Synthesis and surfactant properties of nonionic biosourced

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Synthesis and surfactant properties of nonionic biosourced alkylglucuronamides. Raphaël Lebeuf, Chih-Yin Liu, Christel Pierlot, and Véronique Nardello-Rataj ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b04456 • Publication Date (Web): 08 Jan 2018 Downloaded from http://pubs.acs.org on January 13, 2018

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ACS Sustainable Chemistry & Engineering

Synthesis and surfactant properties of nonionic biosourced alkylglucuronamides. Raphaël Lebeuf,* Chih-Yin Liu, Christel Pierlot, Véronique Nardello-Rataj* Univ. Lille, CNRS, Centrale Lille, ENSCL, Univ. Artois, UMR 8181 - UCCS - Unité de Catalyse et Chimie du Solide, F-59000 Lille, France.

E-mail: [email protected] and [email protected] Mailing: UCCS-CISCO, Bâtiment C6, Avenue Mendeleiev, Cité scientifique, F-59655 Villeneuve d’Ascq CEDEX, France

ABSTRACT: A series of thirteen well-defined alkylglucuronamides [AGUA] surfactants with hydrophobic chains consisting in fatty alcohols (C8, C12 or C16) involved in a glucosidic bond has been synthesized in two steps from glucuronic acid or glucuronolactone. The nonionic polar heads are a sugar unit with an amide function derived from five different amines, i.e. methylamine, ammonia, ethanolamine, ethoxyethanolamine and aminopropanediol. Their CMC values present variations from 0.03 to 29 mM and their hydrophilic/lipophilic behaviors have been compared to conventional dodecyl polyethoxylated surfactants (C12Ej) using the PIT-slope

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methodology. They cover a large scale of the corresponding C12Ej with j varying from 5 to 8. Regarding functional properties, the novel alkylglucuronamides exhibit good foaming properties. Last but not least, two of them could be classified as easily biodegradable according to respirometric tests.

KEYWORDS: Glucamide; Glucuronolactone, Nonionic surfactants; PIT-slope; Foaming

Introduction: For sustainable developments, the substitution of petro-based chemicals by bio-based ones is required. Fortunately, it is already ongoing in the surfactant chemistry, especially thanks to the use of sugar derivatives.1,2 Among them, the nonionic sugar esters,3 alkylpolyglycosides4 and glucamides5 have already found commercial applications. However, even if the chemistry of sugars is highly diversified due to their complex stereochemistry, one may expect that the global polarity of the derived surfactants will not be strongly affected by the nature of the sugar for a given class of compounds. Indeed, the CMC of alkylpolyglycosides is more directed by their alkyl chain length than by their number of sugar units.6,7 Also, the direct derivation of sugars is rather problematic for selectivity reasons. More complex carbohydrates-based surfactants like rhamnolipids or sophorolipids have thus been synthetized by biotechnologies, the last ones being now industrially produced despite complex technologies.8,9 Alternatively, sugar-based surfactants can also be designed starting simply from oxidized sugars, either in their anomeric position or their terminal one (i.e. the primary alcohol), or even both, corresponding to aldonic, alduronic or aldaric acids respectively for the aldose series. These acidic functions open the door


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for further functionalizations, such as amides formation as depicted in scheme 1. It is noteworthy that such an approach leads to structures different from the already commercialized N-Methylamino-glucosyl esters 1 (MAGE) surfactants, these last ones being directly obtained from glucose 2 in two steps through reductive alkylation then acylation (path a).5,10

Scheme 1. Pathways to glucamides from glucose 2 or oxidized glucoses 3, 5 and 7. R, R’ = long alkyl chain.

