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Langmuir 2002, 18, 7538-7543
Characterization of Melamine-Formaldehyde Resins by XPS, SAXS, and Sorption Techniques A. Deryło-Marczewska,* J. Goworek, S. Pikus, and E. Kobylas Faculty of Chemistry, Maria Curie-Skłodowska University, 20031 Lublin, Poland
W. Zgrajka Institute of Agricultural Medicine, 20950 Lublin, Poland Received March 5, 2002. In Final Form: June 26, 2002 The mesoporous melamine-formaldehyde resin was synthesized in the process of polymerization in the presence of fumed silica as an inorganic template. To differentiate the surface properties, the polymer sample was modified by applying 2-methyl-1-butanol. The surface and structural characteristics of obtained sorbents were investigated using various techniques: XPS, SAXS, and sorption from gas phase. The porous structure of initial and modified melamine-formaldehyde resins was determined from nitrogen adsorption/desorption isotherms. To study the polymer activity toward organic substances dissolved in water, the isotherms of sorption of organics from aqueous phase were measured for a wide range of solution concentrations. The liquid sorption data were analyzed to study the effect of polymer structure and surface properties on sorption processes.
Introduction During the past three decades, the synthesis and characterization of organic polymers have become one of the most important research areas in polymer science.1,2 The study of polymer surface and geometrical structure is of analytical relevance. Many industrial and analytical applications of polymers require systems with specific surface properties. The studies of gas/liquid- polymer interfaces have received considerable attention because of the related analytical applications of resins in chromatography as column packings and as specific sorbents for removal of organic substances from various gas and liquid media.3-5 Hence, the understanding and characterization of sorption equilibria on various types of polymers are of great importance. The chemical character of polymer surface and porosity determine mainly the sorption abilities of polymers. The resin synthesis can be easily modified to obtain sorbents differentiated with regard to their porous structure and chemical character of the surface groups. Thus, such materials may be utilized in various applications. Moreover, porous polymers are widely used owing to their stability over a wide pH range. Many synthesis techniques are used to obtain porous polymers. As the porosity agent, the inert diluent or inorganic matrix as a template is introduced to the polymerizing mixture.6 The polymers obtained in such a * To whom correspondence should be addresed. E-mail: annad@ hermes.umcs.lublin.pl. Fax: (48-81)-5333348. (1) Belyakova, L. D.; Kiselev, A. V.; Platonova, N. P.; Shevchenko, T. I. Adv. Colloid Interface Sci. 1984, 21, 55-118. (2) Tsyurupa, M. P.; Shabaeva, A. S.; Pavlova, L. A.; Mrachkovskaya, T. A.; Davankov, V. A. In Characterisation of Porous Solids IV; McEneney, B., Mays, T. J., Rouque´rol, J., Rodrı´guez-Reinoso, F., Sing, K. S. W., Unger, K. K., Eds.; The Royal Society of Chemistry: Cambridge, U.K., 1997; pp 398-405. (3) Abe, I.; Hayashi, K.; Hirashima, T. J. Colloid Interface Sci. 1983, 94, 577-579. (4) Itaya, A.; Kato N.; Yamamoto, J.; Okamoto, K. J. Chem. Eng. Jpn. 1984, 17, 389-395. (5) Goto, S.; Goto, M.; Uchiyama, S. J. Chem. Eng. Jpn. 1984, 17, 204-205. (6) Feibush, B.; Li, N.-H. U.S. Patent 4,933,372, 1990.
