Role of Interfacial Tension in the Oil Industry

8/17/2019 Role of Interfacial Tension in the Oil Industry

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Carole Moules, Camtel Ltd

Abstract

This paper concentrates on the principles and measurement of interfacial tension and its value in

predicting the contamination of oil e.g. by organic polar compounds which are formed by theinteraction of oil, water and oxygen and reduce the functionality of the oil. These contaminants

are attracted to the oil/water interface and as a result significantly lower the interfacial tension.A pure interface is compared to one modified by surface active additives for the effect oninterfacial tension, various methods for measuring interfacial tension are described and a method

using interfacial tension to determine the polar and dispersed components of a liquid will beexplained.. The ASTM Standard D971 will be discussed and results shown for both transformer

and lubrication oils. The question ‘The Surface Tension Test - Is it Worth resurrecting?’1 can be

discussed during Question Time.

Introduction

Many industrial processes involve different phases (liquids and/or solids) which come together at

some stage to form an interface e.g. emulsification, flotation, coating, detergency, lubrication,dispersion of powders etc. The physical behaviour of these interfaces is quantified by the valuesof the interfacial tension of the liquids and where solids are involved the contact angles of these

liquids on the solids, both of which can be measured directly. It is possible to breakdown thesurface tension of a liquid and solid (more commonly called surface energy when referring to a

solid) into disperse and polar components using equations found in many text books. Solidinterfaces however are not the subject of this paper.

Terminology

The terms surface, interface and phases are scattered throughout literature, sometimesindiscriminately, hence it is important to ensure that the definitions used here are clarified so that

the reader understands which ‘boundary’ is being described.• Interface – general term to cover the boundary between phases (liquid/gas, liquid/liquid,

liquid/solid etc.) but specifically used for liquid/liquid boundary

• Surface – term normally used for a liquid/gas boundary

• Phase – gas, liquid and solid

• Interfacial Tension – in broad terms the tension at a liquid/gas, liquid/liquid, liquid/solid

interface but normally reserved for the liquid/liquid boundary

• Surface Tension – generally accepted as the tension at the liquid/gas boundary

• Interfacial Free Energy – excess of free energy at the liquid/solid interface

• Surface Free Energy – excess of free energy at the gas/solid interface

• Tension can be shown by either the Greek symbol ! or "

Liquid/Gas and Liquid/Liquid Interfaces

Within the bulk of a liquid the molecules are completely surrounded by other molecules suchthat the forces of attraction are equal in all directions but at the interface there are ‘unsatisfied’forces resulting in an inward attraction. This is what causes small drops to assume a spherical

shape and soap films to contract. The tendency of an interface to contract in order to minimisethe interfacial area leads to a state of tension hence surface tension is the tension along the

surface of a liquid in air. It is the force acting at right angles to a line of unit length in the


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Figure 1. Pure Liquid/Air Interface

Figure 2. Pure Liquid/Liquid Interface

With the formation of any type of interface work is done and there is an associated free energychange i.e. the molecules at the interface have an excess of free energy due to being in the

interface. The units (per unit area) of specific excess free energy are the same as those forinterfacial tension but are only numerically equal for pure liquids in equilibrium with theirvapour or when two pure immiscible liquids make contact at a plane interface:

work/area or force/distance (length)

where

Free Energy = Work/Area (J m-2

) J = m2kg s

-2 & Area = m

2 hence m

2kg s

-2/m

2 = kg s

-2

Tension = Force/Length (N m-1

) N = m kg s-2

& Length = m hence m kg s-2

/m = kg s-2

Generally the interfacial tension of two liquids is less than the highest individual surface tension

of one of the liquids because the mutual attraction is moderated by all the molecules involved.

Some authors consider it sufficient to define both interfacial tension and interfacial free energy

as the work required to increase the area of an interface isothermally and reversibly by unitamount.

It is normal to express interfacial tension in mN/m which is exactly equivalent to the older units

of dyne/cm.

Modified Interfaces

An interface can be modified by the addition of soluble and insoluble materials which results ineither an adsorbed film or a deposited film, see Figure 3.


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Figure 3. Modified Interfaces

Surface active materials have molecules containing polar heads (hydrophilic) and non-polar tails(hydrophobic) which orientate at the interface as a monolayer. This is the more favoured

situation compared with complete solution in one or other of the phases. The effect is to expandthe interface which in turn must be balanced by the tendency for the interface to contract undernormal interfacial tension forces. The result is a lowering of interfacial tension according to the

following:-! = !0 - # (1)

where !0 = initial interfacial tension before addition of surfactant! = final interfacial tension# = expanding (interfacial) pressure

Ideal conditions for spontaneous emulsification and miscibility of liquids exist when # > !0.

