Alkanediyl-r,ω-bis(dimethylalkylammonium bromide). 7.Fluorescence Probing Studies of Micelle Micropolarity and

Microviscosity

Raoul Zana,* Martin In, and Helene Levy

Institut C. Sadron (CNRS-ULP), 6 rue Boussingault, 67083 Strasbourg-Cedex, France

Guy Duportail

Laboratoire de Biophysique, URA 491 (CNRS), Faculte de Pharmacie, BP 24,67401 Illkirch, France

Received April 10, 1997. In Final Form: July 25, 1997X

The microviscosity of micelles of the dimeric (gemini) surfactants alkanediyl-R,ω-bis(dimethyldodecyl-ammoniumbromide), referred to as 12-s-12 (s) carbonnumber of the alkanediyl spacer), has been obtainedfrom measurements of the fluorescence anisotropy of micelle-solubilized diphenylhexatriene (DPH) andof the intensity ratio IM/IE of micelle-solubilized dipyrenylpropane (P3P, IM and IE ) intensities of themonomer and excimer emissions). The micropolarity of these surfactant micelles was characterized bythe value of the fluorescence intensity ratio I1/I3 of the first and third vibronic peaks in the emission spectraofmicelle-solubilized pyrene. Themicroviscosity andmicropolarity of the trimeric surfactant 12-3-12-3-12and of the tetrameric surfactant (12-3-12-4-12-3-12) micelles have also been determined in order to gaininformation on the effect of the degree of oligomerization of the surfactant. The microviscosity andmicropolarity of the asymmetric two-chain surfactants, the dimethyldodecylalkylammonium bromides,referred to as 12-s/2 (s/2 ) carbon number of the variable alkyl chain varying from 1 to 10), which canbe considered as themonomers of the 12-s-12 surfactants, have beendetermined for the sake of comparison.The I1/I3 vs s plots for the 12-s-12 and 12-s/2 series were found to be coincident, except for s ) 2, revealingthat the polarity sensed by pyrene in its micellar solubilization site is not affected by the “dimerization”of the surfactant. The micropolarity goes through a maximum for the two series of surfactants as a resultof the change of location of the spacer or variable alkyl chain as s is increased. The microviscosity in the12-s-12 series decreased rapidly as s increased above 3, then more slowly, and leveled out, but was nearlyindependent of s in the 12-s/2 series, in the range s/2 ) 1-8. For the two series of surfactants themicroviscositydecreasedupon increasing temperaturebut increasedwhenthemicelle shapeof thesurfactantoligomer micelles changed from spherical to elongated. Also, the microviscosity increased significantlyand nearly linearly with the number of dodecyl chains in going from DTAB to the dimer, trimer, andtetramer. These results are used to explain the low values of the quenching rate constants characterizingmicellar solutions of surfactant oligomers and hydrophobic microdomains in solutions of water-solubleamphiphilic polymers (polysoaps), with respect to micelles of the corresponding monomeric surfactants.

Introduction

The dimeric (gemini) surfactants alkanediyl-R,ω-bis-(dimethyldodecylammonium bromide), referred to as 12-s-12 (s ) carbon number of the alkanediyl spacer), arecurrently under extensive investigation in our labora-tory.1-9 Their association behavior in aqueous solution(critical micelle concentration (cmc), micelle ionizationdegree,micelleaggregationnumbers,micelledynamics),1,4solution rheology,5 andmicrostructure,6-9 behavior at theair-water interface,3 and formation ofmesophases2 havebeen recently reported. The aggregation numbers (N) ofdimeric surfactantmicelleshavebeendeterminedby time-resolved fluorescence quenching.8,9 The same studies

yielded values of the intramicellar quenching rate con-stants (kq) in micelles of 12-2-12 and 12-3-12 which weresignificantly lower than those in micelles of DTAB(dodecyltrimethylammonium bromide), which can beconsideredas the “monomer” of 12-2-12and12-3-12), evenwhen taking into account the difference in N valuesbetween the different micelles. Indeed, for spherical orclose-to-spherical micelles, kq is close to inversely pro-portional toN and inversely proportional to the viscosityof themicroenvironment of themicelle-solubilized probe,referred toasmicroviscosity,ηi.10-12 This led to thepresentinvestigation of the microviscosity of 12-s-12 micelles asa function of the spacer carbon number s. For a betterknowledge of the properties of the 12-s-12 micelles wealso determined theirmicropolarity, since it had not beensystematically investigated yet. We also extended themicroviscosity and micropolarity determinations to twonewly synthesized surfactant oligomers, the trimericsurfactant 12-3-12-3-129,13 and the tetrameric surfactant12-3-12-4-12-3-12,14 in order toknowhowthesepropertiesare affected by the surfactant degree of oligomerization.

