Universal Phase Transformation Mechanism and Substituted Alkyl

Article pubs.acs.org/IECR

Universal Phase Transformation Mechanism and Substituted Alkyl Length and Number Effect for the Preparation of Overbased Detergents Based on Calcium Alkylbenzene Sulfonates Zhaocong Chen, Feng Chen, and Dongzhong Chen* Key Laboratory of Mesoscopic Chemistry of Ministry of Education and Department of Polymer Science and Engineering, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, People’s Republic of China S Supporting Information *

ABSTRACT: Overbased calcium sulfonate is one of the largest commercially produced nanomaterials; the phase transformation mechanism involved in the key process of carbonation reaction for overbased detergent preparation has not yet been fully understood. Following our previous investigation based on the heavy alkylbenzene sulfonate (HABS) surfactant of industrial byproduct, two commercial products with well-defined chemical compositions and structures, long-chain alkylbenzene sulfonate (LCABS) and comparative short-chain linear alkylbenzene sulfonate (SLABS), are employed in this study as model surfactants for further mechanism study of this pivotal process by the combination of analytical techniques such as potentiometric titration, DLS, TEM, FTIR, and XRD. It has been demonstrated that amorphous calcium carbonate (ACC) is a prerequisite for the preserving and stabilization of the alkaline reserve, especially under thermal work environment. The phase transformation from ACC to crystalline vaterite polymorph rather than calcite has been unambiguously confirmed as a universal mechanism for all the alkylbenzene sulfonate based systems. Furthermore, the length and number of alkyl tails of alkylbenzene sulfonate surfactants exhibit a strong influence on characteristics of detergent products. The LCABS with long-chain alkyl substituents or HABS with dialkyl substituents plays an important role not only in inhibiting the agglomeration process, but also in protecting the metastable inorganic cores from fusion to avoid phase transformation. Such understanding should be of crucial importance for guiding the preparation of overbased detergents and greases. In addition, the study on the influence of the molar ratio of calcium oxide in the total alkaline calcium salts and the dosage of surfactant LCABS, promoter methanol, and catalyst anhydrous calcium chloride help to determine the suitable work window for detergent production, and moreover, the water content provides a handle for better understanding the reaction process and achieving good quality control for detergent manufacturing. structure of overbased detergents, first proposed by Markovic and Ottewill27,28 and presently widely accepted, is a core−shell type consisting of an alkaline inorganic core of mainly metal carbonate surrounded by a concentric shell of surfactant monolayer. In general, the radius of the inorganic core was measured between 1 and 10 nm, and the thickness of the surrounding surfactant was around 1−4 nm.1,2,27−29 The reverse micellar core was mainly made of amorphous calcium carbonate (ACC), based on X-ray absorption fine structure spectroscopy (EXAFS) evidence.29,30 The existence of calcium hydroxide in the reverse micellar core was further confirmed by X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectroscopy (ToF-SIMS) measurements,31 and also by the quantitative examination of thermal gravimetric analysis (TGA).19 Generally, calcium salt overbased detergents are synthesized in a one-pot process by carbonation of lime in a mixture of surfactant, promoter, and nonpolar solvent. The carbonation reaction (also called overbasing reaction) is the key procedure for the overbased detergent preparation, which takes place within the reverse micellar core with CaCO3 particle nucleating

1. INTRODUCTION Overbased detergents are among the most important additives in lubricant formulations, which typically consist of variant inorganic bases, surfactants, and diluent oil with the oil-soluble colloidal nanoparticles of metal carbonate (and metal hydroxide) as an alkaline reserve to effectively neutralize both inorganic and organic acids produced during combustion, thus to avoid sludge accumulation and prevent internal combustion engine corrosion.1,2 A range of surfactants have been utilized to synthesize overbased detergents for variant applications, typically sulfonates, phenates, salicylates, phosphonates, and others.1−3 Calcium alkylbenzene sulfonates, well-known for their high thermal stabilities, good detergency, rust inhibition, and antiwear properties,4,5 are among the most widely used surfactants for commercial overbased detergent preparation as well as various industrial oil applications. Besides, some novel surfactant systems addressing environmental concerns have been developed recently utilizing calixarene/stearate surfactants to prepare sulfur-free overbased detergents6−8 or employing oleate9−13 or polyol poly-12-hydroxystearic acid surfactants14 to synthesize biodegradable overbased detergents. A large number of modern analytical techniques15−19 and theoretical calculation and modeling20−26 have been adopted to explore their compositions and especially the detailed structures of the overbased detergents. It is extensively supported by both theoretical simulation and experimental evidence that the © 2013 American Chemical Society

Received: Revised: Accepted: Published: 12748

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dosage on phase transformation? (4) How can crystallization of the inorganic core in the reverse micelles be avoided and high quality detergents be attained? The universal mechanism of phase transformation involved in the carbonation reaction is of great importance for achieving an in-depth understanding and process control for overbased detergent preparation. In addition, the promoter methanol, alkaline calcium salt ratio, and calcium chloride involved in carbonation reaction are optimized in view of phase transformation and for product quality control mainly within the ACC region which is essential for commercial detergent preparation.

and growing to form an alkaline inorganic nanoparticle of several to tens of nanometers in size,1,21,29 setting a practical example for surfactant-templated nanomaterial synthesis32 similar to reaction in reverse water-in-oil (w/o) microemulsion nanoreactors.33 The mechanism of inorganic core phase transformation during the carbonation reaction is quite complicated and not yet fully understood, though it is of crucial importance for achieving an enhanced understanding and process control for detergent production, especially for the preparation of high quality overbased detergents originating from industrially valuable alkylbenzene sulfonate. ACC is generally considered to be a prerequisite for the preserving of the alkaline reserve in the preparation of calcium system overbased detergents; otherwise it is believed to be destabilized and resulted in calcite precipitation.2,21,29−31 Our previous work19 mainly concentrated on the preparation of overbased nanodetergent from calcium salts of heavy alkylbenzene sulfonate (HABS); the composition of the nanoparticle amorphous core, especially the polymorphism of further formed calcium carbonate crystals, and their phase transformation mechanism have been investigated by employing various analytical techniques. The main conclusions on the carbonation process and phase transformation of inorganic core in this system are briefly summarized as follows: (1) The starting solution contained preexisting reverse micelles of HABS surfactant. (2) With the addition of lime and continuously supplying with CO2, the carbonation reaction took place within the polar reverse micellar core containing dissolved lime, where CaCO3 nucleated under supersaturation as gaseous CO2 penetrating by diffusion, and colloidal particles grew progressively through intermicellar Brownian collisions and coalescence and produced well-defined amorphous detergents, the inorganic core of which was composed of a mixture of CaCO3 and a minor amount of Ca(OH)2. (3) Upon approaching carbonation completion and depletion of Ca(OH)2, a phase transformation from ACC to vaterite polymorph rather than calcite occurred with a sharp increase in particle size, which was in contrast to the existing cognition. (4) An overbased detergent in vaterite crystalline polymorph of low viscosity and high base number was prepared for the first time. Although HABS were evaluated as excellent starting materials for preparing overbased detergents,19,34−36 the complexity of their chemical compositions19,34,37,38 originated as byproducts during the preparation of linear alkylbenzene might cause concern on whether some conclusions and viewpoints based on HABS systems are universal for all systems using alkylbenzene sulfonate (ABS) as a starting material for preparing overbased detergent, since some complicated unknown ingredients in trace amount might be involved and their impact on the performance could not be absolutely excluded. Consequently, long-chain alkylbenzene sulfonate (LCABS) and comparative short-chain linear alkylbenzene sulfonate (SLABS), two commercial materials with well-defined chemical structures, have been selected as model surfactants to further answer some imperative questions, which have not been elucidated before: (1) Does the phase transformation from ACC to vaterite polymorph rather than calcite occur in all alkylbenzene sulfonate overbased detergent systems? (2) What is the influence of surfactant chemical structure (such as the substituted alkyl length and number of alkylbenzene sulfonate) on detergent properties? (3) What is the effect of reagent

