iii) Carbonyl Overtone.-Since carbonyl groups produce strong absorption bands in the region 1780-1620 cni. -I, weak overtone bands may be expected a t approximately twice this frequency range, D~z., 3560-3240 ern.--'. Such bands overlap the regions of the OH and NH stretching bands and may confuse the interpretation of weak bands in this region, though no cases of this have yet been encountered. (iii) Bands between 1640 and 1550 Cm.-l.Bands of moderate intensity are frequently observed in the region 1640-1550 cm.-' (see Table 111). Such bands generally indicate unsaturation in the molecule (C=C, C=N stretching vibrations), though there is the possibility that they could be caused by NH bending vibration^.^ Phenyl rings generally give absorption bands in the ranges 1640-1615 or 1605-1600 cm.-' while alkaloids containing aromatic nitrogen rings (e. g. pyridine, quinoline rings) generally give bands between 1600 and 1560 cm.-l. Thus the bands a t 1592 and 1376 cm.-' present in the spectrum of anabasine but absent froni the spectrum of piperidine (see Fig. 1) may be assigned to the pyridine ring. Similarly the band between 1570 and 1566 cm.-' found in the spectra of cytisine, S-methylcytisine, thermopsine and anagyrine may be assigned to the a-pyridone ring (11). The observation of bands in this region gives an indication of the types of structure which might be present but does not permit an unambiguous choice to be made owing to the multiple interpretations possible izliz., cevine and jervine). (iv) Bands at Lower Frequencies-The spectra included in this survey do not extend beyond 1340 cni.-' due to the strong absorptions of the chloroform. Satisfactory spectra are obtained, however, by using Nujol mulls2aor thin films of the alkaloids. X great deal of complex structure is , as yet found in the region 1500-GO0 c i i i . ~ ~and the assignment of bands in this region to specific groups in the molecule is comparatively untle-
[CONTRIBUTION FROM
THE
veloped. The chief value of such spectra lies in this high specificity, which offers a very delicate method for the identification of individual alkaloids. Thus they have been used to confirm the identity of monolupine, rhombinine and anagyrine. Acknowledgments.-The authors wish to thank Mr. li. Lauzon for technical assistance in the rneasurenient of the spectra. Summary 1. The infrared absorption spectra of fortyseven alkaloids have been investigated in the region 3700-1 340 cm.-' in dilute chloroform solution. 2. Hydroxyl groups may generally be detected by the appearance of a sharp band in the region 3625-3540 cm. -l with an apparent molecular extinction coefficient of 30-160 though there are exceptions. 3. Imino groups produce a sharp band in the region 3480-3440 cm.-' with an apparent molecular extinction coefficient of 100-210 when the imino group is present in an indole nucleus or secondary amido group. When the imino group is present in a piperidine ring, however, the absorption is much weaker but may be observed a t increased path lengths. 4. Carbonyl groups may be detected by the appearance of a strong band in the region 17801620 cm.-l with an apparent molecular extinction coefficient from 300-1200. Certain correlations have been established between the carbonyl frequency and the type of carbonyl group present. 5 . Phenyl rings generally give absorption bands in the region 1640-1600 ern.-' while aromatic nitrogen rings generally give bands between 1600 and 1560 ern.-'. fi. The presence of a carbonyl group both in ceviiie and jervine is indicated by the infrared absorption spectra. (11) Marion and Ouellet, THISJOURNAL, TO, 3076 (1948). OTTAWA, CANADA
RECEIVED MAP 27, 1950
DEPARTMEST OF CHEMISTRY, ITNIVERSITP OF MICHIGAN]
Energetics of the Adsorption of Aliphatic Amines by Silica Gel1 R Y DONALD G. DO RAY,^ YINGF u
In a previous communication3 adsorption isotherms of diethylamine, n-butylamine and di-nbutylamine on silica gel a t 25' were presented and their significance as relating to the nature of the adsorption process discussed. In the present paper these adsorption systems will be considered from the basis of energy changes. _ _ Isosteric Heats, A H , AF and of Adsorption. -In order to determine the isosteric heats, adsorption isotherms for these amines at 40" were
as
(1) A portion of the material in this paper is from a dissertation submitted to the Horace H. Rackham School of Graduate Studies by Donald G. Dobay in partial fulfillment of the requirement- for the rleFree of Doctor of Philosophy, Septemher, 1948. ( 2 ) Linde Air Products Company, Tonawanda, N Y. ( 3 ) Bartell and Dobay, THISJ O U R N A L , 72, 4388 (19601
AND
F. E. BARTELL
obtained and the pressure range employed was from zero to a value higher than t h a t corresponding to the inception of the hysteresis effect. These data are given in the first column of Table I. The isosteric heats were calculated by means of the Clapeyron equation d Q l d ( x ' ~ )= RTiTZlog, (p,lpJ,'(Ti - T?)
