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Surface activity and micellization parameters of cationic surfactants containing hydroxyethyl group and C9-chain
Surface Activity and Micellization Parameters of Cationic Surfactants Containing Hydroxyethyl Group and C 9 -chain Ziyafaddin H. Asadov, Shafiga M. Nasibova, Ravan A. Rahimov, Gulnara A. Ahmadova, Saida M. Huseynova PII: DOI: Reference:
S0167-7322(16)30863-7 doi:10.1016/j.molliq.2016.11.105 MOLLIQ 6652
To appear in:
Journal of Molecular Liquids
Received date: Revised date: Accepted date:
9 April 2016 1 November 2016 22 November 2016
Please cite this article as: Ziyafaddin H. Asadov, Shafiga M. Nasibova, Ravan A. Rahimov, Gulnara A. Ahmadova, Saida M. Huseynova, Surface Activity and Micellization Parameters of Cationic Surfactants Containing Hydroxyethyl Group and C9 -chain, Journal of Molecular Liquids (2016), doi:10.1016/j.molliq.2016.11.105
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Surface Activity and Micellization Parameters of Cationic Surfactants Containing Hydroxyethyl Group and C9-chain
Ziyafaddin H. Asadov, Shafiga M. Nasibova, Ravan A. Rahimov*,
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Gulnara A. Ahmadova, Saida M. Huseynova
Institute of Petrochemical Processes of Azerbaijan National Academy of Sciences,
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Hojaly ave. 30, Az 1025, Baku, Azerbaijan
Abstract
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Two series of cationic ammonium surfactants with several hydroxyethyl groups, namely, nonyl-(2hydroxyethyl)ammonium-,
nonyl-di(2-hydroxyethyl)ammonium-
and
nonyl-tri(2-
hydroxyethyl)ammonium bromides and iodides were synthesized. On the basis of systematical
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measurements of surface tension and specific electroconductivity, various surfactivity parameters were calculated. It is revealed that the surface activity rises with an increase of the number of hydroxyethyl groups. By determination of binding degrees () of the surfactants counterions it was revealed that the
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value of for iodide-ion is higher than for bromide-ion. The salts synthesized on the basis of
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diethanolamine have a higher petroleum-collecting capacity.
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Key words: cationic surfactant, surface tension, micellization, adsorption, counterion, petroleumcollecting and dispersing.
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*Corresponding author: Dr. Ravan A. Rahimov
Institute of Petrochemical Processes of Azerbaijan National Academy of Sciences, Hojaly ave. 30, Az 1025, Baku, Azerbaijan Tel.: +99450 545 20 48 Fax: +99412 490 24 76 e-mail: [email protected]
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1. Introduction
Tendency of surfactant molecules to self-associate with formation of micelles is characteristic for surfactants. The molecular structure of surfactant determines main properties and spheres of their application. In water, an elongation of the hydrocarbon chain in surfactant lowers such parameters as critical micelle concentration (CMC), a degree of dissociation of counterion (α),
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and the number of aggregation [1–4]. The aggregation, solubilization, and catalytic properties of
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surfactants of cationic nature significantly depend on the kind of head-groups. Herewith, an
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impact of the head-group substituents tending to hydrogen bonding is very important [5−12]. Such studies are stimulated by a wide use of cationic surfactants as detergents, emulsifiers and demulsifiers, corrosion inhibitors, bactericides and others. Below CMC, ionic surfactants in water are supposed to completely dissociate into ions. When micelles are formed, the
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counterions become to be bound to the micelle core. Only a part of these ions may be considered as free ions. This is characterized by the α parameter [13−15]. Besides, different dimensions of
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the head-groups also bring about various micellization particularities [16−17]. Lately, the effect of introducing hydroxyethyl fragment into ammonium salts has been studied. Some researchers found out that hydroxyethyl group contributes to high surfactivity.
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The authors [18] show that replacement of methyl group by hydroxyethyl fragment lowers CMC.
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Ammonium salts having hydroxyethyl groups possess distinctive properties. Such surfactants containing C12-C18 alkyl chains have been studied more widely [3, 19-21]. However, the
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surfactants containing hydroxyethyl groups and alkyl chains shorter than C10 [5] have not been reported yet. So, their synthesis and study attract theoretical and practical interest. The submitted paper is dedicated to synthesis and study of such cationic surfactants as
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nonyl-(2-hydroxyethyl)ammonium bromide (NMEABr), nonyl-di(2-hydroxyethyl)ammonium bromide (NDEABr), nonyl-tri(2-hydroxyethyl)ammonium bromide (NTEABr), nonyl-(2hydroxyethyl)ammonium
iodide
(NMEAI),
nonyl-di(2-hydroxyethyl)ammonium
iodide
(NDEAI) and nonyl-tri(2-hydroxyethyl)ammonium iodide (NTEAI). By surface tension- and electroconductivity measurements their colloidal-chemical parameters have been determined. In the meantime, dependence of the colloidal-chemical parameters and capacity for petroleumcollecting and dispersing from the number of hydroxyethyl groups has been investigated.
2. Materials and methods 2.1.
Reagents and instruments
Bruker TOP SPIN spectrometer (300.13 MHz and 75.46 MHz) was used for recording 1H NMR and 13C NMR spectra. Values of chemical shift () in ppm are registered downfield with regard to TMS. D2O was used as a solvent. IR spectra were recorded (in KBr disks) on an ALPHA FTIR (Bruker) spectrometer. 1-Bromononane (98%), 1-iodononane (95%), monoethanolamine
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(≥98%), diethanolamine (≥98.0%), triethanolamine (98%) were purchased from Sigma-Aldrich®.
2.2.
Synthesis of cationic surfactants
Samples of (0.05 mol) 10.4 g of 1-bromononane and (0.05 mol) 3.1 g of monoethanolamine-
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MEA (or respective amount of diethanolamine-DEA or triethanolamine-TEA in mol) were
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refluxed with 60 mL of dry acetone at 75 °C for 5 h. Interaction of 1-iodononane with ethanolamines is carried out at a relatively moderate temperature (t=50-55 °C, 5 h). After
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completion the reaction, the mixture was cooled down to −28 °C, and white crystals appeared which were recrystallized in acetone at least three times; the salt was then dried in desiccator for 15 h. A white solid product was obtained in 80-87 % yield.
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The interaction between nonyl halide and ethanolamines can be illustrated by the
where X is Br, I.
