Adsorption and micellization behavior of synthesized amidoamine cationic surfactants and their biological activity

Article abstract of DOI:10.1016/j.molliq.2015.12.111

The adsorption and micellization behavior at aqueous surface of synthesized amido-amine cationic surfactants was studied using surface tension and conductometric measurements at three different temperatures of 25, 40 and 60 ^°C. The chemical structure of the synthesized cationic surfactants was confirmed using Fourier transform infrared and proton nuclear magnetic resonance spectroscopy. The synthetic route is simple and comprises two steps. The first is amidation between palmitic acid and dimethyl amino propyl amine, while the second step is quaternization of the first step product with different alkyl bromides. The study evaluated the effect of temperature and the hydrophobic chain length of the synthesized amidoamine cationic surfactants on the studied surface parameters. In aqueous system the adsorption tendency of the synthesized cationic surfactants is higher than micellization and both increase by increasing the hydrophobic character and the solution temperatures. The synthesized surfactants were found to have good antibiotic effect against gram positive and negative bacteria and fungi.

Full text of DOI:10.1016/j.molliq.2015.12.111

Journal of Molecular Liquids 216 (2016) 284–292
Contents lists available at ScienceDirect
Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
Adsorption and micellization behavior of synthesized amidoamine
cationic surfactants and their biological activity
b
c
c
c
Samy M. Shaban a, , A.S. Fouda , M.A. Elmorsi , T. Fayed , O. Azazy
⁎
a
Egyptian Petroleum Research Institute (EPRI), Nasr City, Cairo, Egypt
Department of Chemistry, Faculty of Science, El-mansoura University, Egypt
Department of Chemistry, Faculty of Science, Tanta University, Egypt
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
The adsorption and micellization behavior at aqueous surface of synthesized amido-amine cationic surfactants
was studied using surface tension and conductometric measurements at three different temperatures of 25,
40 and 60 °C. The chemical structure of the synthesized cationic surfactants was confirmed using Fourier
transform infrared and proton nuclear magnetic resonance spectroscopy. The synthetic route is simple and
comprises two steps. The first is amidation between palmitic acid and dimethyl amino propyl amine, while the
second step is quaternization of the first step product with different alkyl bromides. The study evaluated the
effect of temperature and the hydrophobic chain length of the synthesized amidoamine cationic surfactants on
the studied surface parameters. In aqueous system the adsorption tendency of the synthesized cationic
surfactants is higher than micellization and both increase by increasing the hydrophobic character and the
solution temperatures. The synthesized surfactants were found to have good antibiotic effect against gram
positive and negative bacteria and fungi.
Received 22 November 2015
Received in revised form 9 December 2015
Accepted 30 December 2015
Available online xxxx
Keywords:
Surface tension
Conductivity
Cationic amidoamine surfactants
Micellization & adsorption thermodynamics
Biological activity
© 2015 Elsevier B.V. All rights reserved.
1. Introduction
surfactants, capable of forming vesicles, as dialkyl dimethyl ammonium
halides (DDACl, DDAB and DODACl), were also studied [16]. In our
The adsorption and micellization of surfactants at the water–air
interface have an effect on the properties of the water. The adsorption
of the surface-active agent on the surface reduces the surface tension,
which is the major factor for wider applications of surfactants [1–7]. Ad-
sorption is the key factor for various applications like corrosion inhibi-
tors (increasing adsorption, the inhibition efficiency, increase) and as
capping agent in nanotechnology [8–10]. The opposing two parts of
surfactants are the key factor for the adsorption at interfaces and aggre-
gates in the bulk solution. Studying the physicochemical of the synthe-
sized surfactants at the surface is very important in determining the
best application, in which they may be used. Since most surfaces and
natural colloid are often negatively charged, so the cationic surfactants
provide a strengthen adsorption layer [11]. There are many papers
that reported the biological activity of commercially available cationic
surfactants like N-alkyltrimethyl ammonium halides (CTAB, TTAB,
DTAB, CTACl) [12,13] and N-alkyl pyridinium such as cetyl or dodecyl
pyridinium chloride (CPCl, DPCl) [14] and alkyl dimethyl benzyl
ammonium surfactants (benzalkonium and benzethonium) are other
examples of commercially produced surfactants investigated and
widely used as disinfectants in hospitals [15]. Commercial twin-chain
study, we prepared cationic surfactants with twin chain and containing
an amide functional group, which enhances its biodegradability, and re-
ducing its aquatic toxicity. Intermediates produced in the degradation
process of amide were found to be biodegradable and less toxic [17].
