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Naphthalene as a fluorescent probe in micellar systems
NAPHTHALENE M VAN
15 September 1978
CHEMICAL PHYSICS LETTBBS
Volume 58, number 2
AS A FLUORESCENT
BOCKSTAELE,
J_ GELAN,
PROBE IN MKELLAR
H_ MARTENS,
Limburgs Uniwtsitair Centrum. Univerdaire
SYSTEMS
J_ PUT
olmpus. B 3610 Diepenbeek, Bel&m
and J.C. DEDEREN,
N. BOENS and F.C. DE SCHRIJVER
Department of Chemistry. University of Leuven, B 3030 Hewdee,
BeIgium
Received11 April 1978 Revisedmanuscriptreceived12 June 1978
The fluorescencelifetime of naphthalenein some mice&r systemsis measured,ushzgthe singlephoton counting technique. The results obtained are different from those described previously_ A possible explanation is that in sodium dodecylsulfate as in hexadecyltrimethylammonium bromide and hexadecyltrimethylammonium chloride solutions two lifetime components are present corresp_ondkg to naphthalene solubilized in the micelles and to naphthalene in the water phase. Under certain conditions the two components show a single exponential decay.
1. Introduction Fluorescent probes are extensively used for the
study of micellar systems [I] _ They can provide useful information on polarity and viscosity of the micellar interior [2-S] _ Naphthalene is considered as an interesting probe, since it partitions between the micellar and aqueous phases. Fluorescence lifetimes of naphthalene in these phases are resolvable and the two environments can be studied independently. Fluorescence lifetime measurements of naphthalene in various micellar systems were descriied previ” ously by Hautala et al. [6]. In the course of a photochemical study of mice&u systems with naphthalene derivatives, we performed some lifetime measurements which yielded different results from those described by these authors.
2. Materials and methods
Sodium dodecyl sulfate (SDS) was purchased from Merck (for tenside measurements) and was used as such_ HexadecyItrimethylammonium bromide (HDTBr)
was purchased from Merck and recrystallized from methanol/tetrahydrofuran_ Hexadecyltrimetbylammonium chloride (HDTCI) was prepared, using the following technique:-A solution of puritled HDTBr was passed through a column filed with an anion-exchange resin Merck nr. 4767 saturated with OH- ions. The resulting HDTOH solution was free of Br- (no precipitate of AgBr was formed upon testing with an acidified AgNOs solution)_ Titration of the HDTOH solution with hydrogen chloride yielded HDTCl salt solution at the equivalent point. HDTCl was isolated and recrystailized several times from tMeOH/THF before use_ Naphthalene was purchased from UCB (highest purity) and was sublimed twice before use. A degassed (four freeze-thaw cycles) cyclohexane (Aldrich spectrophotometric grade) solution of this purified naphthalene yielded a fluorescence lifetime of 206 ns (cf. literature: 94 ns Birks [7] ; 108 ns Hautala et al_ [6] )_ DoubIy distiiled water was used in all experiments. Inorganic salts were purchased from Merck (PA) and were used as such. Fluorescence decay curves were obtained using the 211
CHEMICAL
Voiume 58, number 2 TabIe I Rrametersof
PHYSICS
10 4 hl) in Hz0 and in azzonic and cationic micelIar systems (air saturated)
fluorescenQdecayofnaphth~e.ll(ZX Medium
Am _-
H20 SDS (5 SDS (5 HDTCI HDTCI HDTBr -
15 September 1978
LETTERS
I -00 1.00 1.00 1.00 0.82
x 1O-2 M) x lO-2 31) + KBr (IO-+ 11) (1O-7- 31) (IO-’ Xl) i- CuC12 (1O-3 M) ( 10m2 hl) ----___---
a) a) a) 3
rm ml
AW
TW (ns)
-
1.00 -
35 -
O-18
28
53 53 55 5.5 7
a) Only one component measurable_
singIe photon counting technique IS]_ Experimental fluorescence deczy curves were anaiysed by a nonlinear Ieast squares curve fitting program [9], with convoIution with the Iamp curve_
3. Resiits Fluorescence decay of naphthalene in solutions of micehe forming detergents can be analysed using the equation, F(E) =A,
exp(--r/rm)
+A=, exp(--t/i,,),
(1)
the fLst term being assigned to emission of naphthalene in the mice&r interior, the second term to emission of naphthalene in the bulk water phase. In eq. (I), A, and A, are the amplitudes in the micelIar and aqueous phase, respectively; r, and rlv are the fluorescence lifetimes respectively in micellar and aqueous phase and t is the time. Using this approach we measured these parameters for naphthalene solubilized in pure water and in water containing either cationic micelies of HDTBr and HDTCl or anionic micelies of SDS. The results are presented in table I _Naphthalene shows the same behavior in the HDTCl system and
lo6 Counts
10‘
103
lo2
i0
50
150
100
200 channel
Fig_ i_ Fluoresaxnce decay curve of nq I.&dene
212
(2 X IO4
250 number
M) in HDTCI (10 -* M). Channel width of 0.4963 ns_
Volume 58. number 2
CHEMICAL
PHYSlCS
LETTERS
15 September 1978
IO5 Count!
