Micellization of cationic surfactants in alcohol — water

Download PDF
1 downloads 0 Views 760KB Size Report
Comment
Jul 26, 2016 - The effect of addition of methanol and ethanol on the micellization of ... solvent surface tension to the limiting surface tension at the cmc has ...
Journal of Molecular Liquids 222 (2016) 906–914

Contents lists available at ScienceDirect

Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq

Micellization of cationic surfactants in alcohol — water mixed solvent media Sujit Kumar Shah a, Sujeet Kumar Chatterjee a, Ajaya Bhattarai a,⁎ a

Department of Chemistry, M. M. A. M. C., Tribhuvan University, Biratnagar, Nepal

a r t i c l e

i n f o

Article history: Received 28 April 2016 Received in revised form 11 July 2016 Accepted 21 July 2016 Available online 26 July 2016 Keywords: Cationic surfactant Methanol Ethanol Surface tension Micelle Conductivity Surface properties Solvent parameter Solvophobic parameter

a b s t r a c t The effect of addition of methanol and ethanol on the micellization of cationic surfactants dodecyltrimethylammonium bromide (DTAB) and cetyltrimethylammonium bromide (CTAB) in aqueous medium have been studied by conductance and surface tension measurements at 298.15 K. Different physicochemical properties such as Gibb's free energy of micellization (ΔG°m), free energy of surfactant tail transfer (ΔG°trans), maximum surface excess concentration (Гmax), area occupied by surfactant molecule (Amin), surface pressure at the cmc (πcmc), packing parameters (P) and standard free energy interfacial adsorption (ΔG°ads) are calculated in water, 0.10, 0.20, 0.30 and 0.40 volume fractions of methanol–water and in water, 0.10, 0.20, 0.30, 0.40, 0.50 and 0.60 volume fractions of ethanol–water respectively at 298.15 K. Addition of alcohol significantly affects the physicochemical properties of both DTAB and CTAB. With increasing concentration of alcohol, cohesive force and dielectric constant decrease that affects the micellization and other physicochemical properties. However, at the higher volume fraction of ethanol–water a slight variation of properties are seen. The micellization of DTAB and CTAB have been assessed in terms of different solvent parameters. The ratio of the solvent surface tension to the limiting surface tension at the cmc has been used as the solvophobic effect. The solvophobic parameter characterized by Gibbs energy of transfer of hydrocarbon from gas into a given solvent can be used to account for the effect of alcohol on the formation and growth of the cationic surfactants aggregate in water. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Surfactants are characterized by having hydrophobic head and hydrophilic tail which when added in water get aggregated, after certain concentration, to form micelle and that concentration is known as critical micelle concentration (cmc) [1]. The importance of surfactant is due to its ability to form micelle. Micellization is governed by balancing of hydrophobic and hydrophilic force, which is greatly affected by additives due to which it finds great importance in science and technology [1–14]. The presence of alcohol in different quantities can strongly alters the physicochemical properties of micellar solution [13]. In these studies, effects of alcohols are explained on the basis of interaction of surfactant aggregates and solvent composition. However, there is lack of detailed understanding of physicochemical properties of methanol and ethanol with cationic surfactant. This paper describes the effect of methanol and ethanol on physicochemical properties of cationic surfactants, mainly dodecyltrimethylammonium bromide (DTAB) and cetyltrimethylammonium bromide (CTAB). In the literature, researcher describes the effect of alcohol on micellization is direct consequence of decrease in polarity of medium and thus decrease in hydrophobic effect. ⁎ Corresponding author. E-mail address: [email protected] (A. Bhattarai).

http://dx.doi.org/10.1016/j.molliq.2016.07.098 0167-7322/© 2016 Elsevier B.V. All rights reserved.

Effect of octan-1-ol (up to 0.0300 mol dm−3) and nonan-1-ol (up to 0.0275 mol dm− 3) on the micellar behavior of CTAB was studied by Dubey [1] by measuring conductivity, density and speed of sound and conclude that there is gradual decrease in cmc. Nazir et al. [3] studied the effect of alcohols (methanol, ethanol and propanol) only up to 20% by volume on micellization of CTAB by conductivity measurement and they found degree of ionization and cmc are affected by addition of alcohol as cosolvent. Similarly, effect of methanol, ethanol and propanol on micellization of cationic gemini and monomeric surfactants was studied by Kumar et al. [5]. The effect of ethylene glycol (up to 60% by weight) on micellization of tetradecyltrimethylammonium bromide was studied by fluorescence anisotropy of several molecules residing in different regions of the micelle and the effect was discussed on the basis of solvent penetration [7]. Ruiz et al. [11] studied thermodynamic and micellar properties of Tween 20 in water–ethylene glycol mixed solvent media up to 40% by weight. The study allowed concluding that ethylene glycol acts as water structure breaker and its interaction with the surfactant hydrophilic are controlling factors of the micellization process. Dubey [13] studied effect of higher chain monohydric alcohol on micellization of aqueous solution of sodiumdodecyl sulfate by measuring density, speed of sound, viscosity and specific conductivities. It is concluded that higher chain alcohol molecules behave as water structure makers. Das et al. [14] selected water–ethylene glycol and water–dimethyl

