PULSE RADIOLYTIC INVESTIGATION OF

PERFLUORINATED SURFACTANTS IN AQUEOUS

SOLUTIONS

EWA SZAJDZINSKA-PIETEK and JERZY L. GEBICKI Institute of Applied Radiation Chemistry, Technical University of Lodz, Wroblewskiego 15, 93-590 Lodz, POLAND

Received 7 April 2000; accepted 19 June 2000

Abstract--Pulse radiolysis studies were performed for aqueous solutions of ammonium

perfluorooctanoate, tetraethylammonium perfluorooctyl sulfonate, and an ammonium salt of a

carboxylic perfluoropolyether derivative. The protiated system of sodium octanoate was also examined for comparison. The rate constants of hydrated electron (eaq) and 'OH radical reactions with these

amphiphiles, and absorption spectra of the intermediate products are reported for the first time. It is demonstrated that the critical micellar concentrations (CMC) can be determined from the kinetic data. The observed differences in e-aq reactivity towards fluorinated amphiphiles above CMC are rationalized in terms of different charge density at the micellar interface.

INTRODUCTION

For several years our research group has focused on the study of aqueous surfactant

micelles [1-6] and reversed micelles [7-10] by means of pulse radiolysis with optical detection of transient species. Our work as well as that from other laboratories

indicate that this technique allows on the one hand, to follow the effects of molecular

organization on the early events (nano- to millisecond time regime) involving radiation produced hydrated electrons, radicals and radical-ions, and, on the other

hand to obtain information on the structure and dynamics of the micellar aggregates and on the distribution and microenvironment of the incorporated solutes; cf. the

recent review [ 1 1 ]. In the present work we extend the pulse radiolytic investigations on aqueous

solutions of perfluorinated surfactants, i.e. surfactants in which all hydrogens in the

hydrophobic alkyl chain are replaced by fluorines. An increasing interest is observed

recently in this class of amphiphiles, as they have numerous applications in industrial

and biomedical fields [12-14]. We have chosen ammonium perfluorooctanoate

(APFO) as a model compound and examined its reactivity towards hydrated electrons

(e'aq) and hydroxyl radicals produced by electron irradiation of the aqueous solutions

below and above the critical micellar concentration, CMC. To evaluate the effect of

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F/H substitution in the alkyl chain, a similar study has been performed for the

protiated derivative, sodium octanoate (SO). Scavenging of e-aq has also been

investigated in micellar solutions of tetraethylammonium perfluorooctyl sulfonate

(TEAPFOS) and ammonium salt of carboxylic perfluoropolyether (PFPE) derivative, to see how the reaction rate depends on the kind of the surfactant and/or structural

differences between the respective aggregates. To the best of our knowledge the pulse radiolytic study of a perfluorinated

surfactant has been only reported by Auodia et al. [15]. The surfactant used in this

work was a sulfonate with a relatively short hydrophobic chain containing a benzene

ring, the moiety highly reactive towards e-aq. Based on the kinetic data the authors

deduced the concentration of the fluorosurfactant monomers in the presence of the

hydrocarbon surfactant, sodium dodecyl sulfate, and established the micellar

pseudophase diagram for such mixed system.

EXPERIMENTAL

APFO (purum >98%), SO (puriss >99%), and TEAPFOS (purum >98%) from Fluka

were used as received. The PFPE surfactant (trade name: Galden MF 300 ammonium

salt) was a gift from Ausimont CRS, Colloid Lab., Bollate, Italy. This material was

a mixture of branched carboxylic perfluoropolyethers with a distribution of molecular

weights as shown in Figure I (A) (by gas chromatography), and the average molecular

weight MW = 550. Its critical micellar concentration can be estimated as CMC ? 1

g/1 at 25 °C (by surface tension), cf. Figure 1 (B).

Aqueous surfactant solutions were prepared using Millipore deionized water.

For experiments where hydrated electrons were followed water was adjusted to pH 11 with sodium hydroxide (analytical grade), tert-butanol (t-BuOH, spectroscopic

grade) in concentration of 0.1 M was usually added as a scavenger of 'OH radicals,

and the solutions were deaerated by careful bubbling with argon (at least 20 min.). For

experiments where 'OH radicals were followed the solutions (pH z 7) were saturated

with N20 to transform hydrated electrons into equivalent amount of 'OH radicals.

