Acid Neutralization by Marine Cylinder Lubricants Inside a Heating

Ind. Eng. Chem. Res. 2006, 45, 5619-5627

5619

Acid Neutralization by Marine Cylinder Lubricants Inside a Heating Capillary: Strong/Weak-Stick Collision Mechanisms Jianzhong Fu,† Yunfeng Lu,† Curt B. Campbell,‡ and Kyriakos D. Papadopoulos*,† Department of Chemical and Biomolecular Engineering, Tulane UniVersity, New Orleans, Louisiana 70118, and CheVron Oronite Company, LLC, 100 CheVron Way, Richmond, California 94802

Base nanoparticles are present in marine cylinder lubricants (MCLs) for controlling cylinder corrosive wear by neutralizing combustion-generated acids. A portion of the surfactants stabilizing these nanoparticles may have dynamic properties, upon which strong/weak-stick collisions are proposed to drive the interaction between base particles and acid drops in oil media. Neutralization of sulfuric acid droplets (150-200 µm in diameter) by MCL samples (with a base number of BN ) 70 mg KOH/g oil) was performed inside a heating-capillary microreactor at 23-27, 110-140, and 140-170 °C. The progress of reactions was recorded in real time through a video-microscopy system and quantified by image analysis. Among four directly observed reaction phenomena, three had never been seen in previous research. These reaction behaviors support the theory of strong/weak-stick collisions and supplement our knowledge to help us better understand the acid neutralization in oil and to propound new strategies for improving the acid-neutralizing performance of overbased detergents. 1. Introduction One of the most important functions of marine cylinder lubricants (MCLs), which are special cylinder oils that are characterized by high alkalinity, is to neutralize acid components formed during fuel combustion and oil degradation, which cause corrosive wear in engine parts. The most effective and commercially practiced means of providing acid-neutralizing function is formulating MCLs with overbased detergent additives.1-4 The key in improving the acid-neutralizing performance of MCLs may lie in (i) the preparation of such overbased detergents and (ii) the facilitation of acid/base transfer in oil media. Research, to understand the synthesis of overbased detergents and the mechanisms of acid neutralization by these overbased particles in hydrocarbon media, has been conducted since the middle of the twentieth century, because of the enormously increased use of sulfur-containing heavy fuels, which are chiefly utilized to curb energy costs. In early research, the base number (BN), which is also called the total base number (TBN) and is defined as the number of milligrams of KOH equivalent to the base content in one gram of an oil sample,5 was one of the most important parameters to monitor. It was found that the basicity of MCLs was the major control for the cast-iron piston ring wear in a modified Petter AV-I test engine.2 Also, a minimum level of BN was determined to be necessary to decrease engine wear and such a BN level was related to the fuel sulfur content.6 However, a BN value alone only provides for an approximation of the acid-neutralizing potential of a MCL. The BN alone cannot indicate the ability or efficiency of a MCL to neutralize acid components, because different MCLs formulated with the same BN may have different behaviors of acid neutralization. The selection or preparation of appropriate overbased detergents has a critical impact on the MCL’s ability to neutralize acids. Because field tests in a full-scale marine engine are slow, expensive, and subject to undesirable variations, which is * To whom correspondence should be addressed. Tel.: +1-504-8655826. Fax: +1-504-865-6744. E-mail address: [email protected]. † Department of Chemical and Biomolecular Engineering, Tulane University. ‡ Chevron Oronite Company, LLC.

especially infeasible for comparing a series of MCL candidates, several laboratory testing techniques have been developed to understand the acid neutralization in MCLs and to evaluate the acid-neutralizing ability of MCL samples. Two commonly used methods monitor the changes of (i) pH7-9 and (ii) the pressure of carbon dioxide10-12 during the process of neutralization. Changes in pH value and carbon dioxide pressure result from the reaction between acid samples and preformed base particles in MCLs. The typical reactions may be expressed as

H2SO4 (l) + Ca(OH)2 (s) f CaSO4 (s) + 2H2O (l) (1) H2SO4 (l) + CaCO3 (s) f CaSO4 (s) + CO2 (g) + H2O (l) (2) These reactions will result in crystal calcium sulfate hydrates and gaseous carbon dioxide. In 1984, Hosonuma and Tamura proposed the first model of acid neutralization in oil media by investigating the neutralization between sulfuric acid emulsions and overbased detergents in a diesel engine oil through the evolution of carbon dioxide.10 This model envisions (i) the stabilizing surfactant molecules attached to calcium carbonate particles or sulfuric acid droplets dynamically exchange with those monomers (calcium sulfonate) in the bulk and (ii) the acid neutralization is dependent on the adsorption of the base particles onto the acid/oil interface and neutralization products (calcium sulfate) are formed inside the acid droplets. Higher reactivity was observed with decreasing acid emulsion size, larger base particle size, and the presence of ashless dispersants. Using a rotating diffusion cell technique, Lewis proposed another neutralization model,13 a “droplet formation and transfer mechanism”, which suggests that (i) a surfactant layer forms at the acid/oil interface when the acid phase is introduced, (ii) spontaneous curvature of the interface occurs, thus forming acid-containing reverse micelles, and (iii) a transient “dimer”, where the neutralization reaction can occur, forms upon the collision between the solubilized acid micelles and the dispersed base particles. This mechanism shows the neutralization reaction occurring in the bulk oil phase and not at the acid/oil interface as Hosonuma and Tamura proposed.10

