- Email: [email protected]
The adsorption of hydrocarbons on cellophane
The Adsorption of Hydrocarbons on Cellophane III. Effect of Relative Humidity S H E L L E Y KATZ AND DEREK G. GRAY Pulp and Paper Research Institute of Canada and Department oJ Chemistry, McGill University, 3420 University Street, Montreal, H3A 2A7 Quebec, Canada Received June 30, 1980; accepted November 4, 1980 Adsorption of mesitylene, n-heptane, n-octane, n-nonane, and n-decane on cellophane at 20°C and at several relative humidities (RH) was studied by elution gas chromatography. The desired humidities were achieved by passing the carrier gas through a thermostated water saturator and then into the chromatographic column containing the cellophane. From the BET analysis of the octane adsorption isotherms, the external specific surface area of the cellophane sample was found to remain constant at 90 m 2 kg-' with changing relative humidities, in contrast to the behavior of cotton fibers and paper. The London dispersion component of the cellophane surface energy, calculated from both zero surface coverage and spreading pressure data, decreased slightly from - 4 2 mN m -1 at 0% RH to - 3 9 mN m -1 at 95% RH. INTRODUCTION
The adsorption of nonswelling hydrocarbon vapors on dry cellophane at zero and finite surface coverages has been previously considered (1, 2). Equilibrium partition coefficients and adsorption isotherms were calculated and correlated to the interactions of the adsorbate with the cellophane substrate and with itself. Using Fowkes' theory (3), the dispersion component of the cellophane surface free energy was estimated from both incremental quantities in the free energy of adsorption of the alkane sorbates and from their spreading pressures. In the present paper, the adsorption of alkanes on cellophane containing varying amounts of sorbed water is considered. The sorption of water by cellulosic materials is technologically important. Changing the moisture content of wood, paper, fiber, or film alters the dimensional, surface, and mechanical properties of the materials. In particular, the sorption of water into cellulose changes the surface area. Care is necessary in interpreting these changes,
however, because surface area may be defined and measured in three distinct manners. (i) Sorbed water swells cellulose by breaking accessible cellulose-cellulose hydrogen bonds and increasing the internal surface area of the cellulose, termed the "contact area" by Stamm (4). The contact area has been determined by the BET analysis (5, 6) and other treatments (7, 8) of the water sorption isotherm. The contact area has also been determined by the BET treatment of nitrogen adsorbed on solvent-exchanged swollen cellulose (9). All methods give values for the cellulose contact surface area of - 2 × 105 m 2 kg -1 (200 m 2 g-l). (ii) The external surface area of a fiber immersed in water is termed the hydrodynamic specific surface (10). This surface area is of practical significance in permeability studies (11, 12). (iii) The surface considered in this study is the external surface of water-swollen cellulose; that is, the surface which is accessible to water-insoluble vapors. Tremaine et al. (13) first investigated the three-component
339 0021-9797/81/080339-13502.00/0 Journal of Colloid and Interface Science, Vol. 82, No. 2, August 1981
Copyright © 1981 by Academic Press, Inc. All rights of reproduction in any form reserved.
340
KATZ AND GRAY
system of water-cellulose fibers-organic vapors using gas chromatography (gc). By measuring the adsorption isotherms for decane at different relative humidities (RH), changes in the cellulose surface were monitored. Water-swollen cotton filter paper was subsequently studied in detail (14-16). For both systems, an overall decrease in surface area with increasing relative humidity was observed. For paper this decrease occurred in two stages: the first, starting at -60% RH was attributed to relaxation of surface fibrillation; the second sudden drop at - 9 0 % RH was attributed to capillary condensation. In this study, the external surface area of water-swollen cellophane film is measured by gc. Unlike the paper and fiber surfaces previously studied, this substrate is virtually nonporous and is nonfibrillar. The previously observed changes in surface area with relative humidity should therefore be absent. A comparison of the results on different cellulosic substrates thus provides a good test of the novel experimental methods employed. The adsorptive properties of the watermodified cellulose can also be studied by gc. The wettability of cellulosic substrates is often determined by solid-liquid contact angle measurements. For example, the contact angles of water and organic liquids on regenerated cellulose film at different relative humidities have been measured (17-19). Since water swells cellulose, the interpretation of this contact angle is questionable. These studies furthermore neglected to account for the finite spreading pressures of the liquid adsorbates, 7r°. The gc adsorption measurements on cellulose paper (16) circumvented these problems. Cellulose paper, however, is essentially a capillary system and consists of 40-80% void volume. The gc study of the adsorptive properties of a wet cellulose surface can therefore be simplified by using the relatively smooth and nonporous surface of cellophane film. In the present work, the variation with moisture content of the external surface area and adsorptive properties of cellophane is Journal of Colloid and Interface Science, Vol. 82, No. 2, August 1981
studied by elution gas chromatography. By saturating the carrier gas with moisture, a dynamic equilibrium between the cellophane and the water vapor is set up. For finite surface coverages, n-octane and mesitylene are used as adsorbates; the zero coverage region (Henry's law region) is studied for these adsorbates and for n-heptane, n-nonane, and n-decane. The treatment of the gc data at low coverages and the determination of the adsorption isotherms were previously discussed (1, 2). The free energy of adsorption per mole of adsorbate is also considered and the dispersion component of the surface free energy is estimated at several water contents. Throughout the investigation, the stationary phase is treated as a single component. EXPERIMENTAL gc Apparatus and Measurement The general apparatus used has been previously described (1). In brief, adsorption data were obtained on a Hewlett-Packard 5711A gas chromatograph equipped with a heated injection port and dual flame ionization detectors. A thermostated, circulating water bath (Haake, model FK) replaced the air oven and maintained the column temperature at 20.0 ___ 0.05°C. For the present work, the N2 carrier gas line was modified so as to achieve constant relative humidities in the column. Dry N2 gas was passed through an external flow controller and then through a water-bubble saturator immersed in a thermostated water bath (Lauda K4R, _0.05°C) prior to entering the column. The column inlet and the saturator were therefore at the same pressure. Suitable temperature adjustments of the saturator water bath allowed the column to be conditioned to the required humidity (38, 61, 88, or 95%). The average percentage relative humidity in the column (RH) was calculated from __
% RH-
W
P°'sate°ut × 100, W " Po,colJ)Osat
[1]
ADSORPTION OF HYDROCARBONS ON CELLOPHANE where pWsat is the saturated vapor pressure of water in the saturator, Po.eol w is the saturated vapor pressure of water at the column temperature, Pout is the column outlet or atmospheric pressure, Psat is the saturator or inlet pressure, and j is the correction for gas compressibility (20). The humidity calculated from Eq. [1] has been found by previous workers (14, 15) to c o r r e s p o n d to the column water content to within _+2%. In this work, the water content at 95% R H was determined by heating the column at 60°C under a dry Nz stream for - 4 8 hr, followed by heating at 105°C for - 2 hr. The eluted moisture was collected at the column outlet in a U tube containing Linde 4A molecular sieves. The relative humidity varied along the column due to the finite column pressure drop. Pressure drops increased with the water content; a maximum drop of 11 kN m -2 was produced. Nitrogen flow rates, measured with an accuracy of 1% at the column outlet with a calibrated soap-bubble flowmeter, were set close to 3.3 x 10 -7 m 3 s e e - ' (20 cm a min-1). Flow rates were corrected for column temperature, gas compressibility, and water vapor in the flowmeter and column according to (Pout - PW,no) , Tno Pout(1 -- pWsat/Psat)
1(7co1 = Fnoj --T
[2]
where Ff~o is the measured flow rate, Fcol is the corrected average volume flow rate, T and Tno are the column and room temperatures, respectively, and Po.fol w is the saturated vapor pressure of water in the flowmeter. A detailed derivation of Eq. [2] was given by Dorris (14). The net retention volume, VN, was then calculated from the product of the corrected flow rate and the net retention times according to Eq. [3], VN = (tR -- tM)P~ol,
[31
where tR and tM are the times required to elute the sample and methane vapor, respectively. The cellophane column was equilibrated at the required R H for at least 2 days before
341
any sample was injected. This time period was believed to be sufficient to attain an equilibrium between the cellophane surface and the water vapor. The hydrocarbon adsorption measurements were made in order of increasing cellophane moisture contents. Isotherms were obtained for n-octane and mesitylene by making a series of injections from - 10-6 to 2.0 mm 3 using 1- and 10-mm a Hamilton syringes. The recorder pen displacement, h, was related to the vapor pressure of probe, p, by mcalJ(RTh
p-
Seal/CooI
[41
where mca~ is a known number of moles of probe, J( is the recorder chart speed, Seal is the peak area produced by the calibrating injection, and R is the gas constant. Equation [4] is based on ideal gas conditions and assumes a linear detector response. The number of moles adsorbed per kilogram of cellophane, q, was then calculated by integrating the gc envelope of the peak maxima at arbitrary intervals o f p / p o according to
q
-
';0
- wRT
V~dp,
[5]
where w is the weight of dry cellophane in the column. Values for Po, the saturated vapor pressure of the probe, were obtained from API tables (21) for n-octane. For mesitylenel Po was obtained from the data of Stuckey and Saylor (22). Injection volumes of the n-alkanes heptane, nonane, and decane were - 1 0 -6 mm 3. The column dead volume was determined by injecting methane. Materials
Mesitylene and the n-alkanes, heptane, octane, nonane, and decane, were obtained from either Polyscience Corporation or Aldrich. The samples were better than 99% pure and were used as received. Triply distilled water was used in the saturator. The unplasticized cellophane, obtained from British Cellophane Limited, was p a c k e d in Journal of Colloid and Interface Science, Vol. 82, No. 2, August 1981
342
KATZ AND GRAY
80
solutions. The sample bottles were then sealed, with the cellophane samples sus0 SALTSOLUTIONS 6O pended above the solutions. The sample A H2SO4 SOLUTIONS bottles were submerged in a thermostated E water bath (Colora Ultra-thermostat) at o (,,,i 40 J 20°C. The water uptake of the cellophane was followed gravimetrically for 1-3 months to ensure that equilibrium had been attained. ~ 20 The H2804 concentrations (0.1-11 N) were .OA....,.~A.,.' A ~ O ~ j determined by volumetric titration with o~o~-~-"~ standard NaOH solutions, and the corre00 100 sponding partial pressures of water were ob% RELATIVE HUMIDITY tained from the literature (23). The proceFIG. 1. Wateruptake at 20°Cdeterminedgravimetri- dure was repeated for cellophane suspended cally for the cellophane sample as a function of the over saturated salt solutions. The following relative humidity above saturated salt solutions and salts were used: Pb(NO~)2, NH4H2PO4, KBr, sulfuric acid solutions. NaC1, N a B r ' 2 H 2 0 , CaC12"6H20, and KC2H302. The corresponding partial presthe column as previously described (1). The sures of water were obtained from the literaweight of the dry cellophane packed was ture (24). 11.957 g; the estimated geometric area of the cellophane was 68 _+ 2 m 2 kg -1 (1). RESULTS
AND DISCUSSION
Source of Errors
Sorption of Water by the Cellophane
The precision of the measured retention volumes at zero coverage is estimated to be 1-2% due to variations in flow rate and temperature. The error in VN of octane introduced by neglect of the sorption at finite coverages is - 0 . 1 % and is therefore insignificant. The octane isotherms are estimated to be precise to _+1%. The reproducibility in the gc peak maxima envelope was poorest for the measurements using mesitylene at low surface coverages, and led to an uncertainty in the adsorption isotherm of - 5 % in this case.
