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2.3. Characterization. 1H NMR and 13C NMR spectra were
recorded on a Bruker Avance 300 spectrometer (300 MHz) in
CD3OD at room temperature. ESI HRMS spectra were taken on a
Bruker Daltonics Data Analysis 3.2 system. HPLC analysis was
performed on a Waters HPLC system equipped with an Alltech 2000
ELSD detector with a reverse phase (C18) column.
On the basis of the above Gu−Sjoblom equation,17,18
̈
Laskowski et al.19 deduced a theoretical relationship between
TK and the hydrophobic tail length of alkyl amine hydro-
chloride, and as electrolyte concentration (Ci) less than 1000
mM:
12-DAS with purity 97.22% (HPLC), 1H NMR (300 MHz,
CD3OD): δ 0.90 (t, J = 6.58 Hz, 3H), 1.29 (m, 16H), 1.61 (m, 2H),
1.99 (m, 2H), 2.18−2.23 (m, 4H), 2.88 (t, J = 6.69 Hz, 2H), 3.10 (s,
6H), 3.25−3.36 (m, 4H), 3.53 (m, 2H). 13C NMR (75 MHz,
CD3OD): δ 14.42, 19.87, 23.71, 23.95, 26.91, 30.39−30.73, 33.05,
37.11, 48.71, 51.56, 63.23, 63.80, 176.69. ESI HRMS [M + Na]+: calcd
for C20H42N2NaO4S, 429.2757; found, 429.2765.
T = 14.05 log C + 8.19n − 46.35
By using such a correlation, they successfully screened
surfactants for potash ore flotation.
(3)
K
i
With quantitative structure−property relationship (QSPR)
molecular simulation, Xu and her co-workers20 established a
general empirical prediction of TK for anionic surfactants. A six-
parameter correlation for a diverse set of 46 hydrocarbon
anionic surfactants, and a three-parameter correlation for 19
linear fluorocarbon anionic surfactants, were, respectively, built
by employing topological, thermodynamic, and structural
descriptors. From these relations, the TK values of the
surfactants that have not yet been synthesized could be
estimated on the basis of their molecular structure.
While these empirical correlations are useful to predict TK for
cationic and anionic surfactants, to the best of our knowledge,
there is little information available to forecast the TK of
zwitterionic surfactants, particularly for those with hydrophobic
tails longer than C18.15 In this context, a series of
amidosulfobetaine surfactants, 3-(N-fattyamidopropyl-N,N-di-
methyl ammonium) propanesulfonates (n-DAS, n = 12, 14, 16,
18, 20, 22, 24, and 28, Scheme 1), were used in this work to
14-DAS with purity 98.91% (HPLC), 1H NMR (300 MHz,
CD3OD): δ 0.90 (t, J = 6.69 Hz, 3H), 1.29 (m, 20H), 1.61 (m, 2H),
1.99 (m, 2H), 2.19−2.24 (m, 4H), 2.88 (t, J = 6.77 Hz, 2H), 3.11 (s,
6H), 3.26−3.38 (m, 4H), 3.54 (m, 2H). 13C NMR (75 MHz,
CD3OD): δ 14.44, 19.89, 23.70, 23.91, 26.90, 30.40−30.77, 33.04,
37.10, 48.72, 51.47, 63.23, 63.82, 176.58. ESI HRMS [M + Na]+: calcd
for C22H46N2NaO4S, 457.3070; found, 457.3087.
20-DAS with purity 99.29% (HPLC), 1H NMR (300 MHz,
CD3OD): δ 0.89 (t, J = 6.56 Hz, 3H), 1.30 (m, 32H), 1.62 (m, 2H),
1.99 (m, 2H), 2.19−2.24 (m, 4H), 2.88 (t, J = 6.68 Hz, 2H), 3.11 (s,
6H), 3.26−3.39 (m, 4H), 3.55 (m, 2H). 13C NMR (75 MHz,
CD3OD): δ 14.44, 19.87, 23.73, 23.96, 26.92, 30.42−30.77, 33.07,
37.12, 48.72, 51.51, 63.21, 63.78, 176.69. ESI HRMS [M + Na]+: calcd
for C28H58N2NaO4S, 541.4010; found, 541.4006.
