Studying surface and thermodynamic behavior of a new multi-hydroxyl Gemini cationic surfactant and investigating their performance as corrosion inhibitor and biocide

Article abstract of DOI:10.1016/j.molliq.2020.113881

Some of commercial materials, like monoethanolamine, chloroacetic acid, triethanolamine amine and vanillin have been used for constructing three Gemini cationic surfactants with different hydrophobic tail length labelled CMTC, LMTC & PMTC. The chemical structure of the new Gemini was confirmed by FTIR & ¹HNMR. The hydrophobic tail of the synthesized Gemini controls their behavior in solution at the three studied temperatures as studied by surface tension measurements. The PMTC Gemini cationic surfactants with longest tail length showed lowest critical micelle concentration; which also showed a decreasing trend with raising the solution temperature. The thermodynamics insight refer the higher tendency of the synthesized surfactants toward adsorption and micellization with both increasing the hydrophobicity and temperature. The prepared electronic rich surfactant showed good inhibiting against the steel corrosion in the aggressive 1.0 M HCl; which is correlated with the surfactant structure and the temperature. The PMTC surfactant showed highest inhibition efficiency 92.09percent at 55 °C, due the longer hydrophobic tail exhibit higher adsorption affinity compared to shorter surfactant tail CMTC. The polarization measurement revealed that the three surfactants behave as mixed type inhibitors. The synthesized three electronic rich Gemini surfactants showed high antimicrobial activity against bacteria & fungi, which was found correlated with the surfactant hydrophobic behaviors.

Full text of DOI:10.1016/j.molliq.2020.113881

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Studying surface and thermodynamic behavior of a new multi-
hydroxyl Gemini cationic surfactant and investigating their
performance as corrosion inhibitor and biocide
Samy M. Shaban, Shrouk Abd Elsamad, Salah M. Tawfik, Adel
A.-H. Abdel-Rahman, I. Aiad
PII:
S0167-7322(20)30018-0
DOI:
https://doi.org/10.1016/j.molliq.2020.113881
MOLLIQ 113881
Reference:
To appear in:
Journal of Molecular Liquids
Received date:
Revised date:
Accepted date:
2 January 2020
17 July 2020
19 July 2020
Please cite this article as: S.M. Shaban, S.A. Elsamad, S.M. Tawfik, et al., Studying
surface and thermodynamic behavior of a new multi-hydroxyl Gemini cationic surfactant
and investigating their performance as corrosion inhibitor and biocide, Journal of
Molecular Liquids (2020), https://doi.org/10.1016/j.molliq.2020.113881
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Studying surface and thermodynamic behavior of a new multi-
hydroxyl Gemini cationic surfactant and investigating their
performance as corrosion inhibitor and biocide
Samy M. Shaban *a, Shrouk Abd Elsamad b, Salah M Tawfika, Adel A-H Abdel-
Rahmanb, I. Aiada
a:Petrochemical Department, Egyptian Petroleum Research Institute, Egypt.
b :Chemistry Department, Faculty of Science, Menoufia University, Egypt.
Abstract
Some of commercial materials, like monoehanolamine, chloroacetic acid,
triethanolamine amine and vanillin have been used for constructing three Gemini
cationic surfactant with different hydrophobic tail length labelled CMTC, LMTC &
PMTC. The chemical structure of the new Gemini was confirmed by FTIR &
1HNMR. The hydrophobic tail of the synthesized Gemini control their behavior in
solution at the three studied temperatures as studied by surface tension measurements.
The PMTC Gemini cationic surfactants with longest tail length showed lowest critical
micelle concentration; which also showed a decreasing trend with raising the solution
temperature. The thermodynamics insight refer the higher tendency of the synthesized
surfactants toward adsorption and micellization with both increasing the
hydrophobicity and temperature. The prepared electronic rich surfactant showed good
inhibiting against the steel corrosion in the aggressive 1.0 M HCl; which is correlated
with the surfactant structure and the temperature. The PMTC surfactant showed
highest inhibition efficiency 92.09 % at 55 ⁰C, due the longer hydrophobic tail exhibit
higher adsorption affinity compared to shorter surfactant tail CMTC. The polarization
measurement revealed that the three surfactants behave as mixed type inhibitors. The
synthesized three electronic rich Gemini surfactant showed high antimicrobial activity
against bacteria & fungi, which was found correlated with the surfactant hydrophobic
behaviors.
Keywords: Gemini Surfactants Surface Parameters; Thermodynamic insight; Steel
Corrosion; Tafel & Impedance; Biological Activity.

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1. Introduction
The surfactant characterize by their unique chemical structure, which make them
very important in many fields either industrials or research, so they attracted the
attention of the researcher. The surfactant is organic compounds contain two different
parts of different polarities attached together; one part high polar called hydrophilic
head while the other characterize by its high hydrophobicity and called hydrophobic
tail [1-5]. The surfactants characterize by its ability to reduce the surface tension
between two different phases, liquid is one of them. The unique structure of
surfactant, give them the ability to either aggregate in micelles forms and/or adsorb at
the interfaces for system stabilization which give them a critical role in vast of
essentials applications such as designing the microemulsion, detergent,
pharmaceutical applications, emulsion polymerization, corrosion inhibitors,
nanotechnology, drug delivery, emulsifier, enhancing oil recovery & biocide [6-12].
The Gemini surfactants are special category surfactant which consist of two
hydrophobic tail and two hydrophilic heads connected together by a spacer, this
structure make an enhancement in their surface activity compared to conventional
monomeric surfactant, where they gain low critical micelle concentration (CMC),
higher rheological properties, characteristic micellar and more efficient in reducing
surface tension [13]. Therefore more attention in both industrial and research
laboratories has been paid for development and synthesizing novel Gemini
surfactants.
Although the hydrochloric acid solution (HCl) is aggressive solution, but it is
commonly involved in vast industrial applications such as pickling, descaling,
cleaning, and etching of mild steel [14, 15]. Therefore using corrosion inhibitors is
very critical issue for saving the equipment, especially they mainly constructed from
mild steel. The performance of the corrosion inhibitors, depends on their chemical
structure, whereas it is preferred to have some electronic rich functional group able to
interact with the vacant d-orbital such as oxygen, sulfur, phosphorous, nitrogen,
double bond, aromatic ring and triple bond for enhancing the corrosion inhibitors
adsorption [15, 16]. Therefore by considering the unique adsorption affinity of the
surfactant on surface, the researchers focused on enrich the surfactant structure with
some active center for using as corrosion inhibitors in addition the hydrophobic

