®
`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
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`14
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`REGEV AND ZANA
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`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
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`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 to I 0 wt% surfactant the micellar
`shape and size do not change much (not shown). However, at
`15 wt%, still in the micellar phase, some regions of ordered
`elongated micelles are observed (Fig. 8). It is tempting to
`believe that these regions are precursors of the hexagonal
`phase, appearing only at 35 wtO/o ( II , 12). Indeed, this result is
`in line with Westesen findings from the evolution of peaks in
`SAXS spectra with increasing surfactant concentration ( 12)
`mentioned as "strong interparticle interference between 10 to
`
`35 wt% Tyloxapol." The d spacing calculated in this work
`(Fig. 2 of Ref. 12), 6 nm, is similar to the d spacing measured
`from our cryo-TEM image (Fig. 8).
`Below 0.1 wt% we are unable to observe any form of
`aggregation in this system. most probably due to the low
`concentration of aggregates and/or their low aggregation num(cid:173)
`ber.
`For the sake of comparison and also because the microstruc(cid:173)
`ture of solutions of TX I 00 has not yet been investigated by
`cryo-TEM, we have examined by this technique a 3 wt%
`solution of TXlOO quenched from 25°C. The micrograph in
`Fig. 9 reveals spheroidal micelles with a diameter of about 5
`nm, crowded in the thicker part of the film, close to the walls,
`similarly to Tyloxapol (Fig. 7). On the basis of the value I 06
`of tbe aggregation number of TXlOO (Table 1), the TXIOO
`micelles were expected to be elongated with an axial ratio of
`about 3.5 (56). Also the approximate value of the aggregation
`number of the micelles of a surfactant with an alkyl chain of
`effective carbon number 8.5, as TX I 00, and of volume 0.348
`nm3
`, close to that of a chain with 12 C atoms, such as TX l 00,
`is expected to be about 22 . Thus for TX l 00, as for several other
`ethoxylated nonionic surfactants, the experimental micelle ag(cid:173)
`gregation numbers are always larger than those for the corre(cid:173)
`sponding minimum spherical micelles, and nevertheless the
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`16
`
`REGEV AND ZANA
`
`FIG. 9. Cryo-TEM micrograph of3 wt% TXIOO quenched from 25°C showing spheroidal micelles 5 nm in diameter. Dark objects are frost particles. Bar,
`200 nm.
`
`micelles are observed to be spheroidal by cryo-TEM (38- 40).
`It has been advanced that the ethoxyleneoxide moieties may
`partially mix with the nonpolar groups (57, 58). If such is the
`case, spheroidal micelles, with much larger aggregation num(cid:173)
`bers than the minimum spherical micelle fonned by a surfac(cid:173)
`tant with the same alkyl chain, may be present in the solution
`as was recently discussed (40). The bulkiness of the head
`groups suggests another, possibly complementary, explanation.
`
`Indeed if one assumes that a uniform shell of polyethyleneox(cid:173)
`ide moieties covers the spherocylindrical micellar core of axial
`ratio 3.5 of the TX I 00 micelles, the thickness ofthis shell can
`be calculatd to be 0.332 nm. The axial ratio seen by cryo-TEM
`is that of the full micelles, core plus shell. Taking into account
`the shell, the axial ratio is then recalculated to be 1.65. Such a
`value is not large enough for the micelles to be seen as prolate
`particles.
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`AGGREGATION BEHAVIOR OF TYLOXAPOL IN AQUEOUS SOLUTION
`
`17
`
`CONCLUSIONS
`
`The aggregation behavior of the nonionic surfactant oligo(cid:173)
`mer Tyloxapol has been investigated and compared to that of
`its corresponding monomer, Triton X-100, in water. It bas been
`found that aggregates that can shie ld pyrene from water are
`present in Tyloxapol solutions at a concentration about I 00-
`fold lower than for TX I 00. However, the results did not permit
`us to exclude that Tyloxapol molecules can fom1 intramolec(cid:173)
`ular micelles or wrap around pyrene at a very low concentra(cid:173)
`tion, shielding it from water. The micropolarity provided by
`Tyloxapol micelles is lower than that of TX I 00 micelles at
`least at temperatures below 25°C whereas their microviscosity
`is about four times larger than that of TX I 00 micelles in the
`temperature range investigated. Finally, the micelles ofTylox(cid:173)
`apol have been found to be smaller than those of TXlOO. This
`behavior is opposite to that found for ionic surfactant o lig(cid:173)
`omers with respect to their corresponding monomers. Cryo(cid:173)
`TEM showed spheroidal micelles in 3 wt% solutions of the two
`surfactan ts.
`
`ACKNOWLEDGMENT
`
`The authors thank Mr. A. Ramcau from the lnstitut C. Sadron (Strashourg)
`for detem1ining the chromatogram of Tyloxapol shown in Fig. I.
`
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