®
`Journal ofC.olloici anci Interface Science 2 10, ll- 11 (1999)
`Article lO jcis.l998.5776. available online at hup://www.idealibrary.com on I 0 E ~ l
`
`Aggregation Behavior of Tyloxapol, a Nonionic Surfactant
`Oligomer, in Aqueous Solution
`
`*Departmem of Chemical Engineering, Ben-Gurion University. 84105 Beershem. Israel: and i·fnslitlll C. Sadron (CNRS).
`6 me Boussingault, 67000 S1rasbourg. France
`
`Oren Regev* and Raoul Zanat·1
`
`Received March 26, 1998; accepted July 29, 1998
`
`The aggregation behavior of Tyloxapol, a nonionic surfactant
`oligomer with a repeating unit close to Triton X-100 (TX100), and
`a maximum degree of polymerization of about 7, has been inves(cid:173)
`tigated in aqueous solution by means of fluorescence probing,
`time-resolved fluorescence quenching (TRFQ) and transmission
`electron microscopy at cryogenic temperature (cryo-TEM). The
`plot of the pyrene fluorescence intensity ratio Jt/I3 against the
`Tyloxapol concentration shows no clear evidence of a critical
`micelle concentration contrary to TXlOO. Nevertheless, the fitting
`of these data, assuming a partition of pyrene between Tyloxapol
`aggregates and water, yields erne values in the micromolar range,
`i.e., about a hundred times lower than for the "monomer" TX100.
`The values of 11/13 at high surfactant concentrations indicate that
`Tyloxapol micelles provide pyrene a less polar environment than
`TX100 micelles. The use of the viscosity-sensitive probe 1,3-
`dipyrenylpropane indicates that the microviscosity of Tyloxapol
`micelles is quite high, three to four times larger than that for
`TXlOO micelles, and decreases rapidly with increasing tempera(cid:173)
`ture. Also the microviscosities of both TXlOO and Tyloxapol mi(cid:173)
`celles are larger than those for the micelles of the nonionic ethoxy(cid:173)
`lated surfactant C 12E9 • The aggregation numbers of Tyloxapol
`and of TX100 micelles measured using TRFQ increase with tem(cid:173)
`perature, with the Tyloxapol micelles being smaller than the
`TX100 micelles. Cryo-TEM shows that the Tyloxapol micelles
`remain spheroidal up to a concentration of about 10 wt%. At 15
`wt%, some regions of ordered elongated micelles are also observed
`which may be the precursors of the hexagonal phase known to
`occur at about 35 wt<>..b. c 1999 Academic Press
`Key Words: nonionic surfactant oligomer; Triton X-100; Tylox(cid:173)
`apol; aggregation in aqueous solution; erne; micelle shape; micelle
`microviscosity.
`
`INTRODUCTION
`
`In recent years a new class of surfactants which can be broadly
`referred to as surfactant oligomers has attracted increasing interest
`(1- 5). Surfactant oligomers are made up of two or more am(cid:173)
`phiphilic moieties connected at the level of, or very close to, the
`head groups by a spacer group. The surfactant dimers, also re(cid:173)
`ferred to as gemini surfactants (2), have been the most investi-
`
`1 To whom correspondence should be addressed.
`
`0021 -97<)7/99 $10.00
`Copyright Q 1999 by Academic Press
`All rights of reproduction in any fonn reserved.
`
`8
`
`in surfactant dimcrs and olig(cid:173)
`interest
`gated. The
`omers is due to the fact that they are characterized by much lower
`critical micelle concentration (erne) values and much stronger
`surface tension lowering action than the corresponding conven(cid:173)
`tional (monomeric) surfactants (1-5). These two properties deter(cid:173)
`mine most uses of surfactants in fotmuJations. Surfactant trimers
`and tetramers are characterized by even lower cmc values than
`surfactant dimers (6-8). The work on surfactant dimers and
`oligomers has been recently reviewed (3-5). Thus far, the reported
`studies have essentially concerned cationic and anionic surfactant
`dimers. There has been no report on the properties of nonionic
`surfactant oligomers, probably because of difficulties in their
`synthesis. However, a nonioruc surfactant oligomer, refetTed to as
`T yloxapol, has been synthesized and described (9, 10) and can be
`obtained from chemical manufacturers. Tyloxapol is very close to
`being an oligomer of the much investigated Triton X- 1 00
`(TXIOO; see chemical srrucrures in Scheme 1). The micelle for(cid:173)
`mation by Tyloxapol and the phase diagrams of TyloxapoVwater
`and TyloxapolffX 1 00/water mixtures have been investigated
`(11-13). Tn view of our current interest in surfactant oligomers
`and the availability of Tyloxapol we decided to investigate its
`aggregation behavior in aqueous solution and to compare it to that
`of its monomer, TX I 00. For this purpose we made use of fluo(cid:173)
`rescence probing using the probes pyrene (erne, micelle polarity)
`and I ,3-dipyrenylpropane (micelle microviscosity) ( I 4-16). We
`also used time-resolved fluorescence quenching (micelle aggre(cid:173)
`gation numbers) (17- 20) and transmission electron microscopy at
`cryogenic temperature (direct imaging of the aggregates in the
`surfactant solutions) (2 I). The results reveal important differences
`between the properties of the micelles of Tyloxapol and TX 1 00
`and also between their aggregation behaviors.
