Langmuir 1994,10, 1741-1748
1741
Factors Affecting Poly(isobuteny1)succinimide Dispersant Adsorption onto Surfactant-CoatedColloidal Particles in Nonaqueous Media B. L. Papke' and L.M. Robinson Texaco Research and Development, P.O.Box 509, Beacon, New York 12508 Received November 15,1993. In Final Form: February 25,1994"
Factors affecting the adsorption of poly(isobuty1ene)succinimidedispersanta onto alkarylsulfonatecolloids were explored,including polarity of the succinimide dispersant headgroup, sulfonate colloid surface area, packing density, and the chemical nature of the adsorption interaction. Langmuirian adsorption behavior was observed in all cases, accompanied by measurable increases in colloid size. However, the degree of succinimide dispersant adsorption was found to increase and become more irreversible with increasing headgroup polarity. Irreversibleadsorption of succinimide dispersants is believed to occur in a two-step process, a rapid and reversible initial physisorption,followed by a slower base-catalyzed hydrolysis of the succinimide to form a carboxylate salt and amide functionalities,resulting in irreversible chemisorption.
Introduction The stabilization of nonaqueous colloidal dispersions by adsorbed surfactants is a subject with wide-ranging applications (i.e. lubricant oils, paints, surface coatings, emulsions, toners, etc.).' Since colloidal dispersions in hydrocarbon and other nonpolar solvents often have no strongly repulsive force to stabilize the attractive van der Waals forces, coagulation is often prevented through steric or surfactant stabilization provided by adsorbed polymers or surfactants, respectively.2~3Consequently, factors affecting the adsorption of Surfactants onto colloidaldispersions are of considerable economic significance. One wellcharacterized nonaqueous colloid is a colloidal dispersion of alkaline-earth carbonates (typically calcium or magnesium) stabilized by an adsorbed monolayer of an alkaline-earth alkarylsulfonate ~urfactant.~ These colloidal dispersions are widely utilized as automotive and diesel engine lubricant additives; the oil-soluble inorganic base is used to neutralize acidic contaminants and oil oxidation products. Automotive and diesel engine lubricants contain complex mixtures of oil-soluble additives which enhance specific chemical and physical properties of the base Most lubricant additives are surfactant-type materials, providing a balance between oil-solubility and chemical activity. Physical and/or chemical interactions between lubricant additives are known to occur,cQ and both synergistic and antagonistic interactions between specific classesof lubricant additives have been Since typical lubricant formulations may contain as many as a dozen different lubricant additives, unraveling the complex *Abstract published in Advance ACS Abstracts, May 1, 1994. (1) Napper, D. Polymeric Stabilization of Colloidal Dispersions; Academic Press: New York, 1983. (2) Chen, Y.-L.; Xu, 2.;Ieraelachvili, J. N. Langmuir 1992, 8, 2966. (3) Witten, T. A,; Pincus, P. A. Macromolecules 1986,19,2609. (4) Roman, J.-P.;Hoomaert, P.;Faure, D.; Biver, C.; Jacjuet,F.; Martin, J.-M. J. Colloid Interface Sci. 1991, 144, 324. (5) 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. (6) Inoue, K.; Watanabe, H. ASLE Trans. 1983,26,189. (7) Inoue, K.; Watanabe, H. J. Jpn. Pet. Inst. 1981,24, 101. (8) Inoue, J.; Watanabe, H. J. Jpn. Pet. Inst. 1982,25, 106. (9) Papke, B. L.; Bartley, L. S.; Migdal, C. A. Langmuir 1991,7,2614. (10) Spikes, H. A. Lub. Sci. 1990,2, 3. (11) Rounds, F. Lub. Sci. 1989, I, 333. (12) Hsu, S. M.; Lin, R. S. Society Automotiue Engineers Paper 1983, No. 831683.
interactions which may occur is indeed a daunting task. Recently, we developed an ultracentrifugation separation technique, which, when combined with an appropriate analysis method (i.e. infrared analysis), enabled us to quantitatively measure lubricant additive interactions using Langmuir adsorption technique^.^ These experiments demonstrated that poly(isobuteny1)succinimide dispersants (a common class of engine lubricant additives) adsorb strongly onto base-containing alkarylsulfonate colloids. Alkaline-earth alkarylsulfonates are oil-soluble salts of high molecular weight (ca. 450) sulfonic acids. These surfactants form inverse micellar structures in hydrocarbon solvents13and are used to stabilize colloidaldispersions of inorganic base (typically carbonates). The synthesis and properties of these colloids have been thoroughly in~estigated.~J~ Colloidal particle sizes vary depending upon the specific composition of the alkarylsulfonate surfactant and the amount of inorganic base incorporated into the colloidalcore; commercial products generally have diameters between 80 and 160 A, which includes the thickness of the adsorbed surfactant layer. For simplicity, these dispersions are often referred to as basic alkalineearth sulfonates, with the calcium sulfonates being the most common. Although the calcium sulfonate colloids are already coated with a strongly adsorbed monolayer of the sulfonate surfactant, other surfactant or functionalized polymers may also interact with and adsorb onto the colloids. For example, weak adsorption of a 25 000 molecular weight end-functionalized hydrogenated polyisoprene onto calcium sulfonate colloids has been r e ~ 0 r t e d . l In ~ these studies, the functional group was either a tertiary amine (a dimethylamine) or a sulfonate-amine zwitterion. The maximum polymer-colloid ratio observed was 6, and the adsorption appeared to be readily reversible. While the observed interaction was weak, it demonstrates that even very simple polar groups can adsorb onto the surface of (13) Singleterry, C. R. J. Am. Oil Chem. SOC. 1955,32,446. (14) Giasson,S.; Espinat, D.;Palermo,T.; Ober, R.; Peaeah,M.; Morizur, M. F. J. Colloid Interface Sci. 1992,153,355. Tricaud, C.; Hipeauz, J. C.; Lemerle, J.Lubr. Sci. 1989,1,207. Manaot, J. L.; Hallow, M.; Martin, J. M. Colloids Surf. A: Physicochem. Eng. Aspects 1993, 71, 123. OSullivan,T. P.;Vickers,M.E. J.Appl. Crystallogr. 1991,24,732. Miller,
J. F.; Clifton, B. J.; Benneyworth, P. R.; Vincent, B.; MacDonald, I. P.; Marsh,J. F. Colloids Surf. 1992, 66, 197. (15) Carvalho, B. L.; Tong, P.; Huang, J. S.; Witten, T. A.; Fetters, L. J. Macromolecules 1993,26,4632.
