Water is one of the most destructive contaminants in engine oil after soot [1]–[3]. Water in engine oil causes a host of problems such as accelerating metal surface fatigue, increasing wear, producing corrosion, reducing film thickness, and inducing oil oxidation and additive depletion [2]–[6]. Water enters the engine oil in different ways and moves in the lubrication system in several physical and chemical states [7]. Water in lubricants is found in three different phases. Dissolved water when the water level in the oil is under saturation level. The molecules of dissolved water disperse in the engine oil in very small invisible sizes. When the amount of water exceeds the saturation level or the level that water can be dissolved, the oil loses its ability to dissolve more water resulting in an emulsified phase. As water content increases in the oil, the extra water over the emulsified water level in the oil separates, forming free water. Emulsified and free water are widely recognised as particularly detrimental phases, significantly impacting lubrication systems and contributing to subsequent failures [8].
Different types of lubricants can manage different levels of dissolved water depending on the physical and chemical properties of the oil and its additives [9]. Dispersant/detergent additives disperse water molecules in the oil and prevent the formation of free water droplets. These additives influence the character of mineral oil to hold the water at different phases in terms of solubility and emulsion [10]. Dissolved water in oils could also be influenced by factors such as relative humidity, pressure, additives, and temperature [10]–[12]. According to Henry's law, the relationship between water content in oil and the amount of water in the air is linearly proportional [13]. Henry illustrates the varied levels of dissolved water in oils that contain different additives [11]. It is commonly agreed that water concentration in automotive oils does not have to exceed 0.2 wt.%. The water concentration in the drain oil was found to be typically below 1 wt.% and variations are widely seen in several studies [6], [13].
The hydrolysis effect on lubricants has been considered the main factor contributing to engine oil's chemical breakdown in the presence of water [14]. The physical and chemical stability of engine oil is threatened by even a small amount of suspended water. Water can hydrolyse, transform, and wash additives out of the lubricant [14]. It is known that water has a density higher than the engine oil. Excessive water beyond the saturation level will result in separation, potentially causing the removal of additives from the oil due to the presence of free water [15]. Dispersants and detergents in oils are used as the first barrier to protect the oil from water contamination. Water, as a polar molecule, is similar to oil additives that have polar heads. The heads of polar additives latch onto water molecules and surround them, forming reverse micelles. The non-polar tails of additives allow the additives to dissolve easily into the oil. When the water content in the oil increases, more reverse micelles are generated until they reach a level where the water amount overwhelms these additives and free water will separate [10]. Considering this, it could be assumed that extracting water molecules through filtration or separation, particularly as free water encapsulated by detergent, dispersant, or other additives, can inadvertently lead to the removal of these additives from the oil, however this is not yet demonstrated experimentally.
Dissolved water in the oils diminishes the additives’ performance and promotes corrosion. Water destroys the Zinc dithiophosphate (ZDDP) additive, which is referred to as antioxidation and antiwear additive, when it reacts with water at a temperature of 60°C and above, resulting in increased wear [16], [17]. Rounds[18] suggested that ZDDP decomposition occurred not only by thermal decomposition but also due to the hydrolysis process. Fuller et al.[19] showed that long-chain phosphates, which result from the decomposing of adsorbed ZDDP on rubbing surfaces, hydrolyse these long chains in the presence of water, causing the formation of short-chain polyphosphates and phosphoric acid as described in Eq. (1) and Eq. (2) [20]–[22].
\({7Zn\left({PO}_{3}\right)}_{2}+6{H}_{2}O \to {{Zn}_{7}\left({P}_{5}{O}_{16}\right)}_{2} \left(short chain polyphosphate\right)+{4H}_{3}{PO}_{4}\) Eq. (1)
\({2Zn\left({PO}_{3}\right)}_{2}+3{H}_{2}O \to {Zn}_{2}{P}_{2}{O}_{7} \left(short chain polyphosphate\right)+{2H}_{3}{PO}_{4}\) Eq. (2)
Water influences the interfacial chemistry of ZDDP tribofilm formation, ZDDP adsorption and tribofilm adhesion to metal surfaces resulting in increased wear [23]. The formation of tribofilm can be influenced by the high polarity of water molecules. Water molecules attach to the metal surfaces and prevent additives from forming tribofilm [24]. The PAO lubricant containing 1 wt.% ZDDP was investigated in the existence of 1 wt.% water. The results demonstrated an increase in wear [24]. Parsaeian et al. [25] studied the effect of humidity conditions on wear depth with different relative humidity levels. The results revealed an increase in wear depth with a higher level of humidity. Parsaeian et al. [26] also investigated different levels of water 0.5, 1.5 and 3 wt.% in the oil. The results reported an increase in wear depth and a decrease in tribofilm thickness with higher water concentration. Cen et al. [27] reported that adding water or adsorbing more water from humid air increases wear. The results agreed with the theory of the hydrolysis effect of ZDDP tribofilm that water can decay the stability of tribofilm by reducing tribofilm adhesion on metal surfaces [28]. Dorgham et al. [29] demonstrated that water in the oil causes a delay in the formation of tribofilm ending in the formation of short-chain phosphates.
While the free water separation occurs when it surpasses the saturation level, it remains uncertain whether the oil can perform similarly to fresh oil after the separation of free water or not. This is due to additional factors, such as additive depletion caused by water or the presence of remaining dissolved water, which can also impact the functionality of the oil. The study aims to understand how separating water in different phases from a fully formulated oil affects its chemical composition and tribological performance.