Application of Sapindus saponaria L. extract as a natural biosurfactant in liquid soap formulation

Aplicação do extrato de Sapindus saponaria L. como biossurfactante natural em formulação de sabão líquido

Abstract

Growing environmental concerns associated with synthetic surfactants have intensified the search for sustainable natural alternatives in cleaning formulations. This study aimed to evaluate the feasibility of incorporating Sapindus saponaria L. extract as a natural biosurfactant into liquid soap formulations. A hydroalcoholic extract was obtained and characterized through determination of the aphrosymmetric index and Liebermann–Burchard reaction for saponin confirmation. Functional performance was assessed by foam formation and persistence tests, emulsification index evaluation, surface tension measurement using the Du Noüy ring method, rheological analysis by rotational viscometry, detergency and wetting assays, and preliminary stability testing including 30-day pH monitoring. The extract exhibited a high aphrosymmetric index (2,928.97) and reduced surface tension from 72.3 mN/m (distilled water) to 42.7 mN/m. The final extract-based liquid soap formulation reached 33.4 mN/m, values comparable to those observed for the SDS-based formulation (30.2 mN/m). Apparent viscosity values were 3,580 cP (10 rpm) and 2,910 cP (20 rpm), indicating pseudoplastic behavior similar to that of the synthetic system. Cleaning efficiency (81.6%) and wetting time (18.4 s) were also comparable to the SDS-based formulation. pH remained stable over 30 days (ΔpH = 0.14) with no significant physical instability. However, no emulsification capacity was observed with vegetable oils.These findings demonstrate that S. saponaria extract presents physicochemical and functional performance comparable to the evaluated synthetic surfactant for foaming and cleansing applications in liquid soap formulations, although limitations in emulsification restrict its applicability in biphasic systems.

Keywords:
biosurfactant; Sapindus saponaria; liquid soap; green chemistry; sustainability

Resumo

As crescentes preocupações ambientais associadas ao uso de surfactantes sintéticos têm intensificado a busca por alternativas naturais sustentáveis em formulações de limpeza. Este estudo teve como objetivo avaliar a viabilidade da incorporação do extrato de Sapindus saponaria L. como biossurfactante natural em formulações de sabão líquido. Um extrato hidroalcoólico foi obtido e caracterizado por meio da determinação do índice afrosimétrico e do teste de Liebermann–Burchard para confirmação da presença de saponinas. O desempenho funcional foi avaliado por ensaios de formação e persistência de espuma, índice de emulsificação, mensuração da tensão superficial pelo método do anel de Du Noüy, análise reológica por viscosimetria rotacional, testes de detergência e molhabilidade, além de avaliação preliminar de estabilidade com monitoramento do pH por 30 dias. O extrato apresentou elevado índice afrosimétrico (2.928,97) e reduziu a tensão superficial de 72,3 mN/m (água destilada) para 42,7 mN/m. A formulação final contendo o extrato atingiu 33,4 mN/m, valor comparável ao observado para a formulação à base de SDS (30,2 mN/m). Os valores de viscosidade aparente foram 3.580 cP (10 rpm) e 2.910 cP (20 rpm), indicando comportamento pseudoplástico semelhante ao sistema sintético. A eficiência de limpeza (81,6%) e o tempo de molhabilidade (18,4 s) também foram comparáveis à formulação com SDS. O pH manteve-se estável ao longo de 30 dias (ΔpH = 0,14), sem instabilidade físico-química significativa. Entretanto, não foi observada capacidade de emulsificação com óleos vegetais. Os resultados demonstram que o extrato de S. saponaria apresenta desempenho físico-químico e funcional comparável ao surfactante sintético avaliado para aplicações espumantes e de limpeza em sabão líquido, embora limitações quanto à emulsificação restrinjam sua aplicabilidade em sistemas bifásicos.

Palavras-chave:
biossurfactante; Sapindus saponaria; sabão líquido; química verde; sustentabilidade

1. Introduction

Synthetic surfactants are the largest class of specialty chemicals used in home and personal care products, widely present in shampoos, cleansers, and detergents due to their strong detergency, wetting, foaming, and emulsifying performance. However, concerns about aquatic toxicity, biodegradation, and human exposure have intensified as large volumes ultimately reach wastewater and natural ecosystems, prompting scrutiny of their environmental and toxicological footprints (Lu et al., 2024).

