Langmuir 1998, 14, 2307-2315
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Surface Activity Properties at Equilibrium of Novel Gemini Cationic Amphiphilic Compounds from Arginine, Bis(Args) Lourdes Pe´rez,† Aurora Pinazo,† Milton J. Rosen,‡ and Ma Rosa Infante*,† Department of Surfactant Technology, CID (CSIC), J. Girona 18-26, 08034 Barcelona, Spain, and Surfactant Research Institute, Brooklyn College of the City University of New York, Brooklyn, New York 11210 Received October 16, 1997. In Final Form: January 9, 1998 A systematic study of the surface active properties at equilibrium (in water and in the presence of 10-2 M NaCl) of a novel class of gemini cationic surfactants from arginine is described. They are NR,Nω-bis(NR-acylarginine) R,ω-alkylidenediamides or bis(Args). Parameters studied include cmc (critical micelle concentration), pC20 (negative log of the surfactant molar concentration, C20, required to reduce the surface tension of the solvent by 20 mN/m), γcmc (the surface tension at the cmc), Γmax (the maximum surface excess concentration at the air/aqueous solution interface), Amin (the minimum area per surfactant molecule at the air/aqueous solution interface), and the cmc/C20 ratio (which measures the tendency of the surfactant to adsorb at the interface relative to its micellization tendency). Data on the synthesis and chemical and spectral characteristics of surfactants are also reported.
Introduction In the past decade an increasing number of papers have been published addressing the synthesis and properties of a new type of bifunctional amphiphilic compounds showing unusual surfactant behavior called dimeric or gemini surfactants.1-5 They are amphiphilic compounds that have two hydrophobic chains and two hydrophilic head groups per molecule connected through a spacer chain. These surfactants are the subject of increasing study, given their unusual physicochemical properties in comparison with those of the corresponding conventional monomeric surfactants (one chain/one head group). In general, they are more effective in adsorbing at the aqueous solution/air interface, reducing the surface tension, and forming micelles.6,7 Owing to their extraordinary activity, they are regarded as an outstanding new generation of surfactants with excellent solubilization, wetting, and rheological properties at low concentrations.2 In this context, our group in the Department of Surfactant Technology has recently synthesized a family of a new class of cationic surfactants of gemini type, derived from the amino acid arginine, with the aim of obtaining surfactants that are environmentally acceptable and have a high degree of performance.8 They are NR,Nω-bis(NRacylarginine) R,ω-alkylidenediamides or bis(Args). They consist of two symmetrical long chain NR-acyl-L-arginine residues of 10 or 12 carbon atoms linked by amide covalent * To whom correspondence should be addressed. Fax: (34-3) 204 59 04. E-mail:
[email protected]. † CID (CSIC). ‡ Brooklyn College of the City University of New York. (1) Menger, F. M.; Littau, C. A. J. Am. Chem. Soc. 1993, 115, 10083. (2) Rosen, M. J. CHEMTECH 1993, 23, 30. (3) Zana, R.; Talmon, Y. Nature 1993, 362, 228. (4) Diz, M.; Manresa, A.; Pinazo, A.; Erra, P.; Infante, M. R. J. Chem. Soc., Perkin Trans. 2 1994, 1871. (5) Song, L. D.; Rosen, M. J. Langmuir 1996, 12, 1149. (6) Devinsky, F.; Lacko, I.; Bittererova, F.; Tomeckova, L. J. Colloid Interface Sci. 1986, 114, 314. (7) Zhu, Y. P.; Masuyama, A.; Kobata, Y.; Nakatsuji, Y.; Okahara, M.; Rosen, M. J. J. Colloid Interface Sci. 1993, 158, 40. (8) Pe´rez, L.; Torres, J. L.; Manresa, A.; Solans, C.; Infante, M. R. Langmuir 1996, 12, 5296.
