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Solubilization of spiropyran in aqueous NaBr solutions of dodecyltrimethylammonium bromide
Collr~itls
21
Solubilization of spiropyran of dodecyltrimethylammonium Shoichi
Ikeda
Department (Received
in aqueous NaBr solutions bromide
and Yoke Saso
qf Chemistq~. 28 October
Faculty ofScience.
1991; accepted
Nagoya
16 January
University,
Chiktrsa. Nagoya
464. Japatl
1992)
Abstract I .X3-Trimcthylindolino-6’-nitrobcnzopyrilospiran is solubilizcd in aqueous NaBr solutions of dodccyltrimcthylnmmonium bromide (DTAB). The dye is soluble in solutions of DTAB having concentrations higher than the critical micellc conccnrration. The solubilization power of DTAB is 2.08. IO-” mol dye per mol DTAB at NaBr concentrations lower than I.84 ill. while it increases linearly with increasing NaBrconccntration from 1.84-4.50 M. The constant solubilizution power of DTAE ,s attributable to the spherical miccllcs. while its higher solubilization power in the prcscncc of NaBr beyond 1.84 M can bc nscribcd to rod-like miccllcs. The solubilization capacity of a spherical miccllc of DTAB ranges from 0.1 l-0.18. dcpcnding on :hc NaBr concentration, where the micellc aggregation number varies from 52-86. The rodlike miccllc has an increasing soiubi!‘ rzation capacity with increasing NnBr concentration. and ir can solubilize the dye to such an amount :haL I36 DTAB molecules along irs c ontour length incorporate one dye molcculc. The solubilizcd dye is partly in the mcrocyaninc rorm. and the solubilization locus has polarity corresoonding _ LOa diclcctric constant = 35. Ke _.WO~~!~: ..I Dodccyltrimethylammonium
bromide:
sodium
bromidr;
introduction The spiropyrans constitute a class of photochromic compounds, generally insoiubie in water, which can be readily converted from the colorless Spiro form (I) to the colored merocyanine form (II) CH3 CH3
through hcteroiytic scission, when irradiated by ultraviolet Iight. The merocyanine form (Ii) reverts to the Spiro form (I) either by illumination with visible light or by thermal relaxation [i-4]. It has been suggested that, from the kinetic standpoint, II is more stable in a more polar environment than i [S-7]. Corrmpordrrm Scicncc, Nagoya
to: S. Ikeda, Dept. University, Chikusa,
Ol66-6622/92/SO5.00
0
1992 -
of Chemistry, Faculty Nagoya 464, Japan. Elsevicr
Science
of
Publishers
solubilization:
spiropyran.
In the present work the soiubiiization equiiibrium of i,3,3-trimethyiindoiino-6’-nitrobenzopyriiospiran (A) in aqueous NaBr solutions of dodecyltrimethyiammonium bromide (DTAB) is investigated, in order to observe the effect of ionic strength, i.e. of miceiie size and shape, on the soiubiiization power as well as on the sohrbilization locus. It is known that the spherical miceiies of DTAB increase their size with increasing NaBr concentration and the sphere-rod transition of DTAB miceilcs occurs at I.80 A4 NaBr [SJ. The rod-like micelles of DTAB grow further with increasing NaBr concentration. Experimental
Dodecyitrimethylammonium grade reagent, was purchased B.V. All rights
reserved.
bromide, special from Tokyo Kasei
Kogyo Co., Inc., Tokyo, recrystallized three times from acetone containing a small amount of ethanol, then dried at 40°C for 8 h and stored in a dcsiccator. NaBr was special grade reagent from I\Jakarai Chemical Co., Inc., Kyo!o. I ,3,3-Trimcthylinaolino-6’-nitrobcnzopyrilospiran ilvas obtained from Tokyo Kasei Co., Inc., and used without further purification. Dioxane was spectroscopic-grade reagent from Nakarai Chemicals Co., Inc. Water was redistilled in a glass still from an alkaline KMnO, solution.
Absorption spectra were measured in the range 200-600 nm on a Shimadzc UV ZOOS spcctrophotometcr and a U-t 25MU recorder, using a quartz cuvcttc of I cm path length. The light source for illuminating with visible light was a 300 W projector lamp (Kondo KP10s) on an Etmo S-30 Prqiector. Ultraviolet light was provided by an 85 W mercury lamp from Japan Storage Battery Co., Ltd, Kyoto.
