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An examination of the surface reactivity of graphite fibers by gas-solid chromatography
An Examination of the Surface Reactivity of Graphite Fibers by Gas-Solid Chromatography 1 C. S. B R O O K S AND D. A. SCOLA United Aircraft Research Laboratories, East Hartford, Connecticut 06108
Received February 3, 1969; accepted November 6, 1969 A study was made of the surface reactivity of "as received" and surface treated graphite fibers for water and several organic vapors (toluene, pyridine, n-decane, isobutyric acid, n-octylamine, and aniline) employing gas chromatography with the graphite yarn used as the adsorbent column. The adsorption affinity of the graphite fiber column for the various vapors was determined by measuring the pulse retention times for the vapors to traverse the column at a given temperature ranging from 70 ° to 225°C. The graphite fibers Thornel 25~ 40, and 50 and HMG 50 derived from rayon precursors were examined. Fib.er surface treatments consisted of (i) heating in hydrogen at 20 tort at 1200°C, (2) heating in one atmosphere of hydrogen at 550°C, (3) air oxidation at 500°C, and (4) nitric acid oxidation. Heats of adsorption calculated from the temperature dependence of the pulse retention times proved to be an insensitive measure for evaluation of surface treatments of the graphite yarn. Adsorption coefficients calculated for the initial vapor contact provide a useful correlation with the enhanced surface reactivity of these graphite fiber yarns. INTRODUCTION Chromatography was applied very early by Cremer and Prior (I), by Greene and Pust (2), and Schay and coworkers (3) to measure gas-solid adsorption effects and to calculate heats of adsorption. Other relevant publications describe the application of pulse chromatography to evaluate gas-solid adsorption equilibria (4-9). There are several published gas-solid chromatographic experiments (I0-13) of special interest because they involve carbon and graphite surfaces. It has been demonstrated (9, ii, 12) that the heats of adsorption for a number of vapors on a carbon surface as determined by calorimetric and isosteric procedures and as determined b y pulse chromatographic procedures are equivalent. T h e application of gas-liquid partition i Presented at the 43rd National Colloid Symposium, Case-Western Reserve University, Cleveland, Ohio, June 23-25, 1969. Copyright O 1970 b y Academic Press, Inc.
chromatography to analytical separations has been the predominant aspect of research in vapor phase chromatography. However, there is increasing recognition that vapor phase chromatography has considerable potential for the measurement of a variety of physicochemical properties such as (1) the equilibrium distribution of volatile solutes between a gas and a fixed phase (liquid, solid, or interfaeial), (2) mass transport properties, such as diffusion coefficients (gas, liquid, and surface), adsorption-desorption kinetics, and interfacial transfer rates, and (3) surface areas, phase transitions, and permeabilities where mobile and fixed phases are involved. Two recent articles by Giddings and Mallik (14) and Kobayashi, Chappelear, and Deans (15) have reviewed a number of these nonanalytieal applications of vapor phase chromatography. The objective of the present study was to examine the chemical reactivity of the graphite fiber by gas chromatography. ExJournal of Colloid and Ynterface Science, VoI. 32, No. 4, April 1970
561
562
BROOKS AND SCOLA TABLE I SUMMARY OF CHROMATOGRAPHICEXPERIMENTSON GRAPHITEFIBER ~ARNB
Fiber
Treatment
rhornel 25 Water-sized "as received"
Surfacearea (cm2/gm X 104)
0.8 a
Phorne140 Water-sized "as received" Phornel 50 Water-sized "as received"
0.55 b
_:IMG50
No sizing "as received"
0.87 b
Phorne125
Oxidized 8 hours at 80°C in 60% HN03
Phorne125
Heated i hour 1200°C in H2 at 20 torr, evacuated to 1 #, cooled to 20°C in vacuum, purged with argon, and opened to atmosphere ['horne140 Heated 2 hours in 1 arm H~ flow at 15 cmS/min and 550°C, cooled in H~ to 23°C Phorne140 Heated 2 hours in 1 atm air flow at 15 cm~/min and 550°C, cooled in air to 23°C ?horne150 Oxidized 8 hr boiling 70% HNO~ ~MG 50
Oxidized 8 hr boiling 70% HNO~
2.5 b 9.7 b
Column 6 sections 12 ft length 12 sections 6 ft length 12 sections 6 ft length 12 sections 6 ft length 6 sections 12 ft length 6 sections 12 ft length
Vapors Toluene, pyridine, water, aniline n-Decane, n-octylamine, isobutyric acid n-Decane, n-octylamine, isobutyric acid n-Decane, n-octylamine, isobutyric acid Toluene, pyridine, water Toluene, pyridine, water
12 sections 6 ft length
n-Decane, n-octylamine
12 sections 6 ft length
n-Decane, n-octylamine
12 sections 6 ft length 12 sections 6 ft length
n-Decane, n-octylamine n-Decane, n-octylamiae, isobutyric acid
a Measured by low-temperature (-196°C) physical adsorption of N~ (18). b Measured by low-temperature (-196°C) physical adsorption of Kr (19). periments were conducted to determine the affinity of several organic vapors and water toward " a s received" graphite yarn and toward yarn samples which had undergone surface treatments. The graphite yarn samples were used as the gas chromatographic column substrates and the activity of these substrates was determined by measuring the pulse retention time required for the vapors to iransverse the yarns at various temperatures (70°-225°C). This study differs from published experiments in that low specific surface area graphite filaments were used as the immobile phase, rather t h a n high specific surface area powders. Application of gas-solid chromatography for analytical separations has been limited to volatile, relatively inert vapors because of irreversible adsorption, variable adsorption activity, and excessive retention of polar Journal Of Colloid and Znterfdce Science, Vol.32, No. 4, April 1970
molecules. I n the present application it is these features t h a t are exploited. In this study adsorption coefficients and heats of adsorption were calculated from the measured pulse retention times in order to evaluate the intrinsic reactivity of the fiber surface. EXPERIMENTAL METHOD The graphite fiber yarns examined in these studies consisted of Thornel 252, Thornel 402, Thornel 502, and H M G 503. The treatments used to modify the surface reactivity of these graphite yarns are summarized in column 2 of Table I. Union Carbide Corporation, Carbon Products Division, 270 Park Avenue. New York, New York 10017. HITCO Materials Division, 1600~West 135th Street, Gardena. California 90249
SURFACE REACTIVITY OF GRAPHITE FIBERS The graphite columns were prepared by making a spiral wrap of graphite yarn sections around a 40-mil stainless steel wire. In most instances the columns were prepared in duplicate. The fiber-wrapped wire was then inserted in }{ inch OD copper or stainless steel tubing. The various column mountings are summarized in Table I. The chromatograph was a Perkin-Elmer Model 154C Fractometer. The signal peaks, obtained from the changes in thermal conductivity of the effluent gas (compared to helium), were detected with a thermistor thermal conductivity sensor and recorded with an L & N Speedomax G strip chart potentiometer. The adsorbate vapors consisted of water, pyridine, toluene, n-decane, isobutyric acid, n-octylamine, and aniline. The column was maintained at fixed temperatures between 70 ° and 225°C for each series of compounds chromatographed. The helium carrier gas passed through the column at flow rates of either 15 or 52 cms NTP per minute. The retention times for water and the organic compounds, toluene, pyridine, and aniline, were measured at column temperatures ranging from 225 ° down to 70°C. The column was first adjusted to the highest temperature (225°C) and the retention times for duplicate pulses of a given vapor were measured. Following these measurements the column temperature was adjusted to the next lower temperature (200°C), and the retention times of the same compound were measured again. The temperature was redueed in decrements of 25 ° to the lowest temperature (70°C). In the case of high-boiling compounds, n-deeane, isobutyrie acid, and n-octylamine, the initiM vapor contact was at 225°C and the temperature was reduced in decrements of 25°C down to 125°C and then raised in increments of 25°C up to 225°C. The order of vapor contacts and the exposure temperature are indicated by the order in which they appear in the data tabulations. In order to standardize the elution conditions the established procedure consisted of waiting 5 min after a return of the signal trace to the base line before injecting a second adsorbate pulse.
