1014
Langmuir 2007, 23, 1014-1017
Self-Assembly Study of Polydisperse Ethylene Oxide-Based Nonionic Surfactants Miklo´s Nagy, La´szlo´ Szo¨llo¨si, Sa´ndor Ke´ki, and Miklo´s Zsuga* Department of Applied Chemistry, UniVersity of Debrecen, H-4010 Debrecen, Hungary ReceiVed July 13, 2006. In Final Form: October 26, 2006 The self-assembly behavior of polyethoxylate-based multicomponent nonionic surfactants was studied. Using the dynamic light scattering method, thermodynamical parameters such as the critical micelle concentration (cmc) and hydrophile-lipophile balance (HLB), as well as the micelle size and micelle size distribution, were determined. The number average molecular weight and number average HLB of the samples were determined by MALDI-TOF-MS and 1H NMR techniques, and the data were evaluated. A connection was found between the HLB and the ln(cmc) value of the samples which can be described by a simple equation. Using this equation and plotting ln(cmc) versus the average number of ethylene oxide units, lines were obtained at different temperatures, and their slope allowed the calculation of the contribution of a single ethylene oxide unit to the Gibbs free energy of micellization.
Introduction Nonionic surfactants are among the most important compounds for industry and research. The ethylene oxide-based surfactants are regularly used as blends for a broad range of molecules with different molecular weights.1 The application of nonionic surfactants covers a wide range of fields including the detergent, pharmaceutical, and petrol industries. One of the most important parameters for this type of surfactants is the hydrophile-lipophile balance (HLB), which was introduced by Griffin.2,3 This number is an empirical expression for the relationship of the hydrophilic and hydrophobic groups of a surfactant. The HLB number provides a semiquantitative description of the efficacy of surfactants with respect to emulsification of water and oil systems. The HLB number for nonionic surfactants can be calculated by the following equation:
HLB ) 20(1 - ML/MT) where ML is the formula weight of the hydrophobic part of a molecule and MT is the total formula weight of the surfactant molecule. Nonionic surfactants are classified by their HLB number since it is fundamental in considering their application. Surfactants with a low HLB number tend to dissolve in nonpolar solvents, while a high HLB number results in good water solubility. When dissolved in water, nonionic surfactants tend to self-assemble in a process leading to the formation of diversely shaped micelles.4-14 * To whom correspondence should be addressed. Phone: 00-36-52512900/22480. Fax: 00-36-52-348-173. E-mail:
[email protected]. (1) Lindgren, M. Sjo¨stro¨m. Chemom. Intell. Lab. Syst. 1994, 23, 179-189. (2) Griffin, W. C. J. Soc. Cosmet. Chem. 1949, 1, 311. (3) Griffin, W. C. J. Soc. Cosmet. Chem. 1954, 5, 259. (4) Tuzar, Z.; Kratochvil, P. AdV. Colloid Interface Sci. 1976, 6, 201. (5) Gref Minamitake, Y.; Peracchia, M. T.; Trubetskoy, V.; Torchilin, V.; Langer, R. Science 1994, 263, 1600. (6) Wilhelm, M.; Zhao, C. L.; Wang, Y.; Xu, R.; Winnik, M. A.; Mura, J. L.; Riess, G.; Croucher, M. D. Macromolecules 1991, 24, 1033. (7) Xu, R.; Winnik, M. A.; Riess, G.; Chu, B.; Croucher, M. D. Macromolecules 1992, 25, 644. (8) Yu, K.; Eisenberg, A. Macromolecules 1996, 29, 6359. (9) Forder, C.; Patrickios, C. S.; Armes, S. P.; Billingham, N. C. Macromolecules 1996, 29, 8160. (10) Lee, S. C.; Chang, Y.; Yoon, J. S.; Kim, C.; Kwon, I. C.; Kim, Y. H.; Jeong, S. Y. Macromolecules 1999, 32, 1847. (11) Tanodekaew, S.; Deng, N. J.; Smith, S.; Yang, Y. W.; Attwood, D.; Booth, C. J. Phys. Chem. 1993, 97, 11847.
