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Electrolyte Effects on the Surface Tension and Micellization of n-Dodecyl β-D-Maltoside Solutions Lei Zhang, P. Somasundaran,* and C. Maltesh† Langmuir Center for Colloids & Interfaces, 911 Mudd Building, Columbia University, New York, New York, 10027 Received August 7, 1995. In Final Form: February 7, 1996X Sugar-based surfactants such as polyglucosides are a relatively new class of nonionic surfactants portrayed to be suitable for high-salinity applications. To better utilize such surfactants, it is useful to elucidate the mechanisms of their interactions at various interfaces in the presence and absence of electrolytes and the reasons for their salt tolerance. In this work, the effect of various salts on the surface tension and critical micelle concentration of the nonionic surfactant n-dodecyl β-D-maltoside (DM) was studied. Interestingly, while the packing of DM molecules at the air-water interface was not affected by the nature of salt added, cations and anions were found to have markedly different effects on the surface activity and critical micelle concentration (cmc) of the surfactant. For the same cation Na+, the effect of anions decreases in the order F- > Cl- > SO42- > Br- > PO43- > citrate > I- > SCN-. For the same anion Cl-, the effect of cations follows the order K+ > Na+ > Rb+ > Li+ > Ca2+ > Al3+. The effectiveness of the salts is correlated with the charge to radius ratios of the ions, and it is clear that cations and anions have disparate effects. The results are discussed in terms of the structure-making and structure-breaking properties and solvation heats of ions.
Introduction Alkyl polyglucosides are nonionic surfactants finding increasing applications in various fields1-4 due to the recent development of economic production techniques. In comparison with ionic and some nonionic surfactants, these reagents lower surface and interfacial tension more effectively and show a greater tolerance for electrolytes. They offer good detergency properties and are very mild to skin. Another important characteristic of these surfactants is that they are environmental friendly, since they are easily biodegradable and are produced from naturally occurring renewable resources such as fatty alcohols and sugars.1,3,5-7 The solution and interfacial behavior of polyglucosides have been investigated using surface tension measurements and phase diagrams.8 These surfactants show two critical micelle concentrations (cmc’s) proposed to result from transitions between two types of micelles. It is probable that the unavoidable structural distribution present in alkyl polyglucosides could also be responsible for the two cmc’s. Interestingly addition of sodium chloride reduces the two cmc’s to one value located between the initial two cmc values. This feature however is not detected for defined isomers such as alkyl β-D-maltosides which could be considered as typical sugar-based surfactants. Several interfacial and aggregation studies have bee conducted using n-dodecyl β-D-maltosides in the past;9,10 however, the effect of salts * To whom correspondence should be addressed. † Current address: Corporate Research Division, Nalco Chemical Co., Naperville, IL 60563. X Abstract published in Advance ACS Abstracts, April 15, 1996. (1) Hughes, F. A.; Lew, B. A. J. Am. Oil Chem. Soc. 1970, 47, 162. (2) Putnik, C. F.; Borys, N. F. Soap, Cosmet., Chem. Spec. 1986, June, 34. (3) Salka, B. Cosmet. Toiletries 1993, 108, 89. (4) Helenius, A.; McCaslin, D. R.; Fries, E.; Tanford, C. Methods Enzymol. 1979, 56, 734. (5) Matsumura, S.; Imai, K.; Yoshikawa, S.; Kawada, K.; Uchibori, T. J. Am. Oil Chem. Soc. 1990, 67, 996. (6) Bjorkling, F.; Godtfredsen, S. E.; Kirk, O. J. Chem. Soc., Chem. Commun. 1989, 934. (7) Adelhorst, K.; Bjorkling, F.; Godtfredsen, S. E.; Kirk, O. Synthesis 1990, 2, 112. (8) Balzer, D. Langmuir 1993, 9, 3375. (9) Drummond, C. J.; Warr, G. G.; Greiser, F.; Ninham, B. W.; Evans, D. F. J. Phys. Chem. 1985, 89, 2103.
