Analysis of Foaming Properties, Foam Stability, and Basic Physicochemical and Application Parameters of Bio-Based Car Shampoos

Abstract

Environmental protection has become one of the key challenges of our time. This has led to an increase in pro-environmental activities in the field of cosmetics and household chemicals, where manufacturers are increasingly trying to meet the expectations of consumers who are aware of the potential risks associated with the production of cosmetics and household chemistry products. This is one of the most important challenges of today’s industry, given that some of the raw materials still commonly used, such as surfactants, may be toxic to aquatic organisms. Many companies are choosing to use natural raw materials that have satisfactory performance properties but are also environmentally friendly. In addition, modern products are also characterized by reduced consumption of water, resources, and energy in production processes. These measures reduce the carbon footprint and reduce the amount of plastic packaging required. In the present study, seven formulations of environmentally friendly car shampoo concentrates were developed, based entirely on mixtures of bio-based surfactants. The developed formulations were tested for application on the car body surface, allowing the selection of the two best products. For these selected formulations, an in-depth physicochemical analysis was carried out, including pH, density, and viscosity measurements. Comparison of the results with commercial products available on the market was also performed. Additionally, using the multiple light scattering method, the foamability and foam stability were determined for the car shampoos developed. The results obtained indicate the very high application potential of the products under study, which combine high performance and environmental concerns.

1. Introduction

The problem of environmental protection has become one of the most important challenges of modern times. Its scale and the growing dangers of environmental pollution have contributed to a significant increase in consumer awareness of the environment and the popularization of environmentally friendly products [1]. Progressively, consumers are looking for alternatives, such as pro-environmental products that are based on natural ingredients and thus friendly to both the user and the environment. This trend has prompted many companies to intensify their pro-environmental efforts, including the cosmetic and household chemical industries [2]. As a result, a dynamic development of natural raw materials can be observed, which provides an alternative option to commonly used synthetic substances, which are often harmful to the environment [3]. At the same time, the growth of innovative and environmentally friendly technological solutions is also observed. Modern products are characterized not only by more sustainable raw materials but also by a reduced consumption of water, reagents, and energy in their production cycle [4,5].

Among household chemicals, concentrated cleaning products, including shampoos for washing cars, are increasingly being used. This results in significantly lower water content compared to commonly used formulations. Moreover, the use of concentrated shampoos allows a reduction in the carbon footprint of the product. Additionally, it reduces the use of plastic packaging, transportation costs, and the need for storage space. This measure has important implications for the detergent industry, which, in response to consumer expectations, is increasingly guided by the canons of sustainability [5,6]. Surfactants are an indispensable element in the formulation of soap products, including shampoos. The proper selection of surface-active compounds significantly affects the physicochemical and application properties of the product, such as foamability [7,8]. Nowadays, mixtures of surfactants are widely used, allowing for improvement in the effectiveness of the product. Studies show that the use of mixtures of at least two surfactants leads to foams that are more efficient compared to formulations based on a single surfactant. Furthermore, by adjusting the concentration and the ratio of surfactants used, the optimization of the properties of the products, such as foaming, washing abilities, or ease of application, can be achieved [9]. In particular, the use of the right combination of surfactant mixtures can further positively influence the performance of the product by exhibiting synergistic effects, enhancing the performance of the entire formulation of the product [10,11].

Despite growing interest in environmental issues, synthetic surfactants are still widely used in many formulations of washing products. However, they show negative effects on the environment, especially on aquatic ecosystems, due to their characteristic features, such as high chemical stability and low biodegradability [12]. A particular threat of synthetic surfactants may come from their effect on microorganisms, which depends on the type of surfactant. Non-ionic surfactants, characterized by antimicrobial activity, can damage bacterial cells by interacting with structural and enzymatic proteins. Anionic surfactants show the ability to bind to DNA and bacterial enzymes and disrupt their function, while cationic surfactants interact directly with the cytoplasmic membrane, leading to its destabilization [13,14]. Consequently, in recent years, biosurfactants, which are alternatives to the commonly used synthetic surfactants, have gained popularity [15]. These are compounds naturally produced by plants, animals, and microorganisms [16]. Their main function is also to reduce the surface tension at the interface, but in many cases, they exhibit greater efficacy than their synthetic counterparts [17]. Biosurfactants are mainly characterized by their environmental harmlessness, as well as by better stability against ambient temperatures and high salinity. Therefore, they are increasingly being used by cosmetics and cleaning products producers, mainly in response to the demands of consumers, who are increasingly supporting the concept of sustainability in the industry [18,19].