Scheme 1. Preparation of Ethylene Oxide-Based Surfactants
The micelle shape is strongly dependent on the surfactant structure. The process is entropy-driven and in some cases shows significant temperature dependence.15 The microstructure of the solution is important for product stability and environmental considerations. Although there are several publications on polyoxyethylated nonionic surfactants, these publications do not give a comprehensive overview of the field.16-20 In the present study we investigate the self-assembly behavior of ethylene oxide-based nonionic surfactants with different hydrophilic-hydrophobic balances. The self-association properties as well as solution stability in aqueous media were investigated by light scattering methods as a function of the concentration and temperature. Materials and Methods Materials. Alcohol ethoxylates were obtained from Shell from the Neodol 91 series and were used without further purification. The carbon chain length of the base alcohols was the same, but the degree of polymerization of poly(ethylene oxide) changed. Their synthesis and general structure are shown in Scheme 1. Methanol (Merck, Germany) was distilled before use. 2,5Dihydroxybenzoic acid (DHB) and sodium trifluoroacetate (NaTFA) (Aldrich, Germany) were used without further purification. Distilled water was filtered through a 0.2 µm filter before solution preparation. Solution Preparation and cmc Determination. All the solutions were prepared from the corresponding Neodol sample by direct dissolution in water. The applied concentration range was from 0.0031 (12) Yu, G. E.; Ameri, M.; Yang, Z.; Attwood, D.; Price, C.; Booth, C. J. Phys. Chem. B 1997, 101, 4394. (13) Ke´ki, S.; Dea´k, Gy.; Kuki, A Ä .; Zsuga, M. Polymer 1998, 39, 6053. (14) Yun, J.; Faust, R.; Szila´gyi, L. Sz.; Ke´ki, S.; Zsuga, M. Macromolecules 2003, 36, 1717. (15) Chen, L. J.; Lin, S. Y.; Huang, C. C.; Chen, E. M. Colloids Surf., A 1998, 135, 175. (16) Rosen, M. J. Surfactant and Interfacial Phenomenon, 2nd ed.; Wiley: New York, 1989. (17) Flockhart, B. D. J. Colloid Sci. 1971, 16, 484. (18) Stead, J. A.; Taylor, H. J. Colloid Interface Sci. 1969, 30, 482. (19) La Mesa, C. J. Phys. Chem. 1990, 94, 23. (20) Schulz, P. C.; Moya, S. Colloid Polym. Sci. 1998, 276, (1) 87.
10.1021/la062032r CCC: $37.00 © 2007 American Chemical Society Published on Web 12/05/2006
Self-Assembly of EO-Based Nonionic Surfactants
Langmuir, Vol. 23, No. 3, 2007 1015
Figure 2. MALDI-TOF-MS spectrum of Neodol 91-6E cationized with sodium ions. Table 1. Number Average Molecular Weights and HLB Numbers of the Neodol Samples Determined by MALDI-TOF-MS and 1H NMR
Figure 1. A representative intensity-log(concentration) plot for cmc determination for the Neodol samples (Neodol 91-5E, in H2O, t ) 35 °C, θ ) 90°). to 0.2 mg/mL for Neodol 91-2.5E, from 0.1 to 2.0 mg/mL for Neodol 91-5E and Neodol 91-6E, and from 0.1 to 4.0 mg/mL for Neodol 91-8E. The count intensity of the scattered light was plotted as a function of the logarithm of the concentration. Two lines were fitted on the initial and final parts of the curve. The cmc was given by the intercept of these lines as shown in Figure 1. Characterization. The MALDI-MS measurements were performed with a Bruker BIFLEX III mass spectrometer equipped with a time-of-flight (TOF) mass analyzer. In all cases a 19 kV acceleration voltage was used with pulsed ion extraction (PIE). The positive ions were detected in the reflectron mode (20 kV). A nitrogen laser (337 nm, 3 ns pulse width, 106-107 W/cm2) operating at 4 Hz was used to produce laser desorption, and 300 shots were summed. The spectra were externally calibrated with a poly(ethylene glycol) standard (Mn ) 1450 g/mol, Mw/Mn ) 1.02) using linear calibration. Samples were prepared with a DHB matrix (20 mg/mL), an analyte solution of 2 mg/mL, and sodium trifluoroacetate (1 mg/mL) as the cationization agent in methanol. The solutions were mixed in a 10: 5:1 (v/v/v) ratio (matrix/analyte/cationiation agent). A volume of 0.5 µL of the solution was deposited onto a metal sample plate and allowed to air-dry. Dynamic Light Scattering (DLS).21 For the DLS experiments a Brookhaven light scattering instrument equipped with a BI-9000 digital correlator and temperature-controlled goniometer was used. The light source was a solid-state vertically polarized laser operating at λ ) 533 nm. Using the methods of cumulants, the effective diameters (deff) of the micelles were determined from the characteristic decay rate (〈Γ〉) of the autocorrelation function of the scattered light at 90°. The diffusion coefficient (D) was calculated as D ) 〈Γ〉/q2, where q is the scattering vector. The effective diameter was determined from the Stokes-Einstein equation given by deff ) kT/ (3πηD), where k, T, and η are the Boltzmann constant, the temperature, and the viscosity of the solvent, respectively. The particle size distribution was determined at a 90° scattering angle and evaluated by the nonnegative constraint least-squares (NNLS) method. The polydispersity of the samples is represented as µ2/ Γ2,where µ2 is the second cumulant of the decay function.14 NMR. The 1H NMR spectra were recorded in CDCl3 at 25 °C on a Bruker AM 360 spectrometer with tetramethylsilane as the internal (21) Berne, B. J.; Pecora, R. Dynamic Light Scattering: With Applications to Chemistry, Biology and Physics; John Wiley and Sons: New York, 1976.