563
THEORY AND DATA TREATMENT With the use of the general chromatographic theory of Martin and Synge (16) and the specific relationships of gas-solid chromatography of Cremer and Prior (1), the adsorption coefficient (K) and the retention volume (Vg) can be calculated from the measured pulse retention time (tr). Vg = K = (tr/tm -- 1 ) ( Y m / W ) ,
[1]
where t~ is the transit time for the carrier gas through the column, V.~ is the total gas free space in the adsorbent column, and W is the absorbent weight. The transit times for the carrier gas through the graphite fiber columns were calculated as follows: t~ = ( ~ 2 L ) - -
where r L V T1
W / p - W~/p~ V(T~/T1)
[21
= = = =
internal column radius in cm, column length in cm, volume flow rate cm3 NTP/sec, absolute temperature at flow meter, T2 = absolute temperature of col umn,
W = weight of graphite in gm, p = density of graphite in gm/cm 3, W~ = weight of steel wire in am, and p~ = density of steel wire in gm/cm a. The unconventional feature of the data treatment here consists of using the width of the peak base as the pulse retention time tr. The width of the peak base represents the total time required from initial entry of the vapor into the detector until the desorbed vapor pulse has been completely eluted from the adsorbent column. In conventional partition chromatography the pulse retention time represents the delay time from injection at the head of the column until the maximum in the signal peak of the solute vapor contacts the detector. In the present study the pulse retention time as conventionally defined would be too short for accurate deterruination. However, the pulse retention time (base width) as defined here is considered to provide a reasonably accurate measure of gas-solid adsorption, providing the flow rate of the elution gas is great enough to minimize diffusion spreading of the signal peaks. Journal vf Colloid and Interface Science, Vol. 32, N0, .4, April 1970
564
BROOKS AND SCOLA
Carrier gas flow rates of at least 50 cma N T P / m i n are necessary to minimize longitudinal diffusion of the adsorbate pulses. For cartier gas flow rates of 50 cma N T P / m i n and above signal pulse broadening can be attributed primarily to adsorption effects (17). The isosteric heats of adsorption were calculated from the temperature dependence of V~ by the relationship
yarn in order to minimize any specific vaporfiber chemicM interaction effects. The average differences between duplicate n-decane pulses on a given column all fell within the range of -4-1.1 to i 2 . 9 sec. A similar comparison of the average differences between duplicate pulses for n-octylamine yielded a range from ±0.7 to ~-2.9 sec, indicating the same precision observed for the n-decane. The precison observed for n-decane on dupliVg = c exp(-- AH~d~/RT). [31 cate columns of this water-sized yarn was approximately twice that obtained for the RESULTS AND DISCUSSION single column (see columns A and B, Fig. 1). Establishment of Procedure Parameters. It The column preparations represented here was necessary to establish the role of several offer acceptable variations in regard to both operating parameters in order to select the pulse repeatability on a given column and most suitable operating procedures. These pulse reproducibility on a duplicate column parameters included the reproducibility of packing, at least for a nonpolar adsorbate the column packing, effect of column length, such as n-decane. Some of the large differthe effect of pulse size, and the effect of ences observed between the first and second carrier gas flow rate. pulse for the polar adsorbates, notably at Column Packing Reproducibility and Pulse initial contact and after surface treatments, Repeatability. Typical pulse retention time can be attributed to chemisorption effects. data for two graphite columns (twelve secEffect of Column Length. Various column tions, each 6 feet in length) illustrating the lengths were examined, ranging from 3 to 12 effect of column packing reproducibility and feet in length. The principal difficulty in of pulse repeatability are shown in Fig. 1. preparation of 12-foot columns was the inserOperating conditions consisted of the use of tion of the yarn-wire columns into the 0.01 cm3 pulse size and helium flow rate of 1/~-inch tubing without "bailing up" of the 52 4- 5 cm8 NTP/min. Comparisons were yarn. The short 3-foot columns, on the other made for a hydrocarbon vapor with the "as hand, were easily prepared with uniform received" water-sized Thornel 50 graphite packing but provided too short vapor contact times. The most successful column preparations, in terms of packing reproducibility, lO 2 0,01 cm3 PULSE TWELVE6'YARN SECTIONS52 ¢m3 NTP/m[nHe FLOW were the intermediate length (6-foot) columns with twelve sections of yarn. Column Adsorption Capacity. The manu: 5 i facturers of fibers such as Thornel and HF[G 50 state that the specific surface areas are of the order of 1 × 104 cm~/gm. On the basis ~== n-OCTYLAMINE ~, / I''~ Z of a cylindrical rod fiber geometry and on the assumption of no internal porosity, the specific surface area (S) of Thornel 50 is / ~, ] 0 -/ ~ , . ~ / * n-DECANE calculated to be 0.37 X 104 cm~/gm:
:" ko"
,,~/o
COLUMN A O
I
r
I
20
22
24
COLUMN B • zx PRECISION I [ 26
28
104/T
FIG. 1. Column packing reproducibility and pulse repeatability for Thornel 50 graphite fiber yarn water-sized ("as received"). Journal of Colloid and Interface Science, Vot. 32, No. 4, A p r i l 1970
S = 2/rp = 2/(3.3 X 10-4 cm)(1.63 gm/cm8) = 0.37 X 104 cm~/gm, [4] where r is the fiber radius and p is the fiber density. ~Ieasurement of the surface areas of these fibers by low temperature (-- 196°C) physical adsorption of N2 (18) and of Kr (19) indi-
SURFACE REACTIVITY OF GRAPHITE FIBERS cates specific surface areas ranging from 0.8 to 9.7 X 104 cm2/gm (Table I). In order to compare the coverage area of typical adsorbate pulses to the total adsorption capacity of representative graphite fiber yarn columns the ratios of pulse coverage area to graphite fiber area were calculated from the relationship Adsorbate coverage area Total fiber area (0.01 cm3pulse)(0.73 gm/cm 3) (6.02 X 10~-3) (16 × 10-16cm2/molecule) = 0.24. (142 MW)(9.7 X 104 cm2/gm) (1.7 gin)
[51