* To whom correspondence should be addressed.X Abstract published in Advance ACS Abstracts, September 15,

1997.(1) Zana, R.; Benrraou, M.; Rueff, M. Langmuir 1991, 7, 1072.(2) Alami, E.; Levy,H.; Zana,R.; Skoulios, A.Langmuir1993, 9, 940.(3) Alami, E.; Beinert, G.; Marie, P.; Zana, R. Langmuir 1993, 9,

1465.(4) Frindi, M.; Michels, B.; Zana, R. Langmuir 1994, 10, 1140.(5) Kern, F.; Lequeux, F.; Zana, R.; Candau, S. J. Langmuir 1994,

10, 1714.(6) Zana, R.; Talmon, Y. Nature 1993, 362, 228.(7) Danino, D.; Kaplun, A.; Talmon, Y.; Zana, R. In Structure and

Flow in Surfactant Solutions; Herb, C., Prud’Homme, R., Eds; ACSSymp. Ser. 578; American Chemical Society: Washington, DC, 1994;p 105.

(8) Danino, D.; Talmon, Y.; Zana, R. Langmuir 1995, 10, 1448.(9) Danino, D.; Talmon, Y.; Levy, H.; Beinert, G.; Zana, R. Science

1995, 269, 1420.

(10) Lianos, P.; Lang, J.; Strazielle, C.; Zana,R.J.Phys.Chem.1982,89, 1019.

(11) Van der Auweraer, M.; De Schryver, F. C. Chem. Phys. 1987,111, 105.

(12) Almgren,M.;Alsins, J.;Mukhtar,E.; vanStam,J.J.Phys.Chem.1988, 92, 4479.

(13) Zana, R.; Levy,H.; Papoutsi, D.; Beinert, G.Langmuir 1995, 11,3694.

5552 Langmuir 1997, 13, 5552-5557

S0743-7463(97)00369-7 CCC: $14.00 © 1997 American Chemical Society

In this work, the micelle microviscosities were deter-mined using two fluorescence probing methods based onthemeasurement either of the fluorescence anisotropy ofmicelle-solubilized DPH (1,6-diphenylhexatriene)15,16 orof the fluorescence intensity ratio IM/IE of micelle-solu-bilized P3P (1,3-bis(1-pyrenyl)propane, IE and IM )fluorescence intensities of the excimer and monomeremissions of P3P).17,18 The micropolarity of the 12-s-12micelles was characterized by the value of the ratio I1/I3of the intensities of the first and third vibronic peaks inthe fluorescence emission spectrumofmicelle-solubilizedpyrene. This ratio is very sensitive to the polarity of thepyrene immediate environment.15,19,20For the sake of comparison, we determined the mi-

cropolarity and microviscosity of the micelles of dimeth-yldodecylalkylammonium bromides, referred to as 12-s/2,where thevariable alkyl chain carbonnumber s/2 variesfrom 1 to 10. These surfactants, which can be consideredas the monomers of the 12-s-12 surfactants, have beenwell investigated.21,22At the outset it should be recognized that the concepts

of microviscosity and micropolarity have been stronglycriticized.16,23 In particular, it has been pointed out thatdifferent values of microviscosities and micropolaritiesare obtained when using a given method and differentprobes, because each probe may be located in a differentmicellar site on a time average basis.23 Also, differentmethods often yield different results.15 Nevertheless,micropolarity and microviscosity measurements shouldretain their interest when investigating a series ofhomologous surfactants as in the present study. Indeed,the solubilization site of the probe is then expected toremain nearly the same for all the homologues, and thevariations of the measured quantities are more meaning-ful.While this study was performed, De et al.24 reported

microviscosities for thedimeric surfactants 16-s-16,whichhave an alkyl chain longer than the 12-s-12 surfactants.Their results are less extensive than the ones reportedbelow, but reveal the same trend, namely, a decrease ofmicroviscosity as the spacer carbon number increases.Our results also show a very large increase of microvis-cosity in going from the monomer (DTAB) to the dimer(12-3-12), trimer (12-3-12-3-12), and tetramer (12-3-12-4-12-3-12). The micropolarity of the 12-s/2 and 12-s-12series appears to go through a wide maximum whenplotted against s.

Experimental SectionMaterials. The samples of surfactants and of pyrene were

the same as those used in other studies.4,8,9,13,14,21,22 The sample

of P3Pwas purchased fromMolecular Probes (Eugene, OR). Thesample of DPH, chromatographically purified for scintillation,was from Koch-Light Laboratories (Colnbrook, Bucks, U.K.).Methods. The fluorescence intensities weremeasured using

a Hitachi F-1040 spectrofluorometer. P3P was excited at 346nmandthe intensites IMand IEweremeasuredat thewavelengthscorresponding to the first vibronic peakand to the excimer,whichwere around 378 and 485-490 nm, respectively, in the P3Pemission spectra. Pyrene was excited at 335 nm and theintensities I1 and I3 were those for the first and third vibronicpeaks locatednear374and385nminthepyreneemissionspectra.A spectrofluorometer type 8000 fromSLM (Urbana, IL) in the

