Spacer Chain Length in Dimeric Micellar Organization
J. Phys. Chem., Vol. 100, No. 28, 1996 11665
SCHEME 1: a: C16H33NMe2 (3.0 equiv), Dry EtOH,
Reflux, 48 h; 70-90% Yield
1.24-1.40 (br m, 52 H, alkyl chain 2 × 13 CH2), 1.75 (br s, 4
H, alkyl chain 2 × 1 CH2CH2N+), 2.64 (br s, 2 H, spacer chain
1 × 1 CH2CH2N+), 3.36 (s, 12 H, 2 × 2 N+CH3), 3.46 (m, 4
H, alkyl chain 2 × 1 CH2N+), 3.77 (m, 4 H, spacer chain 2 ×
1 CH2N+). C,H,N analysis, Calcd. for C39H84N2Br2,2.0H2O:
C 60.29, H 11.42, N 3.60. Found C 60.02, H 11.38, N 3.33.
Bis(hexadecyldimethylammonium)butane (2b). 1H NMR (200
MHz, CDCl3) δ 0.88 (t, 6 H, alkyl chain 2 × 1 CH3), 1.25-
1.40 (br m, 44 H, alkyl chain 2 × 11 CH2), 1.70-2.00 (m, 12
H, alkyl chain 2 × 3 CH2), 2.20 (br s, 4 H, spacer chain 1 × 2
CH2CH2N+), 3.30 (s, 12 H, 2 × 2 N+CH3), 3.40-3.50 (m, 4
H, alkyl chain 2 × 1 CH2N+), 4.00 (br s, 4 H, spacer chain 2
a
Br(CH2)mBr
n-C16H33N+Me2(CH2)mN+Me2n-C16H33
Br– Br–
2a–g, m = 3, 4, 5, 6, 8, 10, 12
m = 3, 4, 5, 6, 8, 10, 12
Although there is lot of current interest in the dimeric micellar
system, much less is known about their micelle structure at
ambient temperatures. Different surfactant systems have been
earlier examined by neutron scattering.12-14 Neutron scattering
has been extensively used also for the examination of different
membrane structures.15 Recently, a report describing the small
angle neutron scattering spectra of a different dimeric surfactant
system, 10-m-10,2Br-, i.e., [C10H21N+Me2-(CH2)mN+Me2C10H21,
2Br-] has appeared in the literature.16 The present work
describes SANS spectra of dimeric micelles composed of 16-
m-16,2Br- surfactants 2, where, m ) 3, 4, 5, 6, 8, 10, and 12.
This chain length is equal to that present in the well-known
monomeric cationic surfactant, cetyltrimethylammonium bro-
mide (1). Since the details of the micellar properties of 1 is
well documented in literature, the present study allows us to
compare these results directly with that of 1. To understand
the role of flexible spacer chain such as length and to gain
adequate insight into their role in determining the microstruc-
tures of dimeric surfactant micelles, we have performed SANS
experiments employing gemini micelles of different spacer chain
length. The effect on SANS spectra upon variation of concen-
tration with dimeric surfactant of specific m value and the effect
of temperatures on the neutron cross sections was also examined.
To combine the information available from SANS studies with
other micellar properties, the effects of m values on the critical
micellar concentrations and microviscosities were also studied
with 16-m-16,2Br- surfactants, 2.
×
1
CH2N+). C,H,N analysis, Calcd. for C40H86N2-
Br2,2.0H2O: C 60.74, H 11.74, N 3.54. Found C 60.65, H
11.41, N 3.42.
Bis(hexadecyldimethylammonium)pentane (2c). 1H NMR
(200 MHz, CDCl3) δ 0.88 (t, 6 H, alkyl chain 2 × 1 CH3),
1.15-1.45 (br m, 42 H, alkyl chain 2 × 10 CH2 and spacer
chain 1 CH2), 1.68 (crude t, 16 H, alkyl chain 2 × 4 CH2),
2.02-2.20 (br m, 4 H, spacer chain 1 × 2 CH2CH2N+), 3.33
(s, 12 H, 2 × 2 N+CH3), 3.45 (crude t, 4 H, alkyl chain 2 × 1
CH2N+), 3.90 (crude t, 4 H, spacer chain 1 × 2 CH2N+). C,H,N
analysis, Calcd. for C41H88N2Br2: C 64.04, H 11.54, N 3.64.
Found C 64.23, H 11.72, N 3.47.
