2614
Langmuir 1991, 7,2614-2619
Adsorption of Poly(isobuteny1)succinimide Dispersants onto Calcium Alkylarylsulfonate Colloidal Dispersions in Hydrocarbon Media B. L. Papke,' L. S. Bartley, and C. A. Migdal Texaco Research Center, P.O. Box 509,Beacon, New York 12508 Received October 22, 1990.I n Final Form: April 19,1991 Ultracentrifugation and Fourier transform infrared techniques were used to quantitatively measure interactions between a basic calcium alkyarylsulfonate colloidal dispersion and poly(isobuteny1)succinimide dispersants. Strong physical interactions were identified and are believed to result from adsorption of the poly(isobuteny1)succinimide onto the calcium alkyarylsulfonate colloid. Langmuir adsorption techniques were used to obtain quantitative information on factors affecting the interaction ratio and strength, including the polyalkenyl polyamine chain length, and the amine/ polyisobutylene ratio used in dispersant synthesis. Poly(isobutenyl)succinimide/calcium sulfonate molar interaction ratios as high as 0.40were observed for low molecular weight succinimide dispersants. Succinimide adsorption was shown to occur through interaction of the polar headgroup with the sulfonate colloid. These interactions appear to be irreversible in nonpolar solvents.
Introduction Oil-soluble lubricant additives are essential to the performance of modern automotive and diesel engine oils and are used to enhance specific chemical and physical properties of the base oi1.I Among the many different types of additives of practical importance, two of the most commonly used lubricant additives are alkaline-earth alkylarylsulfonates and poly(isobuteny1)succinimides. Alkaline-earth alkylarylsulfonates,the salts of high molecular weight (ca. 450)oil-soluble sulfonic acids, are known to form inverted micellar structures in hydrocarbon medias2 These surfactants can be used to stabilize colloidal dispersions of an inorganic base (such as a carbonate or hydroxide), which is the primary reason for their widespread use in engine lubricants. They provide an inexpensive source of oil-soluble base to aid in the neutralization of corrosive acids generated during engine operation. Commercial additive concentrates of alkaline-earth sulfonates may contain as much as 20-35 wt % of inorganic carbonate and 18-30 wt % of the alkylarylsulfonate surfactant in a mineral oil solvent.3 The manufacturing processes used to prepare these colloidal dispersions have been described by Marsh,3and particle sizes and properties have been measured by small angle neutron scattering: transmission electron microscopy,5 and ultracentrifugation sedimentation ratesa6 To avoid confusion, these colloidal dispersions may be referred to as a basic alkylarylsulfonate, or more simply as a basic alkaline-earth sulfonate. Particle sizes vary with manufacturing conditions and the amount of inorganic basic incorporated but generally range from 80 to 160 A in diameter, including the thickness of the adsorbed surfactant layer. The properties of sterically stabilized colloidal dispersions, such as the basic alkaline-earth sulfonates, have been well documented.' (1) Stewart, W. T.; Stuart, F. A. In Advances in Petroleum Chemistry and Refining; Kob, K. A., McKetta, J. J., Jr., Eds.; Interscience: New York, 1963; Vol. 7, pp 3-64. (2) Singleterry, C. R. J. Am. Oil Chemists SOC.1955, 32, 446. (3) Marsh, J. F., Chem. 2nd. 1987, 20, 470. (4) Markovic, I.; Ottewill, R. H.; Cebula, D. J.; Field, I.; Marsh, J. F. Colloid Polym. Sci. 1984, 262, 648. ( 5 ) Reading, K.; Dilks, A.; Graham, S.C. In Petroanalysis '87;Crump, G. B., Ed.; John Wiley and Sons, Inc.: New York, 1988; pp 239-251. (6) Tricaud, C.; Hipeaux, J. C.; Lemerle, J. Lub. Sci. 1989, 1, 207.