Glucose 2 can be oxidized on its anomeric position to provide gluconic acid which lactonizes to give the lactone 3 (also referred as gluconolactone). Such lactone can be opened by an amine to provide 4 (path b)11,12 Starting from glucaric acid 5, the use of two equivalents of



amine allows to get the ammonium carboxylate function and the amide, giving a product which lactonizes (6) under acidic conditions (path c).13 Lastly, glucuronic acid 7 can be used as a precursor to provide glucamides. Direct amidation of 7 has not been reported but compound 8 has been obtained through per-acetylation of glucuronic acid 3 to give the mixed anhydride, followed by amidation and de-acetylation (path d).14,15 Alternatively, hydrophobic alkyl chains can be grafted with fatty alcohols instead of fatty amines onto 7 or the more available glucuronic acid δ-lactone 9, providing the lactone 10 as a precursor of alkylglucuronamides 11a-d (path e).16 It is noteworthy that the variability of the hydrophilic part of 11a-d can be obtained using different amines. For instance, the use of fatty amines provide surfactants like 11a having potential applications in shampoos and detergency,17,18 and the thermotropic properties of the product obtained with octylamine have been studied.19 Use of taurine can also provide the anionic surfactant 11b,20 and cationic surfactants 11c can be obtained by the use of N,Ndimethylethylene diamine followed by quaternarization.21 This study has also reported the use of ethanolamine to give 11d with decyl, undecyl and dodecyl alkyl chains, but no physicochemical characterizations were reported for these compounds. Concerning other sugars than glucose, some glycamides have been obtained from glyconolactones like for the synthesis of copper-chelating macrocycles,22-24 glycosylglycerolipid derivatives25 and hepatoprotective hydrazines.26 With regard to the surfactant applications, Benvegnu et al. have recently described the synthesis of bolaamphiphiles27 and guluronamides from polyguluronic acid or alginates.28 A general trend consists in the replacement of ionic surfactants by nonionic ones, being generally considered milder and having better salt resistance. Among them, polyethoxylated fatty alcohols CiEj still dominate the market but are more and more competing with biobased surfactants such


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as alkylpolyglucosides. It is noteworthy that the thermal behaviors of their solutions differ in the sense that CiEj become insoluble when heated up to a temperature called "cloud point". If this property can be useful for detergency or cloud point extractions, it can also limit some other applications. Also, even if ethylene oxide can be accessible from bioethanol, it is still not competitive to the petro-based one and a bio-based alternative to CiEj in addition to alkylpolyglucosides can only be beneficial. As only few glucamides derived from glucuronic acid with structures differing from MAGE surfactants have been reported, and because of their sourcing, their readily accessibility and the large variability of their hydrophilic part, we designed a series of glucamides similar to compound 11d. Their physicochemical properties have been determined in order to position them in relation to existing commercial surfactants and to highlight possible applications. Thus, water-solubility, CMC, surface activity efficiency (σCMC) have been determined. Their hydrophilic/lipophilic behavior HLB has been assessed with the PIT-slope method29,30 and compared to that of CiEj, alkyldiglycerol and alkylglucoside. Both the hydrophilic and the hydrophobic parts have been varied using respectively different fatty alcohols in C8, C12, and C16 alkyl chain lengths (noted a, b, c respectively) and different amines, i.e. methylamine 12, ammonia 13, ethanolamine 14, ethoxy-ethanolamine 15 and aminopropanediol 16 (table 1).

Results and Discussion Synthesis of compounds 12 to 16 The glucosides with C12 (10b) and C16 (10c) alkyl chains were obtained from glucuronic acid 7 according to the literature in moderate yields (60 and 50% respectively).16 The reaction requires two equivalents of BF3 as Lewis acid and two equivalents of fatty alcohol to ensure good yields. Starting from glucuronolactone 9 for the C8 glucoside 10a provides the product with



