non-aqueous media interface

Colloids and SurJaces, 68 (1992) 12 i - 125 Elsevicr Science Publishers B.V.. Amsterdam

121

An ESIX spectroscopic study of the adsorption of Aerosol QT at the alumina/non-aqueous media interface Ku.rnio Esumi and Takehiro

Kobayashi

Department of Applied Chemistry. Institute of Colioid and interface Science, Science University of Tokyo, Kagurazaka, Shinjuku-ku. Tokyo 162, Japan (Received 6 April 1992; accepted 27 May 1992) Abstract Adsorption of Aerosol OT on alumina particles. preheated at 2S”C or 3OO”C, from hcxane and cyclohexanone was studied using two types of nitroxide probes. From the electron spin resonance spectra, the interaction between alumina surfaces having different amounts of water and Aerosol OT was discussed. Keywords:

Adsorption;

Aerosol

OT; ESR probe; non-aqueous

dispersion.

Introduction The dispersion of particles in non-aqueous media has been extensively studied in order to understand the interactions at the solid/liquid interface Cl]. It is known [Z-4] that water, especially on the particle surface, affects the dispersion of particles in non-aqueous media very significantly. Recently, electron spin resonance (ESR) spectroscopy has been used to investigate the structure of the solid/liquid interface on a molecular scale [S-9]. Spectroscopic techniques for studying interaction at the solid/liquid interface have some merits: in situ changes are monitored directly and a wide flexibility is available in the experimental conditions employed. In this work, the adsorption behavior of Aerosol OT on alumina from non-aqueous media was studied using the ESR probe technique. Further, Corresponderrce ro: K. Esurni, Dept. of Applied Chemistry. Institute of Colloid and Interface Science, Science University of Tokyo, Kagurazaka, Shinjuku-ku, Tokyo 162, Japan. 0166-6622/92/SOS.O0

0

1992 -

Elsevier

Science

Publishers

the effect of water at the solid/liquid interface was elucidated using alumina preheated at 25” C or 300°C under vacuum before use.

Experimental Materials Sodium bis(Zethylhexyl)sulfosuccinate (Aerosol QT) was supplied by Nikko Chemical Co. Ltd., and purified as follows [lo]. Aerosol OT was dissolved in methanol and the precipitate produced was removed with a fine glass filter, followed by drying. Alumina was r,‘repared as follows [l 11. Excess aqueous methanol (weter : methanol, 4 : 1 by weight; was added to a solution, in butanol, of aluminum butoxide, and stirred vigorously for 5 h at 90°C. The precipitate obtained was separated by centrifuging, washed with methanol, finely ground, and then evacuated at 100°C. Finally, the dried products were calcined for 2 h at 500°C in B.V. All rights

reserved.

172

an electric furnace. The crystal structure of alumina was that of ;I-alumina and the surface arca dctcrmined by adsorption of nitrogen was I65 m” g- ‘. 3,2,6,6_TetramethylI -piperidinylosy (TEMPO) and 4-amino-2,2,6.6-tetramethyl-I-pipcridinyloxy (TEMPAMINE) wcrc obtained from Aldrich and used as received. it-Hexane and cyclohcxanonc were used as solvents and wcrc dchydratcd with Molecular Sicvc 4A (Wako Pure Chemicals Co.).

The special ESR tube used in this study is shown in Fig. 2. Sample preparation was as follows: alumina (0.03 g) was piaccd in the ESR tube and a non-aqueous solution of Aerosol OT i I cm”) containing an ESR probe (1 * lO-5 mol dm-“) was placed in the side arm. After this solution was frozen with liquid nitrogen, the vessel was attached directly to a vacuum lint: and outgassed at 25 C or 300-C and IO-” ?brr for 3 h. Then, !hc tube was cooled down to room temperature and the To vacuum syslcm sam~lr prcparalion

Alumina (0.3 g) was placed in an L-shaped test tube which was attached directly to a vacuum line and outgasscd at 25’C or 3OO’C and IO-” Torr for 3 h and cooled to room temperature in vacua prior to Aerosol OT adsorption. A non-aqueous solution of Aerosol OT (10 cm”) was then poured into the L-shaped test-tube under nitrogen atmosphere. The concentration of Aerosol OT was determined by the Mcthylcne Blue method [I 2-J. The Aerosol OT-Methylenc Blue complex was extracted into chloroform and the complex concentration was determined by measuring absorbances at 652 nm with a UV/vis spectrophotometcr (DOA., Hitachi Cc.).

derring

+

Teflon stopcock

J

Fig. I. The special

non-aqueous solution and the alumina wcrc mixed in vacua. At the end of this procedure. the alumina surf&c bccamc ESR sensitive. ESR spectra wcrc recorded on a JEOL JES FE 3-X spectrometer operating with an X-band microwave 100 kHz field modulation. Both suspension and supcrnatant were used for the ESR measurements.

ESRtuhc

ESR tube used.