`
`EXPERIMENTAL
`
`Materials
`
`The sample of TXIOO (Aldrich Chemicals, Europe) was
`used as received. A sample of Tyloxapol (Sigma) was exten(cid:173)
`sively dialyzed against water using dialysis bags with a cutoff
`of about 1500 Da and the surfactant was recovered by exten(cid:173)
`sive vacuum rotatory evaporation first under 15 mm mercury
`
`

`
`AGGREGATION BEHAVIOR OF TYLOXAPOL IN AQUEOUS SOLUTION
`
`9
`
`OH
`I
`
`~-
`
`CH3- <(- CH3
`yH2
`CH3- y - CH3
`CH3
`
`Triton X- 100
`
`X =9-10
`
`?"
`
`J::_ ¥
`
`c~
`
`CH3- y - CH3
`CH2
`I
`CH3-y-CH3
`CH3
`
`CH3- T- CH3
`CH2
`I
`CH3- y - CH3
`CH3
`
`n
`
`CH3- y - CH3
`CH2
`I
`CH3- T - CH3
`CH3
`
`Tyloxapol
`
`x = 8- 10 n < 6
`
`SCHEME I. Chemical structures of Triton X-1 00 and of Tyloxapol (EO = -CH2CH20-).
`
`then under I mm mercury, at 40°C. The water content of the
`samples of raw and dialyzed Tyloxapol and of TX I 00 were
`measured using Karl- Fisher titration and found to be below I
`wt%. The concentrations given below have been corrected for
`that water content. The raw Tyloxapol was examined by size
`exclusion chromatography (solvent: tetrahydrofuran; four col(cid:173)
`umns filled with beads of polystyrene cross-linked by divinyl(cid:173)
`benzene, of diameter I 0 J.Lffi and of pore sizes 5, 1 0, 50, and
`100 nm; detection: UV absorption or index of refraction). The
`chromatogram is shown in Fig. I. Using the method reported in
`(22), this chromatogram was decomposed in a sum of four
`components corresponding to the monomer, dimer, b·imer, and
`
`E
`
`higher oligomers with percentages in weight of about 4.4, I 0,
`16.9, and 68.7, respectively (Fig. !). The nonionic surfactant
`nonaetbyleneglycol monododecyl ether {C12E9) was purchased
`from Fluka and used as received. The clouding temperatures of
`aqueous solutions of C 12E9, TXJ 00, and Tyloxapol at a con(cid:173)
`centration of3 wt% were found to be 83 ::!:: 0.5, 65.9 ::!:: 0.2, and
`90 ::!:: I °C, respectively.
`The fluorescence probes pyrene (purified by zone melting)
`and 1,3-dipyrenylpropane (DPP, from Molecular Probes, used
`without further purification) were the same as in previous
`studies (7). The quenchers of the pyrene fluorescence used for
`aggregation number detenninations were dimethylbenzophe(cid:173)
`none (DMBP) (23) and tetradecylcyanopyridinium chloride
`(TCNPC) (24).
`Water purifed using a Millipore Milli-RO 3Pius (resistance
`> 18 mil) was used for the preparation of all solutions. The
`surfactant concentrations are expressed in wt% or in moVL on
`the basis of molecular weights of 624 g/mol for TX I 00 and 636
`g/mol for the repeating unit ofTyloxapol. These values assume
`an average of 9.5 ethoxy units per molecule or repeating unit
`{1 1- 13).
`
`Methods
`
`Spectrojluorometry. The pyrene emission spectra were re(cid:173)
`corded using a Hitachi F40 I 0 spectrofluorometer and used to
`obtain the ratio / rf/ 3 of the intensities of the first and third
`vibronic peaks in the emission spectra of pyrene solubilized in
`the micellar solutions. The excitation wavelength was set at
`335 nm. For conventional surfactants the variation of / t//3
`with the surfactant concentration C pennits one to obtain the
`erne. In addition, the value of I rf13 at C ;;::.. erne gives a
`measure of the polarity sensed by pyrene at its mjcelle solu(cid:173)
`bilization site ( 14, 15).
