H. Akba s¸ et al. / Spectrochimica Acta Part A 75 (2010) 671–677
673
Scheme 2. Synthesis of gemini cationic surfactants types 16-6-16 and 16-10-16.
1
3
C NMR (75 MHz, CDCl ): ı = 14.37, 22.91, 23.28, 26.45, 29.53,
2.3. Determination of melting temperature and Krafft
temperature
3
2
9.72, 32.09, 51.32, 57.02, 66.12.
Melting temperatures (TM) were measured using a Gallenkamp
apparatus. The sample of solid surfactant was set in a capillary tube
and examined under a microscope. The temperature was progres-
sively increased until melting was visually observed. All surfactant
samples were dried under vacuum at room temperature in the pres-
ence of phosphorous pentoxide prior to measurement. The error of
2
.2.1.2. Spectral characteristics for 12-2-12. 1H NMR (300 MHz,
CDCl ): ı = 0.86 (t, 6H), 1.17–1.30 (m, 36H), 1.81 (m, 4H), 3.47 (s,
3
1
2
2H), 3.70 (m, 4H), 4.70 (s, 4H).
13
C NMR (75 MHz, CDCl ): ı = 14.38, 22.92, 23.28, 26.48, 29.42,
3
9.60, 29.77, 29.80, 29.88, 32.15, 51.30, 57.02, 66.09.
◦
the reported values of TM is about 3 C.
The Krafft point of a surfactant is the temperature above which
the solubility of the surfactant increases dramatically in an aque-
ous solution. Thus, a solution containing surfactant is cloudy below
the Krafft point, but becomes clear at temperatures above its Krafft
point. The Krafft temperatures (TK) of cationic surfactants were
determined using surfactant solutions of 1 wt% (i.e., well above the
CMC of gemini surfactants) via the conductometric method [24,25],
as well as by visual observation of the clarity of the system. The
solutions were prepared as reported by Zana [26], and electrical
conductivities of solutions were measured using a WTW Terminal
2
.2.1.3. Spectral characteristics for 16-2-16. 1H NMR (300 MHz,
CDCl ): ı = 0.86 (t, 6H), 1.23–1.34 (m, 52H), 1.82 (m, 4H), 3.46 (s,
1
3
2H), 3.70 (m, 4H), 4.69 (m, 4H).
13
C NMR (75 MHz, CDCl ): ı = 14.38, 22.93, 23.27, 26.47, 28.40,
3
2
5
9.00, 29.43, 29.60, 29.62, 29.67, 29.79, 29.83, 29.92, 29.98, 32.16,
1.26, 57.07, 66.02.
2
.2.2. Method 2
A mixture of N,N-dimethylhexadecylamine and a corresponding
−
1)
7
40 (cell constant = 0.485 cm
conductometer. The conductivity
alkyl dibromide was refluxed in acetone for 24 h to synthesize 16-6-
6 and 16-10-16 type gemini cationic surfactants (Scheme 2). The
cell was calibrated in KCl solution at an appropriate concentration
range. First, the surfactant was dissolved in water, placed in a closed
1
white solid was filtered after cooling and was then recrystallized
from acetone at least two times.
◦
flask, and then stored in the refrigerator at 3.5 C. After precipita-
tion occurred, the solution remained in the refrigerator for at least
one additional day. In some cases, the solution had to be frozen
◦
.2.2.1. Spectral characteristics for 16-6-16. 1H NMR (300 MHz,
CDCl ): ı = 0.86 (t, 6H), 1.23–1.33 (m, 52H), 1.57 (m, 4H), 1.70 (m,
and then left to melt slowly at about 1.5 C to obtain a precipitate.
2
The conductance measurements were taken from low temperature
to above the clarification temperature. The Krafft temperature was
taken as the temperature where the conductance к-versus-T plot
showed a break. This break usually coincided with the tempera-
ture of full clarification of the system. The Krafft temperatures of
all cationic surfactants determined in this way are summarized in
Table 1, and the variation of conductance with increasing tem-
perature in a 1 wt% solution of 16-10-16 is shown in Fig. 1 for
3
4
H), 2.0 (m, 4H), 3.37 (s, 12H), 3.40–3.47 (m, 4H), 3.69–3.72 (m, 4H).
13
C NMR (75 MHz, CDCl ): ı = 14.36, 21.77, 22.92, 23.10, 24.42,
3
2
2
6.54, 27.73, 27.94, 29.50, 29.59, 29.62, 29.70, 29.83, 29.85, 29.88,
9.91, 32.15, 51.26, 64.40, 65.06.
2
.2.2.2. Spectral characteristics for 16-10-16. 1H NMR (300 MHz,
◦
CDCl ): ı = 0.88 (t, 6H), 1.25 (m, 48H), 1.35 (m, 8H), 1.44 (m, 8H),
only one of them. Krafft temperatures listed below 0 C gave no
3
1
.74 (m, 8H), 3.35 (s, 12H), 3.43–3.49 (m, 4H), 3.75–3.80 (m, 4H).
precipitate when kept in ice water for at least 24 h. As seen in
Fig. 1, for this system, the TK was taken as 26.5 C, the tempera-
13
◦
C NMR (75 MHz, CDCl ): ı = 14.35, 22.70, 22.90, 23.07, 26.07,
3
2
3
6.52, 28.40, 28.65, 29.49, 29.58, 29.64, 29.71, 29.83, 29.87, 29.91,
2.13, 51.24, 64.42, 64.54.
ture above which the solution no longer contained particles of solid
surfactant.