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surfactant tail shown impressive role in protecting surface from corrosion by forming
a protecting layer the aggressive medium and the steel.
Here we focused on preparing some new Gemini cationic surfactant with a specific
designated structure with various functional groups like hydroxyl, carbonyl, aromatic,
methoxy, and quaternary nitrogen for best action as corrosion inhibitors and as
biocide due to the upraising of microbial resistivity to common antibiotic. The surface
parameters of the synthesized Gemini cationic surfactant were studied at different
solution temperatures.
2. Materials and Experimental Techniques
2.1. Materials
All chemical were used without further purification, the octanic acid (99%),
dodecanoic acid (99%), hexadecanoic acid (98%), vanillin and monoethanol amine
were purchased from Sigma Aldrich company, while the, p-toluene sulphonic acid
(PTSA), chloroacetic acid, triethanol amine and (4-hydroxyphenyl) propanoic acid
were obtained from Merck.
2.2. Synthesis of Gemini cationic surfactant
Three different Gemini cationic surfactants varying in their hydrocarbon chain
were prepared and their synthetic procedures outlined in the Scheme 1 through three
steps. The vanillin (1) (0.1 M, 15.21 g) were refluxed with monoetnolamine (2) (0.1
M, 6.11 g) in 100 mL ethanolic solution for 6 hours. The obtained compound from the
previous step after evaporation and purification steps (3) (0.05M, 9.75 g) was
esterified with chloroacetic acid (4) (0.1 M, 9.45 g) in 100 mL xylene as solvent in
presence of 0.01& PTSA as catalyst using dean-stark apparatus for receiving the
theoretical amount of water (1.8 mL) to produce dichloro ester compound (5).
Equimolar from triethanol amine (6) (0.01 M, 1.49 g) was esterified with octanoic
acid (7) (0.01 M, 1.44 g), dodecanoic acid (8) (0.01 M, 2.003 g) and hexadecanoic
acid (0.01 M, 2.56 g) separately in presence of 0.01% PTSA as catalyst [17], to
produce the corresponding hydroxyl ester compounds 10, 11 and 12 respectively as it
is outlined in Scheme 1. The obtained dichloro ester (5) (0.01 M, 3.48 g) was refluxed
with hydroxyl ester compounds compound 10 (0.02 M, 5.51 g), compound 11 (0.02

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M, 6.63 g) and compound 11 (0.02 M, 7.75 g) separately in presence of ethyl alcohol
as solvent for 48 hours. The diethyl ether was used for recrystallization after the
evaporating the solvent and further purification the final synthesized Gemini cationic
surfactant were labeled as CMTC, LMTC and PMTC for the surfactant with eight,
eleven and sixteen carbon atoms as tail as it is outlined in Scheme 1.
Scheme 1: Synthetic pathway of cationic Gemini surfactants.
2.3.Structure characterization:
1
The Fourier transform infrared spectroscopy and H NMR spectroscopy were used
for confirming the chemical structure of the synthesized Gemini cationic surfactants.
The ATI Mattsonm Infinity Series™ instrument (Bench top 961 incorporated in the
Win First™ V2.01 Software, EPRI, Egypt), was established for confirming the main
functional groups initiated. The 1H NMR (Bruker High Performance Digital FT-NMR

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Spectrometer A Vance III 400 MHz, Cairo University, Faculty of Pharmacy), was
used for structure confirmation.
2.4. Measurements
2.4.1. Surface tension measurements (γ)
The surface tension of the prepared cationic gemini surfactant CMTC, LMTC and
PMTC aqueous solution from at 20, 40 and 60 ᴼC were conducted using tensiometer-
K6 instrument (KrÜss Company, Germany). The distilled water has been used for
calibrating. Different concentrations (5x10-3 to 5x10-7 M) from the aqueous CMTC,
LMTC and PTMC surfactant have been prepared and their γ has been measured. Each
run was repeated triplicate and the average has been considered for calculations.
The inflection point in the surface tension (γ) versus [log c] plots of the synthesized
cationic Gemini surfactant CMTC, LMTC and PMTC refer to the corresponding
critical micelle concentrations (CMC). The concentration of the synthesized cationic
Gemini surfactant CMTC, LMTC and PMTC concentration has the ability to suppress
the blank surface tension by 20 mN/m is the surfactant efficiency (C20).
The effectiveness (πCMC) of the CMTC, LMTC and PMTC gemini surfactant is
equal to the difference between the surface tension of blank water (γo) and surfactant
solution at the CMC (γCMC) as described in equation (1):
πCMC = γo – γCMC
(1)
The maximum surface excess (Γmax) has been calculated according to Gibb’s
adsorption equation 2, and correspond to the concentration of the cationic Gemini
surfactant CMTC, LMTC and PMTC unimers at the solution interface per unit area
[18-20].
1
훅후
휞풎풂풙 = (ퟐ.ퟑퟎퟑ 풏푹푻)(훅 퐥퐨퐠 퐜
)
(2)
푇
The R, represent the universal gas constant, the symbol n equal 3 for Gemini cationic
amphipathic and T is the absolute solution temperature. (δ γ/δ log c) is the surface
pressure (slope at the premicellar region of surface tension – concentration curve;
linear function).
the minimum surface area (Amin) has been measured through the Gibb’s adsorption
equation 3 [21] and equal to the average area in square angstrom occupied by the
adsorbed Gemini cationic surfactant CMTC, LMTC and PMTC unimers at the surface
(the symbol N is equal to Avogadro’s number).