A gram of spiropyran ground by a spatula was added to, usually, 5 cm3 of aqueous NaBr solution of DTAB in a stopper-cd test-tube (1.5 cm diameter x IO cm), and the test-tube was shaken in the dark in a Yamato BT-22 thermostat at 25 rfi 0.1 “C. The suspension was shaken. usually for 2 days, and then hltcrcd through a Millipore hltcr, FGLP 01300, having pore size, 0.2 urn. The pink color of the solution faded during or after the filtration. Some of the filtrates were kept in the dark for 30 min, and the absorption spectra of the colored solutions wcrc immediately measured. For the determination of the amount of solubiIized dye, the filtrate was diluted by adding an equal volume of dioxane and bleached by illuminating with visible light for I min, and then the absorption spectra of the solutions were recorded again. In the presence of NaBr more concentrated
than 4.50 114, precipitation occurred when dioxane was added to the filtrate. The optical density, A, was read at the peak around 350 nm and twice this value was divided by the value of the molar cxtinclion cocfhcicnt, c. The solubility of dye in the miccttar solution, S (mol I -’ or bI). is given by S=2A/(
(1)
Spiropyran was dissnlvcd in a mixture of equal volumes of dioxane and w~tcr or aqueous NaBr solution, and the solution was irradiated by visible light for I min. Then the absorption spectra of the solutions were measured. The molar extinction coefficient of the peak around 350 nm did not change with the NaBr concentration. but its wavclength shifted from 352 nm in water to 348 nm in 4.0 A,I NaBr. For the calibration of dye concentration in dioxanc-water or dioxanc-aqueous NaBr solutions, a value, ( =9530 I mol- ’ cm-‘, at the peak of the absorption band was obtained and this value is assumed below. Results
The solubilization equilibrium of spiropyrnn in the miccllar solution could be attained within a day or two, and the micettar solution containing sotubilizcd dye was colored pink, indicating partial conversion of the Spiro form (I) into the merocyanine form (II). The spectrum of the aqueous micellar solution of solubilized dye is illustrated in Fig. I. The absorption bands arc manifest at 240, 260, 350 and 525 nm. The molar extinction coeficient of the visible band at 525 nm is 2000 I mol- ’ cm- ‘. When this solution is illuminated by visible light, its pink color fades readily and the visible band disappears, but the ultraviolet bands remain nearly unaltered. The spectrum of the solution after illumi-
s. Ikccla. Y. Stl.w/Collf~irls Slcr:firlT.s67 ( 1992) 2 I-17 I
D.6 -
Fig. 2. The Fi_r. I. Absorption DTAB solution
containing filtcrcd
spectra
of aqueous
solubilizcd after shaking
(I) kept in lhc dark
spiropyran:
miccllar C=O.O40
solutions
of
M. (I) The
Tar 7 days at 25 C. (II)
Solution
for 3Omin.
nation is also shown in Fig. 1. The absorption band at 350 nm has nearly the same molar extinction cocficicnt before and after illumination by visible light.
The rate of solubilization was measured by following the optical density of the miccllar solution treated as described above. It was found that a nearly constant value of optical density was approached or attained within 24-48 h, and that thereafter a much slower process of increasing optical density proceeded. This is ilIustrated in Fig. 2. The initial rapid increase in optical density at 350 nm rcprescnts the solubilization process, but the later, slower increase in optical density finally reveals a change in color to yellow as well as the appearance of a new band at 390 nm. Thus it can be concluded that the solubilization equilibrium is attained within 24-48 h, and thereafter some photochcmical changes occur in the merocyanine form (II ).
Figure 3 shows the solubility of spiropyran in aqueous NaBr solutions of DTAB. In the absence
miccllar
incrcasc
solution
ity is rcprcsentcd miccllnr
solutions
equal volume
in solubility
or DTAE
\.ith
by the optical of DTAB
of dioxane
of spiropyrun time; C=O.O40 density
after
at 350 nm of aqueous
filtration.
and illumination
in an aqueous IV. The solubildilution
with
an
by visible light.
of NaBr, and in its presence at low concentrations, it can be seen that the solubility of the dye remains zero below a certain concentration, which can be identified with the critical micelle concentration. Co (M). Above the critical micelle concentration the solubility increases linearly with increasing DTAB concentration. Values of the critical micelle concentration are given in Table 1, ‘together with those reported previously [9-141. Values of the critical micclle concentration derived from the solu-
Fig. 3. Solubility DTAB (A).
of spiropyran
at 75 ‘C. C, (M):
2.00; (0).
3.00; (0).
in aqueous
NaBr
solutions
(C). 0: (A), 0.05; (Ll). 0.50: (0). 4.00: (I), 4.50.
or
1.00:
TABLE
I
Critical
rniccllc concentration
C. (bf 1
of DTAB
in aqueous
NaBr
solutions
C” (!\I) Prcscnt work
Ref. [?I”
0
0.01 I7
0.0
0.05
0.003x
0.50
0.00
“Rcfractivc ‘Surfxc
164
Rcl-. [IO]”
Rcl-. [I
0.015x
0.0147
I J’
121”
Ref. [
Ref. [ 143’
0.0138 0.0057 I
0.0070
I2
0.0 I47
0.0 15s 0.00575
0.00 I46
index. hElcctric
conductivity.
‘Lielit
scattering.
“Electric
conductivity
in water.
0.00
and light scattering.
