Colloidal lignin particles for eco-friendly and fully emulsified Pickering emulsions: physical stability and interfacial properties

Abstract

Colloidal lignin particles (CLPs) are emerging as eco-friendly alternatives to molecular fossil-based emulsifiers, also offering an effective way to incorporate lignin’s multifunctionality. In this context, this work aims to demonstrate the role of CLPs in effectively stabilizing Pickering emulsions, thereby expanding the potential use of lignin in high-performance delivery systems, such as cosmetic formulations. For this, softwood kraft lignin CLPs (10–50 g/L) were used to create oil-in-water Pickering emulsions with an oil volume fraction of 0.3–0.7. Optimization was performed by response surface methodology with the emulsified layer (EL) and the emulsion droplet size selected as the objective responses to be maximized and minimized, respectively. The increase in CLPs concentration and oil volume fraction has enhanced the EL magnitude and reduced the oil droplet size, resulting in fully emulsified and stable systems. The optimized formulation was evaluated for storage stability, stabilization mechanism, and rheological behavior. The emulsion exhibited excellent long-term stability, with no significant changes in droplet size, morphology, or homogeneity observed over one month. Confocal microscopy and Cryo-Scanning electron microscopy revealed that CLPs bridged between droplets, resulting in highly efficient packing of oil droplets. Furthermore, the emulsion exhibited a gel-like behavior, which is favorable for applications requiring enhanced stability and ease spreading. Overall, the obtained results provide insights into the formulation of completely emulsified systems using lignin-based stabilizers, highlighting favorable attributes for applications in eco-friendly cosmetic and personal care products.

Introduction

Emulsifiers play a crucial role in emulsion stabilization by reducing the interfacial tension between immiscible phases, making them widely applied in various products across the cosmetic, food, and pharmaceutical industries [1]. However, many conventional emulsifiers have been linked to environmental impacts, including water pollution, harm to marine ecosystems, and poor biodegradability, as well as adverse health effects, such as endocrine disruption and allergic reactions [[1], [2], [3]]. Pickering emulsions, i.e., particle-stabilized emulsions, have been recognized as a safer and more sustainable alternative to conventional emulsions due to their emulsifier-free nature [2,4].

Particles from both inorganic and organic sources have been used for Pickering emulsion stabilization [5]. However, with increasing consumer demand and industry regulations favoring eco-friendly and sustainable ingredients, there is a strong interest in utilizing particles of biological origin, including proteins [[6], [7], [8]], carbohydrates [[9], [10], [11], [12]], polyphenols [13,14], and biomass components [[15], [16], [17], [18]]. The main challenge is to find particles that match or surpass the performance of traditional emulsifiers while being widely available, cost-effective, and not competing with food resources.

Lignin, a complex macromolecule present in all vascular plants, is abundantly found as a by-product of industrial processes, the so-called technical lignins [19,20]. Lignin’s eco-friendly nature comes from its natural, renewable source, widespread availability, and non-toxicity; therefore, its valorization promotes sustainable and circular practices. Besides this, the lignin’s random structure, composed of phenylpropanoid units, confers unique properties, making it a versatile material with the potential to replace petrochemical-based ingredients and, therefore, reducing the carbon footprint of manufactured products [21]. For example, lignin has proven its potential in several strategic fields, including biofuels, binders, dispersants, polymeric materials, and packaging, in addition to being a renewable source of aromatic compounds [22,23]. More recently, lignin’s remarkable functional properties, such as biocompatibility, antioxidant, antimicrobial, UV shielding, and its ability to adsorb at oil-water interfaces, have attracted significant interest for its use as an eco-friendly ingredient in emulsion formulations [17,24].

The presence of nonpolar groups (e.g., aromatic rings and aliphatic chains) and polar groups (e.g., hydroxyl and carboxylic acid functionalities) in lignin macromolecules can impart natural amphiphilicity to their particulate forms, especially following molecular rearrangement [25]. For this reason, colloidal lignin particles (CLPs) started to be explored as Pickering stabilizers. Relevant examples include Pickering emulsions stabilized by CLPs from softwood kraft lignin, prepared with different vegetable oils (orange, coconut, and paraffin), which reveal synergistic effects between lignin and the oils in enhancing UV protection [24]. The resultant oil-in-water emulsions were considered promising formulations for skincare applications [24]. In another study, CLPs obtained from lignin derived from the enzymatic hydrolysis of corn stover successfully stabilized kerosene-in-water Pickering emulsions, where the pH-responsive nature of lignin was used in designing recyclable emulsion systems with high performance for enhanced oil recovery (EOR) [26]. CLPs from eucalyptus, obtained through different extraction procedures (enzyme, alkali, and organosolv), were successfully applied as Pickering stabilizers to protect curcumin from UV degradation in drug-loaded emulsion systems [27]. From this perspective, CLPs have proven their inherent potential as Pickering stabilizers. However, to effectively replace traditional emulsifiers, CLPs must demonstrate similar or better stabilizing performance [28]. For this reason, the storage stability of lignin-based Pickering emulsions requires deeper investigation, as studies addressing this issue are still limited.

Moreover, the focus on the ability of CLPs to form fully emulsified systems is only sparingly explored, as most studies dealing with CLPs’ performance as Pickering stabilizers result in systems with two separate phases, an emulsified layer and a particle-rich aqueous suspension [16,26,[29], [30], [31], [32]]. In conclusion, the design of fully emulsified and stable systems is crucial to prove the capacity of CLPs to replace conventional emulsifiers.

The present study aims to investigate the development of highly stable lignin-based Pickering emulsions, enhancing the application of lignin in high-performance systems, such as cosmetic products. For this, particle properties and emulsion formulation were comprehensively investigated. First, tailored CLPs were produced and fully characterized in size, morphology, and interfacial properties. Then, the effect of the CLPs concentration and oil volume fraction on the formation of a fully emulsified system and the resultant emulsion droplet size was investigated using the surface response methodology. The optimized emulsion formulation was characterized in terms of stability and rheological behavior. Furthermore, the stabilization mechanisms of CLPs in the emulsion structure were clarified using confocal and cryo-SEM microscopy. This study distinguishes from the existent literature by applying a systematic statistical approach to evaluate the effects of particle concentration and oil volume fraction on CLP-stabilized Pickering emulsions, and by successfully demonstrating that CLPs can form fully emulsified systems that remain stable over time.

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Materials

Indulin AT, a purified form of kraft pine lignin (purity 97 %), was kindly supplied by Ingevity™. Ethanol absolute (purity ≥99.8 %) was purchased from Honeywell (Germany). Di‑sodium hydrogen phosphate anhydrous was purchased from Panreac (Spain) and citric acid from Merck (Germany). Miglyol 812 was purchased from Acofarma (Spain). Distilled water was used in all the processes.

Giovana Colucci, Liandra Gracher-Teixeira, Samara Cristina Silva-Pituco, Barbara Miqueletti de Oliveira, Andreia Ribeiro, Arantzazu Santamaria-Echart, Alírio Egídio Rodrigues, Maria Filomena Barreiro, Colloidal lignin particles for eco-friendly and fully emulsified Pickering emulsions: physical stability and interfacial properties, Journal of Molecular Liquids, Volume 441, 2026, 129038, ISSN 0167-7322, https://doi.org/10.1016/j.molliq.2025.129038.

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