J Solution Chem (2011) 40: 608–620 DOI 10.1007/s10953-011-9676-4
Micellization of Metallosurfactant N-Dodecyl/Hexadecyl/Octadecyl Salicylaldimine Cobalt(III) Complexes in Nonaqueous Media S. Caleb Noble Chandar · D. Sangeetha · M.N. Arumugham
Received: 15 February 2010 / Accepted: 30 August 2010 / Published online: 23 March 2011 © Springer Science+Business Media, LLC 2011
Abstract Abstract The critical micelle concentrations (CMC) of three metallosurfactant Schiff base cobalt(III) complexes of the type [Co(trien)(C19 H30 NO)]Cl2 , [Co(trien) (C23 H38 NO)]Cl2 and [Co(trien)(C25 H42 NO)]Cl2 , where trien = triethylenetetramine, have been studied in n-alcohol and in formamide at different temperatures by the electrical conductivity method. CMCs have also been measured as a function of percentage concentration of alcohol in the mixed solvents with formamide. Specific conductivity data (at 303–323 K) served for the evaluation of the temperature-dependent CMC and thermodynamic parame◦ ), and enters such as the standard Gibbs energy changes (G◦mic ), enthalpy changes (Hmic ◦ tropy changes (Smic ) of micelle formation. It is suggested that addition of an alcohol leads to increased penetration of formamide into the micellar interface, the extent depending on the alcohol’s chain length. The results have been discussed in terms of the solvophobic interaction, dielectric constant of the medium, the chain length of the alcohol, and the surfactant in the solvent mixture. Keywords Cobalt(III) · CMC · Metallosurfactants · Micellization · Thermodynamics of micellization
1 Introduction The aggregation behavior of surfactants in nonaqueous polar solvents and aqueous–organic mixed solvents has been the subject of much attention in the past few years [1–12]. This is due to two reasons: (i) the necessity of elucidating the effects of solvent quality on the S. Caleb Noble Chandar · D. Sangeetha SAS, Chemistry Division, VIT University, Vellore 632 014, Tamil Nadu, India M.N. Arumugham () Department of Chemistry, Muthurangam Government Arts College, Vellore 632 014, Tamil Nadu, India e-mail:
[email protected] S. Caleb Noble Chandar Present address: Department of Chemistry, Voorhees College, Vellore 632 001, India
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609
Scheme 1 Structural comparison of a metallosurfactant with an organic surfactant
nature of the self-assembly of amphiphiles, and (ii) the appearance of certain applications of surfactants where the presence of water is undesirable. Polar organic solvents with properties resembling those of water, such as ethylene glycol, glycerol, and formamide, have been the most widely investigated. These solvents share three physical properties [6]: high cohesive energy, high dielectric constant, and hydrogen bonding. Studies on the effect of alcohols on micellar properties of surfactants were initiated by Ward in 1940 [13], who found that the critical micelle concentration (CMC) of SDS passes through a minimum on addition of ethanol. Such behavior was confirmed later for the first three homologous alcohols in various micellar systems. Addition of long-chain alcohols was always found to decrease the value of the CMC. Extensive studies on the effect of linear alcohols (ethanol to hexanol) on the CMC, micellar molecular weight, and degree of ionization of the micelles of homologous alkyltrimethylammonium bromides were reported by Zana and co-workers [14–16]. The concentration at which micelles appear in solution is termed the critical micelle concentration (CMC). Experimentally, the CMC can be determined from the inflection plots of some physical property of the solution as a function of concentration. This cooperative process is characteristic of the surfactant species and is influenced by several factors such as temperature and pH of the medium [17]. The CMC can serve as a measure of micelle stability in a given state, and the thermodynamics of micellization can be determined