‘Dye
I -Is
solubilhtion.
tension.
bilization of spiropyran are. however, those obtained by other methods. The slope of the Iincar relation solubility and miccllc concentration
smaller
than
between
dye
C, (M), the solubilization
power
of DTAB
is given
by
s, = S/(C - c-0)
(3)
gives the solubilization power of DTAB in the micellar form, and this is plotted as a function of NaBr concentration in Fig. 4. The solubilization power of DTAB toward spiropyran is constant (independent of the NaBr concentration) if the NaBr concentration is lower than 1.84 M. At NaBr concentrations higher than 1.84 M, the solubilization power of DTAB increases with increasing NaBr concentration. Its values arc listed in Table 2. If the NaBr concentration is represented by
s,=2.08-lo--3 s, = (0.88 + O.GC,)*
10 - 3
c, d 1.84
(3a)
C, 2 1.84
(3b)
In the previous work on light scattering in aqueous NaBr solutions or DTAB [8], it was demonstrated that DTAB forms spherical micelles when the NaBr concentration is lower than 1.80 M whi!c it forms rod-like micelles at higher NaBr concentrations. The rod-like micelles are in equilibrium with the spherical micelles. The threshold NaBr concentration for the sphcrc-rod transition, 1230 M, is very close to the NaBr concentration whcrc the solubilization of DTAB suddenly changes. Thus the observed threshold NaBr concentration for diflixent solubilization behaviors. I.S4 M, can be
4-
TABLE
3-
2
Solubilizatic~~
;
a
N:lBr
0 2B
conccn:
r spiropyran
in miccllcs
of DTAB
at dilfcrcnt
t)IlS
0
vi I-
L Fig. 4. Solubilization toward spiropyran.
power
of
DTAB
in miccllar
solutions
,r
C,
SC
(A# )
(IO-J Al)
0.00 0.05
2.08
52. I
0.105
2.07
59.4
0. I33
0.50
2.0 I
0.144
I .OO
2.15
71.7 76.0
2.00
2.18
3.00
2.94
88.1 138.1
0.192 0.407
4.00
3.3 I
190.1
0.629
4.50
3.57
217.0
0.775
/?l
(mol dye per mol miccllc)
0. I63
identified with that for the sphere-rod transition of micelles. The solubilization power of DTAB is higher when it is in a rod-like micelle, as compared with when it is in a spherical micelle. This fact has been observed for solubilization of Sudan Red B with other surfactants [ 13,15,16]. The solubilization power of DTAB can be converted to the solubilization capacity of the DTAB micelle, if the micelle aggregation number is known. It has been established that linear, doublelogarithmic relationships hold between micelle molecular weight, M,, and ionic strength, (C, + C,), for spherical and rod-like micelles, respectively, and they are expressed by
m
Fig. 5. Solubilization capigcity of DTAB micclles pyran z! different NaBr concentrations.
toward
spiro-
log M, = 0.085 log(C, + C,) + 4.37 c, < 4.00 A4
(4a)
log M,V=l.l I log(C, + C,)+4.10 C, 2 1.80 M
(4b)
for DTAB in aqueous aggregation tt1 =
number
NaBr
of both
solutions micelles
(5)
Assuming that the micelle size is not influenced by the presence of solubilized dye, the number of dye molcculcs solubilized in a miccllc, i.e. the solubilization capacity of a micelle, Z (mol dye per mol micelle), is given by YX-- Ins,
(6)
Its values are also shown in Table 2. Figure 5 shows the solubilization capacity of a DTAB micelle as a function of micelle aggregation number. The solubilization capacity of a DTAB micelle increases linearly with increase in its aggregation number, but the relationships are different for spherical and rod-like micelles. They are expressed by C = 0.00208111
52
(W
.Z = O.O044ttl- 0.198
86
(7b)
respectively.
The aggregation
number,
to the micelle
Soluhilizctior~
that
is formed
in 1.80 M
kmts
[Xl. The
is
MJ308.8
ponds NaBr.
86, corres-
Since the micellar solution is colored when spiropyran is solubilized, the dye molecules in a micelle must be, at least partially, in the merocyanine form (II). To estimate the polarity of the solubilization locus, the visible absorption band of the solubilized dye in micelles was compared with those of the dye in dioxane-water mixtures of different compositions. It was found that the band shifts from 518 nm in the 4: 6 v/v dioxane-water mixture to 549 nm in the 9‘: 1 v/v dioxane-water mixture, and the wavelength is linearly related to the volume fraction of dioxane in the mixture. Its molar extitiction coefficient decreases sharply with increasing dioxane content: c = 11000 in 4: 6 v/v dioxane-water mixture and c = 112 in 9: 1 v/v dioxanewater mixture. However, the absorption band of spiropyran in the micellar solutions is located at 525 nm, nearly independent of NaBr concentration. This wavelength is equal to that observed ir “le 1 : I v/v dioxane-water mixture. Thus the solubilized spiropyran is situated at a locus having a dielectric constant of around 35 [l73.
