stay at
Air/Water Interface
Time(s)
No
rma
lize
dS
urf
ac
eP
res
su
re(m
N/m
)