For cleaning products such as shampoos, a foam plays an important role, both from the user’s point of view and from the product’s effectiveness. Foam is a dispersed system in which the gas phase, in the form of bubbles, is dispersed in a continuous liquid phase. The type of surfactant used and its concentration have a significant effect on the formation and properties of the resulting foam [20,21]. In the literature on foam properties, two key parameters are often pointed out, namely foamability and foam stability [8]. Foamability refers to the ability of a solution of a single surfactant or a mixture of them to form a foam, while stability describes the length of time the foam persists until it completely degrades. Both parameters are dependent on the adsorption properties of the surfactants, their concentrations used, and the method of foam formation [8]. Foam stability can be limited by the occurrence of different phenomena such as liquid drainage, Ostwald ripening, or bubble coalescence [20,21]. Depending on the type of surfactant used, a completely different effect can be obtained with regard to the stability of foam. In the case of non-ionic surfactants, foam is stabilized by steric repulsion. The reduction of surface tension and the suppression of bubble coalescence result from the accumulation of surfactant molecules in the surface layer of the bubbles [8]. It is most effective at high surface coverage of at least 90%. As a result, the foam obtained in its presence tends to be less persistent at lower concentrations but has more favorable application properties [22,23,24]. Ionic surfactants, on the other hand, stabilize foam through electrostatic repulsion, reducing the coalescence of foam bubbles and allowing foam volume to be maintained longer [25]. Understanding the mechanisms mentioned above allows the creation of more tailored formulations, depending on individual requirements for the product and customer expectations [26].

The formulation of car shampoo can be enhanced by the addition of co-surfactants, which typically exhibit amphoteric character. This amphoteric nature imparts greater tolerance to environmental variations, such as changes in pH. Co-surfactants not only reinforce the performance of the primary surfactant but also influence other key formulation properties. For instance, a reduction in micellar charge density resulting from co-surfactant addition contributes to decreased irritancy of the shampoo. Furthermore, co-surfactants can increase the viscosity of the formulation by promoting electrostatic interactions that alter micellar structure. The amphoteric character of these compounds also enhances foaming capacity and foam stability. The presence of both positively and negatively charged groups within the same molecule facilitates stronger interactions at the air–water interface, leading to a reduction in interfacial tension. This, in turn, promotes more efficient foam formation and improved stability [27].

Fatty alcohol ethoxylates (FAEs) are nonionic surfactants characterized by a hydrophilic segment composed of repeating ethylene oxide (EO) units terminating in a hydroxyl group. This structure enables their solubility in water without requiring ionization. FAEs are mainly produced from palm oil and other renewable sources. They are distinguished by excellent emulsifying properties, high biodegradability, low toxicity, low production cost, and strong compatibility with other surfactants, making them well-suited for use as co-surfactants. The critical micelle concentration (CMC) of FAEs is influenced by the length of their alkyl chains; FAEs with longer alkyl chains exhibit lower CMC values compared to those with shorter chains. This is due to stronger van der Waals interactions among the longer hydrophobic chains, which enhance micelle stability and facilitate its formation at lower surfactant concentrations. Conversely, surfactants with shorter alkyl chains have higher CMC values because the weaker hydrophobic interactions hinder micelle assembly. Additionally, an increased degree of ethoxylation raises the CMC by enhancing the molecule’s hydrophilicity, thereby requiring higher concentrations to achieve micelle formation [28].

Alkyl glucosides are a class of surfactants obtained through the condensation of long-chain fatty alcohols and glucose derived from renewable plant sources. They are also characterized by their ecological nature, low irritancy potential, and good wetting properties. Alkyl glucosides exhibit significantly lower CMC values compared to many traditional surfactants, contributing to their mildness and reduced skin irritation. Their applications have expanded beyond cosmetics to include industrial cleaning products and personal care formulations. Due to their biodegradability and favorable safety profile, alkyl glucosides are increasingly recognized as environmentally friendly and safer alternatives to conventional surfactants [29,30].

Taking all these aspects into consideration, the aim of our work is to develop environmentally friendly car shampoo formulations in concentrated form. To prepare these formulations, raw materials with a high degree of biodegradability were selected. Mixtures of bio-based surfactants were employed to ensure both the efficacy and sustainability of the resulting products. Seven bio-based car shampoo concentrates, formulated with biodegradable raw materials, were designed not only to be environmentally safe and non-hazardous to human health but also to demonstrate satisfactory cleaning and foaming performance.

All formulations were initially tested by application on the surface of the car body, which allowed us to specify two variants with the best foaming and washing properties. The selected shampoos and surfactant mixtures used in their preparation were subjected to physicochemical analysis, including measurement of pH, density, and viscosity, the results of which were compared with commercially available products. Furthermore, the foamability and stability of the foams were also evaluated. To our knowledge, no similar research results have been published to date that present physicochemical parameters, with particular emphasis on foam properties, and indicate the feasibility of using shampoos based solely on biodegradable raw materials for effective car body washing.

2. Materials and Methods

2.1. Reagents

A selected group of chemical reagents was used for the preparation of the car shampoos, as shown in

Table 1. Chemicals and their mixtures used during the formulation of car shampoos.

