Functionalized Metal Oxide Nanoparticles to Reduce Polyester Microfiber Release During Laundry Washing

Abstract

The release of microplastic fibers from synthetic textiles during domestic laundering is a major contributor to aquatic pollution. Nanomaterial-based surface treatments have recently emerged as a potential route for minimizing microfiber shedding. This study investigates the use, for the first time, of metal oxide nanoparticles (TiO₂, ZnO, MgO) functionalized with fatty acids (oleic acid (OA) and stearic acid (SA)) as microfiber-retaining agents. The nanoparticles were modified via a simple adsorption process at room temperature, monitored by zeta potential analysis, and confirmed by DSC-TG and FTIR-ATR analysis. When applied to polyester fabrics during simulated washing cycles, the hydrophobicity of the polyester surface coated with functionalized nanoparticles was assessed via contact angle measurements, and the effect on microfiber shedding was evaluated by the filtration of wastewater and by weighing the mass of fibers retained in the filters. ZnO and MgO nanoparticles treated with stearic and oleic acid demonstrated a significant reduction in fiber shedding compared to commercial laundry detergent (approximately 46–70%). In contrast, fatty acid adsorption onto TiO₂ was less efficient (reduction in microfiber release ~23%), and the TiO₂-based systems showed limited improvement in microfiber shedding, possibly due to insufficient hydrophobic interaction. These results demonstrate that fatty acid functionalization of low-cost inorganic nanoparticles is a promising strategy for mitigating microfiber pollution in laundry effluents.

1. Introduction

The continuous and ubiquitous release of microplastics into aquatic and terrestrial ecosystems represents one of the most pressing environmental challenges of the 21st century [1]. Microplastics, defined as plastic particles smaller than 5 mm, are now detected in virtually every environmental compartment, including surface waters, deep-sea sediments, soils, the atmosphere, and even within biological tissues of humans and animals. Their persistence, combined with their capacity to absorb toxic contaminants and host pathogenic microorganisms, poses significant ecological and human health risks [2,3]. Several studies have reported microplastic ingestion across trophic levels, from plankton to fish and mammals, with potential implications for food safety, bioaccumulation of hazardous compounds, oxidative stress, and inflammation in exposed organisms. The annual global input of microplastics into the oceans is estimated to exceed 1.5 million tons, with domestic sources accounting for a major fraction of this load [4].

Among the many pathways by which microplastics enter the environment, laundry washing of synthetic textiles has emerged as a dominant contributor. Polyester (polyethylene terephthalate, PET), the most widely produced synthetic fiber, represents more than 60% of the global textile market. During washing, mechanical agitation, thermal stress, and detergent-induced surface erosion promote the detachment of microfibers (typically ranging from 10 µm to 5 mm in length). Once released into wastewater, these fibers are only partially retained by conventional treatment process, allowing substantial quantities to reach rivers and marine environments [4].

It is estimated that a single 5 kg domestic wash of polyester garments, with conventional cycles (40 °C, 1:30 h) can release up to 6 million microfibers, depending on the type of fabric and washing conditions (temperature, duration, load weight) [5]. Lant et al. also reported that use of colder and faster wash cycles (versus conventional 40 °C long cycles) reduced microfiber generation by about 30% (for certain test conditions) and that the fiber release tends to stabilize after multiple cycles [5].

Overall, microfiber release depends on a complex combination of fabric attributes (fiber type, construction, finish), washing process (temperature, time, mechanics) and chemical interactions during laundering [6].

Efforts to mitigate the problem of microfiber shedding have been made essentially by three main strategies: (1) physical filtration or containment, (2) modification of textile surface (on the production line) and (3) chemical surface treatments.

Physical filtration (or interception) includes installation of filters in washing machines, using external lint filters, retrofitting filters or in-line capture devices, or using laundry bags engineered to trap fibers before they enter the wastewater stream. While effective to some extent, their efficiency depends on mesh size, maintenance frequency, and user compliance, and very fine fibers may escape [7,8]. Moreover, their real-world efficacy is limited by their inability to trap ultrafine fibers below the mesh threshold and the need for periodic cleaning or disposal of these released fibers once they risk re-entering aquatic systems [9].

The structural modification of textiles consists of designing fabrics to intrinsically resist fiber loss and can be achieved by increasing fabric tightness, optimizing yarn twist, reducing surface “hairiness”, or modifying fiber cross-sectional geometry to reduce mechanical interactions that promote fiber detachment [10,11,12]. However, structural changes often trade off with textile comfort, breathability, drape, or cost and may not fully eliminate fiber release under repeated mechanical stress [9].

