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Thermodynamics of Micelle Formation of the Counterion Coupled Gemini Surfactant Bis(4-(2-dodecyl)benzenesulfonate)-Jeffamine Salt and Its Dynamic Adsorption on Sandstone Annamária B. Páhi,† Zoltán Király,*,† Ágnes Mastalir,‡ József Dudás,§ Sándor Puskás,| and Árpád Vágó| Department of Colloid Chemistry, UniVersity of Szeged, Aradi Vt. 1, H-6720 Szeged, Hungary, Department of Organic Chemistry, UniVersity of Szeged, Dóm tér 8, H-6720 Szeged, Hungary, Research Institute of Chemical and Process Engineering, UniVersity of Pannonia, Egyetem u. 10, H-8200 Veszprém, Hungary, and MOL Hungarian Oil and Gas Plc, E&P, New Technologies and R&D, P.O. Box 37, H-6701 Szeged, Hungary ReceiVed: July 23, 2008; ReVised Manuscript ReceiVed: September 24, 2008
A novel counterion-coupled gemini (cocogem) surfactant, DBSJ, was synthetized via the 2:1 coupling reaction between 4-(2-dodecyl)benzenesulfonic acid (Lutensit A-LBS) and polypropyleneglycol-bis(2-aminopropyl) ether (Jeffamine D230). The surfactant had a polydispersity index of Mw/Mn ) 1.04, as determined by electrospray-ionization mass spectrometry. The micellar properties of DBSJ in water were investigated in the temperature range 283-348 K by conductometry and titration microcalorimetry. The critical micelle concentration (cmc) of the cocogem was found to be more than 1 order of magnitude less than that of monomeric sodium 4-(2-dodecyl)benzenesulfonate (SDBS). The mean degree of dissociation in the temperature range studied proved to be R ) 0.39. The calorimetric enthalpies of micelle formation agreed well with the enthalpies calculated via the van’t Hoff relation. The cmc versus T curve passes through a minimum just below room temperature, after which the micelle formation changes from endothermic to exothermic. The Gibbs free energy of micelle formation was nearly constant as the temperature was increased, due to enthalpy/ entropy compensation. The isotherm for DBSJ adsorption from aqueous solution onto sandstone was determined by continuous flow frontal analysis solid/liquid chromatography at 298 K and 60 bar. The adsorption of DBSJ on sandstone followed an S-type isotherm. Surface aggregation occurred over an extended range of concentration. Surface saturation was reached at a solution concentration more than 1 order of magnitude less than for monomeric SDBS. This finding is a point of concern in the chemical flooding of oil reservoir rocks to enhance oil recovery. Introduction Gemini (dimeric) surfactants are composed of two amphiphilic moieties having the structure of conventional (monomeric) surfactants connected by a spacer group close to the head groups.1-3 To date, various kinds of gemini surfactants have been synthetized (anionic, cationic, zwitterionic, and nonionic) with a large variety of spacer groups (hydrophilic or hydrophobic, short or long, rigid or flexible). Because of their highly effective performance, gemini surfactants are of potential use in the chemical industry for the production of pharmaceuticals, cosmetics, household materials, medical science and agricultural chemicals, and in enhanced oil recovery (EOR).1-3 Gemini surfactants possess advantages which are superior to those of their single-chain counterparts, including greater interfacial activities and much lower critical micelle concentrations (cmcs). For hydrophilic spacers, the cmc increases slightly with progressive spacer length.4,5 For hydrophobic spacers, the cmc passes through a weak maximum with increasing spacer length.6-9 For a given spacer, the cmc decreases with increase in the length of the alkyl chains.10,11 Calorimetric studies have * To whom correspondence should be addressed. E-mail: zkiraly@ chem.u-szeged.hu. † Department of Colloid Chemistry, University of Szeged. ‡ Department of Organic Chemistry, University of Szeged. § University of Pannonia. | MOL Hungarian Oil and Gas Plc.