2. EXPERIMENTAL SECTION 2.1. Materials. The long-chain alkylbenzene sulfonic acid (LCABSA, >90%) was provided by Jintung Petrochemical Corp. Ltd. The molecular structure of LCABSA is R−Ph− SO3H; R = C18H37−C26H53. The composition provided by the supplier was determined mainly from mass spectroscopy analysis in weight percentage: C18, 1.8%; C20, 69.6%; C22, 22.0%; C24, 4.9%; >C24, 1.7%. ASTM (American Society of Testing and Materials) test methods were applied for its composition analysis: sulfonic acid, 1.988 mmol/g (thus LCABSA with an average molecular weight of Mn ≈ 500); inorganic sulfuric acid, 0.76% (w/w) (ASTM D664); water content, 1.47% (w/w) (ASTM D1533). The short-chain linear alkylbenzene sulfonic acid (SLABSA, 97%) was purchased from Alfa Aesar (CAS No. 68584-22-5). The molecular structure of SLABSA is R′−Ph−SO3H; R′ = C10H21−C13H27. The composition in weight percentage was C10, 1%; C11, 40%; C12 28%, and >C12 31%. Sulfonic acid was 3.037 mmol/g (therefore SLABSA with an average molecular weight of Mn ≈ 330) and inorganic sulfuric acid was 1.22% (w/w) (ASTM D664); water content was 1.36% (w/w) (ASTM D1533). Diluent oil (150 SN) was of technical grade and provided by PetroChina Dalian Petrochemical Corp. Ltd. Carbon dioxide (99.8%) was received in cylinders from Qixia Industrial Gases Co. Other materials were of analytical grade and purchased commercially, such as methanol (99.5%), heptane (95%), anhydrous calcium chloride (96%), calcium hydroxide (95%), and calcium oxide (98%). The solid calcium salts were ground into powder of micrometer size before use. 2.2. Typical Syntheses. A 2000 mL round-bottomed flask equipped with an oil-bath jacket, a thermocouple, an overhead mechanical stirrer, a condenser set, and an inlet for gaseous CO2 injection was set up. The outlet of the condenser was sealed with a U-type sealing pipe filled with silicone oil to airproof the system and monitor the CO2 absorbability. As a typical preparation, into this flask were added LCABSA or SLABSA surfactant (150 g), heptane (1.1 L), 150 SN diluent oil (140 g), methanol (35 mL), and calcium oxide (13 g for SLABSA or 9 g for LCABSA to neutralize sulfonic acid to produce calcium sulfonate surfactants). The mixture was heated to and held at 45 °C for 45 min so as to transform the sulfonic acids into their calcium salts. Then, methanol (70 mL), anhydrous calcium chloride (3.5 g), and alkaline calcium salt (0.87 mol, a mixture of CaO and Ca(OH)2 in a predetermined ratio) were added, and CO2 was introduced at a rate of 280 cm3/min for a time period ranging from 10 to 100 min. Finally, the residual solids were removed by centrifugation and filtration, and the volatile accessory ingredients and solvent such as heptane, methanol, and water were evaporated to obtain the overbased detergents, which appeared as a pale 12749

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Figure 1. Instantaneous temperature of the reaction system and the product TBN versus carbonation time with a constant gaseous CO2 sparging rate of 280 cm3/min.

core, where it reacts with the preexisting lime (Ca(OH)2 and/ or CaO) to generate CaCO3, which then nucleates under supersaturation and consumes the lime gradually.19,21,29 It is a critical reaction procedure for the preparation of desired overbased detergents; however, many aspects involved in this process remain unclear. Our previous work19 utilized a number of analytical techniques to explore the phase transformation mechanism during the preparation of overbased detergents based on calcium heavy alkylbenzene sulfonate (CaHABS). However, the influence of the surfactant chemical structure (such as the length of the hydrophobic alkyl chain) on detergent properties and the effect of reagent dosage on phase transformation were not covered, which are crucial for the quality control of detergent production. Meanwhile, the complexity of the HABS chemical composition may disturb involved mechanism understanding. Consequently, LCABS and SLABS with clear compositions and structures were selected as model surfactants for further study of this key process of industrial interest. 3.1.1. Real-Time Temperature Changes of Carbonation Reaction System and Total Base Number (TBN). Since carbonation is an exothermic reaction,19 the real-time system temperature change and the CO2 gas absorbability were conveniently measured to monitor the reaction process. For the LCABS system, as shown in Figure 1a, a notable temperature increase was observed within the initial 30 min with the ventilation of gaseous CO2, and then the temperature remained almost constant around 53 °C for a while before a remarkable temperature drop occurred with reaction around 57.5 min with CO2 gas escaping from the sealing pipe indicating the incomplete absorption of CO2 thereafter. Finally, the CO2 ebullition rate reached 280 cm3/min, the same as the input rate around 70 min, revealing the complete nonabsorption and the carbonation reaction coming to an end. The evolution of the total base number (TBN) during the carbonation increased gradually and leveled off around 245 mg KOH/g after reaction for 70 min, which agreed well with the reaction progress revealed by the real-time temperature changes (Figure 1a). The change trend of the instantaneous temperature and TBN versus carbonation time of the comparative SLABS system was generally quite similar to that of the LCABS system with a temperature drop occurring around 58.0 min, and the CO2 ebullition rate reached 280 cm3/min around 70

yellow or brown clear oily liquid in SLABS and LCABS systems, respectively. 2.3. Characterization. Dynamic light scattering (DLS) was performed using a Brookhaven BI-200SM laser light scattering spectrometer equipped with a digital detector (BI-PAD) and a semiconductor laser light source operating at 532 nm. The viscosity of detergents was measured at 25 °C using a Brookfield DV-II+Pro viscometer equipped with an LV4 spindle (60 rpm). Fourier transform infrared (FTIR) spectra were recorded on a NICOLET TNEXUS 870 infrared spectrometer by applying a thin detergent additive film on KBr crystal disk in the wavenumber range 400−4000 cm−1. Xray diffraction (XRD) analysis was conducted on a Bruker D8 Advance X-ray diffractometer with detergent liquid films applied directly in the sample groove using Cu Kα radiation (40 kV and 40 mA) at room temperature. After depositing the diluted detergent solution on a cupreous grid coated with carbon film, the dried samples were directly observed using a JEOL JEM-2100 transmission electron microscope (TEM) under 200 kV. 2.4. Measurement of Total Base Number. The total base number (TBN), which represents a measure of the potential of the detergent to neutralize acid and is one of the most important parameters for specifying an overbased detergent, is defined as the amount of potassium hydroxide that would be equivalent to 1 g of the material and expressed in the unit of milligrams of KOH per gram. TBN was determined by a standard potentiometric titration method according to method ASTM D2896. 2.5. Thermal Stability Assessment. Samples were diluted to 10% (w/w) with 150 SN base oil. The diluted solutions were stripped off residual volatiles with a rotary evaporator under reduced pressure and then put in an oven at fixed temperature of 100 °C to evaluate their thermal stabilities. Samples were taken out for DLS and FTIR analyses in a predesigned time interval ranging from 1 to 20 days.