`The emission spectra of DPP were recorded using the same
`spectrofluorometer in the range from 350 and 550 nm, using an
`excitation wavelength of 346 nm. These spectra showed an
`
`30
`
`20
`
`22
`
`28
`26
`24
`Elution volume (ml)
`FIG. l. Chromatogram of a 3 wt% solution of raw Tyloxapol in tetrahy(cid:173)
`drofuran. The signal (ordinate scale) is proportional to the refractive index of
`the eluted solution. The chromatogram has been analyzed according to the
`method in Ref. (22). Curve E is the experimental chromatogram. Curves I, 2,
`3, and 4 correspond to the monomer, dimer, trimer, and higher oligomer,
`respectively. The broken line curve is the sum of curves I to 4.
`
`

`
`10
`
`REGEV AND ZANA
`
`...
`5-
`
`2.0
`
`1.9
`
`1.8
`
`1.7
`
`1.6
`
`1.5
`
`1.4
`
`1.3
`
`1.2
`
`emission due to the monomeric form of DPP in the range
`360-400 nm with four vibronic peaks (/M, intensity of the first
`peak located near 378 nm) and a broad emission maximum
`arising from the excimer fonn of DPP, centered at 485-490 nm
`(intensity I r;_). The product of the intensity ratio I M/ I E by
`fluorescence lifetime ,.E of the DPP excimer is proportional to
`the microviscosity of the surrounding medium (25, 26). The
`intensities I M and 1 E were measured using air-saturated solu(cid:173)
`tions. The lifetimes in the same solutions were measured using
`a single photon counting apparat1ts (23, 24) at an excitation
`wavelength of 335 urn and monitoring the emission using a
`high-pass Kodak gelatin filter 4 (A. > 460 nm).
`Time-resolved fluorescence quenching. We attempted to
`measure the aggregation numbers of Tyloxapol and TX J 00
`micelles in aqueous solution using the time-resolved fluores(cid:173)
`cence quenching method (TRFQ) ( 15, 17- 20) with pyrene as
`the fluorescent probe and DMBP or TC PC as quenchers. The
`fluorescence decay curves were recorded using the same single
`photon counting apparatus which included a Type 7450 EGG(cid:173)
`Ortec multichanne l analyzer witb a time base of 1024 channels
`(23, 24). As usual, the pyrene concentration was adjusted to be
`of about I to 2% of the micelle molar concentration, [M),
`whereas the quencher molar concentration, [Q], was adjusted
`to be close to [M] (i.e., an average of one quencher per
`micelle). Prior to each experiment, the solutions investigated
`were deaerated by three successive freeze-pump-thaw cycles.
`The following fluorescence decay equations ( 17- 20) were fit(cid:173)
`ted to the experimental fluorescence decay curves of pyrene in
`the absence and in the presence of the quencher, respectively,
`using a weighted least-squares procedure (23, 24):
`
`1(1) = l(O) exp( -tl -r)
`l (t ) = /(O)exp{-A 2t - A3[ 1 - cxp( l - A4t)]}.
`
`[l]
`
`[2]
`
`In these equations /(0) and 1(1) are the fluorescence intensities
`at time 0 and t, ,. is the pyrene excited state lifetime in the
`absence of the quencher, and A 2 , A 3 , and A 4 are three fitting
`parameters. The micelle aggregation number, N, the rate con(cid:173)
`stant for intramicellar quenching, k0 , and the rate constant for
`probe and/or quencher exchange between micelles, ke> may
`then be determined from
`
`N = A3{(C - cmc)/[Q]}[l + E)l
`k0 =A i( l+E]
`k. = A4 - kQ = A4el(l + e),
`
`[3]
`
`[4]
`
`(5]
`
`where C is the surfactant concentration in mole per dm3 (15,
`17-20), with
`
`[6]
`
`10 ...
`
`104
`
`10"2
`c (%wt)
`FIG. 2. Variations of f / / 3 with the surfactant concentration at 10°C (0 ,
`e ), 25°C (D, • ), and 40°C (6, .A) for Tyloxapol (0 , 6, D ) and TXlOO (e , .A,
`•>· The solid lines going lhrough the data points are only guides to the eye.
`
`10"1
`
`10°
`
`The overa ll error on the aggregation number is estimated to
`be of:±: 10% .
