Mixed micellization and interfacial properties of alkanediyl-α, ω-bis(dimethylcetylammonium bromide) in the presence of alcohols

Journal of Molecular Liquids 162 (2011) 113–121

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Journal of Molecular Liquids j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m o l l i q

Mixed micellization and interfacial properties of alkanediyl-α, ω-bis(dimethylcetylammonium bromide) in the presence of alcohols Riyaj Mohammad a, Iqrar Ahmad Khan a, Kabir-ud-Din a,⁎, Pablo C. Schulz b a b

Department of Chemistry, Aligarh Muslim University, Aligarh-202 002, U.P., India Department of Chemistry and INQUISUR, Universidad Nacional del Sur, Bahía Blanca, Argentina

a r t i c l e

i n f o

Article history: Received 21 May 2011 Received in revised form 19 June 2011 Accepted 20 June 2011 Available online 13 July 2011 Keywords: Gemini surfactant Alcohol Mixed micelle Synergism

a b s t r a c t Mixed micellization and interfacial properties of cationic gemini surfactants alkanediyl-α, ω-bis(dimethylcetylammonium bromide) (16-s-16, s = 4,5,6) have been studied in the presence and absence of various alcohols (1,2-butandiol, 2-methyl-1-butanol, 2-ethyl-1-butanol, 2-butene-1,4-diol). Parameters studied include cmc (critical micelle concentration), C20 (concentration required to reduce the surface tension of the solvent by 20 mN/m), Γmax (maximum surface excess concentration at air/solution interface), and Amin (minimum area per surfactant molecule). These parameters indicate mixed micellization between the surfactants and alcohols. The theories of Rosen and Rubingh have been used to investigate the interactions between the constituents at the interface and in the micellar solution. The micelle aggregation numbers (Nagg) of mixed systems have been obtained using the steady state fluorescence quenching technique. The micropolarity of the systems was also evaluated from the ratio of intensity of peaks (I1/I3) of the pyrene fluorescence emission spectrum. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Gemini surfactants are an interesting class of surfactants whose ability to strongly reduce surface tension makes them potentially useful as detergents. They are made up of two amphiphilic moieties connected by a spacer group of varied nature at or near the polar head groups. The polar head groups can be ionic/nonionic/zwitterionic or a combination thereof [1,2]. In comparison to their conventional analogs (single head/ single tail), they form micelles at very low concentrations and also have score of better properties. Due to this, gemini surfactants have potential applications in many fields, including environmental protection [3–5]. Because of possessing low viscosity, tendency of forming small aggregates, easy mode of preparation and long lasting shelf-life, micelles have been widely accepted as agents for targeted drug delivery. However, in most of their applications, surfactants are used in the presence of additives. Among various additives, alcohols hold a special place as they are the most common co-surfactants used with surfactant + oil systems to generate microemulsions. The role of an alcohol in microemulsions is multiple. First, it delays the occurrence of liquid crystalline phases. Second, it decreases the binding modulus and increases the fluidity of the mixed surfactant + alcohol interfacial layers separating oil and water. Third, it decreases the interfacial tension between the microemulsion phase and excess oil and water [6].

⁎ Corresponding author. Tel.: + 91 571 2703515; fax: + 91 571 2708336. E-mail address: [email protected] (Kabir-ud-Din). 0167-7322/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.molliq.2011.06.011

With the ever increasing demand of the surface active compounds with superior surface activities, attempts have been made to explore widely the mixed micellar systems [2]. The mixed micellization involving geminis is also a topic of burgeoning interest: the interactions with cationic, nonionic, anionic, zwitterionic, sugar based and gemini surfactants have been studied for their use in different formulations [5,7–15]. Synergistic interactions in gemini mixed micelle formation make them more efficient and economic in different applications such as, protein refolding, protein aggregation for the cure of diseases, colloidal nanoparticle formation, foaming, cosmetics, solubilization, etc. Because of the uniqueness of geminis, the interaction of proteins and polymers with gemini surfactants has been proven better than that with conventional surfactants [16–19]. Recent micellization studies on the cationic gemini micelles with MEGA 10 [20], isopropyl myristate [21], and organic solvents [22,23] show that these types of systems are still a subject of keen interest to the researchers working on the third generation surfactants. In continuation to our own studies on the solution properties of geminis in the presence of co-surfactants (simple linear chain alcohols [24]/amines [25,26], special amines [27]) we report herein, for the first time the mixed micellar/interfacial behavior of cationic geminis alkanediyl-α, ω-bis(dimethylcetylammonium bromide) (16-s-16, s = 4, 5, 6) surfactants with different mole fractions of a variety of special alcohols at 303.15 K. Due to the presence of longer alkyl chain length in 16-s-16 series, hydrophobicity increases which results in lower values of cmc's in 16-s-16 series as compared to 12-s-12 (ref. # 1). As a result, more advanced properties are shown by surfactant solutions at quite low concentrations of 16-s-16—this is an important point regarding the

