A STUDY OF THE FINE STRUCTURE OF CARBONACEOUS SOLIDS BY MEASUREMENTS OF TRUE AND APPARENT DENSITIES PART 1. COALS By Rosartnp E, FRANKLIN Received 18th November, 1948 The true density of a series of coals was measured with helium gas and apparent densities were measured with methanol, water, #-hexane and benzeng, liquids. From the results obtained, the following conclusions were drawn, Helium fills rapidly and completely the pore space of coals ground to pass: a 72 B.S. sieve, and measures the true density of the coals. The pore space of: coals ground to pass a 72 B.S. sieve is filled by methanol almost completely in q few hours, There is a contraction of 2-6 x ro7® cm.? for each cm.? of surface covered by methanol. Water, n-hexane and benzene fill the pores space of somé low rank coals practically completely. The apparent densities of such coals in these liquids are high owing to the contraction which accompanies adsorption; n-Hexane and benzene penetrate only very slowly into the pore space of some coals owing to the relatively large diameters of the molecules of these liquids. There is no appreciable volume of closed pores in coals. The accessibility of the pore space of a coal to liquids and gases varies with, rank in a manner sitnilar to the porosity and adsorptive properties, Coals of high porosity have the most open pore structure. The pores in coals contain numerous fine constrictions, and the variation in the accessibility of the pore space from one coal to another is related to a variation in the width of thesa constrictions rather than in the mean diameter of the pores. The width of the constrictions is of the same order as the diameters of simple molecules and js smallest in coals containing between 89 % and 93 % carbon. The Apparent Density of Porous Solids.—-It is well known that the apparent density of finely porous solids and of solids possessing large specific surfaces is highly dependent on the method of measurement The variation: may be attributed im the main to two factors which In- fluence the results in opposite directions. Values greater than the true’ density of the solid may result from the decrease in volume which a6- companics adsorption of the filling fluid; alternatively, slow or in- complete penetration of the pores by the fluid may lead to low appareit: density values. In the latter case a density drift (or increase of apparetits density with time) is frequently observed. These considerations apply ta a wide range of organic and inorganic colloidal materials and are well illustrated by the many measurements which have been made on charcoals. A selection of these is given in Table I; it is clear that very varied results may be obtained with a single charcoal, and also that the values obtained with a given series of liquids may fall into different order when different charcoals are used. ; The results quoted in Table I show that both incomplete penetration. of the pores and the contraction due to adsorption may be important Thus the apparent density values are intimatcly related to the fine structure of the solid investigated; they depend not only on the pro- perties of the liquid and the density of the solid, but also on the natute and extent of the surface of the solid, the size and accessibility of. the. pores, and the extent to which the accessibility of the pores and inner surfaces may be influenced by deformation of the solid resulting from interaction with the liquid. 274 ROSALIND E. FRANKLIN 275 ‘he object of the present work was to use measurements of true and ent densities to investigate the inner structure of coals, the vari- of structure with rank (see later), and the nature of the interaction en coal surfaces and certain liquids. TABLE I.—APPARENT DENSITIES OF CHARCOALS IN LIQUIDS Harkins Cude Culbertson and Firth? and and Corriez 5 Ewing? Hulett3 Weber# Liquid Activated Coconut Sugar Coconut Active Sugar charcoal charcoal charcoal charcoal charcoal charcoal tater. « . 1-84 Igo 17g 185 1°86 1°88 Zoform . . 1°99 2°18 2:22 a -— —_ fel. . . 2°04 1:96 1:98 180 201 T'65 vdisulphide. 2°06 nae — 1-98 2°03 197 é. . . 211 — —- _ 2°05 — $tbwi alcohol . . — 2°00 1:96 — 2-08 — Significance of Density Measurements made with Helium.