`Cly o-transmission electron microscopy (c1y o-TEM). The
`specimens are prepared by blotting a 5-p.l drop of the sample
`on a lacey carbon substrate (300 mesh, Ted Pella Inc.) in a
`controlled environment vitrification system (CEYS) (27),
`where temperature is controlled by a bulb and relative humidity
`is kept above 95% by a wet sponge to prevent sample evapo(cid:173)
`ration. The specimen is blotted by a filter paper, resulting in the
`fonnation of a very Lhi11 sample fil m suspended o ver the grid,
`which is then vitrified very quickly in liquid ethane maintained
`at the liquid nitrogen temperature, 83 K ( - 190.15°C). The
`CEVS operation retains the original composition of the sample
`so that the original microstructures remain unaltered in the
`vitrified specimen. The vitrified specimen is then transferred by
`means of a Gatan cryoholder into a JEOL 1200EXll electron
`microscope operating at I 00 kV in the conventional TEM
`mode with nominal under focus of about 4 J..tm. The images are
`recorded at 83 K on S0-163 film. The low-dose mode of the
`electron beam is used to reduce radiation damage by the
`electron beam.
`
`RESULTS AND DISCUSSION
`
`The cmc and Micelle Micropolarity
`
`Figure 2 shows the variations of the 1/ 13 ratio with the surfac(cid:173)
`tant concentration, C, for Tyloxapol and TX I 00 at l 0, 25, and
`40°C. The Jt/13 vs C plots for the raw and dialyzed samples of
`Tyloxapol were determined at 25°C and found to be coincident.
`TI1is is not surprising in view of the very low fraction of monomer
`(4%) contained in tbe raw sample and wlticb may be eliminated
`by dialysis. All of the measurements below refer to the raw
`sample. In all instances the intensity ratio shows a sigmoidal
`
`

`
`AGGREGATION BEHAVIOR OF TYLOXAPOL IN AQUEOUS SOLUTION
`
`11
`
`decrease upon increasing C, revealing that pyrene goes from a
`hydrophilic to a hydrophobic environment. The latter is provided
`by surfactant aggregates when present in the solution with a
`sufficient volume fraction to solubilize pyrene. The two sets of
`results show important differences.
`First, for Tyloxapol, the concentration range where most of
`the decrease of I / /3 occurs is about 1 0-fold lower than that for
`TXlOO, showing that this surfactant oligomer provides pyrcne
`with hydrophobic microdomains at a much lower concentration
`than its monomer. The erne range of TX 100 is seen to be
`around 0.01 wt%, i.e., 0.15 mM. The cmc of TXlOO can be
`obtained using two approaches.
`
`I. It is taken as the concentration corresponding to the
`intercept of the extrapolations of the high concentration part
`and of the rapidly varying part of the / 1/13 vs C plot (28). The
`erne values are then found to be 0.012, 0.024, and 0.024 wt%
`which is about 0.19, 0.38, and 0.38 mM at 10, 25, and 40°C,
`respectively.
`2. For several ionic and non ionic surfactants with the erne in
`the 0. 1 mM range the comparison of the electrical conductance
`or surface tension vs C plots and of the I 1 II 3 ratio vs C plots
`showed that the erne, obtained for the conductance or surface
`tension plots (true erne), closely corresponds to the concentra(cid:173)
`tion at the inflection point of the / .f/ 3 ratio vs C plot (29- 31 ).
`This concentration is also very close to that at mid-decrease of
`the / 1 I I3 ratio. Using this procedure, the TX 100 erne values
`were found to be 0.05, 0.10, and 0.12 wt% (that is, 0.08, 0.16,
`and 0.19 mM) at I 0, 25, and 40°C, respectively.
`
`The cmc ofTX 100 has been repo•ted to be between 0.25 and
`0.27 mM at 25°C (32-34), a value between the ones given
`above at this temperature.
`The second difference between Tyloxapol and TX l 00 is that
`the decrease of 1/ / 3 is much less steep for the fonner, stretch(cid:173)
`ing over nearly two decades of concentration for Tyloxapol
`against about one decade for TXlOO. In addition, there is no
`really sharp change at the end of the decrease for Tyloxapol,
`contrary to TXlOO. This complicates the determination of the
`"cmc" for Tyloxapol. Indeed, pyrene reports a partly hydro(cid:173)
`phobic environment already at a concentration of, say, 3 X
`I 0- 4 wt% (i.e., 5 X 10- 6 M). However, the erne, if it exists,
`may be located at an even lower concentration. Indeed at such
`a low amphiphile concentration the volume of the hydrophobic
`pseudo-phase is very small and a significant fraction of pyrene
`remains in the water, thereby reporting a partly aqueous envi(cid:173)
`ronment even at concentrations above the em c. The I 1 II 3 plots
`for Tyloxapol look vc1y much like those characterizing sys(cid:173)
`tems with very low or zero erne, where pyrene is progressively
`partitioned into hydrophobic aggregates, as the concentration
`of the amphiphile is increased (35). In such systems, the
`experimental fluorescence intensity ratio can be written in the
`form (35)
`
`10·1
`
`100
`
`10 ..