The microorganism gained self-immunity from conventional antibiotic,
so the researchers focused on searching new antibiotic like surfactants.
The research aimed to prepare cationic surfactant containing twin chain
and amide group from the low price material. The physicochemical and
thermodynamic of the aqueous solution of surfactants were determined
from surface tension and conductance measurements at three differ-
ent temperatures. The antibiotic effect of the synthesized cationic
surfactants was determined using filter paper disk agar method
against gram positive and negative bacteria and fungi.
2. Materials & experimental
2.1. Materials
Dimethylaminopropylamine, hexadecanoic acid and 1,3-dibromo
propane were purchased from Sigma Aldrich Company. Decyl bromide,
dodecyl bromide and hexadecyl bromide were purchased from Merck
Company and used as it without any purification. All the used organic
solvents were purchased from Algomhoria Chemical Company.
⁎
Corresponding author.
E-mail address: dr.samyshaban@yahoo.com (S.M. Shaban).
http://dx.doi.org/10.1016/j.molliq.2015.12.111
0167-7322/© 2015 Elsevier B.V. All rights reserved.

S.M. Shaban et al. / Journal of Molecular Liquids 216 (2016) 284–292
285
2.2. Synthesis of amidoamine cationic surfactants
2.3. Structure confirmation
2.2.1. Synthesis of N-(3-(dimethylamino) propyl) palmitamide
The synthetic routes of novel cationic surfactants were trappable
by Fourier transform infrared (FTIR) and proton nuclear magnetic
resonance spectroscopy (1HNMR). The FTIR analysis was done in
Egyptian Petroleum Research Institute using ATI Mattsonm Infinity
Series™, Bench top 961 controlled by Win First™ V2.01 Software
while 1HNMR was done in National Research Institute using GEMINI
200 (1H 200 MHz) in DMSO-d6.
The first step is amide formation through reaction of 0.03 mol from
palmitic acid (7.7 g) with 0.1 mol from dimethylamino-1-propylamine
(3.06 g) in 130 mL toluene. p-Toluene sulphonic acid (0.01%) was added
as dehydrating agent to the reaction mixture. The reaction was stopped
after complete removal of the reaction water (0.03 mol, 0.54 mL) using
Dean–Stark system. The solvent was removed using vacuum rotary
evaporator. The catalyst was extracted from the reaction medium
using petroleum ether. Subsequent purification was done by means of
vacuum distillation to remove the excess and residual materials [18].
2.4. Measurements
2.4.1. Surface tension measurements (γ)
2.2.2. Synthesis of N-(3-(dimethyl alkyl ammonio) propyl) palmitamide
bromide derivatives
The surface tension of aqueous solution of the novel amidoamine
twin chain cationic surfactant were measured by a platinum ring
detachment method using a K6 Krüss (Hamburg, Germany)
tensiometer at three different temperatures 25, 40 and 60
0.1 °C. The accuracy of the measurements was 0.5 mN·m−1.
The platinum ring was cleaned before each measurement with diluted
chromic acid mixture solution and washed with double distilled
water. Each concentration was measured three times and the average
was recorded and used without correction. The critical micelle concen-
tration (CMC) was determined from the break point in surface tension
(γ) versus [log c] plots [19].