10.1021/ie051209u CCC: $33.50 © 2006 American Chemical Society Published on Web 05/24/2006

5620

Ind. Eng. Chem. Res., Vol. 45, No. 16, 2006

In 1998, Roman first developed a laboratory technique that enables acid neutralization to react in a confined lubricant film and to avoid external stirring,12 which is a vital factor that impacts experimental results, because of various viscosities of oils and the acid/oil systems. Roman’s experiments showed that the neutralization reaction occurred in three stages by monitoring the evolution of formed carbon dioxide, and a detailed mechanism was illustrated to account for such pressure behaviors: (i) the solubilization of sulfuric acid into lubricant film and the formation of acid droplets cause the initial pressure drop; (ii) frequent collisions between acid droplets and overbased particles result in the fast neutralization, thus rapidly increasing the pressure, and the formed calcium sulfate particles have a tendency to precipitate or be solubilized by dispersants in the bulk; and (iii) when the relative concentrations of acid and base fall below a critical value, the neutralization in the oil film is stopped, even if acid droplets are still present; at this stage, reaction occurs only at the wall, corresponding to a slowly increasing pressure plateau. Using our capillary video-microscopy imaging technique, our laboratory proposed an “interfacial neutralization mechanism”,14,15 envisioning that the overbased detergent particles approach and collide with the acid/oil interface by Brownian motion. A successful stick collision results in the “adsorption” of an overbased detergent particle onto the interface, then a channel forms, which allows the water and acid to be transferred into the core of the collided reverse micelle where the reaction occurs. After that, the reacted base particle solubilizes the reaction products and desorbs back into the bulk oil. At almost the same time as our studies, using a stop-flow technique, Hone and co-workers suggested that no water (vide acid) was transferred to the base particles,9,16 which was apparently at odds with the research of Wu et al., in which acid was transferred to base particles.14,15 Hone et al.’s experiments were designed to neutralize microemulsion acid droplets with a diameter of ∼2 nm, using overbased detergent additives. Therefore, a “base transfer channel” was proposed, which suggests that the mechanism of acid neutralization in hydrocarbon media should involve base transfer from the base particles into the core of acid droplets, where acid neutralization occurred. Hone and coworkers assumed that engine oil would disperse the aqueous acid present in an engine in a similar way to their model microemulsion droplets and that the dynamic exchange of surfactant molecules probably existed between the acid droplets and the overbased detergent particles. In this paper, we are presenting strong-stick and weak-stick collision mechanisms of acid neutralization in oil media, based on a hypothesis that a portion of dynamically exchangeable molecules exists among the stabilizing surfactant layer of an overbased carbonate particle. 2. The Dynamic Hypothesis of “Rigid-Structured” Carbonate Reverse Micelles Base particles in MCLs are nanometer-sized colloids that are preformed as overbased reverse micelles dispersed in proper hydrocarbon media.17,18 These overbased particles consist of a base core that is stabilized by a monolayer of surfactant molecules. The base core is a mixture of metal carbonate/ hydroxide, and the stabilizing layer is the metal salt of a surfactant acid, typically alkylbenzene sulfonates. The interactions between stabilizing surfactant molecules and carbonate base cores of overbased reverse micelles were reported to be stronger than electrostatic attraction,19 to be chemically bound,20 and to have strong Coulombic forces.21 It has been suggested

that the stabilizing surfactant molecules of a carbonate reverse micelle are tightly bound to the particle surface and do not take part in any exchange with those surfactant molecules in the bulk;20 however, other studies have also reported that a dynamic equilibrium possibly exists.22 Overbased detergent particles can remain stable when properly extracted from their original media and diluted, thus making it possible to formulate various engine oils with different levels of basicity. If all the stabilizing surfactant molecules of an overbased carbonate reverse micelle were tightly bound to its carbonate core, it would be impossible for the micelle to grow in size and increase its base capacity to the required BN level during a synthesis process. Because water, polar solvents, and metal hydroxides were all involved or were essential in modern manufacturing processes of overbased detergents,3,18 some of them may remain in the base core of the overbased reverse micelle; surfactant molecules attached to these residues may be dynamically exchangeable. Figure 1 suggests schematically the dynamic exchange sites of a “rigid-structured” carbonate reverse micelle. The inner core of the reverse micelle (the gray regions with spherical particles in the figure) consists of a calcium carbonate base, and its outer core (the ring-shaped region in the figure) consists of surfactant polar heads, as well as carbonates, hydroxides, polar solvent residues, and water. A monolayer of surfactant molecules with their hydrophobic tails constitutes the shell of the reverse micelle. Surfactant molecules indicated with a cross (×) are tightly bound to the core of calcium carbonate, while those indicated with a circle (O) are attached to calcium hydroxide or those polar residues or water. The tightly bound molecules contribute to the “rigid structure” of overbased carbonate micelles, which enables these base particles to remain stable when extracted from their original media, whereas those molecules attached to calcium hydroxide or polar residues or water are dynamically exchangeable. A dynamic exchange ratio (σ) of dynamic exchangeable sites “O” (nO) to the total molecules of the surfactant shell (nO + n×), may be used to characterize the dynamic exchange extent of an overbased detergent. Some evidence of dynamic exchange in “rigid-structured” carbonate reverse micelles can also be found in the results of earlier research. It has been reported that the non-carbonate base is more accessible than the carbonate base during neutralization,10,23 and Papke concluded that the former (hydroxide) must be located within the outer regions of the polar core.23 This can be attributed to the fact that the hydroxide base may be dynamically exchangeable, whereas the carbonate base may not. The small amount of polar solvents added in the manufacturing processes of overbased additives has been determined to have an effect on loosening the rigidity of the overbased micelles19,24 and thus may be key to their dynamic properties. The growth of calcium carbonate colloids has also been determined to be dependent on the dynamic exchange of the necessary calcium hydroxide during the carbonation process in the synthesis of carbonate overbased detergents.18 3. Strong/Weak Stick Collision Mechanisms Logically, acid neutralization in MCLs must be dependent on how acid droplets and base particles encounter each other and on the relative transport of acid and base contents. Upon an effective or stick collision that results in the occurrence of neutralization, whether the collision area involves the exchangeable sites (indicated with a circle, “O”, in Figure 1) of the base particle determines the exposure area of the base core and the transport of acid and base contents. An effective or stick