3. RESULTS AND DISCUSSION 3.1. Carbonation Process of Calcium System Overbased Detergents Based on LCABS and Comparative SLABS. Carbonation, also called overbasing reaction, can be briefly described as that the sparged CO2 dissolves in the organic phase and diffuses into the swollen reverse micellar 12750

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Figure 2. (a) FTIR spectra, (b) XRD profiles marked with “v” for vaterite peaks and the contrast XRD patterns of calcite and standard vaterite CaCO3, and (c) the viscosity measured at 25 °C of CaLCABS overbased detergents of various carbonation times.

form within reaction for 70 min as shown in Figure 2a (see also Figure S1 in the Supporting Information), the phase transformation ended in a complete crystalline vaterite state for more than 80 min with the disappearance of the 862 cm−1 band, a sharp and strong ν2 peak at 877 cm−1, and a notable ν4 absorption at 745 cm−1 (marked by an downward-pointing arrow). After that scarcely any changes were observed in the FTIR spectra on lengthening the carbonation duration even to 100 min. It is worthwhile to note that the persistently existing peaks at 831 and 722 cm−1, which were respectively assigned to C−H plane bending vibration of para-substituted benzene and rocking vibration of −(CH2)n− (with n ≥ 4) of the alkyl chain,37,45 and some other bands with almost unchanged intensities were attributed to the alkylbenzene sulfonate surfactants as observed in the initial system (0 min). XRD analyses further confirmed the calcium carbonate polymorphs and phase transformation. Figure 2b presents some typical XRD patterns of detergents for different carbonation times in the LCABS system together with the calcite and standard vaterite patterns for contrast (behavior for SLABS similar to that shown in Figure S2 in the Supporting Information). No crystalline peaks emerged in the XRD profiles of those products stopped before 50 min revealing the ACC character, while one could clearly observe a series of gradually enhanced diffraction peaks of 2θ at 20.9, 24.9, 27.1, 32.8, 43.9, 49.9, and 55.8° corresponding to Miller indices (004), (110), (112), (114), (300), (118), and (224), typical of vaterite (JCPDS No.33-0286 in space group P63/mmc)42−44 in those XRD profiles of detergents for more than 55 min

min, except with a noticeably slow temperature increase within the initial 40 min and thus a narrowed constant temperature region and slightly decreased platform TBN value around 230 mg KOH/g after reaction for more than 70 min (Figure 1b). 3.1.2. Fourier Transform Infrared (FTIR) Spectroscopy, Xray Diffraction (XRD), and Viscosity. FTIR spectroscopy and XRD measurements were convenient and useful to monitor the phase transformation during carbonation. Some characteristic carbonate adsorption bands can be utilized to monitor the phase transformation of calcium carbonate and determine their polymorphs, such as the broad out-of-plane bending absorption of carbonate ions at 865 cm−1 (ν2) in ACC changes into peaks around 875 cm−1 upon crystalline phase formation,39−44 and the in-plane bending absorption (ν4) at 745 cm−1 for vaterite and 712 cm−1 for calcite has been employed to conveniently distinguish these different crystalline polymorphs of calcium carbonate even for their quantitative assessment.42−44 The FTIR spectra of obtained detergents with different carbonation reaction times ranging from 0 to 100 min are representatively shown in Figure 2a (see also Figure S1 in the Supporting Information). In both LCABS and SLABS systems, a broad peak around 862 cm−1 featuring the typical ν2 absorption of ACC appeared for carbonation time of 10 min as the carbonation reaction proceeded. Then this peak increased gradually with increasing carbonation time, and an obvious shoulder at 877 cm−1 emerged (marked by an upwardpointing arrow) after 55 min of reaction, which indicated the partial transformation into crystalline form. On further carbonation, after a mixture state of ACC and partial crystalline 12751

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Figure 3. Visual appearance comparison of products (a) CaLCABS based and (b) CaSLABS based systems after carbonation reaction with time ranging from 0 to 100 min.

Figure 4. Number average diameter dn and PDI of (a) CaLCABS overbased detergents and (b) comparative CaSLABS based products versus carbonation duration measured by DLS.

vaterite and reached the minimum viscosity 899 mPa·s at 70 min upon almost carbonation completion with ACC transforming into crystalline vaterite form. Finally, a gel-like product of high viscosity was obtained upon complete transformation into vaterite polymorph. The gelation behavior upon carbonation completion or postcarbonation was in sharp contrast to the HABS system where a vaterite detergent product with a uniform particle size around 35 nm and low viscosity was attained, which was presumably ascribed to the more powerful stabilizing effect of the HABS surfactant with two alkyl chains compared with the single alkyl chain LCABS or SLABS systems, which resulted in aggregation among particles leading to gel-like products due to their insufficient protecting capability for crystalline vaterite inorganic cores upon postcarbonation. The comparison of the visual appearance of the obtained products with increasing carbonation duration as shown in Figure 3 gave a straightforward understanding of the inorganic core phase transformation and product attributes. By comprehensively considering the product characteristics and inorganic core phase state, an about 25 min (from reaction 30 to 55 min) suitable “work window” for clear

carbonation as shown in Figure 2b for 60, 70 and 100 min, respectively. By contrast, no any signals of calcite polymorph were observed. Therefore, a phase transformation from ACC into crystalline vaterite was unambiguously demonstrated again by the combination of FTIR and XRD evidence with a transition region in the carbonation duration ranging between 55 and 80 min in both LCABS and SLABS systems, which could completely dispel the concern on the possible impact of trace unknown ingredients in the HABS system19 and confirm the universal character of the phase transformation into vaterite crystalline form upon completion of carbonation reaction generally for the alkylbenzene sulfonate overbased detergent systems. Moreover, as shown in Figure 2c, the viscosity changes of the system versus carbonation time also clearly supported the occurrence of phase transformation. For the LCABS system, after a viscosity decrease during the initial 20 min of the heterogeneous region with suspending lime solids, as the carbonation reaction proceeded, the viscosity increased gradually to the maximum 2214 mPa·s at reaction for 55 min, and then decreased in the mixture region of ACC and 12752

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Figure 5. Typical TEM image of CaLCABS overbased detergents with carbonation duration of 60 min and the histogram of particle sizes.

size by statistical measurements of TEM images was 13.5 ± 1.9 nm, about 3 nm smaller than the diameter measured by DLS, which was in reasonable agreement, since the results obtained from TEM images mainly reflecting the inorganic core size were usually on average 1−4 nm smaller than those hydrodynamic diameters measured by DLS.1,2,27−29 For the SLABS system, dn increased gradually from 1.9 to 8.9 nm during the first 55 min accompanied by decreasing PDI from around 0.55 to 0.25 (Figure 4b). Then, it was followed by a rapid increase in both dn and PDI during carbonation reaction for longer than 55−70 min in the phase transition from ACC to vaterite region. Last, upon completion of carbonation reaction for more than 70 min, a notable fusion of the vaterite crystalline cores and agglomeration process occurred, which resulted in the gelation and sharply increased particle size of over 100 nm in the postcarbonation region of 80−100 min, and wider double distribution bands were observed in the DLS measurements. By comparison of the CaSLABS and CaLCABS systems, it could be deduced that the length of the alkyl chain had less effect on the reaction process. The performance for the SLABS system in real-time temperature change, the phase transformation process, and TBN evolution was largely similar to that shown by the LCABS system. However, it exhibited a notable influence on the product characteristics, particularly the size and its stability of reverse micellar inorganic core nanoparticles. The CaLCABS overbased detergents in both the ACC state and vaterite polymorph possessed narrower size distributions compared with the comparative CaSLABS products with the same carbonation time. In addition, although both systems suffered from gelation due to remarkable agglomeration processes upon phase transformation approaching an end, pronounced fusion of the crystalline nanoparticles was only observed in the CaSLABS system (Figure 4b). Moreover, remarkable differences were manifested for comparative CaSLABS based samples as compared with CaLCABS overbased detergents in product thermal stabilities and inorganic core phase transformation as presented in section 3.2, which was not quite unexpected for short-chain alkylbenzene sulfonates such as sodium dodecylbenzene sulfonate, and mixtures of related sulfonates usually serve as major components of laundry detergents with water detergency.47,48 Herein the adoption of well-defined shortchain SLABS as a comparative surfactant with notable differentiation facilitated the reaction process investigation and in-depth understanding of the mechanism, especially the inorganic core phase transformation implicated in the