way have a dual nature of pore system. The small pores (micropores) are formed on polymer surface and among spherical particles in polymer aggregates during a crosslinking process. The wider pores (mesopores) arose from the presence of the porosity agent. Thus, porous polymers exposed a highly developed surface area. Absorption and adsorption occur simultaneously during sorption of liquids and vapors on polymers. Absorption results in the swelling of polymer skeleton in contact with various adsorbates.7-10 The swelling process starts simultaneously with the sorption before pores of the dry sorbent are filled. As a result, the space accessible for sorbate molecules increases; the sorbate molecules do not only fill the pore system but also penetrate the polymer skeleton. Thus, the textural characteristics of the resins are quite different in dry and wet (in contact with liquid media) states.11 Saturation of the polymer with organic solvents and water results in changing substantially pore diameters and pore size distributions. Thus, the sorption process is a combination of adsorption and absorption phenomena and the proportion of both these effects is determined by the degree of the swelling process in contact with gas and liquid media and interactions of sorbate molecules with a solid surface. In the present work, the studies of surface and structure properties of cross-linked porous melamine-formaldehyde resin are presented. The polymer MEA was synthesized in the process of condensation of formaldehyde with melamine; the fumed silica was used as a template forming the pore structure of cross-linked polymer. The obtained polymer sample was also chemically modified by applying 2-methyl-1-butanol to differentiate its surface properties (7) Dufka, O.; Maslinsky, J.; Churac`ek, J.; Komare, K. J. Chromatogr. 1970, 51, 111-117. (8) Omidian, H.; Hashemi, S. A.; Sammes, P. G.; Meldrum, I. Polymer 1998, 39, 6697-6704. (9) Chuang, W.-Y.; Young, T.-H.; Wang, D.-M.; Luo, R.-L.; Sun, Y.M. Polymer 2000, 41, 8339-8347. (10) Soles, Ch.; Yee, A. F. J. Polym. Sci., Part B: Polym. Phys. 2000, 38, 792-802. (11) Goworek, J.; Stefaniak, W.; Zgrajka, W. Mater. Chem. Phys. 1999, 59, 149-153.
10.1021/la0202172 CCC: $22.00 © 2002 American Chemical Society Published on Web 08/31/2002
Characterization of Melamine-Formaldehyde Resins
(sample MEM). The surface characterization of the initial sample and the chemically modified one was made by means of the SAXS and XPS methods. The pore structure of initial and modified resins was studied on the basis of low-temperature nitrogen isotherms. The resulting adsorption isotherms were used to evaluate the standard quantities characterizing the surface and structural features of studied polymers: BET specific surface area, the total and mesopore volumes, and the external surface area. The mesopore volumes and external surface area were calculated by applying the Rs-method; a nonporous melamine-formaldehyde polymer synthesized in the absence of silica modifier was used as a reference material. The pore size distributions were obtained from desorption branches of nitrogen isotherms by using the BarrettJoyner-Halenda (BJH) method. The sorption activity of melamine-formaldehyde resins toward organic substances dissolved in water was studied for two chosen compounds. Experimental Section Materials. The components for polymer synthesis and modification: melamine (purity 99%), paraformaldehyde (purity 95%), fumed silica (primary particle size 7 nm, SBET ) 370 m2/g), and 2-methyl-1-butanol (purity 99%) were purchased from Aldrich. The organic adsorbates of commercially available quality: nitrobenzene and 4-nitrophenol were supplied by Merck and Aldrich. The liquid mixtures were prepared with bi-distilled water. Methods. Resin Synthesis. The synthesis of hyper-cross-linked melamine-formaldehyde resins was performed by contacting a reacting mixture of melamine and paraformaldehyde with fumed silica as the inorganic template at controlled pH.11,12 This mixture was aged during three days and was heated. Next, the template silica particles were removed from the cross-linked copolymer by dissolution in alkaline solution (NaOH) having a pH at about 12 without destruction of the polymerizate. Prior to experiment, the final product was washed with hot water and acetone. Chemical character of the ME resin surface, which is in contact with adsorptives, is determined mainly by the presence of amine groups, hydrophobic hydrocarbon segments, and nitrogen atoms in triasine rings and additionally by the hydroxyl groups in the modified sample. The modification of obtained polymer was performed with 2-methyl-buthanol following hydroxylation of ME resin with formaldehyde. The resulting character of the surface and surface chemical stoichiometry were determined using X-ray photoelectron spectroscopy (XPS) and small-angle X-ray scattering (SAXS) techniques. Gas Adsorption Experiments. Nitrogen adsorption/desorption isotherms at 77 K were determined volumetrically using ASAP 2405N analyzer (Micromeritics Corp., U.S.). Before the experiment, the adsorbents were degassed (∼10-2 Pa) at 493 K. XPS Measurements. XPS spectra were obtained using ESCALAB-210 electron spectrometer (VG Scientific Ltd., Sussex, U.K.) employing Mg KR X-rays. The measurements were performed at base pressure lower than 8‚10-7 Pa. The X-ray gun was operated at 15 kV and 20 mA. Survey scans were collected from 0 to 1200 eV with the pass energy of 50 eV for each sample followed by the C1s, N1s, and O1s regions. Each spectral region was scanned between 10 and 20 times depending on the signal intensity. Data processing was performed using ECLIPSE program applying Shirley-type background subtraction. Curve fitting was performed using the nonlinear least-squares algorithm and assuming a mixed Gaussian/Lorentzian peak shape of variable proportion. This peak-fitting was repeated until an acceptable fit was obtained. XPS data were also used to examine the atomic composition and surface species of initial and modified polymer. Atomic percentage values and elemental ratios were calculated from the peak-area ratios after correcting with the (12) Deryło-Marczewska, A.; Goworek, J.; Zgrajka, W. Langmuir 2001, 17, 6518-6523.