Poly(isobuteny1)succinimides are a second widely utilized class of engine lubricant additives. These materials, commonly referred to as succinimide dispersants, are oilsoluble surfactants prepared by reaction of low molecular weight polyisobutylene (ca. 1000-2500 number average molecular weight) with maleic anhydride and a polyethylene polyamine having the generalized structure shown in Figure 1. For amines having more than one primary nitrogen, such as triethylenetetramine, either bis(imide) or mono(imide) structures may be synthesized. Poly(isobuteny1)succinimides prevent the formation of oilinsoluble sludge in engine operation, and are believed to function through a steric stabilization mechanism involving adsorption of the polar portion of the succinimide dispersant molecule on engine sludge particulates.8 Succinimide dispersants may associate in hydrocarbon solvents to form weakly bound micelles, but micelle formation occursmuch more readily in the presence of polar materials (such as acids or water).g The solubilization of acids (organic and inorganic) by succinimide dispersants in hydrocarbon solvents is well documented.lOJ1 Both alkaline-earth sulfonates and poly(isobuteny1succinimides) are used together in commercial engine lubricants, and it is of interest to understand whether physical interactions occur between these two materials. Lubricant additive interactions are known to affect engine performance; both synergistic and antagonisticinteractions have been reported,12J3and a better understanding of factors affecting engine lubricant additive interactions is needed. The present study describes the novel application of an ultracentrifugation/ infrared technique to identify direct physical interactions between poly(isobuteny1)succinimide dispersants and basic calcium sulfonate additives. This technique was used both as a rapid screening method to identify potential additive interactions and as a quantitative technique to obtain detailed information on (7) Napper, D. H. InPolymeric Stabilization of Colloidal Dispersions; Academic Press: New York, 1983. (8)Forbes, E. S.; Neustadter, E. L., Tribology 1972,5, 72. (9) Inoue, K.; Watanabe, H. ASLE Trans. 1983,26,189. (10) Fontana, B. J. Macromolecules 1968, 1,139. (11) Bradley, R. V.; Jaycock, M. J. Prep.-Am. Chem. Soc., Diu. pet. Chem. 1972, 17, G101. (12) Spikes, H. A. Lub. Sci. 1990, 2, 3. (13) Rounds, F. Lub. Sci. 1989, I , 333.
0743-7463/91/2407-2614$02.50/00 1991 American Chemical Society
Adsorption of Dispersants onto Colloidal Dispersions
2060 av. M.W.
"
polyamine
Figure 1. Idealized poly(isobutenyl)bis(succinimide) dispersant structure. molar interaction ratios and relative interaction strengths for various succinimide dispersant structures.
Experimental Section Materials. The calciumalkylarylsulfonateused in the present study had a base:sulfonate molar ratio of 201 and contained approximately 38 wt % calcium carbonate, 18 wt % calcium sulfonate, and 44 wt % of a nonpolar hydrocarbon carrier oil. The sulfonate surfactant was a mixture of approximately 65 wt % petroleum sulfonate (a largely monoalkylarylsulfonate in the C-20 to C-26range) and 35 w t % synthetic (mostlydialkyl (C-12) benzenesulfonates). Amore detailed description of this sulfonate is given elsewhere." A strictly neutral version of the same alkylarylsulfonate surfactant was also used. The neutral sulfonate exists as an inverse micellar structure rather than as a colloidal dispersion, since it contains no solubilized inorganic base. Physical properties of the neutral calcium sulfonate have been des~ribed.'~ Heptane (Aldrich, 99% spectrophotometricgrade) was used as a nonpolar solvent for both the sulfonate and succinimide additives. Succinimide dispersants used in this study were prepared by the reaction of poly(isobuteny1)succinic acid anhydride (derived from polyisobutylene (PIB, Indolpol H-1500, Amoco Chemical Company, number average molecular weight 2060) and maleic anhydride) with a polyethylene polyamine, generally in a 2:l molar ratio. The preparation of these materials is well documented in both patent and open literature.8J6 Succinimidedispersants used in the present study contained approximately30% unreacted polyisobutylene due to incomplete conversion of the original polyisobutylene to the poly(isobuteny1)succinic acid anhydride. The