a better yield (76%) under the same conditions. For all the products, only the beta isomer on the anomeric position was obtained according to the 13C NMR shift at 110 ppm instead of 104 ppm for the alpha isomers in the furanoside series.31 Another reported protocol using supported sulfuric acid on silica may offer 10% of the alpha isomer.32 The lactones 10 were then opened with methylamine or ammonia at room temperature, and at 50°C for 6 hours with the other primary amines (ethanolamine, ethoxyethanolamine and aminopropanediol) using microwaves, to give the glucuronamides 12-16 in various yields from 8 to 97% (Table 1). Since full conversions were obtained in all cases, the lower yields for compound 13a (8%) and 13b (18%) are probably due to loss of the product on chromatography due to their low solubility. Moreover, opening of lactones 9 by secondary amines seems to be more complicated. As an example, diethanolamine did not lead to the corresponding glucuronamide due to a degradation pathway, the acetal function of 9 being cleaved, even at room temperature, as observed by the recovery of the fatty alcohol. This may be ascribed by the higher basicity of the secondary amine, which may induce the opening of the THF ring and regeneration of the aldehyde function as previously reported with basic conditions.33 Since analytically pure grade products were obtained by chromatography for physicochemical characterizations, no optimization of the synthesis was undertaken in view of pilot scale development. Nevertheless, since the direct glycosylation is being developed by heterogeneous catalysis32 and amidation is a 100% atom efficiency step, such surfactants can be produced in a green way and purified by crystallization, avoiding reagents excesses and chromatographies.


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Table 1. Synthesized glucuronamides (12-16) with different polar headgroups and alkyl chains C8 (a), C12 (b) and C16 (c). Yields (%) are given from 9 after purification through silica gel chromatography and water-solubility at 25 °C are indicated in brackets.

R’ R

n-C8H17

n-C12H25

n-C16H33

Me

12a, 97% (0.6-0.8 M) 13a, 8% (>1 M) 14a, 59% (>1 M) 15a, 61% (>1 M) 16a, 51% (>1 M)

12b, 94%, (0.25 mM) 13b, 18% (10 mM) 14b, 92% (0.1 M) 15b, 59% (>1 M) 16b, 51% (>1 M)

/

H -CH2CH2OH -CH2CH2O(CH2)2OH -CH2CH(OH)CH2OH

/

14c, 79% (0.015 mM) 15c, 87% (0.25 mM) 16c, 78% (0.1-0.2 mM)

Aqueous phase behavior of alkylglucuronamides 12a-c to 16a-c

The nature of both the alkyl chain and the amine of the polar head have a strong influence on the solubility of the synthesized glucamides (Table 1). For a given glucuronamide (12 to 16), the solubility decreases when the number of carbon atoms of the alkyl chain increases (a vs c). As an example, for ethanolamine-based glucuronamides 14, 14a is readily water-soluble (> 1 M) whereas 14b and 14c have a limited solubility of 0.1 M and 0.015 mM respectively. Moreover, since the solubility dramatically decreases from C12 (14-16, b) to C16 (14-16, c), products 12c



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and 13c are expected to not be soluble enough in view of the results obtained with 12b (0.25 mM) and 13b (10 mM) and have thus not been synthesized. Changing ammonia by methylamine (path g) with the C12 alkyl chain (13b vs 12b) strongly decreases their solubility. Indeed, whereas 13b is soluble up to 10 mM using a gentle heating, the water-solubility of 12b is less than 0.25 mM. On the contrary, just one more hydroxymethylene group to methylamine increases dramatically the solubility (e.g. 14b is still water-soluble above 0.1

M).

As

expected,

compounds

derived

from

ethoxyethanolamine

15a-c)

and

aminopropanediol (16a-c) are the most water-soluble because of the presence of additional ethoxy or hydroxy functions, allowing fair solubility of compounds with C16 alkyl chain length (still limited to 0.25 mM, 0.12 g/L). Amazingly, the ethoxyethanolamine part provides a slightly better solubilization than the aminopropanediol one. This was confirmed by the Krafft temperature of 0.1 wt.% solutions of compounds 14c, 15c, 16c with TKrafft of > 90 °C, 26 °C and 33 °C respectively. For shorter C8 and C12 alkyl chains, the Krafft temperature is below 0 °C even at 1 wt.%. Finally, except for 14c, all compounds in Table 1 present water-solubility that allows critical micelle concentration (CMC) determination at 25 °C. In addition, all the compounds did not exhibit cloud point under heating up to 90 °C.

Surface activity of alkylglucuronamides 13a-b to 16a-c The CMCs of the various alkylglucuronamides obtained from the surface tension isotherms have been measured (Figure 1) and all CMCs are reported in Table 2 as well as the surface tensions at CMC (σCMC).


