Synthesis and characterization of asymmetrical gemini surfactants

Article abstract of DOI:10.1039/c6cp07688b

The effect of variation in the length of surfactant hydrocarbon tail groups was tested in a series of dissymmetric gemini surfactants (N¹-alkyl N¹,N¹,N³,N³-tetramethyl-N³-(6-pyren-6yl)-hexyl)propane-1,3-diammonium dibromide designated as C_mC₃C_nBr, with m = hexyl pyrene, and n = 8, 12, 14, 16, and 18. The aggregation properties of these surfactants have been investigated by means of ¹H NMR, fluorescence spectroscopy, surface tension and electrical conductivity measurements. The critical micelle concentration (CMC) was determined using surface tension and confirmed using the specific conductance method. Krafft temperatures and the degree of micelle ionization (α) were obtained from specific conductance measurements. With an increase of the dissymmetry (m/n) ratio, the CMC decreased linearly and an increase in the Krafft temperatures was observed for all of the gemini surfactants. α values for the dissymmetric GS were higher than those of the m-3-m counterparts, which may be attributed to enhanced micelle-micelle interactions that arise from increased hydrophobicity of the hydrocarbon chains. The introduction of the bulky pyrenyl tail group resulted in much lower CMC values compared to their symmetrical counterparts affecting the packing of these surfactants at the air/water interface, which resulted in high-ordered structures (lamellar and inverted micelles). This in turn affected the thermodynamic parameters of the micellization.

Full text of DOI:10.1039/c6cp07688b

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DOI: 10.1039/C6CP07688B
Journal Name
ARTICLE
Synthesis and Characterization of Asymmetrical Gemini
Surfactants
Received 00th January 20xx,
Accepted 00th January 20xx
a,b
a
a
a,c
a
Aula Al Muslim, Dana Ayyash, Sarbjeet Singh Gujral , George M. Mekhail , Praveen P. N. Rao ,
a,b,
and Shawn D. Wettig
†
DOI: 10.1039/x0xx00000x
The effect of variation in the length of surfactant hydrocarbon tail groups was tested in a series of dissymmetric gemini
www.rsc.org/
1
1
1
3
3
3
surfactants (N -alkyl N ,N ,N ,N - tetramethyl-N -(6-pyren-6yl)-hexyl) propane-1,3-diammonium dibromide) designated as
m
C C
3
C
n
Br, with m=hexyl pyrene, and n= 8,12,14,16, and 18. The aggregation properties of these surfactants have been
1
investigated by means of HNMR, fluorescence sepctroscopy, surface tension and electrical conductivity measurements.
The critical micelle concentration (CMC) was determined using surface tension and confirmed using the specific
conductance method. Krafft temperatures and the degree of micelle ionization (α) were obtained from specific
conductance measurements . With an increase of the dissymmetry (m/n) ratio, the CMC decreased linearly and an
increase in the Krafft temperatures was observed for all of the gemini surfactants. α values for the dissymmetric GS were
higher than those of the m-3-m counterparts, which may be attributed to enhanced micelle-micelle interactions that arise
from increased hydrophobicity of the hydrocarbon chains. The introduction of the bulky pyrenyl tail group resulted in
much lower CMC values compared to their symmetrical counterparts afftecting the packing of these surfactants at the air/
water interface, which resulted in high-ordered structures (lamellar and inverted micelles). This in turn affected the
thermodymanic parameters of the micellization.
important properties of gemini surfactants is their ability to
form aggregates of various morphologies in aqueous solutions,
which increases their potential utility for drug entrapment and
Introduction
Gemini surfactants are amphiphilic compounds composed
of two hydrophilic heads and two hydrophobic tails, which are
connected by a spacer group at the head area or near it. In
aqueous solution, amphiphilic molecules associate to minimize
the exposure of the hydrophobic components to the water
phase forming hydrophobic micro-domains (known as
micelles) which are stabilized in solution by the hydrophilic
components. Gemini surfactants are attractive compounds
which can be utilized in various applications, including soil
remediation, soap and pigment industries, drug entrapment,
[2, 3]
gene delivery applications.
concentration (CMC) is two to three orders of magnitude
Their low critical micelle
[4]
lower than the conventional surfactants , which can lead to
lower surfactant concentration in drug and gene delivery
applications.
The most well- studied category of GSs are the N,N-
bis[(dimethyl
alkyl)-α,ω-alkanediammonium
dibromide]
surfactants, known generally as m-s-m type gemini surfactants,
where m represents the carbon chain length of the alkyl tail
and s the number of carbon atoms in the polymethylene
[
1]
and recently, in gene delivery.
Gemini surfactants possess
[
5]
spacer.
physiochemical properties of the symmetrical gemini
The effect of the alkyl tail chains on the
unique characteristics that make them of substantial interest
in the aforementioned applications. One of the most
[6]
surfactants has been the focus of numerous studies , but little
work has been carried out to examine the self-assembly and
[7]
micellization of the dissymmetric gemini surfactants. The
micellization of dissymmetric m-s-n surfactants in solution is
highly governed by the total hydrophobic chain length (m + n)
[8]
and the extent of dissymmetry (m-n). With an increase in the
m/n ratio of asymmetry, the CMC is observed to decrease
linearly with the degree of micelle ionization (α) being only
slightly affected and the Gibbs free energy of micellization
becoming more negative, with a correspondingly more
[9]
negative enthalpy.
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3
Scheme 2: Synthesis of 12-3-n surfactants. Reagents and Conditions: (i) CH CN,
8
bromooctadecane, with n=12, and n= 14, 16, and 18
3
9C˚, (ii):CH CN, 1-bromotetradecane, 1-bromohexadecane and 1-
octadecane, were purchased from Aldrich Inc. All solutions
were prepared using ultrapure water obtained from a
Millipore Milli-Q filtration system. The synthesis of the
surfactants, as well as intermediates, are described in detail in
the following sections. The general reaction schemes are
shown in Schemes 1 and 2.
5
-Bromohexane-1-pyrene ketone(2): The synthesis was
[10]
carried out as described by Wang et al.