Sodium thiocyanate (analytical grade) was used as a competitive scavenger of 'OH

radicals, to determine the rate constant of the reaction of these species with the

surfactants.

The linear electron accelerator, ELU-6E (6 MeV electron beam), and the

equipment for pulse radiolysis with the optical detection have been described [16].

Electron pulses of 7 ns and 17 ns delivered doses in the range 15 - 20 Gy and 40 -

50 Gy, respectively; the longer pulses were used in experiments aimed to detect

transient absorption spectra. 0.01 M NaSCN solution was used for the dosimetry. The

measurements were performed at room temperature (ca 24 °C).

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Figure 1. Physico-chemical properties of PFPE surfactant used in this work. A - Distribution of molecular weights obtained by gas chromatography; B - CMC (indicated by the arrow) determination

by surface tension measurements at 25 °C. The data supplied by A. Chittofrati (Ausimont CRS, Colloid

Lab., Bollate, Italy).

RESULTS

Transient Absorption Spectra For argon saturated surfactant solutions (concentrations 5 -10 mM) the spectra recorded

immediately after the ionizing pulse reveal the intense absorption band typical for e-aq,

900

with the maximum at 720 nm. The decay of e-aq in SO, APFO and TEAPFOS

systems is accompanied by the appearance of new transient absorption maxima, as

shown in Figure 2(A, B and D). In SO solutions there are two distinct peaks, at

Àmax=310 nm and Àmax = 430 nm. In APFO solutions a broad absorption band of

relatively low intensity is clearly seen with the maximum around 330 nm, but no

maximum is detected at longer wavelengths. In TEAPFOS solutions the transient

absorption maximum appears at Àmax = 345 nm. The spectra recorded for PFPE

surfactant solutions, however, are almost identical as those for water in the absence of

the surfactant, cf. Figure 2(C); only an increasing absorption below 400 nm is observed

which is mainly due to the radicals formed in 'OH reaction with tert-BuOH. We note

that the "blue" edge of the spectrum of the blank sample is not significantly affected by the presence of APFO, but for TEAPFOS solutions a significant absorption increase is

clearly seen in this region; the absorbance recorded for TEAPFOS at 260 nm is about

2.5 times larger than that in water at the same time after the irradiation pulse.

Figure 3 presents the spectra of SO and APFO solutions saturated with N,O

recorded just after the irradiation pulse. In the APFO solution, similarly as in neat water

the absorption is observed below 400 nm, increasing towards blue, which can be

ascribed to 'ooh radicals. The spectrum of the SO solution is qualitatively similar, but

more intense in the low wavelength region. In the time scale of microseconds the

absorption decay only has been observed with no maxima built up.

Kinetic Behavior- of e-u'J and 'OH Species The decay of e-aq was followed at 600 nm for the surfactant solutions of different

concentrations, below and above CMC. The decay curves, absorbance vs time, were

approximated by the pseudo-first order kinetic equation. As shown in Figure 4, for the SO solution k' increases linearly in a wide range

of surfactant concentrations. The corresponding second order rate constant of e-aq reaction is 3.3'106 M-ls-', and it does not change in the vicinity of the critical micelle

concentration, CMC = 0.4 M [17]. Within the uncertainty <10% the kinetics of

decay parallels that of the growth of the product observed at Àmax = 310 nm.

The results obtained for APFO, in the presence and in the absence of t-BuOH,

are presented in Figure 5. For samples containing no t-BuOH the kinetic fits were

somewhat poorer and the respective values of the pseudo-first order rate constants, k', were higher, as part of hydrated electrons was scavenged by 'OH radicals present in the

system. However, the dependencies of k' on the surfactant concentration are parallel for

the two series of experiments. Each plot of Figure 5 consists of two linear sections, and

the concentration corresponding to the break point can be identified with the CMC (see Discussion section). The CMC values obtained from our data are 0.027 M and 0.032

M APFO in the presence and in the absence of t-BuOH, respectively. The difference

