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Aug 27, 2008 - The efficiency of a series of nonionic dispersants at stabilizing carbon-rich particles in oil was evaluated. The dispersants were synt...
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Langmuir 2008, 24, 10665-10673

10665

Micellization and Adsorption of a Series of Succinimide Dispersants Yu Shen and Jean Duhamel* Institute for Polymer Research, Department of Chemistry, UniVersity of Waterloo, Waterloo, Ontario N2L 3G1, Canada ReceiVed May 7, 2008. ReVised Manuscript ReceiVed June 16, 2008 The efficiency of a series of nonionic dispersants at stabilizing carbon-rich particles in oil was evaluated. The dispersants were synthesized by coupling a polyamine to two polyisobutylenes terminated at one end with a succinic anhydride (PIBSA). Their chemical composition was characterized by FTIR absorption. The associative strength of the dispersants was determined from their ability to self-associate in solution into reverse micelles. Their critical micelle concentration (CMC) and aggregation number (Nagg) were determined by fluorescence in an apolar solvent, namely hexane. The associative strength of the dispersants was found to increase with increasing number of secondary amines in the polyamine core. The adsorption isotherms describing the adsorption of the dispersants onto carbon black particles were determined and showed the existence of two binding regimes for the dispersants having more amines in their polar core. Much stronger binding occurred at dispersant concentration lower than the CMC. The weaker binding observed at dispersant concentrations larger than the CMC is believed to result from the dispersants having to compete between micellization and adsorption at concentrations above the CMC.

Introduction Ultrafine particles (UFPs) refer to particles having a diameter smaller than 100 nm.1 As the link between UFP inhalation by humans and health deterioration becomes ever stronger,2 governments around the world issue ever more stringent requirements aimed at reducing UFP emission into the air.3 This affects directly the car and truck industry since car and truck engines generate UFPs which are either metallic or carbon-rich in nature depending on whether they result from engine wear or combustion, respectively.4 The new engines designed to meet the most recent government regulations happen to lead to higher soot concentrations in the engine oil.5 This, in turn, is detrimental to the good operation of the engine, since increased levels of soot in the engine are known to thicken the oil to the point where it can no longer be pumped by the engine.6,7 This extreme thickening of the engine oil is due, in part, to the aggregation of carbonaceous UFPs into large particulate aggregates (LPAs) having a diameter greater than 1 µm. Furthermore LPAs precipitate out of solution and contribute to increased sludge production in the engine oil.8 The association of UFPs into LPAs is caused by the presence of polar groups on the UFP surface resulting from oxidation of the oil during normal engine operation.6,8 In apolar oil, UFPs minimize exposure of their polar surface to the solvent by undergoing aggregation into LPAs. In order to minimize the detrimental production of LPAs, dispersants are added to the oil. Over the years, numerous oil-soluble dispersants have been * To whom correspondence should be addressed. E-mail: jduhamel@ uwaterloo.ca. (1) Gwinn, M. R.; Vallyathan, V. EnViron. Health Perspect. 2006, 114, 1818– 1825. (2) Nel, A. Science 2005, 308, 804–806. (3) Reish, M. S. Chem. Eng. News 2005, 5, 20–22. (4) Ritter, S. Chem. Eng. News 2006, 13, 38. (5) Jackson, M. M.; Arters, D. C.; Macduff, M. G. J.; Mackney, D. W. Pat. Int. WO/2003/091365, 2003. (6) Gallopoulos, N. E. Soc. Autom. Eng. 1970, 700506. (7) Edmisten, W. C.; Peterson, J. V.; Sholts, R. A. Soc. Autom. Eng. 1970, 700509. (8) Harrison, P. G.; Creaser, D. A.; Perry, C. C. Lub. Eng. 1992, 48, 752–758. (9) Chevalier, Y. Curr. Opin. Colloid Interface Sci. 2002, 7, 3–11. (10) Jao, T.-C.; Passut, C. A. In Handbook of Detergents Ed. Showell, M. S., NY, 1999, pp 437-471. (11) Liston, T. V. Lub. Eng. 1992, 48, 389–397.

designed and commercialized and their study has been the focus of a number of reviews.9-11 Oil-soluble dispersants are categorized according to whether they contain metal (metallic dispersants such as alkaline-earth alkylarylsulfonates) or not (ashless dispersants). The most successful ashless dispersants used by the oil-additive industry are the succinimide dispersants which were introduced more than 40 years ago.12 Succinimide dispersants are obtained by coupling a polyamine with one or several low molecular weight polyisobutylenes (750 < Mn < 4000 g/mol) terminated at one end with a succinic anhydride function (PIBSA).9-11 The polyisobutylene ensures that the dispersant be soluble in the apolar oil while the polar polyamine can latch onto the polar surface of UFPs. Adsorption of the dispersant onto the UFP surface stabilizes the particles in solution and prevents their aggregation into LPAs. The self-association of succinimide dispersants in apolar solutions has been investigated and evidence of reverse micelle formation has been reported.13-15 Small micellar aggregates have been observed with aggregation numbers (Nagg) smaller than 10 units14,15 and a CMC of 0.4-1.0 g/L for a poorly characterized succinimide dispersant has been determined.13 As any detergents, succinimide dispersants are surface active and this property has been the object of numerous studies aiming at characterizing the behavior of succinimide dispersants at an air-water interface,16-18 at the surface of carbon black particles used as mimic for the carbon-rich particles (CRPs) found in soot,19-27 and more (12) Stuart, F. A.; Anderson, R. G.; Drummond, A. Y. US Pat. 3,202,678, 1965. (13) Vipper, A. B.; Krein, S. E.; Bauman, V. N. Neftekhimiya 1968, 6, 922– 926. (14) Inoue, K.; Watanabe, H. ASLE Trans. 1982, 26, 189–199. (15) Ganc, J. P.; Nagarajan, R. Soc. Autom. Eng. 1991, 912397. (16) Ghaı¨cha, L.; Leblanc, R. M.; Chattopadhyay, A. K. Langmuir 1993, 9, 288–293. (17) Tomlinson, A.; Danks, T. N.; Heyes, D. M.; Taylor, S. E.; Moreton, D. J. Langmuir 1997, 13, 5881–5893. (18) Reynolds, P. A.; McGillivray, D. J.; Gilbert, E. P.; Holt, S. A.; Henderson, M. J.; White, J. W. Langmuir 2003, 19, 752–761. (19) Pugh, R. J.; Matsunaga, T.; Fowkes, F. M. Colloids Surf. 1983, 7, 183– 207. (20) Pugh, R. J.; Fowkes, F. M. Colloids Surf. 1984, 9, 33–46. (21) Pugh, R. J.; Fowkes, F. M. Colloids Surf. 1984, 11, 423–427. (22) Bezot, P.; Hesse-Bezot, C.; Rousset, B.; Diraison, C. Colloids Surf. A 1995, 97, 53–63.

