Synthesis of monoethers of sugar comprising a long alkyl chain and uses thereof as a surfactant

Abstract

A process for obtaining a mixture of C4-C8 and C9-C18 alkyl monoether of saccharide, comprising: a) a first step of acetalization or trans-acetalization of a saccharide or of a mixture of saccharides with a C4-C8 aliphatic aldehyde or the acetal thereof, b) a second consecutive or simultaneous step of acetalization or trans-acetalization of the product obtained in a) of the saccharide or mixture of saccharides with a C9-C18 aliphatic aldehyde or the acetal thereof, c) a step of catalytic hydrogenolysis of the saccharide acetals obtained, and d) a step of recovery of a mixture of C4-C8 and C9-C18 alkyl saccharide monoethers. The invention further relates to a mixture of C4-C8 and C9-C18 alkyl saccharide monoethers and the use thereof as a surfactant.

Claims

1. A process for obtaining a mixture of C4-C8 alkyl monoether of saccharide and/or of saccharide derivative and of C9-C18 alkyl monoether of saccharide and/or of saccharide derivative, said saccharide derivative being a glycosylated and/or hydrogenated and/or dehydrated saccharide, said process comprising: a) a first step of acetalization or trans-acetalization of a saccharide, of saccharide derivative or of mixtures thereof with a C4-C8 aliphatic aldehyde or the acetal thereof, b) a second consecutive or simultaneous step of acetalization or trans-acetalization of the product obtained in a), of the saccharide, of the saccharide derivative or of mixtures thereof, with a C9 to C 18 aliphatic aldehyde or the acetal thereof, c) a step of catalytic hydrogenolysis of the acetals of saccharide and/or of saccharide derivative obtained in b), and d) a step of recovery of a mixture of C4-C8 alkyl monoether of saccharide and/or of saccharide derivative and of C9-C18 alkyl monoether of saccharide and/or of saccharide derivative.

2. The process as claimed in claim 1, in which said saccharide derivative is a monoanhydrosaccharide or a C1-C4 alkyl glycoside.

3. The process as claimed in claim 1, in which said saccharide derivative is an anhydrosaccharide or an alkyl glycoside.

4. The process as claimed claim 1, in which said saccharide is a monosaccharide, a disaccharide or a trisaccharide.

5. The process as claimed claim 1, in which said saccharide and/or derivative of saccharide comprises from 4 to 7 carbon atoms.

6. The process as claimed claim 1, in which said saccharide derivative is a sugar alcohol selected from the group consisting of erythritol, threitol, arabitol, xylitol, ribitol, mannitol, sorbitol, galactitol, iditol, volemitol, isomalt, maltitol, lactitol, maltotriitol, maltotetraitol, polyglycitol and the process further comprises a dehydration step, before said step a) of first acetalization or trans-acetalization.

7. The process as claimed in claim 1, in which said saccharide derivative is an alkyl glycoside selected from a group consisting of methyl glucoside, ethyl glucoside, propyl glucoside, butyl glucoside, methyl xyloside, ethyl xyloside, propyl xyloside, butyl xyloside, methyl mannoside, ethyl mannoside, propyl mannoside, butyl mannoside, methyl galactoside, ethyl galactoside, propyl galactoside and butyl galactoside.

8. The process as claimed in claim 1, in which the C4-C8 aliphatic aldehyde is a C5 aliphatic aldehyde or the acetal of the latter and/or the C9 to C18 aliphatic aldehyde or the acetal of the latter is a C12 aliphatic aldehyde or the acetal thereof.

9. The process as claimed claim 1, in which the molar ratio (C4-C8 or C9-C18 aliphatic aldehyde or acetal thereof):(saccharide, the derivative of saccharide or mixtures thereof) is between 5:1 and 1:5.

10. The process as claimed claim 1, in which the first and/or second step of acetalization or trans-acetalization is carried out in the presence of an acid catalyst, wherein the first and/or second step of acetalization or transacetalization is carried out in conditions without solvent or in the presence of a polar solvent.