Surfactant solutions reach equilibrium (static interfacial tension) when all the surfactant

molecules are aligned and orientated at the interface. Once the monolayer is saturated excessmolecules aggregate to form micelles. At this critical micelle concentration (CMC) the

interfacial tension remains constant and cannot be further reduced by the addition of more of thesurfactant, Figure 4. The CMC occurs at the point when the minimum interfacial tension for aparticular surfactant or surfactant mix is reached, the value of which determines its potential use

as a wetting agent for a particular application. Knowledge of the CMC can act as a measure toprevent unnecessary additional amounts of expensive ingredients from being used in aformulation.


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dissolve into the other until saturation is reached thus lowering interfacial tension, e.g.water/hexane falls from ~ 49 to 47 mN/m in 1 hour. This phenomenon should not be mistaken as

the dynamic tension which is a measure of the tension as new interface is created. By definitionthe dynamic surface tension relates to the liquid/gas interface and the dynamic interfacial tensionto the liquid/liquid interface.

The rate at which new interfaces are formed can be controlled and changed to study theefficiency of individual surfactants and mixtures of surfactants to reach the surface to lower

interfacial tension. For pure liquids the measurement of interfacial tension is independent of therate at which the interface is created.

Measurement Techniques

The following section describes the most used methods for measuring static surface andinterfacial tension which can be found in standard text books.

Du Noüy Ring for Liquid/Gas & Liquid/Liquid Interfaces

The universally accepted technique for measuring interfacial tension is by the Du Noüy ringwhich is a precise geometry made of Pt-Ir. International standards for both the liquid/gas and

liquid/liquid interface, from a range of industries e.g. electrical insulation & electronics, water &environmental, rubber and surfactants, are based on this technique. The methods for cleaning the

ring are described in these standards but it should be noted that new health and safety rules maypreclude their use. Depending on the test material, water or a ‘legal’ solvent should be used toremove the test liquid from the surface followed by an acetone rinse and a final clean water rinse

before flaming to red heat.

The measurement simply requires the ring to be wetted by the liquid and then pulled through the

interface while measuring the force exerted on the ring. The ring must sit square and parallel tothe interface as failure to do so will result in errors in the measurements. The maximum force ofthe vertical constituent is directly proportional to the surface tension. The measuring sensor of

the Camtel range of Tensiometers is an electronic balance so the weight is monitored andconverted to force i.e. 1cm = 9.81mN. Calibration is done using traceable weights.

Figure 5. Schematic of the Du Noüy Ring Method

When the interface is liquid/liquid the ring should ideally be zeroed in the light phase. The ring

is then cleaned and heavy phase poured into the sample vessel. The ring is positioned under thesurface of the heavy phase to completely wet it and while in this position the light phase is


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The equation to calculate interfacial tension assumes that the contact angle of the liquid on the

ring is zero hence cos $ = 1 (the condition which exists when the force vectors, as the ring ispulled out of the liquid, are vertical), see Figure 5.

The wetted length, L = 4#R, is obtained from the circumference of the inner + outer (2*mean)radii of the ring:-

! = F/L cos$ (2)

Unfortunately with this method, due to the volume of the liquid hanging from the ring as it is

pulled above the surface, a correction needs to be applied:-! = (Fmax – Fv)/L cos$ (3)

where Fv = gV (%L - %A)V = volume of liquid at F = Fmax

Work by Harkins & Jordan2 (table of empirical values) and Zuidema & Waters

3(algorithm based

on2) allows a correction to be made for the additional volume of liquid raised above the

interface:-

where

R is mean ring radius & r is mean wire radius& is heavy phase density & % is light phase density

The time to reach maximum pull on the ring depends on the speed with which the ring is pulledthrough the interface but is typically 20-60 seconds. If surface tension changes as a function of

time then a single measurement with the ring will not show the ageing effect hence it isnecessary to lower the ring into the liquid before the lamella breaks to relieve the force and thenrepeat the measurement. This sequence must be conducted many times to acquire a series of data

points over the specified time period. Obviously every time the ring is moved it creates an‘artificial’ dynamic movement of molecules within the interface which can mask naturally

occurring time dependent behaviour. It is therefore better to use the Du Noüy ring technique forpure liquids and the Wilhelmy plate or pendant drop methods for solutions containing surfactantsor impurities.