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80

14b

80

13a 14a

13b

15c

70

70

60

60

σ (mN/m)

σ (mN/m)

50 40 30

15a

15b

50 40 30

20 0,00000010,000001 10-7 10-6 0,00001 10-5

0,0001 10-4

0,001 10-3

0,01 10-2

0,1 10-1

20 0,00000010,000001 10-7 10-6 0,00001 10-5

1

[glucuronamide] (M)

0,0001 10-4

0,001 10-3

0,01 10-2

0,1 10-1

[glucuronamide] (M)

80

16b

16a

16c

70 60

σ (mN/m)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


50 40 30

20 0,00000010,000001 10-7 10-6 0,00001 10-5

0,0001 10-4

0,001 10-3

0,01 10-2

0,1 10-1

1

[glucuronamide] (M)

Figure 1. Surface tension (σ) plotted against surfactant concentration for compounds 13, 14, 15 and 16 at 25 °C (a = C8, b = C12 and c = C16).


1


Table 2. CMC (mM), σCMC (mN/m) and PIT-slope (dPIT/dx2 in °C) for alkylglucuronamides 1316. CMC (mM) 13 0.082 16 0.13 n.s.a 29 0.17 0.027 4.8 0.18 0.034

Compounds 13a 13b 14a 14b 14c 15a 15b 15c 16a 16b 16c a

σCMC (mN/m) 29.7 35.2 31.4 29.9 n.s.a 30.3 31.3 32.5 29.3 30.8 30.4

dPIT/dx2 (°C) 34 3 67 37 16 82 50 37 107 76 40

Not soluble enough

The formation of aggregates in aqueous solution is driven by the so-called “hydrophobic effect”, which depends on the alkyl chain length.34 On the other hand, the hydration of the hydrophilic head group increases the affinity of the molecule for water, which increases the CMC. In fact, the hydrophobic effects prevail when comparing nonionic surfactants with different polar head groups. For example, alkylglycosides from glucose, mannose and galactose with an octyl chain have a CMC ranging from 6 to 20 mM, whereas the ones with a decyl chain have a CMC from 0.25 to 0.80 mM and the one with a dodecyl chain from 0.05 to 0.20 mM.35 Also, changing the glucamide part of the C12-MAGE by xylamide and glyceramide weakly lowers the CMC from 0.35 mM to 0.33 mM and 0.23 mM respectively.36 These effects are also reflected in our results since the CMC decreases about 100 times from C8 to C12 for each series of glucuronamides but are rather closed together for a given alkyl chain. However, the addition of 4 carbon atoms on the tail from dodecyl to hexadecyl allows lowering the CMC by only 10


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folds. This does not exactly follows the Klevens’ equation giving a linear variation of the logarithm of the CMC as a function of the alkyl chain carbon numbers,37 but this is in agreement with the behaviors of glucamides 4 obtained from gluconolactone.11 The surface tension reduction ranging from 29.7 to 35.2 mM/m for most of the glucuronamides is quite good compared to the above mentioned alkylglucosides (28 to 30.5 mN/m).35 According to Gibb’s adsorption equation (1), the amount of surfactant molecules adsorbed at the water/air interface per unit area (Гmax) can be calculated with R the ideal gas constant (J.mol-1.K1

), T the temperature (K), γ the superficial tension (N.m-1), and C the concentration (mol.l-1)

respectively. ߁max= −

1 ݀ߛ lim 2.3ܴܶ ‫ ݀ ܥܯܥ→ܥ‬log ‫ܥ‬ Eq. (1)

The minimum area (A0 in Å2) per surfactant molecule which gives information of the polar head group size, is obtained from the relation (2) with Гmax the amount of surfactant molecules adsorbed on the water/air interface per unit area, and N the Avogadro number.38

1020 ‫ܣ‬0= ܰ ∙ ߁max

Eq. (2)

These values are reported in Table 3 for the glucuronamide surfactants having the same dodecyl chain length, and compared to their corresponding CiEj.