10.1021/la801416a CCC: $40.75  2008 American Chemical Society Published on Web 08/27/2008

10666 Langmuir, Vol. 24, No. 19, 2008 Scheme 1. Synthesis of Dispersants from the Reaction of 2 mol of PIBSA with 1 mol of Polyamine

recently at the surface of carbon nanotubes.28,29 Interestingly, there has been no report in the literature where the ability of succinimide dispersants at forming reverse micelles and at adsorbing onto CRPs have been studied in parallel and where the effect of one property over the other has been investigated. The present work addresses this issue by investigating the behavior in solution of a series of succinimide dispersants (Scheme 1) prepared by reacting a polyamine core with two polyisobutylene chains terminated at one end with a succinic anhydride (PIBSA). The self-assembly of the PIBSA-based dispersants into reverse micelles was characterized by determining the critical micelle concentration (CMC) and aggregation number (Nagg) of the dispersants in hexane by steady-state fluorescence. The polar interior of the micelles was probed with bis-(2,2′-bipyridine)ruthenium(II)-5-amino-1,10-phenanthroline hexafluorophosphate (RuNH2). RuNH2 was used instead of the better known tris(2,2′bipyridyl)dichlororuthenium(II) hexahydrate (Ru(bpy)3) to take advantage of its free amine that could enhance the association of RuNH2 with the amine-rich core of the reverse micelles and eventually provide a means of covalent attachment to the PIBSAbased dispersants. The CMC of the dispersants was determined by monitoring the dispersant concentration above which the chromophores naturally insoluble in hexane would be taken up by the dispersant micelles. Quenching experiments with potassium iodide (KI) yielded the Nagg values. The adsorption process was investigated by using carbon black particles (CBPs) as model CRPs. The adsorption isotherms were constructed by monitoring the concentration of dispersant not adsorbed onto the CBPs by UV-vis spectrophotometry. Analysis of the adsorption isotherms provided information on the influence that the dispersant chemical structure has on the effectiveness of its binding. Most interestingly, this study represents the first example in the literature where the influence that dispersant micellization has on the binding of the dispersants onto CBPs could be inferred.

Experimental Section Materials. The polyamines diethylenetriamine (DETA), tetraethylenepentamine (TEPA), and pentaethyelenehexamine (PEHA) were purchased from Aldrich and used as received. Polyisobutylene (23) Bezot, P.; Hesse-Bezot, C.; Diraison, C. Carbon 1997, 35, 53–60. (24) Tomlinson, A.; Scherer, B.; Karakosta, E.; Oakey, M.; Danks, T. N.; Heyes, D. M.; Taylor, S. E. Carbon 2000, 38, 13–28. (25) Dubois-Clochard, M.-C.; Durand, J.-P.; Delfort, B.; Gateau, P.; Barre´, L.; Blanchard, I.; Chevalier, Y.; Gallo, R. Langmuir 2001, 17, 5901–5910. (26) Cox, A. R.; Mogford, R.; Vincent, B.; Harley, S. Colloids Surf. A 2001, 181, 205–213. (27) Won, Y.-Y.; Meeker, S. P.; Trappe, V.; Weitz, D. A. Langmuir 2005, 21, 924–932. (28) Yang, Y.; Grulke, E. A.; Zhang, Z. G.; Wu, G. J. Appl. Phys. 2006, 99114307-1-8. (29) Yang, Y.; Grulke, E. A.; Zhang, Z. G.; Wu, G. Colloids Surf. A 2007, 298, 216–224.

Shen and Duhamel succinic anhydride (PIBSA) was supplied by Imperial Oil. Sodium 1,4-bis(2-ethylhexyl)sulfosuccinate (AOT) was purchased from Aldrich and purified according to Craig and Linda’s method.30 Bis(2,2′-bipyridine)-dichlororuthenium(II) hydrate used in the preparation of RuNH2 or tris(2,2′-bipyridyl)dichlororuthenium(II) hexahydrate (Ru(bpy)3) were purchased from Aldrich. The solvents xylene (ReagentPlus, 99%, Aldrich), tetrohydrofuran (THF, distilled in glass, Caledon), methanol (HPLC grade, EMD), and acetonitrile (HPLC grade, Caledon) were used as received. Hexane (HPLC grade, EMD) was distilled before use to minimize the presence of residual water in hexane. Methylene blue (Fisher Scientific) and activated carbon black (Aldrich DARCO KB-B 100 Mesh powder) were dried in vacuum overnight at 100 °C to remove moisture. The pH indicator, tetrabromophenolnaphthalein ethyl ester (TBPE) was purchased from Chemika. KI bought from Aldrich was dissolved in Milli-Q water and freeze-dried overnight. Molecular Weight Distributions. Apparent molecular weights were determined by Gel Permeation Chromatography (GPC) with a Waters system using THF as an eluent and a Jordi linear DVB mixed-bed column. The instrument was coupled with a DRI detector. The column was calibrated using known molecular weight polystyrene standards. These experiments were carried out at room temperature. Purification of PIBSA. PIBSA was supplied by Imperial Oil. Characterization of PIBSA by gel permeation chromatography (GPC) was carried out in THF (Figure SI.1 in the Supporting Information). The presence of a low molecular weight impurity was observed at a high elution volume in the GPC trace (31 mL in Figure SI.1). Although information on the synthesis of PIBSA was not provided, such products are often obtained by an Alder-ene reaction.31 The impurity found in the GPC traces was assumed to be unreacted PIB. Column chromatography was used to separate PIBSA from its impurity by taking advantage of the difference in polarity between PIBSA and the nonpolar PIB. The PIBSA sample was passed through a silica gel column using a 10:1 hexane/ethyl acetate mixture as the eluant. After the apolar impurity had eluted off the column, the more polar PIBSA was flushed from the column with THF. Fractions of the eluted PIBSA were collected and their molecular weight distribution was obtained by GPC. No impurity was detected after purification via column chromatography (Figure SI.1). Synthesis of Polyisobutylenesuccinimide Dispersants. The three dispersants PIB-DETA, PIB-TEPA, and PIB-PEHA were obtained according to a similar procedure based on reacting a given number of succinic anhydrides from PIBSA with a slight molar excess of primary amines of the polyamine. The synthesis of PIB-DETA is presented in more detail as an example. Purified PIBSA (2 g, 1 mmol succinic anhydride equivalent) was dissolved in 18 mL xylene and placed into a two-neck round-bottom flask equipped with a Dean-Stark apparatus to remove water generated during the reaction. A slight molar excess of DETA (0.058 g, 0.56 mmol) was added to the PIBSA solution to ensure that all PIBSA molecules would react with a polyamine molecule. The apparatus was heated with an oil bath at 170 °C and the solution was left refluxing under nitrogen for 10 h (Scheme 1). After reaction, the reaction mixture was cooled to room temperature. Removal of unreacted polyamines was accomplished by subjecting the reaction mixture to three acid, base, and neutral washes by mixing the dispersant solution in xylene with 30 mL of 0.5 M HCl, 30 mL of 0.5 M NaHCO3, and 30 mL of Milli-Q water, respectively. The product was then precipitated with acetone, redissolved in hexanes, precipitated with acetone three times, and dried under vacuum overnight. Coupling of two PIBSA molecules with one polyamine core was evidenced by GPC analysis as the GPC peak characteristic of the dispersant shifted to smaller elution volumes in Figure SI.1. The syntheses of the dispersants were conducted either with purified or nonpurified PIBSA making sure that the molar ratio of succinic anhydride to secondary amines was kept constant in each coupling reaction. If nonpurified PIBSA was used to prepare the (30) Craig, A. M.; Linda, J. M. J. Phys. Chem. 1981, 85, 3938–3944. (31) Walch, E.; Gaymans, R. J. Polymer 1994, 35, 1774–1778.