Finally, chemical surface treatments in the finishing phase of the production process, with application of protective layers or functional coatings that strengthen the fiber surface or reduce friction, are being used to shed microfibers. Some approaches include polymeric coatings (e.g., silicones, polyurethane), hydrophobic finishes, crosslinkers or nanoparticle-based layers [13,14]. Although surface coatings and chemical finishes can significantly reduce microfiber shedding by improving fabric cohesion and lowering surface friction, this approach faces notable limitations. Most polymeric coatings (e.g., silicones, polyurethanes, fluoropolymers) are themselves synthetic and non-biodegradable, which raises concerns about secondary microplastic generation as these coatings wear off during repeated laundering and have limited washing durability (coatings may deteriorate or detach after repeated cycles) and due to the fact that coating layers may alter textile aesthetics, flexibility, and breathability, compromising comfort and consumer acceptance [9,15].

While these strategies have demonstrated partial success, they remain limited by high cost, user compliance, and long-term durability. Consequently, there is a critical need for innovative, sustainable, and cost-effective approaches to mitigate microfiber emissions directly at the source.

An interesting, yet still technically demanding, approach involves forming a coating in situ during the washing process that effectively deposits protective agents onto the textile fibers under wash conditions. Such coatings would ideally self-assemble or dynamically bond during washing, reinforcing the fiber surface as mechanical forces act, thus combining flaking mitigation with operational convenience.

In this context, nanoparticles have emerged as promising functional additives for textile applications. Metal oxide nanoparticles, including TiO₂, ZnO, and MgO, have been widely explored in nanotextiles to provide properties such as UV protection, antimicrobial activity, self-cleaning behavior, and water repellency [16,17]. In many of these applications, however, the final performance depends not only on the intrinsic properties of the nanoparticles but also on the coating formulation, textile substrate, binders, and finishing conditions used to immobilize them on the fiber surface. Beyond these established functionalities, metal oxide nanoparticles may also play a role in reducing microfiber shedding, based on their high specific surface area and ability to interact with fiber surfaces via van der Waals forces, hydrogen bonds, or electrostatic interactions, creating a reinforced interface between the nanoparticle and fiber [18,19]. For polyester microfiber mitigation during washing, the key requirement is not only the functional activity of the nanoparticle but also its ability to interact with and remain associated with the hydrophobic polyester surface under mechanical agitation and aqueous conditions. Since many unmodified metal oxide nanoparticles are intrinsically hydrophilic, their direct compatibility with polyester may be limited, leading to weak adhesion or poor retention during washing. [20,21]. An effective strategy is functionalization with long-chain fatty acids, such as oleic or stearic acid. Oleic acid and stearic acid were selected as model long-chain fatty acids because they share the same C18 hydrocarbon length and terminal carboxylic acid group but differ in chain saturation and molecular geometry. Stearic acid is a saturated and linear C18:0 fatty acid, which favors dense molecular packing and the formation of more ordered hydrophobic layers on solid surfaces. In contrast, oleic acid is a monounsaturated cis-C18:1 fatty acid; the presence of the cis double bond introduces a kink in the hydrocarbon chain, reducing packing efficiency and increasing conformational disorder. These structural differences are expected to influence the organization, stability, and hydrophobic character of the fatty acid layer formed on the metal oxide nanoparticles and consequently their affinity toward hydrophobic polyester fibers during washing. These acids can chemically bond (via carboxylate groups) to the nanoparticle’s metal core, while the hydrophobic alkyl chains remain oriented outward. This organic shell effectively transforms the nanoparticle surface to be more compatible with hydrophobic polymer fibers, enabling stronger van der Waals interactions and hydrophobic–hydrophobic affinity and potentially reducing interfacial friction [22,23].

The present work focuses on the evaluation of functionalized TiO₂, ZnO, and MgO nanoparticles as a strategy to reduce polyester microfiber shedding during simulated washing. By comparing nanoparticles modified with oleic and stearic acids, this study aims to clarify how surface functionalization and nanoparticle–fiber interfacial affinity affect adhesion to polyester fibers and, consequently, microfiber release. In this way, the work proposes a potentially scalable approach to mitigate textile-derived microplastic pollution while recognizing that its full environmental profile requires further assessment.

2. Materials and Methods

2.1. Materials

Nanoparticles (TiO₂, MgO and ZnO) were purchased from Nanografi (Ankara, Turkey) and were used without any further modifications. TiO₂ nanoparticles (99.8% purity) used had particle sizes of 20–30 nm, MgO nanoparticles (99.5% purity) had particle sizes of 35 nm, and ZnO nanoparticles (99.5% purity) had particle sizes of 30–50 nm. Oleic and stearic acids were both purchased in Sigma-Aldrich (St. Louis, MO, USA), with purities of ≥99.0% and ≥98.5%, respectively, and ethanol (96% v/v) was acquired from José Manuel Gomes dos Santos, Lda (Odivelas, Portugal). Distilled water was used in the functionalization reactions. The commercial laundry detergent used for comparison of released microfibers was Skip Active Clean ^® (Unilever, Seville, Spain).

The polyester fabrics (100% PET—Minimate (Porto, Portugal)) used for washing trials were cut (5 × 5 cm) on a laser cutting machine (CO₂ 130 W laser) with a velocity of 100 mm/s and 40% power (software RD Works V8) (Shenzhen RuiDa Technology Co., Ltd., Shenzhen, China).