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following reaction scheme 1:
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Scheme 1. Synthesis of cationic surfactants
FTIR spectra of the synthesized cationic surfactants (NDEABr was taken as a
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representative for the synthesized surfactants) showed the following absorption bands (cm1): 3332 (OH), 2954, 2923 and 2854 (CH), 2365 (N+H), 1464 and 1376 (CH), 1246 (C-N), 1071 (C-O), 722 (CH2)x. In surfactant NDEABr 1H-NMR (300.13 MHz, D2O), δ (ppm): 0.78 (CH3), 1.20 (CH2 chain), 1.67 (CH-CH2-N+H), 3.17 (CH2-N+H), 3.31 (CH2-CH2-OH), 3.75 (CH2-CH2-OH), 3.85 (OH), 8.50 (N+H). 2.3.
Surface Tension Measurements
Surface tension was measured on a DuNouy ring KSV Sigma 702 tensiometer (Israel). The sample under measurement was placed in the glass cell (with a double jacket) using water bath for thermostating. A Pt wire ring was placed inside the solution of the sample and then gradually pulled through the liquid-air border. Average value of surface tension was calculated on the basis of 3 readings with 3 min interval. The Pt wire ring was rinsed with water and flamed using Bunsen burner between experiments. The deviation of the surface tension of distilled water from 72.0 (25 C) was not more than ±0.2 mN/m.
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2.4.
Electroconductometric Measurements
Specific electroconductivity of the surfactant solutions was measured by “ANION 4120” conductometer (Russia). The range of measurements is 104 S/m – 10 S/m, the range of measurement temperature being 0 – 100 C and relative error not exceeding ± 2%.
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30-40 ml of the surfactant solutions are prepared at various concentrations (0.001–5%).
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The prepared solutions are thermostated at a water bath (0.1 C). Before measurements solutions are agitated. At this moment there must not be gas bubbles in the solution and level of
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the solution must be higher than electrodes. The electrode is kept the solution during 3–5 min and the value of the specific electroconductivity is read. At the end of each measurement, the water must be in the interval 2-5 μS/cm.
Study of petroleum-collecting and petroleum-dispersing properties
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2.5.
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electrode is rinsed with distilled water and dried. The specific electroconductivity of distilled
Studies of petroleum-collecting and petroleum-dispersing properties of the synthesized cationic
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surfactants have been performed in parallel for pure-state reagents and their 5% wt. aqueous
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solutions. As an example of crude oil, Pirallahy petroleum (from the oil field near Baku, Azerbaijan) was used, its density being 0.9244 g cm3 (20 C) and kinematic viscosity equalling
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1.05 cm2 s1 (30 C) 0.02 g of the surfactant (or its solution) was added to a thin film (0.150.16 mm thickness) of the crude oil on the surface of water in Petri dishes. Petrocollecting coefficientK was calculated by the formula K=So/S, where So is the initial area of the crude oil surface at
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the beginning of the test, and S is an area of the surface of petroleum transformed in a thickened spot. During the test, the surface area of the spot was measured at certain time to intervals () and respectively values of K were computed. Petroleum-dispersing capacity was evaluated by a degree of cleaning of polluted water surface from petroleum –KD which was found as a ratio of the area of the surface of cleaned water and the initial surface area of the petroleum slicks.
3. Results and Discussion
3.1.
Effect of the Number of the Hydroxyethyl Groups and Counterion of the Surfactants on Their Surface Properties
The surfactants obtained on the basis of MEA are solids at negative temperatures and liquids at ambient conditions. They have a good solubility in ethanol, isopropanol, acetone, ethyl acetate
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and water. An increase in the number of hydroxyethyl groups in the molecule of surfactant raises solubility in water. Figures 1 and 2 illustrate the effect of varying the concentration of the surfactants on the surface tension of their aqueous solutions at the border with air at 25 °C. These results show that, for each type of surfactant, there is a gradual decrease in the surface tension with an increase of
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the solution concentration up to a certain constant value. These constant values of surface tension
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are 30; 27.9 and 24.1 mN/m for the surfactants based on nonyl bromide and containing one, two, and three hydroxyethyl groups attached to the N-atom, respectively. In the case of the surfactants
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based on nonyl iodide, constant values of surface tension are 30.6; 28.8 and 28.4 mN/m, respectively, having one, two, and three hydroxyethyl groups bonded to the N-atom. The CMC values are 1.40103; 1.19103 and 1.04103 moldm3 for the surfactants
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based on nonyl bromide and having one, two, and three hydroxyethyl groups linked to the N atom, respectively (Table 1). The values of CMC for the nonyl iodide-based surfactants are
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correspondingly 1.19103; 1.05103 and 0.93103 moldm3. This decrease in the CMC values demonstrates the effect of an increase in the number hydroxyethyl groups of the surfactants on their surface tension-concentration plot. Rising the number of hydroxyethyl groups on the N
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atom of N-nonylammonium halide salts from one to three causes a gradual decrease in the
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corresponding CMC and in the surface tension. As is seen from Table 1, when bromide anion of the surfactant is substituted by iodidethe
CMC
value
decreases.
In
the
case
of
gemini
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counterion
[ethanediylbis(dimethyltetradecylammonium)] [21], alkylpyridinium [23] and other kinds of surfactants [24-26], a similar tendency is noticed. Table 1 shows that the anions are important for maximum values of surface excess
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concentration - max, as well as the minimum surface area per molecule of surfactant – Amin at the border of water solution with air. The values of (mol/cm2) were calculated by the formula [6, 26]: = (1/nRT)(/lnC)
(1)
where (/lnC) is surface activity (it is found as slope of =f(lnC) at constant T) and R is universal gas constant. It is clear that depends on the concentration of surfactant and max is reached at CMC. The coefficient-“n” characterizes the number of ions generated by a surfactant molecule. In the case of the synthesized salts, the value of “n” may be equalled to 2. The max value is needed for determining the minimum area (denoted by Amin, nm2 ) of a molecule at the border of aqueous solution with air in accordance with the equation [6, 26]: Amin = 1016/NAmax
(2)
where NA is Avogadro's number. The values of max and Amin are dependent on the molecular structure. As is evident from the table, upon substituting bromide-anion by iodide-counterion the max value becomes lowered, whereas Amin rises. For MEA-based surfactants, upon substituting
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bromide-anion by iodide-counterion max increases, while Amin decreases. From the other side, when the number of hydroxyethyl groups linked to N atom rises, max increases and Amin diminishes. A substantial decrease in the area occupied by a molecule (Amin) in the case of the surfactants having three hydroxyethyl groups, as compared with the surfactants containing one or two hydroxyethyl groups, may be attributed to the enhanced intra-hydrogen bonding between
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three hydroxyl groups and inter-hydrogen bonding with the aqueous solution.