Reagent	Composition	Concentration [%]	Supplier
Mixture 1	D-pentose and D-glucose, oligomeric, C8-10-alkyl glycosides; D-pentose, oligomeric C10 and C12 alkylglycosides	18.0–30.0 12.0–24.0	WHEATOLEO, Pomacle, France
Mixture 2	D-pentose and D-glucose, oligomeric, C8-10-alkyl glycosides	40.0–44.0	WHEATOLEO, Pomacle, France
Mixture 3	D-pentose and D-glucose, oligomeric, C8-10-alkyl glycosides	58.0–62.0	WHEATOLEO, Pomacle, France
Mixture 4	D-pentose and D-glucose, oligomeric, C8-10-alkyl glycosides; glycine betaine ester	>45.0 >3.0	WHEATOLEO, Pomacle, France
Mixture 5	ethoxylated octyl alcohol; ethoxylated octan-1-ol	70.0–90.0 10.0–20.0	Croda Poland Sp z o.o., Krakow, Poland
Supporting substance 1	1-Propanaminium, 3-amino-N-(carboxymethyl)-N,N-dimethyl-, N-(C12-18(even numbered) acyl) derivatives, hydroxides, inner salts	30.0–60.0	Innospec, Cheshire, United Kingdom
Supporting substance 2	3-Methoxy-3-methylbutan-1-ol	≥98.0	Kuraray Co., Hattersheim am Main, Germany
Supporting substance a 3	Glycerol	>99.5	Brenntag Polska Sp. z o.o., Kedzierzyn-Kozle, Poland
Supporting substance 4	Reaction mass of trisodium salts of N,N-bis(carboxylmethyl)-(2S)-alanine and N,N-bis(carboxymethyl)-(2R)-alanine	***	BASF, Warszawa, Poland
Supporting substance 5	2-Phenoxyethanol	100.0	HSH Chemie, Warszawa, Poland
Supporting substance 6	Water		-
Supporting substance 7	Parfum		Dullberg, Hamburg, Germany
Supporting substance 8	Parfum		Iberchem S.A., Murcia, Alcantarilla, Spain
Supporting substance 9	C.I. Acid Violet 126 dye	70.0–90.0	Heubach GmbH, Frankfurt, Germany
Supporting substance 10	Sanolin Lave Green G liquid VP 5225 dye		Heubach GmbH, Frankfurt, Germany

***—data not available.

.

2.2. Preparation and Characteristics of Car Shampoos

Car shampoo formulations were developed and prepared in the laboratory of the Department of Applied Chemistry of the Faculty of Chemistry at Adam Mickiewicz University in Poznan. Each of the originally developed seven car shampoos was characterized by carefully selected raw materials that, according to the Organisation for Economic Co-operation and Development standardized methods (OECD 301 test) [31], show very high biodegradability within 28 days (90% or higher).

Briefly, all car shampoo formulations were prepared by mixing distilled water with a previously prepared surfactant mixture (Table 1 and

Table 2. Qualitative composition of the surfactant mixtures used in the car washing shampoos developed during the study.

Formulation	Mixtures of Surfactants *
	Mixture 1	Mixture 2	Mixture 3	Mixture 4	Mixture 5
Shampoo 1	+	-	-	+	-
Shampoo 2	+	+	-	-	-
Shampoo 3	-	+	-	+	-
Shampoo 4	-	-	+	+	-
Shampoo 5	-	-	+	-	+
Shampoo 6	-	+	-	-	+
Shampoo 7	+	-	-	-	+

* Chemical composition of mixtures is shown in Table 1. + mixture used in the formulation. - mixture not used in the formulation.

). Subsequently, the formulations were supplemented with auxiliary substances (Table 1) and thoroughly mixed again.

The formulations are based on fully biodegradable, non-ionic surfactants and are developed by focusing on a combination of appropriate detergent properties and environmental care. In addition, the car shampoos obtained were characterized by a significantly lower water content compared to standard formulations, which typically contain 50–90 wt.% of water. This resulted in the form of shampoo concentrates.

Several commercial car shampoos were also randomly selected to enable comparative analysis at later stages of the study. The content of the individual surfactant mixtures in the developed formulations is shown in Table 2. All tested formulations contained the same contribution of supporting substances and equal concentrations of surfactants, differing only in the type of surfactant mixture.

2.3. Application Tests of Prepared Car Shampoos

The application tests started with the preparation of a series of solutions of the tested shampoos, taking into account their nature as concentrates. In order to further evaluate the properties of the shampoos tested, two dilution variants were made: the first containing 20 cm³ of shampoo concentrate per 5 L of water, and the second with 40 cm³ of shampoo concentrate per 5 L of water. Each solution was stirred mechanically for a period of 2 min. To ensure comparability of results, application tests were performed on the same car with black paint, evenly covered with atmospheric pollutants and road grime. A different section of the car body was assigned to each shampoo to avoid overlapping application zones. The solutions were applied manually to the pre-wetted surface of the car, using a specialized sponge to facilitate even distribution of the product. After the pre-washing process, each of the shampoos was left on the paint surface for five minutes to check the tendency to leave traces resulting from water evaporation. The residual product was then rinsed with water, and the final effect was visually assessed. Analogous test procedures were used for both concentrations of shampoo solutions. Based on the analysis of the application properties, with the organoleptic evaluation of foaming, foam stability, and washing ability, the best two formulations of car shampoos were selected for further study.

2.4. Characterization of Physicochemical Properties of Car Shampoos

2.4.1. pH

The pH of the prepared products was tested at room temperature (25 °C). EcoSense^® pH 10 pH/Temperature Meter, Pen Style (VWR International, Radnor, PA, USA) was used for these tests. For each measured sample, the measurement procedure was repeated three times, and the arithmetic mean value was calculated along with the standard deviation from the results obtained.

