Adsorption and Surface Tension of Silica in Transformer Oil

7/30/2019 Adsorption and Surface Tension of Silica in Transformer Oil

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Adsorption and surface properties of silica withtransformer insulating oilsq

Amane Jadaa,*, Abdelouahed Ait Chaoua, Yves Bertrandb, Olivier Moreauc

aInstitute de Chimie des Surfaces et Interfaces, 15 rue Jean Starcky, B.P. 2488, 68057 Mulhouse, FrancebDivision Recherche et Developpements, Electricite de France, CIMA 8, Les Renardieres, BP 1. F-77818 Moret-sur-Loing Cedex, France

cDepartement Machines Electriques, Division Recherche et Developpements, Electricite de France, Service Materiel Electrique,

1 Avenue du General de Gaulle, 92 141 Clamart Cedex, France

Received 22 January 2001; revised 19 December 2001; accepted 7 January 2002; available online 12 February 2002

Abstract

The presence of polar species in transformer insulating oil may cause degradation and electric discharges in the power transformer. Such

oil polar fraction can originate either from the neat oil and/or from its oxidative degradation in the power transformer. The aim of this study is

to examine the relation between the insulating oil and the electrical properties of its polar fraction in order to predict eventual failure in the

power transformer.

We investigate at ambient temperature the adsorption of the polar fractions of various transformers insulating oils (a new and two used

oils) from the neat oils onto silica particles. The adsorbed amount was higher for the used oils as compared to the new one. Infrared spectra of

the polar fraction indicate the presence of hydroxyl, aromatic and carboxyl functional groups that are found in the asphaltenes compounds.

Microelectrophoresis study of the oil polar fraction covered silica particles gives negatively charged oil polar fraction. Such oil surface

charge depends on the pH and results from the ionisation of the oil acidic surface groups. Finally, we obtain a good correlation between the

amount of the oil polar fraction and the magnitude of the zeta potential at the water/oilsilica interface. q 2002 Elsevier Science Ltd. All

rights reserved.

Keywords: Asphaltenes; Adsorption; Silica; Zeta-potential

1. Introduction

Insulating oils are used in the electric power transformer

mainly to transfer heat. However, static electrication due

to the insulating liquid ow causes failure and electric

discharges in the power transformers [111]. Further, the

presence of small amount of sulphur in the oil can promote

dissolution of copper (cupric corrosion by sulphur), which

functions as degradation catalyst of the oil. In order to over-

come static electrication and oil degradation, we add the

benzotriazole (BTA) to the insulating oil in some manufac-tures researches [2]. This molecule adsorbs easily from the

oil onto the pressboard and metallic cooper. Hence, the

addition of the BTA to the oil inhibits charge separation

on the pressboard and reduces the oil degradation [2].

The static electrication, which is due to the liquid ow,

results from the charge generation at oil/pressboard

interface. In this process, it is generally assumed that the

pressboard and the oil acquire, respectively, a negative and a

positive charge, due to preferential adsorption onto the

pressboard of negative ions present in the oil (impurities,

additives). Static electrication occurs if the energy of oil

ow is sufcient to separate these ions from the oil/press-

board interface. Previous simulation studies [12], on static

electrication, have shown that charge generation at oil/

pressboard interface results not only from the transfer of

ionic species from the oil to the pressboard, but also from

the pressboard to the oil. In that work, the authors consider

two modelling approaches for simulation studies. Accordingto these models, charge generation at oil/pressboard inter-

face can be due either to adsorption onto the pressboard of

negative ions present in the oil, or to diffusion in the oil of

positive ions coming from the pressboard. Such positive

ionic species may be the protons H1 that result from the

dissociation of the alcohol radicals in the cellulose. The two

modelling approaches considered in the simulation studies

[12] were in accordance with the experimental observations.

Several authors [1322] have studied the process of the

electrical charge generation at solid/organic liquid interface.

From these works, it was concluded that the solid acquires

Fuel 81 (2002) 12271232

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* Corresponding author. Tel:133-3-896-08709; fax: 133-3-896-08799.

E-mail address: [email protected] (A. Jada).q Published rst on the web via Fuelrst.comhttp://www.fuelrst.com


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electrical charges either by preferentially adsorbing ions,

possibly still associated in structures, or by ion formation

resulting from surface groups dissociation, which are being

held in some lyophilic structure. Therefore, the occurrence

of the static electrication in power transformer suggests the

presence of an electrical double layer at the oil/pressboard

interface.

Mineral transformer oil is mainly a mixture of hydrocar-

bon compounds of three classes: alkanes, naphtenes and

aromatic hydrocarbon. Polar compounds found in the trans-

former oil are a minor part of the constituents, and usually

contain heteroatoms, such as, oxygen, nitrogen, or sulphur,

which may greatly inuence the chemical and electrical

properties of the oil. These heteroatoms are mainly

associated with the oil aromatic structures, such as pyridine,

pyrazine, pyrrole, carbazole, indole, or benzoic acid groups

and can result in their stabilisation in aqueous medium.