— hoice of helium for the measurement of the “true ’’ density of a er of porous solids has been dictated by its small molecular diameter ould enable it to penetrate into very fine pores, and by its small ex Waals’ field resulting in negligible adsorption on solids at room # pe ature. It is not, however, self-evident that a ‘‘true’’ density : ‘assigned to a complex and fine-structured material such as coal, ‘solid contains closed pores which are inaccessible to all fluids ing helium, then it may be necessary to define arbitrarily whether ; such spaces are to be included in the “ true’ volume of the material. ér,as has been well emphasised by Hermans,® density is essentially sroscopic property, and irregularities of molecular dimensions cannot sured by attempting to pack molecules into the available space. ithe structure of the solid is such that “ pores” of a few Angstréms sroccupy an appreciable fraction of the total volume, then measure- sinade with helium (molecular diameter 2A) will give an apparent which has no precise or fundamental significance.. Hermans misidérs':that in the case of cellulose “there is no longer a conclusive om t6 "believe that any special preference should be given to the ty found in helium, or that it would have any particular physical AIL the evidence resulting from the present work, however, low that for coals helium may be used to measure a well-defined hich may reasonably be called the true density. In par- 1ay be mentioned here that aithough the fine structure and volume are known to vary widely from one coal to another, “measured in helium is a function of chemical composition her evidence that helium measures the true density of coals ater sections of this paper. ‘hysical Structure of Coal.—Coals of different rank form a elated materials representing different (though not necessarily Sand Ewing, J. Amey. Chem. Soc., 1921, 42, 1787. vans, Faraday Soc., 1923, 19, 444. dear d. Hulett, J. Amer, Chem. Soc., 1920, 42, 391. ertson and Weber, ibid., 1938, 60, 2695. Thesis (Paris, 1937). 18; Contribution to the Physics of Cellulose Fibres (Elsevier Publishing 2,02, ti Fuel, 1948, 27, 46. 276 FINE STRUCTURE OF CARBONACEOUS SOLIDS successive) stages in the process of ‘ coalification ’’ which started with the decay of plant material in a peat-bog and led to the formation of lignites, sub-bituminous coals, anthracites and peranthracites, In- vestigations of the colloidal structure of coals are described in a series of papers presented to a Conference on the ‘ Ultra-fine Structure of Coals and Cokes,’’ and the subject has also been reviewed by Bangham.s Griffith and Hirst ® measured the heat of wetting in methanol of some two hundred coals, and, with the aid of adsorption data due to Maggs,10 gave evidence that the surface area of coals accessible to methanol varies from about 20 to 200 m.2/g. The relation between the surface areca and rank of coals is shown.in Fig. 1, where the heat of wetting in methanol is plotted against the volatile content of the coal.* Heat of 7 x wetting | l l cal./g. 24 | 22 x x x : x 20 K x |x 18 | , x . | , |? 16 t r xx x x ~ : . a 4+ / x . ‘| x x 12 | a — x x 1. ro x x wa % 8 x a x * cad x x are 6 «| x Se nN | . xO * x * “x xe ee x * x « Bs J -—- yj — — fe oreo 4 xe x x Mae * x x x x” x A 2 xe ee ~ — 7 x | | [. : ° 5 10 5 30 35 qo 45 50 3 20 25 3 35 °, Volatile Matter Dry, Ash-free Fic. 1.—The heat of wetting—volatile matter classification. The porosity of a large number of coals was measured by King and Wilkins, who uscd mercury to measure the lump density and water for the “true’’ density. Porosity values ranged from about 2% to 20 %, the variation of porosity with rank being similar to that of the heat of wetting in methanol. The highest porosities were found among the low rank coals, and the lowest in coals of about 89 % carbon content. Measurements by Duningham }* again reveal a similar relationship between inherent moisture content and rank. 8 Bangham, Ann. Reports, 1943, 40, 29. . ® Griffith and Hirst, Conference on the Ultrafine Structure of Coals and Cokes, (B.C.U.R.A., 1944), p. 80. , 10 Maggs, tbid., (B.C.U.R.A., 1943), Pp. 95. a * The volatile content of a coal is closely related to its rank, and cal. of; heat of wetting in methanol is equivalent to approximately 1o m.? of surface {see ref. 8). 11 King and Wilkins, Conference on the Ultrafine Structure of Coals and Cokes. (B.C.U.R.A., 1943), p. 46. 22 Dunningham, ibid., p. 57. ROSALIND E. FRANKLIN 277 Bangham *. 13 has postulated that coals have a micellar structure, the micelles being bound to one another principally by van der Waals’ forces. Evidence of this is drawn from the behaviour of coal in solvents,!4 from adsorption and surface area measurements,*: 1° from the rheological behaviour of coal?3 and from the low-angle scattering of -rays.35 The decrease in surface area and porosity with increasing rank (up to about 89 % carbon content) is attributed to a process analogous to syn- eresis. In the present work, measurements of true and apparent densities have yielded further information concerning the size and distribution of the pores in coals, and, on the basis of the results, the micellar theory is further developed. Experimental Materials.