0743-7463/94/2410-1741$04.50/00 1994 American Chemical Society
Papke and Robinson
1742 Langmuir, Vol. 10, No. 6,1994
otherwise stable calcium sulfonate colloids. Stronger adsorption was observed for polyfunctionalizedpolymers (ca. 80 OOO molecular weight) grafted with an average of seven maleic anhydrides per chain and derivatized with primary amines.16 In this study, adsorption of polymer functional groups onto the calcium sulfonate colloid resulted in “bridging” interactions and large solution viscosity increases. The polymer adsorption showed moderate shear stability, suggesting the interaction was considerable stronger than that observed for the simpler end-functionalizedpolymers. Finally, as mentioned previously, strong adsorption of poly(isobuteny1)succinimide dispersants onto calcium sulfonate colloids has been reported.gJ7 In the present study, factors affecting the reversibility and nature of the dispersant-colloid interaction are explored in detail.
Experimental Section Materials. Three basic calcium alkarylsulfonates of differing colloidal size were used in the present study. All three were prepared from the same sulfonate surfactant, a mixture of approximately 55 wt % petroleum sulfonate (a monoalkarylsulfonate) and 45 w t % synthetic sulfonate (a dialkyl C-12 benzenesulfonate); the mixed calcium sulfonates had an average molecular weight of approximately 1OOO. The alkarylsulfonate anion, the effective surfactant, had a molecular weight of approximately 450. The synthesis of basic calcium sulfonate colloids is described in the literature;‘ a more detailed description of the sulfonate surfactant used in these studies is given elsewhere.’@J9 The three calcium sulfonates in this study differed in the amount of inorganic base incorporated into the polar core. The first sulfonate (referred to as a “neutral” sulfonate) had a base:sulfonate ratio of 0.51. The second calcium sulfonate had a colloidal core of amorphous calcium carbonate,” and a base: sulfonate ratio of approximately 12:l and an average particle size of 110A (determined by light scattering); the third sulfonate was similar to the second but had a still larger base:sulfonate ratio of approximately 201 and an average particle size of 120130 A. Commercial calcium sulfonates contain approximately 50 w t % of a carrier oil. Results from adsorption studies are corrected for the presence of this carrier oil and are reported for adsorption onto oil-free calcium sulfonate colloids. Poly(isobuteny1)succinimide dispersants used in the present study were prepared by the reaction of poly(isobuteny1)succinic acid anhydride with a poly(ethy1eneamine)in a 21 molar ratio (referred to as a bis(succinimide)dispersant structure). The poly(isobuteny1)succinicanhydride was obtained from the reaction of polyisobutylene (PIB) and maleic anhydride. Two different molecular weight polyisobutylenes manufactured by Amoco Chemical Co. having number average molecular weights of 960 and 2060,respectively, were used to prepare the dispersants in the present study. These succinimide dispersants have the idealized structure shownin Figure 1A;the synthesis and physical properties of these dispersants have been des~ribed.~ A variety of polyethylene polyamines were used in dispersant synthesis, including pentaethylenehexamine (PEHA, Dow Chemical Co.), triethylenetetramine (TETA, Aldrich, 98% ), diethylenetriamine (DETA, Aldrich, 99%), and ethylenediamine (EDA, Aldrich, 99% ). Succinimide dispersants containing secondary nitrogens, such as those prepared using PEHA, TETA, or DETA, can be further derivatized by reaction with various carboxylicacids (i.e. acetic acid, Aldrich, 99.8% ; hexanoic acid, Aldrich, 99.5% ; octanoic acid, Aldrich, 99.5%; phenylacetic acid, Sigma; oleic acid, Aldrich, 99%; hydroxyacetic acid, Du Pont, 70% aqueous (16) Papke,B.L.;Rubin,I.D.SocietyAutomot.Eng.,Paper1992,No. 922281. (17) Papke, B. L. In Mixed Surfactant Systems; Holland, P. M., Rubingh, D. N., Eds.;ACS Symposium Series 501; American Chemical Society: Washington, DC, 1992; p 377. (18) Jao, T.X.; Kreuz, K. L. In Phenomenon in Mixed Surfactant Systems; Scamehorn, J. F., Ed.;ACS Symposium Series 311; American Chemical Society: Washington, DC, 1988; p 90. (19) Jao, T.-C.;Joyce, W. S. Langmuir 1990,6,944. (20) Papke, B. L. Tribol. Trans. 1988, 31, 420.
