Evaluation of Sargassum spp. Oil as a Potential Additive for Biolubricant Formulations

Abstract

Macroalgae-derived oils offer a sustainable and environmentally friendly alternative to conventional mineral oils and additives in lubricant formulation. Their favorable fatty acid profiles can contribute to mitigating the environmental impacts caused by using fossil-based products. This study evaluates the potential of Sargassum spp. oil, collected from the Mexican Caribbean, as a lubricity-enhancing additive in synthetic base oils. The impact of Sargassum spp. oil on viscosity and tribological performance was analyzed. The results indicated that Sargassum spp. oil has potential as an anti-wear additive since a formulation using 10% v/v Sargassum spp. oil in a synthetic lubricant (PAO6) resulted in a wear scar diameter value of 703.03 ± 15.56 µm, which is about 10% lower than PAO6 used as commercial synthetic control. Additionally, the formulation significantly enhanced the viscosity index, with a value of 169, which was notably higher than the control (137). The parameter of the coefficient of friction was significantly reduced from 0.1 to 0.08 (about 27%) when using the formulation with 10% v/v Sargassum spp. oil. These findings underline the potential of algae oils as additives in synthetic lubricants, promoting a transition towards greener products and reducing the environmental impact derived from conventional formulations of polyalphaolefins and esters.

1. Introduction

In recent decades, the search for sustainable, renewable, and ecological alternatives to gradually reduce the use of fossil resources and thus mitigate greenhouse gas emissions has greatly intensified [1]. In the particular case of petroleum-derived lubricants, used lubricants present serious environmental challenges due to their chemical makeup and the risks they pose to ecosystems, wildlife, and human health. Fossil-derived lubricants are particularly problematic because they contain harmful substances such as heavy metals (e.g., lead, zinc, copper), hydrocarbons, additives, and other toxic chemicals [2]. When spilled or improperly disposed of, these contaminants can infiltrate soil and water sources, degrading groundwater and surface water quality. They can be toxic to aquatic life, disrupting fish reproduction, harming other aquatic species, and damaging ecosystems. Additionally, spilled lubricants on soil can hinder plant growth by blocking roots and reducing soil fertility, creating a barrier that prevents water and nutrients from reaching plant roots [3]. Among the options to reduce and/or replace the use of petroleum-derived products, algal biomass stands out as one of the most promising resources for producing next-generation biofuels and synthesizing new high-value bioproducts [4,5]. Algae are photosynthetic organisms divided into macroalgae (multicellular) and microalgae (unicellular) both with high growth rates, with harvest times between 1 and 4 months. They are up to four times more efficient in carbon dioxide (CO₂) capture and photosynthesis than terrestrial plants, producing and accumulating lipids with favorable fatty acids through controllable environments. Another advantage over biomass obtained from terrestrial plants to obtain byproducts is that they do not require arable land or fresh water for their cultivation, which avoids competition with food production [6,7].