and stable ACC product preparation was observed for LCABS or SLABS based systems. 3.1.3. Dynamic Light Scattering (DLS) and Transmission Electron Microscopy (TEM). The evolution of particle size during carbonation reaction is of crucial importance for the detergent preparation and also for understanding the reaction mechanism, which can reveal chemical composition change of the reverse micellar core19 or reflect on the effect of surfactants. Samples were diluted to 0.5% (w/w) in heptane before dynamic light scattering (DLS) measurements, and the particle size was expressed in terms of number average diameter dn. The polydispersity index (PDI) reflecting the size distribution of particles was also determined, which was another important parameter for evaluating a colloidal dispersion solution.46 For the LCABS system, dn increased quite quickly from about 3 nm at carbonation 10 min to 18.1 nm at carbonation 55 min (Figure 4a). Then, dn maintained constant values around 18 nm only with small undulations between the carbonation durations of 55 and 70 min, in good agreement with the reaction slowing down and approaching the end around reaction for 70 min. The diameter increased slightly to about 20 nm upon further postcarbonation reaction to 100 min, which manifested that the gelation was a result of conglomeration of distinct nanoparticles. As shown in Figure 4a, the PDI decreased sharply from about 0.6 to 0.2 during the first 20 min of the carbonation accelerating region, which was in good agreement with the reaction progress undergoing a transition from the initial heterogeneity with the presence of larger residual alkaline calcium salt particles to the formation of uniform ACC detergents. After maintaining a constant low value around 0.17 in the ACC product region between 20 and 55 min, the PDI increased rapidly from 0.17 to 0.52 with the carbonation time extending from 55 to 80 min; this sharp increase in PDI coincided with the phase transition of the inorganic core from ACC into vaterite polymorph. Furthermore, as shown in Figure 5, the particle morphology of CaLCABS overbased detergents with carbonation duration 60 min was investigated by transmission electron microscopy (TEM), showing mainly uniform spherical and some irregular structures for the partially crystallization transformed sample. It was worthwhile to note that the pure ACC nanoparticles of carbonation reaction for less than 55 min in this system failed to be imaged by TEM with many attempts of varied operational conditions such as staining with uranyl acetate or lowering the voltage. Thus partially or completely transformed detergents in vaterite crystalline form were adopted for TEM investigations here and in the following other samples. The average particle 12753

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Figure 6. FTIR spectra after different periods of thermal testing of CaLCABS overbased detergents with carbonation times of (a) 35, (b) 60, and (c) 70 min and CaSLABS based samples with carbonation times of (d) 35, (e) 60, and (f) 70 min, all with 10% (w/w) dispersion in 150 SN base oil.

Table 1. Number Average Diameter dn (nm) of Overbased Product Nanoparticles with Different Carbonation Times versus Thermal Trial Periods thermal trial period surfactant species

carbonation time (min)

0 day

1 day

2 days

4 days

8 days

12 days

16 days

20 days

LCABS

35 60 70 35 60 70

6.3 15.4 17.2 6.5 14.8 16.9

7.0 16.2 52.9 123 315 742

6.7 20.6 177 468 a a

7.2 241 a a a a

10.1 a a a a a

9.5 a a a a a

10.8 a a a a a

9.5 a a a a a

SLABS

a Forming turbidity or precipitation, the measured particle size was over 1 μm and varied, due to the agglomeration and instability originating from heterogeneity.

carbonation reaction of a complex heterogeneous process with multicomponents involved. 3.2. Thermal Stability Assessment of CaLCABS and CaSLABS Based Overbased Products. Thermal stability is one of the most important properties of a lubrication detergent, since internal combustion engines run at high temperatures. Thus, a thermal stability evaluation experiment was conducted for both CaLCABS and CaSLABS based overbased products to undergo thermal testing at 100 °C for periods ranging from 1 to 20 days. Samples of different reaction stages with variant carbonation times were chosen for this assessment.

1. ACC detergent of carbonation for 35 min: The carbonation time was less than the onset phase transformation time (55 min in this case), and the micellar inorganic cores of samples were completely composed of amorphous calcium carbonate (ACC). 2. ACC with a small quantity of vaterite detergent of carbonation for 60 min: Beyond the onset phase transformation time of 55 min, the inorganic cores of products were composed of mainly ACC and also a small quantity of crystalline vaterite polymorph. 3. Vaterite with a small quantity of ACC detergent of carbonation for 70 min: The phase transformation process 12754

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Figure 7. Change trends of dn, PDI measured by DLS, and TBN and viscosity of LCABS based detergent carbonation for 35 min versus variant volume of methanol (a, b), the molar ratio R of CaO in the total alkaline calcium salt (c, d), and the amount of anhydrous calcium chloride (e, f).

SN base oil. The number average diameter (dn) changes after thermal testing measured by DLS are summarized in Table 1, and the visual appearance comparison after thermal trials of different periods is provided in the Supporting Information (Figure S3). For CaLCABS overbased detergents, ACC detergent of carbonation for 35 min showed excellent thermal stability; the nanoparticle size dn’s were quite stable around 6−10 nm (Table 1) and no any turbidity occurred within 20 days of thermal trial (Supporting Information, Figure S3). However, those detergents including nanoparticles partly in vaterite crystalline form of different contents such as sample carbonation for 60 and 70 min suffered from generating turbidity or precipitation upon

almost approached the end (exact completion would cause gelation such as carbonation for 80 and 100 min), and the inorganic cores consisted of primarily vaterite CaCO3 and a small amount of ACC. Figure 6 shows the FTIR spectral comparison of CaLCABS and CaSLABS samples in 10% (w/w) dispersion of 150 SN base oil undergoing different periods of thermal testing. It is obvious that compared with Figure 2a and Figure S1 (Supporting Information), a much stronger peak at 722 cm−1 of rocking vibration of −(CH2)n− (with n ≥ 4) and relatively diminished bands related to alkylbenzene sulfonates were observed for all samples, which was quite reasonable for these 10% solution samples dispersed in hydrocarbon solvent of 150 12755