Note that measurements of interfacial tension are affected by temperature and hence literaturevalues for pure liquids will always make reference to the temperature e.g. water is 72.8 mN/m at

20oC, see Figure 6.

S u r f a c e T e n s i o n o f P u r e W a t e r v s . T e m p e r a t u r eS u r f a c e T e n s i o n o f P u r e W a t e r v s . T e m p e r a t u r e

5 05 0

6 06 0

7 07 0

8 08 0

- 2 0- 2 0 00 2 02 0 4 04 0 6 06 0 8 08 0 1 0 01 0 0 1 2 01 2 0

T e m p e r a t u r e , d e g r e e s CT e m p e r a t u r e , d e g r e e s C

S

u r f a c

e

t e

n s

i o n , m

N

S

u r f a c

e

t e

n s

i o n , m

N

)/(

679.104534.0

)(*)2(

.*452.1725.0

2 r R R

ST RawF v '+

'&+=

!"

(4)


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,

ideally from micro-roughened platinum but can be pre-wetted paper or glass with a hydrophyliccoating. In this technique the plate is suspended at the interface and maintained in this position

by a force which balances the weight of the plate and the meniscus force of the liquid acting onthe lower edge of the plate. Again this force is proportional to the interfacial tension asdescribed by Equation 2 where the wetted length is the circumference of the plate, Figure 7.

The contact angle of the liquid with the ‘high surface energy’ Pt plate is assumed to be zerohence no corrections for additional forces are required. With the platinum plate it is easy to see

if liquid ‘jumps’ up the plate as it is immersed into the liquid, a sign of total wetting or contact

angle equal to zero.One major difference between the ring and plate is the way in which the measurement is carried

out i.e. the ring moves through the interface whereas the plate is static at the interface. Thereforethere is no ‘artificial’ disturbance of the interface and no increase in the time to reach equilibrium

when using the plate method. Any time dependent effects are true effects hence the plate is therecommended geometry for studying time dependent characteristics.

When using a plate to determine interfacial tension between two liquids it is necessary to zero

the balance with the plate completely under the surface of the light phase. The plate should thenbe cleaned and re-positioned at the surface of the heavy phase and air before adding sufficient

light phase to completely cover the plate otherwise the measurement will include a contributionfrom the light phase/air interface.

Both ring and plate geometries can be used with the force balance type of tensiometer.

Figure 7. The Wilhelmy Plate Technique

Pendant or Hanging Drop for Liquid/Gas and Liquid/Liquid Interfaces

Drop Shape analysis of a hanging or inverted bubble is a text book method for determininginterfacial tension based on the following assumptions:

• The drop is symmetric about a central vertical axis i.e. the drop can be viewed from any

angle.

• The drop is not in motion i.e. interfacial tension and gravity are the only forces affecting

the shape and viscosity and inertia play no part in determining the shape


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balancing pressure as a consequence of interfacial tension which is greater on the concave side,is:-

(()

*++,

-+=.

21

11

rrP # (5)

In a column of fluid of density % and height h, .P = %gh where g is acceleration due to gravity.

The equatorial diameter of the drop and other related geometrical parameters are measured and

the experimental shape is fitted to theoretical profiles derived from Bashforth and Adams tablesand other workers such as Andreas and Padday, see Figure 8.

A video system is used to capture the drop and embedded algorithms in the software compute the

interfacial tension. Like the Wilhelmy plate technique it is possible to monitor the change ofdrop shape over time (time dependent interfacial tension) when surface active agents are present.

Figure 8. Pendant drop Profile

Calibration with this type of instrument is simple requiring only optical magnification accuracywhich is easy to a trace to national standards.

The volume (or more correctly the weight) of a pendant drop which can be supported on a roundtip is described by Tate’s Law as:-

#"rW 2= (6)

where W = weight of drop and r = radius of wetted tip

It is based on the theory that the interfacial tension of a liquid acting vertically around the rim of

a tube holds the drop up against gravity although Tate did not state that the drop weight isproportional to the interfacial tension.

