Langmuir 1990,6, 522-524
522
Notes The important experimental tool here is the ratio ZE/ ZM of the pyrene monomer emission intensity to the pyrene
Interaction of Fluorescent Dye Labeled (Hydroxypropy1)cellulose with Nonionic Surfactants Francoise M. Winnik Xerox Research Center of Canada, 2660 Speakman Drive, Mississauga, Ontario, L5K 2Ll Canada Receiued April 14, 1989. In Final Form: July 17, 1989 Introduction Recently, Brackman et al. reported the formation of complexesbetween a neutral water-soluble polymer, poly(propylene oxide) (PPO, MW 1OOO) and a neutral surfactant, n-octyl 0-D-thioglucopyranoside (OTG).' An important feature of this work is that it is the first account of the unambiguous detection of complexes between neutral polymers and neutral surfactants. Until now it was generally believed that neutral surfactants are indifferent toward neutral polymers.'12 Brackman et al. detected polymer/micelle complexes by microcalorimetry and turbidity measurements. They also reported that the surfactant has the same critical micelle concentration (cmc) in the presence of PPO as it does in water. This is in contrast to the situation encountered with all ionic surfactants, for which association with neutral polymers is always accompanied by a decrease in cmc.' It is only for the PPO-OTG system that Brackman et al. reported unequivocal evidence for polymer/micelle complexation, although they carried out measurements also with poly(ethy1ene oxide), poly(N-vinylpyrrolidone), poly(vinyl alcohol)-poly(viny1acetate) copolymer, and (hydroxypropy1)cellulose (HPC). They found that the cmc of the surfactant did not change in the presence of those polymers. Nevertheless, it is not excluded that complexes were formed but remained undetected by the measurements. This note reports evidence for the formation of complexes between HPC and two neutral surfactants: OTG and n-octyl 0-D-glucopyranoside(OG). The evidence stems from experiments with a pyrene-labeled (hydroxypropy1)cellulose (HPC/Py). OH
OH
I
ow
OCH2CHCH3
AH
&H2CHCH3
I
I
ocn2cncn3
I
OH
OCH2y13
(1) Brackman, J. C.; van Os, N. M.; Engberta, J. B. F. M. Langmuir 1988,4,1266. (2) For two studies of neutral polymer/neutral surfactant interactions, see: Szmerekovi, V.; Krtilik, P.; Berek, D. J . Chromutogr. 1984, 285, 188. Boscher, Y.; Lafuma, F.; Ouivoron, C. Polym. Bull. 1983, 9, 533. (3) For reviews, see, for example: Goddard, E. D. Colloids Surf. 1986, 19, 255.
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excimer emission intensity and its changes as a function of surfactant concentration. Pyrene excimer emission from HPC/Py in water originates mostly from preformed pyrene dimers or aggregates stabilized by hydrophobic b ~ n d i n g .They ~ may occur both within a polymer chain and between pyrenes from different chains. The addition of ionic surfactants to aqueous solutions of HPC/ Py causes a disruption of the pyrene aggregates, resulting in a decrease in pyrene excimer emission together with an increase in pyrene monomer emission.' Hence, a transition in a plot of ZE/ZM as a function of surfactant concentration signals interaction of the surfactant with the pyrene label and consequently binding of the surfactant to the polymer. In this study with OG and OTG, two kinds of experiments were performed. In one, dilute solutions of the labeled polymer were treated with increasing amounts of surfactants, and changes in ZE/ZM were monitored. In a second experiment, small amounts of HPC/Py were added to a concentrated solution of unlabeled HPC. The fluorescence of this solution was monitored as a function of surfactant concentration. The first experiment probes the interactions between the labeled polymer and the surfactants. In the second experiment, HPC/Py is a polymeric probe. It reports on interactions between the surfactants and HPC itself. Experimental Section Materials. Ethyl 8-D-thioglucopyranoside(ETG), n-octyl 0-D-thioglucopyranoside (OTG), and n-octyl 0-D-glucopyranoside (OG)were