The research on algae oils has gained attention; the vast majority has been concentrated on microalgae oil because some species may contain more than 30% of the total lipids based on their dry weight (DW) [8]. The use of microalgae for bio-oil production, both on a small and pilot scale, has been widely studied. Different metabolic processes of lipid storage in algae such as stimulating their production by nutritional stress have shown the potential to overcome key obstacles in downstream unit operations and improve the economic efficiency of various microalgae-based products [1,9]. Different biofuels from microalga oil have been widely studied through various thermochemical conversions [10], i.e., biodiesel [11], biogas, biochar, bio-oil [12], and jet fuel [13]. In contrast, marine and freshwater macroalgae are not considered a sustainable source of oil-based products due to most of the species have <5% total lipid content DW. However, macroalgae play an important role in the development of bio-based substances under the concept of biorefinery, but their use as raw material to produce bio-oils has yet to be investigated [14]. Macroalgae are commonly termed seaweed and are divided into red (Rhodophyta), green (Chlorophyta), and brown (Phaeophyceae) algae. Among macroalgae, Chlorophyta and Phaeophyceae are notable for their higher content of polyunsaturated fatty acids, such as alpha-linolenic acid and linoleic acid [15]. There are some investigations into the production of bio-oil from macroalgae. For example, Wang et al. [16] studied the pyrolysis temperature effect of microwave-assisted pyrolysis on the production of bio-oil obtained from green seaweed Ulva Lactuca and they showed that the maximum bio-oil yield was 18.4 wt.% at a temperature of 500 ºC. Choi et al. [17] obtained bio-oil from Saccharina japonica by pyrolysis on a fixed-bed reactor at 450 ºC and reported a yield of 47%; also, they reported a yield of biochar of 33%. Similarly, Kim et al. [18] obtained 28.78 wt.% of bio-oil from Saccharina japonica by pyrolysis at 380 ºC for 5 min in a micro-tubing reactor. Recently, there has been a sudden proliferation of brown macroalgae, mainly the Sargassum genus, in the Mexican Caribbean, favored by the tropical location with massive arrivals of Sargassum, ranging between 10,105 and 40,932 m³ per kilometer of coast. Although macroalgae generate unfavorable impacts on the economy of the tourism sector and the local environment, the rapid collection of these “golden tides”, traditionally considered a waste, offer a constant source of raw material with potential for valorization for various bioprocesses [19,20]. Sargassum has garnered significant scientific attention due to its diverse applications and composition, which is abundant in bioactive compounds. Research highlights its potent antioxidant, antimicrobial, and anticancer activities [21]. Species like Sargassum muticum are particularly notable for their role in alginate extraction, with seasonal variations in yield and rheological properties, making them valuable for biopolymer production [22]. Furthermore, this brown macroalgae shows great promise for third-generation biofuel production via hydrothermal carbonization, with byproducts demonstrating potential as agricultural biostimulants [23]. Its high phenolic and flavonoid content underscores its untapped potential in nutraceutical development [24]. Therefore, the use of these residual macroalgae could boost the development of biorefineries and promote the circular economy, thus promoting the production of bio-oils and other bioproducts, as well as contributing to regional economic growth.

In general, one of the least explored algae byproducts, in particular those based on algae oils, are lubricants [25,26]. However, considering the environmental benefits and the growing interest in replacing petroleum-based products (i.e., fuels and lubricants) with renewable oil sources, new eco-friendly lubricant formulations can be developed by algae oils due to their natural fatty acid compositions that are well known to enhance lubricant lubricity properties [26]. Algae oils, like other bio-oils, have distinct properties important for lubrication, including good lubricity, certain viscosity (this depends on each machinery application), high viscosity index, low pour point, and excellent oxidation stability [26]. These properties depend on fatty acid factors, such as carbon chain length, saturation level, and polarity. Oils with long carbon chains (n > 9), high levels of saturation (comprising saturated fatty acids (SFAs), monounsaturated fatty acids (MUFAs), and polyunsaturated fatty acids (PUFAs)), and high polarity are considered ideal for lubrication, especially in machining applications [27]. Algae oils can potentially revolutionize the industry by providing greener and more sustainable solutions. However, key challenges must be addressed, including scaling up algae biomass production and oil extraction processes, ensuring a consistent supply of the product with the required lubricant properties for specific applications, and maintaining its biodegradability.

Lubricants are substances that reduce friction, heat, and wear between surfaces in relative motion. These products play a fundamental role in all industries because they are used in any kind of machinery and processes to guarantee their proper operation and extended maintenance periods. According to their main composition, lubricants can be classified as mineral (of petroleum origin), synthetic (polyalphaolefins and esters), and biolubricants if there is at least 25% oil content from biomass [28,29]. As a reference for lubricant consumption worldwide, around 40 million metric tons of mineral lubricants are produced annually, from which approximately half is settled in water bodies due to spills or improper disposal after use, representing a major environmental concern due to the toxicity of these products. For this reason, lubricant industries have been urged to adopt bio-based and ecological production processes to mitigate the associated environmental impacts [30]. Lipid composition analysis of bio-oils is imperative for the optimization of bioprocesses in both biolubricant formulation and production, as it determines the quality of physicochemical properties and the behavior in industrial applications, especially under specific temperature conditions [31]. The fatty acid content directly influences the density, melting point, and kinematic and dynamic viscosity of bio-oils. As the chain of saturated fatty acids increases, a reduction in density and an increase in viscosity and melting point are observed. On the other hand, unsaturated fatty acids tend to have higher viscosities and lower melting points, which help to balance the properties of blended lubricants [32]. In addition, longer carbon chains form thicker films, and their dense packing and intermolecular interactions reinforce the film structure, decreasing wear and improving strength [33].

