†To whom correspondence should be addressed. Fax: +81-75-753-6285; E-mail: adachi＠kais.kyoto-u.ac.jp
Biosci. Biotechnol. Biochem., 66 (8), 1723–1726, 2002
Solubility of Saturated Fatty Acids in Water at Elevated Temperatures
Pramote KHUWIJITJARU, Shuji ADACHI,†and Ryuichi MATSUNO
Division of Food Science and Biotechnology, Graduate School of Agriculture, Kyoto University,
Sakyo-ku, Kyoto 606-8502, Japan
Received March 20, 2002; Accepted April 25, 2002
The solubility in water of saturated fatty acids with
even carbon numbers from 8 to 18 was measured in the
temperature range of 60 to 2309 C and at a pressure of 5
or 15 MPa. The pressure had no signiˆcant eŠect on the
solubility. The solubility of the fatty acids increased
with increasing temperature. At temperatures higher
than about 1609 C, the logarithm of the solubility in
mole fraction was linearly related to the reciprocal of
the absolute temperature for each fatty acid, indicating
that the water containing solubilized fatty acid molec-
ules formed a regular solution at the higher tempera-
tures. The enthalpy of a solution of the fatty acids in
water, which was evaluated from the linear relationship
at the given temperatures, increased linearly with the
carbon number of the fatty acid
Key words: subcritical water; fatty acid; solubility;
Water at a signiˆcantly high temperature below the
critical point (3749 C) under enough pressure to main-
tain its liquid state is called subcritical water. Subcrit-
ical water has characteristics diŠerent from those of
ambient water. For example, the dielectric constant
of water at 259 C is about 80, but that of water at
2009 C decreases to about 35, which is approximately
equal to that of ambient ethanol.1)The ion product
changes from an order of 10„14for ambient water to
an order of 10„11for subcritical water;2)that is, water
is prone to dissociation at high temperatures. Due to
these properties of subcritical water, attention has
recently been paid to the utilization of subcritical
water as an extraction or reaction medium in food
and its related processing. Many researchers have
reported the extraction of natural substances from
plant sources with subcritical water.3–6)Kabyemela
et al. have investigated the epimerization and decom-
position of glucose7)and the decomposition of
cellobiose8)in sub- and supercritical water. Yoshida
et al.9)have reported the production of organic acids
and amino acids through the hydrolysis of ˆsh meat
by subcritical water. Holiday et al.10)have used sub-
critical water for hydrolyzing vegetable oils to free
fatty acids, and Møller11)has patented the process for
splitting fat by hydrolysis at a near-critical tempera-
ture and supercritical pressure.
Thus, subcritical water seems to be a promising
medium for fat and oil processing. The solubility of
oil, fat and fatty acids in water would be an im-
portant and basic parameter. However, there are
only a few reports on such solubility at temperatures
lower than 609 C.12–14)In this context, we measured
the solubility of fatty acids with even carbon num-
bers of 8 to 18 in water at 609 C to 2309 C under pres-
sures of 5 and 15 MPa.
Materials and Methods
(À98z), palmitic (À95z) and stearic (À95z) acids
were purchased from Wako Pure Chemical Indus-
tries (Osaka, Japan). Caprylic (À98z) and lauric
(À99z) acids were respectively purchased from
Tokyo Kasei Kogyo (Tokyo, Japan) and Sigma-
Aldrich Japan (Tokyo, Japan). The distilled water
used in this study had a conductivity of less than
2.0 mSWcm. Chloroform (Wako, 99z) was used as
the solvent to collect a solubilized fatty acid.
Solubility measurement. The solubility of each
fatty acid in water was measured according to the
method of Miller and Hawthorne,15,16)which had
been used to determine the solubility of various
organic substances in sub- and supercritical water,
with some modiˆcations. Figure 1 schematically
shows the apparatus used in this study. A high-
pressure-resistant vessel (20 mm i.d.×120 mm, TVS-
N2 model with some modiˆcations, enabling 20 MPa
and 2609 C to be used, Taiatsu Garasu Kogyo,
Osaka, Japan) was packed with 3-mm diameter solid
glass beads to facilitate the contact of water with the
fatty acid. Five milliliters of water was ˆrst put into
the vessel, before 10 ml of the fatty acid was added.