Discussion We have found that a spiropyran dye, 1,3,3trimethylindolino-6’-nitrobenzopyrilospiran (A), can be solubilizcd in both kinds of miccllcs of DTAB to different degrees. Linear increase of the soiubilicy above the critical micellc concentration has been observed for other micellar solutions with different dyes [ 13,15,16,18,19]. l-lowcver, values of the critical micelle concentration assigned to DTAB here arc somewhat lower than those obtained by other methods, suggesting some interaction between spiropyran and DTAB. possibly involving the mcrocyaninc form (II). The micellar solutions containing solubiIizcd spiropyran are colored, and accordingly spiropyran is solubilized in the micclles in both the Spiro (I) and mcrocyaninc (II) forms. If it is assumed that the molar extinction coeficicnt of the visible absorption band of the mcrocyanine form (II) is determined by the polarity of the medium, its value in polar media would be 35 000 1 mol- * cm - ’ [S]. Then the fraction of spiropyran in the mcrocyaninc form (II) is found to he as low as 6% in the miccIle. The merocyanine form (II) is zwitterionic and its p!:enolatc part can be strongly attracted by the cationic micciles of DTAB. If the counterion of dodccyltrimethylammonium (DTA ‘) is changed ion, the critical from Br- ion to the phenolate micelIe concentration could change, possibly becoming lower. This would be the reason for the low values of the critical micel!c concentration as observed. Some af the spiropyran moiecules are incorporated in micciles in this “counterion” form, however, most of them would be accommodated at the locus of solubilization which matches the hydrophobicity of the Spiro form (I). The colored micellar solutions can bc readily bleached by illumination with visible light, and the bleached solutions are reversibly colored during illumination with ultraviolet light. The solubilization loci of spiropyran could be different for the Spiro form (I) and the merocyanine form (II). Nevertheless, the illumination with visible light appears not to influence the soiubility of spiro-
pyran. so that the Spiro form (I) can be solubilized in the micelles to the same degree as (or more than) the merocyanine form (II). It seems probable that the solubilization locus would not c!lanse by the conversion of the merocyaninc form (11) to the Spiro form (I), and both forms of dye would accommodate near the miccllc surface so that they arc readily convertible. The solubilization loci have equal polarity, irrespective of whether they arc in a spherical micelle or in a rod-like micelle. Spiropyran can accommodate only at the locus having polarity equivalent to a dielectric constant of 35. Extcnsivc studies on the polarity of the microenvironment of the solubilizcd probe were pcrformcd by using 2,6-diphcnyl-4-(2,4,6-triphcnyl-1 -pyridiso-called E,(30) 1201. The results no)phenoxidc. in micclles of showed that the dye is soiubilized DTAB at a locus having a dielectric constant of 35-36 [Z&21]. It was also observed [IS] that Sudan Red B is solubilized in the micelles of dodccyldimcthylammonium bromide at a locus having polarity corresponding to the 5 : 2 v/v acetone-water mixture, which has a dielectric constant of 38. The solubilization of spiropyran occurs in a spherical micelle among ten to six micelles, depending on the micellc size. The low solubilization may to the low hydrophobicity and the be attribu. large molecular size of the spiropyran. The spiropyran is incorporated in a rod-like micclle in such a way that one dye n~olecule is solubilized by, on average, 136 DTA+ ions; thus two solubilized spiropyran molecules are about 82 A apart from each other along the contour length of a rod-like micelle, assuming the DTA+ ion to be 0.60 A thick.
References I 2 3 4
E. Fischer and Y. Hirshbcrg. J. Chcm. Sot.. (1952) 4517. Y. Hirshbcrg. J. Am. Chcm. Sot., 7X (1956) 2304. 0. Chaudc and R. Rumpf. CR. Acnd. Sci. (Paris). 236 (I 953) 697. 1. Bcrtclson. Photochromism: in G.H. Brown (Ed.),
7
Techniques ol Chemistry, Vol. III, Wiley-lnterscicncc, New York. 1971, Chapter 3. p. 45. J.B. Flanncry, Jr., J. Am. Chcm. Sot., 90 (1968) 5660. 1. Shimizu, H. Kokado and E. !nouc, Bull. Chcm. Sot. Jpn.. 42 ( 1969) 1730. H. Scki snd K. Ichimura. J. Colloict lntcrfacc Sci.. I29
8
(I9S9) 353. S. Ozcki and
5 6
9
IO II 12
S. IkcdqJ. Colloid lntcrfacc Sci.. 87 (1981) 424. H.B. Klcvcns, J. Phys. colioid Chcm.. 52 (1948) 130. A.B. Scott and H.V. Tartar, J. Am. Chcm. Sot.. 65 ( 1943) 192. H.J.L. Trap and J.J. Hcrmans. Proc. K. Ned. Akad. Wet.. Ser. B. 58 ( 19.55) 9i. M.F. Emerson and A. Holtzcr. J. Phys. Chcm., 71 (1967) I SY8.
13 14 15 I6 !7
I8 I9 20
21
T. Imae, A. Abe. Y. Taguchi and S. !kcda. J. Colloid Intcrl& Sci.. 109 (1956) 5G7. A. Tanaka and S. Ikcda, Colloids Surfxcs. 56 (1991) 217. S. OzcLi and S. ikeda. J. Phys. Chcm., 89 (1985) 5088. A. Abe. T. lmac and S. Ikeda, Colloid Polym. Sci., 265 (1987; 637. J. Timmcrmans. Physical Chemical Constants of Binary Systems, Vol. 3. Intcrscicncc. New York. 1960. p. 16. G.S. Hartlcy. J. Chcm. Sot., (1938) 1968. I.M. Koltholl and W. Stricks. J. Phys. Colloid Chcm., 51 ((94X) 915. K.A. Zachariasse. N. Van Phuc and B. Kozankicwicz. J. Phys. Chcm., 85 (I 981) 2676. C.J. Drummond, F. Gricser and T.W. Hcaly. Faraday Discuss. Chcm. Sot., Yl (1986) 95.