S.K. Shah et al. / Journal of Molecular Liquids 222 (2016) 906–914

sulfoxide mixed solvent media of varied compositions on DTAB, SDDS and Tween-20. Recently, effect of methanol on physicochemical properties of DTAB in aqueous solution was studied by measuring electrical conductivity and surface tension. The studied was limited up to 0.4 volume fraction of methanol–water system [15]. Similarly, the effect of methanol on CTAB system is fixed up to 0.4 volume fraction of methanol–water system. The literature shows that very few investigations are carried out with limited discussions on physicochemical properties of DTAB and CTAB in ethanol–water system [3,16]. This article intends to discuss minutely about the effect of methanol and ethanol–water system on micellization of DTAB and CTAB at 298.15 K. The results have been analyzed in terms of the solvent parameters, viz, permittivity (D), Reichardt's parameter (ET(30)), Gordon parameter (G), viscosity (η0) and Hildebrand parameter (δ). The solvophobic effect can be described from the ratio of solvent surface tension to the limiting surface tension at the cmc. The obtained results have also been explained from the solvophobic parameters of hydrocarbon in the aqueous organic solutions. 2. Experimental section 2.1. Materials Methanol and Ethanol (99.0% pure) were obtained from Merck, India. The methanol had a density of 0.7872 g/cm3 and coefficient of viscosity of 0.5440 mPa·S at 298.15 K; these values are in good agreement with those found in the literature [17]. The ethanol had a density of 0.7850 g/cm3 and coefficient of viscosity of 1.0990 mPa·S at 298.15 K; these values are in good agreement with those found in the literature [18]. The cationic surfactants DTAB and CTAB were purchased from Loba Chemie Private Limited (Mumbai, India). DTAB and CTAB were highly pure (N99.0%) and used after drying for 1 h. All solutions were prepared in triply distilled water with a specific conductance of b 0.6 μS/cm at 298.15 K. The cmc of DTAB and CTAB were measured at 298.15 K. The observed cmc of DTAB and CTAB by conductivity are 15.1 mM and 0.9 mM which were matched with the literature [19]. The methanol– water and ethanol–water mixtures were prepared up to 0.4 volume fraction and 0.6 volume fraction at 298.15 K respectively by maintaining at constant temperature in thermostat [20]. The mixed solvents were further used after 24 h to make the solutions of DTAB and CTAB. Again, the solutions were kept for 24 h and the volume make up of the solutions were done with the solvents at constant temperature 298.15 K.

907

conductance of DTAB and CTAB in methanol–water and ethanol– water mixtures at 298.15 K are presented in Figs. 1 to 4 respectively. For each plot conductivity increases with increase in concentration with certain slope and at certain concentration, the slope changes. The intersection of these two lines having different slopes is estimated as critical micelle concentration (cmc). Ratio of premicellar and postmicellar slopes give degree of ionization (α). In this method, properties of surfactant solutions are mainly exhibited by the plots of conductance with concentration of solution where there are different variations of pre- and post-micellar slopes. With the addition of additives (methanol and ethanol in this case) these two slopes are varied which leads to difference in physicochemical properties of solution. It is seen from Figs. 1–4, that conductivity of DTAB and CTAB decreases with increase in methanol and ethanol content in water. These figures also illustrate that the slopes in pre- and post-micellar region also decreases considerably. This phenomenon is explained on the basis of ability of alcohol to break the water structure and increase in the viscosity of medium [5]. Tables 1 and 2 display the values of premicellar (S1) and postmicellar (S2) slopes drawn from the plots of conductivity and concentration of DTAB and CTAB solutions at different volume fractions of methanol–water and ethanol–water respectively. It is seen from the data of Tables 1 and 2 that there is significant decrease in slopes of premicellar region than that in postmicellar region with the increase in methanol and ethanol content. Due to this variation in slopes there is change in physicochemical properties of surfactant in methanol–water and ethanol–water system. Bhattarai [23] described the physicochemical properties of cationic-anionic surfactants mixture on the basis of variation of slopes with increase in methanol content. Tables 1 and 2 also display the values of cmc and degree of ionization of DTAB and CTAB in water and methanol–water mixtures and in water and ethanol–water mixtures at 298.15 K respectively. α values of DTAB and CTAB are in close agreement with literature [19,24]. It is seen from the Tables 1 and 2 that cmc as well as degree of ionization (α) increase with increase in methanol and ethanol content. However, the interesting thing is that cmc seems a slight decrease after 0.40 volume fractions of ethanol–water. Increase in degree of ionization (α) with increase in alcohol volume fraction of ethanol is explained on the basis of two factors. First is due to intercalation of alcohol molecules between surfactant ions in micelle which leads to decrease in surface charge density which make more friction between ionic heads and counterions stay dissociated. Second factor is due to reduction of the relative permittivity with addition of alcohol [14–16].

2.2. Methods The specific conductivity measurements of freshly prepared solutions were measured by digital conductivity meter from Systronics India Ltd. with dip type conductivity cell having cell constant 1.002 cm−1 and having an uncertainty of 0.01%. The conductivity cell was calibrated with KCl solution (0.1 M and 0.01 M) at 298.15 K [21]. The surface tension of freshly prepared solutions was measured with a calibrated du Nouy tensiometer (Kruss, Germany) by platinum ring detachment method [22]. The uncertainty of the measurements was within ±1 × 10−3 N·m−1. The tensiometer was connected to a water-flow cryostat to maintain the temperature equilibrium. Prior to each measurement, the ring was heated briefly by holding it above a Bunsen burner until glowing. The temperature control had an accuracy of ±0.1 °C. 3. Results and discussion 3.1. Conductometric studies Conductivity study is one of the important tools to explore the physicochemical properties of surfactant solutions. Plots of specific

Fig. 1. Conductivity versus concentration of DTAB solution in solutions in water (○), 0.10 volume fraction (●), 0.20 volume fraction (∇), 0.30 volume fraction (□), 0.40 volume fraction (Δ) methanol–water mixed solvent media at 298.15 K. Copyright permission from Journal of Surfactant and Detergents 19 (2016) 201–207.

