Langmuir 1993,9, 3039-3044
3039
Inverse Gas Chromatography Characterization of Birch Wood Meal Donatien Pascal Kamdem* Forestry Department, Michigan State University, 120 Natural Resources, East Lansing, Michigan 48824
Samar Kanti Bose and Philip Luner Empire State Paper Research Institute (ESPRI), State University of New York, Syracuse, New York 13210 Received September 17, 1992. In Final Form: July 8, 199P Inverse gas chromatography (IGC) was applied to characterize the surface of birch wood meal. The isosteric heat of adsorption, qst, the dispersive component of the surface energy, y.D, and the acidlbase character of birch wood meal surface were estimated by using the retention time of different nonpolar and polar probes at infinite dilution. The specific interaction parameter Isp, the specific enthalpy of adsorption AHA'P, and the enthalpy of acid-base interaction A H A of ~ ~polar probes on wood meal surface were determined. A H Aand ~ AHA*~ were correlated with the donor (DN)and modified acceptor (AN*) numbers of the probes to quantify the acidic K Aand the basic KBparameters of the substrate surface. The values of K A and KB suggest that the extractives free wood meal surface is amphoteric, with predominantly acceptor electron sites.
Introduction In the last twenty years, IGC (inverse gas chromatography) has been widely used to study the thermodynamics of adsorption and the surface properties of organic and inorganic materials.' This technique is based on the physical adsorption of a well-known probe a t different concentrations by a solid surface. From the elution peak, the retention time and the peak shape give information about the adsorption process. IGC allows the determination of enthalpy, free energy, and entropy of adsorption. The London dispersive component of the surface energy,2J the adsorption isotherm,4t5 the surface area of an adsorbent? the Flory-Huggins X parameters? the diffusion coefficient of a probe into the nonmobile phase,' and the acid-base surface properties7*Softhe nonmobile phase can be also obtained by IGC. These thermodynamic functions can be estimated a t infinite dilution and a t finite concentration region by varying the amount of injected probe.' The IGC technique can be conducted over a wide temperature range, a t different moisture contents of the nonmobile phase, with mild and corrosive substances. Although this technique has been used to study polymers,4I6 cellulose,3*swood fibers from pulping process,2~5~9 carbon,la
* To whom correspondence should be addressed. *Abstract published in Advance ACS Abstracts, September 1, 1993. (1) Conder,J. R.; Young,C. L. Physicochemical Measurement by Gas Chromatography; Wiley-Interscience: New York, 1979. (2) Kamdem, D. P.; Riedl, B. J. Wood Chem. Technol. 1991,11,57. (3) Lee, H. L.; Luner, P. Nord. Pulp Pap. Res. J. 1989,2, 164. (4) Gray, D. G.; Guillet, J. E. Macromolecules 1972,3, 316. (5) Dorris, G. M. Characterizationof Low Energy Surfaces by Inverse Gas Chromatography. Ph.D. Thesis, McGill University, Montreal, 1979. (6) Munk, P.; Hattam, P.; Du, Q.; Abdel-him, A. A. J . Appl. Polym. Sci.: Polym. Symp. 1990,45, 289. (7) Saint-Flour, C.; Papirer, E. Znd. Eng. Chem. Prod. Res. Deu. 1982, 21,666. (8) Lavielle, L.; Schultz, J.; Nakajima, K. J. Appl. Polym. Sci. 1991, 42, 2825. (9) Gurnagul, N.; Gray, D. G. J. Pulp Pap. Sci. 1985,11,598. (lO)Vukov, A. J.; Gray, D. G. In Inverse Gas Chromatography: Characterization of Polymers and Other Materials. ACS Diu.Polym. Mater. Sci. Eng., Proc. 1988,58, Chapter 14, 185.