`
`10-4
`
`10-'
`10-2
`c (%wt)
`FIG. 3. Variations of the nonnaiized fluorescence intensity ratio R with
`the surfactant concentration for Tyloxapol at 10°C (0 ), 25°C (0 ), and 40°C
`(L\.). The curves in solid lines I , 2, and 3 have been calculated using Eq. [8]
`with the values K = 1434, 934, and 480 (wt%)- 1
`, and erne = 1. 1 X 10- 4
`,
`1.4 X 10- 4
`, and 0.7 X 10- 4 wt%, at 10, 25 and 40°C, respectively.
`
`where (111/ 3)w and (/1/J3 )Ag are the values of the intensity
`ratio in water and in the aggregate (that is, at C ~ cmc), and
`K is an apparent binding constant of pyrene to the aggregates
`(K/55.5 is in fact the partition coefficient of pyrene bet\veen
`the hydrophobic domains considered as a pseudo-phase and
`water). The data in Fig. 2 have been normalized and replotted
`in Fig. 3 as R against C using a semi-logarithmic scale for the
`concentration, with R given by
`
`R = [11/J3 -
`
`(/I/13)Ag]
`(/.f/3)AJ I[(I.I/J)w -
`= 1/[1 + K(C- cmc)].
`
`[8]
`
`The quantity R varies between I and 0 upon increasing C. It
`represents the fraction of pyrene solubilized in the aggregates.
`Its variation with log C is an S-shaped curve and R = 0.5 for
`erne = I I K. The R vs log C plots permit an easier
`C -
`comparison between surfactants of differing (/1//3 )Ag and also
`an easy visualization of the quality of the fit of Eq. [8] to the
`data. The plots l-3 have been calculated using Eq. [8] with
`appropriate values of the binding constant and of the erne given
`in the legend to Fig. 3. These plots are seen to provide a good
`fit for the Tyloxapol results at 25 and 40°C. The fit is somewhat
`less satisfactory at I 0°C. The differences which may exist
`between the calculated curves and the data have been discussed
`and attributed to the large change of pyrenc concentration in
`the aggregates at concentrations close to the cmc (35). The erne
`values thus obtained are very inaccurate, around I o-4 wt%,
`which is 1.6 p,M, at the three temperatures investigated. This
`value is very low, much lower than that for TX I 00. However,
`our results do not permit one to conclude whether the aggre(cid:173)
`gates capable of binding pyrene in Tyloxapol solutions result
`from an aggregation of Tyloxapol around the pyrene mole-
`
`

`
`12
`
`REGEV AND ZANA
`
`-............ _
`--
`
`.....................
`
`10
`
`_ w
`
`8 -
`-:IE
`... -
`- w
`.:
`
`7
`
`6
`
`5
`
`...
`lol
`c
`-;
`
`16.4
`
`16.0
`
`15.6
`
`15.2
`
`cules, or reflect the solubilization or binding ofpyrene to single
`Tyloxapol molecules which would be able to form intramolec(cid:173)
`ular micelles, or arise from a true intermolecular association of
`Tyloxapol molecules as for nonnal amphiphiles at above the
`erne. ln the latter case our erne value would be much smaller
`than that advanced by Westesen and Koch (12). ote that ionic
`surfactant oligomers have consistently been found to have
`much lower erne values than the corresponding monomers. A
`similar behavior is expected for Tyloxapol with respect to
`TX100.
`The third difference between TXlOO and Tyloxapol is that at
`C ~ erne, as the temperature is decreased from 40 to 1 0°C, the
`value of / 1/ / 3 increases from about 1.32 to 1.44 for TXl 00, but
`remains nearly constant at about 1.32 for Tyloxapol. At 25°C
`the values of I/ 13 are 1.32 and 1.4 for Tyloxapol and TX I 00,
`respectively, indicating that the micelles ofTyloxapol provide
`pyrene with an environment of lower polarity than TX I 00
`micelles (14, 15). However, at 40°C, Tyloxapol and TX I 00
`mjcelles are characterized by about the same value of / 1/ / 3 •
`
`Micro viscosity
`
`The values of the DPP excimer lifetime and of the ratio
`/ M/ I E were determined for Tyloxapol and TXlOO solutions at
`different concentrations and temperatures. We did not attempt
`to calculate absolute microviscosities from tbe results. Indeed
`this involves the use of a calibration plot (plot of TE/M/ / E vs
`viscosity for a series of liquids of known viscosity) and the
`microviscosities thus obtained depend on the caljbration curve
`used (36). Since we were essentially interested in comparing
`the micellar microviscosities of Tyloxapol and of its monomer
`TX I 00, all the results below concern the values of the product
`TE/M/ J E which is referred to as microviscosity even though it is
`realized that it is only proportional to the microviscosity. At the
`outset it should be stated that the measurements of I M/ 1 E for
`Tyloxapol solutions at temperatures below 40°C were difficult
`
`1600 K_
`1200 ~ ~ ~
`
`0-_
`
`~
`- w
`...
