Langmuir 1995,11, 1464- 1467
1464
Self-Diffusion Studies on cu-MethoxyPolyethylene Glycol Macromonomer Micelles by Using Cyclic Voltammetric and Fourier Transform Pulsed Gradient Spin-Echo Nuclear Magnetic Resonance Techniques Bhaskar Geetha and Asit Baran Mandal" Chemical Laboratory, Physical and Inorganic Chemistry Division, Central Leather Research Institute, Adyar, Madras 600 020, India Received May 3, 1994. I n Final Form: January 17, 1995@ The self-diffusioncoefficient,D,, interaction parameter, Kf,and hydrodynamic radius, Rhm,of the acrylic ester of o-methoxy polyethylene glycol macromonomer micelles were determined by a cyclic voltammetric technique using Co(sep)Cls as the redox-active electrochemical probe. A self-diffusion coefficient of the above macromonomer was also determined by Fourier transform pulsed gradient spin-echo (FT-PGSE) NMR technique. It has been found that the D, is useful in measuring the critical micelle concentration of the macromonomer.
Introduction We have recently synthesizedl w-methoxy polyethylene glycol based macromonomer and characterized its various physicochemical properties including micelle forming ability i n aqueous solution using various techniques. We have determined the critical micelle concentration (cmc)l of this macromonomer in aqueous solution in the absence and presence of methyl orange probe during UV-vis spectroscopic measurements. The aggregation number of the above macromonomer i n aqueous solution was determined by a fluorescence technique1 and found to be 20 at 25 "C. We have also made use of a cyclic voltammetric (CV) technique2 in order to substantiate the cmc value of the macromonomer, and t h e partition coefficients of the electrochemical (EC) probe between macromonomer and water in nonmicellar and micellar states. Certain surface active monomers like sodium undecanoate are found to result i n polymers of anticipated hydrated size equal to the micelle size of the monomer, by the method of photophysical p ~ l y m e r i z a t i o n Recently, .~ a self-diffusion study of poly(ethy1ene oxide)-block-poly(dimethylsi1oxane) diblock copolymers in organic solvents and in poly(ethy1ene oxide) melts by Fourier transform pulsed gradient spinecho (FT-PGSE) NMR technique has been r e p ~ r t e d . ~ However, to the best of our knowledge the self-diffusion study of the macromonomer has not been reported so far. As the macromonomer is extensively useful in designing various polymeric surfactants, therefore, it is worthwhile to study the self-diffusion coefficient of the individual macromonomer and, thus, a measure of its micelle size. Recently, we have employed different electrochemical techniques5 to determine the self-diffusion coefficient of various surfactant micelles on glassy carbon and Pt
* To whom correspondence should be addressed.
Abstract published in Advance A C S Abstracts, April 15,1995. (1)Geetha, B.; Mandal, A. B.; Ramasami, T. Macromolecules 1993, 26,4083-4088. (2) Geetha, B.; Mandal, A. B. Proceedings of the ZUPAC Sponsored International Symposium on Surface and Colloid Science and Its Relevance to Soil Pollution and 6th National Conferenceon Surfactants, Emulsions and Biocolloids; Madras, India, 1994,pp 317-325. (3) Paleos, C. M.; Stassinopoulou, C. I.; Malliaris, A. J.Phys. Chem. 1983,87,251. (4)Kirfily, Z.;Cosgrove, T.; Vincent, B. Langmuir 1993,9,1258. (5) (a) Mandal, A. B.; Baranski, A. S.; Verrall, R. E. Measurements of self-diffusion coefficient of various micelles by cyclic voltammetric, chronocoulometric and square-wave voltammetric techniques. Manuscript in preparation. (b) Mandal, A.B. Langmuir 1993,9,1932-1933. @
working electrodes using ferrocene as a n electrochemical probe. Co(sep)Cl3 is a n excellent reversible EC probe whose utility to determine the cmc and self-diffusion coefficient of the surfactants and macromonomer micelles has not been examined. Therefore, in view of the current interest in methods to characterize micelles sizes, intermicellar interactions, and self-diffusion coefficient of the micelles, we felt it worthwhile to report i n this paper the results of CV using Co(sep)C13as a n EC probe. We also report the results of the FT-PGSE NMR technique for the determination of the self-diffusion coefficient as well as cmc of the above macromonomer.