908

S.K. Shah et al. / Journal of Molecular Liquids 222 (2016) 906–914

Fig. 2. Conductivity versus concentration of CTAB solution in solutions in water (○), 0.10 volume fraction (□), 0.20 volume fraction (∇), 0.30 volume fraction (●), 0.40 volume fraction (■) methanol–water mixed solvent media at 298.15 K.

Alcohol has lower dielectric constant than that of water. When alcohol is mixed with water, the dielectric constant of alcohol–water medium decreases. Reduction of dielectric constant causes an increase in ionic repulsion between cationic heads of surfactant molecules. This increase in repulsion can be quantitatively explained by Debye length — the thickness of the ionic atmosphere around each ion in solution [25], sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1 εr ε0 RT X ¼ κ 4π F 2 C i Z 2i

Fig. 4. Plot of specific conductance of CTAB solutions in water (○), 0.10 volume fraction (□), 0.20 volume fraction (∇), 0.30 volume fraction (●), 0.40 volume fraction (■), 0.50 volume fraction (Δ) and 0.60 volume fraction (+) of ethanol–water mixed solvent media at 298.15 K.

16]. A slight decrease in cmc after 0.40 volume fraction of ethanol–water might be due to penetration of ethanol molecule into the micelle [3]. 3.2. Thermodynamics of micellization According to pseudo-phase separation model [28], the standard Gibb's free energy of micellization ΔG°m, is calculated from the relation

ð1Þ

where κ1 is the Debye length, εr is the dielectric constant of the solution, ε0 is the dielectric constant of a vacuum, R is the universal gas constant, T is the absolute temperature, F is the Faraday constant, Ci is the concentration of an ion in solution, and Zi is charge on the ion. Assuming all the variables constant, a ratio of square roots of dielectric constants can be used to determine the effect of different solvents on Debye length. In general, a reduction in Debye length will result in an increase in electrostatic repulsion of head groups [26] due to which hydrophobic interaction becomes less [27], which eventually increases the cmc [14–



ΔGm ¼ ð2−αÞRT lnX cmc

ð2Þ

where Xcmc is mole fraction of surfactant at cmc, R is universal gas constant and T is the temperature. The effect of alcohol on micellization process can be studied by means of free energy of surfactant tail transfer, ΔG°trans, which is defined by [15]     ΔGtrans ¼ ΔGm

alcoholþwater

   − ΔGm

ð3Þ

water

Gibb's free energy of micellization (ΔG°m) and free energy of surfactant tail transfer (ΔG°trans) values of DTAB and CTAB in water, 0.10, 0.20,

Table 1 Values of cmc and degree of ionization of DTAB and CTAB by conductivity and surface tension measurements in water different volume fractions of methanol–water at 298.15 K. Volume fraction of methanol

Fig. 3. Plot of specific conductance of DTAB solutions in water (○), 0.10 volume fraction (□), 0.20 volume fraction (∇), 0.30 volume fraction (●), 0.40 volume fraction (■), 0.50 volume fraction (Δ) and 0.60 volume fraction (+) of ethanol–water mixed solvent media at 298.15 K.

CMC/mM

Premicellar Postmicellar Degree of slope (S1) slope (S2) ionization Conductometry Tensiometry (α)

DTAB 0.0 0.1 0.2 0.3 0.4

14.5 17.5 20.5 25.1 34.5

14.6 17.1 19.2 25.3 34.1

116.7 96.3 80.7 61.4 45.4

24.5 23.1 22.6 22.1 21.8

0.21 0.24 0.28 0.36 0.48

CTAB 0.0 0.1 0.2 0.3 0.4

0.98 1.20 1.70 3.80 6.81

0.97 1.22 1.78 3.55 6.43

152.1 135.7 112.6 75.9 57.1

33.5 31.2 30.4 29.6 29.2

0.22 0.23 0.27 0.39 0.51

cmc and degree of ionization of DTAB in water and methanol–water system were taken from [15].

S.K. Shah et al. / Journal of Molecular Liquids 222 (2016) 906–914

909

Table 2 Values of cmc and degree of ionization of DTAB and CTAB by conductivity and surface tension measurements in water different volume fractions of ethanol–water at 298.15 K.

after which surface tension remains almost unchanged. This break point gives cmc [30]. Tables 1 and 2 list out the cmc of DTAB and CTAB by surface tension method which is very close to that obtained by con-

Volume cmc (mM) Premicellar Postmicellar Degree of slope (S2) ionization fractions slope (S1) Conductometry Tensiometry (α) of ethanol

Þ of sigmoidal curve, where C is conductivity method. The slope ðd dγ logC

DTAB 0.00 0.10 0.20 0.30 0.40 0.50 0.60

15.1 15.5 21.4 32.5 46.2 43.3 42.2

15.4 15.7 20.5 34.7 42.7 39.1 36.3

102.1 72.1 52.1 37.4 31.4 27.2 28.5

24.5 23.8 22.4 22.1 20.4 20.1 20.0

0.24 0.33 0.43 0.59 0.65 0.74 0.70

CTAB 0.00 0.10 0.20 0.30 0.40 0.50 0.60

0.9 1.3 2.7 3.1 4.9 3.8 3.4

1.0 1.2 2.2 2.9 4.5 3.8 3.2

152.1 83.1 68.7 41.2 32.1 32.3 37.5

31.9 29.1 28.2 27.6 27.3 27.1 27.0

0.21 0.35 0.41 0.67 0.85 0.84 0.72

centration of surfactant solution in mol L−1, give various information about the surface properties of surfactant solution [30]. Variation of Þ of DTAB and CTAB solutions with volume fraction of methslope ðd dγ logC anol–water and ethanol–water mixed solvent are shown in Figs. 9 and 10. It is seen that the plots look similar with slight difference that might be due to different in chain length. Þ of sigmoidal curve, the maximum surface excess From slope ðd dγ logC concentration at the air/alcohol–water interface can be calculated by applying Gibb's isotherm, Гmax ¼ −

