Aggregation behavior in inverse micellar systems: spectroscopic

Aggregation behavior in inverse micellar systems: spectroscopic evidence for a unified model ... Reverse Micelles: A Closed or Open Association Model?...
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Langmuir 1986, 2, 448-456

448

essary to pump the new solution through the ATR spectroscopic cell. Summary a n d Conclusions The present investigation has demonstrated the feasibility of studying the dissociation behavior of ionizable surfaces, even at very low surface charge. In the study of HC monolayers it has been demonstrated that above an cr of approximately 0.2 these surfaces obey a simple Gouy-Chapman charge, potential-electrolyte relationship. However, in the low-ionization regime the monolayer exhibits marked deviations from this theory, with the pK, showing a pronounced minimum, the shape of which is strongly dependent on the electrolyte concentration. This behavior is remarkably similar to that of the partially

insoluble polymer polyglutamic acid. Despite the similarity in results it is difficult to account for the monolayer results from the theories applied to the polymer titrations. The results presented in this study have established that the rearrangement of the monolayer or the sensitivity of the technique to error in the low-charge region is an unlikely cause of the observed dissociation behavior. Clearly this low-ionizationphenomenon requires further study to establish its origin and to clarify the general process of dissociation of ionizable surfaces. In a future paper a full discussion of the electrolyte dependence of the pK, minimum, together with a theoretical analysis of all the results on HC monolayers, will be presented. Registry No. HC, 26038-83-5; PVOE, 9003-96-7; NaC1, 7647-14-5; eicosanol, 629-96-9.

Aggregation Behavior in Inverse Micellar Systems: Spectroscopic Evidence for a Unified Model A. Verbeeck, E. Gelade, and F. C. De Schryver* Department of Chemistry, Katholieke Uniuersiteit Leuven, Celestijnenlaan 200 F, B-3030 Leuuen, Belgium Received December 9, 1985. I n Final Form: March 12, 1986 Neutral 1-naphthaleneaceticacid and the ionic sodium 1-pyrenesulfonateand 1-naphthylmethylammonium chloride were used as probes in both fluorescence decay and UV-absorption measurements to obtain information on the aggregation mechanism of the anionic surfactant sodium bis(2-ethylhexy1)sulfosuccinate (AOT) and the cationic surfactantsdodecylammonium propionate (DAP) and didodecyldimethylammonium chloride (DDDAC)in cyclohexane. In contrast with results in literature, a unified aggregation mechanism is proposed: starting from linear aggregates, cyclic inverse micelles are formed by a structural reorganization in a small concentration range, indicated as the “operational cmc”. The same spectroscopicmethods were used to determine the influence of water and temperature on the aggregation process. Water molecules stimulate aggregation of monomers at low surfactant concentration and stabilize the cyclic inverse micelles, especially if the polar head groups interact strongly with water molecules. Addition of water, however, has no influence on the cmc. Introduction Three models have been proposed to describe the aggregation of surfactants in apolar solvents.’ In the mass-action model (MA model) aggregation starts at a certain concentration of surfactant, the critical micellar concentration (cmc). At the cmc, n monomers aggregate to form a micelle (with aggregation number n) (eq 1). n monomers * micelle

(1)

Below the cmc only monomers exist in the solution, above that concentration monomers and rather monodisperse micelles are present. In the stepwise aggregation model (SA model), aggregation of surfactants starts at very low concentrations (IO+ to 1O-I M).2 In an infinite process, monomers, one after another, are added to form a n-mer (eq 2). Such a process monomer

dimer e trimer

*

+ n-mer

(2)

excludes a cmc and predicts at every concentration more (1)Eicke, H. F., Ed. Topics in Current Chemistry; Springer-Verlag: Berlin, Heidelberg, New York, 1980; Vol. 1. (2) (a) K e d , S.;Gutmann, H. Surf. Colloid Sci. 1976,8,193-296.(b) Kertes, A. S. In Micellization, Solubilization and Microemukions; Mittal, K. L., Ed.; Plenum Press: New York, 1977;pp 445-457.

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than two species in the solution. Aggregation of surfactant molecules in apolar solvents were often explained by one of these model^.^ A logical classification of surfactant molecules in two groups according to their aggregation mechanism failed.3i4 A third model was introduced by Eicke et al. (E model) (Figure l).5 They accepted units of three monomers as nuclei for the micellization process.6 From these nuclei, via a kind of SA mechanism, linear aggregates are formed. At a certain concentration, these linear aggregates structurally reorganize to cyclic ones. The E model also excludes a cmc. However, the surfactant concentration where linear aggregates transform to cyclic structures (inverse micelles) is often called the “operational cmc”. Ache7 and Zanaa proposed the E model as a possible explanation for (3)Gelad6, E.;Verbeeck, A.; De Schryver,F. C. Proceedings of the 5th Symposium on Surfactants in Solution; July, 1984. (4) Muller,N.J. Colloid Interface Sci. 1978,63,383. (5)(a)Eicke, H. F. In Micellization, Solubilization and Microemulsiom; Mittal, K. L., Ed.; Plenum Press: New York, 1977;Vol. 1. (b) Eicke, H. F.; Hopmann, R. F. W.; Christen, H. Ber. Bumenges. Phys. Chem. 1975,79,667. (6)Eicke, H. F.; Christen, H. J.Colloid Interface Sci. 1974,48,281. (7)(a) Jean, Y.; Ache, H. J. J. Am. Chem. SOC.1978,100, 984. (b) Fucugauchi, L. A.; Djermouni, B.; Handel, E. D.; Ache, H. J. J. Am. (c) Chem. Soc. 1979.101.2841. . . . . Diermouni.. B.:. Ache. H. J. J.Phvs. Chem. 1979,83,2476.

