Langmuir 1995,11, 767-773
767
Solution Properties of Hydrophobically End-Capped Low Molecular Weight Polyethylene Glycols? N. Ch. Padmavathi and P. R. Chatterji" Division of Organic Coatings and Polymers, Indian Institute of Chemical Technology, Hyderabad 500 007, Andhra Pradesh, India Received January 18, 1994. I n Final Form: December 7, 1994@ The solution behavior of a series of polyethylene glycols (EO 4-35) esterified at both ends with behenic acid, H&(CH2)2oCOOH,was evaluatedin comparison with their monoesters. Understandably the solubility patterns of these esters in water are clearly governed by their hydrophilidipophilic balance values. The gel permeation chromatographyand viscosity data strongly suggest that the disubstituted homologues are more compact than the monosubstituted counterparts both in aqueous and organic media. The ability of the diesters to aggregatein aqueous medium is establishedfrom surfacetension measurements and pyrene fluorescence probe methods. The molecular area calculation based on surface tension values indicates that, at the air-water interface, the area occupied by the monoesters and diesters are comparable.
Introduction Polyethylene glycol (PEG) with a hydrophobic moiety at one end and a free -OH group a t the other constitute the basic architecture of a nonionic amphiphile. They are emperically written as C,E,, where C stands for the hydrophobic part, n signifylngthe number ofcarbon atoms, and E represents the ethylene oxide (EO) chain, CH2CH20, m denoting the number of EO units. The nonionic surfactants are low molecular species, where m 8 and n 10-12. Very comprehensive and systematic analyses of the low molecular weight nonionic surfactants are available in the literature.' Such studies have also been conducted for higher molecular weight amphiphilic diblock copolymers. Recently Abe et a1.2studied the micelle formation in systems with general formula C,E,, where 12 < n < 16 and 10 < m < 40. Bahadur et al.3 have reported the aggregation behavior of Pluronic P-94, which is a [polyethylene oxide-polypropylene oxide1 block copolymer of average molecular weight 4600. The aqueous micellization of [polystyrene-polyethylene glycol] diblock copolymers has been studied extensively by a combination of methods such as fluorescence probe and QUELS.4-7 All these investigations clearly indicated that the diblock copolymers formed spherical micelles in water when the length of the EO chain was significantly longer than the insoluble hydrophobic portion. Functionally, triblock amphiphiles of the C,E,C,, or the E,C,E,, class should also be surfactants. However hardly any information is available on such low molecular weight triblock systems. Most ofthe amphiphilic triblock systems studied are ofhigh molecular weight (Mw = lo3lo4) with polyethylene oxide forming either the middle
-
-
IICT Communication No. 3304.
* Author to whom correspondence should be addressed. Abstract published in Advance A C S Abstracts, February 1, 1995. (1) Schick, M. J. Nonionic Surfactants: Physical Chemistry, Vol. 23. Marcel Dekker, Inc.: New York, 1987. (2)Abe, M.; Uchiyama, H.; Yamaguchi, T.; Suzuki, T.; Ogino, K. Langmuir 1992,8, 2147. (3) Bahadur, P.; Pandya, K.; Langmuir 1992,8,2666. (4) d'olivera, J. M. R.; Xu, R.; Jensma, T.; Winnik, M. A.; Hruska, Z.; Hurtrez,G.;Reiss, G.;Martinho, J. M. G.; Croucher,M. D. Langmuzr 1993,9,1092. (5) Wilhelm, M.;Zhao, C. L.; Wang,Y.;Xu, R.; Winnik, M.A.;Mura, J. L.; Riess, G.; Croucher, M. D. Macromolecules 1991,24,1033. (6)Xu,R.; Winnik, M. A.; Hallet, F. R.; Riess, G.; Croucher, M. D. Macromolecules 1991,24,87. (7) Hruska, 2. H.; Piton, M.; Yekta, A.; Dehamel, J.; Winnik, M. A,; Riess, G.; Croucher, M. D. Macromolecules 1993,26,1825. @
Table 1. Characteristics of PEGS and Their Esters molecular weight by
polydispersity of esters
hydroxyl value"
hydroxyl value
NMR ofdiesters
mono
sample
M
di D
PEG(2OO) PEG(300) PEG(400) PEG(6OO) PEG(1000) PEG(1500)
507 366 314 172 95 74
22 1 306 357 652 1178 1511
219 302 355 681 1176 1467
1.037 1.080 1.090 1.050 1.040 1.040
1.033 1.090 1.100 1.030 1.090 1.090
a
Milligram of KOH per gram of sample. b By GPC.