`
`800
`
`400
`
`0
`
`10
`
`20
`c (%wt)
`FIG. 4. Variations of the micelle microviscosity with the surfactant con(cid:173)
`centration for Tyloxapol (0 ) and TXIOO (e ) at 45°C.
`
`30
`
`3.1
`
`3.2
`
`3.4
`
`4
`3.5
`
`3.3
`103fT
`FIG. 5. Variations of the micelle microviscosity (solid lines) for Tylox(cid:173)
`apol (0 ), TX 100 (0 ), and C,2 E9 (~), and corresponding variations of the
`lifetime of the DPP excimer (broken lines) for Tyloxapol (• ). TX 100 (e ), and
`cl2~ (.A.), with temperature.
`
`because of the extremely low values of the excimer intensity
`which reflect extremely large values of the microviscosity. The
`TE/ M/ I E values for Tyloxapol solutions below 40°C must be
`considered lower bound values.
`Figures 4 and 5 show the variations of TEI M/JE with con(cid:173)
`centration and temperature (1), respectively, for Tyloxapol
`and TX 100. In all instances the microviscosity of Tyloxapol
`aggregates is larger than that of TXI 00. A similar difference
`has been found between the microviscosity values of ionic
`surfactant oligomers and their conesponding monomers (7).
`Figure 4 shows that the microviscosity is nearly independent of
`the TX I 00 concentration but decreases somewhat as the Tylox(cid:173)
`apol concentration is increased. Figure 5 shows that the mi(cid:173)
`croviscosity of both Tyloxapol and TX I 00 aggregates de(cid:173)
`creases steeply as the temperature increases. In the semi(cid:173)
`logarithmic representation adopted in Fig. 5 (In( T El M/ 1 E)
`against I / 1) the plots are linear; that is, one can write
`
`[9]
`
`The slopes of the plots in Fig. 4 yield the values 61.2 and 62.2
`kJ/ mol for the activation energy E~ of the microviscosity for
`TX I 00 and Tyloxapol micelles, respectively. Also represented
`in Fig. 5 are the semi-logarithmic variations of the lifetime 'TE
`of the DPP excimer with I I T. The plots are linear and yield the
`values 16.8 and 11 .8 kJ/mol for the activation energy E:E of TE
`for TX I 00 and Tyloxapol.
`For the sake of comparison we have also represented in Fig.
`5 the variations of the values of TE and of the micellar micro(cid:173)
`viscosity for the nonionic ethoxylated surfactant C 12 E9 . It is
`seen that the microviscosity of C 12E9 micelles is lower than
`that for TX I 00 and Tyloxapol micelles in the entire range of
`temperatures investigated. The activation energies for TE and
`
`

`
`AGGREGATION BEHAVIOR OF TYLOXAPOL IN AQUEOUS SOLUTION
`
`13
`
`TABLE 1
`R esults of TRFQ Studies of Tyloxapol and TXIOO Solutions
`
`Surfactant
`
`C {wt%)
`
`T (•C)
`
`Quencher
`
`-. (nst
`
`A21 (ns)
`
`AJ
`
`10- 6 A 4 (ns)
`
`Jllr
`
`10- 6 kQ (s- 1)
`
`10- 5 k. (s- 1}
`
`TXIOO
`TXIOO
`TXIOO
`Tyloxapol
`Tyloxapol
`Tyloxapol
`Tyloxapol"
`C 12E9
`C 12E9
`
`3.436
`3.436
`3.436
`3.418
`3.456
`3.418
`3.35
`3.2
`3.2
`
`25
`25
`40
`25
`40
`40
`55
`25
`40
`
`0.463 mM TCNPC
`0.528 mM DMBP
`0.528 mM DMBP
`0.570 M TPCNC
`0.590 mM DMBP
`0.570 mM TPCNC
`1.236 mM TPCNC
`No quencher
`No qucnchcr
`
`348.2 (241.1]
`348.2 (241.1)
`329.2 (19l.Oh]
`349.5 [281.0]
`328.1 (225.4b]
`328.1 (225.4b]
`[192.7]
`345.6 (181.4)
`321.3 [ 135.8]
`
`317.1
`310.2
`268.4
`
`285.6
`292.1
`152.8
`
`0.750
`0.850
`1.176
`
`0.427
`0.420
`1.68
`
`4.18
`4.78
`4.57
`
`3.88
`4.20
`4.22
`
`106
`104
`155
`
`63.8
`58.3
`99
`
`3.95
`4.40
`4.07
`
`3.04
`3.46
`3.52
`
`2.3
`3.8
`5.0
`
`8.4
`7.4
`7.2
`
`a Values in brackets are for aerated solutions. The other values are for deaerated solutions.