Pyrene (49.4 mmol)
3
and AlCl (60mmol) were dissolved in 80 mL of precooled
Scheme 1:. Synthesis of Pyr-3-m surfactants. Reagents and conditions: (i) Br
(
CH
2
)
5
COCl, AlCl
3
,
CH
tetramethylpropane- diamine, CH
2
Cl
2
,
-78C°, (ii) Et
3
SiH, TFA, 0C°, (iii) N,N,N`,N`- CH Cl (-78C˚). Another 80 mL of CH Cl containing 6-
2 2 2 2
CN, (iv) 1-bromooctane, 1-bromododecane,1-
3
bromotetradecane, 1-bromohexadecane, 1-bromooctadecane with m= hexyl bromohexanoyl chloride (24.7 mmol) was added in a drop wise
pyrene, and m= 8, 12, 14, 16, and 18
fashion to the mixture and the reaction continued for 3h at -
7
8˚C. The reaction was quenched with 1M HCl and neutralized
with NaHCO , and washed using a saturated aqueous solution
of NaCl. The oil layer was separated and dried with anhydrous
Na SO overnight. The excess solvent was removed via rotary
In this current study, we aimed to synthesize and
characterize two groups of dissymmetric gemini surfactants,
referred to as m-s-n. The first group consisted of m-s-n
surfactants with one hexyl pyrene tail and one alkyl tail (n=
3
2
4
evaporation and gave a yellow/greenish paste that was further
8,12,14,16 and 18). Meanwhile the second m-s-n had two
purified by soxhlet extraction with pentane to yield a light
1
yellow solid (yield= 80%). HNMR (CDCl
alkyl tails, where: m=12 and n=14, 16, and 18. We also aimed
to investigate the physiochemical properties of these GSs using
surface tension, and specific conductance to determine the
critical micelle concentration, surface tension. Krafft
temperature, Krafft point, head group area, packing
parameter, degree of micelle ionization, and thermodynamic
properties of micellization.
3
, 300MHz) δ(ppm):
8
.84 (d, 1H, pyr-H), 8.31-8.01 (m, 8H, pyr-H), 3.45 (t, 2H,
CH Br), 3.24 (t, 2H, α-CH ), 1.98-1.84 (m, 4H, β-CH and δ-CH ),
.65-1.55 (m, 2H, γ-CH ).
-(1-Pyrene bromohexane)(3): This reaction was carried out as
2
2
2
2
1
2
6
[
11]
previously reported.
Trifluoroacetic acid (15 mL) and 5-
bromohexane-1-pyrene ketone (6.0 mmol) were dissolved in
5mL CH Cl which was precooled to 0˚C. 2.7 mL of
6
2
2
triethylsilane was added, dropwise, to the mixture and stirred
under an Argon atmosphere for three days. The reaction was
Materials and Methods
Materials
3
neutralized using saturated aqueous NaHCO and the organic
Pyrene, 6-bromohexanoyl chloride, anhydrous aluminum layer was separated and dried overnight with anhydrous
chloride, triethylsilane, trifluoroacetic acid, N,N,N’,N’- Na SO . The product was then purified using soxhlet extraction
2
4
tetramethyl propanediamine, 1-bromododecane, 1-bromo- with pentane (100%). A final yield of 92% was produced.
1
tetradecane, 1-bromohexadecane, 1-bromooctane, 1-bromo-
HNMR (CDCl , 300MHz) δ(ppm): 8.27-7.83 (m, 9H, pyr-H).
3
2
| J. Name., 2012, 00, 1-3
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DOI: 10.1039/C6CP07688B
ARTICLE
3
.41-3.30 (superimposed triplets, 4H-α-CH
.81 (4H, β-CH , ε-CH ), 1.514-1.42(m, 4H, γ-CH
Pyr-3: (N-(3-dimethylaminopropyl)-N,N-dimethyl-6-(pyren-6- spacer), 3.41-3.22 (m, 6H, pyr-H & chain-H), 3.18 (S, 6H, N -
2
, pyr-CH
2
), 1.90- treated as above for pyrene-3-12 with a yield of 1 g (35%).
1
1
2
2
2
, δ-CH
2
).
3
HNMR (CDCl ) δ(ppm): 8.24-7.80 (m, 9H, pyr-H), 3.72 (t, 4H,
+
+
yl)-hexane-1-ammonium bromide)(4): 6-(1-pyrene)-bromo- (CH
hexane (10.4 mmol) and N,N,N’,N’- tetramethylpropane 1.21 (m, 36H, octadecyl chain and hexyl chain), 0.84 (t, 3H,
diamine (13.2 mmol) were dissolved in 50 mL of anhydrous CH -chain).
3 2 3 2 2
) ), 2.70 (S, 6H, N -(CH ) ), 2.11 (m, 2H- CH -spacer), 1.79-
3
’
acetonitrile and the mixture was stirred at 45˚C for three days. 12-3-14: (N-dodecyl-N -tetradecyl-1,3-propanediammonium
The solvent was removed using rotary evaporation leaving a dibromide): 1-bromododecane (8.02 mmol) and N,N,N`,N`-
light yellowish precipitate. This crude product was further tetramethylpropane diamine (1.34 mL) were added to 40 mL of
recrystallized from acetonitrile to yield an off-white solid anhydrous acetonitrile and the mixture was stirred for 24h at
1
40%). HNMR (CDCl
(
3
, 300MHz) δ(ppm): 8.20-7.78 (m, 9H, pyr- 89C°. The product was recrystallized and 1-bromotetradecane
+
+
2 2 2
H), 3.43 (m, 2H-N -CH ), 3.40-3.16 (m, 4H, py-CH & CH -N ), (2.39ml) was then added to the 12-3 intermediate and the
+
3
.05 (S, 6H, N -(CH
N(CH ), 1.87-1.51 (m, 10H, (CH
Pyrene-3-12:
3
)
2
), 2.36 (broad, 2H, CH
2
-N), 2.23 (S, 6H, reaction was continued at 89C° for another 24h. The product
3
)
2
2
)
4
1
& β-CH
1
2
3
.
was recrystallized from hot acetonitrile and ethyl acetate
1
3
3
(N -dodecyl-N ,N ,N ,N -tetramethyl-N -(6- (1:1ratio), and dried in vacuum for two days to give a yield of
1
pyren-6yl)-hexyl) propane-1,3-diammonium dibromide(5): (1g, 50%). HNMR (CDCl
Pyrene-3 (4 mmol) and 1-bromododecane (9.0 mmol) were spacer, 3.50-3.46 (t,2H, ʏ-(CH
added to 40 mL of anhydrous acetonitrile and the mixture was 3.02-2.97(m, 4H, (CH ), 1.82-1.78 (m, 4H, (CH
-spacer), 0.88-0.83(t, (CH
(N-dodcyl-N’-hexadecyl-1,3-propanediammonium
3
) δ(ppm): 3.88-3.83 (t,2H, ɑ (CH
)-spacer, 3.38(s,12H, (CH
), 1.34-1.23(
).