21
Solubilization of spiropyran of dodecyltrimethylammonium Shoichi
Ikeda
Department (Received
in aqueous NaBr solutions bromide
and Yoke Saso
qf Chemistq~. 28 October
Faculty ofScience.
1991; accepted
Nagoya
16 January
University,
Chiktrsa. Nagoya
464. Japatl
1992)
Abstract I .X3-Trimcthylindolino-6’-nitrobcnzopyrilospiran is solubilizcd in aqueous NaBr solutions of dodccyltrimcthylnmmonium bromide (DTAB). The dye is soluble in solutions of DTAB having concentrations higher than the critical micellc conccnrration. The solubilization power of DTAB is 2.08. IO-” mol dye per mol DTAB at NaBr concentrations lower than I.84 ill. while it increases linearly with increasing NaBrconccntration from 1.84-4.50 M. The constant solubilizution power of DTAE ,s attributable to the spherical miccllcs. while its higher solubilization power in the prcscncc of NaBr beyond 1.84 M can bc nscribcd to rod-like miccllcs. The solubilization capacity of a spherical miccllc of DTAB ranges from 0.1 l-0.18. dcpcnding on :hc NaBr concentration, where the micellc aggregation number varies from 52-86. The rodlike miccllc has an increasing soiubi!‘ rzation capacity with increasing NnBr concentration. and ir can solubilize the dye to such an amount :haL I36 DTAB molecules along irs c ontour length incorporate one dye molcculc. The solubilizcd dye is partly in the mcrocyaninc rorm. and the solubilization locus has polarity corresoonding _ LOa diclcctric constant = 35. Ke _.WO~~!~: ..I Dodccyltrimethylammonium
bromide:
sodium
bromidr;
introduction The spiropyrans constitute a class of photochromic compounds, generally insoiubie in water, which can be readily converted from the colorless Spiro form (I) to the colored merocyanine form (II) CH3 CH3
through hcteroiytic scission, when irradiated by ultraviolet Iight. The merocyanine form (Ii) reverts to the Spiro form (I) either by illumination with visible light or by thermal relaxation [i-4]. It has been suggested that, from the kinetic standpoint, II is more stable in a more polar environment than i [S-7]. Corrmpordrrm Scicncc, Nagoya
to: S. Ikeda, Dept. University, Chikusa,
Ol66-6622/92/SO5.00
0
1992 -
of Chemistry, Faculty Nagoya 464, Japan. Elsevicr
Science
of
Publishers
solubilization:
spiropyran.
In the present work the soiubiiization equiiibrium of i,3,3-trimethyiindoiino-6’-nitrobenzopyriiospiran (A) in aqueous NaBr solutions of dodecyltrimethyiammonium bromide (DTAB) is investigated, in order to observe the effect of ionic strength, i.e. of miceiie size and shape, on the soiubiiization power as well as on the sohrbilization locus. It is known that the spherical miceiies of DTAB increase their size with increasing NaBr concentration and the sphere-rod transition of DTAB miceilcs occurs at I.80 A4 NaBr [SJ. The rod-like micelles of DTAB grow further with increasing NaBr concentration. Experimental
Dodecyitrimethylammonium grade reagent, was purchased B.V. All rights
reserved.
bromide, special from Tokyo Kasei
Kogyo Co., Inc., Tokyo, recrystallized three times from acetone containing a small amount of ethanol, then dried at 40°C for 8 h and stored in a dcsiccator. NaBr was special grade reagent from I\Jakarai Chemical Co., Inc., Kyo!o. I ,3,3-Trimcthylinaolino-6’-nitrobcnzopyrilospiran ilvas obtained from Tokyo Kasei Co., Inc., and used without further purification. Dioxane was spectroscopic-grade reagent from Nakarai Chemicals Co., Inc. Water was redistilled in a glass still from an alkaline KMnO, solution.
Absorption spectra were measured in the range 200-600 nm on a Shimadzc UV ZOOS spcctrophotometcr and a U-t 25MU recorder, using a quartz cuvcttc of I cm path length. The light source for illuminating with visible light was a 300 W projector lamp (Kondo KP10s) on an Etmo S-30 Prqiector. Ultraviolet light was provided by an 85 W mercury lamp from Japan Storage Battery Co., Ltd, Kyoto.