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Figure 2. Transient absorption spectra recorded for argon saturated surfactant solutions and for neat

water, containing t-BuOH, after complete decay of hydrated electrons: A - 5 mM SO (below CMC); B - 5mM APFO (below CMC); C - 10 mM PFPE (o, above CMC) and neat water D - 10 mM TEAPFOS (above CMC). The spectra were recorded at 83 ps, 17 40 ps and 6 ps after the end of the pulse for A, B, C and D, respectively, and are normalized for the irradiation dose of 45 Gy; optical path: 1.5 cm.

is close to the experimental uncertainty; the errors of the slopes of the straight lines for

APFO/t-BuOH samples are as high as ca 10% in the region of low concentrations and

20% in the region of high concentrations. It is noteworthy, however, that the effect of

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Figure 3. Transient absorption spectra recorded for N20 saturated solutions of SO, dotted line, and

APFO, solid line, (concentration 10 mM, below CMC) immediately after the pulse. Irradiation dose:

45 Gy; optical path: 1.5 cm.

the medium chain-length alcohols on lowering surfactant CMC values has been reported

[18]. The present CMC estimation is in a good agreement with the results obtained by others using different techniques: surface tension and ESR [19], NMR [20], and

conductivity measurements [21 ]. Data in the range 0.025 to 0.033 M have been

reported for the unbuffered APFO solutions [ 19-21 ], and a 3-fold lower values in the

presence of NH4CINH4OH buffer (pH = 8.8) [22], at ambient temperatures. The

average value CMC = 0.030 ± 0.03 M can be evaluated from the previous and the

present data for APFO solutions with no salt added. From the results presented in

Figure 5 the second order rate constants of e-aq reaction with APFO are determined as

5, 1 . 10 M-1s-l and 1 .3.10 M-'s-' for the free surfactant below CMC and for the

micellized surfactant, respectively. The other two perfluorinated surfactants examined have their CMCs much

lower than that of APFO: CMC = 1'10-3 M has been reported for TEAPFOS [23], and

for the PFPE surfactant CMC = 1,8'10-3 M is obtained, with respect to the average

molecular weight, from the data presented in Figure 1. At and below these

concentrations the observed decay of e-aq is not markedly faster than in the blank sample

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Figure 4. Dependence of the pseudo-first order rate constant, k', of e-aq decay (0, data corrected for the decay in blank sample) and of the product growth (o) on the concentration of SO in argon saturated solutions containing t-BuOH. The straight line represents the least square fitting to e-aq decay data. The CMC value indicated by the arrow was reported earlier [17]. Inset: An example of the pseudo-first order kinetic fits (smooth lines) to the experimental traces (noisy lines) for the 0.095 M SO solution.

(water/t-BuOH). In the PFPE system it remains slow with increasing surfactant

concentration up to =13 mM. TEAPFOS, however, appears a relatively good e-aq

scavenger. From the data presented in Figure 6 the second order rate constants of e-aq reaction with micellized surfactants are determined as M-'s' and 7.3'107 M-IS-

I

for PFPE and TEAPFOS, respectively. The kinetics of 'OH radical reaction with the surfactant was examined for

APFO and SO solutions using the competition method, with thiocyanate ions as

competitors. They scavenge 'OH radicals according to the reaction: _

whose rate constant is kSCN = 1.1'1010 M-1S.1 [24]. The absorbance of the thiocyanate radical anions was followed at 475 nm as a function of concentration of the added

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Figure 5. Dependence of the pseudo-first order rate constant of e'aq decay, k', on the concentration of

APFO; argon saturated solutions in the absence (o) and in the presence (D) of t-BuOH. The straight lines represent the least square fitting. See text for details.

surfactant (the second 'OH scavenger). The rate constant of the surfactant reaction with

'OH radicals, ksurt? can be obtained from the relation:

where Ao and A are the absorbances of the thiocyanate radical anion, just after the

ionizing pulse, in the absence and in the presence of the surfactant (Surf), respectively; and the square brackets stand for concentrations.

Figure 7 presents the results obtained for SO solutions in the concentration

range up to 1 M, with the constant concentration of thiocyanate ions [scan = 0.1 M.

The dependence Ao/A - 1 on the [SO]/[SCN ] ratio exhibits two linear sections. The

intersection point corresponds to [SO] = 0.33 M, which can be identified with the CMC.