Figure 1. Molecular structure of bis(Args).
bonds to an R,ω-alkylidenediamine spacer chain of various lengths (Figure 1). They are codified as Cn(LA)2 when x ) 10 and Cn(CA)2 when x ) 8. This novel family arises from the appropriate linkage of two long chain NR-acyl-L-arginine single-chain surfactants (XAM), LAM (NR-lauroylarginine methyl ester) and CAM (NR-caproylarginine methyl ester), whose syntheses and properties have been the object of intense study by our group since 1984 (Figure 2).9-12 In this paper we report a systematic study of the surface active properties at equilibrium for Cn(LA)2 and Cn(CA)2 (with n ) 2, 3, 4, 6, 9, and 10) in water and in the presence of 10-2 M NaCl. Parameters studied include cmc (critical micelle concentration), pC20 (negative log of the surfactant molar concentration, C20, required to reduce the surface tension of the solvent by 20 mN/m, which measures the efficiency of adsorption), γcmc (the surface tension at the cmc), Γmax (the maximum surface excess concentration at the air/aqueous solution interface), Amin (the minimum area per surfactant molecule at the air/aqueous solution interface), and the cmc/C20 ratio (which measures the (9) Infante, M. R.; Garcia Dominguez, J. J.; Erra, P.; Julia´, M. R.; Prats, M. Int. J. Cosmet. Sci. 1984, 6, 275. (10) Solans, C.; Infante, M. R.; Azemar, N.; Warnheim, T. Prog. Colloid Polym. Sci. 1989, 79, 70. (11) Solans, C.; Pe´s, A.; Azemar, N.; Infante, M. R. Prog. Colloid Polym. Sci. 1990, 81, 144. (12) Kunieda, H.; Nakamura, K.; Infante, M. R.; Solans, C. Adv. Mater. 1992, 4, 291.
S0743-7463(97)01135-9 CCC: $15.00 © 1998 American Chemical Society Published on Web 04/11/1998
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Figure 2. Molecular structure of the single-chain surfactants XAM.
tendency of the surfactant to adsorb at the interface relative to its micellization tendency). Data on the synthesis and chemical and spectral characteristics of surfactants are also reported. Results and Discussion Cn(LA)2. Plots of the surface tension (γ) at 25 °C of aqueous solutions of Cn(LA)2 homologues versus the log of their bulk phase concentration in moles per liter (log C) in water and in 10-2 M NaCl are shown in Figure 3 and Figure 4, respectively. Although bis(Arg) compounds have been carefully purified (see Experimental Procedure), there is an uncertainty e 0.1%. However, given the high surface activity of bis(Args), we assume that the potential impurities (if present) would have similar or less surface activity, and therefore the adsorbed layer at the surface is mainly controlled by molecules of bis(Args).13 The critical micelle concentration (cmc) of these compounds was determined from the break point of the surface tension versus concentration curves. The saturation adsorption values, Γmax, at the air/water interface and the minimum area per molecule, Amin, were calculated using the Gibbs adsorption equation, as usual14,15
Γmax )
-1 ∂γ 2.303nRT ∂ log C
(
)
T
Amin ) (NAΓ)-1 × 1016 where R ) 8.31 J‚mol-1 K-1, NA ) Avogadro’s number, Γmax is in mol/cm2, and Amin is in (nm/molecule) × 102. The value of n for the Gibbs equation is the number of species whose concentration at the interface alters with changes in the surfactant concentration in the solution. For bis(Arg) surfactants, n ) 3 (the dicationic amphiphile species and two chloride ions). Since the surface tension values used for the Γm calculation in water (below the cmc) were all at 6, Am decreases when n increases, mainly due to the decrease of the intramolecular head group distances as a consequence of a change in the localization of the spacer chain as n increases. The spacer becomes too hydrophobic to remain in contact with water and may form a loop which reinforces the hydrophobic inter- and intramolecular interactions, allowing the heads to get closer together, and consequently Am decreases. This has also been observed for other bis(Quat) analogues at large spacer chain lengths.3 The pC20 values of the Cn(LA)2 compounds in water are also about 3 units larger than that of LAM, indicating that the C20 values of the former (or their efficiency of adsorption at the air/aqueous solution interface) are 3 orders of magnitude greater than that of the latter. This dimeric structure, therefore, appears to cause a more closepacked arrangement at a water/air interface and a more efficient adsorption there. However, the increase in the cmc/C20 ratio to a maximum when the number of polymethylene groups in the spacer is 4-6 indicates that the increased bulkiness of the surfactant molecule in this range inhibits micellization more than it does adsorption at the aqueous solution/air interface. In fact, the relatively large pC20 values when the number of polymethylene groups in the spacer exceeds 4 indicate that adsorption at the aqueous solution/air