succinimide dispersants were present in a nonpolar hydrocarbon carrier oil to facilitate handling; the active dispersant concentration (excluding unreacted PIB) was about 3790. The following describes a typical dispersant synthesis. Poly(isobuteny1)succinicacid anhydride (1759.0g,0.50mol, prepared from a polyisobutylene of approximately 2060molecularweight) was charged into a 5-Lthree-neck flask equipped with a mechanical stirrer, thermometer, and nitrogen inlet. A nonpolar hydrocarbon oil (100 P Pale oil, 1470.8 g) was added, a nitrogen flow started, and the mixture heated to 60 "C. Pentaethylenehexamine (PEHA, Dow E-100, 72.60 g, 0.275 mol) was added, and the temperature increased to 120 OC. After 1.0 h, the temperature was increasedto 160"C and maintained for an additional 2.0 h. The hot mixture (ca 100 "C) was filtered through a diatomaceousearthfilter aid material. The product (approximately a 37% concentrate) analyzed as follows: Calcd for N, 0.70. Found: N, 0.65. Total Acid Number (TAN, ASTM D974) = 0.6. Total Base Number (TBN, ASTM D 2896) = 16.1. This general procedure was repeated using triethylenetetramine (TETA, Aldrich, 98% ), diethylenetriamine (DETA, Aldrich, 99%), and ethylenediamine (EDA, Aldrich, 99%). A mono(imide)using ethylamine (EA, Aldrich) was also prepared using a similar procedure with approximately twice the molar ratio of amine as was the case for the bis(imide) dispersants. A dodecylbis(succinimide)was prepared starting from 2-dodecen1-ylsuccinicanhydride (Aldrich,97 % ) and triethylenetetramine (TETA, Aldrich, 98%) using similar procedures. Calculated molecular weights (based on starting material specification and final product composition) and percent active materials for the basic calcium alkylarylsulfonate and poly(isobuteny1)succinimidesused in the present study are summarized in Table I. For example,the calciumalkylarylsulfonate has (14) Jao, T. C.; Kreuz, K. L. In Phenomenon in Mixed Surfactant
Systems; Scamehom,J. F., Ed.;ACS Sympaium Series 311; American
Chemical Society: Washington, DC, 1988; p 90. (15) Jao, T.-C.; Joyce, W. S.Langmuir 1990,6,944. (16) Nalesnik, T. E. U.S.Patent 4,636,322, Jan 13, 1987.
Langmuir, Vol. 7, No. 11,1991 2615
Table I. Average Molecular Weights for Calcium Sulfonate and Poly(isobuteny1) succinimide Additives additive av mol wt % active material neutral Ca sulfonate 1000 45 basic Ca sulfonate 55 3000 H-l500/PEHA/ bis(imide) 37 4575 H-lWO/TETA/bis(imide) 37 4490 H-lW/DETA/ bis(imide) 37 4390 H-l500/EDA/bis(imide) 37 4400 H-l500/EA/mono(imide) 37 2225 dodecyl/TETA/ bis(imide) 687 100 a molecular weight of about lo00 g/mol so the basic calcium sulfonate (containing 20 mol of calcium carbonate) has a molecular weight of 3000 g/mol. Measurement Techniques. Ultracentrifugation separations were conducted using a Beckman L8-55M ultracentrifuge equipped with a Beckman 50.2 Ti fixed anglerotor. All additives or additive combinations were diluted 1/6 (w/w) with distilled heptane and centrifuged in polypropylene tubea at 20 OC. Fourier transform infrared spectra were obtained using a Nicolet 510 infrared instrument with 0.05 cm KCl fixed path length solution cells. In a typical adsorption experiment, a weighed amount of the overbased sulfonate additive was mixed with the dispersant additive in petroleum ether, stirred for 30 min at room temperature, and sonicated briefly to ensure the solution was completely homogeneous. The petroleum ether was removed at 80 OC under Ns, and the dispersant/detergent additive mixture was placed in a thermostated oven at 100 OC for 4.0 h. After cooling, the additive mixture was diluted 1/6 (w/w) with distilled heptane, stirred for 30 min, briefly sonicated, and ultracentrifuged for 3.0 hat 18 OOO rpm. Translucent polypropylenetubes were marked to show solvent volumes prior to centrifugation; the top eighth of the ultracentrifuged solution was discarded and a cut was taken of the next quarter of the tube volume (using a flat-tip syringe). The heptane solvent in this fraction was stripped under vacuum, and a solution cell infrared spectrum was obtained on the remainingoil solution. The intensity of thesuccinimide C - 0 stretching adsorption band at 1705 cm-' was recorded and compared with dispersant standard calibration curve to quantitatively determine the amount of remaining dispersant.
