886
R. I. RAZOUK AND R. SH. MIKHAIL
Vol. 61
But, if the surface is completely covered with specifically adsorbed chloride, further change of dz causes no change in q~-z or $lJ ie., equation 21 approaches the limiting case
It is therefore concluded that the existence of a linear Tafel region of slope 0.06-0.09, succeeded a t higher overpotentials by transition to a slope of about 0.11, for hydrogen evolution on metals having e.c.m. potentials in the region of low slope,,2 is qualitatively explained by the effect of d42/dAp4 and that the order of magnitude of the effects given by 15 and 21 are consistent with experiment. Using C z - b = 72pf. cm.-2for 0.1 NHCI, and the The Tafel slope on W and Ag (Fig. 8, Table IV) e.c.m. and Km-l as 53 pF cm.-2 (for 0.1 N HC1 at, are hence consistent (cf., eq. 15) with a rate-deterHg and the e.c.m.), ddz/dAd 0.7, which value mining proton discharge or electrochemical desorpsubstituted in eq. 15 gives b = 0.07 (cf., the experi- tion. The value of v (Table IV) of about unity mental 0.06). If the suggestion adduced of chlo- supports the view that the rate determining step is ride saturation adsorption is accepted, the weakest electrochemical desorption. numerical part of this calculation is the value of Acknowledgments.-I.A.A. wishes to thank the Km-1, which is taken as the same as that for a cor- Egyptian education committee, and A.K.M.S.H. responding system involving mercury. If K , - is the Pakistanian government for grants held during actually 1/2-53, dAp/dln& = 0.08; or if K,-I the work. is actually 2.53, dA+/dlni, = 0.05. These extreme (42) The e.c.p.m.p.’s on the N-H scale for Ag and W are, respecvalues of K,-1 hence give results for Tafel line tively, about -0.05 and -0.2 v.84 On this scale, the potential ranges slopes to within some 25% of the experimental in which the 0.06 slopes are observed lie, respectively, in the ranges values. -0.10 to -0.20 and -0.10 to -0.16 e.c.m.’s.
SURFACE PROPERTIES OF MAGNESIUM OXIDE BY R. I. RAZOUK AND R. SH. MIKHAIL Contribution from the Department of chemistry, Faculty of Science, Ain-Shams University, Cairo, Egypt Received November 18, 1966
An attempt to study some surface properties of magnesium oxide has been made by measuring the adsorption of n-hexane and cyclohexane on the products obtained by the dehydration of brucite in the presence of air and under vacuum. The effect of the temperature of dehydration as well as the duration of heating has been examined in each case. It is found that calcination in air for 5-24 hours gives rise to products which increase in surface area with rise of temperature of calcination until a maximum is reached with the product prepared at 500”. Further rise of temperature leads to a decrease in the surface area, which ultimately becomes very small when the calcinat,ion temperature is 1100”. Dehydration under vacuum for 5 hours gives rise to products of greater surface areas than the corresponding products obtained in the presence of air. The surface area changes very little up to a dehydration temperature of 650’ and then falls sensibly with further rise of temperature and becomes negligible at llOOo also. The effect of duration of heating is to decrease the area of the oxide prepared by calcination in air above 500’. When dehydration is under vacuum, the same effect is observed at the higher temperatures, but at lower temperatures an increase in the surface area with time is noticed as a direct consequence of further decomposition of brucite. The results have been discussed in the light of present knowledge of the sintering process.