10”
IO”
IO’
10
0 channel
Fig. 2.
number
Fiuorescencedecay curve of naphthalene(2 X IO4 M) in HDTBr (10e2 M)_ Channel width of 0.9950 ns. systems, but is hidden under the intense long lived fluorescence of naphtbalene in the micehes. For this reason we measured fluorescence lifetimes for a series of solutions, containing different amounts of counterionquencher. These results are summarized in table 2. The anionic micellar system was made by using mixtures of HDTCI and HDTBr, while for the cationic mice&r system, we added Cuzf ions. As can be seen, the second component shows up in both systems, as soon as the fluorescence of the m.iceJlarnaphthalene is sufficiently quenched_
in the SDS system. Only one component is detected, assigned to naphthalene in the mice&r interior (fig- 1) since it is not quenched by addition of quencher ions with opposite charge to the micelIes (table I)_ In HDTBr, the lifetime of this component is strongly shortened, and a second component shows up, due to naphthalene in the water phase (fig. 2). This sborterring of the lifetime can be interpreted as being due to heavy atom quenching by the bromide counterions flO] _ These results suggest that a seccnd component is probably also present in the other micellar Table 2 Parameters of the fluorescence decay of naphthalene (2 X 10” tedon quencher added (air saturated solutions)
M) in aqueous micelIar systems with different amounts of coun-
Medium
Am
HDTCI
1.00 =I 1.00 a) 0.89 0.82 1.00 a) 0.91 0.88
HDTCUHDTBr 9/l <10m2 ,M) HDTCI/HDTBr 3/7 (10m2 M) HDTBr (LOe2 XI) SDS (5 x LO-? hi) SDS (5 X IO" M) + C&l2 (5 x 1O-3 M) SDS (5 x 1O-2 M) + CII~I~ (IO-~ AM)
55 33 9 7 53 2.0 1.8
-
-
0.11 0.18
29 28 -
0.09 0.12
22.1 21.4
a) Only one component measurable.
213
Volume 58, number 2
CHEMKAL
15 September 1978
PHYSICS LET-ERS
Tabie 3 Parametersof ffuorescence decay of naphthaienein aqwous micelIarsystems,as descriied by Hautzla et al. [6]
Hz0
HDTBr (IO-* M) HDTcl (lo-2 M) SDS (3 x lo-* %I) SDS (3 X 10m2M) f Br- (4 X lo-* M)
4. Discussion
Some remarkable differences are observed when comparing our rest&s with those of Hautala et al_ 163, which are summarized in tabIe 3. (i) On ‘he bssis of their rest&s, the authors argued that the rather farge difference in lifetime, observed for the cationic versus the anionic detergent system, could reflect a more polar character, a greater oxygen
-
1 0.14 050 -
-
0.86 050 1.00 1.00
11 23 60 64
39 35 39 -
time of the intense component. On the basis of the experimental parameters A,. A,, T,, T, for the_ EIDTBr system presented in table 1 the percentage of naphthalene present in the micehe is found to be 85% using the equation [6] : =rnic
-=
c
tot
I+---
[
AW
iW
‘mA
1 mewatfx water %lic
&e
(PF
--I
(2)
-
SoIubility or both, in the former system_ Our results show no difference between these two mice!& sys-
This result can be compared with that obtained by assuming a simple two-phase distriiution of naph-
tems_ A possiiIe explanation might be that the HDTCI surfactant used by the other authors contained a small amount of Br- ions. This would explain the rather short fluorescence lifetime of naphthalene they measured in HDTCI (23 ns) as compared to our result (55 ns)_ A small amount of Br- will indeed quench very efficiently the naphthalene dissolved in the micehe, as is ihustrated by our quenching experiments summarized in table 2_ (ii) Hautala et al. give no explanation for the obse.qation of only one component in the anionic SDS system, while they observe two components in the cationic system. Our results show one component as well for *he cationic as for the anionic system without quencher_ Upon addition of quenching counterions, a second component appears in both systems. This indicates that the decay component, assigned to naphthalene in water, is always present but only resolvabie when the intense component from naphth2Iene in the micellar interior is sufficiently short. Computer simulated curves, combining one Iow-intensity component with an intense short lived or long lived one, support this view_ This would explain then the appearance of the low intensity component of naphthalene in Hz0 when quenching counterions are added reducing the life-
thalene. Taking the partition coefficient of naphthalene between water and hexane as l/250& a micellar radius of 20 A and an aggregation number 80, we find for the HDTBr system 86% of mice&r naphthaIene_
214
5 conchlsions
-
As proposed by Hautrda et al. [6] naphthaiene partitions between the micellar and the water phase as could be predicted by the partition coefficient. Without counterion quencher, the emission from naphthalene in bulk water is not measurable_ This component appears as well in anionic as in cationic micehar systems, upon addition of quenching counterions. No significant difference between the anionic and cationic miceliar systems studied, has been observed.