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stability assessment demonstrated that the LCABS based system exhibited a much better thermal stability and the ACC phase of the inorganic micellar core was a prerequisite for the preparation of high quality products. Consequently, a series of comparative experiments with variant promoter methanol dosage, alkaline calcium salt composition, and the catalyst calcium chloride dosage were performed, adopting LCABS as the surfactant and conducting carbonation reaction for a fixed 35 min with CO2 introduced at a rate of 280 cm3/min. 3.3.1. Promoter Methanol. The promoter is a crucial reaction element which acts as solubilizing agent for the lime metal compounds and promotes their conversion into calcium carbonate colloidal particles.1,2,9,12,13,35 Methanol is the most widely used one among different promoters adopted for carbonation reaction. It is believed that methanol locating at the interfacial zone of the surfactant layer acts as a cosurfactant and can markedly lower the rigidity of micelles.29,49 The influences of methanol dosage on amorphous detergent product quality (TBN, viscosity, particle size, and PDI) with the carbonation reaction for 35 min are shown in Figure 7a,b. First the TBN of samples rose rapidly from almost vacancy to 170 mg KOH/g as the promoter methanol increased from 0 to 42 mL. Afterward, TBN leveled off around 180 mg KOH/g with further increase in methanol volume. On the other hand, particle size measured by DLS as shown in Figure 7a helped one to better understand the reaction process which might be divided into two regions as follows. In the region of methanol volume less than 42 mL, poor detergents obtained with ACC nanoparticles smaller than 6 nm in low yield, together with the high PDI and low TBN as shown in Figure 7a,b, it was obvious that a poor lime utilization and low conversion were achieved. Since methanol as a promoter could noticeably reduce micellar rigidity29,49 to accelerate reactant mass transport and increase the reaction rate, thus a deficiency in methanol resulted in the poor carbonation efficiency. In the region ranging from 42 to 180 mL, the volume of methanol was suitable with the TBN remaining steady (Figure 7b). The DLS measurement showed that the particle diameter gradually increased from about 7 to 12 nm with the increase in methanol volume. That might be presumably attributed to the reduction of micellar rigidity with the increase in methanol amount,29,49 which consequently promoted the mass transport, coalescence, and growth during collision to generate larger ACC particles similar to the behavior of the HABS-based detergent system in forming vaterite particles.19 Furthermore, as observed from the HABS-based detergent system, excess in methanol volume resulted in micellar instability and agglomeration;19 here upon exceeding 105 mL of methanol, the noted increase in particle size, PDI, and also viscosity, and slight decrease in TBN, indicated a little agglomeration. 3.3.2. Alkaline Calcium Salt. Alkaline calcium salts including calcium oxide and/or calcium hydroxide were adopted to produce the colloidal particles of calcium carbonate for overbased detergent preparation.2 Sometimes Ca(OH)2 was exclusively utilized to simplify the reaction;7,29,33 however, it suffered from the disadvantage that excessive solid wastes were generated in the crude reaction products. Thus industrial practice is to employ a mixture of Ca(OH)2 and CaO as the alkaline calcium salt reagents to improve lime utilization and reduce the viscosity of the products.9,50−52 Holding the total molar amount of alkaline calcium salt at a fixed 0.87 mol and carbonation for 35 min, a series of comparative experiments,

cooling to ambient temperature after different periods of thermal trials (Supporting Information, Figure S3). The particle size changes measured by DLS for the detergents of carbonation for 60 and 70 min (Table 1) could lead to good understanding and interpretation of the visual appearance changes of turbidity or precipitation at 8 days and 1 day, respectively (Supporting Information, Figure S3 ). For comparative CaSLABS based products, all the tested samples suffered from a serious precipitation generation after short periods of thermal trials (Supporting Information, Figure S3). It was also revealed by DLS measurements that a sharp increase in dn occurred after only thermal trial for 1 day (Table 1). These experimental results indicated that, although unnoticed differences in the preparation process were displayed for the comparative CaSLABS based products compared with the CaLCABS based detergents, they exhibited a much poorer thermal stability even for the CaSLABS sample in pure ACC state of carbonation for 35 min. Furthermore, the polymorph changes of the inorganic cores (mainly consisting of CaCO3) during thermal assessment were monitored by both FTIR (Figure 6) and XRD measurements (data not shown). First, all of the ACC overbased samples prepared from both LCABS and SLABS surfactants were still in amorphous state; no signal of crystalline CaCO3 emerged even after the longest 20 days thermal trial employed here (Figure 6a,d). Second, the vaterite polymorph contents of CaLCABS overbased detergents with carbonation durations of 60 and 70 min remained almost unchanged in the thermal stability assessment (Figure 6b,c). Third, as shown in Figure 6e,f, the FTIR spectra of CaSLABS based samples with carbonation durations of 60 and 70 min revealed an obvious phase transformation. With elongation of the thermal test time, the remarkable increase in the ν2 peak at 877 cm−1 of crystalline CaCO3 together with the decrease in 862 cm−1 band of ACC, even the absence of the 862 cm−1 band after 20 days of thermal trial for the carbonation 70 min sample (Figure 6f), and also the gradually enhanced shoulder ν4 absorption at 745 cm−1 demonstrated clearly the transformation from ACC to vaterite polymorph upon thermal trial. It is worthwhile to note that, under thermal trial, the deterioration of the overbased products of different carbonation degrees especially based on CaSLABS surfactants usually went through three stages of turbidity− flocculation−solid precipitation (Supporting Information, Figure S3), and precipitation even occurred in the amorphous state for the ACC sample in the CaSLABS system. Such behavior together with the particle size changes measured by DLS (Table 1) manifested the agglomeration with inorganic core fusion and the quite weak protecting ability of the comparative short-chain surfactant CaSLABS. In conclusion, the thermal stabilities of CaLCABS overbased detergents were much better than those of CaSLABS products. The long alkyl chain of alkylbenzene sulfonate surfactants played an important role not only in inhibiting the agglomeration occurrence, but also in protecting the metastable ACC micellar cores against phase transformation into crystalline state CaCO3 under a thermal environment. 3.3. Influence of Important Ingredient Dosage on Characteristics of ACC Detergents. Understanding of the influence of important ingredient dosages on characteristics of ACC products remains limited although it is of fundamental importance for high quality detergent preparation. As presented above, the comparison investigations for SLABS and LCABS based systems on both the carbonation process and thermal 12756

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Table 2. Reagent Dosage Adopted for the Series of Comparative Experiments and the Nongel Product Range and the Onset Time of Phase Transformation Determined with a Fixed Amount of 0.87 mol of Total Alkaline Calcium Salt and Pumped CO2 Gas at a Rate of 280 cm3/min group no. 1

LCABSA (g)

methanol (mL)

97 150 200

105

150

R (CaO ratio)

CaCl2 (g)

H2O added (g)

1

3.5

0

56 105 175

1

3.5

0

150

105

0 0.5 1

3.5

0

150

105

1

0 1.4 3.5

0

150

105

1

3.5

0 4 16

2

3

4

5

onset time of phase transformation (min) nongel product range (min) 65 55 55 55 55 65 55 55 55 b 55 55 55 45 35

10−30 20−70 30−80 45−100 20−70 20−100a 30−55 20−60 20−70 b 10−100 20−70 20−70 10−55 20−30

a

A high viscosity gel-like product obtained between carbonation for 40 and 65 min. bThe micellar core is still in ACC phase after carbonation reaction for 100 min but with very low conversion.