Micellization and Adsorption of a Series of Succinimide Dispersants

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Table 1. Succinimide Content of Dispersants dispersant

absorption ratio Abs(1717 cm-1)/Abs(1390 cm-1)

Nsuccinimide/NIB

PIB-DETA PIB-TEPA PIB-PEHA

0.87 0.89 0.89

1:33 1:32 1:33

dispersant, the unreactive low-molecular weight PIB impurity was removed from the reaction mixture after the precipitation step with acetone through column chromatography according to the same procedure used to purify PIBSA. FT-IR Absorption. Since the molecular weight of PIBSA was not provided, the composition of PIBSA was determined by FTIR absorption using a Spectrum RX I PERKIN Elmer spectrophotometer. The succinic anhydride (SA) content of PIBSA was obtained by taking the ratio of the absorptions at 1785 and 1390 cm-1 in the FTIR spectrum and using these values in eq 1.31 The absorptions at 1785 and 1390 cm-1 characterize the carbonyls of the succinic anhydride moieties and the methyls of the PIB backbone, respectively.

NSA abs(1785 cm-1) ) 0.024 × NIB abs(1390 cm-1)

(1)

The NSA/NIB ratio was found to equal 0.03 ()1:33 ) 3 mol %), which, assuming that each PIBSA molecule is terminated by a single SA unit, yields an Mn value of 2070 g/mol. FT-IR was also applied to determine the succinimide content of the dispersants through a calibration curve which was generated in the laboratory. Known amounts of methyl succinimide were mixed with a matrix of polyisobutylene and their absorption was measured by FT-IR. The ratio of the absorbance at 1717 cm-1 characteristic of the succinimide carbonyls over that at 1390 cm-1 was plotted as a function of the methyl succinimide content in Figure SI.2. A straight line could be drawn through the resulting data points which relates the FT-IR absorption ratio Abs(1717 cm-1)/Abs(1390 cm-1) to the ratio of the number of moles of succinimide units (NSI) over that of isobutylene (NIB):

NSI abs(1717 cm-1) ) 0.035 × NIB abs(1390 cm-1)

Figure 1. Fluorescence spectrum of RuNH2 in a PIB-PEHA solution. (1): Spectrum 1; [PIB-PEHA] ) 5 g/L, [RuNH2] ) 4 µM. (2): Spectrum 2; Fluorescence spectrum of PIB-PEHA in hexane normalized at 510 nm. (3): Spectrum 1 - Spectrum 2. λex ) 452 nm.

(2)

The FT-IR spectra of the dispersants were acquired and their succinimide content was determined using eq 2. As shown in Table 1, the succinimide content of all dispersants equaled 1:33 ) 3.0 mol %, which is the same composition as the one obtained for the succinic anhydride content of PIBSA purified by column chromatography. This result suggests that all succinic anhydride moieties of PIBSA have reacted to form succinimide moieties. Synthesis of Ru-bpy. The probe Ru-bpy was synthesized according to an original procedure by Ellis et al.32 which was slightly modified by Quinn.33 cis-bis(Bipyridyl)dichlororuthenium(II) dihydrate (Ru(bpy)2Cl2, 100 mg, 0.2 mmol) and 5-amino-1,10phenanthroline (50 mg, 0.24 mmol) were dissolved in 5 mL of hot Milli Q water and 10 mL of hot ethanol, respectively. The two solutions were mixed and deaerated under N2 for 20 min, and then refluxed under N2 for 4 h. Most of the ethanol was removed by rotary evaporation and 1.0 mmol (162 mg) of ammonium hexafluorophosphate salt was added to the solution. The solution was cooled on an ice bath for 20 min before the precipitate was filtered and washed with Milli Q water (2 × 0.5 mL). After the crystals were dried under vacuum, they were redissolved in a minimum amount of acetone and purified with an alumina column 5 cm high × 1.5 cm wide, using a 1:2 toluene/acetonitrile mixture as eluant. The fractions were collected, dried and further purified by recrystalllization from cold diethyl ether by the dropwise addition of the dissolved (32) Ellis, C. D.; Margerum, L. D.; Murray, R. W.; Meyer, T. J. Inorg. Chem. 1983, 22, 1283–1291. (33) Quinn, C. DeVelopment of a Water-Soluble Dye/Quencher System to Study Polymer Chain Folding in Water by Fluorescence M. Sc. Thesis at the University of Waterloo, 2007.