Results and Discussion Ultracentrifugation as a Technique To Measure AggregationPhenomenon. Direct physical interactions between basic calcium sulfonate colloidal dispersions and poly(isobuteny1)succinimides were quantitatively measured using a Langmuir adsorption approach.17 Ultracentrifugation was used in the present study as a physical separation technique to study interaction of the succinimide dispersant with the basic calcium sulfonate colloidal particles. This technique was feasible because a large differencein ultracentrifugation sedimentation rates exist between the calcium sulfonate colloids and the succinimide dispersants. Basic alkaline-earth sulfonates can be removed from dilute low-density hydrocarbon solutions within a few hours through ultracentrifugation.6 The succinimide dispersants in the present study were essentially unaffected under similar conditions. If interactions occur between the basic sulfonate and the succinimide dispersant, ultracentrifugation should cause the succinimidedispersant to be physically removed from solution along with the basic sulfonate colloid. A pictorial representation of the rationale behind this approach is shown in Figure 2. This method can be used to identify physical interactions between any two materials where a large difference in sedimentation rates exist. The essential difference between this approach and the classical adsorption tech(17) Adamaon, A. W. The Physical Chemistry of Surfaces, 5th ed.; John Wiley and Sons, Inc.: New York, 1990; Chapter XI.
2616 Langmuir, Vol. 7, No. 11,1991 a) No Interaction
Papke et al. 140s
b) Strong Interaction
Removed for FTIR Analysis
Before
After
Before
1 i:
After
18,000 RPM, 3.0 hrs, 48,000 g max.
Figure 2. Ultracentrifugation/FTIR method for screening potential additive interactions; pictorial representation.
B # 2000.0
V
JJ
178s
1800.0
1600.0
-
1400.0
1200.0
WavenumbDr
niques described by Adamson" is that in the present studies the adsorbent is a colloidal dispersion rather than a solid adsorbent. The minimum ultracentrifugation conditions required to remove all of the basic sulfonate from the top threequarters of the centrifuge tube were determined experimentally. Heptane was used as a solvent throughout because of its low density (0.684 g/cm3 a t 20 "C) and moderatelyhigh boiling point (98.4 "C). Translucent polypropylene ultracentrifuge tubes were used to facilitate the removal of solution fractions after centrifugation. Centrifugation of a 1/6 (w/w) dilution of the basic calcium sulfate in heptane at 18 OOO rpm for 3.0 h at 20 O C was sufficient to meet the minimum separation requirements. The basic calcium sulfonate used in this study contained no "free" sulfonate surfactant, as determined by infrared examination of the top tube fractions after centrifugation. Strictly neutral calcium alkylarylsulfonates form inverse micelles and have much slower sedimentation rates than the basic calcium sulfonate colloidal dispersions used in the present study. Poly(isobuteny1)succinimide dispersants were also ultracentrifuged under conditions identical to those established for removal of the basic calcium sulfonate from heptane solution. In contrast to results with the basic calcium sulfonates, no significant separation of the succinimide dispersant in heptane solvents was observed under these conditions, and even increasing the ultracentrifugation conditions 15-fold resulted in only small dispersant sedimentation from heptane solutions at 20 "C. Fourier transform infrared spectroscopy (FTIR) was used to follow the separation of basic calcium sulfonates and dispersants from solution and to quantitatively measure interactions between these two materials. Representative infrared spectra for the basic calcium sulfonate and a poly(isobuteny1)succinimide dispersant are shown in Figure 3. In addition to the usual hydrocarbon bands (associated with C-Hstretching and bending modes, etc.) the basic calcium sulfonate has two infrared bands due to the carbonate species (1495cm-l, asymmetric CO, stretch; 865 cm-l, symmetric cos stretch) and two bands assigned to the sulfonate moiety (ca. 1200 cm-l, a combination of several asymmetric so3 stretching bands, and 1065 cm-I, symmetric 803 stretch). The poly(isobuteny1)succinimide dispersant has distinctive infrared bands at 1785 and 1705 cm-1 (assigned to the succinimide carbonyl asymmetric and symmetric stretch) and at 1230 cm-l (assigned to the gem-dimethyl structure on the polyisobutylene chain). Since some unreacted polyisobutylene is present, the ratio of the 1705/1230 cm-' bands can be used as an indication of whether the succinimide dispersant is behaving differently from the unreacted polyisobutylene.
