JOURNAL OF OLEO SCIENCE Copyright ©2006 by Japan Oil Chemists’ Society J. Oleo Sci., Vol. 55, No. 3, 99-119 (2006) JOS REVIEW Amine Oxides: A Review ＊ Sudhir Kumar SINGH, M. BAJPAI and V.K. TYAGI Department of Oil and Paint Technology, Harcourt Butler Technological Institute (Kanpur-208 002, INDIA) Edited by M. Iwahashi, Kitasato Univ., and accepted September 20, 2005 (received for review August 29, 2005) Abstract: Amine oxides are amine-based surfactants, represent one of the smaller classes of surfactants as compared to alcohol ethoxylates and sulfonated and sulfated anionic surfactants. However, the uniqueness of the hydrophile in such surfactants provides specific properties that are difficult, if not impossible, to replicate by the use of classic nonionic and anionic surfactants. The aim of the present paper is to survey the most important developments and understandings of the chemistry of amine oxide production, it’s physico-chemical studies, applications and environmental properties. Key words:amine oxide, amine-based surfactant, hydrophile, physico-chemical, environmental dispersant, and in deodorant bars (anti-bacterial agent), 1 Introduction due to their compatible synergistic effect and environ- Although, amine oxides were known and studied ment friendly nature. Amine oxides are exothermic, before 1900, it was not until 1939, with the issuance of second order reaction products of tertiary amines and an I.G. Farbenindustrie patent that material such as hydrogen peroxide (3). The nature of tertiary amine in dimethyldodecyl amine oxide were recognized as sur- amine oxides may be aliphatic, aromatic, heterocyclic, factant. After a further 22 years their utility in liquid alicyclic or combination thereof. In current amine household formulations was disclosed and widespread oxides the surfactant precursor is generally a C12 - C18 interest was generated. The substitution for the tradi- alkyldimethyl amine (4). tional fatty alkanolamides as foam booster in dishwash- Amine oxides come under the special class of surfac- ing was the specific example which brought recognition tant known as amphoteric surfactant. The basic reason to the amine oxides. Their favourable weight/effect behind that is, amine oxide changes from net cationic ratio offsets the considerably higher cost in this applica- via zwitterionics to nonionics on going from low to tion (1). high pH; which confirms it’s amphoteric nature. Reaction of hydrogen peroxide with secondary or This paper presents a detailed review on amine primary amines does not result in commercially useful oxides, with special emphasis on the chemistry of oxi- materials (2), but with tertiary amines a variety of com- dation, physico-chemical studies, various applications mercial useful materials obtained which are used not and anti-microbial activity with variation in chain only in various types of cleaning formulations but also, length. The last part of this paper is briefly focused on proving their utilities in liquid bleach products (surfac- the safety of amine oxides. tant basis), textile industry (anti-static agent), rubber industry (foam stabilizer), polymer industry (polymer- ization catalyst), anti-corrosion compositions, lime soap ＊ Correspondence to: V.K. TYAGI, Department of Oil and Paint Technology, Harcourt Butler Technological Institute, Kanpur - 208002, INDIA E-mail: [email protected], [email protected] Journal of Oleo Science ISSN 1345-8957 print / ISSN 1347-3352 online 99 http://jos.jstage.jst.go.jp/en/ S.K. Singh, M. Bajpai and V.K. Tyagi During the synthesis of amine oxides information on 2 The Chemistry of Oxidation for Amine the amount of unreacted tertiary amine present is need- Oxide Formation ed in order to follow the reaction. A number of analyti- The generally accepted mechanism for oxidation of cal procedure including chromatographic procedures tertiary amines with hydrogen peroxide involves the have been devised to obtain this information. But all of ammonium peroxide as an intermediate followed by these procedures have some limitations. Wang and Met- splitting off water. Previous work (3) strongly suggests calfe (6) developed a simple, rapid, non aqueous titra- the reversibility of the formation of the ammonium per- tion procedure that makes use of the “anamalous salt” oxide. The proposed mechanism is as follows: behaviour of amine oxides. A modified solvent and k1 ∗ k3 titrant is used to obtain two potential breaks in the titra- RN3223223+⇔ HO() RNHO. →+ RNO HO 2 k2 rxn.intermediate tion. The first break corresponds to half of the amine From the above proposed mechanism, the rate of for- oxide. The second break represents the second half of mation of amine oxide can be derived; using “steady- the amine oxide plus any unreacted amine. With this state approximation”. The rate of formation of amine information the amine oxide and unreacted amines can oxide can be written as, be calculated. γ = []∗ ()RNO3 kRNHO33. 