(1)
where dQ/d(x/m) represents the differential isosteric heats of adsorption and the p-values represent the pressures a t which equal amounts of the amines were adsorbed by the solid at the two different temperatures. This method of calculating the isosteric heats requires that the surface area of the solid be considered as being independent of the temperature. While this is not strictly true, the
ENERGETICS OF THE ADSORPTION OF ALIPHATICAMINESBY SILICA GEL
Jan., 1951
TABLE I _ _ ISOSTERIC HEATS, AH,A F AND ii3OF ADSORPTION OF AMINEVAPORS DIFFERENTIAL (ALL I N KILOCALORIES/MOLE) PI/PI, (40")
x/m mdg.
PZ/P?, (250)
d
dQ
O.OOLi3 0041 ,062.0 ,125 ,229 297 ,339 ,493 ,519 .572
38.1 37.2 22.4 15.21 11.34 10.03 9.58 8.71 8.57 8.36
0.014 ,068 ,191 ,468 ,490 ,800
18.50 17.92 13.26 9.31 9.22 9.07
0.048 ,125 ,298
o.nx
,471
.45"
,608
.5g" ,703
25.8 13.9 14,4 12.8 12.6 12.4
81.7 86.4 95.5 100.8 108.2 114.2 118.4 142.4 148.2 163.3
0.020 ,048
118.9 127,s
0.0315 ,146 ,281 ,500
.170 ,224 .307 ,358 .394 ,534 ,556 ,602
-
o
-AH
Diethylamine 30.4 29.5 14.7 7.51 3.64 2.33 1.88 1.01 0.87 0.66
-
~-
BY
300
SILICAGEL.
- APZ,O
- AFmo
- 43,,&
- A.7, h
3 .7ti 3.21 1.75 1.24 0.87 ,72 .64 .42 ,39 .33
2 43 1 89 1.10 0 93
89 . 3 87.1 43.2 21.0 9.3 5.5 4,2 2.0
XX.7
2.52 1.59 0.98 .45 ,422 ,302
2.15 1.20 0.79 .43 ,406 ,287
.73 .64
.58 .39 ,365 ,315
1.1
87.0 43.3 20.8 9.2 L5. 3 4.1 1.9 1.5 1.0
25.2 26.4 12.8 1.4 1.2 1.1
24.6 26.0 12.7 1.3 1.1 1.o
37.0 1.0 4.3 0.1 -0.04 -0.2
37.3 1.3 4.5 0.3 0.4 -0.2
1.6
%-Butylamine
137.5 157.2 160 2 182 5
? nnd AF do not actually become zero but assume very sinall values can be easily explained by assum-
and that there was no chemical reaction between the adsorbate and the adsorbent. Since this leads to impossible values for the entropy change, the only possible explanation is that the actual process does not correspond to that postulated. It must logically follow, then, that a chemical reaction was taking place along with the physical adsorption which caused the energy values obtained, especially the g? value, to be so high. These unusually high values may. then, be considered as conclusive evidence for the chemical nature of the adsorption in these systems. In the previous paper3 i t was suggested that the BET constant n represents not the total number of layers in the capillaries but the total number of adsorbed layers, i. e., total number of layers minus the layers due to condensation. That being the case, the energy and entropy changes after 2 layers have been adsorbed should have very low values as the n values for these systems were found to be
A s
ENERGETICS OF THE ADSORPTION OF ALIPHATICAMINESBY SILICA GEL
Jan., 1951
about 2. From the monolayer values for these amines, the amount of adsorbates in the first two layers were deduced to be approximately as 180 mg./g. 220 mg./g. 250 mg./g.