indicated that micelle formation becomes more exothermic with increasing alkyl chain length11 and rising temperature.7,9 The enthalpy of micellization displays a minimum with increase of the spacer length.4,8,9 The adsorption of cationic gemini surfactants from water onto hydrophilic silica and silicate surfaces has been extensively studied.12-17 The initial, direct adsorption of the organic cations onto the negatively charged surface is followed by the formation of surface aggregates via a co-operative adsorption mechanism until surface saturation is reached close to the cmc. The maximum surface coverage decreases with increasing spacer length. AFM studies have indicated the formation of bilayers, spherical, or cylindrical surface aggregates, depending on the surface charge density of the underlying substrate.17,18 The surface micelles become more flattened as the spacer length of the amphiphile increases. Little attention has been paid so far to the adsorption of anionic gemini surfactants at solid/solution interfaces. It has been reported that the amount of disodium didecyldiphenyl ether disulfonate adsorbed on limestone is about half-that of monosodium monodecyldiphenyl ether monosulfonate.15 The extents of adsorption of both the monomeric and the dimeric derivatives from aqueous solution onto sand have been found to be negligible.15 However, conventional anionic surfactants, such as sodium alkylbenzene sulfonates, display appreciable adsorp-
10.1021/jp806522h CCC: $40.75 2008 American Chemical Society Published on Web 11/07/2008
Bis(4-(2-dodecyl)benzenesulfonate)-Jeffamine Salt
J. Phys. Chem. B, Vol. 112, No. 48, 2008 15321
Figure 1. The synthesis of bis(4-(2-dodecyl)benzenesulfonate)-Jeffamine salt (DBSJ) via the 2:1 coupling reaction between 4-(2-dodecyl)benzenesulfonic acid (Lutensit A-LBS) and polypropyleneglycol-bis(2-aminopropyl) ether (Jeffamine D230).
tion on sandstone.19,20 This is a matter of concern in surfactant flooding as a method of EOR from partially depleted reservoir rocks.19-23 In counterion-coupled gemini surfactants (cocogems), two surfactant tails are bound via a geometrically well-defined functional counterion.24 The present study describes an economical synthesis of a novel cocogem surfactant, bis(4-(2dodecyl)benzenesulfonate)-Jeffamine salt (DBSJ). The exact composition and the polydispersiy of DBSJ were determined by electrospray-ionization mass spectrometry (ESI-MS). The thermodynamics of micelle formation of DBSJ in water was investigated over a range of temperature on the basis of conductometric and thermometric titration experiments. The dynamic adsorption of the cocogem from aqueous solution onto sandstone was investigated by continuous flow frontal analysis solid/liquid chromatography.25,26 The adsorption isotherm was compared with those for two closely related monomeric surfactants, sodium 4-(2-decyl)benzene-19 and sodium 4-(2dodecyl)benzenesulfonates.20 The micellar behavior and the adsorption properties of surfactant solutions play important role in EOR applications. Experimental Section Materials. 4-(2-Dodecyl)benzenesulfonic acid (Lutensit ALBS) was a product of the BASF Group and had a quoted purity of 97%. Polypropyleneglycol-bis(2-aminopropyl) ether (trade name Jeffamine D230) was a product of Huntsman Corporation and had a quoted purity of 97%. The sandstone was a sedimentary rock from the Algyo˝ oil field, Hungary. A piece of this sandstone was crushed and fractionated, and the sieve fraction 100-250 µm was selected for use. The BET specific surface area of the sandstone was 2.7 m2 g-1 to N2 at 77 K, as determined with a Micromeritics Gemini 2375 gas sorption apparatus. The percentage elemental composition of the sample was determined by energy-dispersive X-ray analysis (EDX; ¨ NTEC QX2) to be Si ) 24.0, O ) 72.4, Al ) 1.4, Na ) RO 1.0, Mg ) 0.8, Fe ) 0.2, K ) 0.1, and Ca ) 0.1. Ultrapure water with a typical resistivity of greater than 18 MΩ cm was produced with a Milli-Q purification system (Millipore). Surfactant solutions were made up volumetrically. Surfactant Synthesis. DBSJ was synthetized via the endcoupling reaction between the difunctional primary amine Jeffamine D230 and Lutensit A-LBS. The dropwise addition of the amine to the acid under stirring was conducted in methanol solution at 273 K until neutralization was reached (pH ) 7). The progress of the reaction was also monitored by thinlayer chromatography (silica plate; 5% methanol in chloroform). After the completion of the reaction, the solvent was removed by rotary evaporation. The product, a dark-red waxy material, was dried and stored in a vacuum desiccator until use. The onespot synthesis of DBSJ is outlined in Figure 1. The surface