purchased from Sigma Chemical Co. and used without further purification. (Hydroxypropy1)cellulos.e (HPC, Aldrich, average MW 100000) was used as received. Pyrene-labeled HPC (HPC/Py) was prepared from HPC (MW 100 000) as previously described.6 It contains 5.33 X lo-' mol of Py/g of polymer or on average 1 pyrene for 56 glucose units. Water was deionized with a Millipore Milli-Q water purification system. Fluorescence Measurements. The fluorescencespectra were recorded at 25 *C on a SPEX Fluorolog 212 spectrometer equipped with a DM3000F data system. Emission spectra were not corrected. The excimer to monomer intensities ratio (IE/ ZM) was calculated by taking the ratio of the emission intensity at 477 nm to the half-sum of the emission intensities at 376 and 396 nm. Solutions for analysis were prepared by allowing HPC/Py (ca. 60 mg) to dissolve in water (100 mL). Aliquots (10 mL) of this solution were added to a sufficient quantity of OTG, OG, or ETG to yield ca. 4 X lo-' M surfactant concentration when diluted to 25 mL. Solutions of lower surfactant concentrations were obtained by diluting these solutions with an aqueous HPC/Py solution of identical polymer concentration. Samples with the labeled (HPC/Py, 25 ppm) and unlabeled (HPC, 5 g L-') polymers were prepared 24 h prior to surfactant addition. All the surfactant/polymer solutions were allowed to stay at room temperature for 2 h prior to fluorescence measurements. (4) Winnik, F. M.; Winnik, M. A.; Tazuke, S.; Ober, C. K. Macromolecules 1987, 20, 38. Yamazaki, I.; Winnik, F. M.; Winnik, M. A.: Tazuke, S. J. Chem. Phys. 1987, 91, 4213. (5) Winnik, F. M.; Winnik, M. A.; Tazuke, S.J. Phys. Chem. 1987, 91, 594. (6) Winnik, F. M. Macromolecules 1987,20, 2745.
0 1990 American Chemical Society
Langmuir, Vol. 6, No. 2, 1990 523
Notes
-
WAVELENGTH (nm)
-4.0
-3.0
-2.0
- 1.0
log [additive]
Figure 1. Fluorescence spectra of HPC/ (25 ppm) in water for a series of n-octyl 8-D-glucopyranosi e (OG) concentrations: ,A, = 330 nm; shaded spectrum, [OG]= 0.
3
0.8
0
,
I
Figure 3. Plot of I,/I for aqueous HPC/Py (25 ppm) a6 a function of added 8-r&oglucopyranoside: (a) OTG in water; (b) OTG in water containing HPC (5 g L-'); (e) ETG in water.
log [OG]
Figure 2. Plot of IE/I for aqueous HPC/Py (25 ppm) as a function of added n-octy&Dglucopyranoside (OG):(a)in water; (h) in water containing HPC (5 g L-9.
Results and Discussion Fluorescence spectra of HPC/Py are shown in Figure 1for solutions in water and in the presence of n-octyl 0o-ghcopyranoside. In pure water, the emission is characterized by a broad excimer emission (of intensity IE) centered at 477 nm and a well-resolved pyrene "monomer" emission (of intensity 1,) with the [O,O]band at 376 nm. Addition of OG has no effect on the spectrum of HPC/Py until its concentration exceeds 2.5 X lo-' M. Then it causes a decrease in the intensity of the excimer and a small shift in its maximum, as well as an increase in pyrene monomer emission intensity. All these changes occur over the narrow OG concentration range of 2.5 X lo-' to ca. 4 X lo-' M, that is, just above the cmc of OG (2.3 X lo-' M).' Similar phenomena take place with solutions of unlabeled HPC containing small amounts of HPC/ Py. In this case though, in the absence of surfactant, the excimer emission relative to monomer emission is weaker than in dilute solutions of HPC/Py alone. One way to explain this is that in the presence of large amounts of HPC interpolymeric pyrenelpyrene aggregates are mostly broken, and only intrachain aggregates persist. When OG is added to this solution, it causes a drop in IE/IM of smaller amplitude than in dilute solutions. Nevertheless, the transition takes place at the same surfactant concentration (ca. 2.5 X lo-' M) as in the dilute case (Figure 2). When solutions of HPC/Py mixed with HPC are subjected to increasing amounts of the sulfur-containing surfactant OTG, a sharp decrease in IE/IM occurs for OTG (7) de Grip, W. J.; Bavee-Geurls, P. 1979,23, 321.