2.2. Nanoparticle Functionalization Reaction with Fatty Acids

The procedure for the surface treatment of nanoparticles using oleic and stearic acids was adapted from Liu et al. [24]. The nanoparticles (0.5% w/w) were solubilized in 10 mL of water, and the fatty acids (0.5% w/w) were dispersed in 10 mL of water. Fatty acid solutions were added to the nanoparticle solution and maintained under magnetic stirring at 1000 rpm at room temperature, and the progress of the reaction was monitored by zeta potential analysis. When the reaction was complete, the white solid was washed twice with ethanol to remove unbound acid, filtered through filter paper (5 µm), and dried at 40 °C overnight.

2.3. Experimental Techniques

The success of the adsorption reaction was monitored by zeta potential analysis, performed in a Zetasizer Ultra supplied by Malvern Panalytical (Malvern, UK), and pH measurements were conducted with a Consort C1010 electrode at 25 °C. The reaction products were analyzed by DSC-TG, performed in an STA 449 F5 Jupiter, obtained from Netzsch (Selb, Germainy), with temperature ranges from 25 to 600 °C at 0.5 °C/min and with aluminum crucibles. The IR spectra were obtained with an FTIR-ATR spectrometer, supplied by Thermo Nicolet 380 (Madison, WI, USA), with smart orbit diamond ATR, with a 4 cm⁻¹ resolution and 64 scans.

The effect of functionalized nanoparticles on the adhesion and retention ability of synthetic microfibers during washing was evaluated by simulating the washing of 100% polyester fabrics with the different functionalized nanoparticle systems, and these assays were conducted in triplicate. The fabrics (3 pieces of 5 × 5 cm/wash) were washed using a stirring plate at 100 rpm, 25 °C, for 1 h, with a water–functionalized nanoparticles ratio of 1:200. After washing, the fabrics were removed and the washing solution was filtered through fiberglass filters (0.45 µm). These were dried at 105 °C for 2 h, cooled to room temperature, and weighed in an analytical balance (Model AS 220.R1. PLUS) with a sensitivity of 0.1 mg. The quantification of released microfibers was performed by mass difference based on ISO 4484-1:2023—Textiles and textile products—Microplastics from textile sources; Part 1: Determination of material loss from fabrics during washing (International Organization for Standardization, 2023).

The surface of the polyester after washing with commercial detergent and with different functionalized nanoparticles was analyzed by determining the water contact angle (WCA (°)) using a Biolin Scientific optical tensiometer, model Theta Flex (Göteborg, Sweden). The measurements were made with distilled water (drop volume of 4 µL) with image acquisition for 5 s.

3. Results

3.1. Zeta Potential and pH Measurements During Functionalization Reactions

The functionalization of nanoparticles with long-chain fatty acids is a widely established strategy to tailor interfacial properties such as hydrophobicity, dispersibility in organic media, and compatibility with polymer matrices [25]. In the case of TiO₂, MgO, and ZnO nanoparticles, the interaction with carboxylic acids such as oleic acid and stearic acid occurs predominantly through coordination of the carboxylate group to surface metal centers, leading to the formation of M–OOC–R bonds (M = Zn, Mg, Ti) [26]. Monitoring such surface modification requires complementary techniques, among which zeta potential (ζ) measurements play a particularly relevant role as an indirect probe of surface chemical evolution.

The results of ζ and pH measurements of the functionalization reaction of TiO₂, MgO and ZnO nanoparticles with oleic and stearic acid at 0, 3 and 96 h are presented in

Table 1. Zeta potential and pH variations with functionalization reaction time (hours) with oleic and stearic acids of TiO₂ nanoparticles.

TiO2 Nanoparticles	Reaction Time (h)	Zeta Potential (mV)	pH
Oleic acid	0	−7.93 ± 1.2	7.9
	3	−12.3 ± 2.4	5.7
	96	−30.5 ± 0.6	6.0
Stearic acid	0	−7.93 ± 1.2	7.9
	3	−11.4 ± 0.4	6.2
	96	−21.1 ± 2.8	6.0

,

Table 2. Zeta potential and pH variations with functionalization reaction time (hours) with oleic and stearic acids of MgO nanoparticles.

MgO Nanoparticles	Reaction Time (h)	Zeta Potential (mV)	pH
Oleic acid	0	−0.7 ± 0.1	10.8
	3	−5.7 ± 0.9	10.7
	96	−25.6 ± 1.0	10.4
Stearic acid	0	−0.7 ± 0.1	10.8
	3	−3.7 ± 0.3	9.7
	96	−12.4 ± 0.9	10.6

and

Table 3. Zeta potential and pH variations with functionalization reaction time (hours) with oleic and stearic acids of ZnO nanoparticles.

ZnO Nanoparticles	Reaction Time (h)	Zeta Potential (mV)	pH
Oleic acid	0	0.8 ± 0.2	7.8
	3	2.1 ± 0.3	7.2
	96	−36.7 ± 4.2	7.1
Stearic acid	0	0.8 ± 0.2	7.8
	3	−1.7 ± 0.3	7.3
	96	−27.8 ± 2.6	8.6

, respectively.

