Indian Journal of Chemistry Vol. 4.'iA. April 2006. pp. XS2 - S86 Oxidation of nonionic surfactant (Tween-80) by chromium(VI) in acidic medium Syed Mi sbah Zahoor Andrabi & Zaheer Kh an* Dcpartment of Chemistry. Jamia Millia Islamia. lamia Nagar. New Delhi 110 Email : drkhanchem@yahoo.co.in and Kabir-ud-Din Department of Chemistry. Aligarh Mu slim Univ er si ty. Aligarh 202002. India Recei l'ed]/ }mlll(// }' 2005: revised J] Februarv 2006 The oxidation of Twccn-SO by ch ro mium(VI) in aqueo us acidic so lutions has bccn st udi ed at diff erent temperat ures (30-60 °C). The rat es of disappea rance of chromiull1( VI ) absorban ce follow sccond ord er kineti cs: fir st order eac h in ICr( VI )1and rTw-8 01 at constant ac idity. The rcaction ord er with re spect to rl-n increases from one to higher 'lv ith increasc in rH C10 4 1. Interes tingly. Tw- 80 is not oxidi sc d in absence of acid. The rcaction is considered to proceed through th e formati on of a chromate es ter bctween th e chromiull1 ( VI) and primary - 01-1 group of polyoxyethylene chain of Tw- SO. T he activation parameters ca l cu lated using Arrhcnius and Eyring equ: lIi ons are: E" = 46 kJ mol I. /,;J-f' = 44 kJ mol-I and /';:}' = - 200 JK- I mol- I. A mcc hani sm co nsistent with the res ults obtaincd has been proposed with foll owing rate equ ation: (kIKc, IK"d 1-1 +1 + k2Kc"K,,2 K" dH + f) rTw-80J - dICrvIJ / cll = (I+K"III-I+J + K"IK,dI-l+J 2) IPC Cod,,: Inl. C I. R C07I D3/00 The ro le of surfactants in bulk solution and at interfac es IS of grea t importance in in dustrial processes involvi ng pa int s, detergents, fo od che mi stry and oil recovery. Kin et ic and mec hani stic studi es of a va ri ety of che mi cal reactions in presence of surfactant agg re gates (m ic e ll es) have been ca rri ed o ut to und erstand th e role of mi celles I.,. However, th e re are few repo rt s ava i la ble wi th su fficient info rl11ation to reveal how sur fac tant molecules act as a reactant (r ed ucta nt ) .j· 5. There are no reports on the oxidation of T\ veen-80 by chromium(VI). Kin etic studies in surfactant med ia s an provide mechanistic deta il " abo ul hydrophobic and electrostat ic influences on nol o nl y inorganic redox reactions but also on biological electron tran sfers whi ch take place on membrane surfaces or at prote in -s ub strate interfaces. We have re po rt ed ea rli er the ox idations of no n-i o ni c surfactant (TX - IOO ) an d anI o nIc surfactant (S DS ) by cerium(lV)4 and pennangan ate 6 respectively, wh ere in we observed tilat th e monomer a nd the aggregates of th ese surfactants are ox idi sed. The main objective in thi s st ud y is to ex te nd th e inves ti ga ti on to other surfac tants. elucidate suitable mecha ni sm and put forward th e rate equation to shed li ght on th e ox id a ti ve-degradation mechanis m. Mnterials and Methods Reage nt grade potassium dichromate (99%, Merck. India), manganese(ll) c hl oride (99%, Qual igen s. India) and Tween-80 (99 %, Merc k, Indi a) were used to prepare their stock so lu tions. To mai nt ain th e lH+l constant, perchl oric ac id (Thomas Baker, 70% reage nt) was L1 se d. The so luti on of potassi um dichl J mate was stored in a da rk glass bottle. Doubly distilled (first time from alkaline KMnO.j) . CO 2 -free deionised water was L1sed for dil ut io n and for th e preparation of stock solution of a ll th e reactants. The reaction was carried o ut in a glass-stoppered three-necked fla sk fitted with a doubl e-wall ed condenser to check evaporation. A mixture co nt a ini ng required amounts of chromium(V I), and H2 was th ermally equilibrated at th e req uired te mp erature (3 0-60°C) and to thi s was added a measured amount of Tw-80 solution, pre-equi librated at th e sa me tem perature. The reaction vo lume was always 50 cm J Th e co urse of th e reac ti on WJ" monitored by measuring th e absorbance of th e re lll Htllln g chrornium(Vl) at definite time inter e ll s at 370 11m Oil a Spectronic 20-0 spectrophotomele r. The re actio n was norma ll y fo ll owed up to 80% completion. The pseudo-first o rd er rate constants (k ob ,. s" ) were