Diethyl n-Butyl Di-n-butyl
Inspection of the curves in Figs. 1 and 2 shows this is the case. In a system consisting of b bulk and s surface phases the variation of the total energy is given by
+
ThdSbb
PbdVb b
n1.9
prhdn," b
+
+
+ cc P,'d?t,' l
Y9dU~
s
s
(4)
1
wliere the symbols T , P, V , .?, y,a, p and n are, respectively, temperature, pressure, volume, entropy, surface tension, surface area, chemical POtential and number of moles, &, Z,, and 2 , signify summation over all bulk phases, components and surface phases. From this i t follows that with a single vapor in equilibrium with an adsorbent a t constant temperature the familiar Gibbs equation becomes -n
JasdYs = 2 , J n d d p a = f n d r
(5)
as the chemical potentials in the different phases are equal and each is equal to the potential in the vapor phase, and 2,ns = n, the total number of nioles of vapor adsorbed by the solid. The above equation can be utilized for the determination of the free surface energy decrease per unit area when the solid is wetted by a liquid. First, consider the solid as a non-porous substance of area a,. If the temperature is much below the critical temperature and the pressure is suf. ficiently high, the adsorbed vapor will then condense into liquid covering the entire solid surface, provided there is no contact angle between the solid and the liquid. If the surface tension of the condensed layer is the same as that in the bulk phase. then the total change in free surface energy is -Q.(Y,I TI, - Y 8 d = JndiJ or
+
-Pel
+
71"
- yso) = Sndw'a,
=
Srdr
(6)
where r is the surface excess per unit area and the subscripts sl, so and lv denote the solid-liquid, solid-vacuum and liquid-vapor interfaces, respectively. From this relationship the adhesion tension of the solid against the liquid, defined by A = Y,, - Y&
is then given by A =
fr+
+
the determination of adhesion tension, because = a,, and whatever area of liquid-vapor interface that may have been created during the filling process, will have disappeared completely a t the saturation pressure when the capillaries become filled as the external area of the solid is negligible compared with the internal. The final result, then, is
a,I
--(LB(YOI
-
73")
- aY1\
+
nlvYlv
= -as(Yd
- Y,,,)
= Jndp
Free Surface Energy Changes
tlE =
311
(7)
The same method can be applied to porous solids. One of the differences between the adsorption by a non-porous and by a porous solid is that in the latter case the area of the adsorbing surface is continuously decreasing during the process, hence n/a, no longer represents the surface excesses. Furthermore, in the capillaries liquidvapor interfaces were created and were destroyed during the progress of adsorption. These differences, however, will not cause any complication in
or '1 = Y,,, - Y,I = f iKiiJjU5
(8)
Thus the determination of the adhesion tension of a porous solid is even simpler than that for nonporous solids. I t is only when one wishes to compare the results obtained in the intermediate pressure ranges for these two kinds of adsorbents the above mentioned differences will cause any cornplication. I n order to simplify the calculation and interpretation, the following hypothetical process may be substituted for the actual process. Let the porous solid be spread out into a planc surface of the same area as that on which adsorption takes place until a layer of the condensed liquid appears. Assume that the surface tension of the condensed layer decreases with increasing adsorption and that it reaches zero value a t the maximum adsorption, i. e., when the pressure reaches the saturation value. As far as the total energy effects are concerned, the actual and the hypothetical processes are equivalent as the following comparison will immediately show : The actual case: -uS(Ysi - Yea) - 071, = the hypothetical case. -uS(Y,1 - Y,,J - u,O