A gram of spiropyran ground by a spatula was added to, usually, 5 cm3 of aqueous NaBr solution of DTAB in a stopper-cd test-tube (1.5 cm diameter x IO cm), and the test-tube was shaken in the dark in a Yamato BT-22 thermostat at 25 rfi 0.1 “C. The suspension was shaken. usually for 2 days, and then hltcrcd through a Millipore hltcr, FGLP 01300, having pore size, 0.2 urn. The pink color of the solution faded during or after the filtration. Some of the filtrates were kept in the dark for 30 min, and the absorption spectra of the colored solutions wcrc immediately measured. For the determination of the amount of solubiIized dye, the filtrate was diluted by adding an equal volume of dioxane and bleached by illuminating with visible light for I min, and then the absorption spectra of the solutions were recorded again. In the presence of NaBr more concentrated
than 4.50 114, precipitation occurred when dioxane was added to the filtrate. The optical density, A, was read at the peak around 350 nm and twice this value was divided by the value of the molar cxtinclion cocfhcicnt, c. The solubility of dye in the miccttar solution, S (mol I -’ or bI). is given by S=2A/(
(1)
Spiropyran was dissnlvcd in a mixture of equal volumes of dioxane and w~tcr or aqueous NaBr solution, and the solution was irradiated by visible light for I min. Then the absorption spectra of the solutions were measured. The molar extinction coefficient of the peak around 350 nm did not change with the NaBr concentration. but its wavclength shifted from 352 nm in water to 348 nm in 4.0 A,I NaBr. For the calibration of dye concentration in dioxanc-water or dioxanc-aqueous NaBr solutions, a value, ( =9530 I mol- ’ cm-‘, at the peak of the absorption band was obtained and this value is assumed below. Results
The solubilization equilibrium of spiropyrnn in the miccllar solution could be attained within a day or two, and the micettar solution containing sotubilizcd dye was colored pink, indicating partial conversion of the Spiro form (I) into the merocyanine form (II). The spectrum of the aqueous micellar solution of solubilized dye is illustrated in Fig. I. The absorption bands arc manifest at 240, 260, 350 and 525 nm. The molar extinction coeficient of the visible band at 525 nm is 2000 I mol- ’ cm- ‘. When this solution is illuminated by visible light, its pink color fades readily and the visible band disappears, but the ultraviolet bands remain nearly unaltered. The spectrum of the solution after illumi-
s. Ikccla. Y. Stl.w/Collf~irls Slcr:firlT.s67 ( 1992) 2 I-17 I
D.6 -
Fig. 2. The Fi_r. I. Absorption DTAB solution
containing filtcrcd
spectra
of aqueous
solubilizcd after shaking
(I) kept in lhc dark
spiropyran:
miccllar C=O.O40
solutions
of
M. (I) The
Tar 7 days at 25 C. (II)
Solution
for 3Omin.
nation is also shown in Fig. 1. The absorption band at 350 nm has nearly the same molar extinction cocficicnt before and after illumination by visible light.
The rate of solubilization was measured by following the optical density of the miccllar solution treated as described above. It was found that a nearly constant value of optical density was approached or attained within 24-48 h, and that thereafter a much slower process of increasing optical density proceeded. This is ilIustrated in Fig. 2. The initial rapid increase in optical density at 350 nm rcprescnts the solubilization process, but the later, slower increase in optical density finally reveals a change in color to yellow as well as the appearance of a new band at 390 nm. Thus it can be concluded that the solubilization equilibrium is attained within 24-48 h, and thereafter some photochcmical changes occur in the merocyanine form (II ).
Figure 3 shows the solubility of spiropyran in aqueous NaBr solutions of DTAB. In the absence
miccllar
incrcasc
solution
ity is rcprcsentcd miccllnr
solutions
equal volume
in solubility
or DTAE
\.ith
by the optical of DTAB
of dioxane
of spiropyrun time; C=O.O40 density
after
at 350 nm of aqueous
filtration.
and illumination
in an aqueous IV. The solubildilution
with
an
by visible light.
of NaBr, and in its presence at low concentrations, it can be seen that the solubility of the dye remains zero below a certain concentration, which can be identified with the critical micelle concentration. Co (M). Above the critical micelle concentration the solubility increases linearly with increasing DTAB concentration. Values of the critical micelle concentration are given in Table 1, ‘together with those reported previously [9-141. Values of the critical micclle concentration derived from the solu-
Fig. 3. Solubility DTAB (A).
of spiropyran
at 75 ‘C. C, (M):
2.00; (0).
3.00; (0).
in aqueous
NaBr
solutions
(C). 0: (A), 0.05; (Ll). 0.50: (0). 4.00: (I), 4.50.
or
1.00:
TABLE
I
Critical
rniccllc concentration
C. (bf 1
of DTAB
in aqueous
NaBr
solutions
C” (!\I) Prcscnt work
Ref. [?I”
0
0.01 I7
0.0
0.05
0.003x
0.50
0.00
“Rcfractivc ‘Surfxc
164
Rcl-. [IO]”
Rcl-. [I
0.015x
0.0147
I J’
121”
Ref. [
Ref. [ 143’
0.0138 0.0057 I
0.0070
I2
0.0 I47
0.0 15s 0.00575
0.00 I46
index. hElcctric
conductivity.
‘Lielit
scattering.
“Electric
conductivity
in water.
0.00
and light scattering.