Neodol 91-2.5E
Neodol 91-5E
Neodol 91-6E
Neodol 91-8E
parent alcohol C9-C11 C9-C11 C9-C11 C9-C11 Mn (g/mol), MALDI/1H 429/257 506/370 590/410 614/486 NMR HLB, MALDI/1H 12.8/7.7 13.8/11.5 14.6/12.3 14.8/13.5 NMR
standard. Samples of 10 mg/mL concentration were prepared and were measured using the following experimental parameters (d1 ) 1 s, SFO1 ) 360.13216078 MHz, TD ) 16384, sw ) 15 ppm, rg ) 64, pw ) 0, signal-to-noise ratio >5000).
Results and Discussion Characterization of the Neodol Samples. The Neodol samples under investigation were multicomponent systems; therefore, thorough characterization was carried out. For the structure determination of the Neodol samples MALDI-TOFMS experiments were performed. The resulting spectrum with the assignations is shown in Figure 2. The mass series (M) appearing in the MALDI-TOF-MS spectra can be expressed as
M ) Mcat + Mendgroups + nMEO + Ma
(1)
where Mcat, Mend groups, MEO, and Ma are the masses of the cation, end groups, repeat unit (i.e., ethylene oxide, EO), and end group, respectively, and n stands for the number of EO units. As can be seen in Figure 2, there are three series in the spectrum. The main peak series can be assigned to the oligomers with a decyl (C10) alcohol end group, while the two additional series originated from surfactant molecules with undecyl (C11) and nonyl (C9) alcohol end groups. The mass difference between the identical series is 44 Da, which corresponds to the mass of an ethylene oxide (C2H4O) unit, while the mass difference between two adjacent peaks is 14 Da, which is the mass of a methylene (CH2) unit. Since MALDI is capable of recording individual oligomer chains present in the sample, the number average molecular weight and the HLB values can be calculated from the MALDI-TOF spectrum according to eq 2, where I is the intensity and Mi is the molecular weight of a peak in the spectrum, and eq 3, where Mn,PEO represents the average molecular weight of the poly(ethylene oxide) (PEO) segment in a given surfactant molecule
1016 Langmuir, Vol. 23, No. 3, 2007
Nagy et al.
EO units) can be ionized more effectively; i.e., the ionization efficiency is higher as compared to that of the oligomers with short EO chains. This conclusion is also supported by the observation that in the case of Neodol 91-2.5E peaks at m/z 255, 269, and 283 (corresponding to the sodiated adduct of Neodol 91-2.5E with two EO units) are completely absent from the MALDI-TOF-MS spectrum, indicating very poor ionization efficiency for these oligomers. Interestingly, while both Mn and HLB depend significantly on the EO chain length, the relative peak intensities (r(C9), r(C10), r(C11)) of oligomers with C9, C10, and C11 alcohol end groups (defined by eqs 6-8, where IC9, IC10, and IC11 are the MALDITOF-MS intensities of the oligomers with C9, C10, and C11 alcohol end groups) are independent of the number of EO units as shown in Figure 2. Figure 3. Relative intensities (r) of the MALDI-TOF-MS peaks of Neodol 91-5E and 91-8E as a function of the number of EO units (nEO).
and Mn is the average molecular weight of the same molecule.