T-format was used for measurements of the steady-statefluorescence anisotropy ofDPH. The excitationwavelengthwas360 nm, and the emission was monitored at 435 nm. Bothwavelengthswere selected using interference filters fromSchott(Mainz, Germany). A home-built device ensured the automaticrotation of the excitation polarizer, allowing continuous mea-surements of the fluorescence anisotropy, r.25 The cell holderwas maintained at a constant temperature to within 0.1 °C andmonitored by a thermocouple inserted in the cell, above the lightbeam. The relationship between microviscosity ηi and fluores-cence anisotropy r is26

where R is the gas constant, T the absolute temperature, τ theprobe lifetime in its environment, Vm the probe effective molarvolume, and r0 the fluorescenceanisotropy inamediumof infiniteviscosity. ForDPHthevalue r0)0.362wasmeasured inglycerolat-60 °C.27,28 Thevaluesof theDPHlifetimes in the investigatedmicellar solutionsweremeasuredwith thesinglephotoncountingapparatus used in our previous studies of micelle aggregationnumbers (excitation wavelength: 360 nm; emission monitoredin the range 390-490 nm, using the gelatin Kodak bandpassfilter 98).4,8,13 We did not attempt to calculate absolute valuesof themicroviscosities. Ratherallmeasuredvalueswere referredto that of DTAB micelles, thus obtaining microviscosity valuesrelative to that of DTAB micelles. This procedure permitted usto eliminate the difficulty associated with the value of Vm.In the case of P3P, the quantity τEIM/IE, (τE ) excimer

fluorescence lifetime) is theoretically proportional to the micro-viscosity.29,30 The measured values of this quantity were alsoreferred to that for DTAB, thus yielding relative microviscosityvalues. The excimer fluorescence lifetime was measured at anexcitation wavelength of 335 nm and at an emission monitoredat λ > 460 nm, using the gelatin Kodak filter 4.The solutions for fluorescence probing studies were prepared

by injecting the appropriate amount of a probe stock solution,in ethanol for pyrene and tetrahydrofuran for P3P and DPH, inthe investigated aqueous surfactant solution which was thenstirred at 40 °C at least for 12 h. The injected volume amountedto less than 0.5% of the volume of solution. We checked that thissmall amount of solventhadnoeffect on themeasuredproperties.This procedure gave more reproducible results, particularly inthe caseofP3P, than theusual one (introductionofanappropriatevolume of probe solution in an organic solvent into an emptyflask, evaporation of the solvent, introduction of the surfactantsolution, andprobe solubilizationby stirring). The solubilizationof very hydrophobic probes may require days or weeks, andincomplete solubilization cancause severe errors in themeasuredquantities.31,32

(14) In, M.; Zana, R. in preparation. The chemical formula of thesurfactant tetramer is

(15) Zana, R. InSurfactant Solutions.NewMethods of Investigation;Zana, R., Ed.; M. Dekker: New York 1987, Chapter 5 and referencestherein.

(16) Hare, F.; Amiell, J.; Lussan, C. Biochim. Biophys. Acta 1979,555, 388 and references therein.

(17) Zachariasse, K. A. Chem. Phys. Lett. 1978, 57, 429.(18) Zachariasse, K. A.; Duveneck, G.; Busse, R. J. Am. Chem. Soc.

1984, 106, 1045.(19) Kalyanasundaram, K.; Thomas, J. K. J. Am. Chem. Soc. 1977,

99, 2039.(20) Dong, D. C.; Winnik, M. A. Photochem. Photobiol. 1982, 35, 17.(21) Zana, R. J. Colloid Interface Sci. 1980, 76, 330.(22) Lianos, P.; Lang, J.; Zana, R. J. Colloid Interface Sci. 1983, 91,

276.(23) Henderson, C. N.; Selinger, B. K.; Watkins, A. R. J. Photochem.

1981, 16, 215.(24) De, S.; Aswal, V. K.; Goyal, P. S.; Bhattacharya, S. J. Phys.

Chem. 1996, 100, 11664.

(25) Beck, A.; Heissler, D.; Duportail G. Chem. Phys. Lipids 1993,66, 135.

(26) Shinitzky,M.;Dianoux,A.C.;Gitler,C.;Weber,G.Biochemistry1971, 10, 2106.

(27) Shinitzky, M.; Yuli, I. Chem. Phys. Lipids 1982, 30, 261.(28) Shinitzky, M. In Physical Methods on Biological Membranes

and their Model Systems; Plenum Publishing Corp.: New York 1984;p 237.

(29) Miyagishi, S.; Suzuki,H.;Asakawa,T.Langmuir1996,12, 2900.(30) Turley, W. D.; Offen, H. W. J. Phys. Chem. 1985, 89, 2933.(31) Winnik, F. M.; Winnik, M. A.; Ringsdorf, H.; Venzmer, J. J.

Phys. Chem. 1991, 95, 2583.(32) Kabanov, A. V.; Nazarova, I. R.; Astafieva, I. V.; Batrakova, E.

V.; Alakhov, V. Y.; Yaroslavov, A. A.; Kabanov, V. A. Macromolecules1995, 28, 2303.