lignin,11J2silica,13 and calcium carbonate,14 information on wood meal is currently unavailable. The surface of wood, in terms of surface energy and acid-base property, plays an important role in pulping processes and in wood utilization, especially in wood composites. The dispersive component of the surface energy could be obtained from classical contact-angle measurements and from adsorption isotherms by the Gibbs e q ~ a t i 0 n s . l ~These two methods are time-consuming. Further, surface roughness, the presence of pores, and surface energy gradients make the contact angle method dubious.16 The same problems have been reported on carbon fibers.l0 In the conventional method for wood acidity characterization, the aqueous extract from wood meal is titrated with an alkali. The so-measured acidity or basicity of wood arises from the cumulative contribution of chemicals present a t the surface and in the bulk of wood meal. This information is inadequate to characterize the wood surface property and to predict the work of adhesion as defined by Fowke~.'~ An alternative route for estimating the surface energy and the acid-base surface properties is based on IGC technique. IGC involves surface and bulk adsorption of a probe by a substrate. At different amounts of injected probes, the retention time remains constant for surface adsorption and varies with bulk adsorption. Retention times of polar and nonpolar probes injected a t infinite dilution were used to calculate the isosteric heat of adsorption, the London dispersive component of the surface energy, and the specific interaction parameter, The acid-base character of the adsorbent was estimated (11) Rials, G. T. In Wood and Pulping Chemistry. TAPPI Conf.Proc. 1989, 29. (12) McArthur, S . L. ThermodynamicPropertieeof Solventaatlnfmite Dilution in Lignin. M.S. Thesis, Michigan State University, 1987. (13) Papirer, E.; Balard, H.; Vidal, A. Eur. Polym. J. 1988, 24, 783. (14) Colenutt, A. B. A.; Sing, K. S. W. J. Chromatogr. 1989,479, 17. (15) Adamson,A. W. Physical Chemistry ofsurfaces;John Wdeyand Sons: New York, 1976; p 568. (16) Riedl, B.; Kamdem, D. P. J. Adhes. Sci. Technol. 1992,9,1053. (17) Kamdem, D. P.; Riedl, B. J. Colloid Interface Sci. 1992,150,507. (18) Fowkes, F. M. J. Adhes. Sci. Technol. 1990,4, 669.
0743-7463/93/2409-3039$04.00/0 0 1993 American Chemical Society
Kamdem et al.
3040 Langmuir, Vol. 9, No. 11, 1993 Table I. Properties of the Probe7aa*3m probe n-hexane n-heptane n-octane n-nonane n-decane
cc4
CHCla benzene ethyl acetate toluene ethylbenzene diethyl ether diisopropyl ether THF
AN*
YLD
DN (kcaUmol)
(A?
(mJ.m-2)
AN
51.5 57 63 69 75 46 44 46 48
0 0
0 0 0 8.6 25.1 8.2 9.3
0.7 5.4 0.17 1.5
0 0 0 0.1 1.7 0.1
0 0 0.59 11.4
47
18.4 20.3 21.3 22.7 23.4 26.8 25.9 26.7 16.5 27.4 29.2 15.0
3.9
1.4
19.2
13.7
45
22.5
8.0
0.5
20.1
40.2
a
using the acceptor and donor numbers concept correlated to the enthalpy of specific interaction of adsorption as defined by Papirer e t al.13 and the enthalpy of acid base interaction suggested by Riddle and Fowke~.~O
Experimental Section Apparatus and Measurement. A Hewlett Packard 5700A gas chromatograph equipped with flame ionization detector were maintained at 473 K. Hewlett Packard (HPChem)software was used to obtain the retention time for symmetrical peaks. For the asymmetrical peaks, the retention times were estimated by using the fiist-order moment method.17 Prepurified nitrogen was used as carrier gas at a flow rate ranging from 10 to 30 mL/min. Appropriate corrections for the pressure drop alongthe column and for the soap bubble flowmeter temperature were made on the flow rate. A steel column (4.0mm i.d. and 1.0 m long) previously washed with methanol and acetone was packed with 5.6g of 40/60,60/80, and 40180mesh size white birch (Betulapapyrifera)wood meal. The white birch wood meal was extracted according to the standard TAPPI" method. Probes. Chromatographically pure alkanes ranging from n-hexane to n-decane, aromatic compounds from benzene to ethylbenzene, tetrahydrofuran (THF),and ethers were used as probes. All probes were analytical grade purchased from Aldrich and used as received. Table I lists the properties of the probt~PJe*fl used in this study. Experiments were performed at a temperature range between 323 and 340 K. A small volume of vapor probe (less than 1 pL) was injected into the column with 1-pL Hamilton syringes. Retention volume and peak width recorded in the range of the flow rate variation were constant within a margin error of 5 % . The retention volume was also independent of the amount of probe injected. Since the temperature range considered in this study is below the glass transition temperature of the wood constituent, the adsorption process should be considered to be instantaneous. The adsorption process taking place in the conditions of this study involves mainly surface; bulk adsorption is,negligible. Data Analysis. The net specific retention volume per gram of adsorbent at 273.15K,V," was obtained by using the following equation.'