Calculations based on Eq. [5] show that the total pulse adsorbate for n-deeane coverage area ranges from about one-fourth the total area of the oxidized HMG-50 graphite fiber to more than a monomolecular layer for the other fibers. It is evident from these calculations that with the columns used in the present experiments and a pulse size of 0.01 cm 3 the total adsorption capacity of the graphite fiber yarn column is exceeded in most instances. This means that almost instantaneous break-through at the detector occurs for at least a portion of the unadsorbed 0.01 cm3 vapor pulse which is transported at the same transit speed as the carrier gas. The signal peaks demonstrated tailing owing to adsorption on the fiber surfaces. For flow rates of 52 cm 3 NTP/min the 0.01 cm3 signal peaks were found to be relatively symmetrical (similar to peak A, Fig. 6 in reference 12), whereas the larger signal pulses, 0.1 em 3, showed very strong tailing (similar to peak B, Fig. 6 in reference 12). The faster carrier flow rate (52 cm3 N T P / min) combined with the smaller vapor pulse (0.01 em 3) is the preferred combination in order to provide the more favorable ratio of column adsorption capacity to vapor pulse coverage area and to minimize diffusion effects which would obscure resolution effects attributable to vapor-fiber adsorption interaction. In the present study the object was to obtain a measure of surface reactivity for a
565
major fraction of the fiber surface. In accomplishing this latter objective with the use of adsorbate pulses which are large relative to the column adsorption capacity, column overloading is probable; this condition is further aggravated by the difficulty of attaining a uniform column packing with a fibrous packing. Although these disadvantages apply to the present unconventional procedure of using large adsorbate pulses and peak widths as a measure of adsorbate residence time, this procedure is considered to provide an adequate measure of the average adsorption coefficient corresponding to a relatively large fraction of the fiber surface. Retention Volumes. The retention volumes (Vu) have been calculated from Eq. [1] and are plotted versus 1/T for Thornel 25 (Fig. 2), Thornel 50 (Fig. 3), and H M G 50 (Figs. 4.4 and 4B). It is evident from the plots in Figs. 3, 4A, and 4B that the polar vapor noctylamine is adsorbed most retentively on initial contact. In practically all instances the V~ vs. temperature plot demonstrated a normal temperature dependence after going
O.l cm3
PULSE
15 cm3 NTP/mln
SIX12'YARNSECTIONS HeFLOW 7 / ANILINE
> 10 3
'
/
[~/
W ~ ~ATER
5 /V
Z
/
PYRIDINE
0 2 102
5
20
r
22
r
24
I
r
26 28 104/T
i
30
J
32
34
FIG. 2. R e t e n t i o n v o l u m e s f o r t o l u e n e , p y r i dine, w a t e r , a n d a n i l i n e o n T h o r n e l 25 w a t e r - s i z e d ("as received"). Journal o/Colloid and Interface Science, Vol. 32, No. 4, April 1979
566
BROOKS AND SCOLA
through the entire temperature cycle from 225°C down to 125°C and back up to 225°C. The failure to repeat the large initial Vg is evidently the result of partial poisoning or deactivation of the fiber surface, presumably by chemisorption of adsorbate. Adsorption Coegicients at Initial Vapor Contact. In view of the occurrence of this initial deactivation at initial contact, notably with the more polar vapors, it was considered most revealing to make comparisons of the 0.01 cm3 PULSE
TWELVE6' YARNSECTIONS 52 cm3 NTP/mln He FLOW
"/
®
e / / n - G C T Y LAMINE
103 ~.,,i, n-DECANE
LU 0 >
5
Z 0 E-Z
-
©. / 5 INITIAL CONTACT @® DECREASING TEMP. o • INCREASING TEMP. ~ J
'~
2
10
i 20
I 22
I 24
I 26
I 28
104/T
FIG. 3. Retention volumes for n-decane and n-octylamine on Thornel 50 water-sized ("as received"). 2
(A)
2
).01 cm3 PULSE TWELVE6' YARNSECTIONS 52 cm3 NTP/min He FLOW ~/
m 10 3
initial adsorption coefficients (K). In Figs. 5, 6, and 7 the adsorption coefficients at initial vapor contact are shown for Thornel 25, Thornel 40, Thornel 50, and HMG 50. The vapors consisted of toluene, water, pyridine, analine, n-decane, n-octylamine, and isobutyric acid. Graphite fiber surface treatment conditions consisted of exposure to hydrogen at 20 torr at 1200°C, hydrogen treatment at 550°C, air oxidation at 550°C, and nitric acid oxidation. The adsorption coefficients (K) show ap. preciably stronger adsorption of the polar adsorbates water, pyridine, and aniline compared with toluene on the "as received" Thornel 25 (Fig. 5). It is evident also that hydrogen treatment at high temperature (see Table I) enhances surface reactivity for all the adsorbates relative to either the "as received" or the nitric acid oxidized Thornel 25 graphite fiber. A second exposure of the hydrogen-treated column to these same vapors shows that an appreciable deactivation has occurred. Similar conclusions apply to the data for Thornel 40 presented in Fig. 6. Both in situ hydrogen treatment at 550°C and air oxidation at 550°C enhance reactivity of Thornel 40 toward n-octylamine. These in situ treatments resulted in larger initial K values than the high-temperature treatment (1200°C) inasmuch as the former system (Thornel 25)
[]
Y •
~
~> u2
n-OCTYLAMINE
[]
D
O
[]
Z
INCREASING TEMP. i r .~
Z
"~ 2
Z
~
I
I
22
2r4
E
26
~/
[
n-OCTYLAMINE
?
n-DECANE
~> -
iNITiAL CONTACT ~ DECREASING TEMP. •
20
/
m 103 n-DECANE
O~_
102
(B)
0.01 cm3 PULSE TWELVE6' YARNSECTIONSS2 cm3 NTP/mln Ho FLOW
I
28
lo 4/T
,/~
2
102
J
. /
INITIAL CONTACT
/~
DECREASING TEMP. ~ iNCREASING TEMP. ~1' fif
]
20
2'2
2~4
~
26
~ ~]
]
28
iO4/T
FIG. 4A. Retention volumes for n-decane and n-octyl~mineo n HMG-50 ("as received"). FIG. 4B. Retention volumes for n-decane and n-ocytlamine on HMG-50 oxidized, 8 hour boiling 70% HNO3. Journal of Colloid and Interface Science, Vol. 32, N o . 4, A p r i l 1970
SURFACE REACTIVITY OF GRAPHITE FIBERS had some exposure to the ambient atmosphere prior to the chromatographic measurements. Examination of the calculated adsorption coefficients for Thornel 50 in Fig. 7 shows that (1) no significant difference between adsorption of amine and acid occurs on the "as received" fiber, (2) nitric acid oxidation treatment enhances the adsorption of ndecane, and (3) nitric acid oxidation makes
3200
2800
15 cm3 NTP/min He FLOW
2600
U
Z 14.1 ___2 0 0 0
,,="
~u o
u
ILl
O 1600 Z
== o
-1200
< F_,
~,o
2~
::"4
C,u o~
o
400
WATER O X I D A T I O N SIZED AS 8 HR RECEIVED B O I L I N G 7 0 %
O
u 1400 Z
HNO 3
_o
600
',9
,L
Z i--
1800
<
i
o I
1OO0
I
800
~, 2 2 0 0
~
HMG 50
THORNEL 50,,, '1
:
o_
~ SIX 12 ~ YARN SECTIONS
TWELVE6' YARN SECTIONS 52 cmJ NTP/m[n He FLOW
v 2400
O
0.1 cm3 PULSE
0.01 cm3 PULSE
567
WATER SIZED AS RECEIVED
OXIDATION 8 HR BOILING 70% HNO3
FIG. 7. Initial a d s o r p t i o n coefficients at 547°I4 for Thornel 50 a n d HMG-50.
no significant change in the adsorption of the amine. 200 In the case of "as received" HMG-50 WATER SIZED H2 TREATMENT OXIDATION there appeared to be preferential adsorption AS RECEIVED AT 1200 C AT 80 C of the amine compared with the acid. HowWITH 60% HNO3 50 proFI~. 5. Initial adsorption coefficientsat 422°K ever, nitric acid oxidation of HMG duced a significant increase in adsorption for Thorne125. retention of n-deeane, n-octylamine, and isobutyric acid compared with the "as 3200 0.01 cm3 PULSE 52 cm3 NTP/mln He FLOW received" fiber. TWELVE 6' YARN SECTIONS zz~
Heats of Adsorption of Several Organic Vapors for Pretreated Graphite Yarn Surfaces.
