Cascading One-Pot Synthesis of Biodegradable Uronic Acid-Based Surfactants from Oligoalginates, Semi-Refined Alginates, and Crude Brown Seaweeds

The present article describes a one-pot and cascade mode process using biocompatible/biodegradable reagents, for simply obtaining surfactant compositions comprising mixtures of d-mannuronic acid and l-guluronic acid directly from oligoalginates or semi-refined alginates (mixtures of alginate, cellulose, hemicellulose, laminaran, and fucan). Simple treatments of partial purification of the reaction crudes (elimination of the salts and/or the residual fatty alcohols) or isolation of the surfactant compositions result in sugar-based compounds having performance levels appropriate to applications in detergency. In addition, the challenging extension of this cascading one-pot synthesis technology to crude milled brown seaweeds was successfully carried out to provide promising surface-active compositions made up of alkyl uronate and alkyl glycoside monosaccharides.


Introduction
Currently, 100% biobased surfactants containing both a hydrophobic tail and a hydrophilic head of plant origin represent 5% of the total surfactants in the market worldwide [1]. This is the most dynamic segment, with growth of more than 20% per year and a high penetration rate in Europe. The introduction of increasingly strict regulatory standards relating to environmental impact and effects on human health should open up new opportunities for these products. On the other hand, the increased demand of consumers for more environmentally friendly products is leading industries to use surfactants or products certified by a label (Ecolabel, COSMOS) which guarantees compliance with this requirement.
Fully 100% biobased surfactants on the market mainly include products derived from sugars, which belong to the non-ionic family. They incorporate a glucose head for alkyl polyglucosides (APGs), sucrose for sucro-esters, and sorbitan for sorbitan esters [2][3][4]. Depending on the number of carbon atoms making up the lipophilic chain (4 to 22), these surfactants may have hydrotropic, foaming, degreasing, wetting, foam-and viscosityboosting, and emulsifying properties, and may lead to good sensory properties. Anionic sugar-based surfactants are present on the market to a much smaller extent compared with their non-ionic homologs. However, anionic surfactants are currently the most used types, being incorporated in the majority of detergent and cleaning-product formulas in daily use. Their most prominent representatives are linear alkylbenzene sulfonates (LAS), alcohol ether sulfates (AES), secondary alkane sulfonates (SAS), and alcohol sulfates (AS)

Preparation of Starting Materials
Poly(oligo)mannuronates and poly(oligo)guluronates are attractive raw materials for the development of original 100% biobased surfactants. They are obtained from fresh or dry algae derived from Laminaria digitata using a process based on a pre-extraction of the alginates, followed by several steps of precipitation by modulating the pH of the reaction medium in order to separate the G blocks and the M blocks constituting the alginate [16,17]. Finally, an acid hydrolysis step affords the oligomannuronate (DP = 4: Table 1) or oligoguluronate (DP = 30: Table 1).  [16,17], oligoalginate (OAlg), and semi-refined alginates (s-r Alg) [18].

Preparation of Starting Materials
Poly(oligo)mannuronates and poly(oligo)guluronates are attractive raw materials for the development of original 100% biobased surfactants. They are obtained from fresh or dry algae derived from Laminaria digitata using a process based on a pre-extraction of the alginates, followed by several steps of precipitation by modulating the pH of the reaction medium in order to separate the G blocks and the M blocks constituting the alginate [16,17]. Finally, an acid hydrolysis step affords the oligomannuronate (DP = 4: Table 1) or oligoguluronate (DP = 30: Table 1).  [16,17], oligoalginate (OAlg), and semi-refined alginates (s-r Alg) [18]. Concerning oligoalginate (OAlg) and semi-refined alginate (s-r Alg) [18], the procedure involves acid leaching of fresh or dry algae derived from Laminaria digitata, followed by dissolution of the sodium alginates by increasing the pH of the medium. Then, a solid/liquid separation is performed in order to remove the algal residues. At this stage, the liquid fraction can be freeze-dried and constitutes the semi-refined alginates (Table 1) in the form of sodium alginates. In order to obtain refined oligoalginates, a purification step is introduced into the previous steps. After separation of the algal residues, the latter purification step includes or consists of precipitation of the alginic acid by reducing the pH, followed by several washes with acidic water in order to remove the co-products. Increasing the pH with Na 2 CO 3 makes it possible to dissolve the sodium alginates again while limiting the salt, compared with the use of sodium hydroxide. Finally, the alginate solution is treated with acid in order to reduce the degree of polymerization, which produces, after a step of freezing and then freeze-drying, the oligoalginate of DP 12.7 (Table 1).

Synthesis from Oligoalginates
In order to achieve the goal of synthesizing novel biobased uronic surfactants from alginates, several uronate sources have been investigated to assess their potential as biomass feedstock for the preparation of such amphiphilic molecules through green synthetic pathways. At first, oligomannuronate (OM) and oligoguluronate (OG) were used in order to define the optimal reaction conditions for the one-pot cascade synthesis of uronate monomeric surfactants. Then the optimized conditions were transposed to oligoalginate with a M/G ratio of 1.4 and a DP of 12.7.

