Micellization and Solvation Properties of Newly Synthesized Imidazolium- and Aminium-Based Surfactants.

This work aimed to study the solvation properties of newly synthesized cationic surfactants: 1-hexyl-1-methyl-1H-imidazol-1-ium bromide (R6Im), 1-dodecyl-1-methyl-1H-imidazol-1-ium bromide (R12Im), N,N,N-tributylhexan-1-aminium bromide (R6N4), and N,N,N-tributyldodecan-1-aminium bromide (R12N4) in water and ethanol-water solvents with a 0.237 mole fraction of ethanol at 298.15 K using conductivity, refractive index, surface tension, and density measurements. Critical micelle concentration (CMC) for the synthesized surfactants was determined and discussed. Thermodynamic parameters including association constant, molal volume, and polarizability were calculated and discussed. Some surface properties of surfactants including excess surface concentration and minimum area per molecule were also calculated and discussed. A good agreement was found between the CMC values obtained from different techniques, such as conductivity, refractive index, and surface tension. Imidazolium surfactants had been proved to decrease the CMC and increase the association constant with the increase of ethanol mole fraction, while tributylamine had been proved to increase the CMC and decrease the association constant with the increase of ethanol mole fraction. Also, imidazolium surfactants had been proved to have higher CMC than tributylamine, which may be related to higher solvation of imidazolium surfactants than that of tributylamine. Both surfactants (R12Im) and (R12N4) were proved to have lesser CMC.

Introduction

Surfactants have vital importance in many industrial fields, especially cationic surfactants which play an effective role in food processing, oil recovery, delivery of genetic material, antimicrobial, cosmetics, and corrosion inhibitors.[1−8] Imidazolium and tributyl cationic surfactants consist of a hydrophilic head which commonly depends on the amine or quaternary ammonium positive unit and a hydrophobic tail squeezed out of water due to the hydrogen bonds between water molecules.

The surfactant in diluted concentration dispersed as individual molecules in solution. As the concentration increases, the properties of the solution change until reaching the concentration where individual molecules associate to form micelles. This concentration is called critical micelle concentration (CMC). CMC is a fingerprint of any surfactant at a given temperature and electrolyte concentration. The size and shape of micelles depend on factors including surfactant concentration, temperature, pH, and solvent properties.[9] The variation of the CMC with chemical and physical parameters provides a perfect view for the nature of the surfactant self-association. Methods for CMC determination include conductivity, viscosity, surface tension, ion activity, dye incorporation, gel filtration spectrophotometrically, counterion magnetic resonance, and refractive index measurements.[10−14] Solvation of different cationic surfactants in different solvents was investigated before,[15−23] where CMC was measured with different methods. Also, density, refractive index, and UV spectra measurements of solutions are expected to shed some light on the solute–solvent interactions and configuration of their mixtures.[26−29] Other thermodynamic properties including molal volume, ionic association, and polarizability were investigated for ionic liquids,[24,25] especially for imidazolium ionic liquid surfactants.[26−29]

The present work aims to study the synthesis and characterization of some cationic surfactants. Determination of accurate values of the CMC for the synthesized surfactants using different techniques, such as conductivity, refractive index, and surface tension was the second aim of the present study. Also, the present work aims to study the solvation thermodynamic parameters including molal volumes, refractive index, and UV–visible measurements of the synthesized surfactants in water and ethanol–water solvents with an ethanol mole fraction of 0.237.

Results and Discussion

Structure Confirmation

The chemical structure of the synthesized cationic surfactants was investigated using IR spectroscopy and 1H NMR spectroscopy.

IR Spectroscopy Analysis

The IR spectrum of R6IM and R12IM ionic liquids is represented in Figures and 2. All vibration bands of functional groups are represented in Table . Both the parent compounds (R6IM and R12IM) are hydrophilic and show the presence of a large amount of water at 3444 and 3433 cm–1, respectively, for R6IM and R12IM.

Figure 1

IR spectra for the R6Im surfactant.

Figure 2

IR spectra for the R12Im surfactant.

