MEASUREMENT OF INTERFACIAL TENSION IN FLUID-FLUID SYSTEMS J. Drelich Ch. Fang C.L. White Michigan Technological University, Houghton, Michigan INTRODUCTION For more than a century, a variety of techniques have been used to measure interfacial tensions between immisci- ble fluid phases. A recent monograph by Rusanov and Prokhorov (1) provides a broad review of the technical literature on the interfacial tension techniques with de- tailed discussion of the theoretical bases and instrumenta- tion. Additional valuable sources of information on the in- terfacial tension measurement methods include selected chapters in Refs. 2–5. In this article, we present a very brief overview of the most common techniques used in interfacial tension measurements. The reader is encou- raged to explore Refs. 1–5 and references therein for fur- ther details. This article is organized as follows. ‘‘Classical Inter- facial Tension Measurement Methods’’ reviews the me- thods that are used in surface chemistry laboratories. A short comparison of these techniques is presented at the end of the section. This comparison has been prepared to guide a selection of the experimental method for mea- surements of interfacial tension in liquid-fluid systems, including systems with surfactants, viscous liquids, or mol- ten metals. Many of the industrial operations involve the liquid-fluid interfaces, for which the composition is cons- tantly refreshed and does not reach equilibrium. The im- portance of such dynamic interfacial tensions is increa- singly recognized to be essential to the understanding and control of interfacial processes in multiphase, multicom- ponent systems. ‘‘Dynamic Interfacial Tension Measure- ments’’ discusses a freshly created interface. In ‘‘Measure- ment of Ultralow Interfacial Tension’’ an example of: when the value of interfacial tension is significantly less than 1 mN/m is discussed. Ultralow interfacial tensions are common in the fluid systems of advanced tech- nologies of liquid-liquid emulsification processes when effective surfactant solutions are used. Finally, in ‘‘Mic- rotensiomery,’’ we discuss the methods of interfacial tension measurements that have been applied (or have potential to be applied) to microinterfaces of microdrop- lets. Fundamental research on the interfacial properties of nanomaterials (materials and particles with microstructur- al features on the micrometer or nanometer scale) and droplets of micrometer-sized or nanometer-sized dimen- sion will be an important challenge in the rapidly deve- loping field of nanotechnology. CLASSICAL INTERFACIAL TENSION MEASUREMENT METHODS Fig. 1 shows a classification of common interfacial ten- sion measurement methods, both classical and modern. Group I represents examples of techniques commonly used for direct measure of the interfacial tension with a microbalance. The techniques in group II are those in which interfacial tension can be determined from direct measurement of capillary pressure. Analysis of equilib- rium between capillary and gravity forces is employed in the techniques of groups III and IV. Group III techniques rely on the balance between surface tension forces and a variable volume of liquid, whereas Group IV techniques fix the volume of a liquid drop and measure the distortion of that drop under the influence of gravity. Group V in- cludes techniques where the shapes of fluid drops are dis- torted by centrifugal forces and are used to measure ultra- low interfacial tensions. Group I: Direct Measurement Using a Microbalance Interfacial tension at fluid-fluid interfaces is a reflection of the excess energy associated with unsaturated inter- molecular interactions at the interface. This excess energy tends to drive interfaces to adopt geometries that mini- mize the interfacial area, and this tendency can be inter- preted as a physical force per unit length (i.e., a tension) 3152 Encyclopedia of Surface and Colloid Science Copyright D 2002 by Marcel Dekker, Inc. All rights reserved.
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MEASUREMENT OF INTERFACIAL TENSIONIN FLUID-FLUID SYSTEMS
J. DrelichCh. FangC.L. WhiteMichigan Technological University, Houghton, Michigan
INTRODUCTION
For more than a century, a variety of techniques have been
used to measure interfacial tensions between immisci-
ble fluid phases. A recent monograph by Rusanov and
Prokhorov (1) provides a broad review of the technical
literature on the interfacial tension techniques with de-
tailed discussion of the theoretical bases and instrumenta-
tion. Additional valuable sources of information on the in-
terfacial tension measurement methods include selected
chapters in Refs. 2–5. In this article, we present a very
brief overview of the most common techniques used in
interfacial tension measurements. The reader is encou-
raged to explore Refs. 1–5 and references therein for fur-
ther details.
