Characterization and Demulsification

8/3/2019 Characterization and Demulsification

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Journal of Colloid and Interface Science 277 (2004) 464470

www.elsevier.com/locate/jcis

Characterization and demulsification of poly(ethyleneoxide)blockpoly(propylene oxide)blockpoly(ethylene oxide)

copolymers

Zhiqing Zhang, G.Y. Xu , F. Wang, S.L. Dong, Y.M. Li

Key Laboratory of Colloid and Interface Chemistry (Shandong University), Ministry of Education, Jinan 250100, Peoples Republic of China

Received 7 February 2004; accepted 22 April 2004

Available online 28 May 2004

Abstract

Four poly(ethylene oxide)blockpoly(propylene oxide)blockpoly(ethylene oxide) copolymers with different molecular weights and

PPO/PEO composition ratios were synthesized. The characterization of the PEOPPOPEO triblock copolymers was studied by surface

tension measurement, UVvis spectra, and surface pressure method. These results clearly showed that the CMC of PEOPPOPEO was not a

certain value but a concentration range, in contrast to classical surfactant, and two breaks around CMC were reflected in both surface tension

isotherm curves and UVvis absorption spectra. The range of CMC became wider with increasing PPO/PEO composition ratio. Surface

pressure A curves revealed that the amphiphilic triblock copolymer PEOPPOPEO molecule was flexible at the air/water interface. We

found that the minimum area per molecule at the air/water interface increased with the proportion of PEO chains. The copolymers with

the same mass fractions of PEO had similar slopes in the isotherm of the A curve. From the demulsification experiments a conclusion

had been drawn that the dehydration speed increased with decreased content of PEO, but the final dehydration rate of four demulsifiers was

approximate. We determined that the coalescence of water drops resulted in the breaking of crude oil emulsions from the micrograph. 2004 Published by Elsevier Inc.

Keywords: Surface tension; UVvis; Surface pressure; Crude oil emulsions; Demulsification

1. Introduction

Amphiphilic block copolymers, which contain hydrophi-

lic poly(ethylene oxide) (PEO) and hydrophobicpoly(propy-

lene oxide) (PPO) blocks, are commercially available and

widely used [1,2]. These macromolecular surface-active

agents can form normal micelles, reverse micelles, mono-layers, star-like structures, and liquid crystals in aqueous

solutions [38]. Variation of the molecular characteristics

(PPO/PEO composition ratio, molecular weight) of the

copolymer during the synthesis allows the production of

molecules with optimum properties that meet the specific

requirements of different applications. As a result, PEO

PPOPEO block copolymers find widespread industrial ap-

plications in detergency, dispersion stabilization, foaming,

lubrication, demulsification, etc.

* Corresponding author. Fax: +86-531-8564750.

E-mail address: [email protected] (G.Y. Xu).

Several aspects of the structural and dynamic properties

of micelles of the PEOPPOPEO block copolymers have

been investigated by a variety of experimental methods in-

cluding surface tension [9], fluorescence [10,11], UVvis ab-

sorption of selected probes [5,12,13], and small-angle X-ray

scattering (SAXS) [14,15]. Interfacial properties and ag-

gregation behaviors of PEOPPOPEO aqueous solutions,including the surface tension, critical micelle concentra-

tion, and aggregation conformation, often play a central role

in determining and controlling copolymer performance in

many practical applications.

The emulsification of crude oil and brine/water is a

common problem in the oilfield industry that is most fre-

quently resolved through the use of chemical demulsifier

additives. Water-in-crude-oil emulsions are formed during

the production of crude oil, which is often accompanied

with water. The emulsions have stability ranging from a

few minutes to years, depending upon the nature of crude

oil and the extent of water. Natural surfactants such as as-

0021-9797/$ see front matter 2004 Published by Elsevier Inc.

