Anionic surfactants, such as sodium linear alkylbenzene sulfonates (NaLAS), are utilized in various fields, including industry, household, and agriculture. The efficiency of their use in aqueous environments is significantly affected by the presence of cations, Ca 2+ and Mg 2+ in particular, as they can decrease the concentration of the surfactant due to precipitation. To understand cation–sulfonate interactions better, we study both NaLAS colloidal solutions in the presence of CaCl 2 and precipitates forming at higher salt concentrations. Upon addition of CaCl 2 , we find the surface tension and critical micelle concentration of NaLAS to decrease significantly, in line with earlier findings for alkylbenzylsulfonates in the presence of divalent cations. Strikingly, an increase in the surface tension is discernible above 0.6 g L –1 NaLAS, accompanied by the decrease of apparent micelle sizes, which in turn gives rise to transparent systems. Thus, there appears to be a second critical concentration indicating another micellar equilibrium. Furthermore, the maximum salt tolerance of the surfactant is 0.1 g L –1 Ca 2+ , above which rapid precipitation occurs yielding sparingly soluble CaLAS 2 ∙2H 2 O.
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The above examples illustrate the high propensity of Ca 2+ for surfactant precipitation, which may in turn lead to a significant loss of sulfonate surfactant. Thus, characterizing Ca 2+ –surfactant interactions is important to describe the overall performance of a given surfactant in industrial systems. To this end, we study the effect of CaCl 2 on the properties of sodium linear alkylbenzene sulfonate (NaLAS) aqueous solutions with varying cation and surfactant concentrations in a wide range, both below and above CMC. We find upon increasing the concentration of Ca 2+ ions a marked decrease in the air–water surface tension as well as CMC in dilute solutions (0.01–0.1 g L –1 Ca 2+ ). Interestingly, there appears to be a second critical concentration at high surfactant concentrations, associated with another chemical equilibrium. Further enhancement of the amount of CaCl 2 yields the sparingly soluble CaLAS 2 ∙2H 2 O salt, suggesting the maximum salt tolerance of NaLAS to be ~0.1 g L –1 Ca 2+ .
As for DBS – , ions with higher charge density tend to salt out the surfactant at a lower salt concentration. That is, they more readily induce micellization and, eventually, precipitation [ 13 , 24 , 28 , 29 , 30 ]. Indeed, the order of NH 4 + < Na + < Mg 2+ < Ca 2+ has been found for precipitating NaDBS from an aqueous solution [ 15 ], which is a reversal of the cationic Hofmeister series. The solubility of Ca 2+ alkyl aryl sulfonates below CMC has been quantified in terms of thermodynamic products [ 38 , 39 , 40 ]. Based on the thus-obtained equilibrium solubility of Ca 2+ , ca. 1.3–1.7∙10 –4 M [ 38 , 39 , 40 ], the calcium salt has a markedly lower solubility as compared to the Na + one, 1.4–4.6∙10 –3 M [ 39 , 40 ].
In particular, a marked decrease in the surface tension as well as CMC with increasing salt concentration is observed at the presence of electrolytes at a fixed temperature [ 5 , 9 , 12 , 13 , 14 , 15 , 30 , 31 , 32 , 33 , 34 , 35 ]. For NaDBS as well as other sulfonates, this stems from (1) the reduction of repulsive forces acting between negatively charged head-groups as a result of cation–sulfonate interactions, and from (2) the decreased hydration of surfactant molecules owing to strong cation hydration, which in turn promotes hydrophobic interactions [ 14 , 15 , 31 , 34 , 35 , 36 ]. The interplay of these two effects leads to the well-known order of ‘salting-out’, observed first for proteins by Hofmeister [ 15 , 37 ].