This article is organized as follows. ‘‘Classical Inter-
facial Tension Measurement Methods’’ reviews the me-
thods that are used in surface chemistry laboratories. A
short comparison of these techniques is presented at the
end of the section. This comparison has been prepared to
guide a selection of the experimental method for mea-
surements of interfacial tension in liquid-fluid systems,
including systems with surfactants, viscous liquids, or mol-
ten metals. Many of the industrial operations involve the
liquid-fluid interfaces, for which the composition is cons-
tantly refreshed and does not reach equilibrium. The im-
portance of such dynamic interfacial tensions is increa-
singly recognized to be essential to the understanding and
control of interfacial processes in multiphase, multicom-
or other quasi-static techniques may be more appropriate.
This consideration applies to any fluid-fluid system in
which kinetically limited processes (adsorption, viscous
flow, etc.) take place.
Because of the relatively high temperatures involved
and their reactivity with many gases and solids, surface
tension measurements on liquid metals pose special chal-
lenges. The four principal techniques that have been em-
ployed are the maximum bubble pressure method, the
sessile drop method, the drop volume or weight method,
and the pendant drop method (26, 27). It is important that
measurements on liquid metals be carried out in inert
Table 2 Accuracy and suitability of classic techniques used in interfacial tension measurements
Method
Accuracy
½mN=m�
Suitability
for surfactant
solutions
Suitability
for two-liquid
systems
Suitability for
viscous liquids
Suitability for
melted metals
Commercial
availability
Wilhelmy plate �0.1 Limited Good Very good Not recommended Yes
Du Nouy ring �0.1 Limited Reduced accuracy Not recommended Not recommended Yes
Maximum bubble 0.1–0.3 Very good Very good Not recommended Yes Yes
pressure
Capillary rise <<0.1 Very good Very good, Not recommended Not recommended Not
experimentally
difficult
Drop volume 0.1–0.2 Limited Good Not recommended Yes Yes
Pendant drop �0.1 Very good Very good Not recommended Yes Yes
Sessile drop �0.1 Good Very good Very good Yes Not
M
Measurement of Interfacial Tension in Fluid-Fluid System 3159
gas environment to avoid reactions with gases and other
phases. Oxygen and other reactive gases are known to exert
strong effects on the surface tension of selected metals,
even when present in parts per million concentration (27).
DYNAMIC INTERFACIALTENSION MEASUREMENTS
In fluid-fluid systems containing interfacially active so-
lutes, a freshly created interface will not generally be in
compositional equilibrium with the two immiscible fluids
it separates. It is only after solute redistribution from one
or both phases (i.e., adsorption) has occurred that this
interface will achieve its equilibrium state. It is sometimes
important to measure the interfacial tension of freshly
created interfaces, and such measurements yield what is
known as ‘‘dynamic surface tension.’’ A detailed review
of experimental techniques, theoretical background, and
literature on the measurements of dynamic interfacial ten-
sions was recently published by Dukhin et al. (3). Another
valuable source of analysis of adsorption at the interface
and dynamic interfacial tension is the book published by
Joos (28).
Table 3 provides a characteristic time range for the
selected interfacial tension measurement techniques. Of
different techniques already discussed in this article, the
capillary rise method is not recommended for dynamic
interfacial tension measurements. The techniques dis-
cussed in the next sections are not very suitable for exa-
mination of dynamic effects at interfaces either.
Interfacial tension changes that occur over time in-
tervals of at least several seconds (and continue over se-
veral minutes, hours, or days) can be studied by using
most of the classical techniques discussed in the previous
section. For example, Fig. 9 shows the results of inter-
facial tension relaxation between bitumen and water of
varying pH value recorded with the Wilhelmy plate ins-
trument (30). In this bitumen-water system, the dynamic
character of the interfacial tension is caused by diffusion
of natural surfactants from the bitumen to interface and
aqueous phase, and surfactant reaction with ions dissolved
in water (31).