—Coats. A list of the twelve coals used, together with their carbon contents and heats of wetting in methanol is given in Table II. All the samples were of “ bright ”’ coals (i.e. they contained a preponderating amount of vitrain), Except where otherwise stated, all measurements were made on coals ground to pass a 72 B.S. sieve, precautions being taken to avoid an undue proportion of fines. TABLE II.—Anaryses or Coars at of Wetting Coal Locality Capon Hes Methanol - ° cal. fg. A Ireland 95°2 6-2 B S. Wales 94:7 V7 c _ 3. Wales 94:2 7:6 D S. Wales 9L-7 2:8 E _ Kent 90-9 25 F S. Wales 89-7 20 G Derbyshire 84:6 598 H Yorkshire 83°5 8-8 J Staffordshire 82-9 1074 K Northumberland 82-4 16-6 L Nottinghamshire 81-3 ri-8 M Northumberland 80-6 17 Hetium, “ Spectroscopically pure " helium was supplied by British Oxygen CorLtd. ‘Liguips. Water was freshly distilled ; m and n-hexane boiling at 67°-69° C were used. Measurement of Densities with Welium.—A diagram of the apparatus given in Fig. 2. In essentials it was the same as that of Smith and Howard 18 ‘CHE operation consisted in (1) thorough evacuation of the apparatus, (2) measure- of the temperature and pressure of a quantity of helium in the calibrated ethanol and benzene of a.r. grade bE, (3) transfer of the helium through the capillary tube G into the calibrated bulb A which contained the sample, and (4) measurement of the temperature art Tessure of the helium in the bulb A. This gave the free volume in the bulb A ence the volume of the sample. The sample was weighed after the bulb Wasttemoved from the apparatus at the end of the experiment, ie bulb F, at room temperature, was surrounded by a water jacket fitted thermometer and stirrer; the bulb A was in a water-bath supported on Platform P and maintained at 25 cb OroL° C. During the preliminary evacu- nthe water bath was replaced by an electric oven. Pressure measurements made by adjusting the mercury level to a fixed mark B (viewed through a B Bangham, Conference on the Ultvafine Structure of Coals and Cokes A, 1943), p. 18. i Kiebler, ind. Eng. Chem., 1940, 32, 1380. t eRiley, Conference on the Ulivafine Structure of Coals and Cokes (B.C.U.R.A,, NAB). P. 232. “he. Smith and Howard, Ind. Eng. Chem., 1942, 34, 43. 278 FINE STRUCTURE OF CARBONACEOUS SOLIDS telescope) and observing the height of the mercury in the capillary tube G. The capillary tube was carefully selected for uniformity of bore (mean diam. 1-16mm, maximum deviation o-oo, mm.) and mounted against an engraved steel scale which was read to o-r mm. odo. To Mcleod G 7o diffusron Gauge. Pump & Ay vac. LEE D EL | { fe To waler purnp. ca VY Worn f Orcs pO Sa: lo wale pup | CHI atmosphere. § j pry a ee hi a ae fue lndareret “ey , K LQ - 2 Fic, 2, When coal is heated in vacuo, evolution of gas continues, at a steadily de- creasing rate, for long periods of time. Further increase of temperature is always accompanied by further evolution of gas. It was therefore necessary to select arbitrarily an evacuation schedule which, whilst heating the coal sufficiently to ensure that measurable quantities of gas would not be disengaged subsequently at room temperature, yet would not cause significant structural changes. In practice, the sample was heated to go°-100° C until a pressure of less than to-* mm. (with the pumps running) was obtained, the minimum period of heating being 16 hr. The sample was previously evacuated elsewhere in order to reduce its tendency to scatter when evacuated in the density apparatus. Tt was established that minor variations in the above procedure did not influence the results, The first reading of the pressure of helium in the sample bulb was made as rapidly as possible; it was generally completed 30-40 sec. after the gas first entered the bulb. Further readings were taken at intervals until no furthér change was observed in 24 hr. In general, coals showed no density drift in helium provided sufficient time was allowed for the sample to come to the temperature of the thermostat before the gas was admitted. Duplicate determinations of the free-space in the bulb did not differ by more thano-t %. The recorded density values are therefore accurate to sb 012 Ye Measurement of Apparent Densities in Liquids-—The coal samples, contained in calibrated glass bulbs, were evacuated as described