On the effects of head-group volume on the adsorption and aggregation of 1-(n-hexadecyl)-3-Cm-imidazolium bromide and chloride surfactants in aqueous solutions

Article abstract of DOI:10.1016/j.molliq.2021.115478

The effects of the length of alkyl side chain (C_m) of ionic liquid-based surfactants (ILBSs) on their adsorption at the water/air interface, and aggregation in aqueous solutions were investigated for the series 1-(n-hexadecyl)-3-C_m-imidazolium bromides and chlorides, where C_m = C₁-C₄ for the bromides, and C₁-C₅ for the chlorides. These physicochemical properties were calculated from surface tension, conductivity, and fluorescence data. It was found that increasing the length of C_m (i.e., volume of the head group) leads to enhancement of surface activity, increase in the area per surfactant molecule at the water/air interface (A_min) and the degree of counter-ion dissociation (α_mic). Our data also indicated that increasing the volume of the head group results in a decrease of the critical micelle concentration (cmc), Gibb's free energy of adsorption and micellization, and microscopic polarity of interfacial water. In order to delineate the effects of the presence of unsaturation in the HG, we included members that carry C_m = vinyl and allyl in the bromide series. The effect of these groups was found to be similar to removing a methylene group from C_m. The dependence of the solubilization of a lipophilic dye (Sudan IV) and a drug (nitrendipine) on the length of C_m was also studied.

Full text of DOI:10.1016/j.molliq.2021.115478

Journal of Molecular Liquids 328 (2021) 115478
Contents lists available at ScienceDirect
Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
On the effects of head-group volume on the adsorption and aggregation
of 1-(n-hexadecyl)-3-Cm-imidazolium bromide and chloride surfactants
in aqueous solutions
Nicolas Keppeler a, Paula D. Galgano a, Soraya da Silva Santos a, Naved I. Malek b, Omar A. El Seoud a,
⁎
a
Institute of Chemistry, University of São Paulo, São Paulo, SP 05508-000, Brazil
b
Applied Chemistry Department, Sardar Vallabhbhai National Institute of Technology, Surat, Gujarat 395 007, India
a r t i c l e i n f o
a b s t r a c t
Article history:
The effects of the length of alkyl side chain (Cm) of ionic liquid-based surfactants (ILBSs) on their adsorption at
the water/air interface, and aggregation in aqueous solutions were investigated for the series 1-(n-hexadecyl)-
3-Cm-imidazolium bromides and chlorides, where Cm = C1-C4 for the bromides, and C1-C5 for the chlorides.
These physicochemical properties were calculated from surface tension, conductivity, and fluorescence data. It
was found that increasing the length of Cm (i.e., volume of the head group) leads to enhancement of surface ac-
tivity, increase in the area per surfactant molecule at the water/air interface (Amin) and the degree of counter-ion
dissociation (αmic). Our data also indicated that increasing the volume of the head group results in a decrease of
the critical micelle concentration (cmc), Gibb's free energy of adsorption and micellization, and microscopic po-
larity of interfacial water. In order to delineate the effects of the presence of unsaturation in the HG, we included
members that carry Cm = vinyl and allyl in the bromide series. The effect of these groups was found to be similar
to removing a methylene group from Cm. The dependence of the solubilization of a lipophilic dye (Sudan IV) and
a drug (nitrendipine) on the length of Cm was also studied.
Received 8 December 2020
Received in revised form 17 January 2021
Accepted 22 January 2021
Available online 30 January 2021
Keywords:
Ionic liquid-based surfactants
Surfactant head-group volume
Adsorption and aggregation parameters
Green synthesis
Polarity of micelle interfacial water
Micelle solubilization
© 2021 Elsevier B.V. All rights reserved.
1. Introduction
structures, ILBSs have higher surface activity and lower critical micelle
concentration (cmc) as compared with the structurally related “conven-
The unique physicochemical properties (high polarity, chemical and
thermal stability, and negligible vapor pressure) of ionic liquids (ILs) led
to their widespread use of in different research areas, as well as in the
chemical and pharmaceutical industries [1–3]. The molecular structural
versatility of ILs means that many useful properties, including polarity,
miscibility with molecular solvents, and electrochemical behavior can
be tailored to the researcher's need by a judicious choice of the struc-
tures of the cations and anions [4–6]. For example, ILs markedly im-
prove the pharmacokinetic and pharmacodynamic properties of drugs,
overcoming the limitations in drug delivery of water insoluble, or spar-
ingly soluble drugs [5]. Additionally, enzymatic reactions were success-
fully carried out in IL based aqueous microemulsions with high
conversion rates, high enantioselectivity, and better enzyme stability,
and recyclability [7–9].
tional” surfactants [10–15]. The enhanced surface activity of ILBSs is im-
portant because less surfactant is required for the end-use. Therefore,
knowledge of the adsorption/aggregation behavior of ILBSs is required
to develop their applications.
The useful applications of ILBSs, e.g., in nanotechnology, drug deliv-
ery, and decontamination depend on their molecular structures, includ-
ing the hydrophobic chain (HC) length, charge and size of the head
group (HG), and nature of the counter-ion. In nanotechnology, for ex-
ample, the ease of tailoring the properties of ILBSs through molecular
modification turns them into efficient templates to control the average
size, porosity and morphology of new nanomaterials [16]. Previously,
we studied the effect of the length of the HC on the adsorption and mi-
cellization properties in water of imidazole-based ILBSs, namely
CnMeImCl, where n = 10, 12, 14 and 16 [17]. The effect of HG size
was not studied for many ILBSs, although this structural variable bears
on the packing parameter of the monomers in the micellar aggregates,
hence their geometries [18]. Effects of this structural variable was eval-
uated for conventional surfactants, e.g., alkyltrialkylammonium bro-
mides and chlorides. It was found that the values of the cmcs and
aggregation numbers (Nagg) decrease as a function of increasing the
number of methylene groups in the HG [19–22].
Ionic liquid-based surfactants (ILBSs) are ILs with colloidal proper-
ties due to their long-chain hydrophobic “tails”. The favorable charac-
teristics of ILs are carried over to ILBSs. Depending on their molecular
⁎
Corresponding author.
E-mail address: elseoud.usp@gmail.com (O.A. El Seoud).
https://doi.org/10.1016/j.molliq.2021.115478
0167-7322/© 2021 Elsevier B.V. All rights reserved.

N. Keppeler, P.D. Galgano, S. da Silva Santos et al.
Journal of Molecular Liquids 328 (2021) 115478
The objective of the present work is the synthesis and determination
of adsorption/aggregation properties of the imidazole-based ILBSs
depicted in Fig. 1. As shown, the hydrophobic tail was fixed as n-
hexadecyl, whereas the other substituent in the imidazolium ring
(R) was varied from methyl to n-pentyl; the unsaturated groups vinyl
and allyl were also included. Data on the adsorption and aggregation
properties in water areavailableonly for the followingILBSs: C16MeImBr
[23–25], C16EtImBr [12], C16BuImBr [26], C16VnImBr [12] and C16MeImCl
[27–29] (Bu, Et, Me, and Vn refer to n-butyl, ethyl, methyl, and vinyl
group, respectively). The colloidal properties of the other six members
of this ILBS series were not evaluated, while two of them (C16PrImCl
and C16PnImCl) are novel (Pn and Pr refer to n-pentyl and n-propyl
group, respectively). Similar dialkyl imidazolium-based ILs have been
synthesized and studied [30], showing superior surface activity and
lower cmcvalues as compared to single chain imidazolium counterparts.
We used surface tension and conductivity to probe the surfactant
adsorption at the water/air interface and its aggregation in aqueous so-
lution. The quantities calculated include: cmc, surface tension at cmc
(γcmc), surface tension reduction efficiency (by 20 mN m−1; pC20), sur-
face excess concentration at the interface (Γmax), minimum area per
molecule at the water/air interface (Amin), Gibb's free energy of adsorp-
tion (ΔG0ads) and micellization (ΔGm0 ic), degree of counter-ion dissocia-
tion (αmic). We used the fluorescence spectra of micelle-solubilized
pyrene to calculate the concentration of water in the interfacial region.
