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Gas-Solid Chromatography. A Method of Measuring Surface Free Energy Characteristics of Short Glass Fibers. 1 Through Adsorption Isotherms Carol Salnt Flour and EugOne Paplrer' Centre de Recherches sur la Physicc-Chimie des Surfaces SolMes, 24, Avenue du R6sMent Kennedy, 68200 Mulhouse, France
Adsorption isotherms of n-alkanes and polar adsorbates on short glass fibers were determined by a gas chromatwaphic technique. The isotherms were well described by the BET equation. Spreading pressures at saturation were deduced from the ndecane and benzene isotherms and analyzed in terms of Fowkes' theory. The London dispersion force contribution to the surface free energy of the short glass fibers was shown to be 50 f 6 mJ m-2, while the specific interaction contribution to the work of adhesion between benzene and the glass fibers was shown to be 16 f 2 mJ m-2.
Introduction Asbestos owes its numerous uses to its remarkable physicochemical and mechanical properties, at a relatively low cost. In particular, industrial composite materials often include asbestos as a reinforcing agent, associated either with an organic or a mineral matrix (resins, polymers, cement, ...). However, asbestos is now generally recognized as a health hazard and much effort is being displayed to find alternative replacement fibres. Preliminary investigation (Langley, 1973) has shown the potential interest of short glass fibers combined with phenolic resins in friction materials (brake pads, clutches, ...). Excellent mechanical properties are usually, obtained in dry conditions a~ expected, since glass fibers are high energy solids. However, environmental exposure to humidity causes a considerable decrease of the fracture strength of such composites. While a variety of procedures have been adopted to prevent this premature failure, among them the use of silane (Subramanian et al., 1977) and titanate (Monte et al., 1977) coupling agents, an adequate and exhaustive explanation of the resulting phenomena has not yet been given. Besides the chemical bonding theory, little attention has been paid to the variations of the surface free energy components, i n h e d b y a surface treatment. The aim of this contribution is to look for a method of measuring the surface free energy of short glass fibers. In a future paper we shall examine its dependence on surface treatment. Current methods of determining surface free energy, for example the procedures based on the interpretation of solid liquid contact angles, do not apply to short glass fibers as a result of their geometric characteristics (250 pm in length, 6 pm in diameter). Now the interaction potential of solid surfaces can be analyzed through adsorption isotherms, the latter usually being obtained by means of volumetric or microgravimetric techniques. However, the very low value of the specific surface area of the particular solid under investigation renders such methods impracticable if reasonably good precision is looked for. Gas-solid chromatography appears particularly well suited under those conditions; in fact, it has already been applied successfully to low specific surface area substrates, wood fibers, for instance (Dorris and Gray, 1979). Experimental Section The Solid Phase. Short glass fibers from Rockwool International were used for column packing. Bulk chem-
Table I. Composition of the Glass Fibers oxide
wt%
mol %
SiO,
46 16 17 9 12
48.7 18.1 10.6 3.6 19.0
GO
MgO
Table 11. Diameter Distribution
4,
0.5
Ltm ni,
8
1.5 2.5
3.5
4.5 5.5 6.5 7.5 9.5 11.5
10.5 20.5 15.5 22.5 7
10
4
1
1
% no.
ical analysis of the fibers (Table I) reveals a composition quite different from that of common glass fibres. It is worth noting that the fibers were in fact obtained from rock melts of Danish origin. The diameter distribution measured on macrophotographs is given in Table II. The geometric specific surface area calculated from the above distribution, by assuming that fiber length is proportional to diameter, is equal to 0.25 m2 g-l. The specific surface area was also measured by krypton adsorption and application of the BET equation. The mean value from ten determinations is equal to 0.30 f 0.05 m2 g-' (with AKr= 0.195 nm2). The short glass fibers used exhibit a pronounced tendency to form agglomerates when submitted to mechanical vibrations. Figure 1 shows the structure of such an agglomerate, suggesting a rather low compactness (-60%). Good mechanical strength results from the tangled structure and enables easy sieving and column packing without causing damage to the agglomerates. In a typical run, chromatographic measurements were obtained with a 1.0 m long steel column of 6.35 mm external diameter packed with 7.783 g of agglomerates having a mean diameter in the range 0.5-1 mm. Adsorbates. The solute probes were reagent analytical grade from Fluka and were used without further purification. Chromatographic Measurements. Measurements were carried out with a commercial Intersmat IGC 12 gas chromatograph fitted with a katharometer. The original air oven was only slightly modified by introducing a secondary forced air convector, in order to control column temperature to f l "C when operating near room temperature. The injection port was maintained at least 50' above the boiling point of the adsorbate so as to ensure
O196-432~/82/~221-O337$01.25/0 0 1982 American Chemical Society
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Ind. Eng. Chem. Prod. Res. Dev.. Vol. 21. No. 2. 1982
Figure I . Scanning electron micrograph of a glass fibers agglomerate (XIOO).