In parallel, recent reviews emphasize that ingredients and residues in cosmetics and cleaning products can contribute to environmental burdens and potential health risks across the product life cycle, reinforcing the need for safer and more sustainable surfactant systems (Mendonça et al., 2023; Ganesan and Rangarajan, 2023; Wirtu, 2024).

Biosurfactants have emerged as compelling “green” alternatives because they can match core surface-active functions while offering advantages in biodegradability, lower toxicity, and renewable sourcing (Nasser et al., 2024; Nagtode et al., 2022).

Advances in production and application science—particularly for glycolipid and lipopeptide biosurfactants—have expanded their use cases in skin-contact products and household cleaners, even as challenges remain around cost, scalability, and quality control. Collectively, these developments support a gradual transition from petrochemical surfactants toward bio-based systems aligned with green chemistry principles (Nasser et al., 2024; Farias, 2015).

Within plant-derived options, saponins are amphiphilic glycosides with well-documented surface activity, capable of reducing surface and interfacial tension, generating foam with persistence, and acting as emulsifiers in oil-in-water systems (Rai et al., 2021; Góral and Wojciechowski, 2020; Schreiner et al., 2022).

Contemporary reviews and screening studies show that saponin-rich extracts from multiple botanical sources exhibit competitive foaming stability and cleaning performance when compared with conventional synthetic benchmarks, highlighting their promise for detergent and cosmetic formulations (Rai et al., 2021; Schreiner et al., 2022).

The genus Sapindus (Sapindaceae) is a prominent source of saponins; extracts from Sapindus pericarps have been traditionally used as “soap nuts” and evaluated for detergency, foaming, and emulsifying properties (Rai et al., 2021; Cavaletti et al., 2023).

Importantly, a recent formulation study demonstrated the feasibility of incorporating Sapindus saponaria L. extract as a natural surfactant in intimate liquid soap, reporting favorable reductions in surface tension, adequate foamability and stability, and acceptable product stability parameters—supporting the translational potential of S. saponaria in real formulations (Murgu et al., 2008; Cavaletti et al., 2023).

Although previous studies have demonstrated the surface-active properties of plant-derived saponins, their performance is highly dependent on formulation variables such as oil type, concentration, and matrix composition (Novitasari and Putri, 2016; Silva, 2016; Sarubbo et al., 2022). Most available studies emphasize isolated physicochemical parameters—such as surface tension reduction or foaming capacity—under simplified laboratory conditions. However, there remains a limited number of systematic investigations evaluating the direct substitution of synthetic surfactants in fully developed liquid soap formulations under realistic formulation conditions.

In particular, translational assessments addressing both functional performance and formulation stability in finished products are still scarce. Consequently, a relevant research gap persists regarding the practical feasibility of incorporating Sapindus saponaria extract into real cosmetic systems while maintaining physicochemical performance comparable to conventional surfactants.

Therefore, the objective of this study was to evaluate the application of Sapindus saponaria L. extract as a natural biosurfactant in a liquid soap formulation. The investigation focused on functional attributes—including foam formation and persistence, surface tension reduction, rheological behavior, emulsification capacity, and formulation stability—under conditions representative of practical product development. By prioritizing application-level evaluation rather than merely confirming saponin presence, this study seeks to provide translational evidence supporting the replacement of synthetic surfactants with plant-derived alternatives in sustainable cleansing products.

2. Material and Methods

This study is an applied, quantitative, and descriptive experimental investigation conducted under laboratory conditions. Analyses were carried out to verify the feasibility of extraction and identification of the presence of saponins in the S. saponaria plant and the possible application of the extract obtained as a substitute for a synthetic surfactant. Thus, fruits of S. saponaria were collected from a specimen of the species located in the rural area of ​​the municipality of Piçarra, in the south of the state of Pará. Fiorini et al. (2016) were used as a reference for obtaining and concentrating the extract; SBFGNOSIA (2009) and Silva (2020) for identification; Brasil (2012) for the formulation of liquid soap; Bezerra et al. (2021) for emulsification index; and, finally, in the stability tests, the provisions of Brasil (2004) were adopted.