As emphasized in the previous section, examination of
the interfacial tension for surfactant solutions using clas-
sical techniques should be carried out with caution. Sur-
factants often adsorb on the solid surfaces of equipment
used in measurements and change the wetting character-
Table 3 Characteristic time range for common interfacial tension measurement techniques
Method Time rangea Comments
Wilhelmy plate >10 s Some of the surfactants might alter the wetting properties
of the plate, causing the change of measurement
conditions (possible source of error)
Du Nouy ring >30 s Same as above
Pendant drop >10 s Strongly surface active chemicals might cause the release
of pending drop before completion of the measurement
Sessile drop >10 s Some of the surfactants might alter the wetting properties
of a solid support substantially changing the shape of
the sessile drop
Drop volume/weight 1 s–20 min Hydrodynamic effects associated with releasing liquid
volume and circulation of liquid inside the drop
sometimes significantly reduce the accuracy of the
interfacial tension measurements
Maximum bubble pressure 1 ms–100 s Difficulties with determination of the real surface age and
problems with hydrodynamic effects in the vicinity
of interface
Growing drop/bubble >10 msb Not available commercially
Oscillating jetc 1–10 ms Not available commercially
Pulsating bubblec 5 ms–0.2 s Not available commercially
aBased on Dukhin et al. (3).bIt is claimed by MacLeod and Radke (29) that the dynamics interfacial tension can be measured for several hours, although Ref. 3 specifies the upper
limit as 600 s.cMethods not discussed in this article.
3160 Measurement of Interfacial Tension in Fluid-Fluid System
istics of a solid surface. This effect usually causes expe-
rimental problems in the Wilhelmy plate, du Nouy ring,
and sessile drop techniques (see comments in Table 3).
Short-time interfacial tension and wetting effects play
important roles in high–volume industrial processes such
as froth flotation of particles and droplets, detergency,
foam or froth generation, and stability (3). In these pro-
cesses, dynamic interfacial tensions become more crucial
to the success of the technology than the equilibrium (or
near-equilibrium) interfacial tension. This issue has been
emphasized in the literature (3, 32, 33), but straightforward
links between dynamic interfacial tensions and (e.g., pro-
cess efficiencies) have not yet been well established. This
research area is just evolving, and the continued funda-
mental research will probably establish a better connection
of dynamic interfacial phenomena with practical needs.
Four basic techniques for measurements of the dynamic
interfacial tension at short intervals include the maximum
bubble pressure, growing drop (bubble), oscillating jet,
and pulsating bubble methods (Table 3). Neither the os-
cillating jet technique nor the pulsating bubble technique
is discussed in this article. Bases of these techniques are
provided in Refs. 1–3 and references therein.
The maximum bubble (drop) pressure method and its
modifications have been the most popular techniques used
in research conducted in recent years. Apparatus for ma-
king these measurements are also available commercially.
The maximum bubble pressure technique was briefly des-
cribed in ‘‘Classical Interfacial Tension Measurement Me-
thods.’’ Following is a short description of a modification
of the maximum bubble pressure technique that is suitable
for dynamic surface tension measurements.
Growing Drop (Bubble) Method
Modern instrumentation permits the pressure inside a bub-
ble or drop to be precisely and continuously measured as it
forms and detaches from the end of a capillary (3, 29, 34,
35). The geometry of a drop or bubble can also be mo-
nitored during growth and detachment by using advanced
videographic equipment. This ability to simultaneously
monitor both pressure and geometry (size and shape) of
bubbles or drops as they form allows dynamic interfacial
tensions to be evaluated over a range of growth rates.
Furthermore, the same experimental approach allows mea-
surement of surface tensions using approaches normally
applied to systems in (or near) equilibrium, such as the
drop volume and maximum pressure drop techniques.
Fig. 10 describes an experimental approach used by
MacLeod and Radke (29) for measurement of dynamic
interfacial tension using the growing drop technique. In
Fig. 9 Dynamic interfacial tension measured between bitumen
and water at 60�C using the Wilhelmy plate technique. The pH
values for water at the beginning (t = 0) and at the end of the
measurements (t = 90 min) are listed in the figure legend. (The
results are from Ref. 30.)
Fig. 10 Schematic of the growing drop apparatus used by MacLeod and Radke. (From Ref. 29.)
M
Measurement of Interfacial Tension in Fluid-Fluid System 3161
this apparatus, a liquid drop or gas bubble is formed and
released from a capillary by using a precise micropump to
carefully control the growth rate of the drop or bubble. A
pressure transducer is used to simultaneously monitor and
record the internal pressure in the drop or bubble, while
its size and shape are recorded by using a video camera.
These experiments can be carried out for a range of flow
rates ranging from near equilibrium growth (very low
flow rates) to highly nonequilibrium growth conditions
(very rapid bubble or drop growth).
