Gemini surfactant electrospun membranes

Article abstract of DOI:10.1021/la902287b

Our research demonstrates electrospun nonwoven fibrous scaffolds from a low molar mass gemini ammonium surfactant, N,N'-didodecyl-N,N,N',N'-tetramethyl-N, N'-ethanediyldiammonium dibromide (12-2-12). Cryogenic transmission electron microscopy (cryo-TEM) and solution rheological experiments revealed micellar morphological transitions of 12-2-12 in water and watenmethanol (1:1 vol). The microstructure of 12-2-12 in water transitioned from entangled, cylindrical, threadlike micelles to branched threadlike micelles, and a viscoelastic, entangled, highly branched network of threadlike micelles with increasing concentration finally formed. In sharp contrast, the solution behavior of 12-2-12 in watenmethanol produced a drastically different micellar microstructure compared to that in water, and the morphology transitioned from partitioned, globular micelles to overlapped micelles at an overlap concentration (C*) of 11 wt %. Electrospinning 12-2-12 from water did not produce fibers at any concentration; however, electrospinning 122-12 in watenmethanol at concentrations greater than 2C* produced hydrophilic continuous fibers with diameters ranging from 0.9 to 7 μm. High surface area scaffolds with hydrophilic surfaces offer potential as charged controlledrelease membranes, tissue engineering scaffolds, and coatings for biologically compatible devices.

Full text of DOI:10.1021/la902287b

pubs.acs.org/Langmuir
©
2009 American Chemical Society
Gemini Surfactant Electrospun Membranes
†
‡
‡
†
,†
Matthew P. Cashion, Xiaolin Li, Yan Geng, Matthew T. Hunley, and Timothy E. Long*
†
Department of Chemistry, Macromolecules and Interfaces Institute, Virginia Tech (0212), Blacksburg, Virginia
2
‡
4061, and Department of Chemistry, University of Georgia, 1001 Cedar Street, Athens, Georgia 30602
Received June 25, 2009
Our research demonstrates electrospun nonwoven fibrous scaffolds from a low molar mass gemini ammonium
0
0
0
0
surfactant, N,N -didodecyl-N,N,N ,N -tetramethyl-N,N -ethanediyldiammonium dibromide (12-2-12). Cryogenic
transmission electron microscopy (cryo-TEM) and solution rheological experiments revealed micellar morphological
transitions of 12-2-12 in water and water:methanol (1:1 vol). The microstructure of 12-2-12 in water transitioned from
entangled, cylindrical, threadlike micelles to branched threadlike micelles, and a viscoelastic, entangled, highly branched
network of threadlike micelles with increasing concentration finally formed. In sharp contrast, the solution behavior of
1
2-2-12 in water:methanol produced a drastically different micellar microstructure compared to that in water, and the
morphology transitioned from partitioned, globular micelles to overlapped micelles at an overlap concentration (C*) of
1
2
1 wt %. Electrospinning 12-2-12 from water did not produce fibers at any concentration; however, electrospinning 12-
-12 in water:methanol at concentrations greater than 2C* produced hydrophilic continuous fibers with diameters
ranging from 0.9 to 7 μm. High surface area scaffolds with hydrophilic surfaces offer potential as charged controlled-
release membranes, tissue engineering scaffolds, and coatings for biologically compatible devices.
Introduction
coils in solution, and exhibit contour and persistence lengths of
approximately 1 μm and 10 nm, respectively. The mass-action
7
Electrospinning biologically derived, low molar mass amphi-
philes represents an innovative strategy for biocompatible, high
surface area fibers for tissue engineering, drug delivery, and
law controls the uniaxial growth of WLMs, and as the lipid
volume fraction increases, the persistence length becomes suffi-
ciently large for WLMs to behave as flexible dynamic analogues
1
biomedical membrane technologies. Traditionally, the forma-
8
,9
to polymers in solution. Droplets were formed when asolectin
tion of electrospun fibers requires viscous, entangled high molec-
ular weight polymers in solution or in the melt, since sufficient
viscosities are necessary to withstand electrostatic forces and
whipping instabilities of the charged electrospun jet.
laboratories were the first to electrospin low molar mass phos-
was electrospun at C < C due to the absence of WLM entangle-
e
ments, and beaded fibers were collected when electrospun at C =
2
-5
C . Electrospinning continuous fibers from asolectin solutions
e
Our
required a sufficient solution viscosity in order to suppress the
3
,10
6
Rayleigh instabilities and prevent breakup of the electrified jet.
pholipids based on solutions of asolectin in organic solvents, and
The three-dimensional, entangled asolectin physical networks at
highsurface area membranes with fiber diameters less than2.8 μm
were obtained. However, this initial discovery was limited to
phospholipids in organic solvents, and our challenge remained to
demonstrate the versatility of electrospinning low molar mass
amphiphiles in aqueous solutions.