ηi ) RTτ/[Vm(r0/r - 1)] (1)

Micelle Micropolarity and Microviscosity Langmuir, Vol. 13, No. 21, 1997 5553

Results and DiscussionMicropolarity. Tables 1 and 2 list the values of the

pyrene intensity ratio I1/I3 for the surfactant micellesinvestigated. The measurements were performed atsurfactant concentrations ranging between 2 and 3 wt %,where the probe is fully partitioned in the micelles. Wechecked that the I1/I3 value did not vary upon dilution to0.5 wt %.The I1/I3 data are plotted against s in Figure 1. Within

the experimental error ((2%) the data points for the 12-s-12 and 12-s/2 series fall on the same curve, except forthe value s)2discussed below, having a broadmaximumstretching from s ) 3 to 8 and then decreasing at larger

values of s. Recall that at s/2 > 4 the cmc of the 12-s/2surfactants starts decreasing rapidly,21 while that of the12-s-12 surfactants is a maximum at s ) 10-12.1 Thesevariationswere interpretedas indicating that thevariablealkyl chainof the12-s/2 surfactants21 and the spacergroupof the 12-s-12 surfactants1 go from a mostly aqueousenvironment to the micelle core, at above these values ofs. A regular trend was also observed for the micelleaggregation number N of the 12-s/2 surfactants: at amoderate concentration (about 50 mM)N first decreasedas s/2 increased from 1 to 4 and then increased withincreasing s/2.22 Vesicles were observed for the 12-8 and12-10 homologues and for the corresponding dimericsurfactants 12-16-12and12-20-12at concentrations closeto those used in the present study.7,8 Last, the micelleionization degree was found to increase with s.1,21 Thevalue of themicelle micropolarity is dependentmostly onthemicelle composition at the solubilization site of pyrene(and of other aromatic compounds), that is, the palisadelayer,15,33 since pyrene is known to interact with quater-nary ammoniun head groups,34,35 which is made up ofsurfactantR-methylenegroups, headgroups, counterions,andwater. Thus, the decrease of I1/I3 as s increases abovethe value where the spacer starts being part of the coreis probably mainly due to the replacement of some of thewater molecules present in the palisade layer by themethylene groups of the increasingly longer spacers andalso to the decrease of bromide ion concentration in thislayer. Thus, from the point of view of micropolarity, thepyrene microenvironment in micelles of 12-s/2 and of 12-s-12 surfactants appears to be nearly the same, that is,independent of the surfactant chemical structure (mono-mer or dimer) at a given value of s, for the cationicsurfactants investigated. This conclusion is also consis-tent with the fact that the measured micropolarity is anequilibriumpropertywhich involvesnomotionof theprobein its microenvironment, contrary to the microviscosity(see below).The results in Table 1 show that the change of I1/I3 in

going from the monomer (DTAB), to the dimer (12-3-12),trimer (12-3-12-3-12), and tetramer (12-3-12-4-12-3-12)is relatively small and almost within the experimentalerror. The observed near constancy of micropolarityprobably arises from the existence of two compensatingeffects relating to surfactant packing, as s is increased.At the concentrations used, one goes from sphericalmicelles (DTAB) to slightly elongated micelles (12-3-12)8and to very long worm like micelles (trimer,9 and also forthe tetramer in view of the high viscosity of its solution).14These changes of micelle shape reveal an increase in thesurfactant packing parameter36 with the degree of oligo-merization which is expected to result in a decrease ofmicropolarityowingto the tighterpackingof thesurfactantalkyl chains. However, this tighter packing may bringpyrene slightly closer to the micelle surface, on a time-averaged basis. The locally stronger micelle surfacecharge density (locally is used to refer to the surface areaoccupied by one surfactant at the micelle surface) mayalso contribute to this change of pyrene location. Indeed,pyrene has been shown to interact with quaternaryammoniumhead groups.34,35 Such a change, evenminor,of pyrene solubilization site toward the micelle surfacewould bring about an increase of micropolarity, whichcompensates the first effect just discussed. In the case of

(33) Macanita, A.; Costa, F.; Costa, S.; Melo, E.; Santos, H. J. Phys.Chem. 1989, 93, 336.

(34) Lianos, P.; Zana, R.; Viriot, M.-L. J. Phys. Chem. 1984, 88,1098and references therein.

(35) Ghosh, S.; Maki, A.; Petrin, M. J. Phys. Chem. 1986, 90, 5210.(36) Israelachvili, J.N.;Mitchell, J. D. ;Ninham,B.W.J.ChemSoc.,

Faraday Trans. 1 1976, 72, 1525

Table 1. Values of the Pyrene I1/I3 Ratio for theInvestigated Surfactant Dimer, Trimer, and Tetramer

Assembliesa

surfactant concn (wt %) I1/I3 at 25 °C I1/I3 at 40 °C

12-2-12 0.2-5 1.29 1.2612-3-12 2.0-5.2 1.48 1.4312-4-12 2.0 1.46 (1.52)b 1.4112-5-12 2.0 1.5112-6-12 2.0 1.48 (1.53)b 1.4512-8-12 2.2 1.45 1.4212-10-12 2.2 1.37 1.3212-12-12 2.4 1.34 1.3112-14-12 2.5 1.32 1.2812-16-12 2.5 (1.31)d 1.2812-20-12 2.8 (1.24)d 1.22DTAB 2.7 1.41 1.3712-3-12-3-12c 3.0 1.44 (1.48)b12-3-12-4-12-3-12 2.0 1.45 1.40

a Pyrene concentration between 1 and 10 µM, depending on thesurfactant concentration. The absence of excimer emission waschecked in all instances. b The values in parentheses are for thecorresponding chloride surfactants. c Fromref13. d These solutionswere turbid at 25 °C and the listed values have been obtained byextrapolating the data to 40 °C on the basis of the T-dependenceof the values of I1/I3 for the other surfactants in this table.