2.4. Determination of Krafft Temperature. TK is normally
determined by electrical conductivity of saturated surfactant solution
as a function of temperature. However, such a process is time-
consuming22 and not suitable for the uncharged species such as
amidosulfobetaine surfactants in this work. The more easily accessible
methods, for example, visual observation23 and spectrophotome-
try,14,24 have been proposed to determine the temperature at which
the surfactant solution with concentration far above the cmc (normally
1 wt %) suddenly becomes clear. The accuracy of visual observation
for surfactants being rather low, the spectrophotometric technique is
more suitable for metastable crystalline phase rather than solution with
solid precipitate. Therefore, the TK of n-DAS in this work was
determined photometrically for the metastable crystalline solutions,
and by visual observation for the others.
Stock sample solutions with concentration of 1 wt % were prepared
by dissolving surfactant powders in distilled water or desired brine,
followed by gentle agitation while mildly heating for some long-chain
surfactants. When completely solubilized at high temperatures, the
solutions were cooled to induce precipitation, and then left to stand
overnight for equilibrium prior to measurements. The TK was
determined with a Unico UV/vis-4802 spectrophotometer and/or by
visual observation. With the optical method, the transmittance was
measured at a fixed wavelength of 500 nm unless stated otherwise, and
the temperature of the measured solution in the cell compartment was
controlled by an external Julabo circulating bath. While the
transmittance was being recorded, the corresponding temperature
was instantaneously monitored with an electronic filament probe
(accuracy of 0.1 °C) inserted into the cell through a small hole on a
plastic cap used to prevent evaporation. For the visual inspection, the
TK was determined by heating 10 mL of surfactant solution in a sealed
tube until a clear solution was obtained;23,25 and the reproducibility of
three times for the same measurements was 0.1 °C.
Scheme 1. Chemical Structure of the Series of
Amidosulfobetaine Surfactants n-DAS
relate their TK with hydrophobic length n, and the effects of
salinity and ionic species on the TK of these surfactants were
also examined.
2. EXPERIMENTAL SECTION
2.1. Materials. Surfactants n-DAS for n = 16, 18, 22, 24, and 28
with purity higher than 97% (HPLC) were synthesized previously.8,21
Lauric acid (Fluka, ≥99.0%), myristic acid (Sigma, ≥99.0%),
eicosanoic acid (J & K Chemical, ≥99.0%), N,N-dimethyl-1,3-
propanediamine (DMPDA, Alfa Aesar, ≥99.0%, GC), and 1,3-
propanesultone (Alfa Aesar, ≥99.0%, GC) were used as received. All
other chemicals were analytical grade, and the water used was triply
distilled by a quartz water purification system.
2.2. Synthesis of Surfactants 12-DAS, 14-DAS, and 20-DAS.
These analogues were prepared with a similar protocol for the
synthesis of 18-DAS.8 A typical procedure is listed as follows: 200
mmol of fatty acid, 240 mmol of DMPDA, and 0.30 g of NaF were
introduced into a three-necked flask. The reaction mixture was
refluxed at 160 °C under N2 atmosphere for several hours, during
which the byproduct H2O was absorbed continuously by Al2O3 placed
in a solvent still head. The excess DMPDA was removed, and the
residues were washed repeatedly with acetone. The purified product
was dried to obtain the tertiary ammine intermediate. Along with 500
mL of ethyl acetate, the obtained intermediate and 300 mmol of 1,3-
propanesultone were introduced into a round flask, and refluxed
around 80 °C for several hours. The occurred white precipitate was
washed repeatedly with acetone (mixture of acetone and 2-propanol
for 20-DAS) and dried under a vacuum to obtain the final pure
amidosulfobetaine. Detailed synthesis and characterization for these
surfactants can be found in the Supporting Information.
3. RESULTS AND DISCUSSION
3.1. Effect of Wavelength on TK. Exhibited in Figure 1 is a
typical example of temperature dependence of light trans-
mittance for 1 wt % 20-DAS in 500 mM NaCl solution at
varied wavelengths. With increasing temperature, the light
transmittance at a fixed wavelength maintains a constant value
close to 0 at first, but increases drastically to a maximum value
in a narrow region between the critical lower (T1) and upper
(T2) temperatures, then it changes negligibly when the
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dx.doi.org/10.1021/la204316g | Langmuir 2012, 28, 1175−1181