∑IiMi)/(∑Ii) HLB ) 20[1 - (∑Mn,PEO)/(∑Mn)] Mn ) (
(2) (3)
However, there may be one serious drawback when one uses a MALDI spectrum to determine the Mn and HLB values. In eqs 2 and 3 it is assumed that all oligomer chains can be ionized with the same efficiency, i.e., that the ionization efficiency does not depend on the chain length. This assumption may be valid for longer EO chains, but for oligomers with relatively short EO chains the ionization efficiency may be highly chain length dependent. Therefore, to test the applicability of MALDI-TOFMS for the molecular weight determination of the Neodol samples, 1H NMR measurements were performed. 1H NMR spectroscopy is a widely used and accepted method for the determination of the number average molecular weights (Mn) and the HLB value. The molecular weights from the 1H NMR spectrum were calculated according to eq 4, where SCH3 stands for the signal intensity of the terminal methyl (0.9 ppm) group and SOCH2 for
Mn ) [(SOCH2/SCH3)(3/4)MEO] + Malc
(4)
that of the OCH2 groups of the ethylene oxide units (3.2 ppm), MEO is the molecular weight of the ethylene oxide repeating unit (44 g/mol), and Malc is the molecular weight of the base alcohol (158 g/mol average). The number average HLB values were determined from the NMR spectrum according to eq 5.
HLB ) [(SOCH2/SCH3)(3/4)MEO]/Mn
(5)
The Mn and HLB values determined by the 1H NMR and MALDI-TOF-MS methods are summarized in Table 1. The data of Table 1 show that there are significant differences in the values of Mn and HLB determined by 1H NMR and MALDI-TOF-MS. The Mn and HLB values calculated from the MALDI-TOF-MS spectra are significantly higher than those determined by 1HNMR for each sample. On the other hand, the differences between the corresponding Mn and HLB values determined by the two methods decrease as the number of EO units increases. This result suggests that signals from oligomers with longer EO chains dominate over those containing shorter EO chains, causing an apparent shift to higher Mn values. The reason for this shift is that the oligomers with a longer hydrophilic segment (i.e., more
r(C9) ) IC9/(IC9 + IC10 + IC11)
(6)
r(C10) ) IC10/(IC9 + IC10 + IC11)
(7)
r(C11) ) IC11/(IC9 + IC10 + IC11), i.e., r(C11) ) 1 - r(C9) - r(C10) (8) The results presented in Figure 3 demonstrate that approximately the same relative intensities can be found for the oligomers with C9, C10, and C11 alcohol end groups at low (4-5 EO units) and at high (18-19 EO units) EO contents independently of the nature of the Neodol samples. According to Figure 3, and assuming that a small modification of the end groups by one and two CH2 units does not affect the ionization efficiencies significantly, it can be concluded that each Neodol sample contains about 50%, 30%, and 20% (n/n) C10, C11, and C9 alcohol end groups. Since the 1H NMR signal of a proton is widely accepted to be proportional to the concentration of that proton, we accepted the results of 1H NMR as the correct values for Mn and HLB, and these values were used in further investigations. However, it should be noted that although MALDI-TOF-MS failed to give the correct Mn and HLB values for these low molecular weight Neodol samples, it proved to be capable of determining not only the end groups but also the composition of a multicomponent mixture. Self-Assembly Behavior of the Neodol Samples. The variation of the cmc with the HLB number of the Neodol samples was investigated. It is expected that samples with a larger hydrophilic segment, i.e., with a higher HLB number, are better solubilized in water as compared to those with shorter hydrophilic segments (at a constant length of the lipophilic segment). Therefore, an increase of the cmc with the HLB number can be expected for the Neodol samples. Indeed, the cmc increases with the HLB number as shown in Figure 4. To explain the dependence of the cmc on the HLB number, we first considered the thermodynamics of micellization. The Gibbs energy of micellization is given by the following equation:
∆Gmic° ) RT ln(cmc)
(9)
∆Gmic° can be expressed as
∆Gmic° ) ∆Glip° + ∆Ghyd°
(10)
where ∆Glip° and ∆Ghyd° are the contributions of the lipophilic and the hydrophilic segments to the total Gibbs energy. If ∆GEO° denotes the contribution of the free energy of one EO unit to ∆Ghyd° for the hydrophilic segment consisting of n
Self-Assembly of EO-Based Nonionic Surfactants
Langmuir, Vol. 23, No. 3, 2007 1017 Table 2. Effective Diameters of Micelles at 25 and 35 °C Determined by DLSa
effective diameter (nm), T ) 25 °C effective diameter (nm), T ) 35 °C a
Figure 4. cmc values for the Neodol samples as a function of the HLB number at 25 and 35 °C.