The vessel was then placed in a gas chromatographic
oven (GC-17A, Shimadzu Seisakusho, Kyoto, Japan)
to control the temperature during the solubility meas-
urement. Stainless steel tubing for HPLC (1W16 inch
o.d.) was used for connecting each part together. An
Fig. 1.Schematic Diagram of the Experimental Apparatus.
○ 1 reservoir of water, ○ 2 -○ a and ○ 2 -○ b HPLC pumps for
respectively delivering water and chloroform, ○ 3 preheating coil,
○ 4 high-pressure-resistant vessel for measuring the solubility of
fatty acids in water, ○ 5 gas chromatographic oven as an isother-
mal chamber, ○ 6 reservoir of chloroform as a collecting solvent,
○ 7 3-way joint, ○ 8 cooling coil, ○ 9 back-pressure valve, and ○
test tube for collecting the eluate.
in the Eluate at () 60, (#) 80, and () 1209 C and 15 MPa.
Change with Time in the Concentration of Caprylic Acid
P. KHUWIJITJARU et al.
HPLC pump (LC-10Atvp, Shimadzu) was used to
deliver water at a constant ‰ow rate of 0.1 mlWmin
through a pre-heated coil into the vessel. Water
passed down through the fatty acid phase to reach
equilibrium solubility. The water phase ‰owed out of
the bottom of the vessel and was mixed with
chloroform, which was delivered at a ‰ow rate of
0.2 mlWmin by another LC-10Atvp HPLC pump, at a
3-way joint. The chloroform was introduced as the
collecting solvent to prevent clogging of the tubing by
the fatty acid at low temperatures. The mixture of
water and chloroform was passed through a cooling
coil and sampled into a test tube. A back-pressure
regulator (70 MPa maximum pressure, Tescom, Elk
River, MN, U.S.A.) was connected to the line after
the cooling coil to control the pressure in the system.
The pressure was regulated to 5 or 15 MPa. After 150
minutes had elapsed from the start of the operation,
3 to 5 samples of 1 ml of water (3 ml in total volume)
were taken in test tubes and used for a gas chromato-
graphic determination of the fatty acid.
Gas chromatographic analysis. An internal stan-
dard solution dissolved in chloroform (0.5 or 1.0 ml)
was added to a test tube containing an eluted
sample. The internal standards were lauric acid (1 or
2 mgWml) for caprylic, capric and myristic acids,
capric acid (0.2 mgWml) for lauric acid, and myristic
acid (0.1 mgWml) for palmitic and stearic acids. The
mixture was well mixed in a vortex mixer, and the test
tube was allowed to stand until the two phases had
separated well. One or two milliliters of the chlo-
roform phase was removed, and the fatty acids in the
method of Hashimoto et al.17)The methylesteriˆed
fatty acids were determined with a GC-7A gas chro-
according to the
matograph (Shimadzu) equipped with a ‰ame ioniza-
tion detector and a 3.1-m glass column of 3.2 mm i.d.
containing Thermon-3000 5z Shincarbon A 60–80
mesh packing material (Shimadzu). A ˆve-point
calibration curve was generated for each fatty acid,
and the solubility was calculated by assuming that
each fatty acid had been completely partitioned in the
Results and Discussion
Solubility of the fatty acids in water
Figure 2 shows the change in concentration of
caprylic acid in the eŒuate with time at diŠerent tem-
peratures and at 15 MPa. At any temperature, the
concentration reached a constant value after 150 min
or longer. Therefore, 3 to 5 samples were collected
after 150 min or longer in the subsequent experi-
ments, and the fatty acid concentrations in the sam-
ples were determined by gas chromatography. The
standard deviation was within 10z of the mean value
in most cases.
Figure 3 shows the solubility of the fatty acids in
water as a function of temperature. The open and
closed symbols represent the solubility at 5 MPa and
15 MPa, respectively. There was no signiˆcant eŠect
of pressure on the solubility at the pressures tested.