Drop Weight/Drop Volume Method

There is an ASTM Standard Test for measuring the Interfacial Tension of Electrical InsulatingOils, D2285-99, by the Drop Weight technique

4. A relatively simple apparatus is described

which uses a piston which has a scale, graduated in mN/m, attached to it to force water into oilcontained in a glass beaker. Calibration of a parameter R2 is performed by expelling from a


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( ) (()

*++,

-'=

2

1 R

S d D R# (7)

where R1 = scale divisions per drop of water in oil

R2 = scale divisions per drop of water in air

D = density of water and d = density of oil

S = surface tension of water

75% of a drop (found by expelling a trial drop) is expelled and allowed to age suspended from

the needle for 30s and then the volume of the drop is increased slowly to allow it to fall after afurther 45-60s. The volume of water in terms of scale divisions gives the interfacial tension

assuming an average oil density. A correction curve for different oil densities can be made fromrunning tests with different oils and applying the factor.

The method is described as rapid and for field use, giving a semi-quantitative answer subject to

significant error.

In a book by Adam5 the drop volume/weight method is described and much detail is given

regarding the shortfalls of the technique e.g. only part of a drop falls and is smaller than Tate’s

crude formula by on average 40%. Harkins and Brown6 did much work with different tip sizesand found that the weight of a drop is a function of the radius of the tip and the square root of the

capillary constant. For greater description of the problems associated with this technique pleaserefer to the literature. For now accept that this technique requires laborious standardisation with

liquids whose surface tension has been found by a more accurate method using carefullyregulated conditions and tubes with sharp tips (not blunt ends) and time to form a drop of around10 minutes.

Determination of the Disperse and Polar Components of a Liquid

In order to determine the disperse and polar components of the surface energy of a solid it is

necessary to use mathematical models which require the total surface tension (Liquid/Airinterface) of the test liquids and their disperse and polar components to be known. For thepurpose of this document it is assumed the reader is aware of surface energy models such as

Owens/Wendt and Wu and also that the contact angles of the test fluid on the solid are known.

When the test liquid values are unknown then by using the theory of Fowkes regarding theinteractions between two fluids it is possible to calculate both the disperse and polar components.

Background to the Theory of Fowkes

The surface tension of a liquid is a direct measurement of the intermolecular forces and whenonly dispersion forces are involved then:

!total = !disperse (6)

where ! = surface tension

However this is not always the case and when polar forces are present the equation becomes

!total = !disperse + !polar (7)

When two immiscible liquids such as oil and water are in contact each individual liquid issubjected not only to forces from within its bulk but also from the second liquid which lowers

the tension. The magnitude of the oil/water interfacial tension is a function of the dispersionforces of the oil and water molecules and Fowkes showed that the geometric mean of thecontributions to these forces describes the force of interaction:


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where o = oil, w = water, ow = oil/water interface, d = dispersion contribution

In general for any two immiscible fluids, 1 and 2, the equation is:

( )])([2 21212112 p pd d ####### +'+= (9)

where p = polar contribution

Experimental Method

It is necessary to use a known test liquid with no polar component e.g. n-hexane and measure theinterfacial tension between this liquid and the unknown sample. The surface tension and

disperse component of n-hexane which in this case are equal, can be found in the literature and inthe Database of the Camtel software. It is also necessary to know or use your instrument tomeasure the surface tension of the unknown liquid.

Example Assume the 2 liquids used are n-hexane and water measured at 20ºC.

For n-hexane, !total = 18.4 mN/m = !disperse and !polar = 0

For water, !total = 72.8 mN/m = !disperse + !polar

Interfacial tension between n-hexane and water = 51.0 mN/m

Therefore the disperse component of water can be found by re-arranging Equation (9):

2

0

2*1 (

)*+

,- '+= oww

o

d

w

###

## (10)

2

2

518.724.18*

4.18

1()

*+,

- '+=

d

w#

= 21.85 mN/m

ASTM Standard: D 971-99a for Electrical Insulating Oils7

The Du Noüy ring method described in the Standard clearly states that the interfacial tensionbetween a mineral oil and water is obtained under strict non-equilibrium conditions whereby the

measurement is completed within 60 seconds after the formation of the oil/water interface. Theforce used to calculate the non-equilibrium tension is the force required to ‘break’ the liquid

lamella from the ring rather than the maximum force. A correction factor by Zuidema & Waters3

based on Harkins & Jordan’s2 work is applied.

It should be noted that the work of the latter is the basis for measuring the absolute or

theoretically correct value of interfacial tension. The force used must be the maximum pull onthe ring which theoretically is proven to be proportional to interfacial tension and is the point

where the contact angle of the liquid attached to the ring is zero and not the break point as theliquid detaches itself from the ring which is a lower value, see Figure 5.