Asolectin is a soy-derived natural surfactant mixture composed
of charged and neutral lipids, and the morphology of asolectin
transitions from reverse, spherical micelles into elongated, worm-
like micelles (WLMs) with increasing concentration in nonpolar
solvents. Asolectin WLMs form entanglements above the entan-
C > C provided the viscosity and physical associations necessary
e
to form continuous electrospun fibers from low molar mass lipids.
Electrospinning asolectin scaffolds proved that entangled high
molecular weight polymers were not essential to the formation of
electrospun fibers; however, some degree of supramolecular
association was needed to form continuous fibers from low molar
mass precursors.
In this study, we report the aqueous solution behavior and
0
electrospinning of an ammonium gemini surfactant, N,N -dido-
0
0
0
0
0
decyl-N,N,N ,N -tetramethyl-N,N -ethanediyldiammonium di-
bromide (12-2-12), as shown in Figure 1. Gemini surfactants,
which are also known as dimeric surfactants, are an interesting
glement concentration, C . In chloroform/N ,N -dimethylforma-
e
mide (70/30 by weight), the C was 35 wt % for asolectin, and the
e
scaling relationship of specific viscosity (η ) with concentration
sp
2
.4
8.4
class of amphiphiles due to their low critical micelle concentration
transitioned from η ∼ C at C < C to η ∼ C at C > C . Aso-
sp
e
sp
e
11-15
(
cmc) and intriguing supramolecular assemblies in solution.
lectin WLMs are known to entangle above C , similar to polymer
e
*
(
Corresponding author. E-mail: telong@vt.edu.
1) Singh, G.; Bittner, A. M.; Loscher, S.; Malinowski, N.; Kern, K. Adv. Mater.
008, 20(12), 2332–2336.
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(
(
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810.
(
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6
78 DOI: 10.1021/la902287b
Published on Web 08/14/2009
Langmuir 2010, 26(2), 678–683

Cashion et al.
Article
investigation correlates solution rheology with cryo-TEM for 12-
2-12 in water and water:methanol in order to understand the
electrospinning performance of low molar mass surfactants for
functional amphiphilic membranes.
Materials and Methods
0
0
0
0
Figure 1. N,N -Didodecyl-N,N,N ,N -tetramethyl-N,N -ethane-
diyldiammonium dibromide (12-2-12) gemini surfactant.
Materials. 1-Bromododecane (97%), 1,2-bis(dimethylamino)-
ethane (99%), ethanol (ACS grade anhydrous), ethyl acetate
(
HPLC grade), ethyl ether (anhydrous), methanol (HPLC grade),
Gemini surfactants contain two hydrophobic tails and two
hydrophilic headgroups covalently connected with a spacer.
Synthetic gemini surfactants include cationic, anionic, zwitterio-
nic, and nonionic functionalities, depending on the incorporated
and cetyltrimethylammonium bromide (CTAB) (>98%) were
purchased from Sigma-Aldrich Chemical Co. 1,2-Bis(dimethyl-
amino)ethane and 1-bromododecane were passed through a silica
2
gel column prior to use, and ethanol was distilled from CaH under
1
2
spacer. Consequently, gemini surfactants have received signifi-
cant industrial attention as emulsifiers and dispersants in deter-
gents, cosmetics, personal hygiene products, coatings, and paint
reduced pressure. Aqueous solutions were prepared using de-
ionized (DI) water. All reactions were performed in flamed glass-
ware equipped with a magnetic stir bar under argon pressure
(6-8 psi), unless otherwise noted.
1
6-18
formulations.
Gemini surfactants have also received more
1
13
recent interest as templates for the synthesis of metal
Instrumentation. H and C NMR spectroscopy were per-
1
9-21
nanoparticles
and as novel gene transfection agents due to
formed on a Varian Unity 400 spectrometer at 400 MHz in
their superior surface-active properties and DNA binding
capabilites.
3
CDCl . FAB mass spectrometry was performed on a JEOL
2
2-24
HX110 dual focusing mass spectrometer. Strain-controlled solu-
tion rheology was performed on a TA Instruments AR-G2
rheometer using a concentric cylinder geometry at 25 °C. Steady
The propensity for supramolecular assembly and WLM growth
of gemini surfactants strongly depends on surfactant composition,
2
5,26
shear ramp experiments were performed at shear rates between
solution temperature, concentration, and solvent.
Zana, Tal-
-1
0
.1 and 1000 s . The specific viscosity (η ) was calculated as
sp
mon, and Danino explored the influence of spacer group on the
self-assembly of quaternary ammonium gemini surfactants, and
the resulting aqueous microstructures were examined using cryo-
follows: ηsp = (η
the apparent viscosity determined from the Newtonian zero shear
region.
0
s s s 0
- η )/η , where η is the solvent viscosity and η
27,28
genic transmission electron microscopy (cryo-TEM).