The solubility at 609 C was almost the same as or
slightly lower than the previously reported value at
ambient pressure.13)The solubility of every fatty acid
increased with increasing temperature and decreased
with increasing carbon number. The dependence of
solubility on the carbon number of the fatty acids at
100, 180 and 2309 C is shown in Fig. 4. The logarithm
of the solubility decreased linearly with increasing
carbon number at all temperatures. The slope of the
line in Fig. 4 tended to decrease with increasing tem-
Enthalpy of the solution
The solubility in units of moleWkg of water, which
Solubility of the Fatty Acids in Water at Various Temper-
(, ) caprylic, (#, $) capric, (, ) lauric, (, ) myris-
tic, (%, &) palmytic and (<,
closed symbols represent the solubility at 5 and 15 MPa, respec-
tively. The curves were empirically drawn.
) stearic acids. The open and
Fatty Acid at (, ) 100, (#, $) 180 and (, ) 2309 C.
The open and closed symbols represent the solubility at 5 and
15 MPa, respectively.
Dependence of the Solubility on the Carbon Number of a
Fatty Acids and the Reciprocal of the Absolute Temperature.
The symbols are the same as those deˆned in Fig. 3. The
curves and lines were empirically drawn. The slope of a line
gives the enthalpy of the solution as a regular solution. The
broken lines indicate the hypothetical solubility drawn by ex-
trapolating the solid lines.
Relationships between the Mole Fraction Solubility of the
in Water, D ˜ HF, on the Carbon Number of the Fatty Acid.
The D ˜ HFvalues were estimated from the results shown in
Fig. 4 at temperatures above 150–1609 C.
Dependence of the Enthalpy of a Solution of a Fatty Acid
Solubility of Fatty Acids in Water
is shown in Fig. 3, was converted to the mole frac-
tion, XF, and is plotted against the reciprocal of the
absolute temperature, T, in Fig. 5. DiŠerentiation of
the logarithm of XF with respect to 1WT gives the en-
thalpy of solution, D ˜ HF:18)
D ˜ HF＝„R(& ln XFW&(1WT))sat
where R is the gas constant, and the sat subscript
represents ``saturated''. The plot for each fatty acid
gave a straight line at temperatures above 150–
1609 C, indicating that the aqueous solution of the
fatty acids behaved as a regular solution at these tem-
peratures. The enthalpy of the solution of a fatty acid
in the liquid state in water, D ˜ HF, which was esti-
mated at these temperatures, is plotted against the
carbon number of the fatty acid in Fig. 6. The en-
thalpy was linearly related to the carbon number.
The D ˜ HF value at temperatures below 1509 C,
which was calculated according to Eq. 1, decreased
with decreasing temperature, and the experimental
solubility was higher than the hypothetical solubility
expressed by the broken line in Fig. 5. Similar
phenomena have been observed for the solubility of
alkanes19)and normal alcohols18)in water. These ab-
normal phenomena can be explained by the iceberg
formation around the solute molecules in water. In
other words, a large positive enthalpy of mixing of a
non-polar solute with water was compensated by a
large negative enthalpy of iceberg formation that
resulted in a higher solubility than that for a regular
solution.18,19)This explanation should be applicable
to the dissolution of a fatty acid in water at a temper-
ature below 1509 C.
1726 1726P. KHUWIJITJARU et al. Download full-text
Another aspect that we can observe from Fig. 5 is
that the solubility of the long-chain fatty acids such
as palmitic and stearic acids tended to increase more
dramatically with increasing temperature than that of
the short-chain fatty acids, and if we extrapolate the
line to higher temperatures, the solubility of the long-
chainfatty acids would be greater than that of the
short-chain fatty acids at these temperatures. Based
on the molecular dynamic simulations of dilute aque-
ous solutions, Yezdimer et al.20)have suggested that
alkanes with longer chains had greater solubility than
those with shorter chains near the critical point of
water. Their simulation appears to agree with the ex-
trapolated notion, although the solubility at much
higher temperatures should be experimentally meas-
Part of this study was ˆnancially supported by
Grant-aid for exploratory research from Japan Soci-
ety for the Promotion of Science. K. P. gratefully ac-
knowledges a Monbukagakusho Scholarship from
the Japanese government.
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