Where Tois the column temperature, W is the amount in grams of adsorbent packed into the column, Q is the corrected flow rate of the nitrogen carrier gas: and t, and t, are the retention time of probe and methane, respectively. From Vgo,some thermodynamic functions can be obtained by using the following equations:
(kcaVmo1)
DN/AN*
0 0 0 0
Mw 86.2 100.2 114.2 128.2 142.2 153.8 119.4 78.1 88.1 92.1 106.2 74.2 102.2 72.1
(3)
-AGAo= RT In Veo + C
AS,'
= ( A H A " - AGA")/T
(5)
where AHA" is the enthalpy of adsorption, qd is the heat of adsorption, qh is the isosteric heat of adsorption, R is the gas constant, T is the temperature, AGA" is the free energy of adsorption, C is the constant depending on the reference state of the probe, and MA" is the entropy of adsorption. The interaction between an adsorbate and an adsorbent could be separated into two components, one dispersive and the other specific. Dispersive Component. The dispersivecomponentis defined as the contribution of dispersion forces such as the London force. The adsorption of saturated normal alkanes by an adsorbent involves mainly dispersive force. The surface energy is also the sum of these two components Y.
=Y P +YP
where y, is the totalsurfaceenergy,Y , the ~ dispersivecomponent, and 7.P the specific component. r B D is computed using the Dorris and Gray6 semiempirical equation: (7)
where Y(-cH~)is the surface tension of a methylene group and AG(-CHr) is the free energy of a methylene group. 7 ( a F )is equal to 35.6 mJ.m-2 at 293.15 KPZ1 AG(-CHr) is obtained from the slope of In Vg0versus the number of carbon atoms of a series of n-alkanes. N is the Avogadro number. The estimated value of the surface occupied by a methylene group, a, is 0.06 nmz. Specific Interactions. The specific interactions represent the contribution of forces other than dispersives. This includes hydrogen, polar, and acid-base interaction. The specific interaction can be estimated theoretically from the retention data of polar probes. The interaction of polar probes and substrates involves dispersive and specific forces. Several approaches have been proposed to differentiate the respective contribution of the dispersive component and the specificinteracti0ns~aJ8*~from retention data of polar probes. The specific interaction can be calculated by using the methods proposed by Papirer et al.7 and Lavielle et a l . 8 3 as expressed in the following equation:
IaP= AGAsp = RT In
.,V
(8) vg
V,* and V, are the net specific retention volume of the alkane (21)Panzer, U.;Schreiber, P. Macromolecules 1992,25, 3633. (22)Lavielle, L.;Schultz, J. Langmuir 1991, 7, 978.
(19)Schultz, J.; Lavielle, L.; Martin, C. J. Chim. Phys. 1987,84, 2. (20)TAPPI Test Methoab 1991, I , T264 om 88.
(4)
(23) Schreiber,H.P.; Weitheimer,M. R.;Lambla,M. J.J.Appl.Polym. Sci. 1982, 27, 2269. (24)Chen, F.Macromolecules 1988,21, 1640.
