Mixed micellization behavior of Gemini (Cationic Ester-Bonded) surfactants with conventional (Cationic, Anionic and Nonionic) surfactants in aqueous medium

Article abstract of DOI:10.1524/zpch.2013.0288

Two cationic ester-bonded cleavable gemini surfactants of different hydrophobic chain length ethane-1,2-diyl bis(N,N-dimethyl-N- alkylammoniumacetoxy)dichloride, C_nH_2n+1 (CH ₃)₂N+ (CH₂COOCH₂) ₂N+(CH₃)₂C_nH_2n+1. 2Cl- (n-E2-n, n = 12, 16), having ester linkage in the spacer, were synthesized adopting the reported procedure. Physicochemical properties of the single and binary gemini-conventional mixed micelles of different mole fractions were studied by conductivity measurements at 30°C. The conventional surfactants used were: DTAC (dodecyltrimethylammonium chloride), CTAC (hexadecyltrimethylammonium chloride), CPC (cetylpyridinium chloride), SDS (sodium dodecyl sulfate), SDBS (sodium dodecylbenzene sulfonate), TX-100 (t-octylphenoxypolyethoxyethanol) and Brij 58 (polyoxyethylene (20) cetyl ether). Whereas the critical micelle concentration (cmc) values for the dicationic geminis (12-E2-12 and 16-E2-16) were found to be very low as compared to the respective monomeric surfactant with the same number of carbon atoms in the hydrophobic chain per hydrophilic head group, those for all the binary systems were found to be less than the ideal cmc values studied at different mole fractions of the geminis. This synergistic interaction between the surfactants has been analyzed in the light of various theoretical models such as Clint, Rubingh, Motomura and Maeda. by Oldenbourg Wissenschaftsverlag, Mu?nchen.

Full text of DOI:10.1524/zpch.2013.0288

Z. Phys. Chem. 227 (2013) 133–149 / DOI 10.1524/zpch.2013.0288
by Oldenbourg Wissenschaftsverlag, München
©
Mixed Micellization Behavior of Gemini (Cationic
Ester-Bonded) Surfactants with Conventional
(Cationic, Anionic and Nonionic) Surfactants
in Aqueous Medium
∗
∗
By Nazish Fatma, Wajid Husain Ansari , Manorama Panda , and Kabir-ud-Din
Department of Chemistry, Aligarh Muslim University, Aligarh-202002, India
(
(
Received June 2, 2012; accepted in revised form August 12, 2012)
Published online October 29, 2012)
Cleavable Gemini Surfactants / Cmc / Mixed Micelles / Synergistic Interaction
Two cationic ester-bonded cleavable gemini surfactants of different hydrophobic chain
+
length ethane-1,2-diyl bis(N,N-dimethyl-N-alkylammoniumacetoxy)dichloride, C
n
3 2
H2n+1(CH ) N
+
−
(
CH
2
COOCH
2
)
2
N (CH
3
)
2
C
n
H
2n+1. 2Cl (n-E2-n, n = 12, 16), having ester linkage in the spacer,
were synthesized adopting the reported procedure. Physicochemical properties of the single and
binary gemini-conventional mixed micelles of different mole fractions were studied by conductivity
◦
measurements at 30 C. The conventional surfactants used were: DTAC (dodecyltrimethy-
lammonium chloride), CTAC (hexadecyltrimethylammonium chloride), CPC (cetylpyridinium
chloride), SDS (sodium dodecyl sulfate), SDBS (sodium dodecylbenzene sulfonate), TX-100
(
t-octylphenoxypolyethoxyethanol) and Brij 58 (polyoxyethylene (20) cetyl ether). Whereas the
critical micelle concentration (cmc) values for the dicationic geminis (12-E2-12 and 16-E2-16)
were found to be very low as compared to the respective monomeric surfactant with the same
number of carbon atoms in the hydrophobic chain per hydrophilic head group, those for all the
binary systems were found to be less than the ideal cmc values studied at different mole fractions
of the geminis. This synergistic interaction between the surfactants has been analyzed in the light
of various theoretical models such as Clint, Rubingh, Motomura and Maeda.