Ammo-
Electrospun fibers were analyzed using a Leo 1550 field emis-
sion scanning electron microscope (FESEM). Electrospun fibers
nium gemini surfactants with shorter spacer lengths, e.g. two or
three methylene groups formed entangled threadlike micelles at
exceptionally low concentrations, while gemini surfactants with a
spacer length of four methylene groups formed spheroidal mi-
celles. The earlier literature has revealed that short spacer lengths
covalently restrain the charged ammonium groups at a closer
proximity than electrostatic repulsive forces allow for monomeric
1
were collected on a /
1
4
in. ꢀ /
4
in. stainless steel mesh and adhered
to a SEM disk. The mounted fibers were sputter-coated with an 8
nm Pt/Au layer to reduce electron charging. Surface tension
measurements were performed using a Kruss (Hamburg,
Germany) DSA100. Each pendant drop was created manually
and the surface tension measured and recorded, and the drop
image was also recorded digitally. The measured surface tension
of DI water was 70 mN/m, and the surface tension of water:
methanol (1:1 vol) was 38 mN/m. An Oakton (Vernon Hills, IL)
Con 6 conductivity meter was used to measure conductance and
temperature of the surfactant solutions. The conductance of DI
water was 4.8 μS/cm, and water:methanol (1:1 vol) was 9.8 μS/cm
at 25 °C.
2
8
surfactants. Therefore, ammonium gemini surfactants with
short spacers efficiently pack in cylindrical geometries due to a
higher packing parameter and lower spontaneous curvature.
The supramolecular microstructure of 12-2-12 in water and
water:methanol (1:1 vol) was probed with solution rheology and
cryo-TEM, and the electrospinning capabilities were investigated.
While others have examined the solution behavior and micro-
0
0
0
Synthesis of N,N -Didodecyl-N,N,N ,N -tetramethyl-N,
0
1
3,26,29-31
N -ethanediyldiammonium Dibromide (12-2-12). Zana and
structure of ammonium gemini surfactants in water,
our
2
6
co-workers pioneered the synthesis of 12-2-12, and our proce-
dure is an adaptation of this earlier procedure. 1,2-Bis(dimethyl-
amino)ethane (0.052 mol, 6.09 g) and 1-bromodocecane (0.109
mol, 27.1 g) were dissolved in ethanol (0.551 mol, 32.2 mL) in a
250 mL round-bottomed flask. A condenser was connected to the
reaction flask, and the flask was placed in an 80 °C bath for 48 h.
The reaction mixture was precipitated into ethyl acetate, and
a white solid was collected, redissolved in chloroform, and
reprecipitated into ethyl ether. The white solid product was
isolated and dried at 40 °C under reduced pressure. The
(
16) Xia, J.; Zana, R. Applications of Gemini Surfactants. In Gemini Surfac-
tants: Synthesis, Interfacial and Solution-Phase Behavior, and Applications; Zana,
R., Xia, J., Eds.; Marcel Dekker: New York, 2004; pp 301-321.
(
(
17) Chen, L.; Xie, H.; Li, Y.; Yu, W. Colloids Surf., A 2008, 330(2-3), 176–179.
18) Zhou, M.; Nemade, P. R.; Lu, X.; Zeng, X.; Hatakeyama, E. S.; Noble, R.
D.; Gin, D. L. J. Am. Chem. Soc. 2007, 129(31), 9574–9575.
19) Bakshi, M. S.; Sharma, P.; Banipal, T. S. Mater. Lett. 2007, 61(28), 5004–
009.
(
5
2
1
(
20) Lu, T.; Huang, J.; Li, Z.; Jia, S.; Fu, H. J. Phys. Chem. B 2008, 112(10),
909–2914.
(
12(22), 8259–8265.
(
21) Bakshi, M. S.; Possmayer, F.; Petersen, N. O. J. Phys. Chem. C 2008,
1
13
product was characterized with H NMR, C NMR, and
1
22) Kirby, A. J.; et al. Angew. Chem., Int. Ed. 2003, 42(13), 1448–1457.
23) Luciani, P.; Bombelli, C.; Colone, M.; Giansanti, L.; Ryhanen Samppa, J.;
3
mass spectrometry. H NMR (CDCl ): δ (ppm) 4.78 (4H,
þ
þ
(
N -CH -CH -N ), 3.71 (4H, 2(N-CH -CH )), 3.53 (12H,
2
2
2
2
Saily, V. M. J.; Mancini, G.; Kinnunen Paavo, K. J. Biomacromolecules 2007, 8(6),
999–2003.
24) Vongsetskul, T.; Taylor, D. J. F.; Zhang, J.; Li, P. X.; Thomas, R. K.;
Penfold, J. Langmuir 2009, ACS ASAP.
25) Rosen, M. J. Surfactants and Interfacial Phenomena, 3rd ed.; Wiley-Inter-
science: Hoboken, NJ, 2004.
þ
þ
2
1
((CH ) -N )), 1.85 (4H, 2(N -CH -CH -CH )), 1.20-
.45 (36H, 2(N -CH
2((CH
2(N -CH -CH )), 59.1 (2C, N -CH -CH -N ), 52.0 (4C,
((CH
15.0 (2C, 2((CH
MS MW 454.2 g/mol.