2800
c, 2400
9 <
2000
U
1600
--
~
~
12OO
800
40O
WATER SIZED AS RECEIVED
H2 TREATMENT 2 HRS AT 5 5 0 C
AIR O X I D A T I O N 2 HRS AT 5 5 0 C
FI(~. 6. I n i t i a l a d s o r p t i o n coefficients a t 547°I4 for T h o r n e l 40.
Heats of adsorption were calculated from Eq. [3] for these organic vapors and water for Thornel 25, Thornel 40, Thornel 50, and HMG 50 (Table II). All the heats of adsorption calculated for the hydrocarbons (toluene, n-hexane, n-octane, and n-decane) for these graphite fibers lie within the range of 2.6 to 4.1, keal/mole and are appreciably lower than published values ranging from 4.0 to 13.0 kcal/mole for low molecular weight hydrocarbon vapors on graphitized carbon blacks. The polar organics (aniline, n-octylamine, and isobutyric acid) yield heats of adsorption within the range of 3.5 to 6.8 kcal/mole, which is somewhat lower than the published values ranging from 5 to 10 kcal/mole for Journal of Colloid and Interface Science, Vol. 32, No. 4, April i970
BROOKS AND
568
SCOLA
TABLE II ISOSTERIC I-IEATS OF ADSORPTION FOR VARIOUS VAPORS ON GRAPHITE SURFACES AH Adsorption (kcal/mole) Graphite fibers
rhornel 25 rhornel 40 rhornel 50 [tMG 50
Vapor
Temp. range
(°K)
"As Oxidized8 hours Boilin. Received" 70% HNOa
Toluene, water, aniline n-Hexane, n-octane, n-decane
343-422 343-422 343-422 371-499 371-499 371-499
4.1+0.5 4.5 6.8 2.94-0.5 3.4 3.8
n-Decane, n-octylamine, isobutyrie acid n-Decane, n-octylamine, isobutyrie acid
395-547 395-547 395-547 395-547 395-547 395-547
3.64-1.2 3.9 5.4 2.64-0.5 3.5 4.3
--
Graphitized carbon blacks (pub. data)
Graphon Sterling MT-G Sterling 3100
--
3.44-1.5 5.3
Sterling 3100
Temp. range (°K)
Vapor
Benzene, water
AH adsorption Sourc{ (kcal/ ref. mole)
i00-490 9.1 305-369 4.5-6.0
Ethane butane, hexane, heptane Ethanol, butanol, hexanol
4.0 7.5 10.0 13.0 5.0 8.0 10.0
9 12 11 11 11 11 11 11 11
3.14-1.6 6.3 3.8
low molecular weight alcohols (ethanol, butanol, and hexanol) on Sterling 3100. I t is worthy of note t h a t in the published d a t a essentially the same heats of adsorption were obtained for alkanes and the corresponding alcohols on Sterling 3100, indicating t h a t the adsorptive forces are predominantly dispersive in character. A closer agreement was observed in the case of water. A value of 4.5 kcal/mole was calculated for Thornel 25, whereas a range of 4.5 to 6.0 kcal/mole has been reported for water on Sterling M T - G (12). T h e heat of adsorption data in Table I I are lower t h a n those obtained b y other workers (9, 11, 12) for the same organic vapors on graphitized carbon blacks. This m a y be due in p a r t to some deactivation or poisoning of the graphite yarns b y the initial v a p o r contact at the highest t e m p e r a t u r e used. An additional factor m a y be t h a t the published heat values correspond to partial coverages of the adsorbent b y the adsorbate, whereas in the present study surface coverage in most instances approximates a complete monomolecular layer of adsorbate. The comparisons with published d a t a are primarily to demonstrate the effects of varying polarity of the adsorbate for other graphite adsorbents. There was no significant change within experimental uncertainty in the heats Journal of Colloid and Interface Science, Vol. 32, l~lo. 4, April 1970
of adsorption on the graphite Thornel 50 and HMG 50 fibers after a surface treatment such as nitric acid oxidation (see Table II), so this precludes the use of the heat values for evaluation of the effects of such a surface treatment. It is apparent that the adsorption coefficients calculated for the initial vapor contacts at 225°C remain the most significant criterion of intrinsic surface reactivity and the most reliable measure of the effects of a treatment on the graphite fiber surfaces. ACKNOWLEDGMENTS This work was conducted under the sponsorship of the United Aircraft Research Laboratories and the Air Force Materials Laboratory, WrightPatterson Air Force Base, Ohio (AF contract 33 (615)-5046). The authors express appreciation to both sponsors for support and permission to publish. The authors appreciate the contribution of Dr. Malcolm Basche for the high-temperature surface treatments, of Mrs. Evelyn Malloy for the krypton surface areas, and of Mrs. C. Livermore for technical assistance.
REFERENCES 1. CREMER,E., A~D PRIOR,F., Z. Elektrochem. 55, 66 (1951). 2. GREENE, S. A., AND PUST, H . , J . Phys. Chem.
62, 55 (1958).
SURFACE REACTIVITY OF GRAPHITE FIBERS 3. SCHAY,G., FEJES, P., HALASZ,I., ANDKIRALY, J., Magy. Kern. Folyoirat 63, 143 (1957). 4. HANLAN,J. F., AND FREEMAN, M. P., Can. J. Chem. 37, 1575 (1959). 5. BEEBE, R. A., AND EMMETT, P. H., J. P h y s . Chem. 65, 184 (1961). 6. CREMER,E., Monatsh. Chem. 92, 112 (1961). 7. CARBERRY,J. J., Nature 189, 391 (1961). 8. CREMER,E., AND HUBER, H. F., I.S.A. Proc. 1961, In~. Gas Chromatography Symp., p. 117; Angew. Chem. 73, 461 (1961).
9. ROSS,S., SAELENS,J. K., AND OLIVIER, J. P., J. Phys. Chem. 66, 696 (1962). 10. HABGOOD,H. W., AND HANLAN,J. F., Can. J. Chem. 37, 843 (1959). 11. BELYAKOVA, L. D., KISELEV, A. V., AND KOVALEVA, ]-~. V., Anal. Chem. 36, 1517 (1964).
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12. GALE,R. L., ANDBEEBE, R . A., J . Phys. Chem.
68, 555 (1964). 13. BEEBE, R. A., EVANS, P. L., KLEINST]~NBER, T. C. W,, AND RICItARDS, L. W., J". Phys. Chem. 70, 1009 (1966). 14. GIDDINGS,J. C., ANDMALLIK,K. L., Ind. Eng. Chem. 59, 18 (1967). 15. KOBAYASHI, R., CHAPPELEAR, P. S., AND DEANS, H. A., Ind. Eng. Chem. 59, 63 (1967). 16. MARTIN, A. J. P., AND SYNGE, R. L. M., Biochem. J. 35, 1358 (1941). 17. EBERLY, JR., P. E., AND SPENCER, E. m., Trans. Faraday Soc. 57, Part 2, 289 (1961). 18. DECRESCENTE, M. A., SCOLA, D. A., AND
BaOOKS, C. S., AFML-TR-67-218, Part 1, (July 1967). 19. BEEBE, R. A., BECKWITH,J. B., AND HONIG, J. M., J. Am. Chem. Soe. 67, 1554 (1945).
