Synthesis and surface active properties of gemini cationic surfactants and interaction with anionic azo dye (AR52)

Article abstract of DOI:10.1007/s11743-011-1260-9

A homologous series of new gemini cationic surfactants were synthesized and characterized using micro elemental analysis, FTIR, 1H-NMR and mass spectra. The surface activities of these amphiphiles were determined based on the data of surface tension. Critical micelle concentration, effectiveness of the surface tension reduction, efficiency of adsorption, maximum surface excess, minimum surface area and critical packing parameter were evaluated. The effect of cationic micelles on solubilization of anionic azo dye, sulforhodamine B (Acid Red 52) in aqueous micellar solution of the synthesized gemini cationic surfactants was studied at pH 6.9 ± 0.5 and 25°C. The results showed that the solubility of dye rose with increasing surfactant concentration as a consequence of some association between the dye and the micelles. It was also observed that the aggregation of surfactant and dye takes place at a surfactant concentration below the CMC of the individual surfactant. The partition coefficients between the bulk water and surfactant micelles as well as the Gibbs energies of distribution of dye between the bulk water and surfactant micelles were calculated using a pseudo-phase model. The effect of the hydrophobic chain length of Gemini cationic surfactants on the distribution parameters was also reported. The results show favorable solubilization of dye in cationic micelles. AOCS 2011.

Full text of DOI:10.1007/s11743-011-1260-9

J Surfact Deterg (2011) 14:371–379
DOI 10.1007/s11743-011-1260-9
ORIGINAL ARTICLE
Synthesis and Surface Active Properties of Gemini Cationic
Surfactants and Interaction with Anionic Azo Dye (AR52)
Fatma H. Abdel-Salam Asrar G. El-Said
•
Received: 12 July 2010 / Accepted: 20 February 2011 / Published online: 11 March 2011
Ó AOCS 2011
Abstract A homologous series of new gemini cationic
surfactants were synthesized and characterized using micro
1
elemental analysis, FTIR, H-NMR and mass spectra. The
Introduction
Gemini surfactants are a novel type of surfactant synthe-
sized in recent years. Its advent has greatly broadened the
perspective of interfacial science [1–3]. Compared with the
conventional surfactants, gemini surfactants are more
efficient in lowering surface tension and have a much lower
critical micelle concentration (CMC). Despite their recent
development, gemini surfactants are already a hot subject
in colloids and surface science and are considered to be a
type of surfactant that will be most widely used in the
twenty-first century [4–6].
surface activities of these amphiphiles were determined
based on the data of surface tension. Critical micelle con-
centration, effectiveness of the surface tension reduction,
efficiency of adsorption, maximum surface excess, mini-
mum surface area and critical packing parameter were
evaluated. The effect of cationic micelles on solubilization
of anionic azo dye, sulforhodamine B (Acid Red 52) in
aqueous micellar solution of the synthesized gemini cat-
ionic surfactants was studied at pH 6.9 ± 0.5 and 25 °C.
The results showed that the solubility of dye rose with
increasing surfactant concentration as a consequence of
some association between the dye and the micelles. It was
also observed that the aggregation of surfactant and dye
takes place at a surfactant concentration below the CMC of
the individual surfactant. The partition coefficients between
the bulk water and surfactant micelles as well as the Gibbs
energies of distribution of dye between the bulk water and
surfactant micelles were calculated using a pseudo-phase
model. The effect of the hydrophobic chain length of
Gemini cationic surfactants on the distribution parameters
was also reported. The results show favorable solubiliza-
tion of dye in cationic micelles.
Gemini cationic surfactants have been extensively
studied due to their easy preparation. This has also made
them particularly interesting from an industrial point of
view, and much of the published information concerning
the solution properties of gemini surfactants is based on
cationic species [7–9].
Surfactant–dye associations are subjects of numerous
investigations [10–13]. Surfactants are used as auxiliaries
in most textile finishing processes such as washing, wet-
ting, dispersing, dyeing and finishing. Also, the interaction
between dyes and surfactants have been studied to provide
some insight into small molecule–macromolecule interac-
tions, which is of major importance in biochemistry [14].