Figure 5. Variation of ln(cmc) with the average number of EO units at 25 and 35 °C.
EO units, ∆Ghyd° becomes
∆Ghyd° ) n∆GEO°
(11)
Substituting eqs 10 and 11 into eq 9, eq 12 is obtained. Equation 12 suggests that by plotting ln(cmc) as a function of the number of EO units (n) a linear relationship with a slope of ∆GEO°/(RT)
ln(cmc) ) ∆Glip°/(RT) + n∆GEO°/(RT)
(12)
should be obtained (note that the length of the lipophilic segment does not vary). Indeed, the ln(cmc) values change linearly with the average number of EO units as shown in Figure 5. From the slopes of the lines depicted in Figure 5, the values of ∆GEO° [0.9 kJ/mol (at 25 °C) and 0.3 kJ/mol (at 35 °C)] can be determined. On the basis of these results, the dependence of the cmc on the HLB number (Figure 4) can now easily be interpreted by (22) Alexandridis, P., Lindman, B., Eds. Amphiphilic Block Copolymers; Elsevier: Amsterdam, Lausanne, New York, Oxford, Shannon, Singapore, Tokyo, 2000; p 13.
Neodol 91-2.5E
Neodol 91-5E
Neodol 91-6E
Neodol 91-8E
326 (0.29)
135 (0.33)
60 (0.20)
5.4 (0.48)
232 (0.49)
78 (0.29)
73 (0.18)
6.6 (0.31)
The numbers in parentheses stand for the micelle size distribution.
taking into account that the average number of EO units is proportional to the value of HLB/(20 - HLB). The sizes of the micelles formed from the Neodol samples were also investigated, and most systems showed a unimodal size distribution. The average diameters of the micelles with the micelle size distributions are summarized in Table 2. According to the data of Table 2, the micelle size decreases with the HLB number. The largest micelle sizes were measured for the Neodol 91-2.5E sample and the smallest for Neodol 91-E. It is also interesting to note there is approximately a 10-fold increase in the micelle size in the case of Neodol 91-6E as compared to Neodol 91-8E. This large increase is more surprising when considering that there is only an approximately 25% decrease in the average number of EO units. It is known that when a block copolymer is dissolved in a block-selective solvent both the chain composition and the architecture affect micellization.14 As it turns out, in the case of the Neodol samples the micelle size is highly dependent on the length of the hydrophilic segment. It was shown that the short, well-dissolving segment results in the formation of vesicle-like structures.22 As the chain length is increased, formation of core-shell-type micelles can be expected. In the case of the Neodol samples the well-dissolving chain is the EO chain; therefore, its length determines the architecture and the size of the aggregates formed in water. The aggregate size also varies with the temperature, most probably due to the temperature-dependent EO chain-water interaction. Raising the temperature may result in an induced conformational change; i.e., the more polar conformers dominate at lower temperatures and the less polar at higher temperatures. This type of rearrangement gives a good explanation for the micelle size variation with the temperature in the case of the Neodol samples.
Conclusion The self-assembly of polydisperse industrial-type ethoxylatebased nonionic surfactants was studied. Important solution parameters such as the critical micelle formation concentration, micelle size, and size distribution were measured. It was found that the cmc value increases with the HLB number, and a linear correlation was found between ln(cmc) and the average number of EO units. From the slopes of the ln(cmc) versus average number of EO units the contribution of a single EO unit to the total free energy of micellization was deduced. The samples were characterized by MALDI-TOF-MS and 1H NMR methods, and the number average molecular weight and HLB numbers were determined from the corresponding spectra. There were significant differences between the values of both Mn and HLB determined by means of the two methods. The reason for these differences was thoroughly discussed. Acknowledgment. This work was financially supported by Grant Nos. K 62213, M 28369, and M 36872 given by OTKA (National Fund for Scientific Research Development, Hungary), RET 006/2004, and the Bolyai Ja´nos Fellowship. LA062032R