0.01 mol from the synthesized amide in the first step (3.41 g) was
refluxed with 0.01 mol from the alkyl halides decyl bromide (2.21 g),
dodecyl bromide (2.49 g), and hexadecyl bromide (3.05 g) separately
in the presence absolute ethyl alcohol as a solvent for 25–30 h depend-
ing on the alkyl halide. After evaporating the absolute alcohol, the
residual was purified with diethyl ether. The obtained product labeled
DMOPP, DMDPP and DMHPP for cationic surfactant with decyl, dodecyl
and hexadecyl alkyl chain respectively. The synthetic routes are
represented in Scheme 1.
Scheme 1. Synthetic route of novel cationic amidoamine surfactants.

286
S.M. Shaban et al. / Journal of Molecular Liquids 216 (2016) 284–292
2.4.2. Conductivity measurements
The conductivities of the studied aqueous solution of the synthe-
sized cationic surfactants were measured at three different tempera-
2400 to 3400 cm−1 (broad band) and appearance band for amide NH at
3422.10 cm−1 and shifting the band of carbonyl from acid region to
amide region at 1665.25 cm−1. The prepared cationic surfactants
show stretching vibration band of –C–H aliphatic symmetric and asym-
metric at 2851.32 and 2920.17 cm−1 respectively in addition –CH2
bending at 1377 cm−1, −CH3 bending at 1468.41 cm−1 and absorption
band at 1255.55 cm−1 corresponding to C–N bond.
tures 25, 40 and 60
0.1 °C using a digital conductivity meter Cond
3210 SET 1, Probe tetra corn 325 (Wissenschaftlich Technische
Werkstattern), having a sensitivity of 1 μS cm−1 and an accuracy
of 0.5%. The solutions were prepared from deionized double distilled
water having a conductivity of 1.9 μS cm−1. The electrodes were
washed, after each reading, several times with deionized water [20].
3.1.2. 1HNMR spectra
The number and distribution of proton in the prepared amido-
amine cationic surfactant were confirmed by 1H-NMR spectra.
Fig. 2 shows the 1H-NMR spectra of N-(3-(dimethyl octyl ammonio)
propyl) palmitamide bromide (DMOPP) showing signals at: δ =
0.8 (t,6H, 2CH3 alkyl chain); δ = 1.19 (m,34H, –COCH2CH2(CH2)12CH3,
N⨁CH2CH2 (CH2)5CH3); δ = 1.42 (m,2H, COCH2CH2(CH2)12CH3); δ =
1.72 (m,2H, N⨁CH2CH2(CH2)5CH3); δ = 2.02 (m,2H, N⨁CH2CH2CH2NH);
2.4.3. The biological activity evaluation
The biological activity of the synthesized double chain cationic
surfactant containing the amide group was evaluated using filter
paper disc agar against some pathogenic bacteria and fungi. The gram-
positive bacteria were Bacillus subtilis and Staphylococcus aureus while
gram-negative were Escherichia coli and Pseudomonas aeruginosa. The
Candida albicans and Aspergillus flavus were used as an example for
fungi. The source of the microorganism was micro analytical center,
Cairo University [21]. The procedures were as follows:
δ
= 2.47 (t,2H, COCH2CH2(CH2)12CH3); δ = 2.69 (t,4H,
−CH2N⨁(CH3)2CH2–); δ = 2.97 (t,2H, CONHCH2); δ = 3.21 (s,6H,
−CH2N⨁(CH3)2CH2–) and δ = 7.97 (m,1H, CH2CONHCH2).
1. Inoculate flask of melted agar medium with the organism to be
tested.
3.2. Specific conductivity study
2. Pour this inoculated medium into a petri dish.
3. After the agar has solidified, a multilobed disc that impregnated with
different antibiotics laid on top of agar.
4. The antibiotic in each lobe of disc diffuses into medium and if the
organism is sensitive to a particular antibiotic, no growth occurs in
a large zone surrounding that lobe (clear zone).
5. The diameters of inhibition zones were measured after 24–48 h at
35–37 °C (for bacteria) and 3–4 days at 25–27 °C (for fungi).
6. Measure each clear zone and compare between them to determine
the antibiotic, which is more effective.
Studying the effect of chain length of the synthesized double chain
cationic surfactant and the solution temperatures on the specific con-
ductivity were clarified in Figs. 3, 4 and Table 1. Fig. 3 shows the effect
of changing the alkyl chain length on the conductivity of the solution.
