Mechanism of Acid Neutralization by Overbased Colloidal Additives in

reaction between overbased commercial detergent additives, used as crankcase lubricants, and aqueous strong acids (e.g., HCl and H2SO4) dispersed in t...
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Langmuir 2000, 16, 340-346

Mechanism of Acid Neutralization by Overbased Colloidal Additives in Hydrocarbon Media Duncan C. Hone, Brian H. Robinson,* and David C. Steytler School of Chemical Sciences, University of East Anglia, Norwich, Norfolk NR4 7TJ, U.K.

Roger W. Glyde and Jane R. Galsworthy Infineum UK Ltd., Milton Hill Business and Technology Centre, P.O. Box 1, Abingdon, Oxfordshire OX13 6BB, U.K. Received April 12, 1999. In Final Form: August 16, 1999 Stopped-flow kinetic studies have been employed to study the mechanism of acid neutralization for the reaction between overbased commercial detergent additives, used as crankcase lubricants, and aqueous strong acids (e.g., HCl and H2SO4) dispersed in the form of water-in-oil microemulsion droplets. Three water-soluble pH indicators, including methyl orange, 4-nitrophenyl-2-sulfonate, and nile blue have been used to monitor the change in pH accompanying the neutralization reaction. This process represents a model reaction involving communication between a nanometer-sized colloidal particle and a similar-sized aqueous droplet with the reactants solubilized inside the respective colloidal speciessdynamic processes in such systems have rarely been studied previously in a systematic way. The state of the system at the end of the neutralization reaction was monitored by the techniques of small angle neutron scattering (SANS) and ultracentrifugation. The rate was studied as a function of the initial concentrations of overbased additive and droplets and the initial acid concentration. The effect of temperature and the nature of the hydrocarbon oil were also systematically investigated, and in addition, the surfactant stabilizing the droplet was also changed. Through these studies, it was found that the model reaction provides fundamental information concerning the mechanism of acid neutralization by detergent additives. It is found, rather surprisingly, that the mechanism involves base transfer from the particle into the water cores of the microemulsion droplets, where neutralization of the solubilized acid occurs.

1. Introduction In lubricating oils, as employed in automobile engines, a range of acids can be produced as a result of oxidative degradation of the hydrocarbon base oil and hydrolysis reactions involving acid-precursor impurities in the fuel supply. One type of acid is reflected in the general formula for a carboxylic acid, RCOOH. In addition, gases which are formed in the combustion process react with water to produce acidic species such as H2SO4, HCl, and HNO3; this latter reaction, to produce strong acid species, is particularly important in the case of diesel formulations. For example, fuel supplied for marine diesel engines is normally rich in sulfur, leading to the facile production of sulfuric acid. Unless neutralized, these acids will readily attack metal surfaces leading to rapid corrosion and wear of engine components. Polymerization of organic species can also occur, resulting in resin formation and the buildup of deleterious deposits. Consequently there is a requirement to continuously neutralize these acids as they are formed during the operation of an engine. The problem is particularly acute at ambient temperatures, such as under engine start-up conditions. Since it is necessary to neutralize large amounts of acid, it is convenient to supply “basicity” to the oil by incorporation of an overbased additive into the engine oil formulation, which then provides a convenient reservoir supply of base. This usually takes the form of a colloidal particle containing a concentrated core of base, e.g., CaCO3, surrounded by a shell of surfactant, which solubilizes the inorganic core in a hydrocarbon media. This class of additive provides the means to disperse a large amount of basic colloid in the oil, and nowadays significant amounts of these * To whom correspondence should be addressed. E-mail: [email protected].