This CMC value is somewhat lower than that reported for pure SO (0.4 M [17]), and the

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Figure 6. Dependence of the pseudo-first order rate constant ofc-aq decay, k', on the concentration of TEAPFOS (o) and PFPE (I ,) surfactants in argon saturated solutions containing t-BuOH. The straight lines represent the least square fitting.

difference can be ascribed to the presence of added salt in the system examined by us

(0.1 M NaSCN). The SO solutions of concentrations below CMC were also examined

using lower thiocyanate concentration, 0.005 M, and the results are shown in the inset

of Figure 7. The average value of the rate constant of scavenging of 'OH radicals by

nonmicellized SO obtained from these two experiments is M-'s-'.

Micellization leads to a decrease of the scavenging rate constant; from the data of Figure

7 its value above CMC can be estimated (with the uncertainty ±18 %) as 2.6.109

The competition experiments with APFO have indicated that this perfluorinated surfactant remains relatively unreactive towards *OH radicals; no lowering of the

absorbance of thiocyanate radical-anion was observed even for the ratio [APFO]/[SCN ] = 350; the upper limit of the rate constant can be thus estimated as 3.107 M-'s-'.

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Figure 7. Competition kinetics of SO and thiocyanate anion in reaction with *OH radicals. Concentration of SCN- was kept constant, 100 mM (main plot) and 5 mM (inset); the straight lines

represent the least square fitting. See text for details.

DISCUSSION

In this section we will comment separately on reactions of e-aq and 'OH radicals with

perfluorinated surfactants as compared to protiated surfactants. The present results (rate constants and spectra) will be compared with the available literature data for similar

systems, and the observed differences in e-aq behavior in the three perfluorosurfactant solutions above CMC will be discussed in terms of different charge density at the

micellar surface.

Scavenging of Hydrated Electrons

Our experiments reveal that the hydrated electron does react with the negatively charged

carboxylic and sulfonic surfactants, the rate constants of the reaction are of the order 106 - 10' M-1s-\ and, with the exception of micellized PFPE carboxylate, they are higher for

perfluorinated derivatives than for SO.

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In the case of SO the scavenging of e-aq leads to the transient product exhibiting a dual absorption in the UV region (Figure 2A), as indicated by the coincidence of the

rate constants of e aq decay, observed at 600 nm, and of the product growth, observed at

310 nm (Figure 4). Unfortunately, the growth could not be followed in a

straightforward way in the wavelength region of the second maximum of the product, since above 400 nm the extinction coefficient of e-aq is relatively high and we observe

a superposition of the absorptions due to the product and e-aq. However, as shown in

Figure 8, the observed kinetic traces are well reproduced as a sum of the electron decay curve and the product growth curve, calculated with the use of the pseudo-first order rate

constants determined from measurements at 600 nm and 310 nm, respectively. We infer that in SO solutions the primary attack of e-aq is on the carboxylic

surfactant head-group, and assign the scavenging product to R-CO22-' radical-anion, where R stands for the alkyl chain. In agreement with such interpretation is the

observed continuous increase of the rate constant of e aq decay through the CMC region

(vide infra). For APFO, the perfluorinated analogue of SO, the rate constant of ear

scavenging is more than an order of magnitude higher, and at the same time the

Figure 8. Fitting the kinetic trace recorded at 450 nm for argon saturated 0.095 M SO solution (noisy curve) by the sum of the electron decay curve and the product growth curve, calculated with the use of the pseudo-first order rate constants determined from measurements at 600 nm and 310 nm respectively.

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absorbance of the RF-C02 2-. radical-anion (where RF denotes perfluoroalkyl chain)

decreases, cf. Figure 2 A and B. Note that due to the F/H substitution the respective maximum position is shifted towards longer wavelength, and the second maximum

is not resolved. We conclude that in the case of APFO there is an additional reaction

path, most probably the dissociative electron attachment to fluorine atoms of the chain

leading to elimination of F- and formation of a carbon-centered radical. Similar

reaction was suggested to occur under laser irradiation of fluoropolymer films in

contact with water [25]. The perfluoroalkyl-carboxylate radicals formed in APFO

solution do not exhibit a pronounced absorption in the examined wavelength range; the "blue edge" of the spectrum is comparable to that observed in the blank sample