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ퟏퟔ
ퟏퟎ
푨풎풊풏
=
(3)
휞
푵
풎풂풙
2.4.2. Specific conductivity
The specific electrical conductivity of the synthesized Gemini cationic surfactant
CMTC, LMTC and PMTC at 20, 40, and 60 ᴼC were performed using a Cond-3210
SET Tetra-Con 325 (1 Probe) device and the data were for determining both CMCs
and degree of counter ion dissociation (α). The CMC correspond to the intersection
point in the curve specific conductivity concentration curve, while the (α) was
determined according to Frahm´s method which equal to the ratio between the
postmicellar to premicellar region slops [22].
2.4.3. Corrosion measurement
The performance of the synthesized gemini cationic surfactant CMTC, LMTC and
PMTC to inhibit the corrosion of the mild steel in aggressive hydrochloric acid
medium (1.0 M) has been determined electrically at 25 ᴼC and gravimetrically by
weight loss method at 20, 35, 50 and 65 ᴼC. The weight loss experiments were
conducted using mild steel specimens with fixed dimension of 3 cm x 6 cm x 0.4 cm.
Each steel coupon was treated as following before conducting the measurements, the
coupon was abraded using different grade emery papers from 400 to 1200, then it
degreased with acetone and kept for drying before weighting using a digital balance.
The specimens were dipped in the corrosive solution either with or without the
synthesized CMTC, LMTC and PMTC Gemini cationic inhibitor for a day (24 hrs).
After 24 hrs, the specimens were removed from the corrosive solution and washed,
then dried before reweighting again. A thermostat water bath was utilized during the
experiment [23, 24].
The electrochemical measurements, were conducted on Voltalab 40 Potentiostat
instrument PGZ 301 model equipped with Voltamaster 4 software. Three electrodes
platinum counter electrode, working electrode and a saturated calomel electrode
(SCE) were equipped in the glass cell with three neck. The exposed surface area of
working electrode is 0.7 cm2 was treated as mentioned previously in weight loss
experiment. The Tafel was conducted by altering the external applied potential
automatically from −800 to −200 mV with 0.1 mVs−1 scan rate [25]. For the

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electrochemical impedance (EIS), the frequency range was adapted from 100 kHz to
50 MHz with a 5 mV [26]. The chemical composition of the working electrode for
both electrochemical & weight loss measurements is C (0.17%), P (0.02%), Mn
(0.472%), Si (0.25%). S (0.018%) and the rest percent is iron.
2.4.4. Biological Activity
The biological activity of the synthesized gemini cationic surfactant CMTC, LMTC
and PMTC has been determined using a modified agar diffusion method [27] at
operation and development department (EPRI). The test were conducted on Gram +ve
bacteria (Bacillus subtilis ATCC 6633, Staphylococcus aureus ATCC 35556), Gram -
ve bacteria (Escherichia coli ATCC 23282 and Pseudomonas aeruginosa ATCC
10145), Yeast (Candida albicans IMRU 3669) and Filamentous Fungus (Aspergillus
niger ATCC 16404) following the same procedure mentioned in details in previous
work [28]. The tested bacteria and fungi strains have been poured with the prepared
nutrient agar (czapek dox and wickerham) medium into the plates. In each plate, a
sterile 10-mm cork borer was used to cut three equidistant wells, then 100 μL of the
prepared CMTC, LMTC and PMTC Gemini surfactant (1 mM) has been injected into
°
the wells. Finally the bacteria was incubated for overnight at 37 C, while both yeast
°
and the fugi incubated for 48 hours at 30 C. The inhibition zone diameter (mm) was
considered as an indicator for their antimicrobial activity. The Erythromycin
(bacteria) and Metronidazole (yeast and fungi) were used as positive control.
3. Results and Discussion
3.1. Structure elucidation of the synthesized polymeric cationic surfactant:
The FTIR showed in Fig. 1a, confirm of the chemical structure of the prepared
PMTC as an example, band at 1738.84 cm-1 refer to the new formed ester group
whereas the broad hydroxyl and carbonyl groups regarding to acid disappeared. Some
other bands like at 2930 and 2850.6 cm-1 for asymmetric and symmetric stretching
aliphatic (CH), respectively, whereas bands at 1465, 1403 & 1032 cm-1, refer to C=C,
bending aliphatic CH2 and C-O, respectively.
1
Fig. 1b shows the H NMR spectra of the PMTC surfactant. δ 0.86 (t, 6H, 2CH3
alkyl chain), 1.1–1.24 (m, 48H, 2 (OCOCH2CH2(CH2)12CH3)), 1.48 (m, 4H, 2