additives are to be found in nearly all modern oils formulated for crankcase applications (e.g., automobile, truck, and marine engines). In addition to their fundamental role in providing a protective lubricating film for engine components and dissipating the heat generated in an engine, lubricating oils have a variety of other tasks to perform. Preventing acid-induced metal corrosion and lubricant degradation, as described above, is just one of these tasks. Others include the dispersion of considerable quantities of soot generated in diesel engines and the control of friction to improve the fuel efficiency of the engine. Crankcase lubricants are therefore produced with a balanced formulation of a cocktail of colloidal chemical additives,1 in addition to the overbased detergents. It is essential that interactions between these additives do not interfere adversely with the performance of any of the individual additives (i.e., negative synergy). Correctly formulated, the additive package will provide long-term optimum engine performance over a wide range of operating conditions. The structure of a detergent additive is generally believed to fit the model shown in Figure 1.2 The particle consists of a core of basic metal carbonate or a mixture of metal carbonate and metal hydroxide, depending on the cation, and the process employed in their production. The “metal” cation is Ca2+, Mg2+ or Na+ or a mixture of these. The core is stabilized by a monolayer skin of surfactant. Surfactants used can include sulfonates, phenates, carboxylates, and salicylates; the surfactant counterion is again typically calcium, magnesium, or (1) Glyde, R. W. Chem. Br. 1997, 39-41. (2) Roman, J. P.; Hoornaert, P.; Faure, D.; Biver, C.; Jacquet, F.; Martin, J. M. J. Colloid Interface Sci. 1991, 144, 324-339.

10.1021/la9904354 CCC: $19.00 © 2000 American Chemical Society Published on Web 10/13/1999

Mechanism of Acid Neutralization

Figure 1. Schematic diagram of an overbased additive particle.

sodium. However, both the type of surfactant and the metal used depend on the specific application required. The core radius of the overbased particle is in the size range 15100 Å (1.5-10 nm). The overall dimension is then determined by the additional contribution of the alkyl chain length of the stabilizing surfactant (approximately 10-25 Å depending on the surfactant employed). Because the particles are nanosized, a large proportion of the core molecules will necessarily be located at the surface layer. The structure of the particle core is generally thought to be amorphous, and some water may be present in the core. Particle sizes are readily determined by means of small angle neutron scattering (SANS) or small-angle X-ray scattering (SAXS),3-5 dynamic light scattering (DLS), ultracentrifugation and viscometry studies. Various synthetic routes to these overbased products have been described by Marsh.6 A typical procedure is as follows: The acid form of the surfactant is neutralized by the addition of metal hydroxide to form the neutral metal salt of the surfactant. The system is then carbonated by injection of CO2(g) in the presence of excess metal hydroxide and a polar solvent to give particles with a metal carbonate core stabilized by the metal surfactant. The polar solvent is then removed to give the final additive in the form of a viscous concentrate containing approximately 30-40 wt % of colloidal particles. The base content of the overbased additive is defined by its total base number (TBN). The TBN is the equivalent milligrams of KOH per gram (of the prepared concentrate) and is usually determined by means of potentiometric perchloric acid titration.7 For example, a concentrated overbased additive formulation with a TBN of 300 contains basic material equivalent to 300 mg of KOH/g of concentrate. The aim of the fundamental study reported in this paper is to obtain kinetic and mechanistic information on a model system, which is relevant to the acid neutralization process occurring in a high-performance engine lubricant. In an engine, the neutralization reaction will occur over a wide range of operating conditions, e.g., different shear rates, pressures, and temperatures. The neutralization of different mineral acids has been investigated, particularly HCl and H2SO4 (although similar results are obtained with other strong acids), as a function of temperature and oil viscosity. The acid has been studied in a microemulsion medium because, in an engine, excess free surfactant and water are usually present, which may be expected to lead to microemulsification of the acid/water in the oil. Surfactants which have been used to stabilize the droplets in this model study are those which are known to form well(3) Markovic, I.; Ottewill, R. H. Colloid Polym. Sci. 1986, 264, 6576. (4) Markovic, I.; Ottewill, R. H.; Cebula, D. J.; Field, I.; Marsh, J. F. Colloid Polym. Sci. 1984, 262, 648-656. (5) Delfort, B.; Born, M.; Chive, A.; Barre, L. J. Colloid Interface Sci. 1997, 189, 151-157. (6) Marsh, J. F. Chem. Ind. 1987, 470-473. (7) American Society for Testing and Material (ASTM) D-2896, 1995.