Experimental Section Synthesis, Purification, and Characterization of the Macromonomer. The macromonomer,acrylic ester of methoxy polyethylene glycol, 1,has been synthesized by reacting 0.1 mol of methoxy ether of polyethylene glycol of molecular weight 398 (received from Aldrich) with 0.3 mol of synthesized acryloyl chloride in CHzClz in the presence of triethylamine. The purity and molecular weight of synthesized macromonomer have been
determined from its lH NMR spectrum in CDC13 performed on a 300-MHz Bruker MSL-300P spectrometer. The molecular weight of the investigated macromonomer was calculated using the procedure based on the integrals for the vinyl protons and the backbone unit and found to be 460. The purity of the macromonomerwas found to be 98%. The availability of methoxy ether of polyethylene glycol in a narrow molecular weight distribution has enabled the use of such species with polydispersities in the range 1.05-1.25. Therefore, we have chosen to synthesize a macromonomer with a hydrophilic polyethylene oxide block of low polydispersity index. The polydispersity (Le., the ratio ofweight Cvergge molecular weight to number average molecular weight, M,/M,) of our investigated macromonomer is 1.05. Details regarding synthesis, purification, and characterization of the macromonomer refer to our recent pub1ication.l Synthesisand Purification of Co(sep)Cls. The compound [Co(sep)lCls, 2, was prepared by modifying the published procedure.6 A 20-gportion of tris(ethy1enediamine)cobaltchloride was dissolved in 80 mL of 37%(w/v)formaldehyde solution. Then dry ammonia was bubbled through the resulting solution. The temperature ofthe system was maintained at room temperature by coolingthe reaction vessel. The excess ammonia was removed by aeration after completion of the reaction. The resulting mixture was filtered and the pH of this filtrate adjusted to -3 with HC1. Following rotary evaporation and removal of the hexamethylene tetramine side product, a crude product was obtained. The pure compound of Co(sep)C13,2,was obtained by ~
(6) Farraudi, G. J.; Endicott, J. F. Inorg. Chim. Acta 1979,37,219.
0743-746319512411-1464$09.00/00 1995 American Chemical Society
Self-Diffusion Studies of Micelles
Langmuir, Vol. 11, No. 5, 1995 1465 H
\JH
II
CH H CO-(CH
3
2
-CH20)n-CM
2
-CH -0-C 2
0
Hacromonomer, 1
recrystallization from aqueous HCl solution. For details regard~ ing synthesis and purity of 2, refer to an earlier r e p ~ r t .The structural details ofboth the macromonomer and Co(sep)Clahave also been given in our recent publications.1r2 Self-DiffusionMeasurements by CV Technique. In the electrochemicalmeasurements, the solutions were deoxygenated by bubbling argon for at least 10 min. The measurements were performed using a cyclic voltammeter (Princeton Applied Research, Model 173 with a PARC Model 175 Universal Programmer) and a three-necked electrolytic cell. A saturated calomel reference (SCE) electrode was employed in this study, and all potentials that are quoted are with reference to the SCE. The working electrode was Pt with a Pt counter electrode. Regarding experimental details for the determination of the self-diffusion coefficient of the micelles by CV technique, refer to our previous publications.8 It is generally believed that the excess ofthe probe concentration may perturb the micelle structure. Therefore, in the self-diffusioncoefficient measurements by CVtechnique, the concentration of the EC probe was varied accordingly in order to maintain the constancy of the ratio R (=CdCmic)at 0.5 so that the question of perturbation of the micellar structure by the probe does not arise at all. C, is the concentration of the probe and Cmic = (C, - cmc)/N is the micelle concentration, where C, is the concentration of the macromonomer and N is the aggregation number of the micelles. Self-DiffusionMeasurements by FT-PGSENMR Technique. The self-diffusionmeasurements were made on protons at 300 MHz on a Bruker MSL-300P spectrometer. The temperature of the nonspinning samples (in 5-mm tubes) was kept at 20 f 1"C. The imaging probe with actively shielded gradient coils was used for the measurements. The FT-PGSE technique employed here is well described by S t i l b ~The . ~ duration of the two gradient pulses (6) was varied while the time between them (A) was kept constant. The gradient strength was calibrated by measuring the frequency width at the base of a Wilmad 5-mm NMR tube whose inner diameter is 4.1 mm. The diffusion coefficient of pure waterloat 20 "C was found to be 2.08 x cm2s-l. The pulsed gradient is in the range 0-35 G cm-l in the above spectrometer. After calibration of the gradient pulse the molecular self-diffusioncoefficient (0)for a given strength (G) species diffusing isotropically was determined via the intensity of its frequency-resolved signal I (2r)in the spin-echo spectrum. The signal decays with increasing 6 according to eq 19J1
I ( 2 t ) = I(0)exp[-2r/Tz - D(yGd)'(A - 6/31]
(1)
(7)Govindaraju, K.Ph.D. Thesis, Madras University, India, 1986, Chapter 2. (8) (a)Mandal, A. B.; Nair, B. U.; Ramaswamy, D . Langmuir 1988, 4,736.(b)Mandal,A.B.;Nair,B.U.;Ramaswamy, D.Bul1.EZectrochem. 1988,4,565. (c) Mandal, A. B.; Nair, B. U. J . Phys. Chem. 1991,95, 9008. (d) Mandal, A. B.; Nair, B. U. J . Chem. SOC.,Faraday Trans. 1991,87,133.(e) Mandal, A. B.; Nair, B. U.Aduances in Measurement and Control of Colloidal Processes; Williams, R. A,, de Jaeger, N. C., Eds.; Buttexworth Scientific Publications: London, 1991; Chapter 2, nn 136-149 = = -- - -- . (9) Stilbs, P. Prog. NMR Spectrosc. 1987,19,1. (10)Mills, R. J . Phys. Chem. 1973,77,685. (11)Stejskal, E. 0.;Tanner, J. E. J . Chem. Phys. 1965,42,288.