Table 2. Solubility Characteristics of Esters of PEG solubility
a
sample
HLB
200D 300D 400D 200M 600D 300M 400M lOOOD 600M 1500D lOOOM 1500M
4.226 5.867 7.199 7.389 9.229 9.364 10.804 11.823 12.764 13.714 14.926 16.306
- = insoluble,
HzO
THF
+ + + + + + + + + + + +
+ = soluble, (+) = soluble with difficulty.
segment or the two terminal b l o ~ k s . ~ -We ' ~ have been particularly interested in the solution behavior of low molecular weight triblock systems of the type C,E,C,. In this paper, we report the properties of the system where n = 22 and m varies from 4 to 35.
Experimental Section PEGSwere purchased from Sigma and Aldrich. Behenic acid was from BDH Chemicals Ltd. Doubly distilled water was used throughout. The solvent tetrahydrofuran (THF) was of HPLC ( 8 ) Kwon, G.; Naito, M.; Yokoyama, M.; Okano, T.; Sakurai, Y.; Kataoka, K. Langmuir lS93,9,945. (9) Yekta, A.; Dehamel, J.;Brochard, P.;Adiwidijaja, H.; Winnik,M. A. Macromolecules 1993,26,1829. (10)Yekta, A.; Dehamel, J.: Adiwidiaia, _ _ H.; Brochard, P.; Winnik, M. A. Langmuir 1993,9,881. (11)Richev, B.; Kirk, A. B.:Eisenhart, E. K.; Fitzwater, S.;Hook, J. Langmuir 1692,63,31. (12) Strasser, C. M.; Francois, J.; Clouet, F.; Tripette, C. Polymer 1992,33,627.
0743-746319512411-0767$09.00/00 1995 American Chemical Society
768 Langmuir, Vol. 11, No. 3, 1995
Padmavathi and Chatterji
Esterification uith behenic acid
No e s t e r formation
J
Hydrolysis
3 HO+CH2-CH2-0*
0 II C- iCH2120-CH3
i HO-+CH2-CH2-0
+
0 II f C-tC$la-CH3
HO+W2-CH2-0~
H
Figure 1. Possible structures of PEG-borate complexes and their behavior during esterification. grade. The PEGS were fractionated according to the method of Suzuki et al.13 to a final polydispersity index of 1.0-1.2 by gel permeation chromatography(GPC). The exact molecular weights were calculated both from the hydroxyl values,14which showed 2 mol of hydroxyl groups/mol of PEG, and also the lH NMR of the diesters.l5 The calculations are explained later in the text. Table 1lists the values obtained. GPC was run on a Shimadzu unit, fitted with an RI detector using a Waters 100 A styragel column and a flow rate at 0.5 mumin. THF was the eluent used. The purity of the samples was confirmed by the single, sharp peak for each sample in the chromatograms. The concentrations were 50 mg/mL. IR spectra were run on a Perkin-Elmer model 882 instrument and 'H NMR spectra were produced on a Gemini 200 MHz spectrometer. Viscosity measurements of 1mM solutions were taken in THF and doubly distilled water at 27 "C in a Schott-Geratte automatic viscometer with Ob capillary. The intrinsic viscosities (7) were calculated using the following single-point equation?