-(13(Y3i
- Ys,,)
= --ch(YsI
- Y,,J
The advantages of the hypothetical process arc that it is not necessary to know how the area of the adsorbing surface changes with adsorption, and a single integration is sufficient to evaluate the energy changes, because in this process the change in the xvailable surface energy is, by hypothesis, solely due to the decrease of interfacial tensions. The individual values of the integral J r d p a t different pressures will then correspond to the energy changes per unit area. If one wishes to convert these hypothetical into the actual values, it can be accomplished by the method indicated in a later section. I n order to evaluate JI'dk a graphical method was used in conjunction with the usual simplifying assumptions, i. e . , identifying dp with RT d log. p , and r with xlma,, a, being 320 m.?/g. in the present case. Both of these simplifications will undoubtedly introduce some inaccuracies into the calculations, but it is believed that they will not be serious, as the pressures are fairly low, the highest being 23s mm. for diethylamine. The results of these calculations are given in Table I1 and plotted vs. relative pressures in Figs. 3 and 4, the latter being the log-log plot of Fig. 3. From these calculations the adhesion tensions of silica gel against the three amines a t 25' are found to be Diethylamine L)i-n-butyl,iiniiic n-Butylamine
100 ergs/sq cm 91 c r g s j q ciii 119 ergs/sq
ctii
DONALD G. DOBAY, YINGFu
312
TABLEI1 FREE SURFACEENERGYCHANGES OK Sios GEL Diethylamine v
x ioio
(moles, sq. cm )
3.89 3 98 4 02 4 07 I 10 4 27 4 36 4 53 4.i o 4 91 5 09 6 38 3 73 6 I5 t i 70 7 39 8 55 10 5 13 2 20 9 31 0 35.2 35 4 3,5 5
- 2.7
iergsj sq. cm.)
AT
~
31.9 38.3 42.5 4-5.0 47.5 34.8 59.1 62.3 64.4 67.0 68.8 70.6 72.3 73.8
3.09 3.11 3.16 3 . 18 3.21 3.38 3.55 3.72 3.88 4.10 4.34 4,6:3 5.06 5.60 I D :% 6.64 76.7 8.24 78.2 10.86 79.8 14.49 81.9 19.3 8 4 . 3 21.4 88.3 21.5 9 2 . 3 21.6 95 9 2 1, 65 100 '1 21.7
--
--c___
DE
08
05 R E L A T YE PRESSURE
E. BARTELL
Vol. '73
25"
Di-n-butylamine ?!-Butylamine r 1. - A*, -A*, x IO'* x i0'U (molesj (ergs1 (ergs/ (moles/ sq. cm.) sq. cm.) sq. cm.) jq. cm.)
33 0 37.4 40.5 42.7 44,3 30.2
53,7 56 , 3 58.4 60.2 61.8 tj3.2 ii4.7 tiij. 1
67.5 69.1 71.0 Z3.3 76.2 79.1 82.3 85.1 88. 1. 81).$1
3,04 5.Ifi 3.21
t?.29 3, 5 .t j l 5.70 J . 89 6.02 6.19 6.32 6.45 63.62 6 92 7.30 8.07 8.96 10.4 12 5 20.3 34,2 :36.3 Xj .tj 3ti X
3 4 . (j 43.2 .jO. 5 X .5 58, Y ti3 . 0 70.2 74.4
-
v, I . (
80.4 82.8 81.7 86.8 888 90.4 92.0 93.7 95, ,5 97.4 101).0 103.9 109.2 113.9 118.5
The absolute values in the above list may be in error by as much as 10%. I n addition to the uncertainty inherent in the BET method for the surface area determinaTb tion, the other difficulty is to locate the linear portion of the adsorption isotherms in order to evaluate the integral from zero to the lowest pressure ac* tually measured. A s the initial rise of the isotherms 1 of these amines wds exI tremely rapid, i t was not possible t o evaluate the initial slope very accu- I rately, and an error of about five ergs may result I from this source. The I relative values, however, 20IO are more reliable. It may be mentioned that these Fig. 3 . three systems probably represent the extreme cases. \T'ith other systenis. alcohol-silica, for example, the errors due to integration would be much smaller. 111order to obtain reliable results by this niethod, the primary reyuirenients are thdt the vapors must have been condensed into liquids to fill up the capillaries completely and that these liquids must have the same properties, especially the surface tension, as that in the bulk. Questions covering both these points c u i bc. answered from an inspection of the curves in Fig. 3 .I11 these curves have the same general features. .It low pressures the surface ciicrqy clroi) 15 vvry rapid; 10-
AND F.