26]
(3)
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CMC = 0 CMC
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The effectiveness of surface tension decrease (CMC) is determined from the equation [6,
where 0 is the surface tension at the border of water with air, CMC is the surface tension at the interface of surfactant aqueous solution with air at CMC. The CMC values for the synthesized
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salts at 298 K temperature fall into the interval 41.4 - 47.9 mN m–1. When Br-anion is replaced by I, CMC is lowered, but with substitution of H atom by hydroxyethyl group, CMC increases.
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Further, the adsorption efficiency (pC20), values are helpful for comparison of the efficiency of adsorption of surfactant at air/water interface [5]. The greater the pC20 value, the larger the efficiency of the surfactant adsorption at the border and the larger the decrease of
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surface tension. Thus, pC20 increases when Br is replaced by I but, with the surfactants based
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on MEA, the opposite situation is seen.
3.2.
Conductometric Measurements
Plots illustrating a dependence of specific electroconductivity from the surfactant concentration
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at 25 C were built (Fig. 3-5). As is evident, specific conductivity increases with a rise in concentration. On the basis of slopes of electroconductivity/concentration plots, both above and below the CMC, the values of ionization degree () were determined [4]: α=S2/S1
(4)
where S1 and S2 are slopes of the mentioned dependence at the concentrations smaller and larger than CMC. The degree of counterion binding () was found as [4]: β=1-α
(5)
The β values are given in Table 1. The CMC values found from intersection point of two straight lines are slightly different from those determined on the base of measurements of surface tension. As may be seen from Table 1, the values of β for both types of surfactants rise with an increase of the number of hydroxyethyl groups bound to N-atom in the head group of the surfactants. The value of β decreases with an increase of the number of hydroxyethyl group, which is consistent with observations made by Chatterjee et al. [18] in their studies of the series of hydroxyethyl-replaced cetylammonium bromide salts.
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If Br-anion in the surfactant becomes substituted by I, increases. Similar tendency was
registered
for
tetramethylammonium
halides
[27],
ethanediylbis(dimethyltetradecylammonium) halides [21], and for cetyltrialkylammonium halides [28].
Thermodynamic properties of the synthesized salts
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3.3.
In the case of these surfactants, the standard Gibbs free energy change of micelle formation
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(Gmic) may be calculated by Eq. 6 [26].
Gmic = (2)RTlnCMC
(6)
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where R is universal gas constant, and T is standard absolute temperature (298.15 K). The standard Gibbs free energy change for adsorption process (Gad) for the obtained salts were computed by Eq. (7) [26]:
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Gad = (2)RTlnCMC 0.6023CMCACMC
(7)
where ACMC has the unit Å2 per molecule, and CMC denotes the surface pressure (in mN/m) at CMC at the border of surfactant water solution with air.
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The Gmic and Gad values are presented in Table 1. It is seen that both micelle formation
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and adsorption processes are spontaneous because the Gmic and Gad values are negative. Meanwhile, Gad is more negative. So, it may be concluded that adsorption is preferential than
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micellization. As is seen from Table 1, in the surfactants containing Br and I, with an increase of the number of hydroxyethyl groups, the values of Gmic and Gad increase. Therefore, hydroxyethyl groups are unfavorable for micelle formation and adsorption processes. However,
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in the surfactants containing C12 chains, with an increase of the number of hydroxyethyl groups, the values of Gmic and Gad decrease [1, 19, 21]. The opposite situation in the synthesized nonyl halide-based surfactants may be explained by the circumstance that, because of a shorter alkyl chain, the more the number of hydroxyethyl groups, the better the watersolubility. As a result, micelle formation and adsorption processes are less spontaneous.
3.4.
Petroleum-collecting and petroleum-dispersing properties of the synthesized salt
Petroleum-collecting and petroleum-dispersing capacity of the surfactants has been investigated regarding thin films of Pirallahy crude oil on the surface of distilled, fresh and sea waters. The surfactants were used as 5% wt. aqueous solutions. The cationic surfactants based on DEA exhibit a higher petroleum-collecting capacity. Thus, 5% wt. aqueous solution of NDEAB has the value of Kmax in fresh water 60.8, being longer than 4 days. In the sea water, 5% aqueous solution of NDEAI is more effective as a petrocollector (Kmax=40.5, =7 days). This salt shows
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the same effect in the fresh water. In distilled water, the effectiveness of this surfactant is higher (Kmax=46.0, =7 days). Among the MEA-based surfactants, NMEAB demonstrates the highest petrocollecting capacity. In all waters, Kmax equals 30.4 (5 days). For NMEAI, Kmax is nearly 23.0. The TEA-based surfactants manifest mainly mixed petroleum-collecting and dispersing
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properties.
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4. Conclusion
Ammonium salts containing hydroxyethyl-group have been synthesized on the basis of nonyl
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bromide (or iodide), MEA, DEA and TEA. They have been characterized by IR- and NMRspectroscopy. Colloidal-chemical parameters of the obtained cationic surfactants have been computed. The influence of the number of hydroxyethyl groups and the nature of the counterion
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(Br and I) on these parameters has been elucidated. It has been established that, due to a shorter hydrocarbon chain, in the synthesized surfactants, with an increase of the number of
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hydroxyethyl groups, in distinction to the similar surfactants with a longer alkyl chain, Gmic and Gad rise. In the sea water, 5% aqueous solution of NDEAI exhibits the highest petroleum-
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collecting capacity.