Further, nitrogen and oxygen atoms can be involved in

various acid and basic functional groups, such as carboxylic

acid, ether, ester, aldehyde, ketone, amine and amide[2325]. In addition, Waldo et al. [26] showed that sulphur

atom is essentially present in thiophene and organic sulphur

form.

The insulating oil polar fraction can result either from the

neat oil and/or from its oxidative degradation in the power

transformer. It is mainly made of charged particles in the oil,

which deteriorates transformer insulation by decreasing

electrical strength. It is useful to know the amount and the

charge of this polar fraction to predict future failure in

power transformer. The object of this work is to establish

a relationship between the chemical and the electrical prop-

erties of the transformer insulating oil polar fraction. Forthis purpose, we have extracted the polar fractions of

various transformers insulating oils from their neat oils by

using silica gel as solid support, and we have examined their

surface compositions and charges.

2. Experimental

2.1. Materials

Electricite de France supplied the three oils used in this

work. Two oils, respectively, UO1 and UO2, were used in

power transformers, while the third oil, NO, is new oil. Thesilica substrate used is a silica gel 60 for column chromato-

graphy purchased from Merck, having specic surface

area 480540 m2 g21. The solid particles size 40

60 mm, which are aggregates of primary particles having

size of about 12 nm.

2.2. Extraction and adsorption onto silica of the oil polar

fraction

The silica gel 60 was preliminary heated at 100 8C during

1 day, a weight amount (2 g) of the dried silica was placed in

a stoppered bottle and a known volume (50 cm3) of the neat

oil was added. The resulted dispersion was allowed to stand

few days at ambient temperature, until the adsorption equi-

librium was reached. Then, the oil fraction covered silica

substrate was separated from the dispersion and washed

several times with n-hexane to remove the residual non-

polar oil fraction adsorbed on silica. Finally, the oil polar

fraction covered silica solid substrate was placed in an oven,

until all the residual n-hexane was driven off into vapour.

2.3. Preparation of the oil polar fraction covered silica

particles aqueous dispersions

The silica dispersions were prepared by introducing a

given amount of the oil covered silica particles (0.1%) in

the 1023 M NaCl aqueous solutions. The resulted disper-

sions were ultrasonicated for few minutes and the pH values

of the systems were varied in the range 39 by adding to the

dispersions small amounts of sodium hydroxide (NaOH) or

hydrochloric acid (HCl) aqueous solutions. The nal silica

dispersions were then shaken for few days, until electricalequilibrium was reached. The nal pH values were

measured prior the zeta potential measurements.

2.4. Infrared measurements

Infrared spectra of the sample were recorded on

BRUKER (IFS 66, IFS 66/S, IFS 48) apparatus with a

golden gate single reection diamond attenuated total

reectance (ATR) accessory, purchased from GRASEBY

SPECAC. In this technique, the sample (the neat oil or the

oil polar fraction) is held in intimate contact on an ATR

crystal and internal reection occurs, when the infrared

radiation enters the ATR crystal (diamond). The crystaldesign enables total internal reection of the radiation that

creates an evanescent wave at the crystal surface. Such

evanescent wave extends into the sample, which is in

contact with the crystal. The spectra were an accumulation

of 100 scans and ranged from 4000 to 600 cm21.

We have recovered the oil polar fraction from the oil

polar fraction covered silica solid substrate by using ethanol

as eluent. The resulted ethanol solution was then placed in a

vacuum rotary evaporator to drive out the residual solvent

and to yield the pure oil polar fraction, which was investi-

gated by the infrared spectroscopy.

2.5. Zeta potential measurements

The electrokinetic potential or zeta potential of the silica

dispersions was measured at ambient temperature using the

microelectrophoresis method. The measurements were

made with a Zetaphoremeter II model Z3000, having cell

section of 0.5 0.2 cm2, apparent cell dimension of

1504 mm and a micrometer calibration with objective

20.X: 50 mm. This apparatus purchased from SEPHY,

converts the electrophoretic mobility Ue into the zeta poten-

tial z according to the Smoluchowski's equation [27,28]:

z h=1Ue; where h and 1 are, respectively, the viscosity

A. Jada et al. / Fuel 81 (2002) 1227 12321228


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and the permittivity of the aqueous medium. At least, we

achieve three experiments with each dispersion. The values

of the zeta potential were reproducible within ^2 mV.