Figure 2. Absorption spectra of (1) 0.5 g/L PIB-PEHA in hexane with 60 µM of KI and 4 µM of RuNH2; (2) 0.5 g/L PIB-PEHA in hexane with 4 µM of RuNH2; and (3) ) (1) - (2).

product in a minimal amount of acetone. The precipitate was dried under vacuum. Steady-State Fluorescence Measurements. The fluorescence spectra were obtained with a Photon Technology International LS100 steady-state fluorometer. The Ru(bpy)3 and RuNH2 solutions were outgassed for 45 min under a gentle flow of nitrogen due to the sensitivity of the long-lived dyes to quenching by oxygen. The emission spectra were acquired from 470 to 700 nm by exciting the samples at 452 nm. Ru(bpy)3 and RuNH2 were employed to probe the dispersant micelles at the molecular level by fluorescence. The probe was introduced into the dispersant solutions as follows. A stock solution (0.21 mM RuNH2 in acetonitrile or 0.25 mM Ru(bpy)3 in methanol) was prepared. An aliquot of the stock solution (∼60 mg) was weighed and added into the vial and the acetonitrile or methanol was evaporated under nitrogen. Solutions of the PIBSAbased dispersants or AOT in hexane at different concentrations were added to the dried vials and stirred overnight. After stirring for 24 h, no change in the fluorescence signal of the solution could be detected over time indicating that equilibrium had been reached. Analysis of the fluorescence spectra was complicated by the unwanted emission of the dispersants at 510 nm.34 The dispersant emission was subtracted from the spectra to quantify the emission intensity of the ruthenium complex (Figure 1). KI was used as the RuNH2 quencher. Before use in the quenching experiments, KI was dissolved in water and freeze-dried. Each quenching experiment was conducted by keeping the dispersant and RuNH2 concentrations constant while increasing the amount of quencher added to the solution. The KI concentration in the dispersant solution was determined by UV-vis absorption as follows. An aliquot of the solution containing KI, RuNH2, and the dispersant in hexane was placed in a vial and the hexane was evaporated off under a gentle flow of N2. A known amount of polar tetrahydrofuran (THF) was added to the dry film of KI, RuNH2, and dispersant to break up the reverse micelle and minimize light scattering. The absorption spectrum of that solution was acquired (Spectrum 1 in Figure 2). A similar treatment was applied to switch the solvent of the RuNH2 (34) Pucci, A.; Rausa, R.; Ciardelli, F. Macromol. Chem. Phys. 2008, 209, 900–906.

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and the dispersant solution from hexane to THF. The absorption spectrum of the resulting solution in THF was recorded (Spectrum 2 in Figure 2). Subtracting Spectrum 2 from Spectrum 1 in Figure 2 yielded the absorption spectrum of KI in THF whose concentration could be determined from its extinction coefficient equal to 6310 M-1 cm-1 in THF at 362 nm. Time-Resolved Fluorescence Measurements. The fluorescence decays of RuNH2 were obtained with a time-correlated single photon counter purchased from IBH Ltd. using a pulsed xenon flash lamp. All solutions were degassed for 45 min. under nitrogen, excited at 452 nm, and their emission was monitored at 601 nm with the right angle geometry to acquire the fluorescence decays over 1024 channels. All decays had a minimum of 15 000 counts at the decay maximum to ensure a high signal-to-noise ratio. Data Analysis of the Fluorescence Decays. The assumed fluorescence response of the solutions, g(t), was convoluted with the instrument response function L(t) to fit the experimental decays G(t) according to eq 3. Due to the existence of residual light scattering, a light scattering correction was also added to the analysis.35

G(t) ) L(t) X g(t)

(3)

The symbol X in eq 3 indicates the convolution between the two functions. The sum of exponentials shown in eq 4 was used to fit the fluorescence decays. i)n

g(t) )

∑ Aie-t⁄τ

i

(4)

i)1

The fits of the decays with eqs 3-4 were good with χ2 < 1.3 and the residuals and autocorrelations of the residuals were randomly distributed around zero. The number-average decay time was obtained i)n i)n Aiτi ⁄ ∑ i)1 Ai. as ) ∑ i)1 Scanning Electron Microscopy (SEM). SEM images of the carbon black (CB) sample were taken by using a high resolution LEO 1530 field emission scanning electron microscope. The CB sample was dried in the oven at 100 °C and mounted to a thin layer on a plug using conductivity tape. Characterization of the Carbon Black Particles. As commonly done by others,19-27 CB particles (CBPs) are used as a substitute for the carbon-rich particles (CRPs) generated in engine oil. The CBP size distribution was estimated from a scanning electron micrograph shown in Figure SI.3. The CBPs appear to have a broad size distribution with length scales ranging from 1 to 10 µm. Two procedures were applied to estimate the surface area of the CBPs. In the first procedure, the adsorption of methylene blue (MB) to CBPs was determined. As discussed in an earlier report,24 MB was used because its dimensions are closer than those of a N2 molecule used in a Brunauer-Emmett-Teller (BET) experiment to that of the polar head of a PIBSA-based dispersant. MB (0.7 g) was dissolved in 200 mL of Milli-Q water to make a stock solution. Aliquots of 5 mL MB solution were added into 10 vials to which increasing amounts (9-30 mg) of activated carbon black were added. All the samples were thoroughly shaken for 14 h until equilibrium was reached. The solutions were centrifuged and the concentration of MB in the supernatant, Ceq, was determined by UV-vis absorption using the extinction coefficient of 54 700 ( 300 dm3/mol-1 cm-1 for MB in water. The knowledge of Ceq yielded the number of adsorbed MB, nads. The ratio of nads over the mass of CBPs in the solution (m) was plotted as a function of Ceq in Figure SI.4. At high MB concentration, all sites on the surface of the CBPs available for the binding of MB are occupied and nads/m plateaus. The specific area of carbon black was calculated from the plateau value of nads/m in Figure SI.4. At saturation, the nads/m ratio equaled 0.94 mmol/g. Assuming that each MB molecule laid flat on the CB surface and occupied an area of 1.35 nm2, a CB surface area of 764 m2/g was obtained.24 The specific area of CBPs was also determined by the BET method using a GEMINI III 2375 Surface Area Analyzer. A specific area (35) Demas, J. N. Excited-State Lifetime Measurements, Academic Press, New York, 1983, pp 102-111.

