Micellar For ma tion Study of Crown Ether Surfactants

Transcription

1 Jour nal of the Chi nese Chem i cal So ci ety, 2002, 49, Micellar For ma tion Study of Crown Ether Surfactants Fang-Fei Chou ( ) and Jeng-Shong Shih* ( ) De part ment of Chem is try, Na tional Tai wan Nor mal Uni ver sity, Tai pei, Tai wan 116, R.O. C. Flu o res cence probe and nu clear mag netic res o nance (NMR) meth ods were em ployed to in ves ti gate the micellation of pre pared crown ether sur fac tants, e.g. decyl 15-crown-5 and decyl 18-crown-6. Pyrene was em - ployed as the flu o res cence probe to eval u ate the crit i cal micellar con cen tra tion (CMC) of these sur fac tants in aque ous so lu tions while spin lat tice re lax ation times (T 1) and chem i cal shifts of H-1 NMR were ap plied in non-aqueous so lu tions. Decyl 15-crown-5 with lower CMC forms mi celles much eas ier than decyl 18- crown- 6 with higher CMC in aque ous so lu tions, whereas decyl 18-crown-6 forms mi celles eas ier than decyl 15-crown-5 in nonaqueous so lu tions. Com par i son of the CMC of crown ether sur fac tants and other polyoxy - ethylene sur fac tants such as decylhexaethylene gly col was made. Ef fects of salts and sol vents on the micellar for ma tion were also in ves ti gated. In gen eral, ad di tions of both al kali metal salts and po lar or ganic sol vents into the aque ous surfactant so lu tions in creased in the CMC of these sur fac tants. The for ma t ion of mi celles in or ganic sol vents such as meth a nol and acetonitrile was suc cess fully ob served by the NMR method while it was dif fi cult to study these sur fac tants in or ganic so lu tions by the pyrene flu o res cence probe method. The NMR study re vealed that the for ma tion of mi celles re sulted in the de crease in all H-1 spin lat tice re lax ation times (T 1) of hy dro pho bic groups, e.g. CH 3 and CH 2, and hy dro philic group OCH 2 of these sur fac tants. How - ever, upon the micellar for ma tion, the H-1 chem i cal shifts ( ) of these surfactant hy dro pho bic groups were found to shift to downfield (in creased ) while the chem i cal shift of the hy dro philic group OCH 2 moved to up - field. Com par i son of the spin lat tice re lax ation time and H-1 chem i cal shift meth ods was also made and dis - cussed. IN TRO DUC TION Ar ti fi cial macrocyclic polyethers such as crown ethers and cryptands have a dual func tion al ity, with a lipophilic seg - ment on the outer sur face of the ring and a po lar seg ment on the in ner sur face that can form sta ble com plexes with metal ions 1-4 and po lar or ganic spe cies. 5-7 Attaching a lipophilic long-chain alkyl group to a crown ether or cryptand can con - vert it into a surfactant, which is ca pa ble of form ing mi celles in a man ner anal o gous to clas si cal non-ionic sur fac tants In this study the crown ethers, decyl 15-crown-5 and decyl 18-crown-6, were pre pared. Like com mon sur fac tants, the macrocyclic polyether surfactant can pro duce foams and can be ap plied in the sep a - ra tion of ions by flo ta tion. 10 Fur ther more, the long-chain macrocyclic polyether surfactant can be em ployed as an ionophore to carry var i ous al kali metal ions through liq uid mem branes In gen eral, a pyrene flu o res cence probe is em ployed to mea sure the crit i cal micellar con cen tra tion (CMC) of sur fac tants in clud ing the macrocyclic polyether surfactant in the aque ous so lu tions. Upon for ma tion of mi - celles, pyrene will move into the in side of the mi celles from the aque ous phase which re sults in an al ter ation in the in ten - sity ra tio (I 1 /I 3 ) of pyrene flu o res cence bands I (373 nm) and III (383 nm). 