2.4.2. Density

For density measurements, 5 cm³ of undiluted car shampoos were taken using a LABMATE Pro LMP 1000–10,000 µL automatic pipette (PZ HTL S.A., Warsaw, Poland). Samples were sequentially placed in a glass beaker located on a tared ONYX 220 analytical balance (FAWAG S.A., Lublin, Poland). Density measurements were performed in triplicate for each of the car shampoos tested. Based on the data obtained, the arithmetic mean and standard deviation values were calculated.

2.4.3. Viscosity

The viscosity of the products was also measured. An IKA ROTAVISC me-vi viscosity meter (IKA Poland Sp. z o.o., Warsaw, Poland), with a VOL-SP-6.7 measuring spindle (IKA Poland Sp. z o.o., Warsaw, Poland), was used to carry out the measurements. A total of 6.7 cm³ of shampoo was taken during sample preparation and analyzed in automatic mode. All measurements were made in triplicate at an ambient temperature of 23 °C. The arithmetic mean and standard deviation values were calculated for the obtained results.

2.4.4. Foamability

For foamability measurements, a 250 cm³ measuring cylinder was used, in which 50 cm³ of a 1 wt.% car shampoo solution was placed. The cylinder content was then manually shaken for a period of 30 s, with the inlet tightly covered. Shortly after the shaking process, the height of the resulting foam was measured, taking into account only the dense, stable foam layer. The measurements were carried out sequentially 5, 10, and 15 min after the end of the shaking process. For comparison, the tests were conducted using both distilled water (<1.0 mgCaCO₃/L [<0.1 °dH]; 0.06 μS/cm) and tap water (280 mgCaCO₃/L [16 °dH]; 690 μS/cm). For each of the shampoos tested, measurements were made in triplicate, the results were averaged, and the foam stability index (X_a) was calculated according to Equation 1 [32].

$$X_a=\mathrm{ }{\displaystyle \frac{V_2}{V_1}}\cdot 100\%$$

(1)

where X_a—average foam stability index [%]; V₁—initial foam height, measured immediately after the end of shaking [cm]; and V₂—foam height measured after 15 min [cm].

2.4.5. Foam Stability

Stability studies of the created foam were carried out using the multiple light scattering (MLS) method with the Turbiscan Lab Expert apparatus (Formulation, L’Union, France). The method is based on the multiple scattering of photons by particles suspended in the dispersion before the signal is recorded by a backscattered light detector. Changes in signal intensity over time reflect the degree of instability of the foam formed in the analyzed samples [33]. This analysis allows for the rapid and sensitive detection of destabilization mechanisms, such as the coalescence of foam bubbles. For measurement purposes, solutions containing 50 wt.% of the tested car shampoo concentrates were prepared with an IKA Yellow Line DI 25 Ultra Turrax basic homogenizer (IKA Poland Sp. z o.o., Warsaw, Poland) operating at a speed of 9500 rpm. The homogenization and simultaneous foaming process lasted 2 min for each sample. After this time, MLS measurements were immediately performed in the form of scans measured every 35 s for a total measurement period of 29 min and 45 s. This allowed a total of 51 scans to be obtained. The temperature of the measurement system was maintained between 27 °C and 29 °C.

2.4.6. Microbiological Tests

Microbiological tests of the two selected car shampoo formulations were conducted in accordance with relevant international standards to assess the presence and quantity of microorganisms in the developed products. The specific analyses performed are summarized in

Table 3. List of microbiological tests performed.

Test	Standardized Method
Presence of Staphylococcus aureus in 1 g	[34]
Presence of Escherichia coli in 1 g	[35]
Presence of Pseudomonas aeruginosa in 1 g	[36]
Presence of Candida albicans in 1 g	[37]
Count of aerobic mesophilic bacteria at 32.5 °C	[38]
Count of yeasts and molds at 25.0 °C	[39]

.

2.4.7. Contact Angle

Two car shampoo formulations (S01 and S02), along with three commercially available car shampoos, were selected for the study. Contact angle measurements were performed using a Krüss DSA100 goniometer (Krüss Optronic GmbH, Hamburg, Germany). The analysis was conducted on a single white lacquered car panel. All tests were carried out at room temperature using water as the test liquid.

3. Results and Discussion

3.1. Application Tests and Optimization of Final Formulations

During application tests, properties such as ease of application, foaming of the shampoo, stability of the foam produced, the ability to form so-called water spots (i.e., deposits formed on the surface of the car after water evaporation), as well as the washing abilities of the tested car shampoos were evaluated. The results for both shampoo concentrations tested are summarized in

Table 4. Results of the application test of car shampoos prepared with a dilution of 1:250 and 1:125.

Formulation	Dilution Factor	Properties Tested
		Ease of Application	Foamability	Foam Stability	Ability to Form Water Spots	Washing Abilities
1	1:250	-	-	-	+	+
2		-	-	-	+	-
3		+	+	-	+	+
4		-	+	-	+	-
5		+	+	+	-	+
6		+	+	+	-	+
7		+	-	-	-	+
1	1:125	-	-	-	+	+
2		-	-	-	+	-
3		+	+	+	+	+
4		-	+	-	+	-
5		+	+	+	-	+
6		+	+	+	-	+
7		+	-	-	-	+

+ signifies a positive result in the test. – signifies a negative result in the test.

.