above. The bulbs were then scaled, the scaled tips were broken off while immersed in the liquid, and the bulbs placed in a thermostat at 25 -k 0-02° C before levelling the liquid and weighing. The density drift was often considerable, and in order to obtain comparative data it was necessary to make measurements after standard periods of immersion. Except where otherwise stated, all densities recorded were Ineasured after 24 hr. immersion and all density drifts are given as the percentage difference between the density values obtained after 2 hr. and after 24 hr. immersion. Duplicate measurements did not differ by more than 05% even when large density drifts occurred. ee Lump Density.—For the measurement of the lump density of coals, single lumps were dried to constant weight in an air-oven at 105° C, smeared with @ ROSALIND E. FRANKLIN 279 thin film of vaseline, and re-weighed rapidly in air and in water. The possi- ‘bility of errors due to large ash inclusions or other irregularities was climinated ‘by making measurements on five or six different lumps of each coal. Correction of Densities for Mineral Matter.—An approximate cor- rection for mineral matter was applied to all the density values recorded. Wandless and Macrae,!?7 who made a detailed study of one coal, showed that he error involved in correcting for mineral matter on the basis of ash density And ash content is not appreciable so long as the ash content is small. The %orrected density is given by d= ad’ (x00 - A) rooa — Ad’ ’ nyhere d’ is the observed density, a the density of the ash and 4 the ash content yer cent. The correction amounts to less than 2 % for all the coals used in the ‘present investigation. Measurement of Heat of Wetting.—tlleats of wetting of coals in liquids swefe measured by the routine method developed in the B.C.U.R.A. laboratories thv Griffith and Hirst,9 Results Control. Experiments with Helium.—Preliminary experiments showed ‘that-under the conditions of the density measurements helium penetrates rapidly and completely into the pores of all the coals investigated. Equilibrium was always rapidly established, and, below a certain size limit, the density was in- dependent of the particle size of the material (Table IV, column 3). Further, the results were not altered when the period of evacuation at 90°-100° C was ‘extended from 16 hr. to 10 days. TABLE III.—Aprarent DENSITIES OF THE COALS Helium Methanol Water n-Hexane Benzene Coal Density Drift Density Drift Density Drift Density Drift Density | Drift after fe after ‘ i after 3 after Ae (g.fom.3) } (%) 24 hr. to. 24 hr. to 24 hr. to 24 hr. to (g. fom.) 24 hr.) (g./om.3) 24 br) (gz. /em.3) 24 br.) (g./cr.3) 24 hr.) A 1645 | Oro | 1-700 | 0-3 | 1630 | a-o 1-497 | o8 | 1-578 | 1-5 B E517 | oo | 1:556 | or r488 | oro | 17433 Trg | 1450 | 2:9 c 497 | OO | 1549 | oT I°475 | OO | 1°425 | 20 —- — D 1362 | Og | 1-374 | org | 1°31r8 | Oro | 1-300 | 6-3 T'293 | O73 E 1337 | OO ; ¥°352 | ov 19305 ; oO | F207 | a3 | re2agg | 0-6 F E°31E oo 17333) o6 291 | oo T'286 | 0-3 1-297 | 0-3 & T:293 | oo 17334 | G6 1297 | 0-0 17276 | ov 1°286 I°3 H 301 | OO | 1357 | OG | 1307 | Oro | 1-262 | 18 | 1-276 | a5 J 1-302 | OO | I-4O2 | O-F | 1°333 | oo | 1-297 | o-8 T-292 | 13 K 1°305 | oro 4-387 | og 1-326 | oo 1+329 1-8 I*342 I-4 al e307 | Oro | 4387 | or5 | 1-328 | OO | T272 | 2-2 | re321 | 2-6 M In34l | oo 1-327 | a3 1-370 | 0-0 1374 29 a _ ' fhe True and Apparent Densities of Twelve Coals.—The apparent deities and density drifts of the twelve coals described in Table II were measured in methanol, water, #-hexane and benzene, and true densities were ured with helium. ‘he results are given in Table III, and in Fig. 3 the ties are plotted against the carbon content of the coals. The main feature € results are— (t) The density in methanol is in every case the highest. as (2) Densities in n-hexane and in benzene are in general lower than the true Yensity (measured with helinm). ‘wo low rank coals are exceptions. (3). Where the apparent densities in -hexane and in benzene differ appreciably, thervalue given by n-hexane is the lower. ) Apparent densities in water are lower than the truce density for high rank 5 and higher for low rank coals. The change occurs between 82 % and 85 %.carbon content. 17 Wandless and Macrae, Fuel, 1934, 13,4. 280 FINE STRUCTURE OF CARBONACEOUS SOLIDS TABLE IV.--INFLUENCE OF PARTICLE SIZE ON APPARENT DENSITY — Helium Methanol Water n-Hexane Coal Size Density | Drift | Density | Drift | Density | Drift Density | Drift after (%, 2 after (%, 2 after (%, 2 (g.fem.3) | (%) 24 hr, hr, to 2a4hr. | hr. to 24 hr. br. to (g./om.3) }24br.