The behavior of surfactants in an aqueous environment depends on a number of physicochemical parameters, such as temperature and salt concentration. Sulfonates can be tolerant to very high temperatures (up to 200 °C) as compared to sulfate detergents [ 1 , 19 ]. However, their usage is restricted to rather low-salinity systems due to their propensity for precipitation [ 1 , 19 , 20 , 21 , 22 , 23 ]. Beside soap formation, salts have a large impact also on other critical surfactant properties, such as adsorption [ 24 , 25 , 26 ], transport [ 27 ], as well as bubble and foam stability [ 28 , 29 ]. Thus, understanding salt–surfactant interactions is indispensable when assessing the efficiency of surfactants in altering certain physicochemical properties.
One of the most commonly applied anionic surfactants are long-chain alkyl aryl sulfonates [ 1 ], as they can be synthesized from cheap raw materials [ 1 , 2 ] and their design can be tailored to the purpose of application. Among sulfonates, commercially available sodium dodecyl benzenesulfonate (NaDBS) is by far the most studied. It is environmentally advantageous for its faster biodegradation as compared to branched isomers owing to its long aliphatic chain [ 3 ], which also results in a very low critical micelle concentration (CMC), i.e., 1.2–2.9 mM [ 4 , 5 , 6 , 7 , 8 , 9 , 10 , 11 , 12 , 13 , 14 , 15 ]. Moreover, its ability to effectively decrease the interfacial tension (IFT) between an apolar and polar phase, e.g., oil and water, has been reported in earlier works [ 13 , 15 , 16 , 17 , 18 ].
The Ca 2+ :Na + molar ratios in the solids were determined by energy-dispersive X-ray (EDX) spectroscopy. To record the spectra, a Röntec QX2 spectrometer (Berlin, Germany) equipped with Be window and coupled to a Hitachi S-4700 scanning electron microscope (Tokyo, Japan) was used at 18 kV acceleration voltages. For each solid, spectra were taken at least four different spots to obtain realistic representation of the elemental distribution.
The average micelle size in a set of samples was determined with a Malvern NanoZS dynamic light scattering (DLS) instrument (Malvern, UK) operating with a 4 mW helium–neon laser light source (λ = 633 nm). The measurements were performed in back-scattering mode at 173° and at (25.0 ± 0.1) °C. The samples were stirred for 24 h prior to the measurements. For each sample, three to four repetitions were carried out and the size was taken as the arithmetic mean of the volume-averaged hydrodynamic diameters.
The surface tensions at the air–water interface and the critical micelle concentration (CMC) values of a set of surfactant solutions with different electrolyte concentration were determined at (25.0 ± 0.1) °C using a tensiometer (type K100; Krüss, Hamburg, Germany), with the aid of the Wilhelmy plate method. In the tensiometer, a platinum–iridium ring is suspended from a torsion balance, and the force (in mN m –1 ) required to pull the ring off the surface is measured. Here, additional experiments were performed for solutions prepared from MgCl 2 ∙6H 2 O as described above.
The composition of the surfactant was deduced using a 1260 Infinity II HPLC setup coupled to a G6125B LC-MSD (mass selective detection) from Agilent (Santa Clara, CF, USA), applying electrospray ionization (ESI). For analysis, an ~0.05 g L –1 (50 ppm) NaLAS aqueous solution was prepared and a water: methanol mixture at 85:15 volume ratio was used as eluent. The mass spectrum was carried out in negative-ion mode, scanning the m/z region of 150–445.
First, solution with an appropriate concentration of Ca 2+ -ion (0.01–5.00 g L –1 ) was prepared from CaCl 2∙ 2H 2 O. This solution was then filled into a volumetric flask containing the necessary amount of solid NaLAS. The thus-obtained aqueous mixtures (c NaLAS = 0.05–5.00 g L –1 ) were stirred for 24 h at room temperature. To avoid the incidental adsorption of either Na + , Ca 2+ , Cl – , or LAS – ions onto the glass surface of the flask, plastic vessels were used to store all colloidal solutions and filtrates. In case of precipitate formation, the samples were filtered, washed with 20 mL water, and dried. In addition, several precipitates were calcined at 900 °C for 24 h or at 1000 °C for 16 h under air in a Nabertherm L9 furnace (Lilienthal, Germany).