Figure 3. Absorption spectra of TBPE (50 µM) in THF in the presence of dispersant PIB-PEHA. The absorption bands centered at 412 and 610 nm are characteristic of free TBPE and TBPE complexed with the dispersant, respectively. The PIB-PEHA concentrations for the absorption at 412 nm equal from top to bottom: 0.0, 0.32, 0.39, 0.45, 0.52, 0.63, 0.78, 0.89 mM.

of 1175 m2/g was found using N2 as the analysis gas. Compared to the specific area measured by the BET method (1175 m2/g), the specific area measured by MB adsorption (764 m2/g) is smaller. This difference is due to the molecular size difference between MB (1.35 nm2) and N2 (0.16 nm2) with nitrogen molecules being able to enter pores three times smaller than MB can.24 Since the polar head of the dispersant is expected to have a size more similar to that of MB than to that of N2, the specific area measured by MB adsorption is chosen as a measure of the surface area accessible to the dispersant so that A ) 764 m2/g was used in this study. Adsorption Isotherms. The construction of the adsorption isotherms of the dispersant molecules onto CBPs required the determination of the concentration of free dispersant, i.e. the dispersant not bound to CBPs. Unfortunately, the concentration of free dispersant was more complicated to obtain than that of free MB because the extinction coefficient of the dispersant was much smaller than that of MB. To circumvent this complication, an indirect method was developed by using a pH indicator tetrabromophenolphthalein ethyl ester (TBPE) as done earlier in apolar solvents.20,25 In the present study, THF was used instead of an apolar solvent to break up the dispersant aggregates and ensure that all secondary amines were exposed to the solvent. In THF, TBPE exhibits a strong absorption band at 410 nm (Figure 3). Upon the addition of the basic dispersant, the peak at 410 nm diminishes and a new peak at 612 nm appears as the acidic dye is neutralized. Interestingly for analytical purposes, the absorption of the 50 µM TBPE solution at 612 nm increases linearly with increasing dispersant concentration (Figure 4). This feature enabled the construction of a calibration curve that related the absorption of TBPE at 612 nm to the concentration of each dispersant. These calibration curves established for each dispersant in THF were used to determine the concentration of free dispersant in the CBPs solution in hexane as follows. A 2 g/L solution of dispersant in hexane was prepared and masses of 9-300 mg of CB were added. The samples were agitated for 14 h. After equilibrium had been reached, the solids were filtered through 0.2 µm Millipore filters and each sample was weighed to estimate the solution volume from the density of hexane (0.67 g/mL). Hexane in the samples was evaporated under a gentle flow of nitrogen and replaced by a same volume of a 50 µM TBPE solution in THF. The UV-vis absorption at 612 nm of the TBPE solution in the presence of dispersant yielded the free dispersant concentration, Ceq, thanks to the calibration curves given in Figure 4. The amount of adsorbed dispersant at equilibrium Γ expressed in µmol/m2 was calculated using eq 5:

Γ)

(Co - Ceq) × V m×A

(5)

where C0 and Ceq are, respectively, the initial concentration of dispersant solution and the equilibrium concentration of dispersant

Micellization and Adsorption of a Series of Succinimide Dispersants

Figure 4. TBPE absorption as a function of dispersant concentration in THF. [TBPE] ) 50 µM. (0) PIB-PEHA, ()) PIB-TEPA, (∆) PIBDETA.

in the supernatant after adsorption, V is the volume of the solution and m and A ()764 m2/g) are the mass and surface area of the CBPs, respectively.

Results The micellization of the oil-soluble dispersants PIBSA-DETA, PIBSA-TEPA, and PIBSA-PEHA and their adsorption onto CBPs were investigated in hexane. The results corresponding to each aspect of the study are presented in two separate sections hereafter. Micellization. Fluorescence is a well-established method that yields reliable quantitative information on the organization of macromolecular assemblies such as reverse micelles.36,37 The critical micellar concentration (CMC) at which dispersants start to form micelles, and Nagg, the number of dispersant molecules constituting a micelle are the two most important parameters used to characterize surfactants in terms of their micelle formation and structure. However, prior to using RuNH2 to determine the CMC and Nagg of PIBSA-based dispersants in an apolar solvent, its applicability as a probe for reverse micelles must be verified. In particular, an excited RuNH2 must be sufficiently long-lived to ensure that the quenching of RuNH2 occurs on a time scale much shorter than its own lifetime. Under such conditions, the classic Turro-Yekta procedure38 can be applied to determine Nagg of surfactant micelles. To this end, fluorescence decay measurements were conducted on RuNH2 in acetonitrile and in a dispersant solution in hexane. These decays are shown in Figure 5. RuNH2 exhibits a long-lived decay in acetonitrile. The decay of RuNH2 in a hexane solution of PIB-PEHA is bimodal with a very fast component due to the short-lived emission of the dispersant34 and a slower component due to the long-lived RuNH2. Omitting the short component in the analysis of the fluorescence decays, the decays in Figure 5 were fitted to a sum of two exponentials (n ) 2 in eq 4). The results of the biexponential fits are listed in Table 2. The number-average decay time of RuNH2 was found to equal 1.1 µs, much larger than the lifetime of other probes commonly used to study micellar systems such as pyrene (450 ns in hexane).39 Consequently, the lifetime of RuNH2 is suitable to determine the Nagg values of micelles, in (36) Gehlen, M. H.; De Schryver, F. C. Chem. ReV. 1993, 93, 199–221. (37) Silber, J. J.; Biasutti, A.; Abuin, E.; Lissi, E. AdV. Colloid Interface Sci. 1999, 82, 189–252. (38) Turro, N. J.; Yekta, A. J. Am. Chem. Soc. 1978, 100, 5951–5952. (39) Birks, J. B. Photophysics of Aromatic Molecules. Wiley: New York, 1970, p 352.

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Figure 5. Time-resolved fluorescence decays of RuNH2 in acetonitrile and in a hexane solution of the dispersant PIB-PEHA. [RuNH2] ) 4 µM, [PIB-PEHA] ) 3 g/L, λex ) 452 nm, λem ) 601 nm. Table 2. Parameters Retrieved from the Bi-exponential Fit of the RuNH2 Decays RuNH2 solution in acetonitrile in hexane with dispersant

τ1 (ns) 1030 600

a1 0.98 0.80

τ2 (ns) a

6000 3000

a2

(ns)

χ2

0.02 0.20

1130 1080

1.09 1.43

a This long decaytime might be a result of the inherent presence of a non-negligible background noise in the instrument function (Figure 5).