where Z(0) is the NMR signal intensity without gradient application. The time between rf pulses ( r )was kept constant, typically 100 ms, to keep the contribution from transverse relaxation (2'2) and scalar coupling to the echo decay constant.12 The plots of lnZ(2z)vs @(A - 8/31showed excellent linearity over 2 decades of Z(22). Normally, 10 data points were collected for each frequency-resolved signal over a 10-foldintensity change. At least two determinations were made for each D . The most prominent lH NMR signals of the macromonomer in the FTPGSE spectra of the samples studied here are those of the poly(oxyethylene) groups (chemical shift at 3.5 ppm), the signals of which decay slowly, and are easily distinguished from faster diffusing species of water (chemical shift is 4.9 ppm).
Results and Discussion Figure 1shows a typical CV curve of a Co(sepICl3probe in the presence of a macromonomer solution. Scan rates of potential in the range 0.02-0.2 V s-l were used for the macromonomer systems. With micellar systems containing a n EC probe completely solubilized in micelles, the diffusion coefficient of the probe, D, would correspond to the micelle diffusion coefficient D,, whereas C would still be the probe concentration. The diffusion-controlled nature of the peak current, i,, was verified by performing the experiments a t various scan rates in micellar environments at various macromonomer concentrations C,. D, a t various C, were determined from the slope of i, vs u1I2 plots (Figure 1). TheEvz value ofthe Co(sep)Cla probe obtained in the presence of our macromonomer systems with reference to saturated calomel electrode (SCE) is in the range of -0.51 to -0.56 V. The decrease of D , upon increasing C, has been observed5s8J3when investigating self-diffusion of the micelles as in the present study (cf. Figure 2). In the concentration range where the intermicellar interactions are not too large, D, can be written as14
D, = D,"/[l
+ IzdC, - cmc)l
where kf is the interaction coefficient and D," the selfdiffusion coefficient in the absence of intermicellar interactions, which can be used to calculate the hydrodynamic radius of the micelles Rhm from the following Stokes-Einstein equation
D," = kT/6nr,@,"
(3)
where 70 is the viscosity of the solvent medium. A plot (12) Stilbs, P. J . Colloid Interface Sci. 1982,87,385. (13)(a)Zana, R.; Mackay, R. A. Langmuir 1986,2,109.(b) Cebula, D.J.; Ottewill, R. H.; Ralston, J.; Pusey, P. N. J.Chem. Soc., Faraday Trans. 1 1981,77,2585. (14) Mackay, R. A. Microemulsions; Robb, I., Ed.; Plenum Press: New York, 1982; p 207.
Geetha and Mandal
1466 Langmuir, Vol. 11, No. 5, 1995
Table 1. Self-Diffusion Coefficient (D,"), Intermicellar Interaction Parameter (kf), and Hydrodynamic Radius of the Micelles (Rhm) by Using a CV Technique at 25 "C with Co (sep) C13 as an Electrochemical Probe solvent
HzO-0.1 M KCl
svstem macromonomer
D,o/10-6 cm2.s-1
RhVA
1.0," 1.Mb 24.53,a 20.8b
kflM-' 60 5 5
This CV work. Literature values from transport studies (ref 15).
7
v
2 00
0
0.1
0.2
-+ "'/2/
(V
0.3
0.4
0.5
5-1 )'I2
Figure 1. (a) CV of Co(sep)Cl3 probe (1.0 mM) in 0.04 M macromonomer at 25 "C. [KC11 = 0.1 M, u = 20-200 mV/s, E = -0.7 to -0.4 V. (b)Plot ofi,vs u1/2. [KC11 = 0.1 M (constant). Curves 1-5: 0.5 mM Co(sep)Cl3 0.02 M macromonomer; 0.75 mM Co(sep)C13 0.03 M macromonomer; 1.0 mM Co(sep)Cl3 0.04 M macromonomer; 1.25 mM Co(sep)Cls 0.05 M macromonomer, and 1.5 mM Co(sep)Cl3 0.06 M macromonomer, respectively. Area of the Pt working electrode = 0.0918 cm2. Counter electrode is also Pt. All potentials were measured
+
+
+
+
+
with reference to saturated calomel electrode (SCE).