Introduction Results on the calcination of magnesium hydroxide and carbonate, and on the surface properties of the produced oxide, do not seem to be in agreement, especially regarding the conditions of maximum activity of the product. This may be due to the use of precipitated hydroxide or carbonate , a s a parent substance, leading to an activity of the calcination product which depends to a great extent on details of preparation such as the nature of the starting materials, the introduction of foreign ions during precipitation, aging, particle size and so f0rth.l As is shown in the present investigation, the presence of air during calcination may also be an important factor in determining the surface properties of the product. I n most of the earlier work, a series of calcination experiments was performed in which the duration of heating was fixed arbitrarily and the calcination temperature increased in steps. However, it has been shown recently that the effect of temperature on the surface properties of active solids may not be (1) S. J. Gregg, “The Surfaoe Chemistry of Solids,” London, 1951.
merely quantitative, but also qualitative, so that a rise of temperature may influence the extent of sintering as well as its mechanism. A study of the activity of solids obtained by calcination a t a series of fixed temperatures and for varying lengths of time both in the presence and in absence of air would thus be of interest. The present work is an attempt to examine the effect of (i) the temperature of calcination, and (ii) the duration of heating at fixed temperatures, on the activity of magnesium oxide prepared by the dehydration of brucite in the presence of air and under vacuum. The calcination of magnesium carbonate and prepared hydroxide is being studied. I n the absence of a more quantitative measure of the activity of the oxide, the adsorption capacity of the solid to vapors of organic non-polar compounds such as n-hexane and cyclohexane has been determined. Experimental Materials.-The magnesium oxide used in the present investigation was prepared from a piece of brucite described (2) 9. J. Gregg, J. Chem. Soc., 3940 (1953).
SURFACE PROPERTIES OF MAGKESIUM OXIDE
July, 1957
in earlier work.8 The naturally occurring crystalline brucite was chosen as parent substance in order to eliminate as far as possible the influence of factors which may be introduced in the preparation of magnesium hydroxide in the laboratory, and because the kinetics of its thermal decomposition has already been ~ t u d i e d . ~ The n-hexane was prepared from a product supplied by the Shell Co., Cairo, and was purified by the method of Castille and Henri6; the fraction used had a b.p. 68.668.8' under 760 mm. The cyclohexane was prepared from a product supplied by Carlo Erba, Milano, and was fied by the methods of Hinrichsen and Kempfee and merman and Martin'; the b.p. of the fraction used was 80.880.9 ' under 760 mm Apparatus and Procedure.-The adsorption system used for the determination of the isot.herms was R volumetric one similar to that described by Razouk and Salem.* The bulb containing the specimen was made of silica in order to withstand the higher tem eratures of dehydration and was sealed to the Pyrex-male apparatus through a graded silicaPyrex joint. The system was placed in a thermostat kept a t 30 f 0.01'. All measurements of pressure were taken by means of a cathetometer reading to 0.02 mm., and were corrected for mercury depression and for temperature. Two series of experiments were carried out in which brucite was dehydrated either in the presence of air or under vacuum, and in each case the effect of the dehydration temperature and the duration of heating on the adsorption isotherm was studied. In the first series, a weighed specimen of brucite was enclosed in a bulb attached to ,a capillary tube fitted with a stopcock and a ground joint. The bulb was heated a t the required temperature, with the stopcock open to air, for 5 hours, care being taken to ensure that the required temperature was reached in one hour, the object being to standardize the technique as far as ossible. Then the stopcock was closed and the bulb weigKed again after cooling in order to determine the loss in weight. The bulb was next fixed to the adsorption system through the ground joint and adsorption isotherm was measured on the calcined product after being outgassed. For studying the effect of duration of heating, the brucite was "shockheated" under the same conditions by surrounding the bulb with a tubular furnace previously heated to the required temperature of dehydration, and the heating was continued for varying lengths of time. The product was finally "shock-cooled" by removing the furnace, and the adsorption isotherm determined. In the second series, the experiments were conducted in the same manner except that the brucite was dehydrated under vacuum.
887
0.06
A
0.04
2 v
ul 0.02
.
0
0
0
0
0
0
0.2 0.4 0.6 0.8 1.0
PlPo.
Fig. 1.-Adsorption isotherms of cyclohexane a t 30" on the products obtained by the calcination of brucite during 5 hours a t various temperatures.