3.4. Influence of Reagent Dosage on Effective Work Window for Detergent Preparation. For the preparation of calcium system overbased detergents, amorphous calcium carbonate (ACC) is generally considered to be a prerequisite for preserving the alkaline reserve as also strongly supported by the above thermal trial results. Therefore how to keep the ACC character and avoid phase transformation into crystalline forms of inorganic micellar cores is of crucial importance for detergent preparation. Furthermore, the detergents themselves after removal of volatile solvents should be in a liquid state and avoid gelation. As manifested in the above sections, keeping the carbonation duration before the onset of phase transformation can effectively prevent crystalline polymorph formation. Thus the onset of phase transformation is certainly an important index for the production of detergents. Then the questions follow: Is there any effect of variant reagent dosage on the onset time of phase transformation, and what is the influence? By comprehensively considering the onset of phase transformation and the nongel product range, effective work windows for the detergent preparation can be determined. Hence a series of carbonation experiments have been designed to further probe into the phase transformation process and could help to ascertain suitable work windows for detergent production. Comparative carbonation reaction experiments of variant reagent dosage were performed with a fixed amount of 0.87 mol of total alkaline calcium salt (besides the stoichiometric quantity of CaO added previously for neutralizing the LCABSA in the neutralization reaction) and pumped CO2 gas at a rate of 280 cm3/min. The visual appearance changes of investigated systems and FTIR, XRD, TEM, and viscosity analyses were utilized to evaluate the liquid (nongel) product range, explore the phase transformation process, and determine the onset time of phase transformation. The comparative reagent dosage adopted for different experimental groups and the resulting onset time and nongel product range are summarized in Table 2. 3.4.1. Amount of Surfactant LCABSA. The varying amounts of surfactant LCABSA of 97, 150, and 200 g were first investigated with 150 mL of unaltered promoter methanol, 0.87

varying the molar ratio of CaO to the sum of CaO and Ca(OH)2 (R = [CaO]/[CaO + Ca(OH)2]), were conducted in this study. It is well-known that CaO consumes one more water molecules than Ca(OH)2 does, so increased CaO content could decrease the size of reverse micelles through depleting the extra water generated during carbonation, which might reduce the particle size. As shown in Figure 7c, for CaLCABS overbased detergents in the ACC state, with the increase in R there was a generally slightly decreased trend of the particle sizes from 10 nm to around 8 nm measured by DLS, which was in contrast to the remarkable diameter decrease from about 50 to 20 nm for the vaterite detergent based on the CaHABS system.19 Other parameters such as the PDI (Figure 7c), TBN, and viscosity (Figure 7d) exhibited indistinctive changes with small undulations. These results demonstrated that the CaO ratio R had a weak influence on characteristics of ACC detergents; in other words, there was a broad tolerance for the alkaline calcium salt raw material choice for ACC detergent preparation, which was a favorable feature for detergent manufacture. 3.3.3. Catalyst Anhydrous Calcium Chloride. Anhydrous calcium chloride (CaCl2) is also an important additive called a catalyst for carbonation reaction;9,52 it can restrain the initial content of water through formation of pentahydrate. Comparative experiments were performed to investigate the effect of variant amount of CaCl2 on ACC detergent preparation. As shown in Figure 7e,f, a fairly sharp rise in the particle size and TBN, together with a rapid decrease in PDI and increase in viscosity, was observed, when the added CaCl2 was less than 1.4 g with other reactants and conditions as described for the typical synthesis procedure. It was implied that the shortage of CaCl2 would result in poor lime utilization and low conversion. All parameters investigated including particle size, PDI, TBN, and viscosity remained steady and desired detergents were achieved with the catalyst dosage ranging from 1.4 to 5.0 g. Excessive catalyst dosage would result in a remarkable rise in viscosity as a 3-fold increase from around 1500 to 4500 mPa·s was observed as CaCl2 further increased from 5.0 to 8.4 g (Figure 7f). 12757

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Figure 8. Typical TEM images of CaLCABS overbased detergents at carbonation for 100 min with different CaO ratios of (a) R = 0, elongated kernel-shaped morphology with average particle size 97.5 × 46.0 nm; (b) R = 0.5, particle average size 24.7 nm; and (c) R = 1, particle average size 15.8 nm.

Information, Figure S7). On the other hand, a slight increase in the onset time of phase transformation from 55 to 65 min (Table 2) for the system of maximum volume methanol used was confirmed by FTIR and XRD, which indicated that excessive methanol locating at the polar micellar cores might keep inorganic core disorder to slow down the crystallization process. While the excessive methanol induced a remarkable viscosity increase just before the phase transformation, the viscosity decrease after the transformation occurrence as also observed in other cases was a result of the markedly lowered viscosity of vaterite detergents as manifested in the HABSbased system.19 Also, a slight TBN decrease with the excess of methanol was exhibited (Supporting Information, Figure S8, S9); therefore, a moderate volume of methanol was a sensible choice. 3.4.3. Alkaline Calcium Salt Composition Ratio. It was found that the molar ratio of CaO to the sum of CaO and Ca(OH)2 (R) had an insignificant effect on the estimated onset time of phase transformation (Table 2) and also the TBN change trends versus carbonation time (Supporting Information, Figure S10). However, the CaO ratio R exhibited an important influence on the properties of obtained products, typically on their particle sizes and viscosity. On the one hand, a remarkable decrease in particle size was manifested with R increasing as shown in Figure 8 with some typical TEM images. The vaterite detergents obtained at 100 min carbonation duration with R = 1 and R = 0.5 were small grain-like particles with average sizes around 15.8 and 24.7 nm, respectively. When the CaO ratio decreased to 0, that is Ca(OH)2 was exclusively used as the alkaline calcium salt, the particles displayed an elongated kernel-shaped morphology of average size around 97.5 × 46.0 nm. As mentioned in section 3.3.2, the composition of the alkaline calcium salt played an important role in adjusting the extra water generated during carbonation, which also accounted for this performance. Consequently, the smaller CaO ratio in alkaline calcium salt composition would allow the formation of larger reverse micelles due to unrestrained water quantity generated during carbonation, which thus resulted in increased inorganic core size. On the other hand, a remarkably different visual appearance of samples was observed with R changing. The products with lower CaO ratio turned turbid and became gelatinous or even solid at ambient temperature after the removal of volatile solvents (Table 2, Supporting Information, Figure S11). It was demonstrated that the lower CaO ratio in the total alkaline calcium salt would result in notedly increased particle size leading to undesired gel-like products; therefore, suitable CaO content in the alkaline calcium salt composition was required for a qualified detergent preparation. 3.4.4. Quantity of Catalyst Anhydrous Calcium Chloride. As reported in our previous work, there was no inhibiting effect

mol of CaO (R = 1), and 3.5 g of CaCl2, and without additional water added. The carbonation reaction duration to get a liquid state product without gelation is called the nongel product range. The nongel product ranges were directly found out from the comparison of visual product appearances as provided in the Supporting Information (Figure S4). After removal of the volatile solvents used during the detergent synthesis, the ranges were determined to be 10−30, 20−70, and 30−80 min for 97, 150, and 200 g amounts of LCABSA, respectively, as listed in Table 2. The onset time of phase transformation from ACC to crystalline vaterite form was determined to be 55 or 65 min as listed in Table 2 based on FTIR and XRD analyses (data not shown). The particle sizes of obtained products with carbonation 100 min were estimated from the statistical measurements of some typical TEM images (Supporting Information, Figure S5) to be 29.9, 15.3, and 14.7 nm for products with 97, 150, and 200 g of LCABSA surfactant added, respectively. The decrease in particle size with increased feeding surfactant was reasonable considering that the particle size of preexisting reverse micelles in the carbonation system was mainly dependent on the molar ratio of added surfactant to water. Increase of the ratio would result in reduction in the micellar size to realize a larger surface area.53,54 With lime converted into alkaline reserve in the reverse micellar cores, TBN rose rapidly in the first 40 min and then increased modestly with carbonation time elongated to around 100 min (Supporting Information, Figure S6). A slight increase in TBN was observed when the surfactant LCABSA dosage decreased; a larger particle size of lowered micellar rigidity was thought to account for the higher TBN. However, a too low surfactant dosage would result in poor product properties and stability. For the lowest surfactant amount of 97 g adopted in this study, only in the initial stage of reaction less than 30 min could be a nongel product obtained, characterized by a low conversion and inefficient lime utilization and resulting in heavy solid sediment. As the carbonation reaction time lengthened to more than 35 min, a gelatinous even solid product was obtained. Thus a moderate amount, 150 g, of surfactant LCABSA was used in the comparative experiments following other groups. 3.4.2. Promoter Methanol Dosage. The interfacial zone of the surfactant layer and the polar core of reverse micelles were thought of as two sites for methanol to mainly exist and take effect.55 In the former case, methanol acted as a cosurfactant, markedly reducing the rigidity of micelles to accelerate reactant mass transport and raise the reaction rate.29,49 As a consequence, when the methanol volume increased from 56 to 105 mL and to 175 mL, a gradual increase in particle size from 13.6 to 15.3 nm and then to 20.4 nm was observed from TEM images with 100 min carbonation duration (Supporting 12758