Many techniques have been used for qualitative and
quantitative description of dye–surfactant interactions, i.e.,
potentiometry [15, 16], ordinary conductimetry [17, 18], or
with ion selective electrodes [19, 20], but the most widely
used are spectroscopical methods [21–26].
Keywords Gemini cationic surfactant ꢀ Synthesis ꢀ
Structure analysis ꢀ Surface activity ꢀ Acid Red 52 ꢀ
Partition coefficient ꢀ Solubilization
Surfactants are able to affect the electronic absorption
spectra of many dyes. Consequently, in the presence of a
surfactant, new bands in the electronic absorption spectra
of many dyes can appear, and the stronger the mutual
interaction between dye and surfactant is, the greater the
F. H. Abdel-Salam (&) ꢀ A. G. El-Said
Department of Chemistry, Faculty of Science (Girls Branch),
Al-Azhar University, P.O.Box, 11754, Nasr City, Cairo, Egypt
e-mail: prof_drfatma@yahoo.com
1
23

3
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J Surfact Deterg (2011) 14:371–379
Scheme 1 Synthetic route for
,2-bis-diquaternary ammonium
gemini cationic surfactants.
Where R: a = -(CH CH
b = -(CH , c = -
, d = -
P.T.S.
i) ClCH COOH + ROH
ClCH COOR
1
2
2
benzene
I
a-d
)
2 9
3
,
2 3
)11CH
CH3
CH3
CH3
CH COOR
. 2Cl
CH3
(CH
CH
2
)
)
15CH
17CH
3
3
+
+
ClCH COOR
+
N
CH CH2
N
ROOCH C
2
ii)
N.CH .CH .N
2
2
2
(
2
2
2
CH3
CH3
CH3
I
CH3
a-d
IIa-d
observed change [25]. In systems containing an ionic dye
and an oppositely charged surfactant, electrostatic inter-
actions appear. The thermodynamic process involving the
transfer of the dye between micellar and bulk water phases
is characterized by free energy changes and partition
coefficients, respectively. The partition coefficient depends
on the structure of the additive, the surfaces that constitute
the micelles and other physicochemical variables such as
the concentration and temperature. Among the various
factors that contribute to the solubilization in micellar
systems, hydrophobic contributions are quite significant
used as purchased. Dye Acid Red 52 was obtained from
Sigma. Triple-distilled water was used in all measure-
ments. The characterization of dye is listed in Table 1.
Methods
Melting points were determined on a Reichert hot stage and
were uncorrected. FT-IR spectra were recorded on a Perkin
-
Elmer spectrometer in the 4,000–400 cm range using
1
1
KBr discs. H-NMR spectra were recorded on a Joel-AX
500 MHZ with TMS as an internal standard and mass
spectra were recorded on a Joel- AX 500 (EI and FAB).
Elemental analyses (C, H, N, Cl) were performed with a
Varian elemental apparatus and in satisfactory agreement
with the calculated values (Table 2). UV–visible spectra of
AR52 solutions were recorded on Perkin Elmer-Lambda
spectrophotometer model 201. Stock solutions of surfac-
[
27].
In this paper we offer a simple method for synthesizing
gemini cationic surfactants based on 1,2-bis [N,N-dime-
thyl-N-carbalkoxy methyl ammonio] ethane dichloride (m-
2
-m; where m represents the hydrocarbon ‘‘tail’’ chain with
a length of 10, 12, 16 and 18 carbon atoms). The surface
tension and critical micelle concentration (CMC) of these
surfactants were measured, and some of surface parameters
were obtained on the basis of surface tension measurement.
Their properties were compared with a monoalkyl quater-
nary ammonium salt (dodecyl trimethyl ammonium chlo-
ride DTAC) which has been presented in previous articles.
Also, the solubilization and interaction of azo dye sulfo-
rhodamine B (Acid Red; AR52) with the synthesized sur-
factants were investigated by UV–visible spectroscopy
measurements. Absorption spectrophotometry was used to
-
1
tants and dye used in this study were 1 9 10
and
-
2
-1
1 9 10 molL , respectively. The variation of absor-
bance of the dye solution by increasing surfactants con-
centrations was recorded at maximum absorption
wavelength (kmax) using water as a blank.