The analysis of the results of Table 1, Table 2 and Table 3 shows that the functionalization of the three types of metallic nanoparticles with fatty acids progressively changed the chemical surface and the interfacial medium of the nanoparticles.

The three systems show the same overall trend: with increasing reaction time, the zeta potential progressively shifts towards more negative values, especially between 3 and 96 h, which is consistent with a progressive functionalization of the surface by carboxylate species from fatty acids. In metal oxide nanoparticles, the surface charge results from the acid–base balance of surface hydroxyl groups and the specific adsorption of ligands. Therefore, a continuous decrease in ζ towards more negative values over time is a strong indication of increased surface coverage by adsorbed fatty acids and not just a momentary change in the medium [27].

In the case of TiO₂, the initial value is already negative (−7.93 mV at pH 7.9), which is consistent with the fact that the pH is above the typical isoelectric point of TiO₂, frequently reported to be around 6.2 [28]. After modification, ζ becomes much more negative, reaching −30.5 mV with oleic acid and −21.1 mV with stearic acid at 96 h. This behavior can be explained by the effect of the specific adsorption of the carboxylic group to the oxide surface. ZnO shows an even more pronounced evolution: it starts from a nearly neutral/slightly positive ζ (+0.8 mV at pH 7.8), consistent with an initial pH below or near its typical isoelectric point (≈8.7–9.5) [29], and evolves to a strong charge inversion at 96 h, especially with oleic acid (−36.7 mV). This indicates that, as the reaction progresses, the adsorbed fatty acid layer dominates the interfacial behavior and overrides the native charge of ZnO. MgO nanoparticles exhibit the most peculiar behavior because despite starting almost neutral at ζ (−0.7 mV), they always maintain a very high pH (9.7–10.8), reflecting the high surface basicity and their very high isoelectric point, typically ~12 [30]. However, a clear shift towards negative values is observed, especially with oleic acid (−25.6 mV at 96 h), which shows that the interaction with the fatty acid occurs. In this case, the most plausible interpretation is that the strong basicity of MgO favors the acid–base interaction between the –COOH and –OH groups on the surface, but the strongly basic and reactive character of MgO partially dampens the charge inversion when compared to ZnO.

Comparing the adsorption behavior of the two acids, an unequivocal trend is observed: in all nanoparticles, oleic acid generates more negative final ζ values than stearic acid. Since both have the same carboxylic functional group, the difference in results is mainly from the hydrocarbon tail: oleic acid possesses a cis double bond and tends to produce less crystalline and more dynamic adsorbed layers, with better interfacial reorganization and, in many systems, greater coverage ability. On the other hand, stearic acid, being saturated and linear, tends to pack more rigidly and aggregate more easily.

3.2. FTIR-ATR Analysis of Functionalized Nanoparticles

The products of the different functionalization reactions were analyzed by FTIR-ATR spectroscopy because it is a fast and robust tool for qualitatively confirming the efficiency of functionalization of metal oxide nanoparticles (MgO, TiO₂, and ZnO) with fatty acids by simultaneously identifying the presence of the organic ligand and the type of chemical interaction established with the surface. For clarity, free fatty acid was identified by the C=O band (~1690–1710 cm⁻¹), while oxide-associated species were indicated by its reduction and the appearance of COO⁻ bands (~1540–1560 and ~1390–1460 cm⁻¹). Overlapping signals prevent precise assignment of coordination geometry. The marked decrease in the intensity of fatty acid bands in the functionalized samples is expected, since these materials are predominantly inorganic and contain only a surface-bound organic fraction. Moreover, ethanol washing removes free or weakly adsorbed acid, leaving mainly the oxide-associated fraction. Thus, the weaker C–H and other fatty acid signals reflect the lower relative organic content of the functionalized nanoparticles compared with the pure acids rather than the absence of fatty acid. FTIR-ATR was therefore interpreted qualitatively, considering the combined changes in the C=O, COO⁻, and C–H regions.

Figure 1 and Figure 2 show the infrared spectra of TiO₂ nanoparticles functionalized with oleic and stearic acids, respectively.

The comparison of the FTIR-ATR spectra of non-functionalized TiO₂, pure fatty acids (OA and SA), and the materials obtained after functionalization (TiO₂ + OA and TiO₂ + SA) allows for a critical evaluation of the incorporation of an organic phase on the oxide surface and the interaction mode of the carboxylic group with TiO₂. Non-functionalized TiO₂ exhibits the typical behavior of an inorganic oxide, with low absorbance throughout most of the 4000–1000 cm⁻¹ range and an intense contribution in the low-wavenumber region (<700–500 cm⁻¹) [31].