I
I
I
40
40
I
I
I
I
I
b
400 600 800 1000 1200 Temp. of dehydration ('C.). Fig. 2.-Effect of temperature of calcination of brucite during 5 hours on (a) loss in weight, curve I ; (b) specific surface area of product, curve 11; (c) specific surface area of MgO, Curve 111. 0
200
calcination until a maximum is reached with the product obtained a t 500°,and the adsorption then falls progressively with further rise of temperature. The temperature of maximum activity is slightly higher than that obtained by Gregg and Packerlo Results and Discussion using precipitated magnesium hydroxide as the I. Calcination in Air.-Preliminary experiments parent substance, whereas the surface area of showed that slight changes in the conditions of cal- their oxides is much higher than that of dehydrated cinabion influence the surface properties of the brucite. product and, therefore, these conditions were The surface area of the various products has standardized in the manner described above. been estimated from the B-point" of the isotherms, (i) Effect of Temperature of Calcination.taking the molecular area of cyclohexane as 39 The results of experiments on the adsorption of 8.2,12 and the values obtained are plotted in Fig. 2, cyclohexane on six specimens of brucite calcined curve 11,as a function of the dehydration temperain the presence of air for 5 hours at 350, 500, 650, ture. The estimates of the surface area are found 800, 950 and 1100" are shown in Fig. 1. The iso- to agree with the average values obtained by the therms are Type I1 of the Brunauer classificationg method of Brunauer, Emmett and Teller,13 whenand the adsorption seems to be purely physical in ever their equation is obeyed by the experimental nature since outgassing a t room temperature data, and by the method of Gregg.14 As calcination brings about complete removal of the adsorbate. in air for 5 hours a t temperatures below 650" does The isotherms indicate that the adsorptive capacity not bring about complete dehydration of brucite, of the product increases with rise of temperature of the percentage loss of water a t the various temperatures of dehydration is also inserted in Fig. 2, (3) R. I. Razouk and R. Sh. Mikhail, THIS JOURNAL, 69, 636 curve I. It is evident that the surface area of the (1955). (4) 8. J. Gregg and R. I. Razouk, J . Chem. Xoc., S 1, 36 (1949). products prepared below 650" will increase if (5) A. Castille and V. Henri, Bull. soc. chim. biol., 6 , 299 (1924). (6) F.W.Hinrichsen and R. Kempfe, Ber., 46, 2106 (1912). (7) J. Timmerman and F. J. Martin, J . chim. phgs., 23, 733 (1926). (8) R. I. Razouk and A. Sh. Salem, THISJOURNAL, 62, 1208 (1948). (9) 8. Brunauer, "Physical Adsorption of Gases and Vapors," Oxford Press, New York, N. Y., 1945.
(10) 9. J. Gregg and R. K. Packer, J . Chem. Soc., 51 (1955). (11) 8. Brunauer and P. H. Emmett, J . A m . Chsm. Soc., 59, 1553, 2682 (1937). (12) N. Smith, C.Pierce and H. Cordes, ibid., 72, 5595 (1950). (13) S. Brunauer, P. H. Epmett and E. Teller, kbid., 60, 309 (1938). (14) 8. J. Gregg, J. Chsm. Soc., 696 (1942).