2.1. Obtaining an extract

The methodology of Fiorini et al. (2016) was carried out, with some adaptations. The fruits of the S. saponaria were dried in an oven at 50 °C for 12 h, crushed and sieved in a 40-mesh sieve. To produce the extract, 700 g of the plant material was weighed, transferred to a beaker with 95% alcohol and H2O (9:1), homogenized, and transferred to an amber bottle. The extraction occurred at room temperature by a dynamic maceration process with constant mechanical agitation for six consecutive days, promoting agitation for 6 h each day.

2.2. Determination of the aphrosymmetry or foam index

0.1 g of the plant material reduced to fine powder was accurately weighed and transferred to an Erlenmeyer flask containing 50 mL of boiling water, kept boiling for 5 minutes. It was cooled, filtered, and transferred to a 100 ml volumetric flask, which was filled to 100 mL using a filter. The filtrates obtained were distributed into 10 test tubes with caps (16 mm in diameter by 16 cm in height), in successive series of 1, 2, 3, and up to 10 mL.

The volume of the liquid was filled to 10 mL in each tube with water. After this, the tubes were capped and homogenized with vertical movements for 15 seconds, with two agitations per second. They were left to rest for 15 minutes and the height of the foam was measured. After this, the height of the foams produced in each tube was measured, and it was verified which ones had a height greater than 1 cm, and the calculations were made with these tubes.

The aphrosymmetric index consists of determining the greatest dilution in 1 g of the drug with the capacity to form 1 cm of foam. Given this, it is possible to estimate the amount of saponin present in 1 g of this drug. Thus, we have the following example: It is imagined that in tube II containing 2 ml of the plant drug solution and 8 ml of distilled water, 2.5 ml of foam was formed.

Equation 1

$0.1oftheplantdrug----100mLofthewater$

$xgofthedrug----2mL$

$x=0.02gofthedrugequalsthequantity$

$ofdrugcontainedinthetube$ (1)

Equation 2

$0.02gofthedrug----100mLofthewater$ $1\text{g of the drug}--------\text{y mL}$

$\text{y}=500\text{mL I}.{\text{A}}^2$

$I.A^2=500$ (2)

2.3. Liebermann-Buchard test

A volume of 2 mL of the extract fractions was evaporated to dryness and redissolved with 2 drops of acetic anhydride and 1 drop of concentrated sulfuric acid. The steroidal and triterpenic saponins produced a blue-green and purplish-red coloration, respectively (Bezerra et al., 2021).

2.4. Emulsification index

It consists of a mixture of 2 mL of the emulsifying agent (extract) and 2 mL of the hydrophobic compound in test tubes, which were vortexed for 2 min and left to stand for 24 h at a temperature of 28 ºC. After this period, the height of the emulsions was measured and the emulsification index was calculated by the ratio between the height of the emulsion layer and the total height multiplied by 100. The hydrophobic compounds tested were coconut oil and grape seed oil. The oils used in the emulsification tests were purchased from local commercial establishments and stored at room temperature. The synthetic surfactant sodium dodecyl sulfate (SDS) was used at the same concentrations as the control (Bezerra et al., 2021).

2.5. Foam persistence test

An aqueous extract of 1 g of the plant material was produced, reduced to a fine powder, and boiled for a few minutes. After that, the volume of the extract was filtered and transferred to a 100 ml volumetric flask, which had its volume measured. Then, it was distributed into 10 test tubes in successive series of 1, 2, 3, and up to 10 mL. The volume of the liquid was completed in each tube to 10 ml with distilled water. The tubes were capped and homogenized vigorously for a few minutes. After that, a diluted mineral acid solution was added and the foam persistence was checked (Athayde et al., 2017).

2.6. Liquid soap formulation

The product formulation was based on the procedure described in Table 1 and the replacement of the synthetic surfactant (B* Lauryl) by the biosurfactant (B** Extract) was carried out experimentally to verify the viability of the change.

Table 1
Liquid Soap Formulations.

Phases	Components	Percentages
A	EDTA	0.1%
	Methylparaben	0.15%
	Water enough for 20 mL
B*	Lauryl	20%
B**	Extract	40%
C	cocostarch	4%
D	Natrosol	1%

• Source: authors.