0.30 and 0.40 volume fractions of methanol–water at 298.15 K are displayed in Table 3. Gibb's free energy of micellization (ΔG°m) and free energy of surfactant tail transfer (ΔG°trans) values of DTAB and CTAB in water, 0.10, 0.20, 0.30, 0.40, 0.50 and 0.60 volume fractions of ethanol–water at 298.15 K are displayed in Table 4. ΔG°m values of DTAB and CTAB are in close agreement with literature [19,24]. ΔG°m values signify the spontaneity of micellization. More negative the values of ΔG°m means the micellization are more spontaneous. It is seen from the Tables 3 and 4 that ΔG°m values become less negative with increase in volume fraction of methanol–water and ethanol– water respectively. This illustrates micellization becomes less favorable with increase in methanol and ethanol content up to 0.40 volume fraction. However, at 0.50 and 0.60 volume fractions of ethanol–water, there are slight decrease in negative values of ΔGom, indicating a little favorable condition for micellization. This slight variation of cmc at 0.50 and 0.60 volume fractions of ethanol–water may be due to critical aggregation concentration (cac) of ethanol and which starts forming mixed micelle with cationic surfactants [29].

It is seen from Figs. 5 to 8 that the surface tension of DTAB and CTAB decreases with increase in methanol and ethanol content in water. Surface tension of alcohol–water mixed solvent medium is reduced by adsorption of surfactant at the interface, and a sigmoidal curve between surface tension (γ) and log [surfactant] is produced by distinct break

ð4Þ

where R is the gas constant (8.314 J mol−1 K−1). The constant n takes the values 2 for conventional surfactant where the surfactant ion and the center line are univalent [15]. The area occupied by surfactant molecule (Amin) at the air/alcohol– water interface has been obtained by, A min ¼ 1=NГmax

ð5Þ

where N is Avogadro's number. The value of the surface pressure at the CMC (πcmc) is obtained as πcmc ¼ γ o −γ cmc

ð6Þ

where γo and γcmc are the values of surface tension of water and the surfactant solution at the cmc respectively. Israelachvili et al. [31] proposed that the micellar shape is mainly governed by the geometry of the surfactant and its packing. The surface area of amphiphiles in mixed micelles and micellar growth (sphericalnonspherical) can be used to find out the packing parameters, P, P¼

3.3. Surface tension measurement and surface properties

  1 dγ 2:303nRT d logC T;P

Vo A min lc

ð7Þ

where, Vo is the volume of exclusion per monomer in the micelle, given by Tanford's formula [32]. Vo = [27.4 + 26.9(nc − 1)]2 Å3, lc = [1.54 + 1.26(nc − 1)] Å, is the maximum chain length and nc is the number of carbon atoms in the hydrocarbon chain.

Table 3 Thermodynamic and surface properties of DTAB and CTAB in methanol–water mixed solvent media at 298.15 K. Volume fractions of methanol

ΔG°m (kJ mol−1)

ΔG°trans (kJ mol−1)

Γmax106 (mol m−2)

Amin (Å2 molecule−1)

πCMC (mN m−1)

P

ΔG°ads (kJ mol−1)

DTAB 0 0.1 0.2 0.3 0.4

−36.58 −34.90 −33.17 −30.54 −26.85

– 1.68 3.41 6.04 9.73

2.85 2.18 1.35 0.82 0.64

58.15 76.14 122.54 200.97 258.98

34.43 19.99 13.03 7.87 5.32

0.36 0.27 0.17 0.10 0.08

−48.63 −44.06 −42.78 −40.06 −35.14

CTAB 0.0 0.1 0.2 0.3 0.4

−49.90 −48.44 −45.53 −38.79 −33.41

– 1.45 4.36 11.10 16.48

3.01 2.10 1.72 0.94 0.77

54.98 78.73 96.33 175.71 213.26

33.59 18.09 10.23 8.07 4.32

0.38 0.26 0.21 0.11 0.09

−61.02 −57.01 −51.46 −47.32 −38.95

Thermodynamic and surface properties of DTAB in methanol–water mixed solvent media at 298.15 K were taken from [15].

910

S.K. Shah et al. / Journal of Molecular Liquids 222 (2016) 906–914

Table 4 Thermodynamic and surface properties of DTAB and CTAB in different ethanol–water mixed solvent media at 298.15 K. Volume fractions of ethanol

ΔG°m (kJ mol−1)

ΔG°trans (kJ mol−1)

Γmax106 (mol m−2)

Amin (Å2 molecule−1)

πcmc (mN m−1)

P

ΔG°ads (kJ mol−1)

DTAB 0.00 0.10 0.20 0.30 0.40 0.50 0.60

−35.8 −33.5 −29.9 −25.1 −22.6 −21.0 −21.4

– 2.3 5.9 10.7 13.2 14.8 14.4

2.8 1.5 0.8 0.2 0.1 0.03 0.003

57.8 104.1 189.5 634.1 938.5 4299.2 45,034.5

34.1 11.8 6.7 3.7 1.5 1.5 0.4

0.36 0.20 0.11 0.03 0.02 0.004 0.0004

−47.6 −40.9 −37.6 −35.5 −31.1 −59.8 −129.8

CTAB 0.00 0.10 0.20 0.30 0.40 0.50 0.60

−48.4 −43.2 −38.5 −31.4 −25.6 −26.3 −29.0

– 5.2 10.1 17.0 22.8 22.1 19.4

3.0 1.6 0.4 0.1 0.04 0.001 0.0006

55.1 100.3 364.6 1281.1 3391.6 121,535.6 267,035.9

32.9 11.2 5.2 1.5 0.8 0.4 0.4

0.38 0.21 0.06 0.01 0.006 0.0001 0.00008

−59.4 −50.1 −49.3 −43.1 −42.0 −319.0 −672.2

The standard free energy interfacial adsorption at the air/saturated monolayer interface can be evaluated from the relation [31]. 