R. I. RAZOUK AND R. SH. MIKHAIL
888
Vol. 6 1
referred to the weight of oxide actually present and piece of brucite, originally calcined in air a t 500' not to the weight of the calcination product, yet for 3.5 hours, was evacuated a t the same temperathis increase does not lead to the disappearance of ture for 5 hours, and the surface area was found to the maximum surface area but simply flattens the increase from 53.6to (30.1 m2./g., an increase which peak of the curve as is shown in Fig. 2, curve 111. corresponds closely to the loss of water accomThe same behavior was observed with products panying the evacuation and which amounted to formed by calcination at various temperatures 3.8%. On the other hand, the surface area of an during periods reaching up t o 24 hours and a active oxide of magnesium prepared by the demaximum surface area was always obtained when hydration of brucite under vacuum at 500" for 5 the dehydration temperature was 500". hours is reduced from 114 to 60.6 m2./g. on being It was thought useful to check the values of the heated in an air at the same temperature for 3.5 surface area as measured from the adsorption of hours. Both experiments point definitely to the cyclohexane with a standard method of surface permanent injury of the surface during calcination area determination. Such measurements were or heating in the presence of air. kindly undertaken by Mr. M. Perrin in Professor (ii) Effect of Time of Calcination.-Owing to M. Prettre's laboratory in Lyon (France) on speci- the complications which arise from the presence of mens prepared under almost identical conditions, air, the study of the effect of duration of heating using the BET method for the low temperature on the adsorptive properties of the calcination adsorption of nitrogen.16 Comparison of the products was confined mainly to the product of results obtained with cyclohexane and from low maximum surface area obtained by calcination at temperature nitrogen adsorption is shown in Table 500". Figure 3 shows the isotherms of cyclohexane on the products prepared by calcination durI; the agreement is seen to be satisfactory. ing various intervals of time and also the percentTABLE I age loss in weight in each case as a function of the PRODUCTS OF BRUCITE duration of heating. The calculated surface areas SURFACE AREA OF CALCINATION Exot. with Expt. with are given in Table 11. Temp. of calcination, OC.
25 350 500 650 800 950 1100
cydiohexane Surface area b2/gJ
Loss in wt., % 0
8.8 27.1 30.0 30.9 30.9 30.9
0 8.7 3G. 1 21.8 12.8 10.0 3.0
nitrogen Surface area % (m.*/g.)
Loss in wt.,
0
10.4 28.2 30.7 30.9 30.9 30.9
1.0 7.5 37.0 28.0 18.0 12.0 G.0
TABLE I1 EFFECT OF TIME ON THE SURFACE AREA Calcination temperature 500" Time, hr. 0.5 1 2 3.5 5 % ' decomposition 81.9 8 3 . 8 8 6 . 7 87.4 87.7 Surface area (m.*/g.) 93 75.7 5 8 . 2 5 3 . 6 36.1
'
Inspection of Fig. 1 shows further that in the neighborhood of saturation, the adsorption increases sharply indicating the occurrence of appreciable capillarv condensation. It is of interest also t o note that the monolayer capacity represents only a small fraction of the amount of hydrocarbon taken up by capillary condensation, and that this amount depends very little on the temperature of calcination. Comparison with the results of experiments on magnesium oxide prepared by the dehydration of brucite under vacuum (vide infra) shows that calcination in air gives rise to products possessing surface areas smaller than the corresponding areas of the oxides formed under vacuum. This reduction in surface area seems to be a direct consequence of the onset of sintering rather than the result of an adsorbed film of air or one of its constituents formed at the surface of the oxide during or after calcination; for the physical adsorption of oxygen and nitrogen is negligible a t the temperature of the experiment and a chemisorbed film of oxygen on magnesium oxide, which is unlikely, would probably not affect the surface area. I n agreement with this, it has been found that continued outgassing of the calcination product results in a slight change of the surface area as a consequence of further decomposition. Thus in a typical experiment, a
It is clear that the maximum activity is obtained under these conditions with the product obtained by calcination during half an hour or less, as compared with about 4 hours observed by Gregg, Packer and Wheatley16using precipitated magnesium hydroxide calcined a t 400". It is to be noted that this behavior differs from that of the oxide formed by dehydration under vacuum a t the same temperature (vide infra) where the surface area increases slightly with time of heating as a result of further decomposition. It seems thus that the contact of air at 500" brings about appreciable sintering and a consequent rapid fall of the adsorption capacity, whereas in the dehydration under vacuum a t the same temperature no sintering occurs. 11. Dehydration under Vacuum. (i) Effect of Temperature of Dehydration.-The adsorption isotherms of cyclohexane and n-hexane on seven specimens of magnesium oxide prepared by the dehydration of brucite at 350, 500, 650,800, 950, 1020 and 1100' for 5 hours have been determined. The isotherms are also Type I1 of the Brunauer classification9 and there occurs slight hysteresis which continues to low pressures, though evacuation a t room temperature brings about complete removal of the adsorbate, as is shown in Fig. 4 for cyclohexane. The presence of this hysteresis may be explained in terms of the generalized domain
(15) The lack of a regular supply of liquid nitrogen in Cairo a t the present monient made such measurements materially impossible.
