J . Phys. Chem. 1984, 88, 2228-2235
2228
quantum yield of fluorescence is so large relative to that for phosphorescence, O2 quenching does not significantly alter the resulting spectrum and, therefore, the energetic onset appears unchanged. The same set of circumstances must obtain in other molecular systems as well. Acknowledgment. The authors acknowledge the contributions of Dr. Frederic Stoeckel, Dr. Sigeo Okajima, and Mr. Warren
Beck to the early instrumental aspects of this work. The work was supported by grants from the National Science Foundation and the donors of the Petroleum Research Fund, administered by the American Chemical Society. M.D.S. also acknowledges support from the Research Corporation and the North Carolina Board of Science and Technology. Registry No. C H 3 C H 0 , 75-07-0; CD,CDO, 1632-89-9.
Fluorescence Decay Study of the Adsorption of Nonionic Surfactants at the Solid-Liquid Interface. 1. Structure of the Adsorption Layer on a Hydrophilic Solid Pierre Levitz,* Henri Van Damme, and Didier Keravis C.N.R.S., Centre de Recherches sur les Solides, Organisation Cristalline Imparfaite, 45045 Orleans Cedex, France (Received: July 25, 1983)
We have extended to the adsorbed phase formed by a nonionic surfactant (Tritron X 100 (TX 100)) at the solid (silica)-aqueous solution interface the fluorescence decay methods developed by others for the determination of micellar aggregation numbers. The method is based on the exclusive solubilization of a strongly hydrophobic fluorescent probe molecule within the hydrophobic regions of the condensed molecular assemblies of the adsorbed layer. Pyrene is shown to be well suited for this purpose. The simultaneous analysis of the time laws for the pyrene monomer and excimer decays has been performed along the TX 100 adsorption isotherm from 0 = 0.07 to 0 = 1 (0 being the apparent surface coverage, i.e., the ratio “amount adsorbed/amount adsorbed at the plateau of the isotherm”). The main result is that from 0 = 0.07 to 0 = 0.8, the adsorbed layer has a fragmented (or micellar) structure whereas, beyond 0 = 0.8, it is better described as an “infinite”, or continuous, medium in which large diffusion paths exist for the probe molecules. The parallel determination of the average aggregation number (N)and aggregate number density (Na8)in the fragmented-phase domain shows three distinct regimes: (i) a growing regime, from 6’ = 0 to 0 = 0.17, where the size of the surface aggregates increases at constant number density; (ii) a self-repeating regime, from 6 = 0.17 to 0 0.5, where the number density increases, while the size of the aggregates remains constant; the surface aggregates in this regime are undistinguishable from the regular micelles formed by TX 100 in aqueous solution (N= 110); (iii) again a growing regime, up to a point (0 = 0.8) where the adsorbed phase can be described as an assembly of surface micelles (N= 200) in close packing. At this point, the transition from a fragmented-phase model to a continuous-phase model occurs.
-
I. Introduction The structure of the adsorbed phases formed by surfactant molecules on a solid surface, at the solid-solution interface, is a problem which, in spite of a rather large number of investigations, has not yet received a satisfying answer. Most studies in this field have indeed been restricted to the determination of the adsorption isotherms and, although several models were proposed on the basis of these adsorption data, very limited spectroscopic investigation of the organization of the adsorbed layer has been performed yet. The reason for this lack of information is undoubtedly that a direct spectroscopic study of molecules physically adsorbed at a solidsolution interface raises serious difficulties which are related, on the one hand, to the presence of the solution which introduces its own contribution to the observed signal and, on the other hand, to the dynamic character of adsorption which may average this contribution with that of the adsorbed phase. In the case of ionic surfactants, all the models proposed on the basis of the adsorption data admit the existence of condensed (or organized) molecular assemblies on the surface, either in the form of micellar-like aggregates (“hemimicelles”’) or in the form of more extended lamellar phase^.^,^ One of the strongest arguments in favor of the existence of such organized assemblies is the normalization (superposition), in the 0 vs. C/cmc plane, of all the adsorption isotherms obtained within a homogeneous series of surfactants, with variable alkyl chain length (0 being the ratio of the amount of adsorbed molecules to the amount adsorbed at (1) Gaudin, A. M.; Fuerstenau, D. W. Trans. AIME 1955, 202, 958.