The equilibrium sorption isotherm for water on the sample of unplasticized cellophane is shown in Fig. 1. The results are in good agreement with literature data (25). Similar water sorption isotherms have been obtained for cotton cellulose (15).
Water Sorption Apparatus and Measurement Glass weighing bottles, containing known weights of cellophane, were suspended inside glass sample bottles by means of a nickelchromium wire attached to the plastic caps of the sample bottles. Glass marble weights were placed in the bottom of the sample bottles which also contained aqueous HzSO 4 Journal of Colloid and Interface Science, Vol. 82, No. 2, August 1981
Effect of Humidity on the Cellophane Surface Area The surface area of the cellophane sample at different relative humidities was calculated from a series of isotherms for n-octane using the BET theory. The adsorption isotherms were calculated from the gc retention volumes corrected for column temperature, pressure drop, and the presence of water vapor, by means of Eq. [5]. The validity of this peak maxima method for calculating adsorption isotherms was previously discussed (2). The peak fronts for n-octane superimposed well for RH >- 61%. This indicated that equilibrium conditions were attained at all partial pressures of octane (26). A series
ADSORPTION OF H Y D R O C A R B O N S ON C E L L O P H A N E I
1.25
I I z
I
2
i
=
I
uuJ
I
343
E Z
0.75
2
I
0.25
>~
i
5
10 15 20 NET RETENTION VOLUME 1 0 6 / m 3
30
F i e . 2. Superimposed gas chromatographic peaks of n-octane on cellophane at 95% RH and 20°C. The heavy line indicates the envelope of the peak maxima. VM is the volume of carrier gas required to elute the noninteracting methane vapor.
of gc peaks for octane at 95% RH and 20°C is given in Fig. 2. At 61% RH, the mesitylene peaks were less reproducible than for octane; mesitylene isotherms at higher humidities were therefore not studied. Curiously, at 38% RH, the peak profiles for both hydrocarbons were erratic and did not superimpose with the usual precision; hence, no adsorption isotherms were calculated at this RH. A similar effect was observed on cellulose paper at - 6 5 % RH (15) and on dry lignin-rich wood fibers (27). The reason for this is not yet known.
Isotherm points in the relative partial pressure range 0.05 < P/Po < 0.3 were fitted by a least-squares method to the linear form of the BET equation, as shown in Fig. 3. In general, the linearity was excellent. The standard deviation of the slope for 95% RH was 2%; smaller deviations were observed for the other lines. Although the shortcomings of the BET model are well known (28, 29), it remains useful as a means for obtaining a monolayer coverage. Table I lists the monolayer coverages, qm, and BET constants, C, obtained from the slopes and in-
28
[] 95 % RH 0 88°1o RH
~.
C]~'~ i~'[~F,~
O
24
b
°22 os
~
0~0
t , f
20
Q. 18
16
I
0
O.1
0.2
0.3
P/Po FIG. 3. BET plots for n-octane on cellophane at 20°C at three relative humidities. The data points were obtained from the smoothed adsorption data at arbitrary intervals ofp/po. Journal of Colloid and Interface Science, Vol. 82, No. 2, A u g u s t 1981
344
K A T Z AND GRAY TABLE I
BET Results for the Surface Adsorption of n-Octane on Cellophane at 20°C and at Different Relative Humidities
(%)
qm × 104/mole kg-1
C
0 61 88 95 Redried
2.27 __. 0.11 a 2.27 2.33 2.29 1.87
2.9 ~- 0.2 ~ 2.3 2.3 2.1 2.5
a Standard deviations refer to the reproducibility from triplicate measurements (2).
tercepts of the corresponding BET plots. Maximum uncertainties of qm and C were estimated from the deviations from linearity of the plots to be - 3 % . These data show that the monolayer coverage is independent of the relative humidity in the column. In order to obtain the surface area from the values for the monolayer coverage, the area occupied by an n-octane molecule is required. On dry cellophane, this molecular area was determined to be 0.655 nm z from the monolayer capacity and the BET nitrogen surface (90 m 2 kg-1). We assume that this molecular area remains the same when the cellophane is exposed to relative humidities up to 95%. (Constant molecular areas of the n-alkanes on cotton filter paper up to at least 65% RH were previously obtained (14, 15).) Making this assumption, then the constant monolayer coverage indicates that the cellophane surface area is constant at 90 m 2 kg -1 as the relative humidity is changed. This result contrasts with that previously obtained (13-16) on paper and fiber surfaces. The surface area of cotton cellulose paper, for example, decreased significantly after exposure to 65% RH. The postulated explanation was that moisture allowed the paper structure to relax to an equilibrium state. Over 85% RH, the paper surface area dropped drastically due to water condensation in the large pores. In contrast to the rough, porous nature of fibers and paper, scanning electron Journal of Colloid and Interface Science, Vol. 82, No. 2, August 1981
micrographs (30) show that cellophane is a relatively smooth film. It is virtually free from pores when dry (31). Hence a constant surface area for cellophane is reasonable. In fact, a slight increase in the cellophane surface area might be expected due to swelling. When submerged in water, however, cellophane swells predominantly in the thickness direction (31). Since the maximum moisture content in the gc column was - 3 5 % (see Fig. 1) and since the cellophane disk edges contribute only 2% to the geometric area (1), this contribution to the external surface area is minor. One anomaly is apparent in Table I. The monolayer coverage for n-octane on the redried cellophane is lower than on the initial dry sample and corresponds to a surface area of 73.7 m 2 kg -1. This is due to the sticking together of cellophane disks on wetting and redrying; the formation of cellulosecellulose bonds decreases the accessible surface area in the column.
Retention at Zero Coverage Very small injections of mesitylene and the n-alkanes gave fairly symmetrical peaks on the moist cellophane. An example is shown in Fig. 4. The retention volumes were
°],
o
i
5
J/ lO
i
15
20
NET RETENTION VOLUME. IOe/rn 3
FIG. 4. Chromatographic peaks in the Henry's law region for n-octane on cellophane at 20°C and 61% RH. The recorder response is proportional to the hydrocarbon concentration.
345
A D S O R P T I O N O F H Y D R O C A R B O N S ON C E L L O P H A N E T A B L E II Variation of Hydrocarbon Retention Volumes with Relative Humidity on Cellophane at 20°C V~/w ~ × 10z
m3kg 1 (%)
n -Heptane
0 38 61 88 95 Redried
0.351 + 0.01 0.345 0.329 0.301 0.284 0.270
n-Octane
1.12 1.08 1.00 0.901 0.819 0.823
_+ 0.5 _+ 0.01 _+ 0.03 _+ 0.03 _+ 0.001
n -Nonane
n -Decane
Mesitylene
3.51 -+ 0.1 3.37 3.17 2.83 2.51 + 0.1 2.49
11.3 _+ 0.3 10.6 9.82 8.71 7.82 7.80
6.90-+ 0.04 6.51 5.95 -+ 0.1 5.49 5.13 --
Per unit weight of dry cellophane,
also independent of sample size, indicating that the amount of probe retained varied linearly with the partial pressure. The net retention volumes for the n-alkanes and mesitylene on cellophane at four relative humidities, expressed as VN/w, where w is the total weight of dry cellophane in the column are listed in Table II. The standard deviations reported are believed to be due to variations in the column water content. Also included in Table II are the Vr~/w values previously obtained on dry cellophane (1,2). The retention volumes of hydrocarbons on dry cellophane result only from surface adsorption of the hydrocarbons as the cellophane bulk is inaccessible to these nonswelling vapors. However, it is necessary to consider if bulk sorption of the hydrocarbons might occur for cellulose at higher relative humidities. Combined surface and bulk retention mechanisms have been observed for several polymer solvent systems (32). In this case, the retention volume is expressed ac-
cording to
VN = KsA + KLVL,
[6]
where K~ and KL are the surface and bulk partition coefficients, respectively, A is the total surface area of the stationary phase and VL is the total volume. The separation of the retention volume into contributions from surface adsorption and bulk sorption normally requires measurements on stationary phases with different surface-to-bulk ratios. This is not possible for a single cellophane film sample. The probable effect on hydrocarbon retention by water-swollen cellophane may, however, be estimated from a model where the stationary phase consists of two distinct components: cellophane and water. The bulk sorption term of Eq. [6], KLVL, is therefore assumed to arise from the solubility of the probe in the water phase only. Assuming further that the solution isotherm of the hydrocarbon probe in water is linear up to P/Po = 1, and that the probe vapor is ideal, then,
concentration of probe in stationary phase (mole m -3) KL =
concentration of probe in gas phase (mole m -3)
= RT/Mpo" solubility of probe in water, where M is the molar mass of the probe. The solubilities of n-octane and mesitylene (in unit of kg m -3) were obtained from the literature (33) for 25°C. For a water volume
[7]
of 10-~ m 3 (an intermediate water content in the column) KLVL is approximately 1.6 × 10-8 m a for heptane, 1.0 x 10-g m 3 for Journal o f Colloid a n d Interface Science, Vol. 82, No. 2, August 1981
346
KATZ AND GRAY TABLE III
Surface Partition Coefficients for Hydrocarbons on Cellophane at 20°C and at Different Relative Humidities Ks × 10S/rna RH (%)
n-Heptane
0 38 61 88 95 Redried
3.9 _+ 0.1 3.83 3.66 3.34 3.16 3.66
n-Octane 12.4 12.0 11.1 10.0 9.1 11.2
_+ 0.6 ± ± ± ±
0.1 0.3 0.3 0.01
n-Nonane
n-Decane
39.0 ± 1 37.4 35.2 31.4 27.9 z 1 33.8
126 -2_ 3 118 i09 96.8 86.9 106
Standard deviations reflect variations in VN/w values given in Table II.
