Sulfidogenic-corrosion inhibitory effect of cationic monomeric and gemini surfactants: Planktonic and sessile diversity

Article abstract of DOI:10.1039/c6ra02393b

A cationic monomeric surfactant (CMS) and a cationic gemini surfactant (CGS) were successfully synthesized and characterized. A comparison of the CMS and CGS metal corrosion inhibitory effects was carried out on the basis of protecting a metal surface from cultivated salinity (3.18% NaCl) and an environmental sulfidogenic bacterial activity in a reactor's bulk phase (planktonic) and on a metal surface (sessile). Environmental sulfidogenic bacterial consortia were originated from an oil-field water tank. The sulfidogenic bacterial activities were evaluated based on redox potential and sulfide concentrations in the reactor's bulk phase. In addition, changes in biofilm structures and constituents and the metal corrosion rate were determined to estimate changes on metal surfaces. Results in the reactor's bulk phases showed that at high surfactant (CMS, CGS) concentrations, a considerable decline in the redox potential was observed and the sulfide productivity was completely suppressed. The synthesized surfactants showed the highest metal corrosion inhibition efficiencies of 92% and 94% at concentrations of 10 mM and 1 mM for the CMS and the CGS, respectively. In addition, the synthesized CMS and CGS showed nonspecific antibacterial activity against Gram-positive and Gram-negative bacterial-standard strains.

Full text of DOI:10.1039/c6ra02393b

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Sulfidogenic-corrosion inhibitory effect of cationic
monomeric and gemini surfactants: planktonic and
sessile diversity
Cite this: RSC Adv., 2016, 6, 42263
a
b
c
a
A. Labena,* M. A. Hegazy, H. Horn and E. Muller
¨
A cationic monomeric surfactant (CMS) and a cationic gemini surfactant (CGS) were successfully
synthesized and characterized. A comparison of the CMS and CGS metal corrosion inhibitory effects was
carried out on the basis of protecting a metal surface from cultivated salinity (3.18% NaCl) and an
environmental sulfidogenic bacterial activity in a reactor's bulk phase (planktonic) and on a metal surface
(sessile). Environmental sulfidogenic bacterial consortia were originated from an oil-field water tank. The
sulfidogenic bacterial activities were evaluated based on redox potential and sulfide concentrations in
the reactor's bulk phase. In addition, changes in biofilm structures and constituents and the metal
corrosion rate were determined to estimate changes on metal surfaces. Results in the reactor's bulk
phases showed that at high surfactant (CMS, CGS) concentrations, a considerable decline in the redox
potential was observed and the sulfide productivity was completely suppressed. The synthesized
surfactants showed the highest metal corrosion inhibition efficiencies of 92% and 94% at concentrations
of 10 mM and 1 mM for the CMS and the CGS, respectively. In addition, the synthesized CMS and CGS
showed nonspecific antibacterial activity against Gram-positive and Gram-negative bacterial-standard
strains.
Received 27th January 2016
Accepted 13th April 2016
DOI: 10.1039/c6ra02393b
www.rsc.org/advances
application is the most practical strategy for microbial corro-
sion prevention. In a particular system, a surfactant (surface
1
. Introduction
5
Metals deteriorate in aqueous environments not only by active agent) is dened as a substance that at a specic
chemical or electrochemical reactions but also by microbial concentration adsorbs partially or completely to an interface
6
metabolic activities in a process called microbially inuenced (metal/liquid). A monomeric surfactant composed of a hydro-
1
corrosion (MIC). MIC is a well-documented phenomenon that philic polar head is linked to a hydrophobic non-polar tail.
causes a plethora of economic and environmental problems When surfactant molecules are added to the water, it escapes
such as corrosion of pipelines and tanks, plugging of injection from bulk phase to an available interface as a result of its
2
or disposal wells, and souring of oil reservoirs. Suldogenic hydrophobic nature. Surfactants have been abundantly applied
microorganisms are known to be implicated in many cases of to protect metal surfaces from corrosive environments (corro-
3
7,8
MIC. The corrosion activity of the suldogenic microorganisms sion inhibitors) and corrosive microorganisms (biocides).
is attributed mainly to a process called cathodic depolarization. Improving the structure of a monomeric surfactant leads to
These suldogenic microorganisms can induce corrosion either enhanced surface properties and thus improves metal corrosion
9
in a bulk phase (planktonic cells) or localized in the form of inhibition and biocidal efficiencies. The new gemini surfac-
a layer consisting of a multispecies microbial community, tants (also called dimeric) are composed of at least two hydro-
extracellular polymeric substances (EPS), and inorganic mate- philic heads and two hydrophobic chain tails connected by
10
rial and water (biolms). The hypothesized mechanism of the a spacer group. Many studies revealed that gemini surfactants
suldogenic biolms-induced corrosion has been attributed to possess many physicochemical properties that are superior to
many factors such as electron transfer processes within a bio- those of corresponding monomeric surfactants. Gemini

lm matrix, and metal ions bound by EPS that can catalyze the surfactants show a remarkably low critical micelle concentra-
cathodic reactions. In the oil and gas industry, biocide tion (CCMC), unusual aggregation morphologies, high depen-
4
dence on spacer structure, and strong hydrophobic micro-
11
domain, therefore giving high metal surface protection. The
oil and gas industries in Egypt still suffer from corrosion
problems induced by environmental factors such as high water
salinity and suldogenic microbial activities, although corro-
sion inhibitors and biocides are widely applied. A comparative
a
Technical University of Munich, Urban Water Systems Engineering, Am Coulombwall,
8
b
5748 Garching, Germany. E-mail: labena.labena@gmail.com
Egyptian Petroleum Research Institute (EPRI), Nasr City, Cairo, Egypt
c
Karlsruhe Institute of Technology, Engler-Bunte-Institut, Water Chemistry and Water
Technology, Engler-Bunte-Ring 1, 76121 Karlsruhe, Germany
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study between a cationic monomeric surfactant (CMS) and of species ions in solution and
a cationic gemini surfactant (CGS) was carried out in Egypt. concentration.