Additionally, we studied the dissolution of a hydrophobic dye (Sudan
IV) and nitrendipine, a drug employed for treatment of hypertension
[31], because the association of certain substrates, e.g., cyclodextrins
that complex drugs show dependence on the structure of the HG [32].
Gas chromatography: Shimadzu 17A-2 gas chromatograph, equipped
with an FID detector and Supelcowax 10 capillary column.
Melting points: IA 6304 apparatus (Electrothermal), the melting
points (m.p.'s) were not corrected.
CHN elemental analyses: Perkin Elmer 2400 CHN apparatus. These
analyses were performed in the Analytical Center of this Institute.
1H and 13C NMR spectra: Avance 300 (Bruker), 300 MHz for 1H and
75 MHz for 13C NMR. All samples were dissolved in CDCl3 containing
TMS as internal reference.
Conductivity measurement and chloride ion titration: Mettler-Toledo
S400 SevenExcellence pH/mV meter with InLab 710 electrode 710
(K = 0.8 cm−1), attached to Lauda E200 thermostat, and Schott Titronic
T200 programmed burette.
Solution surface tension: Lauda TVT 2 drop volume tensiometer, at-
tached to Lauda E300 thermostat.
UV–Vis spectra: Shimadzu UV-2550 Spectrophotometer, provided
with thermostated cell holder and 4000A digital thermometer (Yellow
Springs Instruments).
Fluorescence spectra: Hitachi F-7000 fluorescence spectrophotome-
ter, provided with thermostated cell holder. We used 1 cm path length
quartz cuvettes with PTFE stoppers.
2.3. Synthesis of the ILBSs
The ILBSs were synthesized by the reaction schemes shown in Fig. 2.
Both series of ILBSs were synthesized via microwave-assisted reaction.
2.3.1. 1-Alk(en)ylimidazole
We dissolved sodium metal (0.30 mol) in 100 mL of dry 2-propanol
under reflux and magnetic stirring (approx. 2 h). After cooling to room
temperature, we added to the reaction flask, under stirring, a solution of
imidazole (0.30 mol) in 40 mL of 2-propanol, stirred the mixture for ad-
ditional 30 min, removed the solvent to yield a yellowish paste, and
suspended the latter in 120 mL of dry acetonitrile (MeCN). We put the
reaction flask in an ice bath, agitated the suspension vigorously, and
used a peristaltic pump to add a solution of 1-bromoalkane or 1-
bromoalkene (0.33 mol) in 25 mL of MeCN during 6 h, and then kept
the mixture stirred overnight at room temperature. We filtered the pre-
cipitated solid, removed the solvent and distilled the product under re-
duced pressure (~1 mmHg). In all cases we obtained colorless liquids;
boiling points at 1 mmHg are 52 °C, 65 °C, 70 °C, 72 °C and 61 °C for 1-
R-imidazole, with R = ethyl, n-propyl, n-butyl, n-pentyl, and allyl, re-
spectively. The yields of 1-R-imidazole were from 80 to 90%, based on
imidazole. 1-Methylimidazole and 1-vinylimidazole are commercially
available.
2. Materials and methods
2.1. Materials
The reagents were purchased from Acros, Sigma-Aldrich, Merck or
Synth (São Paulo) and were purified as recommended elsewhere [33].
We purified 1-bromo-n-hexadecane and 1-chloro-n-hexadecane by re-
peated fractional distillation in a 50 cm long Vigreux column, under re-
duced pressure. Satisfactory purity was achieved (> 99.9%) as shown by
gas chromatographic analysis. Deionized (Milli-Q) water was used
throughout.
2.2. Equipment
We used the following equipment:
Synthesis of the ILBSs: Masterflex model 77,390–00 metering pump
and CEM Discover (CEM Co.) microwave (MW) equipment, provided
with pressure accessory (for chloride ILBSs).
2.3.2. Bromide ILBSs
We slowly added a solution of 1-bromehexadecane (0.055 mol) in
70 mL of dry ethyl acetate, with efficient stirring to 0.050 mol of the ap-
propriate 1-alk(en)ylimidazole and heated the reaction mixture for 5 h
at 70 °C in the microwave equipment (60 W irradiation power). We re-
moved the solvent, washed the resulting white solid with n-hexane
(3 × 25 mL) and dried it under reduced pressure over P4O10 until con-
stant mass. The products were characterized by melting point, 1H
NMR, and elemental analysis, vide item 1.1 of Supplementary Material
(SM). The 1H NMR spectra of the products are presented in Fig. SM1
to SM6.
2.3.3. Chloride ILBSs
The ILBS C16MeImCl was available from a previous study [17]. We
synthesized the other chlorides by reacting 0.050 mol of 1-
alkylimidazole with 0.055 mol of 1-chloro-n-hexadecane in the pres-
sure reactor (50 mL) of the microwave equipment, under the following
conditions: no solvent; irradiation power = 200 W; 170 °C; 20 bar; 5 h.
After cooling to room temperature, we washed the product with n-hex-
ane (3 × 25 mL) and dried it under reduced pressure over P4O10 until
Fig. 1. Molecular structures of the synthesized ILBSs.
2

N. Keppeler, P.D. Galgano, S. da Silva Santos et al.
Journal of Molecular Liquids 328 (2021) 115478
Fig. 2. Reaction schemes for the synthesis of the ILBSs.
constant mass. These surfactants were characterized by their melting
point, 1H and 13C NMR, and Volhard titration method for chloride anal-
yses [34]. The results of the analyses are shown in SM and the corre-
sponding 1H and 13C NMR spectra are presented in Fig. SM7-SM14.
Previous studies showed that water/1-propanol binary mixture is a good
model for interfacial water [38].
2.4.4. Dye and drug solubilization
We added an excess of finely powdered Sudan IV or nitrendipine to
the surfactant solution (ca. 8 x cmc), agitated the suspension for 5 min
(vortex mixer) and then overnight using the above-mentioned tube ro-
tator (60 rpm). We removed the undissolved solid by filtration for
Sudan IV, using 10 μm PTFE membrane (0.45 μm, 13 mm; Gelman
4422) and centrifugation for nitrendipine (ThermoCientific Sorvall Leg-
end X1R). We diluted the filtrate 5 times with the respective ILBS solu-
tion and measured the absorbance of dissolved solubilizate using the
above-mentioned Shimadzu spectrophotometer. We recorded the
UV–Vis spectra from 440 to 600 nm for Sudan IV and from 300 to
420 nm for nitrendipine solutions at 25.0 0.1 °C and then calculated
the concentrations of solubilized dye and drug from the appropriate
Beer's law plot.
2.4. Experimental method
2.4.1. Surface tension measurements
For each surfactant we prepared 12 solutions, whose concentrations
were from ca.5timesbelowthecmcupto15timesabovethecmc.Values
ofthelatterwereavailablefromtheconductivitymeasurements,detailed
inSection2.4.2.Werecordedthesurfacetension(γ)ofeachsolutionafter
10 min temperature equilibration at25 0.1 °C on the drop volumeten-
siometer. The outer and inner radius of the steel needle were 1.39 and
1.08 mm, respectively. The measurements of the ILBSs solutions were
done with 10 different drop formation velocities (0.5 s mL−1 to
5 s mL−1); average = 3 drops at each velocity.
2.4.2. Conductivity measurements
3. Results and discussion
We calibrated the conductivity meter with 0.01 mol L−1 KCl aqueous
solution. The measurements were made at 25 0.1 °C for all ILBSs, ex-
cept for C16MeImCl, whose data were published elsewhere [17]. We
added 100 μL aliquots of the ILBSs aqueous solutions (ca. 5–10 times
above cmcs, based on literature values, e.g., of C16MeImBr and
3.1. General
We synthesized pure chloride ILBSs by a novel, fast, solventless,
microwave-assisted route, instead of using literature procedures that
require reflux of both reactants in a solvent for 24–48 h [39,40]. This
synthesis, besides being greener than its traditional counterparts, gave
good yield of pure ILBSs. Use of elemental analysis for assessing the pu-
rity of chloride ILBSs was not feasible because of their hygroscopic char-
acter, and the waxy, difficult-to-dry nature of the corresponding iodides
and perchlorates, therefore we used the Volhard titration method.