3
2
2
2
2
1
þ
3
2
-CH
2
-(CH
2
)
9
-CH
3
)), 0.88 (6H,
(
1
)). C NMR (CDCl
2
)
9
-CH
3
3
): δ (ppm) 64.0 (2C,
2
þ
þ
þ
(
2
2
2
þ
þ
2
3
)
2
-N )), 20.0-32.0 (20C, 2(N -CH
2
-(CH
2
)
3
10-CH )),
(
(
(
(
(
26) Zana, R.; Benrraou, M.; Rueff, R. Langmuir 1991, 7(6), 1072–1075.
27) Zana, R.; Talmon, Y. Nature 1993, 362(6417), 228–230.
2
)
9
-CH )). Calculated MW 454.6 g/mol, FAB
3
28) Danino, D.; Talmon, Y.; Zana, R. Langmuir 1995, 11(5), 1448–1456.
29) Han, L.; Chen, H.; Luo, P. Surf. Sci. 2004, 564(1-3), 141–148.
30) Weber, V.; Narayanan, T.; Mendes, E.; Schosseler, F. Langmuir 2003,
Cryo-TEM of 12-2-12 Solutions. The 12-2-12 solutions
were examined on a Tecnai 20 microscope, operated at 200 kV
using a Gatan cryo-holder. A small droplet of the suspension
(5-10 μL) was placed on a carbon film supported on a TEM
1
9(4), 992–1000.
31) Kern, F.; Lequeux, F.; Zana, R.; Candau, S. J. Langmuir 1994, 10(6), 1714–
723.
(
1
Langmuir 2010, 26(2), 678–683
DOI: 10.1021/la902287b 679

Article
Cashion et al.
copper grid. Following the plunge-freezing technique for sample
preparation, the suspension on the TEM grid was blotted and
immersed into a liquid ethane reservoir cooled with liquid nitro-
3
2
gen. The vitrified solutions were transferred to a Gatan 626
cryo-holder and cryo-transfer stage cooled with liquid nitrogen.
During observation of the vitrified samples, the cryo-holder
temperature was maintained below -175 °C to prevent sublima-
tion of vitreous water. The images were recorded digitally using a
Gatan low-dose charge-coupled device (CCD) camera with soft-
ware package.
Electrospinning Process. Electrospinning of 12-2-12 from
water did not produce fibers at any surfactant concentration.
Electrospun fibers of 12-2-12 were collected from water:methanol
(
1:1 vol) solutions following our earlier electrospinning
2
,6,33-38
Figure 2. Cartoon of gemini surfactant entangled threadlike mi-
celles in a polar solvent.
procedure.
various concentrations. The solutions were transferred into a
0 mL syringe with an 18-gauge needle, and the syringe was
12-2-12 was dissolved in water:methanol at
2
mounted in a KD Scientific Inc. syringe pump. The positive lead
of a Spellman High Voltage Electronics Corp. CZE1000R high-
voltage power supply was connected to the syringe needle, and the
stainless steel collection target was grounded and placed 12 cm
from the needle tip. The fluid was pumped at a constant flow of
5
mL/h, and 25 kV of voltage was applied.
Results and Discussion
1
2-2-12 Solution Microstructure in Water. The cmc, sur-
face tension, and interfacial properties of quaternary ammonium
2
6
gemini surfactants were previously reported. In polar solvents,
the hydrophilic regions of gemini surfactants orient toward the
polar environment, with the headgroups located at the interface
of the hydrophobic tails and polar solvent. Surfactant spherical
micelles transition one-dimensionally into threadlike micelles
with increasing concentration, and entanglements form, as shown
Figure 3.
ηsp vs concentration for 12-2-12 in water at 25 °C. The
surfactant microstructure transitioned from linear threadlike
micelles to branched threadlike micelles at 1.5 wt % and into a
highly branched entangled network at 2.7 wt %.
micelles with a low concentration of branch points, rings, and
end-caps at 1.0 wt %, and a branched threadlike micellar network
at 1.5 wt %.
2
8,39,40
in the cartoon in Figure 2.
On the basis of a theoretical
3
1
model of micellar growth, Kern and co-workers reported the
possibility for three surfactant concentration regimes: (1) a dilute
regime of slow increase in micellar size with increasing concentra-
tion, (2) a semidilute regime of rapid micellar growth, and (3) a
concentrated regime where the aggregation number depends on
the net charge of the end-caps. The transition from the dilute
regime to the semidilute regime occurs at the overlap concentra-
tion, C*. The rate of micellar growth at C* increases due to
higher concentrations of bound ions screening the electrostatic
interactions between headgroups, and the value ofC* decreases as
Strain controlled solution rheology provided the zero shear
viscosityfor12-2-12inwateratarangeofconcentrations. Figure3
summarizes the relationship between ηsp and concentration for
1
2-2-12 in water, where ηsp was calculated from the apparent
33
viscosity in the Newtonian zero shear region. The solution
rheological scaling behavior was correlated to cryo-TEM micro-
graphs as shown in Figure 4, and the microstructural changes
were consistent with the observed solution rheological scaling
behavior. Although rheology is a dynamic experiment and cryo-
TEM is static, the visualization of surfactant solutions using cryo-
TEM provides an acceptable method to correlate micellar micro-
8
2
40,41
the end-cap energy, E , increases, C* ∼ 1/E .