Journal of Colloid and Interface Science, Vol. 32, No. 4, April 1970
Received February 3, 1969; accepted November 6, 1969 A study was made of the surface reactivity of "as received" and surface treated graphite fibers for water and several organic vapors (toluene, pyridine, n-decane, isobutyric acid, n-octylamine, and aniline) employing gas chromatography with the graphite yarn used as the adsorbent column. The adsorption affinity of the graphite fiber column for the various vapors was determined by measuring the pulse retention times for the vapors to traverse the column at a given temperature ranging from 70 ° to 225°C. The graphite fibers Thornel 25~ 40, and 50 and HMG 50 derived from rayon precursors were examined. Fib.er surface treatments consisted of (i) heating in hydrogen at 20 tort at 1200°C, (2) heating in one atmosphere of hydrogen at 550°C, (3) air oxidation at 500°C, and (4) nitric acid oxidation. Heats of adsorption calculated from the temperature dependence of the pulse retention times proved to be an insensitive measure for evaluation of surface treatments of the graphite yarn. Adsorption coefficients calculated for the initial vapor contact provide a useful correlation with the enhanced surface reactivity of these graphite fiber yarns. INTRODUCTION Chromatography was applied very early by Cremer and Prior (I), by Greene and Pust (2), and Schay and coworkers (3) to measure gas-solid adsorption effects and to calculate heats of adsorption. Other relevant publications describe the application of pulse chromatography to evaluate gas-solid adsorption equilibria (4-9). There are several published gas-solid chromatographic experiments (I0-13) of special interest because they involve carbon and graphite surfaces. It has been demonstrated (9, ii, 12) that the heats of adsorption for a number of vapors on a carbon surface as determined by calorimetric and isosteric procedures and as determined b y pulse chromatographic procedures are equivalent. T h e application of gas-liquid partition i Presented at the 43rd National Colloid Symposium, Case-Western Reserve University, Cleveland, Ohio, June 23-25, 1969. Copyright O 1970 b y Academic Press, Inc.
chromatography to analytical separations has been the predominant aspect of research in vapor phase chromatography. However, there is increasing recognition that vapor phase chromatography has considerable potential for the measurement of a variety of physicochemical properties such as (1) the equilibrium distribution of volatile solutes between a gas and a fixed phase (liquid, solid, or interfaeial), (2) mass transport properties, such as diffusion coefficients (gas, liquid, and surface), adsorption-desorption kinetics, and interfacial transfer rates, and (3) surface areas, phase transitions, and permeabilities where mobile and fixed phases are involved. Two recent articles by Giddings and Mallik (14) and Kobayashi, Chappelear, and Deans (15) have reviewed a number of these nonanalytieal applications of vapor phase chromatography. The objective of the present study was to examine the chemical reactivity of the graphite fiber by gas chromatography. ExJournal of Colloid and Ynterface Science, VoI. 32, No. 4, April 1970
561
562
BROOKS AND SCOLA TABLE I SUMMARY OF CHROMATOGRAPHICEXPERIMENTSON GRAPHITEFIBER ~ARNB
Fiber
Treatment
rhornel 25 Water-sized "as received"
Surfacearea (cm2/gm X 104)
0.8 a
Phorne140 Water-sized "as received" Phornel 50 Water-sized "as received"
0.55 b
_:IMG50
No sizing "as received"
0.87 b
Phorne125
Oxidized 8 hours at 80°C in 60% HN03
Phorne125
Heated i hour 1200°C in H2 at 20 torr, evacuated to 1 #, cooled to 20°C in vacuum, purged with argon, and opened to atmosphere ['horne140 Heated 2 hours in 1 arm H~ flow at 15 cmS/min and 550°C, cooled in H~ to 23°C Phorne140 Heated 2 hours in 1 atm air flow at 15 cm~/min and 550°C, cooled in air to 23°C ?horne150 Oxidized 8 hr boiling 70% HNO~ ~MG 50
Oxidized 8 hr boiling 70% HNO~
2.5 b 9.7 b
Column 6 sections 12 ft length 12 sections 6 ft length 12 sections 6 ft length 12 sections 6 ft length 6 sections 12 ft length 6 sections 12 ft length
Vapors Toluene, pyridine, water, aniline n-Decane, n-octylamine, isobutyric acid n-Decane, n-octylamine, isobutyric acid n-Decane, n-octylamine, isobutyric acid Toluene, pyridine, water Toluene, pyridine, water
12 sections 6 ft length
n-Decane, n-octylamine
12 sections 6 ft length
n-Decane, n-octylamine
12 sections 6 ft length 12 sections 6 ft length
n-Decane, n-octylamine n-Decane, n-octylamiae, isobutyric acid
a Measured by low-temperature (-196°C) physical adsorption of N~ (18). b Measured by low-temperature (-196°C) physical adsorption of Kr (19). periments were conducted to determine the affinity of several organic vapors and water toward " a s received" graphite yarn and toward yarn samples which had undergone surface treatments. The graphite yarn samples were used as the gas chromatographic column substrates and the activity of these substrates was determined by measuring the pulse retention time required for the vapors to iransverse the yarns at various temperatures (70°-225°C). This study differs from published experiments in that low specific surface area graphite filaments were used as the immobile phase, rather t h a n high specific surface area powders. Application of gas-solid chromatography for analytical separations has been limited to volatile, relatively inert vapors because of irreversible adsorption, variable adsorption activity, and excessive retention of polar Journal Of Colloid and Znterfdce Science, Vol.32, No. 4, April 1970
molecules. I n the present application it is these features t h a t are exploited. In this study adsorption coefficients and heats of adsorption were calculated from the measured pulse retention times in order to evaluate the intrinsic reactivity of the fiber surface. EXPERIMENTAL METHOD The graphite fiber yarns examined in these studies consisted of Thornel 252, Thornel 402, Thornel 502, and H M G 503. The treatments used to modify the surface reactivity of these graphite yarns are summarized in column 2 of Table I. Union Carbide Corporation, Carbon Products Division, 270 Park Avenue. New York, New York 10017. HITCO Materials Division, 1600~West 135th Street, Gardena. California 90249
SURFACE REACTIVITY OF GRAPHITE FIBERS The graphite columns were prepared by making a spiral wrap of graphite yarn sections around a 40-mil stainless steel wire. In most instances the columns were prepared in duplicate. The fiber-wrapped wire was then inserted in }{ inch OD copper or stainless steel tubing. The various column mountings are summarized in Table I. The chromatograph was a Perkin-Elmer Model 154C Fractometer. The signal peaks, obtained from the changes in thermal conductivity of the effluent gas (compared to helium), were detected with a thermistor thermal conductivity sensor and recorded with an L & N Speedomax G strip chart potentiometer. The adsorbate vapors consisted of water, pyridine, toluene, n-decane, isobutyric acid, n-octylamine, and aniline. The column was maintained at fixed temperatures between 70 ° and 225°C for each series of compounds chromatographed. The helium carrier gas passed through the column at flow rates of either 15 or 52 cms NTP per minute. The retention times for water and the organic compounds, toluene, pyridine, and aniline, were measured at column temperatures ranging from 225 ° down to 70°C. The column was first adjusted to the highest temperature (225°C) and the retention times for duplicate pulses of a given vapor were measured. Following these measurements the column temperature was adjusted to the next lower temperature (200°C), and the retention times of the same compound were measured again. The temperature was redueed in decrements of 25 ° to the lowest temperature (70°C). In the case of high-boiling compounds, n-deeane, isobutyrie acid, and n-octylamine, the initiM vapor contact was at 225°C and the temperature was reduced in decrements of 25°C down to 125°C and then raised in increments of 25°C up to 225°C. The order of vapor contacts and the exposure temperature are indicated by the order in which they appear in the data tabulations. In order to standardize the elution conditions the established procedure consisted of waiting 5 min after a return of the signal trace to the base line before injecting a second adsorbate pulse.