11. The process as claimed claim 1, in which hydrogenolysis is carried out at a temperature between 80 and 140 C. and/or at a pressure between 15 and 50 bar, in the presence of a catalyst based on precious metals, on base metals in the group of ferrous metals.

12. A composition comprising a mixture of positional isomers of C4-C8 alkyl monoether of saccharide or of saccharide derivative and of positional isomers of C9-C18 alkyl monoether of saccharide or of saccharide derivative, in which the saccharide derivative is a glycosylated and/or hydrogenated and/or dehydrated saccharide, and the saccharide is a hexose.

13. The composition as claimed in claim 12, in which the C4-C8 alkyl group is a C5 alkyl and the C9-C18 alkyl group is a C12 alkyl, the saccharide derivative being selected from monoanhydrosorbitol or alkylglucoside.

14. The composition as claimed claim 12, in which the ratio of C5 alkyl monoether of saccharide or of saccharide derivative/C 12 alkyl monoether of saccharide or of saccharide derivative is between 5:95 and 95:5.

15. The process as claimed in claim 5, wherein said saccharide and/or saccharide derivative is selected from: a hexose selected from a group consisting of glucose, mannose, galactose, allose, altrose, gulose, idose or talose, and a hexitan selected from a group consisting of 1,4-anhydro-D-sorbitol; 1,5-anhydro-D-sorbitol; 3,6-anhydro-D-sorbitol; 1,4 (3,6)-anhydro-D-mannitol; 1,5-anhydro-D-mannitol; 3,6-anhydro-D-galactitol; 1,5-anhydro-D-galactitol; 1,5-anhydro-D-talitol; and 2,5-anhydro-L-iditol.

16. The composition as claimed in claim 12, wherein the saccharide derivative is methyl glucoside.

17. A method for reducing the surface tension of a liquid, said method comprising: contacting a liquid with: a composition comprising a mixture of C4-C8 alkyl monoether of saccharide and/or of saccharide derivative and of C9-C18 alkyl monoether of saccharide and/or of saccharide derivative, or a composition comprising a mixture of positional isomers of C4-C8 alkyl monoether of saccharide or of saccharide derivative and of positional isomers of C9-C18 alkyl monoether of saccharide or of saccharide derivative, in which the saccharide derivative is a glycosylated and/or hydrogenated and/or dehydrated saccharide, and the saccharide is a hexose, or a mixture of C4-C8 alkyl monoether of saccharide and/or of saccharide derivative and of C9-C18 alkyl monoether of saccharide and/or of saccharide derivative, said saccharide derivative being a glycosylated and/or hydrogenated and/or dehydrated saccharide, obtained by a process comprising: a) a first step of acetalization or trans-acetalization of a saccharide, of saccharide derivative or of mixtures thereof with a C4-C8 aliphatic aldehyde or the acetal thereof, b) a second consecutive or simultaneous step of acetalization or trans-acetalization of the product obtained in a), of the saccharide, of the saccharide derivative or of mixtures thereof, with a C9 to C 18 aliphatic aldehyde or the acetal thereof, c) a step of catalytic hydrogenolysis of the acetals of saccharide and/or of saccharide derivative obtained in b), and d) a step of recovery of the mixture of C4-C8 alkyl monoether of saccharide and/or of saccharide derivative and of C9-C18 alkyl monoether of saccharide and/or of saccharide derivative.

18. The method as claimed in claim 17, wherein the mixture of C4-C8 alkyl monoether of saccharide and/or of saccharide derivative and of C9-C18 alkyl monoether of saccharide and/or of saccharide derivative is a mixture of C5 alkyl monoether of saccharide and/or of saccharide derivative and of C12 alkyl monoether of saccharide and/or of saccharide derivative and in which said saccharide derivative is selected from monoanhydrosorbitol or alkyl glucoside.