c
E 20
I
where c is the concentration in g/dL. Surface tension measurements were taken in water at 25 "C on a Du Nouy tensiometer. The critical micellar concentrations (cmc)were determined from the plots of surface tension ( y ) versus concentration. The surface excess concentration ( r )and the area per molecule (A) were evaluated from Gibb's adsorption isotherm:17
*=---
1
dr nRT d In c
(2)
Okoo
I
400
2;oo
A00
2doo
,700
lql00
1100
WAVENUMBER (CM''1
Figure 2. Infrared spectra: (a)monoester, (b) diester of poly For fluorescence measurements, different concentrations of sample solutions were prepared in water, and the concentration ofpyrenewaskeptat 5.0 x 10-7Mineachsolution. Onemilliliter (13)Suzuki,T.;Koataka, T. Macromolecules 1980, 13, 1495. (14) Nonionic Surfactants: Chemical Analysis; Marcel Dekker, Inc.: New York, 1987; Vol. 19. (15)Greff,R. A.; Flangen, P. W. J . Am. Oil.Chem. SOC.1963,40,118. (16) Rao, R. M.;Yaseen, M. J . Appl. Polym. Sci. 19&3,31,2501. (17) Osipaw, L. I. Surface Chemistry; Reinhold Publishing Corp.: New York, 1962.
ethylene glycol. of each solution was placed in a 10 mm rectangular quartz cell, and the spectra were run in a SPEX fluorolog spectrophotometer in right angle geometry using slit openings of 2 mm and 339 nm excitation. The emission spectra were accumulated with an integration time of 1 d0.5 nm. Synthesis. Preparation of Monoesters of PEG. In order to avoid the esterification on both sides of the PEG molecule, the hydroxyl group on one side was blocked by complexing it with boric acid. Boric acid complexation is a common method for the
Langmuir, Vol. 11, No. 3, 1995 769
Solution Properties of Polyethylene Glycols
70
bO
so
LO
Figure 3. lH NMR spectrum of diester of PEG (200). preparation of monoesters of diols.18-20 For this, 1mol of PEG was treated with 0.33 mol ofboric acid under continuous vacuum at 80-90 "C until the evolution of water ceased. Possible structures for the PEG-borate complexes are shown in Figure 1. To this reaction mixture were added behenic acid (0.5 mol) and p-toluenesulfonic acid as catalyst. A vacuum of 7 mm at 110 "C was maintained throughout. The reaction mixture was then refluxed with water for 1h to hydrolyze the borate complex and then neutralized with sodium acetate. The underivatized PEG set free from I1 and III in Figure 1was washed away with saturated brine. The washings were done till the product responded negatively to the presence of free PEG with Dragendroff reagent. The unreacted behenic acid was removed by passing the reaction mixture through alkaline silica gel with a chloroform-ether mixture as elutant. The yield ofthe product is about 70%. Preparation of diesters of PEG. Well-ground behenic acid (2 mol) was refluxed with distilled thionyl chloride (3 mol) for 3 h. Excess thionyl chloride is distilled off. To the remaining acid chloride was added PEG (1 mol) and the mixture refluxed for about 2 h. The product, the diester of PEG, was extracted from the reaction mixture as described for monoesters. Characterizationof Mono- and Diesters. The IR spectra of the above samples provided qualitative proof of esterification (Figure 2). The absence of a peak at 1700 cm-l confirmed the absence of free acid in the product. The peak at 1740 cm-l is indicative of the formation of an ester linkage. The bifurcated peak between 2800 and 2900 cm-l is due to the alkyl group (stretching) of PEG. The broad peak at 1100 cm-l is that of the ether linkage (CH2CH20)of PEG. The broad peak between 3200 and 3600 cm-l suggests the hydroxyl group in the monoester at one terminal (Figure 2a). The total disappearance of the same in the diester confirms the esterification of PEG on both sides (Figure 2b). The hydroxyl values were determined for both mono and diesters. While monoesters registered a value equivalent to one hydroxyl group per mole of PEG, the diesters showedno hydroxyl content. More detailed structural information could be obtained from proton NMR measurements.15 The NMR spectra of the diesters contain an internal standard of six equivalent protons on the terminal methyl groups of the behenyl chain (Figure 3). This enabled us to calibrate the number of protons responsible for resonance at 1.2-1.5 ppm (behenyl chain) and at 3-3.9 ppm (ethoxide chain), thus categorically establishing the identity of the diester. The same strategy was followed for the monoesters too. The polydispersity index of the mono and diesters were (18)Councler Ber. 1878,11,1106. (19) Hartman, L.J. Chem. SOC.Part II 1957,1918. (20) Rao, T. C.;Sastry, Y . S. R.; Rao, R. S. J.Am. Oil Chem. SOC. 1977,54, 15.