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(OCOCH2CH2(CH2)12CH3)), 2.18 (t, 4H, 2 (OCOCH2CH2(CH2)12CH3)), 2.2 (t, 8H, 2
N+CH2CH2OH), 2.33 (t, 4H, 2 N+CH2CH2OCO), 2.6 (s, 4H, 2 N+CH2CH2OH), 2.9 (s,
2H,
CH2CH2OCOCH2N+CH2CH2OCO(CH2)14CH3), 3.36 (t, 8H, 2 N+CH2CH2OH), 3.6 (t,
PhOCOCH2N+CH2CH2OCO(CH2)14CH3),
CH2CH2OCOCH2N+CH2CH2OCO(CH2)14CH3), 3.9 (s, 3H, PhOCH3), 4.05 (t, 2H,
CH=NCH2CH2OCOCH2N+CH2CH2OCO(CH2)14CH3), 7.3-7.9
PhOCOCH2N+CH2CH2OCO(CH2)14CH3),
3.1
(s,
2H,
CH=N
4H,
2
3.8 (t, 2H, CH=N
(s, 3H, aromatic
protons), 8.15 (s, 1H, CH=NCH2CH2OCOCH2N+CH2CH2OCO(CH2)14CH3),
Fig. (1)
3.2.Surface and thermodynamic study:
The surface tension was used to study the surface behavior of the synthesized
CMTC, LMTC and PMTC Gemini surfactant in aqueous medium at 20, 40 & 60 ⁰ C.
With increasing the surfactant concentration the surface tension decrease dramatically
(Fig. 2a-c), till reaching to a steady state, where no more reduce in surface tension,
this point is known by critical micelle concentration (CMC). The CMC values were
calculated based on conductivity measurements (Figs. S1-S3, supporting information
and the data was outlined in the Table (1).
Decreasing the surface tension with increasing the surfactant concentration is due
to the migration of the surfactants unimers from solution bulk to the surface, therefore
the surface tension is reduced. At maximum surfactant saturation at surface, the
surfactant unimer start to aggregate together forming micelles instead of migration to
surface, therefor no more reduction in surface tension. This concentration at which the
micelle start to form is known by critical micelle concentration (CMC) which mainly
depend on the chemical structure of the synthesized Gemini surfactant, their affinity
with the solvent and temperature.
The depicted CMC values of CMTC, LMTC and PMTC surfactant in Table (1)
showed a decreasing trend with both increasing the surfactant hydrophobicity and
rising the solution temperature from 20 to 60 ⁰ C. Increasing the length of the

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hydrophobic tail of CMTC, LMTC and PMTC surfactant, lead to an increasing the
solution hydrophobicity so more repulsion between the surfactant tail and water,
consequently destroying the water structure, therefore the surfactant unimer favor to
aggregate in in solution bulk forming micelles, or migrate to the surface. Based on
that, increasing the hydrophobic carbon tail, the affinity toward the micelle formation
increase and can be formed at lower concentration [29-31]. The CMC values of the
ᴼ
synthesized CMTC, LMTC and PMTC surfactant at 20 C, are equal to 2.19, 1.45 &
0.6 mM, respectively (Table 1).
The calculated CMC values in Table 1, showed a decreasing trend with
o
increasing the solution temperature from 20 to 60 C (Fig. 2d). Increasing the
temperature, decrease the hydration around the hydrophilic head of CMTC, LMTC
and PMTC surfactant, which enhance the micellization affinity. At the same time,
rising the solution temperature destroy the water structure around the hydrophobic
tail, which disfavor the micelliztion and delay the micelle formation [32, 33].
Therefore, the temperature has two opposing effect on CMC, and net magnitudes of
the two aforementioned factors control in the behaviors of the CMC with temperature
[34-37]. The experimental data (Table 1) reflect a decreasing in CMC with raising the
solution temperature.
The Γmax & Amin of the prepared CMTC, LMTC and PMTC surfactant were
calculated according to Equations 2 & 3, respectively and the data were exploited in
Table 1. The data revealed that both Γmax & Amin are correlated with both surfactant
tail and temperature. The Γmax of the prepared CMTC, LMTC and PMTC surfactant
decreases with increasing number of carbon atoms from eight to sixteen, while the
Amin increase. Increasing the length of the hydrophobic tail, is followed by increasing
their occupied area at the interface, due to increasing the affinity of the surfactant tail
bend and curl, so Amin increase [38]. Increasing the Amin, suggesting the surfactant
unimers oriented horizontally on the surface, so amount of the accumulated surfactant
unimers is decreased at the interface. For example, the Amin of the CMTC, LMTC &
⁰
PMTC surfactant equal to 296.2, 303.5 & 318.9 A2 at 60 C respectively.
Table 1 outlines the temperature effects on both Γmax and Amin, where Amin
increased while Γmax decreas as the solution temperature increase. As the solution
temperature increased, the activation energies of CMTC, LMTC & PMTC unimers
also increased, which increased the repulsion induced between the double
hydrophobic tails and the aqueous medium. This behavior force the unimers to orient

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more horizontally at the surface, leading to increasing Amin, which promote a decrease
in the amount of accumulated CMTC, LMTC & PMTC unimers at surface, such that
Γmax decreased [39, 40].
The πCMC values of the synthesized aqueous CMTC, LMTC & PMTC at 20, 40,
and 60 ᴼC (Table 1), revealed that the PTMC surfactant is most effective surfactant in
reducing the surface tension among the three synthesized surfactant. The πCMC values
ᴼ
of CMTC, LMTC & PMTC at 60 C were 33.03, 36.18, and 37.44 dyne/cm,
respectively (Table 1). Generally the most effective surfactant is characterized by its
high CMC/C20 value [41, 42], which are confirmed by our results, where the PTMC
surfactant has higher CMC/C20 ratio compared to CMTC & LMTC surfactant as
depicted in the Table (1) at all tested temperatures.
The C20 values of the CMTC, LMTC & PMTC increase with both elongating the
hydrophobic tail of the surfactant & with the temperature (Table 1). Increasing the
hydrocarbon tail prompt the surfactant unimers migration affinity to the surface, so
more reduction in the surface tension take place [43]. with raising the temperature, the
CMC decrease as mentioned previously, due to the temperature effect on the
interaction between the surfactant and water and the predominant effect is decreases
the hydration around the hydrophilic head than that around the hydrophobic, so the
temperature has a direct relation with increasing the hydrophobicity, consequently
enhancing the migration to surface and more reduction in surface tension.
The krafft temperature of the synthesized Gemini cationic surfactant were determined
based on visual observation, where higher surfactants solution than the CMC was
prepared and clearly dissolved and then kept for 24 hrs at 0 ⁰C, then we raise the
temperature slowly for clearly dissolving the precipitated crystals [44]. The krafft
temperature of CMTC, LMTC and PMTC are 12, 15 and 21 ⁰C, respectively
indicating the good solubility of the synthesized surfactant. The in-cooperated
functional groups like four hydroxyl and two quaternary groups help greatly in
increasing the solubility of the synthesized inhibitors and consequently increasing the
area of their application.
Fig. (2)
Table 1