Synthesis
Synthesis of Alkyl Chloroacetate Esters (Ia–d
)
quantify surfactant/water partition coefficients K - and KS
X
Alkyl chloroacetates were prepared by refluxing chloro-
acetic acid (0.5 mol) with corresponding fatty alcohols
(decyl, dodecyl, hexadecyl and octadecyl) in presence of p-
toluene sulfonic acid (0.1%) and benzene (100 mL) as
solvent, until the desired amount of water (0.1 mol,
1.8 mL) was removed. The mixture was extracted with two
as well as the Gibbs energies of distribution of dye by
applying the mathematical models that consider partition-
ing of the dye between the micellar and aqueous pseudo-
phase. Schemes 1 describe the synthetic route of gemini
cationic surfactants.
50 portions of 25% sodium carbonate, and the benzene
layer was dried over anhydrous sodium sulfate and the
Experimental
Table 1 Characterization of AR 52
Materials
C.I
45100
N,N,N,N-tetramethyl 1,2-ethylene diamine and fatty alco-
hols (decyl, dodecyl, hexadecyl and octadecyl) were
obtained from Sigma Co., and were used without purifi-
cation. Chloroacetic acid and p-toluene sulfonic acid were
obtained from Aldrich. All solvents were of high purity and
Mol. formula
Formula weight
Appearance
27 29 2 7 2
C H N NaO S
580.66
Red brown powder
554 nm
k
max
1
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J Surfact Deterg (2011) 14:371–379
373
Table 2 Analytical and
physical data of the synthesized
1,2-bis-diquaternary ammonium
gemini cationic surfactants
Comp no. Empirical
formula
Formula wt. M.P. (°C) Yield (%) Calc./found
C
H
N
Cl
II
a
II
b
II
c
II
d
C
C
C
C
30
H
34
H
42
H
46
H
62 2
N O
70 2
N O
86 2
N O
94 2
N O
4
Cl
4
Cl
4
Cl
4
Cl
2
2
2
2
585.25
641.29
753.38
809.42
80–82
85
87
92
96
61.57 10.59 4.79 12.12
1.21 10.32 4.51 11.90
63.68 10.92 4.37 11.06
3.39 10.66 4.12 10.86
6
118–120
124–126
140–142
6
66.96 11.42 3.72
6.60 11.19 3.48
68.26 11.61 3.46
8.01 11.48 3.29
9.41
9.19
9.01
8.90
6
6
solvent distilled off afterward. The esters of chloroacetic
acid were obtained with a yield high as 95–97%. Decyl and
dodecyl esters were obtained as liquids, while hexadecyl
and octadecyl esters were solid. M.P. 30–32 and 37–39 °C,
1
respectively. H NMR for C (CDCl , TMS): d 0.86(t,3H);
1
2
3
d 1.18-1.35(m,18H); d 1.64(m,2H) and d 3.81(s,2H); d 4.10
t,2H).
(
Synthesis of 1, 2-Bis[N,N-Dimethyl-N-Carbalkoxymethyl
Ammonio] Ethane Dichloride (IIa–d
)
Sodium hydroxide (0.05 mol) was added into alkyl chlo-
roacetates (0.2 mol) in 50 mL ethanol. Afterwards,
N,N,N,N,-tetramethyl ethylene diamine (0.1 mol) and 1 g
KI were added to the solution. The mixture was refluxed
for 8 h, then cooled to room temperature and filtered to
remove insoluble materials. The filtrate was evaporated
then the residue was recrystallized twice from ethyl acetate
giving IIa–d (m-2-m, m = 10, 12, 16 and 18) as a highly
hygroscopic white solid (Table 2). A typical IR spectra for
Fig. 1 Mass spectra of II
b
-
1
C : 2,919 cm for CH–stretching, t C=O at 1,628, t CH
1
2
+
-
1
-1
at 1,485 cm
t C–O at 1,467 cm
and
N
at
υ
-
1 1
3
1
,050 cm . H NMR(CDCl , TMS): d 0.87 (t, 6H, 2CH ),
.26 (m, 36H, 2 (CH ) ; d1.62 (m, 4H, 2(CH ) CH );
3
3
2
8
2 9
2
d3.35 (s, 12H, 4CH ); d 4.20 (t, 4H, OCCH C); d 4.34 (s,
3
2
þ
þ
?
4
H, 2COOCH N ) and d 4.60 (s,4H,H; N CH CH N).