14,15 The pyrene flu o res cence probe has been dem on strated as a good tool for the de ter mi na tion of the CMC of var i ous sur fac tants in aque ous so lu tions; thus, in this study, the pyrene flu o res cence probe method was ap plied to de ter mine the CMC of these crown ether sur fac tants in aque - ous so lu tions. How ever, the pyrene flu o res cence probe method the o ret i cally can t work for the surfactant in or ganic sol vents or or ganic/wa ter mixed sol vents, be cause the pyrene mol e cules place in or ganic sol vent whether the mi celle of surfactant forms or not. In other words, the pyrene can t move into the surfactant mi celles in or ganic sol vents which leaves the in ten sity ra tio (I 1/I 3) of pyrene flu o res cence un - changed. It means that it is dif fi cult to use the pyrene flu o res - cence probe to de ter mine the CMC of sur fac tants in or ganic sol vents. There fore, in this study, we have de vel oped the NMR method for the de ter mi na tion of CMC of these crown ether sur fac tants in or ganic so lu tions. Spin lat tice re lax ation times (T 1) and chem i cal shifts of H-1 NMR have been suc - cess fully ap plied to de ter mine the CMC of crown ether sur - fac tants in non-aqueous so lu tions. EX PER I MEN TAL Syn the sis of Crown Ether Surfactants Crown ether surfactant, decyl 15-crown-5, was syn the -

2 600 J. Chin. Chem. Soc., Vol. 49, No. 4, 2002 Chou and Shih sized by the re ac tion of 1,2-epoxydodecane with tetra ethyl - ene gly col to get decyl pentaethylene gly col as an in ter me di - ate fol lowed by the cyclization of decyl pentaethylene gly col as shown in Fig. 1. Firstly, tetraethylene gly col (0.5 mole) was added in drops into a 150 ml so lu tion of so dium hy dride (0.25 mole) in 1,4-dioxane and mixed with 1,2- epoxy dode - cane (0.24 mole). The in ter me di ate decyl pentaethylene gly - col was ob tained from the re ac tion of 1,2-epoxydodecane and tetraethylene gly col by re flux for 24 hr and acid i fi ca tion with 6 N HCl fol lowed by vac uum evap o ra tion at 200 C/2 mm Hg. The prod uct of decyl 15-crown-5 was ob tained by ad di - tion of p-toluenesulphony chlo ride (0.12 mole) in drops into a mix ture of decyl pentaethylene gly col (0.12 mole) with NaOH (0.48 mole) in 1,4-dioxane (600 ml) and vac uum evap o ra tion at 140 C/2 mm Hg. The fi nal prod uct of decyl 15-crown-5 iden ti fied with mass spec trom e try (M + = 361) and H-1 NMR spec trum with (ppm), 0.90 (t, CH 3, 3H), 1.26 (s, CH 2, 18H), (m, OCH 2CH 2O, OCHCH 2O, 19H). The syn thetic pro ce dure for decyl 18-crown-6 is also de - picted in Fig. 1. The fi nal prod uct of decyl 18-crown-6 was ob tained by vac uum evap o ra tion at 160 C/2 mm Hg and also iden ti fied with mass spec trom e try (M + = 405) and H-1 NMR spec trum with (ppm), 0.86 (t, CH 3, 3H), 1.28 (s, CH 2, 18H), (m, OCH 2CH 2O, OCHCH 2O, 23H). Crit i cal Micellar Con cen tra tion (CMC) Mea sure ments In this study, pyrene flu o res cence probe and NMR meth ods were em ployed to mea sure the CMC of crown ether surfactant in aque ous and non-aqueous so lu tions, re spec - tively. In the pyrene flu o res cence method, the CMC of sur - fac tants was mea sured at var i ous aque ous con cen tra tions with M pyrene added. The con cen tra tions where a sig nif i cant slope change in the plot of I 1 (373 nm)/i 3 (383 nm) in ten sity ra tio against surfactant con cen tra tion are de noted as CMC. In the NMR method, the CMC of the crown ether sur - factant was de ter mined as the con cen tra tion at a point where a sig nif i cant change in the plot of spin lat tice re lax ation times (T 1) or chem i cal shifts of H-1 NMR against surfactant con - cen tra tion. The spin lat tice re lax ation time T 1 was mea sured by the in ver sion re cov ery method 16 with [180 - (De lay time)-90 ] n pulse se quences as shown in Fig. 2. Ap pa ra tus A JASCO FP770 fluorimeter was used to mea sure the flu o res cence of pryrene, whereas a JEOL JNM-FX100 Fou - rier trans form NMR was em ployed to mea sure spin lat tice re - lax ation times (T 1) and chem i cal shifts of H-1 NMR. The JEOL Fou rier trans form NMR and JEOL JMS-D300 mass spec trom e ters were ap plied to iden tify the in ter me di ates and fi nal prod ucts of the crown ether sur fac tants. Chem i cals Pentaethylene gly col and 1,2-epoxydodecane were ob - tained from Aldrich Chem i cal Co. and tetraethylene gly col was pur chased from Janssen Chem i cal Co. All other chem i - cals used were of an a lyt i cal grade. RE SULTS AND DIS CUS SION Flu o res cence Study on CMC of Crown Ether Surfactant Pyrene flu o res cence probe method was ap plied to study the for ma tion of crown ether sur fac tants, e.g. decyl 15- crown- 5 and decyl 18-crown-6 in aque ous so lu tions. The Fig. 1. Sche matic syn the sis di a gram of decyl 15- crown- 5 and decyl 18-crown-6. Fig. 2. H-1 Spin lat tice re lax ation time (T 1) mea sure - ment for decyl 15-crown-5 in acetonitrile.