All samples were made by using deionized water, which was produced by reverse osmosis and was further purified by UV irradiation, using a Puranity TU3 UV/UF+ system (VWR). Furthermore, samples were always freshly prepared before tensiometric and dynamic light scattering (DLS) measurements, since long-term aggregation of micelles or particle cohesion may alter the samples properties significantly.
As for NaDBS, it is known to be composed of sulfonates with different chain lengths [ 34 ]. Thus, its exact composition was determined via high-performance liquid chromatography coupled with mass spectrometry (HPLC-MS; for technical details, see below). Based on the relative peak counts, the highest fraction is undecylbenzene sulfonate (43%) followed by dodecylbenzene sulfonate (27%) with the average molar mass being 336.81 g mol –1 for the sodium salt; see the mass spectrum in . Therefore, this compound will be referred to as sodium linear alkylbenzene sulfonate, NaLAS, throughout this work.
We started our study with scoping experiments to separate the concentration regions based on the appearance of precipitate. Accordingly, the samples can be divided into two groups: at cCa2+ ≤ 0.1 g L–1 for all surfactant concentrations (0.01–5.00 g L–1), the solutions are transparent or opalescent without the formation of filterable precipitates, whereas at cCa2+ > 0.1 g L–1 for all surfactant concentrations, a solid appears immediately or after a few hours of stirring (the total equilibration time was 1 day). In possession of this information, we analyzed the colloidal and solid systems according to different aspects to get insights into the behavior of the overall system.
The impact of the binding of Ca2+ ions on the average particle sizes in the colloidal region has been probed by dynamic light scattering, depicted as volume-averaged average diameters in a. In neat surfactant solutions, we obtained very large values (~200 nm) for cNaLAS ≤ 0.3 g L–1, which seem to be inconsistent with previous experimental and simulation results suggesting the diameter of NaDBS micelles to be 4–6 nm [34,53,54]. It is possible that the high values are experimental artefacts, probably associated with the very low surfactant concentrations not suitable for DLS detection.
Open in a separate windowIn the concentration range of cNaLAS = 0.45–0.67 g L–1, the obtained diameters drop significantly, which is in broad agreement with the determined CMC (668 mg L–1, b). At higher concentrations, we observe the diameter to further decrease, reaching ~5 nm at cNaLAS = 1.1 g L–1. Such a continuous decline has already been observed for NaDBS, i.e., ~16 nm (cNaDBS = 6.3 g L–1) to ~10 nm (cNaDBS = 69 g L–1), in the presence of 0.2 M NaCl [53]. However, this decrease was assigned to the breakdown of the Stokes–Einstein equation used to evaluate the DLS data. Indeed, the micelle size tends to increase with concentration for surfactants as a result of increased aggregation [34,55]. Nevertheless, the diameters obtained in this work agree qualitatively with the expected micellar dimensions [34,53,54]. We also note that the corresponding polydispersity indices are rather high (0.3–0.5; see Figure S2), reflecting the somewhat broad size distribution of the surfactant owing to the different alkyl chains.
In the presence of CaCl2, the samples become opaque already at low surfactant concentrations with particle diameters clearly exceeding ~200 nm. That is, dissolved species aggregate to a high degree signaling the onset of precipitation, which in turn can be explained by the very low thermodynamic solubility of alkyl aryl sulfonates [38,39,40]. This observation, however, appears to contradict our surface tension measurements, where no precipitation was observed. This contradiction can be resolved by the different timescales of the two experiments: during surface tension measurements—carried out as titrations—the solution was equilibrated only for a short time at each composition, whereas all samples were stirred for one day prior to light scattering detection, allowing the particles to aggregate. This is in line with a previous observation that precipitation starts only after several days at low metal-ion concentrations [38]. In addition, the lowest surfactant concentrations might be problematic to obtain accurate values with DLS.