flash vaporization a t each injection. Helium was used as carrier gas with a flow rate of about 20 cm3 min-', measured at the column outlet by means of a soap bubble flowmeter. Correction was allowed for saturated vapour pressure of water, pressure drop across the column (0.03MPa), and the difference in temperature between column and flowmeter. Before each series of measurements the column was conditioned a t 150 OC during 24 h under helium flow. Hamilton 1-, lo-, and 100-fiL. Each injection was in fact repeated several times and showed elution peaks to be perfectly reproductible. The 'air peak" method (Conder and Young, 1979) provided a rapid and simple means of determining gas holdup, i.e., column dead volume, necessary for isotherm calculation. T h e Adsorption Isotherm. The retention theory at finite concentration (Conder and Young, 1979) provides a simple and reasonably accurate means of determining adsorption isotherms from a series of chromatograms corresponding to discrete sample injections of known volume. From a practical standpoint, considering only gas compressibility and isotherm curvature, while neglecting sorption effects (as pressure drop is small), nonideal peak spreading (Saint Flour and Papirer, 1982), and other gas-phase imperfections, the adsorption isotherm, q = f(p) is obtained from the equations h
4 = (PU/mS,,k)x
(Zh -20)
dh
P = (PUhupRn/FNSpar
(1) (2)
The integrated quantity appearing in eq 1 should be handled with care whenever there is evidence of an existing point of inflexion in the adsorption isotherm. Isotherm curvature and the shape of the elution profile are indeed closely related (see Figure 2). Therefore, when one proceeds to use eq 1, one should bear in mind that (zh- zo) is the distance to the diffuse side of the elution peak which can either be a front boundary or a rear boundary, depending on the isotherm curvature. Besides, when using eq 1and 2 it is important to make sure that the "coincidence phenomenon" does occur. Asymmetrical peaks in which the diffuse sides are not superimposable cannot be used for the calculations; they
0
0.2
0.4
0.6
0.8
0
Figure 2. n-Decane adsorption isotherm. at 65 'C, on short glass fibers.
represent a case of nonideality which is too complex to deal with. For this reason, n-decane and benzene isotherms had to be extrapolated in the region near the point of inflexion so as to cover the entire range of @/pa). Results a n d Discussion (1)n-Decane as t h e Solute Probe. Cross-Sectional Area of the n -Decane Molecule. The experimental isotherm at 65 "C of n-decane on short glass fibers is given in Figure 2. For low volume injection (0.70 pL), the diffuse side is a front boundary, suggesting that the isotherm curvature changes at some point. For reasons already developed, the isotherm had to he extrapolated in the region of the point of inflexion. The shape of the isotherm indicates its belonging to type I1 of the Brunauer classification. The theoretical BET (Brunauer et al., 1938) isotherm in its linear form is expressed by
where qm is the monolayer capacity and C is a constant related to the heat of adsorption. A plot of Lp/p~I/[q(l- P/PO)I against @/Po) gives a straight line whose slope and intercept enable the estimation of C and qm. The absence of experimental data in the vicinity of the inflexion point was not a real handicap in locating the monolayer capacity. A plot in the range 0.01 < p/po < 0.20 by means of a least-squares technique was indeed sufficient to yield reasonable values of qmand C (Figure 3). The results are qm = 100 fig g-I and C = 4.8. The low C value is in agreement with the experimentally observed weak knee isotherm. The normal procedure would be to use the monolayer capacity to determine the specific surface area of the short glass fibers, provided the cross-sectional area of the ndecane molecule is known. Inversely, knowing the specific surface area from Krypton adsorption, adjustment can he made to determine the cross-sectional area of the probe
Id.Eng. Chem. Rod. Res. Dev., Vol. 21, No. 2, 1982 359
3000
slope
;
0.2
0.1
000
7874
1
(PIP,)
I
I
Figure 3. BET transform of the n-decane isotherm on short glass fibers (at 65 "C).