tensions of a set of surfactant solutions were determined at 298.15 K by using a Du Nou¨y tensiometer (Kru¨ss K10T). The O-ring method provided a surface tension of 28.8 mN m-1 at a nominal cmc of 0.065 mM. The interfacial tension at the water/ gasoline interface was determined by using a spinning drop tensiometer (Kru¨ss SITE 04) to be 0.039 mN m-1 at the cmc. Methods. Mass Spectrometry. The molar mass distribution of the polydisperse DBSJ sample was determined by ESI-MS27,28 (Agilent 6120). The mobile phase was Milli-Q water percolated at a flow rate of 0.33 mL min-1 by using an HPLC pump (Agilent 1200). Ten microliters of sample solution (2 ppm in water) was introduced into the spray chamber by using an electric injection valve. The spraying conditions were the following: capillary voltage, 3 kV; ionization temperature, 573 K; drying gas (N2) flow rate, 10 mL min-1; nebulizer pressure, 1810 Torr. Positively and negatively charged analyte species were generated in consecutive runs by applying positive and negative potentials, respectively, on the electrospray needle facing the counter electrode. The ESI source was connected to the quadrupole analyzer of the MS, which in turn was connected to the multichannel detector, both held under a pressure of less than 5 × 10-5 Torr. The mass range was scanned from 100 to 1500 Da. Solution ConductiWity. Specific conductance measurements were performed in the temperature range 288.15-328.15 K with a PC-interfaced OK-114 conductometer (Radelkis) equipped with an OK-0907P platinum electrode with sheet plates, having a cell constant of κcell ) 1.0 cm-1. The glass, double-walled measuring vessel was fitted with a magnetic stirrer and thermostatted at T (0.05 K. Twenty-five milliliters of distilled water was titrated with a concentrated surfactant solution (c . cmc) in 1 min intervals in aliquots of 0.5 mL by using a PC-controlled Titroline-96 volumetric titrator (Schott AG). Titration Microcalorimetry. Thermometric titration experiments were performed in the temperature range 288.15-348.15 K with a computer-controlled VP-ITC power-compensation microcalorimeter (MicroCal). Water (1.4 mL) in the sample cell was titrated under constant stirring with 280 µL of concentrated surfactant solution (about 10 times the cmc) in aliquots of 10 µL in periodic time intervals of 5 min. The heat evolved or absorbed during the stepwise dilution experiment was recorded in the form of a series of calorimeter peaks. The enthalpogram (calorimeter power signal vs time) was evaluated by means of Origin Microcal 7.1. software. Dynamic Adsorption Measurements. The experimental setup of the flow adsorption apparatus is outlined in Figure 2. This is an improved version of that applied in our previous studies.25,26,29 Surfactant solutions were prepared by mixing a stock solution (L2) with distilled water (L1) in the desired proportions, controlled by an HPLC pump (Agilent 1200). The binary pump was supplied with a low-dead-volume mixing chamber (BPM).
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Figure 2. Experimental setup of continuous flow frontal analysis solid/ liquid chromatography. L1, eluent reservoir (water); L2, surfactant stock solution (0.2 mM); D, online solvent degasser ( 0 and ∆micH > 0), after which both the entropy and enthalpy terms favor micelle formation (T∆micS > 0 and ∆micH < 0). Linear extrapolations suggest that micellization becomes purely enthalpically driven (T∆micS < 0 and ∆micH < 0) above 370 K. Figure 8 illustrates well the delicate balance between the change in enthalpy and the change in entropy as the temperature is raised: large changes in the two quantities result in a moderate decrease in the Gibbs free energy due to
Bis(4-(2-dodecyl)benzenesulfonate)-Jeffamine Salt
Figure 9. Break-through curves of the adsorption of a series of aqueous DBSJ solutions on Algyo˝ sandstone recorded by the UV detector (λ ) 225 nm). Adsorption is followed by elution with pure water for each step.
Figure 10. Isotherm of adsorption aqueous DBSJ solution onto Algyo˝ sandstone as compared with the isotherms of adsorption of aqueous SDeBS solution onto Bentheim sandstone19 and of SDBS solution onto Fontainebleau sandstone.20 Solid symbols, adsorption; empty symbols, desorption. Solid lines are drawn to guide the eye.