Increasing the length of the hydrophobic chain was accompanied by a
decreasing in the solution conductivity as cleared from decreasing
the values of the degree of counter ion dissociation (α) recorded in
Table 1. The α values of the synthesized amidoamine cationic surfac-
tants DMOPP, DMDPP and DMHPP were 0.267, 0.254 and 0.244 at
25 °C respectively. The degree of counter ion dissociation (α) obtained
using Frahm's method and equal to the ration between the postmicellar
to premicellar region slops. The decreasing in conductivity (decreasing
values of counter ion dissociation) with increasing chain length is a
result of two factors, the number of counter ion and the strength of
the bond between the counter ion and the head of the surfactant.
Increasing the hydrophobic chain length of the synthesized amido-
amine surfactants, the molecular weight increase so, the number of
dissociated ions decrease. Also increasing the hydrophobicity of,
the hydration decrease and so the charge density formed around
the micelle increased so specific conductivity decreased [22–25].
Fig. 4 shows the effect of solution temperature on the specific con-
ductivity of synthesized cationic amidoamine surfactants. The specific
3. Results and discussion
3.1. Structure confirmation
3.1.1. FTIR spectra
The chemical structures of the amido-amine cationic surfactants se-
ries were confirmed using FTIR spectroscopy. The three amido-amine
cationic surfactants show nearly the same bands in infrared spectra, so
we will explain the spectra of DMDPP for examples. Fig. 1 show the
FTIR of DMDPP which confirm the conversion of acid to amide through
disappearance the hydroxyl group of carboxylic acid which ranged from
Fig. 1. IR spectrum of N-(3-(dimethyl octyl ammonio) propyl) palmitamide bromide (DMDPP).

S.M. Shaban et al. / Journal of Molecular Liquids 216 (2016) 284–292
287
Fig. 2. 1H-NMR spectrum of N-(3-(dimethyl octyl ammonio) propyl) palmitamide bromide (DMOPP).
conductivity found to increase with rising the solution temperature as
indicated by increasing values of α recorded in Table 1. The degree of
counter ion dissociation of an aqueous solution of DMOPP were 0.267,
0.283 and 0.3 at 25, 40 and 60 °C respectively. The behavior of conduc-
tivity with temperature return to increasing the dissociation of the
counter ion from the head of synthesized double chain surfactants
monomer or their micelle with rising temperature and this effect is
major than the columbic attraction force between the head and its
counter ion [26–29].
intersection point in Figs. 3 and 4 between the two lines which repre-
sent the relation between the specific conductivity of synthesized cat-
ionic surfactant aqueous solution and their concentration correspond
to the critical micelle concentration. The abrupt change in surface
tension curves represented in Figs. 5–7 refers to the CMC for the synthe-
sized double chain surfactants at specified temperature. The calculated
critical micelle concentration from the two techniques were recorded
in Table 1 and they were found nearly similar but the CMC obtained
from conductance measurements are higher than that obtained from
surface tension due to premicellar region [30–32].
3.3. Critical micelle concentration (CMC) and surface activity
The increasing chain length of the synthesized double chain cationic
surfactant has a decreasing effect on their critical micelle concentration
as appeared in the data recorded in Table 1 and shown in Fig. 8. The
critical micelle concentrations of aqueous solution of DMOPP, DMDPP
and DMHPP were 1.806, 1.0 and 0.651 mM/L at 25 °C respectively.
The critical micelle concentrations of the aqueous solution of
the synthesized double chain cationic surfactants at three different tem-
peratures 25, 40 and 60 0.1 °C were determined from two different
techniques; surface tension and conductivity measurements. The
Fig. 3. The plots of specific conductivity against concentrations of the prepared cationic
Fig. 4. The plots of specific conductivity against concentrations of the prepared cationic
surfactants in distilled water at 60 °C.
surfactants (DMOPP) in distilled water at 25, 40 and 60 °C.

