Factors affecting Water Solubility in Oils
Water can occur in three phases within an oil system, depending
on the chemistry of the oil in question. This goes for both mineral
and synthetic oil. In general, oils dissolve some water. However,
each oil has its specific water-saturation point beyond which excess
water becomes either emulsified or free. Therefore in various
oil-systems, one may have to deal with dissolved, emulsified,
A solution is a thermodynamically stable state, where solvating
forces homogeneously mix all the molecules present in the solution.
The type and amount of additives mostly determine the water
solubility of new oils, whereas oxidation products have a remarkable
effect on the solubility of aged oils.
Pure base oils have very limited solubility, which is related to the
ratios of paraffin, naphthenic, and aromatic compounds. The saturation
point at 20 °C varies from approximately 30 parts per million (ppm)
of paraffin oils to over 200 ppm of fully aromatic liquids, but it is typically
between 40 to 80 ppm. Solubility may increase significantly with the
use of additives. The typical value for new lubrication oil is <500 ppm.
Oxidation products also increase solvating efficiency. Mineral-based
transformer oils typically have very little additives and therefore have
low solubility like base oil, whereas lubrication oils with greater amounts
of additives generally have much higher solubility (Figure 1).
Figure 1: Average water solubility of mineral base oil and
one lube oil as function of temperature.
The overall absorption forces and water content of the solution in the
equilibrium state are determined by Gibb's energy of mixing. On the
molecular level, the absorption forces are binding forces between the
water molecules and the molecules in the oil matrix. Water molecules
are polar by nature, so the interaction forces increase with the
increasing polarity of the matrix molecules, such as additives and
The dependence of solubility on temperature is almost always exponential
(Figure 1). Hot oil dissolves greater amounts of water. The hotter the
oil, the greater the water absorption from air to oil in the same humidity
conditions. This should be noted in any system setup, as airborne moisture
contamination is one of the most common water sources.
Ageing of oil
Lubricating oil circulating in high-speed systems deteriorates with time
due to oxidation. Oil characteristics - presence of oxygen, catalysts
resent, and the temperature levels to which the oil is exposed - determine
the rate of the aging process. In lubrication oil systems, air is always
present, and the metal debris from machine construction and the moisture
present are catalysts for the aging, that is, oil deterioration process.
Ageing processes are equilibrium reactions, and therefore the decay
rate of oil is a function of activity of water rather than absolute water
content. High temperatures and mechanical stresses, e.g. in the bearings,
also accelerate the process.
Free water / emulsion
When water content in oil reaches the saturation point of that oil, it
separates out and free water is formed, resulting in a two-phase system.
Free water is commonly considered as the number one contaminant of oil.
Water corrosion and cavitation type damages are mainly consequences
of the free water phase.
Under a high mixing ratio or presence of surfactant additives (i.e. wetting
agents that lower the surface tension of a liquid, allowing easier spreading,
and the interfacial tension between two liquids), water may form an
emulsion with oil. An emulsion is a mixture of two unblendable substances,
where one substance is dispersed in the other. Because of surfactants,
micro-size water droplets are homogeneously mixed in oil, forming an
emulsion. The surfactants are chemicals that have both hydrophilic (a
molecule that can bond with water) and hydrophobic (a molecule that
is repelled by water) natures and thus are soluble to both phase water
and oil. They form micelles (liquid particles) over the water droplets that
convert the droplets "soluble" in oil. Surfactants may be added to oils
to form emulsion, or some additives may act as emulsifiers, although
added for different purposes.
Traditionally, water in oil has been measured by Karl Fisher titration
(a method for determining the moisture content of a sample) and
expressed in ppm, which is the total absolute water content, thus
not giving any indication whether water is dissolved or free. However,
due to differences in oil types and difficulty in predicting aging effects,
ppm values are often not sufficient. Therefore, relative values like
water activity (aw) are useful parameters for setting alarms in control
Capacitive thin film sensors give this value without temperature
corrections or oil-type calibrations. The active film of the sensor
absorbs water molecules, which change the dielectric constant (i.e.
a measure of the ability of a material to resist the formation of an
electric field within it) of the film. The absorption is proportional to
the equilibrium relative humidity of oil, thus indicating the margin to
The Vaisala HUMICAP® thin film polymer sensor products are beneficial
in applications where the water amount must not exceed solubility
limit, i.e. free water has to be avoided. The sensor is very sensitive
even to negligible amounts of water and other small polar molecules.
Therefore, the active polymer film and the sensor structure have to
be such that the additives and oxidation products in oil do not disturb
the measurement. The latest generation of the Vaisala HUMICAP®
sensor is developed for demanding moisture measurement in liquid
hydrocarbons. The sensor's excellent chemical tolerance provides
accurate and reliable measurement over a wide measurement range.
If the solubility of a specific oil is known through the whole operating
temperature range, the measured relative moisture value can be
converted to absolute water content (ppmw). However, we must
note that the conversion is valid only if the water solubility of the
oil does not change. In lubrication systems the solubility changes
with time regardless of the system maintenance (Figure 2). In such
cases, conversion to fresh oil does not give a true value of the water
amount. However, an oil regeneration process may fully restore the
original water solubility level of the oil.
Figure 2: Solubility of fresh and used engine lubrication oil.
Article presented by...
Senja Paasimaa - Application Manager
Vaisala Helsinki, Finland
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