Kern and co-
c
c
workers observed a C* of 1.55 wt % for 12-2-12 in water at 25 °C;
however, the concentrated regime was not easily distinguished in
their plot of zero shear viscosity vs volume fraction. Bernheim-
Groswasser and co-workers used cryo-TEM to explore the
sphere-to-cylinder transition of 12-2-12 in water at concentrations
up to 1.5 wt %. The researchers observed elongated threadlike
42
structure to rheology. Gonz ꢀa lez and Kaler published a review
describing the complementary characterization of cryo-TEM and
solution rheology.
The first transition at 1.5 wt % in Figure 3 represents C*, and
3
9
this value agreed with the transition that others reported for 12-2-
31,43
1
2 in water at 25 °C.
The scaling relationship above C*
1
.7
4.8
increased from η ∼ C at C < C* to ηsp ∼ C at C > C*, indi-
sp
(
32) Cui, H.; Hodgdon, T. K.; Kaler, E. W.; Abezgauz, L.; Danino, D.;
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006, 39(2), 575–583.
34) Hunley, M. T.; Harber, A.; Orlicki, J. A.; Rawlett, A. M.; Long, T. E.
Langmuir 2008, 24(3), 654–657.
catinga much higher rate of micellar growth. Figure4adepicts the
cryo-TEM image of 12-2-12 in water at 1.0 wt % and indicates the
presence of entangled, linear, threadlike micelles with long con-
tour lengths and a low concentration of end-caps. The classical
definition of C* for polymers, which is the concentration when
polymeric coils begin to overlap, is ambiguous for the discussion
of noncovalent supramolecular assembly of 12-2-12. Clearly,
entanglements are present between the threadlike micelles below
(
2
(
(
35) Hunley, M. T.; Long, T. E. Polym. Int. 2008, 57(3), 385–389.
(
36) Hunley, M. T.; McKee, M. G.; Long, T. E. J. Mater. Chem. 2007, 17(7),
6
05–608.
(
37) McKee, M. G.; Elkins, C. L.; Long, T. E. Polymer 2004, 45(26), 8705–8715.
38) McKee, M. G.; Park, T.; Unal, S.; Yilgor, I.; Long, T. E. Polymer 2005, 46
(
(
7), 2011–2015.
39) Bernheim-Groswasser, A.; Zana, R.; Talmon, Y. J. Phys. Chem. B 2000,
04(17), 4005–4009.
40) Oda, R.; Huc, I.; Homo, J.-C.; Heinrich, B.; Schmutz, M.; Candau, S.
Langmuir 1999, 15(7), 2384–2390.
41) MacKintosh, F. C.; Safran, S. A.; Pincus, P. A. Europhys. Lett. 1990, 12(8),
97–702.
1.5 wt %, as shown in Figure 4a, and the size of the elongated
(
1
(
(42) Gonzalez, Y. I.; Kaler, E. W. Curr. Opin. Colloid Interface Sci. 2005,
10(5,6), 256–260.
(43) Oelschlaeger, C.; Waton, G.; Candau, S. J.; Cates, M. E. Langmuir 2002,
18(20), 7265–7271.
(
6
6
80 DOI: 10.1021/la902287b
Langmuir 2010, 26(2), 678–683

Cashion et al.
Article
Figure 4. Cryo-TEM micrographs of 12-2-12 in water: (a) 1.0 wt %, linear, entangled, threadlike micelles, (b) 2.4 wt %, branched threadlike
micelles, (c) 4.1 wt %, highly branched, entangled, threadlike micelles.
micelles are concentration dependent. The abrupt change in
scaling behavior at C* is indicative of the transition from the
dilute to the semidilute concentration regime, which corresponds
to the regime of increased micellar growth.
Figure 4b displays the cryo-TEM micrograph of 12-2-12 in
water at 2.4 wt %, C > C*, and the presence of “Y”-shaped
branch points are distinguished between the threadlike micelles.
If the junctions in Figure 4b were entanglements, then the contrast
would darken from the increased electron density of the over-
lapping threadlike micelles; however, the contrast does not
4
2
darken, and the junctions are considered branch points. The
second transition in Figure 3 at 2.7 wt % represents the transition
into the concentrated regime, C**, and the scaling relationship
increased from ηsp ∼ C at C* < C < C** to ηsp ∼ C at C >
C**. The cryo-TEM in Figure 4c displays the formation of a
highly branched network of entangled, threadlike micelles at
Figure 5.
ηsp vs concentration for 12-2-12 in water:methanol at
25 °C. Individual globular micelles initiate overlap at a surfactant
concentration of 11 wt %.