Fig. 4
a t medium pressures the rate of decrease slackens. This portion of the curve is entirely analogous to those curves depicting the variation of surface tension of water with the concentration of capillarily active substances. As the pressure becomes still higher, the rate of energy drop becomes large again until the saturation pressure is reached. This second drop has no analog in the liquid-vapor systems. The logical explanation of this second drop is to assume that there is a layer of liquid condensed on the solid surface within the capillaries. -is more vapor condenses, the area of this liquid surface will, of course, decrease, or, the surface tension decreases according to the hypothetical process, hence the available surface energy also decreases. If there is no condensation of the adsorbed vapors, then i t is impossible to explain this second drop. Comparison of the pore volume of the gel with the volume of the liquid condensed shows that the capillaries are completely filled up. The surface tensions of the condensed liquids can also be estimated from these curves. If the radii of the capillaries are the same, then condensed films should be formed on all the walls a t the same time. Since the second energy drop has been attributed to the liquid film, then for gels with uniform radii the - A y vs. p/po curve should follow oabc of Fig. 3 where b is the interaction of oa and dc produced and can be accurately located from Fig. 4. As stated previously, the ordinates of the curves in Fig. 3 represent the change of available surface energy per unit area, the difference between the ordinates b and c is then the surface tension of the liquid in the capillaries. Accepting this explanation, the surface tension of these three amines can be read off from Fig. 3 as From curves
T)iethyl Dibutyl n-Butyl
20 0 dytie\/ciii
22 7 tiyiit.\/cin 22 1 dyiies,/ciii
Experimental
19 1 tiynes/cni. 29 (i riynes/cm. 22 9 dynes/ciii
Considering all the uncertainties and assumptions involved in the calculation, the agreement between the estimated and the experimental surface tensions is satisfactory. As these surface tensions are close together, it is desirable to be able to check the premise by using some other system for which the surface tension value is quite different from those
Jan., 1951
ENERGETICS OF THE ADSORPTION OF ALIPHATICAMINESBY SILICA GEL
listed above. For this purpose the system water vapor-silica gel investigated by Andersons can be used. It was found for this system the simple BET equation is applicable to about P / P o = 0.3. From the vm value, the number of molecules required t o form a complete monolayer, so obtained, the specific area of Anderson’s silica was estimated to be0434sq. m./g. using Livingston’s’ value, 10.6 sq. A., for the molecular area of water on silica, Using the same procedure as before Fig. 5 was obtained for water. From this figure the surface tension of water a t 15’ was found t o be 73, the accepted value being 73.5. While this close agreement niay well be fortuitous, i t does lend support to the above argumentas Thus i t is shown that the surface tensions of liquids in the capillaries do have practically the same values as in bulk, a t least for such pore size as for this silica gel. The actual curves, however, follow adc instead of abc. This can be easily understood. The silica gel used contains, of course, pores of various sizes. Since the condensation of the vapor depends on the pore radius, i t must start in the smaller pores before point b is reached. As the rate of change of - A y with respect to pressure is highest a t the inception and lowest a t the end of the process, the energy drop due to the change of surface energy of the film is greater than that due to the similar change of the solid-liquid interface. Therefore, the course of the actual curve adc lies above the idealized curve abc. This also explains why the dibutylamine curve starts to deviate from its normal course a t a lower relative pressure, because the reduction of the pore radius depends on the size of the adsorbate molecule and dibutylamine has the largest size among the three amines. According to the above argument the choice of point b to indicate the inception of condensation of vapors is equivalent to using the average radius in place of the actual radii. Even though the procedure adopted to locate point b, the intersection of the two logarithmic curves, seems reasonable, i t would be very desirable if i t were possible to check the premise b y some other methods. This can be accomplished by calculating the area of the liquid surface in two ways only one of which involves point b. The change of surface energy due to the change of surface area is given by
313
RELATIVE PRESSURE.
