pubs.acs.org/Langmuir © 2010 American Chemical Society
Cationic Ester-Containing Gemini Surfactants: Physical-Chemical Properties A. R. Tehrani-Bagha†,‡ and K. Holmberg*,† †
Department of Chemical and Biological Engineering, Chalmers University of Technology, SE-412 96 G€ oteborg, Sweden, and ‡Institute for Color Science and Technology, Tehran, Iran Received January 11, 2010. Revised Manuscript Received March 11, 2010
Three ester-containing cationic gemini surfactants, two with decanoyl chains and either a three-carbon or a six-carbon spacer unit and one with dodecanoyl chains and a three-carbon spacer, were synthesized and evaluated. A corresponding monomeric cationic ester surfactant was used for comparison. This type of amphiphile, a so-called esterquat, is known to undergo rapid hydrolysis above the critical micelle concentration because of micellar catalysis. The esterquat geminis of this work were found to be much more susceptible to hydrolysis than the esterquat monomer. This difference is believed to be caused by anchimeric assistance by the second cationic headgroup in the gemini amphiphiles. However, there is no correlation between the rate of chemical hydrolysis and the rate of biodegradation. The monomeric esterquat, which is the most stable in the chemical hydrolysis experiments, was the only surfactant that passed the test for “readily biodegradable”. We also observed a considerable difference in the hydrolysis rate within the small series of gemini surfactants. The amphiphile with two decanoyl chains and a three-carbon spacer, N,N0 -bis(2-(decanoyloxy)ethyl)-N,N, N0 ,N0 -tetramethyl-1,3-propanediammonium dibromide, had the fastest rate of hydrolysis. This surfactant also exhibited a considerably lower degree of micelle ionization than the other surfactants, which is believed to be due to the closer proximity of the charged groups on the micelle surface. A small distance between headgroups will give more pronounced neighboring group participation, accounting for the increased rate of hydrolysis. An interesting property of the surfactant that is the most susceptible to hydrolysis is that it gives rise to an extremly stable foam. We propose that the foam stability is a result of the partial hydrolysis of the surfactant generating sodium decanoate, an anionic surfactant, that forms a mixed film with the starting cationic gemini surfactant. It is known that mixed monolayers in which there is a strong attractive interaction between surfactant headgroups can lead to stable foams.
1. Introduction The search for novel surfactants with higher efficiency and effectiveness gave birth to the concept of gemini surfactants. Gemini surfactants are a new generation of surfactants composed of two monomeric surfactant molecules chemically bonded together by a spacer at or near their headgroups. Thus, gemini surfactants possess two hydrophilic and two hydrophobic groups. They are more surface-active and have much lower critical micelle concentration (cmc) values than their monomeric counterparts.1-3 Because of their unique physical-chemical properties, gemini surfactants continue to gain widespread interest in the scientific community and for various applications.4,5 Environmental concern has become one of the major driving forces for the development of new surfactants, and aquatic toxicity and the rate of biodegradation have become major issues. One of the main approaches to designing readily biodegradable surfactants is to insert a bond with limited stability between the *Corresponding author. E-mail:
[email protected]. Tel: þ46 31 772 2969. Fax: þ46 31 16 0062.
(1) Menger, F. M.; Littau, C. A. J. Am. Chem. Soc. 1991, 113, 1451. (2) Zana, R.; Benrraou, M.; Rueff, R. Langmuir 1991, 7, 1072. (3) Rosen, M. J. CHEMTECH 1993, 23, 30. (4) Zana, R. Adv. Colloid Interface Sci. 2002, 97, 203. (5) Menger, F. M.; Littau, C. A. J. Am. Chem. Soc. 1993, 115, 10083. (6) Overkempe, C.; Annerling, A.; vanGinkel, C. G.; Thomas, P. C.; Boltersdorf, D.; Speelman, J. Esterquats. In Novel Surfactants: Preparation, Applications, and Biodegradability; Holmberg, K., Ed.; Marcel Dekker: New York, 2003; p 347. (7) Stjerndahl, M.; Lundberg, D.; Holmberg, K. Cleavable Surfactants. In Novel Surfactants: Preparation, Applications, and Biodegradability; Holmberg, K., Ed.; Marcel Dekker: New York, 2003; pp 317. (8) Tehrani-Bagha, A. R.; Holmberg, K. Curr. Opin. Colloid Interface Sci. 2007, 12, 81.