from a study of the temperature dependence of the CMC. Metallosurfactants have been shown to display typical surfactant-like behavior [18], e.g., self-association into complex structures, aggregation at interfaces, surface tension lowering, etc., as well as exhibiting some rather interesting trends, most notably in terms of the variation of the critical micelle concentration across a homologous series. These trends arise largely from the the unique structural features of the metal ion-organic framework head group. The chemical characteristic of the metal ion brings a novel set of properties to this class of materials, e.g., fluorescence, catalysis, etc. In these surfactants, the central metal ion with its primary coordination sphere acts as the head group and the hydrophobic entity of one or more ligands acts as the tail group (Scheme 1). Schiff bases have played an important role in the development of coordination chemistry as they readily form stable complexes with most of the transition metal ions. Transition metal complexes with Schiff base amphiphiles as ligands are of interest [19, 20]. Studies on the micelle formation of metallosurfactants in nonaqueous solvents have attracted little attention [21, 22] as compared to the vast number of extensive studies that have been reported in the literature dealing with simple surfactants. Increased interest in these
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surfactants can be attributed to the ease with which desired physicochemical properties can be obtained just by varying the solution composition. The value of the CMC depends mainly on the structure of the amphiphile, temperature, and the presence of additives [23]. The effects of additives on the CMC of surfactants have been widely studied [24–27]. Some of the most studied solubilizers are alcohols, because of the important role they have in the preparation of microemulsions [28–33]. Micelles formed due to “solvophobic interactions” in polar nonaqueous solvents are similar in many respects to the micelles that are formed in aqueous media, although in general, micelle formation is not as favored in nonaqueous solvents (low dielectric constant) as they are in water for a given surfactant [34]. In previous papers [35–38, 48] we reported the influence of temperature on the behavior of some novel Co(III) and Cr(III) metallosurfactants in aqueous and nonaqueous solutions. In continuation of our works on metallosurfactants, a systematic attempt has been made to study the effect of alcohols of varying polarities on the micelles of various metallosurfactants in formamide, using the conductivity method. Various aspects of the interaction of these surfactants with formamide during micelle formation in the presence of alcohols of various chain lengths are also discussed. Thermodynamic parameters of micellization have also been obtained by determining the critical micelle concentration (CMC) as a function of temperature.
2 Experimental 2.1 Materials The metallosurfactant Schiff base Co(III) complexes [Co(trien)(C19 H30 NO)]Cl2 , [Co(trien) (C23 H38 NO)]Cl2 , and [Co(trien)(C25 H42 NO)]Cl2 , used in the present work, were prepared as reported earlier [38]. Their purity was tested by physico-chemical and spectroscopic methods. All other chemicals were obtained from Merck and were used without further purification. 2.2 Methods 2.2.1 Electrical Conductivity Measurements CMC values were measured conductometrically using a specific conductivity meter. The conductivity cell (dip type with a cell constant of 1.0 cm−1 ) was calibrated with KCl solutions in the appropriate concentration range. The cell constant was calculated using molar conductivity data for aqueous KCl published by Shedlovsky [39] and Chambers et al. [40]. Various concentrations of the complexes were prepared in the concentration range 10−5 to 10−1 mol·dm−3 . The conductivities were measured at 303, 308, 313, 318 and 323 K. The temperature of the thermostat was maintained constant within ±0.01 K. At least one set of 50 specific conductance readings at 50 different concentrations of the complex was recorded in order to get the CMC values for each system.