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The Gibbs equations (4-9) were used for predicting the thermodynamic performance
of the synthesized CMTC, LMTC & PMTC Gemini cationic surfactant in the aqueous
ᴼ
phase at 20, 40, and 60 C [18, 45-48], and the calculated thermodynamic parameters
were depicted in Table 2.
표
(
)
훥퐺푚ꢀ푐 = 2 1.5 − 훼 푅푇 푙푛(푋퐶푀퐶
)
(4)
(5)
휋퐶푀퐶
훥퐺푎표푑푠 = 훥퐺푚표 ꢀ푐 − 6.023푋10−2 (
⁄
ᴦ푀푎푥
)
표
훥퐺
푚푖푐
)
∆퐻푚ꢀ푐 = −푇2 (휕(
)
(6)
(7)
푇
p
p
휕푇
표
훥퐺
푎푑푠
)
∆퐻푎푑푠 = −푇2 (휕(
)
푇
휕푇
표
푚푖푐
∆퐻
−훥퐺
푚푖푐
∆푆푚ꢀ푐
∆푆푎푑푠
=
=
(8)
(9)
푇
표
푎푑푠
∆퐻
−훥퐺
푎푑푠
푇
Here, XCMC is the molar fraction of the CMC which is determined from surface tension
measurements and α is the degree of counter ion dissociation of the synthesized
Gemini cationic surfactants CMTC, LMTC and PMTC which is calculated from the
conductivity measurements (Figs. S1-S3, supporting information) according to the
Frahm’s method.
The calculated change in micellization free energy (ΔGomic) and adsorption (ΔGoads) of
the synthesized Gemini cationic surfactant (Table 2), are negative values, referring to
the both CMTC, LMTC and PMTC micelliztion and adsorption process are favorable
and spontaneous, where both adsorption at solution surface and micellization process
in bulk are a spontaneous trend in order to decrease the acquired solution energy as a
result of dissolving surfactant in the aqueous solution [49]. The data in Table 2,
revealed that, the ΔGo is more negative than that corresponding ΔGomic, reflecting
ads
the synthesized CMTC, LMTC & PMTC Gemini cationic surfactant prefer adsorption

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at solution surface first till maximum saturation at surface is attained, then they start
to aggregate together in micelles structure.
Both ΔGo and ΔGo are greatly influenced with the surfactant tail and with
ads
mic
solution temperature; whereas the adsorption and micellization and adsorption affinity
are enhanced with increasing the both hydrocarbon chain length and temperature
(Table 2). Elongation hydrocarbon chain length enhance disruption of the water
structure, leading to increasing the system free energy; which in-turn force the
CMTC, LMTC & PMTC unimers to migrate to surface (where the hydrophobic part is
oriented away from the solution and the hydrophilic head be in contact with the
aqueous phase) and/or aggregate forming micelles (where the hydrophobic part is
oriented interior the micelle structure away from the aqueous solution, while the
hydrophilic head be in contact with the aqueous phase as outer face) in a way to
decrease unfavorable increasing in the free energy [49, 50]. At the same time, with
raising the temperature, the water hydration around the CMTC, LMTC & PMTC
Gemini hydrophilic head decreased, which finally led to an increase in the free energy
of the system, so surfactant tend to aggregate in micelle or adsorb at the surface to
make stabilization for the system [51-53].
The change in the entropy of micellization ΔSmic and adsorption ΔSads (Table 2),
are positive implying more surfactant ordering during both micellization and
adsorption process respectively. The ΔSads is more positive than ΔSmic which refer to
more freedom of hydrophobic part during adsorption than micellization [54, 55]. The
positive value of changes in micellization enthalpy (ΔHmic) and adsorption enthalpy
(ΔHads), disclose that both micelliztion and adsorption process are totally controlled
by the entropy change, where transferring the methylene group from the interior of the
micelle in bulk or transfer from adsorption at surface into inside solution, lead to a
decrease in the entropy values and so less ordering [56-58].
Table 2
3.3. Corrosion inhibition performance:
I. Gravimetric study

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The corrosion inhibition efficiency of the synthesized CMTC, LMTC & PMTC
o
Gemini surfactant against steel corrosion in 1.0 M HCl at 20, 35, 50 and 65 C was
estimated throughout weight loss measurements following the protocol described in
material and experiments. The corrosion parameters corrosion rate (k), surface
coverage (θ) & the weight loss inhibition efficiency (ɳw) were calculated according
to equations 10-12.
∆푊
푘 =
(10)
(11)
(12)
푆푡
푊 −푊
휊
휃 =
푊
휊
푊 −푊
휊
휂 =
× 100
푤
푊
휊
The variables 푊 & 푊휊 refer to the experimental weight loss in the mild steel coupon
in the presence and absence of the prepared CMTC, LMTC & PMTC, while the 훥푤
correspond the average weight loss before and after immersion in 1.0 M HCl for 24
hrs, S refer to coupon surface area (cm2) and t refer to the immersion time (hrs). The
calculated corrosion parameters were listed in the Table 3. The data point to presence
a direct relation between the 휂 and the concentration of the prepared CMTC, LMTC
푤
& PMTC, due to increasing the surfactant adsorption at the steel surface with
increasing the concentration constructing a barrier between the aggressive medium
and steel, so the θ and 휂 increase and [7, 59, 60].
푤
The data in Table 3 and Fig. 3, point to an enhancement in the 휂 with increasing
푤
the length of the synthesized Gemini surfactant hydrophobic tail, where the ղw of
CMTC, LMTC & PMTC at 50 ᴼC, are equal to 87.19, 90.64 & 92.09 % at 1000 μM,
respectively. The surface study of the synthesized surfactants disclosed that, with
increasing the surfactant carbon chain length, the adsorption affinity of the surfactant
increase, so the covered 휃 enhanced, leading to enhancing the inhibition efficiency
[61-63].
The 휂 results in Table 3, revealed that, raising the solution temperature from 20
푤
to 50 ⁰C, the 휂 then it decrease again at 65 ᴼC. This behavior reflect that the
푤
synthesized CMTC, LMTC & PMTC inhibitors can adsorb on the steel surface