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S.M. Shaban et al. / Journal of Molecular Liquids 216 (2016) 284–292
Table 1
The surface properties of synthesized gemini cationic surfactant at various temperatures.
Temp.
C20 ∗ 10−5
πCMC
(mN·m−1)
/
Г
max ∗ 10−10
Amin
A2
/
Comp.
CMCa/(mM·L−1
)
CMCb/(mM·L−1
)
α
CMC/C20
°C
25
(mol·L−1
)
(mol·cm−2
)
1.806
1.168
0.759
1.0
0.713
0.464
0.651
0.331
0.215
1.810
1.233
0.863
1.200
0.847
0.621
0.803
0.479
0.289
0.267
0.283
0.300
0.254
0.269
0.274
0.244
0.257
0.265
2.512
1.318
0.858
1.278
0.782
0.558
1.000
0.423
0.273
37.91
39.50
37.00
39.85
40.75
38.00
35.23
37.50
35.80
1.62
1.50
1.33
1.55
1.42
1.32
1.35
1.21
1.09
102.52
110.53
124.90
107.13
116.64
126.17
123.32
136.82
152.93
71.90
88.63
88.44
78.22
91.20
83.18
65.06
78.22
78.80
DMOPP
DMDPP
40
60
25
40
60
25
40
60
DMHPP
a
The values obtained from surface tension measurements.
The values obtained from conductometry measurements.
b
Increasing the hydrophobicity of the synthesized cationic surfactants,
lead to increasing the free energy of the aqueous system and so the
surfactant monomers aggregate into clusters. In clusters the hydropho-
bic tail becomes interior the clusters to avoid energically unfavorable
contact with aqueous medium, while the hydrophilic head becomes in
contact with aqueous medium, thus the free energy of the system
decreased and consequently CMC decrease [33–35].
In micelle formation, the hydration around the hydrophilic head
increase compared to unimers which observed by an abrupt increase
in conductivity as in Figs. 3 and 4, due to increasing the hydration
decreases the binding between the counter ion and head group. When
micelle starts to be formed, we notice steady in the values of surface
tension as shown in Figs. 5–7, due to the micelle be formed in the bulk
not surface.
Elevating the aqueous solution temperature, the critical micelle
concentration decrease in as shown in Fig. 8 and Table 1. The CMC of
the synthesized double chain cationic surfactant DMOPP were 1.806,
1.168 and 0.759 mM/L at 25, 40 and 60 °C respectively. The temperature
has two opposing effects, decreasing the hydration around the hydro-
philic head which enhance micelle formation. The second effect is
disrupting the water structure around the hydrophobic tail by which
the surfactants disfavor micellization, therefore the net effect is the
magnitude of the two opposing effects. From the obtained data in
Table 1, the predominant effect decreasing the hydration around the
hydrophilic head group, hence CMC decreased [36–38].
The critical micelle concentration of the synthesized amidoamine
surfactants showed higher critical micelle concentration than CMC
of Gemini surfactant with the similar number of carbon atoms prepared
by S.M. Shaban et al. [39]. The CMC of DMHPP at 25 °C was 0.651 mM/L
while it was 0.327 mM/L for the surfactants C16-S3-C16 [39]. This can
be attributed to increasing the hydrophobicity by the spacer [40].
The effectiveness of synthesized double tailed cationic surfactants
(πCMC) in reduction of the surface tension has been determined from
surface tension measurements using the following equation:
πCMC ¼γ
γ
ð1Þ
o
−
CMC:
The effectiveness is the difference in the values of surface tension at
the critical micelle concentration (γCMC) and at blank water without
surfactants (γo). The calculated (πCMC) values were recorded in
Table 1. The best effective surfactant is one, which has a higher ability
to reduce the surface tension that has higher CMC/C20 value. By in-
spection data in Table 1, it is clear that increasing the chain length
of synthesized double chain cationic surfactant, the values of CMC/C20
increase then it decreased at the surfactants with higher chain length
(DMHPP). So the synthesized double chain amidoamine cationic surfac-
tant DMDPP is the most effective one in the reduction the surface
tension where it has the larger CMC/C20 which equal to 78.22, 91.2
and 83.18 at 25, 40 and 60 °C respectively. The higher value of effective-
ness is indicative of the condensed nature of synthesized double tailed
surfactants unimers at the aqueous medium/air interface and the
lower value refer to that the formed monolayer from the monomers is
more expanded [41–43].