Figure 2 shows the adsorption isotherms of Aerosol OT from hexanc and cyclohcxanonc solutions on alumina prchcated at 25°C and 300°C. According to Morimoto et al. [l3], the amounts of water bonded to y-alumina treated at given tcmpcratur:s, cxprcssed as the number of hydroxyl groups per 100 A’, were as follows: 18.66 at 30°C; 6.04 at 300°C. Peri and Hannan [I 41 assumed a value of 12.5 OH groups per 100 A’ as the surface density of hydroxyl groups on the fully hydroxylated surface of ;r-alumina. Since the samples tested here can be expected to bc almost the same as has been cited above, it is reasonable to consider that alumina preheated at 25°C contains physically adsorbed water molecules. but after preheating at 3OO’C only chemically adsorbed hydroxyl groups remain on alumina. In hexane, the affinity of Aerosol OT for alumina preheated at 300°C is rather high since the maximum adsorption is

123

0

10

20

Equilibrium concn. of AOT / mmol dm-3 Fig. 2. Adsorption isotherms of Aerosol OT on alumina from non-aqueous media: (0), 300”C-preheated alumina/hexanc: (e), X‘C-preheated alumina!hcxanc; (G), 300”C-prchcated alumina,kyclohcxanone; (A), X”C-preheated alumina/cyclehcnanonc.

reached before any residual surfactant concentration is detected in the supernatant. However, the affinity of Aerosol OT for alumina preheated at 25’ C is low, but the maximum adsorbed amount is high compared to that for the 300”C-preheated system, probably due to physically adsorbed water on alumina which forms a polar environment for adsorption of Aerosol OT. In cyclohexanone, solvent molecules are incorporated into the polar environment formed by hydroxyl groups or physically adsorbed water on alumina and interfere with adsorption of Aerosol OT. It is reasonable to suppose that this effect is greater for the 25”Cpreheated system, so that the maximum adsorbed amount of Aerosol OT for the 25”C-preheated system is lower than that for the 300”C-preheated system. Further, the maximum adsorbed amount of Aerosol OT from hexane solution is higher than that from cyclohexanone, indicating that the adsorbed Aerosol layer formed on alumina from cyciohexanone may be loose compared with that from hexane. Analysis of the anisotropic features of the ESR

spectra gives information on the rotational mobility of the nitroxide probes. An isotropic spectrum characterized by a sharp three-line absorption is obtained when the nitroxide is tumbling in a nonviscous fluid. However, line broadening occurs when the probe is immobilized. Thus, the relative anisotropy observed in an ESR spectrum is directly related to the rotational mobility of the probe which is proportional to the microviscosity of the The rotational correlation probe environment. times, which reflect the microviscosity, have been calculated by Mazzoleni et al. [15]. The change in the 14N hyperfine splitting constant (AN) can be correlated with the reduced micropolarity at the binding sites of the probe in the adsorbed layer. In other words, A,, which is proportional to the unpaired spin density on the nitrogen nucleus, is larger in polar solvents, changing from about 14 G in apolar solvent to 17 G in water. Figure 3 shows the AN of TEMPO at the alumina/liquid interlace as a function of the concentration of Aerosol OT added. In hexane, AN is quite large in the absence of Aerosol OT, decreases with an increase of Ae.osol OT concentration and

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30

40

Additive

concn.

of AOT / mmol dm-3

Fig. 3. Plots of the A, value of TEMPO at the alumina/liquid interface vs the concentration of Aerosol OT added; (0), 3OO”Cpreheated alumina/hexane; (0). X”C-preheated alumina/ hexane; (A), 300”C-preheated alumina/cyclohexanone; (A), 35”C-preheated alumina/cyclohcxanonc.

K. Eszmzi. T. Kobayashi/Colloids

124

then reaches a constant value, approximately 15 G. This value of 15 G is almost the same as that in the supernatant. Since it is known [16] that TEMPO is essentially hydrophilic, TEMPO locates around hydroxyl groups or physically adsorbed water on the alumina surface in the absence of Aerosol OT. TEMPO moves to sites of lower polarity on the alumina/hexane interface when Aerosol OT is added. These sites are probably on an adsorbed layer of Aerosol OT because the partition of TEMPO is significantly larger at the interface than in the supernatant. However, in cyclohexanone, AN shows a constant value, 15.4 G, which is mainly controlled by the polarity of cyclohexanone itself, for both preheated systems. Figure 4 shows the rotational correlation time (rn) of TEMPO at the alumina/liquid interface. In hexane, in the absence of Aerosol OT, 58 for the 300”C-preheated system is very large, about 8 times that for the 25”C-preheated system. With increasing Aerosol OT concentration, rB for both systems decreases and becomes constant at 15 mrnol drnmJ or above. This suggests that TEMPO is bound strongly to hydroxyl groups on the alumina surface preheated at 3OO”C, but

