Solvo-Surfactant Properties of Dialkyl Glycerol Ethers: Application as

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Solvo-surfactant properties of dialkyl glycerol ethers: Application as eco-friendly extractants of plant material through a novel Hydrotropic-Cloud-Point-Extraction (HCPE) process. Raphaël Lebeuf, Estelle Illous, Corentin Dussenne, Valerie Molinier, Eric Da Silva, Marc Lemaire, and Jean-Marie Aubry ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/acssuschemeng.6b01101 • Publication Date (Web): 15 Jul 2016 Downloaded from http://pubs.acs.org on July 17, 2016

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ACS Sustainable Chemistry & Engineering

Solvo-surfactant properties of dialkyl glycerol ethers: Application as eco-friendly extractants of plant material through a novel Hydrotropic-CloudPoint-Extraction (HCPE) process. Raphaël Lebeuf,*[a] Estelle Illous,[a] Corentin Dussenne,[a,b] Valérie Molinier,[a] Eric Da Silva,[b] Marc Lemaire,[b] and Jean-Marie Aubry.*[a] [a]

-Univ. Lille, CNRS, Centrale Lille, ENSCL, Univ. Artois, UMR 8181 - UCCS - Unité de

Catalyse et Chimie du Solide F-59000 Lille, France. [b]

-Univ Lyon, Université Lyon1, CNRS, INSA, CPE-Lyon, ICBMS, UMR 5246, 43 Bd du 11

novembre 1918, F-69622 Villeurbanne cedex, France E-mails: [email protected], [email protected]

KEYWORDS : solvo-surfactant, hydrotropes, glycerol, solubilization, extraction, cloud point, piperine, binary diagram.

ABSTRACT. Six dialkyl glycerol ethers with short alkyl chains have been synthesized and their solvo-surfactant properties have been compared to those of the mono- and tri- substituted

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isomers. The dialkyl ethers are completely miscible with water at room temperature and some of them show a cloud point. In aqueous solution, they are much better solubilizers of Disperse Red 13 hydrophobic dye than the trialkyl ethers. They also have much lower boiling points than the isomeric mono-substituted counterparts. These features make the dialkyl glycerol ethers attractive for aqueous extraction of hydrophobic material. Their volatility allows an easier recovery of solute compared to when non-volatile non-ionic surfactants are used, as in the standard Cloud Point Extraction (CPE) process. The effectiveness of the novel HydrotropicCloud-Point-Extraction (HCPE) method proposed in this work is illustrated by extracting piperine from black pepper using an aqueous solution of 1-butyl-3-methyl glycerol ether that is an attractive candidate in the series because of its ease of synthesis, volatility, middle-range Cloud Point and high solubilizing power.

INTRODUCTION Hydrotropes are small amphiphilic compounds composed of a ionic or non-ionic polar head and a short aromatic or alkyl hydrophobic tail (≤ C6) that distinguish them from surfactants which possess longer tails (≥ C8). They can be used for many applications such as solubilization of high value hydrophobic compounds for cosmetic or pharmaceutical formulations,1 microemulsification of oils in the presence of surfactants or even alone,2,3,4 and fluidification of aqueous solutions of lyotropic liquid crystals. Hydrotropes can also be used for extraction processes as shown in the 40’s for the separation of lignin from cellulosic materials,5 a revival issue in the context of biorefinery development.6 Extractions of other natural compounds by hydrotropes have been recently described in the literature, mainly by using alkylbenzene


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sulfonates or sodium salicylate.7,8 Indeed, hydrotropes improve safety of extraction processes owing to the lower inflammability of water-based extractants compared to organic solvents. They can also avoid solubilizing undesired constitutive plants materials like carbohydrates, gums… Moreover products can be easily recovered by dilution of the hydrotropic solution since diluted hydrotropes are unable to solubilize the solute. This last point is however counterbalanced either by the loss of the hydrotrope or a costly further concentration.

Hydrotropes can be ionic such as sodium xylenesulfonate (NaXS), or non-ionic, like 2butoxyethanol or alkyl glycosides.9 Some non-ionic hydrotropes with low boiling points may be referred as solvo-surfactants,10 emphasizing their solubilizing properties and volatility. Ongoing our efforts to develop biosourced hydrotropes,11,12,13 we focused on short chain dialkyl glycerol ethers, a neglected class of compounds in the literature (Table 1), especially with regard to their physico-chemical properties14,15 and to their expected low toxicity.16 Actually, they should exhibit higher hydrophilicity than trialkyl glycerol ethers, which behave as ordinary solvents,17 but also stronger lipophilicity than monoalkyl glycerol ethers, that should confer them improved solubilizing power. Moreover, due to the similarities of their structures with alkyl poly(ethylene oxide) non-ionic surfactants, cloud points are also expected. This rather unusual phenomenon is already exploited for Cloud Point Extractions (CPE), which is used as analytical methodology18 for metals complexes19 or proteins.20 This could also be potentially applied for the preparative extraction of active ingredients from biotransformations21 or microalgaes22 in situ. The CPE process has for main advantage a simple separation of the solute by gentle heating, followed by decantation or centrifugation. Indeed, when a homogeneous aqueous solution of a surfactant having a cloud point is heated above a precise temperature (Tcp), the surfactant becomes


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insoluble and is released from water (generally as lower phase). If the surfactant was used to solubilize a hydrophobic compound, this latter will preferentially stay in the surfactant-rich phase during the decantation step. So far, CPE has only been performed using surfactants (mainly Triton X-100 or X-114), which have to be separated from the solute if one wish to recover them for a preparative application. We thus examine the possibility to use volatile hydrotrope for CPE that would allow combining the good extraction ability of hydrotropes and the easiness of solute recovery as well as hydrotrope recycling (scheme 1).