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applications of geminis (cost as well as environmental toxicity) which prompted us to choose the 16-s-16 series for the present studies. 2. Materials and methods 2.1. Materials 1,4-Dibromobutane (≥ 98%, Fluka, USA), 1,5-dibromopentane (≥98%, Fluka, USA), 1,6-dibromohexane (≥97%, Fluka, USA), N, Ndimethylcetylamine (≥95%, Fluka, Switzerland), ethyl acetate (HPLC and spectroscopic grade, ≥99.7%, Merck, Mumbai) and ethanol absolute (99.8%, E. Merck, Germany) were used without further purification. The special alcohols 1,2-butandiol (≥98%), 2-methyl-1-butanol (≥99%), 2-ethyl-1-butanol (≥98%), and 2-butene-1,4-diol (≥95%) (Scheme 1) were Sigma-Aldrich, Germany, products and were used as received. The gemini surfactants (Scheme 1) were synthesized by refluxing the corresponding α, ω-dibromoalkane with N, N-dimethylcetylamine (molar ratio 1:2.1) in dry ethanol with continuous stirring at 353.15 K for 48 h. The progress of the reaction was monitored by using the TLC technique and the reactions were completed in a single batch. After completion of the reaction the solvent was removed under vacuum and ethanol/ethylacetate mixture was used for recrystallization (at least five times). After recrystallizations, all the three surfactants were characterized by 1H NMR and FT-IR (Supplementary material) which were in agreement with the literature data [28]. The purity of the gemini surfactants was further ensured by the absence of minimum [2] in surface tension versus log of concentration of surfactant plots (Fig. S1 of Supplementary material). 2.2. Surface tension measurements The critical micelle concentrations (cmc's) of the gemini surfactants (with and without additives) in aqueous media were determined by measuring the surface tension of the pure gemini as well as of 16-s-16/additive (alcohol) solutions of various mole fractions at 303.15 K. The surface tension values were measured by the ring detachment method using an S. D. Hardson tensiometer (Kolkata, India). For each set of experiments, the ring was cleaned by heating it in alcohol flame. The cmc values were obtained from surface tension (γ) versus logCt plots. The γ values decrease continuously and then become constant along a wide concentration range (Fig. S1 of Supplementary material). The point of break, when the constancy of surface tension begins, was taken as the cmc of the system. The variation of C20 (the efficiency of a surfactant in reducing the surface tension of water is the surfactant concentration required to reduce the surface tension by 20 mN/m), the cmc/C20 ratio, and ΠCm12 (the surface pressure at the cmc), Γmax (the maximum surface excess), Amin (the 0 minimum surface area per molecule) and ΔGads (the standard Gibbs energy of adsorption) values, obtained at different mole fractions of the added alcohols in 16-s-16 (s = 4, 5, 6) solutions, are also collected in Table 1. 2.3. Steady-state fluorescence measurements The micellar aggregation numbers (Nagg) of pure surfactants and mixed systems were determined by the steady-state fluorescence quenching measurements. The fluorescence measurements were taken on a Hitachi F-2500 fluorescence spectrometer (Japan) with excitation and emission slits widths of 2.5 nm. Excitation was done at 337 nm and emission was recorded at 350–450 nm. All the spectra had one to five vibronic peaks (Fig. S2 of Supplementary material). Pyrene and cetylpridinium chloride were used as probe and quencher, respectively. An aliquot of the stock solution of pyrene in ethanol was transferred into a standard volumetric flask and the solvent was

Scheme 1. Molecular structure of (A) gemini surfactants (butanediyl-1, 4-bis (dimethylcetylammonium bromide), 16-4-16; penanediyl-1, 5-bis(dimethylcetylammonium bromide), 16-5-16, hexanediyl-1, 6-bis(dimethylcetylammonium bromide), 16-6-16) and (B) alcohols.

evaporated with N2 stream. The surfactant solution was then added and pyrene concentration was kept constant at 2 × 10 − 6 mol·L − 1. In order to ensure a Poisson distribution, the variation in quencher concentration was made carefully (quencher concentration range = 0.50–3.44 × 10 − 5 mol·L − 1). The micellar aggregation numbers (Nagg) were deduced using Eq. (1) [29]

lnI0 = lnIQ +

Nagg ½Q  ½S–cmc

ð1Þ

where [Q] and [S] are the concentrations of quencher and total surfactant, respectively, and Io and IQ are the respective fluorescence intensities in the absence and presence of the quencher. 3. Results and discussion 3.1. Various surface parameters and Gibbs energies Representative plots of the surface tension (γ) versus log of the total surfactant concentration of 16-4-16 are shown in Fig. S1 of Supplementary material (similar plots resulted for the 16-5-16 and 16-6-16 surfactants). The critical micelle concentrations (cmc's) of the mixed systems as well as of pure components were determined from the break points of such plots. Clearly, the surface tension decreases as the concentration of the additive increases. The surfactant molecules at low concentrations get adsorbed at the liquid/air interface until the surface is completely occupied; thereafter, the excess molecules tend to self-associate in the solution to form micelles and the surface tension becomes constant. This break point in the plot represents the cmc and all the so-obtained cmc values are presented in Table 1. The cmc values of the geminis are in fair agreement with the literature data [28] (literature values: 2.7 × 10 − 5 mol·L − 1 for 16-4-16, 3.6 × 10 − 5 mol·L − 1 for 16-5-16, 4.3 × 10 − 5 mol·L − 1 for 16-6-16). The cmc values decrease with increasing the alcohol concentration whereas an increase in the spacer chain length (s-value) of the gemini

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115

Table 1 0 (s) Effect of additive concentrations on the cmc (determined by surface tension measurements), C20, cmc/C20, ΠCm12, Гmax, Amin, ΔGads and Gmin values of cationic gemini surfactants: 16-416 (butandiyl-1, 4-bis(dimethylcetylammonium bromide)), 16-5-16 (pentanediyl-1, 5-bis(dimethylcetylammonium bromide)) and 16-6-16 (hexanediyl-1, 6-bis(dimethylcetylammonium bromide)) in aqueous solutions at 303.15 K. C20.105 (mol·L− 1)

cmc/C20

ΠCm12 (mΝ·m− 1)

Гmax.106 (mol·m− 2)

Amin (Å2)

0 − ΔGads (kJ·mol− 1)