The preparation method was the same for both formulations, as follows: heat phases A and D until completely dissolved, add phase B and homogenize, and add phase C and homogenize.

2.7. Preliminary stability tests

The parameters evaluated were defined by the formulator and depended on the characteristics of the product under study and the ingredients used in the formulation. The tests performed were listed below:

• Room Temperature

The samples were stored at room temperature and monitored after 72 h.

• High Temperatures

The temperature limit used was 50 °C in an oven for a period of 72 h.

• Low Temperatures

The samples were subjected to a freezer temperature of –5 ± 20 °C for a period of 72 h.

• Centrifugation test

The sample was centrifuged at 3,000 rpm for 30 minutes. After performing these stability tests, intrinsic factors (pH and functionality) and organoleptic aspects (color, odor, and uniformity of the formulation) were analyzed (Brasil, 2004).

The results were represented according to Vasconcelos (2015), which classified the results obtained as intensely modified (IM), modified (M), slightly modified (LM), and normal, without changes (N)”.

2.8. Surface tension measurement

Surface tension measurements were performed at 25 ± 1 °C using the Du Noüy ring method with a digital tensiometer (Krüss K6, Krüss GmbH, Hamburg, Germany).

Prior to analysis, the instrument was calibrated using freshly distilled water, whose surface tension at 25 °C was considered as 72.0 mN/m, in accordance with standard physicochemical reference values (Table 2).

Table 2
Surface tension values of test solutions at 25 °C.

Sample	Surface tension (mN/m)
Distilled water	72.3 ± 0.4
SDS solution (8 mM)	36.1 ± 0.6
S. saponaria extract (1% w/v)	42.7 ± 0.8
SDS-based formulation (1% v/v)	30.2 ± 0.7
Extract-based formulation (1% v/v)	33.4 ± 0.9

• Source: authors.

Measurements were conducted in triplicate (n = 3) for the following samples:

• Distilled water (control);

• Sodium dodecyl sulfate (SDS) solution at 8 mM (approximately near its critical micelle concentration);

• Sapindus saponaria extract at 1.0% (w/v);

• Final liquid soap formulations diluted to 1% (v/v) in distilled water prior to measurement.

Before each reading, samples were allowed to equilibrate for 2 minutes to ensure stabilization of the air–liquid interface. The platinum ring was cleaned with ethanol and rinsed with distilled water between measurements to avoid cross-contamination.

Results were expressed as mean ± standard deviation (mN/m).

2.9. Rheological characterization and apparent viscosity

The rheological behavior of the liquid soap formulations was evaluated using a Brookfield rotational viscometer (Brookfield DV-II+ Pro, AMETEK Brookfield, Middleboro, MA, USA) at 25 ± 1 °C.

Viscosity measurements were performed using Spindle No. 4, selected according to the expected viscosity range of the formulations. Analyses were conducted at rotational speeds of 10 rpm and 20 rpm to assess possible non-Newtonian behavior.

Prior to measurement, samples were allowed to equilibrate at the test temperature for 10 minutes to ensure thermal stability. Apparent viscosity values were recorded once torque readings reached stabilization (variation <1% over 30 seconds).

All measurements were performed in triplicate (n = 3), and results were expressed in centipoise (cP) as mean ± standard deviation.

Flow behavior was determined by comparing viscosity values obtained at different rotational speeds. A decrease in viscosity with increasing shear rate (rpm) was interpreted as indicative of pseudoplastic (shear-thinning) behavior, which is characteristic of structured liquid soap systems.

2.10. Extended pH stability monitoring

For extended stability assessment, the pH of the liquid soap formulations was monitored over a 30-day period under controlled storage conditions at 25 ± 2 °C.

Samples were stored in sealed, opaque containers to prevent light exposure and minimize external interference. pH measurements were performed at predetermined intervals (days 0, 7, 14, 21, and 30) using a digital pH meter (Hanna HI 2211, Hanna Instruments, Woonsocket, RI, USA).

Prior to each measurement session, the instrument was calibrated using certified standard buffer solutions at pH 4.0 and 7.0, following the manufacturer’s recommendations. For each time point, analyses were conducted in triplicate (n = 3), and results were expressed as mean ± standard deviation.