Strikingly, the samples become transparent upon further increasing the surfactant concentration, which is consistent with the well-known redissolution of poorly soluble sulfonate salts in more concentrated surfactant solutions [38,39,40]. For instance, we observe transparency at cCa2+ = 0.01 g L–1 and cNaLAS = 0.625 g L–1, which was confirmed by the drop in the absorbance as well; see Figure S3. At a tenfold concentration of Ca2+, however, the sample turns transparent only around cNaLAS = 5 g L–1. That is, higher Ca2+ concentrations required larger amounts of surfactant for the cloudiness to disappear. This trend is similar to the above regarding the reversal of the surface tension: higher cCa2+ required higher cNaLAS for γ to increase ( a, inset). The similarity between the two trends suggests that there appears to be a second critical surfactant concentration, corresponding to another chemical equilibrium. As for the tensiometric curves, we estimate this critical concentration as the minimum of γ ( a, inset). In the case of the DLS samples, we take this concentration as the average of those belonging to the most concentrated cloudy sample and the most dilute transparent one. A comparison of these estimates from the two methods indeed shows strong correlation (except for cCa2+ = 0.075 g L–1); see b.
In conclusion, the two phenomena, i.e., the increase in the surface tension and the disappearance of cloudiness, stem probably from the same molecular process, likely to be associated with the collapse of large micelles or aggregates. As such, the particle sizes decrease significantly and become very similar to those obtained in the absence of salt at high surfactant concentration ( a). (Nevertheless, these values should be taken with a grain of salt as the measurements yielded very high polydispersity indices and poorer fits of the correlograms; see Figure S2.)
Having the aqueous phase analyzed, we now turn to the characterization of solids forming at higher metal-ion and surfactant concentrations. We find that above cCa2+ = 0.1 g L–1, precipitation occurs readily at all surfactant concentrations. That is, the maximum salt tolerance of NaLAS (up to 5 g L–1) is 0.1 g L–1 Ca2+, i.e., 0.28 g L–1 CaCl2.
The thus-obtained precipitates are all largely amorphous, similar to the poorly crystalline sodium salt, as shown in the X-ray diffractogram in Figure S4. The infrared spectrum of the neat sodium salt matches well the one reported earlier [56], with the following characteristic vibrations in cm–1: 2956 and 2870 (–CH3 asymmetric and symmetric stretching), 2924 and 2854 (–CH2– asymmetric and symmetric stretching), 1601, 1496 and 1408 (aromatic –C=C– stretching), 1461 (–CH2 scissoring), 1378 (–CH3 symmetric bending), 1182 and 1042 (–S=O asymmetric and symmetric stretching), 1129 and 1012 (aromatic =CH in-plane bending), 831 (aromatic =CH out-of-plane bending), 688 (–SO3 bending); see .
Open in a separate windowUpon binding of Ca2+ ions, most peak positions remain unaltered (within 4 cm–1), with the exception of the –S=O stretching bands which shift from 1182 to 1992 cm–1 and 1042 to 1049 cm–1, respectively. That is, Na+/Ca2+ exchange affects mostly the sulfonate moiety, consistent with this group being the metal-ion coordination site. This is consistent with previous calculations for the DBS– anion in the presence of Na+, Mg2+, and Ca2+ ions [52,57], and with Ca2+ having a high affinity toward oxygen-donor ligands. In addition, the variation of the –S=O bands are in line with those reported for the incorporation of the DBS– anion in the interlayer gallery of Mg-Al- [56,58], as well as Zn-Fe layered double hydroxides (LDHs) [59]. Nevertheless, the shift to higher wavenumbers is the opposite to that found for LDHs [56,58,59]. Most likely, this is due to the difference between the binding interactions: the sulfonate anion is bound directly to the Ca2+ ion in the precipitate, whereas for LDHs, it is connected to the hydrated metal ions via hydrogen-bonding [56,58,59,60]. Moreover, it is seen in that the distance between the positions of the asymmetric and symmetric vibration does not differ significantly (140 vs. 143 cm–1), in line with previous findings [56], suggesting similar coordination modes for Na+ and Ca2+.
Furthermore, the very broad peak around 3000 cm–1 in the spectrum of NaLAS, which corresponds to the –OH stretching region, becomes much more intensive for the calcium salt. In parallel, a peak at 1653 cm–1 shows up, which belongs to the scissoring mode of water [61,62]. Consequently, the calcium salt is more strongly hydrated than NaLAS. As for calcium salts precipitating from solutions with very different metal-ion-to-surfactant ratios, the spectra are again very similar indicating similar stoichiometries.