46 (1955).
(16) 9. J . Gregg, R . K. Packer and K. H. Wheatley, J . Chem. SOC..
,
I
SURFACE PROPERTIES OF MAGNESIUM OXIDE
July, 1957
889
Time (min.). theory developed by Everett, et al.,17 in which 80 160 240 320 the phenomenon is attributed to the existence of a 0.08 large number oi independent domains some of which can exist in metastable states, and to the persistence of these metastable states in the change of either one physical state to the other or in both 0.06 changes. It is probable that in the dehydration of brucite, the magnesium oxide formed possesses a highly strained lattice as a result of the formation M 0.04 of a new phase having a different molar volume and 2 v a different crystalline structure, and that the do- a mains may be closely related to this state of strain 0.02 as well as to the cracks and faults in the structure of the parent substance. The surface areas of the various products of dehydration have been estimated by the method described above, taking the molecular area of 0 0.2 0.4 0.G 0.8 1.0 cvclohexane and of n-hexane (PIPO). respectively, and the values obtained are given' in Fig. 3.-, adsorption isotherms of cyclohexane a t 30" Table 111. on the products obtained by the calcination of brucite a t 500' for varying intervals of time: 0 , loss in weight as TABLE rII a function of length of time of calcination. SURFACE AREA OF BRUCITEDEHYDRATED UNDER VACUUM h
FOR
5 HOURS
Temp. of dehydration, "C. 350 500 650 800 950 1020 1100 14 3 (cyclohexane) 119 114 109 75 28 16 3 Surface (m,*/g.) (n-hexane) 124 115 108 74 31
{
Table 111shows that the specific surface areas of the different dehydration products evaluated from the adsorption of cyclohexane and n-hexane are in close agreement. Reference has already been made to the agreement between the values obtained from measurements of the adsorption of the former and by the BET method of low temperature nitrogen adsorption. It is evident that the oxide prepared a t the lowest temperature, viz., 350" possesses the maximum surface area and is thus the most active, whereas the temperature of maximum activity is 500" for the products of calcination for intervals varying between 5 and 24 hours in the presence of air. The activity is little affected by rise of the temperature of dehydration under vacuum until 650" is reached when the surface area of the oxide begins to diminish sensibly with increase of temperature and ultimately t,he oxide obtained by dehydration a t 1100" possesses a very small surface area. The variation of the specific area of the different oxides with dehydration temperature is in qualitative agreement with the behavior of dehydrated brucite on sorption of water vapor. For it has been found by the authorsa that the oxides obtained by dehydration of brucite under vacuum at temperatures below 650" take up water stoichiometrically a t saturation vapor pressure, but that the equilibrium sorption values a t saturation fall rapidly as the temperature of dehydration is raised, becoming almost nil for the oxide dehydrated a t 1100". However, no direct relation has been found between the estimated surface area and the water sorbed a t saturation, or between the area and the amount of water removed by or retained after evacuation&. room temperature. (17) D.11. Everett and W. I. Whitten. Trans. Faraday Soc., 48,749 (1952); D. H.Everett and F. W. Smith, kbid., 60, 187 (1954). (18) F. A. P. Maggs, Proceedings of a Conference on the Ultra-fine Struotirre of Coals and Cokes, B.C.U.R.A., London, 1944, p. 95.