For brevity, we use in the discussion below the term HG volume to
denote the increase in the length of the side chain attached to the
imidazolium ring from methyl- to n-pentyl. We use the general struc-
ture C16RImX to denote the series studied in the present work, where
R refers to the substituent group (saturated and unsaturated) and X re-
fers to Br and Cl. In the first part of each discussion, we consider the re-
sults of the saturated groups in the HG, designating them as Cm. The
effect of presence of unsaturation is discussed subsequently by compar-
ing C16EtImBr with C16VnImBr (2 carbons), and C16PrImBr with
C
16MeImCl [35,36]) to 10 mL of water, and registered the conductivity
after each addition. We used a home-developed software, both for pro-
grammed addition of the concentrated surfactant solution to water and
acquisition of conductivity data.
2.4.3. Fluorescence measurements
We pipetted 200 μL of 10−4 mol L−1 methanolic solution of pyrene
into 10 mL volumetric flasks, evaporated the solvent, added the appro-
priate volume of the surfactant solution and water. The final (fixed)
concentrations of pyrene and surfactant were 2.0 × 10−6 mol L−1 and
10 x cmc, respectively. To ensure complete dissolution of pyrene, we ag-
itated the volumetric flasks overnight (Glas-Col model 099A RD4512
tube rotator; 60 rpm); sonicated the solutions for 15 min (Fritsch,
Laborette 17 sonicator) and then waited for another 15 min before re-
cording the spectra at 350–500 nm, using an excitation wavelength of
334 nm, and slit widths of 2.5 nm.
C16AlImBr (3 carbons; Al = allyl group).
We calculated the effective water concentration as follows: (i) the
values of fluorescence intensities ratio of the first and third vibronic
peaks (I1/I3) for pyrene in the micellar pseudo-phases were calculated;
(ii)acalibrationcurveofI1/I3 versus[water]inwater/1-propanolmixtures,
outside the micellar domain, was plotted according to literature data [37];
(iii)theI1/I3values(inpresenceoftheILBS)wereconvertedintointerfacial
water concentration using the above-mentioned calibration curve.
We discuss the dependence of solution physicochemical properties
on the volume of the HG in the following order: adsorption at water/
air interface, micelle formation, and concentration of interfacial water.
Where appropriate, we compare the effects of increasing HG volume
with those caused by increasing the length of hydrophobic chain (Cn),
e.g., from n-decyl to n-hexadecyl, keeping the HG structure constant
(i.e., C10MeImCl to C16MeImCl [17]). Additionally, some aggregation
3

N. Keppeler, P.D. Galgano, S. da Silva Santos et al.
Journal of Molecular Liquids 328 (2021) 115478
Table 1
properties of the ILBSs series were compared with those of structurally
related conventional surfactants. Finally, we discuss the effects of in-
creasing the HG volume on the solubilization of hydrophobic nonionic
dye (Sudan IV) and the drug nitrendipine, a dihydropyridine calcium
channel-blocker employed in the treatment of primary hypertension
[31].
The equations to calculate the parameters of the surfactant adsorp-
tion at the water/air interface and aggregation as micelles are standard
[18]. Therefore, we show only the results of these calculations; the cor-
responding equations are presented in supplementary material, SM.
Dependence of surfactant adsorption parameters at 25 °C on head group volume of the
studied ILBSs.
ILBS
γcmc
pC20
Γmax
Amin (nm2)
ΔG0ads
(mN m−1
)
(μmol m−2
)
(kJ mol−1
)
Bromide ILBSs
C16MeImBr
38.7
39.8
39.7
40.3
37.7
38.7
3.55
3.65
3.80
3.92
3.63
3.74
3.0
2.7
2.3
1.9
3.1
2.6
0.54
0.61
0.73
0.86
0.53
0.63
−36.83
−38.19
−40.46
−42.73
−37.09
−38.90
C
C
C
C
C
16EtImBr
16PrImBr
16BuImBr
16VnImBr
16AlImBr
3.2. Adsorption at water/air interface
Chloride ILBSs
C
C
16MeImCl
16EtImCl
40.9
35.4
35.2
38.0
39.6
3.39
3.62
3.82
3.89
4.15
3.4
2.6
2.2
2.1
1.6
0.49
0.63
0.75
0.80
1.06
−35.64
−38.29
−40.78
−41.86
−46.37
It is important to measure solution surface tension values after the
surfactant adsorption has reached equilibrium; this requires unknown
time and can be long for ionic surfactants [41]. Therefore, we measured
the surface tension as a function of drop formation time (t), as exempli-
fied in Fig. SM15 for C16CmImBr. For each surfactant solution, we calcu-
lated the surface tension at infinity time (γinf) by regression analysis of
γ vs. t. For the studied ILBS series, the difference between the initial sur-
face tension (γ0) and γinf reaches 8.5%; this difference affects the calcu-
lated adsorption parameters values by up to 20%.
C16PrImCl
C
C
16BuImCl
16PnImCl
Amin with Cm is clearly shown in Eqs. (1) and (2) for the C16CmImX (X =
Br and Cl) series, whereas Eq. (3) shows the effect of increasing Cn of the
CnMeImCl series [17]. Eqs. (1), (2) and (3) are linear regressions of the
lines in Fig. SM16.
We plot in Fig. 3 the extrapolated values of the surface tension (γinf
)
as a function of log [ILBS]. As expected, values of γinf decrease fast as a
function of increasing the surfactant concentration until the cmc, and
then decrease much slower. The absence of “dips” in these plots show
that the ILBSs are surface active pure [42]. Demonstrating this purity is
important, because uncertainty in the cmc value (due to presence of
the dip) bears on the calculated adsorption and micellar properties
[43,44]; removing surface-active impurities from the solution is, at
best, time-consuming and laborious [45].
C16CmImBr : Amin=nm2 ¼ 0:42 ðꢀ0:03Þ þ 0:108 ðꢀ0:010Þ Cm, R2
¼ 0:975
ð1Þ
ð2Þ
ð3Þ
C16CmImCl : Amin=nm2 ¼ 0:35 ðꢀ0:06Þ þ 0:131 ðꢀ0:017Þ Cm, R2
¼ 0:934
CnMeImCl : Amin=nm2 ¼ 1:46 ðꢀ0:08Þ−0:062 ðꢀ0:006Þ Cn, R2
¼ 0:969
We employed the surface tension data to calculate the values of cmc
and adsorption parameters, including γcmc, pC20, Γmax, Amin, and ΔGa0ds
,
see Eqs. SM1 to SM4. We present the values of these parameters in
Table 1. The cmc values, given by the break in plot of γinf versus log
[ILBS], are presented and discussed in Section 3.3.
Eqs. (1) and (2), and Table 1 show the following consequences of in-
creasing the HG volume on the calculated values of the adsorption pa-
rameters of C16RImX: (i) an increase in Amin, due to steric repulsions
between the increasingly voluminous Cm groups [46]; (ii) a corollary
to this increase in Amin is the presence of less surfactant molecules at
the water/air interface which agrees with the decrease of Γmax and in-
crease in γCMC; (iii) the adsorption of a more hydrophobic HG is favored
Where γcmc, pC20, Γmax, Amin and ΔG0ads stand for surface tension at
cmc, surface tension reduction efficiency, surface excess concentration
at the interface, minimum area per molecule at the water/air interface
and Gibb's free energy of adsorption, respectively.
As seen in Fig. 3, the slopes of the straight lines in the region below
the cmc decrease as a function of increasing the HG volume, this corre-
sponds to an increase in the Amin and a decrease in Γmax. The increase of
as shown by the increase in |ΔG0ads| and pC20
.