4
.8
14.0
4
.1 wt % of 12-2-12 in water. The microstructure in Figure 4c is
more difficult to distinguish due to the higher concentration and
entangled threadlike micelles. The concentration of cross-links
at C** increased, and the microstructure transitioned from a
branched structure to an entangled highly branched network with
a higher concentration of threadlike micelles.
This article reports the concentrated regime of 12-2-12 in water
and depicts the highly branched network of threadlike micelles
4
4
with cryo-TEM. Drye and Cates theoretically explored the
network formation of “living” cross-linked surfactant solutions.
As expected, the degree of branching was dependent upon the free
energy relationship between forming a cross-link and forming an
end-cap. The cross-linking energy was considerably higher for
conventional surfactants than the energy required to form a
hemispherical end-cap. However, the researchers theorized that
Figure 6. Cryo-TEM micrographs of 12-2-12 in water:methanol:
(a) 10 wt %, individual globular micelles, (b) 12 wt %, overlapped
micelles.
12-2-12 Solution Microstructure in Water:Methanol. The
solution behavior of 12-2-12 in water:methanol was explored to
compare the influence of a solvent mixture to water on the
microstructure and electrospinning behavior of 12-2-12. Metha-
nol has a lower surface tension than water and higher volatility,
8
,45
lowering the micelle curvature with the addition of salt
provided a strategy to increase the E and decrease the cross-link
c
energy. The 12-2-12 gemini surfactant did not require additional
electrolyte to decrease the mean curvature of the growing thread-
like micelles. The covalent linkage between the ammonium
groups maintained the headgroups at a closer proximity than
electrostatic repulsive forces allow for conventional ammonium
surfactants. The gemini microstructure grew uniaxially into
elongated, threadlike micelles that formed entanglements as the
4
7
which is beneficial for electrospinning fibers. The solution
behavior of 12-2-12 in water:methanol demonstrated drastically dif-
ferent microstructures compared to that in water. The ηsp depen-
dence on concentration for 12-2-12 in water:methanol is depicted in
Figure 5, and only two concentration regimes were observed.
The micellar morphology transitioned from partitioned globular
micelles into overlapped micelles at C* = 11 wt %. Figure 6a
is a cryo-TEM micrograph of 12-2-12 in water:methanol at
1
2-2-12 concentration increased. The short covalent linkage
between the ammonium sites decreased the area of the gemini
headgroup, thereby lowering the micellar spontaneous curvature
and allowing the surfactants to efficiently pack into a cylindrical
geometry. As the number of linear, threadlike micelles increased,
the concentration of cylindrical end-caps increased and branch
10 wt %, C < C*. Individual irregular globular micelles are
distinguished below C* with an average diameter of 100 nm. The
micelles fused together from the overlapping of individual micelles at
C >C*, as shown for 12 wt % in Figure 6b. Cryo-TEM micro-
graphs at concentrations 2C* and 4C* were indistinct due to
the high concentration of surfactant microstructures; nonetheless,
4
6
points formed between the threadlike micelles to minimize Ec.
(
(
44) Drye, T. J.; Cates, M. E. J. Chem. Phys. 1992, 96(2), 1367–1375.
45) Candau, S. J.; Khatory, A.; Lequeux, F.; Kern, F. J. Phys. IV 1993, 3(C1),
1
97–209.
(47) Nagarajan, R.; Drew, C.; Mello, C. M. J. Phys. Chem. C 2007, 111(44),
16105–16108.
(
46) Lin, Z. Langmuir 1996, 12(7), 1729–1737.
Langmuir 2010, 26(2), 678–683
DOI: 10.1021/la902287b 681

Article
Cashion et al.
low-resolution threadlike micelles were distinguished at 2C*. Low
resolution was expected for highly concentrated samples because
cryo-TEM is more suitable for low concentration solutions where
3
2
individual microstructures are more clearly observed.
The solution rheology of CTAB in water:methanol was also
explored for comparative purposes, and the C* was measured at
1
9 wt %. The lower C* concentration for 12-2-12 (C* = 11 wt %)
in water:methanol compared to CTAB resulted from the in-
creased surface activity of gemini surfactants. The micellization
of surfactants in a polar environment involves two competing
forces: (1) the interaction of the hydrophobic chains with
the polar solvent, which drives the formation of micelles, and
(
2) the electrostatic repulsive forces between the charged head-
4
8
groups, which opposes micellization. The dual hydrocarbon
tails of gemini surfactants provide a larger thermodynamic
contribution for micellization compared to single tail monomeric
1
1
surfactants. Also, gemini surfactants have a smaller headgroup
area contribution because the gemini charged groups are cova-
lently confined at a closer proximity than electrostatic repulsive
forces allow for single head conventional surfactants like
2
8
CTAB.