2
2
2
?
EI/FAB (m/z: 640 (M –1) (1.13%) and the following
?
?
,
abundant peaks: 86(23.69%) for [CH (CH ) ]
3
?
16.19%) for [(Me) NCH CH N(Me) ] 142(100%) for
116
2 5
,
(
[
2
2
2
2
?
CH (CH ) ] and 144(10.37%) for [CH (Me) NCH
2
3
2 9
3
2
?
Fig. 2 Surface tension versus log C of 1,2-bis-diquaternary ammo-
nium gemini cationic surfactants at 25 °C
CH N (Me) CH ] ; Fig. 1.
2
2
2
Surface Tension Measurements
distilled water. The platinum ring was thoroughly cleaned
and dried before each measurement. Each experiment was
Surface tension measurements were performed for freshly
prepared solutions of surfactants in the concentration ran-
repeated several times until good reproducibility was
-1
achieved. The results was accurate within ± 0.1 mNm ;
-
1
-5
-1
ges of 10 –10 mol L using Du-Nouy tensiometer at
5 °C. The tensiometer was calibrated against triple
2
Fig. 2.
1
23

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J Surfact Deterg (2011) 14:371–379
Table 3 Surface properties of 1,2-bis-diquaternary ammonium gemini cationic surfactants at 25 °C and monododecyl trimethylammonium
chloride for comparison
-1
-6
max 9 10 mol/m
2
2
2
Comp
CMC molL
cCMC mN/m
pCMC mN/m
C
A
min 910 nm
pc20
CMC/C20
CPP
-
-
-
-
-
3
3
3
4
II
II
II
II
a
b
c
d
6.31 9 10
4.47 9 10
2.81 9 10
9.55 9 10
1.20 9 10
25.0
22.0
26.0
28.5
39.0
47.0
50.0
46.0
43.5
33.0
3.97
4.43
5.42
5.55
3.42
41.81
37.47
30.63
29.91
49.00
2.20
2.75
2.85
2.90
2.60
1.12
2.51
1.99
0.76
0.50
0.56
0.69
0.70
2a
a
b
c
a
c
b
b
DTAC
3.16
0.43
DTAC Dodecyl trimethyl ammonium chloride
See references a [41], b [calculated]; c [42]
Micelle/Water Partition Coefficients (K and K )
s
8
7
6
5
4
3
2
1
0
x
K and K were determined from the absorbance of a series
s x
of solution containing a fixed concentration of dye (Cdye;
1
-
3
0 lmolL ) and the surfactants concentration were
changed in the range of 10 –10 molL in CMC scale.
-
1
-5
-1
All tests were conducted at 25 °C and pH 6.9 ± 0.0.5.
Results and Discussion
C10
C12
C16
C18
Cn
Surface Properties
Fig. 3 Critical micelle concentration of the homologous series of 1,2-
bis-diquaternary ammonium gemini cationic surfactants versus there
number of carbon atoms in the alkyl substituent (R)
Surface Tension c and Critical Micelle Concentration
(
CMC)
Surface Excess Concentration (Maximum Surface
The surface tension of aqueous solutions of gemini sur-
factants were measured at 25 °C. The results of surface
tension measurements decrease with increasing concen-
tration, and then reached to constant values within the
narrow concentrations. The break points in Fig. 2 are taken
as CMC. The values of CMC of each gemini surfactant are
shown in Table 3. Conventional surfactant dodecyl tri-
methyl ammonium chloride (DTAC) was used as a control
compound. According to Table 3, CMC values of gemini
Excess) Cmax and Minimum Surface Area; Amin
The maximum surface excess of the diquaternary amphi-
philes (Table 3) was calculated using Gibbs adsorption
isotherm [30, 31].
Cmax ¼ ꢁ1=2:303 n RT ðoc=o log CÞT
ð1Þ
According to Zana [32, 33], the constant n takes a value
of 2 for an ionic surfactant when the surfactant ion and the
counterion are univalent, and a value of 3 for a dimeric
surfactant made up of a divalent surfactant ion and two
univalent counterions, in the absence of a swamping
electrolyte. In order to extract the minimum surface area
-
3
-3
-3
IIa–d are 6.31 9 10 , 4.47 9 10 , 2.81 9 10
-
and
.55 9 10 molL , respectively, while for the conven-
4
-1
9
-
2
-1
molL . As
tional surfactant DTAC it is 1.20 9 10
expected from recent publications, the CMCs of these
gemini surfactants are lower than that of DTAC, because
the two cationic head groups are connected by a spacer and
the electrostatic repulsion between head groups is hindered.