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polar headgroup and the hydrophobic tail of the surfactant. Such surfactants are often referred to as cleavable surfactants.6-8 Cationic surfactants, with a consumption of around 700 000 tons per year, have many applications, such as fabric softeners, asphalt additives, corrosion inhibitors, biocides, textile auxiliaries, and so forth. They adsorb with a very high substantivity on various substrates, and by doing so, they change the surface properties, which is important in many applications.9-11 However, cationic surfactants in general have higher aquatic toxicity than other surfactants and they are more irritating to the skin and to the eyes. The toxicity of these surfactants is believed to result from their tendency to interact strongly with negatively charged surfaces, including the lipid membranes of biological cells.11,12 Among all cationic surfactants, the so-called esterquats are the most important because they dominate the fabric softener market today. Esterquats were described in the patent literature in the 1930s, and an early use of esterquats was as textile auxiliaries. The breakthrough for esterquats in fabric care applications came in 1991, when European detergent manufacturers reformulated their rinse cycle softeners because of pressure from environmental authorities.6 An attractive feature of the esterquats is their decomposition upon hydrolysis into fatty acid salts and a (9) Steichen, D. S. Cationic Surfactants. In Handbook of Applied Surface and Colloid Chemistry; Holmberg, K., Ed.; Wiley: New York, 2001; Vol. 1, p 309. (10) Steichen, D. S.; Gadberry, J. F. Current Developments in Cationic Surfactants. In New Products and Applications in Surfactant Technology; Karsa, D. R., Ed.; Sheffield Academic Press: Sheffield, England, 1998; p 59. (11) Cross, J. Environmental Aspects of Cationic Surfactants. In Cationic Surfactants; Cross, J., Singer, E. J., Eds.; Marcel Dekker: New York, 1994; p 3. (12) Huber, L.; Nitschke, L. Environmental Aspects of Surfactants. In Handbook of Applied Surface and Colloid Chemistry; Holmberg, K., Ed.; Wiley: Chichester, England, 2002; Vol. 1, p 509.
Published on Web 04/13/2010
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small non-surface-active moiety, which are both readily biodegradable.13,14 The purpose of this work was to prepare and characterize cleavable cationic gemini surfactants with ester bonds inserted between the hydrocarbon tails and the positively charged headgroups and to evaluate their physical-chemical properties. The susceptibility of the esterquat gemini surfactants to chemical hydrolysis was investigated, and their rate of biodegradation was determined. The values obtained were compared to those of the corresponding monomeric surfactants.
2. Experimental Section 2.1. Materials. Decanoyl chloride, 2-(N,N-dimethyl)aminoethanol, bromoacetyl bromide, dichloromethane, acetone, 1,3dibromopropane, and 1,6-dibromohexane were all purchased from Sigma-Aldrich. Dodecyltrimethylammonium bromide was obtained from Fluka. Deuterium oxide (99.8 atom % D) and 1 M DCl in D2O were purchased from Dr. Glaser AG (Basel, Switzerland). Methyl bromide (99%) was from Air Liquid Co.
2.2. Syntheses. 2.2.1. N-(2-(Decanoyloxy)ethyl)-N,Ndimethylamine. Decanoyl chloride (200 mmol, 38.14 g) in dichloromethane (30 mL) was added dropwise to a stirred solution of 2-(N,N-dimethyl)aminoethanol (400 mmol, 35.65 g) in dichloromethane (70 mL). The reaction mixture was stirred for 4 h at room temperature. The quaternary salt was removed by washing with a 5% solution of sodium hydrogen carbonate (3 70 mL), and the organic phase was dried over magnesium sulfate and filtered. The colorless oily product was isolated after evaporation of the solvent (46.23 g, 95% yield). 1H NMR (400 MHz, CDCl3): δ 0.87 (t, 3H), 1.1-1.3 (m, 12H), 1.61 (m, 2H), 2.28 (s, 6H), 2.32 (t, 2H), 2.56 (t, 2H), 4.17 (t, 2H).
2.2.2. N-(2-(Decanoyloxy)ethyl)-N,N,N-trimethylammonium Bromide (Decyl Esterquat Monomer). Methyl bromide (280 mmol, 26.58 g) was placed into a reactor containing N-(2-(decanoyloxy)ethyl)-N,N-dimethylamine (70.0 mmol, 17.11 g) in isopropanol (150 mL). The mixture was stirred at 50°C for 3.0 h. The product was recovered by filtration and recrystallized from isopropanol/ethyl acetate (23.68 g, 85% yield). 1H NMR (400 MHz, CDCl3): δ 0.87 (t, 3H), 1.1-1.3 (m, 12H), 1.6 (m, 2H), 2.35 (s, 2H), 3.57 (s, 9H), 4.14 (t, 2H), 4.57 (t, 2H).