3. Results and discussion

3.1. Infrared spectra of the neat oil and its polar fraction

Table 1 presents the amounts of the oil polar fractions

found in various insulating oils. As can be observed in the

table, the amounts of the polar fractions are higher in the

used oils as compared to the new one. The Fourier transform

infrared spectra (FTIR) of the neat oils NO, UO1 and UO2,

obtained in the region 4000600 cm21, are presented,

respectively, in the Fig. 1(a)(c). As can be seen in the

gures, the neat oils from different origins display similar

infrared spectra and are of high aliphatic index. These

spectra show three stretching and two bending absorption

peaks, respectively, in the regions 30002800 and 1450

1370 cm

21

, due to the alkyl CH bonds. In addition,Fig. 1(a)(c) indicates that the used oils UO1 and UO2 as

compared to the new one NO, are not degraded.

It is useful to know the structure of the insulating oil polar

fraction, which is the oil active specie that can cause electric

discharges and failure in the power transformer. Knowing

the structure of such oil polar fraction will allow us a better

understanding of its adsorption properties and the polarisa-

tion that occurs at the pressboard/insulating oil interface.

Fig. 2(a) and (b) shows infrared spectra of, respectively,

UO1 and UO2 polar fractions. Hence, we can observe differ-

ent features, when we compare the spectra to those of the

neat oils, in the region 18001600 and 34003100 cm21.

The spectra in Fig. 2(a) and (b) indicate also that the oilpolar fractions contain associated phenolic groups due to

hydrogen bonding, carboxylic groups, and aromatic groups.

These groups absorb, respectively, in the regions, 3400

3100, 18001700, and around 1600 cm21. Moreover,

Fig. 2(a) and (b) shows that the amount of the polar groups,

i.e. phenolic, carboxylic and aromatic groups, increase from

the UO1 to UO2 oil. On the other hand, the infrared spectra

of the asphaltenes as studied by others authors [2931],

present various absorption bands, which are found in the

spectra of oil polar fractions investigated in the present

work. These absorption bands are usually assigned to the

CH bonds of alkyl, the CH and the CyC bonds of

aromatic compound, the CyO bonds of carbonyl groups,

and the OH bonds of alcohol. Accordingly, the infrared

spectra of the UO1 and UO2 oil polar fractions presented in

Fig. 2(a) and (b) are consistent with the structure of the

asphaltenes. Thus, the asphaltene compounds contain

oxygen atoms, which are involved in various acidic func-

tional groups of linear or aromatic structures analogous to

carboxylic acid, and phenolic acid groups. Such groups, asdetermined by infrared spectroscopy, refer to the family of

compounds containing the carboxylic or phenolic, which are

bounded by alkyl chains to the condensed aromatic rings of

asphaltenes.

3.2. Electrical properties of the oil polar fraction

The electrical properties of the oil polar fractions were

determined by measuring the zeta potential of the oil polar

fraction covered silica particles. The analysis of the oil polar

fraction electrical properties provides a way of examining

charged particles in the oil, when the transformer is in equi-librium operation. Such charged particles can adsorb from

the oil onto the pressboard and hence cause failure in the

transformer.

Further, varying the pH of the aqueous dispersion allows

us to control the polarity of the oil polar fraction covered

silicawater interface and its inuence on the acidity of the

oil surface functional groups. Fig. 3 indicates the variation

of the zeta potential versus the pH, for various oil polar

fraction covered silica aqueous dispersions. The negative

values of the zeta potential of the aqueous dispersions

observed in the pH range 39 indicate negative oil polar

fraction surface charges. For the bare silica, the negative

surface charge observed is due to the ionisation of thesurface hydroxyl groups. The properties of such silica are

determined by the surface chemical activity, which in turn

depends on the concentration, the distribution, the type of

hydroxyl groups, the presence of siloxane bridges; and on

the porous structure of the silica. Several studies [32,33]

conrm the presence of silanol (SiOH) groups on silica

surface. In addition, in the capillary electrochromatography

on silica columns, used for separation of organic

compounds, the efciency depends on the silica particle

size [34].

The mechanism of the surface charge generation at the

A. Jada et al. / Fuel 81 (2002) 1227 1232 1229

Table 1

Amounts of the oil polar fractions adsorbed on the silica and maximum values of the zeta potential in water of the oil covered silica particles

Insulating oil Reference in

the text

Amount of the polar fraction

adsorbed on the silica (in

gram of the polar fraction per

100 cm3 of the neat oil) (%)

Maximum value of the oil

polar fraction covered

silica particles zeta

potential (mV)

New oil NO 0.052

35Used oil UO1 0.36 2 45

Used oil UO2 0.20 2 70


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A. Jada et al. / Fuel 81 (2002) 1227 12321230

Fig. 1. (a) Infrared spectra of the NO neat oil. (b) Infrared spectra of the UO1 neat oil. (c) Infra red spectra of the UO2 neat oil.