It also can be seen from Table 3 and Fig. 3 that the
CMC values of prepared gemini surfactant decrease with
an increase in the hydrophobic carbon chain from 10 to 18
as for common surfactants in a homologous series [28].
Gemini surfactants have a greater tendency to form
micelles with ethylene as spacer compared to the rigid
spacer [29].
2
occupied by a surfactant molecule, Amin (nm ), at the air–
water interface when the surface adsorption was saturated,
Eq. 2 was used:
1
6
A ¼ 10 =N:C
where N is the Avogadro number, R = 8.314 J mol
ð2Þ
k ,
max
-
1 -1
T is absolute temperature and (qc/qlog C) is the slope of the
c versus log C plot at 25 °C.
A substance which lowers the surface tension is thus
present in excess at or near the surface, i.e. when the
1
23

J Surfact Deterg (2011) 14:371–379
375
surface tension decreases with increasing the activity of the
surfactants, Cmax is positive. The results given in Table 3
indicate that increasing of the hydrophobic chain length led
to a decrease in Amin values. Crowding occurred at the
interface as a result of increasing Cmax, which consequently
decreased the area occupied by surfactant molecules at the
interface. Also, comparing the values of Amin for all
investigated surfactants and DTAC which are listed in
Table 3, we can see that all compounds have a smaller Amin
value than monomeric surfactant DTAC.
VH
CPP ¼
ð3Þ
aoxLC
where VH is the volume occupied by a saturated
hydrocarbon chain containing n carbon atoms and L is
the length of hydrocarbon groups in the micelle core. At
c
C
saturation, a can be replaced by A and V and L can be
o
H
C
calculated by the following relation [40]:
VH ¼ 27:4 þ 26:9 nc
LC ¼ 1:5 þ 1:26 nc
Surface Tension Reduction cCMC
The packing parameter calculated for the gemini
surfactant is less than 1, indicating the probable
formation of lamellar vesicles.
The effectiveness of surface tension reduction cCMC for all
investigated compounds is smaller than that (39.0 mN/m)
of DTAC. This is in line with previous work showing that
the presence of two hydrophobic groups in the gemini
molecule leads to greater surface activity [31, 34]. Com-
paring the values of cCMC for all investigated compounds
with other gemini surfactants, we can find that these salts
produce a larger decrease in surface tension at the air–
water interface than the other Gemini surfactant [35, 36].
Interaction of Acid Red 52 with Gemini Cationic
Surfactants
Figure 4 is shown as an example for studying the effect of
different concentration of the Gemini surfactant on the
absorption spectrum of AR 52. From Fig. 4, it can be seen
that the addition of gemini surfactant (12-2-12) at con-
centration below CMC caused a bathochromic shift of
about 20–30 nm with the appearance of a short peak at
Efficiency of Adsorption PC20
400 nm. This hyperchromic shift is characteristic of AR52
The values of efficiency of the prepared surfactants are
shown in Table 3, where the efficiency is slightly increased
with increasing molar ration of methylene units. This is due
to the fact that the efficiency of adsorption at interfaces
increases linearly with an increase in the carbon atoms of
hydrophobic group [37, 38]. Moreover, the bis-alkyl qua-
ternary ammonium salts in this investigation adsorb more
efficiently at the interface than do the monoalkyl quater-
nary ammonium salt DTAC.
bound to cationic micelles of surfactant and the intensity of
the maximum absorption band increased with increasing
surfactant concentration after the CMC [43]. In submicellar
regions, the dye formed a sparingly soluble precipitate
which became soluble as the gemini (12-2-12) concentra-
tion reached its CMC. The loss of absorption band in the
dye solution is due to this precipitate. The electrostatic
forces between positive charges of cationic surfactants and
anionic sulfonate groups of the dye and hydrophobic forces
cause the formation of the dye-surfactant complex. An
interesting and most likely explanation involves the for-
mation of dye aggregates in parallel orientation, the so,
called H-aggregates [25, 44, 45].