Surprisingly, the double concentration of the shampoo did not significantly affect its properties in the context of the application. In both cases, comparable foam quality and similar overall evaluation values were obtained. This outcome may be attributed to the surfactant concentrations exceeding the critical micelle concentration (CMC). However, further experimentation is required to determine the exact CMC values of the surfactant mixtures used. Based on the analysis of the results and observations during testing, two formulations, namely formulations No. 5 and No. 6, were identified as having the most favorable application properties. The foam produced by both shampoos was thick (Figure 1), durable, and stable over time, outperforming the other formulations.

In addition, a high level of ease of use was noted during the application test—the shampoos were easily spread on the car surface, without the need to squeeze the solution from the sponge. The products could be applied without difficulties, providing a smooth sponge glide and a lack of streaks (Figure 2).

In contrast, a tendency was observed to form a light, oily film on the car body for formulations 1–4, which only disappeared after a more intensive spreading of the shampoo. These formulations also had a greater potential for the formation of traces of evaporated water (water spots), which was not observed for shampoos No. 5 and No. 6. This tendency may be due to the presence of oligomeric alkyl glycosides as the main surfactants in the product formulation. The emulsifying properties characteristic of this group of compounds probably contributed to the formation of a thin, slightly oily film on the surface of the car body after application of the product [40]. On the other hand, formulations containing ethoxylated alcohols, namely those labeled as No. 5 and No. 6, showed markedly improved application properties and a reduced tendency to form water spots.

3.2. Analysis of Basic Physicochemical Parameters

New labeling was adopted for the two top car shampoo formulations, which were selected for further testing based on the application tests shown in the previous section. Shampoos prepared according to recipes No. 5 and No. 6 were further identified as samples S01 and S02, respectively. Both formulations remained unchanged in terms of the composition of key ingredients but were enriched with appropriately selected fragrances and coloring agents. Shampoos and surfactant mixtures used in their preparation were tested for mean pH, density, and viscosity values (

Table 5. Results of the basic properties measurements performed for two developed car shampoos (S01–S02), the respective surfactant mixtures, and for the commercial shampoo formulations (S03–S25).

Sample	Parameter
	pH (±SD)	Density (±SD) [g/cm3]	Viscosity (±SD) [mPa·s]
Shampoo S01	7.00 ± 0.01	1.0081 ± 0.0054	26.5 ± 0.00
Mixture of surfactants S01	5.89 ± 0.00	1.0476 ± 0.0229	135.4 ± 0.00
Shampoo S02	7.24 ± 0.01	1.0142 ± 0.0133	22.3 ± 0.14
Mixture of surfactants S02	5.77 ± 0.01	1.0301 ± 0.0080	139.5 ± 0.00
S03	7.35 ± 0.08	0.9340 ± 0.0047	54.7 ± 0.00
S04	6.65 ± 0.13	0.9259 ± 0.0053	22.3 ± 0.14
S05	5.03 ± 0.03	1.0080 ± 0.0004	496.5 ± 6.18
S06	8.66 ± 0.06	0.9729 ± 0.0047	50.4 ± 0.63
S07	6.23 ± 0.07	0.9973 ± 0.0037	-
S08	4.00 ± 0.01	0.9968 ± 0.0066	90.1 ± 0.07
S09	7.94 ± 0.04	0.9721 ± 0.0061	210.8 ± 0.00
S10	7.05 ± 0.09	0.9432 ± 0.0056	243.0 ± 8.70
S11	9.64 ± 0.04	1.0277 ± 0.0049	-
S12	3.82 ± 0.05	0.9634 ± 0.0024	266.2 ± 94.30
S13	4.39 ± 0.06	1.0390 ± 0.0000	8194.0 ± 178.20
S14	4.87 ± 0.07	0.9898 ± 0.0069	82.3 ± 11.10
S15	6.75 ± 0.10	1.0070 ± 0.0007	243.9 ± 45.00
S16	7.43 ± 0.04	1.0122 ± 0.0073	440.0 ± 15.84
S17	3.42 ± 0.03	0.9537 ± 0.0062	491.2 ± 33.94
S18	6.82 ± 0.04	0.9694 ± 0.0064	77.3 ± 17.11
S19	6.79 ± 0.05	0.9911 ± 0.0087	104.9 ± 1.77
S20	6.66 ± 0.05	1.0433 ± 0.0021	-
S21	4.24 ± 0.03	0.9709 ± 0.0038	22.4 ± 0.14
S22	7.46 ± 0.03	1.0037 ± 0.0013	-
S23	6.58 ± 0.04	1.0054 ± 0.0015	-
S24	7.30 ± 0.04	1.0278 ± 0.0003	1751.0 ± 1.41
S25	7.12 ± 0.03	1.0071 ± 0.0009	-

). For both shampoo formulations, the hydrophilic-lipid balance (HLB) value calculated on the basis of the surfactant mixtures they contained was 13.3, which is within the typical range for detergent products (13–15) according to the Griffin scale. HLB is one of the key indicators determining the suitability of a surfactant for a particular application. It determines the tendency of a given surfactant to separate between the aqueous and oil phases, which is of direct relevance to the formulation of detergent-based products [41].

In addition, the results obtained were compared with selected commercial products available on the market, including some of them advertised as biodegradable products (samples S18, S22, and S24). The results of the analysis of commercial shampoos and their comparisons with the formulations developed during our study are shown in Table 5 and Figure 3. For six samples of commercial shampoos, namely S07, S11, S20, S22, S23, and S25, no viscosity analysis could be performed due to the too low values of this parameter, which were beyond the measurement range of the viscometer used and prevented a reliable measurement.