The pure acids show the expected organic bands: in oleic acid, –CH₂ stretches typically appear near 2922 and 2853 cm⁻¹, and the C=O band of the carboxylic group appears near 1708 cm⁻¹. In stearic acid, these bands typically appear around 2916, 2849 and 1696 cm⁻¹. On TiO₂ surfaces, carboxylic acids can interact with surface Ti–OH/Ti sites mainly through chemical adsorption/coordination involving the carboxylic group. This interaction may involve partial deprotonation of the –COOH group and formation of surface-bound carboxylate species coordinated to Ti surface centers. Therefore, a decrease or modification of the free C=O stretching band, together with the appearance of bands in the characteristic COO⁻ region, supports the formation of Ti–carboxylate-like surface species rather than simple physical adsorption [32].

Comparing the two functionalized systems, TiO₂ + OA shows only very weak organic signals, with a discrete band near 1471 cm⁻¹, suggesting low surface coverage or weak ligand retention after purification. Conversely, TiO₂ + SA exhibits much more evident organic bands, particularly in the ~2900–2850 cm⁻¹ region, in addition to the band at ~1464 cm⁻¹, indicating a greater amount of acid retained on the surface. Thus, based on FTIR-ATR, the functionalization of TiO₂ was clearly more efficient with stearic acid than with oleic acid, in agreement with the literature, which reports that the adsorption of stearic acid on TiO₂ is superior to that of oleic acid and that the CH₂ bands between 2800–3000 cm⁻¹ are a good indicator of this surface adsorption [32]. This difference arises from the molecular structures of the fatty acids: stearic acid, being linear and saturated, packs more efficiently and forms a more ordered layer on TiO₂, enhancing retention, while oleic acid’s cis double bond disrupts packing, leading to a less organized layer and weaker retention detectable by FTIR-ATR.

The MgO system, and the products of its functionalization with OA and SA, were also analyzed by FTIR-ATR, the results of which are presented in Figure 3 and Figure 4 respectively.

In the spectra of non-functionalized MgO nanoparticles, bands are observed in the region of ~1480 and ~1421 cm⁻¹, consistent with surface carbonate/bicarbonate species formed by contact of MgO with atmospheric moisture and CO₂, something very common in this oxide due to its high surface reactivity [33]. The broad signal above 3500 cm⁻¹ is assigned to O–H stretching vibrations from surface hydroxyl groups and/or adsorbed water, consistent with the high surface reactivity and partial hydration tendency of MgO nanoparticles.

For the MgO + OA system, the maintenance of the aliphatic chain bands and, above all, the strong reduction/disappearance of the free acid carbonyl band accompanied by the appearance of a new band in the ~1560 cm⁻¹ region indicate that the OA is no longer mostly in its free molecular form and interacts with the MgO surface in the form of a surface carboxylate. This behavior is consistent with the literature for OA-modified MgO, where the binding of the carboxylic group to the oxide surface is interpreted as evidence of effective chemical bonding between the acid and the support [34,35].

In the MgO + SA system, the ~1500–1400 cm⁻¹ region was treated cautiously because it may contain overlapping contributions from COO⁻ stretching vibrations and CH₂ deformation/scissoring modes. Therefore, magnesium-stearate-like species were not assigned based on this region alone but on the combined decrease in the free acid C=O band, the appearance/intensification of carboxylate-region bands, the persistence of aliphatic C–H stretching bands, and comparison with the spectra of pure stearic acid and non-functionalized MgO. This interpretation is supported by studies showing that stearic acid adheres effectively to MgO surfaces and can originate an interfacial layer of magnesium stearate, as well as by FTIR data of magnesium stearate, which typically show carboxylate bands in the ~1577 and ~1466 cm⁻¹ zone and C–H bands in ~2917–2850 cm⁻¹ [36].

The FTIR-ATR results of ZnO nanoparticles functionalized with oleic and stearic acids are presented in Figure 5 and Figure 6, respectively.

In the spectra of non-functionalized ZnO, the oxide signature is essentially observed, with a broad band in the region of ~3400 cm⁻¹, normally associated with adsorbed –OH/water, and a low-frequency Zn–O band, here near 437 cm⁻¹, in line with the typical behavior of ZnO reported in the literature [37]. The broad band around 3300 cm⁻¹ is mainly attributed to O–H stretching vibrations from ZnO surface hydroxyl groups and/or adsorbed water. Since this region is not specific, it was not used as direct evidence of functionalization; ZnO–OA interaction was instead inferred from the reduction in the free C=O band and the appearance of COO⁻ bands.

After functionalization, in both the ZnO + OA and ZnO + SA systems, the main evidence of surface modification is the disappearance of the C=O band of the free acid and the appearance of bands in the typical coordinated carboxylate region. In the case of ZnO + OA, bands clearly appear at ~1546 and ~1398 cm⁻¹, consistent with the asymmetric and symmetric stretching of COO⁻, indicating that the carboxylic group of the oleic acid has begun to interact with the ZnO surface instead of remaining predominantly in its free molecular form. For ZnO + SA, the same overall behavior is observed, with an intense band at ~1560 cm⁻¹ and a response in the ~1420–1400 cm⁻¹ zone, equally consistent with the formation of zinc-carboxylate-like species [38,39].