octane, and 4.9 × 10-6 for mesitylene, or 0.3, 0.07, and 6% of their respective retention volumes. The solubility of nonane and decane in water is even lower than that of octane, so bulk sorption may also be ignored for these alkanes. For mesitylene, however, a small contribution to the retention volume from bulk sorption cannot be ruled out. A similar conclusion was reached by Karger (34) in his study of hydrocarbon adsorption on water-coated silica and Chromosorb P. The decreasing retention volumes of the alkane probes given in Table II indicate an increasing antipathy between the hydrocarbons and the surface as the cellophane takes up water. These values for VN/w may be combined with the specific surface area of the cellophane, discussed in the previous section, to give the partition coefficients for surface adsorption, Ks. These Ks values, listed in Table III, are of the same order of magnitude as those on moist cotton paper reported at 10°C (16). Interpolation of the In Ks vs 1/T plot given in Ref. (16) for 20°C and -95% RH gives, for example, a Ks for octane of 6.1 × 10-6 m; the corresponding value for Ks on cellophane at 95% RH is 9.1 × 10-6 m. These values of Ks are still, however, a factor of 10 greater than those for n-alkanes on bulk water (34-36), suggesting that a larger force field of attraction exists for the moist cellophane surface. It should be noted that for the redried Journal of Colloid and Interface Science, Vol. 82, No. 2, August 1981
cellophane, the lower VN/w values, combined with the smaller surface area discussed in the previous section, lead to partition coefficients that are smaller than for the initial dry cellophane. In fact, the partition coefficients are close to those for 61% RH. Furthermore, on redrying the cellophane in the gc column, only 26% water by weight was recovered, whereas about 35% by weight was expected from Fig. 1. It seems likely that the redrying conditions were not sufficiently severe to remove all the sorbed water.
Free Energy of Adsorption and of the Cellophane Surface Some adsorption isotherms for n-octane are shown in Fig. 5. Isotherms for mesitylene were similar in shape. A weak-kneed type-II isotherm was observed for n-octane on dry cellophane. As the humidity increased, the isotherms approached type-III behavior although the type-II character was still retained with C values greater than 2 (Table I). Similar values for the surface excess, F, and for C on cellophane were observed for the n-alkanes on cotton paper (16) at about 90% RH and on fibers (14). Note that adsorption of n-alkanes and aromatic hydrocarbons on the surface of water gives type-III isotherms, with values for F smaller by an order of magnitude (35, 37, 38). Thus, the surface properties of the water-swollen gel still differ from those of bulk water. The free energy of adsorption, A(~A, of n-octane as a function of surface coverage at three humidities is illustrated in Fig. 6. The AGAvalues were calculated from AGA = RT In P/Ps,g,
[8]
where ps,g, the vapor pressure of a molecule in the gaseous standard state, is 101 kN m-2; the p values at each coverage were obtained directly from the adsorption isotherms (Fig. 5). Errors in AGA thus arise from errors in the isotherm determinations. The shape of the free energy of adsorption curves in Fig. 6 remains almost constant with increasing
ADSORPTION OF HYDROCARBONS ON CELLOPHANE n-OCTANE 40
,
# ~ 2
///3
o O,o . .
2.,
O,o. .
////
3. 95 % RH E
347
/
/
/
30
E ~
20
r-10
0
,
0
0.1
0.2
0.3
0.4
0.5
0.6
p/~ FIG. 5. Adsorption isotherms for n-octane on cellophane at 20°C at three different relative humidities. Fm indicates the monolayer coverage.
relative humidity. At high coverages, the free energy of liquefaction of octane, AGL, is approached. Although a decrease in the absolute values of AGA with increasing cellophane water content occurs, this decrease is relatively small. Decreases with increasing moisture content were also observed in the standard free energy of adsorption of the n-alkanes, AG~, calculated from zero coverage measurements according to AG~ = -RT
In (KsPs,g/Trs),
[9]
where ~s, the spreading pressure in the standard adsorbed state, is 0.338 mN m -1 (39). The values for AG~ on dry cellophane are only slightly larger (<0.1%) than those at 38% RH; the difference is close to the experimental error. As shown in Fig. 7, a linear relationship between AG~ and the number of carbon atoms in the n-alkane was observed. This constant increment may be related to the London component of the surface free energy, y~, of the cellophane. Values for y~ were calculated using the approach of Dorris and Gray (40). The assumption was made that the work of adhesion, WA, between a liquid CH2 surface and a solid is approximately equal to the increment per methylene group in the free energy of desorption, AG~ H21, expressed per unit
area of one mole of CH2 group; that is, [10]
WA = -AG~H21/Na(cH21,
where N is Avogadro's constant. The area per CHz group, a(cn~), was assumed to equal 0.06 nm ~. For a liquid alkane-solid interface, Fowkes (41) proposed that WA may also be given by WA = 2(y~7~) 1/',
[1l]
where yl and ys are the surface energies of the liquid and solid phases, respectively, and
2G 18
I,(D~ 14
12
10 0
95°/o RH ~ 6 1 % R - H I
1
I 2
'
I I 3 4 T' " 106/mol m-2 /...
-TG[" I 5
FIG. 6. T h e d i f f e r e n t i a l free e n e r g y o f a d s o r p t i o n for n - o c t a n e o n c e l l o p h a n e at 20°C at t h r e e r e l a t i v e h u m i d i t i e s as a f u n c t i o n o f the s u r f a c e e x c e s s . AGL is the free energy of liquefaction of n-octane. Journal of Colloid and Interface Science, Vol. 82, No. 2, August 1981
348
KATZ AND GRAY
26
O
mole-a). Values ofy~ are therefore precise to _+1 m N m -a at best. A second route to y~ is based on values of 7r°, the equilibrium spreading pressure of the a d s o r b e d v a p o r atp/po = 1, determined from the complete adsorption isotherm for a single h y d r o c a r b o n . The w o r k of adhesion required to separate a solid and a liquid is given by WA = Ys + Yl -- %1, [14]
38 % R'H U
24
////"
[] 9s~ ~
} ,o
16 I 7
I 8
I 9
I 10
ALKANE CHAIN LENGTH
FIG. 7. T h e standard free energy o f adsorption for the n-alkanes on cellophane at 20°C at four relative humidities, as a function of the n u m b e r o f carbon a t o m s in the alkane chain.
the superscript L indicates the L o n d o n dispersion c o m p o n e n t . For the particular case of a hypothetical CH2 liquid on a solid, Eq. [11] b e c o m e s , W A = 2(3/(CH2),)/sL) 1/2
= -AG~n2)/Na(cH2).
[12]
The surface energy of a pure CHz surface, y(cH~), was taken to equal 35.6 m N m -~, the surface tension of a linear polyethylene liquid at the experimental t e m p e r a t u r e (42). Average incremental values in AG~ at each moisture content were taken from Fig. 7, and from Ref. (1) for dry cellophane. The results are listed in Table IV along with the values of y~ calculated from Eq. [12]. Also included in Table IV are values for ?~ based on an arithmetic m e a n approximation for WA (43), i.e., Y~ = -(Y(cH~> + AG~H2)/Na(cH~)).
where Y~z is the interfacial tension. Combining Eq. [14] with Y o u n g ' s equation, a relationship between the contact angle 0 of a liquid on a solid and WA can be obtained: ylcos 0 = - y z + WA -- 7r°.
[15]
Since the n-alkanes studied spread on cellophane, cos 0 = 1 and Eq. [15] reduces to WA = 7r° + 2yz.
[16]
T h e dispersion c o m p o n e n t of the cellophane surface energy can then be determined f r o m Eq. [17], using F o w k e s ' geometric m e a n approximation for WA,
y~ = (~o + 2yt)z/4y~
[17]
or from Eq. [18], using the arithmetic m e a n approximation for WA, y~ = 7r° + 2yl - y~.
[18]
The spreading pressure 7r° for n-octane on cellophane at each humidity was determined from an analytical integration of the correT A B L E IV Variation with Humidity of the L o n d o n C o m p o n e n t of the Cellophane Surface Free Energy B a s e d on Zero Coverage M e a s u r e m e n t s at 20°C
[13]
Although the standard deviations in the incremental free energies of adsorption are large, the decrease in magnitude of A G ~ n~) as the humidity increases from 38 to 95% R H is thought to be real; the standard deviations in A G ~ H~) based on m e a s u r e m e n t s made on the same day to avoid variations in water content were smaller (_+0.02 kJ Journal of Colloid and Interface Science, Vol. 82, No. 2, August 1981
(%)
0 38 61 88 95
(kJ mole 1)
2.80 2.80 2.76 2.74 2.69
_+ 0.06 _+ 0.06 _+ 0.05 -+ 0.05 _+ 0.09
finN m -1)
(mN m 1)
42.(2) 42.(2) 41.(0) 40.(4) 38.(9)
41.(9) 41.(9) 40.(8) 40.(3) 38.(9)
" Calculated from the geometric m e a n , Eq. [12]. b Calculated from the arithmetic m e a n , Eq. [13].