ΔG

ads



¼ ΔG

m−

πcmc Γ max

ð8Þ

The maximum surface excess concentration at the air/alcohol–water interface (Гmax), area occupied per surfactant molecule (Amin) at the air/ alcohol–water interface, surface pressure at the cmc (πcmc), packing parameters (P) and standard free energy interfacial adsorption (ΔG°ads) are calculated respectively by Eqs. 4 to 8 for DTAB and CTAB solutions in water, methanol–water and ethanol–water at 298.15 K and are displayed in Tables 3 and 4. These data show that Гmax as well as πcmc decreases with increase in alcohol content indicating less population and surface pressure due to surfactant molecules. However, Amin values are increasing with increase in alcohol content which indicates that surfactant molecules are occupying more surface area at air/alcohol–water interface. The value of packing parameter (P) suggests the shape of micelle acquired. Israelachivili et al. [31] suggested that in general micelles are spherical for P b 1/3. In our investigation, from Tables 3 and 4, it is seen that in absence of methanol and ethanol, P values for both DTAB and CTAB are 0.3 indicating spherical shape. However, when volume fractions of

Fig. 5. Plot of surface tension versus concentration of DTAB solution at 298.15 K solutions in water (○), 0.10 volume fraction (□), 0.20 volume fraction (∇), 0.30 volume fraction (●), 0.40 volume fraction (■) methanol–water mixed solvent media at 298.15 K. Copyright permission from Journal of Surfactant and Detergents 19 (2016) 201–207.

methanol and ethanol are varied the values of P decrease for both DTAB and CTAB. Similar behavior was observed for the effect of ethylene glycol and dimethyl sulphoxide on DTAB, SDDS and Tween-20 [14]. It is seen from Table 4 that surface properties of DTAB and CTAB at higher volume fractions of ethanol (at N 0.40) are very different from that of lower volume fractions. It is due to solubilization of micelle in higher alcohol content. It is further supported by the evidence of very small sigmoidal curve between surface tension (γ) and log [surfactant] as shown in Figs. 7 and 8 for DTAB and CTAB respectively. It is revealed that at higher concentration of alcohol formation of micelle is very difficult to predict due to solubilization of micelle [22,32]. Nishikido et al. [33] studied variation of cmc of polyoxyethylene lauryl ethers having various ethylene chain lengths on addition of short chain alcohols to the surfactant solution by surface tension method. They found the increase in cmc with increase in concentration of methanol and ethanol whereas the cmc found to decrease in case of increasing the concentration of higher alcohols. In comparison of methanol and ethanol, the later was found to increase the cmc in greater extent than the former. The most remarkable thing in Figs. 3, 4, 7 and 8 is that the slopes are decreasing and due to which breakage points in both the types of graphs in conductivity and surface tension which represent the location of micelle are diminishing with the increase in amount of ethanol in water. This may due the reason that alcohols (ethanol in this case) are surface active which enhances the surface activity along with surfactant [33].

Fig. 6. Plot of surface tension versus concentration of CTAB solution in water (○), 0.10 volume fraction (□), 0.20 volume fraction (∇), 0.30 volume fraction (●), 0.40 volume fraction (■) methanol–water mixed solvent media at 298.15 K.

S.K. Shah et al. / Journal of Molecular Liquids 222 (2016) 906–914

Fig. 7. Plot of surface tension with concentration of DTAB solutions in water (○), 0.10 volume fraction (□), 0.20 volume fraction (∇), 0.30 volume fraction (●), 0.40 volume fraction (■), 0.50 volume fraction (Δ) and 0.60 volume fraction (+) of ethanol–water mixed solvent media at 298.15 K.

911

Fig. 10. Variation of ðd dγ Þ with volume fractions of ethanol–water for DTAB (○) and CTAB logC (□) at 298.15 K.

-20

ΔG°m / kJ mol

-1

-30

-40

-50 55.0

57.5

60.0

62.5

ET(30)

Fig. 8. Plot of surface tension with concentration of CTAB solutions in water (○), 0.10 volume fraction (□), 0.20 volume fraction (∇), 0.30 volume fraction (●), 0.40 volume fraction (■), 0.50 volume fraction (Δ) and 0.60 volume fraction (+) of ethanol–water mixed solvent media at 298.15 K.

Fig. 11. ΔG°m vs ET(30) at 298.15 K.

-20

-1 ΔG°m / kJ mol

-30

-40

-50

1.0

1.5

η0 / mPa.s

Fig. 9. Variation of ðd dγ Þ with volume fractions of methanol–water for DTAB (○) and CTAB logC (□) at 298.15 K.

Fig. 12. ΔG°m vs η0 at 298.15 K.