C
is the surfactant
Corrosion inhibition activity and effectiveness of the synthe-
2.2.4. Minimum surface area (Amin). A minimum surface
sized surfactants were investigated to determine level of area (Amin) is dened as an area occupied by one molecule in
2
protection of metal surfaces not only from corrosive environ- nm at an interface and was elucidated for the CMS as follows:
ment (high water salinity) but also from the activity of envi-
ronmental suldogenic bacteria. Biocidal and corrosion
inhibition activities were evaluated by measuring change in
1
4
1
0
A
min ¼ N
(3)
A
G
max
ꢁ
2
A
redox potential, microbial sulde production, biolm structure where N is the Avogadro's number and Gmax (mol m ) is the
and consistency and metal corrosion rate calculation.
maximal surface excess of the adsorbed surfactant molecules to
the interface.
6
2
.2.5. Electrical conductivity (K). An electrical conductivity
2
. Materials and experimental
measurement was carried out using a conductivity meter
LF 191 WTW) at 25 C to determine the C
degree of counter ion dissociation (b) of the CMS. The CCMC
value calculated from the electrical conductivity was used to
conrm the CCMC value detected from the previous surface
ꢀ
(
value and the
methods
CMC
2.1. Surfactant synthesis
2.1.1. Cationic monomeric surfactant (CMS). CMS in this
study was synthesized through two main reactions. The rst was
quaternization between 1 mol of N,N-dimethylpropan-2-amine
and 1 mol of 1-bromododecane. The reactants were allowed to
reux in ethanol at 70 C for 12 h, and then the reaction mixture
was le to cool. In the second reaction, 1 mol of the reactant was
tension result.
o
2.2.6. Standard free energy of micellization (DGmic). A
o
standard free energy of micellization (DGmic) per mole of the
CMS was evaluated as follow:
ꢀ
12
o
allowed to react with 1 mol of potassium hydroxide. Next, it was
mic
DG ¼ (2 ꢁ b)RT ln CCMC
(4)
ꢀ
reuxed in ethanol at 70 C for 2 h. The reaction mixture was le
to cool for 1 h. At the end of the synthesis, the obtained white where b is the degree of counter ion dissociation, R is the gas
precipitate was recrystallized twice from ethanol.
.1.2. Cationic gemini surfactant (CGS). The CGS in this
constant, T is the temperature, and CCMC is the molarity of the
synthesized surfactant.
2
The surface active characteristics of the CGS were previously
calculated and evaluated by Labena et al.
study was synthesized as it has been previously reported by
Labena and co-workers. The chemical structure of the synthe-
8
8
sized surfactant (CMS) was conrmed by Fourier transform
infrared (FTIR) and nuclear magnetic resonance (NMR) spec- 2.3. Comparative study of biocidal and metal corrosion
troscopy (Bruker, Vortex 70 and Bruker, 400 MHZ NMR spec- inhibitory effect of CMS and CGS on environmental-
trometer, Avance DRX 400 for FTIR and NMR, respectively).
suldogenic bacteria cultivated at high medium salinity
(3.19% NaCl)
2.2. Surface active characteristics
2.3.1. Environmental suldogenic bacterial community
and growing conditions. The water tank of an Egyptian petro-
leum company, located in the western desert of Egypt, Qarun
2
.2.1. Surface tension (g). A surface tension value was
detected at different concentrations of the CMS according to the
De N ¨o uy ring method using a Tensiometer (Kruss Type 6).
Calibration was carried out using pure distilled water with

eld, the Karama southwest (KSW), was selected as a case study
in this work. Although corrosion inhibitors and biocides are
applied in the company, water tank corrosion problems still
occur. Therefore a water sample was collected from a water tank
outlet. The water sample named KSW was characterized by high
salinity (3.19% NaCl) and slightly acidic water (pH 6.2). The
KSW water sample was inoculated on site in a modied Post-
ꢁ
1
ꢀ
a surface tension of 72 mN m at 25 C.
2.2.2. Surface pressure at CCMC (pCMC). Surface pressure of
6
the CMS at the CCMC point was obtained from eqn (1):
pCMC ¼ g ꢁ g
(1)
o
CMC
13
gate's-B medium as reported by Postgate. Modication of the
where g
distilled water and surface tension value at the CCMC point, salinity (3.19% NaCl) and pH (6.2) during medium preparation.
respectively.
The medium was prepared, sparged with nitrogen gas accord-
.2.3. Surface excess concentration (Gmax). A surface excess ing to the modied Hungate's technique for anaerobes. In
o
and gCMC are the surface tension value of pure Postgate's-B medium was obtained by using the original water
14
2
concentration of the CMS was calculated according to Gibbs' order to conrm anaerobic cultivation, a resazurin indicator
6
adsorption equation:
(0.0002% w/v) was used. The medium was inoculated with the
water sample 10% (v/v) and incubated for 14 days at 37 C. The
(2) inoculated medium was enriched three times and thus used as
inocula for further experiments.