349
A D S O R P T I O N O F H Y D R O C A R B O N S ON C E L L O P H A N E
sponding adsorption isotherm (2, 27). These values are listed in Table V. Errors in 7r° are estimated to be at least - 1 mN m71. Therefore the increase in ~.0 at 95% R H is not likely to be real. Evidently, 7r° is of significant magnitude in this system and should not be neglected. The values are furthermore real; they cannot be ascribed to capillary condensation as is sometimes suggested (19, 44). The corresponding y~ values for cellophane were calculated from Eqs. [17] and [18] using the tabulated values of ~.0 and 21.62 m N m -1 (45) for the surface tension of n-octane (7l = 7~), at 20°C. These results are included in Table V. Unlike y~ values calculated from zero coverage data, these values are surface area dependent. Differences in the two methods of calculation may then be due to errors in the surface area value, errors in the extrapolation required to measure 7r° (2), and errors in the free energy increment, AG~ H2). The 7) values in Table V calculated using the arithmetic mean method are closer to the values calculated from zero coverage data than are the y ) values calculated from Eq. [17]. This suggests that the arithmetic mean is a better approximation than the geometric mean in calculating the work of adhesion. A third method, based on the reciprocal or harmonic mean (46) gave unreasonably large values for y ) (64 --~ 53 mN m -1 with increasing humidity). All estimates of 7), however, indicate that as the cellophane water content increases, the L o n d o n force field of attraction of the surface for the n-alkanes diminishes. It should be noted that surface properties which are independent of water content were not reached and that the values of y ) are still significantly larger than the value for bulk water of 22.0 mN m -1 (41). The effect of relative humidity on the cellophane surface has been studied by measurement of the contact angle of water. Since water swells cellulose, the interpretation of this contact angle is difficult. Borgin (17) attempted to circumvent the bulk sorption and hysteresis effects by measuring the ad-
TABLE V Variation with Humidity of the L o n d o n C o m p o n e n t of t h e Cellophane Surface Free E n e r g y B a s e d on Finite Coverage M e a s u r e m e n t s RI-I (%)
~r (raN rn-')
7k" (raN m-t)
Z'k~ (raN m-1)
0 61 88 95
21.(3) 19.(1) 18.(2) 18.(8)
48.(2) 44.(9) 43.(6) 44.(5)
42.(9) 40.(7) 39.(8) 40.(4)
Calculated from the geometric m e a n , Eq. [17]. b Calculated from the arithmetic m e a n , Eq. [18].
vancing contact angle as a function of time. He found that between 0 and 50% relative humidity, the water contact angle remained constant. At higher moisture contents, the angle decreased linearly with humidity up to 100% RH. According to Luner and Sandell (19), an increase in the relative humidity from 2 to 84% caused a decrease in the initial contact angle of water on cellulose from 40 to 18° . The decrease corresponding to the humidity increase from 2 to 32% R H was, however, only 6 °. Furthermore, the contact angle of tetrabromoethane on the same cellulose film surface was unchanged in the humidity range of 2 to 32% RH. These essentially constant contact angles on cellophane up to - 5 0 % R H suggest that the first 12% of water is sorbed without affecting the surface properties of the film. Borgin (17) proposed that this water is strongly bound to sites in the cellulose. When the available sites are saturated, further adsorption of water sharply changes the surface properties. The maximum change occurred at - 6 0 % R H corresponding, according to Borgin, to the formation of the first layer of loosely adsorbed water molecules. Further water adsorption increased this amount of loosely bound water with a smaller effect on the surface properties. At 100% RH, a minimum value of 10.2 ° for the w a t e r - w e t celluloseair contact angle was reached, rather than the 0 ° expected for a surface covered with a continuous layer of water. Journal of Colloid and Interface Science, Vol. 82, No. 2, August 1981
350
KATZ AND GRAY
The decrease in 3/L values for cellophane given in Tables IV and V corresponds well to the trend in water contact angles. At 38% RH, the cellophane surface appeared to be the same as when dry. It should be noted that the cellophane at 0% RH, obtained by drying the cellophane column at 105°C (1), still contains some moisture (47, 48). Prolonged drying at 105°C or drying at higher temperatures results in a continuous loss of weight; however, degradation of the cellulose may also occur. Another estimate for the effect of relative humidity on ~/L of cellulose was based on contact angles of methylene iodide and tetrabromoethane (49). Swanson and Brown resolved the surface free energy into polar and dispersion components according to the method of Owens and Wendt (50). The decrease in 3/) on going from 0 to 50% R H was 2 mN m -1. The value for 3/) at 95% RH is high compared to that for pure water ( - 3 9 mN m -1 vs 22 mN m-~). Dorris and Gray (16) found a similar effect for moist cellulose paper, where the lowest value for 3,L was 34 mN m -1, even at 140% water by weight. These values also agree qualitatively with Borgin's observation of a nonzero water contact angle on cellophane at 100% RH. In general, greater decreases in 3/) with relative humidity were observed for cotton cellulose paper than for cellophane, suggesting that, at equivalent relative humidities, more water is surface adsorbed on cotton cellulose than on regenerated cellulose. For both moist celluloses, however, the surface properties of bulk water are not reached. CONCLUSION
By modifying the carrier gas line of the gc apparatus so that it passed through a water saturator, the adsorption of hydrocarbons on water-swollen cellophane film was studied. Due to the experimental difficulties introduced by the addition o f water, such as the increased pressure drop, as well as the theoretical complication of water interacJournal of Colloid and Interface Science, VoI. 82, No. 2, August 1981
tions, the precision in the retention measurements was lower than for the dry cellophane column. Adsorption isotherms were, however, successfully determined for the n-alkanes at humidities greater than 40%. Estimation of the corresponding monolayer coverages by the B E T equation indicated the the cellophane surface area is independent of moisture content. This confirms that the cellophane possesses a geometrically homogeneous surface, in contrast to that of cotton paper. From the isotherms, it was found that the n-alkanes adsorb more on dry cellophane than on wet. This decrease in a d s o r b a t e - a d s o r b e n t attraction was illustrated by the decrease in Ks values with humidities greater than - 4 0 % . Apparently, the first - 1 2 % of water sorbed did not affect the surface as indicated by the L o n d o n component of the surface free energy 3,L, which was - 4 2 mN m -1 for both dry cellophane and cellophane at 38% RH. At 95% RH, this value decreased to 39 mN m -1. The bulk water state, where 3,L = 22 mN m -1, was not attained. ACKNOWLEDGMENTS Thanks are due the Natural Sciences and Engineering Council of Canada and the Government of Quebec for partial support of this work, and for Postgraduate Scholarships (S.K.). REFERENCES 1. Katz, S., and Gray, D. G., J. Colloid Interface Sci. 82, 318 (1981). 2. Katz, S., and Gray, D. G., J. Colloid Interface Sci. 82, 326 (1981). 3. Fowkes, F. M., "Chemistry and Physics of Interfaces II" (S, Ross, Ed.). Amer. Chem. Soc., Washington, D. C., 1971. 4. Stamm, A. J., "Wood and Cellulose Science," p. 192. Ronald Press, New York, 1964. 5. Babbett, J. D., Canad. J. Res. A20, 143 (1943). 6. Stamm, A. J., Tappi 40(9), 761 (1957). 7. Hagamassy, J., Brunauer, S., and Mikhail, R. S., J. Colloid Interface Sci. 29, 485 (1969). 8. Weatherwax, R. C., J. Colloid Interface Sci. 49, 40 (1974). 9. Stone, J. E., and ScaUan, A. M., Pulp Pap. Mag. Canad. 66, T-407 (1965). 10. Mason, S. G., Tappi 33(8), 403 (1950).
ADSORPTION OF HYDROCARBONS ON CELLOPHANE 11. Archer, W. L., and Mason, S. G., Canad. J. Chem. 37, 1655 (1959). 12. Kamiya, Y., and Takahashi, F., J. Appl. Polym. Sci. 23, 627 (1979). 13. Tremaine, P. R., Mohlin, U.-B., and Gray, D. G., J. Colloid Interface Sci. 60, 548 (1977). 14. Dorris, G. M., Ph.D. thesis, Chap. IV. McGiU University, 1979. 15. Dorris, G. M., and Gray, D. G., J. Chem. Soc. Faraday 1 77, 713 (1981). 16. Dorris, G. M., and Gray, D. G., J. Chem. Soc. Faraday 1 77, 725 (1981). 17. Borgin, K., Nor. Skogind. 13, 429 (1959). 18. Borgin, K., Nor. Skogind. 14, 485 (1960). 19. Luner, P., and Sandell, M., J. Polym. Sci. Part C 28, 115 (1969). 20. James, A. T., and Martin, A. J. P., Biochem. J. 50, 679 (1952). 21. "Selected Values of Properties of Hydrocarbons and Other Related Compounds," American Petroleum Institute Project 44, Thermodynamic Research Center, College Station, Texas, 1974. 22. Stuckey, J. M., and Saylor, J. H., J. Amer. Chem. Soc. 62, 2922 (1940). 23. "International Critical Tables," 1st ed., Vol. 33, p. 303. McGraw-Hill, New York, 1928. 24. Lange, N. A., (Ed.), "Handbook of Chemistry," 10th ed., pp. 1420-1423. McGraw-Hill, New York, 1961. 25. Stamm, A. J., J. Phys. Chem. 60, 83 (1956). 26. Kiselev, A. V., and Yashin, Y. I., "Gas-Adsorption Chromatography," p. 113. Plenum, New York, 1969. 27. Dorris, G. M., and Gray, D. G., J. Colloid Interface Sci. 71, 93 (1979). 28. Gregg, S. J., and Sing, K. S. W., "Adsorption, Surface Area and Porosity," Chap. 2. Academic Press, London, 1967. 29. Adamson, A. W., "Physical Chemistry of Surfaces," 3rd ed., p. 603. Interscience, New York, 1976.
351
30. Katz, S., M.Sc. thesis, Chap. I, Fig. 1. McGill University, 1979. 31. Stamm, A. J., J. Phys. Chem. 60, 76 (1956). 32. Gray, D. G., "Progress in Polymer Science," Vol. 5. Pergamon, New York, 1977. 33. McAuliffe, C., J. Phys. Chem. 70, 1267 (1966). 34. Karger, B. L., Sewell, P. A., Castells, R. C., and Hartkopf, A., J. Colloid Interface Sci. 35, 328 (1971). 35. Hartkopf, A., and Karger, B. L., Accounts Chem. Res. 6, 209 (1973). 36. Jones, D. C., and Ottewill, R. H., J. Colloid Interface Sci. 34, 473 (1970). 37. Karger, B. L., Castells, R. C., Sewell, P. A., and Hartkopf, A., J. Phys. Chem. 75, 3870 (1971). 38. King, J. W., Chatterjee, A., and Karger, B. L., J. Phys. Chem. 76, 2769 (1972). 39. de Boer, J. H., "The Dynamical Character of Adsorption," p. 112. Oxford Univ. Press, (Clarendon), London/New York, 1953. 40. Dorris, G. M., and Gray, D. G., J. Colloid Interface Sci. 77, 353 (1980). 41. Fowkes, F. M. in "Chemistry and Physics of Interfaces" (S. Ross, Ed.). Amer. Chem. Soc., Washington, D. C., 1965. 42. Roe, R. J., J. Phys. Chem. 72, 2013 (1968). 43. Zettlemoyer, A. C. in "Hydrophobic Surfaces" (F. M. Fowkes, Ed.). Academic Press, New York, 1969. 44. Blake, T. D., Cayias, J. L., Wade, W. H., and Zerdecki, J. A., J. Colloid Interface Sci. 37, 678 (1971). 45. Ref. (21), 1955. 46. Wu, S., J. Polym. Sci. Part C, 34, 19 (1971). 47. Stamm, A. J., "Wood and Cellulose Science," Chap. 8. Ronald Press, New York, 1964. 48. Borgin, K., Not'. Skogind. 15, 384 (1961). 49. Swanson, J. W., and Brown, P. F., Tappi 54, 2012 (1971). 50. Owens, D. K., and Wendt, R. C., J. Appl. Polym. Sci. 13, 1741 (1969).