563
THEORY AND DATA TREATMENT With the use of the general chromatographic theory of Martin and Synge (16) and the specific relationships of gas-solid chromatography of Cremer and Prior (1), the adsorption coefficient (K) and the retention volume (Vg) can be calculated from the measured pulse retention time (tr). Vg = K = (tr/tm -- 1 ) ( Y m / W ) ,
[1]
where t~ is the transit time for the carrier gas through the column, V.~ is the total gas free space in the adsorbent column, and W is the absorbent weight. The transit times for the carrier gas through the graphite fiber columns were calculated as follows: t~ = ( ~ 2 L ) - -
where r L V T1
W / p - W~/p~ V(T~/T1)
[21
= = = =
internal column radius in cm, column length in cm, volume flow rate cm3 NTP/sec, absolute temperature at flow meter, T2 = absolute temperature of col umn,
W = weight of graphite in gm, p = density of graphite in gm/cm 3, W~ = weight of steel wire in am, and p~ = density of steel wire in gm/cm a. The unconventional feature of the data treatment here consists of using the width of the peak base as the pulse retention time tr. The width of the peak base represents the total time required from initial entry of the vapor into the detector until the desorbed vapor pulse has been completely eluted from the adsorbent column. In conventional partition chromatography the pulse retention time represents the delay time from injection at the head of the column until the maximum in the signal peak of the solute vapor contacts the detector. In the present study the pulse retention time as conventionally defined would be too short for accurate deterruination. However, the pulse retention time (base width) as defined here is considered to provide a reasonably accurate measure of gas-solid adsorption, providing the flow rate of the elution gas is great enough to minimize diffusion spreading of the signal peaks. Journal vf Colloid and Interface Science, Vol. 32, N0, .4, April 1970
564
BROOKS AND SCOLA
Carrier gas flow rates of at least 50 cma N T P / m i n are necessary to minimize longitudinal diffusion of the adsorbate pulses. For cartier gas flow rates of 50 cma N T P / m i n and above signal pulse broadening can be attributed primarily to adsorption effects (17). The isosteric heats of adsorption were calculated from the temperature dependence of V~ by the relationship
yarn in order to minimize any specific vaporfiber chemicM interaction effects. The average differences between duplicate n-decane pulses on a given column all fell within the range of -4-1.1 to i 2 . 9 sec. A similar comparison of the average differences between duplicate pulses for n-octylamine yielded a range from ±0.7 to ~-2.9 sec, indicating the same precision observed for the n-decane. The precison observed for n-decane on dupliVg = c exp(-- AH~d~/RT). [31 cate columns of this water-sized yarn was approximately twice that obtained for the RESULTS AND DISCUSSION single column (see columns A and B, Fig. 1). Establishment of Procedure Parameters. It The column preparations represented here was necessary to establish the role of several offer acceptable variations in regard to both operating parameters in order to select the pulse repeatability on a given column and most suitable operating procedures. These pulse reproducibility on a duplicate column parameters included the reproducibility of packing, at least for a nonpolar adsorbate the column packing, effect of column length, such as n-decane. Some of the large differthe effect of pulse size, and the effect of ences observed between the first and second carrier gas flow rate. pulse for the polar adsorbates, notably at Column Packing Reproducibility and Pulse initial contact and after surface treatments, Repeatability. Typical pulse retention time can be attributed to chemisorption effects. data for two graphite columns (twelve secEffect of Column Length. Various column tions, each 6 feet in length) illustrating the lengths were examined, ranging from 3 to 12 effect of column packing reproducibility and feet in length. The principal difficulty in of pulse repeatability are shown in Fig. 1. preparation of 12-foot columns was the inserOperating conditions consisted of the use of tion of the yarn-wire columns into the 0.01 cm3 pulse size and helium flow rate of 1/~-inch tubing without "bailing up" of the 52 4- 5 cm8 NTP/min. Comparisons were yarn. The short 3-foot columns, on the other made for a hydrocarbon vapor with the "as hand, were easily prepared with uniform received" water-sized Thornel 50 graphite packing but provided too short vapor contact times. The most successful column preparations, in terms of packing reproducibility, lO 2 0,01 cm3 PULSE TWELVE6'YARN SECTIONS52 ¢m3 NTP/m[nHe FLOW were the intermediate length (6-foot) columns with twelve sections of yarn. Column Adsorption Capacity. The manu: 5 i facturers of fibers such as Thornel and HF[G 50 state that the specific surface areas are of the order of 1 × 104 cm~/gm. On the basis ~== n-OCTYLAMINE ~, / I''~ Z of a cylindrical rod fiber geometry and on the assumption of no internal porosity, the specific surface area (S) of Thornel 50 is / ~, ] 0 -/ ~ , . ~ / * n-DECANE calculated to be 0.37 X 104 cm~/gm:
:" ko"
,,~/o
COLUMN A O
I
r
I
20
22
24
COLUMN B • zx PRECISION I [ 26
28
104/T
FIG. 1. Column packing reproducibility and pulse repeatability for Thornel 50 graphite fiber yarn water-sized ("as received"). Journal of Colloid and Interface Science, Vot. 32, No. 4, A p r i l 1970
S = 2/rp = 2/(3.3 X 10-4 cm)(1.63 gm/cm8) = 0.37 X 104 cm~/gm, [4] where r is the fiber radius and p is the fiber density. ~Ieasurement of the surface areas of these fibers by low temperature (-- 196°C) physical adsorption of N2 (18) and of Kr (19) indi-
SURFACE REACTIVITY OF GRAPHITE FIBERS cates specific surface areas ranging from 0.8 to 9.7 X 104 cm2/gm (Table I). In order to compare the coverage area of typical adsorbate pulses to the total adsorption capacity of representative graphite fiber yarn columns the ratios of pulse coverage area to graphite fiber area were calculated from the relationship Adsorbate coverage area Total fiber area (0.01 cm3pulse)(0.73 gm/cm 3) (6.02 X 10~-3) (16 × 10-16cm2/molecule) = 0.24. (142 MW)(9.7 X 104 cm2/gm) (1.7 gin)
[51