1. Introduction
Surfactants comprising the hydrophilic head group and hydrophobic hydrocarbon
chain [1,2] show self-aggregation with a high degree of cooperativity with the aque-
ous medium result in micelle formation which occurs at a threshold concentration (i.e.,
critical micelle concentration, cmc) and depends upon several factors, such as the rela-
tive size of the hydrophilic/hydrophobic parts, the presence of charges and the degree
of hydration [3]. Being one of the most important representative classes of industrial
products, surfactants have manifold applications in every sector of industry such as
*
Corresponding authors. E-mail: wajidhusain.chem@gmail.com; manoramapanda@yahoo.co.in
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N. Fatma et al.
fabric softener [4], emulsifiers, wetting agents, pesticides, agricultural sprays, domes-
tic products, bio-catalysis or bio-processing [5], in pharmaceuticals or drug delivery
systems [6,7] and others. Gemini surfactants are made up of two monomeric units con-
nected at the level of head group by a spacer that can be hydrophilic or hydrophobic [1].
It is well known that the traditional amphiphiles (generally cationic) show good stability
towards chemicals and microbes that gives chance to accumulate in the soil, water or in
environment for a long period of time with slow degradation. Therefore, readily cleav-
able gemini surfactants are most suitable for use instead of the conventional surfactants.
Gemini surfactants with cleavable bond in spacer (like amide or ester) possess superior
properties such as lower cmc, much greater efficiency in reducing the surface tension of
water, reduced interfacial tension, special rheological properties (viscoelasticity, shear-
thickening) at relatively low concentration than the conventional ones [8–10]. Ester
bonded gemini surfactants are found to have good biodegradability as the polar bond
contributes to the higher water solubility making them easily degradable [11–16].
Surfactant mixtures, in which one of the components is a biodegradable gemini, are
of considerable interest for various applications as they perform better than the solu-
tions containing their monomeric surfactant counterparts. In mixed micelles not only
the properties of the individual components are combined, but also synergism is ob-
served in properties like cmc, surface tension, etc. Although several researchers have
chosen the mixed micelles for different kind of studies [17–23], reports regarding the
nature of the gemini-conventional mixed micellar solutions are less. Because of the lack
of work on the mixed micellization study of biodegradable gemini with ester group
functionality in the spacer part as one of the components, we have studied the physico-
chemical properties of the pure and mixed micellar solutions of different cationic,
anionic and non-ionic surfactants with two cationic biodegradable geminis (n-E2-n,
n = 12, 16) of different hydrophobicity. The main inducement for the synthesis of such
gemini surfactants is their eco-friendly nature. In this paper, we report the results of
◦
investigations carried out by conductivity measurements in aqueous solutions at 30 C.
2. Materials and methods
2.1 Materials
The cationic surfactants DTAC (≤ 98%, Aldrich), CTAC (99%, Acros) and CPC
(Merck), anionic surfactants SDS (99%, Sigma) and SDBS (TCI), nonionic surfactants
Brij 58 (Merck) and TX-100 (Fluka) were used as received. N,N-dimethyldodecylami-
ne (Acros Organic), N,N-dimethylhexadecylamine (≥ 95%, Aldrich), ethylene glycol
(99%, Sigma Aldrich) and chloroacetyl chloride (98%, Loba chemie) were also used as
received.
The melting points were visually determined by the microscopic melting point
apparatus Reichert Thermovar Jung, Austria. The preliminary characterization of the
synthesised gemini surfactants was carried out by IR spectroscopy using Interface 2020
1
FT-IR Spectrophotometer, U.K. H-NMR spectra were recorded on 300 MHz Bruker
1
Avance NMR Spectrometer, CDRI, Lucknow in CDCl with H chemical shifts relative
3
to internal standard TMS. Electrospray ionization mass spectra (ESI-MS, positive) were
obtained on an ESI, LC/MS.
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Cleavable/Ester-Bonded Gemini-Conventional Surfactant Mixtures
135
Scheme 1. Synthesis of cationic gemini surfactant ethane-1,2-diyl bis(N,N-dimethyl-N-alkylammonium-
acetoxy) dichloride (n = 12, 16).
