Synthesis of ethylolammonium salts of diethylolamido phosphate surfactant based on the flaxseed oil and their corrosion inhibitor properties

3902 | P a g e O c t o b e r 1 7 , 2 0 1 5 Synthesis of ethylolammonium salts of diethylolamido phosphate surfactant based on the flaxseed oil and their corrosion inhibitor properties V.M.Abbasov, Z.H.Asadov, S.S.Suleymanova, R.A.Rahimov, E.Sh.Abdullayev Institute of Petrochemical Processes of Azerbaijan National Academy of Sciences, Hojaly ave. 30, Az 1025, Baku, Azerbaijan revan_chem@mail.ru ABSTRACT


INTRODUCTION
Carbon steel as a relatively cheap material having very high mechanical characteristics is widely used in practically all fields. Regrettably, this alloy undergoes corrosion in practice under the action of such aggressive components as hydrochloric and hydrosulphuric acids, carbon dioxide and many others [1]. Hydrochloric acid is very effective for dissolving calcium carbonate scale inside the pipelines and finds a large application with this goal. There are several ways for decreasing the rate of metals corrosion. One of them is an introduction of inhibitors into acidic systems. The most preferable inhibitors for protection of metals against corrosion are of organic nature and contain such heteroatoms as nitrogen, oxygen, phosphorus and sulphur as well as aromatic rings and multiple bonds. Among various groups of inhibitors surface-active inhibitors are characterized by high levels of inhibition efficiency, cheapness, relative harmlessness and easiness of production. Inhibition was reported to be realized as a result of adsorption of the surfactant molecules, with an aid of their functional groups, to the metal surface [2][3][4][5].
Presence of such heteroatoms as nitrogen, sulfur and oxygen, because of their tendency to be adsorbed at the metal/media interface with formation of protective layer, is very important [6][7][8].
Inhibition of iron corrosion by surfactants may occur via two phenomena consisting in blocking and hydrophobing effects [9].
As is known, inhibitors are considered to be optimal if they are based on relatively cheap and non-toxic initial components and contain the above-shown heteroatoms and fragments. From this standpoint vegetable oils are very convenient raw materials as they are restorable and ecologically clean [10][11][12][13].
In the present study weight loss measurements were performed to examine the H2S acid corrosion inhibition process of carbon steel by some surfactants synthesized on the basis of flaxseed oil and such ethanolamines as monoethanolamine (MEA), diethanolamine (DEA) and triethanolamine (TEA) for the range of concentrations below the critical micelle concentration. Similar surfactants were synthesized and studied as chemicals for removing thin petroleum films from water surface [14]. Meanwhile, the values of interfacial tension at the water-kerosene border in the presence of the synthesized surfactants have been determined and respective colloidal-chemical parameters have been computed.

Synthesis of diethylolamide
The reaction was carried out in a round bottom Pyrex glass reaction flask submerged in an oil bath. The reaction flask was equipped with a mechanical stirrer, thermometer and condenser. DEA was reacted with the flaxseed oil at a molar ratio of 3:1. The reaction was carried out at a temperature of 145-155 C and the formation of the diethylolamide was monitored with FTIR. At the end of the reaction, the reaction mixture was allowed to cool and was later dissolved in diethyl ether in a separating funnel. The ether phase was washed with 5% aqueous hydrochloric acid. The ether layer was separated, washed with water and passed over sodium sulphate. The resulting ether layer was later concentrated using a rotary evaporator.

Synthesis of ethylolammonium salts of ethylolamide phosphate
Ortho-phosphoric acid (0.135 mol) was added to 400 mL of anhydrous benzene at 50 C in a three-necked round-bottom flask equipped with a thermometer sensor and a magnetic stirrer. The diethylolamide (0.135 mol) was gradually introduced into the flask and allowed to react with the phosphoric acid until a complete conversion was achieved.

Interfacial tension measurements
All the interfacial tension measurements [15] were carried out using kerosene to make the solutions. The solutions kept at the desired temperature were measured 60 s after transfer to the thermostated measuring dishes. The actual temperature within the dishes was controlled prior to and after the measurement by means of a thermocouple.
Deviations from the desired temperature were ±0.2C. The interfacial tension as a function of concentration was measured at 20 C using a drop volume Traube stalagmometer (Russian Federation). Interfacial tension values from the two measurements varying by no more than 0.2 mN m1 were averaged and reported. The interfacial tension of pure kerosene at the border with water was 46.5 mN m1.

Weight Loss Measurements
The weight loss experiments were performed with carbon steel specimens having a composition of (wt%): 0.21- where W is the average weight loss of three parallel carbon steel sheets, A is the total surface area of the specimen,  is immersion time, R0 and R are the values of the corrosion rate without and with addition of the inhibitor, respectively.

Synthesis of ethylolammonium salts
The reaction was carried out using flaxseed oil as the starting material. The oil was reacted with DEA at molar ratio of 1:3 (flaxseed oil:DEA) (Scheme 1): Glycerol was produced as the by-product of the reaction.

