Journal of Photochemistry and Photobiology, A: Chemistry, 48 (1989) 435 - 446

PHOTORHEOLOGICAL EFFECTS IN CATIONIC AND

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NON-IONIC MICELLAR SOLUTIONS AND THEIR APPLICATION TO ACTINOMETRY

THOMAS WOLFF, FRIEDRICH SCHMIDT and GUNTHER VON BUNAU

Physikalische Chemie, UniversitLt Siegen, D 5900 Siegen (F.R.G.)

(Received August 22, 1988; in revised form February 24, 1989)

Summary

The bulk viscosities of aqueous micellar solutions of cetyltrimethyl- ammonium bromide (CTAB) and Triton X-100 change considerably in the presence of certain aromatic solubilizates on photoconversion of the solu- bilizates. This photorheological effect appears in the systems CTAB-4 carboxystilbene, CTAB-phenylglyoxylic acid and Triton X-lOO-N-methyl- N,N-diphenylamine. Addition of either 4-hydroxyazobenzene or 2-carboxy- 4’-hydroxyazobenzene (25 mM) to aqueous CTAB solutions (0.25 M) increases the viscosity by about one order of magnitude, but because of the low photoisomerization yields no photorheological effect is observed. Viscosities were measured with capillary viscometers since the flow be- haviour of the systems investigated is newtonian. The viscosities of the systems containing either phenylglyoxylic acid or one of the azobenzene derivatives change with temperature T more rapidly than with exp(B/T). A linear dependence of viscosity on the number of absorbed quanta is observed when less than 10% of the aromatic solubilizates are photoconverted. Quantum yields of the photoconversions are given and possible applications of the photorheological effects to actinometry are discussed.

1. Introduction

It is well known that the addition of small amounts of suitable sub- stances to dilute aqueous micellar solutions has a drastic influence on the viscosity and the flow behaviour of these systems. Substances which exhibit these rheological effects may be called “rheologically active”, while most others are “rheologically inactive” and have little influence on the viscosity.

We have found that 4-hydroxy-cis-stilbene and some g-substituted anthracenes are rheologically active and can be photochemically transformed into inactive or less active products (“photorheological effect”) [ 1 - 41. The anthracenes are photodimerized, whereas the cis-stilbene derivative is con- verted into its trans isomer. In view of the possible applications, we carried

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out a search for other systems which show photorheological effects. Per- tinent results were obtained with the trans-cis photoisomerization of 4- carboxystiIbene and azobenzene derivatives, the photodecarboxylation of phenylglyoxylic acid, and the photocyclization of N-methyl-N,N-diphenyl- amine. The latter reaction was studied in micellar solutions of the non-ionic surfactant Triton X-100, whereas the others were carried out in cetyltri- methylammonium bromide (CTAB) solutions.

In this paper we report on the photochemistry and the rheological effects observed in these systems and discuss possible applications to acti- nometry.

2. Experimental details

2.1. Compounds N-Cetyl-N,N,N-trimethylammonium bromide and chloride (Merck,

p-a., and Henkel), Triton X-100 (Merck, p.a., molecular weight 646.84 g mol-‘) and phenylglyoxylic acid (Fluka, p.a.) were used as supplied. 4- Hydroxyazobenzene and 2-carboxy-4’-hydroxyazobenzene (both Janssen) were recrystallized twice from ethanol under red light. N-Methyl-N,N- diphenylamine (Kodak) was fractionated in uucuo. 4-Carboxystilbene was prepared as described in the literature [ 51: melting point, 244 “C (literature, 244 - 245 “C).

2.2. Solutions The preparation of the samples was carried out as described in refs.

1 - 4. Surfactant concentrations of 0.125 - 0.25 M and solubilizate concen- trations up to 100 mM were used.

2.3. Irradiations Stirred samples were irradiated in thermostatted cylindrical quartz

cells (length, 2 cm; containing 1.5 ml of solution). A 100 W high pressure mercury lamp (Osram HBO) was employed as the irradiation source. Wave- lengths were selected using interference filters (Oriel, halfwidths approxi- mately 10 nm). Irradiated samples were protected from diffuse daylight and scattered radiation. Stirred samples of N-methyl-N,iV-diphenylamine in Triton X-100 solutions were irradiated in open vessels through the air- solution interface in order to ensure and maintain air saturation and to prevent the deposition of irradiation products on windows.

The progression of the photoreactions was monitored by UV-visible spectroscopy using a Beckman Acta MVII spectral photometer. For this purpose samples were diluted or cuvettes with small optical path lengths were used.