IGC of Birch Wood Meal
Langmuir, Vol. 9, No. 11, 1993 3041
and the polar probe, respectively. Normal alkane is considered as reference capable of only dispersive contribution during the adsorption process. IlPor AGA'P is determined graphically by orthogonal projection of the representative point of the polar probe onto the straight line of normal alkanes. I,pis plotted versus the logarithm of the saturation vapor pressure for the Papirer' method and versus [ a ( y ~ ~ for ) ~ the l ~ Schultza l method. a is the probe crosssectionand y~~is the surfacetension dispersive component of the probe. The enthalpy of specific interaction of adsorption of apolar probe, AHA", can be determined by plotting AGA'P versus 1/T according to eq 5. The Schultz and Papirer methods have been reported to be inapplicable for high-energysurface materials.%*=It was found that some polar probes fell below the alkane reference line. This theoretically suggests an interaction lower than that involved with the dispersive forces of normal alkanes. Such data are thermodynamically difficult to interpret. This finding was reported on materials such as carbon fibersF6 graphite?' and sepioliteM with yaDclose to 100 mJ.m-2. In order to elucidate the situation, Donget al.=have introduced another model. In this model,RT In V," was represented versus PD. PDis the molar deformation polarization parameter and it is a function of the refractive index and the molar volume of the probe. The main assumption of all these three models is that the specificinteractions are in addition to the dispersive interaction. These three methods are equivalent since they used similar RT In V," values for the ordinate but different abscissa parameters. The abscissa parameters are the vapor pressure in Papirer's mode1,1J9.28 surface tension and cross section in the Schultz mode1,8JB22and deformation polarizability in the Dong model.% Another model proposed by Fowkes assumes that specific interactions are essentially acid-base.'a In this former model, acid-base character is used as defined in Gutmann's approach.28 An acid has the ability to attract electrons whereasa base releases electrons. The DN (donor number) is the molar enthalpy of interaction between a base and a reference acceptor SbCls, in a dilute solution of 1,2-dichloroethane. DN is expressed in kilocalories per mole. Unlike the DN, the AN (acceptor number) is an arbitrary unit set at 0 for the shift induced by hexane and at 100 for SbC13 in a dilute solution of 1,2-di~hloroethane.~ DN and AN are expressed in different units and therefore any comparison is inaccurate. Recently, Riddle and Fowkes%redefined a new AN* on the same scale and with the same units as DN. This new AN* takes into consideration the amphoteric character of molecules.% Fowkes' assumptions's stipulate that an enthalpy has two Components, one dispersive HA^ and the other acid-base A H A ~ ~ . The enthalpy of interaction between an acid and a base should also have two components. The enthalpy of adsorption could be expressed by the following relation:
where AHA" is the enthalpy of adsorption obtainable by IGC, A H Ais~the enthalpy of adsorption due to dispersiveforces,AHAab is the enthalpy of adsorption due to acid-base forces, and AHvap is the enthalpy of vaporization. For probes involving only dispersive forces, AHA*~ = 0 and eq 9 becomes (10) AHA" is the excess heat of adsorption frequently reported in the literature16.17 between a normal alkane and an adsorbent. Its (26) Morales, E.; Dabrio, M. V.; Herrero, C. R.; Acoeta, J. L. Chromutographia 1991, 31, 357. (26) Dong, S.; Brendle, M.; Donnet, J. B. Chromatographia 1989,28, '469.
(27) Donnet, J. B.; Park, S.J.; Balard, H. Chromatographia 1991,31, 434. (28) Papiper, E.; Perrin, J. M.; Siffert, B.; Philipponeau, G. J. Colloid Interface Sci. 1991, 144, 263. (29) Gutmann, V. The Donor-Acceptor Approach t o Molecular Interactiom; Plenum: New York, 1978. (30) Riddle, F. L.; Fowkes, F. M. J. Am. Chem. SOC.1990,112,3269.
3.5
2.5
%--
1.5
4 d
0.5
2.95
3.00
3.05
3.10
1000/T (K-')
Figure 1. Plot of In V," versus 1000/Tfor normal alkanes probes. value depends on the nature of the adsorbent and on the molecular property of the adsorbate. For polar probes with contributions of both dispersive and specific interactions, the determination of the dispersive component of A H A o is tedious. By assuming that A H A O is proportional mainly to the molecular property of a probe, a relation between the molecular weight and AHA" of a series of normal alkanes and a specific adsorbent can be obtained graphically in the form AH,~=AM
(11)
where A is the slope and M the molecular weight.80 This assumption allows the estimation of A H A O and AHA*using eqs 9, 10, and 11. From the values of AHAab, AHA'P(P),DN, and AN*, K Aand KB values were obtained by using the following expression:l AHAeb = AH;' = KA DN + KB AN* (12) K Aand KB are the acceptor or acidic constant and the donor or basic constant, respectively. In this work, retention times of nonpolar and polar probes were used to calculate the thermodynamics functions ( A H A O , qd, A H A ~and ~ , AHAOP), the London dispersive component y,D, and the specificinteraction parameter Zv of the wood meal substrate. I,p was obtained by two methods: Papirer'sl method and Schultz's8method. The enthalpy of acid-base interaction AHA* as suggested by Gutmanna was also calculated. K Aand KB of the substrate were estimated by using both AHp(P) and AHA&.