}) (g./cm.3) | 24 hr.) } (g-/om.8) | 24 br.) r | (z) Through 14 B.S. on 1/32 in. 2. . | £298 | oro | 19315 | 2-2 | 1-285 | Og | 1'273 | Ovo (2) Through 72 B.S. on roo BS... «| 1299 | oo | 1°33E | IE | 1-283 | oro | T°27r | Og (3) Through 72 BS... . | r31r | oo | 1-333 | 06 | 4-289 | oo | 1-281 | 03 (4) Through 240 B.S. 1307 | oro | 17335 | Or2 | 1285 | O10 | T29T | 0-6 K =} (zt) Through 14 B.S. on 1/32 in, . .) | 1292 | oO | 1373 | Og | TBE | O'O | 1316 | 22 (2) Through 72 B.S. on 100 BS... . | 12904 | oo | 1-371 | 0-6 | 1-318 | Go | 1-318 | ar (3) Through 72 BS... .| 1305 | oo | 17386 | og | 1°327 | Oro | 1330 | 1:8 (4) Through 240 BS. 1-305 | oo | 1°380 | o-5 | 17336 | OO | 1339 | 18 7 —y— HELIUM —O-— METHANOL. —--- WATER ~~ a—— 31 -HE KANE: veeon BENZENE 6 m. we § = Ss & N Q i4 PS Carbon per cent: 55 glo 80 Fic, 3. ROSALIND E. FRANKLIN 281 (5) (a) There is, in general, no density drift in helium. One coal showed a small drift. (b) There is no density drift in water. (c) Density drifts in methanol are generally small (0-2-0-6 %). (d) Considerable density drifts in n-hexane and benzene are observed with some coals. (6) TRUE Densities.—The significance of the values obtained for the true densities of coals is discussed elsewhere.? It is shown that for all the coals in- vestigated the true specific volume is a linear function of the hydrogen content, extrapolation gives 1-85 g./cm.* for the density of a “‘ coal ”’ of zero hydrogen tent. This relationship indicates that there is a characteristic molecular atomic packing which is common to all coals and very different from that of graphite (density, 2-26 g./cm.’). ‘These measurements provide a general survey of the way in which the ap- giarent densities of coals vary with rank and with the nature of the fluid used to jake the measurement. The following series of subsidiary measurements were de to assist in elucidating the results. (1). To ascertain the significance of the low apparent densities of some coals in water, n-hexane and benzene, the Inmp densities of the coals c, b, F, H and K were measured as described above, and from the lump densities and the densities in helium the true porosities were calculated. The results are given in Table IV. To investigate further the density drifts and low apparent density values, the dependence of the results on the particle size of the coal was studied. Results obtained with coals F and x prepared in four size grades are given in Table V. S TABLE V.—-Porosiry or Coars Density * of | Porosity Calc. Coal | Dry Lump | from Density in {g./om.9) Helium (%) c 1364 Iovl D 1343 4°7 F I+301 372 H 1-214 TT K L151 12:3 * Densities not corrected for mineral matter. The strong influence of the chemical character of the coal surface on the values of the apparent densities in liquids was confirmed by measurements made on oxidised coals. Samples of coals F and x, ground to pass 72 B.S.S., were spread in thin layers in a well-ventilated air oven at 105°C for 36 days. The densities and heats of wetting before and after oxidation are given in Table VI. sah EABLE VI.—INELUENCE OF OXIDATION ON APPARENT DENSITIES AND Heats oF WETTING Helium Methanol Water | n-Hexane Serial Drift |Heat of Drift [Heat off | ett Heat Matera Density Density sel of Density | Drift) after (% 2) Wet- after {% 2| Wet- | Density | (% Wet- 2 hr. to| ting hr. to} ting after [hr. to (g./com.2), (% ) | 24 hr. 3 24 br. ting 4 | (cal.f 24 | (cal.{ | 24 hr, 24 efom ney |g.) [elem 8) gy | ME) hr.) (call Go P3IE | 00 | 1333) 06] 2:0] 294} oo | 0-6} 1-286 | 0-3 | 1-8 ee I°359 | ovo F400 O7]; 3°4 1357, OL 35 I*321 | o-o 23 Goal 1-305 | 0-0 | 4-387 | 0-4 | 16:6 | 1-326 | oro | 9-5 1329 | 1:84 4°9 oxidised 1427 | 0-0 | 1577 | Tex | 22°6 | 1-479 | o-g | ra+3 | 2-372 | 1-4 | 15 te 282 FINE STRUCTURE OF CARBONACEOUS SOLIDS Discussion Apparent Densities in Methanol: Compression of the Adsorbeq Liquid.—The apparent density in methanol exceeds the density jp helium for all the twelve coals investigated. It has been shown above that helium penetrates rapidly and completely into the pores of coals ground to pass a 72 B.S. sieve. Moreover, measurements on the two widely different coals r and K (Table V) show that the pores of coals ground to pass a 72 B.S. sieve are also completely filled by methanol iy 24 hr. ; density drifts in methanol are small and decrease with decreasing particle size, and density values after 24 hr. are practically independent of particle size. The difference between the density values obtained with helium and with methanol must therefore be attributed to the contractiog, which accompanies adsorption. The contraction per g. of coal is given by (Vue—Vueon), where V,, is the specific volume in helium and Vyeox the apparent specific volume in methanol. This function is found to be directly proportional to the heat of wetting of the coal in methanol, for eleven oF the twelve coals investigated (Fig. 4), the contraction being o-0026 cm.? per calorie heat of wetting. Only coal J is represented by a point lying well above the line, but the heat of wetting of this coal was liberated exceptionally slowly and the value recorded is known from other evidence to represent a gross under-estimate.