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Langmuir, Vol. 2, No. 4, 1986 449

Aggregation Behauior in Inverse Micellar Systems 1

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their experimental results on aggregation of ionic surfactants in apolar solvents. In this work, the aggregation of cationic and anionic surfactants in cyclohexane (c-C6H12)is examined. The methods used are absorption spectroscopy and fluorescence decay measurements. The probes used are naphthaleneacetic acid ( N U ) , sodium 1-pyrenesulfonate (PSA-Na+), and naphthylmethylammonium chloride (NMA'Cl-) (Figure 2). In literature, a lot of data about the aggregation mechanism are obtained from changes in UV absorption, fluorescence intensity, and fluorescence decay of solubilized chromophores as a function of the surfactant c~ncentration.~ This work shows that only a combination of such methods and the use of several probes can give reliable results. The same methods were used to study the influence of watere and temperature on the aggregation of surfactants in apolar solvents. From the reporta in literature about the association and the micellization of surfactants as a function of temperature, it is concluded that aggregation in apolar medium is enthalpy-controlled.' Values for the enthalpy and the enthropy of micellization are obtained by using the expression In cmc = AHO/RT- A S o / R (3) Because of a lack of knowledge about the aggregation mechanism, the thermodynamic functions thus obtained do not always correspond to the formation of cyclic inverse micelles. Additionally, eq 3 is only valid if the aggregation number (N,) of the aggregates studied remains constant on increasing the temperature.' For AOT and most anionic surfactants Nag remains unchanged over a wide temperature range." 'however, for cationic systems N~~ varies slightly with temperature.l As a result of the variation of NWgas a function of temperature, the experimentally determined values for AHo and ASofrom eq 3 differ from the real ones. The influence of water on the aggregation in apolar solvents is of interest since water molecules can interact with the polar head groups of the detergent molecules (hydratation). Because association in apolar medium is (8)Zana,R. In Solution Chemistry of Surfactants; Mittal, K. L., Ed.; Plenum Press: New York, 1979; Vol. 1. (9) (a) Jean, Y. C.; Ache,. J. J. Am. Chem. SOC.1978,100,6320. (b) Eicke,H.F.; Dennis, A. J. Colloid Interfoce Sci. 1978,64, 386. (10) Muller, N. In Micelliration, Solubilization and Microemulsions; Mittal, K. L., Ed.; Plenum Press: New York, 1977; Vol. 1. (11) (a) Peri, Y.B. J. Colloid Interface Sci. 1969,29, 6. (b) Zulauf, M.; Eicke, H. F. J . Phys. Chem. 1979,83,480.

PSA' N a

Figure 2. Probes used for the absorption and fluorescence decay measurements in the ionic surfactant solutions.

enthalpy controlled, interactions between the surfactant molecules, especially between their polar head groups (Hbridges, ion-dipole interactions, ...), are responsible for the formation of the aggregates. Therefore, water molecules are thought to be important for the formation of aggregates and for the stability of the aggregates formed. A study of the influence of temperature on the aggregation gives information about the stability of the aggregates. The results are qualitatively interpreted. Especially the role of water and the difference between cationic and anionic surfactants are evaluated.