As a result of the analysis of physicochemical parameters, it can be concluded that the developed formulations of car shampoos showed the properties expected of this type of product. In pH measurements, both shampoos achieved values close to the neutral pH, especially sample S01, for which the pH value was exactly 7.00 (Figure 3a). On the contrary, some commercial products showed pH values that deviated from neutral levels. Shampoos S5, S8, S12, S13, S14, S17, and S21 showed markedly lower pH values, ranging from 3.42 to 5.03. These products have a noticeably acidic character, which is consistent with the information provided by the manufacturers. These lower pH values may be, among others, due to the presence in the formulation of additional pH-lowering substances, such as citric acid or lactic acid. At the same time, it is worth noting that none of the listed products is classified as readily biodegradable, further indicating the use of typical surfactants, which are probably not based on sustainable synthesis processes. In contrast, the highest pH values were recorded for the S6, S9, and S11 shampoos. Furthermore, for some of these products, these values were above the range declared by the manufacturers, which could be the result of, for example, the addition of other substances that raise the pH of the product and thus stabilize some components of the formulation. An example of a surfactant that is stable at high pH values (in the presence of alkali) and salinity is alkyl (or alcohol) ethoxy sulfate (AES) [42].

In the case of density measurements, the formulations developed were in the middle range of the list, covering products with a density slightly higher than 1 g/cm³ (Figure 3b), which should be considered a positive result, as these values are typical for concentrated products. It should be noted that the developed shampoo formulations did not contain any thickening systems that could significantly affect their density and viscosity.

In contrast, the developed shampoos were characterized by lower viscosity compared to most commercial products (Figure 3c). However, the values obtained do not negatively affect the quality assessment of the products and, in fact, reflect our desire to fit into the idea of more environmentally balanced formulations, where the desired physicochemical parameters can be achieved only by a specific surfactant mixture, without the use of unnecessary excipients, such as thickening agents. The low viscosity value can also contribute to an easier application of the shampoo to the surface of the car body, which significantly increases its application qualities.

3.3. Foamability of the Developed Car Shampoos

The results of the analysis of the foaming properties of the two car shampoos, namely S01 and S02, prepared according to the developed recipes, are summarized in

Table 6. Foamability and foam stability index for samples S01 and S02 in distilled water.

Sample/Measurement	Height of Foam Measured at the Specified Time Period [cm]				Foam Stability Index [%]
	0 min	5 min	10 min	15 min
S01/1	19.0	8.0	6.0	5.0
S01/2	21.0	12.0	7.5	6.0
S01/3	19.0	9.0	6.0	5.0
Average value (±SD)	19.7 ± 1.16	9.7 ± 2.08	6.5 ± 0.87	5.3 ± 0.58	26.9
S02/1	20.5	11.5	9.0	6.5
S02/2	19.0	10.5	8.0	5.5
S02/3	19.0	11.0	8.5	5.0
Average value (±SD)	19.5 ± 0.87	11.0 ± 0.50	8.5 ± 0.50	5.7 ± 0.76	29.2

and

Table 7. Foamability and foam stability index for samples S01 and S02 in tap water.

Sample/Measurement	Height of Foam Measured at the Specified Time Period [cm]				Foam Stability Index [%]
	0 min	5 min	10 min	15 min
S01/1	21.0	15.0	6.0	5.5
S01/2	22.0	15.0	7.0	5.5
S01/3	21.5	13.0	6.0	5.0
Average value (±SD)	21.5 ± 0.41	14.3 ± 0.94	6.3 ± 0.47	5.3 ± 0.24	24.8
S02/1	20.0	12.0	6.0	5.0
S02/2	22.0	15.0	7.0	4.5
S02/3	23.0	14.0	6.5	5.0
Average value (± SD)	21.7 ± 1.25	13.7 ± 1.25	6.5 ± 0.41	4.8 ± 0.24	22.3

. Based on the data, the average foam stability index was calculated according to Equation (1).

Figure 4 shows a comparison of the foam stability indexes calculated for developed car shampoos (samples 1–2) and selected commercial products (samples 3–25). Measurements were performed in distilled water.

As shown by the data presented in Table 6 and Table 7 and Figure 4, both formulations developed showed high foam stability in distilled water compared to their commercial counterparts. As expected, for the tests conducted in tap water, slightly lower foam stability indexes were observed, as foamability is strongly influenced by the hardness of water. Only four of the commercial products analyzed had higher foam stability, i.e., samples S08, S09, S14, and S15, for which the foam stability indexes were 63.00%, 52.38%, 41.60%, and 37.50%, respectively. Other commercially available car shampoos achieved lower values for this index, with the lowest foam stability observed for samples S19 (3.92%) and S06 (4.16%). Therefore, the results obtained confirm the high quality and stability of the foam produced by the shampoos prepared during our investigation, indicating their significant application potential, also in terms of market competitiveness.