2.2.3. N,N0 -Bis(2-(decanoyloxy)ethyl)-N,N,N0 ,N0 -tetramethyl-1,3-propanediammonium Dibromide (Decyl Esterquat Gemini, s = 3). 1,3-Dibromopropane (25 mmol, 5.04 g) in dry
acetone (30 mL) was added dropwise to N-(2-(dodecanoyloxy)ethyl)-N,N-dimethylamine (55 mmol, 13.3 g) in dry acetone (50 mL). The mixture was refluxed for 48 h. A white precipitate was filtered out of the solution and recrystallized from ethanol/ ethyl acetate (13.77 g, 80% yield). 1H NMR (400 MHz, CDCl3): δ 0.87 (t, 6H), 1.1-1.3 (m, 24H), 1.58 (m, 4H), 2.35 (t, 4H), 2.77 (m, 2H), 3.51 (s, 12H), 3.98 (t, 4H), 4.07 (t, 4H), 4.64 (t, 4H).
2.2.4. N,N0 -Bis(2-(decanoyloxy)ethyl)-N,N,N0 ,N0 -tetramethyl-1,6-hexanediammonium Dibromide (Decyl Esterquat Gemini, s = 6). 1,6-Dibromohexane (25 mmol, 6.09 g) in dry ace-
tone (30 mL) was added dropwise to N-(2-(dodecanoyloxy)ethyl)-N,N-dimethylamine (55 mmol, 13.3 g) in dry acetone (50 mL). The mixture was refluxed for 48 h. A white precipitate was filtered out of the solution and recrystallized from ethanol/ ethyl acetate (15.34 g, 84% yield). 1H NMR (400 MHz, CDCl3): δ 0.87 (t, 6H), 1.1-1.3 (m, 24H), 1.58 (m, 8H), 2.03 (m, 4H), 2.35 (t, 4H), 2.77 (m, 2H), 3.46 (s, 12H), 3.82 (t, 4H), 4.03 (t, 4H), 4.55 (t, 4H). The syntheses of dodecyl esterquat monomer and gemini (s = 3) were reported in our previous paper.14 (13) Miaoa, Z.; Yanga, J.; Wanga, L.; Liub, Y.; Zhanga, L.; Lia, X.; Penga, L. Mater. Lett. 2008, 62, 3450. (14) Tehrani-Bagha, A. R.; Oskarsson, H.; vanGinkel, C. G.; Holmberg, K. J. Colloid Interface Sci. 2007, 312, 444.
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2.3. Physical-Chemical Characterizations. 2.3.1. CMC Measurements. The cmc values of the surfactants were determined by the conductivity method. Measurements were performed with a CDM 210 conductometer (Radiometer, France) using a water bath with stirring to control the temperature. For each series of measurements, an exact volume of 25 mL of Millipore water (resistivity ∼18.2 MΩ) was introduced into the vessel, and the specific conductivity of the water was measured. The solution was then titrated with the surfactant solution, and the conductivity was measured after each addition. The concentration at which there was a break on the curve of conductivity versus surfactant concentration was taken as the cmc. 2.3.2. Degree of Micelle Ionization (R). The counterion distribution in a micellar solution can be assessed from electrical conductivity versus surfactant concentration plots. The counterion binding to micelles was determined from the ratio between the slopes above and below the cmc.15 2.3.3. Krafft Temperatures. To determine the Krafft temperature, a clear aqueous solution of the surfactant (at a concentration well above the cmc) was prepared and placed in a refrigerator at around 5 °C for at least 48 h. In case of precipitation, the temperature of the precipitated system was raised gradually under constant stirring in a water bath, and the conductance was monitored using a CDM 210 conductometer (Radiometer, France).16 2.3.4. Biodegradability Tests. The closed-bottle test, a standardized method (OECD 301 D), was used to evaluate the biodegradability of the test substances. This method has been described in detail before14 and will be discussed only briefly here. Activated sludge obtained from a plant treating predominantly domestic wastewater was used as the inoculum. Prior to inoculation of the bottles with 2.0 mg/L of dry weight, the activated sludge was aerated by continuous stirring for 1 week to reduce endogenous respiration. The concentration of the test substance was 2.0 mg/L, and the incubation temperature was 20 °C. Biodegradation was measured by following the course of the oxygen decrease in the bottles. To this end, dissolved oxygen in the bottles was monitored with an oxygen electrode (WTW Trioxmatic EO 2000) and an oxygen meter (WTW OXI 530, Retsch, Ochten, The Netherlands). The biodegradation was expressed as the percentage of the biological oxygen demand with respect to the theoretical oxygen demand. Inherent biodegradability was determined by the modified semicontinuous activated sludge (SCAS) test performed according to OECD guideline 302 A.17 The experiment was conducted in 0.15 L test vessels dosed with domestic wastewater spiked with 50 mg/L of the test substance (test unit) and domestic wastewater (control unit). The SCAS test was operated semicontinuously with one cycle of fill and draw per day. Activated sludge was not deliberately wasted. The SCAS tests operated at 20 °C had a hydraulic