We checked the supposed Ca2+:LAS– = 1:2 molar ratio by calcining numerous precipitates both at 900 °C (for 24 h) and at 1000 °C (for 16 h). The X-ray diffractograms in Figure S5 show unambiguously that the only crystalline solid phase is CaSO4 (JCPDS No. 74-2421), with some amorphous CaSO4 or residual organics in a few cases (for instance, the precipitate obtained at 5 g L–1 Ca2+ and NaLAS and calcined at 900 °C; see Figure S5). Consequently, the addition of Ca2+ ions to NaLAS solutions yields CaLAS2. This is supported by the EDX elemental analyses, showing that—within experimental uncertainty—the solids are essentially free of Na+; hence, Na+ ions are fully exchanged with Ca2+ in the solids. The obtained Na+:Ca2+ molar ratios are shown in . (The atomic fraction obtained directly from the measurements are listed as atomic percentages in Table S1 in the Supplementary Materials).
These findings are also corroborated by the satisfactory agreement between the mass of the calcined solids and the ‘theoretical’ one, assuming the exclusive formation of CaSO4 (which can be calculated by multiplying the weight of the precipitates by the CaSO4:CaLAS2 molar mass ratio, 136.14/667.72). Nevertheless, differences of 2–11% still remain. Based on the infrared spectra, this difference can be attributed to the presence of hydrating water in the solid phase. Accounting for this water fraction, we obtain 2.3 ± 1.6 water molecules per surfactant unit and thus an average stoichiometry of CaLAS2∙2H2O. These calculations together with the weight losses are listed in .
Neutral sulfurized calcium dodecyl phenate was prepared via reaction of calcium hydroxide with dodecyl phenol and ratio of sulfur (0.15) mole product (A) is illustrated in Scheme 1.
Scheme 1Reaction of formation of neutral calcium salt of sulfurized dodecyl phenol
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The infrared spectrum of product (A) is given in Fig. 1 which illustrates the following:
Strength of OH band decreases in case of the products. This may be due to the presence of little amounts of unreacted reactant.
Appearance of C–S peak at 729–758 cm−1 in product (A).
Appearance of S–S peak at 557–563 cm−1 in the product (A).
The infrared spectrum of product (A)
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The physico-chemical properties of the compounds prepared are shown in Table 2.
Table 2 The physico-chemical characteristics of the prepared compoundsFull size table
It’s obvious from the data given in Table 2, that the experimental and theoretical values of molecular weights are not identical which may be due to the formation of compounds other than the expected compounds as by-products.
The general formulas of these compounds are:
The experimental formula of the prepared additive (A) is shown as the following: (C40.06H43S1.12O3.58Ca0.56)
Neutral sulfurized calcium salt of dodecyl phenol (A) was used to prepare both basic and overbasic salts. Compounds (A1, A2) are illustrated in Schemes 2, 3.
Scheme 2Reaction of formation of basic calcium dodecyl phenate
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Scheme 3Reaction of formation of overbasic calcium dodecyl phenate
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The H1NMR spectra of compounds (A, A1, and A2) are shown in Figs. 2, 3, and 4, which illustrate the following:
1.
A peak appears at 8–9 ppm in case of compounds (A, A1, A2), which indicates the presence of phenolic OH. The intensity of this peak extremely decreases in case of compounds (A1) and (A2); this may be due to the presence of percentage of unreacted dodecyl phenol in the preparation of the neutral compound.
2.
A peak appears at 0–2 ppm in case of three compounds (A, A1, and A2) which indicates the presence of alkyl hydrogen.
3.
A peak appears at 6–8 ppm in case of three compounds (A, A1, and A2) which indicates the presence of benzene ring.
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4.
A peak appears at 3–4 ppm in case of three compounds (A, A1, and A2) which indicates the presence of Ar–CH group.
