Water Vapor Adsorption on NH41and NH,CI
The Journal of Physical Chemistry, Vol. 82, No. 13, 1978 1521
Adsorption of Water Vapor on Ammonium Iodide and Ammonium Chloride Peter G. Hall” and Mark A. Rose Chemistry Department, University of Exeter, Exeter EX4 SQD, England (Received November 29, 1977)
The adsorption of water vapor on powdered samples of ammonium iodide and ammonium chloride has been investigated at 273, 283, and 293 K. Specific surface areas were determined by krypton adsorption at liquid nitrogen temperature. Although NH41and NH4C1are both water soluble their interaction with water vapor is different. The H20-NH41 isotherms are of BET type I1 shape showing the formation of a well-defined monolayer. A t relative pressures of ca. 0.6, NH41takes up water at constant pressure, as solution occurs. NH4Cl adsorbs more water than NH41but shows no inflection point correspondingto completion of a monolayer. Isosteric heats of adsorption of water vapor on NH41increase from about 40 kJ mol-I at low coverage to about 55 kJ mol-l at higher coverage. Adsorption energies for water adsorption on alkali halides and NH41have been calculated theoretically for the (100) face using four different potential functions. The position above the cation is the adsorption site most favored energetically by all the functions, which also predict reasonable values for the equilibrium distance of the adsorbed molecule from the surface. For adsorption above the cation, the electrostatic and induced interaction terms together contribute as much as 75% of the total attractive interaction.
This paper concerns measurements of the adsorption of water vapor on ammonium iodide and ammonium chloride, and also the theoretical calculation of adsorption energies for water on alkali halides, including “,I. The adsorption of water vapor on ammonium salts is of particular interest in the fertilizer industry in relation to the caking of ammonium fertilizers. T o our knowledge no previous water adsorption measurements on NH41and NH&1 have been reported. The a phase of NH41 has the NaCl structure while the p phase (formed below 255.4 K) and NH4Cl both have the CsCl type of structure. Davis and Mageel obtained experimental and theoretical isosteric heats for adsorbed neopentane on NH41for phases a and p. The energetically most favored site was above the center of the lattice square. Previous theoretical calculations2 of the potential energy of adsorption of water on NaC1, LiF, and NaF showed good agreement with experimental isosteric heats for NaCl but poor agreement in the case of the fluorides. However, the limitation of these calculations was that they were carried out using only one type of function for the dispersion/ repulsion terms. In this paper, we report the results of more detailed calculations covering all the halides of sodium and potassium; for each system we have compared four different potential functions. Comparatively few theoretical calculations of water adsorption potential energies have been reported previously. The adsorbents investigated include silver bromide; fluorides4 of lithium and sodium, rutile: and silver iodide.6 In contrast, the simpler inert gas/alkali halide systems have received much more detailed attention, for example, by Van Dongen,6 and House and J a y c ~ c k .We ~ have generally adopted the procedures and nomenclatures of these worker^,^-^ particularly Van Dongen.8
Experimental Section A conventional volumetric adsorption apparatus, very similar to that used by Barraclough and was used. Details of the procedure have been described.2 Ammonium iodide (Analar) was supplied by BDH Chemicals Ltd., and ammonium chloride was supplied by Hopkin and Williams. The solids were each ground in a pestle and mortar, ball-milled to a maximum particle size of 250 pm, and then dried in an oven a t 373 K. Krypton adsorption a t liquid nitrogen temperature was
used to measure specific surface areas.