Despite the great effort to incorporate a vast variety of bio-oils into lubricant formulations, most bio-oils result from vegetable sources [29], while scarce research has been focused on the use of bio-oils extracted from macroalgae biomass for these purposes. Nonetheless, the potential for the production of bio-oil from macroalgae biomass has been demonstrated by different researchers. For example, Osman et al. reported in 2020 the production of bio-oil from 22 species of macroalgae, selecting the most promising ones by their lipid content for biodiesel production. Among the most effective, Ulva intestinalis (Chlorophyta) biodiesel presented 73.4% of saturated fatty acids, mainly palmitic acid (43.8%), granting high oxidative stability and lubricity with a bioproduct recovery of 3.2% in dry biomass [34]. In another research work, Jeliani et al. in 2021 analyzed and compared the fatty acids of various macroalgae as feedstock for biodiesel production. They identified C. sertularioides with more than 5% lipid content suitable for bio-oil extraction. The authors found that macroalgae composition has a high content of saturated fatty acids, such as palmitic acid (C16:0) and oleic acid (C18:1), which may confer upon biolubricants good thermal stability and resistance to oxidation [35].

Hence, the above reports and literature support the hypothesis that bio-oils from marine macroalgae can be used as promising additives or even base oils for lubricant production. Therefore, this investigation aims to evaluate the potential of Sargassum spp. macroalgae, collected from the Mexican Caribbean, for the extraction of bio-oils suitable for formulating sustainable lubricants. The obtained bio-oils were characterized by analyzing their fatty acid profile through derivatization to fatty acid methyl esters (FAMEs). Subsequently, the bio-oil was separated and then incorporated into a commercial synthetic base oil at 5% and 10% (v/v) concentrations, obtaining two new formulations to evaluate their viscosity, viscosity index (VI), and tribological properties.

2. Results and Discussion

2.1. Evaluation of the Total Lipid Content, Oil Yield Extraction, and Fatty Acid Profiles of Sargassum spp.

Sargassum spp. samples analyzed in the present study resulted in a total lipid content of 23 ± 0.5 mg·g⁻¹ DW, which is comparable with the values reported by Melchor-Martínez et al. in 2023 [36], where the authors reported content of 28.16 ± 3.25 mg·g⁻¹ DW in Sargassum spp. samples collected from the Mexican Caribbean coast. This similarity in lipid contents suggests that the environmental and physiological conditions of the Sargassum species at both locations could be comparable. However, it is important to note that the variability in the FAMES profile may be influenced by factors, such as the season of year in which the collection was carried out and the nutritional conditions of the environment [37]. In this context, Saldarriaga-Hernandez et al., in 2021 [38], demonstrated that the lipid concentration of Sargassum spp. collected in the Mexican Caribbean varied between different values according to the season, obtaining 33.0 ± 1.2 mg·g⁻¹ DW in May and 26.0 ± 5.7 mg·g⁻¹ DW in September.

Once the lipid content was measured, 20.1 kg of dry biomass was processed to extract the oils using Soxhlet equipment, followed by the transesterification reaction to obtain FAMEs. Notably, the oil yield was lower than expected as compared to the total lipid content previously analyzed, with only 0.43 ± 0.02%, equivalent to over 8.6 mL of extracted oil. This result suggests that the Soxhlet method was not completely efficient for oil extraction from this biomass. This inefficiency may be due to physical or chemical barriers within the cellular matrix; specifically, the cellular matrix of certain algae species may have structural components, such as complex polysaccharides and/or thick cell walls, which can limit the efficiency of solvent extraction [39]. Other methods for extracting algae oils include physical wall-breaking treatment, supercritical fluid extraction (SFE), and biological extraction. Currently, the advances in green technology extractions consider SFE a suitable method for the recovery of nonpolar molecules using carbon dioxide that can be transformed into a supercritical fluid at normal temperature and high pressure. The advantages of SFE are high efficiency, high stability of the sample, avoiding contact with oxygen, and high temperature. However, in some instances, the oil yields from SFE are comparable to those with Soxhlet extraction. Therefore, strategies in which a physical treatment is used, such as ultrasonication or microwave-assisted SFE, could improve the overall oil extraction rate [40].