Langmuir, Vol. 18, No. 20, 2002 7539 sensitivity factors determined by Scofield and are reliable to within (10%. SAXS Measurements. SAXS measurements were performed on a slit-collimated Kratky camera using a Cu KR radiation. The scattered intensity measurements were carried out for each of the investigated samples as well as for the empty cuvette (background scattering). The background scattering curve was each time subtracted from the scattering curve for an investigated sample. The absorption coefficient was also measured for each sample. Subsequently, the SAXS curves were recalculated considering the differences in absorption coefficient. Consequently, the curve, the intensity scattering I(q) versus module of scattering vector q (where q ) 4πsin θ/λ, 2θ-scattering angle, λ- wavelength), for each sample was obtained. The geometry of SAXS camera and other conditions of the SAXS experiments allowed for treatment of the scattering curves as slit-smeared data for a beam of infinite length. Liquid Adsorption Experiments. The experimental isotherms for nitrobenzene and 4-nitrophenol sorption from dilute aqueous solutions on both investigated melamine-formaldehyde resins were measured at 293 K at constant pH (pH ) 2.2) and ionic strength (I ) 0.1 mol/dm3) by using a static method. Adding HCl and NaCl solutions adjusted the pH and ionic strength, respectively. The known amount of polymer was first contacted with 5 cm3 of water and degassed under vacuum in Erlenmeyer flasks. It was kept in water to swell and then the adsorbate solution of known concentration was added. The vessels were thermostated at 293 K and agitated until the equilibrium was attained. Finally, the equilibrium solute concentrations were measured by using the UV-vis spectrophotometer Specord M40 (Carl Zeiss, Jena). The adsorbed amount of organic compound was calculated from the material balance. For planning the liquid adsorption experiments, the special simulation procedure was used.13
Results and Discussion Structure Characteristics from Nitrogen Adsorption. To characterize the pore structure of synthesized resins, the low-temperature adsorption/desorption isotherms of nitrogen were measured. The adsorption data were used to evaluate the BET specific surface area, SBET, and the total pore volume, Vt, by applying the standard methods (SBET from the linear BET plots and Vt from the adsorption at the relative pressure p/po ) 0.975).14 The external (macropore) surface area, Sext, and the mesopore volume, Vmes, were obtained from the Rs plot method.14-16 This method is based on the comparison of nitrogen isotherm on a studied solid with the standard isotherm on a reference nonporous adsorbent. This standardreduced isotherm Rs is defined as the ratio of adsorption value corresponding to a given relative pressure p/po and the adsorption value at the point p/po ) 0.4 (po is the saturation pressure, p/po ) 0.4 is a starting point of isotherm hysteresis loop for nitrogen adsorption). As the reference adsorbent, the specially synthesized nonporous melamine-formaldehyde resin ME-0 was used.12 The mesopore structure was characterized by the distribution function of mesopore volume calculated by applying the Barrett-Joyner-Halenda (BJH) method.17 In Figure 1, the adsorption/desorption isotherms of nitrogen on the initial (MEA) and modified (MEM) polymers are presented. In both resins, a similar course of the isotherms is observed: a monotonic increase of adsorption over the pressure range (0; 0.7), stronger growth for p/po > 0.7, and its stabilization for pressures (13) Deryło-Marczewska, A. Langmuir 1993, 9, 2344-2350. (14) Gregg, S. J.; Sing, K. S. W. Adsorption, Surface Area and Porosity; Academic Press: London, 1982. (15) Sayari, A.; Liu, P.; Jaroniec, M.; Kruk, M. Chem. Mater. 1997, 9, 2499-2506. (16) Kaneko, K.; Ishii, C.; Ruike, M.; Kuwabara, H. Carbon 1992, 30, 1075-1088. (17) Barrett, E. P.; Joyner, L. G.; Halenda, P. P. J. Am. Chem. Soc. 1951, 73, 373-380.