J 1000.0
L 000.00
1 600.00
1Cm-l)
Figure 3. Representative infrared spectra: (a) basic calcium sulfonate colloid; (b) poly(isobutenyl)bis(succinimide) dispersant. (bl (C)
(a)
(d)
Dispersant Conc (moles / L x 10-1 Figure 4. Adsorption of poly(isobutenyl)bis(succinimide) dispersanta on a basic calcium sulfonate colloid from n-heptane solution: H-l500/PEHA/bis(imide)(a);H-l500/TETA/bis(imide) (b); H-l500/DETA/bis(imide)(c); H-l500/EDA/bis(imide) (d).
Basic Calcium Sulfonate/Poly (isobuteny1)succinimide Aggregation Behavior. Quantitative information on interactions between basic calcium sulfonate colloids and poly(isobuteny1)succinimide dispersants was obtained by considering the sulfonate colloid to be a solid adsorbent. Adsorption curves are shown for four bis(succinimide)dispersants interacting with the same basic calcium sulfonate colloid in Figure 4. The isotherms are all Langmuirian in shape, that is, they followed the equations n=n
bC "l+bC
-
and -=-
1 +-c
(2) n n,b n, where C is the equilibrium dispersant concentration (in moles per liter) and n is the moles of succinimide dispersant adsorbed per gram of basic calcium sulfonate colloid (containing no carrier oil). As the dispersant concentration is increased, the amount of adsorbed succinimide dispersant will approach some limiting value, n,. This value represents the maximum calcium sulfonate/succinimide interaction ratio. The value of the constant "b" is a measure of the intensity of adsorption, or adsorption strength. If true Langmuir adsorption behavior is observed, then a plot of C / n vs C should be linear, the slope is equal to l / n m ,and the y intercept equal to l/n,b. As
Langmuir, Vol. 7, No. 11, 1991 2617
Adsorption of Dispersants onto Colloidal Dispersions
"'i b0l
V
/
409
300 20e '5
lee
-r-----*---2
e
Dispersant Concentration (Moles/L x ltJ)
Figure 6. Adsorption data for poly(isobutenyl)bis(succinimide) dispersants on a basic calcium sulfonate colloid from n-heptane solutionplotted accordingtoeq 2 H-1500 EDA/bis(imide) (a); H-l500/PEHA/bis(imide) (b); Dodecyll ETA/bis(imide)
4
0
le
+ 28
shown in Figure 5 for three representative dispersants, Langmuir adsorption behavior was observed, and the various b and n, values are tabulated for all the succinimide dispersants in this study in Table 11. The data in Figure 4 demonstrate that succinimide dispersant molecules interact strongly with the calcium sulfonate colloidal particles and that the interactions are weaker when all the secondary nitrogens are eliminated (Le. compare the DETA dispersant (one secondary nitrogen) with the EDA dispersant (no secondary nitrogens) in Figure 4). This evidence suggests that the succinimide dispersant interacts with the calcium sulfonate colloid through the polar succinimide/amine moiety. Evidence that the polyisobutylene polymer itself is noninteracting was obtained by substituting a sample of the underivatized polyisobutylene polymer (Indopol H-1500, 2060 average molecular weight) in place of the succinimide dispersant in the ultracentrifugation/FTIR procedure. The 1230cm-l IR band of the polyisobutylene sample was used to follow the interaction. The polyisobutylene concentration was varied and adsorption curves were obtained. The results are compared with the Langmuir adsorption curves obtained for the H-l500/PEHA/bis(imide) dispersant in Figure 