Journal of Colloid and Interface Science, Vol. 82, No. 2, August 1981
The adsorption of nonswelling hydrocarbon vapors on dry cellophane at zero and finite surface coverages has been previously considered (1, 2). Equilibrium partition coefficients and adsorption isotherms were calculated and correlated to the interactions of the adsorbate with the cellophane substrate and with itself. Using Fowkes' theory (3), the dispersion component of the cellophane surface free energy was estimated from both incremental quantities in the free energy of adsorption of the alkane sorbates and from their spreading pressures. In the present paper, the adsorption of alkanes on cellophane containing varying amounts of sorbed water is considered. The sorption of water by cellulosic materials is technologically important. Changing the moisture content of wood, paper, fiber, or film alters the dimensional, surface, and mechanical properties of the materials. In particular, the sorption of water into cellulose changes the surface area. Care is necessary in interpreting these changes,
however, because surface area may be defined and measured in three distinct manners. (i) Sorbed water swells cellulose by breaking accessible cellulose-cellulose hydrogen bonds and increasing the internal surface area of the cellulose, termed the "contact area" by Stamm (4). The contact area has been determined by the BET analysis (5, 6) and other treatments (7, 8) of the water sorption isotherm. The contact area has also been determined by the BET treatment of nitrogen adsorbed on solvent-exchanged swollen cellulose (9). All methods give values for the cellulose contact surface area of - 2 × 105 m 2 kg -1 (200 m 2 g-l). (ii) The external surface area of a fiber immersed in water is termed the hydrodynamic specific surface (10). This surface area is of practical significance in permeability studies (11, 12). (iii) The surface considered in this study is the external surface of water-swollen cellulose; that is, the surface which is accessible to water-insoluble vapors. Tremaine et al. (13) first investigated the three-component
339 0021-9797/81/080339-13502.00/0 Journal of Colloid and Interface Science, Vol. 82, No. 2, August 1981
Copyright © 1981 by Academic Press, Inc. All rights of reproduction in any form reserved.
340
KATZ AND GRAY
system of water-cellulose fibers-organic vapors using gas chromatography (gc). By measuring the adsorption isotherms for decane at different relative humidities (RH), changes in the cellulose surface were monitored. Water-swollen cotton filter paper was subsequently studied in detail (14-16). For both systems, an overall decrease in surface area with increasing relative humidity was observed. For paper this decrease occurred in two stages: the first, starting at -60% RH was attributed to relaxation of surface fibrillation; the second sudden drop at - 9 0 % RH was attributed to capillary condensation. In this study, the external surface area of water-swollen cellophane film is measured by gc. Unlike the paper and fiber surfaces previously studied, this substrate is virtually nonporous and is nonfibrillar. The previously observed changes in surface area with relative humidity should therefore be absent. A comparison of the results on different cellulosic substrates thus provides a good test of the novel experimental methods employed. The adsorptive properties of the watermodified cellulose can also be studied by gc. The wettability of cellulosic substrates is often determined by solid-liquid contact angle measurements. For example, the contact angles of water and organic liquids on regenerated cellulose film at different relative humidities have been measured (17-19). Since water swells cellulose, the interpretation of this contact angle is questionable. These studies furthermore neglected to account for the finite spreading pressures of the liquid adsorbates, 7r°. The gc adsorption measurements on cellulose paper (16) circumvented these problems. Cellulose paper, however, is essentially a capillary system and consists of 40-80% void volume. The gc study of the adsorptive properties of a wet cellulose surface can therefore be simplified by using the relatively smooth and nonporous surface of cellophane film. In the present work, the variation with moisture content of the external surface area and adsorptive properties of cellophane is Journal of Colloid and Interface Science, Vol. 82, No. 2, August 1981
studied by elution gas chromatography. By saturating the carrier gas with moisture, a dynamic equilibrium between the cellophane and the water vapor is set up. For finite surface coverages, n-octane and mesitylene are used as adsorbates; the zero coverage region (Henry's law region) is studied for these adsorbates and for n-heptane, n-nonane, and n-decane. The treatment of the gc data at low coverages and the determination of the adsorption isotherms were previously discussed (1, 2). The free energy of adsorption per mole of adsorbate is also considered and the dispersion component of the surface free energy is estimated at several water contents. Throughout the investigation, the stationary phase is treated as a single component. EXPERIMENTAL gc Apparatus and Measurement The general apparatus used has been previously described (1). In brief, adsorption data were obtained on a Hewlett-Packard 5711A gas chromatograph equipped with a heated injection port and dual flame ionization detectors. A thermostated, circulating water bath (Haake, model FK) replaced the air oven and maintained the column temperature at 20.0 ___ 0.05°C. For the present work, the N2 carrier gas line was modified so as to achieve constant relative humidities in the column. Dry N2 gas was passed through an external flow controller and then through a water-bubble saturator immersed in a thermostated water bath (Lauda K4R, _0.05°C) prior to entering the column. The column inlet and the saturator were therefore at the same pressure. Suitable temperature adjustments of the saturator water bath allowed the column to be conditioned to the required humidity (38, 61, 88, or 95%). The average percentage relative humidity in the column (RH) was calculated from __
% RH-
W
P°'sate°ut × 100, W " Po,colJ)Osat
[1]
ADSORPTION OF HYDROCARBONS ON CELLOPHANE where pWsat is the saturated vapor pressure of water in the saturator, Po.eol w is the saturated vapor pressure of water at the column temperature, Pout is the column outlet or atmospheric pressure, Psat is the saturator or inlet pressure, and j is the correction for gas compressibility (20). The humidity calculated from Eq. [1] has been found by previous workers (14, 15) to c o r r e s p o n d to the column water content to within _+2%. In this work, the water content at 95% R H was determined by heating the column at 60°C under a dry Nz stream for - 4 8 hr, followed by heating at 105°C for - 2 hr. The eluted moisture was collected at the column outlet in a U tube containing Linde 4A molecular sieves. The relative humidity varied along the column due to the finite column pressure drop. Pressure drops increased with the water content; a maximum drop of 11 kN m -2 was produced. Nitrogen flow rates, measured with an accuracy of 1% at the column outlet with a calibrated soap-bubble flowmeter, were set close to 3.3 x 10 -7 m 3 s e e - ' (20 cm a min-1). Flow rates were corrected for column temperature, gas compressibility, and water vapor in the flowmeter and column according to (Pout - PW,no) , Tno Pout(1 -- pWsat/Psat)
1(7co1 = Fnoj --T
[2]
where Ff~o is the measured flow rate, Fcol is the corrected average volume flow rate, T and Tno are the column and room temperatures, respectively, and Po.fol w is the saturated vapor pressure of water in the flowmeter. A detailed derivation of Eq. [2] was given by Dorris (14). The net retention volume, VN, was then calculated from the product of the corrected flow rate and the net retention times according to Eq. [3], VN = (tR -- tM)P~ol,
[31
where tR and tM are the times required to elute the sample and methane vapor, respectively. The cellophane column was equilibrated at the required R H for at least 2 days before
341
any sample was injected. This time period was believed to be sufficient to attain an equilibrium between the cellophane surface and the water vapor. The hydrocarbon adsorption measurements were made in order of increasing cellophane moisture contents. Isotherms were obtained for n-octane and mesitylene by making a series of injections from - 10-6 to 2.0 mm 3 using 1- and 10-mm a Hamilton syringes. The recorder pen displacement, h, was related to the vapor pressure of probe, p, by mcalJ(RTh
p-
Seal/CooI
[41
where mca~ is a known number of moles of probe, J( is the recorder chart speed, Seal is the peak area produced by the calibrating injection, and R is the gas constant. Equation [4] is based on ideal gas conditions and assumes a linear detector response. The number of moles adsorbed per kilogram of cellophane, q, was then calculated by integrating the gc envelope of the peak maxima at arbitrary intervals o f p / p o according to
q
-
';0
- wRT
V~dp,
[5]
where w is the weight of dry cellophane in the column. Values for Po, the saturated vapor pressure of the probe, were obtained from API tables (21) for n-octane. For mesitylenel Po was obtained from the data of Stuckey and Saylor (22). Injection volumes of the n-alkanes heptane, nonane, and decane were - 1 0 -6 mm 3. The column dead volume was determined by injecting methane. Materials
Mesitylene and the n-alkanes, heptane, octane, nonane, and decane, were obtained from either Polyscience Corporation or Aldrich. The samples were better than 99% pure and were used as received. Triply distilled water was used in the saturator. The unplasticized cellophane, obtained from British Cellophane Limited, was p a c k e d in Journal of Colloid and Interface Science, Vol. 82, No. 2, August 1981
342
KATZ AND GRAY
80
solutions. The sample bottles were then sealed, with the cellophane samples sus0 SALTSOLUTIONS 6O pended above the solutions. The sample A H2SO4 SOLUTIONS bottles were submerged in a thermostated E water bath (Colora Ultra-thermostat) at o (,,,i 40 J 20°C. The water uptake of the cellophane was followed gravimetrically for 1-3 months to ensure that equilibrium had been attained. ~ 20 The H2804 concentrations (0.1-11 N) were .OA....,.~A.,.' A ~ O ~ j determined by volumetric titration with o~o~-~-"~ standard NaOH solutions, and the corre00 100 sponding partial pressures of water were ob% RELATIVE HUMIDITY tained from the literature (23). The proceFIG. 1. Wateruptake at 20°Cdeterminedgravimetri- dure was repeated for cellophane suspended cally for the cellophane sample as a function of the over saturated salt solutions. The following relative humidity above saturated salt solutions and salts were used: Pb(NO~)2, NH4H2PO4, KBr, sulfuric acid solutions. NaC1, N a B r ' 2 H 2 0 , CaC12"6H20, and KC2H302. The corresponding partial presthe column as previously described (1). The sures of water were obtained from the literaweight of the dry cellophane packed was ture (24). 11.957 g; the estimated geometric area of the cellophane was 68 _+ 2 m 2 kg -1 (1). RESULTS
AND DISCUSSION
Source of Errors
Sorption of Water by the Cellophane
The precision of the measured retention volumes at zero coverage is estimated to be 1-2% due to variations in flow rate and temperature. The error in VN of octane introduced by neglect of the sorption at finite coverages is - 0 . 1 % and is therefore insignificant. The octane isotherms are estimated to be precise to _+1%. The reproducibility in the gc peak maxima envelope was poorest for the measurements using mesitylene at low surface coverages, and led to an uncertainty in the adsorption isotherm of - 5 % in this case.
