Long-chain alkylglucosides, ie. alkylglucosides where the alkyl chain numbers 8 or more carbon atoms, are used in industry as nonionic surfactants of high wetting power, for example in highly alkaline industrial cleansers. They are also increasingly used as constituents in skin and hair care agents. This use, however, requires particularly pure products.
Long-chain alkylglucosides are prepared for example by reacting monosaccharides, or reducing sugars which are hydrolyzable to monosaccharides, and alcohols of 8 or more carbon atoms in the presence of acidic catalysts. However, short-chain alkylglucosides, for example C.sub.1 -C.sub.6 -alkylglucosides, can also be transacetalated with alcohols of 8 or more carbon atoms in the presence of acidic catalysts. In either case, however, an appreciable excess of long-chain alcohol is required. The products are thus always reaction mixtures consisting of the long-chain alkylglucoside and the excess long-chain alcohol. The long-chain alkylglucoside must be isolated from these mixtures since otherwise, if the long-chain alkylglucosides are used in aqueous media, unclear solutions are formed. Processes for preparing long-chain alkylglucosides are known; cf. for example U.S. Pat. Nos. 3,219,656, 3,598,865, 3,839,318 and 3,547,828.
To isolate the long-chain alkylglucoside from the reaction mixture, the practice has been to distill off the long-chain alcohol at around 140.degree. C. under much reduced pressure. The high thermalstress on the alkylglucoside gives rise to resinification and increasingly, especially with alkyl glucosides having comparatively long chains, to the formation of dark products.
U.S. Pat. No. 3,547,828 discloses the purification of neutralized mixtures of long-chain alkylglucosides and long-chain alcohols by treating the mixtures with acetone. However, such a method of purification is very expensive to practice in industry. German Laid-Open Application DOS 3,001,064 discloses a process for purifying C.sub.8 -C.sub.16 -alkylglucosides obtained by reacting short-chain alkylglucosides or hydroxyalkylglucosides with C.sub.8 -C.sub.16 -alcohols in the presence of acidic catalysts. In this process, the reaction mixture is neutralized and the long-chain alcohol is separated off by distillation, the distillative removal of at least the last fractions of unconverted long-chain alcohol being carried out in the presence of glycols whose boiling points are not more than 10.degree. C. above and not more than 30.degree. C. below the boiling points of the long-chain alcohols to be separated off. It is true that the addition of glycol brings about a reduction in the viscosity of the mixture to be purified by distillation, but on the other hand it requires the removal of the added glycol. U.S. Pat. No. 3,839,318 also discloses separating the excess long-chain alcohol from mixtures of long-chain alcohols and long-chain alkylglucosides by means of a solvent. The solvent used is for example heptane. However, this process has the disadvantage that it requires the additional use of a solvent which needs to be recovered.