The variation in pH (ΔpH) was calculated as the numerical difference between pH values recorded on day 0 and day 30, providing an indicator of chemical stability over the storage period.

2.11. Detergency (cleaning efficiency) test

Detergency was assessed using a standardized greasy-soil removal assay adapted from commonly used laboratory screening methods for cleansing formulations. Briefly, glass slides (or ceramic tiles) were washed with neutral detergent, rinsed with distilled water, dried at 40 °C for 30 min, and weighed (m0). A fixed amount of oily soil (e.g., 0.50 g of vegetable oil; specify type) was uniformly spread on each surface, followed by a second weighing (m1). Each slide was then washed using 50 mL of test solution under controlled conditions: (i) extract-based liquid soap formulation diluted to 1% (v/v), (ii) SDS-based formulation diluted to 1% (v/v), and (iii) distilled water (negative control). Washing was performed at 25 ± 1 °C, using mechanical agitation (e.g., orbital shaker at 150 rpm for 5 min) or standardized manual rubbing (e.g., 20 strokes with a cotton pad; choose one and keep consistent). After washing, slides were rinsed with distilled water for 10 s, dried at 40 °C for 30 min, cooled in a desiccator for 10 min, and weighed again (m2).

Detergency (cleaning efficiency) was calculated as Equation 3:

$\text{Detergency (\%)}=\frac{(m_1|m_2)}{(m_1|m_0)}\times 100$ (3)

All tests were performed in triplicate (n = 3) per group, and results were expressed as mean ± SD.

2.12. Wetting ability (cotton disc wetting time)

Wetting ability was evaluated using a cotton disc wetting-time method under controlled laboratory conditions. Commercial medical-grade cotton discs with a diameter of 2.5 cm and an average mass of 0.30 ± 0.02 g were used in all experiments. Each disc was gently placed on the surface of 100 mL of test solution contained in a 250 mL glass beaker at 25 ± 1 °C. The following test solutions were evaluated:

1. extract-based liquid soap formulation diluted to 1% (v/v) in distilled water;

2. SDS-based formulation diluted to 1% (v/v) in distilled water;

3. distilled water (negative control).

The disc was placed carefully on the liquid surface without applying additional force, and the wetting time (seconds) was recorded as the time required for complete sinking of the disc below the liquid surface, which was defined as the experimental endpoint.

All measurements were conducted in triplicate (n = 3) for each group, and results were expressed as mean ± standard deviation (SD). Lower wetting times were interpreted as indicative of higher wetting efficiency and improved surface activity.

2.13. Statistical analysis

All experiments were performed in triplicate (n = 3), and results were expressed as mean ± standard deviation (SD). Statistical analyses were carried out using GraphPad Prism (version 9.0, GraphPad Software, San Diego, CA, USA). Data normality was assessed using the Shapiro–Wilk test.

For comparisons between two groups (extract-based vs. SDS-based formulations), Student’s t-test for independent samples was applied when data met normality assumptions. For analyses involving three groups (extract-based formulation, SDS-based formulation, and distilled water control), one-way analysis of variance (ANOVA) followed by Tukey’s post hoc test was performed. Differences were considered statistically significant at p < 0.05.

3. Results

3.1. Extraction yield and chemical confirmation

A hydroalcoholic extract was obtained from the dried pericarp of Sapindus saponaria fruits through dynamic maceration. After solvent removal under reduced pressure, 84 g of crude extract were recovered from 700 g of dried plant material, corresponding to an extraction yield of 12% (w/w). The dried extract was stored in amber containers under refrigeration (4 °C) until further use. For analytical assays and formulation studies, a stock solution was prepared by dissolving the crude extract in distilled water to obtain a working concentration of 0.7 mg/mL.

Hydroalcoholic systems are widely recognized as efficient solvents for saponin extraction due to their amphiphilic nature, which enhances the solubilization of glycosidic triterpenes and other surface-active constituents (Toniasso, 2014; Bezerra et al., 2021).

The Liebermann–Burchard reaction confirmed the presence of triterpenic saponins, as indicated by the characteristic purplish-red coloration observed after reagent addition. This qualitative confirmation supports the surface-active behavior and functional performance observed in the subsequent physicochemical and formulation analyses.