Table 1

IR Bands of R6IM and R12IM Surfactants with Their Vibration Names

wavenumber (cm–1)
R6IM	R12IM	Vibration
523	523	C–Br (halogen group)
621	622	imidazole C2–N1–C5 bending
757	749	out-of-plane C–H bending of the imidazole ring
844	860	C–N stretching vibration
1167	1170	imidazole H–C–C and H–C–N bending
1463	1464	asymmetrical bending vibration of the methyl group (C–H bond)
1570	1570	ring stretching vibration of the imidazole group
1628	1631	O–H bending (physically adsorbed water)
2861	2854	stretching vibrations of aliphatic (C–H) sym
2930	2924	stretching vibrations of aliphatic (C–H) asym
3087	3085	stretching vibrations of alkyl groups at the (N) atom of the imidazolium ring
3146	3149	stretching vibrations of alkyl groups at the (N) atom of the imidazolium ring
3444	3433	stretching of N–H bond water

The IR spectrum of R6N4 and R12N4 ionic liquids is represented in Figures and 4. All vibration bands of functional groups are represented in Table . Both the parent compounds (R6N4 and R12N4) are hydrophilic and show the presence of a large amount of water at 3444 and 3433 cm–1, respectively, for R6N4 and R12N4.

Figure 3

IR spectra for the R6N4 surfactant.

Figure 4

IR spectra for the R12N4 surfactant.

Table 2

IR Bands of R6N4 and R12N4 Surfactants with Their Vibration Names

wavenumber (cm–1)
R6N4	R12N4	vibration
560	560	C–Br (halogen group)
1063	1061	asymmetric stretching N+–C
1382	1380	C–H bending for aliphatic hydrocarbon chain (CH2)n
1466	1464	methyl group of hydrocarbon chain
1624	1624	O–H bending (physically adsorbed water)
2872	2855	–CH2	stretching vibrations of alkyl groups at the (N) atom
2960	2926	–CH3	stretching vibrations of alkyl groups at the (N) atom

1H NMR Analysis

The general structure of R6Im and R12Im with position numbers (a–h) corresponding to different shifts is shown in Scheme . Resonance signals corresponding to the presence of functional groups at different positions are shown in Figures and 6 and represented in Table , where the composition of the imidazolium ring and alkyl chain for both surfactants was explained because of their chemical shifts.[30−32]

Scheme 1

Chemical Structure of R6Im and R12Im with Numbers at Different Positions

Figure 5

1H NMR spectrum of the R6Im surfactant in D2O.

Figure 6

1H NMR spectrum of the R12Im surfactant in D2O.

Table 3

1H NMR Chemical Shifts and Molecular Group Names of Both R6Im and R12Im Surfactants

			chemical shift (ppm)
group name	molecular group	position number	R6Im	R12Im
triplet signal due to the presence of the terminal methyl (−CH3) protons at the ends	–(CH3)–	(h)	0.753	0.676
		(h)	0.766	0.690
		(h)	0.780	0.702
cumulative influence of (−CH2−)n groups attached on the side to the terminal methyl groups	–(CH2)n–	(g)	1.192	1.096
		(g)	1.224	1.201
	–CH2–	(f)	1.793	1.752
methyl protons attached to the nitrogen of imidazolium	–N–CH3–	(a)	2.891	3.797
	–N–CH2–	(e)	4.203	4.138
methine protons of imidazolium side groups	–N–CH–	(c)	7.521	7.460
		(d)	7.473	7.458
	–N–CH–N–	(b)	8.819	8.905

The general structure of R6N4 and R12N4 with position numbers (a–e) corresponding to different shifts is shown in Scheme . Resonance signals corresponding to the presence of functional groups at different positions are shown in Figures and 8 and represented in Table , where the composition of alkyl chain and tributylamine of both surfactants was explained because of their chemical shifts.[31−33]

Scheme 2

Chemical Structure of R6N4 and R12N4 with Numbers at Different Positions

Figure 7

1H NMR spectrum of the R6N4 surfactant in dimethyl sulfoxide (DMSO).

Figure 8

1H NMR spectrum of the R12N4 surfactant in DMSO.

Table 4

1H NMR Chemical Shifts and Molecular Group Names of Both R6N4 and R12N4 Surfactants

			chemical shift (ppm)
group name	molecular group	position number	R6N4	R12N4
triplet signal due to the presence of the terminal methyl (−CH3) protons at the ends	–(CH3)–	(a)	0.862	0.837
		(a)	0.874	0.839
		(a)	0.889	0.897
cumulative influence of CH2 groups attached on the side to the terminal methyl groups	–(CH2)n–	(c)	1.285	1.270
	–CH2–	(b)	1.554	1.554
	–CH2–	(d)	3.339	3.350
methyl protons attached to the atom	–N–CH2–	(e)	3.166	3.165

Micellization and Solvation Studies

CMC Measurements

The CMC of synthesized surfactants was determined by using different techniques including conductivity, refractive index, and surface tension.