Experimental Methods The probe 1-naphthaleneaceticacid ( N U ,Aldrich 997'00)was sublimed at 130 O C after recrystallization from water. Sodium 1-pyrenesulfonate (PSA-Na+)was synthesized by a modification of the method of Tietze et al.12 Solubilizationof l-naphthylmethylamine (Aldrich) in aqueous HC1 (pH 3) yielded the ionic 1-naphthylmethylammoniumchloride (NMA'Cl-). The latter two probes were purified by using classical methods.13 The preparation and/or purification of dodecylammonium propionate (DAP) and sodium bis(2-ethylhexyl)sulfosuccinate (AOT) has been described e1~ewhere.l~Didodecyldimethylammonium chloride (DDDAC)was obtained in the same way as CTAC.16 The solvent cyclohexane (c-c6H12) (Fluka puriss.) was distilled from sodium wires and kept over Lynde-type 4 A molecular sieves. Fluorescence decays of degassed solutions were determined by using a single photon counting apparatuscoupled to a PDP 11/23 computer. To judge the goodnesa of fit several statistical criteria were used.16 A Perkin-Elmer Lambda 5 UV/vis spectrophotometer was used for the UV-absorption measurements. In all cases, from a stock solution of the probe in methanol p.a., the appropriatevolume (microliters)was pipetted in 5-mL flasks and the solvent evaporated. The surfactant was then added by appropriate dilution of a stock solution in c-C6HlZ Finally, these (12) Tietze, E.;Bayer, 0. Ann. Chem. 1939, 540, 189. (13) Gelad€, E.Ph.D. Thesis, K. U. Leuven, 1983. (14) GeladC, E.; De Schryver, F. C. In Reverse Micelles: biological and technological releuance of amphiphilic structures in apolar media; Luisi, P. L., Straub, B. E., E&.; Plenum Press: New York, 1984, pp 143-164. (15) Roelanta, E.;GeladC, E.; Van der Auweraer, M.; Croonen, Y.; De Schryver, F. C. J. Colloid. Interface Sci. 1983, 96,288. (16) Boens, N.;Van den Zegel, M.; De Schryver,F.C . Chem. Phys. Lett. 1984,111, 340.

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Figure 3. Influence of the AOT concentration in CBHIZ on (a) the fluorescence decay parameter ( 7 ) of NAA, (b)the abeorbance (A) of PSA-Na', and (c) the absorbance ( A ) of NMA'Cl-.

solutions were sonicated and degassed.

Results and Discussion Three different surfactant systems have been investigated: the anionic AOT and the cationic DAP and DDDAC. A combination of UV-absorption and fluorescence decay parameter measurements was used to study their aggregation in cyclohexane. Three different probes were used the neutral NAA, the anionic PSA-Na+, and the cationic NMA+Cl-. The ionic probes are insoluble in the solvent, c-C6HI2,but can be solubilized when surfactant is added. Therefore, these probes are adequate to follow the change of absorbance as a function of surfactant concentration. Because the fluorescence decay parameters of NAA and PSA-Na+ are dependent on the environment, fluorescence decay measurements of these probes are used to follow environmental changes due to aggregation. 1. Aggregation in Apolar Solvents. 1.1. Results. 1.l.a. AOT. Aggregation of this surfactant in c-C6H12is studied by measuring the fluorescence decay parameters of NAA and the UV absorption of PSA-Na+ and NMA+Cl-. No fluorescence decay parameters of PSA-Na+ could be measured, because of the small absorbance of this probe at AOT concentrations below M. Results are shown in Figure 3. Fluorescence decays of NAA in this surfactant system are one exponential. Changes in lifetime of NAA plotted against the surfactant concentration show two discontinuities. One occurs at a concentration that is in agreement with the "operational cmc", determined by Eickes using electric field measurements. The other discontinuity is situated at lower AOT concentrations. The change of absorbance of the UV probe NMA+Clas a function of surfactant concentration is strongly different from that of PSA-Na+. The cationic probe reaches maximal absorption a t a concentration of about 2 X lo4 M. The probe PSA-Na+ starts absorbing at 8 X 10"' M. The absorbance is very small at low concentrations but increases abruptly when [AOT] equals the operational cmc, as determined by E i ~ k e . ~ 1.l.b. DAP. Aggregation behavior of DAP in c-CeH12 is studied by measuring the fluorescence decay parameters

ACHE E T A L

Figure 4. Influence of the DAP concentration in c-CSH12on (a) the fluorescence decay parameter ( 7 ) of NAA, (b)the absorbance (A) of PSA-Na+, and (c) the absorbance ( A ) of NMA'Cl-.

of NAA and PSA-Na+ and the absorbance of PSA-Na+ and NMA+Cl-. Results are given in Figure 4. They are very analogous to those obtained fromihe aggregation study of AOT. The fluorescence decay of NAA and PSA-Na+ in DAP/c-C6H12system are one exponential. A plot of the lifetime of NAA against the surfactant concentration shows two discontinuities: one at 2 X M and the other at 2X M of DAP. This last concentration coincides with the concentration where the lifetime of PSA-Na+ suddenly decreases and coincides with the concentration where Ache' found an abrupt decrease of the probability of thermal o-positronium formation in positron annihilation measurements. For the absorbance measurements of PSA-Na+ and NMA+Cl- in DAP, the results were also very analogous to those in AOT. While NMA+Cl- is already totally associated to the aggregates at a surfactant concentration of 2 X M, PSA-Na+ starts to associate at 8 X lo4 M and M. reaches its maximum of absorbance at 8 X 1.1.c. DDDAC. The results of the fluorescence decays of NAA and PSA-Na+ against the concentration of DDDAC are shown in Figures 5 and 6, respectively. Absorption of PSA-Na+ and NMA+Cl- in DDDAC solutions are also given in Figure 6. In contrast to the other surfactant systems, the decay curves of NAA and PSA-Na+ in DDDAC are two exponential and can be described by the following equation: It = A , exp(-t/r,) + AI exp(-t/rJ (4) where A indicates the contribution of the short (s) or the long (1) decay parameters ( 7 ) . The sum of A , and AI is always 100%. From other measurements" it is known that this is due to two different association forms (AF) of the aromatic (17) Viaene, K.; Verbeeck, A.; Gelad6, E.; De Schryver, F. C., unpublished results.