Table 3. Parameters Evaluated from Surface Tension (ST)and Fluorescence (Fl) Measurements sample 400M 600M 600D lOOOM lOOOD 1500M 1500D
5 x 10'0 (mM.cm-2) 3.337 3.170 3.004 1.078 0.906 0.784 0.731
A (A2) 49.75 52.37 55.27 154.04 183.24 211.77 227.13
cmc (mM) ST F1 0.041 0.067 0.043 0.045 0.044 0.065 0.060 0.048 0.040 0.150 0.140 0.070 0.065
ascertained by GPC (Table 1). These values together with the corresponding molecular weights clearly establish the purity of the esters. The Hydrophilic/LipophilicBalance. The hydrophilid lipophilic balance (HLB) of all samples was calculated according to the following equation:
HLB = 20kfH/(kfH -4-
kfL)
(4)
where MH = the molecular weight of the hydrophilic component and M L = the molecular weight of the hydrophobic component.
Results and Discussion The ability of triblock copolymers of ABA type to aggregate in solvents which preferably dissolve either the middle or the terminal blocks has been reported. KrauseZ1 was the first to report the possibility that triblock copolymers with poorly solvated end groups could aggregate. Theoretical aspects of micelle formation by such a system were discussed in detail by ten Brinke et a1.22 Recently Balsara e t aLZ3 have reported their investigations on a set of [polyvinylpyridine-polystyrene-polyvinylpyridine] triblock copolymers. They found that the molecular weight of t h e middle block played a crucial role in determining t h e micellization behavior. However, it is possible that t h e low molecular weight systems may behave differently, due to the finite length ofthe segments. Table 2 presents the solubility characteristics of t h e mono- a n d diesters of the PEGS of different molecular weights. While both mono- a n d diesters of all PEG samples a r e soluble in THF, their solubility in water clearly depends on t h e comparative lengths of the hydrophile and hydrophobe. Only those esters in which t h e PEG chain length is greater than the total alkyl chain (21) Krause, S. J. Phys. Chem. 1964,68, 1948. (22) tenBrinke,G.; Hadziioannou, G. Macromolecules 1987,20,486. (23) Balsara, N.P.;Trirrell,M.; Lodge, T. P. Macromolecules 1991, 24,1975.
Padmavathi and Chatterji
770 Langmuir, Vol. 11, No. 3, 1995 . .
127126-
125124I23
-
I22
-? “
120
T
:
R.T.
104-
(b’
I
4
103I 02
-
I01
,
1
IO-
No. o f EO units
-
1
I
Figure 5. Relative viscosities ( p e l ) of esters of PEG in THF: 0, monoesters, 13,diesters.
IO
10.5
lo4
I
11.0
11.5
R .T.
3
-
r-l
lo-
t
0908-
07-
2 IO
06-
IO5
11.0
11.5
R.T. (min.)
Figure 4. Variation of retention time (RT)versus M, (data from GPO: (a) both mono- and diesters of PEG, (b) only monoesters, (c) only diesters; (1) 1000D,(2)1000M,(3)600D, (4)400D,(5) 300D,(6)600M,(7)400M,(8)300M.
length are soluble in water. When interpreted in terms of the HLB values, we note that only those with HLB > 10 are water soluble. The 600D and 300M samples are clearly border line cases, because if we compare the effectivemolecular weights of the hydrophilidhydrophobic segments, these turn out to be 6001680 and 3001340.With the hydrophobe slightly outweighing the hydrophile, these two diesters form an emulsion in water. Figure 4 shows the GPC elution pattern of the monoand diesters ofthe PEG. Since THF is the common solvent for all samples, we carried out GPC measurements in THF. GPC is based on principles of hydrodynamics in which molecules are sorted out according to their size. It is interesting to note that the points for mono- and diesters fall on two distinctly separate curves, with the monoesters eluting earlier than diesters. The medium being THF, no
0504-
::I 03-
0
,
,
5
IO
,
, 20 N O . O ~[EO]
30
units
-c
Figure 6. Intrinsic viscosities (7)of esters of PEG in THF: 0,
monoesters, I3, diesters.