First, using conductivity measurements, the conductivity for surfactants in water and ethanol–water mixed solvents with a 0.237 ethanol mole fraction has been measured at 298.15 K, as described before in the Experimental Section. By plotting the concentration in (mol L–1) against specific conductance in (μS/cm) as shown in Figures –12, the CMC of surfactants in water and ethanol–water mixed solvents was estimated.

Figure 9

Conductance vs concentration for (R6IM) in water and ethanol–water mixture with an ethanol mole fraction of 0.237 at 298.15 K.

Figure 12

Conductance vs concentration for (R12N4) in water and ethanol–water mixture with an ethanol mole fraction of 0.237 at 298.15 K.

Conductance vs concentration for (R12IM) in water and ethanol–water mixture with an ethanol mole fraction of 0.237 at 298.15 K.

Conductance vs concentration for (R6N4) in water and ethanol–water mixture with an ethanol mole fraction of 0.237 at 298.15 K.

Second, using refractive index measurements, the refractive indices for the surfactant solution in water and ethanol–water mixed solvents with a 0.237 ethanol mole fraction have been measured at 298.15 K, as mentioned in the Experimental Section. By plotting the concentration in (mol L–1) against refractive indices as shown in Figures –15, the CMC has been estimated.

Figure 13

Refractive index vs concentration for (R6Im), (R12Im), and (R6N4) in water at 298.15 K.

Figure 15

Refractive index vs concentration for (R12N4) in water and ethanol–water mixed solvents with an ethanol mole fraction of 0.237 at 298.15 K.

Refractive index vs concentration for (R6Im), (R12Im), and (R6N4) in ethanol–water mixed solvent with an ethanol mole fraction of 0.237 at 298.15 K.

Finally, using surface tension measurements, the surface tension of surfactants in water was measured, as mentioned in the Experimental Section. By plotting the concentration (mol L–1) against surface tension as shown in Figure , the CMC for surfactants in water solvent was estimated at 298.15 K.

Figure 16

Surface tension vs concentration for all surfactants under study in water at 298.15 K.

The CMC values for surfactants (R6Im) and (R12Im) in water and ethanol–water mixed solvents with a 0.237 ethanol mole fraction as estimated from different techniques (conductivity, refractive index, and surface tension) are summarized in Table .

Table 5

CMC Values of the Surfactants under Study in Water and Ethanol–Water Mixed Solvents at 298.15 Ka

		CMC (mol/L)
surfactant	ethanol mole fraction	from conductivity	from refractive index	from surface tension
R6Im	Water	0.0220	0.0195	0.0210
	0.237	0.0088	0.0085
R12Im	Water	0.0122	0.0124	0.0120
	0.237	0.0075	0.0090
R6N4	Water	0.0035	0.0031	0.0029
	0.237	0.0082	0.0075
R12N4	Water	0.0018	0.0019	0.0019
	0.237	0.0022	0.0022

Standard uncertainties, u, of CMC are u(CMC) = 0.0002 mol L–1.

While inspecting the CMC values in Table , we can note that there is good agreement between the CMC values obtained using three different methods (conductivity, refractive index, and surface tension). This indicates that the suitable value of CMC for surfactants under study is reported in Table . In the solvation and micellization processes, we can see that as the hydrophilic nature increases, solvation increases and micellization decreases, and thus, high concentration is needed, that is, high CMC and vice versa. Based on this, the CMC of surfactants (R6Im) and (R12Im) was found to decrease with the increase of ethanol mole fraction, indicating more micellization and less solvation, which may be related to the increase of hydrogen bond formation in ethanol–water mixed solvent.[34] On comparing the CMC of both surfactants, it was found that the CMC of (R6Im) in both water and ethanol–water mixed solvents is more than the CMC of (R12Im), indicating an increase in micellization (low CMC) with an increase in hydrocarbon chain.[35]