2.4. Conventional actinometry For gauging and for determination of previously unknown quantum

yields the method of Hatchard and Parker [6] was used.

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2.5. Rheologicai experiments Samples were checked for flow behaviour using a Haake CV 100

rotating viscometer equipped with a newly developed digital control and data evaluation unit [4]. Newtonian flow behaviour was identified by the linear dependence of shear stress on shear rate at shear rates between 0 and 290 s-l, and by the fact that the same viscosity values were obtained from the rotating instrument and from capillary (Ostwald) viscometers which operate at very high shear rates (more than 1000 s-l). Systems which proved to be newtonian were measured with capillary viscometers. Since the den- sities of the micellar solutions are not significantly different from that of water, viscosity values obtained from capillary viscometers were taken as absolute viscosities. Measured values were reproducible within 2%. The aging of micelles which is known to affect lifetime measurements [7] did not affect the viscosities of ionic micellar systems over several months.

2.6. pH Measurements A Philips PW 9414 pH meter was used to measure pH values at 25 +

0.5 “c!.

3. Results

3.1. 4Carboxystilbene in CTAB solutions Like many other stilbene derivatives, 4-carboxystilbene undergoes

photochemical cis-trans and trans-cis isomerizations as shown in Fig. 1. Depending on wavelength, photostationary ratios of cis and trans isomers may be established on extended irradiation. These ratios were determined by absorption spectroscopy. Since the pure cis compound was not available we used the method proposed by Fischer [S] to determine the cis spectrum from two extended irradiations at different wavelengths (334 and 366 nm) in CTAB solutions (see Fig. 2). The photostationary cis fractions were 81% at 334 and 86% at 366 nm.

COOH

9 0 /” c=c

Hi 4 0

hv -

hV.A

‘COOH

Fig. 1. Isomerization of 4-carboxystilbene.

A solution containing 20 mM of trans-4-carboxystilbene and 0.25 M aqueous CTAB has a higher viscosity than the pure CTAB solution (1.7 mPa s); it is raised further on irradiation at 313, 334 and 366 nm, i.e. on photoconversion of the trans isomer into the cis compound. This increase is shown in Fig. 3 as a function of the quanta absorbed per unit volume

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Fig. 2. Absorption spectra of tram- and cis-4-carboxystilbene and spectra obtained after exhaustive irradiation at 334 nm (----n-. ) and 366 nm (- - -) in aqueous CTAB (0.25 M).

Fig. 3. Viscosity r) at 25 f 0.5 “C as a function of the number of quanta 9, absorbed in unit volume of an aqueous solution containing 0.25 M CTAB and 20 mM 4-carboxystil- bene: X, o and A correspond to irradiation at 313, 334 and 366 nm respectively.

(determined by the method of Hatchard and Parker). Figure 3 shows that the dependence is linear until more than 10 mM of the 334 and 366 nm photons are absorbed or until more than 30 mM of the 313 nm photons are absorbed. On further irradiation the viscosity starts to level off because of the interfering photochemical back isomerization. At the photostationary equilibrium (at 366 nm) a viscosity of 13 mPa s was measured.

Initial slopes dr)/dQ, of the curves in Fig. 3 are given in Table 1 to- gether with the corresponding quantum yields @ of the respective photo- reactions. The quantum yields are the same at 334 and 366 nm (@ = O-4), whereas at 313 nm @ = 0.09, limiting slopes change accordingly. The value Q, = 0.4 agrees well with the photoisomerization yields of other stilbenes [9]. The quantum yields for the back reaction (cis-trans isomerization) (@ = 0.27 at 334 nm and Q, = 0.28 at 366 nm) are identical within error limits.

In order to make sure that thermal cis-trans isomerization does not interfere with the quantum yield determination, we measured the rate constant of the thermal cis-trans isomerization at 60 “C by monitoring absorption changes as a function of time. The observed low value of 2 X IO-” s-l rules out any significant influence on the yields obtained at 25 “C.

From pH measurements it follows that only 7% of the 4-carboxy- stilbene is dissociated in 0.25 M CTAB solutions (see Fig. 4). This indicates,

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TABLE 1

Actinometric sensitivities dq/dQv and quantum yields @ of the corresponding photo- reactions in various surfactant-solubilizate systems

System drl/dQ, @ h (J s mol-‘) (nm)

4-Carboxystilbene in CTABa -33 -150 -150

Phenylglyoxylic acid in CTABb -280 -150

-90

Phenylglyoxylic acid in CTACb -

N-Methyl-N,N-diphenylamine in Triton X-l 00’ 8

0.09 313 0.4 334 0.4 366

0.13 313 0.05 334 0.03 366

0.24 313

0.05 Polychromatic

aTrans-cis photoisomerization. bPhotodecarboxylation. ‘Photocyclization.