* Maggs ” has shown that the heat of wetting of coals in methanol is rt cal./ro m.? of surface, and this is the same as the figure obtained by K'COALS <72 BSS /o G COAL < 240 / © OXIDISED COALS / & WOOD CHARCOALS 7 50 x 7 by f of & S a 040 AR " / Q y * M7 FPSO 7 oO xv 2 / = 7 yy” . ; x 7, 2 7 / { 7 a! vA / 5 io iS 20 25 HEAT OF WETTING IN METHANOL, ca/_/g. Fic. 4. Bangham ® for charcoal. The heat of wetting in methanol may therefore be taken as a measure of the specific surface. of the coals, modified coals. * Owing to the limitations of the calorimeter used, heats of wetting could only be measured during the first ro to 20 min. after immersion. Speci ic volumes measured after 2 hr. immersion rather than after 24 hr., were theretoié used in obtaining the values of (Vue—Vaecou) shown in Trig. 4. ROSALIND E. FRANKLIN 283 charcoals discussed here. The linear relationship in Fig. 4 then shows “the contraction due to adsorption is proportional to the surface area ared. The fact.that the line passes through the origin confirms that hanol and helium penetrate to an approximately equal extent into pore structure. {t is important to note that the contraction per unit area of adsorbing ace is found to be of equal magnitude for charcoals, anthracites, 3 of low rank and highly oxidised coals in spite of the widely differing nysical and chemical properties of these matérials. The contraction “4nnot, therefore, be attributed either to incipient solution or to contrac- fio jn the. solid substance. . ~ Taking a heat of wetting of 1 cal, to represent 10 m.? of surface the observed contraction is 2°6 x Io7® cm.? per cm.? of surface, or about Z,.of the volume which would be occupied by a monolayer of methanol the surface of the solid if no contraction occurred. Thus, even if the traction were spread evenly throughout the absorbed material (coals sorb the equivalent of from 3 to 5 molecular layers of methanol at iration pressure at. 25°C), it is surprisingly large. Measurements olving charcoals and cellulosic materials indicate, however, that the fe, contraction observed when methanol is adsorbed on coals is not exceptional. Several authors have calculated the hydrostatic pressure required to produce the observed contraction in the adsorbed film, but it seems that h calculations can have little meaning since the adsorbed film and the yeges responsible for its formation are essentially anisotropic, and the cture and properties of the film differ from those of the bulk liquid. Digect. measurements of the volume occupied by ‘water vapour on @éllulose * and by organic vapours on charcoal have shown that the greatest contraction occurs for small quantities of vapour adsorbed, when the‘Jateral film pressure is very small and the volume change must be ibuted almost entirely to the close approach of the adsorbate mole- es to the surface atoms of the adsorbent. It seems probable, therefore, iat-the greater part of the large contraction observed when methanol ig,adsorbed on coal occurs in the direction perpendicular to the adsorbing Surface. Apparent Densities in m-Hexane and Benzene.—The apparent densities of coals in m-hexane and benzene show considerable drifts, and the yalues obtained after 24 hr. immersion are lower than the correspond- ing densities in helium for all except two of the coals investigated. Pene- tration. of the pores by these liquids is slow and incomplete, and the ent of penetration varies with the rank of the coal. In the coals b, nd ¥, the liquids are almost completely excluded from the pores: the its-are exceptionally slow, and for coal p the apparent density in gehexane after 2 hr. immersion is still very mear the lump density. The Censities of the anthracites in n-hexane are much lower than in helium, but-comparison with the lump density shows that the hquids penetrate ee considerablé part of the pore space. The coals kK and mM, for which Re-apparent densities in -hexane are higher