Regarding the effect of unsaturation in the HG, comparison of Amin of
C16EtImBr with C16VnImBr, and C16PrImBr with C16AlImBr shows that
Fig. 3. Surface tension for t → infinite (γinf) as a function of logarithm of ILBS concentration, at 25 °C. Part A (C16CmImBr) and Part B (C16CmImCl). The straight lines before the cmc were used
to calculate the minimum area per molecule at the water/air interface.
4

N. Keppeler, P.D. Galgano, S. da Silva Santos et al.
Journal of Molecular Liquids 328 (2021) 115478
introduction of unsaturation is akin to removal of one methylene group.
In this case, the double bond presumably increases the dipole-induced
dipole intermolecular forces between the HGs and water molecules,
leading to the HG approximation [12], hence to a smaller value of
Amin. Comparison of γcmc, pC20, and Γmax of unsaturated and saturated
HGs (Table 1) agrees with closer approximation between the molecules
of the former surfactants. Comparison of the series C16CmImX and
CnMeImCl shows the opposite effects of increasing the number of car-
bons in the HG and HC, respectively. The behavior in the CnMeImCl se-
ries is explained by a concomitant closer packing of monomers at the
micellar interface as a function of increasing the length of Cn [17,46–48].
The values of ΔG0ads can be divided into the contributions from the
surfactant segments, namely, the n-hexadecylimidazolium ring + the
adsorption parameters are small; they become more apparent in the
micellar aggregates, vide infra.
3.3. Micelle formation
Values of the cmcs were calculated from conductivity and surface
tension data; in both cases from the intercept of two straight lines
were calculated from the plotted data. Surface tension data were ana-
lyzed in Section 3.2.
As expected, the specific conductance increases fast until the cmc,
and then less after micelle formation, because conductivity of the latter
species (a macroion) [49] is less than the free, i.e., dissociated surfactant
monomers. Fig. 4 shows typical conductivity data for both series, the
calculated values of cmc and other micellar parameters are listed in
Table 2. The calculation methods of the micellization parameters are
discussed at Section 3.2. of SM. Carpena's method, see Eq. SM5, is espe-
cially adequate for the analysis of cases where the change in the slopes
before and after the cmc is not sharp. This gradual change in conductiv-
ity may be associated with the formation of premicellar aggregates and
is more pronounced for ILBS with Cm = 4 and 5, vide infra.
counter-ion (ΔGa0ds C16+Im+X
ries (Eq. (4)) and methylimidazolium ring + the counter-ion (ΔGa0ds
Me+Im+X), and Cn (ΔG0ads Cn
) for the CnMeImCl series (Eq. (5)).
,
) and Cm (ΔG0ads Cm
, ) for the C16CmImX se-
,
,
ΔG0 ¼ ΔG0
þ ΔG0
Cm
ads,Cm
ð4Þ
ð5Þ
ads
ads,C16þImþX
ΔG0 ¼ ΔG0
þ ΔG0
Cn
ads,Cn
ads
ads,MeþImþX
The cmc values calculated from the data of both techniques are in
excellent agreement, considering that these techniques are sensitive
to different aspects of the micellization process, as discussed in detail
elsewhere [51]. In most cases, close to room temperature, the driving
force for micelle formation is entropic. The reason is that the transfer
of the HC from water into the micelle, and the HG into (less polar) inter-
facial water is associated with release of the highly-ordered water mol-
ecules around the hydrocarbon chains of both surfactant segments.
Further entropy gain on micellization is due to the increase in the de-
grees of freedom of the HC inside the micelle, relative to bulk water
[52]. Therefore, it is expected that values of the cmc decrease as a func-
tion of increasing both Cn and Cm, as shown in Table 2. Stauff-Klevens
rule [53] (Eq. (9)) predicts a linear relationship between the log cmc
and Cn:
Following these equations, the dependence of ΔG0ads on HG volume,
and the length of the hydrophobic chain [17] is described by Eqs. (6),
(7) and (8).
C16CmImBr : ΔG0ads=kJ mol−1 ¼ −34:6 ðꢀ0:4Þ−2:0 ðꢀ0:2Þ Cm, R2
¼ 0:982
ð6Þ
ð7Þ
ð8Þ
C16CmImCl : ΔG0ads=kJ mol−1 ¼ −33:1 ðꢀ0:9Þ−2:5 ðꢀ0:3Þ Cm, R2
¼ 0:955
CnMeImCl : ΔG0ads=kJ mol−1 ¼ −21:8 ðꢀ0:5Þ−0:85 ðꢀ0:04Þ Cn, R2
¼ 0:994
The results of the regression analysis show that the change of ΔGa0ds
per CH2 group in the HG and HC are 2.0–2.5 kJ mol−1 and 0.85 kJ mol−1
,
log cmc ¼ A–B Cn
ð9Þ
respectively. This difference in ΔG0ads maybe related to changes in solva-
tion entropy of the appropriate segment of both ILBSs series. Note that
the methylene groups of CnMeImCl are certainly less solvated (by
water) than the Cm groups of C16CmImX. Consequently, the expected
order is |ΔS|CnMeImCl > |ΔS|C16CmImCl, in agreement with the calculated
ΔG0ads of Table 1. Finally, the effects of the counter-ions on the
where A is a constant that depends on the structure of the surfactant
monomer, and B reflects the effect of each additional CH2 on cmc. We
applied this equation to Cm and Cn; the results of the regression analysis
are shown in Eqs. (10)–(13), and plotted graphically in Fig. SM17.
Fig. 4. Specific conductance (κ) as a function of surfactant concentration. The red straight lines are drawn to determine the cmc values visually and the black curve represents the regression
by the Carpena equation. Part A and Part B are representative plots of bromide and chloride ILBSs, respectively.
5

N. Keppeler, P.D. Galgano, S. da Silva Santos et al.
Journal of Molecular Liquids 328 (2021) 115478
Table 2
(Cm).Thistypeofcorrelationwasalsodeductedfortheconventionalsur-
factant series C16(Cm)3NBr (cmc data from Ref. [54]), Eq. (12). The lower
value of the constant (A) for the C16CmImBr series shows that the struc-
tureofILBSsmonomer(notconsideringthesidechain)favorstheforma-
tion of micelles at smaller concentrations than the conventional
surfactantseries. Tocompare thevaluesof(B) for bothseries, itis impor-
tant to consider that for the C16(Cm)3NBr series there is an increase of 3
CH2 groupsforeachoneoftheC16CmImBrseries.Therefore,itisnecessary
to divide the values of (B) of Eq. (12) by 3. We found that ∣B;
Values of micellization parameters, including cmc, Nagg and αmic determined by experi-
mental and theoretical techniques for ILBSs, at 25 °C.
a
d
ILBS
cmc (mmol L−1
)
Nagg αmic
αmic
ΔG0mic
(Evans)b (Frahm)c (kJ mol−1
)
Surface
tension
Conductivity
Bromide ILBSs
C
C
C
C
C
C
16MeImBr 0.71
16EtImBr 0.55
16PrImBr 0.44
16BuImBr 0.40
16VnImBr 0.60
16AlImBr 0.51
0.65
0.52
0.41
0.30
0.52
0.48
64e
60f
52
0.133
0.150
0.178
0.198
0.154
0.168
0.283
0.344
0.440
0.498
0.338
0.366
−52.59
−53.09
−53.41
−54.30
−53.01
−52.98
C
16CmImBr∣ > ∣B; C16(Cm)3NBr/3∣, the former surfactant series is more
45
surface active.