3.4. Stability Characteristics of Foams in Prepared Car Shampoos

Additional information on the stability of the foams obtained for two new car shampoo recipes was obtained by the multiple light scattering (MLS) method, using the Turbiscan Lab Expert apparatus. As a result of the analysis, plots of the changes in transmission (T %) and backscattering (BS %) of light were obtained for each of the samples tested, i.e., S01 and S02, as shown in Figure 5.

These profiles present the unprocessed T % and BS % data for two car shampoo-derived liquid foams, plotted as functions of sample height (represented along the horizontal axis) over time (indicated by the color gradient of the curves, ranging from blue at 0 min to red at 29 min 45 s). The scans allow us to observe changes in the main parameters of foam, namely air bubble size, foam loss, and air bubble coalescence rate [33].

As can be observed in Figure 5, for both samples, the intensity of transmitted light increases with time near the bottom of the sample. It suggests that the foam starts to vanish with time due to the naturally occurring drainage process caused by gravity. Drainage occurs at the moment the foam is formed and leads to the thinning of the liquid film that separates the bubbles [43]. In the case of the tested car shampoo formulations, drainage occurred more slowly for sample S01, indicating a slightly higher stability of the foam formed. For sample S02, the phase separation process started earlier, which may indicate that the foam is less resistant to changes over time. Evolving peaks of T % are accompanied by decreasing BS % signals that become deeper and broader with time, indicating an increase in the size of the scattering species (bubbles). Furthermore, in the middle part of the sample, the backscattering signal decreases over time, which also corresponds to the increase in the size of bubbles. The lack of increased light transmission in the middle part of the sample indicates that the drainage process does not occur in this region, and all fluctuations in the foam stability should be connected with changes in the size of foam bubbles. It can result from Ostwald ripening of the foam, followed by coalescence of the liquid film at a later stage of measurement. Ostwald ripening arises in multi-bubble systems as a result of the pressure differential between bubbles. The driving force is the reduction in the interfacial energy, which, in turn, causes minimization of the total surface area of the bubbles. This phenomenon facilitates gas diffusion across lamellar structures and leads to gas redistribution followed by a shift in bubble size distribution [43]. Coalescence, on the other hand, is initiated when two bubbles approach within a critical proximity, leading to the rupture of the intervening lamella and subsequent merging. This process results in an increase in bubble size and is accompanied by a concurrent reduction in the total number of bubbles [44]. Similar observations were made by Kang et al. [45] and Delgado-Sánchez et al. [46], who tested the stability of foams enhanced with viscoelastic zwitterionic–anionic surfactants and tannin-based foams, respectively.

Ostwald ripening, usually followed by coalescence, is a phenomenon that is accelerated by decreasing film thickness. They result in collapse and coarsening of the foam, which can be observed in the middle section of the sample at measuring times of >13 min and >5 min for the S01 and S02 samples, respectively, indicating good foam stability of the tested systems (Figure 5). The evolution of the bubble size was also confirmed by changes in the average BS % over time, which is depicted in Figure 6b. Although a drainage process was observed in both samples, manifested by initially low T % values that increase over time (Figure 6a), the averaged backscattering values for both samples were in the range of 50%–60% (Figure 6b), which is characteristic of good quality foam.

Using the raw data, the Turbiscan Stability Index (TSI) can be calculated, which gives an insight into the global stability of the sample by summarizing all the variations of the potential destabilization. In general, the higher the TSI value at a given time, the worse the stability of the sample. Therefore, an increase in TSI values indicates that the foam becomes more unstable. Moreover, the faster the shift, the more rapidly the destabilization occurs. According to the TSI plots presented in Figure 7a, sample S02 shows slightly lower stability, as its TSI values are higher than in sample S01. It is in good agreement with the changes described above in the values of T % (Figure 6a), indicating that in the case of sample S02, the drainage process starts earlier.

The TSI destabilization profiles for the middle parts of samples S01 and S02 (Figure 7b,c) clearly show differences in their behavior with respect to the changes in the bubble sizes caused by the Ostwald ripening and coalescence phenomena. For sample S01, the TSI parameter in the middle part of the measuring cell is stable up to 13 min, while for sample S02, it starts to change rapidly just after five minutes. It represents different properties of the intervening lamella and outer film of bubbles created by the surfactants applied during the formulation of the car shampoos tested.

Despite using different mixtures of surfactants, both formulations showed quite similar properties of the foams obtained. This confirms the accuracy of the shampoo formulations developed in terms of their applicability. For both samples, namely S01 and S02, high foam volume and stability were observed, with the foam of sample S01 having slightly higher density and better stability over a measurement time of almost 30 min. For cleaning products such as shampoos, foam stability lasting more than 10 min can be considered a very satisfactory result.

Comparable foam behavior could be observed for selected commercial shampoos, for which light transmission, backscattering, and TSI profiles have been presented in Figures S1 and S2. This confirms the validity of the shampoo formulations we developed, as the appropriate selection of biodegradable ingredients enabled the creation of products with foam stability comparable to that achieved in formulations based on less environmentally friendly components, such as ionic surfactants, derived from non-renewable sources via conventional chemical synthesis.

3.5. Microbiological Analysis of the Developed Products

The results of the microbiological tests for the S01 shampoo are presented in

Table 8. Microbiological tests results for shampoo S01.