19. The method as claimed in claim 17, wherein said saccharide derivative is methyl glucoside.

20. The composition as claimed in claim 12, said composition being a surfactant composition, and wherein said surfactant composition is selected from a detergent, an emulsifier, an emulsion stabilizer, a foaming agent, a foam stabilizer, a liposome stabilizer, a dispersant, and a wetting agent.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) FIG. 1 shows the surface tension (mN/m) of the sorbitan ether derivatives as a function of the concentration (g/L). The critical micelle concentration (CMC) is indicated by small arrows, C5EthSorb is represented by black diamonds, C12EthSorb by black dots and the C5/C12EthSorb mixture by empty triangles.

(2) FIG. 2 shows the surface tension (mN/m) of the ether derivatives of methyl glucoside as a function of the concentration (g/L). C5EthMeGlu is represented by black stars, C12EthMeGlu by black diamonds, and the C5/C12EthMeGlu mixture by black crosses.

EXAMPLES

Example 1: Materials and Methods

(3) General Procedure for Measurements of Surface Tension

(4) Surface tension was measured at (25.00.1) C. with a Krss K100MK2 tensiometer using a platinum rod as probe. A total of 1.0 ml of water was added to the measuring vessel. When the surface tension is stable (standard deviation of the last five measurements of 0.2 mN/m), manual dilution is carried out with a concentrated solution of surfactant to its limit of solubility while maintaining constant volume.

(5) Analytical Methods

(6) All the reactants and solvents used for the synthesis are commercial products (supplied by SIGMA-ALDRICH and ACROS ORGANICS), which were used without further purification. All the new compounds obtained were characterized by spectroscopic data. The reactions were monitored by thin-layer chromatography using silica gel on aluminum plates (60F254). The nuclear magnetic resonance (NMR) measurements were recorded on a Bruker DRX 300 or 400 or Bruker ALS 300 spectrometer. The chemical shifts are given by reference to the central peaks of d.sub.6-DMSO or CDCl.sub.3 (39.5 and 77.0 ppm respectively) for .sup.13C NMR, and (2.50 and 7.26 ppm respectively) for .sup.1H NMR. The coupling constants J are given in hertz (Hz). The abbreviations are to be understood as follows: s=singlet, d=doublet, dd=doublet of doublets, t=triplet, q=quadruplet, m=multiplet. The separations by flash chromatography were carried out using Grace Davisil LC60A silica gel (40-63). The analyses by high-performance liquid chromatography were carried out on a column using a C18 column (SPHERISORB C18, 5 m, 250 mm20 mm) using MeCN-water (20/80) eluent and detection by refractive index (RI).

(7) The mass spectra were acquired in positive ion mode using a spectrometer (MicroTOFQ-II, BrukerDaltonics, Bremen) with electrospray ionization (ESI). The flow of atomizing gas is at 0.6 bar and the capillary voltage is 4.5 kV. The solutions were injected at 180 L/h in a mixture of solvents (methanol/dichloromethane/water 45/40/15). The mass range of the analysis is 50-1000 m/z and calibration was performed with sodium formate.

(8) All the acetals were dried over magnesium sulfate (MgSO.sub.4), and filtered before hydrogenolysis.

Example 2: Synthesis of the Sorbitan Acetals

(9) Firstly, the sugar acetals were prepared by acetalization or trans-acetalization of sugars following the procedure described in patent application FR 3 007 031. The ratio of 2:1 between the sugar and the aldehyde was not changed but 0.5 equivalent of a long-chain alkyl aldehyde or of its acetal was replaced with 0.5 equivalent of a short-chain alkyl aldehyde or of its acetal to aid dissolution of the reactants (scheme 1). The desired products were obtained in the form of a mixture of short-chain and long-chain acetals with improved yields for long-chain alkyl acetals, relative to the conventional procedure.