doi:10.1016/j.jcis.2004.04.035
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Z. Zhang et al. / Journal of Colloid and Interface Science 277 (2004) 464470 465

phaltenes, resins, carboxylic acids, and solids such as clay

and waxes stabilize these emulsions. It is essential to break

these emulsions before transportation through pipelines and

prior to refining [1619]. Commercial demulsifiers are poly-

meric surfactants such as copolymers of polyoxyethylene

and polypropylene or alkylphenolformaldehyde resins or

blends of different surface-active substances. PEOPPOPEO block copolymers can be used as demulsifiers. These

demulsifiers are surface-active agents and develop high sur-

face pressures at crude oil/water interfaces. This results in

replacement of rigid films of natural crude oil surfactants by

a film which is conducive to coalescence of water droplets.

The efficiency with which a surfactant acts as a demul-

sifier depends on many factors related to the structure of

the surfactant. Such factors include the distribution of the

demulsifier throughout the bulk volume of the emulsion,

the partitioning of the demulsifier between the phases, and

the temperature, pH, and salt content of the aqueous phase.

Other factors of importance are the mode of injection of thedemulsifier, the concentration of the demulsifier, the type of

solvent carrier, the amount of water in the emulsion, and the

age of the emulsion [20,21].

The main purpose of this present work is to study

the character and demulsification of poly(ethylene oxide)

blockpoly(propylene oxide)blockpoly(ethylene oxide)

copolymers, setting up the relationship between the mole-

cular structure of PEOPPOPEO and the demulsification

of crude oil emulsions. In order to achieve this purpose, four

triblock copolymers were synthesized. We found that there

are significant effects of the molecular structure, including

molecular weight and PPO/PEO composition ratio, on thecharacterization and demulsification. These results are very

important for the applications of the PEOPPOPEO block

copolymers.

2. Experimental

2.1. Synthesis of triblock copolymer

Four triblock copolymers, BP74, BP73, BP64, and BP63,

were synthesized through anion polymerization with differ-

ent proportions of propylene oxide and ethylene oxide, andtrimethylene glycol was used as an initiator in the presence

of potassium hydroxide (see Scheme 1). The reactions were

carried out at temperatures from 120 to 140 C and a pres-

sure of 0.3 MPa in a pressure kettle with an agitator.

Their structural descriptions are presented in Table 1.

Molecular weight was measured by end group analysis and

the content of PEO was validated by 2H NMR quantita-

tive analysis. Triblock copolymers (BP74, BP73) and (BP64,

BP63) have the same size PPO block and decreasing size

PEO blocks, respectively, whereas (BP74, BP64) and (BP73,

BP63) have the same content of PEO and PPO/PEO weight

ratio, respectively.

2.2. Cloud points

Cloud points of 1% PEOPPOPEO solutions were de-

termined visually by testing the temperature at which turbid-

ity was observed. We also noted the temperature at which

turbidity disappeared on cooling. The average of the three

results was taken as the cloud point of the system.

2.3. Surface tension measurements

The surface tension was measured at 25.0

C using theWilhelmy plate method with a K12 processor tensiometer

(Switzerland, Krss Co.; the precise degree of measurement

is 0.01 mN m1).

Scheme 1. Synthesis of the PEOPPOPEO triblock copolymers.

Table 1

Triblock copolymers structural information

PEOPPOPEO MWa NPPO NPEO

b PEO wt% Compositionc Cloud point (C)

BP74 6960 72 32 40 (EO)32(PO)72(EO)32 16.2

BP73 5960 72 20 30 (EO)20(PO)72(EO)20 11.2

BP64 5800 60 26 40 (EO)26(PO)60(EO)26 20.6

BP63 4970 60 17 30 (EO)17(PO)60(EO)17 13.5

a Measured by end group analysis.b Number of PEO monomers in each of the two hydrophilic chains.c

Calculated according to the amount of adding monomer.


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466 Z. Zhang et al. / Journal of Colloid and Interface Science 277 (2004) 464470

2.4. UVvis experiment

The solubilization of I2/I in the PEOPPOPEO mi-

celles was studied by UVvis spectroscopy. The UVvis

spectra of the solution were monitored at wavelengths from

200 to 800 nm at a scan rate of 300 nm min1. All samples

were recorded at room temperature using a Hitachi U-4100UVvis spectrophotometer (resolution 0.1 nm) after baseline

correction.