2.0

65.0

912

S.K. Shah et al. / Journal of Molecular Liquids 222 (2016) 906–914 -15

-20

-30

ΔG°m / kJ mol

ΔG°m / kJ mol

-1

-1

-30

-40

-45

-50

-60 0.0100

0.0125

0.0150

0.0175

0.0200

0.0225

-60

Fig. 13. ΔG°m vs 1/D at 298.15 K.

49

56

Fig. 15. ΔG°m vs δ at 298.15 K.

Wei et al. [34] investigated cmc of CATB in 0.1 and 0.5 fraction of ethanol–water and in pure ethanol by steady-state fluorescence techniques in order to investigate the effect of self-assembling properties of the surfactant on the template synthesis of porous inorganic materials and they found that the cmc increases with increase in ethanol concentration and the maximum cmc was found in case of pure ethanol (0.24 mol L−1). Furthermore, from dissipative particle dynamics (DPP), they observed that as the ethanol fraction increased in the solvent mixture, the size of micelles decrease gradually and incase of pure ethanol there is small amount of tiny micelles. They concluded that main driving force for the formation of micelle was attributed to the hydrophobic effect among the surfactant molecules. It is seen from Figs. 3, 4, 7 and 8 that the break point of the pre and postmicellar micellar regions are decreasing with increase in the concentration of ethanol. Similar observations are seen by Li et al. [35] in thermodynamic modeling of CTAB aggregation in ethanol–water mixed solvent media. Their modeling estimated that with increase in concentration of ethanol, size distribution of micelle decreases as well as aggregation number also decreases. Studies on the isotherms of surface tension, density, viscosity and conductivity of CTAB in short chain alcohols (viz. methanol, ethanol and propanol) show that CTAB forms micelle in whole range of methanol but there is a difference in case of ethanol and propanol, where ethanol present some contrast behavior [36].

4

3

-1 ΔG°m / kJ mol

42 δ

1/D

2

3.4. Correlation of ΔG°m with solvent parameters It is interesting for us to correlate ΔG°m with different solvent parameters namely dielectric constant, viscosity, Reichardt's parameter [37], the Hildebrand parameter [38–40], the Gordon parameter [41]. Because micellization consists of molecular association, the fluidity, polarity and solvent structure are expected to influence the process [22]. A singlesolvent property may not be sufficient to guide the process; so here we use methanol–water and ethanol–water mixed solvent media to see the effect of CTAB and DTAB on the ΔG°m values. ΔG°m is the best thermodynamic parameter. Especially for ionic surfactants, the counterion binding influences ΔG°m. In Figs. 11–15, the dependence of ΔG°m (CTAB and DTAB) on the solvent parameters, ET(30), η0, D, G and δ of methanol–water and ethanol– water mixed solvent media are presented. The ET(30), η0 and D were taken from the literature [42], G values were determined by us (Tables 5 and 6) and δ values were calculated with the empirical relationship given elsewhere [39,40] between δ and dielectric constant which works best for hydrogen bonded liquids: δ ¼ 0:45D þ 18:5

ð9Þ

All parameters produced curvilinear correlations with ΔG°m. For CTAB and DTAB curves were concave in methanol–water where as convex in ethanol–water. Such types of trends were also observed by Pan et al. [22] for ΔG°m with solvent parameters in SDS in methanol–water and dioxane–water at 303.15 K. Although some researchers [43,44] reported linear relations between ΔG°m and G for different surfactants in mixed aquo-organic solvents, the points in the plots were widely scattered for witnessing poor correlations. Mukhim and Ismail [45] proposed that the ratio of the solvent surface tension to the limiting surface tension at the cmc, γ/ γlim, can be used as a probable new scale to describe the solvophobic effect [46]. The Fig. 16 shows that the variation of the ratio of the solvent surface tension to the limiting surface tension at the cmc with volume fraction of alcohol. 3.5. Correlation of ΔG°m with solvophobic parameter (Sp)

1

0 -50

-40

-30 G /J m

-3

Fig. 14. ΔG°m vs G at 298.15 K.

-20

According to the principle of solute-solvent interactions, the interaction of alkyl chain with solvents can be characterized by the solvophobicity of hydrocarbon [47]. The solvophobic power of hydrocarbons can be described by their solvophobic parameter, Sp, calculated by Gibbs energies of transfer of hydrocarbons from gas in to a given solvent [47]. For the binary solvent systems studied in this work, the Sp values are available for the solvent mixtures of water–methanol,

S.K. Shah et al. / Journal of Molecular Liquids 222 (2016) 906–914

913

Table 5 Various physicochemical parameters of the mixed solvents (methanol–water) at 298.15 K. Volume fraction Dielectric of methanol constant methanol

m−1)

Water 0.1 0.2 0.3 0.4 Methanol

71.99 56.19 47.53 41.17 36.72 22.51

Solvent molar volume Gordon parameter G (J Vm/(dm3 mol−1)

Solvent surface tension γo/(mN

80.2 75.09 71.61 67.65 63.53 33.00

ΔG°m (DTAB) (kJ mol−1)

ΔG°m (CTAB) (kJ mol−1)

Coefficient of Reichardt's viscosity (mPa·s) parameter (ET/kcal

m−3) 18.07 19.47 20.86 22.26 23.66 32.04

mol−1)

2.74 2.09 1.73 1.46 1.28 0.71

−36.58 −34.90 −33.17 −30.54 −26.85

−49.90 −48.44 −45.53 −38.79 −33.41

0.8959 1.0844 1.3106 1.4712 1.4475 0.5440

ΔG°m (DTAB) (kJ mol−1)

ΔG°m (CTAB) (kJ mol−1)