than in helium, are both law Mink coals of high porosity. It appears, therefore, that the accessibility SE the pores of coals to n-hexane and benzenc is at a minimum for coals pontaining between 89 % and 93 % carbon, and increases for coals of higher or of lower rank, being greatest for low rank coals of high porosity. Ehat. is, the accessibility is closely related to the porosity. _ w-Hexane and benzene readily wet the surfaces of coals, and the vis- sity of n-hexane is very low. The poor penctrating power of the liquids cannot, therefore, be attributed cither to a wetting angle greater than 90° BW ‘Stamm and Scborg, J. Physic. Chem., 1935, 30, 133. Stamm and Hanson, wM.,.1937, Al, 1007. “sDanforth and Devries, J. Amer. Chem. Soc., 1939, 61, 873. 284 FINE STRUCTURE OF CARBONACEOUS SOLIDS or to high viscosity, but must be associated rather with the relatively large size of the benzene and m-hexane molecules. The molecule of n-hexane is considerably larger than that of benzene, and, where the apparent densities of a coal in the two liquids differ appreciably, the n-hexane value is the lower. The above results therefore suggest that the width of the pores or. of constrictions in the pore system in coals is of the same order as the diameters of the molecules used for the density measurements. Thege. fine pores or constrictions are smallest in coals containing between 89 Of and 93 % carbon. In the coal p the pores or constrictions are so narrow: that even helium (molecular diameter 2 A) penetrates somewhat slowly’ a small density drift being observed. It may be noted that the low apparent densities in n-hexane or benzene are always associated with long, slow drifts. This was observed algg with other organic liquids of relatively large molecular size; even the heavy oil supplied for a Hyvac rotary pump showed a prolonged density, drift; indicating slow penetration. This behaviour may probably ‘be attributed to slow distortion of the coal substance by the spreading pressuté in the adsorbed films, and is in sharp contrast with that of carboniséd coals; the more rigid structure of the latter resists deformation by aids sorbed films at 25°C, and, if the molecules of a given liquid are too largé to enter the pores, then the liquid is totally excluded and there is no density drift.?° Apparent Densities in Water.—The apparent density in water 4s greater than the density in helium for coals of less than about 84 % carbdy! content. For high rank coals, on the other hand, it is intermediate between the values obtained with helium and x-hexane, being close‘ tg the low, #-hexane value for coals containing between 89 % and 93% carbon, and nearly equal to the helium value for the highest rank anthracite (Fig. 3). Thus, as in the case of m-hexane and beuzene, the accessibility of the pores and inner surfaces of coals to water is greatest for low rank coals of high porosity, passes through a minimum for coals containing between 89 % and 93 % carbon, and increases again with increasing rank among the anthracites. The failure of water to fill completely the pores of high rank codls cannot be due to the size of the water molecule, since the molecule of methanol is larger, and apparent densitics in methanol are high. Mores over, the absence of any density drift suggests that the cause of Low apparent densities in water is different from that of the low values: ob* tained with n-hexane and benzene. This difference is also revealed “by: the measurements made on samples of coals ¥ and K ground to various sizes (Table V). Finer grinding increases the apparent density of both coals in n-hexane, and the drift for coal ¥ is also increased. In water, on the other hand, the apparent density of coal F remains constant ‘and: that of coal K is increased, but in neither case is a density drift introduced. It seems, therefore, that the low apparent densities of high rank coals in water is an effect associated with their poor wettability. That ‘the external surfaces of coals p and ¥ (91-7 % and 89-7 % carbon respectively) are not easily wetted by water was shown when attempts were made't¢: measure their densities by immersing the sample (not previously evacuate | in the boiling liquid in a simple density bottle. The particles were 10! wetted even after prolonged boiling, and apparent densities as low.# o-7 g./em.? were recorded.