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1. R.Wang, Y. Li, Q. Li, Physicochemical Properties of Quaternary Ammonium Surfactants with Hydroxyethyl Groups, Tenside Surf. Det. 51 (2014) 54-58. 2. Deepti, K.K. Ghosh, Micellization of Cetyldiethylethanolammonium Bromide in Mixed Aqueous Organic Solvents, Journal of Dispersion Science and Technology 31 (2010) 1249-1253. 3. Zh. Zhang, H. Wang, W. Shen, Densities, Conductivities, and Aggregation Numbers of Aqueous Solutions of Quaternary Ammonium Surfactants with Hydroxyethyl Substituents in the Headgroups, J. Chem. Eng. Data 58 (2013) 2326-2338. 4. A.R. Glennie, M.M. Mohareb, R.M. Palepu, Thermodynamic and Related Properties of Alkyl Cationic Surfactants Based on Dimethyl and Diethyl Ethanol Amines, Journal of Dispersion Science and Technology 27 (2006) 731-738. 5. Z.H. Asadov, R.A. Rahimov, Sh.M. Nasibova, G.A. Ahmadova, Surface Activity, Thermodynamics of Micellization and Adsorption Properties of Quaternary Salts Based on Ethanolamines and Decyl Bromide, J Surfact Deterg 13 (2010) 459-464. 6. Z.H. Asadov, R.A. Rahimov, Kh.A.Mammadova, A.V. Gurbanov, G.A. Ahmadova, Synthesis and Characteristics of Dodecyl Isopropylolamine and Derived Surfactants, J Surfact Deterg 19 (2016) 145-153. 7. D.Jurašin, I.Habuš, N. Filipović-Vinceković, Role of the alkyl chain number and head groups location on surfactants self-assembly in aqueous solutions, Colloids Surf. A 368 (2010) 119-128. 8. G.Para, A.Hamerska-Dudra, K.A.Wilk, P.Warszyński, Surface activity of cationic surfactants; influence of molecular structure, Colloids Surf. A, 365 (2010) 215-221. 9. J.Harkot, B. Jańczuk, Surface and volume properties of dodecyldimethylammonium bromide and benzyldimethyldodecylammonium bromide. I. Surface properties of dodecyldimethylammonium bromide and benzyldimethyldodecylammonium bromide, J. Colloid Interface Sci. 331 (2009) 494-499.
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10. Y.Hayami, A.Wataya, T.Takiue, N.Ikeda, H.Matsubara, M.Aratono, Adsorption and micelle formation of alkylammonium chloride-alkyltrimethylammonium chloride mixtures, J. Colloid Interface Sci. 310 (2007) 240-245. 11. M.S.Borse, S.Devi, Importance of head group polarity in controlling aggregation properties of cationic gemini surfactants, Adv. Colloid Interface Sci. 123−126 (2006) 387-399. 12. A.B.Mirgorodskaya, L.R.Bogdanova, L.A.Kudryavtseva, S.S.Lukashenko, A.I. Konovalov, Role of surface potential in the catalytic action of micelles of cationic surfactant with a hydroxyalkyl fragment in the head group, Russ. J. Gen. Chem. 78 (2008) 163-170. 13. B.F.B.Silva, E.F.Marques, Thermotropic Behavior of Asymmetric Chain Length Cationic Surfactants: The Influence of the Polar Head Group, J. Colloid Interface Sci. 290 (2005) 267274. 14. M.D. Trone, M.G. Khaledi, Characterization of Chemical Selectivity in Micellar Electrokinetic Chromatography. 4. Effect of Surfactant Headgroup, Anal. Chem. 71 (1999) 1270-1277. 15. S.P. Stodghill, A.E. Smith, J.H. O’Haver, Thermodynamics of Micellization and Adsorption of Three Alkyltrimethylammonium Bromides Using Isothermal Titration Calorimetry, Langmuir 20 (2004) 11387-11392. 16. H.Xing, P.Yan, K.Zhao, J.Xiao, Effect of Headgroup Size on the Thermodynamic Properties of Micellization of Dodecyltrialkylammonium Bromides, J. Chem. Eng. Data 56 (2010) 865873. 17. S.A. Buckingham, C.J. Garvey, G.G. Warr, Effect of Headgroup Size on Micellization and Phase Behavior in Quaternary Ammonium Surfactant Systems, J. Phys. Chem. 97 (1993) 1023610244. 18. A. Chatterjee, S. Maiti, S.K. Sanyal, S. P. Moulik, Micellization and Related Behaviors of NCetyl-N-ethanolyl-N,N-dimethyl and N-Cetyl-N,N-diethanolyl-N-methyl Ammonium Bromide, Langmuir 18 (2002) 2998-3004. 19. W.Tong, Q. Zheng, Sh. Shao, Q. Lei, W. Fang, Critical Micellar Concentrations of Quaternary Ammonium Surfactants with Hydroxyethyl Substituents on Headgroups Determined by Isothermal Titration Calorimetry, J. Chem. Eng. Data 55 (2010) 3766–3771. 20. B. Song, Sh. Shang, Zh. Song, Solution behavior and solid phase transitions of quaternary ammonium surfactants with head groups decorated by hydroxyl groups, J. Colloid Interface Sci. 382 (2012) 53–60. 21. Zh. Fan, W. Tong, Q. Zheng, Q. Lei, W. Fang, Surface Activity and Micellization Parameters of Quaternary Ammonium Surfactants Containing a Hydroxyethyl Group, J. Chem. Eng. Data 58 (2013) 334−342. 22. S. Manet, Y. Karpichev, D. Bassani, R. Kiagus-Ahmad, R. Oda, Counteranion effect on micellization of cationic gemini surfactants 14-2-14: Hofmeister and other counterions, Langmuir 26 (2010) 10645-10656. 23. K. Bijma, J.B.F.N. Engberts, Effect of counterions on properties of micelles formed by alkylpyridinium surfactants. 1. Conductometry and 1H-NMR chemical shifts, Langmuir 13 (1997) 4843-4849. 24. N.M. Vaghela, N.V. Sastry, V.K. Aswal, Surface active and aggregation behavior of methylimidazolium-based ionic liquids of type [Cnmim] [X], n=4, 6, 8 and [X]=Cl− , Br− , and I− in water, Colloid Polym Sci 289 (2011) 309-322. 25. L. Kellaway, G.G. Warr, The Effect of Head-Group on Selective Counterion Binding to Cationic Surfactants, J. Colloid Interface Sci. 193 (1997) 312-314. 26. M.J. Rosen Surfactants and interfacial phenomena, (2004) 3rd Edn, John Wiley and Sons Inc., New York 27. J. Keiper, L.S. Romsted, Arenediazonium salts: new probes of the interfacial compositions of association colloids. 6. Relationships between interfacial counterion and water concentrations
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and surfactant head group size, sphere-to-rod transitions, and chemical reactivity in cationic micelles, Langmuir 16 (2000) 59-71. 28. H.N. Patrick, G.G. Warr, S.Manne, I.A. Aksay, Surface micellization patterns of quaternary ammonium surfactants on mica, Langmuir 15 (1999) 1685-1692.