2.5. Surface pressure

The surface pressure was determined on an NIMA 601

monolayer balance (MLB, NIMA Co., UK). The chloro-

form was used as a spreading agent with a concentration of

0.05g l1. A 20-l solution was spread on the subphase (wa-

ter) in the LB trough slowly and uniformly with the syringe,

and the chloroform was allowed to evaporate for 30 min. Af-

ter the stabilization of the initial surface pressure, the mono-layer was compressed and the A isotherm was recorded.

2.6. Demulsification of emulsion and demulsifier

effectiveness

The emulsions were prepared by mixing water and crude

oil (from the Shengli oilfield) in HT-II homogenizer. In all

the emulsions the water/oil ratio was 1:1. The speed was ca.

1200 rpm for 5 min. Demulsification was studied at 50 C

and 100 mgl1 doses of the demulsifiers. Water separation

was recorded after various intervals of time, till 3 h.

3. Results and discussion

3.1. Surface activity

A parameter of great fundamental value is the critical

micellization concentration, CMC, the copolymers concen-

tration at which micelles start forming. The micellization of

block copolymers is inherently more complex than that of

conventional, low-molecular-weight surfactant. The CMC of

conventional surfactants is a constant at a certain tempera-

ture, while block copolymers have a wide CMC range in the

surface tension isotherm. A large difference is often notedbetween the CMC values determined by difference methods

because their sensitivity to the quantity of monomers present

in solution may vary. Furthermore, composition polydisper-

sity, batch variations, differences in the concentration ranges

covered in CMC experiments, or the lack of sufficient tem-

perature control may be responsible for the observed varia-

tions [22,23].

We determine the surface tensions of aqueous solu-

tions of PEOPPOPEO copolymers over the range 0.1

10,000 mg l1. From Fig. 1 we can see the CMC range of

BP74 and BP64 is from 10 to 80 mg l1, while BP73 and

BP63 are about from 3 to 100 mg l1

. The range of CMC

becomes wider with decreasing content of PEO. The wide

molecular-weight distribution of the copolymers studied and

the presence of hydrophobic impurities have been singled

out in the past as possible factors that caused differences in

the CMC values obtained by surface tension methods. This

behavior was attributed to the replacement of highly surface-

active, short-PEO-chain-length molecules (contributing to alow surface tension value) on the interface by the major com-

ponent, when these highly surface-active molecules were

solubilized in micelles formed by the major component.

At low concentrations the surface tension decreased with

increasing concentration for all copolymers, in accord with

the Gibbs adsorption isotherm. A change in slope was ob-

served in the surface tension curve at a characteristic copoly-

mer concentration, after which the surface tension values

continued to decrease until a plateau was reached. The two-

breaks behavior was observed in the surface tension vs log

concentration plots for all four copolymers studied, as shown

in Fig. 1.

3.2. UVvis results

UVvis spectra of I2/I treated with different concentra-

tion PEOPPOPEO solutions are presented in Fig. 2. In

solutions there exists the reaction I2 + I I3 ; I

3 has two

absorption peaks at 288 and 350 nm [12,24]. As the copoly-

mer concentration increases, the absorption band shifts to

longer wavelengths, and at C > CMC the band intensity be-

gins to increase. The max value is 350 nm in the initial state

and red-shifted with concentrations C > CMC, which indi-

cates that micelles are formed. The increased intensity arises

from the fact that I2 is gradually dissolved in PEOPPO

PEO micelles with higher concentration, when 2I I2occurred and more I3 emerged because of I2 + I

I3 .

For BP74 and BP64 treated at low concentrations (C 10 mgl1 the spectral shape returns with increased

intensity, whereas the peak intensity of BP73 and BP63 in-

creased when C > 3 mgl1. In this concentration, micelles

are formed. The result is in accordance with the surface ten-

sion measurement.