Test	Result
Presence of Staphylococcus aureus in 1 g	Absent
Presence of Escherichia coli in 1 g	Absent
Presence of Pseudomonas aeruginosa in 1 g	Absent
Presence of Candida albicans in 1 g	Absent
Count of aerobic mesophilic bacteria at 32.5 °C	<1.0 × 101 CFU/g
Count of yeasts and molds at 25.0 °C	<1.0 × 101 CFU/g

, and those for the S02 shampoo in

Table 9. Microbiological tests results for shampoo S02.

Test	Result
Presence of Staphylococcus aureus in 1 g	Absent
Presence of Escherichia coli in 1 g	Absent
Presence of Pseudomonas aeruginosa in 1 g	Absent
Presence of Candida albicans in 1 g	Absent
Count of aerobic mesophilic bacteria at 32.5 °C	<1.0 × 101 CFU/g
Count of yeasts and molds at 25.0 °C	<1.0 × 101 CFU/g

. Both formulations yielded identical results, with no detection of the pathogenic bacteria. These findings confirm that both formulations are free from harmful microorganisms that could pose a risk to users. Additionally, both shampoos exhibited very low levels of aerobic mesophilic bacteria, yeasts, and molds, well below the limits established by relevant safety standards. These results demonstrate a high level of microbiological safety, which is essential to ensure the products are suitable for consumer use and do not present a risk of microbial contamination.

3.6. Contact Angle of Car Shampoos

The selection of appropriate surfactants plays a crucial role in defining the contact angle and, consequently, the ability of the cleaning solution to wet and adhere to the treated surface. Formulations exhibiting lower contact angles demonstrate improved spreading behavior across the substrate, resulting in improved surface coverage and overall cleaning performance. In contrast, higher contact angles are associated with poorer wetting properties and reduced cleaning efficiency. The incorporation of amphoteric surfactants has been found to significantly reduce the contact angle, facilitating a more uniform distribution of the detergent on the surface. Additionally, amphoteric surfactants are associated with a lower potential for skin irritation, thereby enhancing user safety and comfort during application [47,48]. Therefore, contact angle measurements were conducted for the five selected car shampoos, and the corresponding results are shown in

Table 10. Results of contact angle measurements.

Shampoo Tested	Contact Angle [°]
S01	40.4 ± 0.3
S02	33.4 ± 0.5
S11	44.7 ± 2.6
S15	38.1 ± 0.5
S23	42.0 ± 2.2

. All measured values fell within the range of 0° to 90°, with each sample exhibiting contact angle values below 45°. The lowest contact angle was observed for shampoo S02. Notably, the proprietary formulations, which are based on highly biodegradable raw materials, demonstrated surface wetting properties comparable to those of commercial products containing synthetic ingredients. The observed contact angle values indicate good wetting behavior on lacquered surfaces, which likely contributes to the overall cleaning efficacy of the formulations.

4. Summary

In the first stage of this research, seven formulations of bio-based car shampoo concentrates were developed with the aim of combining their high effectiveness with environmental concerns. The formulations were characterized by the use of surfactant mixtures based entirely on biodegradable non-ionic surfactants. Additionally, the other raw materials used during formulation were carefully selected and had very high biodegradability values (of at least 90% over 28 days), as determined by the raw material manufacturers, based on OECD 301 test data and available in safety data sheets.

For the seven products, a comparative analysis was performed in terms of the application properties on the surface of the car body, with evaluation of properties such as ease of application, washing abilities, foaming properties, stability of the foam, and the ability to form so-called water spots. Based on the analysis of the results obtained, two formulations with the best application properties were selected and sent for further physicochemical tests, during which the pH, density, and viscosity values were evaluated and compared with a group of more than 20 commercially available car washing shampoos. The pH analysis showed a neutral character of the products, while the density and viscosity tests confirmed the lack of need for thickening systems to achieve satisfactory parameters for the formulations developed. Furthermore, the HLB values calculated for both tested washing systems were within the typical range for this type of product.

The values obtained for the foam stability index indicate that the two developed formulations showed high stability of the foam produced compared to commercially available products. In addition, in-depth tests of the stability of the foam generated for the two developed shampoos were carried out using the MLS technique. They showed that both products had fairly similar foam properties, for which satisfactory volume and stability were recorded. Only typical processes of washing products were observed, such as foam drainage at the initial test time, followed by evolving changes in the bubble sizes that result from Ostwald ripening and coalescence. The high values obtained for foam stability over time and the absence of undesirable phenomena causing rapid foam loss confirmed the correctness of the developed formulations of car shampoos.

Microbiological testing confirmed the high safety of the developed car shampoos. Both formulations were free of pathogenic microorganisms and exhibited very low levels of bacteria, yeasts, and molds, ensuring microbiological safety. The obtained contact angle values confirm that the tested formulations exhibit good wetting properties on lacquered surfaces, supporting their effective cleaning performance. Additionally, the shampoos formulated with biodegradable raw materials demonstrated wettability properties comparable to those of commercial products made with non-biodegradable ingredients.

The next stages of our scientific research on the developed bio-based car shampoos will focus primarily on evaluating their compatibility with packaging materials. In addition, application tests will be conducted to assess the impact of the formulations on the skin barrier function, complemented by consumer testing. Finally, biodegradability assessments of the developed formulations will be performed under aquatic conditions in accordance with OECD 301 standards.