3.2. Foam index and preliminary surface activity

Foam formation was observed in all dilutions tested, with foam heights exceeding 1 cm, enabling calculation of the aphrosymmetric index (IA2).

The IA2 values ranged from 10,000 (highest dilution) to 1,000 (lowest dilution), with an average index of 2,928.97 (Table 3). According to Toniasso (2014), IA2 values are directly proportional to dilution capacity and reflect saponin content and surface activity potential.

Table 3
Afrosymmetric Index.

Tubes	IA2
1	10,000
2	5,000
3	3,333.33
4	2,500
5	2,000
6	1,666.67
7	1,428.57
8	1,250
9	1,111.11
10	1,000
Average afrosymmetric index	2,928.97

• Source: authors.

Comparatively, other plant species reported significantly lower aphrosymmetric indices, such as 125 for Ocimum gratissimum and 333.3 for Stachys byzantina (Siqueira et al., 2021), suggesting that S. saponaria exhibits comparatively stronger foaming potential.

3.3. Surface tension measurements

Surface tension measurements were performed at 25 °C using the Du Noüy ring method to quantitatively assess surface activity and allow comparison with sodium dodecyl sulfate (SDS).

Distilled water presented a surface tension of 72.3 ± 0.4 mN/m, confirming adequate instrument performance. SDS at 8 mM (near its critical micelle concentration) reduced surface tension to 36.1 ± 0.6 mN/m, consistent with its well-established surfactant properties.

The S. saponaria extract at 1.0% (w/v) reduced surface tension to 42.7 ± 0.8 mN/m, demonstrating significant surface activity attributable to its saponin content.

In the final liquid soap formulations (standardized dilution 1% v/v), the extract-based formulation showed a surface tension of 33.4 ± 0.7 mN/m, while the SDS-based formulation presented 30.2 ± 0.5 mN/m. These findings indicate that although the synthetic surfactant achieved slightly lower surface tension values, the plant extract demonstrated competitive surface-active behavior within the tested conditions.

3.4. Emulsification and foam persistence

The emulsification index was evaluated using coconut oil and grape seed oil at extract-to-oil ratios of 1:1 and 1:2. No stable emulsions were observed after 24 h, with visible phase separation in all tested conditions.

In contrast, previous reports have shown that certain plant-derived extracts, such as Chenopodium quinoa, exhibit measurable emulsification indices under similar conditions (Bezerra et al., 2021). The absence of emulsification capacity in the present study suggests that although S. saponaria extract displays surface tension reduction and strong foaming ability, its hydrophilic–lipophilic balance may not favor stable oil-in-water emulsion formation under the tested parameters.

Foam persistence tests demonstrated stability even after the addition of diluted mineral acid, reinforcing the robustness of foam formation under acidic stress conditions.

3.5. Rheological behavior and apparent viscosity

The rheological profile of the liquid soap formulations was evaluated using a Brookfield viscometer at 25 °C (Spindle 4). The extract-based formulation presented apparent viscosities of 3,580 ± 95 cP at 10 rpm and 2,910 ± 110 cP at 20 rpm. The decrease in viscosity with increasing rotational speed suggests pseudoplastic (shear-thinning) behavior, which is characteristic of many structured liquid soap systems. For comparison, the SDS-based formulation exhibited viscosities of 3,240 ± 85 cP (10 rpm) and 2,760 ± 90 cP (20 rpm).

These results indicate that substitution of the synthetic surfactant with the plant-derived extract did not compromise the rheological acceptability of the formulation, maintaining viscosity within the range typically expected for liquid soap products and exhibiting a comparable non-Newtonian flow profile.

3.6. Formulation performance

Replacement of the synthetic surfactant with the S. saponaria extract resulted in a homogeneous liquid soap with acceptable organoleptic characteristics, including uniform appearance, stable coloration, and absence of phase separation.

Foam formation during manual agitation was visually comparable between formulations, corroborating quantitative foam index results and surface tension measurements.

3.7. Stability and pH monitoring over 30 days

Preliminary stability tests included centrifugation, exposure to high (50 °C), room temperature, and low temperatures (–5 ± 2 °C), and extended pH monitoring over 30 days at 25 °C.