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Table 3. Surface excess concentrations and minimum surface areas for dodecylglucuronamide surfactants with different polar headgroups in comparison with some C12Ej.

13a 14a 15a 16a 13b 14b 15b 16b 15c 16c a

ΓMax (µmol·m-2) 3.4 3.5 2.8 4.9 4.2 3.9 3.0 3.6 9.4 7.5

A0 (nm /molecule) 0.49 0.47 0.59 0.34 0.40 0.43 0.55 0.46 0.18 0.22 2

CiEj C8E1 C8E2 C8E3 C8E4 C12E3 C12E5 C12E6 C12E8 C16E6 C16E12

ΓMax (µmol·m-2) 4.7a 4.5a 4.3a 3.1a 3.9b 3.5b 3.1c 2.0c 4.4d 2.3d

A0 (nm /molecule) 0.35 0.37 0.39 0.53 0.43 0.47 0.54 0.81 0.38 0.72 2

ref 39, bbref 40, cref 41, dref 42

The smaller the surface area per surfactant, the more efficient is the surfactant to reduce the surface tension. It increases with the number of ethylene unit in case of the CiEj, meaning that more surfactants are required to reach low surface tension. As observed, for a given polar head, the A0 values decrease with the alkyl chain length due to the hydrophobic effect. For a given alkyl chain length, this value is more related to the size of the polar head than to its hydrophily, explaining the respective values between 15a-b with the aminoethoxyethanol part compared to

16a-b with the aminopropanediol. Indeed, this latter is more hydrophilic (see below) but smaller. The difference is not pronounced between 15c and 16c probably due to the high effectiveness of both compounds to reduce the surface tension.


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Hydrophilic/hydrophobic balance of alkylglucuronamides 12a-c to 16a-c.

To evaluate the hydrophilic-lipophilic balance of the surfactants, Davies or Griffin methods can be used to calculate a HLB. These methods are based on group contribution, and are well adapted for CiEj, sorbitan esters (Span) or ethoxylated sorbitan ester (Tween). However, since contribution of cyclic glucose and amide functions are unknown, the HLB values cannot be calculated with these methods. Thus, the hydrophylic-lipophilic behaviors of the glucamides surfactants have been determined by the PIT-slope method recently developed by Ontiveros et al.29,30 It consists in the perturbation of the phase inversion temperature (PIT) of a 3 wt.% C10E4/water/n-octane reference system by the incremental addition of a second surfactant S2 (less than 1 wt.%). As the variation of the PIT according to the mole fraction x2 is linear for nonionic surfactants, the slope dPIT/dx2 can be calculated. A positive dPIT/dx2 corresponds to a temperature increase and indicates that S2 is more hydrophilic than C10E4 (which has a PIT-slope equal to zero) while a negative dPIT/dx2, i.e. a decrease of the temperature, is attributed to more hydrophobic surfactants. Noteworthy, the dPIT/dx2 values allows ranking the surfactants taking into account a water/oil environment. The PIT-slopes have been determined for the different alkylglucuronamides (Table 2) and compared with that of a series of well defined nonionic C12Ej (with j = 2 to 8) for which the values varies from -34 to 102°C (Figure 2).30 In order to evaluate the influence of both polar head and alkyl chain on the hydrophilic-lipophilic behavior of the glucuronamide surfactants, their PIT-slope representative points have been located using two complementary perpendicular axis for R and R’ (Figure 2). This projection allows easily surfactant behavior comparison with



C12Ej or C12-Glucoside and C12-diglycerol surfactants, and thus hydrophilic/hydrophobic contribution of both polar head and alkyl chain.

Figure 2. dPIT/dX2 (°C) slope values of the alkylglucuronamides compared to the nonionic alkylpolyethoxylated surfactants CiEj.