Table 6

Degree of Ionization (α), Counterion Binding Constant (β), Free Energy Change of Micellization (ΔGmic), Limiting Molar Conductance (Λ°), Association Constant (Ka), and Free Energy Change of Association (ΔGa) for the Surfactants under Study in Water and Ethanol–Water Mixed Solvents with a 0.237 Ethanol Mole Fraction at 298.15 Ka

surfactant	ethanol mole fraction	α	β	ΔGmic (kJ/mol)	Λ° (S cm2 mol–1)	Ka (L mol–1)	ΔGa (kJ/mol)
R6Im	Water	0.0197	0.9803	–14.2285	135.73	80.280	–10.87
	0.237	0.0152	0.9848	–17.6842	61.980	199.13	–13.12
R12Im	Water	0.79112	0.2088	–10.0276	117.36	112.02	–11.69
	0.237	0.13421	0.8658	–17.1856	66.640	498.35	–15.39
R6N4	Water	0.9972	0.0028	–14.057	102.51	138.82	–12.22
	0.237	0.8393	0.1607	–13.821	37.770	33.820	–8.725
R12N4	Water	0.6613	0.3387	–20.971	104.22	664.93	–16.11
	0.237	0.6238	0.3962	–20.875	85.14	99.9	–11.41

Standard uncertainties, u, are u(α) = 0.0005, u(β) = 0.0004, u(ΔGmic) = 0.1, u(Λ°) = 0.01, u(Ka) = 0.18, and u(ΔGa) = 0.13.

On the other hand, the CMC of surfactants (R6N4) and (R12N4) was found to increase with the increase of ethanol mole fraction, indicating less micellization and more solvation compared with that of (R6Im) and (R12Im), which may be due to the dissociation of the inter- and intrahydrogen bonds.[36] On comparing the CMC of both surfactants, it was found that the CMC of (R6N4) in both water and ethanol–water mixed solvents is more than the CMC of (R12N4), indicating an increase in micellization (low CMC) with an increase in hydrocarbon chain.[37]

Thermodynamic Parameters

Concentration Dependence of Conductivity

Degree of ionization (α) and counterion binding, β = (1 – α), for surfactants under study in water and ethanol–water mixed solvents with a 0.237 ethanol mole fraction at 298.15 K were estimated from eqs 1 and 2[38] and represented in Table .where S2/S1 is the ratio between the slope of post- and premicelle regions. The Gibbs free energy of micellization was estimated from 3following equationwhere α is the degree of ionization, R is the universal gas constant, and T is the absolute temperature. The values of the standard free energy change of micellization (ΔGmic) are represented in Table .

Association Constant

Equivalent conductance (Λ) of surfactants in water and ethanol–water mixed solvents with a 0.237 ethanol mole fraction at 298.15 K has been calculated from the conductivity measurements before the CMC value by using eq .

By plotting Λ against in eq , the intercept which is the limiting equivalent conductance (Λ°) has been estimated.

Conductivity data were used to calculate the value of association constant for the surfactant under study in water and ethanol–water mixed solvents with a 0.237 ethanol mole fraction at 298.15 K according to the Shedlovsky extrapolation (eq ).[39]where S(z) is the Shedlovsky function, which can be calculated from 7following equationwhere Ka is the association constant and γi is the activity coefficient estimated from the Debye–Huckel limiting law as modified by Robinson and Stokes.

The standard free energy change of association at 298.15 K can be calculated using 8following equation

The values of the association constant (Ka) and the standard free energy change of association (ΔGa) are represented in Table .

In the association, solvation, and micellization processes, we can see that as the hydrophilic nature increases, solvation increases, micellization decreases (high CMC), and association increases. Based on this, the values of the association constant for both (R6Im) and (R12Im) were found to increase with the increase of ethanol mole fraction, as indicated from the decrease in CMC with the increase of ethanol mole fraction, which may be related to the decrease in the dielectric constant of ethanol than that of water and thus decrease in solvation.[38−41] The association constant values of both surfactants in water and ethanol–water mixed solvents with a 0.237 ethanol mole fraction increase with an increase in hydrocarbon chain length, as indicated from more micellization (low solvation and low CMC) of (R12Im) than that of the (R6Im) surfactant. Negative values of standard free energy of micellization and association processes indicate spontaneous nature of micellization and association processes. More negative ΔGmic and (ΔGa) have more tendencies to form micelles and to associate.[42]

On the other hand, it was found that the values of association constant for (R6N4) and (R12N4) surfactants decrease with the increase of ethanol mole fraction, which can be related to the increase in CMC of both surfactants with the increase of ethanol mole fraction.[43,44] The association constant values of both surfactants in water and ethanol–water mixed solvents with a 0.237 ethanol mole fraction increase with an increase in hydrocarbon chain length, as indicated from more micellization of (R12N4) than that of the (R6N4) surfactant. Negative values of standard free energy indicate spontaneous nature of micellization and association processes.