2
)-
2
3 4
)
),
2
)
2
2 2
)
stirred at 45C˚ for two days. The solvent was removed using m, 40H, (CH
2
)
20 & ß-CH
2
3 2
)
rotary evaporation and the product was recrystallized from a 12-3-16:
mixture of acetone and ethyl acetate (1:2), and dried overnight dibromide): 1-bromododecane and N,N,N`,N`-tetramethyl-
1
under vacuum with a yield of 1.5 g (50%). HNMR (CDCl
3
)
propane diamine were mixed in the same way as for 12-3-14
(above) to produce the 12-3 intermediate and then 1-
+
δ(ppm): 8.23.-7.78 (m, 9H, pyr-H), 3.77-3.7 (4H, t, N -CH
2
+
(
2 2
spacer)), 3.33-3.31(6H, m, pyr-CH & N -CH )- chain)), 3.20 bromohexadecane was also added in the same manner and
+
+
1
(6H, S, N -(CH
3
)
2
- pyr-side), 1.98(6H, S, N -(CH
3
)
2
- dodecyl conditions. A yield of 0.8g (35%) was obtained. HNMR (CDCl
), δ(ppm): 3.88-3.83 (t,2H, ɑ (CH )-spacer, 3.50-3.46 (t,2H, ʏ-
(CH )-spacer, 3.38(s,12H, (CH ), 3.02-2.97(m, 4H, (CH ),
Pyrene-3-8: (N -octyl-N ,N ,N ,N -tetramethyl-N -(6-pyren- 1.82-1.78 (m, 4H, (CH ),1.34-1.28(m, 4H-(CH , 1.25-1.23( m,
-spacer), 0.88-0.83(t, (CH ).
(4 mmol) and 1-bromooctane (9 mmol) were treated as 12-3-18: (N-dodecyl-N’-octadecyl-1,3-propanediammonium
3
)
chain), 1.66(2H, m, CH
.87(3H, t, C-CH ).
2
, spacer), 1.38-1.22(28H, m, CH
2
2
0
3
2
3
)
4
2 2
)
1
1
1
3
3
3
2
)
2
2 2
)
6
yl)-hexyl)-propane-1,3-diammonium dibromide)(6): Pyrene- 42H, (CH
2
)
21 and ß-CH
2
3 2
)
3
1
above for pyrene-3-12 with a yield of 1 g (35%). HNMR (CDCl
+
δ(ppm): 8.17-7.80 (m, 9H, pyr-H), 3.77 (4H, t, N -CH
3
)
dibromide): 1-bromododecane and N,N,N`,N`-tetramethyl-
2
(spacer)), propane diamine were mixed in the same way as for 12-3-14
+
, N -CH
+
-chain), 3.144 (S,6H, N -(CH
3
.61 (m, 6H, pyr-CH
2
2
3
)
2
2
-
-
(above) to produce the 12-3 intermediate and then 1-
+
pyrenyl side), 2.73 (S, 6H, N -CH
2
- octyl side), 1.87 (m, 2H, CH
bromohexadecane was also added in the same manner and
1
spacer), 1.80-1.23 (m, 20H, octyl chain & hexyl at pyrene-end), conditions. A yield of 1g, (50%) was obtained. HNMR (CDCl
3
)
0
.89 (t, 3H, CH
Pyrene-3-14: (N -tetradecyl-N ,N ,N ,N -tetramethyl-N -(6- (CH
pyren-6yl)-hexyl)-propane-1,3-diammonium dibromide)(7): 1.82-1.78 (m, 4H, (CH
3
- octyl chain).
δ(ppm): 3.88-3.83 (t,2H, ɑ (CH
)-spacer, 3.38(s,12H, (CH
2 2
),1.34-1.28(m, 4H-(CH )
2
)-spacer, 3.50-3.46 (t,2H, γ-
), 3.02-2.97(m, 4H, (CH ),
, 1.25-1.23( m,
).
1
1
1
3
3
3
2
3
)
4
2 2
)
2
)
2
Pyrene-3 (4 mmol) and 1-bromotetradecane (9 mmol) were 46H, (CH
treated as above for pyrene-3-12 with a yield of 1.5 g (50%).
2
)
22 & ß-CH
2
3 2
-spacer), 0.88-0.83(t, (CH )
1
Methods
3
HNMR (CDCl ) δ(ppm): 8.19-7.79 (m, 9H, pyr-H), 3.72-3.57 (t,
+
H, N -CH
1
, spacer), 3.31-3.31 (m, 6H, pyr-H & chain-H), 3.11 Nuclear Magnetic Resonance (NMR): H NMR spectra were
4
2
+
+
(
S, 6H, N -(CH
spacer), 1.79-1.19 (m, 32H, tetradecyl chain and hexyl chain), Chemistry department in University of Waterloo. Samples
.85 (t, 3H, CH -chain). were dissolved in CDCl
Pyrene-3-16: (N -hexadecyl-N ,N ,N ,N -tetramethyl-N -(6- Fluorescence studies: Fluorescence emission spectra of
3
)
2
), 2.55 (S, 6H, N -(CH
3
)
2
), 2.11 (m, 2H- CH
2
-
recorded using a Bruker Avance 300MHz spectrometer at the
0
3
3
.