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physically and chemically under the tested condition through some electronic rich
functional groups such as hydroxyl, carbonyl, double bond, aromatic ring, C=N and
methoxy groups, with higher temperature, the weak physical bond destroy leading to
desorption from some active site causing corrosion [64, 65].
Fig. 3
Table 3
The Arrhenius equation (13), was used for studying the corrosion kinetics in
presence of the synthesized CMTC, LMTC & PMTC inhibitors
−퐸
ln 퐾 = 푎 + ln 퐴
(13)
푅푇
The relation between Ln K vs. 1/T (Fig. S4, supporting information) with a slope
−퐸
푎
equal to (
), which used for extracting the corresponding apparent activation
푅푇
energy 퐸 . The determined 퐸 values (Table 4) for the synthesized CMTC, LMTC &
푎
푎
PMTC inhibitors are lower than that in case of uninhibited solution, which could
ascribed to chemical adsorption of the inhibitors [66-68]. The Ea of 1000 μM of the
CMTC, LMTC & PMTC inhibitors equal to 37, 36.46 & 35.64 kJ/mol, respectively
compared to 42.03 kJ/mol for the uninhibited solution.
The activation enthalpy (ΔH*) and entropy (ΔS*) change have been estimated based
on the transition state theory (equation 8):
The activation enthalpy and entropy change (ΔH*, ΔS*) were calculated according to
equation 14 (transition state theory)
∗
∗
푅
Δ푆
∆퐻
ln (퐾푇) = ln (푁 ℎ) +
−
(14)
푅
푅푇
퐴
By plotting ln value (K/T) versus value 1/T (Fig. S5, Supporting information), a
straight lines were achieved with slope & intercept equal to ΔH*/R & ln ((R/NAh)+
ΔS*/R) respectively. The calculated ΔH* value (Table 4) reflect the endothermic
behavior of the tested steel corrosion in 1.0 M HCl [69, 70]. The negative ΔS* values
(Table 4), indicates that the activated complex in the rate-determining step is an
association rather than dissociation [71, 72].

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Table 4
The Langmuir adsorption isotherm (equation 15) showed the best fitted isotherm as
outlined in Fig. 4 with R2 close to 1.
퐶
1
= (퐾 ) + 퐶
(15)
휃
푎푑푠
Where the parameter C & Kads refer to the specified concentration of the synthesized
CMTC, LMTC & PMTC inhibitors and the adsorption equilibrium. The derived
slopes (Table 5) based on the equation 15, were found more than 1, indicating the
that each synthesized CMTC, LMTC & PMTC inhibitors unimer occupy more than
one adsorption active site, which consider obvious deviation in Langmuir theory,
consequently modified Langmuir isotherm (Villamil isotherm) were used based on
equation 16.
퐶
푛
= (퐾 ) + 푛퐶
(16)
휃
푎푑푠
Where n correspond to the slope and point to the displacements water number. The
standard free energy (ΔG°ads) of adsorption regarding to the synthesized CMTC,
LMTC & PMTC inhibitors was calculated according to equation 17 based on the
derived adsorption constant from equation 16 [73, 74].
°
(
)
∆퐺ads = −푅푇 ln 퐾ads
(17)
The parameter R and T resemble to the gas constant and absolute temperature in
kelvin.
the adsorption heat (∆Hoads) calculated according to equation 18 (The Van’t Hoff),
while the ∆Soads (standard adsorption entropy) determined according to equation 19.
°
ln퐾푎푑푠 = (−∆퐻ads) + Constant
(18)
(19)
푅푇
∆퐺a°ds = ∆퐻a°ds − 푇∆푆a°ds
The calculated ∆Goads, ∆So and ∆Ho which describe adsorption of the the
ads
ads
synthesized CMTC, LMTC & PMTC inhibitors on the steel surface have been
depicted in the Table 5. The ∆Goads reflect the adsorption process of the synthesized

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CMTC, LMTC & PMTC inhibitors on the steel surface is spontaneous and mixture
between chemical and physical adsorption, where the ∆Go vary from -39.08 to -
ads
45.39 kJ mol-1. The ∆Hoads depicted in the Table 5, predicted the exothermic nature of
the synthesized CMTC, LMTC & PMTC inhibitors adsorption on the steel surface.
The positive value of ∆Soads is due to the solvent entropy increasing and more water
desorption entropy.
Fig. 4
Table 5
II. Potentiodynamic polarization study
The performance of the synthesized CMTC, LMTC & PMTC inhibitors as
o
corrosion inhibitor of the steel was examined electrically at 20 C by plotting Tafel
curves as a relation between the potential and the corresponding current density as
outlined in the Fig. 5. The electrochemical corrosion parameters extrapolated from of
the Tafel curves have been inspected in Table 6. The 휃 covered by each concentration
of the synthesized CMTC, LMTC & PMTC inhibitors individually and the
corresponding inhibition efficiency from polarization measurements (ηp %) were
calculated using equations 20 & 21, respectively and the data shown in Table 6.
ꢀ
휃 = 1 −
(20)
ꢀ
휊
ꢀ
ղ푝 = ( 1 − ) × 100
(21)
ꢀ
휊
The io and i are the corrosion current densities of uninhibited and inhibited mild steel
with different concentration of synthesized CMTC, LMTC & PMTC inhibitors,
respectively. The 휂 (Table 6) was found to increase with incremental addition of the
푝
synthesized inhibitors as a result of increasing the inhibitor adsorption on the steel
surface so θ become more enhanced and forming a protective layer of the steel
surface. The 휂 shown a direct relation with increasing the length of the synthesized
푝
surfactant hydrophobic tail (Table 6). This could ascribed to increasing the surfactant
hydrophobic tail, the solution hydrophobicity increase, so the surfactant migration to