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Additive

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Surfaces 68 (1992) I21 -125

weakly with physically adsorbed water on the alumina surface preheated at 25°C. By addition of Aerosol OT, TEMPO would move into the layer of adsorbed Aerosol OT. In cyclohexanone, Tg shows a constant value for both systems. Since a significant difference was not observed for the two systems in cyc!ohexanone when using TEMPO, we employed TEMPAMINE as an ESR probe in this solvent. Figure 5 shows the ESR spectra of TEMPAMINE at the alumina (preheated at 25”C)/cyclohexanone interface in the absence and presence of Aerosol OT. Here, the outer low-field peaks (*) in these spectra represent the immobile component, while the inner low-field peaks (**) represent the mobile component. The spectral intensity ratio of the inner low-field peaks to the outer low-field peaks increases with increasing Aerosol OT concentration. Further, the spectra observed were separated into mobile and immobile components by comparison with the spectra obtained from a

40

concn. of AQT / mmol dm-3

Fig. 4. Plots of the rotational correlation time of TEMPO at the alumina/liquid interface vs the concentration of Aerosol OT added. The symbols arc the same as in Fig. 3.

Fig. 5. ESR spectra of TEMPAMINE at the alumina (preheated at 25”C)/liquid interface vs the concentration of Aerosol OT added: (a), 0; (b), 5; (c), 10; (d), 15; (e), 20; (f). 30; (g), 40 mmol dmw3.

h’. Es1trni. T. Koha~as/ri/Colloiffs

!&rjaces

68 (1992) 121-12s

TEMPAMINE/alumina interface at -5O_“C, representing the immobile component and from TEMPAMINE solution, representing the mobile component. Finally, we estimated the mobile fraction (jJ of the observed spectra by using computer matching of the spectra [17,18]. Figure 6 shows fm values of TEMPAMINE in the cyclohexanone system. fm increases with an increase of Aerosol OT concentration: the value of f, for the 300”C-preheated system is larger than that for the 25”C-preheated system. This result indicates that the interaction between TEMPAMINE and hydrophilic sites on alumina decreases

125

by adsorption of Aerosol OT since hydrophilic groups of Aerosol OT will interact with hydrophilic sites on alumina. In addition, the result for alumina/hexane is also given in Fig. 6 and a similar behavior to that in cyclohexanone is observed, though the magnitude ofJm is different. References I

2 3 4 5 6 7 8 9 IO II

I2 13

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10

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30

20 concn. of AOT

/

40

mmol dm-3

Fig.6. Plots of the mobile fraction of TEMPAMINE at the alumina/liquid intcrfacc vs the concentration of Aerosol OT added: (0). 300”C-prchcated alumina/hcxane; (0). 25”C-preheated aIumina/hcxane; (A). 300’C-preheated alumina/cyclohcxanonc; (A),15.C-preheated aiumina/cyclohexanone,

14 15 16 17 18

R.B. McKay, in M.F. Eicke and G.D. Parfitt (Eds), Intrrfacial Phcnomcna in Apolar Media, Surfactant Science Series, Vol. 21, Dekker. New York, 1987, p. 361. D.N.L. McGown, G.D. Parfitt and E. Willis, J. Colloid Interface Sci.. 20 (1965) 650. A. Kitahara, S. Karasawa and H. Yamada, J. Colloid Intcrfacc Sci., 25 (1967) 490. CA. Malbrcl and P. Somasundaran. J. Colloid Interface Sci.. 133 (1989) 404. S. Schrcicr, J.R. Ernandcs, I. Cuccovia and H. Chaimuvich. J. Magn. Rcson., 30 (1978) 283. Y.Y. Lim and J.H. Fcndler, J. Am. Chcm. Sot., 100 (1978) 7490. P. Chandar. P. Somasundaran, K.C. Waterman and N.J. Turro, J. Phys. Chcm., 91 (1987) 148. K. Esumi, H. Otsuka and K. Meguro, J. Colloid Interface Sci., 142 (1991) 582. C.A. Malbrel. P. Somasundaran and N.J. Turro. Langmuir. 5 (I 989) 490. A. Kitahara and K. Kon-no, J. Phys. Chem., 70 (1966) 3394. H. Hosaka and K. Meguro, Bull. Chem. Sot. Jpn.. 44 (1971) 1252. D.C. Abbott, Analyst, 87 (1962) 286. T. Morimoto. M. Nagao and J. Imai, Bull. Chcm. Sot. Jpn., 44 (1971) 1282. J.B. Peri and R.B. Hannan. J. Phys. Chcm.. 64 (1960) 1526. M.F. Ottaviani, M. Romanelli and F. Mazzoleni, G. Martini, J. Phys. Chem.. 92 (1988) 1953. K. Esumi. H. Otsuka and K. Mcguro. J. Colloid Interface Sci., 136 (1990) 224. H. Sakai, T. Fujimoto and Y. Imamura. Bull. Chem. Sot. Jpn.. 53 (1980) 3457. J. Yao and G. Strauss, Langmuir, 7 (1991) 2353.