Chemical structures of the monomeric and dimeric surfactants used in this study are
shown in Scheme S1 (Supporting material).
2.2 Synthesis of the biodegradable gemini surfactants
The cationic ester-bonded gemini surfactants (n-E2-n) were synthesized in two steps
(Scheme 1) following a reported procedure [12].
(
i) In the first step ethane-1,2-diyl bis(chloroacetate) was prepared by heating a mix-
◦
ture of chloroacetyl chloride (0.22 mol) and ethylene glycol (0.1 mol) at 50 C for
8
h in nitrogen atmosphere. The reaction mixture was washed with saturated brine
for complete neutralization and the product was dissolved in ether, dried over mag-
nesium sulfate, and the solvent was then distilled off under reduced pressure. Low
melting colourless needle-shaped crystals of the compound were obtained in good yield
(15.15 g, 65.36%).
(ii) In the second step, the target compound was obtained by heating ethane-1,2-
diyl bis(chloroacetate) with N,N-dimethylalkylamine (dodecyl and hexadecyl, molar
ratio, 1 : 2.1) in ethyl acetate for 10 h. When the solvent was removed under reduced
pressure, white crystalline solid of the cationic gemini surfactant was obtained. It was
further purified by repeated crystallization in ethyl acetate-ethanol mixture (5 : 1, v/v).
The purity of the compound was confirmed by thin layer chromatography (TLC) (silica
gel, CHCl -MeOH, 6 : 4, v/v). The yield of the characterized compounds n-E2-n were
3
3
0.40 g (78.6%) for n = 12 and 36.81 g (78.7%) for n = 16. The structures were con-
1
1
firmed by FT-IR, H-NMR spectra and mass spectrometry. The H-NMR and MS-ESI
+) spectra of n-E2-n are shown in Figs. S1 and S2 (Supporting material).
(
2.3 Characterization
Whereas the characterization of 16-E2-16 has been discussed elsewhere [13], details of
1
2-E2-12 are given below:
◦
m. p.: 191–194 C, R = 0.58
f
−1
FT-IR (KBr, ν cm ): 2922.75, 2855.42 (C-H); 1746.48 (C=O); 1465 (C-O);
1188.06 (C-N); 724.68.
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N. Fatma et al.
1
H-NMR (300 MHz, CDCl , δ scale): 0.86–0.90 (t, 6H, −2 × CH , alkyl chain);
3
3
+
1
(
(
.25–1.34 (m, 36H, −2 × (CH ) , alkyl chain); 1.77 (m, 4H, −2 × N CH CH ); 3.53
2
9
2
2
+
+
s, 12H, −2 × N (CH ) ); 3.79 (s, 4H, −2 × CH O); 4.49 (s, 4H, −2 × N CH ); 5.36
3
2
2
2
+
s, 4H, −2 × N CH COO).
2
−
−
−
MS-ESI (+) m/z: 605.6 (M-Cl ), 555.6 (M-Cl −CH Cl ), 298 (C H CH3)2
3
12 25
+
+
N CH OOCH=CH ), 130 (CH ) N CH OOCH=CH ).
2
2
3
2
2
2
2.4 Cmc determination by conductometric measurements
The mixed micellization study of n-E2-n was done with six different monomeric sur-
factants at different mole fractions. The solutions of gemini (cationic) and conventional
(cationic, anionic, nonionic) surfactants were prepared in terms of the mole fractions
of gemini (α ) equaling 0.2, 0.4, 0.6, 0.8 in the mixed systems, with the total con-
1
centrations of all the mixed solutions remaining same at each mole fraction. For each
surfactant, the cmc of the single and binary mixtures with the above mole fractions
were determined. Each experiment was repeated to achieve good reproducibility. All
the solutions were prepared in double distilled water.
The conductance [24] as a function of surfactant concentration was measured at
◦
3
1
0 C by using ELICO conductivity bridge Model CM82T and dip cell (cell constant
−1
.02 cm ). The temperature was maintained by a circulating water bath. The conduc-
tivity of the single/binary surfactant solutions were found to increase with the aliquot
additions of a known concentration of the surfactant. The cmc values were obtained at
the break points by plotting the specific conductivity vs. concentration of surfactant.