0.08
c
0
Y 7 7
0
0
0 0 0.2 0.4 0.6 0.8 1.0 PIPO. Fig. 4.*, adsorption isotherms of cyclohexane a t 30' on the products obtained by the dehydration of brucite under vacuum during 5 hours a t various temperatures: I, 350; 11, 500; III! 650; IV, 800; V, 050; VI, 1020; VII, 1100O; 0 , desorption isotherms. 0
0
(ii) Effect of Time of Heating.-The effect of the duration of the heat treatment under vacuum on the adsorption of cyclohexane by magnesium oxide has been studied on specimens dehydrated a t various temperatures and for intervals of time varying from 0.5 to 15 hours. When dehydration takes place at 1100 or 950°, the adsorption capacity falls appreciably witth increase of time of heating, but a limiting isotherm is obtained by heating brucite for 5 hours. This isotherm is characteristic of the dehydration temperature and further heat treatment for 15 hours a t the same temperature does not produce any effect. A similar behavior has been observed with the other oxides, although a t certain temperatures of dehydration a shorter time is required for attaining the limiting isotherm. This explains the arbitrary choice of heating brucite for 5 hours under vacuum in studying the effect of the temperature of dehydration described above. It is worth mentioning that the surface area of magnesium oxide prepared by the calcination of precipitated hydroxide has been found by Gregg, Packer and Wheatley16 to diminish continuously with increase of time of heating over a period of more than 300 hours. This disagreement may have its origin in the use of different parent materials and in the varying conditions of calcination. Indeed, it has been found recently in this
R. I. RAZOUK AND R. SH. MIKHAIL
890
Laboratory that in the calcination of precipitated magnesium hydroxide, the time of heating influences the adsorptive properties of the oxide over a period exceeding 5 hours though still far less than 300 hours. The same effect is noticed in the case of magnesium oxide prepared by dehydration at 800" but only to a much less extent. Heating for 0.5,2 and 5 hours gives rise to isotherms which almost coincide up to a relative vapor pressure of about 0.2 and then deviate slightly a t higher pressures so that a fall in adsorption accompanies the increase in the time of heating. Magnesium oxide prepared by dehydration a t 650" presents a special and critical case, for the duration of heating has no effect on the adsorption isotherms on specimens heated for periods varying between 0.5 and 15 hours. Heating under vacuum for 0.5 hour at this temperature is sufficient to bring about complete dehydration of brucite. At lower temperatures, however, brucite is not completely dehydrated in this interval, and it became necessary to determine the amount decomposed in investigating the effect of time on the surface area of the dehydration product. Thus "shock-heating" at 500" for 0.5 hour leads to a loss of 29.1% of water (as compared with the theoretical loss of 30.9y0) and the adsorption isotherm falls slightly below that obtained with oxides prepared by heating for longer periods. However, when the surface area is calculated and corrected for incomplete dehydration, the same value is obtained as with longer periods of dehydration.
r;
I
sb
100 v
50
m5
P
100 200 300 Time (min.). Fig. 5.-Effect of time of heating on the specific surface area of the products of dehydration of brucite under vacuiim a t various temperatures: I, 350; oII, 500; 111, 650; IV, 800; V, 950; VI, 1020; VII, 1100 0
.