54f
47
The fact that the slope per CH2 in the HC series, Eq. (13), is ca. 3
times larger than in the HG series, Eq. (10), is not surprising because
on micellization there is more dehydration of most of the CH2 groups
in HC (whose micelle interior is oil-like) that any CH2 in the HG; the
latter is transferred from bulk- to less polar water, vide infra. The
(slight) difference in polarity between bulk- and interfacial water
also explains the little dependence of ΔG0mic on the presence of
unsaturation in the HG. On the other hand, micellization of
Chloride ILBSs
C
C
C
C
C
16MeImCl 0.87
16EtImCl
16PrImCl
16BuImCl 0.50
16PnImCl 0.35
0.91
0.58
0.46
0.51
0.39
97g
83
71
61
53
0.220
0.274
0.298
0.278
0.304
–
–
–
–
–
−48.63
−49.10
−49.37
−49.52
−49.90
0.88
0.71
a
Values of Nagg were obtained by theoretical calculation, see Section 3.2. in SM.
Calculated from Eq. SM7.
b
c
C
16CmImBr is more favorable than the corresponding C16CmImCl be-
Calculated from Eq. SM8.
cause the formation of micelles is more spontaneous for bromides
due to the larger ion size and polarizability of Br−, which decreases
the repulsion between the cationic head-ions in the formed micelle,
[55] hence lowers the cmc values.
d
Values of Gibb's free energy of micellization (ΔG0mic) calculated with αmic from Evans
method.
e
Value of reference [35], calculated from steady state fluorescence measurements.
Values of reference [12], calculated from steady state fluorescence measurements.
Value of reference [50], calculated from static light scattering measurements.
f
g
As shown in Table 2, the values of αmic increase as a function of in-
creasing HG volume. The increased dissociation of the counter-ion is in-
duced by an increased steric hindrance around the imidazolium ring
which prevents the approach of the halide ion to the cation at the micel-
lar interface [56]. Based on Eq. SM6, this increase in αmic decreases the
corresponding value of ΔG0mic, as shown in Fig. 5. The linear regressions
of the lines in Fig. 5 are shown in Eqs. (14), (15) and (16), and highlights
the fact that the contribution of HG volume increase is much smaller
(10% for the chloride ILBSs) relative to the same increase in the HC.
ꢀ
ꢁ
C16CmImBr : log cmc=mol L−1
¼ −3:075 ðꢀ0:008Þ−0:097 ðꢀ0:003Þ Cm, R2 ¼ 0:997 ð10Þ
ꢀ
ꢁ
C16CmImCl : log cmc=mol L−1
¼ −2:953 ðꢀ0:019Þ−0:092 ðꢀ0:006Þ Cm, R2 ¼ 0:985 ð11Þ
ꢀ
ꢁ
C16CmImBr : ΔG0mic=kJ mol−1
C16ðCmÞ NBr : log cmc=mol L−1
3
¼ −52:0 ðꢀ0:2Þ−0:54 ðꢀ0:08Þ Cm, R2 ¼ 0:936
ð14Þ
¼ −2:75 ðꢀ0:17Þ−0:21 ðꢀ0:06Þ Cm, R2 ¼ 0:851
ð12Þ
C16CmImCl : ΔG0mic=kJ mol−1
ꢀ
ꢁ
CnMeImBr : log cmc=mol L−1
¼ −48:42 ðꢀ0:10Þ−0:30 ðꢀ0:03Þ Cm, R2 ¼ 0:971
ð15Þ
ð16Þ
¼ 1:79 ðꢀ0:04Þ−0:304 ðꢀ0:003Þ Cn, R2 ¼ 0:999
ð13Þ
CnMeImCl : ΔG0mic=kJ mol−1 ¼ 2:1 ðꢀ1:0Þ−3:16 ðꢀ0:08Þ Cn, R2
¼ 0:936
It is interesting that the Stauff-Klevens rule is also applicable for the
HG, resulting in excellent linear correlation between the log cmc and
Fig. 5. Dependence of Gibb's free energy of micellization (ΔGm0 ic) on the number of carbon atom (Cm or Cn), at 25 °C. Part A (C16CmImX) and Part B (CnMeImCl) [57].
6

N. Keppeler, P.D. Galgano, S. da Silva Santos et al.
Journal of Molecular Liquids 328 (2021) 115478
3.4. Microscopic polarity and concentration of the interfacial water
average number of molecules solubilized in each micelle is defined
by Eq. (17) [66].
Fluorescence measurements with pyrene were performed for
C16CmImX to calculate the microscopic polarity, hence the concentra-
tion of interfacial water ([H2O]int). Monomeric pyrene exhibits five
vibronic peaks in the region between 375 and 410 nm due to π-π* tran-
sitions. The ratio of fluorescence intensities of the first and third vibronic
peaks (I1/I3) is sensitive to the microscopic polarity experienced by the
pyrene molecules. Since the third peak shows higher fluorescence
intensity compared to the first one in a hydrophobic environment, an
increase in hydrophobicity is indicated by a relatively lower value of
the I1/I3 ratio [58]. An example of pyrene fluorescent graph is shown
at Fig. SM18. 1H NMR data for sodium dodecylbenzene sulfonate indi-
cated that the average solubilization site of the pyrene molecule is the
interfacial region, close to the surfactant aromatic head-group [59].
Additionally, 1H NMR and fluorescence studies on ILBSs with aromatic
counter-ions [60,61], and interactions of cationic ILBSs with sodium
dodecylbenzene sulfonate [62] clearly showed the preference of the
aromatic rings to associate with the imidazolium cation. Consequently,
we can safely assume that pyrene will be similarly solubilized in the
interfacial region, close to the heterocyclic ring.
Stotal−Swater
½ILBSꢁ−CMC
Σ ¼ Nagg x SP ¼ Nagg
x
ð17Þ
where Stotal and Swater refer to the solubility of the molecule in presence,
and absence of the surfactant, respectively [66].
Surprisingly, the values of SP of Sudan IV and nitrendipine in micellar
16CmImX decreased linearly as a function of increasing Cm, as shown in
C
Table 3 and Fig. SM19. In addition to solute-micelle hydrophobic inter-
actions, other factors should be considered, namely the micelle size, and
steric hindrance. As shown in Table 2, Nagg for these ILBS decreases lin-
early with the increase of Cm; consequently, the micelles become
smaller, leading to a decrease in the number of solute molecules that
can be accommodated. It is also plausible that this effect is coupled to in-
creased steric hindrance to solute penetration in micellar pseudo-phase.
Table 3 shows much higher solubilization capacity for nitrendipine
than for Sudan IV by both C16CmImX. Whether the drug induces micelle
morphology change (e.g., from spherical micelles → vesicles), akin to
other drugs (e.g., cholesterol and diclofenac sodium [15,67]) is an
open question. The difference between solubilization power of bro-
mides and chlorides ILBSs can be disregarded for Sudan IV, as it is
<5%. For nitrendipine the corresponding difference is >25%, clearly
showing that the counter-ion is important. This may be taken as another
indication for the above-mentioned, drug-induced morphology change,
a subject that we intend to investigate. Finally, it is worth mentioning
that the solubilization power of Sudan IV by the present ILBSs is 2 to 3
times higher than CTAB [66], a conventional cationic surfactant. To the
best of our knowledge, no other study has been done with the dissolu-
tion of nitrendipine in micellar ILBSs.
The results of microscopic polarity (I1/I3) (displayed at Table SM2)
show that the interfacial water of the studied ILBSs solutions (I1/
I3 ≈ 1.31) is less polar than bulk water (I1/I3 = 1.84 [37]), the interfacial
water concentration decreases linearly as a function of increasing the
volume of HG, from 41.6 mol L−1 for C16MeImCl to 38.2 mol L−1 for
C
16PnImCl, and there is only a slight dependence on the counter-ion. It
is interesting that this concentration range 40 2 mol L−1 is similar
to that calculated for conventional cationic surfactants using a
solvatochromic probe [38], and that inferred from a distinct approach,
namely, reaction of micelle incorporated diazonium ion with water,
the so-called ion-trapping technique [63]. This agreement is satisfactory
and shows that pyrene is an appropriate probe for calculating [H2O]int
.