I
1 N
I
E
U
-
W
0
\
;E c3
0
001
002
0.03
004
0.05
006
[Macromonomer]/ mol d m - 3
Figure 2. Plot of D,-I vs concentration of macromonomer at 25 "C from CV measurements.
of D,-l vs concentration of the macromonomer is shown in Figure 2. Extrapolation of this plot a t the cmc has been done to obtain D,". This value is in good agreement with that estimated from transport studied5 using its known aggregation number. D," and Rhmvalues obtained for various surfactant micelles by CV technique5ss are in good agreement with our transport studies.16 Recently, we have determined15 the exact shape, size, hydration, correlation times, and thermodynamic and various physicochemicalparameters of macromonomer micelles. Such (15)Geetha, B.; Mandal, A. B. Submitted for publication i n J . Phys. Chem. (16) (a)Mandal, A. B.; Ray, S.; Biswas, A. M.; Moulik, S. P. J . Phys. Chem. 1980,84,856. (b) Mandal, A. B.; Gupta, S.;Moulik, S. P. Indian J. Chem. 1985,24A, 670.
studies15 indicated that the macromonomer in aqueous solutions is spherical and ellipsoidal in nature depending on the temperature and concentration of the macromonomer. However, a t the cmc of the macromonomer a t 25 "C it is spherical even in the presence of 0.1 M KC1. Therefore, extrapolation of D,-l vs C, plot a t the cmc is justified to estimate D," and Rhmfor spherical micelles. Moreover, we have used adequate supporting electrolyte (0.1M KC1) to avoid the controversy between "slow" and "fast" theory5s8 of the diffusion. I t can be seen t h a t the plot is more or less linear (Figure 2) up to 0.055 M macromonomer concentration and the adsorption is much less of a problem. However, a t the macromonomer concentration S0.055 M, there is a deviation from straight line which can reflect the onset of macromonomer adsorption onto Pt electrode a t high macromonomer concentration. Therefore, the maximum macromonomer concentration used in the present study is 0.06 M. Moreover, when the concentration of the macromonomer is >0.06 M, the pH of the resulting solution has been dwindled down to 4 and as a result irreversibility was developed in the system. In the present investigation, the pH of the solutions has been maintained a t 6 . 5 . We would like to mention that the EC probe usedin the CV does not attach to the macromonomer. However, EC probe has been added externally to the macromonomer micelles, and accordingly these systems are presumably in a fast exchange. Therefore, we have already examined the impact of this correction, and in that case the appropriate expression is D = f9, (1fb)Dp where fb is the fraction of bound probe in the m i c e l l e ~ . ~The J ~ D,", Rhm,and kfvalues for macromonomer micelles are depicted in Table 1. Figure 3 shows the FT-PGSE NMR spectra of 2 mM macromonomer solution in DzO with attenuation for poly(oxyethy1ene)and DzO regions only. The experiments were performed at 10 different values of 6. However, it was difficult to accommodate all the spectra with various attenuations in a single plot. Therefore, we have showed the attenuated spectra with seven different 6 values in Figure 3. Figure 4 shows the self-diffusion coefficient values for poly(oxyethy1ene) (POE) and HzO in the presence of various concentrations of the macromonomer determined by the FT-PGSE technique. I t can be seen in Figure 4 that the self-diffusion coefficients for both POE and HzO are increased upon increasing macromonomer concentration (C,) up to its cmc. However, beyond cmc these values are decreased upon increasing C, (see Figure 4).The same trend has also been observed in our CV technique.2 The cmc obtained for the macromonomer from both the selfdiffusion coefficient plots of POE and HzO (cf. curves 1 and 2 of Figure 4) are the same and found to be 1.2 x lo-* M, and is in good agreement with our recent CV and spectroscopic techniques.1,2 Recently, the second cmc values for the mixed surfactant systems (where high surfactant concentrations were employed) determined by
+
(17) Rusling, J. F.; Shi, C. N.; Kumosinski, T. F. Anal. Chem. 1988, 60, 1260.
Self-Diffusion Studies of Micelles
Langmuir, Vol. 11, No. 5, 1995 1467 H00
Ill
DS s I
lllilh
I
z 25
I
I
1
I
I
I
1
I
I
I
I
8.0
7.0
69
5.0
LO
3.0
2-0
10
0
-la
-20
I -3*0
PPM
Figure 3. FTPGSE NMR spectra of 2 mM of macromonomer solution in DzO with attenuations. Temperature is 20 f 1"C. The pulsed gradient = 2.33 G cm-l. A = 100 ms. of macromonomer amphiphile by FT-PGSE technique was not reported so far. I t was not possible to determine the self-diffusion coefficient values a t extremely low concentration of the macromonomer (when [macromonomer]