Results and Discussion Determination of the enthalpy of interaction between an adsorbate and an adsorbent as defined in eq 9 requires computation of the enthalpy of adsorption AHA",the isosteric heat of adsorption qd, and the enthalpy of vaporization. The enthalpy of adsorption A H A O was calculated by using eq 2. Equation 7 was used to compute the dispersive component of the surface tension Y ~ The ~ . specific interaction I,, was evaluated following the Papirer and Schultz protocol. Papirer 18,values were higher than those of Schultz. The difference was probably due to the errors associated with the determination of the molecular area, the dispersive component, and the vapor pressure of the mobile phase. AHA~P(P)was obtained by plotting Is, versus the inverse of the temperature; the slope corresponds to AI&Sp(P) and the intercept to ASA~P(P).A H A ~ ~ and AHA*P(P) were used to compute the acidic and basic constants of the substrate as expressed in eq 12. Isosteric Heat of Adsorption, qlt. Figures 1 and 2 show the plot of Ln Vgoversus the inverse of the column temperature for normal alkane and polar probes, respectively. These curves are linear and their slopes were obtained from the least-squares method with a standard
Kamdem et al.
3042 Langmuir, Vol. 9, No. 11,1993
I
,Ethyl-C,H61
P
3
sopropylethe
0.8
9Ether
I
I// -1.2 2.95
4
40 I 5
3.09
3.02
Number of Carbon Atoms
Figure 2. Plot of In Vg0versus 1000/T for polar probes. Table 11. Isosteric Heat, qlt, and Enthalpy of Vaporization, -AH,.,, at 298.15 K for Different Probes probe
(kJ/mol)
(kJ/mol)
-mw
qst - m " a *
n-heptane n-octane n-nonane n-decane carbon tetrachloride chloroform benzene ethyl acetate toluene ethylbenzene diethvl ether diisopropyl ether
45.8 52.5 58.7 64.8 46.9 50.0 51.3 58.7 58.2 66.6 52.5 56.3 58.0
36.5 41.5 46.5 51.5 32.4 31.4 33.8 33.0 38.0 42.2 26.7 29.3 30.0
9.3 11.0 12.2 13.3 14.5 18.6 17.5 25.7 20.2 24.4 25.8 27.0 28.0
THF
11
g
N
1000/T ( K-l )
qat
I
7
Figure 3. Isosteric heats of adsorption versus carbon number for normal alkanes and benzene compounds.
(kJ/mol)
2.6
I
02-
9
1.0 -
-0.6
8
8
10
N
deviation of 3% and a correlation coefficient of a t least 0.98 in all cases. The linearity of curves confirms the validity of eq 2. The slopes were used to calculate the heat of adsorption AHA'. The isosteric heat of adsorption qst corresponds to the heat developed when 1mol of probe is adsorbed by an infinite amount of solid without any change of the fraction of surface covered by the probe.' Values of qat were computed from eq 3 and are listed in Table 11. Table I1 also contains the values of the enthalpy of vaporization, AHvap, a t 298.15 K.3132 The probes used in this study exhibit a qst higher than AHvav This indicates that the adsorption was exothermic and that the interactions between probe and wood meal surface was higher than that between 2 mol of probes during the vaporization. The difference between get and -AHvapvaries from 9 to 13kJ/molfor n-alkanes and from 14 to28kJ/molfor polar probes (Table 11). This reflects the interaction between the injected probes and the substrate surface. Probes with high DN values such as T H F and ether exhibit a high value of qet-AHvap (Table I). For probes with low DN inferior to 1, such as CCL, CHC13, or C6H6, q,t-AHv, is lower than that with high DN probes. This indicates that interactions between wood meal adsorbent and adsorbate with high DN or (DN/AN*) values such as T H F and ether are more intense than those with n-alkane or low DN value probes. Figure 3 represents the plots of qst versus the number of carbon atoms of normal alkanes and benzene compounds. Since the polarizability of the hydrocarbons increases with the molecular weight, the increasing values (31) Timmermans, J. Physicochemical Constants of Pure Organic Compounds; Elsevier, New York,1965. (32) Dreisback, D. R. Adu. Chem. Ser. 1955, No. 15.