All the synthesized glucuronamides surfactants 13-16 present PIT-slope values (Table 2) from 3°C (13b, the less hydrophilic) to 107°C (16a, the more hydrophilic). These positive values indicate that all the surfactants are more hydrophilic than C10E4, even those with a hexadecyl chain (14c, 15c and 16c). Moreover, the wide range of PIT-slope values (∆ = 104°C) allows to discriminate finely the surfactants and covers a range of equivalent C12Ej from j = 4 to 8. Since


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the PIT-slope increases in the order c < b < a for a given R group (Figure 2), the hydrophobic contribution order of the alkyl chain follows the expected one of C8 < C12 < C16. On the other hand, since the PIT-slope decreases in the order 16 < 15 < 14 < 13 for a given R’ group, the hydrophilic contribution order of the amide function corresponds to amino < ethanolamine < ethoxyethanolamine < aminopropanediol. To estimate the contribution of the amide function of the polar head of glucamides, the PIT-slope values of the b series can be compared with those of C12Ej, C12-glucoside (dodecyl-βglucopyranoside, dPIT/dx2 = 27°C), and C12-diglycerol (1-O-dodecyl-diglycerol ether, dPIT/dx2 = 4°C). The C12-glucoside possesses four hydroxyl groups on the sugar moiety, against three for the C12-diglycerol (Figure 3). In fact, this direct comparison allows to finely distinguish the hydrophilicity of a primary amide function and of an internal one compared to the other functions (Figure 3). Since there is almost no difference in the PIT-slope values between 13b and C12-diglycerol (3 and 4°C respectively) and that both compounds are composed of three hydroxyl groups and two ethers functions, the hydrophily of the primary amide would be only equivalent to the difference of hydrophily between a secondary alcohol and a primary one (Figure 3) if conformational effects and thus intramolecular hydrogen bonding are not taking into account. The internal secondary amide is less hydrophilic. The higher hydrophily of 14b compared to C12-glucoside is due to its amide function since their alcohol function number and nature are equal, and it corresponds to nearly the half of an ethoxy function comparing C12E6 to C12E7. Since six ethoxy functions seem to have almost the same hydrophily than three secondary alcohols comparing C12-glucoside and C12E6, an internal amide function would enhance the hydrophily equivalently to only the quarter of a secondary alcohol.



Figure 3. Classification of the C12-alkylglucuronamides according to their hydrophilicity in comparison to other known C12-surfactants.

As far as the alkyl chain length is concerned, the dPIT/dx2 values of the different series of alkylglucuronamide surfactants versus the carbon atom numbers of their alkyl chain length was plotted (Figure 4). Roughly speaking, an increase of 8 carbon atoms in the alkyl chain (C8 to C16) induces a decrease of about 60°C for the PIT-Slope. Since this range corresponds almost of the same difference obtained from C12E5 to C12E7, this result suggest that 4 additional carbon atoms on the alkyl chain are compensated by approximatively one ethylene oxide group (see also projections in ordinate in Figure 2).


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120 16 100

d(PIT)/dx2 (°C)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


15

80

14 60 40 13 20 0 6

8

10

12

14

16

18

alkyl chain lenght

Figure 4. dPIT/dX2 (°C) slope values of the different series (13, 14, 15 and 16) of the alkylglucuronamide surfactants versus carbon atoms number of their alkyl chain length.

Foaming properties of the alkylglucuronamide surfactants 14 to 16

Foaming properties are desired mainly for personal care products like shampoos, and are generally produced by anionic surfactants like SDS,43,44 but this latter is known to be potentially irritant for the skin and the eyes, either alone or in combination with other components.45 The use of SLES allows to minimize such problem, but the actual trend consist in developing sulfate-free compositions.46 Nethertheless, foaming properties are quite rare for nonionic surfactants.47 Alkylpolyglucosides with long alkyl chain stabilize foams, but suffer of limited solubility.48,49 Even if they are also known to give synergies in combination with cationic or anionic surfactants,49,50 the development of intrinsically better sugar-based foaming surfactants is still of interest.



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The foamability of the C12 and C16 glucuronamides were assessed by measuring the foam volume production and the foam stability after 15 minutes using the Ross-Miles tests.51 Results are reported in Table 4 and compared with those of SDS and the commercially available C10-MAGE glucamide (6). In order to saturated the air/solution interface with the surfactants, their concentration was set at more than ten folds their CMC, i.e. 0.1 wt.% (1 g/L ≈ 2.5 mM) for the C12 and 0.01 % (0.1 g/L ≈ 0.20 mM) for the C16 glucuronamides. Same weight concentration was used for SDS (CMC = 8.2 mM) but since no stable foam was produced at such a concentration with C10-MAGE (CMC = 5.0 mM)52, a higher concentration was also used for this compound.