`b Values at 41•c.
`• Values representing the number of surfactant hydrophobic moieties per micelle.
`" This measurement has been perfonned on an air-saturated solution.
`
`for the microviscosity for C 12E9 are 13.7 and 49.8 kJ/mol,
`respectively.
`Note that at a given temperature, the '~'F values increase in
`the order C12E9 < TXIOO < Tyloxapol, that is, in the order of
`increasing microviscosity, in the temperature range investi(cid:173)
`gated. The values of the pyrene fluorescence lifetimes in aer(cid:173)
`ated micellar solutions of the three surfactants listed in Table 1
`also increase in the order C 12E9 < TXlOO < Tyloxapol.
`However, Table I shows that identical values are found for the
`pyrene lifetime in deaerated micellar solutions, i.e., in the
`absence of solubilized oxygen, of the three surfactants . This
`fact confirms that the differences in DPP excimer and pyrene
`lifetimes found for the aerated solutions of the three surfactants
`are due to a loss of efficiency in the quenching of the probe
`fluorescence by molecular oxygen, which increases with the
`micelle microviscosity, as previously discussed (7).
`The larger microv iscosity ofTXl OO micelles with respect to
`micelles of ethoxylated nonionic surfactants CmEn (E =
`ethoxy group) explains why abnonnally low values of the
`micelle aggregation number were measured in a previous time(cid:173)
`resolved fluorescence quenching study of TX I 00 solutions
`(37). This study used the fonnation of excimer by pyrene ( 15).
`A later study (38) showed that the rate constant for intrami(cid:173)
`cellar pyrene excimer fonnation is generally lower than the rate
`constant for pyrene quenching by alkylpyridinjum ions. Thjs
`fact together with the high microviscosity ofTXIOO resulted in
`decay plots where the linear part was not well developed. The
`fitting of Eq. [2] to the decay data yielded incorrect values of
`the fitting parameters and suggested a very rapid intermicellar
`exchange of pyrene. In turn, the analysis of the fitting param(cid:173)
`eters by means of Eqs. [2]- [5] yielded N values much lower
`than those obtained when using light scattering, for instance,
`particularly at low temperatures where the microviscosity of
`TXl 00 micelles is high.
`The microviscosity ofthe mixtures ofTX IOO and Tyloxapol
`and the DPP excimer lifetime in these mixtures have been
`
`measured at 45°C. The results are represented in Fig. 6 against
`the Tyloxapol weight fraction. The continuous increases of '~'E
`and T FJM//F between pure TX 100 and Ty loxapol indicates the
`fonnation of mixed aggregates. This conclusion is supported
`by the results of Westesen et a/. ( 13).
`
`Aggregation Numbers
`
`As discussed in the preceding paragraph the high microvis(cid:173)
`cosity ofTXlOO and Tyloxapol micelles makes it necessary to
`use very efficient quenchers of pyrene fluorescence in order to
`make rei iable estimates of the micelle aggregation numbers of
`these two surfactants. ln a first attempt we used the tetrade(cid:173)
`cylpyridinium ion as quencher. W ith both TX I 00 and Tylox(cid:173)
`apol the decay plots were nearly linear and did not permit any
`meaningful determination of aggregation numbers. We there-
`
`1600
`
`1400
`
`1200
`
`-
`_,.
`w 1000
`::::::
`""W 800
`"'
`
`600
`
`400
`
`140
`
`-
`.,
`.:..
`100 w
`"'
`
`80
`
`200
`1.0
`0.8
`0.4
`0.6
`0.2
`0.0
`Tyloxapol
`weight fraction
`TX100
`FIG. 6. Variations of the micelle microviscosity (e ) and of the DPP
`excimer lifetime (0) for the TX I 00- Tyloxapolmixrures at 45°C, as a function
`of the Tyloxapol weight fraction in the mixture. Total surfactant concentration:
`about 5 wt%.