The efficiency of the specific surfactant (C20) is the concentration of
the surfactant required to suppress the surface tension of the blank
solution by 20 dyne/cm. The values of efficiency were calculated from
surface tension measurements and recorded in Table 1. By analyzing
these data, we noted that the efficiency of synthesized double tailed
cationic surfactant increase by elevating both temperature and the hy-
drophobicity. By rising the temperature of the synthesized surfactants
aqueous solution from 25 to 60 °C, the decreasing hydration around
the hydrophilic head of surfactants unimers is the predominant effect,
so the rate of migration of the surfactant unimers to surface increase
Fig. 5. The relation between surface tension and log concentration of cationic surfactant
Fig. 6. The relation between surface tension and log concentration of prepared cationic
(DMOPP) at various temperatures.
surfactant (DMDPP) at various temperatures.

S.M. Shaban et al. / Journal of Molecular Liquids 216 (2016) 284–292
289
Table 1, by analyzing these data it was found that the both parameters
are found to depend on solution temperature and the hydrophobic
chain length of synthesized cationic surfactant. Rising the solution tem-
perature and increasing the hydrophobicity leading to increasing the
free energy of the system and enhance the unimers of the synthesized
cationic surfactants to migrate to the surface more rapidly at lower con-
centration so the packing densities of prepared cationic surfactants at
the interface decreased Γmax decrease. The dense packing of cationic
unimers force them to be less perpendicular [49,50].
The maximum surface excess of the synthesized amidoamine surfac-
tants are higher than that in Gemini surfactant with similar number of
carbon atoms prepared by S.M. shaban etals [39]. The Γmax of DMHPP
at 25 °C was 1.35 × 10−10 mol/cm2 while it was 0.43 × 10−10 mol/
cm2 for the surfactants C16-S3-C16 [39]. From which we can conclude
that the synthesized double-tailed surfactants have higher ability to ac-
cumulate at surface with more tightly packed and coherent interfacial
films. Hence, the synthesized surfactants can show higher activity as
emulsifier, foaming, corrosion inhibitors and phase transfer catalyst
[40].
Fig. 7. The relation between the surface tension and log concentration of prepared cationic
surfactant (DMHPP) at various temperatures.
3.4. Micellization and adsorption thermodynamic study
so more reduction at surface tension. Increasing the hydrophobic tail
chain length, the hydrophobicity increase, hence the adsorption at
surface increase is being at more lower concentration [44–46].
The surface excess of synthesized cationic surfactants is expressed as
the concentration of synthesized surfactant unimers at the interface per
unit area. The values of maximum surface excess Γmax calculated using
Gibb's adsorption equation from surface tension measurements [47].
The adsorption and micellization behavior of the prepared
cationic surfactants in aqueous medium were clarified from their
thermodynamic properties, which calculated using the model
proposed by Zana [51].
ΔGꢀ
ΔGꢀ
¼ ð2 − αÞ RT lnðXCMC
Þ
ð4Þ
ð5Þ
ð6Þ
ð7Þ
ð8Þ
ð9Þ
mic
Γ max¼ − ð1=2:303 nRTÞ ðδ γ=δ logcÞT
ð2Þ
¼ ΔGꢀ
− ðπCMC=Γ maxÞ
ads
mic
where R is the gas constant, n is the number of active species (n equal 2
for cationic surfactant with monovalent counter ion), (δ γ/δ log c) is the
surface pressure and T is the absolute temperature.