Aggregation Behavior in Inverse Micellar Systems

Langmuir, Vol. 2, No. 4, 1986 451

"10 K L

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Figure 5. Influence of the DDDAC concentration in c-C6Hlpon the contribution (7%)of the long fluorescence decay parameter (q),the long decay parameter ( T I ) , and the short decay parameter ( T J of NAA.

chromophores with the tetraalkylammonium head group of DDDAC (Figure 7). The one which gives rise to the short decay parameter will be called AF, and the other AF,. Fluorescence decay measurements of NAA and PSA-Na+ show a sudden change in the contribution of q (9671) to the fluorescence decay curves at 2 ' X lo9 M of DDDAC. At the same concentration both U V probes reach their maximum of absorption. The 7 , value of NAA and the T~ value of PSA-Na' show a discontinuity at the same concentration range. In the plots of q against [DDDAC] for both fluorescence probes, a second inflection point is observed at lower concentration. 1.2. Discussion. The results shown in Figures 3 to 6 can only be explained by the E model for aggregation of surfactants in apolar medium. Description of the aggregation by the E model is visualized in Figure 1. At low concentrations linear aggregates are formed (region I); at high concentrations cyclic micelles are present (region 11). The cmc is the detergent concentration where linear structures transform to cyclic ones and is therefore called the "cyclic micellar concentration". Evidence for the association of surfactant monomers at low concentrations to M) is given by measurements of the absorbance of PSA-Na+ and NMA+Cl- and by the fluorescence decay parameters ( T ) of NAA in the surfactant solutions. At the lowest concentrations of AOT and DAP, the lifetime of NAA increases, indicating a decrease of the polarity of the bulk medium. This decrease in polarity results from aggregation of the surfactant monomers. At a certain concentration, the fluorescence decay parameter of NAA suddenly diminishes. This sudden increase in polarity of the microenvironment is due to the association of the probe to the aggregates. This sudden decrease of the decay parameter (7,) of NAA is also noticed in solutions of DDDAC. So, the discontinuity in the changes of fluorescence decay parameter of NAA in the concentration range of 5

Figure 6. Influence of the DDDAC concentration in c-C6H11on the contribution (%) of the long fluorescence decay parameter (73, the long decay parameter ( T J , the short decay parameter (TA of PSA-Na+, and the absorbance (A) of PSA-Na+ (0)and NMA+Cl- ( 7 ) .

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Figure 7. Two possible association forms (AF) of NAA with the tetraalkylammonium head group of DDDAC, resulting in either a short (s) or a long (1) fluorescence decay parameter ( T ) . X lo4 to 5 X M shows the aggregation of the probe with the aggregates. The concentration where this association occurs and the degree of association is a function of the surfactant and the probe used. This is clearly demonstrated by the absorption measurements. Although the association of a probe to an aggregate is specific for each probesurfadant system, the fact that the different probes in the three different surfactant systems absorb at low surfactant concentrations is another indirect proof for the existence of aggregates at these low concentrations. Once the probe associates with the aggregates, the decay time ( 7 , q, T J of the probe keeps changing when the de-

452 Langmuir, Vol. 2, No. 4, 1986 N "0

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tergent concentration is increased. These variations in probe environment indicate the continuous growth of the aggregates on increasing the concentration. Besides aggregation via a SA mechanism starting at low surfactant concentrations, the E model also implies a structural transformation of the aggregates a t a certain surfactant concentration, the cmc. Evidence for this structure transformation is given by fluorescence decay parameters of NAA and PSA-Na+ in DDDAC. The probability of the association form AF1suddenly increases in the concentration range to 5 X M (Figures 5 and 6 ) . This sudden change of % q as a function of the concentration indicates a structure change of the aggregates formed and can only be explained by a transformation of linear aggregates to cyclic ones. In cyclic micelles AF, is much less favorable (cf. NAA in DDDAC) than in the linear aggregates or even not possible (cf. PSA-Na+ in DDDAC) (Figure 8). As shown in Figure 8 a structure transformation changes the microenvironment of the probes and is therefore also detectable in fluorescence decay measurements of NAA in AOT and DAP. This transformation of linear to cyclic aggregates is responsible for the second discontinuity in the curves of the decay time of NAA (7)as a function of the concentration of surfactant. In both cases, the decay time of NAA decreases which shows a closer approach of the chromophore to the polar head groups. Also T, of NAA in DDDAC diminishes at concentrations higher than the