(s) Gmin (kJ·mol− 1)

System: 1,2-butandiol/16-4-16 0 2.72 (2.7)⁎ 0.2 1.38 0.4 1.15 0.6 0.44 0.8 0.27

0.83 0.38 0.26 0.13 0.08

3.27 3.63 4.37 3.46 3.46

30.0 31.0 32.0 36.0 38.0

1.66 1.64 1.75 2.51 2.69

100.0 101.2 94.9 66.15 61.72

44.6 47.1 46.9 45.4 46.4

25.3 25.0 22.9 14.3 12.6

System: 2-methyl-1-butanol/16-4-16 0.2 0.95 0.4 0.57 0.6 0.50 0.8 0.30

0.32 0.17 0.13 0.08

3.02 3.46 3.80 3.63

32.5 34.0 35.5 37.5

2.15 2.27 2.42 2.69

77.22 73.14 68.55 61.72

44.2 45.4 45.4 45.9

15.1 16.7 14.2 12.8

System: 2-ethyl-1-butanol/16-4-16 0.2 0.40 0.4 0.30 0.6 0.23 0.8 0.22

0.15 0.08 0.09 0.08

2.72 3.80 2.51 2.76

32.0 35.5 36.5 37.5

2.33 2.46 2.56 3.23

71.26 67.49 64.85 51.40

45.0 46.5 46.9 44.4

14.2 13.2 13.5 10.7

System: 2-butene-1,4-diol/16-4-16 0.2 0.56 0.4 0.42 0.6 0.36 0.8 0.21

0.20 0.14 0.11 0.10

2.81 3.02 3.30 2.09

32.0 34.0 36.0 37.0

2.30 2.46 2.62 4.13

72.19 67.49 63.37 40.20

44.4 45.0 45.3 41.9

16.5 14.2 13.4 8.5

System: 1,2-butandiol/16-5-16 0 3.80 (3.6)⁎ 0.2 0.71 0.4 0.46 0.6 0.40 0.8 0.23

0.87 0.40 0.20 0.13 0.08

4.36 1.78 2.29 3.16 2.88

27.0 28.0 29.0 29.5 37.5

1.62 1.96 2.27 2.33 2.77

102.7 84.71 73.14 71.26 59.94

42.3 44.2 43.7 43.9 46.2

27.8 20.9 16.7 16.5 12.5

System: 2-methyl-1-butanol/16-5-16 0.2 1.20 0.4 0.81 0.6 0.48 0.8 0.29

0.33 0.28 0.14 0.07

3.63 2.96 3.47 3.98

31.7 32.5 35.5 38.0

1.67 2.37 2.42 2.51

99.42 70.05 68.61 66.15

47.5 43.2 45.5 47.3

24.1 16.7 14.3 13.5

System: 2-ethyl-1-butanol/16-5-16 0.2 0.40 0.4 0.32 0.6 0.25 0.8 0.22

0.20 0.13 0.09 0.08

1.99 2.51 2.88 2.75

29.0 32.5 35.0 36.7

2.44 2.73 2.92 3.30

68.05 60.82 56.86 50.31

43.2 43.8 44.5 43.8

13.5 11.5 11.9 10.7

System: 2-butene-1,4-diol/16-5-16 0.2 0.58 0.4 0.48 0.6 0.36 0.8 0.23

0.33 0.23 0.17 0.11

1.74 2.09 2.19 2.18

26.5 30.0 32.0 36.0

2.62 2.79 2.96 3.98

63.37 59.51 56.09 41.72

40.5 41.6 42.4 41.8

15.5 12.5 11.8 9.0

System: 1,2-butandiol/16-6-16 0 4.57 (4.3)⁎ 0.2 0.50 0.4 0.39 0.6 0.23 0.8 0.14

1.00 0.35 0.20 0.11 0.07

4.19 1.44 1.95 2.08 1.99

22.0 24.0 28.0 29.0 30.0

1.22 2.01 2.29 2.42 3.02

136.1 82.60 72.50 68.61 54.98

43.2 42.7 43.6 44.7 43.9

40.9 22.4 15.3 15.7 13.9

System: 2-methyl-1-butanol/16-6-16 0.2 1.17 0.4 1.38 0.6 0.40 0.8 0.26

0.50 0.53 0.16 0.11

2.34 2.63 2.52 2.34

27.5 28.5 31.0 32.0

1.72 1.83 2.39 2.91

96.53 90.73 69.47 57.05

44.6 43.8 44.3 43.4

20.0 20.5 15.1 13.7

System: 2-ethyl-1-butanol/16-6-16 0.2 0.50 0.4 0.32 0.6 0.27 0.8 0.22

0.25 0.13 0.09 0.07

2.00 2.39 3.16 3.31

26.0 30.0 33.5 35.5

1.70 2.32 2.42 2.57

97.66 71.56 68.61 64.60

46.0 44.9 46.1 46.6

23.5 15.5 13.8 14.2

αalcohol

cmc.105 (mol·L− 1)

(continued on next page)

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Table 1 (continued) cmc.105 (mol·L− 1)

αalcohol

System: 2-butene-1,4-diol/16-6-16 0.2 0.48 0.4 0.36 0.6 0.35 0.8 0.26

C20.105 (mol·L− 1)

cmc/C20

ΠCm12 (mΝ·m− 1)

Гmax.106 (mol·m− 2)

0.32 0.21 0.15 0.10

1.52 1.74 2.30 2.63

24.5 26.0 31.5 36.0

2.32 2.42 2.74 3.23

Amin (Å2) 71.56 68.61 60.59 51.40

0 − ΔGads (kJ·mol− 1)