Figure 6. Plot of the fluorescence intensity of RuNH2 (9) and Ru(bpy)3 (0) in hexane as a function of AOT concentration. [RuNH2] ) [Ru(bpy)3] ) 20 µM.

particular, those reverse micelles made of PIBSA-based dispersants in hexane. The ability of RuNH2 to probe reliably reverse micelles in nonpolar solvents was investigated by using RuNH2 to determine the CMC of a model dispersant (AOT) and comparing it with the CMC values for AOT reported in the literature, as well as the CMC obtained with Ru(bpy)3, a well-known chromophore used to study reverse micelles.36,37 RuNH2 or Ru(bpy)3 (20 µM) were allowed to mix with AOT solutions having AOT concentrations ranging from 0.01 to 50 g/L in hexane. After equilibrium had been reached, a fluorescence spectrum of the supernatant was acquired. The fluorescence intensity at 600 nm characteristic of the ruthenium complexes was plotted as a function of AOT concentration in Figure 6. The RuNH2 and Ru(bpy)3 fluorescence intensity increased sharply at an AOT concentration of 0.4 ( 0.1 g/L (0.9 ( 0.2

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Figure 7. Plot of the fluorescence intensity of RuNH2 as a function of the concentration of PIBSA-based dispersant. (2) PIB-PEHA, (∆) PIB-TEPA, (9) PIB-DETA. [RuNH2] ) 4 µM. Table 3. Critical Micelle Concentration of AOT and PIBSA-Based Dispersants in Hexane dispersant

AOT

CMC (g/L) CMC (µmol/L)

0.4 ((0.1) 900 ((230)

PIB-DETA

PIB-TEPA

PIB-PEHA

-

0.08 ((0.01) 19 ((3)

0.05 ((0.01) 12 ((2)

mM). The break point was taken as the CMC, where either RuNH2 or Ru(bpy)3 starts to solubilize into the AOT micelles. This result indicates that both ruthenium complexes respond similarly to the presence of reverse micelles. Furthermore, a 0.4 ( 0.1 g/L (0.9 ( 0.2 mM) CMC for AOT in hexane is in good agreement with the CMC of 1.09 mM obtained for AOT in hexane by calorimetry.40 Together, these results confirm the appropriateness of RuNH2 to study reverse micelles. The CMC of the PIBSA-based dispersants was then measured in hexane with RuNH2. Plots of the RuNH2 emission as a function of dispersant concentration are given in Figure 7. Except for PIB-DETA, an increase of fluorescence intensity was observed for the dispersants and the onset concentration for this increase was taken as the CMC. The resulting CMC values are listed in Table 3. Among the three dispersants, PIB-DETA, which has the lowest amine content in the polar head core, exhibited no CMC in the measured concentration range, whereas the onset of micellization of PIB-TEPA and PIB-PEHA occurred at 0.08 g/L (19 µM) and 0.05 g/L (12 µM), respectively (Figure 7). This result suggests that the micellization of PIBSA-based dispersants is favored by a larger amine content in the polar core of the dispersant. Those CMCs are also 2 orders of magnitude smaller than that of AOT (∼1.0 mM) suggesting that PIBSA-based dispersants aggregate at low concentrations as was observed earlier with another succinimide dispersant.13 The aggregation number, Nagg, is a measure of the size of the dispersant micelles. To determine Nagg, steady-state fluorescence quenching experiments were carried out using RuNH2 as the chromophore and KI as the quencher. For a given dispersant concentration above the CMC, solutions containing the same concentration of RuNH2 were prepared with increasing concentrations of quencher. As more and more KI was added to the solution, the fluorescence intensity of RuNH2 decreased and the (40) Mukherjee, K.; Moulik, S. P.; Mukherjee, D. C. Langmuir 1993, 9, 1727– 1730.

Figure 8. Plot of Ln(Io/IQ) as a function of KI concentration for different PIB-TEPA and PIB-PEHA concentrations. PIB-TEPA: (9) 0.3 g/L, (2) 1 g/L, (b) 2 g/L; PIB-PEHA: (0) 0.3 g/L, (∆) 1 g/L, (n) 2 g/L. Table 4. Nagg of PIB-PEHA and PIB-TEPA at Concentrations of 0.3, 1.0, and 2.0 g/L in Hexane [Dispersant] Nagg PIB-TEPA PIB-PEHA

0.3 g/L

1 g/L

2 g/L

4.8 2.6

4.7 2.6

4.9 3.1

fluorescence intensity of the solution without (Io) and with KI (IQ) was recorded. Plotting Ln(Io/IQ) as a function of KI yielded straight lines as shown in Figure 8. According to Turro and Yekta,38 these trends are observed when the quenching of a chromophore located in micelles occurs with quenchers randomly distributed in the micelles according to a Poisson distribution. Accordingly, these straight lines can be fitted with eq 6

()

Ln

Io [KI] ) × Nagg IQ [Disp] - CMC

(6)

Equation 6 was applied to the trends obtained in Figure 8. The CMC values listed in Table 3 were employed to determine Nagg which is reported in Table 4. According to the results listed in Table 4, Nagg for PIB-TEPA (4.8 ( 0.1) is greater than that of PIB-PEHA (2.8 ( 0.3). Within experimental error, Nagg appears to be concentration independent for both dispersants in hexane,

Micellization and Adsorption of a Series of Succinimide Dispersants

Langmuir, Vol. 24, No. 19, 2008 10671

adsorbed onto the CB particles, followed by PIB-TEPA, and finally PIB-PEHA. The Langmuir model given in eq 7 was used to fit the data shown in Figure 9. In eq 7, Γmax and K represent the maximum amount of dispersant adsorbed per unit surface and the binding constant, respectively.

Γ)

ΓmaxKCeq 1 + KCeq

(7)

Figure 9. Adsorption isotherms of the dispersants: (2) PIB-PEHA, (∆) PIB-TEPA, (9) PIB-DETA. The lines passing through the points are the fits obtained with eq 10 using the parameters listed in Table 6.

Γmax and K were retrieved by rearranging eq 7 into eq 8. The Langmuir model fitted the data obtained with PIB-DETA well, but unfortunately it failed for PIB-TEPA and PIB-PEHA. An appreciation of the goodness of the fits can be reached in Table 5 that lists the χ2 values43 obtained from fitting the data shown in Figure 9 with the parameters retrieved from a Langmuir analysis (eq 8).