Synthesis from Oligomannuronate (OM)
In the first stage of the study, the reactivity of sodium oligomannuronate (M/G = 2.7, DP = 4, mass = 500 mg) was investigated towards acid hydrolysis, esterification, and Fisher glycosidation conditions. Acid treatment of OM was performed for 7 h with various amounts of methane sulfonic acid (MSA) in the presence of water and butanol (Scheme 1). Based on previous results obtained in the laboratory [16], the volume of butanol was set at 25 mL. The use of larger quantities of butanol did not allow a significant improvement of the yield while presenting a problem of oversizing the production facilities. However, whereas the reactions were previously envisaged in dry butanol [16], in these new experiments it was found that the solubility of the oligomannuronate was low, thus preventing the reactions from carrying out. The addition of water at a rate of 1.7-2 mL/g of algal extract proved to be sufficient for the solubilization of the oligomannuronate in the reaction mixture. The butanol was distilled out during heating and the water initially added to the reaction mixture and formed during the reaction was eliminated using a Dean-Stark apparatus (140 • C). After neutralization and workup, the organic residue was purified using silica gel column chromatography to isolate fractions enriched in a few compounds, which facilitated the identification of the glycoside-esters present in the mixture by 1 H NMR studies (Figures S1-S4, Supplementary Materials). Indeed, 1 H NMR analysis led to the identification of characteristic signals for each compound, as shown in Table 2. Thus, the formation of (n-butyl) n-butyl α-Dmannopyranosiduronate 1α was observed as the major monosaccharide product (52 mol%, isolated as a pure fraction, 33% yield, Table 3) in addition to a mixture of isomers: (n-butyl) n-butyl α-D-mannofuranosiduronate 2α (7 mol%), n-butyl α-D-mannofuranosidurono-6,3lactone 3α (11 mol%), and n-butyl β-D-mannofuranosidurono-6,3-lactone 3β (13 mol%). Products from the guluronate units present in smaller quantities in the raw material (M/G ratio = 2.7) were also identified, i.e., n-butyl-α-L-gulofuranosidurono-6.3-lactone 4α (5 mol%), n-butyl-β-L-gulofuranosidurono-6.3-lactone 4β (5 mol%), and (n-butyl) nbutyl-β-L-gulofuranosiduronate 5β (3 mol%). Alongside these uronate derivatives, a side product 6 was isolated, butyl-5-(dibutoxymethyl)-2-furoate (BDMF) (5 mol%), and it was Molecules 2023, 28, 5201 5 of 25 characterized by NMR (Scheme 1). This furan compound results from the dehydration of mannuronate/guluronate esters and lactone monomers (named butyl uronates) as reported in earlier studies [19]. The total amount of the butylated Man and Gul derivatives 1-5 is quite satisfactory but it was noticed that the increase in MSA quantity would further degrade the butyl uronates ( Table 3). The overall yield could not be calculated as the isomers possess different molecular weights. It should be noted that 99% or 70% MSA can be used indiscriminately since the same results were obtained in both cases.
same results were obtained in both cases.  Next, seeing the possible transformation of oligomannuronates into monomeric derivatives, the optimal conditions (1.7 eq MSA, BuOH (25 mL), H 2 O (1.7-2 mL/g of crude OM), 140 • C, 7 h) were reproduced and the reaction mixture containing the butyl ester glycoside products 1-5 was subjected directly to simultaneous transesterification and transglycosylation with dodecanol (DodOH). Then the replacement of the butyl chains by the C 12 chains was performed in situ (Scheme 2) under reduced pressure (5 mbar) at 70 • C for 2 h 10 m after the addition of an extra equivalent of MSA (99% or 70%). The reduced pressure allows the removal of the butanol and shifts the equilibrium towards the transesterification and transglycosylation products. A variety of reaction conditions with different quantities of dodecanol and MSA were envisaged (Table 4). After a workup of the medium, two successive purifications by column chromatography were performed. The first one allowed the isolation of the mixture of uronate monomers and the second one aimed at obtaining fractions enriched in a few compounds, which facilitated the identification of the glycoside-esters present in the mixture by 1 H NMR studies. After those purification steps, (n-dodecyl) n-dodecyl α-D-mannopyranosiduronate 7α was isolated in addition to several fractions containing mixtures of isomers corresponding to (n-dodecyl) n-dodecyl α-D-mannofuranosiduronate 8α, n-dodecyl α,β-Dmannofuranosidurono-6,3-lactone 9α,β, n-dodecyl-α,β-L-gulofuranosidurono-6.3-lactone 10α,β, (n-dodecyl) n-dodecyl-α,β-L-gulopyranosiduronate 11α,β, and (n-dodecyl) n-dodecylβ-L-gulofuranosiduronate 12β (Scheme 2) ( Figures S5-S8 Next, seeing the possible transformation of oligomannuronates into monomeric derivatives, the optimal conditions (1.7 eq MSA, BuOH (25 mL), H2O (1.7-2 mL/g of crude OM), 140 °C, 7 h) were reproduced and the reaction mixture containing the butyl ester glycoside products 1-5 was subjected directly to simultaneous transesterification and transglycosylation with dodecanol (DodOH). Then the replacement of the butyl chains by the C12 chains was performed in situ (Scheme 2) under reduced pressure (5 mbar) at 70 °C for 2 h 10 m after the addition of an extra equivalent of MSA (99% or 70%). The reduced pressure allows the removal of the butanol and shifts the equilibrium towards the transesterification and transglycosylation products. A variety of reaction conditions with different quantities of dodecanol and MSA were envisaged (Table 4). After a workup of the medium, two successive purifications by column chromatography were performed. The first one allowed the isolation of the mixture of uronate monomers and the second one aimed at obtaining fractions enriched in a few compounds, which facilitated the identification of the glycoside-esters present in the mixture by 1 H NMR studies. After those purification steps, (n-dodecyl) n-dodecyl α-D-mannopyranosiduronate 7α was isolated in addition to several fractions containing mixtures of isomers corresponding to (n-dodecyl) n-dodecyl α-D-mannofuranosiduronate 8α, n-dodecyl α,β-D-mannofuranosidurono-6,3lactone 9α,β, n-dodecyl-α,β-L-gulofuranosidurono-6.3-lactone 10α,β, (n-dodecyl) n-dodecyl-α,β-L-gulopyranosiduronate 11α,β, and (n-dodecyl) n-dodecyl-β-L-gulo-   The comparison of entries 1 and 2 (Table 4) indicates that the use of dodecanol in large excess does not improve the yield. Finally, the comparison of entries 2 and 3 shows that the decrease in the number of equivalents of MSA introduced for the second step disadvantages the formation of 7α. The reaction conditions of entry 2 (4 eq DodOH, 1 eq MSA) that provided dodecyl mannuronate 7α with an overall yield of 25% were thus selected. The molar composition of the mannuronate and guluronate mixture was determined by integrating the characteristic 1 H NMR signals of each compound of the uronate mixture (Table 5) resulting from the first column chromatography ( Figures S5 and S6, Supplementary Materials): (n-dodecyl) n-dodecyl α-D-mannopyranosiduronate 7α (48 mol%), (n-dodecyl) n-dodecyl α-D-mannofuranosiduronate 8α (9 mol%), n-dodecyl α-D-mannofuranosidurono-6,3-lactone 9α (5 mol%), n-dodecyl β-D-mannofuranosidurono-6,3-lactone 9β (22 mol%), n-dodecyl-α-L-gulofuranosidurono-6.3-lactone 10α (3 mol%), n-dodecyl-β-L-gulofuranosidurono-6.3lactone 10β (3 mol%), (n-dodecyl) n-dodecyl-β-L-gulopyranosiduronate 11β (5 mol%), and (n-dodecyl) n-dodecyl-β-L-gulofuranosiduronate 12β (6 mol%). It is noteworthy that (n-dodecyl) n-dodecyl-α-L-gulopyranosiduronate 11α was not detectable in this mixture due to its presence in quantities that were too small. The next challenge was to develop a one-pot three-step cascade for the synthesis of uronate surfactants possessing a single alkyl chain (Scheme 3). For this purpose, saponification (0.4 N NaOH, 3 eq, 70 • C, 1 h) was carried out directly in the reaction mixture obtained after the transesterification/transglycosylation step. Removal of dodecanol was performed at the end of the process through the removal of water by freeze-drying and the addition of EtOAc into the reaction medium followed by a filtration step. A precipitate composed of the uronate derivatives and additional salts was isolated whereas the filtrate contained the entire fatty alcohol. This solid fraction containing the desired products consists of 96-97% dry matter and 36-37% mineral matter for repeated runs (thermogravimetric analyses). The large amount of mineral matter comes from the raw material used, which already contained it (31.9% dry/crude), and from the NaOH used for the saponification reaction. As the significant amount of mineral matter present in the sodium uronate composition is likely to modify the properties of the surfactants, a purification method allowing for their removal was achieved through liquid-liquid extraction using a 1N HCl aqueous phase and EtOAc. After concentrating the organic phase under reduced pressure, the mannuronate and guluronate surfactants were isolated as carboxylic acids and lactones. 1 H NMR analysis revealed the presence of n-dodecyl α-D-mannopyranosiduronic acid 13α (47 mol%), n-dodecyl α-D-mannofuranosidurono-6,3-lactone 9α (4 mol%), n-dodecyl β-Dmannofuranosidurono-6,3-lactone 9β (12% molar), n-dodecyl β-L-gulopyranosiduronic acid 14β (4 mol%), n-dodecyl α-L-gulofuranosidurono-6,3-lactone 10α (9 mol%), n-dodecyl β-L-gulofuranosidurono-6,3-lactone 10β (9 mol%), and n-dodecyl-β-L-gulofuranosiduronic acid 15β (4 mol%), in addition to a non-identified product X (~10 mol%) which could correspond to n-dodecyl D-mannofuranosiduronic acid (Figures S9 and S10, Supplementary Materials). 1 H NMR analysis led to the identification of characteristic signals for each compound, as shown in Table 6. The surface tension measurements obtained with this H-C 12 Man composition are presented in Section 2.5.