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Fig 1. Surface tension vs. ln of the concentration of nonyl bromide-based ammonium salts in
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aqueous solution (25 C). 1- NMEAB, 2- NDEAB, 3- NTEAB.
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Fig 2. Surface tension vs. ln of the concentration of nonyl iodide-based ammonium salts in
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aqueous solution (25 C). 1- NMEAI, 2- NDEAI, 3- NTEAI.
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Fig 3. The plots of specific electrical conductivity against concentration of MEA-based ammonium salts (25 C). 1- NMEAB, 2- NMEAI. The error of electrical conductivity value is
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±0.3 μS/cm
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Fig 4. The plots of specific electrical conductivity against concentration of DEA-based ammonium salts (25 C). 1- NDEAB, 2- NDEAI. The error of electrical conductivity value is
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±0.3 μS/cm
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Fig 5. The plots of specific electrical conductivity against concentration of TEA-based ammonium salts (25 C). 1- NTEAB, 2- NTEAI. The error of electrical conductivity value is
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±0.3 μS/cm
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Table 1. Surface properties of the synthesized cationic surfactants in aqueous solution at 298 K CMC103, max1010, Amin102, CMC, CMC, Gmic, Gad, pC20 nm2 moldm3 molcm2 mNm1 mNm1 kJmol1 kJmol1 NMEAB 0.52 1.40a 1.45b 1.45 114.9 30.0 4.40 42.0 -24.75 -27.65 NDEAB 0.41 1.19 1.22 2.27 73.0 27.9 3.80 44.1 -23.52 -25.46 NTEAB 0.38 1.04 1.07 2.62 63.3 24.1 3.90 47.9 -23.48 -25.31 NMEAI 0.61 1.19 1.25 1.53 108.5 30.6 4.27 41.4 -26.86 -29.57 NDEAI 0.51 1.05 1.10 2.14 77.5 28.8 3.87 43.2 -25.66 -27.68 NTEAI 0.42 0.93 0.95 2.16 77.0 28.4 4.05 43.6 -24.56 -26.58 a b The CMC value was determined by electroconductivity method ( uncertainties are 2.0-2.5%), The CMC β
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Surfactants
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value was determined by surface tension method ( uncertainties are 1.5-2.0%).
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Graphical abstract
Highlights
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New surface-active, ammonium salts based on nonyl bromide (or iodide) and ethanolamines have been synthesized and characterized. With an increase of the number of hidroxyethyl groups, Gibbs free energy changes of micellization and adsorption processes rise.
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These salts exhibit petroleum-collecting capacity.
S0167-7322(16)30863-7 doi:10.1016/j.molliq.2016.11.105 MOLLIQ 6652
To appear in:
Journal of Molecular Liquids
Received date: Revised date: Accepted date:
9 April 2016 1 November 2016 22 November 2016
Please cite this article as: Ziyafaddin H. Asadov, Shafiga M. Nasibova, Ravan A. Rahimov, Gulnara A. Ahmadova, Saida M. Huseynova, Surface Activity and Micellization Parameters of Cationic Surfactants Containing Hydroxyethyl Group and C9 -chain, Journal of Molecular Liquids (2016), doi:10.1016/j.molliq.2016.11.105
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Surface Activity and Micellization Parameters of Cationic Surfactants Containing Hydroxyethyl Group and C9-chain
Ziyafaddin H. Asadov, Shafiga M. Nasibova, Ravan A. Rahimov*,
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Gulnara A. Ahmadova, Saida M. Huseynova
Institute of Petrochemical Processes of Azerbaijan National Academy of Sciences,
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Hojaly ave. 30, Az 1025, Baku, Azerbaijan
Abstract
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Two series of cationic ammonium surfactants with several hydroxyethyl groups, namely, nonyl-(2hydroxyethyl)ammonium-,
nonyl-di(2-hydroxyethyl)ammonium-
and
nonyl-tri(2-
hydroxyethyl)ammonium bromides and iodides were synthesized. On the basis of systematical
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measurements of surface tension and specific electroconductivity, various surfactivity parameters were calculated. It is revealed that the surface activity rises with an increase of the number of hydroxyethyl groups. By determination of binding degrees () of the surfactants counterions it was revealed that the
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value of for iodide-ion is higher than for bromide-ion. The salts synthesized on the basis of
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diethanolamine have a higher petroleum-collecting capacity.
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Key words: cationic surfactant, surface tension, micellization, adsorption, counterion, petroleumcollecting and dispersing.
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*Corresponding author: Dr. Ravan A. Rahimov
Institute of Petrochemical Processes of Azerbaijan National Academy of Sciences, Hojaly ave. 30, Az 1025, Baku, Azerbaijan Tel.: +99450 545 20 48 Fax: +99412 490 24 76 e-mail: [email protected]
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1. Introduction
Tendency of surfactant molecules to self-associate with formation of micelles is characteristic for surfactants. The molecular structure of surfactant determines main properties and spheres of their application. In water, an elongation of the hydrocarbon chain in surfactant lowers such parameters as critical micelle concentration (CMC), a degree of dissociation of counterion (α),
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and the number of aggregation [1–4]. The aggregation, solubilization, and catalytic properties of
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surfactants of cationic nature significantly depend on the kind of head-groups. Herewith, an
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impact of the head-group substituents tending to hydrogen bonding is very important [5−12]. Such studies are stimulated by a wide use of cationic surfactants as detergents, emulsifiers and demulsifiers, corrosion inhibitors, bactericides and others. Below CMC, ionic surfactants in water are supposed to completely dissociate into ions. When micelles are formed, the
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counterions become to be bound to the micelle core. Only a part of these ions may be considered as free ions. This is characterized by the α parameter [13−15]. Besides, different dimensions of
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the head-groups also bring about various micellization particularities [16−17]. Lately, the effect of introducing hydroxyethyl fragment into ammonium salts has been studied. Some researchers found out that hydroxyethyl group contributes to high surfactivity.
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The authors [18] show that replacement of methyl group by hydroxyethyl fragment lowers CMC.
