NATURALLY OCCURRING FAT ACIDS AND THEIR DERIVATIVES. III. VAPOR PRESSURES AND REFRACTIVE INDICES OF THE BINARY MIXTURES: (A) METHYL CAPRYLATE - METHYL CAPRATE, (B) METHYL PALMITATE - METHYL STEARATE, (C) METHYL STEARATE - METHYL OLEATE

86 Pages·2.974 MB·English

by  ACKERMAN, BERNARD

THE PENNSYLVANIA STATE COLLEGE
The Graduate School
Department of Agricultural and Biological Chemistry
NATURALLY OCCURRING FAT ACIDS AND THEIR DERIVATIVES
III. VAPOR PRESSURES AND REFRACTIVE INDICES OF THE
BINARY MIXTURES: (a) METHYL CAPRYLATE-METHYL
CAPRATE, (b) METHYL PALMITATE-METHYL STEARATE,
(c) METHYL STEARATE-METHYL OLEATE
A Dissertation
fey
Bernard Ackerman
Submitted in partial fulfillment
of the requirements
for the degree of
Doctor of Philosophy
June 1952
Approved:
Professor of Agricultural and
Biological Chemistry
TABLE OF CONTENTS
Page
Introduction 1
Historical 3
Statement of Problem 17
Experimental
Preparation and Purification of Esters 18
Preparation of Binary Mixtures 22
Vapor Pressure Determinations 23
Apparatus 23
Procedure 2b
Calibration 26
Vapor Pressure Determinations for the
Methyl Esters and their Binary Mixtures 28
Refractive Index Determinations 39
Apparatus 39
Procedure and Calculations 39
Refractive Index Determinations for the
Methyl Esters and their Binary Mixtures *+0
Chemical Methods of Determination 58
Saponification Equivalent 58
Iodine Number 58
Discussion
Vapor Pressure Determinations 63
Refractive Index Determinations 69
Chemical Determinations ^,72
General Considerations 3<ri>3i<i3 73
TABLE OF CONTENTS
(continued)
Page
Summary 75
Acknowledgements 76
Bibliography 77
-1-
INTRODUCTION
In recent years, many Important and significant
advances have been made in the chemistry of fats and oils.
For example, great improvements have been devised In the
techniques of low temperature crystallization with the
subsequent isolation of certain fatty acids of reasonably
high purity. More efficient fractionating columns have
been used with success for the purification of fatty acid
esters. Paralleling these advances, better methods have
been developed for the evaluation of the physical prop­
erties of such esters. Consequently, those values of
refractive index, density, vapor pressure, viscosity and
surface tension appearing in the literature have begun to
take on a greater significance.
On the other hand, the physical properties have not
been investigated thoroughly with regard to their
adaptability as analytical tools. Certain physical
methods afford a precision which could be utilized to
great advantage in the analysis of mixtures of fatty acid
esters. The analysis of such mixtures Is Important in
two respects. First, it Is of primary concern to the
investigator who must determine the purity of a partic­
ular ester he has prepared. Second, the analysis of such
mixtures is the basis for the quantitative evaluation of
the fatty acid composition of a natural fat. In the past,
chemical determinations such as the saponification
equivalent and iodine number have been utilized almost
exclusively for both the above types of analysis. In
order to supplement the chemical determinations it would
certainly seem worthwhile to attempt to apply various
physical methods to such analytical procedures.
It must also be borne in mind that mixtures do not
always behave as might be predicted from the properties
of their individual components. Thus certain physical
methods of analysis may be utilized effectively to gain
information concerning the nature of mixtures per se.
This is especially Important to the biochemist, since
chemical entities are rarely, if ever, found in the pure
state in living things. Simple fats and oils for example
are complex mixtures even after they have been extracted
from natural sources and subjected to refining procedures
Binary mixtures of fatty acid esters are indeed far
removed from the complexity of a natural fat, but it is
hoped that a study of such mixtures may prove to be a
beginning for future investigations of more complex
systems.
-3-
HISTORICAL
I. Binary Mixtures
A, General Considerations
In order to understand more fully the behavior
of binary mixtures, it would be well to review some of
the basic principles involved. An outline of the most
important concepts will be included here,
1, The Ideal Mixture
The concept of the ideal mixture, or ideal
solution, as it is often designated, has long been
postulated in research concerning the liquid state. This
concept may be considered to be analogous to that of the
ideal gas or the perfect crystal, in that the ideal mixture
conforms to certain rigid mathematical relationships.
Hildebrand and Scott (17) define the ideal solution as
being formed from its liquid components with zero heat of
mixing and no change in total volume. Sameshima (37)
agrees essentially with the above definition, and also
defines what he calls a "quasi-ideal" solution. This is
a solution which satisfies the two conditions above; in
addition, the components A'koitf reversible chemical reac­
tions.
Probably the best known characteristic of the
ideal mixture is the relationship known as Raoult's
law (36). This relationship states that the vapor pres­
sure of each component of a binary mixture is proportional