1
1
1
3
3
3
pyren-6yl)-hexyl)-propane-1,3-diammonium dibromide)(8): pyrenyl gemini surfactants in aqueous solutions and pyrene in
Pyrene-3 (4 mmol) and 1-bromohexadecane (9 mmol) were methanol have been recorded using Spectra max M5
treated as above for pyrene-3-12 with a yield of 1.5 g (50%). microplate reader (molecular devices, USA) at excitation
1
3
HNMR (CDCl ) δ(ppm): 8.23-7.80 (m, 9H, pyr-H), 3.59 (t, 4H, wavelength 363nm. These samples were measured in
+
spacer), 3.38-3.30 (m, 6H, pyr-H & chain-H), 3.11 (S, 6H, N - transparent disposable cuvettes (ZEN0118) (Malvern
+
), 2.59 (S, 6H, N -(CH
(CH
3
)
2
3
)
2
), 2.15 (m, 2H- CH
2
-spacer), 1.83- instruments, UK). The excitation and emission slit-widths were
1.21 (m, 36H, hexadecyl chain and hexyl chain), 0.85 (t, 3H, set at 1 nm. An average of 40–50 scans was recorded.
CH
3
-chain).
CMC determination: The CMC of each surfactant was
1
(N -octadecyl-N ,N ,N ,N -tetramethyl-N -(6- determined using both surface tension and specific
1
1
3
3
3
Pyrene-3-18:
pyren-6yl)-hexyl)-propane-1,3-diammonium dibromide)(9): conductance techniques. Surface tension measurements were
Pyrene-3 (4 mmol) and 1-bromohexadecane (9 mmol) were performed using a Lauda TE3 automated tensiometer (Lauda,
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Germany), applying the Du Nuoy ring method. The Harkins and
Jordan correction factor was applied to all surface tension
[12]
values. 40 mL of milli-Q quality water was placed in a 100
mL vessel and a concentrated (approximately 5 times of the
CMC based on the pyr-3-12) sample of aqueous surfactant
solution was titrated into this. Surface tensions were
measured until the variability between measurements was less
than 0.1 mN/m. All CMC determinations were carried out in
°
duplicate at temperatures of 25 C.
The specific conductivities were determined with a
TM
SevenEasy S30 conductivity meter (Mettler Toledo,
Switzerland) and double-walled glass titration cell (Fisher
-1
Scientific, USA). A cell constant of 0.56cm was determined for
the conductivity probe. Again, a concentrated surfactant
solution was titrated into XX mL of milli-Q water contained in
the titration cell, and the CMC was again determined in
⁰
duplicate at temperatures of 25, 30, and 35 C. The CMC value
was determined from the abscissa of the inter-section of the
trend lines connecting the experimental points before and
after the CMC (observed as a clear break between two linear
[13]
regions) in a surface tension vs. log concentration plot. The
conductivity vs concentration data was fit to Equation 1
[14]
according to the method of Carpena et al.:
ꢅꢏꢐꢈꢑꢒꢑꢓꢑꢋ/ꢔ
ꢖ [1]
ꢅꢕꢐꢈꢒꢑꢓꢑꢋ/ꢔ
κ
ꢀ ꢁꢀκꢂ ꢃ ꢄ ꢆ ꢃ ꢇꢈꢄ ꢊ ꢄ ꢋꢌꢍ ꢎ
ꢅ
ꢉ
ꢅ
o
where
κ
and
κ
respectively), C is the solution concentration, A
are the initial and solution conductivities
and A are
(
1
2
the pre and post CMC slopes of the conductivity vs
concentration curve, and d is the width of the transition
[15]
region.
Krafft Temperature: The Krafft temperature and Krafft point
) for each surfactant were determined by the specific
Fig. 1: Surface tension plots for A) pyrenyl-substituted gemini surfactants;
Pyrene-3-14 (○) B) dissymmetric 12-3-n gemini surfactants 12-3-16. All
measurements were made at 25⁰C. The point of intersection of the linear
regions of the curves corresponds to the CMC, tabulated in Table 1.
(T
k
[16]
conductivity method as previously reported.
The
temperature was controlled with a RE304 circulating water
bath (Lauda, Germany).
Kcal/mol. The type of interaction occurring between the
Interacting pyrene surfactants (i.e. self or intermolecular).
Molecular Modeling: Molecular docking studies for pyrene
based surfactants were performed to give additional
information on how pyrenyl surfactants interact with each
other (either via self and/or intermolecular aggregation) when Results and Discussion
[17]
present in the aqueous phase.
Discovery Studio (DS)
Surface tension plots for of the pyr-3-n and 12-3-n gemini
surfactants are shown in Fig.1A & 1B and the CMC values are
listed in table 1.Figure 1A and 1B, respectively. The CMC
values were determined from the point of intersection of the
two linear regions of the γ vs. log C curves, and are tabulated
in Table 1. It can be seen that the CMC gradually decreases
with an increase in the length of the alkyl tail from octyl to
octadecyl, for both the alkyl and pyrenyl dissymmetric
surfactants. In a logarithmic plot of the CMC as a function of
the hydrocarbon tail, shown in Fig.2 the symmetrical m-3-m
gemini surfactants show a linear pattern decrease with
Structure-Based-Design software (version 4.5, BIOVIA, San
Diego, U.S.A) was used for the docking studies. Briefly, 3D
models of the pyrene based surfactants (surrounded with
water molecules) were built using the Build Fragment tool.
Energy minimization was performed using the steepest
descent and conjugate gradient minimizations for 1000
interactions. The distance dependent dieletric model was
used as the implicit solvent model for the energy minimization
step. The CDOCKER algorithm was used to dock a surfactant
monomer with another monomer after defining a 25Å sphere
radius around the surfactant molecule. The CHARMm force
field was used for the docking studies. The most stable binding
models between the pyrene based surfactants was evaluated
based on CDOCKER energy and CDOCKER interaction energy in
[14],[18]
increasing tail length (with the exception of (18-3-18)
,
consistent with the typical behavior of surfactant molecules.