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bare SiO2water interface is mainly due to the adsorption of

hydroxyls (OH2) onto an amphoteric silica site. The

isoelectric point, i.e. the pH at which the zeta potential z

0; for this system can be obtained only by extrapolation,

since it occurs at about pH 2.5. Further, the acid side of

the z-pH curve is not accessible to electrokinetic

measurements.

On the other hand, the negative surface charge observed

in Fig. 3 for the oil polar fraction covered silicawater inter-

face, is attributed to the ionisation of the oil polar fraction

acidic groups. These groups, the carboxylic acid and pheno-

lic acid groups, have respectively, pKa, 4 and 4, pKa, 9

values [3537]. The important feature exhibited by these

used oil polar fractions, UO1, and UO2, when compared to

the bare silica, is the increase in the magnitude of the zeta

potential at pH values above 4. This behaviour is due to the

increase of the concentration of the oil surface groups, i.e.

the carboxylic acid and phenolic acid. However, the non-

used oil NO shows slight variations in the zeta potential,

within the experimental error range^2 mV. Fig. 3 indicatesalso an increase in the plateau levels zmax (see Table 1) from

the UO1 to the UO2, in a good agreement, with the increase

of their respective polar group amounts, as observed on their

FTIR spectra.

The change in the surface charge observed for the oil

polar fractions in the aqueous medium is related to the

nature and the amount of acidic groups present at their

surfaces. In addition, an arrangement of oil components

on the silica surface will occur in water, which will give

rise to the acidic groups development at the oilwater inter-

face. This surface arrangement is due to the differences in

the afnity of the various oil components for water and

silica.

It is likely that the polarity of charging at the oil polar

fraction/silica interface may resemble to the same charging

that occurs at the oil/pressboard interface in the power trans-

former. In fact, the surface of the paper or on the pressboard,

used in the transformer, contains polar groups, such as

hydroxyl (OH) that we nd also on the silica surface. In

these groups, the hydrogen and the oxygen atoms are,

respectively, positively and negatively polarised, due to

the large electronegativity of the oxygen atom. Such

positive polarised hydrogen has an afnity for the phenolic

groups that belongs to the oil polar fraction. Hence, in the

transformer, the pressboard may selectively adsorb the

family of compounds containing the phenolic groups,

which are bounded by alkyl chains to the condensed

aromatic rings of the oil polar fraction. Further, according

to Fowkes [38], the acidbase interactions between theacidic groups at the surface of a solid and the basic groups

of the molecules of a liquid are very specic. The energy of

such interaction is proportional to the enthalpy of the

complex or adducts formation of the acidbase pair and to

their surface concentration. Thus, in the pH range 39,

acidbase complexes between the acidic groups of the oil

polar fraction and the ions OH2 of the aqueous solution

form at the solidwater interface. Therefore, an increase

of the oil polar fraction surface charge with the increase

A. Jada et al. / Fuel 81 (2002) 1227 1232 1231

Fig. 2. (a) Infrared spectra of the insulating oil UO1 polar fraction. (b) Infrared spectra of the insulating oil UO2 polar fraction.


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of the pH should result from an increase of the concentration

of the acidbase surface complexes. Finally, the two

methods used in this work, microelectrophoresis and infra-

red spectroscopy, seem to be suited to investigate the

surface composition and functionalities of oil polar groups,

which may be present in the transformer insulating oil.Hence, we nd a good correlation between the amount of

the oil polar fraction, the peak area of various oil polar

groups, obtained by FTIR spectra, and the magnitude of

the zeta potential at the water/oilsilica interface. The use

of the two methods to investigate insulating oils is useful to

predict future failure in the power transformers.

4. Conclusions

At ambient temperature oil polar fractions adsorb from

various transformers, insulating oils (a new and two used

oils) onto the silica gel. The microelectrophoresis measure-ments in water of the various oil covered silica particles,

gives negatively charged oil polar fraction that resembles to

asphaltenes molecules. Further, the FTIR analysis of such

oil polar fraction indicates the presence of phenolic

carboxylic and aromatic polar groups. We nd a good corre-

lation between the amounts of the oil polar groups; their

functionalities and the magnitude of the zeta potential at

the water/oil covered silica interface.

In the power transformer, the oil polar fraction may

adsorb from the oil onto the pressboard leading to static

electrication. The use of microelectrophoresis and infrared

methods seems to be a good tool for analysis of the surface

charge and the structure of the particles included in the

insulating oil and hence to prevent electrical discharges in

power transformer.

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Fig. 3. Variation of zeta potential versus the pH for the aqueous dispersions

of the bare silica and the silica covered with insulating oil polar fractions.

Adsorption and Surface Tension of Silica in Transformer Oil

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