to its respective mole fraction in the liquid phase. Thus
in a mixture of liquids A and B:
PA = P£ XA
PB - P£ XB
where and Pg are the partial vapor pressures of A and B
respectively, P£ and P^ are the vapor pressures of the
pure components, and X^ and Xg are the respective mole
fractions. In the case where the vapor pressure of com­
ponent B is negligible compared to that of A, one may
measure the total vapor pressure of the system and consider
it equal to the vapor pressure of A. A more difficult
case is encountered when both components have appreciable
vapor pressures. It is then necessary to determine the
partial vapor pressures of the components by resorting to
a series of distillations with a subsequent analysis of
the distillates.
In 1901, G. N. Lewis (26) suggested the use of
the term "fugacity" to express the escaping tendency of a
substance. This term is now used instead of the vapor
pressure where large deviations from the ideal gas laws
are encountered; it may be looked upon as a ’’corrected"
vapor pressure. When applied to Raoult's law, the expres­
sion becomes:
-5-
where f is the fugacity of the liquid. It should be point­
ed out that the use of vapor pressure values instead of
fugacities in such relations will lead to no serious error
at pressures of one atmosphere or less.
The determination of vapor pressure has been
the most common method of examining the ideality of a
binary mixture. This determination is relatively simple
and is of great value in drawing conclusions concerning
systems, that deviate from the ideal condition. In 1903,
Young and Fortey (*f6) investigated the vapor pressures of
several binary mixtures in which the components were
related structurally. They found that in this type of
system, the observed partial vapor pressures agreed closely
with the values calculated from Raoult's law. In the case
of ethyl acetate and ethyl propionate, a small deviation
in vapor pressure of about 5 Hg from the "ideal" value
was observed, but this was considered insignificant by
these authors. Parks and Schwenk (31*) found that mixtures
of ethyl alcohol and normal propyl alcohol obey Raoult's
law almost exactly.
Beatty and Calingaert (5) determined the total
vapor pressure of a number of hydrocarbon binary mixtures.
These workers tested the ideality of a binary mixture by
a comparison of the observed total vapor pressure with the
total vapor pressure calculated from Raoult's law. They
-6-
found that mixtures of two aliphatic hydrocarbons behaved
ideally vrithin about one per cent deviation from the
calculated total pressure. Hildebrand and Sweney (18)
were interested in a mixture consisting of n-hexane and 7
n-hexadeeane. Despite the large difference in length of
the carbon chain, the partial vapor pressure of n-hexane
was found to be proportional to its mole fraction in any
given mixture, thus showing agreement with Raoult’s law.
2. Non-Ideal Mixtures
When considered from the viewpoint of the total
vapor pressure, deviations from Raoult's law may be
classified into two main categories. 1) Mixtures having
a total vapor pressure greater than that expected from
the ideal condition are said to show a positive deviation
from Raoult's law. 2) Where the total vapor pressure is
lower than the ideal, the mixture is said to show a
negative deviation. Maxima and minima in these vapor
pressure curves are encountered frequently and constitute
a mixture of a definite composition with a constant
boiling point. Lecat (25) has compiled a great deal of
information on such mixtures, which are known as azeotropes.
The cause of these deviations has been the subject
of much research since the appearance of Raoult's original
postulate. Dolezalek (10) in 1908 suggested that all
deviations from Raoult's law could be accounted for by a
change in the total number of moles present. This could
-7-
happen In one or both of two ways: 1) a chemical reaction
occurs between the components or, 2) a change occurs in the
degree of association in one or both of the components.
At about the same time, van Laar (23) proposed
that deviations from ideality could be predicted from an
examination of the critical pressures of the substances.
Thus, compounds having large differences in critical pres­
sure would form mixtures which would show large deviations
from Raoult's law.
In 1916, Hildebrand (16) found fault with both
of the above explanations. He pointed out that the pro­
posals of Dolezalek were inadequate to explain the very
large positive deviations shown by certain binary mixtures
(21). On the other hand the critical pressure theory of
van Laar was shown to be inadequate in cases where one of
the components is known to exhibit some degree of associa­
tion. To account for deviations from Raoult's law,
Hildebrand names two factors which he considers to be of
prime importance. The first of these is the internal
pressure- (attractive force between the molecules) of any
given substance. Thus, only compounds having internal
pressures of the same magnitude may form ideal mixtures.
Where large differences in internal pressure exist, large
positive deviations from Raoult's law will occur. The
second factor to be considered is the polarity of the