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defined spherical droplets over a range of concentrations. The neutralization reaction can take place either following transfer of the acid to the surface of the particle core where neutralization occurs or by dissolution of the base into the acid droplets where the neutralization reactions would then take place. In the former case one can envisage reaction occurring either following an encounter between droplet and particle or by surfactant-facilitated transfer of acid (as RSO3H) from a droplet to the particle. For the surfactants used in this study the pKa of the surfactant is very low, and so this latter mechanism can essentially be eliminated from consideration. To simplify the base reactant, a model overbased system has been used, which was specially prepared by Infineum UK Limited. This has all the essential properties of a commercial detergent additive. In an engine, buildup of acid will not occur while sufficient base additive is present, since neutralization is a continuous process. However, to test the ability of the additive to neutralize acid, we typically used overall concentrations of acid in the presence of excess base such that complete neutralization of the acid was possible, with some residual base remaining at the end of the experiment. The neutralization reactions have been studied in a range of low alkane chain length (C7, C10, C12) hydrocarbon solvents which are model systems for the more complex mixtures of hydrocarbons found in a typical lubricating oil. The reactions have been studied over the temperature range 10-45 °C. This is fairly representative of the operating temperatures in the cooler parts of the engine away from the combustion zone, over a range of situations. The neutralization process under these conditions is found to be rapid, and so the stopped-flow method is appropriate to follow the rate of the neutralization process. With this technique, reactions are typically studied in the 10 ms to 20 s time range. A simple spectrophotometric assay, based on the color change of pH-responsive dye indicators, has been devised to monitor the efficiency of the neutralization process. 2. Experimental Section 2.1. Overbased Additive Particles. The overbased additive used in this study was supplied by Infineum UK Limited as a concentrate in a base oil (TBN ) 300 mg of KOH g-1) and was used without further purification. It contains an amorphous calcium carbonate/hydroxide core stabilized by a high molecular weight alkylbenzene sulfonate surfactant. All additive solutions were diluted in alkane hydrocarbon solvents, supplied by Fisher, before use in kinetic studies. All concentrations (g dm-3) refer to the initial solution prior to 50/50 mixing to start the reaction. 2.2. Water-in-Oil (w/o) Microemulsion Droplets. Waterin-oil (w/o) microemulsion droplets were prepared by dispersing an acidified aqueous solution (typically [H+]aq ) 0.1 mol dm-3) into a surfactant/hydrocarbon mixture. Microemulsions then form spontaneously on gentle shaking. If greater concentrations of acid are employed, the microemulsion may not be single phase. The overall hydrogen ion concentration [H+]ov is given by [H+]aqφ, where φ is the volume fraction of the dispersed droplet phase which is typically 0.01-0.03. In our experiments φ was generally 0.018. All concentrations refer again to the initial solution prior to mixing. For SANS measurements deuterium oxide (C/D/N isotopes Inc., 99.9% d-atom) was used in place of H2O. 2.3. Surfactants. The surfactant used in this work was either the sodium salt of sulfosuccinic acid bis[2-ethylhexyl] ester (Sigma), commonly known as Na-AOT (AOT) or didodecyldimethylammonium bromide (DDAB) (Sigma). The surfactant was generally used at an overall concentration of 0.1 mol dm-3 and w ) 10, where w ) [H2O]ov/[AOT]ov. Both systems form spherical droplets of well-defined water core radius (r), and it has been shown that the radius, measured in nanometers, is given for AOT, by r(nm) ) 0.175w.8 The water core radius of droplets

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TBN* ) 10-3y(g dm-3) TBN(mg/g)/MW KOH (g mol-1) (1)

a

MW KOH ) 56.11 g mol-1

b

e.g., for a TBN of 300 and a dilution factor leading to y ) 5.0 g dm-3

TBN* ([OH-]) ) 0.0267 mol dm-3

c

Figure 2. 2. (a) pH indicator methyl orange shown in unprotonated form with counterion. (b) pH indicator 4-nitrophenyl-2-sulfonate shown in unprotonated form with counterion. (c) pH indicator nile blue shown in unprotonated form with counterion. formed by DDAB is known to be of similar size.9 At lower w values with DDAB, nonspherical reverse micelles are formed.10 2.4. pH Indicators. HCl, H2SO4 (BDH), and AnalaR water (BDH) were used throughout, and pH values were measured with a Radiometer Copenhagen PHM82 standard pH meter. Stock solutions of the pH indicators, methyl orange (BDH) (Figure 2a), 4-nitrophenyl-2-sulfonate (PNPS) (Fluka) (Figure 2b), and nile blue (Hopkin and Williams) (Figure 2c) were prepared in AnalaR water. 2.5. Instrumentation. Spectrophotometric data were obtained at 25 °C on a HP8452A diode-array spectrophotometer. Acid neutralization data were acquired by rapid mixing of microemulsion and overbased solutions in a HiTech stoppedflow apparatus with a dead time of 95% of the dye was retained in the supernatant that contained the droplets.