References ill
M. GrZtzel and J.K. Thomas, in: Madem fluorescence spectroscopy, Vol. 2, ed. E.L. Wehry (Plenuti Press, New York, 1976) p_ 169. and referenais therein.
Vol&xe
58, number 2
CHEMKCALPHYSLCS
[2] M. Shinitzky, AC. Dianoux, C. Gitler and G. Weber, Biochemistry 10 (1971) 2106_ [3] HJ. PownaU and L.C. Smith, 1. Am_ Chem. Sot. 95 (1973) 3136. [4] NJ_ Turro, M-W. Geiger, R-R. Hautala and N_E_ Schore. in: MicelLization, soIubilization and nkroemulsions, VoL 1, ed. KL hiittal (Plenum Press, New York, 1977) p. 75, and references therein. (5 J IL Kalyanawndaram and J.K. Thomas, J. Phys. Chem. 81(1977) 2176. [6 J R.R. HautaIa, N-E. Shore and NJ. Turro, J. Am. Chem. Sot. 95 (1973) 5508.
LETTERS
15 September 1978
i7 j J_B. Birks, Photophysics of aromatic molecules (WiIey, New York, 1970) p. 126. [S J W.A_ Ware, in: Creation and detection of the excited state, Vol. lA, ed. A.A. Lamola (Dekker, New York, 1971). [9j A. Grinva!d and I. Steinberg, Anal. Biochem. 59 (1974) 583. [IOJ hf. Grgtzel and J.K. Thomas, J. Am. Chem. Sot. 95 (1973) 6885.
15 September 1978
CHEMICAL PHYSICS LETTBBS
Volume 58, number 2
AS A FLUORESCENT
BOCKSTAELE,
J_ GELAN,
PROBE IN MKELLAR
H_ MARTENS,
Limburgs Uniwtsitair Centrum. Univerdaire
SYSTEMS
J_ PUT
olmpus. B 3610 Diepenbeek, Bel&m
and J.C. DEDEREN,
N. BOENS and F.C. DE SCHRIJVER
Department of Chemistry. University of Leuven, B 3030 Hewdee,
BeIgium
Received11 April 1978 Revisedmanuscriptreceived12 June 1978
The fluorescencelifetime of naphthalenein some mice&r systemsis measured,ushzgthe singlephoton counting technique. The results obtained are different from those described previously_ A possible explanation is that in sodium dodecylsulfate as in hexadecyltrimethylammonium bromide and hexadecyltrimethylammonium chloride solutions two lifetime components are present corresp_ondkg to naphthalene solubilized in the micelles and to naphthalene in the water phase. Under certain conditions the two components show a single exponential decay.
1. Introduction Fluorescent probes are extensively used for the
study of micellar systems [I] _ They can provide useful information on polarity and viscosity of the micellar interior [2-S] _ Naphthalene is considered as an interesting probe, since it partitions between the micellar and aqueous phases. Fluorescence lifetimes of naphthalene in these phases are resolvable and the two environments can be studied independently. Fluorescence lifetime measurements of naphthalene in various micellar systems were descriied previ” ously by Hautala et al. [6]. In the course of a photochemical study of mice&u systems with naphthalene derivatives, we performed some lifetime measurements which yielded different results from those described by these authors.