Bis(hexadecyldimethylammonium)hexane (2d). 1H NMR (200
MHz, CDCl3) δ 0.88 (t, 6 H, alkyl chain 2 × 1 CH3), 1.15-
1.45 (s + br m, 48 H, alkyl chain 2 × 12 CH2), 1.62 (br m, 12
H, spacer chain 1 × 2 CH2CH2N+ and alkyl chain 2 × 1
CH2CH2CH2N+), 2.08 (br s, 4 H, spacer chain 1 × 2 CH2CH2-
CH2N+), 3.38 (br s, 16 H, 2 × 2 N+CH3 and alkyl chain 2 ×
1 CH2N+), 3.73-3.81 (m, 4 H, spacer chain 1 × 2 CH2N+).
C,H,N analysis, Calcd. for C42H90N2Br2: C 64.43, H 11.58, N
3.58. Found C 64.27, H 11.78, N 3.36.
Bis(hexadecyldimethylammonium)octane (2e). 1H NMR (200
MHz, CDCl3) δ 0.88 (t, 6 H, alkyl chain 2 × 1 CH3), 1.25-
1.90 (s + br m, 68 H, alkyl chain 2 × 14 CH2 and spacer chain
1 × 6 CH2), 3.36 (s, 12 H, 2 × 2 N+CH3), 3.44-3.50 (m, 4 H,
alkyl chain 2 × 1 CH2N+), 3.71-3.76 (m, 4 H, spacer chain 1
× 2 CH2N+). C,H,N analysis, Calcd. for C44H94N2Br2: C
65.16, H 11.68, N 3.45. Found C 65.05, H 11.88, N 3.20.
Bis(hexadecyldimethylammonium)decane (2f). 1H NMR (200
MHz, CDCl3) δ 0.88 (t, 6 H, alkyl chain 2 × 1 CH3), 1.25-
1.50 (s + br m, 64 H, alkyl chain 2 × 13 CH2 and spacer chain
1 × 6 CH2), 1.60 (br t, 8 H, spacer chain 1 × 2 CH2CH2N+,
and alkyl chain 2 × 1 CH2CH2N+), 3.40 (s, 12 H, 2 × 2
N+CH3), 3.46 (t, 4 H, alkyl chain 2 × 1 CH2N+), 3.70-3.85
(m, 4 H, spacer chain 1 × 2 CH2N+). C,H,N analysis, Calcd.
for C46H98N2Br2,3.0H2O: C 61.86, H 11.73, N 3.13. Found C
62.25, H 11.42, N 2.95.
Bis(hexadecyldimethylammonium)dodecane (2g). 1H NMR
(200 MHz, CDCl3) δ 0.88 (t, 6H, alkyl chain 2 × 1 CH3), 1.25-
1.38 (s + br m, 68 H, alkyl chain 2 × 13 CH2 and spacer chain
1 × 8 CH2), 1.65-1.85 (br m, 8 H, spacer chain 1 × 2 CH2-
CH2N+ and alkyl chain 2 × 1 CH2CH2N+), 3.38 (s, 12 H, 2 ×
2 N+CH3), 3.44-3.54 (m, 4 H, alkyl chain 2 × 1 CH2N+),
3.64-3.74 (m, 4 H, spacer chain 1 × 2 CH2N+). C,H,N
analysis, Calcd. for C48H102N2Br2,0.5H2O: C 65.80, H 11.85,
N 3.20. Found C 66.09, H 12.09, N 3.05.
Experimental Section
General Methods. Cetyltrimethylammonium bromide
(CTAB), n-hexadecyl bromide, and R,ω-dibromoalkanes were
purchased from Aldrich Chemical Co. N,n-Hexadecyl-N,N-
dimethylamine was obtained by refluxing n-hexadecyl bromide
with dimethylamine (Merck, 40% solution in water) in dry
ethanol at 80 °C for 24 h. 1H NMR spectra were recorded in
Bruker SEM-200 (200 MHz) NMR spectrometer. Chemical
shifts (δ) are reported in ppm downfield from the internal
standard. Microanalyses were performed on a Carlo Erba
elemental analyzer Model 1106. All the reagents and solvents
were highest grade available commercially and used purified,
dried or freshly distilled as required. Steam-distilled water was
used for all physical measurements.
Dimeric Surfactants. The dimeric surfactants were synthe-
sized as indicated in the following (Scheme 1).
Synthesis of Bis(quaternary ammonium) Surfactants (2a-
2g). The bis(quaternary ammonium) surfactants 2a-2g were
synthesized as described in detail in the following.
All the surfactants 2a-2g were obtained by refluxing the
corresponding R,ω-dibromoalkanes (m ) 3, 4, 5, 6, 8, 10, and
12) with N,n-hexadecyl-N,N-dimethylamine in dry ethanol (at
∼80 °C) for 48 h. The solvent was removed under vacuum
from the reaction mixture and the solids thus obtained were
recrystallized from hexane/ethyl acetate mixture for at least three
times to obtain pure compounds. The overall yields of the
surfactants ranged from 70 to 90%. All the compounds were
Determination of Critical Micellar Concentrations (Cmc).
Fluorescence technique was used to determine the critical
micellar concentrations. 1,6-Diphenyl-1,3,5-hexatriene (DPH,
purchased from Aldrich Chemical Co.), a fluorescence probe
whose emission quantum yield gets enhanced upon incorporation
from water into a micelle,17 was chosen as the probe. Fluo-
rescence measurements were done in a Hitachi F-4500 fluo-
1
characterized adequately and gave satisfactory H NMR and
C,H,N analysis. Pertinent details are given below.
Bis(hexadecyldimethylammonium)propane (2a). 1H NMR
(200 MHz, CDCl3) δ 0.87 (t, 6 H, alkyl chain 2 × 1 CH3),