4. Conclusions
3.5. Solubilization of lipophilic dye and drug
The synthetic routes employed (microwave-assisted; one is solvent-
less) produced surface-active pure surfactants. Information about ad-
sorption, micellization and micelle interfacial water concentration of
The solubilization of Sudan IV and nitrendipine in aqueous solutions
of the series of ILBSs was investigated. Measurements were not done for
C16BuImBr because the surfactant precipitated during dissolution of
both solubilizates. The molar extinction coefficients (ε0) for both mole-
cules in aqueous media are known, 7.5 × 10−4 mol−1 cm−1 for Sudan IV
and 3.2 × 10−3 mol−1 cm−1 for nitrendipine [64,65], therefore the con-
centration of the dissolved solute can be readily calculated from the
Beer's law plot, assuming the same values of ε0.
The molar solubilization power (SP) of a surfactant is defined as
moles of solubilized molecule per mole of micellized surfactant. If
the micellar aggregation number is not influenced by the solubiliza-
tion of the dye or drug, the solubilization capacity (Σ), which is the
C
16RImX (X = Br and Cl) in aqueous solutions were obtained from
surface tension and conductivity data, and fluorescence of micelle-
solubilized pyrene, respectively. Values of γ employed in the calcula-
tions were those after attaining surfactant adsorption equilibrium at
the water/air interface. Use of Carpena's method for the calculation of
the cmc was especially helpful for C16CmImX, Cm = 4 and 5, where
formation of premicellar aggregates probably occurs. Values of cmc
calculated from both techniques were in excellent agreement.
The structural modification of ILBSs caused by introduction of meth-
ylene (CH2) in the HG causes a decrease in polarity, with concomitant
increase in surface tension reduction in efficiency (pC20), Amin, and
α
mic and a decrease in cmc, ΔG0mic, Nagg, and [H2O]int. The presence of
Table 3
unsaturation in the HG proved to be as relevant as the variation in the
number of CH2, being comparable to the removal of one CH2 due to
the less hydrophobic character of the double bond. The changes caused
by increasing the HG volume are less pronounced than those due to in-
creasing the length of HC. Values of [H2O]int (40 2 mol L−1), obtained
by fluorescence of micelle-solubilized pyrene, are similar to those calcu-
lated by different approaches, based on the use of solvatochromic
probes and the reaction of micelle-incorporated diazonium ion with
water. Solubilization of Sudan IV and nitrendipine decreased as a func-
tion of increasing the HG volume, probably reflecting steric hindrance
to the penetration of the solubilizate in smaller micelles. Comparing
the ILBSs of this study with conventional surfactants with the same hy-
drophobic tail showed that the former class has superior surface activ-
ity, lower cmc values, and are more effective in hydrophobic dye
(Sudan IV) dissolution [10,11,66].
Solubilization power, SP, and solubilization capacity, ∑, of ILBSs for Sudan IV and
nitrendipine.
ILBS
Sudan IV
Nitrendipine
SP
∑ (molecule
SP
∑ (molecule
micelle−1
)
micelle−1
)
Bromide ILBSs
C16MeImBr
0.0175
0.0168
0.0161
1.1
1.0
0.8
1.09
0.968
0.743
70
58
39
C
C
16EtImBr
16PrImBr
Chloride ILBSs
C
C
C
C
C
16MeImCl
16EtImCl
16PrImCl
16BuImCl
16PnImCl
0.0166
0.0160
0.0154
0.0149
0.0146
1.6
1.3
1.1
0.9
0.8
1.50
1.32
1.16
0.938
0.727
146
110
82
57
38
7

N. Keppeler, P.D. Galgano, S. da Silva Santos et al.
Journal of Molecular Liquids 328 (2021) 115478
[14] Z.S. Vaid, S.M. Rajput, A. Shah, Y. Kadam, A. Kumar, O.A. El Seoud, J.P. Mata, N.I.
Malek, Salt-induced microstructural transitions in aqueous dispersions of ionic-
liquids-based surfactants, ChemistrySelect. 3 (2018) 4851–4858, https://doi.org/
10.1002/slct.201800041.
[15] Z.S. Vaid, S.M. Rajput, M. Kuddushi, A. Kumar, O.A. El Seoud, N.I. Malek, Synergistic
interaction between cholesterol and functionalized ionic liquid based surfactant
leading to the morphological transition, ChemistrySelect. 3 (2018) 1300–1308,
https://doi.org/10.1002/slct.201702561.
Author statement
Nicolas Keppeler: Data curation; formal analysis; methodology,
writing. Paula D. Galgano: data curation, formal analysis, methodology.
Soraya S. Santos: data curation, methodology. Naved I Malek: formal
analysis, writing. Omar A. El Seoud: conceptualization, writing, formal
analysis, funding aquisition.
[16] I. Pacheco-fernández, V. Pino, J.H. Ayala, A.M. Afonso, Ionic liquid-based surfactants:
A step forward, in: A. Eftekhari (Ed.), Ionic Liquid Devices, Royal Society of Chemis-
try, Croydon 2017, pp. 53–72.
[17] O.A. El Seoud, P.A.R. Pires, T. Abdel-Moghny, E.L. Bastos, Synthesis and micellar prop-
erties of surface-active ionic liquids: 1-Alkyl-3-methylimidazolium chlorides, J. Col-
loid Interface Sci. 313 (2007) 296–304, https://doi.org/10.1016/j.jcis.2007.04.028.
[18] P.C. Hiemenz, R. Rajagopalan, Principles of Colloid and Surface Chemistry, 3rd ed.
Marcel Dekker, New York, 1997.
[19] M. Yasuda, K. Ikeda, K. Esumi, K. Meguro, Effect of head group size on aqueous prop-
erties of α,ω-type surfactant, Langmuir. 6 (1990) 949–953, https://doi.org/10.1021/
la00095a011.
Declaration of Competing Interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influ-
ence the work reported in this paper.
[20] A. Di Michele, L. Brinchi, P. Di Profio, R. Germani, G. Savelli, G. Onori, Effect of head
group size, temperature and counterion specificity on cationic micelles, J. Colloid In-
terface Sci. 358 (2011) 160–166, https://doi.org/10.1016/j.jcis.2010.12.028.
[21] H. Xing, P. Yan, K.S. Zhao, J.X. Xiao, Effect of headgroup size on the thermodynamic
properties of micellization of dodecyltrialkylammonium bromides, J. Chem. Eng.
Data 56 (2011) 865–873, https://doi.org/10.1021/je100598x.
[22] E. Akpinar, E. Guner, O. Demir-Ordu, A.M.F. Neto, Effect of head-group size of some
tetradecylalkylammonium bromide surfactants on obtaining the lyotropic biaxial
nematic phase, Eur. Phys. J. E. 42 (2019) 1–8, https://doi.org/10.1140/epje/i2019-
11806-y.
[23] A. Pal, S. Yadav, Effect of anionic polyelectrolyte sodium carboxymethylcellulose on
the aggregation behavior of surface active ionic liquids in aqueous solution, J. Mol.
Liq. 241 (2017) 584–594, https://doi.org/10.1016/j.molliq.2017.06.063.
[24] L. Qin, X.H. Wang, Surface adsorption and thermodynamic properties of mixed sys-
tem of ionic liquid surfactants with cetyltrimethyl ammonium bromide, RSC Adv. 7
(2017) 51426–51435, https://doi.org/10.1039/c7ra08915e.
Acknowledgments
We thank São Paulo State Research Foundation (FAPESP) for finan-
cial support (grant 2014/22136-4); the National Council for Scientific
and Technological Research (CNPq) for a pre-doctoral fellowship to N.
Keppeler, and a research productivity fellowship to O. A. El Seoud (grant
306108/2019-4). NIM acknowledge the financial assistance of UGC-DAE
for the Collaborative Research Scheme (UDCSR/MUM/AO/CRS-M-276/
2017). We are indebted to Prof. Iolanda M. Cuccovia for making the fluo-
rescence spectrophotometer available to us, and Dr. Paulo A.R. Pires for
his help with theoretical calculations.