When the CMCs of the pyrenyl substituted dissymmetric
surfactants are plotted in this manner, they no longer exhibit a
4
| J. Name., 2012, 00, 1-3
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ARTICLE
Table (1): Critical micelle concentration (CMC), head group area (a˳), packing parameter (P), and the degree of micelle ionization (α)
a
b
2
-6
2
Gemini
CMC (mM)
CMC (mM)
γ
cmc(mN/m)
Γ(mol/m )*10
a
0
(m /mol)
P
α
surfactant
Pyrene-3-8
Pyrene-3-12
Pyrene-3-14
Pyrene-3-16
Pyrene-3-18
-
7
0.178 ±0.02
0.21±0.02
0.15±0.03
0.14±0.03
0.02±0.07
0.32±0.01
0.22±0.01
0.17±0.02
0.13±0.007*
0.16±0.007
0.16±0.028
0.132±0.0028
0.06±0.014
0.34±0.07
27.6±0.14
43.1±0.02
41.2±0.14
43.6±0.07
40.0±0.00
26.2±0.07
32.2±0.07
40.2±0.14
9.47x10 ±0.07
2.97±0.02
1.47±0.14
3.16±0.07
2.38± 0.00
5.74±0.07
8.07±0.07
8.68±0.14
0.35
0.56
1.13
0.53
0.70
0.30
0.21
0.19
1.11
<1
1
0. 42±0.01
0.61±0.00
0.46
0.91
0.65
<1
0.67±0.02
0.61±0.02
0.57±0.01
0.28±0.00
0.39±0.02
0.44±0.02
0.23
1
1
1
1
2-3-14
2-3-16
2-3-18
2-3-12
0.18±0.02
<1
0.18±0.02
<1
0.98±0.04
b
0.98±0.04[2]
0.38
a
from surface tension, from specific conductivity , data obtained at 25C°. *CMC break point in pyr-3-8 conductivity study was not clear.
linear behavior, but rather show a slight decrease from pyr-3-
2 (0.21mM) to pyr-3-14 (0.15mM) then pyr-3-16 (0.113m M),
followed by a greater decrease between pyr-3-16 and pyr-3-18
0.02mM). Most of the pyrenyl gemini surfactants show a clear
break point at the CMC. ; However, with the exception of pyr-
-18 where the CMC was poorly defined, suggesting the
1
(
3
presence of premicellar aggregation, and this is consistent
with this kind of behaviour which was previously reported
before for gemini surfactants with longer alkyl tails, m<18, in
[
17
]
1
2-s-12; where s=12,14, and 20.
The same behavior was reported in dissymmetric gemini
surfactants with bulky unsaturated tails, such as, phytanyl-3-16
[15]
.
For the m-3-n surfactants, as seen in Figure 2A, the CMC of
2-3-14 was significantly less than that of 12-3-12 (0.32mM
1
versus 0.98mM, respectively); however from n = 14 to n = 18,
logCMC did not decrease in a linear fashion. It may be the
case that we did not have enough data points (i.e. different
dissymmetric surfactants) to identify a linear decrease for the
m-3-n surfactants. An additional methylene group added to
one tail in the m-3-n surfactants does have the same effect it
has on the pyrenyl surfactants. However, the effect is more
pronounced in the pyrenyl surfactants due to the presence of
the pyrene moiety within the surfactants, which adds to the
hydrophobicity of the gemini surfactant through the pyrene
stacking, leading to enhanced hydrophobic interactions. The
contribution of additional methylene unit to the CMC value in
both groups of surfactants was more like that of a single-tail
[19]
surfactant. This is attributed to the increased intermolecular
hydrophobic interactions between tails of same length
between the monomers within the micelles. Similar to the
symmetric gemini surfactants, γCMC decreases with increasing
the m-3-n concentration, which indicates the formation of a
monolayer at the air/water interface, which reduces the
surface tension of water. For the pyrenyl surfactants, the γ
values are relatively higher than those for the corresponding
symmetric m-3-m and the dissymmetric m-3-n surfactants,
suggesting an earlier onset of micellization (with the exception
of the pyr-3-8). Pyr-3-8 has a short alkyl tail, rendering the
surfactant more hydrophilic and increasing its solubility in
Fig. 2: Variation of the Logarithm CMC (determined by tensiometery) as a
function of the alkyl tail length (A) and as a function of the degree of
dissymmetry (B) for the gemini surfactants m3-m. (■), pyr-3-n (▲), 12-3-n (●).
The low value for γcmc (27.6 mN/m) is consistent with this
interpretation. However, the CMC value of pyr-3-8 (0.18mM) is
way lower than that of 8-3-8 symmetric counterpart (57mM).
[20]
The CMC value of pyr-3-8 is also lower than that of pyr-3-
2. This can be attributed to the length of the hexyl pyrenyl tail
1
which is equal to 11-11.5 bonds and this makes pyr-3-12 acts
as symmetrical gemini surfactants below the CMC, thus
minimize the effect of the intermolecular hydrophobic
interactions, limiting the pi-pi stacking and enhancing the
interactions with the dodecyl tail.
water,
favoring
the
bulk
over
the
surface.
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Journal Name
structures
within
the
micelles. The pyrenyl surfactants also exhibit a strong
upward curvature in the plot of vs. C in the region of
the CMC, which is an indication of the formation of
κ
[23]
premicellar
aggregates.
Similar
behaviour
was
observed for the 12-3-n surfactants. The CMC values
decreased with the increase of the alkyl tails, and the
increase in the temperature (in the supplementary
data). The decrease in the CMC with the increase in the
temperature is
a
consequence of the decreased
hydrophilicity of the surfactants molecules and this
reduction of hydration favors micellization. Also, the
increase in temperature breaks down the structured
[24]
water surrounding the hydrophobic groups.
The minimum head group areas for gemini surfactants are
also reported in Table. 1. The head group areas can be
calculated from the surface excess concentration according to
equation 2 below:
0 A
a ꢀꢁꢀꢈN
Γꢋ-1 [2]
where NA is Avogadro’s, and
concentration calculated from the Gibb`s adsorption
Γ is the surface excess
[
25]
equation
:
ꢕꢅ
ꢜꢝ
ꢗ ꢁ
ꢈ
ꢋꢀꢀ[3]
ꢉ.ꢘꢙꢘꢚꢛ ꢜꢞꢂꢟꢠ
where
Kelvin,
R
is the gas constant,
T
is the absolute temperature in
-1
γ
is the surface tension in mN.m , (
-logC plot and n = 3 for gemini
surfactants, accounting for 1 dimeric ion and 2 monomeric
counter ions. The head group areas (a ) for the pyrenyl gemini
surfactants are all smaller than that of their symmetrical
dγ/dlog C) is the
slope of the linear part of the
γ
Fig. 3: A) Specific conductance (ҡ) versus Surfactant concentration for pyr-3-
1
4(○) and 12-3-14(○), respecꢀvely.