Table 5. Parameters Γmax, and K Retrieved by Fitting the Data Shown in Figure 9 with Equation 7

1 1 1 + ) Γ ΓmaxKCeq Γmax

dispersant

Γmax (mol/m2)

K (m3/mol)

χ2

PIB-DETA PIB-TEPA PIB-PEHA

18 ((2) × 10-8 9 ((1) × 10-8 6 ((4) × 10-8

41 ((7) 130 ((20) 150 ((100)

9 × 10-5 2 × 10-4 5 × 10-4

at least over the range of dispersant concentrations studied (0.3-2 g/L). It suggests that the micellization of the dispersants takes place according to a close mechanism41 as found earlier for other succinimide dispersants.15 Nagg for the PIBSA-based dispersants was much smaller than the Nagg values typically found for watersoluble surfactants, in agreement with an earlier report14 and as usually observed for reverse micelles in apolar organic solvents.42 As pointed out by an unknown referee, the small Nagg values found in this report suggest that the concentration of unassociated dispersant might be somewhat larger than the CMC. In turn, this might affect the Nagg values which were found by using eq 6 that assumes that the concentration of unassociated dispersants equals the CMC. Nevertheless, this complication does not affect the main conclusion of this study, that the dispersant reverse micelles are small, much smaller than the micelles of surfactants in water. Adsorption. Solutions of dispersants in hexane were prepared with CBPs. After reaching equilibrium, the solutions were filtered letting through free dispersant, i.e., dispersant not bound to the CBPs. The free dispersant solution was placed under a gentle flow of N2 to evaporate hexane. The dry film of dispersant was solubilized in a TBPE (50 µM) solution in THF. The absorption of the TBPE solution with dispersant was measured and it was converted into a dispersant concentration thanks to the calibration curves established for each dispersant (Figure 4). It allowed the original concentration of free dispersant in the CB solution in hexane to be determined. The knowledge of the concentration of free dispersants in the CB solution enabled the construction of the binding isotherms of PIB-DETA, PIB-TEPA, and PIBPEHA shown in Figure 9, where the number of molecules of dispersant adsorbed per unit surface of CB (Γ) was plotted as a function of the concentration of free dispersant in solution (Ceq), i.e., the concentration of dispersant that is not adsorbed onto CB particles. For all dispersants, the amount of dispersant adsorbed onto the CBPs increased as more dispersant was added to the solutions, although the increase was not as pronounced at higher dispersant concentrations. For a given concentration of free dispersant, PIB-DETA had the largest amount of dispersant (41) Gourier, C.; Beaudoin, E.; Duval, M.; Sarazin, D.; Maıˆtre, S.; Franc¸ois, J. J. Colloid Interface Sci. 2000, 230, 41–52. (42) Ruckenstein, E.; Nagarajan, R. J. Phys. Chem. 1980, 84, 1349–1358.

(8)

One explanation for the failure of eq 8 to fit the data obtained with PIB-TEPA and PIB-PEHA could be the existence of two binding sites on CBPs, as has been suggested in the literature for other PIBSA-based dispersants.20,21,25 That two binding regimes might be at play can be inferred from Figure 10 where 1/Γ is plotted as a function of 1/Ceq. In Figure 10, a straight line is obtained for PIB-DETA, as predicted by eq 8, but for PIBTEPA and PIB-PEHA, a distinct kink can be seen in the plots at a 1/Ceq value of about 0.08-0.10 m3/mmol. A dual-site Langmuir model was then introduced by considering that the binding of PIBSA-based dispersants onto CBPs took place according to two binding regimes.25 According to the dual-site Langmuir model, the adsorption was described by two equilibrium constants K1 and K2 with respective maximum numbers of dispersant adsorbed per unit surface Γ1 and Γ2 (eq 9).

Γ)

Γ1K1Ceq Γ2K2Ceq + 1 + K1Ceq 1 + K2Ceq

(9)

Attempts to fit the data in Figure 9 with eq 9 led to the conclusion that K2 was too small to be recovered with accuracy. Consequently eq 9 was approximated to eq 10 that yielded the parameters K1, Γ1 and the product Γ2K2 listed in Table 6.

Γ≈

Γ1K1Ceq + Γ2K2Ceq 1 + K1Ceq

(10)

According to the χ2 values retrieved from fitting the data in Figure 9 with eq 10, the quality of the fits did not improve much when the data for PIB-DETA in Figure 9 were fitted with eq 10. This result confirmed that a single binding site (i.e., eq 7) is sufficient to handle the binding of PIB-DETA to the CB particles. On the other hand, the χ2 values decreased substantially when the data for PIB-TEPA and PIB-PEHA in Figure 9 were fitted with eq 10. This result suggested that the binding of dispersants having a larger number of secondary amines to CB particles occurs via two binding mechanisms with K1 being substantially larger than K2. Fitting the trends in Figure 9 with fixed K2 values led to the conclusion that K2 must be at least 10 times smaller than K1.44 Moreover, the results in Table 6 indicated that as the (43) The following expression was used for χ2: χ2 ) 1/(Ndata - Nparameter)∑i [Γexp(Ceq) - Γc(Ceq)]2 where Ndata and Nparameter represent the number of data points and independent parameters used to fit the data, respectively; Γexp and Γexp are the experimental and calculated amounts of dispersant adsorbed on the CBP surface, respectively. (44) Shen, Y. Synthesis and Characterization of Oil-Soluble Dispersants, M. Sc. Thesis at the University of Waterloo, 2007. ) 1Ndata

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Table 6. Parameters K1, Γ1, and Γ2K2 retrieved by fitting the data shown in Figure 9 with Equation 10a

a

dispersants

Γ1 (mol/m2)

PIB-DETA PIB-TEPA PIB-PEHA

17 ((10) × 6 ((2) × 10-8 3 ((0.4) × 10-8

10-8

K1 (m3/mol) 43 ((22) 193 ((70) 336 ((70)

Γ2K2 (m) 1 ((0.1) × 7 ((4) × 10-7 5 ((1) × 10-7

10-7

R (nm) 1.8 2.9 4.0

χ2 10 × 10-5 1 × 10-5 1 × 10-5

R is the radius of the disk on the CB particle covered by one dispersant molecule.