cmc. This indicates a higher local concentration of polar head groups around the chromophore in association form AF,. These changes of decay time are additional proof for a transformation of linear to cyclic structures. The change in decay time of the probes in the cyclic aggregates when varying the surfactant concentration indicates a further growth of the aggregates on increasing the concentration after the cmc. The concentration of this structure transformation (cmc) can also be determined by association of the probes to the aggregates as a function of the surfactant concentration, measured by absorption spectracopy, but only if the probe is not already totally dissolved at this concentration. The strong increase of absorbance at the cmc can be explained in two ways: cyclic aggregates can solubilize more chromophores than linear ones or at the cmc suddenly more aggregates are formed. On the basis of the above reported measurements, no distinction can be made on this point. Using the different methods, we were able to show there is one universal model for the aggregation of ionic surfactants in apolar medium. Nevertheless, one must be cautious when using only one of these methods to determine the cmc. For instance,absorbance measurements can give false cmc's. If the probe is already totally associated before, only the concentration of association of the probe to the aggregates can be measured, but not the concentration of structure transformation. Also in measurements of decay parameters, the choice of the probe is important. The probe location in the linear and the cyclic aggregates must be sufficiently different to make a precise determination of the cmc possible (see also next section). 2. Influence of Water on the Aggregation. 2.1. Results. 2.l.a. AOT. The effect of the addition of water to the surfactant solutions in c-C6HI2was studied by measuring the absorbance of PSA-Na+ and NMA+Cl- and the fluorescence decay parameters of NAA and PSA-Na+. Three different concentrations of water were examined: R = 3.7, 6.8, and 13.8 (Figure 9) (R = [H20]/[S]). Increasing the concentration of solubilized water, the curves representing the decay time of NAA against [AOT] show some differences. The discontinuity according to the b

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Langmuir, Vol. 2, No. 4, 1986 453

Aggregation Behavior in Inverse Micellar Systems 511 n s l d i v . 1

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Figure 10. Influence of water in solutions of DAP in c-CSHI2on (a) the fluorescence decay parameter ( 7 ) of NAA, (b) the absorbance (A) of PSA-Na+, and (c) the fluorescence decay parameter ( 7 ) of PSA-Na+. Influence of the temperature on the fluorescence decay parameter ( T ) of PSA-Na+ in DAP/c-C6Hl2(d) T = 20 "C (0),T = 40 "C (A),and T = 60 O C (A).

cmc disappears at the two highest water concentrations measured, while the concentration of association of NAA to the aggregates is more pronounced. By use of PSA-Na' as fluorescence probe, the break in the curves of lifetime against concentration that corresponds to the cmc remains visible and unchanged to that in surfactant solutions in dry C-C~HI~. Variation of absorbance of NMA+Cl- is analogous to that of solutions of AOT in dried c-C6HI2. For PSA-Na+, the absorption a t 5 X M increases as the concentration of water increases and reaches ita maximal value for R = 13.8. For the two other water concentrations, the discontinuity according to the cmc remains visible and at the same concentration as for R = 0. 2.1.b. DAP. The influence of water on the aggregation of the system DAP/c-C6H12was studied by measuring the absorbance of PSA-Na+ and the fluorescence decay parameters of NAA and PSA-Na+. Three different water concentrations were examined: R = 1.4, 2.2, and 4.1 (Figure 10). Concentrations of water, higher than R = 4.1, were not soluble in the aggregates. The variations of the lifetime of NAA as a function of the concentration of surfactant show again two discontinuities: one for the association of the probe to the aggregates, the other for the cmc. Both discontinuities occur a t the same concentrations as those for solutions in cC6H12, and this for all measured concentrations of water. The cmc, however, is less pronounced for the highest R values. When measuring lifetimes of PSA-Na+, a much larger discontinuity at the cmc of DAP is observed. Absorbance of PSA-Na+ already reaches a maximum at M of DAP, the concentration of association for PSA-Na+ to aggregates of DAP in c-C6Hlp. 2.l.c. DDDAC. The effect of water on the aggregation of DDDAC in apolar medium was examined by measuring the absorbance and the fluorescence decay parameters of PSA-Na+ in this system. Two different concentrations of water, solubilized in the aggregates, have been studied R = 1 and 40 (Figure 11). Concentrations of water in between these two were not studied since no clear solutions for these R values could be obtained at room temperature (Figure 12).

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Figure 11. Influence of water in solutions of DDDAC in c-C&,z on (a) the contribution (%) of the long fluorescence decay parameter (73, (b) the long decay parameter (TJ, (c) the short decay parameter (T~),(d) the absorbance (A) of PSA-Na', and (e) the absorbance (A) of NMA+Cl-. Three concentrationsof water are examined: R = 0 ( O ) , R = 1 (o),and R = 40 (0).