2. Materials and methods
Sodium dodecyl sulfate (SDS) was purchased from Merck (for tenside measurements) and was used as such_ HexadecyItrimethylammonium bromide (HDTBr)
was purchased from Merck and recrystallized from methanol/tetrahydrofuran_ Hexadecyltrimetbylammonium chloride (HDTCI) was prepared, using the following technique:-A solution of puritled HDTBr was passed through a column filed with an anion-exchange resin Merck nr. 4767 saturated with OH- ions. The resulting HDTOH solution was free of Br- (no precipitate of AgBr was formed upon testing with an acidified AgNOs solution)_ Titration of the HDTOH solution with hydrogen chloride yielded HDTCl salt solution at the equivalent point. HDTCl was isolated and recrystailized several times from tMeOH/THF before use_ Naphthalene was purchased from UCB (highest purity) and was sublimed twice before use. A degassed (four freeze-thaw cycles) cyclohexane (Aldrich spectrophotometric grade) solution of this purified naphthalene yielded a fluorescence lifetime of 206 ns (cf. literature: 94 ns Birks [7] ; 108 ns Hautala et al_ [6] )_ DoubIy distiiled water was used in all experiments. Inorganic salts were purchased from Merck (PA) and were used as such. Fluorescence decay curves were obtained using the 211
CHEMICAL
Voiume 58, number 2 TabIe I Rrametersof
PHYSICS
10 4 hl) in Hz0 and in azzonic and cationic micelIar systems (air saturated)
fluorescenQdecayofnaphth~e.ll(ZX Medium
Am _-
H20 SDS (5 SDS (5 HDTCI HDTCI HDTBr -
15 September 1978
LETTERS
I -00 1.00 1.00 1.00 0.82
x 1O-2 M) x lO-2 31) + KBr (IO-+ 11) (1O-7- 31) (IO-’ Xl) i- CuC12 (1O-3 M) ( 10m2 hl) ----___---
a) a) a) 3
rm ml
AW
TW (ns)
-
1.00 -
35 -
O-18
28
53 53 55 5.5 7
a) Only one component measurable_
singIe photon counting technique IS]_ Experimental fluorescence deczy curves were anaiysed by a nonlinear Ieast squares curve fitting program [9], with convoIution with the Iamp curve_
3. Resiits Fluorescence decay of naphthalene in solutions of micehe forming detergents can be analysed using the equation, F(E) =A,
exp(--r/rm)
+A=, exp(--t/i,,),
(1)
the fLst term being assigned to emission of naphthalene in the mice&r interior, the second term to emission of naphthalene in the bulk water phase. In eq. (I), A, and A, are the amplitudes in the micelIar and aqueous phase, respectively; r, and rlv are the fluorescence lifetimes respectively in micellar and aqueous phase and t is the time. Using this approach we measured these parameters for naphthalene solubilized in pure water and in water containing either cationic micelies of HDTBr and HDTCl or anionic micelies of SDS. The results are presented in table I _Naphthalene shows the same behavior in the HDTCl system and
lo6 Counts
10‘
103
lo2
i0
50
150
100
200 channel
Fig_ i_ Fluoresaxnce decay curve of nq I.&dene
212
(2 X IO4
250 number
M) in HDTCI (10 -* M). Channel width of 0.4963 ns_
Volume 58. number 2
CHEMICAL
PHYSlCS
LETTERS
15 September 1978
IO5 Count!
10”
IO”
IO’
10
0 channel
Fig. 2.
number
Fiuorescencedecay curve of naphthalene(2 X IO4 M) in HDTBr (10e2 M)_ Channel width of 0.9950 ns. systems, but is hidden under the intense long lived fluorescence of naphtbalene in the micehes. For this reason we measured fluorescence lifetimes for a series of solutions, containing different amounts of counterionquencher. These results are summarized in table 2. The anionic micellar system was made by using mixtures of HDTCI and HDTBr, while for the cationic mice&r system, we added Cuzf ions. As can be seen, the second component shows up in both systems, as soon as the fluorescence of the m.iceJlarnaphthalene is sufficiently quenched_
in the SDS system. Only one component is detected, assigned to naphthalene in the mice&r interior (fig- 1) since it is not quenched by addition of quencher ions with opposite charge to the micelIes (table I)_ In HDTBr, the lifetime of this component is strongly shortened, and a second component shows up, due to naphthalene in the water phase (fig. 2). This sborterring of the lifetime can be interpreted as being due to heavy atom quenching by the bromide counterions flO] _ These results suggest that a seccnd component is probably also present in the other micellar Table 2 Parameters of the fluorescence decay of naphthalene (2 X 10” tedon quencher added (air saturated solutions)
M) in aqueous micelIar systems with different amounts of coun-
Medium
Am
HDTCI
1.00 =I 1.00 a) 0.89 0.82 1.00 a) 0.91 0.88
HDTCUHDTBr 9/l <10m2 ,M) HDTCI/HDTBr 3/7 (10m2 M) HDTBr (LOe2 XI) SDS (5 x LO-? hi) SDS (5 X IO" M) + C&l2 (5 x 1O-3 M) SDS (5 x 1O-2 M) + CII~I~ (IO-~ AM)
55 33 9 7 53 2.0 1.8
-
-
0.11 0.18
29 28 -
0.09 0.12
22.1 21.4
a) Only one component measurable.