(s) Gmin (kJ·mol− 1)

41.4 42.3 43.1 43.5

16.6 17.4 13.3 11.1

⁎ Literature value [28].

surfactants produces the opposite effect (Table 1). The trend is presented in Fig. 1 (A–C) plots. The saturation adsorption values, Γmax (in mol·m − 2), at the air/ water interface and the minimum area of a molecule at the interface, Amin (in Å 2), were evaluated using the following Eq. (2) Γmax = −

1 ðdγ=d log Ct ÞT 2:303nRT

Amin = 10

20

ð2Þ

= NA Γmax

ð3Þ

where R = 8.314 J mol − 1 K − 1, NA = Avogadro's number, and T = absolute temperature. The value of n for the Gibbs equation is the

(A)

number of species whose concentration at the interface alters with changes in the surfactant concentration in the solution. In the present case, we used a value of 2 for n to calculate the Γmax [30]. In the presence of alcohols, the repulsion among head groups decreases and more gemini surfactant molecules can be accommodated at the interface. Hence, the value of Γmax increases with increase in the additive (alcohol) concentration (Table 1). However, a reverse trend (i.e., decrease) is seen in Amin. Low values of Amin suggest that the air/ water interface is close-packed and, therefore, the orientation of the surfactant molecules at the interface is almost perpendicular to the interface. Table 1 also contains the values of C20, cmc/C20, ΠCm12 (the surface 0 pressure at the cmc), ΔGads (the Gibbs free energy of adsorption), and

(B)

3.0 4.0

1,2-butandiol 2-methyl-1-butanol 2-ethyl-1-butanol 2-butene-1,4-diol

2.5

1,2-butandiol 2-methyl-1-butanol 2-ethyl-1-butanol 2-butene-1,4-diol

3.5

cmc X 105 (mol.L-1)

1.5

1.0

2.5 2.0 1.5 1.0 0.5

0.5

0.0 0.0 0.0

0.2

0.4

0.6

0.8

0.0

0.2

alcohol

0.4

0.6

0.8

alcohol

(C) 5 1,2-butandiol 2-methyl-1-butanol 2-ethyl-1-butanol 2-butene-1,4-diol

4 cmc X 105 (mol.L-1)

cmc X 105 (mol.L-1)

3.0 2.0

3

2

1

0 0.0

0.2

0.4

0.6

0.8

alcohol

Fig. 1. Values of cmc of the gemini surfactants (16-s-16, s = 4 (A), 5 (B), and 6 (C)) at different mole fractions of alcohols.

R. Mohammad et al. / Journal of Molecular Liquids 162 (2011) 113–121

(A)

(B)

0.9

1.0 1,2-butandiol 2-methyl-1-butanol 2-ethyl-1-butanol 2-butene-1,4-diol

0.8

0.6 0.5 0.4 0.3

1,2-butandiol 2-methyl-1-butanol 2-ethyl-1-butanol 2-butene-1,4-diol

0.8

C20 x 105 (mol.L-1)

0.7

C20 x 105 (mol.L-1)

117

0.6

0.4

0.2

0.2

0.1 0.0

0.0 0.0

0.2

0.4

αalcohol

0.6

0.0

0.8

0.2

0.4

αalcohol

0.6

0.8

(C) 1.0

1,2-butandiol 2-methyl-1-butanol 2-ethyl-1-butanol 2-butene-1,4-diol

C20 x 105 (mol.L-1)

0.8

0.6

0.4

0.2

0.0 0.0

0.2

0.4

αalcohol

0.6

0.8

Fig. 2. Values of C20 of the gemini surfactants (16-s-16, s = 4 (A), 5 (B), and 6 (C)) at different mole fractions of alcohols.

G (s) min (minimum free energy at the surface). The values are obtained at different mole fractions of the added alcohols in 16-s-16 (s = 4, 5, 6). In each case, the C20 value decreases with additive concentration and follows a similar trend for all the alcohols (see Fig. 2 (A–C)). The magnitude of the negative log of the C20 value is 2 or 3 orders smaller than that of comparable conventional cationic surfactants [31] (C20 value of cetylpridinium chloride (CPC) = 39.8 × 10 − 5 mol·L − 1) and also in good agreement with previous work [7]. The higher surface activity may be due to the presence of two hydrophobic groups in the gemini surfactant molecules. The cmc/C20 ratio measures as how far the surface tension of water can be reduced by the presence of a surfactant. The cmc/C20 effectiveness is in the order: 16-6-16 N 16-5-16 N 16-4-16, which supports that the tendency of 16-6-16 to adsorb at the air/water interface is more than that of 16-5-16 or 16-4-16. In a previous study, we found a similar trend of decreasing cmc with increasing amine concentration [27]. To quantify the effect of alcohols in the mixture on the micellization process, the standard Gibbs energy of micellization, 0 0 ΔG m , and the standard Gibbs energy of adsorption, ΔGads , were calculated by using the following equations: 0

m

ΔGm = RT⋅ lnC12

ð4Þ

m (C12 is the cmc of the mixture of the two components at a given mole fraction)

0

0

m

ΔGads = ΔGm −ΠC12 = Γmax :

ð5Þ

0 values decrease with increasing the alcohol concentraThe ΔG ads tions. The standard state for the adsorbed surfactant is a hypothetical monolayer at its minimum surface area per molecule, but at zero surface pressure. The last term in Eq. (5) expresses the work involved in transferring the surfactant molecule from a monolayer at a zero surface pressure to the micelle. In the present case, the last term of 0 Eq. (5) is very small as compared to ΔGm , which indicates that the work involved in transferring the surfactant molecule from a monolayer at zero surface pressure to the micelle is negligible. All 0 the ΔG ads values are negative (Table 1), implying that the adsorption of the surfactants at the air/mixture interface takes place spontaneously and is in the order: 16-4-16 N 16-5-16 N 16-6-16. The ΠCm12 values were obtained by using the equation

m

m

Π C12 = γ0 −γC12

ð6Þ

where γ0 and γCm12 are, respectively, the surface tension of the solvent and of the mixture at the cmc. With increasing the alcohol