Figure 4. n-Decane isotherms at 65 and 74 "C on short glass fibers.
Table 111. Isosteric Heat of Adsorption as a Function of Surface Coverage ala,
0.1
4st/4m,,
kJ mol-
0.3
53 46 42
0.4
41
0.2
e
molecule. Thus, taking the specific surface area of the short glass fibers as 0.25 < SBmm< 0.35 m2 g-', the molecular cross-sectionalarea of n-decane is found to belong to the interval 0.59 < Andecaae < 0.83 nm2. It is interesting to note that the mean value (0.71 nm2) is very close indeed to the generally accepted value of 0.70 nm2 (extrapolation of Kiselev's data (Kiselev, 1957) for n-alkanes adsorbed on graphitized carbon black). For comparison, the cross-sectional area of n-decane, deduced from the liquid density at room temperature (Mohlin and Gray, 1974)by assuming spherical shape and close hexagonal packing, was found to be 0.57 nm.2 Much has been written on the actual meaning of molecular cross-sectional areas calculated with the BET model, particularly when low C values are involved (Sing, 1976). We shall not go further into that discussion. Differential Isosteric Heat of Adsorption. This thermodynamic quantity qat, is defined as the heat absorbed when one mole of adsorbate is adsorbed by an infinite amount of solid, without change in the fraction of surface covered by the adsorbate. Assuming ideal gas behavior and considering the Clausius4apeyron equation for two close temperatures Tl and T2,it can be shown that qst/Qm = ( R T I T ~ / -T TI) ~ log (PZ/PJ (4) Figure 4 gives two isotherms at two neighboring temperatures of n-decane on the short glaas fibers. Graphical determination of the equilibrium pressures p1and p 2 at different surface coverages q / q m , enables the calculation of qnt/qm. The results are given in Table 111. We are aware of the fact that a temperature control of fl "C is too coarse for precise work in the present context, graphical evaluation of p1and p 2 cannot be expected to be very satisfactory either. Nevertheless, the results in Table III show the dependence of q& on surface coverage; low surface coverages lead to high qnt values, suggesting that the energy site distribution on the solid surface is in fact heterogeneous. Spreading Pressure at Saturation and London Component of the Solid Surface Free Energy. When
Figure 5. The n-decane-glass fiber system (65 "C).
a gas is adsorbed on a solid surface, a spreading pressure (Harkins, 1952)is developed which is defined as (5) 7r = 7 8 - Yev where Y~ is the surface free energy of the solid at the solid-vacuum interface and yev is the surface free energy of the solid at the solid-vapor interface; a is therefore the lowering of the surface free energy consecutive to vapor adsorption on the solid surface. The integrated form of the Gibbs adsorption equation enables the calculation of a through
where R denotes the gas constant, T is the absolute temperature, S is the specific surface area of adsorbent, and M is the molecular weight of adsorbate. Spreading pressure at saturated vapour pressure ?ro is of prime importance, since it can be related to the London dispersion force contribution to the surface free energy of the solid (Fowkes, 1964). Figure 5 shows the plot of q / (p/po)vs. ( p / p o )for n-decane adsorption at 65 OC on the short glass fibers. Using a planimeter to measure the area under the curve, in the entire range of ( p / p o ) ,leads to a fair estimate of TO. In fact, the principal source of error arises from imprecision in the specific surface area determination of the short glass fibers. Taking this as 0.25
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Ind. Eng. Chem. Prod. Res. Dev., Vol. 21, No. 2, 1982
Table IV. n-Decine Spreading Pressure at Saturated Vapor Pressure and London Component of the Surface Free Energy of the Short Glass Fibers
s,
L
flo,
mz g-' 0.25 0.35
7s >
mJ m-2
mJ m-'
24.5 17.5
54 44
Table V. Benzene Spreading Pressure at Saturated Vapor Pressure and Specific Interaction Contribution to Work of Adhesion
s,
no,
mz e"
L
7s
ISp,
9
mJm-'
mJm-Z
mJm-'
37.4 26.7
54 44
18.3 14.9
0.25 0.35
Table VI. Adsorption Characteristics Deduced from the BET Equation
I/
0.2
0.4
0.6
(P/P,)
Figure 6. The benzene-glass fiber system (23 "C).