Solution Surface Properties

According to the measurement of surface tension of the surfactant under study in water at 298.15 K, some surface properties such as maximum surface concentration, minimum area per molecules, and effectiveness of reduction of surface areas were calculated as follows:

The effectiveness of surface tension reduction was calculated by using the following equation[45]where γο is the surface tension of pure water at the appropriate temperature and γCMC is the surface tension of the solution at the CMC.

Maximum surface excess concentration (Γmax)[46] considered effective adsorption of the surfactant on the air–water interface. This is defined as the concentration of surfactant molecules in a surface plane, relative to that at a similar plane in the bulk which can be calculated by using the Gibbs adsorption (eq )where R is the universal gas constant, T is the absolute temperature, and (∂γ/∂log C) is the ratio between surface tension values at CMC to concentration at CMC.

The minimum surface area of surfactant molecules at air–water solution interfaces (Amin)[41] can be calculated from the following 11where N is the Avogadro number. The values of effectiveness, excess surface concentration, and minimum surface area are summarized in Table .

Table 7

Maximum Surface Excess Concentration Γmax, Minimum Surface Area (Amin), and Effectiveness of Reduction of Surface (πCMC) in Water Solvent at 298.15 K

surfactant	Γmax × 107 mol/cm2	Amin × 104 nm2/molecule	πCMC dyne/cm
R6Im	7.33	2.27	1.30
R12Im	3.01	5.52	39.0
R6N4	4.11	4.04	13.0
R12N4	3.09	5.37	24.2

The results show an increase in minimum area per molecule of surfactants (R12Im) and (R12N4) than that of (R6Im) and (R6N4), respectively. This may be related to the increase in hydrocarbon chain length and thus increase in the effectiveness of surface tension reduction. This may be related to the increase in the adsorption of the surfactant (R12Im) than that of (R6Im) at the air–water interface, which orients themselves away from the water, leading to a decrease in maximum surface excess concentration. This indicates an increase in the efficiency of reducing surface tension solution of the surfactant with an increase in hydrocarbon chain length.[46]

Molal Volumes

The density of surfactants in molal concentration in water and water–ethanol with a 0.237 mole fraction of ethanol was measured at 298.15 K. Molal volumes (Vφ) of the surfactant under study were then calculated from the following 12(46)where M is the molecular weight of the surfactant; m is the molal concentration of the surfactant in solution; and ρ and ρ° are the densities of solution and solvent, respectively.

The packing density (P) (the relation between the van der Waals volume and the molal volume of relatively large molecules) was found to be constant.[47,48] Therefore, van der Waals volumes (Vw) of the surfactants can be calculated by using 13 following equation[49]

Electrostriction volume which indicated the volume compressed by the solvent can be calculated from 14 following equation[50]

The values of density (ρ), molal volume (Vφ), electrostriction volume (VE), and van der Waals volume (Vw) of the surfactants under study in water and ethanol–water mixed solvent with a 0.237 ethanol mole fraction at 298.15 K are reported in Table .

Table 8

Density (ρ), Molal Volume (Vφ), Electrostriction Volume (VE), and van der Waals Volume (Vw) for the Surfactants under Study in Ethanol–Water Solvent at 298.15 Ka

surfactant	ethanol mole fraction	ρ (g/cm3)	Vφ (cm3/mol)	Vw (cm3/mol)	VE (cm3/mol)
R6Im	Water	1.0280	236.61	156.3992	–80.2108
	0.237	0.9634	254.01	167.9006	–86.1094
R12Im	water	1.0117	325.31	215.0299	–110.280
	0.237	0.9826	332.67	219.8949	–112.775
R6N4	water	1.0090	345.46	228.3491	–117.111
	0.237	0.9501	367.80	243.1158	–124.684
R12N4	water	1.0208	246.56	162.9762	–83.5838
	0.237	1.0168	371.59	245.6210	–125.969

Standard uncertainties, u, are u(ρ) = 0.0005, u(Vφ) = 0.03, u(Vw) = 0.02, and u(VE) = 0.03.

