810
M.L. Satnami et al. / Journal of Molecular Liquids 221 (2016) 805–814
Table 3
Aggregations number (Nagg), the surface area per head group (A), packing parameter (P) and radius (R) of cationic surfactants viz. CTAB, TTAB and DTAB in the presence of
octanohydroxamic acid (OHA).
(CTAB)
(TTAB)
(DTAB)
[OHA], (mM)
Nagg 0.1
A
P
R (nm)
Nagg 0.1
A
P
R (nm)
Nagg 0.1
A
P
R (nm)
nm2
nm2
nm2
0.0
0.1
0.5
0.8
1.0
2.0
63 (62)43
60.85
54.91
46.37
38.23
0.382
0.486
0.539
0.638
0.774
0.887
0.538
0.434
0.391
0.330
0.272
0.237
3.589
2.820
2.543
2.148
1.770
1.371
59 (58)44
53.85
31.53
26.64
21.74
0.391
0.429
0.733
0.865
1.063
1.246
0.522
0.477
0.279
0.236
0.192
0.164
3.099
2.825
1.653
1.401
1.140
0.972
51 (50)45
48.31
30.54
24.65
20.98
0.344
0.566
0.773
0.890
1.108
1.263
0.407
0.372
0.260
0.236
0.212
0.120
3.051
1.855
1.358
1.179
0.947
0.831
33.35
18.54
17.32
4. Discussion
core leads to an increase of area per head group, and in this way, de-
creases the charge density at the micellar surface. Thus, they are expect-
ed to increase the interfacial tension between the micelle, hydrophobic
core and bulk solvent. All these effects could be responsible for the de-
crease in the cmc of cationic surfactant in the presence of
octanohydroxamic acid (OHA).
4.1. Micellization of cationic surfactants with OHA
Fig. 1 indicates that the critical micelle concentration of surfactant
decreases with increasing the concentration of OHA. Generally, the de-
gree of ionization (α), taken through the ratio of the slopes of pre-mi-
cellar and post-micellar by conductivity is linearly correlated with the
surfactant concentrations [33]. The degree of ionization (α) values of
CTAB, TTAB and DTAB increases with increasing the concentration of
OHA (Table S1). It is suggested that an increase in the degree of ioniza-
tion (α) with respect to concentration of OHA is expected due to the de-
struction of water molecule and also decrease in the polarity of the bulk
phase caused by the addition of cosolvents. That is, in order to diminish
the attraction between the ionic head groups, thus prefer a large surface
area to stay the counterions. Essentially, the hydrophobic and solvopho-
bic interactions are considered to be the main driving force for micelli-
zation. The micellization of surfactants in water is driven by the low
solubility of the alkyl chains. Predominantly, an addition of organic sol-
vent improves the solubility of the alkyl chain, and the micellization
process becomes less favorable as compared to water [34]. Moya et al.
have documented that the cmc values sharply increases with increasing
EG (Ethylene Glycol) concentration because EG is highly associative [35]
and behave as cosolvent [36]. It also forms hydrogen-bonded chains
having mainly two-dimensional cooperative domains and has an analo-
gous nature to that of water [37]. The micellization behavior can be
modifying or affected by solvent properties. Significant contributions
in this field are made by Ghosh and their coworkers [38] as they studied
the effects of short chain alcohols upon the micellization of cationic sur-
factant and observed that the cmc values increases with increasing
length of alcohol from methanol to ethanol and decreases slowly with
1-propanol. The decrease in cmc on addition of methanol and ethanol
is due to the increase in solvation power of the surfactant-alcohol mix-
ture. In our case, the cmc values sharply decreased with increasing con-
centration of OHA (Table S1). At 1.0 mM OHA, cmc of CTAB and TTAB are
1.45, 1.34 times lower than the cmc in the absence of OHA, respectively.