‘Dye
I -Is
solubilhtion.
tension.
bilization of spiropyran are. however, those obtained by other methods. The slope of the Iincar relation solubility and miccllc concentration
smaller
than
between
dye
C, (M), the solubilization
power
of DTAB
is given
by
s, = S/(C - c-0)
(3)
gives the solubilization power of DTAB in the micellar form, and this is plotted as a function of NaBr concentration in Fig. 4. The solubilization power of DTAB toward spiropyran is constant (independent of the NaBr concentration) if the NaBr concentration is lower than 1.84 M. At NaBr concentrations higher than 1.84 M, the solubilization power of DTAB increases with increasing NaBr concentration. Its values arc listed in Table 2. If the NaBr concentration is represented by
s,=2.08-lo--3 s, = (0.88 + O.GC,)*
10 - 3
c, d 1.84
(3a)
C, 2 1.84
(3b)
In the previous work on light scattering in aqueous NaBr solutions or DTAB [8], it was demonstrated that DTAB forms spherical micelles when the NaBr concentration is lower than 1.80 M whi!c it forms rod-like micelles at higher NaBr concentrations. The rod-like micelles are in equilibrium with the spherical micelles. The threshold NaBr concentration for the sphcrc-rod transition, 1230 M, is very close to the NaBr concentration whcrc the solubilization of DTAB suddenly changes. Thus the observed threshold NaBr concentration for diflixent solubilization behaviors. I.S4 M, can be
4-
TABLE
3-
2
Solubilizatic~~
;
a
N:lBr
0 2B
conccn:
r spiropyran
in miccllcs
of DTAB
at dilfcrcnt
t)IlS
0
vi I-
L Fig. 4. Solubilization toward spiropyran.
power
of
DTAB
in miccllar
solutions
,r
C,
SC
(A# )
(IO-J Al)
0.00 0.05
2.08
52. I
0.105
2.07
59.4
0. I33
0.50
2.0 I
0.144
I .OO
2.15
71.7 76.0
2.00
2.18
3.00
2.94
88.1 138.1
0.192 0.407
4.00
3.3 I
190.1
0.629
4.50
3.57
217.0
0.775
/?l
(mol dye per mol miccllc)
0. I63
identified with that for the sphere-rod transition of micelles. The solubilization power of DTAB is higher when it is in a rod-like micelle, as compared with when it is in a spherical micelle. This fact has been observed for solubilization of Sudan Red B with other surfactants [ 13,15,16]. The solubilization power of DTAB can be converted to the solubilization capacity of the DTAB micelle, if the micelle aggregation number is known. It has been established that linear, doublelogarithmic relationships hold between micelle molecular weight, M,, and ionic strength, (C, + C,), for spherical and rod-like micelles, respectively, and they are expressed by
m
Fig. 5. Solubilization capigcity of DTAB micclles pyran z! different NaBr concentrations.
toward
spiro-
log M, = 0.085 log(C, + C,) + 4.37 c, < 4.00 A4
(4a)
log M,V=l.l I log(C, + C,)+4.10 C, 2 1.80 M
(4b)
for DTAB in aqueous aggregation tt1 =
number
NaBr
of both
solutions micelles
(5)
Assuming that the micelle size is not influenced by the presence of solubilized dye, the number of dye molcculcs solubilized in a miccllc, i.e. the solubilization capacity of a micelle, Z (mol dye per mol micelle), is given by YX-- Ins,
(6)
Its values are also shown in Table 2. Figure 5 shows the solubilization capacity of a DTAB micelle as a function of micelle aggregation number. The solubilization capacity of a DTAB micelle increases linearly with increase in its aggregation number, but the relationships are different for spherical and rod-like micelles. They are expressed by C = 0.00208111
52
(W
.Z = O.O044ttl- 0.198
86
(7b)
respectively.
The aggregation
number,
to the micelle
Soluhilizctior~
that
is formed
in 1.80 M
kmts
[Xl. The
is
MJ308.8
ponds NaBr.
86, corres-
Since the micellar solution is colored when spiropyran is solubilized, the dye molecules in a micelle must be, at least partially, in the merocyanine form (II). To estimate the polarity of the solubilization locus, the visible absorption band of the solubilized dye in micelles was compared with those of the dye in dioxane-water mixtures of different compositions. It was found that the band shifts from 518 nm in the 4: 6 v/v dioxane-water mixture to 549 nm in the 9‘: 1 v/v dioxane-water mixture, and the wavelength is linearly related to the volume fraction of dioxane in the mixture. Its molar extitiction coefficient decreases sharply with increasing dioxane content: c = 11000 in 4: 6 v/v dioxane-water mixture and c = 112 in 9: 1 v/v dioxanewater mixture. However, the absorption band of spiropyran in the micellar solutions is located at 525 nm, nearly independent of NaBr concentration. This wavelength is equal to that observed ir “le 1 : I v/v dioxane-water mixture. Thus the solubilized spiropyran is situated at a locus having a dielectric constant of around 35 [l73.