ΔSmic ¼ − d ðΔGꢀ mic=ΔTÞ
ΔSads ¼ − d ðΔGꢀ ads=ΔTÞ
ΔHmic ¼ ΔGꢀ mic þ T ΔSmic
Minimum average surface area is the average area (in square
angstrom) occupied by each cationic unimer adsorbed at the system
interface. Amin values give information about the orientation of the
prepared surfactant at the interface [48]. The minimum surface area
(Amin) calculated from Gibb's adsorption equation:
ΔHads ¼ ΔGꢀ ads þ T ΔSads
:
The degree of counter ion dissociation was obtained from con-
ductance measurement while πCMC, Γmax and CMC were determined
from surface tension measurements. The calculated thermodynamic
parameters were recorded in Table 2. The data in Table 2 demon-
strate that the change in the free energy of adsorption and micelliza-
tion are negative at all the tested temperatures, which indicate that
the two processes are spontaneous. The change in free energy of
micellization of the synthesized cationic surfactants DMOPP
are −44.39, −48.08 and −52.65 kJ mol−1 at 25, 40 and 60 °C
respectively. By rising the aqueous solution temperatures, the
change in the free energy of micellization increase in the negative
direction indicating that micellization is favorable with elevating
the temperature. The change in the free energy of adsorption of the
synthesized cationic surfactant increase in the negative direction
Amin¼1016=Γ max
N
ð3Þ
where N is Avogadro's number. The calculated maximum surface excess
and minimum surface area of the synthesized cationic surfactant
at three different temperatures 25, 40 and 60 °C were recorded in
by elevating temperature; for example ΔGo
were −46.73,
ads
−50.71 and −55.44 kJ mol−1 for the synthesized DMOPP at 25, 40
and 60 °C respectively. Elevating the temperature of the surfactant
aqueous 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
[52–54].
By increasing the hydrophobic character of synthesized double
chain cationic surfactants, the change in the free energy of micellization
and adsorption increase in the negative direction, for example the
change in free energy of micellization ΔGo
were −48.08, −50.72
mic
Fig. 8. Temperatures & hydrophobic chain length effect of synthesized cationic surfactant
on critical micelle concentration values of prepared cationic surfactants.
and −54.54 for the synthesized DMOPP, DMDPP and DMHPP

290
S.M. Shaban et al. / Journal of Molecular Liquids 216 (2016) 284–292
Table 2
Micellization and adsorption thermodynamic parameters of the prepared gemini cationic surfactants.
Temp.
°C
ΔG°mic
ΔHmic
ΔSmic
ΔG°ads
ΔHads
ΔSads
Comp
kJ·mol−1
kJ·mol−1
kJ·mol−1·K−1
kJ·mol−1
kJ·mol−1
kJ·mol−1·K−1
25
40
60
25
40
60
25
40
60
−44.39
−48.08
−52.65
−47.27
−50.72
−55.81
−49.42
−54.54
−59.77
–
–
−46.73
−50.71
−55.44
−49.84
−53.58
−58.70
−52.03
−57.64
−63.07
–
–
DMOPP
DMDPP
DMHPP
28.99
23.48
–
21.34
29.04
–
0.246
0.228
–
0.230
0.255
–
32.40
23.26
–
24.56
26.56
–
0.265
0.236
–
0.249
0.256
–
52.56
27.31
0.342
0.261
59.36
27.47
0.374
0.272
respectively at solution temperature 40 °C. Increasing the chain length
of prepared amidoamine cationic surfactants were accompanied by in-
creasing the hydrophobicity of the aqueous system in which the surfac-
tant 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 sys-
tem will be decreased and increased in the negative direction. By com-
paring the change in the free energy of micellization and adsorption, we
note that ΔGoads is more negative than ΔGomic at the same temperature
and chain length indicating that adsorption process is more favorable
than micellization process. From that, we conclude that the synthesized
cationic surfactants tend to adsorb at air-water interface first until
maximum surface saturation then the monomers aggregates in bulk in
cluster forms [55–57].
cationic surfactant with eight carbon atom chain length (DMOPP) is the
most effective one that DMDPP and DMHPP.