micellization is possible and a different conformation or chain orientation for the mono and diesters could be suspected. Indeed, this possibility is corroborated by the viscosity data in THF. The viscosity decreases with an increase in the number ofethylene oxide units and reaches a minimum value with approximately 10units. M e r this the viscosity increases with increasing molecular weight.17 The difference between mono- and diesters shows up irrespective of whether the relative viscosity, rrel,or the intrinsic viscosity, 7, is plotted against molecular weight (Figures 5 and 6). For a comparative evaluation, we also monitered
Langmuir, Vol. 11, No. 3, 1995 771
Solution Properties of Polyethylene Glycols
19-
0 ISOOM 0 15000
I 13
18-
1716-
I514
-
13m
14
D 106-
105104103102-
07-
06-
101-
05-
100-
04
-
1
I
3
0 3 -
Figure 9. Fluorescence intensity ratios (11/13) versus concen-
02-
tration.
0110
20
NO. of EO units
-
30
The inclination of mono- and diesters to aggregate into micellar assemblies with well-defined cmc values is brought out clearly in Figures 8 and 9. The typical pyrene fluorescence spectra for the mono and diesters of PEG(1500) as a function of concentration are shown in Figures 10 and 11.
Figure 7. Intrinsic viscosities (7)of esters of PEG in water:
0, monoesters, m, diesters. the [VI of the few samples which are water soluble (Figure 7). Here again the monoesters behaved distinctly different from diesters. Another point to be noted here is the magnitude of [VI in aqueous and organic media. The values in THF are decidedly higher than those in the aqueous medium. This again reflects the coil dimension; since both PEG and behenyl segments interact with the organic medium with equal facility, the chains are more extended in this medium. In aqueous medium preferential masking of the hydrophobic behenyl segments leads to contraction of coils.
We would like to make an observation here regarding the 11/13 values for pyrene in these systems. The 11/13 ratio is indicative of the polarity of the environment where the probe is located. In a nonpolar environment, as in hydrocarbon solvents, this ratio is in the vicinity of 0.6 and in polar solvents this value is in the range of 1.22.0.24 All the studies which involved use of pyrene fluorescence to probe the micellization of the PEG-based
0 1500M
54
0 15000
53 52
t
--
51 50
8
E
0 e
-0”
49 48 47
“i t
46
421 43
41
401
0
1
I
1
0.1
0.2
03
I
0.4
I
0.5
I
0.6
Figure 8. Surface tension ( y ) at air-water interface versus concentration.
I
0.7
1
0.8
I
1
I
0.9
I .o
1.1
772 Langmuir, Vol. 11, No. 3, 1995 2,801t 06
-
2.100+06
-
Padmavathi and Chatterji
v)
a
u
t
I4O'+O6 1401+06-
6
n 0)
5 6.99.+05-
0.0 360
380
400
420
440
460
Wavelength ( n m )
Figure 10. Fluorescence spectra of pyrene (5 x
M) in 1500M: (1) 1528 x
mM, (2) 3057 x 10-2 mM, (3) 4586 x
mM, (4) 7643 x 10 mM.
36 0
380
400
420
440
460
Wavrlrngth ( n m )
Figure 11. Fluorescence spectra of pyrene (5 x (4) 4866 x
M) in 1500D: (1) 1216 x
mM, (2) 2433 x
mM, (3) 3649 x
mM,
mM.