The FTIR-ATR results indicate that functionalization occurred through partial conversion of the –COOH group into carboxylate species bound to the surface, with this evidence being clearer in systems where the free carbonyl band decreases and bands appear in the typical COO⁻ region. Based on this criterion, the least effective system was TiO₂ + OA, while TiO₂ + SA showed more evident organic retention and surface interaction, in agreement with studies that report superior adsorption of stearic acid compared to oleic acid on TiO₂. For MgO and ZnO, both acids showed much clearer signs of surface carboxylate formation, indicating more effective functionalization than in TiO₂.

Thus, as a global inference from the spectra, the trend of functionalization efficiency was approximately TiO₂ + OA < TiO₂ + SA < MgO ≈ ZnO, with ZnO and MgO being the supports that most favored the formation of surface carboxylates and stearic acid being the ligand that showed the most robust and organized behavior, especially in TiO₂ and ZnO.

3.3. Simultaneous Thermal Analysis (STA) of the Functionalized Nanoparticles

The thermal stability of the functionalized systems was assessed by TGA and the thermal behavior by DSC, and the most relevant parameters of both techniques are summarized in

Table 4. Thermal degradation properties of functionalized metallic nanoparticles (TiO₂, MgO and ZnO) with OA and SA.

System	Td (5%)	Td (10%)	Td (20%)	Tmax DTG (°C)	Residue at 600 °C (%)
NP TiO2	−	−	−	−	100.0
NP TiO2 + OA	−	−	−	−	97.3
NP TiO2 + SA	224	239	317	247	74.9
NP MgO	385	−	−	−	93.1
NP MgO + OA	274	289	334	434	48.8
NP MgO + SA	255	292	325	300	39.7
NP ZnO	−	−	−	−	100.0
NP ZnO + OA	127	330	347	352	53.5
NP ZnO + SA	359	376	394	409	49.2
OA	218	231	246	273	3.7
SA	214	227	242	274	0.8

and

Table 5. Thermal behavior properties of functionalized metallic nanoparticles (TiO₂, MgO and ZnO) with OA and SA.

System	Peak 1 (°C)	Heat Flow 1 (mW/mg)	Peak 2 (°C)	Heat Flow 2 (mW/mg)
NP TiO2	−	−	−	−
NP TiO2 + OA	−	−	−	−
NP TiO2 + SA	59.1	−0.8	234.1	−0.3
NP MgO	322.8	−0.9	392.8	−0.8
NP MgO + OA	291.9	−3.4	424.4	−2.6
NP MgO + SA	297.2	−3.9	432.4	−2.8
NP ZnO	−	−	−	−
NP ZnO + OA	124.9	−2.7	347.4	−1.5
NP ZnO + SA	128.6	−1.9	416.1	−3.0
OA	278	−1.6	−	−
SA	61.8	−3.1	271.8	−1.4

, respectively.

The combined TGA and DSC results (Table 4 and Table 5) confirm that the thermal behavior of the functionalized nanoparticles strongly depends on the nature of the metal oxide and on the fatty acid used. Oleic acid and stearic acid show their main degradation events around 273–278 °C, with very low residual mass at 600 °C, establishing the reference thermal profile of the free ligands. This is consistent with reports in the literature describing the thermal behavior of pure C18 fatty acids and stearic acid decomposition in the high-temperature range typically associated with bond scission and volatilization of the fatty chain [40]. Oleic acid has likewise been reported to undergo thermolysis from roughly the low-200 °C range onward [41].

In contrast, the functionalized systems exhibit clear shifts in degradation temperatures and changes in thermal events, indicating different amounts of retained fatty acid species and distinct interaction strengths at the oxide surface.

Among the three oxides, TiO₂ showed the weakest interaction with oleic acid, as evidenced by the negligible mass loss and absence of DSC peaks, suggesting very low organic loading. Functionalization with stearic acid was more effective, with higher mass loss and two DSC events, indicating the coexistence of weakly adsorbed/free ligand and a more strongly associated fraction. This interpretation is consistent with the literature showing successful stearic acid surface modification of anatase TiO₂ and the formation of an adsorbed stearic layer on TiO₂ [42]. For MgO, both fatty acids led to high organic contents, confirming effective surface functionalization. However, MgO + OA exhibited a particularly strong thermal stabilization, with a markedly high Tmax DTG (434 °C), consistent with the formation of thermally stabilized magnesium-carboxylate-like species. MgO + SA also showed high organic loading, although its main degradation occurred closer to the free acid region, suggesting a larger contribution from less stabilized or more heterogeneous ligand populations. This interpretation is also coherent with the known complex thermotropic behavior of magnesium stearate and with the well-established tendency of MgO surfaces to undergo hydration/carbonation and form reactive surface layers under ambient exposure, which can contribute to multistep thermal responses [43,44].