ꢀ
ꢀ
ꢁꢀ
ꢁ
ꢁ
1
dg
G
max
¼
nRT
dln C
where Gmax is the surface excess concentration of surfactant
2.3.2. Experimental design and monitoring of suldogenic
ions, R is the gas constant, T is the absolute temperature, g is bacterial activity in reactor's bulk phase and on metal surface. A
the surface tension at a specic concentration, n is the number comparative effect of synthesized surfactants on the
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suldogenic microbial activity was carried out on batch-reactor suldogenic bacterial activity in the reactor's bulk phase, the
experiments using a modied Postgate's-C medium as reported mild steel coupons with the attached biolms were taken from
13
by Postgate. Modication was done by applying water salinity reactor B and immersed in a 0.85% NaCl solution to remove the
3.19% NaCl) and pH (6.2) during medium preparation. A mild planktonic cells (unattached cells). For detection/quantication
(
0
0
00
00
steel coupon (CS1018 3 ꢂ 1/2 ꢂ 11/6 strip, Cormon LTD) was of the suldogenic bacteria (nucleic acids) and EPS components
used as a principal iron source for the cultivated suldogenic on the metal surface, a method reported by Labena and
8
bacteria with a chemical composition as previously reported by co-workers was applied. For each biolm, ve microscopic
Labena and co-workers. The present work was carried out using elds were randomly chosen and scanned for nucleic acids
8
three reactors types: (1) blank reactors (containing Postgate's-C (bacteria), EPS glycoconjugates and proteins (EPS components).
medium with salinity of 3.19% NaCl and not inoculated with The coverage (C) of each single image of the image stack was
ꢀ
the environmentally suldogenic bacteria), (2) control reactors quantied via Image J soware. The average coverage (Cstack)
(
containing Postgate's-C medium with salinity of 3.19% NaCl, was calculated for each image stack from the rst slice (n ¼ 1)
inoculated environmentally and without surfactants), and (3) where the average coverage is equal to 0.1% of the last slice (n ¼
19
surfactant reactors (containing Postgate's-C medium with
salinity of 3.19% NaCl, inoculated environmentally and with
different concentrations of the synthesized surfactants). The
experimental reactors (containing 150 ml) were constructed and
each was inoculated with an inoculum of enriched suldogenic
bacteria (3 ml). The rst reactor (reactor A) was cultivated in
order to evaluate the suldogenic bacterial activity in the reac-
tor's bulk phase every 5 days for a 1 month incubation period.
The suldogenic bacterial activity was evaluated by measuring
redox potential using the SenTix ORP electrode, WTW and
sulde concentration according to the German Standard
n
max), where the average coverage is equal to 0.1%, as follows:
n¼n
max
X
1
C
stack ¼ n
C
(5)
max
n¼1
Aerward, the mean average coverage values of the ve
scanned microscopic elds on the same metal coupon were
determined and used to monitor change in the nucleic acids,
EPS glycoconjugates and proteins in different suldogenic
bacterial biolms.
The biolms on the metal surfaces, the metal surfaces (aer
removing the cultivated suldogenic biolms), and the metal
surfaces inoculated with the enriched suldogenic bacteria and
inoculated with the optimum biocide concentration were tested
using the SEM model Leica/Cambridge StereoScan 360 operated
at an acceleration voltage of 20 V and at magnication ranging
from 50ꢂ to 10 000ꢂ. At the end of the incubation period
1
5
Methods. In addition, a separate reactor (reactor B) was
applied to determine changes in suldogenic biolm structure
and constituents using a confocal laser scanning microscopy
(
CLSM) and staining protocol. Furthermore, an additional
reactor (reactor C) was implemented to detect the suldogenic
biolm on the metal surface, the metal surface (aer removing
the suldogenic biolm) and the metal surface inoculated with
the suldogenic bacteria at the optimum surfactant concen-
tration using scanning electron microscopy (SEM). Additionally,
the biocidal effect of the CMS and the CGS against environ-
mental suldogenic-bacterial growth was further monitored by
determining the minimum inhibitory concentration (MIC). MIC
is dened as “the lowest concentration of an antimicrobial
agent that inhibits the development of visible microbial
growth”. Samples were taken from the reactor A bulk phase
every 5 days during the incubation period and evaluated for
viable bacterial counts using the most probable number (MPN)
method. MPN is dened as “a statistical value showing the
viable bacterial counts in a water sample using dilution and
multiple tube inoculations.” Aer the 30 day incubation period,
the metal coupons were taken from reactor A and immersed in
(30 days), the metal coupons were takeout from reactor C, xed
with 3% glutaraldehyde PBS (pH 7.3–7.4) for 4 h, washed twice
with PBS (5 min each), rinsed with distilled water twice (5 min
each) and then dehydrated using an ethanol gradient (50%,
75%, 95% and 99%) for 10 min before storage in a desiccator.
2.3.3. Nonspecic antibacterial activity. Antibacterial
activities of the synthesized CMS were tested against different
bacterial standard strains (Deutsche Sammlung von Mikroor-
ganismen und Zellkulturen [DSMZ]) as follows: Gram-positive
bacteria (Staphylococcus aureus DSM 3463) and Gram-negative
bacteria (Escherichia coli DSM 30083 and Pseudomonas aerugi-
nosa DSM 50071). The antibacterial activity of the synthesized
1
6
17
20
CMS was evaluated via a modied agar well diffusion method.
