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S.M. Shaban et al. / Journal of Molecular Liquids 212 (2015) 907–914
3
.5. Maximum surface excess (Γmax) and minimum surface area (Amin
)
The critical micelle concentration, maximum surface excess and ef-
fectiveness were obtained from surface tension measurements while
the degree of counter ion of dissociation determined from conductance
measurements. By analyzing the data in Table 2, it was found that the
change in the change in the free energy of micellization and adsorption
are negative which indicate that the adsorption and the micellization
behavior of synthesized Gemini surfactants in the solution at the tested
temperatures are spontaneous. The change in free energy of micelliza-
The surface excess of synthesized Gemini surfactants is expressed as
the concentration of Gemini surfactant monomers at the interface per
unit area. The values of maximum surface excess Γmax calculated using
Gibb's adsorption equation from surface tension measurements [36].
Γ max ¼ −ð1=2:303nRTÞðδγ=δ logcÞ
Τ
−
1
tion alters from −38.52 to −45.1 kJ mol for the synthesized C8-S3-
C8 by changing the solution temperature from 25 to 60 °C. i.e., the
change in free energy of micellization ΔG mic increase in the negative
direction by elevating the temperature indicating that process of micell-
where R is the gas constant, n is the number of active species (n equal 3
for Gemini surfactant with monovalent counter ion) and T is the abso-
lute temperature.
Minimum average surface area is the average area (in square
angstrom) occupied by each Gemini monomer adsorbed at the system
interface. Amin values give information about the angle between synthe-
sized Gemini surfactant monomer and the interface [37].
The minimum surface area (Amin) of the synthesized Gemini surfac-
tants calculated from Gibb's adsorption equation:
o
ization is favorable by rising the temperature. The same trend appeared
o
with the change in free energy of adsorption ΔG ads of synthesized Gem-
o
ini surfactants for example ΔG ads of synthesized C8-S3-C8 change from
−
1
−
48.13 to −56.43 kJ·mol by increasing the temperature from 25 to
6
0 °C. By comparing the change in the free energy of micellization and
o
o
adsorption, we note that ΔG ads is more negative than ΔG mic at the
same condition indicating that process of adsorption is more favorable
than micellization process. By increasing the hydrophobic character of
synthesized Gemini surfactants, the change in the free energy of micell-
ization and adsorption increase in the negative direction for example
the change in free energy of micellization were −38.52, −46.91 and
−54.27 for synthesized C8-S3-C8, C12-S3-C12 and C16-S3-C16 respec-
Amin ¼ 1016=ΓMaxN
The calculated maximum surface excess and minimum surface area
for synthesized Gemini surfactants at the three different temperatures
2
5, 40 and 60 °C were listed in Table 1. By analyzing these data it was
tively at solution temperature 25 °C. The same trend was observed in
o
found that both the maximum surface excess and minimum surface
area depend on solution temperature and the hydrophobic chain length
of synthesized Gemini cationic surfactant. Elevating the solution tem-
perature and increasing the hydrophobic chain length lead to an in-
creasing in the free energy of the system and enhance the monomers
of the synthesized Gemini surfactants to migrate to the surface more
rapidly at lower concentration so the packing densities of prepared
Gemini cationic surfactants at the interface decreased consequently,
the surfactant monomers concentration at interface Γmax decreased.
The dense packing of Gemini monomer force them to be less perpendic-
ular so minimum surface area occupied by a surfactant monomer
increase [38–39].
adsorption process, where ΔG
ads
increase in the negative direction
with increasing the chain length as indicated in Table 2. Increasing the
chain length of prepared aminoamine Gemini surfactants were accom-
panied by increasing the hydrophobicity of the aqueous system in
which the surfactant be dissolved in addition the amphipathic structure
of synthesized surfactants which will lead to the destroying the water
structure thus increasing the free energy of the system. Therefore, the
surfactant monomers migrate to surface or aggregate in clusters. The
migration to surface or aggregation in cluster decreases the energy
of the system, so the change in the free energy of the prepared
surfactant-solvent system will be decreased and increased in the
negative direction. Increasing the temperature of the surfactant aque-
ous system cause a decrease of hydration around the hydrophilic
group, so the hydrophobicity of the system increase and accompanied
by increasing the energy of the system, so molecules of surfactant
tend to adsorb and form micelle to decrease the energy of the system.
3
.6. Micellization and adsorption thermodynamic study
The behavior of synthesized Gemini surfactants in solution was de-
o
o
termined from their thermodynamic parameters of adsorption and mi-
cellization using pseudo-phase separation model for Gemini surfactants
proposed by Zana, [40].
On comparing ΔG
and ΔG
in Table 2, we note that the change
ads
mic
o
in the free energy of adsorption ΔG
ads
of any synthesized Gemini sur-
factant at any tested temperature higher than the change in free energy
o
of micellization ΔG mic. From that, we conclude that the synthesized
o
Gemini surfactants tend firstly to adsorb at the air–water interface
until maximum surface saturation then the monomers aggregates in
bulk in clusters forms. The change in the entropy of both micellization
ΔSmic and adsorption ΔSads values was listed in Table 2, and it found to
be positive values indicating the disruption of water structure around
the tail of Gemini surfactant when they transfer from the aqueous
ΔG mic ¼ 2ð1:5−αÞRT lnðXCMC Þ
o
o
ΔG
¼ ΔG mic−ðπCMC =ΓMaxÞ
ads
À
Á
o
ΔSmic ¼ −d ΔG mic=ΔT
À
Á
o
ΔS ¼ −d ΔG
=ΔT
ads
ads
o
ΔHmic ¼ ΔG mic=TΔSmic
o
ΔH ¼ ΔG
=TΔS
ads
ads
ads
Table 2
Micellization and adsorption thermodynamic parameters of the prepared Gemini cationic surfactants.
o
o
Comp.
Temp.
ΔG mic
ΔHmic
kJ·mol
ΔSmic
ΔG ads
ΔHads
kJ·mol
ΔSads
kJ mol−
1
−1
kJ·mol−1·K−1
kJ·mol
−1
−1
kJ·mol−1·K−1
°C
C8-S3-C8
25
−38.52
−42.14
−45.10
−46.91
−52.42
−56.39
−54.27
−61.98
−66.71
–
–
−48.13
−52.60
−56.43
−57.06
−63.43
−68.63
−62.27
−71.12
−77.19
–
–
4
6
0
0
33.49
4.21
–
62.62
9.83
–
0.241
0.148
–
0.367
0.199
–
40.78
7.35
–
69.53
17.98
–
0.298
0.191
–
0.425
0.260
–
C12-S3-C12
C16-S3-C16
25
4
6
0
0
25
4
6
0
0
98.89
12.18
0.514
0.237
113.59
24.03
0.590
0.304