The results of the FAME analysis of the Sargassum spp. oil are presented in

Table 1. Fatty acid profile (expressed as % of total FAME) of the macroalga Sargassum spp.

Compound Name	Carbon Number	Percent (%) of Total Fatty Acids
Myristic acid	C14:0	1.64 ± 0.01
Palmitic acid	C16:0	29.72 ± 0.45
Palmitoleic acid	C16:1	5.86 ± 0.11
Stearic acid	C18:0	3.37 ± 0.14
Oleic acid	C18:1n9	25.45 ± 0.54
Linoleic acid	C18:2n6	2.63 ± 0.05
Alpha-linolenic acid	C18:3n3	12.19 ± 0.33
Arachidonic acid	C20:4n6	0.73 ± 0.01
Eicosapentaenoic acid	C20:5n3	12.04 ± 1.70
Erucic acid	C22:1n9	2.03 ± 0.04
Docosahexaenoic acid	C22:6n3	4.31 ± 0.20
∑SFAs		34.73 ± 0.59
∑MUFAs		33.34 ± 0.69
∑PUFAs		31.91 ± 1.81

Percents expressed as mean ± SD. SFAs (saturated fatty acids); MUFAs (monounsaturated fatty acids); PUFAs (polyunsaturated fatty acids).

.

The SFAs identified in the Sargassum spp. samples were dominated by palmitic acid (C16:0) with a concentration of 29.72 ± 0.45%, followed by stearic acid (C18:0) with 3.37 ± 0.14%. The MUFAs were represented mainly by oleic acid (C18:1 n9), a monounsaturated lipid, which, in general, was the second most abundant, representing 25.45 ± 0.54%, and is suitable for promoting favorable fluidity and lubricity properties in the biolubricant formulations [27]. Both SFAs and MUFAs are the FAs wanted for obtaining good lubricity properties in biolubricant formulations. However, a considerable concentration of polyunsaturated fatty acids, such as α-linolenic acid (C18:3n3) with 12.19 ± 0.33% and eicosapentaenoic acid (C20:5n3) with 12.04 ± 1.7%, was also identified. These findings are consistent with previous studies, in which similar concentrations of fatty acids were reported in several samples of Sargassum species. In particular, the present study highlights that C16:0 and C18:1n9 are the primary fatty acids, which gives the Sargassum spp. oil good thermal stability [41,42,43]. Likewise, the balance between SFAs (34.73%), MUFAs (33.34%), and PUFAs (31.91%) is observed in the Sargassum spp. oil, suggesting a high potential for lubrication applications. SFAs could provide better oxidation stability, while both MUFAs and PUFAs improve low-temperature performance. MUFAs are less prone to oxidation than PUFAs [44]. The Sargassum spp. oil analyzed in this study exhibited a grease-like consistency at room temperature. This phenomenon may be attributed to its high content of SFAs [45] and the presence of high-molecular-weight compounds, such as waxes, sterols, and tocopherols [46,47]. Previous studies have characterized the content of lipophilic metabolites, such as vitamin E, in oils extracted from marine algae, with a concentration of 85 mg 100 g⁻¹ reported [48]. Additionally, sterols were identified as predominant compounds in the Antarctic seaweed species analyzed, with concentrations ranging from 14.74 to 321.25 mg kg⁻¹ [49]. These findings suggest that lipophilic metabolites, including sterols, may be present in oil extracted via Soxhlet.

It is important to note that fatty acid composition can vary depending on factors, such as Sargassum species, geographic location, season of the year, and cultivation conditions [50]. This variability underlines the importance of conducting more comprehensive studies to optimize the control in both cultivation conditions and extraction methods to obtain Sargassum oils with specific lipid profiles for determining applications such as in the development of biolubricants, in the future.

