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

)