Fig. 3 shows that the UVvis absorption at 350 nm has

two breaks with increasing concentration. Meanwhile, we

observed a peculiar phenomenon from the curve; the secondbreak of BP74 and BP64 absorbance is increasing with the

concentration, but BP73 and BP63 are reduced. This differ-

ence could be attributed to the difference in the structure and

conformation of the copolymer. BP73 and BP63 are more

hydrophobic, and the cloud points are lower than those of

BP74 and BP64, so the turbidity conceals the absorbance

of I3 , and the absorbance intensity is decreased.

3.3. Surface pressure

A curve is an important reflection to demonstrate the

physiochemical properties of the monolayer [2527]. The


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Fig. 1. The surface tension isotherms of four PEOPPOPEO copolymers.

Fig. 2. UVvis spectra of four PEOPPOPEO copolymers at different concentrations.


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Fig. 3. UVvis evolution of the absorbance at 350 nm as a function of

copolymer concentration.

Fig. 4. The A curve for monomers of four PEOPPOPEO copolymers

at 25 C.

amphiphilic copolymers that are used as film-forming mate-

rials can spread and disperse on the water surface and form

a stable monomolecular film only if the interaction between

the phase surface and polymer is large than the cohesion of

polymers themselves.

The A curve of PEOPPOPEO at gas/liquid interface

is shown in Fig. 4. Isotherms for each of the PEOPPOPEO

copolymersare qualitatively alike. At low pressure the PEO

PPOPEO molecules adsorb on the air/water interface in anextended configuration. With increasing pressure, a change

in configuration occurs and the copolymer monolayer be-

comes more compact. On water surface the polymers spread

due to the PEO chains, but can easily give rise to aggregates

forming, upon compression, a continuous sticky film, which

is difficult to transfer onto solid substrate.

The minimum areas per molecule at the air/aqueous so-

lutions interface can be obtained from the A curve by

making tangent of linear part of curve. From the intercept

of the -axis the minimum areas per molecule of BP74,

BP73, BP64, BP63 are about 0.55, 0.43, 0.47, and 0.38 nm2,

respectively. Our data indicate that the order of occupied

areas per molecule is BP63 < BP73 < BP64 < BP74. The

copolymers with the same mass fractions of PEO have sim-

ilar slopes, as can be seen in the isotherm of the A curve.

On bases of the same content of PEO, the minimum area per

molecule increases with the molecular weight of the PEO

PPOPEO copolymers. When the hydrophobic fraction PPO

is uniform, the increasing PEO proportion results in the ex-tension of the molecular configuration, so the area occupied

by PEOPPOPEO molecules is increasing. It is evident that

the PEO chain play an important role on the air/water inter-

face and can contribute to the molecular area.

3.4. Demulsification

A stable emulsion exists only when emulsifying agents

are present. Elimination, alteration, or neutralization of the

emulsifying agents will allow immiscible liquids to separate.

Addition of suitable chemicals with demulsifying properties

specific to the crude oil to be treated will generally providequick, cost-effective, and flexible resolution of emulsions.

Success of chemical demulsifying methods is dependent

upon the following:

(1) An adequate quantity of a properly selected chemical

must enter the emulsion.

(2) Thorough mixing of the chemical in the emulsion must

occur.

(3) Adequate heat may be required to facilitate or fully re-

solve an emulsion.

(4) Sufficient residence time in vessels must exist to permit

settling of demulsified water droplets.

Crude oil emulsions are stabilized by high-molecular-

weight surfactants, viz., asphaltenes and resins. They do not

develop high surface pressures, and therefore steric stabiliza-

tion of water-in-crude-oil emulsions is the most plausible

mechanism of stabilization of such emulsions. Demulsifier

molecules and natural surfactants compete with each other

for adsorption onto the water-drop film. When demulsifier

molecules, which lower interfacial tension much more than

the natural surfactants, are adsorbed at the interface, the film

becomes unstable in the direction of coalescence of water

drops.