3 Results and Discussion 3.1 Effect of Solvent on the CMC It is well known that the micellar properties of surfactants are significantly influenced by the solvents used. The physicochemical properties of solvents: dielectric constant, hydrogen-
5.88
4.23
3.43
2.92
2.67
5.30
3.84
3.24
2.73
2.40
1.21
Ethanol
n-Propanol
n-Butanol
n-Pentanol
n-Hexanol
Formamide
4.46
1.65
6.03
4.22
5.62
Methanol
1.87
2.85
3.29
3.81
4.51
6.22
6.42
4.61
313 K
2.21
3.26
3.64
4.23
4.85
6.61
6.92
4.84
318 K
2.73
3.50
3.92
4.49
5.32
6.90
7.41
4.98
323 K 2.59
0.82
1.02
1.37
1.88
2.22
3.45
3.91
1.02
1.24
1.63
2.27
2.41
3.83
4.23
2.81
308 K
1.25
1.51
1.88
1.64
2.73
4.38
4.82
3.01
313 K
303 K
308 K
303 K
(CMC ×10−4 )/mol·dm−3
[Co(trien)(C23 H38 NO)]Cl2
(CMC ×10−4 )/mol·dm−3
[Co(trien)(C19 H30 NO)]Cl2
Water [38]
Solvent
1.49
1.69
2.42
3.23
3.21
4.62
5.20
3.20
318 K
1.66
1.94
2.28
3.45
3.67
4.93
5.44
3.39
323 K
Table 1 Effect of solvent on the critical micelle concentration of metallosurfactant Schiff base cobalt(III) complexes
0.23
0.50
0.91
1.03
1.45
2.35
2.72
1.22
303 K
0.45
0.77
1.24
1.41
1.72
2.64
3.04
1.41
308 K
0.79
1.24
1.43
1.61
2.22
2.85
3.49
1.63
313 K
(CMC ×10−4 )/mol·dm−3
[Co(trien)(C25 H42 NO)]Cl2
1.16
1.49
1.61
1.87
2.71
3.22
3.79
1.79
318 K
1.42
1.70
1.97
2.24
3.04
3.59
4.27
2.02
323 K
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Fig. 1 Electrical conductivity versus [Co(III)] in methanol solutions Fig. 2 Plot of the CMC (×10−4 ) versus temperature for [Co(trien)(C19 H30 NO)]Cl2 in different solvents
bonding ability, and cohesiveness parameters all play vital roles [41–43]. Our metallosurfactants showed an increase in CMC (Table 1, Figs. 1 and 2) for methanol and ethanol and a decrease in CMC for higher alcohols when compared with the CMC values obtained for aqueous solutions [36]. Decreases in the CMC are observed in n-propanol, n-butanol, npentanol, and n-hexanol. The decrease in CMC can be explained as follows: it is known that the major factor that determines the intermicellar solubility of long chain alcohols is the change in hydrophilic balance of the micelle resulting from inclusion of the alcohol [44, 45]. Moreover, the solvophobic effect associated with the solvophobic moiety of al-
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cohol molecules favors micellization, and increases as the length of the hydrocarbon chain of the alcohol increases. This explains the increased lowering of the CMC as the number of carbon atom increases along the alcohol series [46]. The CMCs of metallosurfactant Schiff base complexes in binary n-alcohol–formamide mixtures are reported in Table 2. Figure 3 shows the critical aggregation concentration as a function of molar concentrations of alcoholic hydroxyl groups in methanol, ethanol, n-propanol, n-butanol, and n-pentanol mixtures in formamide (as a representative for C1 –C5 containing one hydroxyl group). Increases of the CMC upon addition of methanol or ethanol is due to the solvent power of the formamide–alcohol mixture. The CMC increases on increasing the alcohol concentration in mixtures with formamide, which can be explained on the basis of the increased solubility of the nonpolar part of the surfactants in the nonaqueous medium. This is because the addition of methanol and ethanol either disrupts the formamide structure or solvates the solute molecules preferentially. Similar behavior was found for simple surfactants in formamide in the presence of alcohols with various chain lengths [44, 45, 47]. 