The biolubricant formulation was successful since apparent miscibility in the prepared blends was achieved. This finding suggests that, at 5 and 10% vol. of Sargassum spp. oil, the intermolecular forces between both oils favor miscibility, despite the Sargassum spp. oil being a solid grease. The ability to form homogeneous mixtures could be attributed to interactions between the molecular chains of the Sargassum spp. oil and PAO6, which overcome the van der Waals forces responsible for maintaining the solid state of the Sargassum spp. oil [51].

2.2. Kinematic Viscosity and Viscosity Index (VI) of the Oil Formulations

The comparison of viscosity data obtained for the oil samples is shown in Figure 1a–c. The results indicate that adding the Sargassum spp. oil leads to a reduction in viscosity at 40 and 100 °C, and an increment in the viscosity index (VI), as compared to neat PAO6. The reduction in kinematic viscosity was significant for the P6S10 formulation, about 15%, which is a positive result because low-viscosity lubricants can promote less flow resistance and less energy consumption due to flow shear in high-speed lubrication applications. In addition, adding 10% of Sargassum spp. oil to PAO6 produced a VI value of about 169, which is 26% larger than that of neat PAO6 (VI = 137). It is worth saying that the high value of VI is desirable since the larger its value, the wider the temperature interval at which the lubricity properties remain constant, according to the regulation ASTM D2270. This suggests that Sargassum oil-formulated lubricants can experience even less viscosity variation at wider temperature intervals, which is needed for high-performance lubricants formulated to operate at variable temperature conditions. To the best of our knowledge, most vegetable oils (naturally composed of triglycerides and fatty acids) exhibit larger VIs than mineral or synthetic base oils [27], as shown in the data of some examples in

Table 2. Comparison of viscosity index (VI) for different bio-oils.

Oil	Viscosity Index (VI)	Reference
Sunflower oil	218	[52]
Jatropha oil	262	[53]
Rapeseed oil	218	[54]
Soybean oil	246	[55]

. It should be noted that the error bars presented for the kinematic viscosity of P6S5 and P6S10 at 100 °C indicate a variation of as much as ±1.5% and may be attributed to possible microbubble formation of even a few traces of volatile substances in Sargassum spp. oil (i.e., water, solvent, chlorophyll, etc.) at such a temperature. Microbubbles can slightly affect the measurement of density and, thus, the value of kinematic viscosity.

2.3. Tribological Behavior

The comparison of wear and coefficient of friction (CoF) obtained for the different oil samples is presented in Figure 2a,b, respectively. The wear scar diameter generated by the four-ball test, expressed in micrometers, is used as a standard measurement of the surface wear generated with the test oil. According to Figure 2a, neat Sargassum spp. oil produced the largest wear scar diameter, which is attributed to the possible decomposition of the natural triglycerides. These triglycerides have several disadvantages, including a low pour point and poor thermal stability. When exposed to high temperatures, the oil undergoes oxidation, releasing carboxylic acids or alkenes. These oxidation products can then react with the metal surface, forming metal oxides that accelerate corrosion, as reported for other vegetable oils [56]. However, wear was reduced when Sargassum spp. oil was blended with PAO6. The formulation with 10% Sargassum spp. oil (P6S10) showed the lowest wear, even when compared with neat PAO6. These results suggest that adding low concentrations of Sargassum spp. oil to the base formulation slightly improves wear resistance. It can be ascribed to the formation of a more effective lubricating film coming from the large content of MUFAs (mainly oleic acid) and PUFAs (particularly alpha-linolenic and eicosapentaenoic acids) in the Sargassum oil, which are formed by long carbon chains (from 18 to 20 carbons) and, according to [33], produce thicker films, a more dense packing with reinforced intermolecular interactions that may contribute to forming a stronger lubricating film structure, thus decreasing wear. The low wear promoted by the addition of the Sargassum spp. oil into PAO6 is consistent with a reduction in the CoF, which is confirmed by the results presented in Figure 2b. It can also be associated with the possibly more effective lubricating film formed by the fatty acids contained in the Sargassum spp. oil. Neat PAO6 presented the highest CoF (0.100 ± 0.005), while the formulation P6S10 showed significantly lower values (0.081 ± 0.002). In turn, the high content of both palmitic acid (C16:0) and oleic acid (C18:1) may confer, upon new lubricant formulations, suitable thermal stability and resistance to oxidation during the lubrication operations. Hence, it can be said that adding 10% of Sargassum spp. oil is sufficient to improve the lubricity properties of petroleum base oils, particularly those of PAO6, which could lead to a significant improvement in machinery service life and energy savings in industrial applications while using a non-edible oil derived from algae biomass considered as waste. Moreover, the use of Sargassum oil can significantly reduce environmental concerns and avoid competition with oils intended for food while contributing to a reduction in the use of petroleum derivatives in lubricant production.