[H’l/mmol I-’

:/.&:y:::y c,mmol,_,

5 10 15 20 25

Fig. 4. Concentration of protons [H+] in aqueous 0.25 M CTAB as a function of the concentration of solubilized imns-4carboxystilbene (A-A-A) and trctns-2-carboxy-4’- hydroxyazobenzene (X-X-X) at 25 “C.

as discussed below, that most of the substance is solubilized in its undisso- ciated form within the micellar phase.

3.2. Azobenzene derivatives in CTAB solutions Azobenzene derivatives undergo photoisomerization as shown in

Fig. 5. This reaction has been studied in homogeneous solutions and has been tested thoroughly for actinometric purposes [ 10 - 121. We investigated dilute aqueous micellar solutions of the polar derivatives 4-hydroxy-trans-

N =N . h * .RQN_NRR~

KA

Fig. 5. Isomerization of azobenzene derivatives.

‘n %L

4

3 / 2

J’ 1 I .A

:&L.-.-.-

I 3 3.2 i.3 3.4

Fig. 6. Viscosity q at 25 k 0.5 “C of aqueous solutions of CTAB as a function of the concentration of solubilized 4-hydroxyazobenzene (A) and 2_carboxy-4’-hydroxyazo- benzene (x).

Fig. 7. Dependence of viscosity v(ln q/qL; qL, viscosity of water) on reciprocal tempera- ture l/T: a, 0.25 M CTAB; X, 0.25 M CTAB + 25 mM phenylglyoxylic acid; A, 0.25 M CTAB + 20 mM Z-carboxy-4’-hydroxyazobenzene.

azobenzene and 2carboxy-4’-hydroxy-trans-azobenzene. Measurement of pH shows that the latter compound is dissolved essentially in its undissociated form (9% dissociation at a concentration of 25 mM CTAB; see Fig. 4). It can be seen from Fig. 6 that the addition of small amounts of both com- pounds leads to a drastic increase in the viscosity rl of these solutions. Furthermore, In 17 values of solutions containing 25 mM 2carboxy-4’- hydroxybenzene (and 0.25 M CTAB) are not linear functions of reciprocal temperature 2’ (see Fig. 7). On irradiation of both compounds at various wavelengths very little isomerization takes place and therefore light-induced viscosity changes cannot be measured.

3.3. Phenylglyuxylic acid in CTAB solutions On UV irradiation of water-containing solutions, phenylglyoxylic acid

is photolysed to yield benzaldehyde and carbon dioxide. Although insensi- tive to oxygen, the photoreaction (Fig. 8) is known to proceed via the triplet state 113, 141 and to have a pH-dependent quantum yield, i.e. below pH 3.2 the reaction progresses with quantum yields between 0.1 and 0.8, whereas at pH > 4 quantum yields are only a few per cent. The presence of water is essential since in its absence the main products are isomeric diphenyltartaric acids [ 13,15 J .

Fig. 8. Photodecarboxylation of phenylglyoxylic acid (PG).

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E t I mol-’ cm-’

1

/nm /mmol I_’

Fig. 9. UV absorption spectra of PG in 0.25 M CTAB (- ), in water at pH 12 (- - -), in water at pH 2 (----..-) and in chloroform (- . -).

Fig. IO. Concentration of protons [H+] as a function of the concentration of solubilized PG in various aqueous solutions: X , 0.25 M CTAB; A, 0.25 M CTAC; a, water.

Figure 9 displays the absorption spectra of phenylglyoxylic acid (PG) in water at two pH values, in 0.25 M CTAB and in chloroform. The spectrum in CTAB solution is distinct but similar to that in alkaline water containing PG anions. Moreover, the absorption coefficients in micellar solution are independent of concentration only at X > 380 nm. These observations and the fact that the degree of dissociation in micellar solution (80%, see Fig. 10) exceeds that in water indicate that three different states of PG are present in micellar solution: free anions PG- in the aqueous phase, counter-ions of the micellar cations CTA+PG- (dominating the UV spectra) and undis- sociated PG molecules in the micelle phase. Changes in the relative amounts of these three states bring about the observed spectral changes.