(cf. Figure 2 B and C). We note that for CF3COO- anion the rate constant of e'aq

scavenging was reported as the value close to our result for SO

(3 .3 . 1 0 M-'s-' ). Thus, the dissociative electron attachment is efficient only for the

longer chain perfluoroalkyl carboxylates. Due to micellization the chains become less accessible for hydrated electron and

the second order rate constant of the scavenging decreases four times (cf. Figure 5). Such inhibiting effect of micellization has been reported for the e-aq reaction with other

anionic surfactants having an acceptor group in the chain, as well as for the reaction with

hydrophobic electron scavengers added to micellar solutions of unreactive anionic

surfactants [1,5,11,15; and references therein]. A spectacular example is the reaction

of e'aq with pyrene, whose rate constant decreases more than 2 orders of magnitude due

to incorporation of the scavenger into sodium dodecyl sulfate micelles [26]. In comparison to APFO micelles, the PFPE surfactant micelles remain

practically unreactive with respect to hydrated electron. In the examined concentration

range no absorption from the product is detected, and the upper limit of the rate constant

of e-aq scavenging is of the order of 106 M-'s-', cf. Figure 2C and Figure 6. This means

that neither carboxylic anions nor the perfluorinated chains of micellized PFPE can

compete efficiently with the spontaneous decay of e-aq. On the other hand, the rate

constant of scavenging by micellized TEAPFOS is even higher than that determined

for APFO below CMC, and we do observe the absorption peak of the radical-anion, in

this case RF-SO32-', cf. Figure 2D. An increased absorbance below 300 nm, can be

assigned to radicals formed by e-aq reaction with the perfluorinated chains. Our results

suggest that for the sulfonate surfactant the absorption of these radicals, as well as the

radical-anion maximum, are shifted towards the red, with respect to the carboxylate surfactant. We note, however that APFO spectrum was recorded below CMC, and that

of TEAPFOS above CMC. While in the case of monomeric surfactant e-aq can attack

with comparable probability all F atoms along the chain, for the micellized surfactant

the reaction is expected to occur mostly with the F atoms closest to the interface, so the

spectrum is dominated by the radicals at a carbon atom which likely absorb in the region of higher wavelengths, as observed for example in solutions of the propionate salt [27].

909

For the TEAPFOS solution we also have to consider a possibility of e-aq

scavenging by tetraethylammonium counterions (TEA+). Such reaction was reported to occur with the rate constant as high as 1 .2. 1 0 M-1s-1 [28]; it would lead to the

formation of triethylamine and the ethyl radicals, but the latter have a weak absorption below 250 nm only. The TEA+ counterions are also reactive towards 'OH radicals (k = 1.3.108 M-'s-') [29]. In our experimental conditions, however, this reaction can be

neglected; the added t-BuOH is ca 50 times more efficient 'OH scavenger (k = 6.0'108

M-1s-1 [24], and concentration an order of magnitude higher than that of TEAPFOS).

Finally, comparing the behavior of e'aq in TEAPFOS solution with that in APFO

solution, we have to consider its reactivity towards the different head-groups (sulfonic vs carboxylic). The rate constants M-'s', and k = 3 . 5 . 1 0 M-'s-' were reported for methane and ethane sulfonate ions, respectively [24]; the latter value would suggest that R-S03- ions are much more reactive than R-C02 ions (cf. our data for SO, 3 .3 . 1 0

However, our results for micellar solutions of perfluorinated ionomer containing sulfonic groups in the pendant chains (Nafion, in the form of lithium salt) indicate the

rate constant of the order of 106 M-'s-' (with respect to molar content of ions) [30], like

in the PFPE surfactant solution. We thus assume that the e'aq reactivity towards the head-

groups is about the same for perfluorinated alkyl-carboxylate and alkyl-sulfonate surfactants. Then, subtracting from the value of the rate constant for TEAPFOS, 7 .3 . 1 0

M-'s-', the contribution of e-aq reaction with the counterions, we conclude that

perfluorooctyl chains of micellized TEAPFOS are much more efficient scavengers in

comparison to perfluoroheptyl chains of micellized APFO (by a factor of 4.7).