The equilibrium sorption isotherm for water on the sample of unplasticized cellophane is shown in Fig. 1. The results are in good agreement with literature data (25). Similar water sorption isotherms have been obtained for cotton cellulose (15).
Water Sorption Apparatus and Measurement Glass weighing bottles, containing known weights of cellophane, were suspended inside glass sample bottles by means of a nickelchromium wire attached to the plastic caps of the sample bottles. Glass marble weights were placed in the bottom of the sample bottles which also contained aqueous HzSO 4 Journal of Colloid and Interface Science, Vol. 82, No. 2, August 1981
Effect of Humidity on the Cellophane Surface Area The surface area of the cellophane sample at different relative humidities was calculated from a series of isotherms for n-octane using the BET theory. The adsorption isotherms were calculated from the gc retention volumes corrected for column temperature, pressure drop, and the presence of water vapor, by means of Eq. [5]. The validity of this peak maxima method for calculating adsorption isotherms was previously discussed (2). The peak fronts for n-octane superimposed well for RH >- 61%. This indicated that equilibrium conditions were attained at all partial pressures of octane (26). A series
ADSORPTION OF H Y D R O C A R B O N S ON C E L L O P H A N E I
1.25
I I z
I
2
i
=
I
uuJ
I
343
E Z
0.75
2
I
0.25
>~
i
5
10 15 20 NET RETENTION VOLUME 1 0 6 / m 3
30
F i e . 2. Superimposed gas chromatographic peaks of n-octane on cellophane at 95% RH and 20°C. The heavy line indicates the envelope of the peak maxima. VM is the volume of carrier gas required to elute the noninteracting methane vapor.
of gc peaks for octane at 95% RH and 20°C is given in Fig. 2. At 61% RH, the mesitylene peaks were less reproducible than for octane; mesitylene isotherms at higher humidities were therefore not studied. Curiously, at 38% RH, the peak profiles for both hydrocarbons were erratic and did not superimpose with the usual precision; hence, no adsorption isotherms were calculated at this RH. A similar effect was observed on cellulose paper at - 6 5 % RH (15) and on dry lignin-rich wood fibers (27). The reason for this is not yet known.
Isotherm points in the relative partial pressure range 0.05 < P/Po < 0.3 were fitted by a least-squares method to the linear form of the BET equation, as shown in Fig. 3. In general, the linearity was excellent. The standard deviation of the slope for 95% RH was 2%; smaller deviations were observed for the other lines. Although the shortcomings of the BET model are well known (28, 29), it remains useful as a means for obtaining a monolayer coverage. Table I lists the monolayer coverages, qm, and BET constants, C, obtained from the slopes and in-
28
[] 95 % RH 0 88°1o RH
~.
C]~'~ i~'[~F,~
O
24
b
°22 os
~
0~0
t , f
20
Q. 18
16
I
0
O.1
0.2
0.3
P/Po FIG. 3. BET plots for n-octane on cellophane at 20°C at three relative humidities. The data points were obtained from the smoothed adsorption data at arbitrary intervals ofp/po. Journal of Colloid and Interface Science, Vol. 82, No. 2, A u g u s t 1981
344
K A T Z AND GRAY TABLE I
BET Results for the Surface Adsorption of n-Octane on Cellophane at 20°C and at Different Relative Humidities
(%)
qm × 104/mole kg-1
C
0 61 88 95 Redried
2.27 __. 0.11 a 2.27 2.33 2.29 1.87
2.9 ~- 0.2 ~ 2.3 2.3 2.1 2.5
a Standard deviations refer to the reproducibility from triplicate measurements (2).
tercepts of the corresponding BET plots. Maximum uncertainties of qm and C were estimated from the deviations from linearity of the plots to be - 3 % . These data show that the monolayer coverage is independent of the relative humidity in the column. In order to obtain the surface area from the values for the monolayer coverage, the area occupied by an n-octane molecule is required. On dry cellophane, this molecular area was determined to be 0.655 nm z from the monolayer capacity and the BET nitrogen surface (90 m 2 kg-1). We assume that this molecular area remains the same when the cellophane is exposed to relative humidities up to 95%. (Constant molecular areas of the n-alkanes on cotton filter paper up to at least 65% RH were previously obtained (14, 15).) Making this assumption, then the constant monolayer coverage indicates that the cellophane surface area is constant at 90 m 2 kg -1 as the relative humidity is changed. This result contrasts with that previously obtained (13-16) on paper and fiber surfaces. The surface area of cotton cellulose paper, for example, decreased significantly after exposure to 65% RH. The postulated explanation was that moisture allowed the paper structure to relax to an equilibrium state. Over 85% RH, the paper surface area dropped drastically due to water condensation in the large pores. In contrast to the rough, porous nature of fibers and paper, scanning electron Journal of Colloid and Interface Science, Vol. 82, No. 2, August 1981
micrographs (30) show that cellophane is a relatively smooth film. It is virtually free from pores when dry (31). Hence a constant surface area for cellophane is reasonable. In fact, a slight increase in the cellophane surface area might be expected due to swelling. When submerged in water, however, cellophane swells predominantly in the thickness direction (31). Since the maximum moisture content in the gc column was - 3 5 % (see Fig. 1) and since the cellophane disk edges contribute only 2% to the geometric area (1), this contribution to the external surface area is minor. One anomaly is apparent in Table I. The monolayer coverage for n-octane on the redried cellophane is lower than on the initial dry sample and corresponds to a surface area of 73.7 m 2 kg -1. This is due to the sticking together of cellophane disks on wetting and redrying; the formation of cellulosecellulose bonds decreases the accessible surface area in the column.
Retention at Zero Coverage Very small injections of mesitylene and the n-alkanes gave fairly symmetrical peaks on the moist cellophane. An example is shown in Fig. 4. The retention volumes were
°],
o
i
5
J/ lO
i
15
20
NET RETENTION VOLUME. IOe/rn 3
FIG. 4. Chromatographic peaks in the Henry's law region for n-octane on cellophane at 20°C and 61% RH. The recorder response is proportional to the hydrocarbon concentration.
345
A D S O R P T I O N O F H Y D R O C A R B O N S ON C E L L O P H A N E T A B L E II Variation of Hydrocarbon Retention Volumes with Relative Humidity on Cellophane at 20°C V~/w ~ × 10z
m3kg 1 (%)
n -Heptane
0 38 61 88 95 Redried
0.351 + 0.01 0.345 0.329 0.301 0.284 0.270
n-Octane
1.12 1.08 1.00 0.901 0.819 0.823
_+ 0.5 _+ 0.01 _+ 0.03 _+ 0.03 _+ 0.001
n -Nonane
n -Decane
Mesitylene
3.51 -+ 0.1 3.37 3.17 2.83 2.51 + 0.1 2.49
11.3 _+ 0.3 10.6 9.82 8.71 7.82 7.80
6.90-+ 0.04 6.51 5.95 -+ 0.1 5.49 5.13 --
Per unit weight of dry cellophane,
also independent of sample size, indicating that the amount of probe retained varied linearly with the partial pressure. The net retention volumes for the n-alkanes and mesitylene on cellophane at four relative humidities, expressed as VN/w, where w is the total weight of dry cellophane in the column are listed in Table II. The standard deviations reported are believed to be due to variations in the column water content. Also included in Table II are the Vr~/w values previously obtained on dry cellophane (1,2). The retention volumes of hydrocarbons on dry cellophane result only from surface adsorption of the hydrocarbons as the cellophane bulk is inaccessible to these nonswelling vapors. However, it is necessary to consider if bulk sorption of the hydrocarbons might occur for cellulose at higher relative humidities. Combined surface and bulk retention mechanisms have been observed for several polymer solvent systems (32). In this case, the retention volume is expressed ac-
cording to
VN = KsA + KLVL,
[6]
where K~ and KL are the surface and bulk partition coefficients, respectively, A is the total surface area of the stationary phase and VL is the total volume. The separation of the retention volume into contributions from surface adsorption and bulk sorption normally requires measurements on stationary phases with different surface-to-bulk ratios. This is not possible for a single cellophane film sample. The probable effect on hydrocarbon retention by water-swollen cellophane may, however, be estimated from a model where the stationary phase consists of two distinct components: cellophane and water. The bulk sorption term of Eq. [6], KLVL, is therefore assumed to arise from the solubility of the probe in the water phase only. Assuming further that the solution isotherm of the hydrocarbon probe in water is linear up to P/Po = 1, and that the probe vapor is ideal, then,
concentration of probe in stationary phase (mole m -3) KL =
concentration of probe in gas phase (mole m -3)
= RT/Mpo" solubility of probe in water, where M is the molar mass of the probe. The solubilities of n-octane and mesitylene (in unit of kg m -3) were obtained from the literature (33) for 25°C. For a water volume
[7]
of 10-~ m 3 (an intermediate water content in the column) KLVL is approximately 1.6 × 10-8 m a for heptane, 1.0 x 10-g m 3 for Journal o f Colloid a n d Interface Science, Vol. 82, No. 2, August 1981
346
KATZ AND GRAY TABLE III
Surface Partition Coefficients for Hydrocarbons on Cellophane at 20°C and at Different Relative Humidities Ks × 10S/rna RH (%)
n-Heptane
0 38 61 88 95 Redried
3.9 _+ 0.1 3.83 3.66 3.34 3.16 3.66
n-Octane 12.4 12.0 11.1 10.0 9.1 11.2
_+ 0.6 ± ± ± ±
0.1 0.3 0.3 0.01
n-Nonane
n-Decane
39.0 ± 1 37.4 35.2 31.4 27.9 z 1 33.8
126 -2_ 3 118 i09 96.8 86.9 106
Standard deviations reflect variations in VN/w values given in Table II.
octane, and 4.9 × 10-6 for mesitylene, or 0.3, 0.07, and 6% of their respective retention volumes. The solubility of nonane and decane in water is even lower than that of octane, so bulk sorption may also be ignored for these alkanes. For mesitylene, however, a small contribution to the retention volume from bulk sorption cannot be ruled out. A similar conclusion was reached by Karger (34) in his study of hydrocarbon adsorption on water-coated silica and Chromosorb P. The decreasing retention volumes of the alkane probes given in Table II indicate an increasing antipathy between the hydrocarbons and the surface as the cellophane takes up water. These values for VN/w may be combined with the specific surface area of the cellophane, discussed in the previous section, to give the partition coefficients for surface adsorption, Ks. These Ks values, listed in Table III, are of the same order of magnitude as those on moist cotton paper reported at 10°C (16). Interpolation of the In Ks vs 1/T plot given in Ref. (16) for 20°C and -95% RH gives, for example, a Ks for octane of 6.1 × 10-6 m; the corresponding value for Ks on cellophane at 95% RH is 9.1 × 10-6 m. These values of Ks are still, however, a factor of 10 greater than those for n-alkanes on bulk water (34-36), suggesting that a larger force field of attraction exists for the moist cellophane surface. It should be noted that for the redried Journal of Colloid and Interface Science, Vol. 82, No. 2, August 1981
cellophane, the lower VN/w values, combined with the smaller surface area discussed in the previous section, lead to partition coefficients that are smaller than for the initial dry cellophane. In fact, the partition coefficients are close to those for 61% RH. Furthermore, on redrying the cellophane in the gc column, only 26% water by weight was recovered, whereas about 35% by weight was expected from Fig. 1. It seems likely that the redrying conditions were not sufficiently severe to remove all the sorbed water.