The ratio CMC/C20 value is a measure of the surfactant
preference for adsorption relative to micellization. The
value of this CMC/C20 ratio determined for the studied
gemini surfactants indicates that this surfactant shows a
great preference for adsorption at the interface.
Further addition of the Gemini (12-2-12) led to a bath-
ochromic shift of about 20–30 nm [43]. This hyperchromic
peak is characteristic of AR52 bound to cationic surfactant
micelles. The intensity of the maximum absorption band
increased with increasing surfactant concentration beyond
the CMC. Figure 4, also indicates that the solubility of the
dye in the micellar medium increases with increasing sur-
factant concentration, probably because of the penetration
of dye molecules into the micelles. This result is in good
agreement with the findings of Ali R. et al. [11]. The effect
of alkyl chain length on the interaction between AR52 and
gemini cationic surfactants was examined and all the
curves obtained are similar to those obtained in the case of
the gemini (12-2-12); hence, the spectra of gemini (12-2-
12) was used only as an example in this work to explain
Effectiveness of the Surface Tension Reduction pCMC
pCMC is the difference between the surface tension of the
pure water c and the surface tension of the surfactant
o
solution c at CMC (see Table 3). The most efficient sur-
factant is the C12 one which results in the largest reduction
of the surface tension at CMC.
Critical Packing Parameter (CPP)
It can provide a good idea of the shape of aggregates
spontaneously forming [39] through the following
equation:
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J Surfact Deterg (2011) 14:371–379
Fig. 4 Influence of Gemini
2-2-12 on absorption spectra of
1
-
1
‘‘AR’’52 30 lmolL : (1)
-
molL ; (2) 5 9 10 ; (3)
1
-5
0
1
5
1
6
-
4
9 10 ; (4) 5 9 10 ; (5)
-4
-
9 10 ; (6) 1 9 10 ; (7)
3
-3
-
2
-1
9 10 molL at 25 °C, pH
.9 ± 0.5
m
Dye
w
Dye
such effect and the data which were obtained from the
spectra of the other surfactants are recorded in Table 4. By
comparing the spectra of AR52 in the presence of varying
amounts of gemini surfactant with different alkyl chains
length, i.e. C10, 12, 16 and 18, it was found that the
intensities of the absorption bands at 554 and 400 nm
increase with increasing alkyl chain length from gemini
C10 to C18. This indicates an increase in the strength of the
interactions with the increase of the alkyl chain length, as
observed in reference [43].
X
X
Kx ¼
ð4Þ
where Xm and Xw are the mole fractions of dye in
Dye
Dye
micellar and aqueous phase, respectively. They are related
with concentration of species in the solubilization system:
m
Dye
C
m
Dye
X
X
¼
ð5Þ
m
Dye
m
surfactant
C
C
þ C
w
Dye
w
surfactant
C
w
Dye
¼
ð6Þ
w
Dye
þ C
þ nw
Determination of the Partition Coefficients (K , K )
x
s
where Cm
and Cw
represent concentrations of
surfactant
surfactant
Absorbance values obtained at kmax 554 nm can be also
used for the calculation of partition coefficient, K , defined
according to the pseudo-phase model as:
surfactant in micellar and monomeric states, and
-
nw = 55.5 mol dm is the molarity of water. Under the
3
x
present experimental conditions Cm ? C
w
surfactant
ꢂ n , if
Dye
w
we express K /n , we get the equation:
w
s
ꢀ
ꢁ
Table 4 The values of the partition coefficient and reacted Gibbs
-1
m
Dye
m
Dye
m
surfactant
energies for interaction and solubilization of AR 52 (30 lmolL
with different 1,2-bis-diquaternary ammonium gemini cationic sur-
)
C
= C þ C
Ks ¼
ð7Þ
w
C
factants at 25 °C
Dye
2
o
S
2
o
X
Surfactant
K
s
9 10
DG
K
x
9 10
DG
The fraction f of the associated dye may be defined as:
m
Dye
C
10-2-10
12-2-12
16-2-16
18-2-18
23.23
25.23
28.83
31.00
-19.15
-19.35
-19.70
-19.88
12.89
14.00
16.00
17.70
-17.71
-17.92
-18.25
-18.50
f ¼
ð8Þ
CDye
At a certain CDye, f is equal to zero in the non-micellar
region up to. CMC and increases with increasing the
1
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J Surfact Deterg (2011) 14:371–379
377
concentration of surfactant above CMC. As Csurfactant
increases up to infinity, f approaches unity, since all
added dye should be solubilized in micelles.