Nutrient agar plates were inoculated with the tested strains by
streaking on the nutrient agar plates. For each agar plat,
a sterile 10 mm borer was used to cut three equidistant wells. In
addition, 0.2 ml of the synthesized CMS was introduced into the
a Clarke solution (1 L 36% HCl (v/v), 20 g Sb
for 10–15 s, washed twice with deionized water and ethanol, and
nally dried in a desiccator. Next, the dried metal coupons were
2 3 2
O and 50 g SnCl )

ꢀ
wells. The agar plates were incubated overnight at 37 C. The
weighed and the weight loss was determined by comparing
antibacterial activity was determined by measuring inhibition
zone diameter (mm). In addition, sterile water was used as
a negative control, and a standard tetracycline solution was
used as a positive control. In addition, the MIC of the synthe-
sized CMS against the test microorganisms was determined as
the lowest concentration of the antimicrobial agent that
inhibited the visible bacterial growth on the nutrient agar plates
their weight aer and before the incubation period (30 days).
ꢁ
2
ꢁ1
The metal corrosion rate (g m
d ) and metal corrosion
inhibition efficiency of the synthesized surfactants were calcu-
18
lated from the weight loss results using ASTM G4/95 2001.
CLSM with staining methods was applied in order to
monitor changes in bacterial cells (nucleic acids) and EPS
distribution within the suldogenic-biolm matrices in the
presence and absence of CMS and CGS. At the highest
ꢀ
incubated at 37 C overnight.
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1
3
C NMR spectrum conrmed the expected carbon distribution
3
. Results and discussion
in the synthesized CMS.
3.1. Chemical structure of CMS
The nomenclature and chemical structure of the synthesized
The chemical structure of the synthesized surfactant CMS was CMS and CGS are presented according to the International
elucidated and conrmed by FTIR and NMR spectroscopy.
.1.1. FTIR spectra. The FTIR spectrum of the synthesized
CMS indicated that the characteristic peaks (see Fig. 1) for the
Union of Pure and Applied Chemistry (IUPAC) (see Fig. 3).
3
ꢁ
1
3.2. Surface active characteristics
alkyl moiety were 2916.77 and 2849.92 cm for asymmetric and
ꢁ
1
symmetric stretching (CH), respectively. While at 1383.32 cm
Changes in the surface tension (g) value of the synthesized CMS
ꢁ
1
for symmetric bending (CH ), 1468.86 cm
for symmetric at different concentrations are presented in Fig. 4 in comparison
3
ꢁ
1
+
8
bending (CH
band appeared at 1093.79 cm . The FTIR spectrum promoted was noted with increasing CMS concentrations. Aerwards, the
the expected functional groups in the synthesized CMS.
curves broke rapidly by decreasing the synthesized surfactant
.1.2. NMR spectra. H NMR (DMSO) spectrum of the concentrations until reaching a plateau region, indicating
2
) and 718.92 cm for –(CH
2
)
n
– rocking, the R
4
N
with the synthesized CGS. A linear decrease in surface tension
ꢁ
1
1
3
synthesized CMS showed different characteristic peaks (see formation of micelles. The concentration corresponding to the
Fig. 2a) at 0.85 ppm (t, 3H, NCH CH (CH ) CH ), 1.23 ppm break point is called the critical micelle concentration (CCMC).
2
2
2 9
3
ꢁ
1
(
(
3
m, 16H, NCH CH (CH ) CH ), 1.73 ppm (m, 2H, NCH CH
The surface tension of the synthesized CMS was 31 mN m in
2
2
2 9
3
2
2
ꢁ1
CH ) CH ), 3.28 ppm (t, 2H, NCH CH (CH ) CH ), 3.52 ppm (s, comparison to the synthesized CGS at 28 mN m . The synthe-
2 9 3 2 2 2 9 3
ꢁ
3
ꢁ3
H, NCH
3
), 3.97 ppm (m, 1H, NCH(CH
3
)
2
), and 1.43 ppm (d, 6H, sized CGS demonstrated a very low CCMC (4.3 ꢂ 10 mol dm
ꢁ3
)
1
ꢁ2
NCH–(CH
3
)
2
). The data for the H NMR spectrum demonstrated compared to the synthesized CMS (1.5 ꢂ 10
mol dm ).
21
the expected hydrogen proton distribution in the synthesized Lowering CCMC leads to better solubility. The synthesized CMS
CMS, indicating no byproduct.
was effective in reducing the surface tension of water to the
) (see Fig. 2b) spectra of the synthesized CMS present value. The synthesized CGS was more effective at the
1
3
C NMR (CDCl
3
displayed different peaks at d ¼ 14.00 ppm (NCH CH CH -
CCMC point than the synthesized CMS. This attitude was related
2
2
2
(
CH ) CH CH CH ); d ¼ 16.82 ppm (NCH CH CH (CH ) CH - to the gemini surfactant's structure in addition to its strong
2
6
2
2
3
2
2
2
2 6
2
CH CH ); d ¼ 22.62 ppm (NCH CH CH (CH ) CH CH CH ); d ¼ binding ability to its head group-counter ion.