Synthesis from Oligoguluronate (OG)
As the alginate biomass selected either oligoalginates or semi-refined alginates, which are composed of both mannuronate and guluronate units, the previously developed synthetic pathway had to be tested from oligoguluronate (OG) as well. It is noteworthy that more MSA (2.2 eq) was required (Scheme 4) for OG, probably due to the α-binding between guluronate units and/or their higher DP. The same workup based on a solid-liquid extraction step with EtOAc was performed to eliminate dodecanol. This uronate-based mixture was then acidified and purified by liquid-liquid extraction in order to obtain the H-C 12 derivatives and remove the salts. 1 H NMR analysis revealed the presence of the following products: n-dodecyl α-D-mannopyranosiduronic acid 13α (15 mol%), ndodecyl β-L-gulopyranosiduronic acid 14β (13 mol%), n-dodecyl α-L-gulofuranosidurono-6,3-lactone 10α (26 mol%), n-dodecyl β-L-gulofuranosidurono-6,3-lactone 10β (26 mol%), and n-dodecyl-β-L-gulofuranosiduronic acid 15β (21 mol%). The presence of n-dodecyl α,β-D-mannofuranosidurono-6,3-lactone 9α,β was also observed in trace amounts but its quantification was not possible, unlike in the case of synthesis from oligoalginate (See Section 2.2.3). 1 H NMR analysis led to the identification of characteristic signals for each compound, as shown in Table 7 (Figures S11 and S12, Supplementary Materials). The surface tension measurements obtained with this H-C 12 Gul composition are presented in Section 2.5. mol%), and n-dodecyl-β-L-gulofuranosiduronic acid 15β (21 mol%). The presence of ndodecyl α,β-D-mannofuranosidurono-6,3-lactone 9α,β was also observed in trace amounts but its quantification was not possible, unlike in the case of synthesis from oligoalginate (See Section 2.2.3). 1 H NMR analysis led to the identification of characteristic signals for each compound, as shown in Table 7 (Figures S11 and S12, Supplementary Materials). The surface tension measurements obtained with this H-C12 Gul composition are presented in Section 2.5. Scheme 4. One-pot and cascade-mode synthesis of the H-C12 Gul-based surfactant and its composition.