retention time of 36 h. The removal of the test substances was assessed by measuring the nonpurgeable organic carbon (NPOC) content in the effluents of both SCAS units. Before the determination of the NPOC, the effluents were filtered using cellulose nitrate filters with 8 μm pores (Schleicher and Schuell, Dassel, Germany) to remove sludge particles. Filtered samples were acidified prior to injection in a total organic carbon analyzer (model VCPN) (Shimadzu, Kyoto, Japan). The dissolved oxygen concentrations were determined electrochemically using an oxygen electrode (WTW Trioxmatic EO 200) and meter (WTW OXI 530) (Retsch, Ochten, The Netherlands). The pH was measured using a Knick 765 calimatic pH meter (Elektronische Messgerate GmbH, Berlin, Germany). The temperature was measured and recorded with a thermocouple (15) Vautier-Giongo, C.; Bales, B. L. J. Phys. Chem. B 2003, 107, 5398. (16) Zana, R. J. Colloid Interface Sci. 2002, 252, 259. (17) OECD. Guidelines for Testing Chemicals, Section 3: Degradation and Accumulation No. 302 A: Inherent Biodegradability; modified SCAS test, Paris, Cedex, France: 1.981.
DOI: 10.1021/la1001336
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Tehrani-Bagha and Holmberg Scheme 1. Synthesis Routes for Decyl Esterquat Monomeric and Gemini Surfactants
connected to a data logger. The dry weight (DW) of the inoculum was determined by filtering 50 mL of the activated sludge over a preweighed 12 μm Schleicher and Sch€ ull filter. This filter was dried for 1.5 h at 104 °C and weighed after cooling. The DW was calculated by subtracting the weighed filters and by dividing this difference by the filtered volume. 2.3.5. Preparation of a Phosphate Buffer Based on D2O. A 100 mM phosphate buffer was prepared by mixing 50 mL of 200 mM potassium dihydrogen phosphate and 32.1 mL of 200 mM sodium hydroxide to a total volume of 100 mL in D2O. The glass electrode pH meter (744 Metrohm, Switzerland), which was calibrated in H2O-based buffers, showed a pH of 7.1, which means that the effective pD value was approximately 7.5.18 2.3.6. Hydrolysis Study. The chemical hydrolysis of the esterquat surfactants was monitored by 1H NMR. Surfactant solutions of different concentrations in phosphate buffer based on D2O were prepared and transferred to NMR tubes. The tubes were held in a water bath at 40 °C under constant stirring. Their proton NMR spectra were recorded every day for 10 days on a Jeol 400 MHz NMR spectrometer. The degree of hydrolysis at different times was calculated from the relative integrals originating from the N-methyl protons of the intact surfactants and of the hydrolyzed product. The trimethylamine protons of the monomeric esterquat surfactants appear as a singlet at δ 3.23. The N-methyl protons of the dimeric esterquat surfactants appear as a singlet at δ 3.30. 2.3.7. Emulsion Stability. The stability of emulsions containing cationic surfactants at different concentrations was determined at room temperature by observing the backscattered profiles along vertical tubes containing the emulsions as a function of time using Turbiscan MA2000 (Formulaction, France). Mixtures of water and toluene (70:30) with different surfactants added were homogenized for 10 min at 26 000 rpm using a homogenizer (Diax 900, Heidolph, Germany). Emulsions with a total volume of 10 mL were prepared just before the experiments. A freshly prepared emulsion was pipetted into a flat-bottomed cylindrical glass tube and placed in the instrument. The sample was scanned from bottom to the top of the tube with a nearinfrared light source (λair = 850 nm) by using two synchronous optical sensors that detected the intensity of light transmitted through (180° from the incident light) and backscattered by the sample (45° from the incident radiation) along the height of the cell. The curves obtained provide the transmitted and backscattered light flux as a function of the sample height. The stability of the emulsions is followed by monitoring the % of backscattering variation versus the height of the sample every 20 min for 13 h. A decrease in the % of backscattering corresponds to a decrease of the (18) Covington, A. K.; Paabo, M.; Robinson, R. A.; Bates, R. G. Anal. Chem. 1968, 40, 700. (19) Macierzanka, A.; Szelag, H.; Szumaza, P.; Pawzowicz, R.; Mackie, A. R.; Ridout, M. J. Colloids Surf., A 2009, 334, 40. (20) Wulff-Perez, M.; Torcello-Gomez, A.; Galvez-Ruı´ z, M. J.; Martı´ n-Rodrı´ guez, A. Food Hydrocolloids 2009, 23, 1096. (21) Wassenius, H.; Nyden, M.; Holmberg, K. J. Dispersion Sci. Technol. 2001, 22, 297.