The power law scaling relationships in water:methanol were
very similar for both 12-2-12 and CTAB and were dramatically
lower than observed for the surfactants in water. In water:
methanol, the concentration scaling relationships below and
above C* were 1.5 and 2.4 for 12-2-12 and 1.5 and 2.7 for CTAB,
respectively. The decreased scaling relationship in water:metha-
nol compared to water (1.7 and 4.8 for 12-2-12) was attributed to
the ability of methanol to more efficiently bridge the interface of
Figure 7. FESEM of electrospun fibers of 12-2-12, electro-
spun from (a) 20, (b) 22, (c) 28, (d) 30, (e) 42, and (f) 44 wt %
1
1
2-2-12 gemini surfactant solutions in water:methanol (C* =
1 wt %).
flow rate, and 12 cm tip-to-screen (TTS) distance. Figure 7 shows
the SEM images of 12-2-12 fibers electrospun from water:metha-
nol. Droplets formed at 20 wt % (Figure 7a) as a result of the low
solution viscosity destabilizing the electrified jet. When the con-
centration was increased to 2C*, beaded fibers formed as shown
in Figure 7b at 22 wt %. Figure 7c displays the formation of
continuous fibers with a low concentration of large beads present
and fiber diameters ranging from 0.9 to 7 μm when electrospin-
ning from 28 wt %. Although fibers were continuous without
beaded defects at 30 wt %, the fiber morphology was irregular
with diameters ranging from 1 to 6 μm, as shown in Figure 7d.
Continuous fibers with average fiber diameters of 4 and 5 μm, as
shown in Figures 7e and 7f, were electrospun from 42 and 44 wt %
solutions, respectively. The threadlike micelle entanglements
stabilized the electrospinning jet and resulted in the transition
from beads to continuous fibers at concentrations greater than
2C*. The 12-2-12 electrospun membranes were completely solu-
4
9
the surfactant hydrophilic and hydrophobic domains. This
increased the effective headgroup size and decreased the packing
parameter, which lead to the formation of globular micelles as
5
0
shown in Figure 6. The larger scaling factors observed for 12-2-
2 in water resulted from the optimal volume balance of the head
1
and tail areas of the gemini surfactant, which resulted in the
formation of flexible threadlike micelles that were sufficiently
long to entangle and form branch points, thus increasing the
solution viscosity. In contrast, the globular micellar structure in
water:methanol did not permit the same degree of intermolecular
associations, and lower viscosities and scaling relationships were
observed compared to pure water solutions.
Electrospinning Gemini Surfactants. High molecular
weight polymers are electrospun from viscous solutions at con-
1
centrations typically above C or from the melt phase, and chain
ble, and H NMR spectroscopy of the gemini surfactant mats
e
entanglements for nonassociating polymers are necessary to
withstand the Raleigh instabilities and stabilize the whipping
confirmed that the electric field did not alter the chemical
structure of the gemini surfactant.
2
,3,5,51
electrified jet to produce continuous fibers.
The supramo-
Asolectin was electrospun from CHCl /DMF (70/30 by
3
lecular microstructure of 12-2-12 in water and water:methanol
lead to highly viscous solutions, which prompted us to investigate
the possibility for electrospinning. In addition, threadlike micelles
were possible for 12-2-12, and our earlier work described the
weight) where the lipid microstructure formed reverse cylindrical
micelles. Electrospun beads of asolectin were collected at con-
6
centrations below C and beaded fibers at C = C . Continuous
e
e
electrospun fibers of asolectin were collected at C g 1.2C , and the
e
6
necessity for entanglements of these WLMs for electrospinning.
fiber diameter increased with increasing concentration. Electro-
spun fibers of 12-2-12, as depicted in Figure 7, displayed a higher
surface roughness than asolectin electrospun fibers. The 12-2-12
fiber diameters also increased with concentration due to the
higher degree of micellar growth and entanglements; however,
smooth fibers were not observed at any concentration. The
irregularities in the 12-2-12 fibers were presumed to result from
the large applied voltage. Asolectin fibers were electrospun from
The electrospinning studies were conducted at the same tempera-
ture and solvent conditions as the solution rheology experiments
in order to ensure similar surfactant microstructures. Moreover,
all 12-2-12 solutions were electrospun at 25 kV, 5 mL/h syringe
(
48) Evans, D. F.; Wennerstrom, H. Solutes and Solvents, Self-Assembly of
Amphiphiles. In The Colloidal Doman Where Physics, Chemistry, Biology, and
Technology Meet, 2nd ed.; John Wiley & Sons: New York, 1999; pp 1-43.
6
an applied voltage of 15 kV; however, 12-2-12 required a voltage
(
49) Anderson, M. T.; Martin, J. E.; Odinek, J. G.; Newcomer, P. P. Chem.
5
2
Mater. 1998, 10(1), 311–321.
50) Fontell, K.; Khan, A.; Lindstroem, B.; Maciejewska, D.; Puang-Ngern, S.
Colloid Polym. Sci. 1991, 269(7), 727–742.
51) Huang, Z.-M.; Zhang, Y.-Z.; Kotaki, M.; Ramakrishna, S. Compos. Sci.