Synthesis from Oligoalginate (OAlg)
As the process has been shown to be applicable to both D-mannuronate and L-guluronate units, the same synthetic pathway has been tested with oligoalginate (OAlg) composed of D-Man and L-Gul units with the same reaction conditions used for oligoguluronate (OG) (Scheme 5). The OAlg starting raw material is economically advantageous as less purification is required in comparison to OM and OG. The mixture obtained after the Scheme 4. One-pot and cascade-mode synthesis of the H-C 12 Gul-based surfactant and its composition.

Synthesis from Oligoalginate (OAlg)
As the process has been shown to be applicable to both D-mannuronate and Lguluronate units, the same synthetic pathway has been tested with oligoalginate (OAlg) composed of D-Man and L-Gul units with the same reaction conditions used for oligoguluronate (OG) (Scheme 5). The OAlg starting raw material is economically advantageous as less purification is required in comparison to OM and OG. The mixture obtained after the saponification step contains 93.4% dry matter and 41.8% mineral matter. The higher percentage of mineral matter compared to oligomannuronate and oligoguluronate is explained by the higher mineral content of the raw material (44.3% dry/crude). Further acidic treatment of the reaction medium was performed as developed for the OM and OG raw materials (Scheme 5). The final H-C 12 OAlg surfactant composition is characterised by the presence of n-dodecyl α-D-mannopyranosiduronic acid 13α (23 mol%), n-dodecyl α-D-mannofuranosidurono-6,3-lactone 9α (4 mol%), n-dodecyl β-D-mannofuranosidurono-6,3-lactone 9β (9 mol%), n-dodecyl β-L-gulopyranosiduronic acid 14β (9 mol%), n-dodecyl α-L-gulofuranosidurono-6,3-lactone 10α (20 mol%), n-dodecyl β-L-gulofuranosidurono-6,3-lactone 10β (20 mol%), and n-dodecyl-β-L-gulofuranosiduronic acid 15β (9 mol%) ( Table 8). The presence of the non-identified product X (6 mol%) formed during the onepot synthesis of the H-C 12 Man composition was also observed. 1 H NMR analysis led to the identification of characteristic signals for each compound, as shown in Table 8 (Figures S13 and S14, Supplementary Materials). The surface tension measurements obtained with this H-C 12 OAlg composition are presented in Section 2.5.

Synthesis from Semi-Refined Alginate (s-r Alg)
In order to further reduce the cost of surfactant production, the process was applied to semi-refined alginate (s-r Alg) extracted from the Laminaria digitata species composed of D-mannuronate and L-guluronate units in addition to neutral L-fucose, D-glucose, and D-xylose sugars. The first step of the synthetic scheme developed for oligosaccharides could not be directly applied. Indeed, modifications had to be made to take into account Scheme 5. One-pot and cascade-mode synthesis of H-C 12 OAlg surfactant composition.