6. The results clearly show that the strong succinimidebasic calcium sulfonate interactions observed result from an interaction of the polar succinimide headgroup with the calcium sulfonate colloid and do not involve the polyisobutylene polymer. Additional evidence that the polar functionality on the dispersant structures (the amine and/or succinimide) is responsible for the strong interaction with the calcium sulfonate detergent can be obtained by following the succinimide/ polyisobutylene ratio in the infrared. The dispersant samples used in the present study all contained unreacted polyisobutylene, arising from an incomplete
4e
50, 5-0,
Dispersant or Polyisobutylene Conc ( moles / L x 10
be de
Figure 6. Adsorption of polyisobutylene and poly(isobutany1)bis(succinimide) dispersants on a basic calcium Sulfonate colloid dodecyl/ TETA/ bis(imide)(a); H-1500/TETA/ bis(imide) (b); H-l500/polyiaobutylene (c).
(C).
Table 11. Langmuir Constants for Adsorption (Interaction) of Succinimide Dispersants with Basic Calcium Sulfonate Detergents dispersant (amine) n,, mol r1 b, M-1 H-1500 bia(imides) a. EDA 4.5 x 10-6 550 b. DETA 7.9 x 10-6 370 c. TETA 8.2 X 1od 620 d. PEHA 7.6 X 1od 1100 H-1500 mono(imide8) a.EA 6.2 X 1od 110 C-12 bis(imide8) a. TETA 13.2 X 1W6 1090
+ 31
dispersant identity H-l500/PEHA/bis
Table I11 disp/sulfonate w t ratio 0.11 0.2.5 0.43
original dispersant
H-l500/TETA/bis
0.11 0.25 0.43
original dispersant
H-lW/EDA/bis
original dispereant
0.11 0.25 0.43
1230/1705IR ratio (ultracentsample) infinite 11.1 2.7 0.94 infinite 5.3 2.4 0.85 33.3 2.1 1.4 0.80
Table IV. Succinimide-Calcium Sulfonate Interaction Ratios dispersant (amine) %, mol gl disp/Ca sulfonate molar ratio H-1500 bis(imides) a. EDA 4.5 x 1od 0.135 b. DETA 7.9 x 10-6 0.238 c. TETA 8.2 X 1od 0.246 d. PEHA 7.6 X 106 0.227 H-1500 mono(imides) a.EA 6.2 X 0.187 C-12 bis(imide8) a. TETA 13.2 x 10" 0.396
thermal reaction of the polyisobutylene with maleic anhydride. If an interaction of the polyisobutylene polymer with the calcium sulfonate colloid is responsible for the observed adsorption curves, then the 1230/1705 cm-' IR band ratio (polyisobutylene/succinimidedispersant ratio) should remain unchanged after the calcium sulfonate is removed by ultracentrifugation. On the other hand, if the interaction occurs through the polar dispersant headgroup, then the polyisobutylene will not be removed and the 1230/1705 cm-l band ratio will approach infinity. The latter is easily shown to be the case in the present experiments (Table 111). Poly(isobuteny1)succinimidexalcium Sulfonate Interaction Ratios. The maximum amount of succinimide dispersant that interacts with the basic calcium sulfonate colloid is suprisingly high. This becomes more apparent if the Langmuir adsorption data (moles of adsorbed dispersant per gram of calcium sulfonate) is converted into a poly(isobutenyl)succinimide/calcium sulfate molar ratio, as shown in Table IV. For the longer chain polyethylenepolyamines(DETA,TETA,andPEHA) this ratio is approximately one bis(succinimide) dispersant molecule adsorbed for every four calcium alkylaryl-
2618 Langmuir, Vol. 7,No. 11, 1991
Calcium Carbonate Colloidal Particle Figure 7. Pictorial representation of hypothesized interactions between a calcium sulfonatecolloidal particle and adsorbed poly(isobutenyl)bis(succinimide) dispersants.