2
3
2
2
2
2 6
2
2
3
2
(
9.40 ppm (NCH
NCH CH CH (CH
CH (CH CH CH
CH CH CH
); d ¼ 48.15 ppm (NCH
NCH(CH
); and d ¼ 18.82 ppm (NCH(CH
2
CH
CH
CH
); d ¼ 64.54 ppm (NCH
2
CH
2
(CH
2
)
6
CH
); d ¼ 26.37 ppm (NCH
CH CH (CH
2
CH
2
CH
3
); d ¼ 26.37 ppm
The synthesized surfactant lowered the surface energy
present in the excess concentration or near the surface. The
2
2
2
)
2 6
2
CH
2
CH
3
2
CH
2
-
-
1
0
)
6
2
2
3
2
2
2
)
2 6
surface excess concentration (Gmax ꢂ 10 ), result of the
2
2
ꢁ2
); d ¼ 62.66 ppm synthesized CMS, was 6.19 mol m
in comparison to the
2
2
3
3
ꢁ
2
(
3
)
2
)
3 2
). The data for the synthesized CGS 6.29 mol m . Results indicated that by
increasing the hydrophobic chain length, as in the case of the
synthesized gemini surfactant, hydrophobicity increases. The
Fig. 1 FTIR spectra of synthesized cationic monomeric surfactant (CMS).
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1
13
Fig. 2 NMR analysis of synthesized cationic monomeric surfactant (CMS). (a) H NMR, (b) C NMR.
8
synthesized gemini surfactant molecules are thus directed previously reported. In addition, the surface pressure (effec-
toward the interface. Therefore, the surface energy of the solu- tiveness) of the synthesized surfactants (pCMC) increases as the
tion decreases in comparison to the synthesized CMS, which in minimum surface area (Amin) of the adsorbed molecules
23
turn leads to an increase in the maximum surface excess as has declines, as previously adduced. The minimum surface area of
2
2
2
2
been previously reported.
The synthesized CGS had the CMS was 0.27 nm in comparison to 0.26 nm for the
o
a distinctly higher surface activity than the synthesized CMS, as synthesized CGS. Moreover, the negative values of DGmic for the
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Fig. 3 IUPAC chemical structures of the synthesized surfactants (CMS and CGS).
Fig. 4 Surface tension of synthesized cationic monomeric surfactant (CMS) in comparison to synthesized cationic gemini surfactant (CGS) at
ꢀ
different concentrations in water at 25 C.
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Fig. 5 Conductivity of synthesized cationic monomeric surfactant (CMS) in comparison to synthesized cationic gemini surfactant (CGS) at
ꢀ
different concentrations in water at 25 C.
synthesized surfactants indicated a tendency of the synthesized composition in the water sample (W) and its enriched and
23
surfactant molecules to be adsorbed at the interface. In cultivated biolm (CB). The DGGE result (data not shown)
addition, the thermodynamically stable micelles were formed indicated that the KSW sample was represented by highly
2
2
o
spontaneously. Next, there is an increase in DG
negative diverse bacterial communities. The results exhibited change in
mic
ꢁ
1
value of the synthesized CGS (ꢁ22.89 kJ mol ) in comparison the suldogenic microbial diversity of the original water sample
ꢁ1
to the synthesized CMS (ꢁ18.28 kJ mol ). This result means (W) and its related CB. The result was represented in 7 DGGE
that the synthesized CGS adsorbs at the interface faster than the bands from W and 10 from the CB. The suldogenic bacterial
synthesized CMS, and thus acts as a greater driving force for communities were related to the Desulfovibrio genus (phylum
micellization. It is well known that conductivity, in both a pre- Proteobacteria, class Deltaproteobacteria) and genus Desulfo-
micellar and a postmicellar area, is linearly correlated to tomaculum (phylum Firmicutes, class Clostridia). The taxo-
24
surfactant concentrations. The intersection point between the nomic affiliation of the genera Desulfotomaculum (phylum
two lines gives the CCMC value. In addition, the ratio between Firmicutes) within the phylum Proteobacteria can be explained
the two slopes gives the b value. The CCMC value determined by the presence of nonorthologous dsr-genes in these species as
from the specic conductivity number (Fig. 5) were in agree- a lateral gene transfer from species belonging to the phylum
27,28
ment, with the CCMC value determined from the surface tension Proteobacteria.
No archaea species were detected in the
value gure (Fig. 4). It is clearly inferred from the surface active DGGE band sequences with the dsrb-gene. In addition, the KSW
8
properties that variation between the synthesized CGS and water sample was evaluated for non-suldogenic-Archaea using
25,26
CMS was related to increases in cation bulkiness.
a 16S rRNA-cloning method. However, the result was completely
Stability and performance of the synthesized cationic negative.
monomeric and gemini surfactants were evaluated in water
with a salinity of 3.19% NaCl as a case study, by measuring
surface tension for 14 days; CCMC values are shown in Fig. 6.
Data in Fig. 6 show that CMS and CGS were stable for more than
1
week and then slightly decreased with time until reaching
a maximum value aer 14 days in the case study water salinity.
It has been observed that the stability of gemini surfactant was
less than the monomeric surfactant. This behavior was attrib-
uted to the degradation increasing as the number of the
6
hydrophobic chain length group (fatty chain) increases.
3.3. Comparison between synthesized CMS and CGS metal
corrosion inhibitory and biocidal effects against respective
cultivated salinity and environmental suldogenic bacterial
activity
Dissimilatory sulte reductase-b subunit (dsrb), based on
denaturing gradient gel electrophoresis (DGGE), was used to
Fig. 6 Stability and surface performance (surface tension) of cationic
monomeric and gemini surfactants (CMS, CGS) in water with salinity of
characterize the suldogenic microbial community's 3.19% NaCl for 14 days.