3.2 Temperature Dependence of CMC The CMCs of metallosurfactants in n-alcohol–formamide mixtures of different compositions and at various temperatures were determined from conductivity measurements. Figure 1 shows representative plots of the specific conductivity corrected for that of the solvent, k − ks , as a function of the surfactant concentration at a fixed solvent composition and at the different temperatures. Similar plots (not shown) were obtained in all cases studied. The specific conductivity of a solution in a n-alcohol increases with increasing surfactant concentration. This increase seems to be due to the tendency of the metallosurfactant Schiff base complexes to form aggregates at higher surfactant concentrations. Similar plots (not shown) were obtained in all cases studied. This may be attributed also to the combined effect of ionic atmosphere, solvation of ions, and decreases of mobility and ionization upon formation of micelles. When the conductivity of solutions is measured with increasing concentration of surfactant, the specific conductivity–surfactant concentration plots show two straight-line regions with different slopes. The first one corresponds to the concentration range below the CMC, when only surfactant monomers exist in the solution. Micelles start to form and a change in slope appears because the conductivity changes (increases) in a different manner. The intersection of these two straight lines was taken as being at the CMC value of the surfactant. The CMC values were computed from the slope of [Co(III)] versus specific conductivity data. At all temperatures, a break in the conductivity versus concentration plots, characteristic of micelle formation, was observed. The CMCs were determined by fitting the data points above and below the break to two equations of the form y = mx + c and solving the two equations simultaneously to obtain the point of intersection. Least-squares analysis was employed, and correlation coefficients were greater than 0.98 in all the cases. The conductivity measurements at different temperatures were repeated three times, and the precision of the CMC values was found to be within ±3%. Tables 1 and 2 report the values of the CMCs of metallosurfactant Schiff base cobalt(III) complexes as a function of temperature. It was found that the CMC values increase with increases in temperature for each system. This behavior may be related to two competing effects. First, a temperature increase causes a decrease in solvation of the hydrophilic group, which favors micellization. Second, a temperature increase also causes disruption of the solvent structure surrounding the hydrophobic group, and this retards micellization. The relative magnitude of these two opposing effects will determine the CMC behavior. It is observed that with an increase in the alkyl chain length on the polar head group, the CMC again
5.10 4.13 3.74 2.42
4.53 3.47 2.65 2.02
3.31 2.23 1.88 1.01
2.55 2.12 1.70 0.81
2.32 1.72 1.21 0.54
Ethanol 80 60 40 20
n-Propanol 80 60 40 20
n-Butanol 80 60 40 20
n-Pentanol 80 60 40 20
2.59 1.85 1.46 0.89
2.84 2.37 1.97 1.27
3.67 2.44 2.04 1.29
4.67 3.66 2.97 2.28
5.32 4.34 3.94 2.64
2.83 2.28 1.83 1.22
3.22 2.82 2.27 1.43
3.83 2.67 2.28 1.43
4.89 3.88 3.24 2.66
5.66 4.73 4.27 2.89
[Co(trien)(C19 H30 NO)]Cl2 (CMC ×10−4 )/mol·dm−3 303 K 308 K 313 K
Methanol 80 60 40 20
Solvent
3.32 2.64 2.29 1.63
3.66 3.21 2.64 1.84
4.23 3.97 2.54 1.81
5.20 4.21 3.67 3.81
5.85 5.29 4.62 3.27
318 K
3.62 2.84 2.47 1.86
4.09 3.62 2.98 2.27
4.66 4.27 3.27 2.48
5.44 4.65 3.88 4.29
6.27 5.62 4.98 3.62
323 K
1.32 1.08 0.78 0.45
1.70 1.41 1.12 0.79
2.02 1.71 1.32 0.83
3.02 2.42 2.10 1.52
3.51 2.97 2.53 1.97
1.71 1.22 1.12 0.51
2.11 1.65 1.27 1.09
2.46 2.04 1.63 1.16
3.53 2.98 2.43 1.17
3.81 3.13 2.81 2.42
2.13 1.47 1.43 0.89
2.47 1.88 1.51 1.28
2.83 2.42 2.11 1.40
3.88 3.41 2.89 1.68
4.20 3.57 3.24 2.87
[Co(trien)(C23 H38 NO)]Cl2 (CMC ×10−4 )/mol·dm−3 303 K 308 K 313 K
2.48 1.86 1.91 1.19
2.83 2.31 1.85 1.63
3.30 2.91 2.41 1.89
4.42 3.87 3.40 2.12
4.67 3.97 3.70 3.25
318 K
2.94 2.25 2.03 1.60
3.31 2.76 2.22 1.85
3.77 3.27 2.69 2.27
4.93 4.22 3.71 2.44
5.08 4.54 4.18 3.71
323 K