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Figure 1. The nitrogen adsorption/desorption isotherms for the initial and modified melamine-formaldehyde resins MEA and MEM.
Figure 2. The Rs plots for nitrogen adsorption isotherms for the initial and modified melamine-formaldehyde resins MEA and MEM. Table 1. Values of Parameters Characterizing Porous Structure of Polymeric Sorbents, Calculated from Nitrogen Adsorption/Desorption Isotherms SBET Vt Vmeso+micro Sext resin [m2/g] [cm3/g] [cm3/g] [m2/g] D [nm] Vmeso+micro/Vt MEA MEM
220 150
0.45 0.33
0.36 0.28
68 45
7.3 6.1
0.80 0.83
higher than 0.9. The mesoporous character of polymer pore structure is evidenced by the appearance of hysteresis loop on nitrogen isotherms, which corresponds to capillary condensation phenomenon. In the resin MEA, higher adsorption values are observed over the whole pressure range. In Table 1, the standard parameters characterizing the porous structure of the melamine-formaldehyde resins MEA and MEM, the BET specific surface area SBET and the total pore volume Vt, are summarized. Moreover, the values of the external surface area, Sext, and the mesopore and micropore volume, Vmeso+micro, calculated from the higher-pressure parts of Rs-plots are also compared in Table 1. The Rs-plots for the studied polymers are drawn in Figure 2. In the MEA and MEM resins, a very similar course of the Rs-plots is observed over the whole range of pressures, evidencing the same type of porosity for both polymers. However, for MEA the Rs-plot corresponds to higher adsorption values. Three main segments can be distinguished on the plots under study. The linear behavior is observed for low (Rs < 1) and high pressures (Rs ≈ 1.8) corresponding to adsorption in micropores and on external surface. The linear low-pressure plots do not intercept the adsorption axis at the zero point evidencing the existence of small amounts of micropores. The second highpressure linear part of Rs-plot (Rs > 1.8) was used to estimate the sum of micropore and mesopore volume (from
Figure 3. The differential distribution functions of mesopore volumes for the initial and modified melamine-formaldehyde resins MEA and MEM calculated from nitrogen desorption isotherms.
the intercept of this straight line with adsorption axis) and the external surface area (from the slope of straight line). The medium range of these plots reflects the capillary condensation in mesopores. Figure 3 presents the distribution functions of mesopore volume calculated for nitrogen desorption isotherms by using the BJH method. For the resin MEM, the main peak is shifted toward lower pore diameters as a result of binding the hydrocarbon phase. In Table 1, the values of BJH average pore diameter, D, are also given. Analyzing the values of presented parameters, one can state that the resin MEA has higher specific surface area (220 m2/g), total pore volume (0.45 cm3/g), mesopore and micropore volumes (0.36 cm3/g), external surface area (68 m2/g), and higher mean mesopore diameter (7.3 nm) in comparison to the modified sample MEM. Surface Characteristics from XPS Spectra. The XPS wide scans for both polymers indicate that the main contribution to the spectrum is the N1s peak (≈400 eV) and C1s peak (≈290 eV).18,19 A lower intensity signal at ≈530 eV corresponds to O1s core level. To compensate for surface charging effect, all binding energies (BEs) were referenced to the C1s peak at 285.00 eV (C-C/C-H for saturated hydrocarbons) for MEM