polyisobu tyiene
simple polyamine
succinimide
A R
polyisobutylene
derivatized polyamine
.,
succinimide
B Figure 1. Idealized poly(isobutenyl)bis(succinimide)dispersant
structures: (A) simple bis(succinimide); (B)derivatized bis(succinimide)
.
solution) to form the idealized type of dispersant structure shown in Figure 1B. Heptane (Aldrich, 99%, spectrophotometric grade) was used as a nonpolar solvent for both the sulfonate and succinimide additives. Calcium hydroxide (Aldrich, 98%) and calcium carbonate (Aldrich, 99%) powders were used as received from freshly opened bottles. Instrumentation. Ultracentrifugation separations were conducted using a Beckman L8-55ultracentrifuge equipped with a Beckman 50.2Ti fixed angle rotor. Fourier transform infrared spectra were obtained using a Nicolet 510 infrared instrument and 0.05cm KC1fiied path length solution cells. Light scattering studies utilized a Nicomp 370 submicrometer particle analyzer supplemented with a 15-mWHeNe Spectra-Physics laser. This instrument utilized a Nicomp software package capable of analyzing Gaussian unimodal and non-Gaussian multimodal particle size distributions; volume-weighted size measurements are reported. While light scattering particle size distributions were used largely to make only relative size comparisons, the accuracy of the software algorithm was confirmed through comparison with spherical NIST polystyrene standards. In addition, the sizes of the sulfonate colloids themselves have been well established through small angle X-ray scattering and transmission electron microecopy.4 Procedures. Direct physical interactions between basic calcium sulfonate colloidal dispersions and poly(isobuteny1)succinimides were quantitatively measured using adsorption technique^.^ Ultracentrifugation was used as a physical separation technique and was feasible because a large difference in ultracentrifugation sedimentation rates exist between the sulfonate colloid and dispersant. Highly alkaline calcium sulfonate colloidscan be removed from dilute hydrocarbon solutions within 1-3 h through ultracentrifugation; succinimide dispersants used in the present study showed very little sedimentation under similar conditions. In a typical adsorption experiment, a weighed amount of the sulfonate colloid was mixed with dispersant in a volatile hydrocarbon solvent (to ensure a completely homogeneous mixture). The solvent was removed under nitrogen, and the sulfonate/dispersant mixture heated at 100O C for 4.0h (sufficient for complete adsorption to occur). These conditions were chosen to simulate typical lubricant process blending operations. After cooling,the mixture was diluted 1/6(wt/wt) with heptane, stirred 30 min, and ultracentrifuged at 18 OOO rpm for 3-4 h (depending on the size of the sulfonate colloid). This dilution resulted in a maximum colloid volume fraction of about 3% The ultracentrifuge conditionswere sufficientto cause completesedimentation of the colloid into the bottom quarter tube fraction. In fact, most of the colloid was compressed into a pellet at the bottom of the centrifuge tube. If strong dispersant adsorption occurred, the dispersant was removed from solution along with the colloid during ultracentrifugation. For infrared analysis, the top eighth of the ultracentrifuged solution was discarded and a cut was carefully taken from the next quarter of the tube volume (using a flat tip syringe). The heptane solvent was removed and a
.
Langmuir, Vol. 10,No. 6,1994 1743
Adsorption of Dispersants onto Colloids solution cell infrared spectrum obtained on the remaining oil solution. Quantitative measurements of dispersant adsorption were obtained by measuring the intensity of the succinimide carbonyl infrared adsorption band at 1705 cm-l. Light scattering particle size measurements were conducted on the calcium sulfonate colloid before and after dispersant adsorption. Mixtures of the sulfonate colloid and succinimide dispersant were prepared as described in the adsorption studies. Dilute heptane solutions were used for light scattering studies; succinimide dispersant adsorption is largely irreversible in most cases (as is described later), and no apparent desorption of the dispersant was observed. Light scattering samples were prepared using filtered heptane (Nucleopore0.44-pm fidters) in concentrations between 0.25 and 3.5 wt %, depending on the scattered light intensity. Care was taken to avoid concentrations where multiple scattering occurred, and the lowest concentration giving acceptable scattering intensity was used in all cases. Concentrations of the solutions were reduced systematically, and light scattering size measurements were conducted until a concentration was reached where the recorded particle sizes were independent of concentration. All samples were filtered immediately preceding analysis (Nucleopore 0.22-pm gravity filtration). Qualitative dispersant adsorption studies on calcium hydroxide or calcium carbonate powders were also conducted. The purpose of these studies was to identify possible chemisorption interactions; the two salts were used since the colloid core is known to contain both hydroxide and carbonate The dispersant was a bis(succinimide) prepared from reaction of a 2:l mole ratio of a low molecular weight maleic anhydride derivatized polyisobutylene (320 av MW) with pentaethylenehexamine. Adsorption studies were conducted at 100 O C ; 4 h adsorption time was sufficient for complete adsorption. Dispersant adsorption was measured by solution infrared measurements on the supernatant solutionafter sedimentation of suspended solids. Infrared spectra were obtained on solids containing the adsorbed dispersant using a fluorolube mull technique after washing the solidswith heptane and drying under nitrogen.