Discussion We have found that a spiropyran dye, 1,3,3trimethylindolino-6’-nitrobenzopyrilospiran (A), can be solubilizcd in both kinds of miccllcs of DTAB to different degrees. Linear increase of the soiubilicy above the critical micellc concentration has been observed for other micellar solutions with different dyes [ 13,15,16,18,19]. l-lowcver, values of the critical micelle concentration assigned to DTAB here arc somewhat lower than those obtained by other methods, suggesting some interaction between spiropyran and DTAB. possibly involving the mcrocyaninc form (II). The micellar solutions containing solubiIizcd spiropyran are colored, and accordingly spiropyran is solubilized in the micclles in both the Spiro (I) and mcrocyaninc (II) forms. If it is assumed that the molar extinction coeficicnt of the visible absorption band of the mcrocyanine form (II) is determined by the polarity of the medium, its value in polar media would be 35 000 1 mol- * cm - ’ [S]. Then the fraction of spiropyran in the mcrocyaninc form (II) is found to he as low as 6% in the miccIle. The merocyanine form (II) is zwitterionic and its p!:enolatc part can be strongly attracted by the cationic micciles of DTAB. If the counterion of dodccyltrimethylammonium (DTA ‘) is changed ion, the critical from Br- ion to the phenolate micelIe concentration could change, possibly becoming lower. This would be the reason for the low values of the critical micel!c concentration as observed. Some af the spiropyran moiecules are incorporated in micciles in this “counterion” form, however, most of them would be accommodated at the locus of solubilization which matches the hydrophobicity of the Spiro form (I). The colored micellar solutions can bc readily bleached by illumination with visible light, and the bleached solutions are reversibly colored during illumination with ultraviolet light. The solubilization loci of spiropyran could be different for the Spiro form (I) and the merocyanine form (II). Nevertheless, the illumination with visible light appears not to influence the soiubility of spiro-
pyran. so that the Spiro form (I) can be solubilized in the micelles to the same degree as (or more than) the merocyanine form (II). It seems probable that the solubilization locus would not c!lanse by the conversion of the merocyaninc form (11) to the Spiro form (I), and both forms of dye would accommodate near the miccllc surface so that they arc readily convertible. The solubilization loci have equal polarity, irrespective of whether they arc in a spherical micelle or in a rod-like micelle. Spiropyran can accommodate only at the locus having polarity equivalent to a dielectric constant of 35. Extcnsivc studies on the polarity of the microenvironment of the solubilizcd probe were pcrformcd by using 2,6-diphcnyl-4-(2,4,6-triphcnyl-1 -pyridiso-called E,(30) 1201. The results no)phenoxidc. in micclles of showed that the dye is soiubilized DTAB at a locus having a dielectric constant of 35-36 [Z&21]. It was also observed [IS] that Sudan Red B is solubilized in the micelles of dodccyldimcthylammonium bromide at a locus having polarity corresponding to the 5 : 2 v/v acetone-water mixture, which has a dielectric constant of 38. The solubilization of spiropyran occurs in a spherical micelle among ten to six micelles, depending on the micellc size. The low solubilization may to the low hydrophobicity and the be attribu. large molecular size of the spiropyran. The spiropyran is incorporated in a rod-like micclle in such a way that one dye n~olecule is solubilized by, on average, 136 DTA+ ions; thus two solubilized spiropyran molecules are about 82 A apart from each other along the contour length of a rod-like micelle, assuming the DTA+ ion to be 0.60 A thick.
References I 2 3 4
E. Fischer and Y. Hirshbcrg. J. Chcm. Sot.. (1952) 4517. Y. Hirshbcrg. J. Am. Chcm. Sot., 7X (1956) 2304. 0. Chaudc and R. Rumpf. CR. Acnd. Sci. (Paris). 236 (I 953) 697. 1. Bcrtclson. Photochromism: in G.H. Brown (Ed.),
7
Techniques ol Chemistry, Vol. III, Wiley-lnterscicncc, New York. 1971, Chapter 3. p. 45. J.B. Flanncry, Jr., J. Am. Chcm. Sot., 90 (1968) 5660. 1. Shimizu, H. Kokado and E. !nouc, Bull. Chcm. Sot. Jpn.. 42 ( 1969) 1730. H. Scki snd K. Ichimura. J. Colloict lntcrfacc Sci.. I29
8
(I9S9) 353. S. Ozcki and
5 6
9
IO II 12
S. IkcdqJ. Colloid lntcrfacc Sci.. 87 (1981) 424. H.B. Klcvcns, J. Phys. colioid Chcm.. 52 (1948) 130. A.B. Scott and H.V. Tartar, J. Am. Chcm. Sot.. 65 ( 1943) 192. H.J.L. Trap and J.J. Hcrmans. Proc. K. Ned. Akad. Wet.. Ser. B. 58 ( 19.55) 9i. M.F. Emerson and A. Holtzcr. J. Phys. Chcm., 71 (1967) I SY8.
13 14 15 I6 !7
I8 I9 20
21
T. Imae, A. Abe. Y. Taguchi and S. !kcda. J. Colloid Intcrl& Sci.. 109 (1956) 5G7. A. Tanaka and S. Ikcda, Colloids Surfxcs. 56 (1991) 217. S. OzcLi and S. ikeda. J. Phys. Chcm., 89 (1985) 5088. A. Abe. T. lmac and S. Ikeda, Colloid Polym. Sci., 265 (1987; 637. J. Timmcrmans. Physical Chemical Constants of Binary Systems, Vol. 3. Intcrscicncc. New York. 1960. p. 16. G.S. Hartlcy. J. Chcm. Sot., (1938) 1968. I.M. Koltholl and W. Stricks. J. Phys. Colloid Chcm., 51 ((94X) 915. K.A. Zachariasse. N. Van Phuc and B. Kozankicwicz. J. Phys. Chcm., 85 (I 981) 2676. C.J. Drummond, F. Gricser and T.W. Hcaly. Faraday Discuss. Chcm. Sot., Yl (1986) 95.