The results for the products prepared by dehydration under vacuum a t 350" are still more interesting. The adsorption isotherms of cyclohexane on specimens of .brucite dehydrated a t this temperature for intervals of 0.5, 1.5 and 5 hours and having lost 6.0, 17.6 and 30.9% of water, respectively, have been determined. The estimated specific surface areas of the three specimens when referred to the weight, of the dehydration product were found to be 23, 64 and 119 m.2/g., respectively, whereas when referred to the weight of magnesium oxide actually present they become 119, 113 and 119 me2/g.,respectively, indicating that the
Vol. 61
development of the surface is a direct consequence of the decomposition of the hydroxide. A summary of the estimated values of the surface area of the products obtained by the dehydration of brucite a t different temperatures and for varying intervals of time is shown in Fig. 5 . General Discussion.-The activity of solids, which reveals itself in such properties as the adsorptive capacity, has been the subject of numerous investigations and has been discussed lately by It is generally believed that the Gregg, et u1.z~10~16~19 phenomena associated with surface activity are primarily determined by the presence of extensive internal surface area in the solid and the existence of lattice strains in crystalline adsorbents. The area may originate from macro-defects such as cracks or faults resulting from crystallites which are slightly disoriented with reference to one another thus leaving fissures which create the internal surface. I n addition, there exist microdefects which exercise perturbations of the order of the unit cell in which the periodic structure is reestablished a t a distance of a few lattice parameters and which include holes or vacant sites, interstitial atoms or ions, ions in heteropolar lattice with normal position but anomalous charge, foreign atoms, and the like. These defects deform a region of the lattice which will normally become associated with higher potential energy and subsequently with greater reactivity of the solid. In the dehydration of brucite, the original substance possesses a very small surface area and has a crystalline structure distinct from that of the oxide formed. But during this process a pseudo-form is first obtained in which the ions of Mg++ and 0-appear to occupy the same positions that they have in brucite. This pseudo-form is highly strained and tends to crystallize out in the normal lattice of the oxide thus giving rise to small crystallites or micelles. Macro-defects soon develop as a result of the difference in specific volume between the two structures as well as from the disruptive effect of the escaping water vapor during the dehydration process, and a big internal surface is thus formed. Moreover, micro-defects exist also as a consequence of stoichiometric excess of oxygen in the oxide, which has been proved by Mansfield,zO as well as of other perturbing effects. When these micelles come in contact with one another, a process of micellar growth may take place which will result in a n increase in their size and a subsequent decrease in their number and in the surface area of the oxide. Such a process is usually described as sintering if it occurs a t high temperatures. The extent of sintering increases with rise of temperature and with duration of heating. According to Huttig's viewsz1 which have been verified by others, sintering is caused by three processes, vix., surface adhesion, surface diffusion and lattice diffusion. The first process may occur e v p at room temperature a t the points of contact (19) S. J. Gregg and K. J. Hill, J . (Ihem. h'oe., 3945 (1953); S. J. Gregg and M. J. Stephens, ibid., 3951 (1953); S. J. Gregg and R. K. Packer,ibid., 3887 (1954). (20) R. Mansfield, Proo. Phgs. Soc., 8 6 6 , 612 (1053). (21) G. F. Htittig, Kolloid-Z., 98,263 (1942); 99, 262 (1942).
*
t
July, 1957
MASSSPECTRUM OF NEOPENTANE
of neighboring micelles. The shearing stress exerted by the micelles near the region of contact leads to the deformation of the micelles and an increase in the area of contact over which adhesion occurs. As rise of temperature reduces the rigidity of solids, adhesion will increase with rise of temperature and so also sintering. This process seems to have very little effect on the surface properties of magnesium oxide prepared by the dehydration of brucite under vacuum a t temperatures below 660" since there is almost no change in the specific surface area of the oxide with increase of time of heating, though the process is probably effective during calcination in the presence of air. On the other hand, surface diffusion resulting from the mobility of the ions of the solid a t its surface becomes sensible a t temperatures about one-third of the m.p. of the solid, while lattice diffusion where the ions can move through the bulk of the micelles occurs only at temperatures above half the m.p. The last effect probably does not play an important part in the present work as the highest temperature used is 1100" which corresponds to 46% of the m.p. Thus in the dehydration of brucite under vacuum
89 1
a t lower temperatures, the development of the surface area of the product is a direct consequence of the loss of.water from the hydroxide, and the time of dehydration will only influence the surface area of the product in as much as the amount of hydroxide decomposed is a function of time. The oxides prepared above 650" will, however, undergo increased sintering with rise of temperature and prolonged heating. The effect of increase of time will be quantitative but that of temperature rise will be qualitative as well, since this will produce different effects on the three factors which determine the rate of sintering. It is important to note that the effect of time and of temperature are so interrelated that increasing the time will not cause a continued decrease of the surface area indefinitely, but that each temperature of dehydration is characterized by a limiting surface area, and further experiments are required before understanding the relation between the rate of sintering and temp eratur e. Acknowledgment.-The authors wish to thank Professor M. Prettre and Mr. M. Perrin for their measurements of the low temperature nitrogen adsorption on calcined brucite.