Synthesis from Semi-Refined Alginate (s-r Alg)
In order to further reduce the cost of surfactant production, the process was applied to semi-refined alginate (s-r Alg) extracted from the Laminaria digitata species composed of D-mannuronate and L-guluronate units in addition to neutral L-fucose, D-glucose, and D-xylose sugars. The first step of the synthetic scheme developed for oligosaccharides could not be directly applied. Indeed, modifications had to be made to take into account the low solubility of the mixture in butanol due to the high degree of polymerization of the polysaccharides. Thus, the synthesis of the butyl C 4 -C 4 derivatives was divided into two steps. First, 2 g of semi-refined alginate containing 810 mg of saccharidic materials was dispersed in water (60 mL) under reflux in the presence of acid (MSA, 5 eq) to reduce the degree of polymerization. After 8 h reflux, butanol (60 mL) was added, and the Dean-Stark set-up was used to allow the removal of water by azeotropic distillation (15 h). The overall reaction time (23 h) and the quantity of MSA (5 eq) were increased to optimize the conversion rate (Scheme 6). Then, transglycosylation and transesterification reactions with dodecanol were carried out in the same pot. The influence of the MSA amount added in the second step was studied. It was found that a better yield was obtained when no additional acid was added. Indeed, with more MSA, there were more degradation products to be formed. As for the oligosaccharides, the one-pot saponification step was achieved with the mixture resulting from the second step. The concentration of the NaOH solution used was decreased (0.2 N NaOH, 3 eq, 70 • C, 1 h) in order to have a larger quantity in the aqueous phase and thus a better dispersion of the organic phase. Water was then eliminated by freeze-drying. Solid-liquid extraction with EtOAc allowing the removal of dodecanol was evaluated from this raw material. Unfortunately, a loss of n-dodecyl α,β D-glucopyranosides 16 and n-dodecyl fucosides derivatives 17,18 was observed. As these compounds are not in salt form like the uronate derivatives, they were solubilized by EtOAc and carried along with the dodecanol in the filtrate. In order to limit the loss of these products of interest, other solvents were tested: isopropanol, anisole, acetone, methyl isobutyl ketone, methyl ethyl ketone, 2-methyltetrahydrofuran, heptane, cyclohexanone, and acetonitrile. The only solvents that did not allow the non-ionic derivatives to be lost completely were acetone and acetonitrile. The best results were obtained with the latter. However, in the interest of environmental compatibility, acetone was finally chosen. The isolated mixture contains 98.5% dry matter and 40.7% mineral matter. At this stage, due to the difficulty of potential scale-up and the risk of emulsion formation, an alternative method for the final purification was developed. The mixture obtained after solid-liquid extraction with acetone was first dissolved in ice-cold water and then acidified with an oxalic acid solution to a pH of about 2. After concentration by freeze-drying, the mixture was taken up in acetone. The products were solubilized while the salts precipitated. After filtration, the products were recovered in the filtrate. Oxalic acid was chosen for its eco-compatibility and its pKa 1 of 1.25, which allows the sodium carboxylate function of the uronate derivatives (pKa ≈ 4) to be protonated without the risk of also protonating the sodium methane sulfonate (pKa = −1.92). It is also to avoid finding methane sulfonic acid in the filtrate that the pH of the aqueous solution should not be decreased below 2. This cascading one-pot method with a final purification step is applicable on an industrial scale and it allows the elimination of fatty alcohols in addition to salts. 1 H NMR analysis of the H-C 12 s-r Alg composition revealed the presence of the following products (Table 9): n-dodecyl α-D-mannopyranosiduronic acid 13α (23 mol%), n-dodecyl α-D-mannofuranosidurono-6,3-lactone 9α (5 mol%), n-dodecyl β-Dmannofuranosidurono-6,3-lactone 9β (13 mol%), n-dodecyl β-L-gulopyranosiduronic acid 14β (11 mol%), n-dodecyl β-L-gulofuranosidurono-6,3-lactone 10β (20 mol%), n-dodecylα-L-fucopyranosides 17α (5 mol%), n-dodecyl-β-L-fucopyranosides 17β (8 mol%), and n-dodecyl-α,β-L-fucofuranosides 18α,β (15 mol%) (Figures S15 and S16, Supplementary Materials). 1 H NMR analysis of the C 12 s-r Alg composition did not allow the identification of signals corresponding to n-dodecyl α,β D-glucopyranosides 16. Furthermore, a significant amount of n-dodecyl α,β D-glucopyranosides 16 and n-dodecyl fucosides derivatives 17,18 was solubilized in acetone which means that the quantity of alkyl glycosides present in the final surfactant compositions has decreased significantly following the purification steps.