Technol. 2003, 63, 2223–2253.
of 25 kV to produce fibers. Dietzel and co-workers correlated
(
(
(52) Deitzel, J. M.; Kleinmeyer, J.; Harris, D.; Tan, N. C. B. Polymer 2001,
42(1), 261–272.
6
82 DOI: 10.1021/la902287b
Langmuir 2010, 26(2), 678–683

Cashion et al.
Article
Table 1. Surface Tension and Conductivity Comparison for 12-2-12 Solutions in Water:Methanol and Water with Comparable Zero Shear
Viscosities at 25 °C
12-2-12 solution concentration
zero shear η
0
(Pa s)
surface tension (mN/m)
conductance (mS/cm)
electrospun fibers
4
2
2 wt % water:methanol
.8 wt %water
0.0353
0.0348
26
28
43
8
yes
no
electrospun fiber morphology to applied voltage and found that
increasing voltage changed the electrospinning droplet and re-
sulted in a higher concentration of fiber defects.
Conclusions
The micellar morphological transitions of 12-2-12 in water
and water:methanol were characterized with cryo-TEM and
solution rheology. The 12-2-12 microstructure in water transi-
tioned from linear, entangled, threadlike micelles to branched
threadlike micelles at 1.5 wt %, and at 2.7 wt % the micro-
structure evolved into a viscoelastic, entangled, highly branched
threadlike network. In water:methanol, 12-2-12 produced a
considerably different micellar microstructure compared
to water, and the micellar morphology transitioned from
partitioned globular micelles into overlapped micelles at C*,
Electrospinning efforts of 12-2-12 in water did not produce
5
3
surfactant fibers at any concentration. Greiner and Wendorff
published an electrospinning review and discussed recent electro-
spinning efforts from aqueous solutions. The electrospinning
droplet forms a Taylor cone in the presence of an applied voltage
and a jet is ejected from the Taylor cone if the applied voltage is
5
4
strong enough to overcome the surface tension of the droplet.
The solution viscosity, surface tension, and conductivity are key
5
1,53,54
solution parameters in forming electrospun fibers.
Electro-
11 wt %.
spinning from aqueous solutions requires a reduction in the
surface tension of water, which is typically accomplished with
The supramolecular microstructures of 12-2-12 in water and
water:methanol lead to highly viscous solutions, and their
electrospinning was explored. Electrospinning 12-2-12 in water
did not produce surfactant fibers at any concentration. However,
for the first time nonwoven fibrous scaffolds were electrospun
from gemini surfactants, with fiber diameters between 0.9
and 7 μm from water:methanol. The 12-2-12 fiber morphology
transitioned from beaded fibers into continuous fibers at
concentrations greater than 2C* in water:methanol. The
entangled supramolecular structure of the low molar mass
surfactant stabilized the electrospinning jet and resulted in con-
tinuous fibers. Electrospinning gemini surfactant fibers from
polar solvents represents an innovative strategy to develop high
surface area scaffolds or membranes with charged hydrophilic
surfaces for the development of charged controlled-release mem-
branes, tissue engineering scaffolds, or for biologically compatible
coatings.
4
7
10,51
the addition of surfactants or cosolvents.
In an effort to
understand why 12-2-12 did not produce electrospun fibers from
water, the solution conductivity and surface tension for 12-2-12 in
water and water:methanol were explored for solutions with com-
parable zero shear viscosities at 25 °C and are shown in Table 1.
Electrospun fibers were collected from 42 wt % 12-2-12 water:
methanol solutions, which had a surface tension of 26 mN/m
and conductivity of 43 mS/cm. Solutions of 2.8 wt % 12-2-12 in
water did not produce electrospun fibers, and the solution
had a surface tension of 28 mN/m and lower conductivity
of 8 mS/cm. The surface tensions and solution conductivities of
1
2-2-12 in water and water:methanol were within an acceptable
4
7
electrospinning range; however, the 42 wt % water:methanol
solution had a higher conductivity due to the much higher
concentration of surfactant. Electrospinning from water was
not successful potentially due to the low concentration of
1
2-2-12 in water. In water, 12-2-12 formed very viscous solutions
Acknowledgment. We acknowledge Dr. Eugene G. Joseph
of the Department of Chemical Engineering at Virginia Tech
for the surface tension measurements and helpful discuss-
ions. This material is based upon work supported in part
by the U.S. Army Research Office under Grant W911NF-
at very low concentrations. Electrospinning requires a jet to eject a
continuousfiber fromthe spinneret droplet and the low surfactant
concentration in water may not have facilitated the formation of
continuous fibers.
0
7-1-0452 Ionic Liquids in Electro-Active Devices (ILEAD)
(
(
53) Greiner, A.; Wendorff, J. H. Angew. Chem., Int. Ed. 2007, 46, 5670–5703.
54) Burger, C.; Hsiao, B. S.; Chu, B. Annu. Rev. Mater. Res. 2006, 36, 333–368.
MURI.
Langmuir 2010, 26(2), 678–683
DOI: 10.1021/la902287b 683