* With coals c and x, on the other hand, the 20 Wranklin, Coal Res., 1946, p. 37. * Striking proof of the way in which poor wettability (due to a large angles contact) may influence apparent densities in water even after evacuation obtained with a sample of the commercial carbon black P33. The material, we evacuated at go°C for three days, after which time the particles adheredt8 one another, and the substance could be shaken about in the glass bulb in He Ot ROSALIND E. FRANKLIN 285 results agreed well with the apparent densities measured in water after evacuation. ; There is an apparent contradiction between these results and some later work by Maggs #4 who found that even high rank coals adsorb at saturation pressure, sufficient water vapour to fill all the pores. The discrepancy tnay perhaps be due to the fact that Maggs used water qapour whereas in the present work the coal was always placed in direct eontact with the liquid. High rank coals contain adsorbed methane, the last traces of which are difficult to remove, and it may be that water vapour can replace the gas molecule by molecule, whereas the presence of liquid water impedes its escape. Dimensions of the Pores in Coals.—Comparison of the porosity of coals as measured by King and Wilkins with the heat of wetting in qiethanol as measured by Griffith and Hirst suggests that the porosity ig. approximately proportional to the surface area, It follows that, for any given structural model, the mean diameter of the pores 1s approxim- ately the same in all coals. Measurements of the porosity of the coals 6D, F, H, and K confirm this observation. Assuming that 1 cal. of heat of wetting in methanol corresponds to 10 m.? of surface, then, if the pore space in coals consists in uniform, cylindrical, non-intersecting pores, #he mean pore diameter in all coals is about 4o A. Alternatively, if the solid consists of equally spaced cubes of coal substance, the distance of Separation between the cubes would be about 12 A. The dimensions of regions of continuous coal substance would, of course, vary with the titface arca of the coal. TE has been shown, however, that coals do not behave as if they all wontain. uniform pores whose size is independent of rank. The pores ‘ef coal containing between 89 % and 93 %, carbon are the least casily penetrated by liquids and among both low rank coals and anthracites rmeability increases with increasing porosity. Similar observations e-been made by Graham #2 who has pointed out that the permeability ‘methane and to moisture varies widely from one coal to another, the permeability of many anthracites and some low rank coals being low ‘yhereas other low rank coals and also the highest rank anthracite have a-high permeability. Suice the mean pore diameter in all coals is approximately equal while the permeability varies widely, it must be presumed that the pores are notuniform along their length but contain fine constrictions, and that accessibility of the pore space to liquids and gases is determined by size and frequency of these constrictions rather than by the mean ediameter, On this hypothesis, the pores are most highly constricted coals containing between 89 % and 93 % carbon, while low rank coals gh porosity have the most open pore structure, The configuration of the fine pores must be intimately related to the ‘am@-avhich the coal micelles are bound together, and the variation of structure with the rank of the coal must be associated with the n micellar structure which occur during coalification. If the dn surface area and porosity which occurs with increasing rank bout 90 % carbon be attributed to an increase in the size of the then the decreasing accessibility of the pores is in no way ex- ‘The results suggest, rather, that the changes in pore structure ‘ota hémispherical lump with a partly shiny surface. Its density in helium zeem.°, When attempts were made to measure the density in water oated, and approximate measurements gave values of about 0-8 g./cm.,%, fiat atmospheric pressure was not sufficient to force the water into the ited’ spaces between the loosely aggegrated particles. ond, Griffith and Maggs, Faraday Soc. Discussions, 1948, 3, 29. am, Conference on the Ultrafine Structure of Coals and Cokes (B.C.U.R.A,, 51. 286 THE KINETICS OF THE TRANSIENT STATE at this stage are due to a closer compacting of the coal micelles. In a later: paper a simple model based on this hypothesis is developed and is shown to be consistent with a wide range of experimental results in addition to those described above. The work was carried out at the British Coal Utilisation Research Association. The author wishes to thank Dr, D. H. Bangham and Dr, W. Hirst for much helpful discussion and advice. Labovatorie Central des Services Chimiques de I’ Etat, r2 quai Henri IV, Paris IV, Frasce.