The densities of all surfactants under study were found to decrease with the increase of ethanol mole fraction. Also, molal volume was found to increase with the increase of ethanol mole fraction. The densities of (R12Im) and (R12N4) are more than those of (R6Im) and (R6N4), which may be related to the higher molecular weight of the R12 surfactant than that of R6. The molal volume of all surfactants under study was found to increase with the increase of ethanol mole fraction, which may be related to the higher density of water than that of ethanol. The molal volume of (R12Im) and (R12N4) is less than that of (R6Im) and (R6N4), which may be related to the higher density of the R12 surfactant than that of R6.

The molal volumes of (R6N4) and (R12N4) were found to be higher than those of (R6Im) and (R12Im), respectively. This may be related to the lower densities of (R6N4) and (R12N4) than those of (R6Im) and (R12Im), respectively.

Refractive Index

Refractive index of all surfactants under study was measured in water and ethanol–water with a 0.237 mole fraction of ethanol at 298.15 K. Molar refraction was calculated from eq by using the values of molal volumes and refractive indices.[51]where Vφ is the apparent molal volume of the surfactant in solution, n is the refractive index of all surfactant solution, (PE) is the electron polarization, and (PA) is the atomic polarization that can be calculated from 16 following equation[52]

The polarizability of surfactants under study in water and ethanol–water with a 0.237 mole fraction of ethanol at 298.15 K was calculated from the following equation[53]where (N) is Avogadro’s number and (α) is the polarizability of all surfactants.

The values of refractive index, molar refraction, atomic polarization, and polarizability are summarized in Table .

Table 9

Refractive Index (nD), Molar Refraction (Rm), Atomic Polarization (PA), and Polarizability (α) of the Surfactants under Study in Water and Ethanol–Water with a 0.237 Mole Fraction of Ethanol at 298.15 Ka

surfactant	ethanol mole fraction	nD	PA	Rm (cm3/mol)	α cm3
R6Im	water	1.3809	2.0022	54.9230	2.1781
	0.237	1.3966	2.0480	61.1150	2.4236
R12Im	water	1.3828	2.0077	75.8478	3.0079
	0.237	1.3907	2.0307	78.9846	3.1323
R6N4	water	1.3811	2.0028	80.2273	3.1815
	0.237	1.4011	2.0612	89.3803	3.5445
R12N4	water	1.3819	2.0051	57.3665	2.2749
	0.237	1.3818	2.0048	86.4367	3.4278

Standard uncertainties, u, are u(nD) = 0.0001, u(PA) = 0.01, u(Rm) = 0.22, and u(α) = 0.02.

The refractive indices were found to increase with the increase of ethanol mole fraction, which may be related to the higher refractive index of ethanol than that of water. The molar refraction and the polarizability are directly proportional to the apparent molal volume. The molar refraction and the polarizability of surfactants under study are increased with the increase of ethanol mole fraction. This may be related to the increase in the apparent molar volume of the two surfactants with the increase in the mole fraction of ethanol.

The molar refraction and polarizability of (R12Im) and (R12N4) are found to be greater than those of (R6Im) and (R6N4), respectively, in water and ethanol–water solvent with a 0.237 mole fraction of ethanol. This may be related to the increase in hydrocarbon chain length which increases the micellization and decreases the solvation.

The refractive index, molar refraction, and polarizability of (R6N4) and (R12N4) are found to be greater than those of (R6Im) and (R12Im), respectively. This may be related to the higher molal volumes of (R6N4) and (R12N4) than those of (R6Im) and (R12Im), respectively.

UV–Visible Spectra

The UV–visible spectra of all surfactants under study with concentration (0.001 M) in water and ethanol–water mixed solvents with different ethanol mole fractions (x1 = 0.0 to x1 = 0.42) were measured and are represented in Figures –20. The values of the absorbance and the wavelength of surfactants are collected in Tables and 11.

Figure 17

UV spectra of R6Im (0.001 M) in ethanol–water mixed solvent with different ethanol mole fractions (x1 = 0.0–0.421 by mass).

Figure 20

UV spectra of R12N4 (0.001 M) in ethanol–water mixed solvent with different ethanol mole fractions (x1 = 0.0–0.421 by mass).