3. Materials and Methods

3.1. Macroalgae Biomass Collection, Pre-Treatment, and Oil Extraction

Sargassum spp. was collected in August 2021 from Chen Rio Beach, Cancun, Quintana Roo. The samples were obtained at a minimum depth of 3 m to ensure their freshness, and specimens without signs of deterioration were selected. The biomass was washed with fresh water to remove sand, salt, and other impurities. After washing, the biomass was immediately stored at −20 °C. After a storage period of three months, it was thawed and dried at 50 °C for 48 h to remove moisture and residual soil. Subsequently, the dried biomass was ground into a fine powder using an Oster^® countertop blender (Oster^® Classic, Model BLST4655-013). Each blending session was conducted at “level 3” for approximately 8 min, ensuring the algal biomass was finely powdered and prepared for future oil extraction.

Approximately 2.01 kg of dried macroalgae biomass collected from salt water was used for the oil extraction. The extraction was performed in parallel using six KIMAX Soxhlet extractors, each equipped with a condenser and heating mantle. For each extraction, 150 mL of n-hexane (95%, Sigma-Aldrich, St. Louis, MO, USA) was used to extract oil from 20 g of Sargassum biomass. The volume-to-mass ratio was previously standardized to minimize extraction time. Extraction was carried out at 60 °C for 4 h. Afterwards, the oil-containing solvent was filtered to remove residues. The solvent was then evaporated using a rotary evaporator at 60 °C and 60 rpm until all pure n-hexane was separated and recovered. The remaining crude bio-oil was placed in a 40 °C oven to ensure complete evaporation of the solvent.

3.2. Evaluation of the Total Lipid Content and FAME Profile of Sargassum spp.

Each of the three replicate samples (150 mg each) of Sargassum spp. biomass powder was subjected to the following procedure: The extracted lipids were gravimetrically weighed to estimate the total lipid content according to the modified Bligh and Dyer method [57]. Briefly, for each sample, a 6.4 mL mixture of methanol, chloroform, and double-distilled water (in a 2:1:0.5 volume ratio) was added and shaken for 30 s. Additionally, 2 mL of chloroform and 2 mL of water were added, followed by centrifugation (SL 40R, Thermo Fisher Scientific, Waltham, MA, USA) at 4800 rpm for 10 min at 4 °C. The upper phase was discarded, and the lower phase was filtered through a hydrophobic polytetrafluoroethylene (PTFE) syringe filter with a pore size of 0.45 μm into preweighed vials. The lipid extract was concentrated at 15 mmHg and 45 °C for 2 h. The lipid yield was then weighed and recorded.

Then, samples of the lipid yield free of organic solvents and water were resuspended in 2 mL of n-hexane for the determination of fatty acids by derivatization to FAMEs. For this, 2 mL of methanol with 10% sulfuric acid and 1 mL of internal standard were added to the samples with triundecanoin (C11:0) at a concentration of 400 ppm. The mixture was heated at 80 °C for 1 h for the transesterification to occur and then cooled down. Immediately after reaching room temperature, a sample containing FAMEs was extracted with 3 mL of n-hexane while stirring for 1 min. The upper phase formed was recovered and the lower phase was extracted again with 3 mL of hexane. The recovered phases were placed in 10 mL volumetric flasks, which were filled with n-hexane and closed with PTFE plugs. The samples were stored at −20 °C for subsequent analysis.