22 ･･･････････････････（I） 3Synthesis of Amine Oxides Since, the concentration of intermediate is so small that it can’t be measured, so, it has to be replaced by the 3･1 Synthesis of Dimethylalkyl Amine concentrations that can be measured. Oxides and Cyclic Amine Oxides From proposed mechanism we have, Friedli et al. (3) prepared dimethyllauryl amine oxide, ∗ N-laurylmorpholine oxide, N-laurylpiperidine oxide γ ()RNHO. ∗= kRNHO13[][] 22− kRNHO23[]. 22 322 ･･(II) and N-lauryl-3- methyl piperidine oxide with their − ∗ kRNHO33[]. 22 respective amines by reacting with 51% aqueous hydro- Now, because the concentration of (R3N. H2O2)* is gen peroxide at 75℃. Their rates of formation indicates always extremely small, one may assume that it’s rate that the reaction is of second order and two piperidine of change be zero {This is called “steady-state approxi- versions form slower than dimethyl-lauryl amine oxide, mation” (5)}. while lauryl morpholine reacts much faster. Hence, γ ()∗ = RN322.. HO 0 ･･･････････････････････････(III) 3･2 Synthesis of 2-alkoxy-N, N- From equations (I), (II) and (III) we get, dimethylethyl Amine N-oxides kk[][] RN HO Hayashi et al. (7) prepared 2-lauryloxy-N,N- γ = 13 3 2 2 RNO3 dimethylethyl amine N-oxide with it’s amine by react- kk23+ ing with 30% aq hydrogen peroxide at room tempera- or ture. The amine oxide was concentrated in a drying box γ = [][] RNO3 KRN322 HO ････････････････････(IV) under reduced pressure resulted into a crystalline solid where K is overall rate constant and amine oxide. Such amine oxides are stable up to 100℃, K = k1k3 / (k2 + k3) but decomposes rapidly to vinyl ethers at 150℃. At low So, from equation (IV) it is evident that overall order temperature they deoxygenated to their tertiary amine. of reaction for amine oxide formation is 2, and is in Hygroscopic property decreases as length of the alkyl complete agreement with the experiment. chain increases. It has been observed that the degree of conversion of tertiary amines to its amine oxides is dependent on the 3･3Synthesis of Alkyl Benzene Derived purity of the tertiary amines. With commercially avail- Amine Oxides able undistilled tertiary amine yields in the range of Marmer and Linfield (8) prepared aromatic amine 85% to 87% are obtained. But with freshly redistilled oxides via a three step route from a variety of pure 1- tertiary amine and with 10% molar excess hydrogen phenylalkanes and also from a commercial detergent peroxide the yield may go upto 99% (1). alkylate mixture. The process includes (i) sulfonation of 100 J. Oleo Sci., Vol. 55, No. 3, 99-119 (2006) Amine Oxides: A Review the phenylalkane with chlorosulfonic acid in 1,2- dichloroethane. (ii) reaction of resulting alkarylsulfonyl chloride with H2N(CH2)3NMe2 (Me is for -CH3 group) or H2N(CH2)3N(CH2CH2OH)2 under anhydrous condi- tions at room temperature, and (iii) oxidation of result- ing tertiary amines with 30% aqueous hydrogen perox- ide at 65℃. Such aromatic amine oxides were found to be thermally stable below 125℃. 3･4 Synthesis of N, N-dimethylalkyl Amine N-oxide by Micellar-Autocatalysis Rathman and Kust (9) investigated the synthesis of N,N-dimethyldodecyl amine N-oxide in aqueous solu- tions by micellar autocatalysis. The lipophilic reactant, dimethyl dodecylamine was initially solubilized in micellar solutions of the amine oxide surfactant, result- ing in substantially higher reaction rates. Amine con- versions of 90-100% were obtained within 2 h at 70℃. The effects of reactant concentrations, temperature, and initial surfactant concentration were studied. This method is important because of two main reasons: (I) Micellar auto catalysis provides a method for synthesiz- Fig. 1 Molecular Structures of Respective Amine Oxides ing surfactants without employing volatile organic sol- of Section 3. vents in the reaction medium, providing potential eco- nomic and environmental benefits and (II) studies of 5 Physico-Chemical Studies on micellar auto catalysis can refine and extend the under- Amine Oxides standing of the other types of reactions in aqueous sur- factant solutions. It is well known that the dissociation constant K of weak electrolytes at the charged interface are different from those in their solutions. Funasaki (12) were meas- 4 Analysis of Amine Oxides ured the dissociation constant K of acid -base indicators Pinazo and Domingo (10) investigated turbidimetric in aqueous solutions of 20 mM dodecyldimethyl amine analysis of amine oxides and amine oxide - anionic sur- oxide (DDAO), 1% Brij 35 (C12H25-(O-CH2-CH2-)23- factant mixtures. This automatic analysis