The apparatus described and shown schematically in the Standard is a manual device which

relies on the dexterity of the operator to lower the platform on which the sample vessel sits andturn a dial with the other hand to maintain the torsion arm in the zero position. The break point

is clearly an obvious event which unskilled operators can be trained to note.

However a variety of means to measure force (torsion wire, load cell or other means to linearlydetermine tension) and raise or lower the platform (mechanical screw, electronic drive or other

means to precisely change vertical position) are allowed. Simply by using a computer controlledsystem the ring can be pulled through the interface at a steady and repeatable user selected speed

time after time while force is constantly monitored thus removing differences in operatortechniques. It is also possible to pick out the maximum force thus eliminating any need todeviate from theoretical accuracy.


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Tensiometer was used to provide the data. Fig. 9 shows the difference in non-equilibriumsurface tension for water measured automatically at two different stage speeds, 66.9 mN/m at

0.2mm/s and 62.9 mN/m at 0.1mm/s. It indicates that the slower the withdrawal of the ring thelonger the lamella stretches before breaking and hence the lower the non-equilibrium interfacialtension. The true surface tension, ~72 mN/m, is obtained at both speeds and is therefore

independent of speed.

The literature value for the interfacial tension of water/n-hexane interface is 51 mN/m. The

maximum force value is displayed in Fig.9 at 50.4 mN/m whereas the non-equilibrium value is45.4 mN/m. The spike in the data at around 20s is because the procedure was deliberately set to

show the values from the start of the stage moving the sample to the ring. It allows the user tosee the contact point of ring and liquid (the spike) which triggers a position reading to ensure thering is immersed to the selected depth before being pulled out through the interface. This option

is normally de-selected but was left on in this case as an example.

Finally a typical clean and dirty electrical insulating oil are also shown in Fig.9 using timescalesfor raising and lowering the stage of the CIT-100 which allow the test to be carried out according

to times stated in the Standard. Again the true and non-equilibrium values are very different,41.6 mN/m and 32.7 mN/m for the clean oil respectively and 9.17 mN/m and 8.14 mN/m for the

dirty oil.

Comparis

on of Actua l Interfacia l Tensi

on t

o Brea k P

oint Tensi

on

& cf. of Water at 2 differen t speeds

0

20

40

60

80

0 20 40 60 80 100 120

Time (s)

I n t e r

f a c i a l

T e n s

i o n

( m N / m )

Water @ 0.2mm / s

Water @ 0.1mm / s

Water-Hexane @ 0.1mm / s

Water-Clean Oil @ 0.1mm / Sl

Water-Dirty Oil @ 0.1mm / S

True IT = 72.8True IT = 72.8

N o n - E q = 66 .9N o n - E q = 66 .9N o n - E q = 62 .9N o n - E q = 62 .9

N o n - E q = 45 .4N o n - E q = 45 .4

N o n - E q = 32 .7N o n - E q = 32 .7

N o n - E q = 8 .14N o n - E q = 8 .14

True IT = 50.True IT = 50.4



Figure 9. Comparison of Maximum Force to Break Force Interfacial Tension using a PCcontrolled Tensiometer

• The clear advantage of the CIT-100 over a manual instrument is that it removes operator

error e.g. the ring is pulled through the interface at a constant speed, manual control willhave a degree of stop/start to the movement as the operator turns the mechanism to raiseand lower the stage which can affect when the lamella breaks and hence the non-

equilibrium value reported.

• In the CIT-100 the force (weight registered on the balance) is constantly monitored and

displayed so it is possible to see clearly the maximum force required to obtain theabsolute tension. This is not possible with the design shown in the Standard.


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Standard calculation of Non-equilibrium surface tension makes no mention that cos$ should = 1. As the height of the lamella increases so the contact angle continues to

change and force drop. Manual control and sample properties can affect the lamellaheight and break point.

• In the CIT-100 water from a bath can be circulated to maintain temperature at 25º C

• In the CIT-100 a stored database contains the test fluids with associated density values

and these are automatically used to compute the Zuidema correction.

Comparison of an Electrical Insulating Oil and an Engine (lubricating) Oil

Interfacial tension measurements against pure water were made on the Camtel CIT-100 tocompare a clean and dirty synthetic electrical insulating oil with two clean and dirty engine oils,one a natural base oil and the other a synthetic base.