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Depending on the CCMC values of synthesized surfactants, reactors supplemented with the CMS and CGS concentrations
synthesized surfactant ranges were detected. The C
for the of 0.01 mM and 0.1 mM were optimal for suldogenic bacterial
CMC
ꢁ3
ꢁ3
ꢁ2
CGS (4.3 ꢂ 10 mol dm ) was lower than the CMS (1.5 ꢂ 10
growth (as an indication for anaerobic bacterial growth and
ꢁ
3
mol dm ). Therefore, the synthesized CGS concentration activity) (see Fig. 7a and b). Moreover, it has been noted that the
ranges selected were 0.01, 0.1, 1 and 5 mM, which were lower suldogenic bacterial activity was gradually inhibited at
than the CMS at 0.01, 0.1, 1 and 10 mM (see Fig. 4). The redox a concentration of 0.1 mM of the CGS; however, aer the 5 day
potential in the different reactors (Fig. 7a and b) was measured incubation period, the suldogenic bacteria began to resist the
in order to estimate the suldogenic bacterial activity in the surfactant inhibitory effect. The redox potential values were
reactor's bulk phase. It is widely known that most suldogenic increased up to ꢁ80 or even higher by increasing the synthe-
bacteria, cultivated under strictly anaerobic conditions, initiate sized surfactant concentrations (1, 10 mM for the CMS and 1, 5
the sulfate reduction reaction at a redox potential below ꢁ140 mM for the CGS). This result was attributed to complete inhi-
13,29
mV.
The redox potential results in control reactors; the bition of the KSW-suldogenic bacterial activity (sulfate
Fig. 7 Redox potential of environmental sulfidogenic bacteria measured in reactors bulk phase: (a) CMS and (b) CGS. Results shown are mean
values of duplicates with corresponding standard deviation.
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reduction reactions) in the reactor's bulk phase and on the surfactant concentrations led to gradually decreasing the reac-
metal surface. tor's bulk-phase sulde productivity. Moreover, it has been
The sulde productivity results (see Fig. 8a and b), detected recognized that the suldogenic bacterial activity was gradually
in the reactor's bulk phase, were completely in agreement with inhibited at a concentration of 0.1 mM of the CGS. Aer the
the suldogenic bacterial activity detected by the redox poten- 5 day incubation period, the suldogenic bacteria started to
tial results. The sulde as a metabolic end product was resist the surfactant inhibitory effect. The surfactant reactors,
measured in order to estimate the suldogenic bacterial supplemented with the CMS at concentrations of 1, 10 mM, and
activity, which are oxidizing organic compounds or utilizing the CGS at concentrations of 1, 5 showed no sulde produc-
hydrogen as an electron donor with sulfate being reduced to tivity. No sulde productivity means that the suldogenic
3
0,31
sulde.
and detected in control reactors and surfactant reactors sup- phase and on the metal surface.
plemented with concentrations of 0.01, 0.1 mM for the CMS and The inhibition of sulde productivity is an important factor
The sulde was produced in substantial quantities bacterial activity was completely inhibited in the reactor's bulk
32
with a concentration of 0.01 mM for the CGS. Increasing in the metal surface protection. It is widely known that
Fig. 8 Sulfide productivity of environmental sulfidogenic bacteria measured in reactors bulk phase: (a) CMS and (b) CGS. Results shown are the
mean values of duplicates with corresponding standard deviation.
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hydrogen sulde can induce metal corrosion by being a source biocidal activity of the different biocides depends on the net
3
8,39
of bound protons, in addition to its precipitation in the form of charge on their molecules.
In has been reported that surface
33
iron sulde (FeS) on a metal surface. When the iron sulde activity is a vital factor inuencing biocidal activity of the
lm precipitates on the metal surface, it leads to initiation of synthesized surfactant, as was clearly observed by applying
pitting metal corrosion. In this regard, the synthesized CGS compared to CMS.
surfactants at high concentrations (1, 10 mM for the CMS and 1, The changes in biolm structure and constituents on the
mM for the CGS) were applied in order to protect the metal metal surface aer applying different surfactant (CMS, CGS)
40

34
5
surface from the suldogenic, bacterial metal corrosive activity. concentrations was estimated using CLSM and the staining
At those concentrations of the both synthesized surfactants, the method. Biolms were only detected in the control reactors, in
sulfate reduction reaction was completely inhibited. Therefore, the surfactant reactors supplemented with synthesized CMS at
a considerable drop in the redox potential in the reactor's bulk concentrations of 0.01, 0.1 mM, and in the surfactant reactors
phase and no sulde production were determined.
supplemented with synthesized CGS at a concentration of 0.1
Minimum inhibitory concentration (MIC) of the synthesized mM (see Fig. 9a and b). Nevertheless, at a concentration of 0.01
surfactants against the enriched suldogenic bacteria was mM of the CMS, no signicant changes in the suldogenic
estimated (see Table 1). The MIC values were determined at biolm components (nucleic acids, EPs glycoconjugates and
a concentration of 1 mM for both synthesized surfactants. At proteins) were observed. Increasing the CMS concentration
low surfactant concentrations of 0.01, 0.1 mM CMS and 0.01 (0.1 mM) caused a signicant reduction in the bacteria (nucleic
mM CGS, there was no signicant biocidal effect against the acid) and EPS components (glycoconjugates and proteins). At
enriched suldogenic bacteria. However, at a concentration of a concentration of 0.01 mM of the CGS, the KSW-suldogenic
0.1 mM of the CGS, the enriched-suldogenic bacterial growth biolm did not show signicant changes in the bacteria
was partially inhibited. This result was attributed to the biocidal (nucleic acids) and proteins; however, EPS glycoconjugates were
activity of the alkyl chain length that is increased in the reduced. At concentrations of 1, 10 mM of the synthesized CMS
35,36
synthesized CGS in comparison to the synthesized CMS.