sample and C1s peak at 286.1 eV (C-OH) for MEA sample or N1s peak at 398.4 eV (dN-) for both polymer samples.20 Two remaining peaks N1s correspond to -NH- group (BE ≈ 399.5 eV) and positively charged nitrogen (BE > 400 eV) (see Figure 4). However, charging for polymer samples was not corrected very precisely, so BE values observed in the present study are reliable only within 0.5 eV. Moreover, because of porosity of the samples, intensity on the spectra is very low in comparison to a smooth, flat surface. The atomic compositions of the surface for porous polymers are given in Table 2. The data suggest that the amount of carbon as well as oxygen at the surface of MEM sample is considerably greater than in the MEA sample. The enrichment of the aliphatic carbons at the particle surface confirms its chemical modification with strongly bound butyl species. The overview spectra do not indicate the presence of silica and sodium atoms introduced to the polymerizing mixture during synthesis. Figure 5 shows C1s spectra, which indicate three kinds of carbon species for initial polymer sample and four carbon (18) Kang, E. T.; Neoh, K. G.; Tan, K. L. In Handbook of Organic Conducting Molecules and Polymers; Nalwa, H. S., Ed.; John Wiley: Chichester, U.K., 1997; Vol. 3, Chapter 3. (19) Beamson, G.; Briggs, D. High-Resolution XPS of Organic Polymers. The Scienta ESCA 300 Data Base; John Wiley: Chichester, U.K., 1992. (20) Lim, S. L.; Tan, K. L.; Kang, E. T. Langmuir 1998, 14, 53055313.
Characterization of Melamine-Formaldehyde Resins
Figure 4. XPS N1s spectra (Gaussian-Lorentzian fitted lines) for the initial and modified melamine-formaldehyde resins MEA (A) and MEM (B). Table 2. Surface Composition of the MEA and MEM Resins as Determined by XPS atom % resin
C1s
N1s
O1s
MEA
50.48 (CH/CCs1.97) 59.80 (CH/CCs6.53)
45.46
4.06
32.00
8.20
MEM
species for modified sample. The peak, which appears in this last case, may be attributed to hydrocarbon chains bounded to the surface. The C1s peak was fitted with four components at binding energies (BE) of 285.0, 286.1, 287.6, and 288.8 eV. The curve fitting was performed assuming a Gaussian (30%)-Lorentzian (70%) peak shape. In the modified surface, these components correspond to hydrocarbon, C-OH, CdN, and carboxyl type functional groups, respectively. A large contribution of the carbon groups at binding energy higher than 285.0 eV reflects oxidation of the surface during chemical treatment. The main contribution to the spectrum is the C1s peak corresponding to -CdN- group present in melamine rings (BE ≈ 287.6 eV). The small signal at 284.1 eV for MEA sample indicates the presence of the so-called “chain” carbon or carbon. The small peak at BE 284.1 eV for the initial sample may represent traces of carbon formed during the treatment of polymer mixture during synthesis with sulfuric acid. For the modified polymer surface, an additional peak appears at 285.0 eV. This indicates a substantial rise in the amount of C-H in comparison with the MEA sample and reflects the presence of C4 hydrocarbon chains at the polymer surface. Surface Characteristics from SAXS. During the past several years, small-angle X-ray scattering (SAXS) and neutron scattering (SANS) have been found to be especially well suited for studying the porous, disordered systems.
Langmuir, Vol. 18, No. 20, 2002 7541
Figure 5. XPS C1s spectra (Gaussian-Lorentzian fitted lines) for the initial and modified melamine-formaldehyde resins MEA (A) and MEM (B).