Table 2. Values of the Pyrene I1/I3 Ratio for theDimethyldodecylalkylammonium Bromide (12-s/2)

Surfactant Assemblies at 25 °Ca,b

DTAB 12-2 12-3 12-4 12-5 12-6 12-8 12-10

I1/I3 1.41 1.46 1.46 1.46 1.37 1.37 1.25 1.25a Surfactant concentration: 2.1-2.6 wt %. b The I1/I3 values for

the 12-s/2 surfactants previously reported22 had been determinedwith amuch less accurate spectrofluorometer but showed the sametrend.

Figure 1. Variation of the pyrene intensity ratio I1/I3 with thespacer carbon number s for the 12-s-12 series (b) and with thecarbon number of the variable alkyl chain for the 12-s/2 series(0) at 25 °C.

5554 Langmuir, Vol. 13, No. 21, 1997 Zana et al.

12-2-12, the first effect may be predominant, resulting ina lower value of I1/I3 than that obtained for DTAB.The values of I1/I3 listed inTable 1 all decrease by 2-3%

as the temperature increases from 25 to 40 °C. Thischange, however, does not reflect a decrease of polarity.Indeed, water, ethanol, and cyclohexane show a quanti-tatively similardecrease of I1/I3 in this temperature range.Thus theeffect of temperatureon themicellemicropolarityappears to be small.Microviscosity. Table 3 lists the values of the

fluorescence anisotropy r and lifetime τ of DPH, theproduct ηiVm (microviscosity × probe effective molarvolume), and the microviscosity of the various surfactantmicelles investigated, relative to that of DTAB micelles,ηi,rel. Tables 4 and 5 list the values of the P3P IM/IE ratio,the P3P excimer lifetime (τE) measured in the samesystems, and the relative microviscosities (ηi,rel ) (τEIM/IE)surfactant/(τEIM/IE)DTAB) at 15, 25, and 45 °C. The com-parison of the two sets of results at 25 °C reveals a rathergood agreement in view of the difference in the methodsused in obtaining the ηi,rel values and the type of probemotion involved (rotation of DPH molecule around itscenter, “butterfly-wings” motion of the two pyrenylmoieties for P3P). The agreement between the twomethods was also good when studying the effect of thesurfactant concentration (see below, Figure 5). In viewof these results most of the ηi,rel values were determinedusing the IM/IE P3P ratio.The values of ηi,rel reveal several important features.(i) The microviscosity of the dimeric surfactants 12-s-

12 is larger than that of the corresponding monomericsurfactants 12-s/2 at all values of s at 15, 25 and 45 °C (seeFigure 2).(ii) In the 12-s-12 series, the microviscosity decreases

as the spacer carbon number is increased, with most ofthe decrease taking place at s > 3. A similar result wasreported for the 16-s-16 series.24 In contrast, the micro-viscosity in the 12-s/2 series is independent of the variablechain carbon number s/2 (see Figure 2).(iii) The microviscosity increases significantly and

almost linearlywith thedegree of oligomerization ingoingfromDTAB, to 12-3-12, to the trimer 12-3-12-3-12, and tothe tetramer 12-3-12-4-12-3-12, as shown in Figure 3, at15, 25, and45 °C. Note that a study of the adsorbed layers

of DTAB, 12-2-12, and 12-2-12-2-12 on silica concludedthat themicroviscosities of the adsorbed layers of 12-2-12and 12-2-12-2-12 were larger than that of DTAB.37(iv) Figure 4 shows that the plots of log ηi,rel vs 1/T are

linear or close to linear, as expected. The slope of theplots is seen to increase, that is, the relativemicroviscositydecreases faster as the temperature increases, as thedegree of oligomerization increases, and as the spacer

(37) Esumi, K.; Goino; M.; Koide, Y. J. Colloid Interface Sci. 1996,183, 539.

Table 3. Fluorescence Anisotropy and Lifetime of DPHin the Investigated 12-s-12 Assemblies and AssemblyMicroviscosity Relative to That of DTAB Micellesa

surfactantconcn(wt %) r

109τ(ns)

106ηiVm(J.s./mol) ηi,relb

12-2-12 2.01 0.111 7.85 8.61 3.110.085c 7.23c 5.72c 2.350.059d 6.56d 3.43d 1.63

12-3-12 1.71 0.103 7.53 7.43 2.6812-4-12 1.58 0.082 7.35 5.34 1.9312-6-12 1.56 0.082 7.33 5.32 1.9212-8-12 1.82 0.080 7.36 5.17 1.8712-10-12 1.73 0.072 7.59 4.67 1.6912-12-12 1.81 0.069 7.69 4.49 1.6212-14-12 1.87 0.066 7.98 4.41 1.59

0.0485c 7.40c 2.95 1.210.0384d 6.86d 2.19 1.04

12-16-12 0.24e 0.065 7.97 4.32 1.56DTAB 2.50 0.050 6.97 2.77 1.00

0.041c 6.56c 2.43 1.000.0365d 6.20d 2.10 1.00

trimer 1.70 0.172 7.45 16.72 6.100.143c 6.98 11.76c 4.830.094d 6.50 6.13d 2.92

a Values at 25 °C unless specified otherwise. b ηi,rel ) (ηiVm)sur-factant/(ηiVm)DTAB c Values at 37 °C. d Values at 50 °C. e The solutionwas turbid at 1% and clear at this concentration.