A rheological investigation of 0.25 M CTAB and CTAC (cetyltrimethyl- ammonium chloride) solutions containing various amounts of PG proved newtonian flow up to PG concentrations of 25 mM in CTAB and CTAC, and structure viscosity at higher concentrations. Only newtonian solutions were used for further studies. As shown in Fig. 11, the viscosities of the CTAB solutions depend sensitively on PG concentration (and on temperature, see Fig. 7), while those of the CTAC solutions do not.

Irradiation of a solution containing PG (25 mM) and CTAB (0.25 M) leads to a decrease in viscosity q which depends on wavelength. According to Fig. 12, q is a linearly decreasing function of the number of quanta &, absorbed per unit volume over a considerable range.

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1

El-- x

14

10 ;I x

6:; I i i 2:$ -A-A-- ---./ s'

I 20 60 100

c

Fig. 11. Viscosity q at 25 + 0.5 “C of aqueous solutions containing 0.25 M CTAB (x) or CTAC (A) as a function of PG concentration C.

Fig. 12. Viscosity v at 25 + 0.5 “C of aqueous solutions containing 0.25 M CTAB and 15 mM PG as a function of the number of quanta Q, absorbed in unit volume. Irradiation wavelengths, 313 nm (x), 334 nm (0) and 366 nm (A).

However, the gradient dr)/dQ, also depends on wavelength. Actino- metry using the Hatchard-Parker method has shown that, in the range investigated, quantum yields of the photodecarboxylation of PG (listed in Table 1) depend on wavelength in a corresponding way (see Fig. 13).

3.4. N-Methyl-N,N-diphenylamine in Triton X-l 00 solutions The photocyclization of N-methyl-N,Ndiphenylamine (MDPA) takes

place in aerated solutions and is known to proceed via an intermediate

Fig. 13. Decrease in PG concentration AC as a function of the number of quanta Q, absorbed in unit volume (symbols as in Fig. 12).

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I h3 s qf) _& Qp3l

I CH3 CH3 CH3

Fig. 14. Photocyclization of N-methyl-N,N-diphenylamine.

zwitterion (see Fig. 14). It has been studied thoroughly in homogeneous solutions [16] as well as in aqueous cationic and anionic micellar solutions of CTAB and sodium dodecyl sulphate [ 171. In solutions of the surfactant Triton X-100 non-ionic micelles are formed. On addition of MDPA (30 mM) to an aqueous solution of Triton X-100 (0.15 M) newtonian flow is ob- served while the viscosity is enhanced by a factor of two (up to 4.5 mPa s, see Fig. 15 at Q, = 0).

I

6

5

L

i

lie ~Il./mmol-’

Fig. 15. Viscosity q at 25 f 0.5 “C of aqueous solutions containing 0.15 M Triton X-100 and 30 mM N-methyl-N,N-diphenylamine as a function of the number of quanta Qv absorbed in unit volume (polychromatic irradiation, see text).

Polychromatic irradiation of these solutions (MDPA absorbs at X < 340 nm) leads to an increase in viscosity which depends linearly on the number of photons absorbed (see Table 1). This is the first example of a photorheological effect observed in aqueous solutions of a non-ionic sur- factant. Under polychromatic irradiation, the quantum yield of the photo- cyclization of MDPA amounts to @ = 0.05; this value was obtained using the Hatchard-Parker method with appropriate corrections for differences in the absorption of the MDPA and Hatchard-Parker solutions (the absorption coefficient of N-methylcarbazole in aqueous solutions containing 0.15 M Triton X-100 is f = 3300 M-l cm-’ at 345 nm).

4. Discussion

Photorheological effects in micellar solutions are highly specific phe- nomena. They rest on the requirement that either the educts or the products of some photochemical reaction have a much larger influence on the viscos- ity of the solution than that given by Einstein’s limiting law (which states that the specific viscosity is proportional to the volume fraction of the

444

solute and that the proportionality factor is 5/2). The systems described in this paper are characterized by newtonian flow which can only be expected for globular micelles which do not aggregate or disintegrate on shearing. Two types of micelle-solute interactions which influence viscosity can occur: changes in size (and shape) of micelles owing to incorporation of solubilizates and changes in the ionic atmosphere of the micelles. The former type alone cannot account for the magnitude of the effects but may in- directly contribute to the latter by supporting counter-ion dissociation. Since the addition of electrolyte generally increases the bulk viscosity of ionic micellar solutions, particularly strong effects are brought about by dissociating solubilizates such as phenylglyoxylic acid (see pH measure- ments, Fig. 10). As the decarboxylation product (benzaldehyde) is a neutral molecule the viscosity is reduced.