on the surface activity has not bee probed in spite of its superior salt tolerance. The stability of these surfactants in the presence of different salts is of considerable interest for developing a better understanding of the behavior of sugar-based surfactants. In the present study the surface activity and aggregation of a typical alkyl polyglucoside surfactant, n-dodecyl β-D-maltoside, have been investigated in the presence of different salts. Materials and Methods Surfactant. n-Dodecyl β-D-maltoside (DM) was obtained from Calbiochem and used as received. The purity determined using TLC analysis was reported to be >98% with dodecanol and residues of maltose as impurities. Elemental analysis showed the surfactant to contain 55.27% carbon and 9.06% hydrogen. Reagents. The salts studied were obtained from the following sources: LiCl, NaCl, KCl, CaCl2, AlCl3, NaF, NaSCN, Na2SO4, and Na3PO4 from Fisher Scientific Co.; RbCl from Alfa Products; NaBr and NaI from Aldrich Chemical Company, Inc.; and sodium citrate from Amend Drug & Chemical Company. They were all used as received. Triple-distilled water was used for making up solutions, and the ionic strength was maintained at 0.75 mol/L. Surface Tension Measurement. The surface tension of aqueous solutions of the surfactant was measured with the Wilhelmy vertical plate technique using a sandblasted platinum plate as the sensor. The pull exerted on the sensor was determined using a Beckman microbalance (Model LM 600). The entire assembly was kept in a draft-free plastic cage at a temperature of 25 ( 1 °C. For each measurement, the sensor was in contact with the surfactant solution for 30 min to allow equilibration.
Results and Discussion The surface tension of aqueous solutions of n-dodecyl β-D-maltoside in the absence and presence of various salts is shown in Figures 1 and 2. The critical micelle concentration (cmc) of n-dodecyl β-D-maltoside (DM) in water is around 1.8 × 10-4 mol/L, and the minimum surface tension is 35.5 dyn/cm in agreement with those reported in the literature.9 The low critical micelle concentration is expected of the nonionic surfactants. The (10) Warr, G. G.; Drummond, C. J.; Greiser, F.; Ninham, B. W.; Evans, D. F. J. Phys. Chem. 1986, 90, 4581.
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Figure 1. Effect of different sodium salts on the surface tension and critical micelle concentration of n-dodecyl β-D-maltoside.
Figure 2. Effect of different chloride salts on the surface tension and critical micelle concentration of n-dodecyl β-D-maltoside. Table 1. Critical Micelle Concentration (Cmc) of n-Dodecyl β-D-Maltoside in Different Salt Solutions salt
conc (M)
cmc (×10-6 mol/L)
LiCl NaCl KCl RbCl NaF NaBr NaI NaSCN AlCl3 Na3PO4 sodium citrate Na2SO4 CaCl2
0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.125 0.125 0.125 0.25 0.25
109 83.3 78.7 95.5 68.3 114 141 136 130 106 108 93.8 118
surface tension as well as the critical micelle concentration of dodecyl β-D-maltoside is reduced by the salts. The cmc values of dodecyl β-D-maltoside in the presence of various salts are summarized in Table 1. Interestingly, electrolytes showed a strikingly different effect on the solution behavior of alkyl polyglucosides.8 Most inorganic salts reduce the lower consolute temperature (cloud point) distinctly, but alkalis having a strong hydrotropic effect due to a negative charge on polyglucoside micelles resulting in specific binding of some alkalis (NaOH). It is to be noted that the effect of electrolytes on n-dodecyl β-D-maltoside is distinct from that on alkyl polyglucosides despite the similarity in structure. This disparity could arise from the fact that polyglucosides invariably contain a structural distribution as well as polyglucoses and fatty alcohols. In comparison n-dodecyl β-D-maltoside has a single structure and is relatively free