Table 2 Critical micelle concentrations of Schiff base cobalt(III) complexes in formamide in the presence of alcohols
1.05 0.71 0.41 0.20
1.32 1.06 0.80 0.42
1.67 1.37 0.92 0.52
1.95 1.67 1.37 0.91
2.34 1.96 1.59 1.18
1.28 1.02 0.83 0.45
1.62 1.22 1.09 0.62
1.88 1.62 1.21 0.75
2.37 2.07 1.61 1.42
2.64 2.23 1.82 1.39
1.66 1.27 1.07 0.68
1.90 1.48 1.27 1.05
2.31 1.86 1.59 1.17
2.79 2.49 2.10 1.79
3.14 2.62 2.25 1.80
[Co(trien)(C25 H42 NO)]Cl2 (CMC ×10−4 )/mol·dm−3 303 K 308 K 313 K
2.04 1.65 1.45 0.99
2.29 1.92 1.59 1.27
2.90 2.33 1.92 1.55
3.28 2.83 2.48 2.14
3.49 3.07 2.70 2.19
318 K
2.47 2.02 1.87 1.43
2.70 2.41 1.92 1.65
3.47 2.81 2.41 1.88
3.69 3.14 2.86 2.43
4.10 3.22 3.02 2.65
323 K
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615
Fig. 3 Effect of different alcohols in formamide on the CMC (×104 ) for [Co(trien)(C19 H30 NO)]Cl2 at 303 K
shows a decrease. This may be due to an increase in hydrophobic character of the molecule in the coordination sphere compared with that of dodecyl/hexadecyl/octadecylamine [48]. 3.3 Effect of Dielectric Constant on the CMC Furthermore, the CMC of the metallosurfactant Schiff base complexes is a function of the dielectric constant of the medium. In alcohols, the CMC values were found to occur in the order methanol > ethanol > n-propanol > n-butanol > n-pentanol > n-hexanol, while the dielectric constant decreases from methanol to n-hexanol [49]. The decrease in the CMC with the change in dielectric constant is due to a less favorable interaction between the metallosurfactant molecules and the solvent, causing micellization to occur at relatively lower concentrations. The effect of van der Waals interactions, which will, of course, be different in each case, is difficult to predict, but it is obvious that it is an interaction between polar groups and solvent (affected by dielectric constant) which mainly controls the variations of the CMC [50]. In alcohol–formamide mixtures, similar variations in the CMC are also observed. The ionization of the surfactant in formamide is higher than in water; therefore, there is a tendency to form micelles at a lower concentrations. Further, the observation that the CMC decreases with decreases in dielectric constant (methanol to n-pentanol) also shows that the degree of ionization of micellar units changes with solvent composition, and in solvents of low dielectric constant, smaller micelles will form [48]. 3.4 Thermodynamics of Micellization Studies of the CMC versus temperature are often undertaken to obtain information on hydrophobic and head group interactions. This involves deriving various thermodynamic parameters for micelle formation. Two models are generally used, the mass-action or equilibrium model and the phase separation or pseudo-phase model [51–53]. According to these models, the standard Gibbs energy of micelle formation per mole of monomer, G◦mic , is given by G◦mic = RT (2 − αave ) ln CMC
(1)
4.56 4.61 5.74 6.94 6.78
3.32 3.75 4.61 5.11 6.52
3.89 4.24 5.83 5.92 7.88
6.04 6.63 7.46 9.21 11.67 9.24 12.91
14.31 14.67 15.12 15.82 16.72
14.55 14.93 15.42 16.22 18.05
14.87 15.57 15.91 16.14 17.27
15.31 15.91 16.82 17.13 17.86
16.17 16.82 16.71 17.28 18.32 17.62 19.43
Methanol 80 60 40 20
Ethanol 80 60 40 20
n-Propanol 80 60 40 20
n-Butanol 80 60 40 20
n-Pentanol 80 60 40 20 n-Hexanol Formamide
3.21 3.33 4.45 5.61 8.35
[Co(trien)(C19 H30 NO)]Cl2 (CMC ×10−4 )/mol·dm−3 ◦ −G◦mic −Hmic
Solvent, % alcohol
10.13 9.71 9.25 8.07 6.65 8.38 6.52
11.42 11.58 10.49 11.21 9.98
11.55 11.46 10.92 11.03 10.75
9.99 10.22 9.68 9.28 11.27
11.1 11.34 10.67 10.21 9.37
◦ T Smic
22.21 22.89 24.22 25.61 26.11 25.13 26.02
21.49 21.95 23.26 24.72 26.41
20.67 21.88 21.88 22.75 24.12
19.65 20.14 21.35 22.18 23.56
18.12 19.21 20.42 21.34 22.11
5.95 4.88 5.73 8.17 10.68 8.31 12.28
5.32 4.76 8.64 9.89 12.94
4.12 4.40 5.31 6.22 9.14
5.91 5.85 6.21 8.89 11.15
5.15 7.27 7.56 7.67 9.12
[Co(trien)(C23 H38 NO)]Cl2 (CMC ×10−4 )/mol·dm−3 ◦ −G◦mic −Hmic
16.26 18.01 18.49 17.44 15.43 16.82 13.74