Number of Carbon Atoms
Figure 4. Plot of In VE0versus the number of carbon atoms of the normal alkanes at different temperatures. Table 111. Dispersive Component Y , ~of the Surface Energy of Extracted White Birch Wood Meal 323.15 K 328.15 K 333.15 K 338.15 K y,D (mJ.m-2)
43.8
42.9
41.1
39.1
of qstfor higher hydrocarbons in a homologous series were expected. The plots of normal alkane and benzene probes are quite parallel. Benzene compounds show a higher qat than normal alkane. This is the contribution of ?r electrons of the benzene ring. The relative error of qat due to the temperature variation, flow rate measurement, retention time measurement, and the standard deviation of the slope of In V,' vs 1/T was estimated to be about 10%.33 Dispersive Component of Surface Energy, ySD. Figure 4 shows the linearity of In V,' versus the number of carbon atoms a t various temperatures for the n-alkane. From the slope of these curves the free energy of adsorption per methylene group is obtained and eq 7 is applied calculate raD, the London dispersive component of the surface energy. Table I11 shows a decrease of ysDa t higher temperatures. yeDvalues are similar to those obtained for organic materials such as cellulose3*33and wood fibers33 Considering the different sources of error on yeD,the maximum error33 is evaluated up to 2 mJ.m-2. The difference in the measured values of ysDin the range of temperature used is negligible and ysD is evaluated to 41 f 2 mJ.m-2. (33) Kamdem, D. P. Etude de la Surface des Fibrea de Bois Modifi4e par Greffage et par Impregnation a h i d e de la chromatopaphie et de la SpectroacopiePhot.&lectronique,Ph.D.Theais,Universit.6LaVal, Quebec, 1990.
Langmuir, Vol. 9, No. 11, 1993 3043
IGC of Birch Wood Meal 9.5
t
200.0
I
I
1
YClO
Ethyl b e n z e n e
THF
h 4
i
150.0
2
/TnF
5.5 -
v
y,,, ,
-2.5 0.5
1.5
ce
2.5
3.5
4.5
0.0
0.0
10.0
Log Po
Figure 5. Plot of RT In Vg0versus [ a ( r ~ ~ )at l / 323.15 ~ ] K for various probes.
20.0
DN/AN'
Figure 7. Plot of AHAabb/AN*versus DN/AN*.
-m*d
probes carbon tetrachloride chloroform benzene toluene ethyl acetate ethyl benzene diethyl ether diisopropyl ether THF
5.5
v
OB-
0
cl
CHCL,
c
1.5
Ether
-2.5 153
228
303
a,(~L)'!z
[I 2
378
(mJ,""m]
Figure 6. Plot of RTln V , O versus log(&) at 323.15 K for various probes. Table IV. Specific Interaction, Z, from Schultz (S)and Papirer (P) at 323 K in kJ/mol probe Itp(S) Imp (P) probe L p (S) &p (P)
cc4 chloroform benzene ethyl acetate toluene
50.0
and AHA'*(P)Obtained from the Papirer Method' at 298.15 K for Various Probes
h
2
40.0
Table V.
9.5
i
30.0
-0.33 4.56 4.06 5.0
0.35 5.50 4.31 6.20 3.40
ethylbenzene diethylether diisopropylether
THF
6.1 6.4 9.65
3.10 7.30 6.70 9.90
Specific Interactions,Ilp.Figures 5 and 6 correspond to the plots of R T In Vgo versus log Po from the Papirer model and RT In Vgo versus a(yLD)1/2from the Schultz m0de1,7J9*2~ respectively. Table IV lists the values of Isp. Isp(S)represents the specific interaction parameter from Schultz model and Isp(P)from Papirer. Isp(S)andl,,(P) show a similar trend. Isp(P)was selected because of the availability of vapor pressure data of the probes over the temperature range involved in this work. The Papirer method gave higher specific interactions than the Schultz method. For a neutral probe such as CCl,, the Schultz method gave a negative specific interaction parameter which is difficult to explain. This could result from the experimental errors and/or from the determination of various constants involved such as the molecular cross section and the dispersive component of the surface energy a t a particular temperature. This is the main reason for the selection of the Papirer interaction parameter for the estimation of the acid/base character of the substrate in this work. The lowest Ispwas observed with the neutral CCL solvent. Ispof low DN chloroform indicates that basic
-mAab
(kJ/mol)
(kJ/mol)
13.0 10.0 6.0 7.5 7.0 8.5 5.0 8.0 6.0
1.5 8.5 11.5 12.7 18.7 15.9 20.8 19.0 22.0
-mA'P(P)
(kJ/mol) 4.5 8.0 9.0 8.0 19.4 6.5 23.0 17.0 19.5
sites are involved in the adsorption process. Probes with high DN numbers, such as T H F and ether, show high Isp values, suggesting an acidic character surface. The IBp of probes with high and low DN numbers suggests that both acidic and basic sites are present on the substrate surface. Since DN and AN* are expressed in the same units, the high value of Iepof probe with a high (DN/AN*) indicates that adsorption sites are mainly acidic.