Mechanism of Acid Neutralization

Figure 3. Schematic diagram of the acid neutralization reaction taking place between an overbased additive particle and an acidified w/o microemulsion droplet.

Figure 4. SANS data for D2O microemulsion droplets (a) before and (b) after reaction with overbased additive particles. Curve b has been displaced by +2 on the y-axis for clarity.

From the SANS data the scattering is consistent with spherical droplets and representative fits12 to a spherical form factor are shown in Figure 4 for the droplet contrast profile in heptane. There is no evidence for destabilization of the droplets in the presence of the particles. However, we cannot rule out the possibility of some exchange between the surfactant coating the droplet and the surfactant coating the particle. As previously found for w/o microemulsions stabilized by AOT, the droplets exhibit a small degree of polydispersity which could be accounted for using a Schultz distribution function13 with width parameter, σ/R h i ) 0.2. The radii obtained for the water core, R h c, before and after mixing with carbonate particles, are 20.3 ( 0.2 and 20.9 ( 0.3 Å, respectively showing that, within error, the droplet structure is maintained. From the amplitude of the scattering curves the droplet concentration also remains the same, so we may conclude that essentially no indicator or water, initially located in the core of the droplets, is transferred to the particles. However, this does not discount the possibility that surfactant molecules have been exchanged to some extent between the droplets and particles. We have carried out two other diagnostic experiments to establish the site of the neutralization reaction, which could either be at the particle core or in the water droplet. (12) Heenan, R. K. FISH Data Analysis Program; Rutherford Appleton Laboratory Report, RAL-89-129, 1989. (13) Kotlarchyk, M.; Chen, S.-H. J. Chem. Phys. 1983, 79, 2461.

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Using nile blue as the pH indicator, it is clear that the water pools of the microemulsion droplets (initially at pH ∼ 1) increase to a pH > 7 when mixed with the overbased particles. This is indicated by the change in color of the nile blue indicator (pKame ∼ 10) corresponding to the change from acid to base form (Figure 8). If an acid transfer process dominated then the pH in the water pools would not increase above 7. We have also carried out experiments involving premixing of particles and droplets, which contained only water, with no acid or indicator. The reaction scheme is shown in Figure 5. During this initial mix, there is the possibility that base could transfer from the particles into the droplets as described above. After 10 min, the dispersion was mixed with the usual acid/indicator droplet microemulsion. Reaction was found to be very much faster than one without the premix step. This is as expected on the basis of previous studies14 where we have shown that the reaction between acid- and base-containing droplets was very rapid, with a second-order rate constant for the neutralization reaction (Figure 6) k of ∼107 dm3 mol-1 s-1 (the concentration term refers here to the concentration of droplets). The ability of water-only-containing droplets to become basic and then react further with acid-containing droplets leads us to the conclusion that the preferred mechanism of neutralization involves base transfer or abstraction from the overbased additive into the water pools of the microemulsion where the acid neutralization reaction occurs. 3.3. pH Indicators. Solubilization of water-soluble pH indicators inside the water pools of w/o microemulsions allows changes in pH of the water to be readily monitored spectrophotometrically. The main indicators used were methyl orange (MO), 4-nitrophenyl-2-sulfonate (PNPS), and nile blue (NB). These indicators were chosen because they are water soluble and together cover a pH range in the microemulsion from 1 to 10. Figure 7 shows the UV-vis spectrum of MO dispersed in a w/o microemulsion stabilized by the surfactant AOT. In aqueous solution, at zero added ionic strength, MO has a pKa of 3.44 at 25 °C, but on solubilization in the microemulsion (pKame) this value is lowered by approximately 2 pKa units. The state of the water present in the microemulsion droplets studied is different from that of the bulk aqueous solution. This results in pKame being different to the pKa in water due to ionic strength and partitioning effects inside the microemulsion droplets. This has been observed previously for pH indicators in other microemulsion systems15-19 and so is accounted for in this study. The visible spectrum of the indicator is also perturbed to some extent on transfer from aqueous solution to the droplets. Data for λMAX values are shown in Table 1. The large change for the base form of MO implies that it is not located in the water core of the microemulsion. This would also help to explain the large change in pKa that is observed. When a diluted solution of overbased additive is mixed with a microemulsion solution, in which the water pool is acidified, and hence contains methyl (14) Fletcher, P. D. I.; Howe, A. M.; Robinson, B. H. J. Chem. Soc. Faraday Trans 1 1987, 83, 985-1006 (15) El Seoud, O. A.; Chinelatto, A. M. J. Colloid Interface Sci. 1983, 95, 163-171. (16) El Seoud, O. A.; Chinelatto, A. M.; Shimizu, M. R. J. Colloid Interface Sci. 1982, 88, 420-427. (17) Oldfield, C.; Robinson, B. H.; Freedman, R. B. J. Chem. Soc., Faraday Trans 1990, 86, 833-841. (18) Amire, O. A. J. Colloid Interface Sci. 1988, 126, 508-516. (19) Nome, F.; Chang, S. A.; Fendler J. H. J. Colloid Interface Sci. 1976, 56, 146-158.