Results and Discussion 1. Factors Affecting SuccinimideDispersant Adsorption. Calcium sulfonate colloids are stable structures, soluble in all proportions in hydrocarbon solvents, containing a strongly adsorbed monolayer of calcium sulfonate surfactant on an amorphous calcium carbonate core structure.14 Therefore, it may be surprising that a second surfactant material (i.e. the succinimide dispersant) can adsorb onto the colloid surface. However, succinimide dispersants have been shown to adsorb strongly onto calcium sulfonate colloids through interaction of the polar succinimide/amine headgroup with the calcium sulfonate colloidal core.9 In fact, these studies have demonstrated that as many as 45 dispersant molecules can adsorb onto a single sulfonate colloidal particle (80 A core diameter). For very small dispersant structures (i.e. succinimides prepared from dodecenylsuccinic anhydride), the maximum adsorption may increase to over 75. Since the poly(isobuteny1)succinimide is a sterically bulky molecule, the observed adsorption behavior suggests that either the adsorbed sulfonate surfactant has considerable surface mobility to allow adsorption to occur, or some desorption of the sulfonate surfactant occurs. Desorbed calcium sulfonate surfactant will form inverse micellar structures in hydrocarbon solvents; since these inverse micelles have slow sedimentation rates relative to the sulfonate colloidal particles, sulfonate surfactant desorption can be easily measured by ultracentrifugation. No sulfonate surfactant desorption was observed using ultracentrifugation/FTIR techniques," therefore it is believed that lateral mobility of the adsorbed sulfonate surfactant occurs to allow dispersant adsorption to occur. Fluorescence studies also provide supporting evidence for surface mobility of the sulfonate surfactant in these systems.18 In addition,
(a)
(bl
(cl (d)
" ( r ( . . . . .. 8
0
0
10
28
38
I0
Equilibrium Dispersant Conc (moles / L x la.)
b0
58
Figure 2. Adsorption of hydroxyacetic acid derivatized poly(isobutenyl)bis(succinimide) dispersants onto 121 calcium sulfonate colloids: (a) derivatized 960 MW/PEHA/bis(succinimide); (b) derivatized 2060 MW/PEHA/bis(succinimide); (c) nonderivatized dispersant (a); (d) nonderivatized dispersant (b). Table 1. Langmuir Constants for Adsorption of Succinimide Disperoants onto a 12:l Basic Calcium Sulfonate Colloid
dispICaSf2 dispersant (amine) 2060 MW PIB Bis(succinimides) a. PEHA b. PEHA (hydroxyaceticacid) 960 MW PIB Bis(succinimides) a. PEHA b. PEHA (hydroxyacetic acid)
h, mol g-1
b, M-1 molar ratio
9.9 X 10-6 1040 13.3 X 1O-a 390
11.5 X 10-6 21.3 X 10-6
0.213 0.286
440
0.247
210
0.458
certain cosurfactants (i.e. methanol) greatly increase sulfonate surface mobi1ity:and it is possible the dispersant itself may have a similar effect. (a) Succinimide Dispersant Structure. Chemical modifications to the polar amine dispersant headgroup may be expected to affect adsorption onto the sulfonate colloid. For example, carboxylic acids will react with the secondary nitrogens on the dispersant polyamine headgroups to from amides; by varying the chemical structure of the carboxylic acid, different pendant functional or sterically blocking groups may be added (Figure 1B). One simple chemical modification is the reaction with hydroxyacetic acid, which has the effect of chemically transformingthe secondarynitrogens to amides and adding a hydroxyl functionality to the dispersant headgroup. Adsorption curves for two dispersants (before and after reaction with hydroxyacetic acid) with the 12:l (base: sulfonate) colloid are shown in Figure 2. Langmuirian behavior is observed for the adsorption curves in Figure 2, that is, the amount of adsorbed succinimide dispersant approaches a maximum limiting value, h, as the dispersant concentration is increased. The Langmuir constants n, (the moles of succinimide dispersant adsorbed per gram of basic calcium sulfonate colloid (free of carrier oil)) and "b" (a measure of the adsorption strength) are calculated in the standard fashion from the data in Figure 29 and are given in Table 1. For dispersants synthesized from the same molecular weight PIB, the hydroxyacetic acid treated analogue, has a significantly stronger adsorption interaction with the colloid, demonstrating that increased dispersant headgroup polarity can significantly increase adsorption. In contrast, if the polarity of the dispersant headgroup is decreased through steric blocking, then adsorption onto the colloid is reduced. Succinimide dispersant polarity was decreased by allowing various carboxylic acids with longer and/or bulkier hydrocarbon groups to react with
Papke and Robinson
1744 Langmuir, Vol. 10, No. 6,1994 WT% D I S P E R S A N T J$O%
1704
-1
VI\
(b)
(el
E
4
(0
15
20
25
38-
Equilibrium Dispersant Conc (moles / L x 10
I 40
35
\
Figure 3. Adsorption of various derivatized 2060 MW PIB/ PEHA/bis(succinimides) onto a 12:l calcium sulfonate colloid: (a)nonderivatized;(b)hexanoic acid derivatized; (c)phenylacetic acid derivatized; (d) octanoic acid derivatized; (e) oleic acid derivatized. (a)
i S O O . 0 i 7 5 0 . 0 i700.0 iSS0.0 iS00.0 i550.0
(b)
Figure 5. Infrared spectra for physical mixtures of a neutral calcium sulfonate with a 2060MW PIB/PEHA/bis(succinimide)
Wavenumber
(cm-1)
dispersant; expanded spectral region.