Calculations based on Eq. [5] show that the total pulse adsorbate for n-deeane coverage area ranges from about one-fourth the total area of the oxidized HMG-50 graphite fiber to more than a monomolecular layer for the other fibers. It is evident from these calculations that with the columns used in the present experiments and a pulse size of 0.01 cm 3 the total adsorption capacity of the graphite fiber yarn column is exceeded in most instances. This means that almost instantaneous break-through at the detector occurs for at least a portion of the unadsorbed 0.01 cm3 vapor pulse which is transported at the same transit speed as the carrier gas. The signal peaks demonstrated tailing owing to adsorption on the fiber surfaces. For flow rates of 52 cm 3 NTP/min the 0.01 cm3 signal peaks were found to be relatively symmetrical (similar to peak A, Fig. 6 in reference 12), whereas the larger signal pulses, 0.1 em 3, showed very strong tailing (similar to peak B, Fig. 6 in reference 12). The faster carrier flow rate (52 cm3 N T P / min) combined with the smaller vapor pulse (0.01 em 3) is the preferred combination in order to provide the more favorable ratio of column adsorption capacity to vapor pulse coverage area and to minimize diffusion effects which would obscure resolution effects attributable to vapor-fiber adsorption interaction. In the present study the object was to obtain a measure of surface reactivity for a
565
major fraction of the fiber surface. In accomplishing this latter objective with the use of adsorbate pulses which are large relative to the column adsorption capacity, column overloading is probable; this condition is further aggravated by the difficulty of attaining a uniform column packing with a fibrous packing. Although these disadvantages apply to the present unconventional procedure of using large adsorbate pulses and peak widths as a measure of adsorbate residence time, this procedure is considered to provide an adequate measure of the average adsorption coefficient corresponding to a relatively large fraction of the fiber surface. Retention Volumes. The retention volumes (Vu) have been calculated from Eq. [1] and are plotted versus 1/T for Thornel 25 (Fig. 2), Thornel 50 (Fig. 3), and H M G 50 (Figs. 4.4 and 4B). It is evident from the plots in Figs. 3, 4A, and 4B that the polar vapor noctylamine is adsorbed most retentively on initial contact. In practically all instances the V~ vs. temperature plot demonstrated a normal temperature dependence after going
O.l cm3
PULSE
15 cm3 NTP/mln
SIX12'YARNSECTIONS HeFLOW 7 / ANILINE
> 10 3
'
/
[~/
W ~ ~ATER
5 /V
Z
/
PYRIDINE
0 2 102
5
20
r
22
r
24
I
r
26 28 104/T
i
30
J
32
34
FIG. 2. R e t e n t i o n v o l u m e s f o r t o l u e n e , p y r i dine, w a t e r , a n d a n i l i n e o n T h o r n e l 25 w a t e r - s i z e d ("as received"). Journal o/Colloid and Interface Science, Vol. 32, No. 4, April 1979
566
BROOKS AND SCOLA
through the entire temperature cycle from 225°C down to 125°C and back up to 225°C. The failure to repeat the large initial Vg is evidently the result of partial poisoning or deactivation of the fiber surface, presumably by chemisorption of adsorbate. Adsorption Coegicients at Initial Vapor Contact. In view of the occurrence of this initial deactivation at initial contact, notably with the more polar vapors, it was considered most revealing to make comparisons of the 0.01 cm3 PULSE
TWELVE6' YARNSECTIONS 52 cm3 NTP/mln He FLOW
"/
®
e / / n - G C T Y LAMINE
103 ~.,,i, n-DECANE
LU 0 >
5
Z 0 E-Z
-
©. / 5 INITIAL CONTACT @® DECREASING TEMP. o • INCREASING TEMP. ~ J
'~
2
10
i 20
I 22
I 24
I 26
I 28
104/T
FIG. 3. Retention volumes for n-decane and n-octylamine on Thornel 50 water-sized ("as received"). 2
(A)
2
).01 cm3 PULSE TWELVE6' YARNSECTIONS 52 cm3 NTP/min He FLOW ~/
m 10 3
initial adsorption coefficients (K). In Figs. 5, 6, and 7 the adsorption coefficients at initial vapor contact are shown for Thornel 25, Thornel 40, Thornel 50, and HMG 50. The vapors consisted of toluene, water, pyridine, analine, n-decane, n-octylamine, and isobutyric acid. Graphite fiber surface treatment conditions consisted of exposure to hydrogen at 20 torr at 1200°C, hydrogen treatment at 550°C, air oxidation at 550°C, and nitric acid oxidation. The adsorption coefficients (K) show ap. preciably stronger adsorption of the polar adsorbates water, pyridine, and aniline compared with toluene on the "as received" Thornel 25 (Fig. 5). It is evident also that hydrogen treatment at high temperature (see Table I) enhances surface reactivity for all the adsorbates relative to either the "as received" or the nitric acid oxidized Thornel 25 graphite fiber. A second exposure of the hydrogen-treated column to these same vapors shows that an appreciable deactivation has occurred. Similar conclusions apply to the data for Thornel 40 presented in Fig. 6. Both in situ hydrogen treatment at 550°C and air oxidation at 550°C enhance reactivity of Thornel 40 toward n-octylamine. These in situ treatments resulted in larger initial K values than the high-temperature treatment (1200°C) inasmuch as the former system (Thornel 25)
[]
Y •
~
~> u2
n-OCTYLAMINE
[]
D
O
[]
Z
INCREASING TEMP. i r .~
Z
"~ 2
Z
~
I
I
22
2r4
E
26
~/
[
n-OCTYLAMINE
?
n-DECANE
~> -
iNITiAL CONTACT ~ DECREASING TEMP. •
20
/
m 103 n-DECANE
O~_
102
(B)
0.01 cm3 PULSE TWELVE6' YARNSECTIONSS2 cm3 NTP/mln Ho FLOW
I
28
lo 4/T
,/~
2
102
J
. /
INITIAL CONTACT
/~
DECREASING TEMP. ~ iNCREASING TEMP. ~1' fif
]
20
2'2
2~4
~
26
~ ~]
]
28
iO4/T
FIG. 4A. Retention volumes for n-decane and n-octyl~mineo n HMG-50 ("as received"). FIG. 4B. Retention volumes for n-decane and n-ocytlamine on HMG-50 oxidized, 8 hour boiling 70% HNO3. Journal of Colloid and Interface Science, Vol. 32, N o . 4, A p r i l 1970
SURFACE REACTIVITY OF GRAPHITE FIBERS had some exposure to the ambient atmosphere prior to the chromatographic measurements. Examination of the calculated adsorption coefficients for Thornel 50 in Fig. 7 shows that (1) no significant difference between adsorption of amine and acid occurs on the "as received" fiber, (2) nitric acid oxidation treatment enhances the adsorption of ndecane, and (3) nitric acid oxidation makes
3200
2800
15 cm3 NTP/min He FLOW
2600
U
Z 14.1 ___2 0 0 0
,,="
~u o
u
ILl
O 1600 Z
== o
-1200
< F_,
~,o
2~
::"4
C,u o~
o
400
WATER O X I D A T I O N SIZED AS 8 HR RECEIVED B O I L I N G 7 0 %
O
u 1400 Z
HNO 3
_o
600
',9
,L
Z i--
1800
<
i
o I
1OO0
I
800
~, 2 2 0 0
~
HMG 50
THORNEL 50,,, '1
:
o_
~ SIX 12 ~ YARN SECTIONS
TWELVE6' YARN SECTIONS 52 cmJ NTP/m[n He FLOW
v 2400
O
0.1 cm3 PULSE
0.01 cm3 PULSE
567
WATER SIZED AS RECEIVED
OXIDATION 8 HR BOILING 70% HNO3
FIG. 7. Initial a d s o r p t i o n coefficients at 547°I4 for Thornel 50 a n d HMG-50.
no significant change in the adsorption of the amine. 200 In the case of "as received" HMG-50 WATER SIZED H2 TREATMENT OXIDATION there appeared to be preferential adsorption AS RECEIVED AT 1200 C AT 80 C of the amine compared with the acid. HowWITH 60% HNO3 50 proFI~. 5. Initial adsorption coefficientsat 422°K ever, nitric acid oxidation of HMG duced a significant increase in adsorption for Thorne125. retention of n-deeane, n-octylamine, and isobutyric acid compared with the "as 3200 0.01 cm3 PULSE 52 cm3 NTP/mln He FLOW received" fiber. TWELVE 6' YARN SECTIONS zz~
Heats of Adsorption of Several Organic Vapors for Pretreated Graphite Yarn Surfaces.