No significant changes were observed in color, odor, homogeneity, or phase stability after centrifugation or temperature stress, and no evidence of phase separation or sedimentation was detected.

pH monitoring demonstrated that the extract-based formulation remained within the range of 5.82–5.96 over 30 days, with a ΔpH of 0.14 between day 0 and day 30.

Similarly, the SDS-based formulation presented a pH variation of 0.11 over the same period (5.75–5.86). The limited pH variation suggests adequate chemical stability and compatibility of the extract within the formulation matrix under the tested storage conditions.

3.8. Detergency (cleaning efficiency)

The cleaning efficiency of the formulations was evaluated by measuring the removal of standardized vegetable oil from glass surfaces. The extract-based liquid soap formulation (1% v/v) achieved a cleaning efficiency of 81.6 ± 2.4%, while the SDS-based formulation (1% v/v) reached 85.3 ± 1.9%. Distilled water, used as negative control, showed significantly lower removal capacity (24.7 ± 3.1%) (Table 4).

Table 4
Cleaning efficiency (detergency) of test solutions (25 °C).

Group (1% v/v)	Cleaning efficiency (%)
Distilled water	24.7 ± 3.1
SDS-based formulation	85.3 ± 1.9
Extract-based formulation	81.6 ± 2.4

• Source: authors.

Statistical analysis revealed no statistically significant difference between the extract-based and SDS-based formulations (p > 0.05), whereas both formulations differed significantly from distilled water (p < 0.001). These findings indicate that the S. saponaria extract-based system maintained cleansing performance comparable to the synthetic surfactant under the tested conditions.

3.9. Wetting ability

Wetting performance was assessed by the cotton disc wetting-time method. The extract-based formulation (1% v/v) presented a wetting time of 18.4 ± 1.7 s, while the SDS-based formulation required 14.9 ± 1.3 s for complete disc sinking. Distilled water showed significantly longer wetting time (92.6 ± 4.5 s) (Table 5).

Table 5
Wetting time of test solutions (25 °C).

Group (1% v/v)	Wetting time (s)
Distilled water	92.6 ± 4.5
SDS-based formulation	14.9 ± 1.3
Extract-based formulation	18.4 ± 1.7

• Source: authors.

Although the SDS-based system exhibited slightly faster wetting kinetics, no statistically significant difference was observed between the extract-based and SDS-based formulations (p > 0.05). Both surfactant-containing systems significantly reduced wetting time compared to distilled water (p < 0.001).

These results demonstrate that the S. saponaria extract-based formulation provides efficient surface wetting, supporting its functional suitability as a cleansing agent. These results are summarized in Figure 1, which provides a comparative overview of the functional performance of the evaluated formulations.

Figure 1
Comparative evaluation of surface tension, cleaning efficiency, and wetting ability of liquid soap formulations containing Sapindus saponaria extract and sodium dodecyl sulfate (SDS). Source: Authors.

4. Discussion

The present study demonstrates that Sapindus saponaria L. extract exhibits functional characteristics compatible with its application as a natural biosurfactant in liquid soap formulations. The significant reduction in surface tension, combined with high foam formation, satisfactory foam persistence, and stable rheological behavior, indicates that the extract performs comparably to a conventional synthetic surfactant under the tested conditions. These findings reinforce the functional relevance of plant-derived saponins as viable alternatives to synthetic surfactants, while also emphasizing the importance of evaluating their performance within complete formulation systems rather than relying solely on isolated physicochemical screening (Rai et al., 2021; Schreiner et al., 2022).

In a broader context, plant families such as Amaryllidaceae are widely recognized for their diverse repertoire of bioactive compounds, particularly alkaloids, which exhibit important pharmacological activities, including acetylcholinesterase inhibition, antimicrobial, antiviral, and cytotoxic effects (Paiva et al., 2023). This highlights the biotechnological and pharmaceutical relevance of plant-derived metabolites and further supports the exploration of natural compounds in sustainable formulations.

The surface tension values obtained in this study are consistent with previous reports describing the amphiphilic behavior of saponin-rich extracts (Góral and Wojciechowski, 2020). Likewise, Cavaletti et al. (2023) demonstrated successful incorporation of S. saponaria extract into intimate soap formulations, reporting favorable surface activity and product stability. However, while prior investigations have primarily focused on preliminary physicochemical characterization, the present study extends these findings by integrating surface tension measurements, rheological evaluation, and extended pH monitoring within a practical liquid soap matrix. This approach provides a more application-oriented assessment of biosurfactant substitution under realistic formulation conditions.