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Figure 9. Typical TEM images of CaLCABS overbased detergents at carbonation for 100 min with different added water amounts of (a) 0 g, particle average size 15.7 nm; (b) 4 g, particle average size 21.9 nm; and (c) 16 g, kernel-shaped morphology with particle average size 53.8 × 22.9 nm.

For elucidating the influence of the water content on phase transformation, comparative experiments with variant added water quantities of 0−16 g were performed. Kandori and co-workers33 and Roman et al.29 demonstrated through DLS characterization that the microemulsion sizes, thus the produced CaCO3 particles, increased with the addition of water. While Dagaonkar et al.53 and Sugih et al.54 confirmed through direct electron microscopy observation that high water-to-surfactant (1,2-bis-(2-ethylhexyl-oxycarbonyl)-1ethane sulfonate, AOT) molar ratios resulted in the formation of larger nanoparticles in gas-reverse micellar systems, colloid particles with diameters ranging from 25 nm to around 200 nm were produced mainly depending on the volume of water added. For the alkylbenzene sulfonate detergent system studied here, first, the excessive water notably enlarged the particle sizes of product. As shown in Figure 9, the particle morphologies of mainly vaterite overbased products after carbonation for 100 min in the absence of additional water or with 4 g of water added were small grain-like particles, with average diameters of 15.7 and 21.9 nm, respectively. Moreover, an elongated kernelshaped morphology with particle size around 53.8 × 22.9 nm was obtained when the water dosage was further increased to 16 g. When Figure 9 was compared with Figure 8, it was very interesting to notice that the evolution tendencies of both the particle sizes and morphologies versus increase of added water were strikingly similar to that with the decrease of CaO ratio in the total alkaline calcium salts. This coincidence indirectly but strongly confirmed that regulating the extra water generated during carbonation reaction accounted for the influence of the calcium salt composition variety on detergent product properties. Second, the added water dosage obviously shortened the onset time of phase transformation. As listed in Table 2, the onset times of phase transformation of formulations with 0, 4, and 16 g of water added were estimated to be 55, 45, and 35 min, respectively, based on FTIR (Supporting Information, Figure S15) and XRD analyses (Supporting Information, Figure S16). Besides, a notably contracted nongel range exhibited with the increase in added water (Table 2; also see Supporting Information, Figure S17), which might be due to the agglomeration resulting from the increased particle size and the early onset time of phase transformation. Mann’s group investigated the water-induced mesoscale phase transformation of alkylbenzene sulfonate surfactant stabilized ACC nanoparticles in reverse microemulsions and observed the obvious morphology changes of CaCO 3 aggregates from monodispersed spheres to spindle-shaped structures as the water-to-CaCO3 ratio increased.56 Xu et al. also demonstrated that added water could effectively accelerate the transformation and crystallization of ACC in water−ethanol mixture solution.41

of CaCl2 to phase transformation from ACC to vaterite of calcium carbonate during HABS based overbased detergent preparation at high carbonation degree.19 Other questions follow, such as can the added CaCl2 slow down the crystallization? What about deficiency or excess in the quantity of anhydrous CaCl2? Experimental evidence from FTIR, XRD, TEM analyses, and TBN measurements would answer these questions. Generally, anhydrous calcium chloride (CaCl2) is considered to be a helpful additive for detergent preparation,52 which can restrain the initial content of water for improving lime utilization. To begin with, a control experiment was conducted without CaCl2 added. The resulted TBN was rather low with the highest value at 53.2 mg KOH/g after 100 min of reaction due to quite deficient lime conversion (Supporting Information, Figure S12). Consequently, the carbonation degree and alkaline reserve of the thus-obtained product was too low to start crystallization, nor was any phase transformation observed in the CaCl2-free system with carbonation duration ranging from 0 to 100 min. The onset time of phase transformation was found to remain almost constant at 55 min when a certain amount of CaCl2 was added (Table 2). Furthermore, the TBN of CaLCABS overbased detergents with added 1.4 or 3.5 g of CaCl2 increased gradually in an almost overlapped trace from near zero to around 240 mg KOH/g within 100 min of reaction (Supporting Information, Figure S12). The irregular particle morphologies were also very similar and with almost the same sizes around 15.5 nm judged from typical TEM images of products with 1.4 or 3.5 g of CaCl2 added, in sharp contrast to the small spherical particles of about 7.5 nm of the CaCl2-free control system (Supporting Information, Figure S13). It was more apparent from the visual appearance comparison that the products with different amounts of CaCl2 added all had quite broad nongel ranges in contrast to the gelatinous appearance of control product in the absence of CaCl2 (Supporting Information, Figure S14). Therefore, anhydrous CaCl2 was demonstrated to be a crucial catalyst component: a little quantity played a key role in the carbonation reaction, phase transformation process, and alkaline reserve of overbased detergent products, while excessive dosage of CaCl2 was not beneficial, and could not slow down the crystallization process, even resulting in increased viscosity at high carbonation degree. 3.4.5. Water Added. It is believed that the nucleation and growth of CaCO3 occur in water-in-oil microemulsion nanoreactors,1,29,33 so water is among the essentials for carbonation reaction during detergent preparation. Generally, the required amount of water is so low that the preexisting water in the raw materials is usually sufficient for the reaction. Thus, anhydrous calcium chloride and calcium oxide were utilized to restrain the initial content of water, and usually no added external water was needed during the preparation.29,50,51 12759

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with ACC and vaterite mixture polymorphs, obvious phase transformation occurred, even transformation into almost complete crystalline vaterite form resulting in serious precipitation due to deprotection and agglomeration. It was interesting to note that the HABS surfactant with substituted dialkyl chains showed a more powerful capability to stabilize the amorphous products or even vaterite detergents with low viscosity and high TBN19 when compared with the single long chain LCABS surfactant. The performance comparison was reminiscent of stable spherical vaterite nanoparticles being attained mediated by polystyrene sulfonate57 or hyperbranched polymer functionalized with sulfonic acid groups43 and an additional alkyl chain in the sulfonate-modified conjugated polymer inducing vaterite crystals.58 Consequently, high quality detergents of improved stability and compatibility have been reported recently by adopting alkyl-aryl sulfonates of alkaline earth metals with a heavy alkylbenzene or tolyl or xylyl with longer C14−C40 alkyl chains.59 Moreover, the influences of some important reagent dosages were investigated based on phase transformation and detergent quality. Water constituted a fundamental ingredient and water content played an important role in carbonation reaction, since the nucleation and growth of CaCO3 occurred in the preexisting water-in-oil microemulsions constructed by surfactants. Thus the reverse micelle size and the produced colloid CaCO3 particles were mainly dependent on the water-tosurfactant molar ratios. Excessive water dosage would reduce the stability of reverse micelles, hence shortened evidently the onset time of phase transformation and accelerated the crystallization process from ACC to vaterite polymorph resulting in high viscosity or gelation. The catalyst anhydrous calcium chloride and the CaO ratio R in the total alkaline calcium salt also took effect significantly in qualified detergent preparation. Methanol, the most widely used promoter, worked as a cosurfactant and could markedly reduce the rigidity of micelles to accelerate reactant mass transport and raise the reaction rate. Shortage of promoter would result in poor utilization of lime and low conversion. On the other hand, the excessive methanol volume would lead to micelle instability and agglomeration, particularly at high carbonation degree.