Figure 10 shows a clean, dirty and 50:50 mix of the clean and dirty synthetic insulating oil fromone of the UK Electricity boards. As predicted the clean oil has the highest interfacial tension at

around 32 mN/m and the dirty oil the lowest, 15 mN/m with the mix falling somewhere inbetween at 20 mN/m. The results agree with the general rule that when interfacial tension fallsbetween 15-20 mN/m deposits which are hydrophilic in nature may be forming and the oil

should be changed.

Figure 10. Comparison of Clean and Dirty Insulating Oils

Figure 11 shows a similar plot for engine oils where an oil with a natural base is compared to asynthetic base. Here the data is totally different compared to the insulating oil as there is nolarge drop in value between a clean oil and a visually blackened dirty oil. In fact the interfacial

tension values of the clean oils were much lower than a dirty insulating oil with the natural baseat 7.4 mN/m and the synthetic base at 8 mN/m. For the dirty oils the interfacial tension actually

rose slightly to 8.4 mN/m and 9.7 mN/m respectively.

From the marketing claims on the cans of oil which offer protection and cleaning of the engineetc. it is apparent that modern engine oils are far from simple natural or synthetic products.

Polymer molecules are added to disperse contaminating particles in order to keep viscosity low,quantities of base are added to soak up corrosion from acid and the oils contain as much as 20%

0 1 0 2 0 3 0 4 0 5 0 6 0 7 0 8 0 9 0 1 0 0

0

5

1 0

1 5

2 0

2 5

3 0

3 5

Time Secs

S u r f .

T e n . m N / m

Interfacial Tension of Synthetic Insulating Oils

N e w O i l . 0 0 I

5 0 % N e w O i l - 5 0 % A g e d O i l . 0 0 I

A g ed O i l . 00 I


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Interfacial Tension of Clean and Dirty Engine Oils

0

2

4

6

8

1 0

0 1 0 2 0 3 0 4 0 5 0 6 0

Time (s)

I n t e r f a c

i a l T e n s i

Synth - C

Synth - D

Nat - C

Nat - D

Figure 11. Comparison of Clean and Dirty, Natural & Synthetic Based Engine Oils

To complete the picture of interfacial tension tests a more ‘natural’ oil/water test was run using asupermarket brand of sunflower oil. In Figure 12 the engine oils can be seen on the same plot asthe sunflower oil which has a much higher value of interfacial tension of 26 mN/m as expected.

Interfacial Tension of Clean and Dirty Engine Oils

0

5

1 0

1 5

2 0

2 5

3 0

0 5 0 100 150 200 250 300 350 400

Time (s)

I n t e r f a c i a l T e n s i

Synth-C

Synth-D

Nat-C

Nat-D

SunOil

Figure 12. Comparison of Data from Figure 11 with Sunflower Oil

Finally oil/air surface tension tests were made to compare the engine oils against each other andagainst the values obtained for interfacial tension. With much higher surface tension values ofaround 30 mN/m for natural and synthetic, clean and dirty engine oils the additives which

improve functionality clearly only migrate to the oil/water (highly polarised) interface.

Conclusion


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interfacial tension but not the surface tension of a clean oil hence the additives must behydrophilic in nature. Impurities which are formed and contaminate oils are also known to be

hydrophilic but with such large amounts of hydrophilic surfactants added in production theinterface is already saturated and no further reduction in interfacial tension is seen i.e. criticalmicelle concentration has been reached.

There is a suggestion from the few tests run for the purpose of this paper that interfacial tensionslightly increases with contamination in engine oils.

Camtel would be more than happy to measure further oils to add to the knowledge base.

References

1 Fitch J, Practicing Oil Analysis, Issue No. 200209

2 Harkins W. D., Jordan F. J. Colloid Int. Sci. 52 (1930) 1751

3 Zuidema H & Waters G, Ind. Eng. Chem. 13 (1941) 312

4 ASTM Designation: D2285-99 Standard Test Method for Interfacial Tension of ElectricalInsulating Oils of Petroleum Origin Against Water by the Drop Weight Method

5 Harkins W. D., Brown, J.A.C.S. (1919), 499

6 ASTM Designation: D971-99a Standard Test Method for Interfacial Tension of Oil

Against Water by the Ring Method

7 Adam N.K., The Physics and Chemistry of Surfaces, 3rd

Edition, Oxford University Press1941

Role of Interfacial Tension in the Oil Industry

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