Synthesis from Crude Brown Seaweeds
The final challenge was to extend the cascading one-pot synthesis technology developed from oligo-and polysaccharide alginates to commercial milled brown seaweeds rich in alginate and fucoidan polysaccharides (Asco T10 from the Ascophyllum nodosum species, Thorverk, Iceland). Indeed, the use of such crude seaweeds could contribute to lowering the production cost of surfactant compositions due to the reduced price of these raw materials compared to refined or semi-refined polysaccharides and to a simplification of the process which no longer requires the prior extraction of alginates.
The process was developed on a 100 g scale of crude seaweeds in a 1 L reactor. It was shown that it was possible to combine the depolymerization, extraction, and butanolysis steps simultaneously by immersing the milled algae in the butanol solution in the presence of the acid catalyst [20]. In this case, it was observed that the addition of water was not necessary and that the residual moisture of the algae (13 wt%) was sufficient to carry out the depolymerization. In practice, the algae behave like sponges by absorbing the butanol. This optimization made it possible to reduce the amount of butanol, to avoid adding water, and to dispense with the Dean-Stark apparatus that was previously required. A standard distillation set-up was used, which greatly simplified the scaling up of the process. Thus, milled seaweeds (100 g) were stirred in butanol (280 mL) at 135 • C for 7 h in the presence of MSA 70% (30 g). The resulting mixture composed of dibutylated uronates (alginate) and butylated fucosides (fucoidan) was then filtered on sintered glass to eliminate the insoluble materials such as salts or the fibers contained in the seaweeds (Scheme 7).
The second step of the process based on transesterification/transglycosylation reactions led to the replacement of butyl chains by longer chains using a fatty alcohol as well as the recycling of butanol through a vacuum distillation. Octanol was selected as the fatty alcohol to obtain C 8 -surfactants. The advantage of this alcohol was its liquid state at room temperature which facilitated its manipulation during the process, and also its capacity to be eliminated by distillation. The C 8 -C 8 derivatives were then obtained after stirring for 7 h at 70 • C under reduced pressure. The anionic charge of the uronate derivatives was recovered after a saponification step with a 5N aqueous solution of NaOH for 1 h at 65 • C. The salts and the excess alcohol were then removed from the final mixture. For that purpose, residual water was first eliminated by evaporation under reduced pressure, followed by acidification of the medium through the addition of concentrated sulfuric acid to a pH value of about 2-3. This acidification step solubilized the uronate and fucoside derivatives in the residual alcohol and precipitated the salts. Thus, the salts were easily removed by filtration in addition to the other insoluble elements in octanol. Finally, the oily filtrate was treated by molecular distillation using a thin-film evaporator under high vacuum (2 mbar) at a temperature of around 90 • C. Using this technology, octanol was recovered in a very efficient way while minimizing the risk of degradation of the sugar derivatives present in the final mixture.
to a pH value of about 2-3. This acidification step solubilized the uronate and fucoside derivatives in the residual alcohol and precipitated the salts. Thus, the salts were easily removed by filtration in addition to the other insoluble elements in octanol. Finally, the oily filtrate was treated by molecular distillation using a thin-film evaporator under high vacuum (2 mbar) at a temperature of around 90 °C. Using this technology, octanol was recovered in a very efficient way while minimizing the risk of degradation of the sugar derivatives present in the final mixture. . Scheme 7. Cascading one-pot process for the production of surfactant composition from crude seaweeds.
This overall process allowed the production of 24 g of a surfactant composition through a solvent-free process that included purification steps limited to acid/base reactions and filtration/distillation steps (Scheme 7). The isolated mixture was subjected to thermogravimetric analysis which revealed the presence of 83.2% organic matter, 10.5% water, and 6.75% ash. A 1 H NMR analysis in CD3OD was also performed, based on the integration of the anomeric sugar proton signals in the 4.2-5.0 ppm region (Table 10) This overall process allowed the production of 24 g of a surfactant composition through a solvent-free process that included purification steps limited to acid/base reactions and filtration/distillation steps (Scheme 7). The isolated mixture was subjected to thermogravimetric analysis which revealed the presence of 83.2% organic matter, 10.5% water, and 6.75% ash. A 1 H NMR analysis in CD 3 OD was also performed, based on the integration of the anomeric sugar proton signals in the 4.2-5.0 ppm region (Table 10), which showed that the surfactant composition ( Figure 2) contained 30 mol% n-octyl α-D-mannopyranosiduronic acid 19α, 6 mol% n-octyl β-L-gulopyranosiduronic acid 20β, 5% n-octyl α-L-gulofuranosidurono-6,3-lactone 21α, 13 mol% n-octyl-α-L-fucopyranoside 22α, 9 mol% n-octyl-β-L-fucopyranoside 22β, 9 mol% n-octyl-α-L-fucofuranoside 23α, and 28 mol% n-octyl-β-L-fucofuranoside 23β (Figures S17 and S18, Supplementary Materials). The integration of the doublet relative to the fucosyl methyl group at 1.19-1.27 ppm confirmed this percentage of fucosides in the final mixture. 1 H NMR study of the surfactant composition did not allow us to identify signals corresponding to n-octyl α,β D-glucopyranosides. The surface tension measurements obtained with this H-C 8 surfactant composition are presented in Section 2.5. 22α, 9 mol% n-octyl-β-L-fucopyranoside 22β, 9 mol% n-octyl-α-L-fucofuranoside 23α, and 28 mol% n-octyl-β-L-fucofuranoside 23β (Figures S17 and S18, Supplementary Materials). The integration of the doublet relative to the fucosyl methyl group at 1.19-1.27 ppm confirmed this percentage of fucosides in the final mixture. 1 H NMR study of the surfactant composition did not allow us to identify signals corresponding to n-octyl α,β D-glucopyranosides. The surface tension measurements obtained with this H-C8 surfactant composition are presented in Section 2.5.

Physico-Chemical Properties of Anionic and Non-Ionic Surfactant Compositions Derived from Oligoalginates, Semi-Refined Alginates, and Crude Brown Seaweed
Surface activities at the air-water interface were investigated for compositions H-C12 Man, H-C12 Gul, H-C12 OAlg, and H-C12 s-r Alg in addition to the composition obtained from the crude seaweeds. Furthermore, biodegradability and aquatic ecotoxicity were evaluated for the H-C12 s-r Alg composition.

Physico-Chemical Properties of Anionic and Non-Ionic Surfactant Compositions Derived from Oligoalginates, Semi-Refined Alginates, and Crude Brown Seaweed
Surface activities at the air-water interface were investigated for compositions H-C 12 Man, H-C 12 Gul, H-C 12 OAlg, and H-C 12 s-r Alg in addition to the composition obtained from the crude seaweeds. Furthermore, biodegradability and aquatic ecotoxicity were evaluated for the H-C 12 s-r Alg composition. values were measured with this H-C12 Gul composition. Similar CMC and γCMC values were obtained with the batch from oligoalginate (0.11 g L −1 , 27.6 mN m −1 ). Finally, the surfactant mixture obtained from semi-refined alginate provided the lowest CMC with a value of 0.04 g L −1 and a γCMC of 29.0 mN m −1 . Tensiometry measurements were also carried out on anionic surfactant, sodium laureth sulfate (SLES). The CMC was determined at 0.13 g L −1 and the γCMC at 31.9 mN m −1 . Thus, all the compounds obtained from the different sources of algae extracts exhibit better performance in terms of CMC and γCMC.