and 1, 5 mM of the synthesized CGS, the metal surface was
The biocidal activity of the synthesized surfactants against completely protected, not only from the planktonic bacterial
suldogenic bacterial cells can be related to two main effects – cells (bacteria in the bulk phase) as conrmed by the fact that
a physical disruption and an electrostatic interaction. A physical there was a considerable drop in the redox potential in addition
disruption effect presents as hydrophobic chains (alkyl groups) to the sulde productivity inhibition, but also from the culti-
penetrating bacterial cell membranes. This is attributed to vated biolms developed on the metal surface. However, the
similarity of bacterial cell membrane constituents and hydro- cultivated biolm development at a concentration of 0.1 mM of
phobic chains of the synthesized surfactants (CMS and CGS). the synthesized CGS was completely inhibited on the metal
Penetration of a bacterial cell membrane causes destruction of surface; nevertheless complete inhibition was not observed in
the cell's selective permeability and thus disturbs the metabolic the bulk phase. This result was attributed to the CGS structure,
pathway within the cell cytoplasm; aerwards, death of micro- which leads surfactant molecules to adsorb to the interface
37
organisms occurred. Moreover, an electrostatic interaction faster than the synthesized CMS.
occurs between the positively charged CMS ammonium group At the end of the incubation period (30 days), the metal
R N ), CGS two ammonium groups (R N ) and the negatively corrosion rate of the coupons was calculated (Fig. 10a and b).
+
+
(
4
4
charged cell membrane (lipoprotein). The electrostatic inter- The higher metal corrosion rate of the blank reactor (without
actions play an important role in the action of cationic biocides. biomass, medium salinity of 3.19%) in comparison to the
Moreover, a decrease in charge density of cationic compounds control reactor (medium at salinity of 3.19% and inoculated
affects adsorption and biocidal efficiency. In addition, the with environmental suldogenic bacterial consortia) was
attributed to medium salinity. Furthermore, the suldogenic
bacterial biolm in the control reactor protected the metal
8
,41
Table 1 Most probable number (MPN) of cultivated reactors
surface from the harmful chloride anion effect. It has been
proposed that the metal corrosion rate increases when high
concentration of aggressive sodium chloride is present in the
Cultivated reactors
MPN cell per ml
42–44
cultivated medium (such as articial seawater).
Metal
KSW-suldogenic bacteria with CMS
6
6
6
Control
>2.4 ꢂ 10
>2.4 ꢂ 10
>2.4 ꢂ 10
corrosion in an aqueous medium containing chloride anion
induces by chemisorption of chloride anion on the metal
surface.
Two theories of chloride anion–metal surface interaction
have been reported. The rst theory is adsorption: the chloride
anion gets in contact with the metal surface as it favors hydra-
tion of metal ions. Hence, it increases the ease with which metal
ions enter into solution, and furthermore induce pit and crevice
0
0
1
1
.01 mM
.1 mM
mM
No growth
No growth
0 mM
KSW-suldogenic bacteria with CGS
6
Control
>2.4 ꢂ 10
6
0
0
1
5
.01 mM
.1 mM
mM
>2.4 ꢂ 10
4
7.8 ꢂ 10
43,44
corrosion.
The second theory focuses on the oxide-lm: the
chloride anion more easily penetrates the oxide lm of the
No growth
No growth
mM
metal surface through pores or defects than other ions.
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Fig. 9 Biofilm structures (nucleic acids, EPS glycoconjugates and proteins) and constituents of environmental sulfidogenic biofilms measured on
metal surfaces: (a) CMS and (b) CGS. Results shown are the average coverage of five randomly selected areas of the environmental sulfidogenic
biofilms with corresponding standard deviation.
Furthermore, it may colloidally disperse the oxide lm and those concentrations, the synthesized surfactants showed
increase its permeability.
a biocidal effect against the environmental suldogenic
The role of chloride anion in the metal corrosion mechanism bacteria in the bulk phase and on the metal surface. In the
is unique. It can be reacted again and again; hence even a small blank reactor where high salinity (3.19% NaCl) and free sul-
number of chloride anions can sustain the metal corrosion dogenic bacterial activity are present, the metal surface
process. Therefore the iron-chloride reaction is self- corrodes via the effect of chloride anions. In this respect, the
perpetuating and the free chloride serves as a reaction cata- synthesized surfactants protect the metal surface from the chloride
44
lyst. Application of the synthesized surfactants at different corrosion effect. Therefore, the hypothesized metal-surface corro-
concentrations gradually decreased the metal corrosion rate. sion inhibition behavior of the synthesized surfactants against the
The lowest metal corrosion rate was detected at concentrations cultivated salinity can be explained as adsorption of surfactant
of 10 and 1 mM with metal corrosion inhibition efficiency of molecules and formation of a tight surfactant layer at the metal–
45
92% and 94% for the synthesized CMS and CGS, respectively. At liquid interface, rather than the micelles.
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Fig. 10 Metal corrosion rate of environmental sulfidogenic bacteria: (a) CMS and (b) CGS. Results shown are mean values of duplicates with
corresponding standard deviation.