3 Micellar For ma tion Study of Crown Ether Sur fac tants J. Chin. Chem. Soc., Vol. 49, No. 4, CMC value of the crown ether sur fac tants was ob tained by mea sur ing pyrene flu o res cence and plot ting I 1 (373 nm)/i 3 (383 nm) in ten sity ra tio vs. surfactant con cen tra tion which shows a dra matic change in slope as shown in Fig. 3. The CMC value is gen er ally es ti mated as the surfactant con cen - tra tion at the sec ond in flec tion point (Points P1 and P2 in Fig. 3) fol lowed by the ob ser va tion of a level-off curve. As shown in Fig. 3, the CMC ( M) of decyl 15-crown-5 is smaller than that of decyl 18-crown-6 ( M) which means that decyl 15-crown-5 forms mi celles eas ier than decyl 18-crown-6 in aque ous so lu tions. It is well known that nonpolar or less po lar sur fac tants form mi celles eas ier in aque ous so lu tions. A decyl 15-crown-5 mol e cule with five ox y gen do nor at oms can be ex pected to be less po lar than decyl 18-crown-6 with six ox y gen do nor at oms re sult ing in the smaller CMC of decyl 15-crown-5. Com par i son of the CMC of these crown ether sur fac tants and decylhexaethylene gly col was also made. Even though a decylhexaethylene gly - col mol e cule has the same num ber of car bons and one more ox y gen atom com pared to decyl 18-crown-6, decylhex ae - thyl ene gly col ex hib ited greater CMC with M than ei ther decyl 18-crown-6 ( M) or decyl 15-crown-5 ( M), which may be at trib uted to the hy dro gen bond ing be tween the OH group of gly col and sol vent H 2O re - sult ing in greater CMC of the gly col. The ef fect of ad di tion of salts, e.g. NaCl and KCl, on the micellar for ma tion of the crown ether sur fac tants was in - ves ti gated. The crown ether surfactant is con sid ered as a non-ionic surfactant, but be comes a pseudo cationic sur - factant af ter complexing with metal ions of salts. Ow ing to the or ganic char ac ter of the crown ether surfactant, the com - plexation be tween metal ion and crown ether surfactant can be ex pected to lead to an in crease in the sol u bil ity of the crown ether surfactant in aque ous so lu tion which re sults in the in crease in CMC of the crown ether surfactant. As ex - pected, the CMC value of decyl 15-crown-5 surfactant in the pres ence of 0.1 M NaCl in aque ous so lu tions is es ti mated from pyrene flu o res cence mea sure ments to be M (Curve B) which is greater than M (Curve A) as shown in Fig. 4 and Ta ble 1. How ever, as shown in Fig. 4 and Ta ble 1, the CMC value of the decyl 15-crown-5 surfactant in aque ous so lu tion de creases at high salt con cen tra tions 1.0 M. This de crease in CMC oc curs pri mar ily due to the salt ing out ef fect on ad di tion of elec tro lyte. 17 The de creased CMC due to the salt ef fect was also re ported for other crown ether sur fac tants, e.g. cryptand sur fac tants. 18 The CMC of the decyl 15-crown-5 surfactant in the pres ence of KCl is also shown in Ta ble 1. The plot of the CMC value against the con - cen tra tions of salts (Fig. 5) re veals that both decyl 15- crown- 5 and decyl 18-crown-6 show an in creased CMC at low salt con cen tra tions 0.1 M while the CMC at high con - Fig. 4. Plot of I 1/I 3 ra tio of pyrene flu o res cence vs con - cen tra tion of decyl 15-crown-5 in aque ous so lu - tions with (A) no salt, (B) 0.1 M NaCl, (C) 1.0 M NaCl and (D) 1.6 M NaCl. Fig. 3. Pyrene flu o res cence micellar stud ies for (A) decyl 15-crown-5 and (B) decyl 18-crown-6 in aque ous so lu tions. Fig. 5. Con cen tra tion ef fect of salts on CMC of decyl 15-crown-5 in aque ous so lu tions.