213
Volume 58, number 2
CHEMKAL
15 September 1978
PHYSICS LET-ERS
Tabie 3 Parametersof ffuorescence decay of naphthaienein aqwous micelIarsystems,as descriied by Hautzla et al. [6]
Hz0
HDTBr (IO-* M) HDTcl (lo-2 M) SDS (3 x lo-* %I) SDS (3 X 10m2M) f Br- (4 X lo-* M)
4. Discussion
Some remarkable differences are observed when comparing our rest&s with those of Hautala et al_ 163, which are summarized in tabIe 3. (i) On ‘he bssis of their rest&s, the authors argued that the rather farge difference in lifetime, observed for the cationic versus the anionic detergent system, could reflect a more polar character, a greater oxygen
-
1 0.14 050 -
-
0.86 050 1.00 1.00
11 23 60 64
39 35 39 -
time of the intense component. On the basis of the experimental parameters A,. A,, T,, T, for the_ EIDTBr system presented in table 1 the percentage of naphthalene present in the micehe is found to be 85% using the equation [6] : =rnic
-=
c
tot
I+---
[
AW
iW
‘mA
1 mewatfx water %lic
&e
(PF
--I
(2)
-
SoIubility or both, in the former system_ Our results show no difference between these two mice!& sys-
This result can be compared with that obtained by assuming a simple two-phase distriiution of naph-
tems_ A possiiIe explanation might be that the HDTCI surfactant used by the other authors contained a small amount of Br- ions. This would explain the rather short fluorescence lifetime of naphthalene they measured in HDTCI (23 ns) as compared to our result (55 ns)_ A small amount of Br- will indeed quench very efficiently the naphthalene dissolved in the micehe, as is ihustrated by our quenching experiments summarized in table 2_ (ii) Hautala et al. give no explanation for the obse.qation of only one component in the anionic SDS system, while they observe two components in the cationic system. Our results show one component as well for *he cationic as for the anionic system without quencher_ Upon addition of quenching counterions, a second component appears in both systems. This indicates that the decay component, assigned to naphthalene in water, is always present but only resolvabie when the intense component from naphth2Iene in the micellar interior is sufficiently short. Computer simulated curves, combining one Iow-intensity component with an intense short lived or long lived one, support this view_ This would explain then the appearance of the low intensity component of naphthalene in Hz0 when quenching counterions are added reducing the life-
thalene. Taking the partition coefficient of naphthalene between water and hexane as l/250& a micellar radius of 20 A and an aggregation number 80, we find for the HDTBr system 86% of mice&r naphthaIene_
214
5 conchlsions
-
As proposed by Hautrda et al. [6] naphthaiene partitions between the micellar and the water phase as could be predicted by the partition coefficient. Without counterion quencher, the emission from naphthalene in bulk water is not measurable_ This component appears as well in anionic as in cationic micehar systems, upon addition of quenching counterions. No significant difference between the anionic and cationic miceliar systems studied, has been observed.
References ill
M. GrZtzel and J.K. Thomas, in: Madem fluorescence spectroscopy, Vol. 2, ed. E.L. Wehry (Plenuti Press, New York, 1976) p_ 169. and referenais therein.
Vol&xe
58, number 2
CHEMKCALPHYSLCS
[2] M. Shinitzky, AC. Dianoux, C. Gitler and G. Weber, Biochemistry 10 (1971) 2106_ [3] HJ. PownaU and L.C. Smith, 1. Am_ Chem. Sot. 95 (1973) 3136. [4] NJ_ Turro, M-W. Geiger, R-R. Hautala and N_E_ Schore. in: MicelLization, soIubilization and nkroemulsions, VoL 1, ed. KL hiittal (Plenum Press, New York, 1977) p. 75, and references therein. (5 J IL Kalyanawndaram and J.K. Thomas, J. Phys. Chem. 81(1977) 2176. [6 J R.R. HautaIa, N-E. Shore and NJ. Turro, J. Am. Chem. Sot. 95 (1973) 5508.
LETTERS
15 September 1978
i7 j J_B. Birks, Photophysics of aromatic molecules (WiIey, New York, 1970) p. 126. [S J W.A_ Ware, in: Creation and detection of the excited state, Vol. lA, ed. A.A. Lamola (Dekker, New York, 1971). [9j A. Grinva!d and I. Steinberg, Anal. Biochem. 59 (1974) 583. [IOJ hf. Grgtzel and J.K. Thomas, J. Am. Chem. Sot. 95 (1973) 6885.