The surfactant layers, which are adsorbed to the metal surfactant, the adsorption of gemini surfactant on a metal
surface, showed an inhibitive barrier and hence protect the surface is more complicated as it contains two hydrophilic
46
metal surface from the surrounding corrosive environment.
groups and two hydrophobic groups linked by a spacer. This
Two types of adsorption may occur (physical and chemical). The structure displayed three adsorption types. (1) At low gemini
mechanism of the physical adsorption needs the presence of an surfactant concentrations, the adsorption occurs by a hori-
electrically charged metal surface and charged species in the zontal binding of the gemini surfactant molecules to the metal
bulk phase. The chemisorption involves charge transfer or surface. This adsorption behavior is favored by an electrostatic
+
sharing between the surfactant molecules and the metal interaction between the two ammonium groups (R
4
N ) and the
surface. The presence of a surfactant molecule that has rela- cathodic sites on the one hand and the hydroxide ion on the
tively loosely bound electrons or heteroatoms with lone-pair anodic sites on the other. (2) Because of increasing gemini
electrons, together with a transition metal that has vacant and surfactant concentration, a perpendicular adsorption occurs as
low-energy electron orbital, promotes this adsorption. The a result of inter-hydrophobic chain interaction. (3) With
synthesized CMS brings about adsorption on the metal surface a further increase of gemini surfactant concentration up to the
47
via an electrostatic interaction between the ammonium group
C
CMC value, an efficiency plateau appears.
Corrosion inhibition efficiency on the metal surface was
proved by applying SEM. Fig. 11a displays SEM images of the
+
(R
4
N ) and the cathodic sites on the one hand and the counter
ion (hydroxide ion) on the anodic sites on the other.
Typically, a monomeric surfactant at a low concentration carbon steel surfaces at the end of the incubation period
forms a monolayer on the metal surface. The increase in the (30 days) with environmental suldogenic bacteria. The images
monomeric surfactant's concentration results in formation of showed that metal surfaces were completely covered with the
micelles in the bulk solution. In contrast to the monomeric suldogenic biolms. By removing the biolms from the metal
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coupons, the surfaces were destroyed due to effects of the
Results reported in Table 2 indicate that the synthesized
cultivated biolms with high medium salinity on the metal surfactants have nonspecic or broad-spectrum antibacterial
surfaces (Fig. 11b). Fig. 11c and d illustrate another metal activity. The synthesized surfactants (CMS and CGS) showed
surface at the end of the incubation period (30 days) in medium antimicrobial activity (biocide) not only against the environ-
containing the synthesized CMS at a concentration of 10 mM or mental suldogenic bacteria but also against the standard
exposed to the synthesized CGS at a concentration of 1 mM and strains of Gram-positive (Staphylococcus aureus) and Gram-
inoculated with the enriched KSW-suldogenic bacteria. The negative (Escherichia coli and Pseudomonas aeruginosa) bacte-
SEM images of the treated metal surfaces show that the surfaces rial species. Results demonstrated that the antibacterial activity
are free from pitting corrosion and completely covered with of the synthesized surfactants increases as surfactant concen-
surfactant molecules. This result conrmed the highest metal tration increases. Gram-negative bacteria were less sensitive to
corrosion inhibition efficiency of the synthesized surfactants the synthesized surfactants than Gram-positive bacteria at low
(CMS and CGS).
concentrations. This result can be explained by the difference in
Fig. 11 SEM images of environmental sulfidogenic reactors on metal surface: (a) sulfidogenic biofilm, (b) metal surface (at salinity of 3.19% NaCl)
after removing biofilm, (c) CMS inoculated with environmental sulfidogenic bacteria, (d) CGS inoculated with environmental sulfidogenic
bacteria. Scale bar ¼ 50 mm.
Table 2 Antibacterial activity of synthesized surfactants
Pseudomonas
Concentration
(mM)
Staphylococcus
aureus (DSM 3463) (mm)
aeruginosa
(DSM 50071) (mm)
Escherichia
coli (DSM 30083) (mm)
Biocide
Tetracycline
CMS
35 ꢃ 0
28 ꢃ 0.5
35 ꢃ 0
0
a
0.01
0
0
0
1
5
0
.1
16 ꢃ 0
0
0
29.5 ꢃ 0.7
35.5 ꢃ 0.7
38.2 ꢃ 0.35
0
20 ꢃ 0
20.75 ꢃ 0.3
24.75 ꢃ 0.3
0
15.5 ꢃ 0
25.75 ꢃ 0.3
28.1 ꢃ 0.2
0
1
CGS
0.01
0
1
5
0
.1
17.1 ꢃ 0.2
30.8 ꢃ 0.2
37.5 ꢃ 0
38.1 ꢃ 0.28
0
0
25 ꢃ 0
28 ꢃ 0
30.1 ꢃ 0.2
20.5 ꢃ 0
25.5 ꢃ 0
30 ꢃ 0
1
a
0
mm ¼ no effect as resistance of cultivated bacteria to antimicrobial agent.
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the outer cell membrane of the two bacterial types. The entire
outer cell membrane of the Gram-negative bacteria consisted of
lipopolysaccharides and proteins that restrict the entrance of
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49
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+
activity depends on the ammonium group (R N ) and hydro-
4
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49
gemini surfactant leads to an increase in biocidal activity. The
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4
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a
One novel cationic monomeric surfactant (CMS) was success-
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protect the metal surface from the medium salinity (inhibitors)
and environmentally suldogenic bacterial activity (biocides) in
comparison to the synthesized gemini surfactant (CGS) in the
bulk phase and on the metal surface. The CGS showed better
surface active properties at a low concentration compared to the
CMS. Biocidal activities of the synthesized surfactants were
achieved by dropping the redox potential and conrmed by
preventing sulde production in the reactor's bulk phase. This
means that all suldogenic bacterial activities were inhibited in
the reactor's bulk phases and on the metal surfaces. The
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