4 602 J. Chin. Chem. Soc., Vol. 49, No. 4, 2002 Chou and Shih Table 1. CMC and I 1 /I 3 of Pyrene Fluorescence for Decyl 15- Crown-5 in Aqueous Solutions with NaCl and KCl Salts [NaCl] CMC (M) I 1 /I 3 [KCl] CMC (M) I 1 /I M M M M M M cen tra tions ( 1.0 M) of salts de creased. Ef fect of mixed sol vents on the for ma tion of mi celles of these crown ether sur fac tants was also in ves ti gated. The com po si tion of con tin uum phase (sol vent) of surfactant so lu - tion al ways shows a sig nif i cant ef fect on the for ma tion of mi - celles. For in stance, the in creased con cen tra tion of meth a nol leads to an in cre ment in CMC of decyl 15-crown-5 as shown in Fig. 6. In pure wa ter, the O/W type (dis per sion phase: or - ganic, con tin uum phase: wa ter) mi celle could be ex pected (Fig. 7). In other words, the O/W type mi celle is sta ble and eas ily formed in wa ter, but un sta ble in or ganic or lipophilic sol vents. In con trast, the W/O type mi celle can be eas ily formed in or ganic or lipophilic so lu tions (Fig. 7). Due to the low sol u bil ity of the crown ether surfactant in pure wa ter, the mixed MeOH/H 2O sol vent was used in this CMC study. By add ing meth a nol into wa ter, the so lu tion be comes more hy - dro pho bic re sult ing in the in sta bil ity of O/W type mi celles and greater CMC of the crown ether surfactant. The hy dro - pho bic ef fect of the sol vent on the CMC of the decyl 15- crown-5 surfactant can also be ob served by add ing acetoni - trile (Ta ble 2). A sim i lar or ganic sol vent ef fect was also re - ported for other sur fac tants In both meth a nol and aceto - nitrile mixed sol vents, the CMC of decyl 15-crown-5 showed a nearly lin ear re la tion ship to the vol ume % of the or ganic sol vents as shown in Fig. 8. Table 2. CMC and I 1 /I 3 of Pyrene Fluorescence for Decyl 15- Crown-5 in MeOH/H 2 O and MeCN/H 2 O Mixed Solvents MeOH (v/v ) CMC (M) I 1 /I 3 MeCN (v/v ) CMC (M) I 1 /I The micellar for ma tion of these crown ether sur fac tants in pure or ganic sol vents, e.g. meth a nol and acetonitrile, was also in ves ti gated with the pyrene flu o res cence method. As shown in Fig. 9, al though the I 1 (373 nm)/i 3 (383 nm) in ten - sity ra tio of pyrene changes with the in creased con cen tra tions of both decyl 15-crown-5 and decyl 18-crown-6 sur fac tants, no sec ond in flec tion point is ob served in each case which in - Fig. 7. Micellar type crown ether surfactant decyl 15- crown-5 with pyrene (py). Fig. 6. Con cen tra tion ef fect of meth a nol (MeOH) on the micellar for ma tion of decyl 15-crown-5 in MeOH/H 2 O (V/V %)mixed sol vents. Fig. 8. Sol vent ef fect on CMC of decyl 15-crown-5 in mixed sol vents. (A) pure H 2O, (B) MeOH/H 2O and (C) MeCN/H 2O.