Analyses of the fluorescence decay curves of PSA-Na+ in the solutions show that the sudden increase of % 71 as a function of [DDDAC] occurs at M for both R = 1 and 40, the cmc for DDDAC in c-C6Hl2. At the same

454 Langmuir, Vol. 2, No. 4, 1986

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surfactant concentration, the discontinuity in the variation of q against the surfactant concentration is more pronounced when the amount of solubilized water increases, while the decrease to T~ about lo-* M of DDDAC disappears at the highest concentration of water. The probe PSA-Na+ already reaches a maximum of absorbance in DDDAC/c-C6HI2/watersolutions at lo4 M of surfactant. 2.2. Discussion. The E model remains adequate upon addition of water to the surfactant solutions to describe the aggregation in apolar solvents. At low surfactant concentrations linear aggregates are formed (region I). These structures cyclize at a certain concentration (cmc) to form inverse micelles (region 11). The added water molecules greatly affect the absorbance measurements of PSA-Na+ in the surfactant solutions. In contrast to the solutions in dry C-C&12, the probe is much more or even totally dissolved at the association point of the probe to the aggregates. A combination of two effects explains this phenomenon. First, the probe is more easily solubilized in the aggregates because of the possibility of H-bridge formation with the added water molecules. Second, an increased degree of aggregation at low surfactant concentrations, caused by the presence of water molecules, is responsible for the high absorbance of PSA-Na' at these low concentrations. Water molecules seem to stimulate the formation of more and/or larger aggregates. This is in agreement with results, published by Wagner.18 Evidence for this is also given by measurements of the fluorescence decay parameters of PSA-Na' on the three surfactant systems. In region I, where linear aggregates are formed, less variation in the lifetime (7 or q) of the probes (NAA and PSA-Na+) is noticed on addition of water. This vaiiation, indicating a change of microenvironment, is due to the growth of initially small aggregates to larger ones (see last section). In the presence of water, large aggregates are formed from the beginning. The water, solubilized in the aggregates, can influence the location of the probes in the linear and/or cyclic aggregates. As a result, the structural transformation has a large or a small influence on the probe environment. This explains why the cmc is less (cf. NAA in DAP or AOT) or more (cf. PSA-Na+ in DDDAC) pronounced compared with that in apolar medium. So, the choice of &probefor thedetermination of the cmc is very important. This fact is clearly illustrated by comparing the determination of the cmc of DAP and AOT in the presence of (18)Wagner, C. Colloid Polym. Sci. 1976, 254, 400.

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parameter (T) of PSA-Na+ as a function of AOT concentration in cyclohexane, for two water concentrations: (a) R = 3.7 and (b) R = 13.8. Three different temperatures were measured: T = 20 "C ( O ) , T = 40 "c (A),and T = 60 "C (A).

water by NAA or PSA-Na+ as probe (Figures 9 and 10). Hence, water molecules stimulate the aggregation of ionic surfactant monomers. As a consequence of the increased ease of aggregation and the affinity of the chromophore PSA-Na' for water molecules, absorbance measurements are mostly useless in the determination of the cmc when water is added. Measurements of fluorescence decay parameters remain useful in the study of the aggregation process. It is however important to chose a probe that is located in a sufficiently different microenvironment in the linear aggregates compared to the cyclic ones to make the determination of the cmc possible. The most suitable probe in our system is PSA-Na+. Measurements of decay parameters of PSA-Na' in the three surfactant systems clearly show that the concentration domain where the linear aggregates cyclize is, within experimental error, not strongly affected by solubilized water in these aggregates. This is true for all ionic surfactants in apolar solvents. 3. Influence of Temperature on the Cmc. 3.1. Results. The effect of temperature on the aggregation behavior of the three ionic surfactants was studied by measuring the fluorescence decay parameters of PSA-Na+ in the systems. Aggregation in dry c-C,$,Z and after addition of water was examined at different temperatures. 3.l.a. AOT. The concentration of PSA-Na+ in solutions of AOT in dry c-C6HI2,at surfadant concentrations below the cmc, was too small for fluorescence decay measurements. So only two water concentrations were studied: R = 3.7 and 13.8 (Figure 13). Measurements at three different temperatures were done. At the low water concentration only a small shift and a broadening of the concentration range for the structural

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Figure 14. Influenceof the temperature on the contribution (%) of the long fluorescence decay parameter (q),the long decay ) PSA-Na+ parameter (q),and the short decay parameter ( T ~of for DDDAC solutions in C - C & at ~ ~two water concentrations: R = 1 (14.a) and R = 40 (14.b). Four temperatures were evaluated T = 20 OC (O),T = 30 "C (A),T=40 OC (A),and T=50 OC (+).