interface is increased rather than inhibited by this structural change, in direct contrast to its effect on micellization. The surface tension measurements of Cn(LA)2 were also carried out in the presence of 10-2 M NaCl. In this case the value of the coefficient in the Gibbs adsorption equation (n) to calculate the value of Amin is 1. Surprisingly, the ability to adsorb at the water/air interface or to form micelles of Cn(LA)2 does not increase significally when an electrolyte such as NaCl is added to the water solutions. Contrary to those for LAM, the single-chain homologue, the values of cmc, γcmc, and pC20 of Cn(LA)2 are comparable in both media. To confirm whether the values of the cmc of bis(Args) were affected by the addition of NaCl, the simple mass action model20 was applied. To this end, β (counterion binding) was determined from chloride measurements in the surfactant solutions.21 Although we do not have an explanation at the molecular level, the cmc’s calculated with this model were consistent with the experimental cmc values of bis(Args). However all Cn(LA)2 homologues have apparently slightly larger Γmax values (and consequently lower Amin values) and larger cmc/C20 values in 0.01 mol/L NaCl than in water, accounted for by the expectation that electrostatic repulsions in the adsorbed monolayer are reduced by the ionic strength effect.22 Although with lower differences than in Table 1, the Amin and cmc/C20 values in Table 2 also increase to a maximum at C4(LA)2 to C6(LA)2. It is noteworthy also that the compounds where the number of CH2 groups in the spacer is odd deviate in their interfacial properties, especially as shown by their cmc/ C20 values, from the expected values based upon the adjacent compounds with even numbers of methylene groups in the spacer. Cn(CA)2. Plots of the surface tension (γ) at 25 °C in 10-2 M NaCl solutions of Cn(CA)2 homologues versus the log of their bulk phase concentration in moles per liter (log C) are shown in Figure 6.Values of cmc, γcmc, pC20, Γmax, Amin, and cmc/C20 in 0.01 mol/L NaCl are listed in Table 3. For comparison, values for CAM obtained at the same experimental conditions are included. As expected, compared to CAM, the compounds Cn(CA)2 also show very small cmc values. Compared to those of (20) Corkill, J. K.; Goodman, J. F.; Harrold, S. P. Trans. Faraday Soc. 1964, 60, 202. (21) Results to be published. (22) Rosen, M. J. Surfactants and Interfacial Phenomena, 2nd ed.; John Wiley: New York, 1989; p 143.
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Figure 6. Surface tension versus log C of Cn(CA)2 in 0.01 M NaCl at 25 °C.
the Cn(LA)2 homologues, the cmc values of the Cn(CA)2 compounds in 0.01 M NaCl increase somewhat less than
1 order of magnitude as the length of the alkyl chain decreases from 12 to 10 carbon atoms in salt solution.
Novel Gemini Cationic Amphiphilic Compounds
Langmuir, Vol. 14, No. 9, 1998 2313
Table 3. Surface Properties of Cn(CA)2 in 10-2 M NaCl at 25 °C Cn(CA)2
cmc (×10-6 mol/L)
γcmc (mN/m)
pC20
1010Γcmax (mol/cm2)
Amin (× 102 nm2/molecule)
cmc/C20
C2(CA)2 C3(CA)2 C4(CA)2 C6(CA)2 C9(CA)2 C10(CA)2 CAM
58 43 48 21 16 7.3 2700
33 32 33 33 35 36 29
5.6 5.8 5.8 6.2 5.9 6.2 3.7
2.6 ((0.16) 2.5 ((0.40) 2.1 ((0.20) 2.1 ((0.21) 2.8 ((0.10) 2.9 ((0.30) 3.6 ((0.10)
64 66 77 79 60 58 47
21.5 31.2 30.0 36.7 13.3 11.4 14.4
Table 4. Chemical Characteristics of the Synthetic Cn(CA)2 Compounds compound
yield (%)
[R]20365 (c ) 1%, MeOH)
mpa (°C)
C2(CA)2 C3(CA)2 C4(CA)2 C6(CA)2 C9(CA)2 C10(CA)2
78 87 82 81 77 80
-0.6 -1.4 -7.9 -0.6 -10.13 -5.9
119-123 108-112 100-104 98-101 80-85 87-92
a Without decomposition. Studies by DSC show the products do not melt reversibly, probably because of their high hygroscopy.