Table 2. Langmuir Constants for Adsorption of Succinimide Dispersants onto a 201 Basic Calcium Sulfonate Colloid
(C)
(d)
dimeraant (amine)
disp/CaSfi ~hn.mol
c1 b. M-1 molar ratio ~
~
~
2060 MW PIE3 Bis(8uccinimides)
0 ! 0
10
20
30
40
58
I
b0
Equilibrium Dispersant Conc (moles / L x
IO’, Figure 4. Adsorption of original and hydroxyacetic acid derivatized 960 MW PIB/PEHA/bis(succinimide)dispersants onto 12:l and 201 calcium sulfonate colloids: (a) 121 colloid w/ derivatized dispersant; (b) 201 colloid w/same dispersant as (a); (c) 121 colloid w/nonderivatized dispersant; (d) 201 colloid w/ same dispersant as (c). the secondary dispersant nitrogen. A steady decrease in dispersant adsorption was observed for alkyl-capped dispersants with increasingly bulky alkyl substituents (Figure 3). (b) Sulfonate Detergent Colloid Structure. A second factor affecting dispersant adsorption is the nature of the adsorbent, that is the calcium alkarylsulfonate colloid itself. The adsorbent in the present study is a colloidal dispersion of calcium carbonate and hydroxide sterically stabilized by an adsorbed layer of calcium sulfonate surfactant. As discussed earlier, for succinimide dispersant adsorption to occur, it is probable that the sulfonate surfactant must reorganize to permit adsorption of the sterically bulky succinimide dispersant. Maximum succinimide dispersant adsorption is limited either by the available surface area on the sulfonate colloid or by steric constraints. In Figure 4, dispersant adsorption is compared for the 12:l and 201 calcium sulfonate colloids. Langmuir constants for adsorption of two dispersants onto the 20:l calcium sulfonate colloid are given in Table 2 and may be compared with those for adsorption onto the 12:l sulfonate colloid (Table 1). The maximum amount of dispersant adsorbed on the 12:l calcium sulfonate colloid is consistently higher on an equal weight basis than for the 201 calcium sulfonate colloid while the dispersant: sulfonate molar ratios are similar,indicatingthat the active
a. PEHA 7.6 X lod b. PEHA (hydroxyacetic acid) 11.2 X 1W 960 MW PIB Bis(succinimides) a. PEHA 8.5 X lod b. PEHA (hydroxyacetic acid) 11.0 X 1od
1110 390
0.228 0.336
1720 1870
0.330
0.255
surface area available for succinimide dispersant adsorption is larger for the smaller diameter colloid. For example, the adsorption of the 2060 MW PIB/PEHA/bis(succinimide) dispersant is 20-30% higher for the 12:l calcium sulfonate colloid than for the 20:l (base:sulfonate) colloid (on an equal weight basis). Therefore, most of the increased adsorption observed for the smaller colloid appears attributable to its higher surface-to-volume ratio. (c) Infrared Band Shifts. Frequency or intensity shifts in the infrared spectrum of the succinimide dispersant may be expected to occur if the succinimide headgroup is located in the highly polar environment near the surface of the sulfonate colloid. However, a very strong and broad carbonate adsorption band at 1500 cm-l makes it impossible to observe shifts in the 1705-cm-lsuccinimide band in simple dispersant/colloid mixtures a t concentration ratios where strong adsorption occurs (due to the large amount of carbonate base present). A shift in succinimide carbonyl infrared band frequencies can be observed for mixtures of dispersant with carbonate-free “neutral” calcium sulfonate inverse micelles (Figure 5). The dispersant carbonyl adsorption band at 1705 cm-1 is shifted toward lower frequencies (1694 cm-l) and considerably broadened in mixtures with the “neutral” calcium sulfonate, providing additional evidence that the succinimide dispersant headgroup is located near or at the surface of the sulfonate colloid. 2. SuccinimideAdsorptiononto Calcium Sulfonate Colloids-Light Scattering Studies. Physical properties of the “mixed” sulfonate/succinimidecolloid are likely to be quite different from the original colloid. The dispersant has a polyisobutylene “tail” that is up to 5 times
Langmuir, Vol. 10, No. 6,1994 1745
Adsorption of Dispersants onto Colloids I
I
8N
1H
1 2 1 Calcium Sulfonate t Diowrsant
Particle Size (Angstroms)
Figure 6. Volume-weightedlight scattering data for 121calcium sulfonate colloid before and after adsorption of a 2060 MW PIB/ PEHA/bis(succinimide)derivatized with hydroxyacetic acid. Table 3. Volume-Weighted Light Scattering Size Measurements for 1 2 1 Calcium Sulfonate-Succinimide Mixed Surfactant Colloids ~
dispersant (amine) 960 MW PIB Bis(succinimides) a. PEHA b. PEHA c. PEHA (hydroxyaceticacid) 2060 MW PIB Bis(succinimides) a. PEHA b. PEHA (hydroxyaceticacid)
size dispjCaSf2 % ofmax (angstroms) molar ratio adsorption
114 143 137
0.15 0.21 0.15
60 85 85
126 162
0.08 0.08
40 30