5 Micellar For ma tion Study of Crown Ether Sur fac tants J. Chin. Chem. Soc., Vol. 49, No. 4, di cates that the CMC of these crown ether sur fac tants can not be de ter mined with the pyrene flu o res cence method. The CMC value is gen er ally es ti mated as the surfactant con cen - tra tion at the sec ond in flec tion point fol lowed by the ob ser va - tion of a level-off curve. In other words, the pyrene flu o res - cence method is not a proper method to de ter mine the CMC of these crown ether sur fac tants in pure or ganic sol vents, be - cause the pyrene mol e cules al ways place in an or ganic con - tin u ous phase whether the mi celle of surfactant forms or not. There fore, in this study, the NMR method with spin lat tice re - lax ation times (T 1) and chem i cal shifts of H-1 NMR for the de ter mi na tion of CMC of these crown ether sur fac tants in or - ganic so lu tions was de vel oped. The CMC of the crown ether sur fac tants can be es ti mated as the con cen tra tion at the sig - nif i cant change point in the curve of the plot of spin lat tice re - lax ation time. The spin lat tice re lax ation time T 1 can be es ti - mated by the BPP (Bloembergen, Purcell and Pound) Equa - tion 22 as fol lows: 1/T 1 = K /( ) (1) Where K and are a con stant and the res o nance fre - quency of the nu cleus, re spec tively, is the cor re la tion time which is de fined as the time of the nu cleus need to ro tate an an gle of one ra dian. For com mon or ganic com pounds, the cor rec tion time is quite short and << 1. Equa tion (1) be - comes: 23 1/T 1 = K (2) Equa tion (2) means that the slower mo tion with greater re sults in the smaller T 1. Upon the for ma tion of mi celles, the mo tions of the H-1 pro tons of these crown ether sur fac - tants can be ex pected to be slower re sult ing in a de crease in the T 1 of the H-1 of these crown ether surfactant mol e cules. As ex pected, all of the H-1 spin lat tice re lax ation times T 1 of OCH 2, CH 2 and CH 3 of decyl 15-crown-5 in acetonitrile were de creased with the in creased con cen tra tion of the surfactant as shown in Fig. 10. All curves for T 1 of OCH 2, CH 2 and CH 3 showed dra matic changes at in flec tion points (Points P1, P2 and P3 in Fig. 10) oc curred at the same con cen tra tion (0.72 M) of the surfactant which can be es ti mated as the CMC of the decyl 15-crown-5 surfactant in acetonitrile. The de crease in T 1 due to the for ma tion of mi celles was also re ported for other sur fac tants. 24,25 Sim i larly, the CMC of the decyl 18- crown-6 surfactant was also de ter mined with the spin lat tice re lax ation time T 1 method. The CMC of decyl 18- crown- 6 can be es ti mated to be 0.3 M as the con cen tra tion at the sig - nif i cant change points (Point P1) in the curves of the plots of T 1 of OCH 2, CH 2 and CH 3 of the surfactant as shown in Fig. 11. It in di cates that decyl 18-crown-6 forms mi celles eas ier than decyl 15-crown-5 in acetonitrile while decyl 15-crown-5 forms mi celles eas ier in wa ter as men tioned above. Once again, be cause decyl 15-crown-5 mol e cule with five ox y gen do nor at oms can be ex pected to be less po lar than decyl 18-crown-6 with six ox y gen do nor at oms which leads to lower sol u bil ity in wa ter and greater sol u bil ity in or ganic sol - vents such as acetonitnile re sult ing in a smaller CMC in wa ter and a greater CMC in acetonitrile than decyl 18- crown- 6. Fur ther more, as shown in Fig. 11, there are other in flec tion points (Point P2) in these curves which may be at trib uted to an other type of mi celle formed or so lid i fi ca tion of the mi - celles at high con cen tra tion 0.67 M. Fig. 9. Flu o res cence micellar stud ies for var i ous sur - fac tants (A) decyl 15-crown-5 and (B) decyl 18-crown-6 in acetonitrile. Fig. 10. H-1 Spin lat tice re lax ation times (T 1) of pro - tons (A) OCH 2 on the crown, (B) CH 2 on the alkyl chain and (C) CH 3 on the end of alkyl chain of decyl 15-crown-5 in acetonitrile.