transformation (cmc) on increasing the temperature are noticed. After addition of more water (Figure 13b), however, the cmc shifts remarkably to higher concentration when increasing the temperature. 3.1.b. DAP. The effect of temperature was only studied for the aggregation in dry cyclohexane (Figure 10). In the temperature range studied (20-60 "C) almost no shift of the cmc to higher concentrations appeared. 3.l.c. DDDAC. The influence of the temperature was examined in the absence and in the presence of water. Measurements at four different temperatures are given in Figure 14, part a ( R = 0) and part b (R = 40). The cmc, clearly observed by the sudden break in the curves that represent T~ against the surfactant concentration and by the concentration where AF,becomes improbable (% T~ = loo%), remains constant on changing the temperature up to 50 "C. The 9%71 in region I gradually increases when the temperature is increased. 3.2. Discussion. In this study we measured the enthalpy change that corresponds to the structure formation of linear to cyclic structures. Especially the role of water in the stabilization of the aggregates formed is focused on. The results are qualitatively interpreted. For the surfactant solutions in dry c-C&Il2no (DDDAC) or only a small (DAP) shift of the cmc on increasing the temperature is noticed. Ache measured on analoguous shift of the cmc for DAP in c-C6H12with positron annih i l a t i ~ n .The ~ same author also reports that the cmc of

AOT in dry cyclohexane remains constant in the temperature range from 20 to 80 "C. These measurements suggest that the sum of interactions for the aggregation of surfactant monomers in linear and cyclic aggregates is almost the same. After addition of water to the solution, a strong difference between cationic and anionic surfactant systems is noticed. For the system AOT/c-C6H12the cmc shifts to higher concentrations when water is solubilized. The larger the volume of water added, the larger the shift of the cmc. For the cationic DDDAC, however, the cmc remains constant at increased temperature, even after addition of 40 molecules of water per detergent monomer. These results indicate that water strongly stabilizes inverse micelles if the polar head group is anionic. For cationic surfactants, water seems to have no influence on the enthalpy of the cyclization process. The reason for the difference between AOT and DDDAC is the interaction possibility of the polar head groups of the surfactant with the added water molecules. For AOT, not only the sulfonate group interacts strongly with water molecules (Hbridges, ion-dipole interactions, ...) but also the ester functions in the apolar tails can form H-bridges with the water. In contrast, the quaternary ammonium group of DDDAC is only very weakly hydrated. Increasing the temperature causes an increase of '30T~ in the fluorescence decay curves of PSA-Na+ at concentrations below the cmc. Probably, this effect is not due to changes in the aggregation process of DDDAC but to the influence of temperature on the complex formation of the chromophore with the quaternary ammonium head group of the surfactant (association form AF,). Complex formation diminishes a t increased temperature^.'^ This effect has no influence on the determination of the cmc, because association form AF, is not possible in the cyclic aggregates.

Conclusion The results illustrate that a combination of measurement of fluorescence decay parameters and W absorbance of several probes enables one to obtain information on the aggregation mechanism of ionic surfactants in apolar medium. For all the surfactant-solvent systems, an aggregation mechanism is proposed which is a function of the concentration of the surfactant, passing three regions: in the first region to M) linear aggregates of increasing length are formed which in a certain concentration range (cmc region: to M) undergo a structural reorganization to form cyclic aggregates of which the aggregation number further increases. The same mechanism was previously proposed by Eicke et al. to fit their electric field effect measurements on AOT.5 Such a mechanism could also explain most of the apparently contradictory results in l i t e r a t ~ r e .Indeed, ~ the constant increase of the number of monomers per aggregate agrees with a SA mechanism. On the other hand, the cyclization process is comparable to a kind of cooperative effect. Only one of these two elements in the aggregation process was noticed, depending on the method used to study the aggregation, leading to false conclusions. The same methods were used to determine the effect of the solubilized water in the aggregates and the temperature on the aggregation process. The measurements show that water stabilizes the inverse micelles, especially when the polar head groups of the surfactant interacts strongly with the water molecules. This interaction is also responsible for the increased tendency of aggregation of the monomers at low concentrations (region I).

Langmuir 1 9 8 6 , 2 , 4 5 6 - 4 6 0

456

The evidence presented in this paper leads to further support of some of the parameters important in inverse micelle formation and is in agreement with one of the present models for such systems. Although the aggregation of ionic surfactants in apolar medium corresponds to the E model, there still is a diversification of the surfactants according their stability. A good criterium for this diversification is the strength of interaction between the polar head groups and the water molecules. The stability of inverse micelles is an important parameter in their practical applications. Till now, AOT was, because of its high stability, the most widely used surfactant in the study of the applications of inverse micelles. Functionalizing the polar head groups of other, cationic,

surfactant molecules with groups that interact strongly with water molecules, through H-bond formation or iondipole interaction, might lead to an increase or even control of the stability of micelles formed with these detergents. Acknowledgment. Financial support by IWONL-Agfa, the FKFO, the University Research Fund, and the ERO Research Fund are gratefully acknowledged. A.V. and E.G. are indebted to IWONL and NFWO, respectively, for a fellowship. Registry No. AOT, 577-11-7; DAP, 17448-65-6; DDDAC, 3401-74-9; I-naphthaleneacetic acid, 86-87-3; sodium l-pyrenesulfonate, 59323-54-5; I-naphthylmethylammonium chloride, 4643-36-1; cyclohexane, 110-82-7.