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Figure 5. Schematic diagram of experimental procedure for premixing experiment. Table 1. Values of λMAX for Different pH Indicators in Aqueous Solution (H2O) and When Solubilized inside w/o Microemulsion Droplets (µem), [AOT]ov ) 0.1 mol dm-3, w ) 10 in n-Heptane λMAX/nm

Figure 6. Schematic diagram of droplet-droplet reaction.

Figure 7. UV-vis spectrum of methyl orange solubilized in an AOT microemulsion, [AOT]ov ) 0.10 mol dm-3, w ) 10, [methyl orange]aq ) 5.0 × 10-4 mol dm-3.

orange in the acid form, there is an observed color change (red to yellow) as the acid is neutralized and the indicator is converted to the base form. This process is taken, following the observations discussed above, to relate to base being released from the overbased particles into the acidified water pools of the microemulsion where neutralization of the acid occurs. The color change can be monitored by recording the change in absorbance of the

H2O

µem

pH indicator

acid

base

acid

base

MO PNPS NB

506 308 580

462 400 500

516 306 628

408 398 476

acid or base form of the indicator with time, following rapid mixing in the stopped-flow instrument (Figure 8). The simplest experiment involves mixing an equal volume of a solution of overbased additive with an acid/ pH indicator microemulsion solution. The absorbance is recorded at λMAX for either the acid or base form of the indicator, as appropriate. Typical traces are shown in Figure 8 for the same acid-base system but different indicators. For MO and NB the change in absorbance is recorded at λMAX for the acid form of the indicator, and so there is a decrease in absorbance with time. For PNPS the change in absorbance is recorded at λMAX for the base form of the indicator, and so there is an increase in absorbance with time. By use of indicators with different pKame, the neutralization process can be followed to different extents. (It should be noted that with nile blue, the droplet system is already basic when the color change occurs.) For a quantitative analysis of the data, the reaction is followed until the absorbance levels off and the indicator has been totally converted to the base form. The total change in absorbance, ∆A, is recorded, and the time taken to reach ∆A/2 is rather arbitrarily designated as t1/2 (s).

Mechanism of Acid Neutralization

Langmuir, Vol. 16, No. 2, 2000 345 Table 4. Values of t1/2 for Increasing Concentration of Droplets at Constant Overall Acid Concentration, [H+]ov ) 1.8 × 10-3 mol dm-3, w ) 10, 2.0 g dm-3 Additive, n-Heptane [AOT]ov/mol dm-3

[droplets]/mol dm-3

t1/2 (s)

0.10 0.20 0.40

1.331 × 10-3 2.662 × 10-3 5.324 × 10-3

4.60 2.28 1.08

Figure 8. Stopped-flow traces for the reaction between overbased additive particles and acid containing w/o microemulsion droplets using the three pH indicators under otherwise identical experimental conditions. Reaction at 25 °C, 2.0 g dm-3 particles and [AOT]ov ) 0.10 mol dm-3, w ) 10, [H+]aq ) 0.10 mol dm-3, in n-heptane. Table 2. Values of t1/2 (s) Obtained Using Different pH Indictors under Otherwise Identical Experimental Conditions (cf. Figure 8)a pH indicator

pKame

t1/2 (s)

MO PNPS NB

1.3 5.1 10.0

0.76 1.61 2.10

a Reaction at 25 °C, 2.0 g dm-3 particles and [AOT] ) 0.10 mol ov dm-3, w ) 10, [H+]aq ) 0.10 mol dm-3, in n-heptane.