Free Energy of Adsorption and of the Cellophane Surface Some adsorption isotherms for n-octane are shown in Fig. 5. Isotherms for mesitylene were similar in shape. A weak-kneed type-II isotherm was observed for n-octane on dry cellophane. As the humidity increased, the isotherms approached type-III behavior although the type-II character was still retained with C values greater than 2 (Table I). Similar values for the surface excess, F, and for C on cellophane were observed for the n-alkanes on cotton paper (16) at about 90% RH and on fibers (14). Note that adsorption of n-alkanes and aromatic hydrocarbons on the surface of water gives type-III isotherms, with values for F smaller by an order of magnitude (35, 37, 38). Thus, the surface properties of the water-swollen gel still differ from those of bulk water. The free energy of adsorption, A(~A, of n-octane as a function of surface coverage at three humidities is illustrated in Fig. 6. The AGAvalues were calculated from AGA = RT In P/Ps,g,
[8]
where ps,g, the vapor pressure of a molecule in the gaseous standard state, is 101 kN m-2; the p values at each coverage were obtained directly from the adsorption isotherms (Fig. 5). Errors in AGA thus arise from errors in the isotherm determinations. The shape of the free energy of adsorption curves in Fig. 6 remains almost constant with increasing
ADSORPTION OF HYDROCARBONS ON CELLOPHANE n-OCTANE 40
,
# ~ 2
///3
o O,o . .
2.,
O,o. .
////
3. 95 % RH E
347
/
/
/
30
E ~
20
r-10
0
,
0
0.1
0.2
0.3
0.4
0.5
0.6
p/~ FIG. 5. Adsorption isotherms for n-octane on cellophane at 20°C at three different relative humidities. Fm indicates the monolayer coverage.
relative humidity. At high coverages, the free energy of liquefaction of octane, AGL, is approached. Although a decrease in the absolute values of AGA with increasing cellophane water content occurs, this decrease is relatively small. Decreases with increasing moisture content were also observed in the standard free energy of adsorption of the n-alkanes, AG~, calculated from zero coverage measurements according to AG~ = -RT
In (KsPs,g/Trs),
[9]
where ~s, the spreading pressure in the standard adsorbed state, is 0.338 mN m -1 (39). The values for AG~ on dry cellophane are only slightly larger (<0.1%) than those at 38% RH; the difference is close to the experimental error. As shown in Fig. 7, a linear relationship between AG~ and the number of carbon atoms in the n-alkane was observed. This constant increment may be related to the London component of the surface free energy, y~, of the cellophane. Values for y~ were calculated using the approach of Dorris and Gray (40). The assumption was made that the work of adhesion, WA, between a liquid CH2 surface and a solid is approximately equal to the increment per methylene group in the free energy of desorption, AG~ H21, expressed per unit
area of one mole of CH2 group; that is, [10]
WA = -AG~H21/Na(cH21,
where N is Avogadro's constant. The area per CHz group, a(cn~), was assumed to equal 0.06 nm ~. For a liquid alkane-solid interface, Fowkes (41) proposed that WA may also be given by WA = 2(y~7~) 1/',
[1l]
where yl and ys are the surface energies of the liquid and solid phases, respectively, and
2G 18
I,(D~ 14
12
10 0
95°/o RH ~ 6 1 % R - H I
1
I 2
'
I I 3 4 T' " 106/mol m-2 /...
-TG[" I 5
FIG. 6. T h e d i f f e r e n t i a l free e n e r g y o f a d s o r p t i o n for n - o c t a n e o n c e l l o p h a n e at 20°C at t h r e e r e l a t i v e h u m i d i t i e s as a f u n c t i o n o f the s u r f a c e e x c e s s . AGL is the free energy of liquefaction of n-octane. Journal of Colloid and Interface Science, Vol. 82, No. 2, August 1981
348
KATZ AND GRAY
26
O
mole-a). Values ofy~ are therefore precise to _+1 m N m -a at best. A second route to y~ is based on values of 7r°, the equilibrium spreading pressure of the a d s o r b e d v a p o r atp/po = 1, determined from the complete adsorption isotherm for a single h y d r o c a r b o n . The w o r k of adhesion required to separate a solid and a liquid is given by WA = Ys + Yl -- %1, [14]
38 % R'H U
24
////"
[] 9s~ ~
} ,o
16 I 7
I 8
I 9
I 10
ALKANE CHAIN LENGTH
FIG. 7. T h e standard free energy o f adsorption for the n-alkanes on cellophane at 20°C at four relative humidities, as a function of the n u m b e r o f carbon a t o m s in the alkane chain.
the superscript L indicates the L o n d o n dispersion c o m p o n e n t . For the particular case of a hypothetical CH2 liquid on a solid, Eq. [11] b e c o m e s , W A = 2(3/(CH2),)/sL) 1/2
= -AG~n2)/Na(cH2).
[12]
The surface energy of a pure CHz surface, y(cH~), was taken to equal 35.6 m N m -~, the surface tension of a linear polyethylene liquid at the experimental t e m p e r a t u r e (42). Average incremental values in AG~ at each moisture content were taken from Fig. 7, and from Ref. (1) for dry cellophane. The results are listed in Table IV along with the values of y~ calculated from Eq. [12]. Also included in Table IV are values for ?~ based on an arithmetic m e a n approximation for WA (43), i.e., Y~ = -(Y(cH~> + AG~H2)/Na(cH~)).
where Y~z is the interfacial tension. Combining Eq. [14] with Y o u n g ' s equation, a relationship between the contact angle 0 of a liquid on a solid and WA can be obtained: ylcos 0 = - y z + WA -- 7r°.
[15]
Since the n-alkanes studied spread on cellophane, cos 0 = 1 and Eq. [15] reduces to WA = 7r° + 2yz.
[16]
T h e dispersion c o m p o n e n t of the cellophane surface energy can then be determined f r o m Eq. [17], using F o w k e s ' geometric m e a n approximation for WA,
y~ = (~o + 2yt)z/4y~
[17]
or from Eq. [18], using the arithmetic m e a n approximation for WA, y~ = 7r° + 2yl - y~.
[18]
The spreading pressure 7r° for n-octane on cellophane at each humidity was determined from an analytical integration of the correT A B L E IV Variation with Humidity of the L o n d o n C o m p o n e n t of the Cellophane Surface Free Energy B a s e d on Zero Coverage M e a s u r e m e n t s at 20°C
[13]
Although the standard deviations in the incremental free energies of adsorption are large, the decrease in magnitude of A G ~ n~) as the humidity increases from 38 to 95% R H is thought to be real; the standard deviations in A G ~ H~) based on m e a s u r e m e n t s made on the same day to avoid variations in water content were smaller (_+0.02 kJ Journal of Colloid and Interface Science, Vol. 82, No. 2, August 1981
(%)
0 38 61 88 95
(kJ mole 1)
2.80 2.80 2.76 2.74 2.69
_+ 0.06 _+ 0.06 _+ 0.05 -+ 0.05 _+ 0.09
finN m -1)
(mN m 1)
42.(2) 42.(2) 41.(0) 40.(4) 38.(9)
41.(9) 41.(9) 40.(8) 40.(3) 38.(9)
" Calculated from the geometric m e a n , Eq. [12]. b Calculated from the arithmetic m e a n , Eq. [13].
349
A D S O R P T I O N O F H Y D R O C A R B O N S ON C E L L O P H A N E
sponding adsorption isotherm (2, 27). These values are listed in Table V. Errors in 7r° are estimated to be at least - 1 mN m71. Therefore the increase in ~.0 at 95% R H is not likely to be real. Evidently, 7r° is of significant magnitude in this system and should not be neglected. The values are furthermore real; they cannot be ascribed to capillary condensation as is sometimes suggested (19, 44). The corresponding y~ values for cellophane were calculated from Eqs. [17] and [18] using the tabulated values of ~.0 and 21.62 m N m -1 (45) for the surface tension of n-octane (7l = 7~), at 20°C. These results are included in Table V. Unlike y~ values calculated from zero coverage data, these values are surface area dependent. Differences in the two methods of calculation may then be due to errors in the surface area value, errors in the extrapolation required to measure 7r° (2), and errors in the free energy increment, AG~ H2). The 7) values in Table V calculated using the arithmetic mean method are closer to the values calculated from zero coverage data than are the y ) values calculated from Eq. [17]. This suggests that the arithmetic mean is a better approximation than the geometric mean in calculating the work of adhesion. A third method, based on the reciprocal or harmonic mean (46) gave unreasonably large values for y ) (64 --~ 53 mN m -1 with increasing humidity). All estimates of 7), however, indicate that as the cellophane water content increases, the L o n d o n force field of attraction of the surface for the n-alkanes diminishes. It should be noted that surface properties which are independent of water content were not reached and that the values of y ) are still significantly larger than the value for bulk water of 22.0 mN m -1 (41). The effect of relative humidity on the cellophane surface has been studied by measurement of the contact angle of water. Since water swells cellulose, the interpretation of this contact angle is difficult. Borgin (17) attempted to circumvent the bulk sorption and hysteresis effects by measuring the ad-
TABLE V Variation with Humidity of the L o n d o n C o m p o n e n t of t h e Cellophane Surface Free E n e r g y B a s e d on Finite Coverage M e a s u r e m e n t s RI-I (%)
~r (raN rn-')
7k" (raN m-t)
Z'k~ (raN m-1)
0 61 88 95
21.(3) 19.(1) 18.(2) 18.(8)
48.(2) 44.(9) 43.(6) 44.(5)
42.(9) 40.(7) 39.(8) 40.(4)
Calculated from the geometric m e a n , Eq. [17]. b Calculated from the arithmetic m e a n , Eq. [18].