To compare the cmc of various compounds, all the conductivity measurements of the
pure/mixed surfactant systems were performed at the same temperature. The specific
conductivity vs. [surfactant] plots for the pure gemini surfactants are shown in Fig. 1
and for the other systems the plots are shown in Figs. S3, S4A and S4B as “Supporting
Material Section”. The cmcs of the pure monomeric surfactants agree with the literature
values [25–31].
2.5 Cleavable properties
The presence of ester linkages in the spacer part of the cationic gemini surfactants sug-
gests that these might be cleavable through chemical means in alkaline condition at
pH 7.4 and pH 12. Using phosphate-buffered saline and sodium hydroxide/potassium
hydrogen phosphate (Ringer Buffer). 0.05 g gemini surfactant was taken in 10 ml buffer
solution [10]. The FT-IR spectra of 12-E2-12 and 16-E2-16 at pH 12 are shown as
Fig. S5A and S5B, respectively.
From the FT-IR spectra it is quite evident that the gemini surfactants get
cleaved by the buffer in 8 h. The absorption bands for the ester groups (C=O) at
−
1
−1
1
746.48/1746.36 cm (12-E2-12/16-E2-16) are shifted to 1640.59/1641 cm and
−1
new absorption band for the –OH group appears at 3503.49 cm . Thus, the formation
of easily degradable compounds such as fatty acid salts and respective diol or com-
pounds with hydroxyl group takes place. This indicates the hydrolysis or cleavability
of the gemini surfactants.
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Cleavable/Ester-Bonded Gemini-Conventional Surfactant Mixtures
137
Fig. 1. Specific conductivity as a function of surfactant concentration for the pure cationic geminis (12-E2-
◦
1
2 and 16-E2-16) at 30 C.
3. Results and discussion
The cmc values of the gemini-conventional mixed surfactant systems of different com-
positions were determined by the conductivity measurements. Although it is not pos-
sible to perform the conductivity experiments for the pure non-ionic surfactants, for the
gemini-conventional surfactant mixtures the conductivity could be measured. The cmc
values of the pure surfactants decrease in the order DTAC > SDS > SDBS > CTAC >
CPC > 12-E2-12 > 16-E2-16. Compared with the corresponding conventional mono-
meric surfactants, the dimeric surfactants have much lower values of cmc. The remark-
ably low cmc values of n-E2-n are due to greater hydrophobicity of the geminis – the
two hydrocarbon chains break more “structured” water (vis-a-vis single head/single
chain conventional surfactant) which is an energetically favored process. Therefore,
the cmc values are always lower for gemini surfactants than their analogous conven-
tional counterparts [7]. The position of functional group also affects the micellization
process. Two ester bonds in the spacer make it more hydrophilic that prompts micelle
formation at low concentration [13,32]. The cmc values, along with other relevant pa-
rameters, such as, ideal cmc (cmcideal), counterion binding (b), standard Gibbs excess
m
free energy (ΔG ), Gibbs free energy of micellization (ΔG ), mole fraction of surfac-
ex
mic
ideal
m
m
m
tant in ideal state (X1 ), interaction parameter (β ) and activity coefficients ( f , f ),
1
2
are recorded in Tables 1–3 for gemini and conventional surfactants in pure and mixed
states.
3.1 Counterion binding
The conductivity data of the pure and mixed systems were used for the determination
of the fraction of counter ions (b) bound to the micelles. The counterions which are
the main contributing parts for the conductivity of the solutions due to their ionic mo-
bility, bind just adjacent to the surface of the micelles, i.e., to the Stern layer. Below
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N. Fatma et al.