Table 4. Foamability and foam stability of the glucuronamides in comparison with SDS and the commercial C10-MAGE. Compound

14b 15b 16b SDS C10-MAGE C10-MAGE 15c 16c a

Conc. (wt.%) 0.1 0.1 0.1 0.1 0.1 1.92 0.01 0.01c

Conc. (mM) 2.5 2.2 2.3 3.5 2.9 55 0.20 0.21

Foamabilitya (mL) 54 42 52 120 0d 0d 14 22

Stabilityb (%) 93 91 88 58 0 0 57 9e

Foam generated after the drop. bRelative remaining foam after 15 minutes. ccloudy

solution. dRapidly vanished after 3 seconds. eSame value after 3 minutes.

Less foam is generated by C12 and C16 glucuronamides surfactants compared to SDS. However, their stability is greater, the reduction of their volume relative to their initial amount being lower


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after 15 min, even if the foams became less dense and the bubbles grew larger. Only the foams generated by 16c disappear rapidly. Noteworthy, with these concentrations, the foam formed with C10-MAGE glucuronamides is very unstable, being completely vanished after 3 seconds, even at the concentration above the CMC. The synthesized glucuronamides are thus complementary to the MAGE glucamides concerning their foaming properties.

Biodegradability of the dodecyl glucuronamides

The biodegradability of compounds has become an essential property of surfactants and should be systematically considered as it is directly linked to its persistence in the environment. It expresses the ability for microorganisms to degrade molecules. According to REACH legislation, new chemicals have to pass ultimate biodegradation tests for further industrial development. The biodegradability of dodecyl glucuronamides 12b, 13b, 14b, 15b and 16b have thus been examined. From this series, the influence of the nature of the polar head on the biodegradability can be discussed. The evolution of CO2 was determined once per day under aerobic condition at 25 ± 1°C following the 301B OCDE standard using sodium acetate as a reference. Results are reported in Table 5 and biodegradations as a function of time are shown in the Supporting information (SI-40).



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Table 5 Biodegradability of the dodecyl glucamides 12b, 13b, 14b, 15b and 16b

Compounds

Sodium acetate 12b 13b 14b 15b 16b a

Latency duration (days) a 1 12 2 3 7 14

Biodegradation 10 daysb (%) 89 41 84 80 23 18

Biodegradation after 28 days (%) 91 46 90 97 27 18

day for which at least 10% of the surfactant has been consumed. bafter the latency period.

We can notice that the most biodegradable glucuronamides are 13b and 14b which are fully biodegradable after 28 days. On the other hand, the less hydrophilic 12b and the more hydrophilic 15b and 16b look more persistent and do not reach the required level (60 %) to be labeled as readily biodegradable, even if they could have been transformed into secondary metabolites. Nevertheless, since dodecanol takes part between 41 and 51% of the weight sample and that a delay is observed for the beginning of the degradation for some of them, the degradation would concern the surfactant as a whole and would not occur after the hydrolysis of the acetal function. In support, the stability toward hydrolysis was checked with 14b (40 mg/ mL, 0.1 M) in buffered citric acid (0.1 M) / Na2HPO4 (0.2 M) solutions at pH = 3 and 7 in D2O at room temperature. No hydrolysis was observed by 1H NMR after 2 weeks. The degradation may occur by oxidation from the ɷ position of the alkyl chain analogously to the MAGE surfactants, for which the C4-glucamide acid was detected as secondary metabolite.53 The latency period corresponds to the phase which starts at the very beginning of the experiment and ends when the degrading microorganisms are acclimated and when the degree of biodegradation of a chemical substance reach a detectable level i.e. 10% in the present test.