`
`

`
`14
`
`REGEV AND ZANA
`
`FIG. 7. Cryo-TEM micrograph of a 0.1 wt"lo Tyloxapol sample quenched from 25°C showing spheroidal micelles 5 nm in diameter. Dark objects arc frost
`panicles. Bar, 200 nm.
`
`fore turned to the dimethylbenzophenone (23) and the tetrade(cid:173)
`cylcyanopyridinium ion (24) which are very efficient quench(cid:173)
`ers of pyrene fluorescence. The results are surrunarized in
`Table I. The surfactant concentration used was fairly high in
`order to ensure that the probe was nearly completely parti(cid:173)
`tioned in the micelles.
`The N values found for TX I 00 with the two quenchers at
`25°C are identical within the experimental error. Also N in(cid:173)
`creases much with temperature between 25 and 40°C, as for the
`nonionic surfactants C111E, (37-40). Our N values are in very
`good aggreement with those reported by Brown eta/. ( 41 ), who
`used TRFQ with the pair pyrene-benzophenone. They also
`agree with the value reported in a recent dynamic light scat(cid:173)
`tering study (42).
`At 25°C the decay plots for Tyloxapol with the DMBP and
`TCNPC quenchers were close to linear and did not permit
`reliable estimates of aggregation number values. Recall that at
`this temperature the microviscosity of Tyloxapol micelles is
`still quite high and much larger than that of TX I 00. At 40 and
`55°C, however, the decays were sufficiently curved to permit
`reliable estimates of N. The N values thus obtained (see Table
`I) show that N increases with temprature as for TX I 00. How(cid:173)
`ever, tbe values for Tyloxapol are lower tban those for TXlOO,
`indicating smaller micelles. This result is at variance with that
`found for ionic surfactant oligomers for which the micelle
`
`aggregation number, expressed as the number of alkyl chains
`per micelle, increased in going from the monomer to the dimer
`and trimer at a constant weight percent of surfactant (6, 43).
`However, Tyloxapol is an oligomer of larger oligomerization
`degree (up to seven) than those used so far. Also, as shown by
`the chromatogram in Fig. l, the sample used is polydisperse.
`Mixed micellization most likely takes place between the vari(cid:173)
`ous oligomers and may result in spherical micelles. Recall that
`the mixture of dodecyltrimethylammonium bromide (DT AB)
`which forms spherical micelles and of its dimer (denoted
`12- 2- 12) which forms giant thread-like micelles yields sphe(cid:173)
`roidal micelles at a relatively low mole fraction ofDTAB (44).
`The diameter of the hydrated Tyloxapol micelles has been
`calculated to be 6.3 nm, using an average value of 60 units for
`the aggregation number at 40°C, assuming a density of I for
`the aggregates (12), and an average hydration number of four
`water molecules per ethoxy group (45- 49). A value of 7 nm
`was previously obtained ( 12) on the basis of distribution func(cid:173)
`tion analysis (50) of small angle X-ray scattering data for
`Tyloxapol micelles at a concentration of 3 wt%.
`It is interesting to note that the values of the quenching rate
`constant, k0 , for the Tyloxapol and TX I 00 micelles are fairly
`close at 40°C (see Table 1). This is so because k0 is inversely
`proportional to the product of the microviscosity by the aggre(cid:173)
`gation number (51-53). In the present case the higher micellar
`
`

`
`AGGREGATION BEHAVIOR OF TYLOXAPOL IN AQUEOUS SOLUTION
`
`15
`
`FIG. 8. Cryo-TEM micrograph of a 15 w1% Tyloxapol sample quenched from 25°C. Dark objecls are frost particles. Bar, 200 nm.
`
`micruviscosity of Tyloxapol with respect to TX I 00 is largely
`compensated by the smaller micelle aggregation number of
`Tyloxapol with respect to TXIOO.
`The values of the exchange rate constant are fairly low for
`both surfactants at 40°C. Exchange probably takes place
`through micelle collisions witb temporary merging, as in mi(cid:173)
`cellar solutions of C111E11 surfactants (37-40).
`
`Microstructure of the Solutions (Cryo-TEM)
`
`Cryo-TEM images ofTyloxapol at 0.1 wt% show spheroidal
`mjcelles 5 nm in diameter (Fig. 7). The diameter obtained
`confinns the preceding TRFQ results and also the small angle
`X-ray scattering (SAXS) results of Westesen and Koch (12).
`ote that in the cryo-TEM micrograph the micelles crowd in
`the thicker part of the vitrified film, close to the polymer
`support (21, 54, 55). Up