cmc the concentration of the counterions increases with the surfactant concentration and
thus the conductivity increases whereas above cmc the dissociation decreases thereby
reducing the concentration of counterions and thus the conductivity of the solution. The
degree of counterion dissociation was determined by taking the ratio of post-micellar
slope (S ) and pre-micellar slope (S ). The ratio of S to S is considered as the frac-
2
1
2
1
tion of counter ions dissociated from the micelles (g), so that the fraction of counterions
bound (b) to the micelle is calculated by subtracting counterion dissociation from unity
b = (1− S /S ) [33,34]. For 16-E2-16 + CTAC/CPC and 12-E2-12 + CPC systems, b
2
1
increases with increasing α (except at 0.8). However, for 12-E2-12 + DTAC, no trend
1
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Cleavable/Ester-Bonded Gemini-Conventional Surfactant Mixtures
139
in b value was observed. For 16-E2-16 + SDS system, the b value increases with the in-
crease of the mole fraction of gemini whereas no trend in b value was found for any of
the gemini + anionic surfactant systems. Higher b values were observed at lower mole
fractions of the gemini in the gemini + nonionic systems (Table 3).
3
.2 Interaction of dimeric surfactants with monomeric surfactants in mixed
micelles
All the theories suggested by Clint, Rubingh, Motomura and Maeda are summarized
(see “Supporting Material Section” for the definition of terms and the equations used
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N. Fatma et al.
m
to evaluate the parameters) to explain the physicochemical parameters such as X
1
m
m
m
(
micellar mole fraction), β (micellar interaction parameter), f1 and f2 (activity
coefficients), ΔG (excess free energy of mixing), ΔG (Gibbs free energy of mi-
m
ex
mic
M
ideal
◦
cellization), X1 and X1 (mole fraction in micellar as well as in ideal state), ΔG
Maeda
(Standard free energy). The following results emerge:
Lower values of cmc (the experimental cmc, Fig. 2, Tables 1–3) than cmc ob-
12
ideal
tained by Clint’s Eq. (S1) [35] indicate the nonideal behavior of the binary systems;
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Cleavable/Ester-Bonded Gemini-Conventional Surfactant Mixtures
141
this is a required condition for the synergism between the constituents in the mixed mi-
celles. Several types of intermolecular forces acting between the surfactant molecules
are responsible for the formation of the mixed micelles.
3.2.1 Regular solution theory
The experimental results can be interpreted quantitatively by considering Rubingh’s
equation [36], which is based on the Regular Solution Theory (RST), and allows
for the calculation of the micellar mole fractions as well as the interaction parame-
ters. The nature and strength of interactions between the surfactant molecules in the
m
mixed micelles have been interpreted in terms of β by Rubingh’s approach. For
m
m
the gemini-conventional mixed micelles X1 and β were calculated with the use of
m
Eqs. (S2) and (S3), and are compiled in Tables 1–3. The β values give an idea about
the type of interaction between the two components in the mixed micelles which
m
leads to the deviation from ideality. As the β values vary with the composition of
m
av
m
the systems, frequently β values are used [37,38]. The higher absolute value of β
implies reduction in free energy of micellization, which makes the system thermo-
dynamically more stable indicating higher synergism or more interactive interaction.
m
When β is positive, there is repulsion between the constituents of the binary sur-
factant system and the interaction is antagonistic in nature. But, in all the systems
m
studied, we have obtained negative β , indicating the presence of synergistic interac-
tion between both the components in the mixed surfactant systems which again favor
m
the micellization process. The order of β values for the mixed systems is gemini-
av
m
anionic > gemini-cationic > gemini-nonionic. β is maximum for 12-E2-12 + SDS
av
(
−13.06) suggesting strong synergism due to electrostatic interaction between the op-
positely charged head groups of anionic SDS and cationic gemini surfactant, whereas
m
for 12-E2-12 + SDBS, the lower value of β implies less surfactant- surfactant in-
teraction due to the molecular structure of SDBS, as it may not be accommodated
properly in the mixed micelles. The mutual interactions among the surfactants in mi-
celles depend on their nature, hydrophilic and hydrophobic parts. 16-E2-16 + CTAC
(
−9.12) shows higher synergism as compared to 12-E2-12 + DTAC (−8.95) due to
the longer hydrocarbon chain in the former system which may facilitate more mi-
cellization. Low hydrophobicity and high cmc also affect the surfactants interaction.