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Ammonium salts having hydroxyethyl groups possess distinctive properties. Such surfactants containing C12-C18 alkyl chains have been studied more widely [3, 19-21]. However, the
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surfactants containing hydroxyethyl groups and alkyl chains shorter than C10 [5] have not been reported yet. So, their synthesis and study attract theoretical and practical interest. The submitted paper is dedicated to synthesis and study of such cationic surfactants as
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nonyl-(2-hydroxyethyl)ammonium bromide (NMEABr), nonyl-di(2-hydroxyethyl)ammonium bromide (NDEABr), nonyl-tri(2-hydroxyethyl)ammonium bromide (NTEABr), nonyl-(2hydroxyethyl)ammonium
iodide
(NMEAI),
nonyl-di(2-hydroxyethyl)ammonium
iodide
(NDEAI) and nonyl-tri(2-hydroxyethyl)ammonium iodide (NTEAI). By surface tension- and electroconductivity measurements their colloidal-chemical parameters have been determined. In the meantime, dependence of the colloidal-chemical parameters and capacity for petroleumcollecting and dispersing from the number of hydroxyethyl groups has been investigated.
2. Materials and methods 2.1.
Reagents and instruments
Bruker TOP SPIN spectrometer (300.13 MHz and 75.46 MHz) was used for recording 1H NMR and 13C NMR spectra. Values of chemical shift () in ppm are registered downfield with regard to TMS. D2O was used as a solvent. IR spectra were recorded (in KBr disks) on an ALPHA FTIR (Bruker) spectrometer. 1-Bromononane (98%), 1-iodononane (95%), monoethanolamine
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(≥98%), diethanolamine (≥98.0%), triethanolamine (98%) were purchased from Sigma-Aldrich®.
2.2.
Synthesis of cationic surfactants
Samples of (0.05 mol) 10.4 g of 1-bromononane and (0.05 mol) 3.1 g of monoethanolamine-
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MEA (or respective amount of diethanolamine-DEA or triethanolamine-TEA in mol) were
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refluxed with 60 mL of dry acetone at 75 °C for 5 h. Interaction of 1-iodononane with ethanolamines is carried out at a relatively moderate temperature (t=50-55 °C, 5 h). After
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completion the reaction, the mixture was cooled down to −28 °C, and white crystals appeared which were recrystallized in acetone at least three times; the salt was then dried in desiccator for 15 h. A white solid product was obtained in 80-87 % yield.
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The interaction between nonyl halide and ethanolamines can be illustrated by the
where X is Br, I.
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following reaction scheme 1:
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Scheme 1. Synthesis of cationic surfactants
FTIR spectra of the synthesized cationic surfactants (NDEABr was taken as a
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representative for the synthesized surfactants) showed the following absorption bands (cm1): 3332 (OH), 2954, 2923 and 2854 (CH), 2365 (N+H), 1464 and 1376 (CH), 1246 (C-N), 1071 (C-O), 722 (CH2)x. In surfactant NDEABr 1H-NMR (300.13 MHz, D2O), δ (ppm): 0.78 (CH3), 1.20 (CH2 chain), 1.67 (CH-CH2-N+H), 3.17 (CH2-N+H), 3.31 (CH2-CH2-OH), 3.75 (CH2-CH2-OH), 3.85 (OH), 8.50 (N+H). 2.3.
Surface Tension Measurements
Surface tension was measured on a DuNouy ring KSV Sigma 702 tensiometer (Israel). The sample under measurement was placed in the glass cell (with a double jacket) using water bath for thermostating. A Pt wire ring was placed inside the solution of the sample and then gradually pulled through the liquid-air border. Average value of surface tension was calculated on the basis of 3 readings with 3 min interval. The Pt wire ring was rinsed with water and flamed using Bunsen burner between experiments. The deviation of the surface tension of distilled water from 72.0 (25 C) was not more than ±0.2 mN/m.
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2.4.
Electroconductometric Measurements
Specific electroconductivity of the surfactant solutions was measured by “ANION 4120” conductometer (Russia). The range of measurements is 104 S/m – 10 S/m, the range of measurement temperature being 0 – 100 C and relative error not exceeding ± 2%.
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30-40 ml of the surfactant solutions are prepared at various concentrations (0.001–5%).
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The prepared solutions are thermostated at a water bath (0.1 C). Before measurements solutions are agitated. At this moment there must not be gas bubbles in the solution and level of
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the solution must be higher than electrodes. The electrode is kept the solution during 3–5 min and the value of the specific electroconductivity is read. At the end of each measurement, the water must be in the interval 2-5 μS/cm.
Study of petroleum-collecting and petroleum-dispersing properties
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2.5.
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electrode is rinsed with distilled water and dried. The specific electroconductivity of distilled
Studies of petroleum-collecting and petroleum-dispersing properties of the synthesized cationic
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surfactants have been performed in parallel for pure-state reagents and their 5% wt. aqueous
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solutions. As an example of crude oil, Pirallahy petroleum (from the oil field near Baku, Azerbaijan) was used, its density being 0.9244 g cm3 (20 C) and kinematic viscosity equalling
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1.05 cm2 s1 (30 C) 0.02 g of the surfactant (or its solution) was added to a thin film (0.150.16 mm thickness) of the crude oil on the surface of water in Petri dishes. Petrocollecting coefficientK was calculated by the formula K=So/S, where So is the initial area of the crude oil surface at
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the beginning of the test, and S is an area of the surface of petroleum transformed in a thickened spot. During the test, the surface area of the spot was measured at certain time to intervals () and respectively values of K were computed. Petroleum-dispersing capacity was evaluated by a degree of cleaning of polluted water surface from petroleum –KD which was found as a ratio of the area of the surface of cleaned water and the initial surface area of the petroleum slicks.
3. Results and Discussion
3.1.
Effect of the Number of the Hydroxyethyl Groups and Counterion of the Surfactants on Their Surface Properties
The surfactants obtained on the basis of MEA are solids at negative temperatures and liquids at ambient conditions. They have a good solubility in ethanol, isopropanol, acetone, ethyl acetate
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and water. An increase in the number of hydroxyethyl groups in the molecule of surfactant raises solubility in water. Figures 1 and 2 illustrate the effect of varying the concentration of the surfactants on the surface tension of their aqueous solutions at the border with air at 25 °C. These results show that, for each type of surfactant, there is a gradual decrease in the surface tension with an increase of
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the solution concentration up to a certain constant value. These constant values of surface tension
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are 30; 27.9 and 24.1 mN/m for the surfactants based on nonyl bromide and containing one, two, and three hydroxyethyl groups attached to the N-atom, respectively. In the case of the surfactants
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based on nonyl iodide, constant values of surface tension are 30.6; 28.8 and 28.4 mN/m, respectively, having one, two, and three hydroxyethyl groups bonded to the N-atom. The CMC values are 1.40103; 1.19103 and 1.04103 moldm3 for the surfactants
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based on nonyl bromide and having one, two, and three hydroxyethyl groups linked to the N atom, respectively (Table 1). The values of CMC for the nonyl iodide-based surfactants are
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correspondingly 1.19103; 1.05103 and 0.93103 moldm3. This decrease in the CMC values demonstrates the effect of an increase in the number hydroxyethyl groups of the surfactants on their surface tension-concentration plot. Rising the number of hydroxyethyl groups on the N
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atom of N-nonylammonium halide salts from one to three causes a gradual decrease in the
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corresponding CMC and in the surface tension. As is seen from Table 1, when bromide anion of the surfactant is substituted by iodidethe
CMC
value
decreases.