(2) Cases, J. M.; Mutafschiev, B. Surf. Sci. 1968, 9, 57. (3) Cases, J. M. Bull. Mineral. 1979, 102, 684.
0022-3654/84/2088-2228$01.50/0
the plateau of the isotherm). This can indeed be readily understood if the local environment of the alkyl chains in the adsorption layer, all along the isotherm, is similar to the aliphatic core of micelles or bilayers. Much less work has been performed on the adsorption of nonionic surfactants, although a number of techniques including adsorption measurement^,^-^ electrophoretic mobility,’ flotation studies,’O and microcalorimetry’ have been used. The possible existence of organized assemblies of these molecules at the solid-solution interface has also to be considered. As far as the system that we will investigate here (poly(oxyethy1ene) alkylphenols adsorbed on silica from aqueous solutions) is concerned, two main arguments are in favor of the existence of such phases. The first, as will be shown in section IV, is the normalizability of the adsorption isotherms, as with the ionic surfactants. The second is spectroscopic in nature. In a previous paperI2 we showed by fluorescence decay spectroscopy (FDS) of the phenoxy group of the surfactant molecules that the local degree of association
’
(4) Hsiao, L.; Dunning, H. N. J . Phys. Chem. 1965, 59, 362. (5) Abe, R.; Kuno, H. Kolloid 2.Z . Polym. 1961, 181, 70. (6) Corkill, J. M.; Goodman, J. F.; Tate, J. R. Trans. Faraday Soc. 1966, 62, 979. (7) Mathai, K. G.; Ottewill, R. H. Trans. Faraday SOC.1966, 62, 750. (8) Fukushima, S.; Kumagai, S. J . Colloid Interjuce Sci. 1973, 42, 539. (9) Von, Rupprecht, H.; Liebl, H. Kolloid Z . Z . polymer 1972 250, 719. (10) ,Dore,, A. Thtse de Docteur es-Sciences Naturelles Appliquhs, University of Louvain, Belgium, 1981. (11) Rouquerol, J.; Partyka, S . J . Chem. Technol. Biotechnol. 1981,31, 584. (12) Levitz, P.; El Miri, A.; Keravis, D.; Van Damme, H., submitted to J . Colloid Interface Sci.
0 1 9 8 4 American Chemical Society
Adsorption of Nonionic Surfactants
The Journal of Physical Chemistry, Vol. 88. No. 11, 1984 2229
"t 100
L -1
I 0
I 11
I
c
r2 Ln ( C i C M C )
Figure 1. Normalized adsorption isotherms of TX 100 and TX 101 at 25 OC on silica gel. 6' is the ratio of the surface coverage to its value on the plateau. cmc(TX 100) = 2.8 X mol/L; cmc(TX 101) = 0.7 X mol/L. A: TX 100. 0 : TX 101.
of the adsorbed molecules (at equilibrium concentrations below the cmc) is independent of surface coverage up to 0 = 1. This is strong support for the existence of condensed assemblies in which the association of the aliphatic tails would create a hydrophobic core. However, our result does not allow for the determination of the size and morphology of these assemblies. It is indeed consistent as well with the existence of a self-repeating micellar-like adsorbed layer beyond some (very low) critical surface concentration, as with the existence of more extended and continuously growing regions. The aim of this paper is to characterize further the structure of the adsorption layer by FDS. Our approach is basically parallel to what has been developed for micellar solution^.'^^^^ The idea is to "solubilize" a hydrophobic fluorescent probe molecule within the hydrophobic regions of the adsorption layer and to determine, from the time law of the decay, the spatial extent of the regions in which the probe is free to move. Pyrene, owing to its combined fluorescent and hydrophobic properties, has been successfully used for this purpose in micellar and we used the same probe in the present work. Two conditions have to be fulfilled in order to obtain clean information about the condensed assemblies of the adsorbed phase by this method. First, the presence of the probe should not perturb the adsorption process. This will be shown to be the case in section IV. Second, the probe should explore, as selectively as possible, the regions of the surface that we are interested in. Partitioning of the probe molecules between two or several hydrophobic environments would clearly be detrimental to the quality of the information. Thus, measurements at equilibrium concentrations above the cmc have to be avoided since, under such conditions, the probe molecules would most probably partition between the condensed assemblies on the surface, on the one