of impurities. In contrast to the case of alkyl polyglucosides, the solution behavior of alkyl polyethoxylates is less sensitive to the salts primarily because their micelles are uncharged. However, polyethylene oxides are known to precipitate in the presence of salts. In a study of the surface properties of n-dodecyl β-Dmaltoside, Drummond et al. suggest that the maltoside moiety be strongly hydrated and that other surfactants cannot approach the head group of the n-dodecyl β-Dmaltoside close enough for strong interactions.9 This could explain the moderate effect of different salts on the surface activity of n-dodecyl β-D-maltoside. Moreover, n-dodecyl β-D-maltoside is nonionic; hence, there are no repulsive electrostatic forces that can be reduced by the salts. The observed salt effect is explained here in terms of the salting out of the hydrocarbon chains of the surfactant and the structure-making and structure-breaking properties of the salts. The salt effects on the hydrophilic moities of the monomer and the micelle are assumed to be almost equal in magnitude and cancel each other. This is in agreement with the estimated effect of NaCl on the cmc of octyl glucoside and minor role of the polar head group.11-13 An important effect to be considered is the hydration of salt ions in the aqueous solution which can alter the solvent properties. Hydration will decrease the “free” water molecules and increase the effective concentration of the surfactant, thus decreasing both the surface tension and cmc.14 From Figures 1 and 2 it can be seen that, at the same ionic strength, different salts have different effects on the surface tension and cmc of the dodecyl maltoside solution. For the same cation Na+, the effect of anions on the cmc follows the order F- > Cl- > SO42> Br- > PO43- > citrate > I- > SCN-. Similarly, for the same anion Cl-, the effect of cations is in the order K+ > Na+ > Rb+ > Li+ > Ca2+ < Al3+. Ions with a high charge/ radius ratio, e.g., Li+, Ca2+, and Al3+, are called structuremaking ions, since they induce a more coherent structure of the water. Conversely, ions with a low charge/radius ratio, e.g., Rb+, are called structure-breaking ions. Because of the more intense electrostatic field in its vicinity, the structure-making ions are more highly hydrated than the structure-breaking ions. Consequently the amount of distortion in the structure of “free” water surrounding the hydrated structure-making ions is much less than that in the case of the weakly hydrated structure-breaking ions.15-17 In the case of a surfactant solution, water molecules are oriented around the structure-making ions, and this leads to a salting-out effect due to the reduction of hydration of the surfactant. The structure-breaking ions increase the ratio of monomeric water molecules in the bulk and promote the hydration of the surfactant, i.e., a salting-in effect.18 The cmc of dodecyl maltoside in various salt solutions is plotted in Figure 3 as a function of the charge/radius ratio of the ions. It is seen that, for the chloride salts, the effect of cations on the depression of the cmc of maltoside is in the order K+ > Na+ > Rb+ > Li+ > Ca2+ > Al3+, whereas for the sodium salts, the effect of anions is in the order F- > Cl- > Br- > I-. In the case of anions, the structure-making anion F- has a large effect compared with that of the structure-breaking anion I-, which can (11) Mukerjee, P. Adv. Colloid Interface Sci. 1967, 1, 241. (12) Mukerjee, P. J. Phys. Chem. 1965, 69, 4038. (13) Mukerjee, P. J. Phys. Chem. 1970, 74, 3824. (14) Shinoda, K.; Yamaguchi, T.; Hori, R. Bull. Chem. Soc. Jpn. 1961, 34, 237. (15) Breuer, M. M.; Robb, I. D. Chem. Ind. 1972, 530. (16) Somasundaran, P.; Hanna, H. S. Soc. Pet. Eng. J. 1979, 19, 221. (17) Wang, J. H. J. Phys. Chem. 1954, 58, 686. (18) Nishikido, N.; Matuura, R. Bull. Chem. Soc. Jpn. 1977, 50, 1690.
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Langmuir, Vol. 12, No. 10, 1996 2373
Figure 3. Dependence of the critical micelle concentration of n-dodecyl β-D-maltoside on the charge to radius ratio of the ions.