Ecotoxicity Studies
A series of ecotoxicity studies have been performed with the uronic surfactant compositions derived from semi-refined alginate and dodecanol (H-C12 s-r Alg) in addition to standard SLES. Experiments to determine the ecotoxicity of products include effects on aquatic organisms, from microalgae to fish. These different pollutant-sensitive organisms serve as controls. Three standardized tests were conducted: an algal growth inhibition test, a microcrustaceous immobilization test, and a lethal toxicity test on freshwater fish. The microalgae used, called Pseudokirchneriella subcapitata, are ubiquitous in the environment and are sensitive to toxic substances. The test is conducted according to the OECD 201 method [21], which corresponds to the percentage of inhibition of the algal growth rate after a 72-h incubation period (CEr50). The microcrustaceans used are Daphnia Magna. They are freshwater crustaceans that are an important nutritional source for many aquatic organisms and their presence in sufficient numbers and in good health helps to maintain a certain balance in their ecosystem. The Daphnia Magna have the distinction of

Ecotoxicity Studies
A series of ecotoxicity studies have been performed with the uronic surfactant compositions derived from semi-refined alginate and dodecanol (H-C 12 s-r Alg) in addition to standard SLES. Experiments to determine the ecotoxicity of products include effects on aquatic organisms, from microalgae to fish. These different pollutant-sensitive organisms serve as controls. Three standardized tests were conducted: an algal growth inhibition test, a microcrustaceous immobilization test, and a lethal toxicity test on freshwater fish. The microalgae used, called Pseudokirchneriella subcapitata, are ubiquitous in the environment and are sensitive to toxic substances. The test is conducted according to the OECD 201 method [21], which corresponds to the percentage of inhibition of the algal growth rate after a 72-h incubation period (CEr50). The microcrustaceans used are Daphnia Magna. They are freshwater crustaceans that are an important nutritional source for many aquatic organisms and their presence in sufficient numbers and in good health helps to maintain a certain balance in their ecosystem. The Daphnia Magna have the distinction of being extremely susceptible to changes, even minor ones, in the composition of their aquatic environment. The OECD 202 test [22] is based on the determination of the CE50 concentration which, in 24 h and/or 48 h, immobilizes 50% of the microcrustaceans experimented on. Finally, the test on freshwater fish is conducted according to the OECD 203 method [23] to determine the concentration for which the sample has lethal toxicity for 50% of a Brachydanio rerio test population after a 96-h exposure period (CL50). Brachydanio rerio is one of the model organisms commonly encountered in research laboratories for fish behavior studies [24]. These tests are based on acute aquatic toxicity tests, i.e., adverse effects on aquatic organisms during short-term exposure. The results observed based on the concentration of the samples to be analyzed allow the substances to be categorized into different categories, as shown in Table 11.
As shown in Table 12, SLES can be considered toxic to algae and is borderline toxic to Daphnia and fish. In comparison, H-C 12 s-r Alg has a reduced ecotoxicity since it is not very toxic to algae and fish and not toxic to Daphnia.

Biodegradability Studies
The comparative aerobic readily biodegradability of compounds H-C 12 s-r Alg and SLES was finally studied using the OECD 301 B method [25]. The objective of this test is to determine the release of carbon dioxide by microbial digestion in the aerobic environment of the compound to be analyzed. During the test, the compound is placed in a watery medium to which is added a mixed seeding of a plant dealing with urban wastewater. The surfactant studied is the only source of carbon and energy and it is introduced at a theoretical concentration of 10 mg L −1 of dissolved organic carbon (COD). The CO 2 formed during degradation is trapped in external containers. The tests are conducted over 28 days, during which the evolution of the biodegradation rate is determined. The OECD 301 B method considers a product to be readily biodegradable if the biodegradation rate has reached at least 60% after 28 days and readily biodegradable with respect to the 10-day period if the biodegradation rate has reached at least 60% rate 10 days after the rate has reached 10%.

Category Acute 3 (poorly toxic)
CL 50 96 h (for the fish) >10 but ≤100 mg L −1 and/or CE 50 48 h (for the shellfish) >10 but ≤100 mg L −1 and/or CEr 50 72 h (for the seaweed) >10 but ≤100 mg L −1 Above 100 mg L −1 , the substance is considered as non-toxic  As the limit of 60% biodegradation was reached, both products are readily biodegradable but H-C 12 s-r Alg , exhibited lower biodegradability rates after 28 days (72%) than SLES (94%). Under the experimental conditions of the test, H-C 12 s-r Alg , (Figure S19, Supplementary Materials) is considered to be readily biodegradable without meeting the 10-day interval. Indeed, the CO 2 release threshold of 60% of theoretical CO 2 was not reached within the 10-day interval, but within the first 28 days of testing (threshold reached on day 21). In contrast, for SLES, the CO 2 release threshold of 60% of the theoretical CO 2 was reached within 10 days ( Figure S20, Supplementary Materials).