0
2
-
counterparts with the exception of pyr-3-14 with 1.13nm
[5]
⋅
Increased asymmetry, enhances the hydrophobic
mol
1
The CMCs for pyr-3-12, pyr-3-14, and pyr-3-16
.
obtained from the conductivity studies are in good in intermolecular interactions, leading to lower CMC values and
[7]
agreement with those obtained from tensiometry, as higher aggregation ability. Also, with longer hydrocarbon
well as with those reported previously in the literature. tails, a˳ values decrease due to higher packing density at the
[5]
Less agreement was observed for pyr-3-18,which is air/water interface in comparison to their symmetrical
common when comparing the results using different counterparts. For pyr-3-18, although a is higher than pyr-3-16,
techniques to measure different properties in self- it is still lower than that of its symmetrical counterpart 18-3-18
0
[
5, 21]
2
-1 [18]
assembly
low CMC values. CMCs for 12-3-14, 12-3-16, and 12-3-18 14 is lower than that of 14-3-14 (1.35nm
were also in good agreement with those obtained from
The variations in head group areas for the m-3-n
; particularly for surfactants having very which has a value of 1.28nm
⋅mol , . Similarly, a for pyr-3-
0
2
-1 [26, 27]
⋅mol ) .
the surface tension measurements. The degrees of surfactants result from differences in the packing of the
micelle ionization parameter (α) is defined as a fraction hydrophobic tails at the air/water interface. The a values for
0
of an ionic surfactant’s counter ions that are dissociated the m-3-n surfactants are lower than those for their
from micelles, leaving the micelles charged, and it can symmetrical m-3-m counter parts, which suggests increased
[
22]
be obtained from the ratio of A2/A1.
pyrenyl and the 12-3-n gemini surfactants,
For both the packing at the air/water interface as a result of the
values are dissymmetry between the alkyl tails. A trend was established
α
higher for the pyrenyl surfactants compared to the m-3- for 12-3-n surfactants. For 12-3-14, 12-3-16, and 12-3-18, a0
m surfactants, despite having low CMC values. Higher α values are smaller than the a0 values of their symmetrical
values correspond to a greater degree of dissociation of counterpart. As the hydrophobicity of the 12-3-n surfactants
the counterions from the surface of the micelles. With increase, the CMC values decrease, and the surfactant
higher α, the repulsive forces between the partially monomers favour adsorption at the air/water interface. This
charged micelles will play greater role in the aggregate
6
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ARTICLE
Table (2): Critical micelle concentration (CMC), head group area (a˳), packing parameter (P), and the degree of micelle ionizaꢀon (α)
o
a
o
m
-1
o
m
-1
o
m
-1
-1
o
m
-1
T ( K)
Surfactant
Pyrene-3-8
Pyrene-3-12
Pyrene-3-14
Pyrene-3-16
Krafft temperature (T
K
)
CMC (mM)
∆G
(kJ mol ) ∆H
(kJ mol ) ∆S
(kJ K mol ) T∆S
(kJ mol )
20
42
45
60
<60
33
35
60
0.13
-49
-56
-53
-57
-64
-73
-70
-66
-70
-51
-46.
-65
-47
-69
-70
-65
-64
35
17
81
28
32
29
3.5
8.6
-
0.28
0.25
0.45
0.28
0.32
0.34
0.24
0.25
-
84
74
0.16
0.15
134
85
96
101
73
75
-
0.13
2
3
98.15 Pyrene-3-18
0.06
1
1
1
1
2-3-14
2-3-16
2-3-18
2-3-12
0.34
0.18
0.18
0.98
03.15
Pyrene-3-8
Pyrene-3-12
Pyrene-3-14
Pyrene-3-16
Pyrene-3-18
0.12
36
18
84
28
33
30
3.7
8.6
0.29
0.21
0.49
0.25
0.33
0.32
0.23
0.24
87
64
149
75
102
99
69
73
0.16
0.075
0.12
0.042
0.25
1
1
1
2-3-14
2-3-16
2-3-18
0.18
0.18
3
08.15
Pyrene-3-8
Pyrene-3-12
Pyrene-3-14
Pyrene-3-16
Pyrene-3-18
0.10
0.12
0.05
0.09
0.039
0.23
0.17
0.16
-52
-51
-67
-61
-61
-67
-69
-68
37
18
87
29
34
31
3.7
8.9
0.29
0.23
0.5
89
69
154
90
95
97
73
77
0.29
0.31
0.31
0.24
0.25
1
1
1
2-3-14
2-3-16
2-3-18
a
from specific conductivity.
leads to a higher degree of intermolecular interactions In equations 5 and 6,
n is the number of carbon atoms in the
between the alkyl tails, resulting in smaller mean molecular hydrocarbon tails of the surfactant. The packing parameter for
area. However, the discrepancy found in the pyrenyl gemini the py-3-m surfactants is dramatically impacted by the
surfactants was reported in other surfactants, such: phytanyl- presence of the bulky pyrenyl ring in the molecule. The
3-m by Wang et al. The head group areas for both of phy-3-12, calculated values of the volume, length, and the packing
and phy-3-16 surfactants were smaller than those of 12-3-12, parameter for the py-3-n are reported in Table 2. The total
2
-1
and 16-3-16 surfactants (phy-3-12=0.78nm .mol , phy-3-16- volume of the hydrophobic tails is given by (v
1
+ v
However, phy-3-18 was length of the hydrophobic group will be equal to the length of
approximately 1.5 times that of 18-3-18 (phy-3- the longest tail. As seen in Table 2, as well as in the literature,
2
) and the
2
-1
[21]
0.91nm .mol ), respectively.