Table 4. Values of the P3P IM/IE Ratio and of theFluorescence Lifetime τE of the P3P Excimer in MicellarSolutions of Various Surfactant Oligomers and MicelleMicroviscosities Relative to that of DTAB Micelles at 15,

25, and 45 °Ca-c

15 °C 25 °C 45 °C

surfactantC

(wt %)IM/IE(τE) ηi,rel

IM/IE(τE) ηi,rel

IM/IE(τE) ηi,rel

DTAB 2.23 8.16 1.00 4.42 1.00 1.73 1.00(83.0) (72.6) (58.3)

12-2-12 2.30 27.0 3.77 11.90 2.87 3.31 1.95(94.5) (77.4) (59.6)

12-3-12 2.00 29.0 3.99 12.0 2.86 3.00 1.72(93.2) (76.5) (57.9)

12-4-12 2.07 18.1 2.47 8.60 2.11 2.83 1.73(92.6) (78.8) (61.6)

12-5-12 2.12 20.0 2.91 9.74 2.50 3.27 2.08(98.4) (82.4) (64.3)

12-6-12 2.29 17.55 2.59 8.68 2.22 3.32 2.08(99.8) (81.9) (63.5)

12-8-12 2.23 14.80 2.15 7.85 2.02 2.65 1.77(98.8) (82.6) (67.4)

12-10-12 2.37 12.43 1.89 6.86 1.77 2.89 1.87(102.8) (83.0) (65.2)

12-12-12 2.49 11.97 1.76 6.69 1.68 2.73 1.67(99.6) (80.7) (61.7)

12-14-12 2.73 13.05 1.96 6.70 1.72 2.61 1.65(102.0) (82.4) (63.8)

trimer 2.34 47.0 6.63 21.8 5.38 5.27 3.56(95.6) (79.2) (61.2)

tetramer 2.06 67.4 10.05 27.9 7.39 7.20 4.80(101.0) (85.0) (67.3)

a The values of τE (in ns) are given in parentheses. b P3Pconcentration: 2-4 µM. c ηi,rel ) (τEIM/IE)surfactant/(τEIM/IE)DTAB.

Table 5. Values of the P3P IM/IE Ratio and of theFluorescence Lifetime τE of the P3P Excimer in

Solutions of 12-s/2 Surfactants and MicelleMicroviscosities Relative to That of DTAB Micelles at 15,

25, and 45 °Ca-c

15 °C 25 °C 45 °C

surfact-ant

C(wt %)

IM/IE(τE) ηi,rel

IM/IE(τE) ηi,rel

IM/IE(τE) ηi,rel

DTAB 2.23 8.16 1.00 4.42 1.00 1.73 1.00(83.0) (72.6) (58.3)

12-2 2.00 8.89 1.15 4.45 1.03 1.87 1.07(87.9) (74.0) (57.8)

12-3 2.21 8.49 1.11 4.60 1.07 1.88 1.11(88.5) (74.7) (59.7)

12-4 2.22 8.06 1.04 4.38 1.01 1.75 1.01(87.8) (73.8) (58.3)

12-5 2.45 7.60 0.95 4.34 0.97 1.93 1.12(84.4) (71.5) (58.5)

12-6 2.62 7.83 0.97 4.31 0.98 1.85 1.04(83.9) (72.6) (56.7)

12-8 2.70 7.53 0.89 4.50 0.96 1.87 1.03(80.3) (68.1) (55.8)

12-10d 0.58 1.94 (1.24)e 1.50e 1.43 (1.05)f 1.77f(49.8, 52)e

a P3P concentration: 1-4 µM. b ηi,rel ) (τEIM/IE)12-s/2/(τEIM/IE)DTAB. c The values of τE (in ns) are given in parentheses. d Thesolution was milky up to 52 °C. e Values at 55 °C. The values 1.24and 52 ns are for DTAB. f Values at 61 °C. The value 1.05 is thatfor DTAB.