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concentration, the value of ΠCm12 increases, indicating that the efficiency of the system increases (Table 1). Also, the values of ΠCm12 decrease with increasing the spacer length of the gemini surfactants. Sugihara et al. [32] have proposed a thermodynamic quantity for the evaluation of synergism in mixing, which is the free energy of the (s) given air/water interface, Gmin , defined as ð sÞ

m

G min = A min ⋅ γC12 ⋅NA

ð7Þ

(s) The Gmin values, listed in Table 1, are found to decrease with (s) increasing the additive (alcohol) mole fraction. The G min is regarded as the work needed to make an interface per mole or the free energy change accompanied by the transition from the bulk phase to the surface phase of the solution components. In (s) other words, the lower the value of Gmin , the more thermodynamically stable surface is formed. Thus, from the above discussion it can be summarized that the dimeric structure of gemini surfactant not only effectively enhances its tendency of reducing interfacial tension, but also greatly enhances the efficiency of the adsorbing tendency of the surfactant at the interface due to its stronger hydrophobicity (Scheme 1). The results show that the cmc for the gemini surfactants exhibits a tendency to decrease with increasing alcohol concentration. Addition of alcohols to the aqueous gemini solutions decreases the electrostatic repulsion between the polar head groups of gemini micelles leading to a tight packing curvature, which is reflected by an increase in value of Γmax with increase in alcohol concentration. Also, the adsorbing tendency at the air/water interface is found to be decreasing with increasing the spacer chain length, which indeed is confirmed by high cmc/C20 values in case of longer spacer chain length geminis. The effectiveness of a surface active molecule is measured by the surface pressure at cmc (ΠCm12), which increases with the increasing alcohol concentration, indicating an increase in the efficiency of the system. However, ΠCm12 decreases with longer spacer gemini, which indeed supports the highest efficiency of 16-4-16-alcohol systems than that of the other two geminis. The changes in the magnitude of the Gibbs-free energy contribution are associated with the spontaneous adsorption of the Gemini (s) surfactant at air/solution interface. Also, a decrease in the ΔGmin with increasing alcohol concentration reveals the fact that work is required for the transfer of the monomeric form of the surfactants (present at the surface) to the micellar form through the aqueous medium. Thus, (s) the lower the value of ΔGmin , the more thermodynamically stable surface is formed.

3.2. Interaction between gemini and alcohol Molecular interactions between two components (amphiphiles) at an interface or in micelles are commonly measured by the so-called interaction parameters, β, which are conveniently obtained from surface (or interfacial) tension or from cmc data by using well known equations [2]. By calculating the values of the β, the nature and strength of the interaction between two components can be ascertained (β m is the interaction parameter for mixed micelle formation in an aqueous medium and β σ is the interaction parameter for mixed monolayer formation at an aqueous solution/air interface). The corresponding activity coefficient (f1m and f2m, f1σ and f2σ) can be calculated using relevant Eqs. (8) and (9) n 2 f1 = exp βð1–X1 Þ

ð8Þ

n o 2 f2 = exp βðX1 Þ :

ð9Þ

For ideal mixing of two components, β assumes a value of zero. A positive β value means repulsive interaction among mixed species, whereas a negative β value implies an attractive interaction; the more negative its value, the greater is the interaction. At all mole fractions of the mixed systems, the β m values are found to be negative (Table 2), suggesting that the interaction is more attractive in between the two components in the mixed micelles than the self-interaction of the two components before mixing. As the mole fraction of alcohols increases, β m values become more negative. This indicates an increase in the interaction with an increase in concentration of alcohol, seemingly due to intercalation of alcohols in the micelles of the gemini surfactants which, in turn, screens out the electrostatic repulsions and also increases the hydrophobic interactions (also evident from the cmc values (Table 1), which decrease with increasing concentration of alcohols). The β σ trend is similar (Table 2), i.e., the mixtures of geminis/ alcohols show stronger attractive interaction at the solution/air interface. The β σ values are more negative than β m (see Table 2), which imply that the interactions at the solution/air interface are stronger than in mixed micelles. This is due to the steric factor which is more important in micelle formation than in monolayer formation at a planar interface. Increased bulkiness in the hydrophobic group causes greater difficulty for incorporation into the curved mixed micelles compared to that of getting accommodated at the planar interface. The values of interaction parameters indicate that the attractive interactions of gemini/alcohol are more in case of longer spacer chain length than that of the smaller spacer chain length of the gemini (Table 2). The values of excess free energy of micellization, ΔGex, calculated by Eq. (10), ΔGex = ½X1 ln f1 + ð1−X1 Þ ln f2 RT

ð10Þ

are listed in Table 2, which are negative for all mole fractions of the alcohols and the magnitude increases (ΔGex becomes more negative) with increasing alcohol mole fraction, indicating stability of the micelles. Thus, with the increasing mole fraction of alcohols, the interaction parameters (β m, β σ) become more negative, indicating stronger interaction at the air/solution interface. The intercalation of alcohols in the gemini micelles causes a screening of the electrostatic repulsion between head groups of the gemini surfactant micelles and also increases hydrophobic interactions. Also, the attractive interactions are more in case of longer spacer chain length of gemini–alcohol systems. More negative values of ΔGex indicate that micelles are stabilized with increasing the alcohol concentration. 3.3. Synergism In mixtures containing two amphiphiles, the existence of synergism has been shown to depend not only on the strength of interaction between them (measured by the values of the β parameter) but also on the relevant properties of the individual amphiphile components of a mixture [33]. The conditions for synergism in surface tension reduction efficiency (when the total concentration of mixed surfactant required to reduce the surface tension of the solvent to a given value is less than that of individual amphiphile) are the following: (a) β σ must be negative (b) |β σ| N |ln (C1S/C2S)| where C1S and C2S are the molar concentrations of amphiphiles 1 and 2, respectively, required to achieve the same surface tension value. The data show that there is good synergism in surface tension reduction efficiency for the gemini surfactant/alcohol mixtures (Table 3).