Ind. Eng. Chem. Res., Vol. 45, No. 16, 2006 5621

Figure 1. Schematically showing the dynamically exchangeable sites of a “rigid-structured” calcium carbonate reverse micelle. Surfactant molecules attached to calcium hydroxide or polar residues or water are dynamically exchangeable (indicated with “O”), whereas surfactant molecules attached to calcium carbonate are tightly bound (indicated with “×”), contributing to its “rigid structure”. A dynamic exchange ratio σ, equal to nO/(nO + n×), may characterize the dynamic exchange extent of an overbased detergent.

collision may be further defined as (i) a strong-stick collision, when dynamically exchangeable surfactant molecules (at the exchangeable sites) desorb from the approaching base particle in the collision area, and as (ii) a weak-stick collision, when such exchangeable sites are not involved in the collision. As shown in Figure 2a, when a strong-stick collision occurs, the dynamically exchangeable molecules desorb from the reverse micelle and the base core becomes directly exposed to acid attack; rapid neutralization destroys the “rigid structure” at the collision interface, and the entire base stock becomes exposed to an acidic environment, thus a relatively fast reaction will be expected and the reaction products may have a chance to nucleate and grow. When a weak-stick collision occurs, as shown in Figure 2b, because no exchangeable sites are involved in the collision, the base core is still covered by the surfactant molecules and the “rigid structure” is maintained. Considering that molecular dynamic simulations have previously demonstrated that ∼25% of the base core is exposure to the bulk oil,25 and that the reaction is dependent on the transfer of acid content into the base core through the interval of surfactant molecules, a relatively slow reaction will be expected. The reaction may continue in the base core after the collided reverse micelle desorbs back into the bulk oil, and the formed calcium sulfate hydrates may be limited by the still-maintaining-a-rigid-structure reverse micelle and are unlikely to nucleate and grow by combining with those formed in other reacted base micelles. Because, based on the proposed hypothesis, strong-stick collisions, which are dependent on the dynamically exchangeable sites, result in fast reaction and may grow crystals, the amount of dynamic exchangeable molecules present among the stabilizing shell of a base particle (and, therefore, the dynamic exchange ratio σ) can be a decisive factor in influencing acid neutralization in oil media. The higher the dynamic exchange ratio, the greater the number of the strong-stick collisions. Assuming that the frequency of strong-stick collisions is high enough to grow crystals when σ > σc (some large dynamic

exchange ratio), an overall or macroscopic reaction behavior of acid neutralization in lubricant oil may be characterized as (i) a crystal-growing reaction, in which the frequency of strongstick collisions is higher than a certain level, corresponding to the dynamic exchange ratio σc; or (ii) a crystal-solubilized reaction, in which the frequency of strong-stick collisions is low or negligible, because σ < σc, i.e., when calcium sulfate hydrates released from strong-stick collisions at the interface are not enough to nucleate and grow. Almost all the produced crystals are solubilized in individual reacted base micelles upon weak-stick collisions. It is important to remember that, because most of the stabilizing surfactant molecules are tightly bound to the base core of carbonate reverse micelles, weak-stick collisions should dominate in either crystal-growing reactions or crystal-solubilized reactions. 4. Materials and Methods 4.1. Materials. The sulfuric acid stock solution was prepared as 50% (in volume), by directly diluting sulfuric acid reagent (Chempure, No. 832-547) into highly purified water that was produced from the Barnstead E-pure system, followed by distillation in a Fistreem III glass still. The MCLs and overbased detergents were provided by Chevron/Oronite, and the model oils were prepared in our laboratory by properly diluting Chevron/Oronite overbased detergents with dodecane (Aldrich, anhydrous, 99+%). All oil samples had a base number of ∼70 mg KOH/g oil. Melting tubes for preparing capillaries and micropipets were Pyrex (9530-3) and Microcaps (1-000-0300), respectively. The pulling procedures have been shown elsewhere.26 Treatment of the heating capillaries and micropipets was accomplished with 1% octadecyl functional silane (Siliclad, Gelest 3J-3793). 4.2. Acid Neutralization Experiment Design. 4.2.1. Analysis of Key Conditions To Simulate Acid Neutralization of Marine Cylinder Lubricants in an Operating Engine. The