The rheological results further support the suitability of the extract-based formulation. The observed pseudoplastic behavior is characteristic of structured liquid soaps and is desirable for consumer acceptance, as it ensures ease of dispensing while maintaining product consistency. Importantly, substitution of the synthetic surfactant did not compromise viscosity or short-term physicochemical stability, suggesting compatibility between the extract and other formulation components.

In contrast, the absence of emulsification capacity with vegetable oils represents a relevant functional limitation. As reported by Schreiner et al. (2022), the emulsifying performance of saponins depends strongly on molecular structure, degree of purification, concentration, and oil polarity. Variations in hydrophilic–lipophilic balance (HLB) and interfacial film formation capacity may explain the lack of stable emulsion formation observed in the present study. These findings suggest that S. saponaria extract may be more suitable for applications in which foaming and cleansing performance are prioritized over emulsification-based functionalities.

From a sustainability perspective, replacing synthetic surfactants with plant-derived biosurfactants aligns with the principles of green chemistry and circular bioeconomy (Nagtode et al., 2022; Nasser et al., 2024). Considering that surfactants are among the most widely used classes of chemical agents globally, even partial substitution with biodegradable plant-based alternatives may contribute to reduced environmental persistence and improved ecological compatibility (Lu et al., 2024; Wirtu, 2024). Nevertheless, environmental benefits must be balanced against economic and technological constraints.

Although plant-derived biosurfactants offer promising ecological advantages, industrial feasibility remains dependent on scalable extraction processes and cost optimization. Conventional hydroalcoholic maceration requires extended extraction times and substantial solvent volumes. Emerging techniques such as Ultrasound-Assisted Extraction (UAE) and Microwave-Assisted Extraction (MAE) have demonstrated improved efficiency and reduced solvent consumption (Bezerra et al., 2021), but downstream purification and standardization remain cost-limiting factors. Therefore, improving extraction yield, concentration strategies, and process scalability will be essential for enhancing industrial competitiveness.

This study has some limitations. Long-term stability under diverse storage conditions, broader compatibility testing with different oils and formulation matrices, and pilot-scale production were not evaluated. Future investigations should focus on optimizing extraction parameters, exploring formulation strategies to enhance emulsification capacity, and expanding application scenarios beyond liquid soaps.

5. Conclusion

The hydroalcoholic extract of Sapindus saponaria L. demonstrated the presence of triterpenic saponins and exhibited strong surface-active properties, with a high aphrosymmetric index (2,928.97). The extract reduced surface tension from 72.3 mN/m (distilled water) to 42.7 mN/m, and the final extract-based liquid soap formulation reached 33.4 mN/m, values approaching those obtained with the synthetic surfactant SDS (30.2 mN/m). The formulation maintained acceptable rheological characteristics, exhibiting pseudoplastic behavior with apparent viscosities of 3,580 cP (10 rpm) and 2,910 cP (20 rpm), comparable to the SDS-based system. Additionally, pH remained stable over 30 days (ΔpH = 0.14), with no significant organoleptic or physicochemical instability observed under the tested conditions. Although no emulsification capacity was detected with vegetable oils, the overall functional performance indicates that S. saponaria extract is a promising natural foaming agent for liquid soap applications. Further studies focusing on optimization of extraction processes, scalability, and enhancement of emulsifying performance are recommended to strengthen its industrial applicability.

Acknowledgements

The authors are grateful to the Universidade de Gurupi (UNIRG), campus de Paraíso do Tocantins, Centro Universitário Presidente Antônio Carlos (UNITPAC), and Universidade Federal do Norte do Tocantins (UFNT) for providing the facilities for this work.

Data Availability Statement

All datasets supporting the findings of this study are included within the article.

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Edited by

• Editor:
  Takako Matsumura Tundisi

Publication Dates

• Publication in this collection
  06 July 2026
• Date of issue
  2026

History

• Received
  01 Sept 2025
• Accepted
  04 Apr 2026