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number of droplets and/or an increase in their size. More detailed information about this technique can be obtained elsewhere.19-21 2.3.8. Foamability and Foam Stability. A calibrated 100 mL glass cylinder with a stopper was used for the measurement of foam stability and foamability. Ten milliliters of surfactant solution was poured into the calibrated cylinder. The solution was shaken vigorously for 10 s, and the height and volume of the foam were monitored at different times by video image analysis. The foam stability was determined to be the time needed for the collapse of the foam to half of its initial height. The initial foam height was reported as the foamability. The experiments were repeated at least five times, and the values given are the means of the experiments.22,23
3. Results and Discussion 3.1. Syntheses and Physical-Chemical Characterization. The syntheses of decyl esterquat monomeric and gemini surfactants are shown in Scheme 1. The routes are straightforward. Decanoyl chloride was first reacted with N,N-dimethyl-2-aminoethanol, and the ester amine formed was subsequently quaternized with methyl bromide to form the monomeric surfactant or with an alkyl dibromide to give a gemini. Both steps proceeded in good yields. The cmc and the degree of micelle ionization (R) for the decyl esterquat monomeric and gemini surfactants were determined at three different temperatures (25, 30, and 40 °C) by conductivity measurements, and the values obtained are reported in Table 1. As can be seen, the cmc values for the gemini surfactants are more than 10 times smaller than that for the corresponding monomeric surfactant. For comparison, we have included the cmc values of dodecyl esterquat monomeric and gemini surfactants from our previous work,14 and for these, the difference in the cmc values is more than 20-fold. The results confirm the strong tendency of gemini surfactants to self-assemble in solution. The decyl esterquat gemini with a shorter spacer unit (s = 3) has smaller cmc values at the different temperatures compared to the gemini with a longer spacer (s = 6). However, the difference between the two geminis with varying lengths of the spacer unit is small. It has been reported in the literature that there is a maximum in the cmc for the conventional m-s-m gemini surfactants with a hydrophobic polymethylene spacer at s = 5 to 6 irrespective of the value of m.24 The effect of the spacer length on the cmc value is believed to be related to the folding of the spacer unit at the micelle surface. The values of the degree of micelle ionization, R, for decyl esterquat gemini with a shorter spacer (s = 3) at different temperatures were considerably lower than those for both the decyl esterquat gemini with a longer spacer (s = 6) and the (22) Borse, M.; Sharma, V.; Aswal, V. K.; Goyal, P. S.; Devi, S. Colloids Surf., A 2006, 287, 163. (23) Borse, M. S.; Devi, S. Adv. Colloid Interface Sci. 2006, 123-126, 387. (24) Zana, R.; Xia, J. Gemini Surfactants; Marcel Dekker: New York, 2004.
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Table 1. Critical Micelle Concentration (cmc), Degree of Micelle Ionization (r), and Krafft Temperature of the Surfactantsa 25 °C surfactant
cmc (mM)
30 °C R
cmc (mM)
40 °C R
decyl esterquat monomer 29.53 0.33 30.84 0.35 decyl esterquat gemini (s = 3) 2.06 0.22 2.1 0.23 decyl esterquat gemini (s = 6) 2.6 0.31 2.7 0.36 7.2 0.28 7.4 0.29 dodecyl esterquat monomerb b 0.31 0.32 0.33 0.31 dodecyl esterquat gemini (s = 3) a The cmc and R values were determined by conductivity measurements. b Data from ref 14.
cmc (mM)
R
Krafft temp (°C)
33.25 2.45 3.04 8.3 0.37
0.38 0.26 0.37 0.35 0.4