[25] G.Y. Wang, Y.Y. Wang, X.H. Wang, Aggregation behaviors of mixed systems for im-
idazole based ionic liquid surfactant and Triton X-100, J. Mol. Liq. 232 (2017) 55–61,
https://doi.org/10.1016/j.molliq.2017.02.044.
Appendix A. Supplementary material
[26] Q.Q. Baltazar, J. Chandawalla, K. Sawyer, J.L. Anderson, Interfacial and micellar prop-
erties of imidazolium-based monocationic and dicationic ionic liquids, Colloids Surf.
A Physicochem. Eng. Asp. 302 (2007) 150–156, https://doi.org/10.1016/j.colsurfa.
2007.02.012.
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.molliq.2021.115478.
[27] C. Jungnickel, J. Łuczak, J. Ranke, J.F. Fernández, A. Müller, J. Thöming, Micelle forma-
tion of imidazolium ionic liquids in aqueous solution, Colloids Surf. A Physicochem.
Eng. Asp. 316 (2008) 278–284, https://doi.org/10.1016/j.colsurfa.2007.09.020.
[28] S. Thomaier, W. Kunz, Aggregates in mixtures of ionic liquids, J. Mol. Liq. 130 (2007)
104–107, https://doi.org/10.1016/j.molliq.2006.04.013.
[29] N.V. Sastry, N.M. Vaghela, V.K. Aswal, Effect of alkyl chain length and head group on
surface active and aggregation behavior of ionic liquids in water, Fluid Phase Equilib.
327 (2012) 22–29, https://doi.org/10.1016/j.fluid.2012.04.013.
References
[1] N.V. Plechkova, K.R. Seddon, Applications of ionic liquids in the chemical industry,
Chem. Soc. Rev. 37 (2008) 123–150, https://doi.org/10.1039/b006677j.
[2] I.M. Marrucho, L.C. Branco, L.P.N. Rebelo, Ionic liquids in pharmaceutical applica-
tions, Ann. Rev. Chem. Biomol. Eng. 5 (2014) 527–546, https://doi.org/10.1146/
annurev-chembioeng-060713-040024.
[3] U. König, B. Sessler, Ionic liquids in plating industry: opportunities and challenges,
Trans. Inst. Met. Finish. 86 (2008) 183–188, https://doi.org/10.1179/
174591908X327509.
[4] P.D. Galgano, O.A. El Seoud, Ionic liquid-based surfactants: Synthesis, molecular
structure, micellar properties and applications, in: B.K. Paul, S.P. Moulik (Eds.),
Ionic Liquid-Based Surfactant Science: Formulation, Characterization, and Applica-
tions, 1st edJohn Wiley & Sons Inc 2015, pp. 63–99, https://doi.org/10.1002/
9781118854501.ch4.
[5] N. Adawiyah, M. Moniruzzaman, S. Hawatulaila, M. Goto, Ionic liquids as a potential
tool for drug delivery systems, MedChemComm. 7 (2016) 1881–1897, https://doi.
org/10.1039/c6md00358c.
[30] S. Chabba, R. Vashishat, T.S. Kang, R.K. Mahajan, Self – aggregation behavior of
Dialkyl Imidazolium based ionic liquids in aqueous medium : effect of alkyl chain
length, ChemistrySelect.
201600301.
1 (2016) 2458–2470, https://doi.org/10.1002/slct.
[31] R. Trouve, G. Nahas, Nitrendipine: an antidote to cardiac and lethal toxicity of co-
caine, Proc. Soc. Exp. Biol. Med. 183 (1986) 392–397, https://doi.org/10.3181/
00379727-183-3-RC1.
[32] A.J.M. Valente, O. Söderman, The formation of host-guest complexes between sur-
factants and cyclodextrins, Adv. Colloid Interf. Sci. 205 (2014) 156–176, https://
doi.org/10.1016/j.cis.2013.08.001.
[33] W.L.F. Armarego, Purification of Laboratory Chemicals, 8th ed Elsevier, 2008.
[34] G.F. Longman, The analysis of detergents, Talanta. 22 (1975) 621–636.
[35] H.-L. Su, M.-S. Lan, Y.-Z. Hsieh, Using the cationic surfactants N-cetyl-N-
methylpyrrolidinium bromide and 1-cetyl-3-methylimidazolium bromide for
sweeping–micellar electrokinetic chromatography, 2017https://doi.org/10.1016/j.
chroma.2009.05.001.
[6] A.A. Shamsuri, Ionic liquids: preparations and limitations, MAKARA Sci. Series. 14
(2011) 101–106, https://doi.org/10.7454/mss.v14i2.677.
[7] M. Moniruzzaman, K. Nakashima, N. Kamiya, M. Goto, Recent advances of enzymatic
reactions in ionic liquids, Biochem. Eng. J. 48 (2010) 295–314, https://doi.org/10.
1016/j.bej.2009.10.002.
[8] M. Kaur, H. Kaur, M. Singh, G. Singh, T.S. Kang, Biamphiphilic ionic liquid based
aqueous microemulsions as an efficient catalytic medium for cytochrome c, Phys.
Chem. Chem. Phys. 19 (2021)https://doi.org/10.1039/d0cp04513f.
[9] M. Kaur, G. Singh, A. Kaur, P.K. Sharma, T.S. Kang, Thermally stable ionic liquid-based
microemulsions for high-temperature stabilization of lysozyme at nanointerfaces,
Langmuir. 35 (2019) 4085–4093, https://doi.org/10.1021/acs.langmuir.9b00106.
[10] X.J. Yang, P. Zhang, W. Lv, T. Zhou, P. Li, M. Zhao, Aggregation behavior of
Imidazolium-based amino acid ionic liquid surfactants in aqueous solution: the ef-
fect of amino acid Counterions, J. Surfactant Deterg. 22 (2019) 515–523, https://
doi.org/10.1002/jsde.12270.
[11] B. Dong, N. Li, L. Zheng, L. Yu, T. Inoue, Surface adsorption and micelle formation of
surface active ionic liquids in aqueous solution, Langmuir. 23 (2007) 4178–4182,
https://doi.org/10.1021/la0633029.
[12] N.I. Malek, Z.S. Vaid, U.U. More, O.A. El Seoud, Ionic-liquid-based surfactants with
unsaturated head group: synthesis and micellar properties of 1-(n-alkyl)-3-
vinylimidazolium bromides, Colloid Polym. Sci. 293 (2015) 3213–3224, https://
doi.org/10.1007/s00396-015-3746-x.
[36] P.D. Galgano, O.A. El Seoud, Micellar properties of surface active ionic liquids: a com-
parison of 1-hexadecyl-3-methylimidazolium chloride with structurally related cat-
ionic surfactants, J. Colloid Interface Sci. 345 (2010) 1–11, https://doi.org/10.1016/j.
jcis.2010.01.078.
[37] Y. Kusumoto, Y. Takeshita, J. Kurawaki, I. Satake, Preferential solvation studied by
the fluorescence spectrum of pyrene in water-alcohol binary mixtures, Chem. Lett.
(1997) 349–350.
[38] E.B. Tada, L.P. Novaki, O.A. El Seoud, Solvatochromism in cationic micellar solutions:
effects of the molecular structures of the solvatochromic probe and the surfactant
headgroup, Langmuir. 17 (2001) 652–658, https://doi.org/10.1021/la001135l.
[39] L. Ye, F. Chen, J. Liu, A. Gao, G. Kircher, W. Liu, M. Kappl, S. Wegner, H.J. Butt, W.
Steffen, Responsive Ionogel surface with renewable antibiofouling properties,
Macromol. Rapid Commun. 40 (2019) 1–6, https://doi.org/10.1002/marc.
201900395.
[40] J. Shen, W. He, T. Wang, Multifunctional amphiphilic ionic liquid pathway to create
water-based magnetic fluids and magnetically-driven mesoporous silica, RSC Adv. 9
(2019) 3504–3513, https://doi.org/10.1039/c8ra10065a.