R. Mohammad et al. / Journal of Molecular Liquids 162 (2011) 113–121

119

Table 2 Micellar compositions (X1m, X1σ), interaction parameters (βm, βσ), and activity coefficient (f1m, f2m, f1σ, f2σ) of binary mixtures of gemini surfactants (16-4-16, 16-5-16, 16-6-16) and alcohols at different mole fractions of alcohols (αalcohol). βm

f 1m

f 2m

ΔGex (kJ·mol− 1)

X 1σ

βσ

f 1σ

f 2σ

− 10.5 − 11.5 − 16.4 − 19.7

0.00253 0.00345 0.00158 0.00116

0.536 0.360 0.102 0.033

− 4.86 − 6.06 − 9.67 − 12.09

0.158 0.251 0.330 0.378

− 9.5 − 12.3 − 16.7 − 20.2

0.00115 0.00101 0.00055 0.00040

0.788 0.461 0.162 0.056

System: 2-methyl-1-butanol/16-4-16 0.2 0.307 − 9.7 0.4 0.371 − 11.8 0.6 0.407 − 12.7 0.8 0.451 − 15.9

0.00958 0.00951 0.01141 0.00824

0.402 0.198 0.121 0.039

− 5.19 − 6.92 − 7.74 − 9.94

0.278 0.315 0.363 0.409

− 11.9 − 12.1 − 14.1 − 16.8

0.00208 0.00349 0.00331 0.00283

0.400 0.302 0.156 0.060

System: 2-ethyl-1-butanol/16-4-16 0.2 0.339 0.4 0.375 0.6 0.405 0.8 0.435

− 15.1 − 16.1 − 17.5 − 18.9

0.00139 0.00188 0.00200 0.00238

0.177 0.104 0.056 0.028

− 8.50 − 9.49 − 10.66 − 11.71

0.313 0.351 0.364 0.397

− 16.5 − 17.9 − 16.9 − 18.2

0.00042 0.00052 0.00109 0.00133

0.199 0.109 0.107 0.057

System: 2-butene-1,4-diol/16-4-16 0.2 0.339 0.4 0.375 0.6 0.405 0.8 0.435

− 17.3 − 18.6 − 20.1 − 23.2

0.00051 0.00069 0.00083 0.00060

0.136 0.073 0.037 0.012

− 9.80 − 10.99 − 12.18 − 14.42

0.256 0.308 0.337 0.373

− 14.5 − 16.6 − 17.4 − 19.1

0.00033 0.00036 0.00047 0.00054

0.386 0.208 0.138 0.069

System: 1,2-butandiol/16-5-16 0.2 0.319 0.4 0.362 0.6 0.39 0.8 0.428

− 14.9 − 16.7 − 17.7 − 21.2

0.00099 0.00114 0.00138 0.00096

0.219 0.113 0.068 0.020

− 8.16 − 9.68 − 10.60 − 13.13

0.276 0.341 0.376 0.407

− 14.5 − 18.3 − 20.8 − 23.1

0.0005 0.00035 0.00030 0.00029

0.331 0.119 0.053 0.022

System: 2-methyl-1-butanol/16-5-16 0.2 0.321 − 9.7 0.4 0.378 − 11.1 0.6 0.423 − 13.7 0.8 0.462 − 16.8

0.01157 0.01327 0.01054 0.00777

0.369 0.203 0.086 0.028

− 5.31 − 6.61 − 8.42 − 10.49

0.302 0.338 0.397 0.436

− 11.7 − 11.8 − 15.8 − 19.1

0.00332 0.00566 0.00315 0.00231

0.343 0.259 0.082 0.026

System: 2-ethyl-1-butanol/16-5-16 0.2 0.357 0.4 0.388 0.6 0.416 0.8 0.444

− 16.0 − 16.7 − 18.0 − 19.5

0.00132 0.00194 0.00212 0.00244

0.129 0.081 0.044 0.021

− 9.28 − 9.98 − 11.05 − 12.14

0.346 0.376 0.395 0.422

− 18.1 − 18.9 − 18.9 − 19.9

0.00044 0.00061 0.00096 0.00131

0.115 0.068 0.052 0.029

System: 2-butene-1,4-diol/16-5-16 0.2 0.325 0.4 0.355 0.6 0.386 0.8 0.422

− 16.5 − 17.0 − 18.7 − 21.8

0.00054 0.00084 0.00087 0.00067

0.175 0.117 0.062 0.020

− 9.12 − 9.82 − 11.15 − 13.47

0.288 0.34 0.366 0.399

− 15.3 − 18.0 − 18.9 − 20.9

0.00042 0.00039 0.00049 0.00053

0.280 0.124 0.079 0.036

System: 1,2-butandiol/16-6-16 0.2 0.344 0.4 0.375 0.6 0.407 0.8 0.438

− 17.1 − 17.9 − 20.4 − 23.6

0.00065 0.00093 0.00076 0.00057

0.133 0.081 0.034 0.011

− 9.69 − 10.55 − 12.42 − 14.63

0.333 0.370 0.393 0.418

− 18.9 − 21.0 − 22.2 − 23.8

0.00023 0.00024 0.00028 0.00032

0.123 0.056 0.032 0.016

System: 2-methyl-1-butanol/16-6-16 0.2 0.338 − 10.4 0.4 0.367 − 9.4 0.6 0.434 − 14.8 0.8 0.468 − 17.6