H.M. Chem. Phys. Lipids
concentrations between lo-' and 3 X lo-' M, that is, just above the cmc of OTG (9 X M).8 No changes in the monomer or excimer emissions are observed a t lower surfactant concentrations (Figure 3). In the case of dilute HPC/Py solutions, however, a small decrease in IE/IM is detected a t lower surfactant concentrations, between 2 X 10' and 9 X lo-' M. This small gradual decrease is then followed by a sharp drop in excimer emission when the OTG concentration exceeds the cmc value (Figure 3). The situation in the HPC-OTG system is more complex than the HPC-OG system because the thioglncopyranoside group is a weak quencher of pyrene fluorescence. Therefore, with OTG the fluorescence experimenta report on two phenomena occurring simultaneously the binding of the surfactant to the polymer and the diffusion-controlled quenching of pyrene emission by the sulfide group. In order to separate the quenching from other effects, control experiments were carried out with ethyl 0-D-thioglucopyranoside(ETG), a short alkyl chain analogue of OTG. Addition of ETG to dilute solutions of HPC/Py results in a ca. 20% decrease in pyrene monomer emission intensity and a ca. 40% decrease in excimer emission intensity, for an ETG concentration of 2 X lo-' M. This quenching is reflected by a small gradual decrease in IE/lMwith increasing amounts of ETG (Figure 3). However, the overall effect of ETG on the fluorescence of HPC/Py is much less pronounced than that of OTG, especially when the additive concentration exceeds 2 X lo-' M. In the case of the HPC-OTG system, association of the surfactant to the polymer is the predominant factor affecting the spectroscopy of the labeled polymer. Monitoring separately the changes in (8) Saito, S.: Tsuchiya, T. Biochem. J. 1981,222,829. Bmckman et al.' report emc values for OTG ranging between 8.05 X 1CP and 9.2 x lo-* M, depending on the measurement technique.
524
Langmuir 1990, 6, 524-525
monomer and excimer emission in this case reveals that for OTG concentrations lower than the cmc the monomer emission remains constant but the excimer emission decreases. It seems that the sulfur-containing surfactant has a tendency to associate with pyrene dimers while leaving isolated pyrenes unaffected. The fluorescence studies described here uncover the existence of interactions between HPC/Py and OG or OTG, either in dilute solutions or in the presence of large amounts of HPC. Addition of the surfactants at concentrations in the vicinity of their cmc has profound effects on the solution properties of the polymers: it disrupts polymer/polymer aggregates and it modifies the conformation of polymer chains (Figure 4). These phenomena
can be ascribed to the formation of complexes between HPC and OTG or OG micelles, as proposed in the case of ionic surfactant^.^ This note brings forward new evidence for the existence of neutral polymer/neutral micelle complexes. It emphasizes the need for a deeper fundamental understanding of the polymer/surfactant interactions in the absence of electrostatic force^.^ Registry No. OG, 29836-26-8; OTG, 85618-21-9;HPC, 900464-2. (9) Theoretical models of polymer/surfactant interactions are described in: (a) Nagarajan, R. Adu. Colloid Interface Sci. 1986,26, 205. (b)Ruckenstein, E.;Huber, G.; Hoffman, H. Langmuir 1987,3, 382.