the length (extendedconformation) of the sulfonatealkaryl hydrocarbon chain (for the 2060 molecular weight polyisobutylene). In addition, the polyisobutylene chain structure is considerably more rigid than for a simple hydrocarbon chain (due to steric repulsions from the gem dimethyl groups on the PIB chain) and consequentlylikely to adopt a more extended chain conformation. Therefore the size of the mixed sulfonate-succinimide colloid should increase relative to that of the originalcolloid. In addition, surface properties of the mixed surfactant colloid are likely to be dominated by properties of the polyisobutylene polymer. Light scattering studies conducted on the sulfonate colloid before and after dispersant adsorption show a measurable increase in apparent colloid size occurs. For example, in Figure 6 the volume-weighted light scattering data from the 12:l (basemlfonate) colloid is compared with the same colloid after adsorption of a 4500molecular weight bis(succinimide)PEHA dispersant derivatized with hydroxyacetic acid. A clear increase in average colloid size from approximately 110 to 160 A is observed. Thus these data provide direct evidence supporting the hypothesis of dispersant adsorption onto the sulfonate colloidal particles. The size of the mixed surfactant colloid should depend upon both the molecular weight as well as the total amount of adsorbed dispersant. As adsorption approaches saturation, the polyisobutylene carbon chains may be forced to adopt a more extended chain conformation. The effect of succinimide dispersant surface coverage on the size of the mixed surfactant colloid is shown in Table 3 for the 960 MW PIB/PEHA/bis(succinimide) dispersant. Dispersant concentrations were maintained well below complete adsorption to avoid complications in light scattering analysis due to the presence of free (nonassociated) dispersant.15 As dispersant adsorption is increased from 60 % to 85% of maximum, the size of the mixed surfactant colloid increased from 114 to 143 A. A size dependence upon PIB molecular weight is also seen in Table 3; higher
molecular weight dispersants lead to larger size increases at a given dispersant/sulfonate molar ratio. Poly(isobutenyl)bis(succinimide) dispersants may be conveniently viewed as short triblock copolymers: PIB/ polyamine/PIB. When this molecule adsorbs on the surface of a colloidal particle, anchored by the polyamine segment, each polyisobutylene chain may be expected to adopt a conformation largely independent of the rest of the molecule. Low molecular weight polyisobutylene (i.e. 1000-2000 number average molecular weight) has a rather broad size distribution, so it is difficultto determine exactly what magnitude of size increase should be expected for a colloid saturated with adsorbed dispersant. However, if one makes the assumption that a single polymer chain with one end attached to the colloid surface has a chain conformation approximately that of a random coil,l5 then a 2100 molecular weight polymer would extend approximately 30 A away from the colloid surface. Since the colloid is already covered by a sulfonate surfactant layer approximately 20 A in thickness,2l the dispersant polyisobutylene chain would extend an additional 10 A if there were no other steric constraints. This increase is considerably less than the observed 25 A increase in colloid radius (Table 3); a combination of steric Constraints (forcing the polyisobutylene into a more extended conformation)combinedwiththe fact that even a few adsorbed high molecular weight polymer chains would significantly increase the colloid's hydrodynamic radius is most likely responsible. The size of the mixed surfactant colloids is also affected by dispersant derivatization (Table 3). In both examples, derivatization of the dispersant with hydroxyacetic acid results in significant size increases at the same dispersant/ sulfonatemolar ratio. As discussed in the following section, adsorption of the nonderivatized dispersant is believed to be partially reversible; in contrast, adsorption of the hydroxyacetic acid treated dispersant is believed to be irreversible. Therefore, it is possible that limited dispersant desorption in the dilute solutions used for light scattering results in smaller sizes for the nonderivatized dispersants. The high level of dispersant adsorption observed raises interesting questions concerning surfactant packing constraints. Using the 20:l colloid as an example, this colloid has an average size of about 120 A and a central inorganic core diameter of approximately 80 A. Assuming an amorphous calcium carbonate core density of 2.3 g/cmS, this colloid contains approximately 190 calcium sulfonate surfactant molecules. For an average dispersant/calcium sulfonate molar adsorption ratio of 0.25 (Table 2), a single dispersant-saturated colloid contains approximately 48 adsorbed dispersant molecules. Using a reasonable dispersant headgroup area of 150 Az (based on molecular models),the area remaining per sulfonate anion is roughly 34 A2. Since this area is still significantly larger than the geometric close-packed area of 15 A2 for the sulfonate anion, it suggests that dispersant polymer chain crowding is the factor limiting further adsorption. This conclusion is supported by the much larger adsorption values observed for short (2-12 dispersants (ca. 75 adsorbed molecules/ c~lloid).~ Colloidal suspensions are stabilized by the adsorption of higher molecular weight polymers, even when the colloid is already stabilized by an adsorbed surfactant.15 Two important factors come into play. The first is the fact the adsorbed polymer (i.e. the dispersant) increases the (21)Markovic, I.; Ottewil, R. H.; Cebula, D. J.; Field, I.; Marsh, J. F. Colloid Polym. Sci. 1984,262, 648.
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1746 Langmuir, Vol. 10, No. 6,1994 ”-, -197
‘1 f / 0l 0
1
2
3
4
5
b
7
8
9 - 1 9
Equilibrium Dispersant Conc (mdes / L x 10
11
12 2
3
Equilibrium Dispersant h c (moles / L x 10)
Figure 7. Time-dependent dispersant desorption study; 12:l calcium sulfonate colloid with 2060 MW PIB/PEHA/hydroxyacetic/bis(succinimide)dispersant: (a)initial mixture;(b)mixture aged 1 week at room temperature; (c) mixture aged 1month at room temperature.