Table 10

Absorbance and the Wavelength (λ) of R6Im and R12Im Surfactants at 298.15 K in Ethanol–Water Mixed Solvent with Different Ethanol Mole Fractions

		peak 1		peak 2
surf.	mole fraction of ethanol	λ, nm	absorbance	λ, nm	absorbance
R6Im	0.000			274	0.202
	0.118	221	0.797	275	0.209
	0.237	224	0.749	267	0.213
	0.421	223	0.645	272	0.281
R12Im	0.000			273	0.502
	0.118	222	1.097	273	0.512
	0.237	224	1.049	264	0.512
	0.421	222	0.941	271	0.581

Table 11

Absorbance and Wavelength (λ) of R6N4 and R12N4 Surfactants at 298.15 K in Ethanol–Water Mixed Solvent with Different Ethanol Mole Fractions

		peak 1		peak 2
surf. name	mole fraction of ethanol	λ, nm	absorbance	λ, nm	absorbance
R6N4	0.000	220	1.5368
	0.118	222	0.3273	296	0.0483
	0.237	222	0.6546	296	0.0966
	0.421	222	0.8183	296	0.1207
R12N4	0.000	220	1.3969	272	0.0833
	0.118	231	0.9015	271	0.1351
	0.237	231	1.8030	271	0.2701
	0.421	231	2.2538	270	0.3377

UV spectra of R12Im (0.001 M) in ethanol–water mixed solvent with different ethanol mole fractions (x1 = 0.0–0.421 by mass).

UV spectra of R6N4 (0.001 M) in ethanol–water mixed solvent with different ethanol mole fractions (x1 = 0.0–0.421 by mass).

For R6Im and R12Im surfactants in ethanol–water solvent, a decrease in UV absorbance (peak 1) was noted with an increase in ethanol mole fraction from x1 = 0.118 to x1 = 0.421. The decrease in the absorption of UV light is called a hypochromic effect. On the other hand, an increase in UV absorbance was noted (peak 2) with an increase in ethanol mole fraction from x1 = 0.118 to x1 = 0.421 in ethanol–water mixed solvent. Increase in the absorption of UV light is called a hyperchromic effect. These effects may be due to the disruption of the hydrogen bonds between the surfactant molecules as a result of the interaction with ethanol than that with water molecules as a result of change in the dielectric properties of the solvent.

For R6N4 and R12N4 surfactants in ethanol–water solvent, an increase in UV absorbance was noted (peaks 1 and 2) with an increase in ethanol mole fraction from x1 = 0.118 to x1 = 0.421 in ethanol–water mixed solvent. This effect may be due to the disruption of the hydrogen bonds between the surfactant molecules as a result of change in the dielectric properties of the solvent.

With respect to the wavelength shift, no shift was found for all surfactants under study.

Conclusions

This study is concerned with the synthesis and characterization of some cationic surfactants: 1-hexyl-1-methyl-1H-imidazol-1-ium bromide (R6Im), 1-dodecyl-1-methyl-1H-imidazol-1-ium bromide (R12Im), N,N,N-tributylhexan-1-aminium bromide (R6N4), and N,N,N-tributyldodecan-1-aminium bromide (R12N4). By using different techniques such as conductivity, refractive index, and surface tension, thermodynamic parameters in water and ethanol–water solvents at 298.15 K were measured, calculated, and discussed.

A good agreement was found between the CMC values obtained from different techniques, such as conductivity, refractive index, and surface tension. Imidazolium surfactants had been proved to decrease the CMC and increase the association constant with the increase of ethanol mole fraction, while tributylamine had been proved to increase the CMC and decrease the association constant with the increase of ethanol mole fraction. Also, imidazolium surfactants had been proved to have higher CMC than tributylamine, which may be related to the higher solvation of imidazolium surfactants than that of tributylamine. Both surfactants (R12Im) and (R12N4) were proved to have fewer CMC values than (R6Im) and (R6N4), respectively, indicating the effect of hydrocarbon chain length, which means the longer the chain length, the more the micellization and the less the solvation.

Experimental Section

Materials

The chemical compounds used in this study were purchased from different international companies with high-purity grade, as shown in Table .