The ZnO systems showed the clearest evidence of metal carboxylate formation, with both acids producing high organic loadings and strong thermal stabilization. This effect was especially pronounced for ZnO + SA, which displayed the highest degradation temperatures and the most stable DSC profile, consistent with the formation of more ordered and thermally resistant zinc-stearate-like domains. This reading agrees well with the literature showing that zinc stearate melts at about 128 °C, remains relatively stable up to 350 °C, and undergoes major decomposition only above 400 °C. The stronger stabilization observed for SA relative to OA is also consistent with the literature comparing zinc stearate and zinc oleate, where the presence of unsaturation in oleate modifies chain packing and melting behavior, whereas saturated stearate favors more ordered assemblies [45,46].

Overall, the joint interpretation of TGA and DSC indicates that surface functionalization was least effective for TiO₂ + OA, moderate for TiO₂ + SA, and highly effective for both MgO and ZnO systems. The strongest thermal stabilization was observed for MgO + OA and ZnO + SA, while the ZnO-based materials in general showed the most characteristic behavior of organized metal carboxylate species. These results demonstrate that both the oxide surface chemistry and the fatty acid structure govern the extent of ligand anchoring, the organic loading, and the final thermal stability of the functionalized nanoparticles.

3.4. Effect of Functionalized Metallic Nanoparticles on Polyester Microfiber Release

The effect of the use of TiO₂, MgO and ZnO nanoparticles functionalized with oleic and stearic acid on microfibers release was evaluated in simulated washes of 100% polyester fabrics. The effect of the presence of functionalized nanoparticles on surface of the polyester fabrics was evaluated through water contact angle measurements (Figure 7), and the released microfibers were quantified by gravimetry and compared with a commercial textile detergent (Figure 8).

The TiO₂, MgO, and ZnO nanoparticles showed notable differences in results, reflecting both the nature of the metal oxides and their interaction with the fatty acids used. A comparison between the three systems shows that the nanoparticles did not function uniformly, with functionalized ZnO being the most effective, followed by MgO and, lastly, TiO₂.

Analyzing the contact angle results, in Figure 7, the commercial detergent made the polyester surface much more wettable, causing the contact angle to drop from approximately 82° to 21°, which is consistent with the effect of surfactants, which decreases the surface tension of water and promotes spreading on the substrate [47].

Among the systems with functionalized nanoparticles, the ZnO and MgO systems showed clearer formation of surface carboxylates and greater organic retention, while TiO₂, especially with oleic acid, showed less efficient functionalization. In practical terms, the more effective the fatty acid anchoring and the greater the exposure of the hydrocarbon chains to the air/solid interface, the greater the surface hydrophobicity tends to be [48].

The creation of a low-energy surface by the long nonpolar fatty acid chain appears to have the effect with the greatest impact on the adhesion of functionalized systems to polyester. SA tends to form more organized layers than OA due to its saturated chain, while the cis double bond of oleic acid introduces conformational disorder and hinders packing [49].

TiO₂ nanoparticles proved less effective in reducing microfiber release. This can be explained by the more reactive nature of the TiO₂ surface, which, although offering many sites for interaction with the ligand (such as fatty acids), may not be efficient enough to stabilize interactions with polyester fibers. TiO₂ is widely recognized for its high surface reactivity, especially due to the presence of hydroxyl groups on its surface, which cannot interact with polyester groups. Once the functionalization reaction with TiO₂ with both acids was less efficient, there were not enough fatty acid molecules on the TiO₂ surface to interact with the polyester surface [50].

Another aspect that should be considered for TiO₂-based systems is the intrinsic photocatalytic activity of TiO₂. Under UV or suitable light irradiation, TiO₂ can generate reactive oxygen species, such as hydroxyl radicals and superoxide species, which are able to oxidize organic substrates. In polyester-based materials, such oxidative processes may potentially contribute to surface degradation, chain scission, increased roughness, or weakening of the fiber surface, thereby increasing the susceptibility of the textile to microfiber formation under mechanical stress. Although the present washing experiments were not conducted under controlled photocatalytic irradiation and therefore do not allow for this mechanism to be confirmed, photocatalytic activity may represent an additional limitation of TiO₂-containing systems for textile applications exposed to light. Thus, the lower performance observed for TiO₂-functionalized nanoparticles is likely mainly related to their lower fatty acid retention and poorer hydrophobic compatibility with polyester, but possible photocatalytic effects under light exposure should also be investigated in future work.

Functionalized MgO nanoparticles demonstrated greater effectiveness in reducing microfiber release, likely due to the more basic nature of the oxide, which favors a stronger interaction with the carboxylate groups of oleic and stearic acids. The highly basic surface of MgO favors the formation of metal–carboxylate complexes, which are more difficult to destabilize during washing [51]. Furthermore, stearic acid, due to its saturated structure and greater capacity to form denser and more organized layers, appears to have generated a more robust interface, preventing microfiber release more efficiently compared to oleic acid.

The system with ZnO was the most effective in reducing microfiber release, with the functionalized nanoparticles showing a high fiber retention ability. This can be attributed to the strong interaction between Zn²⁺ and the carboxylate groups, forming highly stable complexes that hinder microfiber release. The formation of metallic carboxylates (zinc oleate and zinc stearate) is well documented and known to improve the stability and adhesion of nanoparticles to hydrophobic surfaces [39]. The greater thermal stability and organization of the fatty acid layers in ZnO, provided by OA and SA, may have been crucial to the system’s efficiency in reducing microfiber release.