Study of the Interaction between Arenes and Tetraalkylammonium Compounds in Homogeneous and Micellar Solutions K. Viaene, A. Verbeeck, E. Gelad6, and F. C. De Schryver" Department of Chemistry, K . U. Leuven, Celestijnenlaan 200F, B-3030 Heuerlee, Belgium Received December 9, 1985. In Final Form: March 12, 1986 In this work experimental evidence for strong interactions between arenes and quaternary akylammonium groups is presented. This interaction is noticed for trimethyl(cu-naphthylmethy1)arnmoniumchloride (NMeC1) in homogeneous solution. This resulta in a strong quenching of the arene fluorescence and in a modification of the absorption and 'H and I3C NMR spectra. From the temperature dependence of these phenomena, information about the thermodynamic properties of this interaction is gained. This interaction between arenes and quaternary ammonium groups is also observed in reverse micellar solution and allows more information about the location of the probes in the micelle and about the aggregation mechanism of the detergent molecules to be obtained.

Introduction Since 1979 differences between SDS and CTAX concerning solubility of probes,' 'H NMR results,2the 11/13 ratio, and lifetime of solubilized pyrene3 are interpreted as partly due to weak interactions between aromatic compounds and the quaternary ammonium head group (+NR4) of CTAX. In all cases, however, other explanations for the observed results are possible. In this work, experimental evidence for strong interactions between arenes and quaternary alkylammonium groups in homogeneous and heterogeneous medium is presented. This interaction results in a strong quenching of the arene fluorescence and in a change of the chemical shift of the NMR signal of several protons and carbon atoms. Experimental Section Trimethyl(a-naphthylmethy1)a"onium chloride (NMeC1)was prepared from the iodide salt by means of an ion exchanger (Merck (1) Almgren, M.; Grieser, F.; Thomas, J. K. J. Am. Chem. SOC.1979, 101, 279. (2) Fendler, J. H.; Fendler, E. J.; Infante, G. A.; Pong-Su Shih Patterson, L. K. J . Am. Chem. SOC.1975, 97, 89. (3) (a) Lianos, P.; V i o t , M. L.; Zana, R. J.Phys. Chem. 1984,88,1098. (b)Turro, N. J.; Kuo, P. L. Langmuir 1985, I, 170. (c) Kalyanasundaram, K.; Thomas, J. K. J.Am. Chem. SOC. 1977,99,2039. (d) Zachariasse, K. A.; Kozackiewicz, B.; Kuhnle, W. In Surfactants in Solution; Mittal, K. L., Lindman, B., Eds.; 1984; Vol. 1, p 565. (e) Nakajima, A. Bull. Chem. SOC.Jpn. 1977,50, 2473. (f) Deng, D. C.; Winnik, M. A. Can. J . Chem. 1984,62, 2560.

0743-746318612402-0456$01.50/0

4767).4 The iodide salt was synthesized as described in the literat~re.~a-Naphthylammoniumchloride (NHCl) was synthesized by titrating a solution of naphthylmethylamine (Aldrich) with HCl. After lyophilization, the NHCl was washed several times with acetone. Reagent grade 2-(l-naphthyl)aceticacid (NAA) (Aldrich99%) was recrystallized from water, followed by sublimation (130 "C at 5 mmHg pressure). The purity was controlled by HPLC. Sodium 1-pyrenesulfonate(PSA-Na+)was synthesized in our laboratory following the procedure of Tietze and Bayer.6 It was purified by stirring in methanol with Al,O,. The A1203 was filtered and the methanol evaporated. The synthesis of 4-(1naphthy1)butyricacid (NBA)has been described by Huisgen and Rietz.' It was purified by severalrecrystallizationsfrom petroleum ether. Didodecyldimethylammonium chloride (DDAC)was prepared from the bromide derivative (DDAB) (Aldrich)by ion exchange (Merck 4767) and purified by several recrystallizationsfrom ethyl acetate, dried over Na2C03. Distilled and deionized water was used in all experiments. Cyclohexane (Merck P.A.) was passed over a A1203/charcoal (5050) column, followed by distillation over sodium, and kept on molecular sieves (4 A). Absorption spectra were recorded with a Perkin-Elmer550 S spectrophotometer and corrected fluorescence spectra on a Spex-fluorolog. Fluorescence decay parameters were measured (4) Roelants, E.; Gelade, E.; Van Der Auweraer, M.; Croonen, Y.;De Schryver, F. C. J . Colloid Interface Sci. 1983, 96,288. (5) Sommer, H. 2.;Lipp, H. 1.; Jackson, L. L. J. Org. Chem. 1971,36, 824. (6) Tietze, E.; Bayer, 0 Ann. Chem. 1939, 540, 189. (7) Huisgen, R.; Rietz, 0. Tetrahedron 1958, 2, 279.

0 1986 American Chemical Society