5622


Figure 2. Schematics of an effective/stick collision between an acid droplet and a carbonate reverse micelle. (a) Strong-stick collision (involving dynamic exchange sites “O”), where the base stock directly exposed to acid attack, and the “rigid structure” of the carbonate reverse micelle is destroyed at the interface, thus resulting in a fast reaction, which may have a chance to nucleate and grow crystals. (b) Weak-stick collision (involving no dynamically exchangeable molecules of the reverse micelle), where the “rigid structure” of the carbonate micelle is maintained before it desorbs back into the bulk oil; the reaction is dependent on the transfer of acid into the base core of the reverse micelle, thus resulting in a slow reaction and possible inability of nucleating and growing crystals.

marine cylinder lubricant is fed through cylinder liners and only used once during each piston stroke in a crosshead diesel engine.27 Grooves in the liner spread the MCL circumferentially around the liner, and the motion of the piston further assists in spreading the oil. Typically, marine crosshead engines use heavy diesel or bunker fuels that contain high levels of sulfur compounds, which may result in the formation of highly corrosive sulfuric acid, as schematically shown in Figure 3. During fuel combustion, SO2 is initially formed, which is normally oxidized rapidly to SO3, because of catalytic substances that are present in the fuels or on the liner. Temperature is the main factor that determines how much SO3 will be present, although pressure also can affect the formation of SO3. In fact, there is almost no SO3 present during the combustion phase, and it is formed primarily at relatively lower temperatures and pressures that exist during the second half of the piston stroke (before and during the exhaust phase).28 While the combustion gas temperatures are typically in the range of 1600-2500 °C, the temperature ranges for various engine parts have been estimated or measured as follows: the walls of the cylinder liner, 200-400 °C (upper part) and 110-140 °C (lower part); piston crowns and top piston rings, 200-300 °C; and piston

skirt, 140-170 °C.29-31 Upon contact with the relatively cold and injected MCL and the surface of the liner, sulfuric acid vapors may form and condense, together with water vapors, thus producing acidic solutions of various acid concentrations along the wall surface, according to the local conditions, such as the composition of gases and the temperatures. Because the dew point of sulfuric acid is estimated at ∼170 °C and the maximum formation of sulfuric acid occurs at a temperature ∼30 °C below the dew point,30 the condensed sulfuric acid will most probably occur at the lower part of the cylinder liner. The condensed acidic solutions may be either solubilized into MCL films by the excess surfactant molecules and dispersants presented in the MCL or entrained into MCL films or MCL collections in the form of droplets by the motion of the piston. Some researchers have assumed that these acid droplets would exist in MCLs at a magnitude of ∼2 nm.9 However, there are no means to ensure that solubilization is the only way to take acid into MCL films. Other acid formation pathways may exist; for example, carbon particles produced during fuel combustion may adsorb acid and transfer it to the cylinder walls in a crankcase diesel engine.32 It is reasonable to assume that much bigger sizes of acid droplets (in comparison to the ∼2 nm acid


Figure 3. Sulfuric acid may form and condense under favorable conditions in an operating engine. Acid droplets in marine cylinder lubricants (MCLs) may result from (1) the solubilization of condensed acid solutions and (2) the entrainment of the acid solutions by the piston. The enlarged region in panel a shows the different thicknesses of MCL that may be collected between the cylinder liner and a Keystone ring piston used in the Caterpillar 1Y73 engine, modified from McGeehan.6

size in Hone et al.’s experiments) may also exist, because the motion of the piston will assist in entraining these condensed acid solutions into MCL collections (refer to Figure 3). The solubilized and entrained acid droplets are then rapidly neutralized by the overbased additives formulated in MCLs that are confined in the lubricating films or MCL collections. Although the exact amount of sulfuric acid formed may be unknown during a piston cycle, it must be significantly less than the base content present in the injected MCL during each stroke of the piston.2,6 To cause corrosive wear, an acid droplet must reach the walls of the cylinder liner, piston, and/or rings before it is destroyed by base colloids in the MCL. Thus, the most likely reason for the occurrence of corrosive wear, under the protection of a MCL with enough basicity, is that the rate of acid neutralization is not fast enough to destroy those relatively large acid droplets that entrain in MCL collections between the walls of the cylinder and the piston. Acid droplets of nanometer magnitude are

generally neutralized very fast, because of their extremely tiny size, and polycrystalline calcium carbonate antiwear films that are known to form on surfaces of friction from the preformed carbonate colloids in MCLs33 may provide a final defense. Therefore, to obtain results that are more representative of the acid neutralization by MCLs in an operating crosshead engine, laboratory experiments to simulate acid neutralization in oil media should meet the following key criteria: (i) high pressure simulating the engine environment MCL; (ii) samples should be put into a confined space, mimicking the spaces between engine parts; (iii) the sulfuric acid should be produced in the form of droplets and at relatively large magnitudes (micrometers), because larger droplets should be a larger cause of corrosive wear; (iv) the total acid content added in the MCL samples must be much lower than the total base content of the MCL to simulate the cases where corrosive wear would be prevented practically; and (v) the reaction temperature should be adjustable from 110-140 °C and 140-170 °C, which are