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Generally, the shorter the latency period is, the faster the degradation is. This is the case for this series except for 15b for which secondary metabolites may be harder to be degrading rather than being toxic to the microorganisms in view of the slow but regular evolution of its degradation after the latency period. On the opposite, 14b can be easily assimilated by the microorganisms at the very beginning of the experiment.

Conclusions Starting from glucuronic acid, the scope of glucamide surfactants has been extended to novel compounds having up to a hexadecyl alkyl chain with various amino-alcohols as polar headgroups affording thus a tuning of their hydrophilic/lipophilic balance and solubility. For instance, the different polarities obtained by using ammoniac to the more polar aminoglycerol in the C12 series covers the corresponding range of polarity from C12E5 to C12E7 as determined by PIT-slope methodology. The secondary amide functions contribute only slightly to the hydrophily, being nearly equivalent to only the half of the one of an ethoxy group. Generally speaking, their surface activity is not strongly affected by the nature of the polar headgroup. It is finally shown that these novel compounds exhibit very good foaming properties besides a readily biodegradation for some of them 13b and 14b). All these results make this new series of surfactants relevant biosourced alternatives to the petro-based polyethoxylated fatty alcohols, which are still widely used in numerous applications.



Experimental Section Synthesis and characterization of the compounds 12 to 16. Compounds 10a-c were synthesized according to the litterature16 and amidation reactions were performed using a modified procedure described for the synthesis of 11d.21 All the details are provided in the supporting information.

Surface activity determined by tensiometry. The surface tensions were measured at (20 ± 0.1)°C with a K100MK2 Krüss tensiometer using a platinum rod as the probe. For every surfactant, several solutions at different concentrations were prepared in the range 10-1 M to 10-6 M using water purified by Millipore® Simplicity 185 apparatus and collected at the resistivity of 18.2 MΩ.cm and stand for equilibration at least 1 hour. For measurements, a metallic circular container was filled with 2.5 ml of a solution and the surface tension was measured (standard deviation of the 5 final steps less than 0.1 mN m−1). Measurements were performed after reaching equilibrium, i.e. 20 min for surfactants with octyl and dodecyl chain and 40 min with hexadecyl. All the flasks, pipettes, dishes have to be presoaked with the solutions. The probe and the container were washed then flame-dried to destroy any residual surfactants for further measurements.

PIT-slope experiments. The procedure was followed as described previously29 and is given in the supporting information with the evolutions of the PIT of the reference system.


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Foaming properties determination. The foam-ability and foam-stability of surfactants were determined using the Ross-Miles test.51 A column with an opening of 3 mm of diameter is placed at 1.00 meter height from the bottom of a gradient cylinder (250 mL, d = 3.5 cm). A surfactant solution was poured into the gradient cylinder at the bottom (20 mL) and into the closed column at the top (80 mL). The column was opened and all the solution fall down all at once into the cylinder in a straight way. The volume of the produced foam volume was recorded and considered as foam power or foam-ability.

Biodegradation experiments. The experiments were performed at the Institut Français des Matériaux Agrosourcés (IFMAS) following the 301B OCDE standard. The inoculum was activated sludge from the water treatment plant of Douai (France) and sodium acetate was used as reference.

ASSOCIATED CONTENT Details on the synthesis, 1H and 13C NMR characterization, Phase Inversion Temperature experiments, and biodegradation tests are provided in the Supporting Information available free of charge (WORD document). AUTHOR INFORMATION

Corresponding Authors *Raphaël Lebeuf, * Véronique Nardello-Rataj Email: [email protected] and [email protected]



Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT Chevreul Institute (FR 2638), Ministère de l’Enseignement Supérieur et de la Recherche, Région Hauts-de-France and FEDER are acknowledged for supporting and funding partially this work. The authors thank Thierry Delaunay for the biodegradation experiments and Christophe Penverne for HRMS determination.

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GRAPHICAL ABSTRACT:



SYNOPSIS: Good foaming nonionic glucamide surfactants with different Hydrophilic/Lipophilic Balance have been synthesized, and some of them are easily biodegradable.


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Synthesis and surfactant properties of nonionic biosourced

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