2
-1 [28]
1
8=1.92nm .mol ) .
aggregates formed by m-3-m gemini surfactants tend to form
The packing parameter,
P
shape of the aggregates formed by the surfactant in aqueous depending upon the alkyl tail length. The replacement of one
, of a surfactant describes the cylindrical micelles with a P value of approximately 0.35
solution, and can be calculated from:
Pꢀꢁꢀv/a ×lꢀ
of the tail group by a pyrenyl ring dramatically impacts the
hydrophobic volume due to the bulkiness of the fused rings
without impacting the overall length of the hydrophobic group
except for the py-3-8 surfactant. This restricts the geometry of
0
[4]
where
v is the hydrophobic volume of a surfactant molecule the system such that cylindrical, lamellar and inverted micelles
(calculated from Tanford’s equations[29]):
are now the predicted favorable geometry.
These different packing values are affected by the head
group area as well. The head group areas of the pyrenyl
surfactants showed no consistency whatsoever in the data
vꢀꢁ0.0274ꢃ0.0269n [5]
and
l is the length of the hydrocarbon tails (also calculated obtained from the surface tension. In equations 5 and 6, n is
from Tanford’s equations[29]):
the number of carbon atoms in the hydrocarbon tails of the
surfactant. The packing parameter for the pyrene-3-n gemini
surfactants is dramatically impacted by the presence of the
bulky pyrenyl ring in the molecule. The aggregates shapes
predicted by equation 4 are cylindrical (pyr-3-14), vesicles (pyr-
lꢁ0.154ꢃ0.1265n [6]
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3
-16), inverted micelles (pyr-3-8), or lamellar (pyr-3-12 and and this implies the spontaneity of the aggregation process,
pyr-3-18). This may be due to the presence of the bulky pyrene also ΔG⁰ increases for the same surfactant at different
ring which adds to the hydrophobic volume, but not to the temperatures, which lead to lower CMC. The higher the
m
[21]
length of the tail. For 12-3-n surfactants, inverted micelles temperature, the lower the CMC with the increased
were formed. This is largely due to the small head group areas hydrophobicity as a result of increased alkyl tail length.
that were caused by tight packing at the air/water interface as However, the process of micellization is entropy driven as
a result of increased intermolecular hydrophobic interactions ΔH⁰
between the alkyl tails. dissymmetrical surfactants as well and this means that the
Pyrenyl and 12-3-n gemini surfactants behave similarly removal of a surfactant tail from the water into the core is
when measuring the Krafft temperatures. However, at higher endothermic. In the dissymmetrical gemini surfactants ΔG⁰
m
is positive throughout the pyrenyl and the
m
temperatures there was a second break point and we believe values were decreasing from 12-3-14 to 12-3-18 at a specific
it indicates a phase transition and the beginning of the temperature, which suggests that the addition of one
formation of higher and more complicated morphologies. The methylene unit to one tail is endothermic and requires more
values of the Krafft temperatures and the transition energy relative to the pyrenyl and to the symmetrical
temperatures of the eight surfactants were obtained from the counterparts gemini surfactants. This might be attributed to
plots of the temperature- specific conductance and are the increase in the disorder of the solution, leading to a
presented in Table 2 and Figure 4.
decrease in ΔG⁰ . This was confirmed by the plot of the
m
The thermodynamic parameters of micelle formation for variation of (ΔG⁰ (CH )) with the degree of dissymmetry m/n,
m
2
the py-3-n and the 12-3-n gemini surfactants were calculated which shows the relationship between the Gibbs free energy
by applying the pseudo-phase separation model for ionic per mole of CH2 (ΔG⁰m (CH )) with the degree of the
2
[
30]
surfactants according to
:
dissymmetry (see Fig.5). In the pyrenyl surfactants, as the m/n
increases, the ΔG⁰ becomes more negative, which favors the
m
∆
G⁰
ΔH⁰
ΔS⁰
m
ꢁꢀ2ꢈ1.5-αꢋRTlnXcmc [7]
aggregation led by the hydrophobic interactions. Values of
|
TΔS⁰ | for all of the asymmetric gemini surfactants in this
m
2
m
ꢁꢀ-RT xꢀδlncmc/δT
[8]
study were higher than those of |ΔH⁰ |, which again suggests
m
that the aggregation process of these surfactants is entropy-
[31]
m
ꢁꢀΔH⁰
m
ꢀ-ꢀ∆G⁰
m
ꢀ/ꢀδT [9]
driven. However, when comparing the dissymmetric gemini
surfactants to the pyrenyl surfactants, we notice that ΔG⁰m of
where α is the degree of micelle ionization, which was 12-3-14>pyr-3-14, 12-3-16>pyr-3-16, and 12-3-18>pyr-3-18 at
obtained from the conductance titrations, and Xcmc is the the same temperature, although the latter`s difference is not
molar fraction at the CMC, Xcmc= CMC/55.4, the fact that 1 L of that significant as those of 12-3-14 and 12-3-16. This suggests
water corresponds to 55.4 mol of water at 25
℃
is responsible that substituting an alkyl tail with hexyl pyrene increases the
entropy and becomes less spontaneous relative to the
-1
for the value of 55.4, CMC is in mol·L .
The thermodynamic parameters of micelle formation are symmetrical and to the 12-3-n gemini surfactants. This is
listed in Table 2 for both the py-3-n and 12-3-n series of probably due to the geometric restraints created by the
surfactants different temperatures. As the solution decrease, aromatic pyrene.
corresponding to ∆G⁰m becoming more negative, and micelle
Pyrene has been widely used as a fluorescent probe to
formation becoming more spontaneous. For the pyrenyl characterize micro-heterogeneous systems, such as;
surfactants, the results show that at a certain temperature, as micelles.[32] This is largely due to the significant sensitivity to
the m/n ratio increases, ΔG⁰m values become more negative the polarity of the solvent being used. Pyrene shows several
characteristic vibronic bands in the region of 370 – 400 nm in
its fluorescence emission spectrum. The absolute and relative
intensities, and the width and position of these bands depend
[33]
highly on the polarity of the microenvironment. Due to the
low solubility of pyrene in pure water, the efficiency of
excimer formation is low at surfactant concentrations below
the CMC. At concentrations greater than the CMC, pyrene
molecules will be crowded into the micelles, resulting in
excimer formation and the appearance of a broad excimer
[
34]
emission band near 500 nm. In this study, for the py-3-m
surfactants, pyrene is part of the structure of the surfactants
molecules themselves. At concentrations below the CMC,
formation of pre-micellar aggregates, known to commonly
occur, particularly for more hydrophobic gemini surfactants,
could result in the appearance of excimer emission. The
emission spectra for the py-3m surfactants (see Fig. 6) in water
K
Fig 4: Krafft point (T ) determination for the 12-3-16 gemini surfactant.