analogies between bis(Quats) and bis(Args), we will take into account the studies reported by Danino19 for model considerations. Figure 7. Relationship between log(cmc) and the alkyl chain length for bis(Args) and their single-chain homologues.
Figure 7 shows plots of the variation of log cmc with the surfactant chain length, 10 and 12 carbon atoms, for bis(Args) with s ) 2, 3, 6, and 10 and their single-chain homologues CAM and LAM. Although there are only two points per series, the five plots are nearly parallel. These results indicate that the value of ∆G°t(CH2), a parameter which is a measure of the contribution of the hydrophobic interaction to micellization per mole of CH2, is similar for the single and the dimeric surfactants derived from arginine. The influence of the spacer chain length (CH2)n on the cmc values is similar to that in the Cn(LA)2 compounds: the cmc decreases when the number of methylene groups in the spacer chain increases. The Γmax values are lower (and the Amin values are greater) in 10-2 M NaCl when the alkyl chain decreases; this indicates, as in another series of gemini surfactants, that the molecules are more tightly packed at the water/air interface for the larger alkyl surfactants. The efficiencies of adsorption (pC20 values) of the Cn(CA)2 compounds are slightly lower than that of Cn(LA)2, as expected. The cmc/C20 values for the Cn(CA)2 compounds increase to a maximun when the number of methylene units in the spacer is 4-6, as in the Cn(LA)2 compounds. These compounds constitute an interesting environmentally friendly alternative to conventional cationic surfactants and even to bis(Quats), in which the biocompatibility of lipoamino acids and the efficiency of dimeric surfactants are combined. This paper constitutes the first investigation of gemini surfactants derived from amino acids in which they are systematically studied in order to know the effect of the chemical structure on the physicochemical behavior in solution. From these results we can conclude that variations in the alkyl chain lengths have a higher effect on the surface active properties than those in the spacer chain. Further studies on phase behavior including micellar structure characterization are in progress. For these studies and given the structural
Experimental Procedure Materials. The bis(Args) Cn(LA)2 and Cn(CA)2 were synthesized by the method described in ref 8. This consists of three steps (Scheme 1): (1) acylation to prepare the starting materials NR-acyl-L-nitroarginine [XNA]; (2) coupling to prepare the protected NR,Nω-bis(NR-acyl-L-nitroarginine) R,ω-alkylidenediamide [Cn(XNA)2]; and (3) hydrogenolysis to obtain the final bis(Args). The chloride salts of the compounds were obtained by treatment of the corresponding formate salts (obtained after the hydrogenolysis reaction in formic acid medium) with methanol/ HCl. The characteristics for the synthetic, NR,Nω-bis(NR-lauroylarginine) R,ω-alkylidenediamide (Cn(LA)2) have already been reported.8 The characteristics for NR,Nω-bis(NR-caproylarginine) R,ω-alkylidenediamide (Cn(CA)2) are indicated in Tables 4 and 5. To obtain compounds with an absence of surface active impurities, all synthetic bis(Args) were purified following the surfactant purification described by Kunieda23 to clarify the effect of a small amount of impurities on interface properties. First the compound was dissolved in CHCl3/MeOH and this solution was shaken with water to remove water soluble impurities. The extracted CHCl3/MeOH was dried under vacuum and redissolved in pure MeOH; then the oil soluble impurities were extracted three times with petroleum ether. The solvent was evaporated, and finally the compound was dissolved in dried and chilled MeOH and the solution was filtered through 0.5-µm Millipore filters to remove traces of inorganic salts. This procedure was repeated three times. The purity was around 99.9% by HPLC. Surface Tension Measurements. Equilibrium surface tension measurements were made by the Wilhelmy plate technique with a Kru¨ss K12 tensiometer. The instrument was calibrated against ultrapure distilled water (Milli-Q-4) each day that measurements were made. Sets of measurements were taken until the change in surface tension was less than 0.08 mN/m every 15 min. Surfactants in the concentration range investigated here reached their acceptable surface tension values in 30-60 min for the concentrated samples and in several hours for the dilute samples. We normally take one set of readings on the same solution, except when the sample gives abnormal values, in which case we take two or three, if necessary. The different concentrations of surfactants were prepared by sucessive dilutions of a concentrated sample (10-4 M) using water of γ ) 72 mN/m. All sample solutions were aged in appropriate (23) Kunieda, H.; Shinoda, K. J. Colloid Interface Sci. 1979, 70, 577.