(10) ##STR00001##

(11) In order to synthesize the acetal of sorbitan dodecylidene with a better yield, sorbitan was reacted with valeric aldehyde (0.5 equivalent) in the presence of 20 wt % of AMBERLYST 15 (A15) as acid catalyst. The reaction may be carried out using anhydrous tetrahydrofuran (1 M) or cyclopentyl methyl ether (1 M) as solvent or in conditions without solvent. Moreover, sodium sulfate (1.5 equivalent) was also added as a dehydrating agent in order to trap the water formed during the acetalization reaction, which led to an increase in sorbitan conversion. Sodium sulfate may advantageously be replaced with molecular sieves or by azeotropic removal of water. It should be noted that sodium sulfate was not added to the reaction carried out in conditions without solvent, because of the difficulties of stirring. The reaction mixture was heated at 80 C. for 3 hours, then 0.5 equivalent of dodecanal was added and the reaction mixture was stirred at 80 C. for a further 3 hours.

(12) The results are presented in Table 1 and are compared with those obtained with dodecanal alone.

(13) TABLE-US-00001 TABLE 1 Acetalization of sorbitan with a mixture of short-chain and long- chain aldehydes: Isolated Na.sub.2SO.sub.4 Conversion Conversion yield Ratio Entry Aldehyde (eq.) Solvent C5Ald.sup.b (%) C12Ald.sup.c (%) (1 + 2) C5:C12.sup.c Yield 2 1 C12 1.5 THF nd 87 39.sup.d 2 C5/12 1.5 THF 94 86 69 31:69 81.sup.e 3 C5/12 1.5 CPME 76 76 62 46:54 58.sup.e 4 C12 0 without nd 60 (15 h) 36.sup.d 5 C5/C12 0 without 96 94 (3 h) 77 38:62 82.sup.e .sup.aExperimental conditions: Sorbitan (20 g, 122 mmol), valeric aldehyde (3.2 mL, 31 mmol), AMBERLYST A15 (0.5 g, 20 wt %), 80 C., 3 h, then dodecanal (6.9 mL, 31 mmol), 80 C., 3 h. .sup.bThe conversions were determined from the .sup.1H-NMR spectra. .sup.cThe degrees of conversion were determined by HPLC. .sup.dIsolated yield. .sup.eYield calculated from the total mass recovered and the C5/12 ratio as obtained by HPLC.

(14) After reaction, the crude mixture was purified by silica gel column chromatography to obtain a mixture of C5/C12 acetals. The ratio of the C5 and C12 sorbitan acetals was then determined by HPLC. The weight and the yield of sorbitan dodecylidene acetal (2) were calculated from these data. The mixture of 3,5- and 5,6-O-dodecylidene-1,4-sorbitan (2) was obtained with an isolated yield of 39% (entry 1) in the presence of solvent and without surface-active acetal intermediate. However, with the synthesis of the mixture of 3,5- and 5,6-pentylidene-1,4-sorbitan (1) as a surfactant, the desired long-chain alkyl sorbitan acetal (2) was isolated at 81% in THF (entry 2) and at 58% in CPME (entry 3). Moreover, the isolated yield of pentylidene and dodecylidene acetal obtained was 69% of the isolated yield in THF and 62% in CPME. Without solvent, the effect is even greater. In this case, sodium sulfate was not added, with the aim of making magnetic stirring possible. In fact, the isolated yield of sorbitan dodecylidene acetal went from 36% to 82% owing to the surface-active characteristics of pentylidene sorbitan (entries 4-5). More precisely, using pentylidenesorbitan acetal (1) in situ, the degree of conversion of dodecanal went from 60% to 94% and a shorter reaction time is required. These results show that the acetal of sorbitan pentylidene allows dissolution of the long hydrophobic alkyl chains in the medium and display solvo-surfactant properties. The use of intermediate surfactants contributes to better dissolution of the reactants by lowering the surface tension between the polar and nonpolar phases. The desired APG is thus obtained. In fact, butanol serves both as solvent and reactant, to form O-butyl glucoside, which will then be miscible with FOH during the subsequent transglycosidation. Moreover, this method could be carried out on oligosaccharides in one and the same medium, in other words in a single reactor.