The FAME profile was analyzed using a gas chromatograph by coupling it with a mass spectrometer (Perkin Elmer, Clarus 600/560D, Shelton, CT, USA) using an HP-88 capillary column (100 m, 0.25 mm × 0.20 µm). The carrier gas was helium at a constant flow rate of 1 mL/min. The initial oven temperature was 140 °C (held for 5 min) and was increased at 4 °C/min until the final temperature of 240 °C (held for 15 min) was reached. The split ratio was 1:10, the injector temperature was 265 °C, the mass spectrometer analysis was performed with an EI ionization source using an electron energy of 70 eV, the source and interface temperature was 210 °C, and the m/z range was from 30 to 550. For the identification of FAMEs, the retention time of a typical chromatogram of the 37-component FAME reference standard was used, and quantification was performed using a standard internal method, using triundecanoin (C11:0) as an internal standard (AOAC Official Method 996.06).

3.3. Lubricant Sample Preparation

Neat Sargassum spp. extracted oil, named in this work as “Sargassum oil”, was added in 5% and 10% v/v in a polyalphaolefin base oil (PAO6) (SpectraSyn6, Exxon Mobil, Port Allen, LA, USA), which is widely used for producing high-performance low-viscosity lubricants for many different industrial applications. The mixtures were stirred at 300 rpm for one hour at 20 °C until their homogenization using a magnetic stirrer (Vevor, SH-2, Shanghai, China). These parameters were found to be appropriate to form homogeneous blends. The samples were labeled as P6S5 (95% PAO6 + 5% Sargassum spp. oil) and P6S10 (90% PAO6 + 10% Sargassum spp. oil), respectively. Neat PAO6 was used as a control for all the tests. It is important to mention that the limit of miscibility of Sargassum spp. oil in PAO6 base oil was not conducted since the aim of this work was to evaluate the use of Sargassum spp. oil as an additive in the formulation of biolubricants. This is an open question that should be addressed in the future. However, the formation of homogeneous mixtures may be attributed to interactions between the molecular chains of the Sargassum spp. oil and PAO6, which overcome the van der Waals forces [51].

3.4. Determination of Density and Viscosity of the Oils

Densities and shear viscosities of P6S5, P6S10, and PAO6 were concomitantly measured using an automatic kinematic viscosimeter (SVM 3001, Anton Paar, Graz, Austria), which can simultaneously determine the density and shear viscosity because of the coupling of a vibrating U-tube and a Stabinger rotational viscosimeter, both connected in series as stated in the ASTM D7042 method for Newtonian fluids. Using the data of density and shear viscosity of lubricants, determined at 40 and 100 °C, the kinematic viscosities were calculated to obtain the viscosity index as indicated in the ASTM D2270 method. The proper functioning of the SVM 3001 instrument was verified using a low-shear-viscosity standard oil (S60, CANNON Instrument Company, State College, PA, USA), which has a NIST-traceable certificate for viscosity, density, and kinematic viscosity at 25, 40, 75, and 100 °C, respectively. The SVM 3001 instrument has a repeatability and reproducibility of 0.00005 and 0.0001 g/cm³ for density and 0.1 and 0.35% for viscosity, respectively. The temperature was controlled with a Peltier system, which has a repeatability and reproducibility of 0.005 and 0.03 °C, respectively, in the 20 to 100 °C range.