The peculiarity of the solutions is also revealed by the temperature dependence of the viscosity (Fig. 7): the pure CTAB solution behaves like an ordinary solution, i.e. it fulfils an equation of the form 7)/qL = A exp(B/T) where qL is the solvent (water) viscosity, A is a pre-exponential factor and 8 is a characteristic temperature; in contrast, micellar solutions of rheologically active compounds show strongly non-linear plots in the In q/q= us. l/T diagram, indicating that photorheological effects should be maximum at low temperatures (lower limits are given by the KRAFFT points of the systems).

In addition, photorheological effects depend on photochemical yields and mechanisms which are known to be influenced by surfactants and micellar aggregation. For example, aerobic photocyclization yields of MDPA in cationic and anionic micellar solutions and in homogeneous methylcyclo- hexane solutions consistently amount to values around 0.7 [ 17 - 191, where- as quantum yields in the presence of the non-ionic surfactant Triton X-100 are much lower (approximately 0.05). Since no side reactions take place, absorption by the phenyl moiety of the Triton X-100 molecules and/or quenching by the non-ionic aggregates must be responsible for the low quantum yield.

Quenching also occurs in micellar solutions of azobenzene derivatives. Photoisomerization yields @ of these compounds in other environments are typically of the order of 0.5 [lo - 12, 203, whereas efficient quenching is observed in this study (a < O.Ol), probably due to heavy atom interactions with bromide counter-ions [ 211.

Finally, micelles affect the nature of the educts and therefore the yields of photochemical reactions; for example, phenylglyoxylic acid disso- ciates more readily in micellar solutions than in water, and photodecarboxyl- ation is only possible for the undissociated acid [ 13,141, whose absorption in micellar solutions is masked by that of the prevailing anion (see Fig. 9). This effect is also responsible for the observed wavelength dependence of the quantum yield (see Table 1). Table 1 shows that the replacement of bromide counter-ions by chloride counter-ions enhances the quantum yield by a factor of almost two. (Bromide ions are heavy atoms which promote inter- system crossing to the ground state.)

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It is interesting to note that both a decrease and an increase in viscosity can be obtained by irradiating suitably chosen systems. A pertinent applica- tion of either effect is in the measurement of light intensities. Since chemical actinometry suffers, in general, from fairly expensive instrumentation, there is a basic need for readily available actinometers. Capillary viscometers are almost ubiquitous, but photorheological actinometry has obvious limitations that emerge from the results presented here.

The viscometric quantity relevant for actinometric purposes is the slope dq/dQ, (see the linear parts of the curves in Figs. 3, 12 and 15). Conven- tional chemical actinometer systems, such as solutions of the Fe(III)- phenanthroline complex in the Hatchard-Parker method, are much more sensitive than any photorheological system investigated so far. Taking a change in optical density of 0.05 as the lower limit, it is readily calculated from the absorption coefficient of the Fe(III)-phenanthroline complex (E = 1.11 X lo4 1 mol-’ cm-‘) that the detectable number of photons ab- sorbed amounts to 2 PM; it follows from the slopes dq/dQ, listed in Table 1 that a significant viscosity change requires several millimoles of absorbed quanta.

A further requirement of a good chemical actinometer system is that it must be independent of wavelength [22 - 241. Many conventional systems are unsatisfactory in this respect, but the known photorheological systems are no better. The practical application of the viscometer to the measure- ment of light intensities is therefore limited to the independent gauging of photorheological actinometers. Therefore it follows that this new device can only be used for routine work involving the measurement of large light intensities.

5. Conclusions

Light-induced viscosity changes are only possible when either the educts or the products of some photochemical reaction have strong effects on the flow properties of their solutions, Aqueous micellar solutions are suitable photorheological systems, but they may also influence quantum yields such that the effects on viscosity are reduced. A linear dependence of viscosity on the number of absorbed photons is observed in three of the four systems investigated in this paper. However, this photorheological effect also depends on wavelength and can be applied to actinometry only in special circumstances.

Acknowledgments

We thank Mrs. H. Christian for preparing the drawings and Dr. M. Colaneri (Houston, TX) for critical reading of the manuscript. We are in- debted to Firma Henkel, Diisseldorf, who supplied several surfactants.

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Financial support by the Fonds der Chemischen Industrie is gratefully acknowledged.

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