H1NMR spectrum of product (A)
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Fig. 3H1NMR spectrum of product (A1)
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Fig. 4H1NMR spectrum of product (A2)
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The total base number and the percentage of calcium in the prepared basic compound A1 are 140.5, 5.84% but in prepared overbasic compound A2 are 277.9, 8.54% which indicates that an increase in calcium content leads to an increase in the total base number (TBN) of the prepared additives.
Neutral calcium dodecyl benzene sulfonate (B) was prepared by neutralizing dodecyl benzene sulfonic acid with calcium hydroxide, which is illustrated in Scheme 4.
Scheme 4Reaction of formation of neutral calcium dodecyl benzene sulfonate
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Sulfonates act on the surface of metal parts by creating a protective layer by reaction with the metal surfaces. The total base number and percentage of calcium in the prepared additive B are 60, 2.14%.
Basic and overbasic calcium salt of dodecyl benzene sulfonic acid were prepared by addition of an excess of calcium hydroxide with carbon dioxide.
The different mixtures (X, Y, Z) were prepared using different ratio of neutral sulfurized dodecyl phenate with dodecyl benzene sulfonic acid (0.075: 0.025, 0.05: 0.05, and 0.025: 0.075) moles, respectively. The prepared mixtures (X, Y, Z) were mixed with different amounts of calcium hydroxide to prepare basic and overbasic mixtures where X 1, X 2, Y 1, Y 2, Z 1, Z 2 were obtained. The total base number TBN and calcium percentages of the prepared mixtures were measured and are listed in Table 3.
Table 3 TBN and Ca wt% results of the prepared mixturesFull size table
All the prepared compounds were added to a sample of “SAE-30” lube oil free from any additives, in 2% concentration, and the blends obtained were subjected to serve oxidation condition as described previously and the oxidation stability expressed as increase in viscosity ratio (V/V 0) and total acid number (ΔTAN), and optical density (Log I/I 0) that compared with lube oil sample free from additives. Oxidation stability results of lube oil without and with 2% wt additives of compound (A) after different oxidation hours are shown in Table 4.
Table 4 Oxidation stability of lube oil without and with 2% wt. additives of compound (A) after different oxidation hoursFull size table
It was found also that the additive prepared overbasic (A2) are more efficient as antioxidant than other prepared additives. Increasing the proportions of metal in the phenate, increase their antioxidant properties and this may be attributed to the fact that excess base in the additives can neutralize the excess of acids formed during oxidation of lubricating oil [22]. Oxidation stability results of lube oil without and with 2% wt additives of compound (B) after different oxidation hours are shown in Table 5.
Table 5 Oxidation stability of lube oil without and with 2% wt. additives of compound (B) after different oxidation hoursFull size table
It is clear that calcium salt of dodecyl benzene sulfonate gives better results in oxidation stability test.
All the prepared compounds have been added to the oil samples in the concentration of 2% wt, and using spot test method results given in Table 6 show clearly that the prepared compounds have very good and excellent dispersion power (80–95%) for sludge and solid particles formed during lube oil oxidation compared with lube oil only.
Table 6 Percentage of dispersion in the lube oil sample and it blends with the prepared additives (A, A1, A2, B)Full size table
Results obtained in Table 6 shows that addition of different additives raise the dispersion of oil from 32% to approximately 95%, this is because addition of the additive reduces the oxidation of lube oil and therefore reduces the formation of insoluble materials [23], disperses solid particles, and prevents their agglomeration so addition of these additives gave better oil dispersion.
It is clear from the data given here that excess base not only increase the power of phenate to neutralize the corrosive acids but also improve their efficiency as detergent/dispersant additives. The increase in the proportion of calcium expressed in moles calcium hydroxide per mole dodecyl phenol increases detergency and dispersancy. This proportion results in the formation of an additive having a weight percentage of (8.54) for calcium and a total base number (TBN) of about (277.9) for the final additive (A2). It is clear from the data given here that excess base not only increases the power of phenate to neutralize the corrosive acids but also improves their efficiency as detergent/dispersant additives.