5624


Figure 4. Photograph of a heating-capillary microreactor.

the most relevant temperatures at the cylinder liner, on which the sulfuric acid solutions may form and condense or collected by the piston rings. These criteria had never been entirely satisfied in any previous non-engine experimental setup. 4.2.2. Heating-Capillary Microreactor. The thin-wall capillary technique, which combines the use of a video-enhanced microscopy system, has been extensively used in our laboratory since the early 1990s. It has been successfully adopted in our previous work to investigate the acid neutralization of MCLs at room temperature.14 The need to perform acid-neutralizing experiments at high temperatures motivated us to develop effective means to heat the thin-wall capillary on the stage of an optical microscope. The successful methodsone that does

not interfere with the experiments nor does it block light access for microscopysconsists of making the glass capillary itself electrically conductive so that Joule heat can be generated inside it when a current passes. A photograph of an assembled heatingcapillary microreactor is shown in Figure 4. The preparation and characteristics of the heating capillary are reported in our previous work.34 The heating-capillary microreactor satisfies all the key criteria discussed previously, except for the high pressure. The ∼200-µm capillary provides MCL samples a confined space that is similar to that of MCL collections. In this setup, bulk effects such as agitation can be avoided. Acid droplets can be produced individually with a specially prepared micropipet, and their sizes can be accurately measured using ImagePro software. The sizes of acid droplets are typically in the range of 150-200 µm. The injected sulfuric acid is 50% in volume, thus a ∼200-µm sulfuric acid droplet has ∼4 × 10-5 mmol H2SO4, which will consume ∼4.5 µg KOH, whereas a ∼70-BN oil sample that filled in the heating capillary (the narrowest section) has a basicity of >1861 µg, in terms of KOH. The available base content is much higher than the acid amount injected. The heating capillary can boil n-hexadecane (boiling point, 287 °C) and has stunning heating rates of 78-198 °C/ s;34 thus, it can provide the designated temperature ranges (110140 °C and 140-170 °C) almost instantaneously. 4.2.3. Heating-Capillary Video Microscopy. The acid neutralization of MCLs was performed inside the heatingcapillary microreactor that was placed on the stage of an optical microscope. The experimental setup is schematically shown in Figure 5. The high-temperature video-microscopy system consists of (1) a heating capillary microreactor, (2) a voltage

Figure 5. Schematic diagrams of the acid neutralization experiments setup, featuring (a) a high-temperature video-microscopy system and (b) a droplet injection and temperature measurement method. The legend for panel is as follows: (1) heating-capillary microreactor, (2) voltage transformer, (3) injection system, (4) optical microscope, (5) high-performance CCD camera and processor, (6) video/audio recording system, and (7) PC with ImagePro. In the droplet injection and temperature measurement method depicted in panel b, the micropipet is removed after injection and the temperature is measured with a ∼203-µm thermocouple.


transformer (Staco Energy Products Co., output 0-120/140 V), (3) an injection system, (4) an optical microscope (Olympus model IMT-2), (5) a high-performance charge-coupled device (CCD) camera and processor (Optronics DEI-470), (6) video/ audio recording system, and (7) a computer (Intel Pentium 4, WindowsXP) with ImagePro software. The injection system includes two three-dimensional hydraulic micromanipulators (Narishige MO-303) and their controllers (Narishige IM-200), as well as a nitrogen cylinder. The droplets are injected through a prepared micropipet that is connected to the micromanipulators, and its ∼40-µm tip is inserted in the heating capillary, as shown in Figure 5b. Both the injection time and nitrogen pressure are adjustable and are controlled with precision by the controller. The video/audio recording system includes a monitor (Sony model Trinitron PVM-1943MD), a microphone (RadioShack model 33-3004) and two VCRs (Sony model SVO9500MD), which are connected to the camera and the computer. Analog video can be recorded and transferred to digital video, and the snapshots of image frames are captured with ImagePro image analysis system. The spatial and temporal resolutions of this system are ∼1 µm and ∼200 ms, respectively. The required temperatures were generated by applying proper voltages and were measured either by plotting temperaturevoltage working curves for the heating capillaries or through a special thermocouple (Paul Beckman, model 300-B-200-04-K, ∼203 µm in diameter). When using a temperature-voltage curve, only an approximate range of temperatures can be known for a given voltage, but it can be perfectly controlled to fall into the selected temperature ranges of 110-140 °C and 140170 °C;34 when using the Paul Beckman thermocouple for measuring the exact temperature on site, it is limited to capillaries with an inside diameter bigger than 203 µm and must be inserted from the end of the capillary opposite to the micropipet (refer to Figure 5b). 5. Results and Discussion In most of our acid neutralization experiments, reaction products (crystals and gas bubbles) were observed either inside the acid droplets or on the oil side of the acid/oil interfaces of the acid droplets, whereas in some of the experiments, no crystals were visible. All the observed reaction behaviors can be summarized as follows: (i) Phenomenon I, in which the neutralization products, crystals, and gas bubbles were observed on the oil side of the acid/oil interface of the acid droplets, found in whole temperature ranges (25-27 °C, 110-140 °C, and 140-170 °C); (ii) Phenomenon II, in which the crystals and gas bubbles were observed inside the acid droplets, found in high-temperature ranges (110-140 °C and 140-170 °C); (iii) Phenomenon III, in which no crystals but gas bubbles were observed on the oil side of the acid/oil interface, found in 2327 °C temperature range only; and (iv) Phenomenon IV, in which no product but the shrinking of acid droplets was observed, found in all temperature ranges. Please notice that the acid neutralization experiments were repeated until the observed phenomena could be seen at least three times, except for Phenomenon III, which was only observed twice. The reason that we list it here is that our purpose is to learn how acid neutralization behaves in oil media, thus any observed reaction behavior, no matter how many times it appears, must be understood and explainable. Although Phenomenon I at room temperature was reported in our previous work,14,15 the other three phenomena had never been directly observed in any of the early research. The representative photography of Phenomena II, III, and IV are shown in Figures 6, 7, and 8, respectively,