Fig. 11 shows selected results of MacLeod and Radke
(29) for growth of 0.25 mM aqueous decanol droplets for
a range of droplet growth rates between 5 and 100 mm3/
min. The plots of interfacial tension vs. time show ini-
tially increasing, reaching a maximum, and then decrea-
sing as a function of time. The positive slope of the g vs. t
curves for t < 1 results from the depletion of decanol
adsorption due to rapid expansion (stretching) of the
interface when the bubble is small (see Fig. 4A). As the
drop geometry proceeds beyond hemispherical shape
(Fig. 4C), the relative rate of surface area growth decrea-
ses, allowing decanol from the bulk liquid to replenish the
surface adsorption. The decrease in interfacial tension as-
sociated with this increase in adsorption yields the negative
slope observed in the g vs. t curves for t > 1. As expected,
the largest dynamic interfacial tensions, approaching those
for pure water, are observed for droplets formed at the
highest capillary flow rates. Conversely, the lowest values
(approaching the equilibrium interfacial tension for this
solution) were observed for droplets growing at very slow
rates. At drop formation times approaching 100 s, inter-
facial tensions for drops formed at all flow rates approach
the equilibrium value for the 0.25 mM aqueous 1-decanol
solution.
The zero time for each measurement in Fig. 11 cor-
responds to the moment of detachment for the previous
drop. Data points for a given capillary flow rate in Fig. 11
start at the time where the drop reaches a hemispherical
shape (Fig. 4B), which also corresponds to the point at
which the interfacial tension can be evaluated by using the
maximum pressure method. Interfacial tension measure-
ments using the drop-volume method are, of course, de-
termined at the point where the drop detaches from the
capillary and correspond to the last data point in the
corresponding curve for the growing drop method. These
two techniques, therefore, provide ‘‘snapshots’’ of the in-
terfacial tension values in dynamic systems, and they
bracket the family of curves determined by the growing
drop method. The growing drop method provides the very
important advantage of continuously recording the inter-
facial tension throughout the drop formation and allows
the competing kinetic effects of interfacial stretching and
solute transport for adsorption to be explored.
MEASUREMENT OF ULTRALOWINTERFACIAL TENSION
Recovery of petroleum using tertiary oil recovery tech-
nology; cleaning of solid surfaces from dirt, grease, and
oil; formulation of stable emulsions; in situ remediation
of oil-contaminated soil with surfactant solutions; and
other applications often rely on lowering the interfacial
tension between immiscible liquids to ultralow values
(much less than 1 mN/m) using surfactants. The measure-
ments of such low interfacial tensions are extremely dif-
ficult to perform with classical interfacial tension mea-
surement methods reviewed in ‘‘Classical Interfacial
Tension Measurement Methods’’ or the dynamic tech-
niques discussed in the previous section (e.g., see the ac-
curacy values for methods shown in Table 2). For the
measurements of ultralow interfacial tensions, the spin-
ning drop technique has been developed at both laboratory
and commercial scales (36–39). The basis of this tech-
nique is discussed in the following paragraph. An ad-
ditional method designed for measurements of ultralow
interfacial tensions was proposed by Lucassen (40) and
is based on the analysis of the shape of the drop suspended
Fig. 11 Dynamic surface tension of 0.25 mM aqueous decanol
solution droplets growing in air at 23�C. Based on the experi-
mental data presented by MacLeod and Radke. (From Ref. 29.)
3162 Measurement of Interfacial Tension in Fluid-Fluid System
in liquid with a density gradient. The need for a strict
control of liquid density limits this latter technique to rela-
tively few applications.
Spinning Drop Technique
This technique relies on the fact that gravitational ac-
celeration has little effect on the shape of a fluid drop
suspended in a liquid, when drop and the liquid are con-
tained in a horizontal tube spun about its longitudinal axis
(Fig. 12) (36, 37). At low rotational velocities (o), the
fluid drop will take on an ellipsoidal shape, but when o is
sufficiently large, it will become cylindrical. Under this
latter condition, the radius (r) of the cylindrical drop is
determined by the interfacial tension, the density diffe-
rence (Dr) between the drop and the surrounding fluid,
and the rotational velocity of the drop. As the result, the
interfacial tension is calculated from the following equa-
tion (38):
g ¼ 1
4r3Dro2 ð18Þ
The spinning drop method has been very successful in
examination of ultralow interfacial tensions down to 10�6
mN/m (39). For example, Fig. 13 shows the interfacial
tension values for octane drops suspended in an aqueous
phase saturated with ethoxylated alcohols (41). As shown
in Fig. 13, the interfacial tension for the octane-water-
CnE4 system varied from about 1 to 10�4 mN/m and de-
pended on both temperature in the system and length of
the hydrocarbon chain in the chemical structure of etho-