There are some parameters, which responsible for the cut-off effect
critical micelle concentration (CMC) of the synthesized cationic surfac-
tant, the change in the free energy of adsorption ΔGoads at the bacteria
cell membrane, the size of diffused surfactant unimers and their micelle
and the hydrophobicity of surfactant [61–64]. Increasing the chain
length of the hydrophobic tail accompanied by a decreasing in the
CMC as previously discussed, hence the concentration at the membrane
surface become lower, consequently the activity of DMOPP ˃ DMDPP˃
DMHPP. At the same time increasing the hydrophobic character, the ad-
sorption rate at the cell membrane be higher, so it predicted that
DMHPP ˃ DMDPP ˃ DMOPP. Another theory returns the cut-off effect
to a decrease in the perturbation of the membrane at higher alkyl tail,
assuming that the longer the chain, the better mimic molecule in the
lipid layer, leading to disruption in the membrane. Hence the magni-
tude of these effects determines the most effective surfactant, from
the data recorded in Table 3, it was found that the synthesized cationic
surfactants with twelve carbon atoms (DMDPP) has the maximum anti-
biotic effect against the tested bacteria, the surfactant DMOPP has the
best effect against the tested fungi.
The biological activity of the synthesized cationic surfactants is slight
lower than that of Gemini surfactants with similar number of carbon
atoms prepared by S.M. Shaban et al. [39]. For example the inhibiting
effect of DMHPP against B. subtilis, E. coli and S. aureus was 12, 12 and
13 mm/mg while it was 13, 14 and 14 mm/mg for the gemini surfactant
C16-S3-C16 [39], respectively. This can be attributed to that gemini
surfactants contains additional counter ions and positive charge which
enhance the adsorption at the cell membrane and disruption the
selective permeability of the membrane.
The change in the entropy of both micellization ΔSmic and adsorption
ΔSads values were recorded in Table 2, and it found to be positive values
indicating the disruption of water structure around the tail of synthe-
sized surfactant when they transfer from the aqueous bulk to the air-
water interface or to the micellar interior. The change in the entropy
of adsorption ΔSads is more positive than that of micellization ΔSmic
,
this reflect the greater freedom of hydrophobic part through motion
to the interface than to form micelle [58,59].
3.5. Antimicrobial activity
The activity of the synthesized cationic surfactants containing amide
group against some gram positive and negative bacteria and fungi has
been recorded in Table 3. The synthesized surfactants have good activity
against both tested bacteria and fungi. The antibiotic activity was hydro-
phobic alkyl chain dependent, where the activity increase by increasing
the length of alkyl chain then, it decreases, this effect known by the cut-
off effect [58–60]. Regarding to the tested bacteria the synthesized
DMDPP has the maximum activity then, it decreases at surfactant with
sixteen-carbon atom (DMHPP). Regarding to the fungi, the synthesized
Fig. 9 shows the main composition of Gram-positive and Gram-
negative bacteria membrane. The expected mechanism of synthesized
cationic surfactant as the antibiotic is based on the affinity of the surfac-
tants to adsorb on the cellular cytoplasmic membrane and interaction
between the positive head group of surfactants and negatively charged
membrane where the hydrophobic tail penetrates and disturb the
selective permeability of the membrane causing cell death in addition
the counter ion effect [65–70].
Table 3
The antibiotic effect of synthesized gemini surfactants against pathogenic bacteria and fungi.
Microorganism
Gram reaction
Inhibition zone diameter (mm/mg sample)
Used standard reference
Ref. inhibition zone diameter
(mm/mg sample)
DMOPP
DMDPP
DMHPP
Bacillus subtilis
Escherichia coli
Pseudomonas aeruginosa
Staphylococcus aureus
Aspergillus flavus
Candida albicans
G+
12
13
13
14
12
14
14
15
13
15
10
11
12
12
12
13
8
Ampicillin
Ampicillin
Ampicillin
Ampicillin
Amphotericin B
Amphotericin B
20
22
17
18
17
19
G−
G−
G+
Fungus
Fungus
10

S.M. Shaban et al. / Journal of Molecular Liquids 216 (2016) 284–292
291
Fig. 9. The bacterial cell wall structure.
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This work was financially supported by Egyptian Petroleum
Research Institute, Egypt.
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