Figure 10. Plot of 1/Γ as a function of 1/Ceq for (2) PIB-PEHA, (∆) PIB-TEPA, (9) PIB-DETA.

number of secondary amines in the polar core of the dispersant increased, the binding constant K1 increased and the amount of dispersant needed to saturate the stronger adsorption sites given by Γ1 decreased. The trends obtained with Γ1 and K1 imply that the binding of the dispersant is more efficient when the dispersant contains a longer polyamine linker.

Discussion The PIBSA-based dispersants that formed micelles in hexane were found to self-assemble at dispersant concentrations lower than 20 µmol/L. This result suggests that the polyamine core of the PIBSA-based dispersants is much more efficient at inducing self-assembly than the ionic sulfonate core of AOT since the CMC is close to 1 mM for AOT. Furthermore, the associative strength of the dispersants increases with the amine content of the polyamine core. No CMC could be detected for PIB-DETA in hexane at dispersant concentrations lower than 1 g/L (0.24 mM). An increase of the associative strength of the dispersants is reflected in a lower CMC as found when comparing the CMC of PIB-TEPA (CMC ) 80 mg/L ) 19 µmol/L) to that of PIBPEHA (CMC ) 50 mg/L ) 12 µmol/L). Nagg for PIB-TEPA and PIB-PEHA remained constant within experimental error and equal to 4.8 ( 0.1 and 2.8 ( 0.3, respectively. The dispersant micelles are small with their core made of a similar number of secondary amines, 14 and 11 for PIB-TEPA and PIB-PEHA, respectively. The adsorption isotherms obtained for the adsorption of PIBTEPA and PIB-PEHA onto CBPs could not be fitted by the classic Langmuir isotherm (eqs 7 and 8). A kink was found in the isotherms shown in Figure 10 for free dispersant concentrations around 0.04-0.12 g/L (10-25 µmol/L). This result is reasonable since a similar observation had been made earlier for the adsorption of other succinimide dispersants onto CBPs.20,21,25 Since a deviation from a Langmuir isotherm reflects a change in the binding regime, it has been suggested that two binding sites are present at the surface of CBPs so that the binding isotherms should be fitted with a dual-site Langmuir isotherm.25 Fitting our data for PIB-TEPA and PIB-PEHA in Figure 9 with a dual-site Langmuir isotherm (eq 9) improved the fits substan-

tially giving substance to the claim that binding of PIBSA-based dispersants onto CBPs occurs via two binding mechanisms. Currently the existence of a second binding mechanism is assumed to result from the presence of a weak and a strong binding site at the surface of CBPs.25 However, the data gathered in Table 3 suggest that another rationale could account for the change in binding regime observed for PIB-TEPA and PIBPEHA around 10-25 µmol/L in Figure 10 and the absence of a change of binding regime observed for PIB-DETA. On the one hand, analysis of the data in Figure 9 with the Langmuir isotherm (eq 8) works well for PIB-DETA which does not form micelles in the range of dispersant concentration over which the binding isotherms were constructed. On the other hand, the change in binding regime observed for PIB-TEPA and PIB-PEHA occurs when the free dispersant concentration becomes close to the CMC of those dispersants (see Table 3). This observation leads to a novel interpretation for the change in binding regime, namely that micellization of the dispersants impedes binding of the PIBSA-based dispersants to the CBPs. Micelle formation minimizes the exposure of the dispersant polar heads to the apolar solvent, decreases their drive to bind to the CBP surface, resulting in a change in the binding regime. This second interpretation of the trends shown in Figure 9 presents the advantage that a second binding site on the CBP surface is not required to rationalize the binding isotherms. The kink in the binding isotherms shown in Figure 10 reflects simply the competition that exists between the binding of the dispersants to the CBPs and their micellization. The value of the maximum surface coverage, Γ1, listed in Table 6 can be used to determine the radius, R given in Table 6, of the disk of surface πR2 covered by one dispersant molecule. R was found to increase with increasing number of secondary amines in the core. This effect suggests that, as the number of secondary amines in the core increases, the polar head of the dispersant becomes more strongly anchored onto the CBP surface as suggested by the higher K1 values which enables the PIB tails to better cover a larger area of the CBP surface.

Conclusions The aggregation of a series of PIBSA-based dispersants in hexane was investigated by fluorescence. Except PIB-DETA which did not exhibit a CMC in the range of dispersant concentrations studied, it was found that increasing the number of secondary amines in the polyamine core led to an increase of the association strength of the dispersant as the CMC took place at a smaller dispersant concentration. Fluorescence quenching experiments using RuNH2 and KI were conducted to determine Nagg. PIB-TEPA was found to form larger micelles than PIBPEHA. For both dispersants, Nagg was independent of dispersant concentration, suggesting that the dispersant micelles formed according to a close mechanism in hexane. The adsorption of the dispersants onto CBPs was characterized. Two types of adsorption mechanisms needed to be introduced to properly fit the adsorption isotherms for the dispersants with higher secondary amine content. The binding equilibrium constant, K1, and maximum surface coverage, Γ1, corresponding to the stronger binding sites were determined experimentally. K1 was found to increase with increasing number of secondary amines

Micellization and Adsorption of a Series of Succinimide Dispersants

in the polar core, whereas Γ1 decreased. The results suggest that increasing the number of secondary amines in the polar core of the dispersant enables a stronger anchoring of the dispersant onto the CB particle surface which leads to a better coverage of the particle surface. From an application point of view, it appears that a greater number of secondary amines in the polar core of the dispersant results in a more efficient dispersant. Combining the CMC results in Table 3 with the trends observed for the binding isotherms in Figure 9 led to an alternate interpretation to account for the second binding regime observed for PIB-TEPA and PIB-PEHA. It was suggested that the second binding regime could be due to the formation of dispersant micelles which would compete with the adsorption of the dispersants onto the CBP surface. This suggestion provides an

Langmuir, Vol. 24, No. 19, 2008 10673

alternative to the assumption made earlier that the second binding regime is due to a weaker binding site at the surface of CBPs.25 It remains to be seen whether further experiments can be conducted that can establish the validity of one explanation over the other. Acknowledgment. J.D. and Y.S. are indebted to generous funding from Imperial Oil and NSERC. Supporting Information Available: GPC traces for the dispersants; calibration curve for the succinimide content of the dispersants; electron micrograph of the CBPs; adsorption isotherm of MB onto CBPs. This material is available free of charge via the Internet at http://pubs.acs.org. LA801416A