Micelle Micropolarity and Microviscosity Langmuir, Vol. 13, No. 21, 1997 5555

carbon number decreases. Recall that the tendency ofmicelle growth for oligomeric surfactant micelles variesin the same manner with these parameters.8,9,13 At 45°C,where the differences inmicelle size aremuch smallerthan those at 25 °C, the relative microviscosity of the 12-s-12 surfactantmicelles becomesmuch less dependent onthe spacer carbon number. However, the microviscosityremains larger than for DTAB micelles, and the effect ofthe degree of oligomerization is still important.The effect of the surfactant concentration has also been

investigated. The P3P excimer lifetime was found to beindependent of concentration for all the surfactantsinvestigated. ForDTABand12-6-12which formmicelleshavingsizes thatare slightlydependentonconcentration,8the quantity τEIM/IE was found to be independent ofconcentration as shown in Figure 5. A different behavioris seen in Figure 5 for 12-2-12, 12-3-12, the trimer, andthe tetramer,whichare thesurfactants forwhich theeffectof concentration on themicelle size is the largest.8,13 The

results show, at low surfactant concentration, an increasein relative microviscosity which is steeper and occurs atan increasingly lower concentration in the sequencetetramer < trimer < 12-2-12 < 12-3-12.Again, this surfactant sequence is that of increasing

tendency for micelle growth. At high surfactant concen-tration themicroviscosity levelsout. Miyagishi etal.29,38-40

also observed that the microviscosity of different surfac-tantmicelles, evaluatedusingdifferentmethods, increasedwith the surfactant concentration or salt content whenthis increase induced a sphere-to-rod transition ofmicelle

(38) Miyagishi, S.; Asakawa, T.; Nishida,M. J. Colloid Interface Sci.1987, 115, 199.

(39) Miyagishi,S.;Akasohu,W.;Hashimoto,T.;Asakawa,T.J.ColloidInterface Sci. 1996, 184, 527.

(40) Miyagishi, S.; Kurimoto, H.; Asakawa, T. Langmuir 1995, 11,2951.

Figure 2. Variation of the relative microviscosity with thespacer carbon number for the 12-s-12 surfactantmicelles, fromfluorescence anisotropy (O) and fluorescence intensity ratio IM/IE (b), and with the carbon number of the variable alkyl chainfor the 12-s/2 series (0) from fluorescence intensity ratio IM/IE,at 25 °C.

Figure 3. Variation of the relative microviscosity with thenumber of dodecyl chains (degree of oligomerization) forDTAB,12-3-12, 12-3-12-3-12, and 12-3-12-4-12-3-12 at 15 °C (b), 25°C (9), and 45 °C (2), from IM/IE measurements. The symbols(0) represent the results at 25 °Cas obtained from fluorescencepolarization measurements.

Figure 4. Temperature dependence of the microviscosity ofmicelles of 12-2-12 (b, shift + 0.7), 12-3-12 (∆, shift + 0.5),12-4-12 ([, shift + 0.6), 12-5-12 (1), 12-8-12 (0, shift - 0.4);12-10-12 (2); trimer (9, shift + 0.7), and tetramer (], shift +0.5), relative to that of DTAB micelles, from IM/IE data. Someplots have been shifted along the ordinate axis by the amountsindicated, for the sake of clarity.

Figure 5. Concentration dependence of the microviscosity ofmicelles of 12-2-12 (b). 12-3-12 (4), 12-6-12 ([), trimer (9), andtetramer (2), relative to that of DTAB micelles (O), at 25 °C,from IM/IE data. The symbols (0) refer to the microviscosity oftrimer solutions as obtained from the DPH fluorescenceanisotropy.

5556 Langmuir, Vol. 13, No. 21, 1997 Zana et al.

shape. This increaseprobably reflects the tighter packingof thesurfactant chains in thread-likemicelleswithrespectto spherical micelles for a given surfactant.36However, the change of micelle shape appears to be

only partly responsible for thedifference inmicroviscositybetweensurfactantmonomer (DTAB)andoligomers.Thusin Figure 2, the results for 12-6-12, 12-8-12, and 12-10-12refer to micelles which are very close to spherical at theconcentrations used, just like those of DTAB.8 Neverthe-less, theirmicroviscosities areabout two times larger thanthat of DTAB. Similarly, the results in Figure 5 showthat the microviscosities of 12-2-12, 12-3-12, and 12-3-12-3-12micelles decrease as the surfactant concentrationdecreases but remain larger than that of DTAB even atvery low concentrations (0.1-0.2%) where the micellesare close to spherical, on the basis of the value of theiraggregationnumber.8,13 Last, the results inFigure3 showthat the microviscosities of the surfactant oligomermicelles remain larger than that of DTAB micelles evenat 45 °C, where the micelles are close to spherical, asindicated by the viscosity of their solutions which is thenclose to that of water.The increase ofmicroviscositywith the oligomerization

of the surfactant can be readily understood in terms ofthe probe motion involved in the determination of mi-croviscosity. When the probe rotates around its center(DPH) or when its different parts move (P3P) in themicellar environment, the surfactant alkyl chains mustalsomove to allow thismotion. Clearly, the probemotionis easier if each alkyl chain can move independently. Inoligomers with a short spacer, the motion of one chainwill bring about the motion of the other chains of theoligomer, because the various chains are connected andalso tethered at the micelle surface by the head groups towhich they are chemically bonded. This results inmicroviscosities larger than that for the monomer. Thecooperativity in the motion of the various chains of anoligomer, and thus the microviscosity, is expected todecrease as the spacer carbon number increases, as isindeed experimentally observed.The values of the fluorescence lifetime of DPH (τ) and