Fig. 5.
densed liquid has been shown to have the same value as that in the bulk, hence presumably constant. Therefore, after the second integral has been evaluated, the surface area of the liquid layer can be obtained. From the pressure corresponding to point b, p b , the amount of vapor adsorbed can be read off from the adsorption isotherm. By assuming the capillaries to be circular, the thickness t and the area of the liquid layer can be calculated from the area of layer = -T -- t area of solid r
wt. adsorbed a t P b total wt. adsorbed
1
(10)
where 7 is the average radius of the capillaries. The values of the areas of the condensed liquid layers obtained by these two methods are listed below : From Equation (9)
Diethylamine Dibutylamine n-Butylamine
From Equation (10)
2.49 X lo6 sq. cm. 2.55 X 108 sq. cm. 2.24 X lo6 sq. cm. 2.42 X 106 sq. cm. 2.53 X lo6 sq. cm. 2.52 X 106 sq. cm.
The agreements are satisfactory. Equation (10) may be further checked in another way. The thickness of the monolayer can be calculated from rm,the number of moles of the adsorbate required to form a complete monolayer on a unit surface. The same quantity can also be obtained from equation (10). For n-butylamine rmwas 5.1 X 10-’0. From this, the $hickness of the monolayer was found to be 5.08 A., which agrees satisfactorily with 5-04A. obtained from equation (10). As mentioned previously, the ordinate of point b represents ( -yYsl ylV - yso),i.e., when a solid of When the - Ay vs. $/PO curve starts to deviate unit area is in contact with a layer of liquid of the from its normal course, i t indicates the appearance the same area. In the capillaries, however, al, is of the liquid phase. This contention is supported smaller than as. For the present systems the above by the fact that the pressure corresponding to point calculations showed a l v is only 79% of as. Therea agrees closely with that a t the inception of the fore, the actual energy change corresponding to hysteresis loop. This, then, fixes the lower limit of point b for n-butylamine, say, is 100 instead of 96 as the right hand integral, whose upper limit is of indicated in Fig. 3. The actual values a t other course the number of moles adsorbed a t the satura- points, of course, can be obtained in similar manner. tion pressure. The value of the first integral is Finally i t may be noticed that even though the simply - a l V y l v , as the surface tension of the con- present method should be a general one for the de(6) Anderson, Z. p h y s i k . Chon., 88, 191 (1814). termination of adhesion tension of porous solids (7) Livingston, THISJOURNAL, 66, 569 (1944). against liquids, i t does not distinguish between ( 8 ) Recent work in this Laboratory on fifteen other systems with chemical and purely surface energies. In the presvarious solids and liquids whose surface tensions range from 9 t o 30 dynes/cm. also suhstantiates this view. ent case, the values obtained include both kinds of
+
mergy changes. It may be iiientionecl, however, that with systenis such as silica-alcohols, or cellulose-nitrogen, where the possibility of chcniical rcaction is much smaller, cwictly the s,iiiie type of have Ixwi obtained. Summary 1. Isoteric heats of adsorption of three aliphatic miities by silica gel were tleterinitiecl by means of adsorption isotherms obtained at 25' and a t 40". 2 The change in the heat content s j , the free ciiergy IF and the entropy A S per inole of adsorbI te were determined. 3. The isosteric heats 'iiid - 1Sobtaincd for the itiitial stages of atlsorptioii were so high as to indic,tte that adsorption 111 this range is not strictly physical. 4. Froin eiicrqy , t i i d cnlropy coiisitlc.r,itions it ,tppears that