Analyzing the results and comparing the nature of both ligands (OA and SA), we observe that the nanoparticles systems functionalized with OA shows less efficiency in control microfiber release when compared with similar systems functionalized with SA. The OA presents an unsaturated and flexible chain and tends to form less organized and more mobile layers on nanoparticles. This structural flexibility may have led to lower stability of the layer formed on the nanoparticle surfaces and, consequently, to a greater release of microfibers during washing. The unsaturated structure may also result in less interaction and adhesion with the polyester surface, making the layer less effective at retaining microfibers. On other hand, being saturated and linear, SA forms denser and more ordered layers on the nanoparticles. Its rigid structure and greater packing capacity favor the formation of a more stable and well-organized layer on the surface, which may have contributed to reducing the release of microfibers more effectively. The packing of SA molecules may have provided a more resistant and less permeable barrier to fiber release, which is corroborated by the greater effectiveness observed in the ZnO + SA system [46].

The formation of metal–carboxylate interactions between the nanoparticles and the fatty acids contributes to stabilizing the functionalized surface layer during washing, reducing fatty acid desorption and improving the affinity of the nanoparticles toward polyester fibers [51].

This metal–carboxylate bond not only stabilizes the nanoparticle but also creates a functionalized layer on the polyester particles. This layer can act as a physical barrier that helps reduce fiber abrasion. The strong interaction between the carboxylate group and the metal centers prevents the fatty acid from easily releasing from the surface of the nanoparticles, providing a stable protective coating for the polyester fibers, which reduces the release of microfibers.

The hydrophobic effect/interaction of the long-chain fatty acids also plays a crucial role in reducing microfiber release because it contributes to better adhesion of the nanoparticles to the polyester surfaces, helping to create a protective layer that minimizes direct contact between the polyester fibers, which could generate friction and microfiber release. This effect is especially important when the fabric is in contact with washing solvents or soaps, where reducing friction is essential to prevent fiber shedding [52,53].

Although the present results demonstrate the potential of fatty-acid-functionalized metal oxide nanoparticles to reduce polyester microfiber release during simulated washing, the environmental profile of these systems requires further assessment before they can be considered a fully validated sustainable solution. In particular, the possible release of nanoparticles or dissolved metal species into the washing effluent was not quantified in this study. This aspect is especially relevant for ZnO- and MgO-based systems, since detached nanoparticles or leached metal ions may raise environmental and ecotoxicological concerns depending on their concentration, dissolution behavior, wastewater chemistry, and fate in wastewater treatment systems. Therefore, future work should include the quantification of nanoparticle residues and metal ion release in washing effluents, assessment of nanoparticle retention on the textile surface after washing, ecotoxicological screening of the treated wastewater, and broader life-cycle considerations.

4. Conclusions

Overall, this work demonstrates that surface functionalization of low-cost metal oxide nanoparticles with C18 fatty acids is an effective strategy to increase their interfacial affinity toward polyester and, consequently, mitigate microfiber shedding during laundry. The combined zeta potential/pH monitoring, STA, and FTIR-ATR analyses consistently indicate that MgO and ZnO undergo pronounced functionalization with both oleic and stearic acids, forming carboxylate-rich surface layers with outward-oriented hydrocarbon chains, whereas TiO₂ exhibits limited/ligand-dependent modification, particularly in the TiO₂ + OA system, where only a negligible organic fraction is retained after washing/purification.

The application of nanoparticles functionalized with fatty acids such as OA and SA shows promise as a strategy to reduce the release of synthetic microfibers during laundry. Compared with commercial detergent, the results indicate that the use of nanoparticles, especially ZnO and MgO nanoparticles, with saturated fatty acids can significantly contribute to reducing the release of microfibers, one of the main sources of microplastics in the oceans. The ability to form stable protective layers on the surfaces of polyester fibers, combined with the strong interaction with the nanoparticles, is a key mechanism that helps prevent the release of microfibers. The superior performance of ZnO- and MgO-based systems is attributed to the formation of stable metal–carboxylate bonds and the establishment of a hydrophobic interfacial layer through the interaction of the fatty acid chains and the ester groups, which enhances nanoparticle adhesion to polyester and reduces fiber–fiber and fiber–fluid abrasion during washing. Additionally, the more ordered packing typically associated with stearic acid can further stabilize the protective layer, explaining the stronger mitigation trends observed for saturated ligand systems.

Future studies should also quantify nanoparticle and metal ion release into washing effluents and assess the ecotoxicological profile of the treated wastewater to confirm the environmental safety of this microfiber mitigation strategy.

In summary, fatty acid functionalization of MgO and ZnO nanoparticles can emerge as a promising and scalable route to form “in situ” protective coatings on polyester during washing and reduce microfiber release.