Figure 6. Representative snapshots of Phenomenon II: crystals and gas bubbles were observed inside the acid droplets, reacted in a model oil at (a) 110-140 °C and (b) 140-170 °C.

Figure 7. Representative snapshots of Phenomenon III: gas bubbles were observed but no crystals were visible, reacted in a different model oil at 23-27 °C.

which are series of snapshots taken from videotapes that recorded the progress of neutralization through our heatingcapillary video microscopy system. All the four observed neutralization phenomena can be further generalized as two types of reaction: (i) Type A, where crystal products can be microscopically observed (Phenomena I and II; refer to Figure 6) and (ii) Type B, where no crystals can be visible (Phenomena III and IV; refer to Figures 7 and 8). These two types of reactions may be corresponding to the proposed crystal-growing reaction and crystal-solubilized reaction, respectively. In addition, by roughly comparing the neutralization times in Figures 6-8, we found that the rates of Type A reactions were much faster than those of Type B reactions. For instance, in the temperature range of 110-140 °C, ∼23 s was needed for a droplet with a diameter of 177 µm (∼18.9 × 10-4 µL) to break down in Figure 6a (Type A, crystal-growing reaction), whereas in Figure 8b (Type B, crystal-solubilized reaction), the disappearance time for a droplet with 142 µm (∼9.6 × 10-4 µL) was 24 s. In fact, typically, a couple of hours were needed to react at room temperature when no crystal could be observed. These results support our theory of strong/weak stick collision mechanisms, that crystal-growing reactions (i.e., Type A) have relatively fast rates of neutralization and grow crystal products, which may form a protective anti-wear film on engine metal surfaces.14,33 Therefore, engine corrosive wear may mostly occur under conditions that facilitate crystalsolubilized reactions. An effective collision is an essential step for the occurrence of neutralization between acid droplets and base particles in hydrocarbon media. Hosonuma and Tamura10 suggested and Wu et al.14 directly visualized that the acid neutralizations occurred at the acid/oil interface of the acid droplets (at relatively large

5626


Figure 8. Representative snapshots of Phenomenon IV: no neutralization products were observed, but the shrinking of sulfuric acid droplets was observed, reacted in a MCL sample at (a) 23-27 °C and (b) 110-140 °C.

scale), whereas Lewis,13 Roman,12 and Hone et al.9 proposed that the reactions happen in the bulk at the solubilized acid droplets (at relatively small scale). Hosonuma and Tamura10 suggested that the reaction products should be formed inside the acid droplets, and Hone et al.’s “base transfer channel”9 agreed with this, but Papke23 found, and Roman12 proposed, that the reaction products were solubilized in the bulk, whereas Wu et al.14 found that the products were first formed at the acid/oil interface on the oil side and then solubilized into the bulk. The neutralization model of Hosonuma and Tamura10 and the “base transfer channel” of Hone et al.9 may explain Phenomenon II, in which crystals and gas bubbles were observed inside the acid droplets. Wu et al. demonstrated that acid is transferred to overbased detergent particles,14 whereas Hone et al. concluded that no water (vide acid) is transferred to base particles.9 In fact, Wu et al.14 emphasized Hosonuma and Tamura’s model10 on the adsorption of base particles to the acid/ oil interface of acid droplets, but highlighted acid-to-base transfer, whereas Hone et al.9 extended Hosonuma and Tamura’s model10 from emulsion acid droplets to microemulsion acid droplets. Lewis’s model13 and Roman’s mechanism12 may explain, in part, Phenomenon IV, in which we observed no reaction products but the shrinking of acid droplets could be clearly seen using the microscope, because the acid might be solubilized as acid-containing micelles and react with base particles in the bulk. However, the fact that no products were found at the interface or inside acid droplets in Phenomenon IV, such as as those observed in Phenomena I, II, and III, may be due to other reasons, because the frequency of reaction that is occurring at the acid/oil interface should be much higher than that in the bulk, because, in our experiments, the size of acid droplets was typically in the range of 150-200 µm, much bigger than the solubilized acid micelles in the nanometer scale. Roman12 proposed that the neutralization would experience a stage at which the reaction in the lubricating film was stopped and occurred at the wall only, but Roman’s method used excess concentrated sulfuric acid, which should not occur in an operating engine.