Table 3. Stopped-Flow Data for the Reaction between Overbased Additive Particles and Methyl Orange/Acid Microemulsion, [AOT]ov ) 0.10 mol dm-3, w ) 10, [H+]aq ) 0.10 mol dm-3, in n-Heptane additive concn (g dm-3)

t1/2 (s)

temp (°C)

2.0

0.54 0.76 1.09 1.62 2.57 0.34 0.51 0.74 1.07 1.84 0.22 0.31 0.45 0.66 1.04

30 25 20 15 10 30 25 20 15 10 30 25 20 15 10

4.0

8.0

(Note that this is not to be confused with the time for half the acid in the droplet to be neutralized.) However, from knowledge of the pKame and ∆A/2, the extent of neutralization at t1/2 (eq 2) can be determined. Therefore the choice of indicator depends on the “pH window” one wishes to observe during neutralization. The pKame and hence the pH values corresponding to t1/2 are 5.1 (PNPS) and 10.0 (NB). As mentioned previously the value for MO is just beyond the pKame at a pH value of 1.5. Table 2 shows the difference in t1/2 values obtained using the three different pH indicators under otherwise identical conditions. As expected there is a progressive increase in t1/2 as the reaction medium is monitored at higher pH values. 3.4. Effect of Particle and Droplet Concentration on the Neutralization Process. Table 3 shows the effect of increasing the concentration of overbased carbonate particles in the hydrocarbon solvent (n-heptane). As the concentration is increased, more base has effectively been added to the system and hence the value

Figure 9. Arrhenius type plots for the reaction between overbased additive particles, 2.0 g dm-3 and pH indicator/acid microemulsion droplets, [AOT]ov ) 0.1 mol dm-3, w ) 10 in n-heptane. EA ) 54 ((2) kJ mol-1.

of t1/2 becomes shorter, i.e., the neutralization becomes faster. Generally the concentration of droplets is in 100fold excess over the concentration of particles even though there is an excess of base in the system. Over the particle concentration range investigated, there is a linear relationship between (t1/2) -1 and [particles], but interestingly there is not a direct proportionality so a second-order rate constant cannot be derived. We have been able to adjust the concentration of particles so that the amount of overbased additive neutralized is typically in the 10100% region. It appears that for conditions where all base is neutralized, the first 70% or so (presumably located in the surface region) is neutralized more rapidly than the last 30%. The concentration of droplets has also been systematically varied (Table 4). At constant overall acid concentration (equal %N), increasing the concentration of water droplets results in a faster neutralization reaction, i.e., t1/2 decreases. The data are quite difficult to analyze in detail but are consistent with the proposed model in which base is transferred simultaneously into both empty and acid-containing droplets. From the data it does not seem to be easily possible to establish how many base species are transferred from the particle to a droplet during an effective encounter. 3.5. Effect of Temperature on the Neutralization Process. The technique has been applied to study the effect of temperature on acid neutralization by overbased additive particles in these systems. As the temperature of the system is increased, it is found that the value of t1/2 decreases, i.e., the reaction becomes faster. This is expected for a process with a finite energy barrier to reaction. The values of t1/2 were analyzed using a modified Arrhenius plot in which the rate is substituted by the reciprocal of t1/2. This procedure assumes that the pKame is essentially independent of temperature. Figure 9 shows that plots of ln(t1/2)-1 vs 1/T are linear. This is expected for the case where the droplet sizes do not change with temperature, which is the situation over the temperature range studied (10-30 °C). The activation energy, EA, for these reactions