enthalpy/entropy compensation. At a molecular level, micelle formation is promoted by van der Waals interactions between the hydrophobic moieties of the amphiphilic molecules (favored by enthalpy) and the concomitant release of structured hydration water into the bulk (favored by entropy). The extensive hydrogen-bonding in water gradually breaks down with increasing temperature, the importance of the entropic term of hydrophobic hydration therefore decreases, and the dispersion interactions become increasingly predominant.36,43,44 A further explanation at a molecular level would demand a knowledge of the mode of packing of the DBSJ molecules in the micelles. Such an analysis would require the use of structure-sensitive experimental methods, for example, small-angle X-ray or neutron scattering. However, the present study is concerned with phenomenological thermodynamics rather than with microscopic details. The Dynamic Adsorption of DBSJ from Water onto Sandstone. The break-through curves of aqueous DBSJ solutions are displayed for a series of concentration steps in Figure 9. Adsorption was followed by elution before the next step. The concentration profiles were evaluated by using eqs 1 and 2 and the adsorption isotherm Γ versus c was then constructed, as shown in Figure 10. The isotherm of the adsorption of DBSJ from water onto Algyo˝ sandstone proved to be reversible within experimental error. For comparison, the isotherm for the adsorption of sodium decylbenzenesulfonate (SDeBS) on Bentheim sandstone, reported by van Os and Handrikman,19 and that for sodium dodecylbenzenesulfonate (SDBS) on Fontaineb-
J. Phys. Chem. B, Vol. 112, No. 48, 2008 15325 leau sandstone,20 reported by Rouquerol and Partyka, are also included in Figure 10. These authors investigated the adsorption properties of SDeBS and SDBS in relation to their applications in EOR. All three isotherms in Figure 10 are S-shaped. Each isotherm gradually turns from convex to concave through an inflection, which is typical for the aggregative adsorption of surfactants on hydrophilic surfaces.25,26 At low solution concentrations, isolated surfactant monomers are adsorbed on the surface of the sandstone. We suggest that anchoring to the negatively charged surface occurs through diammonium linkages. As the solution concentration is increased, the surface concentration gradually increases via a co-operative adsorption mechanism, facilitating thereby the formation of surface aggregates. Along the rising section of the isotherms, flattened globular surface micelles or a surfactant bilayer are formed 17,18,45 until the adsorption levels off as the plateau is reached close to the cmc. Figure 10 depicts the surface concentration on a reduced concentration scale, c/cmc. The difference in the cmc values of the three surfactants covers nearly 2 orders of magnitude: 4.63 mM for SDeBS, 1.72 mM for SDBS,46 and 0.061 mM for DBSJ. The plateau values, Γmax, are 2.60, 1.65, and 1.45 µmol m-2 for SDeBS, SDBS, and DBSJ, respectively. The course of the isotherm of SDBS is similar both qualitatively and quantitatively to that of DBSJ. SDBS is a single-chain anionic surfactant which may be regarded as the monomeric species of the gemini DBSJ. Although the plateau values of the two surfactants are similar, the molar mass of DBSJ is more than twice of that of SDBS. It follows that the adsorption layer is more densely packed for the cocogem. An adequate explanation is that DBSJ forms a full bilayer, while SDBS forms a patchy bilayer. Alternatively, the number of globular surface aggregates is larger for DBSJ than for SDBS. Comparison of the cmc values of the three closely related surfactants reveal that the cmc, and therefore the amount of surfactant required to reach surface saturation in a flow system, is far less for DBSJ than for SDeBS and SDBS. From this aspect, the cocogem DBSJ appears to be a more suitable candidate for EOR applications than the single-chain alkylbenzenesulfonate derivatives. Conclusions A novel cocogem surfactant, DBSJ, was synthetized via a simple and economical route. The number-average molar mass of the surfactant was determined by ESI-MS to allow calculation of various physicochemical parameters on a per-mol-of-surfactant basis. The cmc passes through a shallow minimum close to room temperature, as determined by conductometric and thermometric titrations. The degree of dissociation of DBSJ displays no significant variation in the temperature range studied. The van’t Hoff enthalpies of micellization agree well with the calorimetric enthalpies. Micelle formation is favored by entropy below the cmc minimum, by enthalpy at elevated temperatures, and by both entropy and enthalpy at intermediate temperatures. The Gibbs free energy decreases only slightly with increase of temperature, due to enthalpy/entropy compensation. The adsorption of DBSJ from aqueous solution onto sandstone is aggregative; the isotherm is S-shaped, as determined by flow frontal analysis solid/liquid chromatography. The maximum amount of DBSJ adsorbed is attained at the cmc, which is markedly less than the cmc of alkylbenzenesulfonate monomers. This finding is of interest from the viewpoint of EOR. Acknowledgment. This work was supported by MOL Hungarian Oil and Gas Plc, E&P, New Technologies and R&D, the Bolyai Ja´nos Foundation, and the Hungarian Scientific Research Fund (OTKA K68152).
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