The percentages of sludge formation during the oxidation of lube oil sample with and without additive are determined and the data are given in Table 7 and confirms that the presence of this additive decreases the percentage of sludge formed to 1.5% compared to 6.6776% for the oil free from any additive as they reduce oil oxidation and this decreases the formation of small amounts of asphaltic substances, which after that leads to sludge formation. The lowest sludge percentage formed in case of addition of additive (A2) equals 0.13.
Table 7 The percentages of sludge formation during the oxidation of lube oil sample without and with additive (A, A1, A2, B)Full size table
The results obtained from the prepared additive (A, A1, A2, B) are shown in Table 8 which indicate that additive (A2) has good efficiency as detergent/dispersant additive which is 91.11% compared to 53.78% for the blank as the additive reduces oil oxidation and hence prevents precipitant agglomeration.
Table 8 The percentages of PDDE during the oxidation of lube oil sample without and with additive (A)Full size table
Oxidation stability results of lube oil without and with 2% wt additives of groups (X, Y, Z) after different oxidation hours are shown in Figs. 5, 6, 7, 8, 9, 10, 11, 12, and 13; this indicated that all the prepared compounds impart better oxidation resistance properties to the lube oil compared with the undoped oil. Mix (Z)2 with TBN equals to (285) and percentage of calcium equals to (8.75) and higher ratio of dodecyl benzene sulfonic acid imparts better oxidation resistance properties to the lube oil, the efficiency increases with the increase in sulfonic acid ratio in mixture as sulfonic group is more electronegative than phenolic group, which makes donation of labile hydrogen better to stabilize the chain radicals; i.e., these inhibitors destroy the peroxide radicals and thus, stop the propagation step and convert highly reactive peroxide radicals to more stable compounds.
Fig. 5Variation of ∆TAN with oxidation time of lube oil without and with additives (X, X 1, and X 2)
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Fig. 6Variation of V/V 0 with oxidation time of lube oil without and with additives (X, X 1, and X 2)
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Fig. 7Variation of Log I/Io with oxidation time of lube oil without and with additives (X, X 1, and X 2)
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Fig. 8Variation of ∆TAN with oxidation time of lube oil without and with additives (Y, Y 1, and Y 2)
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Fig. 9Variation of V/V 0 with oxidation time of lube oil without and with additives (Y, Y 1, and Y 2)
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Fig. 10Variation of Log I/I 0 with oxidation time of lube oil without and with additives (Y, Y 1, and Y 2)
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Fig. 11Variation of ∆TAN with oxidation time of lube oil without and with additives (Z, Z 1, and 2)
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Fig. 12Variation of V/V 0 with oxidation time of lube oil without and with additives (Z, Z 1, and Z 2)
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Fig. 13Variation of Log I/I 0 with oxidation time of lube oil without and with additives (Z, Z 1, and Z 2)
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The results given in Table 9 show clearly that the prepared additives have very good to excellent dispersancy power (70–97%) for the sludge and soil particles formed during lube oil oxidation compared with the lube oil only. It is clear that the addition of these compounds not only disperses solid particles in the oil and thus prevents their agglomeration and precipitation on metallic parts of engines that can cause damage, but also neutralizes some of the acidic products of oxidation due to their basic nature. The efficiency of additive as detergent increases by increasing the basicity of the prepared additives.
Table 9 Percentage of dispersion of the lube oil sample and it is blends with the prepared additivesFull size table
The percentages of sludge formation during the oxidation of lube oil sample with and without additives are determined and the data are given in Table 10. It was found that mixture (Z)2 gives lower quantity of sludge; this is may be due to increasing of the proportion of metal, which increases their antioxidant properties and this may be attributed to the fact that excess base in the additives neutralization of excess acids formed during oxidation of lubricating oil.
Table 10 The percentages of sludge formation during the oxidation of lube oil sample without and with mixture additivesFull size table
The results obtained from the prepared additives [(X–X 2), (Y–Y 2), and (Z–Z 2)] are shown in Table 11.
Table 11 The percentages of PDDE during the oxidation of lube oil sample without and with mixture additivesFull size table
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