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Scheme 1. Synthetic Pathway for the Synthesis of Bis(Args)
Table 5. Spectral Characteristics of the Synthetic Cn(CA)2 Compounds compound
MM (g/mol)
C2(CA)2
752.9
681
C3(CA)2
766.5
695.6
m/e (M+)
C4(CA)2
780.5
709.95
C6(CA)2
808.5
737
C9(CA)2
850.6
780
C10(CA)2
866.1
793
elem anal % calc (found) C 54.22 (54.17) H 9.38 (8.95) N 18.61 (18.46) C 54.79 (54.40) H 9.47 (9.23) N 18.27 (18.13) C 55.35 (55.19) H 9.55 (9.28) N 17.94 (17.69) C 56.40 (56.32) H 9.72 (9.49) N 17.32 (17.24) C 57.84 (57.59) H 9.95 (9.90) N 16.46 (16.21) C 58.24 (57.97) H 10.01 (9.95) N 16.17 (15.96)
cells at room temperature (25 °C) before taking measurements in order to reach the equilibrium state. Aging times of 6, 12, 24, and 48 h were investigated. The best reproducibility was obtained at 24 h for the water and 10-2 M NaCl solutions and at 48 h for
IR (KBr) υ (cm-1)
3298 (NH) 2922 and 2852 (CH2) 1641 (COsN amide I) 1542 (NsCdO amide II)
1H and 13C NMR δ (ppm) (solvent CD3OD) 1H NMR (200 MHz) 0.85 (t, 6H, 2CH3) 1.2-1.7 (m, CH2) 2.1 (t, 4H, 2CH2 -CO) 3.1 (m, CH2 -NH) 4.2 (m, 2H, 2CH) 7.7-8.1 (m, NH, NH2) 8.5 (m, CON-H) 13C NMR (50 MHz) 13.93 (CH3) 22-31.6 (CH2) 52 (CH) 159.3 (C, guanidine group) 172.2
the more dilute samples (10-6 M). During this period of time, the solutions in the cells were covered with Parafilm in order to avoid evaporation of the water. For dilute samples (10-6 M) and taking into account the high surface activity of these cationic
Novel Gemini Cationic Amphiphilic Compounds surfactants, it is possible to make errors in the measurement, due to the adsorption of surfactant on the wall of the container. To handle this, the glass containers used for measurements were presoaked with the surfactant solution during 24 h; then the surfactant solution was replaced with fresh solution (of the same concentration) without rinsing the container and used for surface tension measurements. To confirm whether adsorption in the container did occur, we did some measurements without presoaked containers and calculated the amount of surfactant adsorbed on the walls of the glass containers (assuming monolayer adsorption) and then we recalculated the surfactant concentration remaining in the solution phase. Plots of these new data were consistent with those from the presoaked container method.
Langmuir, Vol. 14, No. 9, 1998 2315 The platinum plate was cleaned with a flame after every set of measurements, and all glassware used in the measurements and preparation of the solutions was scrupulously cleaned with chromic mixture.
Acknowledgment. This work was supported by Grant QUI97/0570 and Laboratorios Miret S.A. The authors thank Dr. Ramon Pons for his help in the discussion and Angela Pascual and Amalia Vilchez for their help in the surface tension measurements and the synthesis of bis(Args). LA971135U