Example 3 Hydrogenolysis of the C5/C12 Acetals

(15) Hydrogenolysis of the mixture of sorbitan acetals of 3,5- and 5,6-pentylidene (1) and of dodecylidene (2) was carried out using the conditions already described by the inventors in a previous French patent application No. 14/01346. The optimized hydrogenolysis conditions [5 mol % of Pd/C (5%), 120 C., 30 bar of H.sub.2, CPME (0.1 M), with a mechanical stirring speed of 800 rev/min] were used for the synthesis of sorbitan monoethers in position 3, 5 and 6 obtained from sugar acetals (scheme 2).

(16) ##STR00002##

(17) TABLE-US-00002 TABLE 2 Hydrogenolysis of sorbitan acetals.sup.a: Sugar Conversion Isolated Ratio Selectivity Entry acetal (1 + 2).sup.b(%) yield (3 + 4).sup.d C5:C12.sup.c Yield 4 (%) 1 C12 97 55.sup.d 57 2* C5/12 65 18 36:64 17.sup.e 26 3 C5/C12 >98 89 48:52 68.sup.e 69 .sup.aExperimental conditions: sorbitan (20 mmol), Pd/C (5 mol %, 5% Pd), 120 C., 30 bar H.sub.2, stirring speed = 800 rpm *magnetic stirring, 15 h. .sup.bConversions determined from the .sup.1H NMR spectra. .sup.dIsolated yield. .sup.eYields calculated with the C5/12 ratio.

(18) The results are summarized in Table 2. Starting from (3,5+5,6)-dodecylidene sorbitan (entry 1), the acetal (2) was transformed completely. However, in these cases, the isolated yield of (6+5+3)-dodecyl sorbitan (4) was only 55%. However, starting from the previous mixture of C5/C12 acetals of sorbitan (entry 2), the conversion is lower (65%). In fact, with mechanical stirring, conversion is total and the selectivity reaches 69% instead of 57% without pentylidene sorbitan acetal, to obtain the desired product with 68% yield.

(19) Based on these results, the inventors have shown that the limiting step of acetalization with the long-chain alkyl aldehyde could be improved by using a short-chain alkyl acetal of sorbitan as an intermediate, which performs the role of solvo-surfactant by dispersion of the hydrophobic reactant in a polar medium. Moreover, this study showed that the long-chain alkyl sorbitan ethers can be synthesized with a better yield starting from the C5/C12 mixture.

Example 4: Physicochemical Properties of the Surfactants Synthesized

(20) Surface-Active Properties

(21) Several approaches may be used for studying the properties of surfactants. Firstly, surface tension tests were carried out between air and water with a view to determining the variation of the properties as a function of the length of the alkyl chain and of the polar head.

(22) In the present study, the general characteristics of the surfactants were evaluated by measuring the reduction in saturated surface tension (.sub.sat) and the critical micelle concentration (CMC) of the mixture (3+4) previously obtained in water at 25 C. The surface-active properties were compared against those of the pure compounds.

(23) The Surface Tension of the Aqueous Solutions

(24) The surface tension of aqueous solutions containing increasing concentrations of each of the compounds was measured by the Du Noy-Padday method using a platinum rod as probe (method described in J. F. Padday, A. R. Pitt, R. M. Pashley, J. Chem. Soc. Faraday. Trans 1, 1975, 71, 1919-1931). The data are presented graphically in FIG. 1, showing analysis of the surface tension as a function of the concentration of the sorbitan ether derivatives (CSEthSorb (black diamonds), C12EthSorb (black lozenges), C5/C12EthSorb (black triangles)). The value of the critical micelle concentration (CMC) is shown with a small arrow for C5EthSorb.

(25) For all the compounds, lowering of the surface tension of water is observed and the saturation value is reached at very low concentrations. According to the curves, the critical micelle concentrations (CMC) and the saturation surface tension are presented in Table 3.