REARRANGEMENT PEAKS IN THE MASS SPECTRUM OF NEOPENTANE TERMINALLY LABELED WITH ONE C13 BY ALOISLANGERAND C. PETERJOHNSON Contribution from the Chemistry Department, Research Laboratories, Westinghouse Electric Gorp., Pittsburgh, Pa.' Received August 0, 196%
Rearrangement peaks were observed in the mass spectrum of neopentane labeled in one methyl group with 0 8 . For the three- and four-carbon fragments, the results indicate they could be formed in the expected manner of carbon-to-carbon bond scission. I n the two-carbon fragments, however, the experiments seem to lead to the conclusion that the fragments result from an almost statistical combination of any two carbon atoms of the molecule.
Introduction When polyatomic molecules dissociate under electron impact in a mass spectrometer, a number of charged fragments generally are observed. The formation of the majority of these ions can be ascribed to the scission of bonds in the molecule, but in certain cases ions have masses, such t,hat their origin must in addition involve some sort of intramolecular rearrangement. Far from being infrequent, such fragments often contribute to the most prominent peaks in the mass spectrum. Suggestions have been presented2 as to the mechanism by which they might be formed. To clarify the means of formation of its so-called rearrangement peak, Honig3 labeled isobutane with carbon-13 and came to the conclusion that the peak could be explained through isomerization of the molecule. However, in isobutane a unique distinction could not be made between this and other possible mechanisms. The present investigation follows the same method. By using neopentane labeled with C13 (1) Paper presented at American Chemical Society Meeting, Chicago, Illinois, 1950. (2) A. Langer, THIS JOURNAL, 54, 618 (1950). (3) R. E. Honig. Bull. A m . Phy8. SOC.,3 4 , R12 (1949).
in one of the four methyl groups, a number of high intensit,y rearrangement peaks can be investigated, and because of the high order of symmetry of the neopentane molecule, a study of the isotopic mass distribution in these peaks allowed a characterization of the prevailing mechanism. Preparation of the Labeled Neopentane.-The terminally C13-labeled neopentane (neopentane-l-CI3) was prepared by allowing C*3-labeled dimethylzinc to react with t-butyl chloride in toluene to form a reaction complex, which was in turn decomposed by water to form labeled neopentane. The dimethylzinc was made by allowing approximately 50% Cia-labeled methyl iodide (Eastman Kodak) to react with a 96% Zn-4% Cu alloy under reflux a t 47' for 48 hours in an all-glass micro apparatus filled with carbon dioxide, then slowly heating the methylzinc iodide to 180' over a period of several hours to decompose it, and finally distilling the resultant dimethylzinc into the reaction vessel. A solution of t-butyl chloride in toluene was then added at Oo, and reaction was allowed to proceed at that temperature for 10 hours. The product was warmed to 50" for 1 hour, and then decomposed with water added dropwise. The neopentane formed was passed through concentrated sulfuric acid, water, 40% potassium hydroxide solution, magnesium perchlorate and silica gel and then condensed by an acetone-Dry Ice-bath. The gas was further purified by several transfers between evacuated flasks kept successively in Dry Ice. On analysis in the mass spectrometer, a small trace of methyl iodide (less than 0.1%) was found as the only detectable impurity.