8
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ARTICLE
resolved from a broad peak in the range of 450–500 nm (data
not shown), resulting from excimer formation. A small red shift
for the excimer peak was observed of the py-3-m surfactants
(as compared to pyrene alone) and is most likely the effect of
increased hydrophobicity environment around the pyrene
group caused by the formation of the micelles. In Fig. 6A,
excimer formation can be observed at surfactant
concentrations below the CMC (0.01mM) for all of the
surfactants except pyr-3-12, indicating an evidence for the
formation of premicellar aggregates possibly due to increased
association of the tail groups of adjacent surfactant molecules
through possible pi-pi interactions.
To further explore the nature of the interaction(s) between
the tail groups of the py-3-m surfactants, docking simulations
were carried out. The docked poses (see Figure 7) obtained
were ranked based on CDOCKER energy and CDOCKER
Fig.5 The variation of (∆G⁰
from the conductivity studies for Pyr-3-n (
m
(CH
2
)) with the degree of dissymmetry m/n at 25ºC
), and 12-3-n (●).
-1
■
interaction energies in kcal mol and the type of polar and
nonpolar interactions observed between the surfactant were
analysed. The results of these studies indicate that the
intermolecular pi-pi stacking interactions between pyrene
rings on adjacent py-3-8 surfactant molecules was the most
favoured type of interaction occurring between the molecules
(Figure 7A). It was also observed that intermolecular and pi-
sigma stacking interactions occurred (Figure 7B) and these
interactions actually gave the most stable bimolecular complex
(Fig. 7B). Similar results were obtained for each of the py-3-m
surfactants, although the tendency for self-aggregation within
a single surfactant molecule increases as the alkyl chain length
increases, as in the case of Pyr-3-18, where very strong
intramolecular cation-pi and pi-sigma interactions were
observed (Fig.7C).
Conclusions
The pyrene-based gemini surfactants, pyr-3-n, (n=8, 12, 14, 16,
and 18), and the dissymmetric gemini surfactants, 12-3-n, n =
(
14, 16, 18) were synthesized and their aggregation properties
were characterized. The Krafft temperatures of pyrene-3-n and
2-3-n increased with the increase in the alkyl tail length. This
1
finding following the general observations with all other
gemini surfactants. Pyr-3-n and 12-3-n showed much lower
CMC values than those of their symmetrical counterparts,
[10]
confirming previous findings with pyr-3-12 , and those of m-
[
31]
6
-n reported by Wang et al.. The low CMC values are due to
increased hydrophobicity of the pyrene-3-n surfactants, and
due to the dissymmetry of the 12-3-n gemini surfactants,
which imparts unique properties on the surfactants. The
docking studies showed that the first interaction happening in
Fig.6: Fluorescence spectra of pyrenyl gemini surfactants, a) before the CMC the pyr-3-n surfactants is the pyrene-pyrene interactions
0.01mM). b) Post the CMC (0.5mM). (Pyrene black, pyr-3-8 blue, pyr-3-12 grey,
(
(
through pi-pi stacking), and this may cause the initiation of
pyr-3-14 pink, pyr-3-16 navy, and pyr-3-18 orange), a) before the CMC. b) Post
the CMC. The intensity of the excimer peak in pre-CMC solution is lower than the
the micelle formation starting at a very high surface tension.
intensity of the peak after the CMC, which indicates the presence of the pyrene The pyrene in here is included as a guest molecule into the
with in the hydrophobic micelles.
micelle and this is due to the high hydrophobicity of the
pyrene molecules, driving them to prefer the micelle core.
both below (Fig. 6A) and above (Fig. 6B) the CMC are
Self-aggregation behaviour was also associated with increased
comparable to the spectrum obtained for pyrene in methanol
length of the alkyl tail, which gave unique and diverse
with monomer fluorescence maxima at 390nm that are well
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indicating enhanced intermolecular hydrophobic interaction,
which results in smaller a˳. However, pyrene-3-14 has a
relatively larger head group area. This may be caused by the
dissymmetry effect appearing at this surfactant, because
before that pyr-3-12 is close to symmetry and pyr-3-8 has very
low surface tension due to their high solubility in aqueous
phase. The packing parameter is calculated for the pyr-3-n and
1
2-3-n compounds are indicative of the formation of various
morphologies, especially in the pyr-3-n surfactants. Pyr-3-8
forms inverted micelles, whereas pyr-3-14 forms spherical
micelles, pyr-3-12 and pyr-3-18 form lamellar micelles, and
pyr-3-16 forms vesicles. Both vesicles and inverted micelles
have been linked to better transfection results in the
literature. Pyr-3-12 reacted differently from the rest of the
pyrenyl surfactants and again this can be attributed to its
unique structure, which caused it to have more negative ΔG⁰m
at 25°C than pyr-3-14. All of the dissymmetric gemini
surfactants in this study showed that the micellization process
is entropy driven, which resonates with Wang et al. findings
with m-6-n, and that the dissymmetry and the length of the
tail require more energy contribution in order to initiate the
micellization process.
Acknowledgements
The authors would like to thank The Saudi Cultural Bureau and
the Ministry of higher education for the financial funding
provided as part of the Scholarship program of the late King
Abdullah Bin AbdulAziz Al Saud. SDW is supported by the
Natural Sciences and Engineering Research Council of Canada
(NSERC Discovery Grant #335626-2011). The School of
Pharmacy, University of Waterloo is gratefully acknowledged
for providing access to the core equipment facility used in this
research.
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