(26) TABLE-US-00003 TABLE 3 Minimum hydrotrope concentration (MHC) or critical micelle concentration (CMC) and surface tension of water (.sub.sat). Ratio (3c:3b:3a)/ CMC Product (4c:4b:4a).sup.a (mg/L) .sub.sat (mN/m) 3c + 3b + 3a 5-,3-,6-regioisomers C5EthSorb 26:33:41 1779 32 C12EthSorb 27:33:40 30.2 31 C5/C12EthSorb (48/52) 37:16:47/52:38:10 29.8 30 .sup.aRatio of measurements obtained by HPLC

(27) We may conclude from these results that for all the compounds, lowering of the surface tension of water is observed and the saturation value is reached at very low concentrations. Moreover, it may be noted that the C5/C12 mixture, in the ratio 48:52, gave surface-active properties similar to those of pure sorbitan dodecyl ether (by CMC for C5/C12 of 29.8 mg/L and 30.2 mg/L for C12). These results demonstrate the synergy of the C5/C12 mixture. Thus, pentyl sorbitan (3) makes it possible to increase the solubility of the mixture while obtaining surface-active characteristics similar to those of the pure dodecyl sorbitan (4). In fact, the dodecyl sorbitans cause a decrease in surface tension of water even at low concentration of the mixture of surfactants. With sorbitan pentyl ether, a concentration of 1179 mg/L is necessary to lower the surface tension by 31 mN/m whereas a concentration of 29.8 mg/L of the C5/C12 mixture with only 48% of pentyl sorbitan is sufficient to reach this same surface tension.

Example 5: Synthesis of C5/C12 Acetals of Methyl Glucopyranoside

(28) 4,6-O-Dodecylidene--D-methyl glucopyranoside was synthesized as in example 2 with valeric aldehyde (0.5 equivalent) in the presence of 20 wt % of AMBERLYST 15 (A15) as acid catalyst (see scheme 3). As mentioned above, the reaction may be carried out using anhydrous tetrahydrofuran (1 M) or cyclopentyl methyl ether (1 M) as solvent or in conditions without solvent. Moreover, sodium sulfate (1.5 equivalent) was also added as dehydrating agent. The reaction mixture was heated at 80 C. for 3 hours, then dodecanal (0.5 equivalent) was added and the reaction mixture was stirred at 80 C. for a further 3 hours.

(29) ##STR00006##

(30) The results are presented in Table 4 and are compared with those obtained with dodecanal alone.

(31) TABLE-US-00004 TABLE 4 Acetalization of methyl glucoside using a mixture of short-chain and long-chain aldehydes Na.sub.2SO.sub.4 Conversion Conversion Isolated Ratio Yield Entry (eq.) Aldehyde C5Ald.sup.a C12Ald.sup.b yield C5:C12.sup.b C12 1 1.5 C12 76 28 2 0.75 + 0.75 C5/C12 64 72 41 45:55 39 .sup.aConversions determined by .sup.1H NMR. .sup.bRatios determined by HPLC

(32) After reaction, the crude mixture was purified by silica gel column chromatography to give a mixture of C5/C12 acetals of methyl glucoside. The ratio of the C5 and C12 acetals of methyl glucoside was then determined by HPLC. The weight and the yield of acetal dodecylidene of methyl glucoside were calculated from these data.

(33) Without an intermediate surfactant, 4,6-O-decanylidene--D-methyl glucopyranoside was obtained with an isolated yield of 28% (entry 1). However, during synthesis of the 4,6-O-pentylidene--D-methyl glucopyranoside, the desired long-chain alkyl acetal of methyl -D-glucopyranoside was isolated in THF with an improved yield of about 39% (entry 2). These results demonstrate that 4,6-O-pentylidene -D-methyl glucopyranoside allows dissolution of the hydrophobic long-chain alkyl.

Example 6: Hydrogenolysis of the C5/C12 Ethers of Methyl Glucopyranoside

(34) Hydrogenolysis of the 4,6-O-dodecylidene--D-methyl glucopyranoside mixture was carried out using the conditions already described in example 3 (see scheme 4).