2800
c, 2400
9 <
2000
U
1600
--
~
~
12OO
800
40O
WATER SIZED AS RECEIVED
H2 TREATMENT 2 HRS AT 5 5 0 C
AIR O X I D A T I O N 2 HRS AT 5 5 0 C
FI(~. 6. I n i t i a l a d s o r p t i o n coefficients a t 547°I4 for T h o r n e l 40.
Heats of adsorption were calculated from Eq. [3] for these organic vapors and water for Thornel 25, Thornel 40, Thornel 50, and HMG 50 (Table II). All the heats of adsorption calculated for the hydrocarbons (toluene, n-hexane, n-octane, and n-decane) for these graphite fibers lie within the range of 2.6 to 4.1, keal/mole and are appreciably lower than published values ranging from 4.0 to 13.0 kcal/mole for low molecular weight hydrocarbon vapors on graphitized carbon blacks. The polar organics (aniline, n-octylamine, and isobutyric acid) yield heats of adsorption within the range of 3.5 to 6.8 kcal/mole, which is somewhat lower than the published values ranging from 5 to 10 kcal/mole for Journal of Colloid and Interface Science, Vol. 32, No. 4, April i970
BROOKS AND
568
SCOLA
TABLE II ISOSTERIC I-IEATS OF ADSORPTION FOR VARIOUS VAPORS ON GRAPHITE SURFACES AH Adsorption (kcal/mole) Graphite fibers
rhornel 25 rhornel 40 rhornel 50 [tMG 50
Vapor
Temp. range
(°K)
"As Oxidized8 hours Boilin. Received" 70% HNOa
Toluene, water, aniline n-Hexane, n-octane, n-decane
343-422 343-422 343-422 371-499 371-499 371-499
4.1+0.5 4.5 6.8 2.94-0.5 3.4 3.8
n-Decane, n-octylamine, isobutyrie acid n-Decane, n-octylamine, isobutyrie acid
395-547 395-547 395-547 395-547 395-547 395-547
3.64-1.2 3.9 5.4 2.64-0.5 3.5 4.3
--
Graphitized carbon blacks (pub. data)
Graphon Sterling MT-G Sterling 3100
--
3.44-1.5 5.3
Sterling 3100
Temp. range (°K)
Vapor
Benzene, water
AH adsorption Sourc{ (kcal/ ref. mole)
i00-490 9.1 305-369 4.5-6.0
Ethane butane, hexane, heptane Ethanol, butanol, hexanol
4.0 7.5 10.0 13.0 5.0 8.0 10.0
9 12 11 11 11 11 11 11 11
3.14-1.6 6.3 3.8
low molecular weight alcohols (ethanol, butanol, and hexanol) on Sterling 3100. I t is worthy of note t h a t in the published d a t a essentially the same heats of adsorption were obtained for alkanes and the corresponding alcohols on Sterling 3100, indicating t h a t the adsorptive forces are predominantly dispersive in character. A closer agreement was observed in the case of water. A value of 4.5 kcal/mole was calculated for Thornel 25, whereas a range of 4.5 to 6.0 kcal/mole has been reported for water on Sterling M T - G (12). T h e heat of adsorption data in Table I I are lower t h a n those obtained b y other workers (9, 11, 12) for the same organic vapors on graphitized carbon blacks. This m a y be due in p a r t to some deactivation or poisoning of the graphite yarns b y the initial v a p o r contact at the highest t e m p e r a t u r e used. An additional factor m a y be t h a t the published heat values correspond to partial coverages of the adsorbent b y the adsorbate, whereas in the present study surface coverage in most instances approximates a complete monomolecular layer of adsorbate. The comparisons with published d a t a are primarily to demonstrate the effects of varying polarity of the adsorbate for other graphite adsorbents. There was no significant change within experimental uncertainty in the heats Journal of Colloid and Interface Science, Vol. 32, l~lo. 4, April 1970
of adsorption on the graphite Thornel 50 and HMG 50 fibers after a surface treatment such as nitric acid oxidation (see Table II), so this precludes the use of the heat values for evaluation of the effects of such a surface treatment. It is apparent that the adsorption coefficients calculated for the initial vapor contacts at 225°C remain the most significant criterion of intrinsic surface reactivity and the most reliable measure of the effects of a treatment on the graphite fiber surfaces. ACKNOWLEDGMENTS This work was conducted under the sponsorship of the United Aircraft Research Laboratories and the Air Force Materials Laboratory, WrightPatterson Air Force Base, Ohio (AF contract 33 (615)-5046). The authors express appreciation to both sponsors for support and permission to publish. The authors appreciate the contribution of Dr. Malcolm Basche for the high-temperature surface treatments, of Mrs. Evelyn Malloy for the krypton surface areas, and of Mrs. C. Livermore for technical assistance.
REFERENCES 1. CREMER,E., A~D PRIOR,F., Z. Elektrochem. 55, 66 (1951). 2. GREENE, S. A., AND PUST, H . , J . Phys. Chem.
62, 55 (1958).
SURFACE REACTIVITY OF GRAPHITE FIBERS 3. SCHAY,G., FEJES, P., HALASZ,I., ANDKIRALY, J., Magy. Kern. Folyoirat 63, 143 (1957). 4. HANLAN,J. F., AND FREEMAN, M. P., Can. J. Chem. 37, 1575 (1959). 5. BEEBE, R. A., AND EMMETT, P. H., J. P h y s . Chem. 65, 184 (1961). 6. CREMER,E., Monatsh. Chem. 92, 112 (1961). 7. CARBERRY,J. J., Nature 189, 391 (1961). 8. CREMER,E., AND HUBER, H. F., I.S.A. Proc. 1961, In~. Gas Chromatography Symp., p. 117; Angew. Chem. 73, 461 (1961).
9. ROSS,S., SAELENS,J. K., AND OLIVIER, J. P., J. Phys. Chem. 66, 696 (1962). 10. HABGOOD,H. W., AND HANLAN,J. F., Can. J. Chem. 37, 843 (1959). 11. BELYAKOVA, L. D., KISELEV, A. V., AND KOVALEVA, ]-~. V., Anal. Chem. 36, 1517 (1964).
569
12. GALE,R. L., ANDBEEBE, R . A., J . Phys. Chem.
68, 555 (1964). 13. BEEBE, R. A., EVANS, P. L., KLEINST]~NBER, T. C. W,, AND RICItARDS, L. W., J". Phys. Chem. 70, 1009 (1966). 14. GIDDINGS,J. C., ANDMALLIK,K. L., Ind. Eng. Chem. 59, 18 (1967). 15. KOBAYASHI, R., CHAPPELEAR, P. S., AND DEANS, H. A., Ind. Eng. Chem. 59, 63 (1967). 16. MARTIN, A. J. P., AND SYNGE, R. L. M., Biochem. J. 35, 1358 (1941). 17. EBERLY, JR., P. E., AND SPENCER, E. m., Trans. Faraday Soc. 57, Part 2, 289 (1961). 18. DECRESCENTE, M. A., SCOLA, D. A., AND
BaOOKS, C. S., AFML-TR-67-218, Part 1, (July 1967). 19. BEEBE, R. A., BECKWITH,J. B., AND HONIG, J. M., J. Am. Chem. Soe. 67, 1554 (1945).
Journal of Colloid and Interface Science, Vol. 32, No. 4, April 1970