Third, TBN of the formulations with variant added water kept almost the same changing trend, except excessive amount of water would result in a slightly decreased TBN value at high carbonation reaction degree (Supporting Information, Figure S18), reminiscent of a similar tendency reported by Eli and coworkers that excessive amount of water resulted in low TBN and turbid appearance of calcium oleate detergents.9 Furthermore, the influence of water dosage on crystallization of micellar cores was further corroborated by a parallel experiment with short-chain surfactant SLABSA replacing the LCABSA. The onset time of phase transformation decreased obviously from 55 to 45 min when 5 g of water was added as revealed by FTIR and XRD analyses (Supporting Information, Figures S1, S2, and S19); meanwhile, the nongel range shortened remarkably from 30−70 min to 30−45 min with an additional 5 g of water added (Supporting Information, Figure S20). Therefore water content exhibits a strong influence on both the phase transformation of inorganic cores and product qualities. The excessive water remarkably reduced the stability of reverse micelles, which would accelerate the phase transformation from ACC to vaterite polymorph and result in gelatinous products at high carbonation degree due to agglomeration. 3.5. General Discussion. Since the invention of overbased detergents several decades ago, numerous formulations have been patented50−52 and recently much simulative and experimental research on the structures of the reverse micelles and their inorganic cores has been conducted.15−26 However, the phase transformation of the inorganic cores, which is of crucial importance for the quality control of products, particularly for the most widely used sulfonate detergents of commercial value and also various industrial oil applications, has not been well understood, even with some specious notions existing. Our previous work19 about the heavy alkylbenzene sulfonate (HABS) overbased detergent system achieved an enhanced understanding of the carbonation reaction process: a certain amount of residual Ca(OH)2 existing in the reverse micellar core was essential for the stabilization of amorphous inorganic nanoparticles. Upon approaching carbonation completion and depletion of Ca(OH)2, a phase transformation from ACC to vaterite polymorph rather than calcite form occurred. Here, by replacing the complicated HABS surfactant with explicit LCABS or SLABS surfactant, the phase transformation has been unambiguously demonstrated again, which indicates the formation of the vaterite crystalline polymorph is of general validity for all the alkylbenzene sulfonate systems. Furthermore, it was found that the alkyl chain length in the alkylbenzene sulfonate surfactant played an important role for the stabilization of the reverse micelles, with short alkyl tail C10− C13 in SLABS compared with long alkyl tail C18−C26 in LCABS; insufficient protecting and dispersing power of SLABS resulted in their deficiency in stabilizing the inorganic nanoparticles and preventing their transformation from ACC into crystalline vaterite form. The thermal test performance for overbased products based on CaLCABS and CaSLABS further validated such a conclusion. All CaSLABS samples suffered from a turbidity or precipitation generation only after the 1 day thermal trial, whereas the CaLCABS detergents in ACC state showed excellent thermal stability. For CaLCABS overbased detergents of a mixture of ACC and vaterite polymorph, no perceptible further phase transformation was observed after the longest 20 days thermal trial, whereas for CaSLABS samples

4. CONCLUSIONS The synthesis and mechanism study of sulfonate overbased detergents is of persistent interest in the field of lubricant additives as well as in various industrial oil applications. Carbonation reaction is the key procedure for the preparation of overbased detergents and is of crucial importance for the quality control of products; however, the mechanism of phase transformation is not yet fully understood due to the complexity of a heterogeneous process with multicomponents involved. Consequently, following our previous investigation based on the heavy alkylbenzene sulfonate (HABS) surfactant of industrial byproducts,19 long-chain alkylbenzene sulfonate (LCABS) and also comparative short-chain linear alkylbenzene sulfonate (SLABS), two commercial products with well-defined chemical compositions and structures, were employed as model surfactants for further mechanism study of this pivotal process. The phase transformation mechanism of calcium carbonate involved in the carbonation reaction and the influence of reagent dosage have been further probed through monitoring the real-time temperature changes of the reaction system and examining the intermediates of variant stages and the final products by combination of various analytical techniques such 12760

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ACKNOWLEDGMENTS We acknowledge financial support from the National Natural Science Foundation of China (NSFC No. 50273013) and Fundamental Research Plan Project of Jiangsu Province (BK2010244), and we also appreciate Jintung Petrochemical Corp. Ltd. for providing us with the surfactant raw materials of long-chain alkylbenzene sulfonic acid (LCABSA).

as potentiometric titration, DLS, TEM, FTIR, and XRD. Several significant conclusions can be drawn as summarized below. 1. Besides from the HABS system, the phase transformation from ACC to vaterite was demonstrated unambiguously again in both the LCABS and SLABS systems, which implied that crystallization into the vaterite polymorph at high carbonation degree was a universal mechanism for all the alkylbenzene sulfonate systems. Such an understanding should be of crucial importance for guiding the preparation, the process, and finetuning the performance of overbased detergents and greases. 2. The amorphous state of calcium carbonate was a prerequisite for the preserving and stabilization of the alkaline reserve. The less amount of crystalline vaterite form existed in the products, the higher thermal stability they possessed, which impelled ascertaining a suitable work window for the production. 3. The length and number of alkyl tails of alkylbenzene sulfonate surfactants exhibited a strong influence on detergent product properties. The long-chain alkyl (LCABS) or dialkyl (HABS) substituted alkylbenzene sulfonate surfactants played an important role not only in inhibiting the agglomeration process, but also in protecting the metastable micellar cores from fusion to avoid phase transformation of CaCO3 under a high temperature work environment. Comparatively, dialkyl substituted ones were even more powerful as manifested by the realization of HABS-based crystalline vaterite detergents with high TBNs and low viscosities. 4. The compositions of alkaline calcium salts and dosages of promoter, catalyst, and surfactant displayed significant effects on the product viscosity and appearance while they exhibited no apparent influence on the phase transformation, which bestowed the detergent formulations with a broad effective work window bearing high tolerance and good manipulability. On the other hand, water played an important role in both phase transformation and product qualities. Excessive water would reduce the stability of reverse micelles, thus quickening the onset time of phase transformation from ACC to vaterite polymorph and resulting in agglomeration. This perception provided a handle for better understanding the reaction process and achieving good quality control for detergent manufacturing.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

Visual appearance comparison of CaLCABS and CaSLABS based products after undergoing different periods of thermal testing at 100 °C and with various reagent dosages; FTIR spectra and XRD profiles of CaSLABS based products with different carbonation durations and added water; TEM images and associated particle size histograms and TBN changing trends of CaLCABS overbased detergents with various reagent dosages. This material is available free of charge via the Internet at http://pubs.acs.org.

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*E-mail: [email protected]. Fax: +86-25-83317761. Tel.: +86-2583686621. Notes

The authors declare no competing financial interest. 12761

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