[13] N. Azum, A.Z. Naqvi, M. Akram, Kmaabir-ud-Din, Studies of mixed micelle formation
between cationic gemini and cationic conventional surfactants, J. Colloid Interface
Sci. 328 (2008) 429–435, https://doi.org/10.1016/j.jcis.2008.09.034.
[41] H. Matsuki, M. Yamanaka, Y. Yamashita, S. Kaneshina, Adsorption-equilibrium sur-
face tension of surfactant solutions; examination by the drop volume method,
Bull. Chem. Soc. Jpn. 79 (2006) 1704–1710, https://doi.org/10.1246/bcsj.79.1704.
8

N. Keppeler, P.D. Galgano, S. da Silva Santos et al.
Journal of Molecular Liquids 328 (2021) 115478
[42] E.E. Dreger, G. Klein, G. Miles, L. Shedlovsky, J. Ross, Sodium alcohol sulfates.proper-
ties involving surface activity, Ind. Eng. Chem. 36 (1944) 610–617, https://doi.org/
10.1021/ie50415a004.
[43] D.J. Lee, Enthalpy-entropy compensation in ionic micelle formation, Colloid Polym.
Sci. 273 (1995) 539–543, https://doi.org/10.1007/BF00658683.
[44] K. Lunkenheimer, R. Miller, A criterion for judging the purity of adsorbed surfactant
layers, J. Colloid Interface Sci. 120 (1987) 176–183, https://doi.org/10.1016/0021-
9797(87)90337-7.
[56] R. Zana, Ionization of cationic micelles: effect of the detergent structure, J. Colloid In-
terface Sci. 78 (1980) 330–337, https://doi.org/10.1016/0021-9797(80)90571-8.
[57] P.D. Galgano, Líquidos Iônicos Tensoativos: Correlação entre Estrutura Molecular e
Propriedades Micelares de Cloretos de 1,3-dialquilimidazólio, 2012.
[58] M. Akram, F. Ansari, Kabir-ud-Din, Biophysical investigation of the interaction be-
tween cationic biodegradable Cm-E2O-Cm gemini surfactants and porcine serum
albumin (PSA), Spectrochim. Acta - Part A. 206 (2019) 520–528, https://doi.org/
10.1016/j.saa.2018.08.043.
[45] K. Lunkenheimer, H.J. Pergande, H. Krüger, Apparatus for programmed high-
performance purification of surfactant solutions, Rev. Sci. Instrum. 58 (1987)
2313–2316, https://doi.org/10.1063/1.1139343.
[46] M.J. Rosen, L. Liu, Surface activity and premicellar aggregation of some novel
diquaternary gemini surfactants, JAOCS, J. Am. Oil Chem. Soc. 73 (1996) 885–890,
https://doi.org/10.1007/BF02517990.
[47] R. Hadgiivanova, H. Diamant, Premicellar aggregation of amphiphilic molecules, J.
Phys. Chem. B 111 (2007) 8854–8859, https://doi.org/10.1021/jp070038z.
[48] P. Lianos, R. Zana, Micelles of tetradecyltrialkylammonium bromides with fluores-
cent probes, J. Colloid Interface Sci. 88 (1982) 594–598, https://doi.org/10.1016/
0021-9797(82)90289-2.
[49] P.C. Schulz, M.E. Hernandez-Vargas, J.E. Puig, Do micelles contribute to the total con-
ductivity of ionic micellar systems? Lat. Am. Appl. Res. 25 (1995) 153–159.
[50] P.D. Galgano, O.A. El Seoud, Surface active ionic liquids: study of the micellar prop-
erties of 1-(1-alkyl)-3-methylimidazolium chlorides and comparison with structur-
ally related surfactants, J. Colloid Interface Sci. 361 (2011) 186–194, https://doi.org/
10.1016/j.jcis.2011.04.108.
[59] X. Yang, G. Lu, B. She, X. Liang, R. Yin, C. Guo, X. Yi, Z. Dang, Cosolubilization of 4,4′-
dibromodiphenyl ether, naphthalene and pyrene mixtures in various surfactant mi-
celles, Chem. Eng. J. 260 (2015) 74–82, https://doi.org/10.1016/j.cej.2014.08.101.
[60] O. Singh, P. Singla, V.K. Aswal, R.K. Mahajan, Impact of aromatic counter-ions charge
delocalization on the Micellization behavior of surface-active ionic liquids, Lang-
muir. 35 (2019) 14586–14595, https://doi.org/10.1021/acs.langmuir.9b02695.
[61] G. Singh, G. Singh, T.S. Kang, Micellization behavior of surface active ionic liquids
having aromatic counterions in aqueous media, J. Phys. Chem. B 120 (2016)
1092–1105, https://doi.org/10.1021/acs.jpcb.5b09688.
[62] S. Chabba, S. Kumar, V.K. Aswal, T.S. Kang, R.K. Mahajan, Interfacial and aggregation
behavior of aqueous mixtures of imidazolium based surface active ionic liquids and
anionic surfactant sodium dodecylbenzenesulfonate, Colloids Surf. A Physicochem.
Eng. Asp. 472 (2015) 9–20, https://doi.org/10.1016/j.colsurfa.2015.02.032.
[63] V. Soldi, J. Keiper, L.S. Romsted, I.M. Cuccovia, H. Chaimovich, Arenediazonium salts:
New probes of the interfacial compositions of association colloids. 6. Relationships
between interfacial counterion and water concentrations and surfactant headgroup
size, sphere-to-rod transitions, and chemical reactivity in cationi, Langmuir. 16
(2000) 59–71, https://doi.org/10.1021/la990336q.
[51] P. Mukerjee, The nature of the association equilibria and hydrophobic bonding in
aqueous solutions of association colloids, Adv. Colloid Interf. Sci. 1 (1967)
242–275, https://doi.org/10.1016/0001-8686(67)80005-8.
[64] S. Kumar, H.N. Singh, Competitive solubilization of Sudan IV and anthracene in mi-
cellar systems, Colloids Surf. A Physicochem. Eng. Asp. 69 (1992) 1–4, https://doi.
org/10.1016/0166-6622(92)80231-P.
[52] M.J. Rosen, Surface and Interfacial Phenomena, 2nd ed. Wiley-VCH Verlag GmbH &
Co, KGaA, New York, 1989.
[65] S. Onoue, N. Igarashi, Y. Yamauchi, N. Murase, Y. Zhou, T. Kojima, S. Yamada, Y.
Tsuda, In vitro phototoxicity of dihydropyridine derivatives: a photochemical and
photobiological study, Eur. J. Pharm. Sci. 33 (2008) 262–270, https://doi.org/10.
1016/j.ejps.2007.12.004.
[66] A.R. Tehrani-Bagha, K. Holmberg, Solubilization of hydrophobic dyes in surfactant
solutions, Materials. 6 (2013) 580–608, https://doi.org/10.3390/ma6020580.
[67] Z.S. Vaid, A. Kumar, O.A. El Seoud, N.I. Malek, Drug induced micelle-to-vesicle tran-
sition in aqueous solutions of cationic surfactants, RSC Adv. 7 (2017) 3861–3869,
https://doi.org/10.1039/c6ra25577a.
[53] H.B. Klevens, Structure and aggregation in dilate solution of surface active agents, J.
Am. Oil Chem. Soc. 30 (1953) 74–80, https://doi.org/10.1007/BF02635002.
[54] M.A. Rodríguez, M. Muñoz, M. del Graciani, M.S. Fernández Pachón, M.L. Moyá, Ef-
fects of head group size on micellization of cetyltrialkylammonium bromide surfac-
tants in water-ethylene glycol mixtures, Colloids Surf. A Physicochem. Eng. Asp. 298
(2007) 177–185, https://doi.org/10.1016/j.colsurfa.2006.10.062.
[55] S. Manet, Y. Karpichev, D. Bassani, R. Kiagus-Ahmad, R. Oda, Counteranion effect on
micellization of cationic gemini surfactants 14-2-14: Hofmeister and other counter-
ions, Langmuir. 26 (2010) 10645–10656, https://doi.org/10.1021/la1008768.
9