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E ffect of Temperature on Interfacial Tension
in Kerosene-Surfactant-Water Systems
Y OSHIRO KITAMURA, 1 QINGFA H U A N G , AKIYOSHI MIYACHI,
K ENJI YOSHIZAKO, AND T E R U O TAKAHASHI
Department of Applied Chemistry, Okayama University, Tsushima-Naka, Okayama 700, Japan
R eceived November 15, 1991; accepted May 13, 1992
T he temperature dependence of interracial tension inoil-surfactant-water systems was investigated
t o evaluate the adsorption of surfactants under superheated conditions; the adsorption under such cond itions is necessary in order to provide fundamental information for discussing the superheat limit of
W / O emulsions. As surfactants, Span 80 and NPEx ( x = 2, 5, and 7.5 ) were used. The interfacial tension
w as measured by the Wilhelmy method in the temperaturerange from 301 to 355 K. The saturation
a dsorption obtained from the interfacial tension data for the kerosene-water system is almost independent
o f temperature, while that for the n-tetradecane-water system decreases slightly with increasing temp erature. The adsorption equilibrium constant is correlated as a function of temperature which contains
t he free energy of adsorption. From theseconstants, the adsorption of the surfactants under superheated
c onditions can be predicted. © 1992AcademicPress,Inc.

INTRODUCTION

U tilization of emulsified fuels is a viable
m eans for pollutant reduction and enhanced
f uel economy (1, 2). Such improvement in
c ombustion arises from microexplosions; the
w ater droplets dispersed in the fuel phase are
s uperheated to a temperature farabove their
b oiling point and then explode violently. This
m icroexplosion is closely related to the superh eat limit of water which is determined by the
r ates of bubble nucleation at the interface (3).
W e have reported the superheat limit of water
i n fuel emulsions and suggested that the bubble
n ucleation is accelerated by the presence of
s urfactants absorbed on the interface (4). Ino rder to relate the nucleation rate to the sup erheat limit of water in the presence of surf actants, we have to know both the interfacial
t ension in fuel-surfactant-water systems and
t he adsorption of surfactants under superh eated conditions. In our previous work (4),
w e measured the superheat limit of the emul1 T o w h o m correspondence should be addressed.

s ions by usingkerosene, diesel fuel, and nt etradecane as the fuel; no significant differe nce among them was observed. Thus in this
s tudy we used mainly kerosene to examine the
i nterfacial behavior of fuel-surfactant-water
i nterface, because fuels usually are not pure
s ubstances but hydrocarbon mixtures. Then
w e used n-tetradecane as a pure substance for
c omparison.
A lthough a large amount o f data oninterf acial tension has been reported, data at higher
t emperatures are not as abundant ( 5 - 8 ) . In
p articular, the adsorption of surfactants at
h igher temperatures has been infrequently
s tudied (9). The adsorption of surfactants can
u sually be evaluated by measuring the interf acial tension as a function o f concentration.
I t is difficult, however, to measure the interf acial tensionof the water-oil interface at
h igher temperatures, especially above the
b oiling point of water. Thus, we first measured
t he temperature dependence of the interfacial
t ension below the boiling point of water and
t hen determined an equilibrium constant as a
f unction of temperature, from which the ad-

2 49

Journalof Colloidand InterfaceScience,Vol. 154,No. 1, November1992

0021-9797/92 $5.00
Copyright© 1992by AcademicPress,Inc.
All rightsof reproductionin any formreserved,

2 50

KITAMURA

s orption of surfactants at the higher tempera tures can be evaluated.
EXPERIMENTAL

M aterials
K erosene and n-tetradecane (better than
99% purity) were used as fuel, and were obt ained from Ishizu Chemical, Ltd., Osaka, Jap an. As surfactants, Span 80 (sorbitan mono...
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