n-E2-n + CPC shows less synergism than straight chain cationic surfactants because of
the bulky pyridinium head group which decreases the surface charge density by form-
ing loose mixed micelle aggregates. Least synergism is observed for 16-E2-16 + Brij
5
8 system (−1.45) (Table 3). Gemini monomers in aqueous solution may exist in
cis conformation to allow the intermolecular interaction between two alkyl chains of
the molecules. 12-E2-12 is more efficient than 16-E2-16 for the formation of mixed
micelles because of least conformational strain in arrangement of alkyl chains ex-
hibiting higher interaction. The presence of ester linkage in one of the components
in mixed micelles may facilitate the association between the surfactant molecules
through dipole-induced dipole and hydrogen bonding interactions with interfacial wa-
m
m
1
ter [39]. In mixed micelles, X2 values are lower than X , showing more propensity
of the gemini surfactants towards micellization (Tables 1–3, Fig. 3). Positive devia-
m
1
ideal
1
tion of X from X
was observed for all the binary systems as shown in Fig. 3. In
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42
N. Fatma et al.
Fig. 2. Variation of cmcideal and cmc12 vs. mole fraction of gemini (α
1
) in surfactant mixtures: 12-E2-12 (ꢀ,
ꢁ); 16-E2-16 (ꢀ, ꢀ). Filled symbols are for cmc12 and open symbols are for cmcideal. Lines are drawn as
a guide to the eye.
m
1
1
α
2-E2-12 + SDS/SDBS and 12-E2-12 + DTAC systems, the X value increases with
, whereas for 16-E2-16 + Brij 58, the contribution of Brij 58 is more in the
1
2-E2-12
mixed micelles.
m
i
The activity coefficient f of individual surfactants, obtained by using the Eqs. (S4)
and (S5), are less than unity indicating the formation of the mixed micelles. The activity
m
1
m
2
coefficients of the gemini ( f ) are reasonably higher than the f (conventional surfac-
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Cleavable/Ester-Bonded Gemini-Conventional Surfactant Mixtures
143
Fig. 2. Continued.
tant) supporting the non-ideality of the mixed micelles and the presence of synergistic
interaction between the surfactants in it.
m
The higher absolute values of ΔG indicates more stable mixed micellar system.
ex
m
The ΔG values listed in Tables 1–3 are negative indicating the higher stability of the
mixed micelles than the single surfactant micelles [23]. The absolute values of ΔG
vary from 0.75 to 5.64 (kJ mol ), being highest for gemini + anionic systems and low-
ex
m
ex
−1
est for gemini + nonionic systems.
For pure gemini surfactants, Eq. (S7) can be used to evaluate ΔG [40].
mic
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44
N. Fatma et al.
Fig. 2. Continued.
The ΔGmic values are negative for all the mole fractions of geminis in the mixed
systems as listed in Tables 1–3, which again supports the mixed micellization. The
higher values of absolute ΔGmic are due to more favorable micellization in case of
12-E2-12 + SDS/SDBS.
3.2.2 Motomura’s model
M
1
ideal
1
The mole fractions in the micellar (X ) as well as in ideal state (X ) have been
computed by applying Motomura’s approximation too. The RST, which treats mixed
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Cleavable/Ester-Bonded Gemini-Conventional Surfactant Mixtures
145
ideal
m
Fig. 3. Variation of X1 and X1 vs. mole fraction of gemini (α
1
1
) in surfactant mixtures: 12-E2-12 (ꢀ, ꢁ);
6-E2-16 (ꢀ, ꢀ). Filled symbols are for Rubingh’s and open symbols are for Motomura’s model. Lines are
drawn as a guide to the eye.
micelles as a regular solution, is used for the evaluation of interaction parameters. It
depends on the micellar compositions and is silent for unlike chain lengths, counte-
rions and ionic strength. The Motomura’s model, which is independent of the nature
of the surfactants and their counterion and considers mixed micelles as a macro-
scopic bulk phase, can be applied for the study. The related energetic parameters of
such systems can be evaluated in terms of excess thermodynamic quantities [41,42].