In
the
case
of
gemini
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counterion
[ethanediylbis(dimethyltetradecylammonium)] [21], alkylpyridinium [23] and other kinds of surfactants [24-26], a similar tendency is noticed. Table 1 shows that the anions are important for maximum values of surface excess
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concentration - max, as well as the minimum surface area per molecule of surfactant – Amin at the border of water solution with air. The values of (mol/cm2) were calculated by the formula [6, 26]: = (1/nRT)(/lnC)
(1)
where (/lnC) is surface activity (it is found as slope of =f(lnC) at constant T) and R is universal gas constant. It is clear that depends on the concentration of surfactant and max is reached at CMC. The coefficient-“n” characterizes the number of ions generated by a surfactant molecule. In the case of the synthesized salts, the value of “n” may be equalled to 2. The max value is needed for determining the minimum area (denoted by Amin, nm2 ) of a molecule at the border of aqueous solution with air in accordance with the equation [6, 26]: Amin = 1016/NAmax
(2)
where NA is Avogadro's number. The values of max and Amin are dependent on the molecular structure. As is evident from the table, upon substituting bromide-anion by iodide-counterion the max value becomes lowered, whereas Amin rises. For MEA-based surfactants, upon substituting
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bromide-anion by iodide-counterion max increases, while Amin decreases. From the other side, when the number of hydroxyethyl groups linked to N atom rises, max increases and Amin diminishes. A substantial decrease in the area occupied by a molecule (Amin) in the case of the surfactants having three hydroxyethyl groups, as compared with the surfactants containing one or two hydroxyethyl groups, may be attributed to the enhanced intra-hydrogen bonding between
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three hydroxyl groups and inter-hydrogen bonding with the aqueous solution.
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(3)
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CMC = 0 CMC
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The effectiveness of surface tension decrease (CMC) is determined from the equation [6,
where 0 is the surface tension at the border of water with air, CMC is the surface tension at the interface of surfactant aqueous solution with air at CMC. The CMC values for the synthesized
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salts at 298 K temperature fall into the interval 41.4 - 47.9 mN m–1. When Br-anion is replaced by I, CMC is lowered, but with substitution of H atom by hydroxyethyl group, CMC increases.
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Further, the adsorption efficiency (pC20), values are helpful for comparison of the efficiency of adsorption of surfactant at air/water interface [5]. The greater the pC20 value, the larger the efficiency of the surfactant adsorption at the border and the larger the decrease of
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surface tension. Thus, pC20 increases when Br is replaced by I but, with the surfactants based
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on MEA, the opposite situation is seen.
3.2.
Conductometric Measurements
Plots illustrating a dependence of specific electroconductivity from the surfactant concentration
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at 25 C were built (Fig. 3-5). As is evident, specific conductivity increases with a rise in concentration. On the basis of slopes of electroconductivity/concentration plots, both above and below the CMC, the values of ionization degree () were determined [4]: α=S2/S1
(4)
where S1 and S2 are slopes of the mentioned dependence at the concentrations smaller and larger than CMC. The degree of counterion binding () was found as [4]: β=1-α
(5)
The β values are given in Table 1. The CMC values found from intersection point of two straight lines are slightly different from those determined on the base of measurements of surface tension. As may be seen from Table 1, the values of β for both types of surfactants rise with an increase of the number of hydroxyethyl groups bound to N-atom in the head group of the surfactants. The value of β decreases with an increase of the number of hydroxyethyl group, which is consistent with observations made by Chatterjee et al. [18] in their studies of the series of hydroxyethyl-replaced cetylammonium bromide salts.
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If Br-anion in the surfactant becomes substituted by I, increases. Similar tendency was
registered
for
tetramethylammonium
halides
[27],
ethanediylbis(dimethyltetradecylammonium) halides [21], and for cetyltrialkylammonium halides [28].
Thermodynamic properties of the synthesized salts
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3.3.
In the case of these surfactants, the standard Gibbs free energy change of micelle formation
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(Gmic) may be calculated by Eq. 6 [26].
Gmic = (2)RTlnCMC
(6)
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where R is universal gas constant, and T is standard absolute temperature (298.15 K). The standard Gibbs free energy change for adsorption process (Gad) for the obtained salts were computed by Eq. (7) [26]:
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Gad = (2)RTlnCMC 0.6023CMCACMC
(7)
where ACMC has the unit Å2 per molecule, and CMC denotes the surface pressure (in mN/m) at CMC at the border of surfactant water solution with air.
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The Gmic and Gad values are presented in Table 1. It is seen that both micelle formation
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and adsorption processes are spontaneous because the Gmic and Gad values are negative. Meanwhile, Gad is more negative. So, it may be concluded that adsorption is preferential than
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micellization. As is seen from Table 1, in the surfactants containing Br and I, with an increase of the number of hydroxyethyl groups, the values of Gmic and Gad increase. Therefore, hydroxyethyl groups are unfavorable for micelle formation and adsorption processes. However,
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in the surfactants containing C12 chains, with an increase of the number of hydroxyethyl groups, the values of Gmic and Gad decrease [1, 19, 21]. The opposite situation in the synthesized nonyl halide-based surfactants may be explained by the circumstance that, because of a shorter alkyl chain, the more the number of hydroxyethyl groups, the better the watersolubility. As a result, micelle formation and adsorption processes are less spontaneous.
3.4.