di- and triblock systems record a value of l.2.3-11 Our investigations also yield a value of about 1.2. This implies that pyrene is in a slightly polar environment. Intrigued by this, we tested the ability of underivatized PEG to dissolve pyrene. The results are shown in Figure 12. Earlier, Balasubramanian et al.25 and Xu et aLZ6also reported the solubilization of perylene and pyrene by PEG. Reviewing the solubilization of aromatic compounds like benzene, naphthalene, and anthracene by the PEG-based nonionics, McKay et al.27suggest that these molecules could be absorbed at the interface of the hydrophile and hydrophobe segments. It is also possible that the amphiphilicity of PEG itself might contribute toward the hydrophilicity of the overall micellar assembly. For EO-based nonionics, the effect of the length of the EO segments on the overall architecture of the molecule has been determined by surface tension measurements. With an increase in the number of EO units the surface excess concentration (t)decreases progressively and the area per molecule (A) increases. The relevant data are
0,071 I
(24) Kalyanasundaram,K.; Thomas, J. K. J . A m . Chem. Soc. 1977, 99, 2039. (25) Balasubramanian, D.;Srinivas, V.; Gaikar, V. G.; Sharma, M. M.J . Phys. Chem. 1989,93, 3865. (26)Xu, W.; Desmas, J. N.; De Graff, B. A,; Whaley, M. J . Phys. Chem. lSS3,97, 6546. (27) Reference 2, Chapter 6.
0
l
l
I
l
t
l
l
l
l
l
l
~
001 0.02 0.03 0.04 QOS 0.06 0 0 7 0.08 0.090.00.11 0.12 I 3
CONC. OF
PEG (1000)(mMl
---L
Figure 12. Solubility of pyrene in water in presence of PEG(1000).
Langmuir, Vol. 11,No. 3, 1995 773
Solution Properties of Polyethylene Glycols
0.15014
-
013
-
0.12
-
-
0.11
-
0.0
/
0.09-
ooa 0.07
-
0.06 0.05 004
-
Hsia et al.= have reported that for nonylphenol derivatives of PEG, the area per molecule increases with the number of the EO units. Thus, while chains with 9.5 and 10.5 EO units occupy an area of 75-92 A2, those with 15 EO units have an area of 110-130 A2. With 20 EO units, the area increases to 135-175 A2,and for chains with 100 EO units, it increases to 1000 Az. Interpreting similar results Schick et al.' ruled out the possibility of a vertical orientation for the EO chain. They proposed that the EO chain penetrated the aqueous phase in the form of coils, the size of the coil increasing with an increase in the number of EO units. The conformation of EO chains has been discussed in terms of the zigzag (fully extended) and meander (expanded helical coil) models.29 There is evidence that the maximal restriction t o the segmental motion is at the hydrophilehydrophobe interface, with the mobility increasing along the hydrocarbonchain toward the core and along the EO chain toward the exterior hydroxyl end. It is assumed from theoretical considerations that the EO chains having > 10 EO units are primarily in the expanded coils. Laser Raman studies of a series of Igepals and Tritons have indicated a dihedral helical c o n f o r m a t i ~ n . ~ ~
o o IO.
20
30
35
No.of EO unit8
Figure 13. cmc (from surface tension) versus the number of EO units: 0 , monoesters, a, diesters.
----WATER
Figure 14. Visualization of the effective area occupied by PEG hydrophobically end capped at (a) one end and (b)both ends: zigzag line, PEG chain; straight line, alkyl chain.
presented in Table 3. While observing the same trend, we note that the monoesters fall into one pattern and diesters fall into another. The increase in surface tension and cmc are sharper for monoesters than for diesters (Figure 13). The striking feature of Table 3 is that the area per molecule of the monoesters is only slightly less than that of the corresponding diesters.
Conclusions The model that emerges from this discussion is schematically shown in Figure 14. The volume space occupied by the EO chain within the aqueous phase will largely be determined by the dimensions of the EO chain and not by end cappings.28 Granting enough space for molecular motion, we can conceive of a solvent column which, when extended up to the air-water interface, translates into the effective area occupied by the molecule. Whether the EO chain is anchored at one end or both will contribute only marginally to the coil dimension, and hence the effective area at the interface happens to be more or less the same for the mono- and diesters.
Acknowledgment. One of the authors, N.Ch.P. is grateful to C.S.I.R. for providing financial support in the form of a research fellowship. LA940058P (28) Hsia, L.; Dunning, H. N.; Lorenz, P. B. J.Phys. Chem. 1966,60, 657. (29) Kalyanasundaram,K.; Thomas, J. K. J. Phys. Chem. 1976,80, 1462.