(35) ##STR00007##

(36) TABLE-US-00005 TABLE 5 Hydrogenolysis of the acetals of methyl glucoside.sup.a: Ratio of Isolated Ratio of Calculated Acetals yield Ethers yield Entry C5:C12.sup.b Conv..sup.c C5/C12.sup.d C5:C12.sup.b C12.sup.e 1 59 51 2 45:55 68 28 46:54 28 .sup.aExperimental conditions: acetals of methyl glucoside (20 mmol), of Pd/C (5 mol %, 5% of Pd), CPME (200 mL), 120 C., 30 bar of H.sub.2, the stirring speed is 800 revolutions per minute, 15 h. .sup.bRatio determined by HPLC. .sup.cConversions determined from the .sup.1H NMR spectra. .sup.dIsolated yield. .sup.eYield calculated with the C5/12 ratio.

(37) The results are summarized in Table 5. Starting from (4,6)-O-dodecylidene--D-methyl glucopyranoside (entry 1), the sugar acetal was converted at 59%. However, in these cases, the calculated yield of (6+4)-O-dodecyl--D-methyl glucopyranoside was only 51%. However, starting from the previous mixture of C5/C12 acetals of methyl glucoside (entry 2), the conversion is higher (68%). In fact, dodecyl--D-methyl glucopyranoside was obtained with only 28% of calculated yield. Use of the glucoside acetal with a short alkyl chain does not allow the yield of long-chain alkyl glucoside ether to be increased.

Example 7: Physicochemical Properties of the Surfactants Synthesized

(38) Surface-Active Properties

(39) The general characteristics of the surfactants were evaluated as in example 4 by measuring the reduction in saturated surface tension (.sub.sat) and the critical micelle concentration (CMC) of the mixture (C5/C12 MeGlu) obtained previously in water at 25 C. The surface-active properties were compared with pure compounds.

(40) The Surface Tension of the Aqueous Solutions

(41) The surface tension of aqueous solutions containing increasing concentrations of each of the compounds was measured by the plate method using a platinum rod as probe (Du Nouy-Padday method as described in J F Padday, A R Pitt, R M Pashley, J. Chem. Soc. Faraday. Trans 1, 1975, 71, 1919-1931). The data are shown in FIG. 2, showing analysis of the surface tension from the concentration of the ether derivatives of methyl glucoside (C5EthMeGlu (black star), C12EthMeGlu (black diamond), C5/C12EthMeGlu (black cross)).

(42) For all the compounds, lowering of the surface tension of water is observed and the saturation value is reached at very low concentrations. According to the curves, the critical micelle concentrations (CMC) and the saturation surface tension are given in Table 6.

(43) TABLE-US-00006 TABLE 6 Minimum hydrotrope concentration (MHC) or critical micelle concentration (CMC) and surface tension of water (.sub.sat) Product CMC (mg/L) .sub.sat (mN/m) C5EthMeGlu 8199 32 C12EthMeGlu 4.5 28 C5/C12EthMeGlu 18.9 30 (46/54)

(44) Based on these results and as mentioned above for the sorbitan ethers, we may conclude that for all the compounds, a lowering of the surface tension of water is observed and the saturation value is reached at very low concentrations. Moreover, similar surface-active properties are also observed between the C5/C12 mixture, in the ratio 46:54 and the pure dodecyl of -D-methyl glucopyranoside (with a CMC of 18.9 mg/L for the C5/C12 mixture and of 4.5 mg/L for C12). As observed previously in example 4 for the sorbitan ethers, these results confirm the synergy of the C5/C12 mixture. In fact, pentyl -D-methyl glucopyranoside performs the role of solvo-surfactant by improving the solubility of the mixture, and more particularly of dodecyl -D-methyl glucopyranoside. Moreover, at low concentration, dodecyl -D-methyl glucopyranoside lowers the surface tension of water. With pentyl -D-methyl glucopyranoside, a concentration of 8199 mg/L is necessary to lower the surface tension by 32 mN/m whereas only 18.9 mg/L of the C5/C12 mixture is necessary to reach a surface tension of 30 mN/m.