Hence, our studies clearly indicate that permeability of the micelles for e-aq increases in the order PFPE<APFO<TEAPFOS. This finding can be interpreted in

terms of decreasing density of the negative charge at the interface. In the PFPE system electrostatic repulsion of e aq from the surfactant carboxylic anions is so strong that it can

hardly penetrate inside the aggregates to react with the chains; in view of our results the

PFPE micelles may be considered as fairly spherical, highly charged particles. In the

case of APFO the electrostatic barrier for e-aq is evidently lower, and this can be related

to higher degree of counterion binding p. As indicated by small angle neutron scattering

study [31] and the recent fluorescence quenching study [32], APFO micelles are

ellipsoidal in shape and have relatively large aggregation numbers increasing with the

surfactant concentration, from 29 at CMC up to N = 109 at [APFO] = 0.5 M.

Similar behavior has been observed for cesium perfluorooctanoate system, and it has

been shown that the growth of nonspherical aggregates is accompanied by increase of

(3 value fromzO.6 to ?0.9 in the surfactant concentration range 0.06 - 0.5 M [33]. For

comparison, in the case of sodium perfluorooctanoate where spherical aggregates are

formed 40 only in 0.5 M solution) (3 is less than 0.6 [34]. The observed highest reactivity of e'aq towards micellized chains in the

TEAPFOS system can be rationalized taking into account a specific effect of TEA+ +

910

counterions, which due to their hydrophobic nature, effectively bind to micelles and act

as spacers between the head groups [35]. Previous studies by pulse radiolysis [6] and

electron spin echo modulation [36] techniques indicated that substitution of

tetraalkylammonium counterion for sodium counterion in dodecyl sulfate surfactant

brings about an increased roughness and wetness of the micellar surface, thus facilitating electron transfer through it. It is relevant to note that in the case of perfluorooctyl sulfate

the presence of TEA+ counterions leads to formation of thread-like aggregates consisting of interconnected spherical or hemispherical units, each containing at least 52 surfactant

molecules [31,37]. The degree of counterion binding to the micelles has been recently

determined as (3 = 0.8 [38].

Scavenging of .OH Radicals

Our results indicate that reactivity of 'OH radicals with the perfluorinated surfactant,

(APFO) is more than two orders of magnitude lower than with its protiated analogue

(SO); the effect of F/H substitution in the alkyl chain is thus opposite that that observed

for e aq reaction. Scavengiilg of 'OH radicals by SO is due to abstraction of H atom from

the surfactant chain, with the formation of carbon centered alkylcarboxylate radicals

which are responsible for the increased absorption in the UV region (in comparison to

APFO solution), cf. Figure 3. The rate constant of the process, M-'s-', is close

to the data reported earlier for other anionic surfactants of comparable chain length, k = 6.5. 1 0 M-'s-' for octyl sulfate, and k = 2.5. 1 0 M-'s-' for hexyl sulfate [24]. In SO

solution the rate constant decreases ca 2-fold when the chains become incorporated into

micelles. Similar effects have been observed for micellar solutions of typical

surfactants, such as sodium dodecylsulfate, hexadecyltrimethylammonium bromide or

Triton. In these systems the lowering of 'OH radical reactivity above CMC was as high

as one order of magnitude [39]. The present result can be rationalized taking into

account that SO micelles are relatively small aggregates, aggregation number N = 11 I

only [17], so their interface is quite permeable for'OH radicals.

CONCLUSIONS

The perfluoroalkyl carboxylate (APFO) is a stronger scavenger of hydrated electrons in

comparison to its protiated analogue (SO); in the former system dissociative electron

attachment to the fluorine atom in the alkyl chain occurs in addition to e aq reaction with

the head group. Although the rate constants of e-aq scavenging are relatively low, of the

order of 106 - 10' M-'s-', the kinetic data for micellized perfluorosurfactants reflect

different charge densities in the head group region of the aggregates in various systems. The perfluorosurfactants, in comparison to hydrocarbon surfactants, remain practically

unreactive towards 'OH radicals.

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Acknowledgements This work has been supported by the Polish Committee of Scientific Research (in part KBN project 3 T09A 034 11). The authors thank A. Chittofrati of Ausimont CRS, Colloid Lab., Bollate, Italy, for characterization of the PFPE surfactant in terms of MW

distribution and CMC.

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