The fundamental equation for the micellar mole fraction of a surfactant in the sur-
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N. Fatma et al.
c)
Fig. 3. Continued.
factant mixture, presuming the miscibility of the surfactants in the mixed micelles,
was determined by the relationship (shown in supporting information: Eq. (S8) to
Eq. (S13)). The micellar mole fractions of surfactants in ideal state were calculated
using Eq. (S14).
M
The X1 values for mixed systems increase with increase in mole fraction of
n-E2-n (Tables 1–3). In case of gemini + anionic systems at lower mole fraction of
SDS/SDBS, the Motomura’s model becomes non-convergent. Similar results have also
been observed for 16-E2-16 + CTAC/CPC. It is clear from Fig. 3 that even at lower
mole fraction of the gemini surfactant, its contribution in mixed micellization is higher
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Cleavable/Ester-Bonded Gemini-Conventional Surfactant Mixtures
147
Fig. 3. Continued.
than that of single chain surfactants. This is because of the presence of two hydrophobic
chains which enhance its hydrophobicity and try to accommodate in mixed micelles at
the same time.
3.2.3 Maeda model
In addition to the electrostatic interactions between the head groups, steric repulsions
due to different hydrophobic chain lengths of the surfactants can also be taken into ac-
count to justify our explanations. According to Maeda [43] and Aguiar and Ruiz [44],
the hydrophobic chain–chain and hydrophilic head group-head group interactions of
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48
N. Fatma et al.
the surfactant components are found in the mixed micelles. Maeda has proposed hy-
drocarbon chain–chain interaction in mixed systems of two amphiphiles including one
ionic and one nonionic surfactant. This model assumes that in the micellar phase the
decrease in head group repulsion for the ionic surfactants in an ionic + nonionic mixed
surfactant system is due to the presence of nonionic surfactant molecules. It is a new
approach for the determination of the standard free energy change during micellization
◦
m
(ΔG
) as a function of ionic mole fraction (X ) in the mixed micelles (Eqs. (S15)
Maeda
1
◦
to (S17)). A higher value of B (a parameter related to ΔG
) indicates more chain-
1
Maeda
chain interaction and higher stability of the mixed micelles. The values of B for
1
1
2-E2-12 + Brij58, −2.58, −2.23, −2.66, −3.56; 12-E2-12 + TX-100, −10.30, −9.69,
−
9.34, −9.98; 16-E2-16 + Brij 58, −3.84, −2.44, −2.89, −4.97 and 16-E2-16 + TX-
◦
1
00, −8.54, −9.30, −10.72, −15.4. The ΔG
values, calculated from Eq. S(15), are
Maeda
−1
recorded in Table 3. The values are negative and vary between 15.63–17.78 kJ mol ,
the highest value was obtained for the 16-E2-16 + nonionic systems.
4. Conclusion
Two dicationic biodegradable gemini surfactants of different hydrophobicity consist-
ing ester bonded spacer have been synthesized and the physicochemical properties
of their binary mixtures with cationic, anionic and nonionic conventional surfactants
have been studied by conductometric method. Various micellar parameters were de-
termined by using theoretical models suggested by Clint, Rubingh, Motomura and
Maeda. All the mixed surfactant solutions show nonideality in the mixed micelles indi-
m
ideal
1
cated by their cmc values depending on the micellar compositions and their X , X
,
1
m
◦
ΔG , ΔGmic, ΔG
values. The gemini + SDS/SDBS mixed system has the highest
ex
Maeda
m
synergism supported by β values. 16-E2-16 + CTAC shows higher synergism than 12-
E2-12 + DTAC which may be due to the difference in their hydrocarbon chain lengths
that favors micellization. The cmc values of the geminis are significantly less than their
corresponding monomeric counterparts and are the key factors which favor the lower
consumption of cationic gemini surfactants.
Acknowledgement
Manorama Panda acknowledges financial assistance under the Department of Sci-
ence and Technology, Government of India (Project No. SR/WOS-A/CS-46/2007) and
Nazish Fatma is thankful to UGC for fellowship and to SAIF, CDRI, Lucknow, for pro-
1
viding H-NMR facility. Kabir-ud-Din thanks the UGC for awarding BSR – Faculty
Fellowship.
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