Figure 8. Competitivedispersant adsorptionreversibility study using a 12:l calcium sulfonatecolloid and a 960 MW PIB/PEHA/ bis(succinimide)dispersant with a neutral calcium sulfonate: (a) no neutral sulfonate, (b) neutral sulfonate added last; (c) neutral sulfonate added first.
surfactant packing density, providing enhanced surface coverage and blocking adsorption of materials which might destabilize the colloid core structure. The second is that adsorbed dispersant may significantly reduce the depletion-induced attraction between colloidal particles which naturally occurs in solutions containing free polymer.15 The degree of stabilization in the second case depends heavily upon the molecular weight of the adsorbed dispersant. A critical factor determining long term stability, however, is the degree of reversibility of the dhpersant-colloid interaction,as discussed in the following section. 3. SuccinimideAdsorptiononto Calcium Sulfonate Colloids-Interaction Reversibility Studies. Two methods were used to measure the reversibility of dispersant adsorption onto calcium sulfonate colloids, timedependent dilution and a competitive adsorption interaction method. While the classic reversibility experiment would be to reequilibrate the centrifuged adsorbent (i.e. the sedimented colloid)with a fresh supernatant solution, recentrifuge, and measure the amount of desorbed dispersant, the high gravitational forces generated during the ultracentrifugation procedure disrupt and at least partially destroy the sedimented sulfonate colloid, rendering this technique useless for reversibility studies. The first method for measuring dispersant adsorption reversibility was to simply dilute the mixture after adsorption with a hydrocarbon solvent and measure the time-dependent reequilibration. If the adsorption is reversible, then desorption of the succinimide dispersant should occur in the more dilute solution. An example of the results from this study is shown in Figure 7 for adsorption of the 2060 MW PIB hydroxyacetic acid derivatized PEHA bis(succinimide) dispersant onto the 12:l (base:sulfonate) colloid. The initial mixture was diluted 10-fold with heptane; no loss of dispersant due to desorption was observed after 1 week, and only a slight loss of dispersant occurred after 1 month at ambient temperatures. This observation suggests that dispersant adsorption is qualitatively irreversible, at least a t ambient temperatures for the dispersant illustrated. A second method for measuring the adsorption reversibility exploited a competitive interaction between a “neutral”calcium sulfonate and the basic sulfonate colloid. The “neutral” sulfonate is an inverse sulfonate micelle containing a very small amount of solubilized base, with a diameter of 50-60 A as determined by fluorescence techniques.18 Due to its small size and relatively low
density, this sulfonate has a very slow sedimentation rate in the ultracentrifuge, similar to that observed for the succinimide dispersants. The 12:l (base:sulfonate) colloid had a particle size of roughly 1108,(volume-weighted size measured by light scattering) and a relatively rapid sedimentation rate. The existence of interactions between succinimide dispersants and “neutral” sulfonate micelles has already been demonstrated (Figure 5), although the relative strength of this interaction has not been determined. Therefore, if dispersant adsorption is reversible, addition of a neutral calcium sulfonate to the dispersantbasic sulfonate colloid mixture should shift the equilibrium, resulting in a net dispersant desorption. The driving force behind the reequilibration (if it occurs) is the interaction between the dispersant and the “neutral” sulfonate. Reversibility studies were conducted for adsorption of either the hydroxyacetic acid derivatized or the original nonderivatized dispersants onto the 12:l colloid using two specific order-of-addition procedures. Either the basic sulfonate colloid and dispersant were mixed and allowed to equilibrate at 100 “C for 4.0 h before addition of the “neutral” sulfonate (followed by reequilibration at 100 “C) or mixing order was reversed (i.e. the “neutral” sulfonate and dispersant were mixed and allowed to equilibrate before addition of the basic sulfonate colloid). The results from these two mixing orders were compared after ultracentrifugation. If dispersant adsorption is reversible, the mixing order should have no effect; irreversible dispersant adsorption is expected to result in significantly greater dispersant removal from solution if the dispersant and basic sulfonate colloid are mixed first (as in the first mixing procedure). As a control experiment, a nonpolar hydrocarbon oil was substituted for the “neutral”sulfonate in the first mixing order. Adsorption curves for a competitive interaction study between the 960 MW PIB PEHA bis(succinimide) dispersant with the 12:l sulfonate colloid and “neutral” sulfonate inverse micelle are shown in Figure 8. Addition of a “neutral” sulfonate surfactant to a preblended mixture of the dispersant and the sulfonate colloid significantly reduces dispersant adsorption, as may be observed by comparing adsorption curve b with the adsorption curve obtained when an equivalent weight of nonpolar oil is used in place of the “neutral” sulfonate, curve a. If the “neutral” sulfonate is mixed first with the dispersant, dispersant adsorption onto the basic calcium sulfonate colloid is reduced by an additional amount, curve c. Therefore,
Adsorption of Dispersants onto Colloids
Langmuir, Vol. 10, No. 6,1994 1747 1703
B
2
I
b
8
1
9
t
2
t
l
t8
1592
29
Equilibrium Dispersant Conc (moles / L x lay
Figure 9. Competitivedispersant adsorption reversibility study using a 12:l calcium sulfonatecolloid and a 960 MW PIB/PEHA/ hydroxyacetic derivatized bis(succinimide) dispersant with a neutral calcium sulfonate: (a) no neutral sulfonate; (b) neutral sulfonate added last. adsorption of the nonderivatized succinimide dispersant is at least partially reversible. Adsorption of a more polar derivatized dispersant showed a higher degree of irreversibility. The experiment described in the preceding paragraph was repeated for the hydroxyacetic acid derivatized dispersant analogue. Adsorption of the derivatized dispersant in this example was essentially irreversible under the conditions of this experiment, as evidenced by the fact that addition of the “neutral” sulfonate surfactant did not reduce dispersant adsorption (compare curves a and b, Figure 9). This observation is consistent with previous comparisons which demonstrated that the hydroxyacetic acid-treated dispersant had consistently stronger adsorption interaction with the sulfonate colloid (Figure 2). Irreversible adsorption that follows Langmuirian behavior, as has been observed between the succinimide dispersants and the calcium sulfonate colloid, presents an intriguing paradox.22 If the adsorption is irreversible (i.e. desorption does not occur), then why does adsorption not continue either until all the dispersant is removed or until the maximum saturation of the calcium sulfonate colloid is achieved? In other words, why is the adsorption Langmuirian (with adsorption gradually approaching a maximum limiting value at high dispersant concentrations) if it is irreversible? One solution to this paradox is that two adsorption interactions may occur-a rapid and labile (reversible) initial adsorption (resulting in the observed adsorption curves) followed by a slower irreversible adsorption involving some type of “aging” transformation.22 In the present study, one plausible model is that the initial reversible interaction involves a simple physisorption of the succinimide dispersant onto the sulfonate colloid, followed by a slower irreversible interaction of the dispersant headgroup with the inorganic colloid core or perhaps with the sulfonate surfactant itself. In this model, the initial interaction is postulated to be a reversible equilibrium, but if the dispersant remains in intimate contact with the colloid for an extended time, an irreversible adsorption occurs. 4. Evidence for Chemical Interactions between Succinimide Dispersants and Sulfonate Detergent Colloids. Irreversible adsorption of the succinimide dispersant suggests that something stronger than simple dispersant physisorption may be involved. Since the (22) Zawadzki, M.E.;Harel, Y.;Adamson, A.