Table 12

Reg. CAS Number, the Supplier, the Purity, and the Purification Methods for Chemicals throughout the Investigation

component	reg. CAS number	the supplier	purity before purification %	purification methods	purity after purification %
1-bromohexane	111-25-1	Alfa Aesar, Germany	99.00	the components were used without further purification	99.00
1-bromododecane	143-15-7	Alfa Aesar, Germany	98.00		98.00
1-methylimidazole	616-47-7	Alfa Aesar, Germany	99.00		99.00
acetone	67-64-1	Aldrich, USA	99.90		99.90
ethanol	64-17-5	Perfect, Egypt	99.90		99.90
Water	7732-18	Bi-distilled

Synthesis of Cationic Surfactants

Synthesis of Cationic Surfactants Based on 1-Methylimidazole

Cationic surfactants based on 1-methylimidazole were synthesized, as illustrated in Scheme . This process was carried out by using the quaternization reaction. 1-Methylimidazole (50 mM) and 1-bromoalkanes, namely, 1-bromohexane and 1-bromododecane (50 mM), were charged individually in a 250 mL round flask in the presence of acetone (100 mL) as a solvent. The reaction mixture was refluxed under stirring for 18 h, and then the reaction mixture was cooled to room temperature. The brown precipitate was filtered, washed twice with diethyl ether, and then recrystallized from acetone to afford the white crystal products of the cationic surfactants. The yields of the brown crystal products ranged between 74 and 83%. The obtained products of quaternary ammonium cationic surfactants were designated as (R6Im) for 1-hexyl-1-methyl-1H-imidazol-1-ium bromide and (R12Im) for 1-dodecyl-1-methyl-1H-imidazol-1-ium bromide.

Scheme 3

Synthetic Route of the Cationic Surfactants Based on 1-Methylimidazole

Synthesis of Cationic Surfactants Based on Tri-n-butyl Amine

Cationic surfactants based on tri-n-butyl amine were synthesized, as illustrated in Scheme . This process was carried out by using the quaternization reaction. Tri-n-butyl amine (50 mM) and 1-bromoalkanes, namely, 1-bromohexane and 1-bromododecane (50 mM), were charged individually in a 250 mL round flask in the presence of acetone (100 mL) as a solvent. The reaction mixture was refluxed under stirring for 18 h, and then the reaction mixture was cooled to room temperature. The brown precipitate was filtered, washed twice with diethyl ether, and then recrystallized from acetone to afford the white crystal products of the cationic surfactants. The yields of the brown crystal products ranged between 78 and 86%. The obtained products of quaternary ammonium cationic surfactants were designated as (R6N4) for N,N,N-tributylhexan-1-aminium bromide and (R12N4) for N,N,N-tributyldodecan-1-aminium bromide.

Scheme 4

Synthetic Route of the Cationic Surfactants Based on Tributylamine

Characterization of the Synthesized Cationic Surfactants

The synthesized surfactants (R6IM) and (R12IM) were characterized by using a Thermo Fisher Nicolet iS10 IR spectroscope in the range of 400–4000 cm–1 and with a resolution of 4 cm–1. The solid (R12IM) was mixed with KBr pellets. The wavelength of the peaks was proved particular functional groups according to the synthesized surfactant structures. Then, both surfactants were characterized using 1H NMR spectroscopy. Samples were prepared using D2O as a solvent and then recorded by using a JNM-ECA Series FT NMR spectrometer with a frequency of 500 MHz. Chemical shifts of samples were expressed in parts per million according to their structures.

Solvation Studies of the Synthesized Cationic Surfactants

The conductivity measurements were carried out using a Jenway conductivity bridge of certainty (±0.025 μS cm–1). The conductivity bridge was calibrated by the determination of the cell constant, Kcell, using different standard potassium chloride solutions.[54] To avoid errors, the concentration of the surfactant solution increased by adding 0.1 mL from the prepared surfactant solution having concentrations of 0.1 M (mol L–1) and 0.01(mol L–1) to 10 mL of pure solvent placed in a double Jacket glass cell at a constant temperature of 298.15 ± 0.1 K using an ultrathermostat of type MLW 3230 (Germany). After each addition, the solution was stirred to maintain homogeneity of the mixer, and then the conductivity of the solution was measured in μS cm–1. Conductivity measurement was used to calculate different thermodynamic parameters. Density measurements with a weight of 1 mL of the pure solvent including water and a 0.237 ethanol mole fraction solvent and surfactant solution in the same solvents with a concentration of 0.01(mol L–1) were carried out. Refractive indices were measured for 0.01 (mol L–1) solutions of surfactants in both water and ethanol mole fraction solvents by putting one drop of the solution under study into a sample tray by using a Digital refractometer (DR101-60-A. KRÜSS Optronic GmbH, Germany). Surface tension measurements were achieved for the surfactant solution in water solvent by using a ring method with a digital tensiometer K9.