Consequently, 1.60 times lower cmc of DTAB than their aqueous medi-
um have been observed. The diminishing of cmc of surfactants by in-
creasing the concentration of OHA is because of the penetration of
OHA in stern layer of micelle and thus destroying the water structure
[39]. There is possibility OHA-surfactant interaction resultant mixed mi-
celle formation (Fig. 3). This effect can be rationalized by considering
the addition of acid to the bulk solvent sphere that surrounds the hydro-
carbon chain of the surfactant. In fact, the carboxylic acid and alcohol
can be considered as cosolvent, which can replace water in the sphere
and thus bind to the surfactant molecules (co-solvent effect). On the
other hand, when increased chain length of acid or alcohol from (C5–
C8) molecules, and they intercalate between the surfactant head
group. The number of water molecules can be decreased by penetration
of acid molecules to stern layer and electrostatic repulsion between
head groups. In addition, acid molecules penetrate into the micellar
core and intercalate between the hydrophobic parts of the surfactant
(co-surfactant effect). Penetration of acid to the stern layer and micellar
The addition of OHA from 0.1–2.0 mM to the conventional surfac-
tants, the order of Γmax values decreases (Table 1). The variation of max-
imum surface excess (Γmax) is caused by intermolecular head group
distance and promotes the increase in the adsorption of surfactants.
The minimum surface area per molecule (Amin) shows elevated value
with 1.0 mM OHA due to bulky hydrophobic part that makes it difficult
to adjust at the air/water interface. The lower Amin of TTAB + OHA
(1.0 mM) system is because of the attraction between the oppositely
charged head groups resulting in the molecule to be more tightly
packed.
Table 2 shows thermodynamic parameter of surfactants. It is found
that at 1.0 mM OHA, micelle formation with CTAB 1.39 and 1.71 kJ/
mol more favorable than TTAB and DTAB respectively. On the other
hand, the micelle formation of DTAB with same concentration of OHA
is less favorable than CTAB and TTAB due to the reduction of hydropho-
bic interaction and increase of electrostatic interaction. The solubility of
hydrocarbon tail increases (C12–C16) [40,41] and bulk phase becomes
better solvent for the surfactant with increasing concentration of the
OHA. According to Nagarajan et al. [42] the involvement of Gibbs energy
to ΔG0m depends on the following circumstances: (a) Surfactant tail
transferred Gibbs free energy ΔG0trans, which is due to the transfer of sur-
factant tail from the bulk phase into the micellar phase (b) The forma-
tion of a micelle creates an interface, resulting in contact between the
hydrophobic core and the bulk phase stand by the aggregate-core sol-
vent interfacial Gibbs free energy, ΔG0intf (c) The electrostatic repulsion
between the surfactant head groups at the micellar surface which is rep-
resent by the head group interaction Gibbs free energy, ΔGe0lect
.
4.2. Aggregation number (Nagg
)
The aggregation number of the conventional surfactants has been
determined in the presence of octanohydroxamic acid by fluorescence
quenching method. Fig. 4 shows the effects of octanohydroxamic acid
on the aggregation number of surfactants. An increase in the content
of the polar organic solvent results in decrease in Nagg for cationic, an-
ionic, non-ionic and zwitterionic surfactants [43]. The decrease of Nagg
was considered by the variations of thermodynamic properties viz.
Gibbs micellization free energy and the interfacial Gibb's energy
(ΔGinterf0). The contribution of interfacial Gibb's energy (ΔGinterf0)
to the Gibb's energy of micellization (ΔGm0) is reduced with the in-
crease in the concentration of octanohydroxamic acid in the medium
as it (i.e. ΔGinterf0) is proportional to bulk phase/micelle core interfacial
tension [44]. This result points out that the addition of
octanohydroxamic acid, the surface tension of solution decreases. As
the digression of surface tension, mixed medium-hydrocarbon interfa-
cial tension becomes smaller than pure water. However, it must be
noted that as the content of octanohydroxamic acid in the media