0.01072 0.02328 0.00856 0.00688

0.306 0.282 0.061 0.021

− 5.84 − 5.49 − 9.19 − 11.05

0.358 0.369 0.418 0.445

− 15.6 − 13.3 − 17.5 − 18.4

0.00161 0.00496 0.00262 0.00344

0.135 0.163 0.047 0.026

System: 2-methyl-1-butanol/16-6-16 0.2 0.358 − 15.6 0.4 0.396 − 17.3 0.6 0.421 − 18.2 0.8 0.45 − 20.1

0.00164 0.00184 0.00226 0.00230

0.136 0.067 0.040 0.017

− 9.01 − 10.39 − 11.16 − 12.53

0.358 0.390 0.415 0.439

− 18.2 − 19.6 − 21.3 − 22.4

0.00056 0.00068 0.00069 0.00083

0.097 0.051 0.026 0.013

System: 2-butene-1,4-diol/16-6-16 0.2 0.341 0.4 0.372 0.6 0.394 0.8 0.426

0.00044 0.00059 0.00082 0.00076

0.124 0.074 0.050 0.019

− 10.10 − 11.09 − 11.63 − 13.44

0.335 0.358 0.385 0.414

− 18.9 − 18.9 − 20.4 − 22.2

0.00023 0.00040 0.00045 0.00049

0.115 0.088 0.049 0.022

αalcohol

X1m

System: 1,2-butandiol/16-4-16 0.2 0.244 0.4 0.298 0.6 0.373 0.8 0.415

− 17.8 − 18.8 − 19.3 − 21.8

120

R. Mohammad et al. / Journal of Molecular Liquids 162 (2011) 113–121

Table 3 Comparison table for synergism of the mixed systems of gemini surfactants (16-4-16, 16-5-16, 16-6-16) and alcohols. ln (C1S/C2S)

αalcohol

ln(C1m/C2m)

βσ-βm

System: 1,2-butandiol/16-4-16 0.2 0.4 0.6 0.8

6.82

5.10

4.83

3.17

+ 0.9 − 0.8 − 0.3 − 0.5

4.13

6.77

5.47

4.10

− 1.4 − 1.9 + 0.7 + 0.7

5.57

4.77

+ 2.8 + 2.1 + 2.6 + 4.1

6.03

2.83

3.80

+ 0.4 − 1.6 − 3.1 − 1.9

m

(a) β must be negative (b) |β m| N |ln (C1m/C2m)| (c) |β σ − β m| N [|ln (C1S/C2S)| – |ln (C1m/C2m)|] C 2m

and are the critical micelle concentrations of where amphiphiles 1 and 2, respectively. All the mixtures of 16-s-16 gemini surfactants are found to exhibit synergism in mixed micelle formation with the alcohols used in this study (Table 3). Thus, the gemini–alcohol mixed systems show synergism with increased alcohol concentration. Rosen's approach reveals an increased synergism in the mixed monolayer in comparison to the mixed micelle.

3.4. Micelle aggregation number The steady state fluorescence method is a convenient method for evaluating micelle aggregation number (Nagg). The micelle aggregation number was determined at different mole fractions of the binary surfactant–alcohol systems (see Table 4). The Nagg values for mixtures are always greater than of pure gemini surfactants. This again supports our earlier explanation that addition of alcohols decreases the repulsion among head groups and hence compact micelles with higher aggregation number are formed. The ratio of intensity of the first (I1) and third (I3) vibronic peaks, i.e., I1/I3, of the pyrene fluorescence emission spectrum in the presence of surfactants is considered to be the index of micropolarity of the system; i.e., it gives an idea of the microenvironment in the micelle. A low value of this ratio (b1) is generally taken as pyrene having nonpolar surrounding, whereas a higher value (N1) is taken as the pyrene having polar surrounding [34]. In our case, for all mole fractions, the ratio comes out to be greater than 1.

5.62

ln(C1m/C2m)

βσ-βm

4.58

− 1.8 − 3.2 − 1.8 − 0.1

System: 2-methyl-1-butanol/16-6-16 − 2.0 − 0.5 − 2.2 − 2.3

3.64

− 2.0 − 2.3 − 0.9 − 0.4

4.37

+ 1.2 − 0.9 − 0.2 + 0.9

5.57

2.65

− 5.3 − 3.9 − 2.7 − 0.8

System: 2-ethyl-1-butanol/16-6-16

System: 2-butene-1,4-diol/16-5-16 5.13

ln (C1S /C2S )

System: 1,2-butandiol/16-6-16

System: 2-ethyl-1-butanol/16-5-16

Analogously, synergism in the mixed micelle formation exists when the cmc of the mixture is less than that of either amphiphile of the mixture. The conditions for this to exist in a mixture of two surfactants are the following [33]:

C1m

βσ-βm

System: 2-methyl-1-butanol/16-5-16

System: 2-butene-1,4-diol/16-4-16 0.2 0.4 0.6 0.8

6.08

− 2.2 − 0.3 − 1.4 − 0.9

System: 2-ethyl-1-butanol/16-4-16 0.2 0.4 0.6 0.8

ln(C1m/C2m)