In our experiments, because the microscope resolution is ∼1 µm, an ∼2-µm CaSO4‚2H2O crystal, which may weigh ∼0.01 µg, can be clearly observed in our system. For reactions with ∼150-µm sulfuric acid droplets, which can result in ∼0.50 µg CaSO4‚2H2O, how could the crystal products grow bigger than the microscope resolution and be observed in Type A reactions but not in Type B reactions? None of the early reaction models except the presented strong/weak stick mechanisms may explain this: as long as the frequency of strong-stick collisions is high enough, the amount of neutralization products are sufficient to nucleate and grow continuously to (or larger than) a microscopically visible level (e.g. ∼2 µm, 0.01 µg). In Type A reactions (crystal-growing reactions where the frequency of strong-stick collision is significant), the difference between Phenomena I and II may have resulted from the different temperatures and various preformed overbased detergents and MCL formulations, which cause different exposure areas when a strong-stick collision occurs. At higher temperatures and with bigger exposure areas, the base stock and neutralization products are more easily brought into the acid droplet, because of the movement of fluid inside the droplet. This can be partly explained by the experiments at temperatures of 110-140 °C and 140-170 °C, where the movement was observed to be very fast in our recorded videos. In Type B reactions (crystal-solubilized reactions where the frequency of strong-stick collisions is negligible), the different behaviors in Phenomena III and IV may be the result of different “residence times”, corresponding to different overbased particles and MCL formulations, as well as reaction temperature. Here, the “residence time” may be defined as a time period that the collided base particle remains on the acid droplet upon a weak-stick collision. A relatively long residence time may result in Phenomenon III, because the yielded gas bubbles may have a chance to coalesce with those produced at neighboring reaction sites (because they have better mobility than crystals), thus gas bubbles can be observed but no crystals are visible (refer to Figure 7). Conversely, a relatively short “residence time” may result in Phenomenon IV, because the yielded carbon dioxide gas may not have sufficient time to grow at the interface, thus only the shrinking of the acid droplets can be observed (refer to Figure 8). The neutralization might continue in the base core of desorbed carbonate micelles in the bulk. Because of thermal energy, the residence time at high temperatures (110-140 °C and 140-170 °C) may be significantly shorter than that at room temperature. This may explain why Phenomenon III was not observed at high temperatures. Although increasing the temperature may affect the reaction phenomena by enhancing the movement of fluid inside an acid droplet and/or by decreasing the residence time of collided base particles, both the crystal-growing reactions and crystal-solubilized reactions were observed in all temperature ranges (2527 °C, 110-140 °C, and 140-170 °C). Also, there is no evidence to show that acid droplet size would affect the types of neutralization, because an observed phenomenon did not change during neutralization; e.g., the shrinking of droplet shown in Figure 8 did not change into a crystal-growing phenomenon during the size-decreasing neutralization process. This also demonstrates that the transport of reacted base particles away from the acid droplet does not play an important role in controlling the types of reaction, so that the reaction conditions may be regarded as being at equilibrium. Therefore, as predicted by our theory of strong-/weak-stick collisions, the types of acid


neutralization in oil media should be dependent upon the dynamic exchange ratio, no/(no + n×), of the preformed overbased detergents, which determines the frequency fraction of strong-stick collisions. The dynamic property hypothesis and strong-stick and weakstick collision mechanisms may also explain the fact that no water (vide acid) is transferred to base particles in a microemulsion system reported by Hone et al.9 Those researchers believed that the dynamic exchange possibly exists between the microemulsion acid droplets and the base particles, but they do not explain why base content can transfer and acid content cannot. Because, in the microemulsion regime, the acid droplets are thermodynamically stable and their micellar structure is maintained after reaction,9,16 a weak-stick collision under such conditions may no longer be an effective collision and only a strong-stick collision can result in a reaction. Because of (i) the exchangeable sites due to overbased additives and (ii) the extreme curvature effect of the microemulsion acid droplets on base particles, most of the encountered collisions involving dynamically exchangeable molecules are possibly strong-stick collisions, thus resulting in macroscopic effects of base transfer and very fast reaction rates (from 10 ms to 20 s).9 6. Summary With the hypothesis that the surfactant molecules among the “stabilizing shell” of an overbased detergent micelle may be tightly bound20 and dynamically exchangeable,22 we propose strong-stick/weak-stick collision mechanisms that drive the interactions between acid droplets and base particles in oil media. The apparent inconsistency between the “dynamic exchange”22 and “rigid structure”20 of overbased carbonate reverse micelles, between the “acid-to-base” transfer14 and “base-to-acid” transfer,9 and among the formation locations of reaction products in acid droplets,10 in overbased reverse micelles,23 and at the acid/oil interface,14 can be reconciled, and the phenomena observed in this research, our previous work, and research results reported by others, may be conceptually explained. The amount of dynamically exchangeable surfactant molecules in a “rigid-structured” overbased reverse micelle is one of the most important factors in determining the ability of overbased detergent particles in marine cylinder lubricants (MCLs) to neutralize acids, because it determines the likelihood of strong-stick collisions, which result in a fast neutralization and grow crystals that may contribute to form anti-wear film. Thus, a new strategy is recommended to synthesize overbased detergent additives: incorporate, as much as possible, exchangeable surfactant molecules in the overbased reverse micelles. Future work should focus on developing theoretical and/or experimental methods to calculate and/or measure the dynamic exchange rate and exchangeable sites for a specific system, and then explore the controlling factors, such as polar component residues, extra calcium hydroxide, surfactant structure, hydrocarbon media, and the addition of dispersants, etc., whatever can contribute to the dynamic exchange properties of overbased additives while maintaining the stability and functionality of MCLs.