The density and shear viscosity of the Sargassum oil, P6S5, P6S10, and PAO6 were measured at temperatures of 25, 40, 75, and 100 °C. The viscosity and density cells of the SVM 3001 equipment were filled with 2.5 mL of the desired oil using an Injekt^® Luer solo syringe (B. Braun Medical, Melsungen, Germany), which was initially filled with 10 mL of the corresponding oil. Then, the temperature of cells was set to 25 °C, and after 7 min of equilibrium, the measurement of density and viscosity was performed using the instrument in the precision and repeated operation mode. In that mode, the instrument performs multiple measurements during a specified time, which, in this case, was 3 min, and the difference between consecutive values of viscosity and density is continuously computed. The default tolerance in the precision mode is 0.0002 g/cm³ and 0.2% for density and shear viscosity, respectively. These tolerance values may or may not be achieved during measurement. When the time was up, the viscosity and density values were stored along with their tolerances (achieved for all the oils and temperatures). This is one measurement. Afterwards, the SVM 3001 prompts the user to refill the vibrating U-tube and the Stabinger rotational viscosimeter with 0.5 mL for renewing the oil in the cells with the fresh sample in the syringe, always connected to the device. Then, two more measurements at the current temperature are carried out by reheating the renewed sample to equilibrium. Subsequently, the system heats up to the next temperature (40 °C), and so on, up to the highest set temperature (100 °C). The measurements were conducted by heating up to avoid bubble formation. Measurement cells were cleaned with heptane before a different oil sample was analyzed. The density and viscosity of the neat Sargassum oil could not be measured at 20 and 40 °C, since at these temperatures, the oil is in its solid state. Despite this, it was soluble in the synthetic oil POA6 and formed the mixtures described in Section 3.3.

3.5. Tribological Test

Standard four-ball tests based on the ASTM D4172 and ASTM D5183 standard methods were conducted to evaluate both wear and coefficient of friction (CoF), respectively, by using the oil samples. The tester used is home-made and was validated to carry out tests under ASTM D4172 and D5183 standard tests. The characteristics of the home-made tester are given elsewhere [58]. These standard tests are widely used together to assess the wear protection and friction reduction (lubricity) properties of lubricating oils under controlled sliding steel-on-steel contact conditions. The test involves generating wear on four AISI 52,100 steel balls (12.7 mm diameter, 64 HRC hardness, 0.02 μm surface roughness) arranged in a tetrahedral configuration inside an oil container, where the four balls are immersed in the test oil. During the 1 h test at a constant temperature, one ball (upper ball) rotates against the three lower balls under a set load and speed. The specific test conditions are detailed in

Table 3. Tribological testing parameters.

Test Parameter	Value
Load [N]	392
Maximum Hertzian contact pressure [GPa]	4.5
Speed [rpm]	1200
Test duration [min]	60
Temperature [°C]	75 ± 2
Humidity [%]	32
Amount of oil per test [mL]	15
Repeats using new samples	4

. The wear produced on the lower balls is quantified by measuring the wear scar diameter (WSD) using an optical microscope (Olympus, SZX7, Hachioji, Tokyo, Japan) with a magnification of 2× and a precision of 1 µm.

4. Conclusions

Bio-oil derived from Sargassum spp. emerged as a viable additive alternative to improve the properties of synthetic base oils, particularly PAO6. In general, adding 10% of Sargassum spp. oil into PAO6 exhibited the best viscosity index (VI) and tribological properties. According to the fatty acid profile of Sargassum spp. collected in the Mexican Caribbean, the species presented a balanced proportion of saturated fatty acids (SFA) at 34.73%, monounsaturated fatty acids (MUFAs) at 33.34%, and polyunsaturated fatty acids (PUFAs) at 31.91%. The viscosity of PAO6 was the highest at both 40 °C and 100 °C, while P6S10 showed the lowest values at 40 °C and 100 °C, respectively. In addition, P6S10 exhibited the highest viscosity index (VI) among all the oil samples tested. The 10% of Sargassum oil added to PAO6 increased the VI by about 25%. Tribological results showed P6S10 as the most effective since it presented the lowest wear scar diameter (WSD) and CoF as obtained by a tribological standard test. The addition of Sargassum oil to PAO6 was able to reduce 10% and 27% of the wear and CoF, respectively. Sargassum spp. oil from the Mexican Caribbean shows potential as a lubricant additive for greener formulations. According to the viscosity and tribological properties found within this research, the Sargassum oil could be used as a lubricating additive for different applications, such as engines, electric drivelines, gears, automatic transmissions, metalworking fluids, compressors, rock drilling machines, etc. Future studies should explore other Sargassum species and address environmental and economic constraints to ensure the viability and sustainability of large-scale applications. Extra chemical conversion processes (epoxidation, hydrogenation, etc.) of neat Sargassum spp. oil should be carried out in order to increase its thermal and oxidation resistance.