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2.7. Salt tolerance
The results of structure characterization show that the target surfac-
tants, i-HPS and i-OPS, are synthesized successfully.
The transmittance of the 1.0 wt% surfactant aqueous solutions con-
taining different amount of salt at 600 nm was carried out via a UV-
1601 UV/VIS spectrophotometer (Beijing Rayleigh Analytical Instru-
ment Co., Ltd, China, photometric accuracy 0.3%T) at 15 2 °C to in-
vestigate the salt tolerance of the target products. The salts involving the
measurements include NaCl, CaCl2, and MgCl2. The solutions with differ-
ent content of salt were stirred before the measurement.
3.2. Krafft point
Krafft point is a specific temperature at which the ionic surfactant
aqueous solution presents an equilibrium among micelles, monomers,
and undissolved surfactant at a certain pressure. In other words, Krafft
point is the temperature where the solubility of ionic surfactant equals
to its cmc at a given pressure. The solubility will have a rapid augmen-
tation and the micelles are formed in a large amount when the temper-
ature of solution slightly exceeds its Krafft point [13,14]. The
temperature involving such useful properties exerts an increasing im-
portant effect on foam flotation and metal cleaning at low temperature.
In general, the Krafft point of surfactant will be viewed as below 0 °C if
its 1.0 wt% solution still remain clarify when ice and water coexist. The
Krafft points of i-OPS, i-OES, i-HPS, and HPS are listed in Table 1 and all
below freezing point.
The introduction of short hydrophobic chain or branched tail to the
target products can reduce the intermolecular force between the surfac-
tant molecules in solid state [15]. The PO units extend the effective
length of carbon chain to a certain degree, but they do not sacrifice
the solubility of surfactant and instead introduce the branching to
weaken the interaction of surfactant. For i-OES, the reasons lead to its
low Krafft point have been discussed in our previous literature [10].
The surfactant with low Krafft point means that it has a high solubility
at low temperature, which can extend its actual application under dif-
ferent environment conditions.
3. Results and discussion
3.1. Structure of the products
Fig. 1 is the FT-IR spectra of the intermediate products and target
products. We emphatically analyze the information shown in Fig. 1
(A) in which reveals the FT-IR of i-HP (a) and i-HPS (b). The strong ab-
sorption peak of hydroxyl stretching region at 3400 cm−1 in Fig. 1(A-a)
is weaken significantly in Fig. 1 (A-b) through sulfating reaction. And
meanwhile, the intense symmetric stretching vibration peak of sulfur-
oxygen double bond at 1245 cm−1 and the broad stretching vibration
peak of C\O\S at 770 cm−1 appear in Fig. 1 (A-b). The appearance of
these two phenomena at the same time indicates that the hydroxyl
has been substituted by sulfate group [9]. In addition, the weak peak
of hydroxyl in Fig. 1 (A-b) could be caused by the sample smeared on
the KBr plate with a residue of ethanol [10].
The 1H NMR spectra of the target products are shown Fig. 2, in which
a number of information revealed in the two spectra overlaps seriously
owing to the surfactants belong to a homologous series. As a result of
the polydispersity of polypropylene segments in the products, the
chemical shifts of hydrogen atoms in molecules are superposed in the
spectra, which would lead to the indistinction of the splitting peaks.
While the number of hydrogen atom of each group can be calculated
by the integrate areas of respective proton peaks. The assignment and
corresponding number of hydrogen atom of peaks in Fig. 2 are listed
below, respectively.
For i-HPS in Fig. 2 (A), δ: 0.835 (6H,\CH3 of the isohexyl), 1.131 (4H,
\CH2\), 1.306 (9H,\CH3 of the PO groups), 1.427 (1H, methylidyne of the
hydrophobic chain), 3.312 (2H,\CH2\O), 3.493 (6H, O\CH2\methylidyne\),
3.916 (2H, O\CH2\), 4.696 (1H, methylidyne of the PO unit closest to the
sulfate group), 7.260 (CDCl3).
3.3. Equilibrium surface tension
The surface activity of i-OPS, i-OES, i-HPS, and HPS is evaluated by
the equilibrium surface tension. In Fig. 3, the γ of the four surfactants
aqueous solutions gradually decrease with the increase of their concen-
trations and finally reach respective plateaus at 25 °C, indicating that the
amphiphilic molecules diffuse to the air/liquid interface and then attain
saturation on the interface [16]. The curves of γ show two inflection
points versus the logarithm of the concentrations of surfactants solu-
tions. The appearance of the first inflection point may be caused by
the formation of the premicellar aggregates [17,18]. The cmc and corre-
sponding γcmc are obtained by the intersection point of fitting straight
lines of the second decreasing stage and the plateau portion. The satura-
tion adsorption values (Гmax) and the minimum area occupied by a sin-
gle amphiphilic molecule (Amin) at the air/liquid interface are derived
by the Gibbs adsorption isotherm equations [16,18] (Eqs. (1) and (2)).
The parameter, pC20 advanced by Rosen et al. [19], is computed by the
For i-OPS in Fig. 2 (B), δ: 0.871 (6H,\CH3 of the tail), 1.142 (8H,\CH2\),
1.241 (9H,\CH3 of the PO units), 1.489 (1H, methylidyne of the isooctyl),
3.331 (2H,\CH2\O), 3.533 (6H, O\CH2\methylidyne\), 3.904 (2H, O\CH2\),
4.719 (1H, methylidyne of the PO group nearest the sulfate group),
7.260 (CDCl3).
Fig. 1. FR-IR spectra of i-HP (A-a), i-HPS (A-b), i-OP (B-a), and i-OPS (B-b).