Conspectus
A key physical property of volatile
liquids is vapor pressure (VP).
Volatile organic compounds (VOCs) are a classification of compounds
directly associated with low boiling points, high rates of evaporation,
and high flammability. The majority of chemists and chemical engineers
were directly exposed to the odor of simple ethers, acetone, and toluene
in the air while taking an organic chemistry laboratory course as
an undergraduate student. These are just a few examples of the numerous
VOCs produced by the chemical industry. When toluene is poured into
a beaker from its reagent bottle, its vapors readily evaporate at
ambient temperature from this open container. When the cap is securely
placed back on the reagent bottle of toluene, a dynamic equilibrium
develops and exists in this closed environment. This chemical concept
is known as a vapor–liquid phase equilibrium. A crucial physical
property of spark-ignition (SI) fuels is high volatility. In the United
States, most of the vehicles traveling on the road today have SI engines.
Gasoline is the fuel used to power these engines. It is a major product
manufactured by the petroleum industry. This fuel is petroleum based
since it is a refined product of crude oil consisting of a mixture
of hydrocarbons, additives, and blending agents. Thus, gasoline is
homogeneous solution of VOCs.
The VP as a function of temperature
of a pure VOC can readily be
measured using an ebulliometer. The VP is also known in the literature
as the “bubble point pressure”. In this investigation,
the VP as a function of temperature was acquired for the VOCs ethanol,
isooctane (2,2,4-trimethylpentane), and n-heptane.
The latter two VOCs are primary reference fuels components found in
87, 89, and 92 grade gasoline. Ethanol is an oxygenate additive of
gasoline. The VP of a homogeneous binary mixture of isooctane and n-heptane was also acquired using the same ebulliometer
and methodology. In our work, an enhanced ebulliometer was used to
collect the VP data in our work. It is known as the vapor pressure
acquisition system. The devices that comprise the system automatically
acquire the VP data and log it into an excel spreadsheet. The data
are readily transformed into information to compute the heat of vaporization
(ΔH
vap
). The results
described in this Account compare quite favorably to the literature
values. This validates our system for performing fast and reliable
VP measurements.