It has also been recently pointed out that SAXS can be used for characterization of interface (boundary) between two phases of different electron density.21-24 The scattering intensity is proportional to the square of the difference of the electron density between the scattering heterogeneities and their surrounding. In porous materials, the pores are treated as the heterogeneities whose electron density differs from that of the materials constituting the porous skeleton. There are many porous materials for which the power law, I(q) d Io‚q-R (R, Io are constants), is fulfilled in a certain q region. The value R ) 4 (3 for slit-smeared data) corresponds to the three-dimensional, nonfractal heterogeneities with a sharp interface border (this relation is known as the Porod law), whereas 4 < R < 6 (3 < R < 5 for slit-smeared data) corresponds to the threedimensional, nonfractal objects with transition layer on the interface border. For R ) 4, the value of the I(q)‚q4 (I(q)‚q3 for slit-smeared data) is constant in the certain q region. Hence, R exceeding 4 (3 for slit-smeared data) can be assumed as a negative deviation from the Porod law. Figure 6 shows the Porod plots (I(q)‚q3 vs q) for the investigated samples. The negative slope of the end of the plot for the sample MEM (in comparison to almost parallel run for the sample MEA) clearly indicates the existence of the surface transition layer. Up to now, a few methods have been presented for analyzing a deviation from the Porod law.21,22,24-26 The thickness of the transition layer was determined by Ruland-Vonk relation (see eq 11 ref 24). (21) Schmidt, P. W.; Avnir, D.; Levy, D.; Ho¨hr, A; Steiner, M.; Ro¨ll, A. J. Chem. Phys. 1991, 94, 1474-1479. (22) Benedetti, A.; Ciccariello, S. J. Appl. Cryst. 1994, 27, 249-256. (23) Pikus, S.; Dawidowicz, A. L.; Kobylas, E.; Wianowska, D. Appl. Surf. Sci. 2000, 156, 189-199. (24) Pikus, S.; Kobylas, E. Colloids Surf. 2002, 208, 219-229. (25) Ruland, W. J. Appl. Cryst. 1971, 4, 70-73. (26) Vonk, C. G. J. Appl. Cryst. 1973, 6, 81-86.
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Figure 6. Porod plots of the small-angle X-ray intensities scattered by the samples MEA and MEM. Table 3. Transition-Layer Thickness Calculated from SAXS Measurements resin
Ra
thickness of transition layer
MEA MEM
3.01 3.10
1 Å (1 Å 5 Å (1 Å
a Values for slit-collimated data, for point-collimated data the value 1 should be added.
Figure 8. The sorption isotherms for nitrobenzene and 4-nitrophenol from dilute aqueous solutions on the initial and modified melamine-formaldehyde resins MEA and MEM. Table 4. Values of Parameters Characterizing Adsorption of Organic Substances from Dilute Aqueous Solutions on Polymeric Sorbents at 293 Ka resin
adsorbate
log xo
m
MEA
nitrobenzene 4-nitrophenol nitrobenzene 4-nitrophenol
-0.83 ( 0.01 -0.55 ( 0.01 -0.88 ( 0.01 -1.14 ( 0.01
0.50 ( 0.01 0.51 ( 0.01 0.53 ( 0.01 0.67 ( 0.02
MEM
a For all data sets, number of experimental points n ) 16, degrees of freedom f ) 14. For confidence level a ) 0.05, the critical Student parameter tcrit,R ) 2.14.
Figure 7. Pore size distribution functions for the initial sample MEA calculated by two methods proposed by Vonk.27
Table 3 collects the values calculated from the SAXS data. As is seen in Table 3, the transition-layer thickness for the sample MEA is practically equal to 0 (1 Å ( 1 Å); however, for the sample MEM the thickness of this layer is much greater (5 Å ( 1 Å). The estimated value correlates quite reasonably with model length of n-butyl hydrocarbon chain. In Figure 7, the pore size distribution function for the sample MEA calculated by two different methods proposed by Vonk27,28 are presented. Both distributions in Figure 7 are very similar, with maximum at pore diameter D equal to about 100 Å. This result correlates quite reasonably with pore size distributions estimated from adsorption data of liquid nitrogen at 77 K. Sorption of Organic Substances from Dilute Solutions. To investigate the differences in sorption capabilities of synthesized and modified samples of melamineformaldehyde polymers toward organic substances dissolved in water, the sorption isotherms for nitrobenzene and 4-nitrophenol were measured. The experimental sorption isotherms of nitrobenzene and 4-nitrophenol from dilute aqueous solutions on the resins MEA and MEM (27) Vonk, C. G. J. Appl. Cryst. 1976, 433, 433-440. (28) Pikus, S. J. Catal. 1992, 136, 334-341.