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Scheme 1. Water–based extraction processes and recovery of hydrophobic compounds using solutions of hydrotropes or surfactants with or without cloud points.


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The so-called “Hydrotropic Cloud Point Extraction” (HCPE) method proposed in this work belongs in fact to the larger class of Phase Transition Extraction (PTE) method, applied so far to solvent-based systems which separate into two phases either on cooling,23 or by addition of a third solvent like MIBK24 or decane25 for water-acetonitrile mixture. Although being a waterbased system, this last method is undesirable in regards to acetonitrile hazard and its lower solubilizing capacities than hydrotropes. In the first step, the synthesis of the mono-, di- and trialkyl glycerol ethers will be presented, followed by the study of their solvo-surfactant properties to check whether they possess suitable cloud points and solubilizing power for extraction as well as adequate volatility for easy recovery. Finally, the presented HCPE concept will be illustrated by extracting piperine from black pepper.

RESULTS AND DISCUSSIONS

Synthesis of alkyl glycerol ethers Monoalkyl glycerol ethers, abbreviated [x.0.0] according to the notation below, have been reported to be water-soluble in all proportions and temperatures when the alkyl chain has 5 or less carbons, whereas n-hexyl glycerol ether [6.0.0] presents a large miscibility gap.11 OCy CxO

OCz

= [x.y.z]


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Since additional alkylation would decrease the water solubility, we investigated series of diand trialkyl glycerol ethers with less than 6 alkyl carbon atoms in total (x+y+z < 6, see table 1). Alkyl glycerol are materials accessible by several synthetic pathways many of them with high yields and selectivity but low ecological impact.26 Monoalkyl glycerol ethers [x.0.0] were synthesized by a one-step catalytic reductive alkylation of glycerol with an appropriate aldehyde (scheme 2)27. Alternatively, carboxylic acids or esters can be used instead of aldehydes.28

Scheme 2. Eco-friendly access to monoalkyl glycerol ethers by catalytic reductive alkylation of glycerol. Dialkyl glycerol ethers can be synthesized by the opening of their corresponding epoxide with alcohols. Currently, butyl glycidyl ether is a readily available building-block as starting material for epoxy resin. Although it is actually synthesized from epichlorohydrin, a greener access may be envisaged from the monobutyl glycerol [4.0.0], either by dehydration or decarboxylation of the corresponding carbonate as reported for the synthesis of glycidol.29 Ring opening of the epoxide with methanol proceeds regio-selectively under basic conditions giving [4.0.1] (scheme 3, path b), whereas Brönsted or Lewis acid catalysis provides the mixture of the two regioisomers [4.0.1] and [4.1.0] in 4:1 ratio (scheme 3, path c).30 Purification by spinning-band distillation only leads to a partial separation of the isomers. They can be fully separated by chromatography on small scales, but the mixture of regioisomers can also be used as such for extraction as shown later. It is worth noting that opening the epoxides by water is an alternative method to obtain monoalkyl glycerol ethers (scheme 3, path a).


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Scheme 3. Large-scale access to [4.0.0] and [4.0.1] alone or with 20% of [4.1.0] by ring opening of butyl glycidyl ether.

The syntheses of the regioisomers [4.1.0] and [0.4.1] have not been reported so far. To be able to accurately study the physicochemical properties of the pure isomers, multistep synthesis were required via regioselective ring opening of the corresponding epoxide, either by methyl or benzyl alcoholate, followed by methylation and deprotection, as shown in scheme 4 (paths d and e). Other dialkyl glycerol ethers were synthesized from the commercially available methyl- and ethyl glycidyl ethers, whereas propyl- and isobutyl glycidyl ethers were synthesized using an efficient, upscalable synthetic pathway by reaction of their corresponding alcohol with epichlorohydrin,31 which can also be derived from glycerol.32 Finally, trimethyl- and triethyl glycerol ethers were obtained by alkylation of glycerol under phase transfer conditions according to known methods (path f and g).17


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Scheme 4. Synthetic pathways to pure [x.0.1], [x.1.0] and [1.y.0] isomers.

Sufficient amount of compounds have been synthesized to allow careful purification by spinning-band distillation. It should be noted that despite the good quality of their 1H NMR spectra, the samples isolated by chromatography exhibit significantly lower cloud points values than the distilled ones, indicating lower purities. All the alkyl glycerol ethers synthesized in this work are listed in Table 1 with their boiling points measured by DSC at normal pressure (1 bar) and calculated under reduced pressure (1 mbar) to show the easiness of hydrotrope recovery by distillation. Their other physico-chemical properties of interest for application as extractants in Hydrotropic Cloud Point Extraction process which were determined below are also listed.


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Table 1. Synthetic pathways and solvo-surfactant properties of regioisomers of mono-, di-, and trialkyl glycerol ethers as well as other solubilizers of Disperse Red 13 dye studied in this work. Hydr.

Nb of ref.

b.p. (°C)[c]

Synth.Path

Aq. sol.

Cld Pt.

(yield %)[a]

1 bar[b]

1 mbar[c]

(Wt.%) [d]

(°C)

[4.0.0]

223

a (78)

254

21

100

no

[5.0.0]

108

a (74)

262

74

100

no

[3.0.1]

3

b (77)

194

31

100

no

[4.0.1]

14

b (82)

212

44

100

39

[4.1.0]

0

d (44)

230

57

100

70

[1.4.0]

0

e (46)

217

46

100

35

[i-4.0.1]

8

b (59)

203

38

100

22

[i-4.1.0]

0

d (37)

213

47

100

49

[1.1.1]

73

f (79)17

150

-1

100

no

[2.1.1]

0

b + f (72)

162

10

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Solvo-Surfactant Properties of Dialkyl Glycerol Ethers: Application as

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