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the steel surface will be enhanced as it discussed in previous section, so more
protection for the steel surface. The 휂 of the synthesized CMTC, LMTC & PMTC
푝
inhibitors at 1000 μM are equal to 79.8 82.1 & 83 % respectively.
The change in the electrode potentials 퐸
of the synthesized CMTC, LMTC &
푐표푟푟
PMTC inhibitors doesn't exceed 85 mV, compared the blank corrosion potential, as
well as that shift is more shifted toward the negative direction indicating that the
CMTC, LMTC & PMTC inhibitors behave as mixed type inhibitors with predominant
cathodic inhibitor effect. Moreover, the synthesized inhibitors caused change in the
anodic (βa) and cathodic (βc) Tafel slopes cause change in the anodic and cathodic
Tafel slopes with no definite trend was observed, suggesting that the CMTC, LMTC
and PMTC behave as mixed-type inhibitors. It is concluded that both the anodic and
cathodic reactions were inhibited by the inhibitor through entirely occupying the
reaction sites of mild steel surface [75-80].
Fig. 5
Table 6
III. Electrochemical impedance study (EIS).
The impedance spectroscopy response (Fig. 6) was used for studying the corrosion
inhibition performance of the synthesized CMTC, LMTC & PMTC inhibitors at 20
⁰C. The obtained Nyquist curves Fig. 6, showed single and not perfect semicircle as a
result of steel roughness, inhomogeneity, and distribution of active sites. Fig. 6, reveal
that the CMTC, LMTC & PMTC inhibitors do not change the impedance shape
indication that, the synthesized inhibitors do not change the corrosion mechanism.
The Nyquist plots obtained in the real system is a general behavior, where the double
layer initiated on the metal/ solution interface does not behave as a real capacitor. As
known, the electrons on the metal side, control the charge distribution, while the ions
in the solution side control charge transfer. The equivalent ions to the charge on the
metal surface will occupy quite a large volume on the solution side of the double layer
due to the ions are much larger than the electrons. So the constant phase element
(CPE) used instead of double layer capacitance, Cdl, to represent the non-ideal
capacitive behavior of the double layer. The simple Randle equivalent circuit
consisting from the solution resistance (푅푠) and constant phase element which is

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parallel to the charge transfer resistance (푅푐푡) was used to fit the obtained Nyquist
curves as it is outlined in the Fig. 6d.
The double layer capacitance 퐶푑푙 values were determined from equation (22) [81, 82].
f (-푍″ꢀ푚ꢁ ) = 1⁄ (2πCdlRct)
(22)
The parameter 푍″ꢀ푚ꢁ refer to the frequency at maximum impedance imaginary
resistance. The inhibition efficiency from impedance measurements 휂 % calculated
푧
from equation (23), where both 푅푐푡 and Roct refer to the charge transfer resistance with
& without different concentration of CMTC, LMTC & PMTC inhibitors.
휂 = ((Rct-Ro )/Rct) x100
(23)
ct
푧
The extracted data from impedance response depicted in Table 7, reflect increasing
the 푅푐푡 values & decreasing the Cdl with the increasing the concentration of CMTC,
LMTC & PMTC inhibitors and with elongation the surfactant hydrophobic tail. The
decrease in 퐶 is due to decreasing the local dielectric constant and/or increasing the
푑푙
thickness of the double layer that initiated by the CMTC, LMTC & PMTC
compounds [83, 84]. The value of 퐶 is in the presence of the synthesized Gemini
푑푙
cationic surfactant inhibitor is smaller than in its absence. This due to the effective
adsorption of the synthesized inhibitors, so the replacement of water molecules at by
inhibitor molecule that adsorbed at the electrode surface. By increasing the
hydrophobic chain length, the adsorption efficiency at steel surface increase, so the
퐶
become reduced, where formed layer by the synthesized Gemini cationic
푑푙
inhibitors CMTC, LMTC & PMTC resist the charge transfer so the 푅푐푡 increases as it
is clearly by increasing the diameter of the semicircle (Fig. 6).
Fig. 6
Table 7
The inhibition efficiency of the synthesized Gemini surfactant are in the same range
with most of other published Gemini surfactants with similar functional groups [14,
26, 70, 79, 85]. The Gemini inhibitor PMTC showed inhibition efficiency 90.17 and
92.09 % at temperature 35 and 50 ⁰C, respectively. A dewale Adewuyia et.al.,
synthesized Gemini surfactant containing 4 ester groups as well as other 5 carbonyl
groups, however its inhibition efficiency was 92 % at 35 ⁰C [85]. M.A. Hegazy et. al.,