Figure 3. For sodium salts, the higher the solvation heat, the larger is the depression of the cmc. On the contrary, for chloride salts, the higher the solvation heat, the lesser is the depression of the cmc. The effect of anions on the surface tension and cmc of n-dodecyl β-D-maltoside in various salt solutions can be understood by considering the effect of ions on water structure. However, the effect of cations is exactly opposite to those of anions. Similar salt effects have been observed for nonionic alkyl polyethylene oxides.18,21-23 The anomalous behavior of the cations is also reflected in the Hofmeister series or the lyotropic series, which expresses the effectiveness of salts in destabilizing colloids. The lyotropic series of ions is of general significance in a large number of phenomena such as salting out, swelling, and gelation of colloidal systems.24,25 However, no general explanation of the observed order is yet available. The experimental results of the salt effect on the surface tension and cmc of n-dodecyl β-D-maltoside approximately follow the lyotropic series with the tendency of the ions closely related to the positions of the anions and cations in them. Ions with a relatively low lyotropic number tend to have a large effect in decreasing the cmc of n-dodecyl β-D-maltoside. From Figures 1 and 2, it is also evident that the slope of the surface tension vs the logarithm of concentration curve does not change appreciably in the presence of various salts. This suggests that the packing area per molecule (and micellar aggregation) of DM is not affected by the nature of the salt added. Summary
Figure 4. Variation of the critical micelle concentration of n-dodecyl β-D-maltoside as a function of the heat of solvation of different salts.
be expected. However, in the case of cations the effect on the cmc of n-dodecyl β-D-maltoside is in the reverse order. Solvation of the salts and their effect on the structure of water offer an alternate way for comparing the salt effect on the cmc of the surfactant. The higher the solvation heat, the greater is the structure-making effect. The general equation for the calculation of solvation energy for a charged species is19,20
∆Gsolv ) -
[
]
(ze)2N 1 1B 8π(rion + 2rw)0
where N ) Avogadro’s number, e ) the charge on an electron, z ) the valency, rion ) the radius of the ions (Å), 0 ) the permittivity of a vacuum, rw ) the radius of the water molecule (1.38 Å), and B ) the dielectric constant of the solvent (water). The cmc of dodecyl maltoside is shown in Figure 4 as a function of the solvation heat of salts used in the experiment. These results also show a difference in the behavior of sodium and chloride salts, as was seen in (19) Bockris, J. O’M.; Reddy, A. K. N. Modern Electrochemistry; Plenum Press: New York, 1970; Vol. 1. (20) Maltesh, C.; Somasundaran, P. Langmuir 1992, 8, 1926.
The effect of a number of salts on the surface activity of n-dodecyl β-D-maltoside is determined. It is found that different salts have different effects on the surface tension and cmc of the dodecyl maltoside solution: at the same ionic strength of salts, for the same cation Na+, the effect of anions on the surface tension and critical micelle concentration follows the order F- > Cl- > SO42- > Br> PO43- > citrate > I- > SCN-, and for the same anion Cl-, the effect of cations follows the order K+ > Na+ > Rb+ > Li+ > Ca2+ > Al3+. The head group of n-dodecyl β-Dmaltoside has been reported to be strongly hydrated, which suggests that its micellization will be affected by the effect salts may have on the water structure. The effect of anions is explained on the basis of their structure-making and structure-breaking properties, while the effect of cations is exactly the reverse. The observed order of the salt effect follows the lyotropic series approximately. This shows the salt order to basically reflect the intrinsic differences in the interaction of various ions with the aqueous solvent molecules. Acknowledgment. The authors acknowledge the financial support of the National Science Foundation (Grant NSF-CTS-9212759) and technical discussions with Dr. Albert Chan (ARCO Oil and Gas Company). LA950670W (21) Ray, A.; Nemethy, G. J. Am. Chem. Soc. 1971, 93, 6787. (22) Schick, M. J.; Atlas, S. M.; Eirich, F. R. J. Phys. Chem. 1962, 66, 1326. (23) Carale, T. R.; Pham, Q. T.; Blankschtein, D. Langmuir 1994, 10, 109. (24) Voet, A. Chem. Rev. 1937, 20 (2), 169. (25) Collins, K. D.; Washabaugh, M. W. Q. Rev. Biophys. 1985, 18 (4), 323.