hand, and the micelles of the solution, on the other hand. Fortunately, the plateau of the adsorption isotherms is reached at equilibrium concentrations close to the cmc (Figures 1 and 2), so that the major part of the development of the adsorbed phase can be safely studied without interference with the micellar phase. Another process which has to be avoided, or at least controlled, is the direct adsorption of the probe molecules on the hydroxylated surface of silica, concomitantly to their solubilization in the condensed surfactant assemblies. In this respect also pyrene is an interesting probe molecule since its steady-state emission spectrum is very sensitive to the polarity of the environment.'* Hence, molecules directly adsorbed on the polar hydroxylated surface can be readily distinguished from those solubilized in the less polar condensed (13) Azzi, A. Q. Rev. Biophys. 1975, 8, 237. (14) Infelta, P.; Gratzel, M. J . Chem. Phys. 1979, 70, 179. (15) Lianos, P.; Zana, R. J . Phys. Chem. 1980, 84, 3339. (16) Lianos, P.; Dinh-Cao, M.; Lang, J.; Zana, R. J . Chim. Phys. Phys.-Chim. Bioi. 1981, 78, 497. (17) Lianos, P.; Zana, R. J . Colloid Interface Sci. 1981, 84, 100. (18) Kalyanasundaram, K.; Thomas, J. K. J . Am. Chem. SOC.1977, 99,
2039.
10 I
1
1
1
I
c
c (dmole/L)
Figure 2. Adsorption isotherm of TX 100 on silica gel at 25 O C . +:
Without pyrene. Sr : With pyrene; the pyrene-to-surfactant ratio goes from
to 2.5
X lo-*.
assemblies. As we will show in section IV, direct adsorption was always found to be negligible. All the elements are therefore assembled for a good characterization of the adsorbed condensed assemblies. 11. Experimental Section
Materials. The silica was a high-purity Sperosil (XOB015) from (Prolabo-Rh8ne Poulenc), with a BET surface area of 25 m2/g, a particle size between 40 and 100 km, and an average pore size larger than 1300 A. The surfactants were the widely studied polydisperse compounds Triton X 100 and Triton X 101 from Rohm and Haas. TX 100 and TX 101 are octyl- and nonylphenol poly(oxyethylenes), respectively, with an average of 9.5 oxyethylene units per molecule. Pyrene from EGA Chemie was used as received (purity >99%). Adsorption Isotherms and Sample Preparation. All the measurements were performed at 25 "C in distilled water (pH 6.5). The adsorption isotherms in pyrene-free solutions were determined by mixing known amounts of silica (- 100-300 mg) with a known volume (- 10-20 mL) of calibrated surfactant solution in a centrifugation cup. The mixture was shaken for 10 h. This was found to be more than enough for the adsorption equilibrium to be reached. The mixture was then centrifuged and the final concentration of the supernatant was determined spectrophotometrically from the optical density at 276 nm. The amount adsorbed was calculated from the difference between the initial and final concentrations. The same general procedure was used with pyrene-containing solutions. A stock solution of pyrene was first prepared by introducing solid pyrene into a concentrated ( mol/L) micellar solution of surfactant. Solubilization in the micelles was obtained by stirring this solution for at least 1 or 2 days. The micellar solutions used for the adsorption isotherms were obtained by diluting the stock solution. The initial and final pyrene concentrations in the supernatant were determined from the optical density at 320 and 337 nm. The surfactant concentration was still determined at 276 nm, but a correction was introduced for the absorbance of pyrene at this wavelength. Calibration curves were therefore established for pyrene at 337, 320, and 276 nm. For each wavelength, two different extinction coefficients were found: one below and another above the cmc. Fluorescence Measurements. Steady-state fluorescence spectra were recorded at a FICA 55 Baush and Lomb spectrometer. The spectrum of pyrene was recorded between 350 and 500 nm, with excitation at 337 nm. The fluorescence decay spectrometer was a single photon counting EGG-Applied Photophysics instrument. The flash lamp was used at a repetition rate of 30 kHz. Excitation was performed at 320 nm. The pyrene monomer emission was monitored at 385 nm and the excimer emission at 490 nm. Time calibration was performed with deaerated aqueous solutions of
Levitz et al.