< SBET < 0.35 m2 g-3 results in 17.5 < ro < 24.5 mJ m-2. We mentioned above, that ro could be related to the London dispersion force contribution to the surface free energy. Indeed, Fowkes (1962) postulates that the free energy of adhesion WA between two phases liable to interact only through London dispersion forces be given by twice the geometric mean of the London component of the surface free energies of the pure phases. For the glass fiber-n-decane system, this leads to WA = 2(YSLy1)'/2 (7) since yL = y1 when n-decane is the test liquid (n = 23.4 mJ m-2 a t 25 "C). The free energy of adhesion can be expressed in a different way by means of DuprB's relation (DuprB, 1869) WA = 7 s + 71 - ysl (8) where yelis the interfacial tension between the solid and the liquid. yd is given by Young's equation (Young, 1805), a relationship between contact angle 0 and the various tensions Ysl = YS" + Y1 cos 0 (9) Now n-decane has a zero contact angle on glass fibres. Combining eq 7, 8, and 9 finally gives (10) (no+ 2%) = 2(Y,LY1)'/2 from which ysLcan be deduced. However, since ro depends on specific surface area, y: will also be influenced by uncertainty in specific surface Y for area determination. The results are listed in Table J the limiting values of the experimental interval. (2) Benzene as the Solute Probe. Cross-Sectional Area of the Benzene Molecule. The adsorption isotherm of benzene on the short glass fibers at 23 "C is given
solute
T,"C
C
uc!g-l
A, nm2
n-octane n-nonane n-decane benzene nitromethane acetonitrile ethyl acetate tetrahydrofuran
24 35 65 23 22 22 22 22
3.8 4.4 4.8 6.1 4.0 6.4 6.0 5.3
96 98 100 103 62 56 129 157
0.60 0.65 0.71 0.38 0.49 0.36 0.34 0.23
qm,
in Figure 6 (curve a). The extrapolated zone is somewhat broader than previously and extends from -0.2p/po to -0.7p/po, as a result of the absence of the coincidence phenomenon for elution peaks corresponding to that region. Once again, the general shape of the adsorption isotherm indicates its belonging to type I1 of the Brunauer classification. The description of the initial portion of the isotherm by means of eq 3 leads to the following results: qm = 103 hg g-' and C = 6.1. Again, taking the specific surface area of the short glass fibers as belonging to the interval 0.25 < SBET,& < 0.35 m2 g-' gives 0.31 < Abnzene< 0.44 nm2. The most widely used literature value (Kunath and Schulz, 1978) is 0.43 nm2; for the sake of comparison, calculation using the liquid density of benzene at room temperature yields 0.34 nm2. Spreading Pressure at Saturation and Specific Interaction Contribution to the Work of Adhesion. When benzene is used as the solute probe, the free energy of adhesion between adsorbate and adsorbent not only arises from London dispersion forces, but also from the ability of r-electrons of benzene to interact with hydroxyl groups present on the glass surface. Equation 10 is therefore inappropriate for describmg the work of adhesion between benzene and the solid surface. The following equation applies instead (TO
+ 271) = (2Y,LYlL)''2 + Isp
(11)
where IBp is the specific interaction contribution to the total work of adhesion. Equation 11 can easily be solved for the sole unknown IsP. Indeed, n-decane adsorption yielded the numerical value of 7:; ro is readily deduced from the benzene isotherm by first measuring the area under curve b given in Figure 6; finally, y1 and ylL values for benzene can be determined experimentally. Room temperature values reported by Schultz et al. (1977) are the following: y1 = 28.4 mJ m-2 and ylL = 26.7 mJ m-2. As before, Table V lists the two limiting cases when considering specific surface area as being equal to 0.25 and 0.35 m2 g-' respectively. (3) Adsorption of Other Solute Probes. Results for the adsorption of some other solutes including n-alkanes as well as "polar" molecules are given in Table VI. Cross-sectional areas of the adsorbate molecules were calculated from the monolayer capacities, assigning the mean value of 0.30 m2 g-l to the specific surface area of the short glass fibers.