Petroleum-collecting and petroleum-dispersing properties of the synthesized salt
Petroleum-collecting and petroleum-dispersing capacity of the surfactants has been investigated regarding thin films of Pirallahy crude oil on the surface of distilled, fresh and sea waters. The surfactants were used as 5% wt. aqueous solutions. The cationic surfactants based on DEA exhibit a higher petroleum-collecting capacity. Thus, 5% wt. aqueous solution of NDEAB has the value of Kmax in fresh water 60.8, being longer than 4 days. In the sea water, 5% aqueous solution of NDEAI is more effective as a petrocollector (Kmax=40.5, =7 days). This salt shows
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the same effect in the fresh water. In distilled water, the effectiveness of this surfactant is higher (Kmax=46.0, =7 days). Among the MEA-based surfactants, NMEAB demonstrates the highest petrocollecting capacity. In all waters, Kmax equals 30.4 (5 days). For NMEAI, Kmax is nearly 23.0. The TEA-based surfactants manifest mainly mixed petroleum-collecting and dispersing
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4. Conclusion
Ammonium salts containing hydroxyethyl-group have been synthesized on the basis of nonyl
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bromide (or iodide), MEA, DEA and TEA. They have been characterized by IR- and NMRspectroscopy. Colloidal-chemical parameters of the obtained cationic surfactants have been computed. The influence of the number of hydroxyethyl groups and the nature of the counterion
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(Br and I) on these parameters has been elucidated. It has been established that, due to a shorter hydrocarbon chain, in the synthesized surfactants, with an increase of the number of
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hydroxyethyl groups, in distinction to the similar surfactants with a longer alkyl chain, Gmic and Gad rise. In the sea water, 5% aqueous solution of NDEAI exhibits the highest petroleum-
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collecting capacity.
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1. R.Wang, Y. Li, Q. Li, Physicochemical Properties of Quaternary Ammonium Surfactants with Hydroxyethyl Groups, Tenside Surf. Det. 51 (2014) 54-58. 2. Deepti, K.K. Ghosh, Micellization of Cetyldiethylethanolammonium Bromide in Mixed Aqueous Organic Solvents, Journal of Dispersion Science and Technology 31 (2010) 1249-1253. 3. Zh. Zhang, H. Wang, W. Shen, Densities, Conductivities, and Aggregation Numbers of Aqueous Solutions of Quaternary Ammonium Surfactants with Hydroxyethyl Substituents in the Headgroups, J. Chem. Eng. Data 58 (2013) 2326-2338. 4. A.R. Glennie, M.M. Mohareb, R.M. Palepu, Thermodynamic and Related Properties of Alkyl Cationic Surfactants Based on Dimethyl and Diethyl Ethanol Amines, Journal of Dispersion Science and Technology 27 (2006) 731-738. 5. Z.H. Asadov, R.A. Rahimov, Sh.M. Nasibova, G.A. Ahmadova, Surface Activity, Thermodynamics of Micellization and Adsorption Properties of Quaternary Salts Based on Ethanolamines and Decyl Bromide, J Surfact Deterg 13 (2010) 459-464. 6. Z.H. Asadov, R.A. Rahimov, Kh.A.Mammadova, A.V. Gurbanov, G.A. Ahmadova, Synthesis and Characteristics of Dodecyl Isopropylolamine and Derived Surfactants, J Surfact Deterg 19 (2016) 145-153. 7. D.Jurašin, I.Habuš, N. Filipović-Vinceković, Role of the alkyl chain number and head groups location on surfactants self-assembly in aqueous solutions, Colloids Surf. A 368 (2010) 119-128. 8. G.Para, A.Hamerska-Dudra, K.A.Wilk, P.Warszyński, Surface activity of cationic surfactants; influence of molecular structure, Colloids Surf. A, 365 (2010) 215-221. 9. J.Harkot, B. Jańczuk, Surface and volume properties of dodecyldimethylammonium bromide and benzyldimethyldodecylammonium bromide. I. Surface properties of dodecyldimethylammonium bromide and benzyldimethyldodecylammonium bromide, J. Colloid Interface Sci. 331 (2009) 494-499.
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and surfactant head group size, sphere-to-rod transitions, and chemical reactivity in cationic micelles, Langmuir 16 (2000) 59-71. 28. H.N. Patrick, G.G. Warr, S.Manne, I.A. Aksay, Surface micellization patterns of quaternary ammonium surfactants on mica, Langmuir 15 (1999) 1685-1692.
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Fig 1. Surface tension vs. ln of the concentration of nonyl bromide-based ammonium salts in
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aqueous solution (25 C). 1- NMEAB, 2- NDEAB, 3- NTEAB.
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Fig 2. Surface tension vs. ln of the concentration of nonyl iodide-based ammonium salts in
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aqueous solution (25 C). 1- NMEAI, 2- NDEAI, 3- NTEAI.
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Fig 3. The plots of specific electrical conductivity against concentration of MEA-based ammonium salts (25 C). 1- NMEAB, 2- NMEAI. The error of electrical conductivity value is
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±0.3 μS/cm
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Fig 4. The plots of specific electrical conductivity against concentration of DEA-based ammonium salts (25 C). 1- NDEAB, 2- NDEAI. The error of electrical conductivity value is
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±0.3 μS/cm
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Fig 5. The plots of specific electrical conductivity against concentration of TEA-based ammonium salts (25 C). 1- NTEAB, 2- NTEAI. The error of electrical conductivity value is
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±0.3 μS/cm
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Table 1. Surface properties of the synthesized cationic surfactants in aqueous solution at 298 K CMC103, max1010, Amin102, CMC, CMC, Gmic, Gad, pC20 nm2 moldm3 molcm2 mNm1 mNm1 kJmol1 kJmol1 NMEAB 0.52 1.40a 1.45b 1.45 114.9 30.0 4.40 42.0 -24.75 -27.65 NDEAB 0.41 1.19 1.22 2.27 73.0 27.9 3.80 44.1 -23.52 -25.46 NTEAB 0.38 1.04 1.07 2.62 63.3 24.1 3.90 47.9 -23.48 -25.31 NMEAI 0.61 1.19 1.25 1.53 108.5 30.6 4.27 41.4 -26.86 -29.57 NDEAI 0.51 1.05 1.10 2.14 77.5 28.8 3.87 43.2 -25.66 -27.68 NTEAI 0.42 0.93 0.95 2.16 77.0 28.4 4.05 43.6 -24.56 -26.58 a b The CMC value was determined by electroconductivity method ( uncertainties are 2.0-2.5%), The CMC β
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Surfactants
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value was determined by surface tension method ( uncertainties are 1.5-2.0%).
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Graphical abstract
Highlights
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New surface-active, ammonium salts based on nonyl bromide (or iodide) and ethanolamines have been synthesized and characterized. With an increase of the number of hidroxyethyl groups, Gibbs free energy changes of micellization and adsorption processes rise.
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These salts exhibit petroleum-collecting capacity.