S D S in water / hexane
Interfacial Tension of Sodium Dodecyl Sulfate Solutions at the Hexane-Water Interface
0(103~/m) 50 1
P. Joos,*-+ D. Vollhardt,*and M. Vermeulen' Departments of Biochemistry and Chemistry, University of Antwerp, Universiteitsplein 1 , 2610 Wilrijk, Belgium, and Akademie der Wissenschaften der DDR, Zentral institut fur organische chemie, Rudower Chaussee 5, 1199 Berlin Adlershof, DDR Received June 7, 1989. In Final Form: November I, 1989
Introduction It is well-known that the surface tensions of aqueous sodium dodecyl sulfate (SDS) solutions show a minimum in the surface tension-concentration curve, due to the presence of a highly surface-active minor component: dode~anol.l-~ However, no minimum is observed in the interfacial tension-concentration curve at the oilwater interface, because of the solubility of dodecanol in the oil Recently we have studied the interfacial behavior of alkanols at the hexane-water interface during their diffusion from one phase to another.6 It is the aim of this paper to study the influence of dodecanol on the interfacial tension of an aqueous SDS solution with hexane, taking into account this study.
Experimental Section The interfacial tensions are measured with the ring method of De Nouy, taking into account the correction factors of Harkin^.^ A pure SDS sample is obtained by the method of foaming.",' No minimum is observed in the surface tensionconcentration curve or in the interfacial tension-concentration curve. This pure SDS sample is contaminated with known amounts (0.1 and 2 mol %) of dodecanol, and the interfacial tension-concentration curve is measured again. Hexane and dodecanol are of analar grade. Water is demineralized and distilled twice, and attention is payed to the cleanness of the glassware,which is treated with sulphochromicacid. University of Antwerp.
* Zentral institut fur organische chemie.
(1)Miles, G. D. J. Phys. Chem. 1945,49,71. (2)Miles, G.D.;Shedlovski, L. J. Phys. Chem. 1944,48,57. (3)Elworthy, P.;Mysels, K. J. Colloid Interface Sci. 1966,21, 331. 1954,50,874. (4)Cockbain, E.G.Trans. Faraday SOC. (5)Hutchinson, E.J. Colloid Sci. 1948,3,521. (6)Van hunsel, J.; Joos, P. Langmuir 1987,3, 1069. (7)Czichocki, G.;Vollhardt, D.; Seibt, H. Tenside 1981,18, 320. (8)Vollhardt, D.;Czichocki, G., to be submitted. (9)Harkins, W.D.;Jordan, H. F. J. Am. Chem. SOC.1930,52,1751. (10)Lucassen-Reynders, E. H.J . Phys. Chem. 1966,70,1777.
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I -7
-6
-5 l o g m
Figure 1. Interfacial tensions of aqueous SDS solutions against SDS purihexane as a function of the SDS concentration: (0) fied by foaming; (+) 0.1 mol % dodecanol added to the pure SDS sample; ( 0 )2 mol % dodecanol added to the pure SDS sample. The solid line is calculated with uo = 5.1 X N/m, "'I = 2.5 X mol/cm2,and a = 2.5 X lo-" mol/cms.
All experiments are performed at room temperature (22 i 2 "C).
Results and Discussion The interfacial tensions of pure and contaminated SDS solutions at the hexane-water interface are measured and the results given in Figure 1. No significant difference is found between the interfacial tensions of pure SDS solutions and those of contaminated SDS solutions. The interfacial tension (a)-concentration curve is described approximately by the Von Szyskowski equation for ionized surfactants: c = a,,- 2RTr" In
a
where a. is the interfacial tension of the hexane-water interface in absence of all surfactants, I"the saturation adsorption, a the Langmuir-Von Szyskowski constant, and C, and C- the concentrations of cations and anions, respectively. Since no indifferent electrolyte is added, C, = C- and eq 1 becomes 0 1990 American Chemical Society