Ind. Eng. Chem. Prod. Res. Dev., Vol. 21, No. 2, 1982 341
An increment of about 0.05 nm2 in the surface area occupied by the n-alkanes is observed which corresponds to the area of a -CH2- group. Cross-sectional areas for the ”polar” molecules listed here may prove useful for comparison with similar work, such data being very scarce at present in the literature. Conclusion Adsorption isotherms for n-alkanes and other molecules on short glass fibers were obtained using the retention theory a t finite concentration. The isotherms had weak knees and belonged to type I1 of the BET classification. Good reproducibility and fit with the BET equation enabled monolayer capacity and molecules area computation. Determination of heat of adsorption for n-decane at various surface coverage showed the solid to be heterogeneous. Spreading pressure at saturation calculated from the n-decane and benzene isotherms were analyzed in terms of Fowkes’ theory, thus enabling the determination of the contribution to the surface free energy of the solid. Work in progress at the present seems to suggest that for the n-alkanes, there might be a linear relationship between the net adsorption heats at monolayer coverage (defined as qstm- L, where L is enthalpy of liquefaction of the adsorbate) and the term A(T?)O.~.The “polar” solutes always have higher net adsorption heats at monolayer coverage than the corresponding n-alkane with the same A(yt)0*5value. This might be the starting point for a new way of characterizing a solid substrate in terms of surface energetics. Indeed we suspect the specific interaction contribution to bear some relationship to the electrondonor ability of the adsorbate, as defined by Gutmann (1978). Nomenclature F, = average volume flow rate free from water vapor, expreased at column temperature and corrected for pressure gradient
by the gas compressibility factor (James and Martin, 1952) h = peak height m = weight of adsorbent in the column M = molecular weight of the adsorbate p = adsorbate partial pressure in the gas phase q = adsorbate surface concentration expressed on a weight per unit weight basis R = gas constant Swd = area under the peak up = recorder chart speed (zh - za) = distances on the recorder chart between the air peak and the diffuse side of the elution peak p = liquid density of the adsorbate Literature Cited Brunauer, S.; Emmett, P. H.;Teller E. J. Am. Chem. Soc. 1938, 60. 309. Conder, J. R.; Young, C. L. “Physicochemical Measurements by Gas Chromatography”; Wiiey: New York. 1979. Dorris, 0.; Olay, D. 0.J. Colbhl Interface Scl. 1970, 77(1) 93. Duprl, A. “Thbrie m h n i q u e de la chaleur”; Gauthier-Viliars: Paris, 1869. Fowkes. F. M. J. h y s . Chem. 1962, 66. 362. Fowkes, F. M. Ind. Eng. Chem. 1964, 56(12), 41. Gutmann, V. “The Donor-Acceptor Approach to Molecular Interactions”; Plenum Press: New York, 1978. Harkins. W. D. “The Physical Chemistry of Surface Films”; Reinhold: New YWk, 1952; pp 211-217. James, A. T.; Martin, A. J. P. Bkxhem. J. 1952, 50, 679. Kiselev, A. V. “Proceedings, International Industrial Congress on Surface Acthrity‘’; Butterworths: London, 1957; p 168. Kunath, D.; Schuir, D. J. Colbhl Interface Scl. 1978, 66, 379. Longley, J. W. G6r. Offen 2254294, 1973. Monte, S.J.; Sugarman, D. J. Presented at the 174th National Meeting of the American Chemlcal Soclety (Rubber Division), Chicago, Aug 1977. Mohiln, U. 6.; Gray, D. 0.J. ColbM Interface Scl. 1974, 4 7 , 747. Saint Flour, C.; Papirer, E. Ind. Eng. Chem. prod. Res. D e v . submitted, 1952. Schultz, J.; Tsutsuml, K.; Donnet, J. B. J. C o l M Interface Sci. 1977, 59 (2), 272. Sing, K. S. W. In “Characterization of Powder Surfaces”; Parfitt, G. D.; Sing, K. S. W., Ed.; Academlc Press: New York, 1976; p 28. Subramanian, I?. V.; Wang, T. J. Y.; Austin, H. G. SAMPEQ. 1977, 8(4), 1. Young, T. Trans. R . Soc. London 1805, 95(3), 65.
Received for review May 4 , 1981 Revised manuscript received February 8, 1982 Accepted February 22, 1982