6 604 J. Chin. Chem. Soc., Vol. 49, No. 4, 2002 Chou and Shih Table 3. Spin Lattice Relaxation Times of Protons on Crown Ether Surfactant with Critical Micellar Concentration in Acetonitrile Surfactant Decyl 15-crown-5 Decyl 15-crown-5 T 1 (OCH 2 ) a T 1 (CH 2 ) T 1 (CH 3 ) T 1(M) (OCH 2 ) b T 1(M) (CH 2 ) T 1(M) (CH 3 ) a T 1 b T 1(M) The change in T 1 from 0.1 M to CMC of the surfactant. The T 1 for the micelle of the surfactant. Upon the for ma tion of mi celles, the T 1 changes for OCH 2, CH 2 and CH 3 of these crown ether sur fac tants in ace - to nitrile was also in ves ti gated. As shown in Ta ble 3, the changes in T 1 (de noted as T 1 ) of CH 2 are larger than that of OCH 2 or CH 3 for both decyl 15-crown-5 and decyl 18- crown- 6 which means that the mo tion of the CH 2 be comes much slower than OCH 2 or CH 3 upon the micellar for ma tion. Fur ther more, the T 1 (de noted as T 1(M)) of CH 2 of a mi celle was also found to be smaller than OCH 2 or CH 3 as shown in Ta ble 3. It is not sur pris ing that the head and tail (OCH 2 or CH 3) of a mi celle still have quite a greater de gree of free dom than the mid dle part (CH 2) of the mi celle which leads to the slower mo tion (larger in Equa tion 2) of CH 2 re sult ing in the smaller T 1(M) of CH 2 of a mi celle ac cord ing to Equa tion 2 and larger change in T 1 of CH 2 upon the for ma tion of the mi celle. The chem i cal shift of H-1 NMR was also ap plied to de - ter mine the CMC of the decyl 15-crown-5 surfactant. As shown in Fig. 12, all the plots of the chem i cal shifts of OCH 2, CH 2 and CH 3 of the decyl 15-crown-5 surfactant ob vi ously have a sig nif i cant change at around 0.6 M of the surfactant and the CMC of the surfactant can be es ti mated to be around 0.6 M while the CMC is es ti mated to be ac cu rately at 0.72 M with the spin-lattice re lax ation time T 1. It seems to in di cate that the H-1 chem i cal shift method is not a very good tool for the de ter mi na tion of the CMC of the crown ether surfactant while the spin-lattice re lax ation time method is dem on strated to be a good tech nique to de ter mine the CMC of these crown ether sur fac tants in or ganic sol vents such as acetonitrile. CON CLU SION Fig. 11. Con cen tra tion ef fect of decyl 18-crown-6 in acetonitrile on spin lat tice re lax ation times of pro tons on decyl 18-crown-6. Pyrene flu o res cence probe and spin-lattice re lax ation time (T 1 ) of H-1 NMR meth ods can be suc cess fully em ployed to study the micellar for ma tion and de ter mine the crit i cal micellar con cen tra tion (CMC) of crown ether sur fac tants in aque ous and or ganic so lu tions, re spec tively. Com paring these crown ether sur fac tants, decyl 15-crown-5 with a smaller CMC forms mi celles much eas ier than decyl 18- crown-6 in aque ous so lu tions while decyl 18-crown-6 forms mi celles eas ier than decyl 15-crown-5 in nonaqueous so lu - tions. AC KNOWL EDGE MENTS Fig. 12. H-1 Chem i cal shifts of (A) OCH 2 on the crown, (B) CH 2 on the alkyl chain and (C) CH 3 on the end of alkyl chain of decyl 15-crown-5 in acetonitrile. The au thors would like to ex press their ap pre ci a tion to the Na tional Sci ence Coun cil of the Re pub lic of China in Tai - wan for fi nan cial sup port. Re ceived De cem ber 18, 2001.

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