Acknowledgment This research was funded by LEQSF of the Louisiana Board of Regents under its ITRS program, Chevron Oronite Company LLC, and Tulane Institute for Macromolecular Engineering and Science (TIMES). Literature Cited (1) Dyson, A.; Richards, L. J.; Williams, K. R. Proc.-Inst. Mech. Eng. 1957, 171, 717-730; 731-740 (discussion). (2) Cook, B. A. J. Inst. Petrol. 1969, 55, 227-236. (3) Marsh, J. F. Chem. Ind. 1987, 470-473. (4) Galsworthy, J.; Hammond, S.; Hone, D. Curr. Opin. Colloid Interface Sci. 2000, 5, 274-279. (5) ASTM D2896-05, Standard Test Method for Base Number of Petroleum Products by Potentiometric Perchloric Acid Titration, ASTM International. (6) McGeehan, J. A.; Kulkarni, A. V. International Fuels and Lubricants Meeting and Exposition; Society of Automotive Engineers: Toronto, Ontario, Canada, 1987. (7) Lowe, W. Acid Neutralizing Accelerating Compositions. U.S. Patent No. 3,856,687, December 24, 1974. (8) Inoue, K.; Mito, T. Nisseki Rebyu 1988, 30, 197-201. (9) Hone, D. C.; Robinson, B. H.; Steytler, D. C.; Glyde, R. W.; Galsworthy, J. R. Langmuir 2000, 16, 340-346. (10) Hosonuma, K.; Tamura, K. Sekiyu Gakkaishi 1984, 27, 108-113. (11) Katafuchi, T. Eur. Patent No. EP 97-117521; 840120, 1998. (12) Roman, J.-P. Proceedings of the 22nd CIMAC International Congress on Combustion Engines; Congres International des Machines a Combustion (CIMAC): Copenhagen, Denmark, 1998. (13) Lewis, J. Ph.D. Dissertation, University of East Anglia, U.K., 1991. (14) Wu, R. C.; Papadopoulos, K. D.; Campbell, C. B. AIChE J. 1999, 45, 2011-2017. (15) Wu, R. C.; Papadopoulos, K. D.; Campbell, C. B. AIChE J. 2000, 46, 1471-1477. (16) Hone, D. C.; Robinson, B. H.; Galsworthy, J. R.; Glyde, R. W. Surfactant Sci. Ser. 2001, 100, 385-394. (17) Markovic, I.; Ottewill, R. H.; Cebula, D. J.; Field, I.; Marsh, J. F. Colloid Polym. Sci. 1984, 262, 648-656. (18) Roman, J. P.; Hoornaert, P.; Faure, D.; Biver, C.; Jacquet, F.; Martin, J. M. J. Colloid Interface Sci. 1991, 144, 324-339. (19) Jao, T. C.; Joyce, W. S. Langmuir 1990, 6, 944-948. (20) Mansot, J. L.; Hallouis, M.; Martin, J. M. Colloids Surf., A 1993, 71, 123-134. (21) Bearchell, C. A.; Edgar, J. A.; Heyes, D. M.; Taylor, S. E. J. Colloid Interface Sci. 1999, 210, 231-240. (22) Miller, J. F.; Clifton, B. J.; Benneyworth, P. R.; Vincent, B.; MacDonald, I. P.; Martin, J. M. Colloids Surf. 1992, 66, 197-202. (23) Papke, B. L. Tribol. Trans. 1988, 31, 420-426. (24) Jao, T. C.; Kreuz, K. L. J. Colloid Interface Sci. 1988, 126, 622628. (25) Tobias, D. J.; Klein, M. L. J. Phys. Chem. 1996, 100, 6637-6648. (26) Villa, C. H.; Lawson, L. B.; Li, Y.; Papadopoulos, K. D. Langmuir 2003, 19, 244. (27) Wilbur, C. T.; Wight, D. A. Pounder’s Marine Diesel Engines; Butterworth: London, U.K., 1984. (28) Vinogradov, T. L. Tr. TsNIIMF 1967, 81, 54. (29) Obert, E. F. Internal Combustion Engines, 3rd Edition; International Textbook Company: Scranton, PA, 1968. (30) Stott, F. H.; Macdonald, A. G. Wear 1988, 122, 343-361. (31) Lansdown, A. R. High-Temperature Lubrication; Mechanical Engineering Publications: Bury St. Edmunds, U.K., 1994. (32) McConnell, G.; Nathan, W. S. Wear 1962, 5, 43-54. (33) Mansot, J. L.; Hallouis, M.; Martin, J. M. Colloids Surf., A 1993, 75, 25-31. (34) Fu, J.; Lu, Y.; Campbell, C. B.; Papadopoulos, K. D. Ind. Eng. Chem. Res. 2005, 44, 1199.

ReceiVed for reView October 31, 2005 ReVised manuscript receiVed April 21, 2006 Accepted April 25, 2006 IE051209U

Acid Neutralization by Marine Cylinder Lubricants Inside a Heating

Recommend Documents