and the solubilization of AR 52 in the micelles of these
compounds.
o
The free energy of solubilization (DG ) and the free
energy to change from bulk aqueous phase to micellar phase
s
The fraction f can be directly calculated from the
experimental data using the following equation:
o
x
(DG ) were negative and their magnitudes increased with
increasing the hydrophobicity of the gemini surfactant.
It was observed that the power of solubility of the
Gemini surfactant increased as the hydrophobicity of sur-
factant increased.
DA
1
f ¼ DA
ð9Þ
1
1
1
where DA = A-Aw and DA = A –A , A being the
w
absorbance of dye completely bounded to surfactant.
By combining Eqs. 6, 7 and 8, a linear form can be
written:
It seems that electrostatic and hydrophobic interactions
between the molecules of dye and the surfactant micelles
play a major role in the penetration of the dye into the
micelles and the solubilization, as well as on the distribu-
tion of dye molecules between aqueous and micellar pha-
ses. A proposed mechanism of this interaction was similar
to that previously mentioned [48].
1
1
1
¼
þ
ð10Þ
1
1
DA DA
KsDA ðCDye þ Csurf: ꢁ CMCÞ
K and K (K = K /n ) were obtained from the slope of
s
x
s
x
w
1
the plot of
Table 4.
versus 1/(CDye ? Csurf. - CMC), Fig. 5,
DA
According to these results, the surfactant which con-
tained longer hydrophobic chain had a stronger tendency to
solubilize the dye; the solubilization constants were higher
than those measured for the surfactant with the shorter
hydrophobic chain.
The Gibbs energy of solubilization of the dye surfactant
micelles is given by [46]:
o
S
DG ¼ ꢁRT ln Ks
ð11Þ
The standard free energy change for the transfer of dye
from the bulk water phase to the micellar phase is given by
Conclusions
[
43, 47].
o
x
DG ¼ ꢁRT ln K
A conclusion that can be drawn from Table 4 is that the
ð12Þ
x
The reported results show that the aggregation of the
synthesized surfactant and anionic azo dye takes place at
surfactant concentrations far below the critical micelle
concentration of the individual surfactants; their kmax have
considerable hypsochromic shift along with a decrease in
their intensities which strongly depend on combination of
bulk hydrophobic and electrostatic interaction. By
increasing cationic surfactants concentration bathochromic
shift occurs due to the increase in the solubility of AR52 in
cationic micelles. The partition coefficient and free energy
charge determined from spectroscopy measurements indi-
cate the incorporation of dye into cationic micelles.
The increase in the alkyl chain length of the gemini
surfactant lead to greater hydrophobic interactions and
consequently enhanced solubility of the dye in the cationic
micelles.
solubilization constant K increase from C to C . Also, by
18
s
10
comparing the values of K for the synthesized gemini
x
cationic surfactants with different alkyl chain lengths, it is
found that the solubilization constant K increases with the
x
alkyl chain. This confirms that the increase in hydrophobicity
strengthens the dye-gemini cationic surfactant interactions
Ia
IIb
IIc
IId
60
50
40
30
20
10
0
Acknowledgments We would like to thank the chemistry depart-
ment at Al-Azhar University for supporting this work.
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Asrar G. El-Said received her B.Sc. with honors in Chemistry (1981)
at Cairo University, and her M.Sc. (1988) and Ph.D. (1993) in
Physical Chemistry, from the Faculty of Science, Al-Azhar Univer-
sity, Cairo, Egypt. She has been Associate Professor at the same
university since 2001. She has supervised M.Sc. and Ph.D. graduate
students working on applied physical chemistry.
Author Biographies
Fatma H. Abd El-Salam received her B.Sc. with honors in
Chemistry (1983), and her M.Sc. (1989) and Ph.D. (1994) in Applied
Organic Chemistry, from the Faculty of Science, Al-Azhar Univer-
1
23