Senja Paasimaa Application Manager Vaisala Helsinki, Finland Factors Affecting Water Solubility in Oils
solution is a thermody- also increase solvating efficiency. Water can occur in three phases namically stable state, Mineral-based transformer oils within an oil system, depending on A where solvating forc- typically have very little additives es homogeneously mix all the and therefore have low solubility the chemistry of the oil in question. molecules present in the solu- like base oil, whereas lubrication This goes for both mineral and tion. The type and amount of ad- oils with greater amounts of ad- synthetic oil. In general, oils dissolve ditives mostly determine the wa- ditives generally have much high- ter solubility of new oils, where- er solubility (Figure 1). some water. However, each oil has its as oxidation products have a re- The overall absorption forc- specific water-saturation point beyond markable effect on the solubility es and water content of the solu- of aged oils. tion in the equilibrium state are which excess water becomes either determined by Gibb’s energy of emulsified or free. Therefore in various Oil composition mixing. On the molecular level, Pure base oils have very limit- the absorption forces are binding oil-systems, one may have to deal with ed solubility, which is related to forces between the water mole- dissolved, emulsified, and/or free- the ratios of paraffin, naphthenic, cules and the molecules in the oil and aromatic compounds. The matrix. Water molecules are po- water. saturation point at 20 °C varies lar by nature, so the interaction from approximately 30 parts per forces increase with the increas- million (ppm) of paraffin oils to ing polarity of the matrix mole- over 200 ppm of fully aromat- cules, such as additives and oxi- ic liquids, but it is typically be- dation products. tween 40 to 80 ppm. Solubility may increase significantly with Temperature the use of additives. The typical The dependence of solubility on value for new lubrication oil is temperature is almost always ex- <500 ppm. Oxidation products ponential (Figure 1). Hot oil dis-
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new 200 lubrication oil 1000 base oil used 0 0 0 20 40 60 80 100 120 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 T (°C) aw
Figure 1: Average water solubility of mineral base oil and one lube oil Figure 2: Solubility of fresh and used engine lubrication oil. as function of temperature.
solves greater amounts of water. sion and cavitation type damages ter content, thus not giving any uid hydrocarbons. The sensor’s The hotter the oil, the greater the are mainly consequences of the indication whether water is dis- excellent chemical tolerance pro- water absorption from air to oil free water phase. solved or free. However, due to vides accurate and reliable mea- in the same humidity conditions. Under a high mixing ratio or differences in oil types and diffi- surement over a wide measure- This should be noted in any sys- presence of surfactant additives culty in predicting aging effects, ment range. tem setup, as airborne moisture (i.e. wetting agents that lower the ppm values are often not suffi- contamination is one of the most surface tension of a liquid, allow- cient. Therefore, relative values ppm conversion common water sources. ing easier spreading, and the in- like water activity (aw) are useful If the solubility of a specific oil terfacial tension between two liq- parameters for setting alarms in is known through the whole op- Aging of oil uids), water may form an emul- control systems. erating temperature range, the Lubricating oil circulating in sion with oil. An emulsion is a Capacitive thin film sensors measured relative moisture val- high-speed systems deteriorates mixture of two unblendable sub- give this value without temper- ue can be converted to absolute with time due to oxidation. Oil stances, where one substance is ature corrections or oil-type cal- water content (ppmw). However, characteristics - presence of ox- dispersed in the other. Because ibrations. The active film of the we must note that the conversion ygen, catalysts present, and the of surfactants, micro-size wa- sensor absorbs water molecules, is valid only if the water solubil- temperature levels to which the ter droplets are homogeneously which change the dielectric con- ity of the oil does not change. In oil is exposed - determine the mixed in oil, forming an emul- stant (i.e. a measure of the abili- lubrication systems the solubili- rate of the aging process. In lu- sion. The surfactants are chem- ty of a material to resist the for- ty changes with time regardless brication oil systems, air is always icals that have both hydrophilic mation of an electric field within of the system maintenance (Fig- present, and the metal debris (a molecule that can bond with it) of the film. The absorption is ure 2). In such cases, conversion from machine construction and water) and hydrophobic (a mole- proportional to the equilibrium to fresh oil does not give a true the moisture present are catalysts cule that is repelled by water) na- relative humidity of oil, thus indi- value of the water amount. How- for the aging, that is, oil deteri- tures and thus are soluble to both cating the margin to saturation. ever, an oil regeneration process oration process. Aging process- phase water and oil. They form The Vaisala HUMICAP® may fully restore the original wa- es are equilibrium reactions, and micelles (liquid particles) over thin film polymer sensor prod- ter solubility level of the oil. ● therefore the decay rate of oil is a the water droplets that convert ucts are beneficial in applications function of activity of water rath- the droplets “soluble” in oil. Sur- where the water amount must not er than absolute water content. factants may be added to oils to exceed solubility limit, i.e. free High temperatures and mechan- form emulsion, or some additives water has to be avoided. The sen- ical stresses, e.g. in the bearings, may act as emulsifiers, although sor is very sensitive even to negli- also accelerate the process. added for different purposes. gible amounts of water and other small polar molecules. Therefore, Free water / emulsion the active polymer film and the When water content in oil reach- Moisture measurement sensor structure have to be such es the saturation point of that oil, Traditionally, water in oil has that the additives and oxidation it separates out and free water is been measured by Karl Fish- products in oil do not disturb the formed, resulting in a two-phase er titration (a method for deter- measurement. The latest genera- system. Free water is common- mining the moisture content of tion of the Vaisala HUMICAP® ly considered as the number one a sample) and expressed in ppm, sensor is developed for demand- contaminant of oil. Water corro- which is the total absolute wa- ing moisture measurement in liq-
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