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Table 5. Stopped-Flow Data for the Reaction between Overbased Additive Particles, 2.0 g dm-3, and Acid/ Methyl Orange Microemulsion, [AOT]ov ) 0.10 mol dm-3, w ) 10, [H+]aq ) 0.10 mol dm-3, in Various Hydrocarbons hydrocarbon solvent

t1/2 (s)

temp (°C)

n-heptane

0.54 0.76 1.09 1.62 2.57 0.13 0.17 0.24 0.33 0.53 0.04 0.07 0.09 0.13 0.20

30 25 20 15 10 30 25 20 15 10 30 25 20 15 10

n-decane

n-dodecane

is approximately 54 ( 2 kJ mol-1, independent of the specific dye being studied. It is interesting to note that the activation energy is relatively low, given that it is assumed that the base is sterically well protected by the surfactant, i.e., essentially complete coverage. It is also independent of the particle concentration used, as expected. The values of EA clearly indicate that the reactions are not diffusion controlled with rate constant kD, since then the EA values should reflect the much lower values for the temperature dependence of the viscosity of the solvent (∼20 kJ mol-1). 3.6. Effect of Solvent on the Neutralization Process. Various systems were studied in which the hydrocarbon chain length of the alkane solvent has been systematically changed from C7 (n-heptane) to C10 (ndecane) and C12 (n-dodecane). Data using the methyl orange system to monitor the change in absorbance are shown in Table 5. As the chain length of the oil solvent is increased from C7 to C10 to C12, the value of t1/2 progressively decreases. It is well established, using SANS, that the size (and hence concentration) of droplets is independent of the solvent for these low w systems in the temperature range studied. However, the upper temperature phase boundary, which leads to clustering of droplets, is lowered as the solvent chain length is increased. This means that interpenetration of the tail groups of the surfactant surrounding the water cores is facilitated and encounters are longer lived. This results in the reaction rate being faster in the higher molecular weight (higher viscosity) solvent. This result is not so readily predictable. We have also studied the reaction in cyclohexane as oil solvent. Previous studies showed that the rate constant for the H+/-OH reaction involving reaction between droplets (Figure 6) was slowest in this system (order of reaction at 25 °C is C12 > C10 > C7 > cyclic C6). This was ascribed to the cyclohexane solvent penetrating most effectively between the tail groups of the surfactant, which reduces surfactantsurfactant interpenetration and results in the encounters being more “hard-sphere-like”.14 3.7. Change of Surfactant Stabilizing the Water Droplets. Most of our work has been carried out using water droplets stabilized by AOT, but some experiments were also performed using DDAB. In cyclohexane as solvent, it was found that DDAB gave faster rates by an order of magnitude or so. However, this is a relatively small effect in free energy terms, given the complexity of the reaction, and probably reflects differences in (a) surface potential of the microemulsion (AOT is anionic where as DDAB is cationic), (b) bending energy needed to create a region of unnatural curvature, and (c) steric

Figure 10. Schematic diagram of base-transfer channel between an overbased detergent particle and microemulsion droplet during an effective collision.

effects between interpenetrating surfactant tail groups stabilizing the droplet and particle (Figure 10). This region allows exposure of the particle to water and allows base transfer to occur. The dissolution of CaCO3 forms basic species such as HCO3- and OH- inside the water cores of the droplets where neutralization of H+ occurs. 3.8. Effect of Changing the Nature of the Overbased Additive. Using this simple methodology, it is possible to systematically vary the nature of the core and the surfactant. The results demonstrate a general trend in that in terms of the metal cation in the core, the order of reactivity is Na > Mg > Ca. Other results suggest that the rate of neutralization is dependent on the type of surfactant stabilizer around the core. 4. Summary The methodology discussed in this paper has been used to study the neutralization behavior of model overbased detergent additive systems. It is thought that the free excess surfactant present in engine oil formulations disperses the aqueous acid present in a similar way to that of the model w/o microemulsion droplets. In this connection, we have found previously20 that there are close parallels between inter droplet acid-base neutralization and neutralization involving particles in an oil and a coexisting bulk aqueous phase. The use of water-soluble pH indicator molecules allows the change in pH of the water to be monitored for these reactions in a facile way. More complex additive systems of commercial importance have also been investigated. These systems are those in which a number of other additives (e.g., dispersant) are blended with the overbased detergent. Using the technique described in this paper, it has been possible to investigate the effect these additional additives have on the acid neutralization properties of the overbased detergent, and the approach provides a new simple and effective diagnostic test for monitoring the performance efficiency of lubricant detergent additives. Acknowledgment. D.C.H. thanks Infineum UK Ltd. for an industrial studentship. We also thank John Lewis, Zeynep Atay, and Neil Perrins for carrying out important preliminary work on these systems. We gratefully acknowledge very useful discussions with John Marsh and John Cleverley. LA9904354 (20) J. Lewis, Ph.D. Thesis, University of East Anglia, 1991.