of the intramolecular excimer of P3P (τE) listed in Tables3-5 depend minimally on s, varying at the most by 10%as s is increased from 2 to 14 in the 12-s-12 series and by20% in going from the monomer to the tetramer. Nev-ertheless, the data show a minimum at around s ) 8 fortheDPH lifetime and an increase of the P3P lifetimewithsup to s)4, beyondwhich τE levels out. Also, τE increaseswith the oligomerization degree from 72.6 ns for DTABto 85.0 ns for the tetramer. These data will not bediscussed here as they were only used to correct themeasured quantities in order to permit comparisons ofthe relative microviscosity values obtained by the twomethods used.Fluorescence Quenching Rate Constants in Mi-

celles of Oligomeric Surfactants and in Hydropho-bic Microdomains in Polysoap Solutions. The mi-croviscosity data can be used to explain why the valuesof the intramicellar fluorescencequenching rate constants(kq) inmicelles of 12-2-12, 12-3-12, and 12-3-12-3-12werefound to be significantly lower than that in DTABmicelles,8,9,13 even after accounting for the difference inNvalues between the different micelles. For instance thevalues of kq for the quenching of Ru(bpy)2(bpy2C17)2+ by9-methylanthracene were found to be 1.7× 107 s-1, 8.5×106 s-1 and 4.65 × 106 s-1 in DTAB, 12-2-12, and 12-3-12-3-12micelles, respectively.41 The ratio between the kqvalues in surfactant oligomerandDTABmicelles is ratherclose to that of themicellemicroviscosities listed inTables3 and 4.

The results in Figure 3 show a nearly linear increaseof the micelle microviscosity of the oligomers relative toDTAB with the degree of oligomerization. On this basisthe microviscosity of the hydrophobic microdomains insolutions of polymeric amphiphiles (polysoaps) is expectedto be very large, in agreement with the conclusions ofseveral qualitative studies.31,42-46 We recently reportedvalues of the microviscosity of microdomains in aqueoussolutions of poly(disodiummaleate-alt-hexadecyl or decylvinyl ether), obtained from DPH fluorescence anisotropymeasurements.47 A direct quantitative comparison be-tweenvaluesofmicroviscosity in cationic surfactants, suchas those investigated in this work, and anionic polysoapscannot be made owing to the difference in the chemicalnature of these two types of compounds. From a qualita-tive viewpoint it remains that the microviscosity of thepoly(sodium maleate-alt-hexadecyl vinyl ether) micro-domains relative to DTABmicelles is 31,47 which is muchlarger than that of the surfactant oligomermicelles. Thisresult is in line with the behavior seen in Figure 3, whichsuggests that the microviscosity will increase further ifthe oligomerization degree is increased above 4 (we arein the process of synthesizing longer surfactant oligomersto check this point). The very high microviscosity of themicrodomains in polysoap solutions explains why in ourearlier studies of these systems the quenching of pyreneby alkylpyridinium ions was nearly completely inef-ficient,42,43 even though these compoundsarevery efficientquenchers in micellar solutions of conventional surfac-tants. The loss in efficiency of pyrene quenching bymolecular oxygen in these systems, resulting in largerpyrene lifetimes than those for aerated solutions ofconventional surfactants, was also explained on thisbasis.47,48

Conclusions

The relative microviscosities of a series of micelles ofquaternary ammonium surfactant dimers (12-s-12) andoligomers (trimer and tetramer) have been obtained frommeasurements of the DPH fluorescence anisotropy andthe intensity ratio of themonomer and excimer emissionsof dipyrenylpropane. The twomethods yielded results ingood agreement. The relative microviscosity in thesurfactant dimer series first decreased then leveled outas the spacer carbon number s was increased. On thecontrary, the relativemicroviscosity of themicelles of the12-s/2 surfactants, which can be considered as themonomers of the 12-s-12 surfactants, showed only littledependence on the variable alkyl chain carbon numbers/2. Most importantly, therelativemicoviscosity increasednearly linearlywith thedegree of oligomerization in goingfromDTABto the surfactant dimer, trimer, and tetramer.This behavior provides an explanation to the very lowquenching efficiency of pyrene by alkylpyridinium ions inthe hydrophobic microdomains formed by polysoaps inaqueous solutions.

LA970369A

(41) The kq values for the 12-2-12 and 12-3-12-3-12 micelles havebeen determined in thework described in ref 13 but not reported. Thesevalues have been multiplied by the ratio between theN values of thesemicelles and ofDTABmicelles, to eliminate the effect ofmicelle size.11,12

(42) Binana-Limbele, W.; Zana, R. Macromolecules 1990, 23, 2731.(43) Cochin,D.;Candau,F.; Zana,R.Macromolecules1993,26, 5755.(44) Chu, D. Y.; Thomas, J. K. Macromolecules 1991, 24, 2212.(45) Yekta, A.; Duhamel, J.; Brochard, P.; Adiwidjada, H.; Winnik,

M. A. Macromolecules 1993, 26, 1829.(46) Aizawa, M.; Komatsu, T.; Nakagawa, T. Bull. Chem. Soc. Jpn.

1977, 50, 3107.(47) Anthony, O.; Zana, R. Langmuir 1996, 12, 3590.(48) Anthony, O.; Zana, R. Langmuir 1996, 12, 1967.

Micelle Micropolarity and Microviscosity Langmuir, Vol. 13, No. 21, 1997 5557