sulfonate molecules. Since the calcium sulfonate is itself adsorbed onto a calcium carbonate colloidal particle, the dispersant molecule must become a part of this structure. One plausible hypothesis is that the succinimide dispersant forms a type of a mixed micellar aggregate with the calcium sulfonate colloid, as depicted pictorially in Figure 7. Basic calcium sulfonates are stable colloidal materials, and as such the sulfonate surfactant surface coverage (packing density) is normally assumed to be fairly high. Thus it may be suprising that such a large amount of additional surfactant (Le. the dispersant) can be incorporated within this structure. However, it can be demonstrated using some simple approximations that sufficient space exists on the calcium carbonate colloid for the dispersant to adsorb in agreement with the model in Figure 7. The basic calcium sulfonate colloid used in the present study had an average micellar size of about 120-130 A, as determined from light scattering. If the adsorbed sulfonate layer thickness is about 20 A,4 then the calcium carbonate core diameter is approximately 80-90 A, assuming a roughly spherical structure. If the additional assumption is made that the calcium carbonate density is approximately 2.3 g/cm3 (calcium carbonate within the micellar core is amorphous18),then the calcium sulfonate aggregation number for an 80-A core diameter is about 190. For a dispersant/sulfonate molar interaction ratio of 0.24 (an average maximum interaction ratio for a strongly interacting dispersant (Table IV)), this means that as many as 45 dispersant molecules may be bound to an individual sulfonate micelle. Assuming a headgroup area of 150 8,per dispersant (a reasonable average value for the succinimide polyamine headgroup), this leaves approximately 35-40 8,for the sulfonate molecule on the carbonate core. This value agrees reasonably well with a calculated sulfonate headgroup area of 40 A,* The large interaction ratio and the intensity of the dispersant interaction suggest that a complexation reaction may be occurring (rather than simple physical adsorption), perhaps involving the amine/succinimide functionalities complexingwith calcium located within the polar sulfonate micelle core. If this is true, different interaction ratios and strengths may be observed when overbased sulfonate detergents other than calcium are tested (such as sodium or magnesium sulfonates). The basic calcium sulfonate/poly (isobuteny1)succinimide aggregate shown in Figure 7 represents in a very real sense a new additive species. The succinimide dispersant has a surfactant "tail" that is approximately 5 times the length (extended conformation) of the sulfonate (for a 2100 average molecular weight polyisobutylene). In addition, the polyisobutylene chain structure is considerably (18) Papke, B.L. Tribol.Trans. 1988, 31, 420.
Papke et al.