are presented in Figure 8. Analyzing the sorption curves in Figure 8, one can state that in the MEA polymer the highest sorption was observed for 4-nitrophenol; however, for MEM nitrobenzene was adsorbed stronger than p-nitrophenol. Comparing the sorption of nitrobenzene on both resins, one can observe only a slight decrease of sorption value on the modified sample MEM. It may be a global effect of the decrease of specific surface area and pore volume for this polymer and the simultaneous increase of its hydrophobic character. In p-nitrophenol, we observe a strong decrease of sorption values for the resin MEM as a result of three above-mentioned effects. The experimental sorption isotherms for nitrobenzene and 4-nitrophenol on the resins MEA and MEM are also presented in the coordinates log a versus log c (Figure 8) of the linear form of Freundlich equation derived in terms of the general theory of adsorption on energetically heterogeneous solids:29
log a ) log xo + mlog c In the above, a is the adsorbed amount in [mmol/g], c is the adsorbate concentration in [mmol/l], xo is the parameter connected with the position of the adsorption energy distribution on the energy axis, and m is the so-called heterogeneity parameter characterizing the shape of the energy distribution. For all sorbates, this simple Freundlich equation was a good description of sorption processes. The values of parameters of the Freundlich isotherm are collected in Table 4. The parameter m is a measure of adsorption system global nonhomogeneity and it reflects the dif(29) Deryło-Marczewska, A.; Jaroniec, M. Surf. Colloid Sci.; Plenum Press: New York, 1987; Vol. 14, pp 301-379.
Characterization of Melamine-Formaldehyde Resins
ferentiation in sorbent pore structure and surface groups as well as in sorbate properties. By analyzing the values of m, one can find that in the MEM polymer higher values are observed indicating smaller heterogeneity effects. The increase of m value may be correlated with a certain unification of the resin surface by binding the hydrocarbon phase. However, this difference seems to be significant for 4-nitrophenol; all other values are in fact very close. It cannot be ruled out that this deviation of m value for 4-nitrophenol is partly a result of its weaker adsorption; thus, the fitting region covers, to a certain extent, a range where the isotherm changes its character tending to Henry region. The adsorbate orientation effects may be also taken into account.30 The molecule of 4-nitrophenol, independently of its parallel or perpendicular orientation in surface phase, interacts through benzene ring and both functional groups (parallel) or only through one of functional groups (perpendicular). However, in nitrobenzene two distinctly different perpendicular orientations are possiblesone interaction with benzene ring, the other with nitro- group. Thus, only low or moderate heterogeneity effects were found for all studied sorption systems indicating that the resins ME have rather uniform pore structure and homogeneous surface chemistry. Despite their relative uniformity, these polymeric sorbents create large possibilities for modification of their structure and surface properties, which opens a way to obtain materials of differentiated surface properties and high selectivity toward various substances.
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organic materials with highly developed surface area. The synthesized materials can be chemically modified to differentiate their surface properties. Simultaneously, this process leads to the changes in polymer surface areas and porosity. The surface characteristics of studied resins were determined from nitrogen adsorption isotherms: the BET specific surface area, total pore volume, mesopore volume, and external surface area. The differences in surface character of the initial and modified polymers were determined satisfactorily by XPS spectra evidencing the presence of the bound alkyl chains for the chemically treated MEM resin. The SAXS measurements allowed estimating the thickness of the surface transition layer on the modified polymer. The analysis of sorption processes of nitrobenzene and p-nitrophenol from dilute aqueous solutions showed the differences among the studied systems, which were related to their pore structure and hydrophobicity of sorbent surface. The changes of sorbed amounts for the initial and modified polymers were regarded as a summary effect of differences in the surface area, pore volume, and hydrophobic character of sorbent surface. The increase of surface hydrophobicity in the melamineformaldehyde resin with bound hydrocarbon phase is confirmed also by gas chromatographic experiments for substances of differentiated polarity presented in our previous paper.31 Acknowledgment. Financial support from the State Committee for Scientific Research (KBN, Warsaw) Project No. 3 T09B007 17 is gratefully acknowledged.
Conclusions The synthesis of melamine-formaldehyde resins in the presence of fumed silica allows obtaining the porous
LA0202172
(30) Marczewski, A. W.; Deryło-Marczewska, A.; Jaroniec, M. J. Colloid Interface Sci. 1986, 109, 310-324.
(31) Goworek, J.; Stefaniak, W.; Deryło-Marczewska, A.; Zgrajka, W. Anal. Bioanal. Chem., submitted for publication.