Enthalpy of Specific Adsorption AHA'P(P)and of Acid/Base AHA"".AHHb, the enthalpy involved in an acid base type interaction was calculated by using eqs 9 and 10. Table V contains the values of AHA~, AHA"~ and AHA~P(P).A H A ~ P ( P )is the enthalpy of specific interaction, obtained by plotting AGA~P(P)/T versus 1/T as reported by Papirer.22-54A H A ~ P ( P and ) AHAab show the same trend. The AHA"~ values of the high DN probes such as THF, diisopropyl ether, and ether are higher than that of the high AN* adsorbates. CCl,is a neutral probe and falls almost on the alkane line.26 The neutral CC4 shows a A H A of~ 1.5 ~ kJ/mol and a AHA~P(P)of 4.5 kJ/mol. The value of A H A and ~ ~ AHA~P(P)of CC4 indicates a weak interaction with the adsorbate. Low DN probes exhibit ~ . value of the both high AHASP(P) and high A H A ~The enthalpies of specific and acid-base adsorption suggest that the substrate is amphoteric and mainly acceptor electrons character. AHA~P(P)is higher than AHA"~for the low DN probes but the inverse is observed for high DN. The estimated error associated with the determination of AHABP(P)and AHA"~ is estimated a t 25 and 1276, respectively. A linear correlation was obtained between or AHASP(P) and (DN/AN*). The slope and intercept of Figures 7 and 8 were used to calculate K A and KB as expressed in eq 13. Table VI lists the values of K A and KB. K A and KB from A H A is~ higher ~ than that from (34)Kuczynski, J.; Papirer, E.Eur. Polym. J. 1991,27, 663.
Kamdem et al.
3044 Langmuir, Vol. 9, No. 11,1993
and KB from both methods confirm that the extracted white birch wood surface has an amphoteric character and that more acidic sites36 are involved in the adsorption process.
eoo.0
150.0
5
1 c1c
Conclusion
100.0
$
60.0
0.0
I
ECll
0.0
10.0
20.0
30.0
40.0
50.0
DN/AN'
Figure 8. Plot of AHA'P(P)/AN* versus DN/AN*. Table VI. Acid/Base Surface Characteristics of Extracted Birch Wood Meal Papirer AHp"P) this study (AHHb)
KA
KB
3.65 0.25 4.40f 0.10
1.65 f 0.65 3.00 f 0.50
*
AHA~P(P).K A is higher than KBfor both methods. The relative error associated with K A is estimated to be less than 10% with AHA~P(P)and less than 5 % with AHHb (Table VI). The relative errors on K B when used with ~ 40% and 20%, respectively. K A AHA'P(P) and A H A " are
The results indicated that IGC a t infinite dilution can characterize the acidlbase nature of wood meal surface. The extracted wood meal has an acidic surface which is consistent with the chemistry of white birch wood. The isosteric heats of adsorption, qst,of n-alkanes were higher than their heat of vaporization, indicating the contribution of dispersive forces. For polar probes the enthalpy of adsorption was higher than that of vaporization. The difference was attributed to acid-base and dispersive interactions between probes and wood meal. The enthalpy of specific interaction suggests a high interaction between wood meal and polar probes compared to normal alkanes. The interaction between the substrate and the high and low DN probes suggesh the presence of both acidic and basic sites on the wood surface. The values of K Aand KBindicate that predominantly acidic sites are involved in the adsorption process.
Acknowledgment. The authors wish to thank the Empire State Paper Research Institute for financial support and the Michigan State University Eastern Hardwood Utilization Program and Dr.E. Time11 for the gift of the extracted white birch wood meal. (35)Krilov, A.; Lasander, W.H.Holzforschung 1988,42, 253.