Discussion
To facilitate the industrial development of innovative surfactants based on carboxylate saccharides, we have proposed a new approach to produce these surfactants without sulfate and without ethylene oxide, according to a smart strategy based on a multi-step process carried out in a cascading single-pot mode without separating and isolating any reaction intermediates and applicable to oligomeric or polymeric alginates in a more or less refined form. The innovation of this research is also based on its positioning in green/blue chemistry (solvent-free reactions with little waste, biodegradable reagents, valorization of marine plant biomass, and eco-compatible products). The syntheses involve a succession of chemical reactions such as depolymerization by acid hydrolysis of oligo-or polysaccharides, esterification and glycosylation with n-butanol, transesterification and transglycosylation with fatty alcohols, and saponification. A final treatment involving mainly solid-liquid extraction, acidification, precipitation in an organic solvent (EtOAc or acetone), and filtration steps allows the final compositions to be enriched with active ingredients by removing (at least partially) residual salts and fatty alcohols. In terms of performance, the carboxylic acid-based surfactant compositions allow surface tension to be lowered to values ≤30 mN m −1 at concentrations equivalent to or lower than that of sodium laureth sulfate. In particular, the compositions derived from semi-refined alginate (s-r Alg) or oligomannuronate (OM) lead to CMC values two to three times lower than that obtained from sodium laureth sulfate. In addition, a significant improvement in ecotoxicity profile was observed for the biodegradable surfactant composition H-C 12 s-r Alg, especially towards Daphnia.
In order to further simplify the process and reduce costs, a novel strategy has been successfully tested based on the direct use of plant materials in their raw state, i.e., without any chemical transformation, in particular solvent extraction or enzymatic extraction, and at most a mechanical and/or physical transformation, such as washing, grinding, and/or drying. A notable advantage of this approach is the use of plant materials with a water content of up to 13% by weight, thus avoiding the addition of water during the depolymerization step by hydrolysis of the polysaccharides. The process has been applied to brown seaweed containing alginates and fucans and/or fucoidans. The process incorporates the same reaction sequences as for polysaccharides (acid hydrolysis, esterification/glycosylation, transesterification/transglycosylation, and saponification) with the addition of a preliminary extraction step carried out in situ as well as filtration, centrifugation, or distillation steps under reduced pressure, allowing the elimination of the residues of plant material not involved in the syntheses and the recycling of the excess alcohols. The obtained composition H-C 8 crude Alg is rich in carboxylic acid sugars, which also contain alkyl glycosides derived from neutral sugars representative of the structure of the polysaccharides present in the starting plant material (D-fucose). These modify the physicochemical properties of the final compositions and optimise their surface-active performance [20]. Indeed, low values of surface tension (27.0 mN m −1 ) and critical micellar concentration (0.007 g L −1 ) were obtained in the case of these mixtures of uronic acid-and fucose-based surfactants.

Chemistry
Oligomannnuronate, oligoguluronate, oligoalginate, and semi-refined alginate were produced from fresh or dry seaweeds (CEVA, 83 Rue de Pen Lan, 22610 Pleubian, France) [16][17][18]. Dried milled brown seaweeds (Asco T10) were purchased from Thorverk (Iceland) and all other commercially available chemicals were used without further purification. All reactions were monitored by thin-layer chromatography (Kieselgel 60F 254 Merck). Compounds were visualized using a H 2 SO 4 solution (5% H 2 SO 4 in EtOH) or a vanillin solution (15 g of vanillin in 250 mL of EtOH and 2.5 mL of conc. H 2 SO 4 ) followed by heating. Geduran 60 (40-63 µm, Merck) was used for column chromatography. NMR spectra were recorded on a Bruker Avance III 400 spectrometer operating at 400.13 MHz for 1 H, equipped with a BBFO probe with a Z-gradient coil and a GREAT 1/10 gradient unit. The standard temperature was adjusted to 298 K. The zg30 Bruker pulse program was used for 1D 1 H NMR, with a TD of 64k, a relaxation delay d1 = 2 s, and 8 scans. The spectrum width was set to 18 ppm. Fourier transform of the acquired FID was performed with an apodization of 0.3 Hz in most of the cases. Chemical shifts are mentioned in parts per million (ppm) with tetramethylsilane as an internal standard. Coupling constants were expressed in Hertz (Hz) and the following abbreviations were used to indicate the multiplicity: s (singulet), d (doublet), t (triplet), q (quadruplet), m (multiplet), dd (doublet of doublets), dt (doublet of triplets), and br (broad signal).

Physico-Chemistry
Critical micelle concentrations (CMC) and interfacial tensions (IFT) were measured on a force tensiometer Krüss K100. The critical micelle concentrations (CMC) were determined using the du Noüy ring method and the Krüss Laboratory Desktop software with the Surfactant Characteristics program. A water solution of surfactant was prepared around 10 g/L. Only 10 mL of this solution was used for the CMC determination. The deionized water was added thanks to an automatic burette controlled by the software. The concentration and tension determination were automatically determined by the software. The CMC was measured at 25 • C.

Conclusions
In this study, it has been proven for the first time that the conversion of oligo-and polysaccharide alginates in refined or semi-refined forms into biodegradable alkyl uronate surfactants can be carried out by one-pot acid hydrolysis, butanolysis, transesterification, transacetalisation, and saponification reactions, thus avoiding the isolation of any reaction intermediates. In addition, an in situ process was developed to manufacture surfactant compositions directly from crude milled brown seaweeds without requiring the standard steps of polysaccharide extraction and purification. These alkyl uronate monosaccharides as isomeric mixtures, with or without alkyl glycoside co-products, exhibit attractive surface activity and reduced aquatic ecotoxicity compared to commercial sodium laureth sulphate. Further physicochemical studies to evaluate the potential of these innovative surfactant compositions in cosmetic formulations are currently under investigation. Moreover, the transposition of the strategy, which consists in using crude vegetable raw materials instead of extracted polysaccharides, was also successfully achieved in the case of sugar beet pulps containing pectins [20,26]: this work, which is not described in this article, will be submitted for publication elsewhere.

Data Availability Statement:
The data that support the findings of this study are available from the corresponding author upon reasonable request.