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prepared Gemini surfactant containing carbonyl and ether groups and showed
inhibition efficiency 92.8 at 40 ⁰C [70]. The synthesized Gemini surfactant with
tertiary amine and aromatic ring (IL1) by M.I. Nessim et. al., showed an inhibition
efficiency of 86.8% [86]. Although our synthesized Gemini surfactants CMTC,
LMTC and PMTC have vast number of functional groups which enhance their
inhibition for steel corrosion, however these Gemini surfactants don’t show an
impressive inhibition efficiency as expected, this may be due to the stereochemistry
and planarity of the surfactant structure on the steel surface.
The synthesized Gemini quaternary surfactant have been used as inhibitors against the
acid corrosion of iron, where these surfactants can be adsorbed on the metal surface
because it can form a bond between the polar head groups which is rich by many of
functional groups and the metal surface, consequently reducing the corrosion attack
on the metal steel surface. The synthesized CMTC, LMTC and PMTC contain
number of electronic rich functional groups like hydroxyl, carbonyl, benzene ring,
double bond, and azomethine (-C=N-) groups. As outlined from the thermodynamic
calculation of the adsorption isotherm (Table 5), the adsorption of the synthesized
inhibitor is mixture of physical and chemical adsorption. The physical adsorption
occur due electrostatic interaction between the positively ammonium group and both
the negatively counter ion that specifically adsorbed on the steel surface and the
cathodic site on the steel surface in the acidic medium. While the chemical adsorption
of the studied inhibitor arises from the donor-acceptor interactions between electronic
rich groups and vacant d-orbitals of iron. The electronic rich groups are free electron
pairs (hydroxyl, nitrogen and oxygen of carbonyl), ℼ-electrons of multiple bonds
(C=C, -C=N- and C=O) as well as phenyl group [80, 87].
3.4. Biological Activity
With the upraising of the microbial resistance, we are need to developing a new
antimicrobial agent like surfactant. The biological activity of the synthesized cationic
Gemini surfactant CMTC, LMTC and PMTC have been determined utilizing the filter
paper disc agar method. The biological activity results which are depicted in Table 8,

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declared that the synthesized surfactant have high biological activity against the test
bacteria and fungi.
The data inspected in Table 8, point to that the biological activity is affected by the
surfactant tail, as seen, the biological activity increased with increasing the surfactant
tail from 8 to twelve carbon atoms then it decreased again (Table 8), where this
phenomena is known by cut-off effect, which mainly depends on the total surface
activity of the synthesized Gemini cationic surfactant CMTC, LMTC & PMTC (like
CMC, Γmax, and ∆Gads), which depend mainly on the chemical structure [73, 88-91].
The maximum antimicrobial activity was observed for the synthesized LMTC Gemini
surfactant this cut-off effect depend mainly the studied surfactant surface parameters.
As previously discussed (Table 1), increasing the hydrophobic chain length of the
surfactant has a dramatic decreasing effect on the critical micelle concentration, so the
surfactant with longer tail (PMTC), has a lower concentration at the bacteria cell
membrane compared to shorter surfactant tail; consequently the expected order of
biological activity is CMTC > LMTC >PMTC. At the same time, with increasing the
surfactant tail, the adsorption affinity at bacteria surface increase as predicted from
increasing the ∆Gads values (Table 2), this mean the surfactant PMTC can be present
on the bacteria cell membrane at more lower concentration, so the antibacterial effect
is predicted to increase, based on that the expected biological activity order is PMTC
> LMTC > CMTC. Finally, the net magnitudes of all the surface parameters are
responsible for the antimicrobial activity of the synthesized surfactant. The
experimental results showed that the synthesized surfactant LMTC was the most
effective. The surfactant LMTC has the highest biological activity, the inhibition zone
diameter o is equal to 35, 35, 18, 19, 26 & 26 mm against the Staphylococcus aureus,
Bacillus subtilis, Escherichia coli, Pseudomonas aeruginosa, Aspergillus niger and
Candida albicans respectively.
The mechanism is mainly depends on the adsorption affinity of the synthesized
Gemini cationic surfactant CMTC, LMTC and PMTC on the cell wall membrane of
the bacteria and fungi, which have a hydrophobic nature. In addition to the main
functional group regarding to the surfactant like oxygen, double bonds, carbonyl
groups, and nitrogen enhance binding to the membrane. As a result of adsorption at
the cell wall membrane, the hydrophobic tail can penetrate into the cytoplasmic cell

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and destroying the bacteria cell wall selective permeability and releasing the
cytoplasm causing cell death [92, 93].
Conclusion
A series of Gemini cationic surfactant with different hydrophobic tail length labelled
CMTC, LMTC & PMTC were synthesized from some of commercial materials. The
1
chemical structure of the new Gemini was confirmed by FTIR & HNMR. The
hydrophobic tail of the synthesized Gemini as well as temperature control their
behavior in solution. The PMTC Gemini cationic surfactants with longest tail length
showed lowest critical micelle concentration; which also showed a decreasing trend
with raising the solution temperature. The thermodynamics insight refer the higher
tendency of the synthesized surfactants toward adsorption and micellization with both
hydrophobicity & temperature. The prepared electronic rich surfactant showed good
inhibiting against the steel corrosion; which is correlated with the surfactant structure
& the temperature. The PMTC surfactant showed highest inhibition efficiency 92.09
% at 55 ⁰C, due to its higher adsorption affinity compared to shorter surfactant tail
CMTC. The polarization measurement revealed that the three surfactants behave as
mixed type inhibitors. The synthesized three electronic rich Gemini surfactant showed
high antimicrobial activity against bacteria & fungi, which was found correlated with
the surfactant hydrophobic behaviors.
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a
b
Fig. 1: FTIR (a) & 1HNMR (b) spectrum of the synthesized PMTC Gemini cationic
surfactant.

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b
a
20 oC
40 oC
60 oC
20 oC
70
60
70
60
50
40
30
40 oC
60 oC
50
40
CMTC
LMTC
30
-7
-6
-5
-4
-3
-2
-7
-6
-5
-4
-3
-2
Log (Conc.)
Log (Conc.)
c
d
70
60
50
40
30
20 oC
40 oC
60 oC
CMTC
LMTC
PMTC
2.0
1.5
1.0
0.5
0.0
PMTC
20
40
60
-7
-6
-5
-4
-3
-2
Temperature (oC)
Log (Conc.)
Fig. 2: (a-c) The relationship between surface tension and log concentration of
Gemini cationic surfactant CMTC, LMTC and PMTC, respectively, at various
temperatures; (d) Temperatures and hydrophobic chain length effect of the
synthesized cationic surfactant.