Fig. 5 shows that the dehydration speed of BP73 andBP63 is higher than that of BP74 and BP64. Many experi-

ments have proved that the dehydration speed increases with

decreasing PEO contentof the demulsifier. But theend dehy-

dration rate of the four demulsifiers is approximate; 90% of

the water was separated from the emulsions by using all four

demulsifiers after 3 h. It is shown that the block copolymer

surfactants are excellent as demulsifiers for breaking crude

oils from the Shengli oilfield (Peoples Republic of China,

Shandong).

The crude oil emulsions were photographedunder a JEM-

100CX microscope and the coalescence of water drops were

observed. Emulsions formed in the petroleum industry are


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Fig. 5. Effect of demulsifiers on the dehydration rate with time.

Fig. 6. The demulsification micrograph of crude oil emulsion (bright droplets are water). (a) Emulsion was mixed 5 min by an HT-II homogenizer with 1:1

water/oil ratio; (b) 30 min after demulsifier was added; (c) 60 min after demulsifier was added.

predominantly water-in-oil or regular emulsions, in which

the oil is the continuous or external phase and the dispersed

water droplets form the dispersed or internal phase.

Fig. 6 shows a demulsification micrograph of the crudeoil emulsion. From Fig. 6 we can see the size of the wa-

ter droplets become bigger with time after demulsifier is

added. The role of the demulsifier is to change the interfacial

properties and to destabilize the surfactant-stabilized emul-

sion film in the demulsification process. In the beginning,

small and uniform drops flocculate and some large drops

begin to form at 30 min. Then two or more large drops con-

tinue to form a single larger drop: coalescence happens. It

is observed that the droplet size grows fast and the droplet

number reduces after demulsifier is added. We concluded

that coalescence of water droplets destroyed emulsions.

Three terms related to stability commonly encounteredin crude oil emulsion are flocculation, coalescence, and

breaking. Although they are sometimes used almost inter-

changeably, those terms are in fact quite distinct in meaning

as far as the condition of an emulsion is concerned.

Flocculation refers to the mutual attachment of indi-

vidual emulsion drops to form flocs or loose assemblies.

Flocculation can be, in many cases, a reversible process,

overcome by theinput of much less energy than was required

in the original emulsification process. Coalescence refers

to the joining of two or more drops to form a single drop of

greater volume, but smaller interfacial area. Although coa-

lescence will result in significant microscopic changes in the

condition of the dispersed phase, it may not immediately re-

sult in a macroscopically apparent alteration of the system.

The breaking of an emulsion refers to a process in which

gross separation of the two phases occurs. In such an event,the identity of individual drops is lost, along with the physi-

cal and chemical properties of the emulsion. Such a process

obviously represents a true loss of stability in the emulsion.

It has been established that the kinetics of chemical

demulsification is complicated by the interaction of three

main effects, according to the micrograph. These are (1) the

displacement of the asphaltenic film from the oil water inter-

face by the demulsifier; (2) flocculation; (3) coalescence of

water drops.

4. Conclusion

PEOPPOPEO triblock copolymers were synthesized

and the properties and demulsification of PEOPPOPEO

were reported. The CMC of PEOPPOPEO is not a certain

value but a range, in contrast to that of classical surfactants,

and two breaks around the CMC were reflected both on the

surface tension isotherm and in the UVvis absorbance spec-

tra. Surface pressures were obtained for spread monolay-

ers for the range of copolymer compositions, which reveals

that the amphiphilic block copolymer molecule PEOPPO

PEO is flexible on the air/water interface. The copolymers

with the same mass fractions of PEO have similar slopes in


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the isotherm of the A curve. PEOPPOPEO is widely

used as a demulsifier. From the demulsification experiments

the conclusion can be drawn that the dehydration speed in-

creases with reduced content of PEO.

Acknowledgment

This work is supported by the Key Technologies R&D

Program Foundation of China (02BA312B07).

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Characterization and Demulsification