System: 1,2-butandiol/16-5-16

System: 2-methyl-1-butanol/16-4-16 0.2 0.4 0.6 0.8

ln (C1S /C2S )

3.62

− 2.6 − 2.3 − 3.1 + 2.3

System: 2-butene-1,4-diol/16-6-16 4.95

− 1.2 − 0.1 − 1.0 − 0.4

The apparent dielectric constant (D) of the medium was also estimated [35] by using the relation I1 = 1:00461 + 0:01253 D: I3

ð11Þ

The values of D obtained from Eq. (11) probably give a measure of the local polarity in which the probe is located, instead of the true dielectric constant. Moreover, as the probe molecule is relatively large, it is probable that different parts of the molecule are in regions of the micelle having different polarities and then D is probably an average value. There is a complementary effect, caused by the separation of the ionic headgroups of geminis. The local dielectric constant on charged surfaces in water is reduced when the surface electrical potential is increased, because of the orientation of water molecules by the electric field [36,37]. Then, the interacalation of alcohol molecules between the surfactant ionic headgroups must increase the value of D with respect to that in the pure surfactant micelles. Among various factors acting on alcohol addition, the formation and growth of micelles are mainly favored by the screening of electrostatic repulsion among the polar head groups. This is evidenced by an increase of the micelle aggregation number. 4. Summary The interaction of cationic gemini surfactants (butanediyl-1, 4-bis (dimethylcetylammonium bromide), pentanediyl-1, 5-bis(dimethylcetylammonium bromide) and hexanediyl-1, 6-bis(dimethylcetylammonium bromide)) and special alcohols have been investigated. The following conclusions are drawn: (a) The trend of the increase in Гmax and decrease of cmc and Amin is due to formation of mixed micelles with the gemini surfactants. (b) As the spacer chain length of gemini surfactants increases, the cmc, C20, and Amin values increase while ΠCm12 and Гmax. values decrease.

R. Mohammad et al. / Journal of Molecular Liquids 162 (2011) 113–121 Table 4 Average aggregation number, micropolarity and apparent dielectric constant for the gemini/alcohol systems evaluated on the basis of steady-state fluorescence quenching technique. αalcohol

Nagg

Ngem

Nalcohol

I1/I3

D

0 10 18 30 172

1.58 1.92 1.96 2.02 2.06

46 73 76 81 84

10 18 46 70

1.67 1.68 1.81 2.09

53 54 65 87

System: 2-ethyl-1-butanol/16-4-16 0.2 124 100 0.4 100 60 0.6 115 31 0.8 148 30

24 40 84 118

1.92 2.96 2.45 3.07

73 103 115 165

System: 2-butene-1,4-diol/16-4-16 0.2 93 73 0.4 96 57 0.6 128 51 0.8 308 52

20 39 77 256

2.17 2.68 2.26 3.57

93 133 100 205

System: 1,2-butandiol/16-5-16 0 39 0.2 53 0.4 63 0.6 94 0.8 172

0 11 22 56 145

1.88 1.44 1.43 0.230 0.272

70 34 33 1.42 1.47

System: 2-methyl-1-butanol/16-5-16 0.2 137 109 0.4 212 127 0.6 220 32 0.8 252 50

28 85 188 202

0.600 1.125 0.229 1.346

1.52 1.34 1.32 2.39

System: 2-ethyl-1-butanol/16-5-16 0.2 65 52 0.4 72 35 0.6 107 43 0.8 176 35

13 37 64 141

0.177 0.167 0.207 0.171

1.41 1.42 1.42 1.47

System: 2-butene-1,4-diol/16-5-16 0.2 140 91 0.4 166 99 0.6 151 60 0.8 189 37

49 67 91 152

0.417 0.587 0.178 0.131

1.25 1.40 1.43 1.51

System: 1,2-butandiol/16-6-16 0 41 0.2 46 0.4 66 0.6 80 0.8 160

0 10 26 48 130

0.244 0.088 0.233 0.101 1.970

2.12 1.48 1.53 1.52 1.54

System: 2-methyl-1-butanol/16-6-16 0.2 156 119 0.4 129 77 0.6 114 46 0.8 252 50

37 52 68 202

0.676 0.357 0.338 1.346

1.55 1.58 1.48 2.39

System: 2-ethyl-1-butanol/16-6-16 0.2 188 150 0.4 185 41 0.6 195 55 0.8 228 45

38 144 140 183

0.792 0.143 0.235 0.293

1.50 1.55 1.53 1.54

System: 2-butene-1,4-diol/16-6-16 0.2 118 94 0.4 115 33 0.6 107 43 0.8 225 50

24 82 64 175

0.372 0.112 0.127 0.177

1.46 1.43 1.42 1.44

System: 1,2-butandiol/16-4-16 0 30 30 0.2 48 38 0.4 45 27 0.6 50 20 0.8 215 43 System: 2-methyl-1-butanol/16-4-16 0.2 48 38 0.4 46 28 0.6 76 30 0.8 88 18

39 42 41 38 27

41 36 40 32 30

121

0 (c) The values of ΔG ads indicate that the adsorption of the surfactant at the air/solution interface takes place spontaneously. (d) The β values (both β m and β σ) indicate the attractive interaction and that the interaction is more in case of smaller spacer chain length. (e) The gemini surfactant/alcohol systems show an increase in synergism with the increase in alcohol concentration. (f) The negative values of ΔGex at all mole fractions of alcohols indicate the stability of the micelles. (g) Values of Ngem and Nalcohol again indicate that at higher alcohol mole fractions, their contribution is more than of the geminis.

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