Surface activity of the synthesized surfactants
The critical micelle concentrations (CMC) were determined by the surface balance method. The CMC values of the prepared ethylolammonium salts were determined at 293 K from the change in the slope of the plotted data of interfacial tension versus the natural logarithm of the solute concentration. Some representative plots (Fig. 3) were illustrated here for brevity. The obtained values of CMC for surfactants at 293 K temperature are tabulated in Table 1, together with values for the interfacial tension at CMC. Generally, the increasing in the number of ethylol groups in the head group of molecule decreases CMC values due to decreasing the solubility of the surfactants in kerosene. Such improved water-solubility lowers the tendency for the surfactant to form micelles.  a CMC, critical micelle concentration;  CMC, equilibrium interface tension at the CMC; max, the maximum surface excess concentration at the kerosene solution/water interface; Amin, the minimum area per surfactant molecule at kerosene solution/water interface;  CMC, effectiveness of interface tension reduction; Gmic, change of standard free energy of micellization; Gad, change of standard free energy of adsorption.
As a result, a plot of interfacial tension as a function of equilibrium concentration of surfactant in one of the liquid phases, rather than an adsorption isotherm, is generally used to describe adsorption at this interface. The concentration of surfactant at the interface may therefore be calculated from interfacial tension data by using the following equation [16]: is the slope of the plot of  versus lnC at a constant temperature (T), and R is the gas constant in Jmol1K1. The surface excess concentration at surface saturation (max) can be readily found and it is a useful measure of the effectiveness of adsorption of surfactant at the liquid-gas or liquid-liquid interface, since it is the maximum value to which adsorption can attain.
From the surface excess concentration, the area per molecule at interface is calculated using Equation (2) max 16 min 10    N (2) where N is Avogadro's number.
The effectiveness of interfacial tension reduction, CMC = 0  CMC, where 0 is the interfacial tension at the kerosene/water border and CMC is the interfacial tension at the kerosene solution of surfactant/water border at CMC, was determined at 293 K temperature. The max, Amin and CMC values were calculated and listed in Table 1. The data listed in Table 1 show that the minimum areas per molecule at the kerosene solution/water interface decrease with the increase of the number of ethylol units in the head group molecule. The data on Amin and max indicate the dependence of the effectiveness of adsorption at the kerosene solution/water interface from the structure of surfactants. It was found that increase in the number of hydroxyethylol groups in surfactant structure appears to have an unusual decrease in Amin at the interface. This can be attributed to the fact that TEA has a branched hydroxyethyl group, which makes coiling of hydrophobic chain with a consequent decrease in Amin [17].
The effectiveness of interfacial tension reduction in these compounds shows a steady rise with increase in the number of ethylol groups.

Standard free energies of micellization and adsorption at the kerosene solution/water interface
Standard free energies of micellization Gmic, for the synthesized surfactants have been calculated by equation [16] Gmic = RT lnCMC (3) Standard free energies of adsorption Gad for these surfactants have been determined by using the relationship [6] Gad = RT lnCMC  CMCACMC (4) The found values of Gmic and Gad are listed in Table 1. From these data it may be concluded that micellization process has a spontaneous character (Gmic0). All values found for Gad are negative. Moreover, they are more negative than those of Gmic, i.e. adsorption of the mentioned surfactants at the kerosene/water interface is associated with a larger decrease in free energy of the system. As the number of ethylol groups of head group rises, the values of Gmic decrease, while the values of Gad increase. This indicates that the increase in the number of ethylol groups at N atom in these surfactants inhibit adsorption at the kerosene solution/water interface.

Effect of inhibitor concentration
It can be seen that the presence of inhibitors results in a high decrease in the rate of corrosion. In the case of these inhibitors, the corrosion rate decreases as the inhibitor concentration increases, getting maximum inhibition efficiency ranged between 97.0 and 99.5% at 100 ppm after 5 hours of exposure (Table 2). This trend may result from the fact that adsorption of these complex surfactants forms thin inhibitor films on the metal surface which relatively isolate the metal surface from the corrosive environment causing much reduced corrosion rates. Inhibition efficiency of these films depends on various factors including but not limited to corrosiveness of the environment, concentration of the surfactant, any synergetic effects with other molecules present in the environment and/or flow/ shear effects. O c t o b e r 1 7 , 2 0 1 5

Adsorption isotherm and thermodynamic considerations
The adsorption isotherms can provide basic information on the interaction of inhibitor and metal surface. In order to obtain adsorption isotherm, the surface coverage values () for different concentrations of the prepared surfactants in H2S solution have been obtained from gravimetric measurements and tested graphically for fitting a suitable adsorption isotherm. The plot of C/ versus C gave a straight line with correlation coefficient of 0.9998 and the slope closed to 1 providing that the adsorption of the prepared surfactants in solution of hydrosulfuric acid on the carbon steel surface obeys Langmuir adsorption isotherm, which is presented by Eq. (5).

C K
where C is the inhibitor concentration,  is the degree of coverage on the metal surface and Kads is the equilibrium constant for adsorption-desorption process (  (6) where Gads is the free energy of adsorption of inhibitor molecules on metal surface, Kads is the equilibrium constant for adsorption-desorption process, R is the gas constant, T is the absolute temperature and 55.55 is the molar concentration of water.