363.
W.Langmuir 1987,3,
ieOO.0 i7SO.O 1700.0 1650.0 1600.0 1550.0 1500.0 W a v e n u m b e r (cm-i)
Figure 10. Infrared spectra of washed calcium hydroxide solids after contact with succinimidedispersants (a)calcium hydroxide solids with adsorbed 320 MW PIB/PEHA/hydroxyacetic derivatized bis(succinimide);(b) original dispersant spectrum. sulfonate colloid contains both calcium carbonate and calcium hydroxide base within the polar core,2Oqualitative adsorption studies were conducted for dispersant adsorption onto each of these two salts. Poly(isobuteny1)succinimide dispersants were found to adsorb strongly onto calcium hydroxide solids; however, no significant adsorption onto crystalline calcium carbonate (calcite)was observed. Although the crystalline nature of the calcium carbonate adsorbent may have affected these results (calcium carbonate within the colloid core is amorphous), it is interesting to note that calcium hydroxide is believed to be more accessible than carbonate in the colloid, since it is preferentially located near the surface of the sulfonate colloid.20 Infrared spectra obtained on the heptane-washed calcium hydroxide solids after the adsorption experiment clearly demonstrate that the succinimide dispersant chemically interacts with calcium hydroxide to give carboxylate salts (1596 cm-’) and an amide-containing product (shoulder a t approximately 1640 cm-’) (Figure 10). These species are believed to result from a base catalyzed hydrolysis of the succinimide, opening the ring structure to form carboxylate and amide structures. This reaction was rapid, even a t room temperature, as determined by infrared analysis of the washed solids. While dispersant adsorption/reaction was easiest to observe for low molecular weight dispersants (presumably due to a larger percentage of active functional groups), analogous evidence for chemical interactions was obtained for higher molecular weight dispersants as well. Chemical interactions between the succinimide dispersant and hydroxidic base in the colloid are believed to explain the irreversible nature of the dispersant adsorption, as well as the paradox of “irreversible Langmuirian” adsorption.22
Summary and Conclusions Although oil-soluble calcium carbonate colloids are stable materials with the sulfonate surfactant strongly bound to the inorganic core, this does not preclude the adsorption of additional surfactant materials onto the polar inorganic core of these colloidaldispersions. In the present study, evidence for the strong and irreversible adsorption of succinimide dispersants onto these colloidal particles was presented and a number of factors affecting the strength and reversibility of succinimide dispersant adsorption were explored, including succinimide headgroup
1748 Langmuir, Vol. 10, No.6, 1994 polarity, sulfonate adsorbent surface area, sulfonate surfactant packing density, and the chemical nature of the dispersant adsorption. Succinimide dispersants were derivatized through reaction with a series of carboxylic acids to alter the polarity of the polyamine headgroup. Increased dispersant headgroup polarity (i.e. through derivatizationwith hydroxyacetic acid) resulted in a higher level of maximum dispersant adsorption, while derivatization with sterically bulky nonfunctionalized acids reduced adsorption. This evidence demonstrated that adsorption occurs through the polar headgroup of the succinimide surfactant. Adsorption of higher molecular weight succinimide surfactanb resulted in measurable increases in colloid size, as determined through light scattering. The reversibility of succinimide dispersant adsorption was measured using both a time-dependent dilution method as well as a competitive adsorption technique.
Papke and Robinson The adsorption of higher polarity dispersants was found to be largely irreversible, while adsorption of dispersants containing simple polyamine functional groups showed a small degree of reversibility. Factors contributing to the irreversibility of dispersant adsorption were explored;the primary cause is believed to be a chemical interaction between the succinimidefunctional group and hydroxidic base on the sulfonate colloid core, resulting in the basecatalyzed hydrolysis of the succinimide ring to form a carboxylate salt and an amide. No evidence for sulfonate surfactant desorption was observed in the course of these studies, and dispersant adsorption is believed to be accompanied by lateral mobility of the strongly adsorbed sulfonate surfactant monolayer. Dispersant adsorption is believed to significantly affect colloid stability.
Acknowledgment. The authors thank Texaco, Inc., for permission to publish this work.