vancing contact angle as a function of time. He found that between 0 and 50% relative humidity, the water contact angle remained constant. At higher moisture contents, the angle decreased linearly with humidity up to 100% RH. According to Luner and Sandell (19), an increase in the relative humidity from 2 to 84% caused a decrease in the initial contact angle of water on cellulose from 40 to 18° . The decrease corresponding to the humidity increase from 2 to 32% R H was, however, only 6 °. Furthermore, the contact angle of tetrabromoethane on the same cellulose film surface was unchanged in the humidity range of 2 to 32% RH. These essentially constant contact angles on cellophane up to - 5 0 % R H suggest that the first 12% of water is sorbed without affecting the surface properties of the film. Borgin (17) proposed that this water is strongly bound to sites in the cellulose. When the available sites are saturated, further adsorption of water sharply changes the surface properties. The maximum change occurred at - 6 0 % R H corresponding, according to Borgin, to the formation of the first layer of loosely adsorbed water molecules. Further water adsorption increased this amount of loosely bound water with a smaller effect on the surface properties. At 100% RH, a minimum value of 10.2 ° for the w a t e r - w e t celluloseair contact angle was reached, rather than the 0 ° expected for a surface covered with a continuous layer of water. Journal of Colloid and Interface Science, Vol. 82, No. 2, August 1981
350
KATZ AND GRAY
The decrease in 3/L values for cellophane given in Tables IV and V corresponds well to the trend in water contact angles. At 38% RH, the cellophane surface appeared to be the same as when dry. It should be noted that the cellophane at 0% RH, obtained by drying the cellophane column at 105°C (1), still contains some moisture (47, 48). Prolonged drying at 105°C or drying at higher temperatures results in a continuous loss of weight; however, degradation of the cellulose may also occur. Another estimate for the effect of relative humidity on ~/L of cellulose was based on contact angles of methylene iodide and tetrabromoethane (49). Swanson and Brown resolved the surface free energy into polar and dispersion components according to the method of Owens and Wendt (50). The decrease in 3/) on going from 0 to 50% R H was 2 mN m -1. The value for 3/) at 95% RH is high compared to that for pure water ( - 3 9 mN m -1 vs 22 mN m-~). Dorris and Gray (16) found a similar effect for moist cellulose paper, where the lowest value for 3,L was 34 mN m -1, even at 140% water by weight. These values also agree qualitatively with Borgin's observation of a nonzero water contact angle on cellophane at 100% RH. In general, greater decreases in 3/) with relative humidity were observed for cotton cellulose paper than for cellophane, suggesting that, at equivalent relative humidities, more water is surface adsorbed on cotton cellulose than on regenerated cellulose. For both moist celluloses, however, the surface properties of bulk water are not reached. CONCLUSION
By modifying the carrier gas line of the gc apparatus so that it passed through a water saturator, the adsorption of hydrocarbons on water-swollen cellophane film was studied. Due to the experimental difficulties introduced by the addition o f water, such as the increased pressure drop, as well as the theoretical complication of water interacJournal of Colloid and Interface Science, VoI. 82, No. 2, August 1981
tions, the precision in the retention measurements was lower than for the dry cellophane column. Adsorption isotherms were, however, successfully determined for the n-alkanes at humidities greater than 40%. Estimation of the corresponding monolayer coverages by the B E T equation indicated the the cellophane surface area is independent of moisture content. This confirms that the cellophane possesses a geometrically homogeneous surface, in contrast to that of cotton paper. From the isotherms, it was found that the n-alkanes adsorb more on dry cellophane than on wet. This decrease in a d s o r b a t e - a d s o r b e n t attraction was illustrated by the decrease in Ks values with humidities greater than - 4 0 % . Apparently, the first - 1 2 % of water sorbed did not affect the surface as indicated by the L o n d o n component of the surface free energy 3,L, which was - 4 2 mN m -1 for both dry cellophane and cellophane at 38% RH. At 95% RH, this value decreased to 39 mN m -1. The bulk water state, where 3,L = 22 mN m -1, was not attained. ACKNOWLEDGMENTS Thanks are due the Natural Sciences and Engineering Council of Canada and the Government of Quebec for partial support of this work, and for Postgraduate Scholarships (S.K.). REFERENCES 1. Katz, S., and Gray, D. G., J. Colloid Interface Sci. 82, 318 (1981). 2. Katz, S., and Gray, D. G., J. Colloid Interface Sci. 82, 326 (1981). 3. Fowkes, F. M., "Chemistry and Physics of Interfaces II" (S, Ross, Ed.). Amer. Chem. Soc., Washington, D. C., 1971. 4. Stamm, A. J., "Wood and Cellulose Science," p. 192. Ronald Press, New York, 1964. 5. Babbett, J. D., Canad. J. Res. A20, 143 (1943). 6. Stamm, A. J., Tappi 40(9), 761 (1957). 7. Hagamassy, J., Brunauer, S., and Mikhail, R. S., J. Colloid Interface Sci. 29, 485 (1969). 8. Weatherwax, R. C., J. Colloid Interface Sci. 49, 40 (1974). 9. Stone, J. E., and ScaUan, A. M., Pulp Pap. Mag. Canad. 66, T-407 (1965). 10. Mason, S. G., Tappi 33(8), 403 (1950).
ADSORPTION OF HYDROCARBONS ON CELLOPHANE 11. Archer, W. L., and Mason, S. G., Canad. J. Chem. 37, 1655 (1959). 12. Kamiya, Y., and Takahashi, F., J. Appl. Polym. Sci. 23, 627 (1979). 13. Tremaine, P. R., Mohlin, U.-B., and Gray, D. G., J. Colloid Interface Sci. 60, 548 (1977). 14. Dorris, G. M., Ph.D. thesis, Chap. IV. McGiU University, 1979. 15. Dorris, G. M., and Gray, D. G., J. Chem. Soc. Faraday 1 77, 713 (1981). 16. Dorris, G. M., and Gray, D. G., J. Chem. Soc. Faraday 1 77, 725 (1981). 17. Borgin, K., Nor. Skogind. 13, 429 (1959). 18. Borgin, K., Nor. Skogind. 14, 485 (1960). 19. Luner, P., and Sandell, M., J. Polym. Sci. Part C 28, 115 (1969). 20. James, A. T., and Martin, A. J. P., Biochem. J. 50, 679 (1952). 21. "Selected Values of Properties of Hydrocarbons and Other Related Compounds," American Petroleum Institute Project 44, Thermodynamic Research Center, College Station, Texas, 1974. 22. Stuckey, J. M., and Saylor, J. H., J. Amer. Chem. Soc. 62, 2922 (1940). 23. "International Critical Tables," 1st ed., Vol. 33, p. 303. McGraw-Hill, New York, 1928. 24. Lange, N. A., (Ed.), "Handbook of Chemistry," 10th ed., pp. 1420-1423. McGraw-Hill, New York, 1961. 25. Stamm, A. J., J. Phys. Chem. 60, 83 (1956). 26. Kiselev, A. V., and Yashin, Y. I., "Gas-Adsorption Chromatography," p. 113. Plenum, New York, 1969. 27. Dorris, G. M., and Gray, D. G., J. Colloid Interface Sci. 71, 93 (1979). 28. Gregg, S. J., and Sing, K. S. W., "Adsorption, Surface Area and Porosity," Chap. 2. Academic Press, London, 1967. 29. Adamson, A. W., "Physical Chemistry of Surfaces," 3rd ed., p. 603. Interscience, New York, 1976.
351
30. Katz, S., M.Sc. thesis, Chap. I, Fig. 1. McGill University, 1979. 31. Stamm, A. J., J. Phys. Chem. 60, 76 (1956). 32. Gray, D. G., "Progress in Polymer Science," Vol. 5. Pergamon, New York, 1977. 33. McAuliffe, C., J. Phys. Chem. 70, 1267 (1966). 34. Karger, B. L., Sewell, P. A., Castells, R. C., and Hartkopf, A., J. Colloid Interface Sci. 35, 328 (1971). 35. Hartkopf, A., and Karger, B. L., Accounts Chem. Res. 6, 209 (1973). 36. Jones, D. C., and Ottewill, R. H., J. Colloid Interface Sci. 34, 473 (1970). 37. Karger, B. L., Castells, R. C., Sewell, P. A., and Hartkopf, A., J. Phys. Chem. 75, 3870 (1971). 38. King, J. W., Chatterjee, A., and Karger, B. L., J. Phys. Chem. 76, 2769 (1972). 39. de Boer, J. H., "The Dynamical Character of Adsorption," p. 112. Oxford Univ. Press, (Clarendon), London/New York, 1953. 40. Dorris, G. M., and Gray, D. G., J. Colloid Interface Sci. 77, 353 (1980). 41. Fowkes, F. M. in "Chemistry and Physics of Interfaces" (S. Ross, Ed.). Amer. Chem. Soc., Washington, D. C., 1965. 42. Roe, R. J., J. Phys. Chem. 72, 2013 (1968). 43. Zettlemoyer, A. C. in "Hydrophobic Surfaces" (F. M. Fowkes, Ed.). Academic Press, New York, 1969. 44. Blake, T. D., Cayias, J. L., Wade, W. H., and Zerdecki, J. A., J. Colloid Interface Sci. 37, 678 (1971). 45. Ref. (21), 1955. 46. Wu, S., J. Polym. Sci. Part C, 34, 19 (1971). 47. Stamm, A. J., "Wood and Cellulose Science," Chap. 8. Ronald Press, New York, 1964. 48. Borgin, K., Not'. Skogind. 15, 384 (1961). 49. Swanson, J. W., and Brown, P. F., Tappi 54, 2012 (1971). 50. Owens, D. K., and Wendt, R. C., J. Appl. Polym. Sci. 13, 1741 (1969).
Journal of Colloid and Interface Science, Vol. 82, No. 2, August 1981