Coefficient of Reichardt's viscosity (mPa·s) parameter (ET/kcal

63.10 62.33 61.56 60.79 60.2 55.40

Table 6 Various physicochemical parameters of the mixed solvents (ethanol–water) at 298.15 K. Volume fraction of ethanol

Dielectric constant methanol

Solvent surface tension γo/(mN m−1)

Water 0.1 0.2 0.3 0.4 0.5 0.6 Ethanol

80.2 73.7 70.2 66.0 59.6 55.0 47.5 24.28

71.99 48.3 41.2 36.1 31.9 30.6 28.3 22.07

Solvent molar volume Vm/(dm3 mol−1)

Gordon parameter G (J m−3)

18.07 22.13 26.18 30.24 34.29 38.35 42.41 58.63

γ o / γcmc

1.75

1.25

0

0.15

0.30

0.45

−35.8 −33.5 −29.9 −25.1 −22.6 −21 −21.4

2.74 1.72 1.39 1.16 0.98 0.90 0.41 0.28

2.25

0.75

mol−1)

0.60

0.75

Volume fraction of alcohol

Fig. 16. γ0/γcmc vs volume fraction of alcohol at 298.15 K.

63.10 61.98 60.86 59.74 58.62 57.50 56.38 51.90 [49]

1.00

0.75

α

0.9

SP

0.8959 1.1441 1.4346 1.7371 2.0019 2.1609 2.1540 1.0995

water–ethanol [47], although their compositions are different from those investigated in the present work. To solve this problem, we used the correlation method developed by Wang et al. [48] with which the Sp values of the mixed solvents can be predicted at any composition from those of the corresponding pure liquids. It is found that there is a good linear relationship between the Sp values and the volume fraction of methanol and ethanol in water as shown in Fig. 17. The Sp values of hydrocarbon in methanol–water mixed solvents do effect the ionization degree of CTAB and DTAB aggregate in the way that the α values decrease with increasing Sp values (Fig. 18) where as in the same Fig. 18, the α values first increase and then decrease in ethanol– water system for CTAB and DTAB with increasing Sp values. This suggests that the increase of the solvophobic power of alkyl chains in methanol–water solvents would favor the formation of micellization where as for ethanol–water solvents there is slight deviation. In addition, it can be seen from Fig. 19 that the ΔG°m values decrease with increasing Sp values in methanol–water solvents where as in ethanol–water solvents, the ΔG°m values first increase and then decrease

1.1

0.7

0.50

0.25

0.5

0.3

−48.4 −43.2 −38.5 −31.4 −25.6 −26.3 −29.0

0

0.2

0.4 Volume fraction of alcohol

Fig. 17. Sp vs volume fraction of alcohol at 298.15 K.

0.6

0 0.30

0.45

0.60

0.75 SP

Fig. 18. Sp vs α at 298.15 K.

0.90

1.05

914

S.K. Shah et al. / Journal of Molecular Liquids 222 (2016) 906–914 -20

References [1] [2] [3] [4] [5] [6]

ΔG°m / kJ mol

-1

-30

[7] [8] [9]

-40

[10] -50

[11]

-60 0.30

0.45

0.60

0.75

0.90

1.05

SP

Fig. 19. ΔG°m vs Sp at 298.15 K.

[12] [13] [14] [15] [16] [17] [18] [19]

with increasing Sp values. This indicates that the ΔG°m values depend strongly on the solvophobic power of hydrocarbon in water–alcohol mixtures. 4. Conclusion The addition of methanol and ethanol in water changes several physical properties of mixed solvent medium due to which physicochemical properties of surfactant solution also changes. The critical micelle concentration (cmc) of both DTAB and CTAB are increased with increase in methanol and ethanol up to 0.40 volume fraction after which cmc is found to decrease for ethanol–water system. This unusual phenomenon of decrease in cmc after 0.40 volume fraction of ethanol– water is due to penetration of alcohol molecule at the core of micelle. Thermodynamic studies show that the hydrophobicity decreases with the increase in methanol and ethanol due to which cmc increase up to 0.40 volume fraction after which there is slight decrease in cmc for ethanol–water system. Investigation of surface properties reveal that present of methanol and ethanol greatly affect the surface phenomena of DTAB and CTAB. It is seen that presence of ethanol in higher volume fractions, surfactants molecules are no more in ideal arrangements. They gets dispersed in the medium due to lack of balancing of hydrophilic and hydrophobic forces. It can be seen that ΔG°m values in both methanol–water and ethanol–water media do not show a straight forward linear correlation with the different studied solvent parameters i.e. D, ET(30), G, η0 and δ. Variation of the ratio of the solvent surface tension to the limiting surface tension at the cmc with the volume fraction of alcohol give the new scale to describe the solvophobic effect. It can be concluded that the aggregation behavior of cationic surfactants in aqueous organic solutions can be modulated simply by the solvophobic parameters of hydrocarbon in the mixed solvents. Acknowledgements The authors are thankful to University Grants Commission, Nepal for financial help to carry out the experiment.