16.17 17.39 15.02 14.83 13.47
16.55 17.02 16.57 16.53 14.98
13.74 14.29 15.14 13.29 12.41
12.97 11.94 12.86 13.67 12.99
◦ T Smic
Table 3 Thermodynamic parameters in kJ·mol−1 for Schiff base cobalt(III) complexes in formamide in the presence of alcohols
22.92 23.52 25.44 27.13 27.87 26.52 28.15
22.12 22.78 23.98 25.25 28.12
20.71 21.82 23.55 23.51 25.51
20.54 21.28 23.12 23.86 24.28
19.89 20.57 21.72 22.35 23.78
7.29 6.21 6.92 9.51 11.39 9.14 13.41
6.46 6.13 7.22 8.89 10.71
5.28 5.71 6.87 7.69 9.42
6.81 8.40 8.92 9.89 12.64
6.21 8.12 8.91 10.47 10.61
[Co(trien)(C25 H42 NO)]Cl2 (CMC ×10−4 )/mol·dm−3 ◦ −G◦mic −Hmic
15.63 17.28 18.52 17.62 16.48 17.38 14.74
15.66 17.25 16.60 16.36 17.41
15.43 16.11 15.28 15.82 16.09
13.73 12.88 14.20 13.97 11.64
13.68 12.45 12.81 11.88 13.17
◦ T Smic
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Fig. 4 Gibbs energy change of Co(III) metallosurfactants in 40% formamide in the presence of 60% of various n-alcohols
where R, T and αave are the gas constant, absolute temperature and average degree of micellar ionization, respectively. The enthalpy of micelle formation can be obtained by applying the Gibbs-Helmholtz equation to Eq. 1: ◦ Hmic = −RT 2 (2 − αave )d(ln CMC)/dT
(2)
Once the Gibbs energy and enthalpy of micelle formation are obtained, the entropy of micelle formation can be determined by Eq. 3: ◦ ◦ Smic = (Hmic − G◦mic )/T
(3)
Also, the low CMC values in n-alcohols may invalidate the use of above-mentioned equations, because the monomer activity could be quite different from the monomer concentration. Moreover, since the changes in CMC with temperature are small, the values of ◦ ◦ , Smic must be rather inaccurate. Therefore, the thermodynamic parameters G◦mic , Hmic in Table 3 must be viewed only as approximate. Nevertheless, from the present data, some generalizations can be extracted. The observed negative Gibbs energy of micellization for the metallosurfactant Schiff base complexes becomes more negative with increasing alkyl chain length, which indicates micellization is more favored. It also suggests that a strong solvophobic interaction takes place for long alkyl chain alcohols in formamide. Our data show that the Gibbs energy of micellization is negative in all cases and becomes less negative with increasing concentration of cosolvent. As can be seen from Table 3, the standard Gibbs energy of micelle formation decreases as the number of carbon atoms in the alkyl chains of the alcohols increases, as also shown in ◦ is negative and becomes Fig. 4. It is also observed that the enthalpy of micellization Hmic smaller as the cosolvent concentration increases. Negative values of enthalpy of micellization indicate an exothermic nature for the micellization process. Nusselder and Engberts [54] have suggested that for negative G◦mic values, the London-dispersion forces play a major ◦ , role in the micellarization process. The entropic contribution to micelle formation, T Smic ◦ presents positive values that are larger than Hmic , showing the importance of solvophobic ◦ values decrease interactions in the formation of micelles. Therefore, the fact that the T Smic
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as the formamide content increases indicates that the presence of formamide in the system produces a weakening of the solvent’s cohesiveness, thereby increasing the solubility of the hydrocarbon tails and decreasing the solvophobic effect. It is evident from Table 3 that, in all cases, the micellization process was exothermic. It is also observed that in the presence of higher alcohols (n-butanol, n-pentanol, and n-hexanol), the enthalpy value is more negative and the entropy is less positive. Therefore, the contribution of the enthalpy to micellization becomes increasingly important with increasing alcohol carbon chain length, in contrast to the predominance of the entropy factor in aqueous solutions [43, 52].
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