2230 The Journal of Physical Chemistry, Vol. 88, No. 11, 1984 tris(2,2'-bipyridine)ruthenium(II) (650 ns)I9 and pyrene ( mol/L, 180 ns).I8 All the solution samples (micellar or not) were thoroughly deaerated prior to measurements by bubbling dry nitrogen into the 10 mm path quartz cell for 30 min. The silica suspensions were placed in a 2 mm path quartz cell, at -45O to the excitation beam, with enough supernatant to keep the silica entirely in the liquid. 111. Pyrene Monomer and Excimer Decay in Continuous and Fragmented Media Before we report and discuss our results, it may be useful to recall some fundamental points about the time laws of the monomeric and excimeric emission of pyrene, evidenced by other authors. The important processes are P hvo P* (1)
+
P*
Pz*
P
+
P* P*
+ -
ki'
ki'"
hvl
P
(3)
+ P == Pz* k, ke
-+ ki
P
P
(2)
+ hvz
(4) (5)
klr, klnr and kzr, kznr are the radiative and nonradiative rate constants for the monomer and excimer, respectively. k, and k, are the rate constants for excimer formation and dissociation, respectively. There is experimental evidence, from the determination of aggregation numbers in micellar solution^,^^-'^^^^ that kZr kznr>> k,. In other words, the nonradiative dissociation of the excimer may be neglected. With this assumption in mind, we now compare the monomer and excimer decays in two different types of medium: continuous or "infinite" systems, on the one hand, and fragmented of "finite" systems, on the other hand. An infinite medium can be defined as a medium formed by only one large system in which the reactants are homogeneously distributed and where classical diffusion-reaction theories apply, or by several subsystems of this type, sufficiently large for the probe concentration variations from one subsystem to another to be negligible. A finite or fragmented medium can be defined as a system formed by several subsystems sufficiently small and disconnected (slow exchange rates between subsystems) to lead to important probe concentration variations from one subsystem to another. Typical examples of infinite media for pyrene would be an ordinary solution in any organic solvent, or a micellar solution containing large lamellar bilayers, each lamella being large enough to contain a large number of pyrene molecules. A typical example of a fragmented system would be an ensemble of pyrene molecules solubilized in an ordinary micellar solution of small spherical aggregates. Continuous Medium. In a continuous medium, the distribution of the fluorescent probe molecules is homogeneous. The pyrene monomer decay law is a single-exponential function of time:
+
-
Z,(t) exp(-klt - k,"[P]t) (7) with k l = klr klnr. k," is the second-order rate constant for dynamic quenching through excimer formation. [PI is the pyrene concentration. The time dependence of excimeric emission is given by the following expression: exp(-k,t - k,"[,P]t) - exp(-k2t)
+
U t )
-
kz - kl
- k,"[P]
(8)
+
with k2 = kzr kznr. Equation 8 accounts for the familiar rise (at short time) and decay (at long time) of excimeric emission.
At low pyrene concentration, when k,"[P] < k, - kl (this condition will be fulfilled in our experiments), the decay at long time is controlled by the excited-monomer population. At long time, eq 8 reduces to the first exponential component and the monomer and excimer decays run parallel to each other (in logarithmic scale). Fragmented Medium. We will be interested in a micellar-like medium formed by molecular aggregates with an average aggregation number N . The distribution of solubilizates (probes) among such aggregates has been extensively studied in recent year^.^'-^^ It is usually assumed that when the average number of probe molecules per aggregate, li, is small (- l), the equilibrium distribution of probe molecules over the aggregates follows Poisson statistics. As pointed out by Lianos et al.,Is the use of Poisson statistics requires that there be no limit to the number of solubilizates which can be accepted by each aggregate and that there be equal probability of occupation for all aggregates. When li is small, close to or smaller than 1, few aggregates contain more than two probe molecules and the first condition is not restrictive. The second condition would only be strictly met in the case of monodisperse aggregates. Under these conditions, the time dependence of pyrene monomer fluorescence following pulsed excitation iszo (k,