[20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39] [40] [41] [42] [43] [44] [45] [46] [47] [48] [49]

N. Dubey, J. Mol. Liq 184 (2013) 60–67. A. Chandra, V. Patidar, M. Singh, R.K. Kale, J. Chem. Thermodyn 65 (2013) 18–28. N. Nazir, M.S. Ahanger, A. Akbar, J. Dispers. Sci. Technol 30 (2009) 51–55. K. Shirahama, T. Kashiwabara, J. Colloid Interface Sci 36 (1971) 65–70. B. Kumar, D. Tikariha, K.K. Ghosh, P. Quagliotto, J. Mol. Liq 72 (2012) 81–87. C. Oelschlaeger, N. Willenbacher, Colloids Surf. A Physicochem. Eng. Asp 406 (2012) 31–37. C.C. Ruiz, J. Colloid Interface Sci 221 (2000) 262–267. C.C. Ruiz, J.A. Molina-Bolivar, J. Aguiar, Langmuir 17 (2001) 6831–6840. J. Aguiar, J.A. Molina-Bolivar, J.M. Peula-Garcia, C.C. Ruiz, J. Colloid Interface Sci 255 (2002) 382–390. J.A. Molina-Bolivar, J. Aguiar, J.M. Peula-Garcia, C.C. Ruiz, Mol. Phys 100 (2002) 3259–3269. C.C. Ruiz, J.A. Molina-Bolivar, J. Aguiar, G. MacIsaac, S. Moroze, R. Palepu, Colloid Polym. Sci 281 (2003) 531–541. S. Chaudhary, A. Pal, J. Mol. Liq 190 (2014) 10–15. N. Dubey, J. Chem. Eng. Data 55 (2010) 1219–1226. S. Das, S. Mondal, S. Ghosh, J. Chem. Eng. Data 58 (2013) 2586–2595. S.K. Shah, S.K. Chatterjee, A. Bhattarai, J. Surfactant Deterg 19 (2016) 201–207. K. Ludzik, K. Kustrzepa, H. Piekarski, M. Jozwiak, J. Chem. Eng. Data 61 (2016) 1047–1053. S.Z. Mikhail, W.R. Kimel, J. Chem. Eng. Data 6 (1961) 533–537. I.S. Khattab, F. Bandarkar, M.A.A. Fakhree, A. Jouyban, Korean J. Chem. Eng. Data 29 (2012) 812–817. M.L. Moya, A. Rodriguez, M.M. Graciani, G. Fernandez, J. Colloid Interface Sci 316 (2007) 787–795. A. Bhattarai, S.K. Chatterjee, T.K. Deo, T.P. Niraula, J. Chem. Eng. Data 56 (2011) 3400–3405. J.E. Jr Lind, J.J. Zwolenik, R.M. Fuoss, J. Am. Chem. Soc. 81 (1959) 1557–1559. A. Pan, B. Naskar, G.K.S. Prameela, B.V.N. Phani Kumar, A.B. Mandal, S.C. Bhattacharya, S.P. Moulik, Langmuir 28 (2012) 13830–13843. A. Bhattarai, J. Solut. Chem 44 (2015) 2090–2105. S. Chauhan, K. Kumar, D.S. Rana, R. Kumar, M.S. Chauhan, J. Surfactant Deterg 19 (2016) 193–200. S. Paria, K.C. Khilar, Adv. Colloid Interf. Sci. 110 (2004) 75–95. K. Roebuck, A.Y. Tremblay, Colloids Surf. A Physicochem. Eng. Asp. 495 (2016) 238–247. H. Yilmaz, Turk. J. Phys 26 (2002) 243–246. H.-U. Kim, K.-H. Lim, Colloids Surf. A Physicochem. Eng. Asp 235 (2004) 121–128. M. Bielawska, B. Janczuk, A. Zdziennicka, J. Surfactant Deterg 16 (2013) 203–212. I. Mukherjee, S.P. Moulik, A.K. Rakshit, J. Colloid Interface Sci 394 (2013) 329–336. J.N. Israelachivili, D.J. Mitchell, B.W. Niham, J. Chem. Soc. Faraday Trans 2 (72) (1976) 1525–1568. C. Tanford, The Hydrophobic Effect — Formation of Micelles and Biological Membranes, second ed. Wiley, New Work, 1980. N. Nishikido, Y. Mori, H. Uehara, R. Matuura, Bull. Chem. Soc. Jpn 47 (1974) 2634–2638. L. Wei, Z. Ming, Z. Jinli, H. Yongcai, Front. Chem. Chin 1 (2006) 438–442. W. Li, Y.C. Han, J.L. Zhang, L.X. Wang, J. Song, Colloid J 68 (2006) 304–310. M. Bielawska, A. Chodzinska, B. Janczuck, A. Zdziennicka, Colloid Surf. A 424 (2013) 81–88. R.M. Fuöss, C.A. Kraüs, J. Am. Chem. Soc. 55 (1933) 2387–2399. J.H. Hildebrand, R.L. Scott, Solubility of Non-Electrolytes, third ed. Reinhold, New York, 1964. A.N. Paruta, B.J. Sciarrone, N.G. Lordi, J. Pbarm. Sci 51 (1962) 704. N. G. Lordi, B. J. Sciarrone, T. J. Ambrosio, and A. N. Paruta. (/bid), 53(1964)463. M. Sjöber, U. Heriksson, T. Wärheim, Langmuir 6 (1990) 1205–1211. Y. Marcus, Ion Solvation, Wiley-Interscience, Chichester, U.K., 1985 C. Das, B. Das, J. Chem. Eng. Data 54 (2009) 559–565. P. Koya, A. Kabir-ud-Din, Langmuir 26 (2010) 7905–7914. T. Mukhim, K. Ismail, J. Surfactant Deterg 15 (2012) 47–51. D. Das, K. Ismail, J. Colloid Interface Sci 327 (2008) 198–203. M.H. Abraham, P.L. Grellier, R.A. McGill, J. Chem. Soc. Perkin Trans 2 (1988) 339–345. J. Wang, L. Zhang, H. Wang, C. Wu, J. Phys, Chem. B 115 (2011) 4955–4962. J.G. Dawber, J. Ward, R.A. Williams, 713-727, J. Chem. Soc., Faraday Trans. 1 (1988) 84(3).