more rigid than a simple hydrocarbon chain (due to steric repulsions from the terminal methyl groups), and hence likely to adopt a more extended chain conformation. Surface properties of the mixed sulfonate-succinimide micelle are therefore likely to be dominated by the properties of the polyisobutylene polymer, and the overall size of the colloidal dispersion should increase. In fact, significant increases in colloid sizes have been observed in preliminary light scattering studies. The basic calcium sulfonate colloid has a diameter of approximately 130 A as measured by light scattering, the basic sulfonate/succinimide aggregate has a size of roughly 160-170 A, depending on the polyisobutylene average molecular weight. In addition, colloid stability and physical properties are likely to be affected by this aggregation phenomenon.7 Another suprising finding from this study was the unexpectedly high interaction ratios observed for dispersants containing only a succinimide functional group (i.e. the bis(imide) prepared using ethylenediamine or the mono(imide)prepared using ethylamine). Since these two dispersants contain no polyamine functionality, it was thought initially that perhaps there would be little interaction with the basic calcium sulfonate colloids. Although the observed interaction was weaker than for dispersant structures containing longer chain polyethylene polyamine (curve D, Figure 4), a significant interaction was still measured. Presumably, the strength of this interaction is due to polarity rather than complexation. Interestingly, the mono(imide) dispersant prepared with ethylamine (containing only one polyisobutylene "tail") has a higher molar interaction ratio than the comparable bis(imide) structure (prepared using ethylenediamine) (Table IV). This suggests that steric constraints may play a role in the amount of succinimide dispersant that can interact with a given basic calcium sulfonate colloidal particle. Similarly, the molar interaction ratio for the low molecular weight dodecylbis(succinimide) (Table IV) is much higher than for any of the other dispersants, suggesting the bulky polyisobutylene chains are the physically limiting factor in the maximum amount of dispersant adsorbed onto the sulfonate colloid. The "b" values from the Langmuir adsorption equations (eqs 1 and 2)are tabulated in Table I1 for the dispersants included in the present study. Since the "b" value is a measure of the adsorption strength,17 some comparisons may be made, taking into consideration the large experimental uncertainty in these values (at least 1300). The interaction strength (not to be confused with the interaction ratio) of the EDA, DETA, and TETA bis(imide) dispersants prepared from the H- 1500 polyisobutylene are identical within experimental uncertainty. The PEHA dispersant has the strongest interaction strength, and the mono(imide) dispersant has the weakest interaction, demonstrating that adsorption strengths increase with increasing polyamine chain length. Although the interaction between poly(isobuteny1)succinimides and a basic calcium sulfonate colloid displays Langmuirian adsorption curves, there is reason to believe that the adsorption may actually be irreversible. The classic reversibility experiment would be to reequilibrate the centrifuged adsorbent (in this case the calcium sulfonate colloid) with a fresh supernatent solution, recentrifuge, and measure the new dispersant concentration. Unfortunately, the gravitational forces are such in the ultracentrifugation experiment that the sedimented calcium sulfonate colloid itself is destroyed to some degree. An alternative approach to measure interaction reversibility is to use a dispersant mixture, measuring whether the first
Adsorption of Dispersants onto Colloidal Dispersions
dispersant can be displaced by the second. For this experiment to succeed, the two dispersants must be distinctly different in some measurable way. The dodecylsuccinimide does not contain a polyisobutylene hydrocarbon structure, and thus has no infrared adsorption a t 1230cm-l (from the gem-dimethyl moiety). Therefore, the dodecylsuccinimide dispersant can be distinguished in the infrared from the poly(isobuteny1)succinimide dispersants, and competitive interactions can be measured. If one dispersant was allowed to interact first with the sulfonate colloid before the second dispersant was added, no equilibration or displacement was observed. In other words, the dispersant-sulfonate colloid interaction appeared to be irreversible rather than an equilibrium. The paradox of irreversible adsorption has been discussed by A d a m s ~ n , ~and ~ J it~ appears that some type of naging" process involving the adsorbed surfactant (Le. the dispersant) is required to adequately explain the observed irreversibility. Additional studies are required to fully (19) Zawadzki, M.E.;Harel, Y.;Adamson, A.
363.
W.Langmuir 1987,3,
Langmuir, Vol. 7, No. 11,1991 2619
explore the irreversibility of the succinimide-sulfonate colloid interactions. Conclusions An ultracentrifugation/FTIR technique has been developed to rapidly screen potential lubricant additive interactionswhere a clear difference in sedimentation rates exists between two individual additives. Strong physical interactionsbetween a basic calcium alkylarylsdfonate colloid and poly(isobuteny1)succinimide dispersants have been identified using this technique. This interaction is believed to result from adsorption of the succinimide dispersant onto the basic calcium alkylarylsulfonate colloid. It was determined that the dispersant headgroup (i.e. the polyamine/succinimide) is responsible for the strong interaction with the sulfonate colloid; complexation between the polyamine and/or succinimide functionalities with the calcium cation may be the driving force behind this interaction. Acknowledgment. We thank Texaco, Inc., for permission to publish this work and wish to acknowledge the assistance of Mr. J. F.Lucas in preparing the dispersant samples used in this study.