Theoretical Calculations Theoretical isosteric heats of adsorption at zero coverage have been calculated for water adsorbed on the (100) face of alkali halides and NH41. The theoretical heats were calculated using a variety of functions. These are as follows: (i) the “6-exp” function, U = -(CKM/r6)+ B exp(-br), in which C is obtained from the Kirkwood-Muller approximation;1° (ii) the “Mason and Rice 6-exp” function, U = -(C/r6) + A exp(-br), where A = (67 exp y)/(y- 6), b = y/rm, and C = 7rm6/(l- 6/y), using values from Mason and Rice;ll (iii) the “6-LJ” function, U = -(CKM/r6)+ (BLJ/r12), where CKMis obtained as in (i) and B” is obtained from second virial coefficient data;12 (iv) the “LJ” function, U = -(C”/r6) + (BU/r12),where CLJ and BLJ are obtained from second virial coefficient data.12 In addition to the dispersion and repulsion terms indicated above, the total adsorption potential energy, UT, includes an induction term (VI = -1/2ciF), where F is the electrostatic field strength a t the surface and ci is the polarizability of the adatom. There is also an electrostatic term, UE, due to the interaction of the water dipole (moment p) with the surface field. If the direction of the dipole is assumed to be normal to the surface, then U, = -pFz, where F, is the z component of the field. The four adsorption sites considered for each adsorbent were (A) above the midpoint of a lattice square (composed of two anions and two cations); (B) above the midpoint of a lattice edge, Le., between a cation and an anion; (C) above a cation; and (D) above an anion. The total potential was obtained using a computer program13 (ADPOT 3) a t various values of p (= z / a , where z is the distance of the adatom from the surface and a is the distance between adjacent, unlike ions) in the range 0.4 < p C 2.0 in 0.05 increments. Near the potential minimum, UT was recalculated in 0.001 increments of p. The values near the minimum were fitted to a quartic polynomial of the form
UT = a 4 r 4 -I-a3r3+ a2r2 + a,r
0022-365417812082-1521$01.00/00 1978 American Chemical Society
+ a,
(1)
1522
P. G. Hall and M. A.
The Journal of Physical Chemistry, Vol. 82, No. 13, 1978
Rose
1
1d,d c
.
:.i; /
3201
1 I
2Lo.
160
1601 c
/
801
02
0 4 PlPo 0.6 08 Figure 1. Adsorption isotherm for Kr-”,I at 77.5 K. Sample (72.53 g) outgassed at 373 K.
and the coefficients were obtained by a least-squares fit. The minimum was obtained using the condition (auT(x,
Y , z)/az)z=z, = 0
Io
adsorption
s“ IC0
’
DINKz
Figure 2. Adsorption isotherm for H,O-NH,I
at 273 K.
n -
(2)
166-1
and solved by the Newton-Raphson method.
I
1201
%(x, Y)’
Y, (3) T o relate the isosteric heat qst to UMwe have, according to Ross and Olivier14 qOSt= U, - Evib 4RT (immobile model) (4)
+
Further details of the method of calculation and the data used are given by Hall and Rose.13J5 For example, CKM values were calculated using the appropriate ~ a l u e s l J ~ - ~ ~ 40 of polarizability and magnetic susceptibility, and with repulsion parameters from Born and MayerZ4and Van Dongen.* For NH4+,B was obtained, following the procedure of Davis and Mageel by plotting Van Dongen’s B Y values of the alkali ions against their ionic radii and obeo PIN cz 240 320 taining a value corresponding to the ionic size of NH4+. Figure 3. Adsorption isotherms for H,0-NH41. For the ions, CLJ and BLJ values were taken from the second virial coefficient data of Cook12on the assumption The adsorption of water was measured at 273, 283, and that the values for the ions correspond to those for the 293 K. The procedure for determining the multitempinert gases with which they are isoelectronic. This aserature isotherms was: dose 273 K 283 K 293 K sumption was also adopted in the case of the Mason and 273 K or 283 K overnight outgassing. Equilibration Rice function. For water, CLJ and BLJwere taken from required 4-5 h after the addition of a dose, and a t least Moelwyn-Hughe~.~~ 2 h for equilibration after a temperature change. The Results and Discussion 273-K isotherm was followed up to a relative pressure Ammonium Iodide. The sequence of adsorption ex(PIP,) of ca. 0.6; Po is the saturation vapor pressure. The isotherms are shown in Figures 2 and 3. periments with NH41was as follows: (i) initial outgassing For the 273-K isotherm a straight line BET plot was a t 373 K overnight, (ii) krypton adsorption, (iii) overnight obtained in the range PIPo = 0.05-0.36. The 283- and outgassing a t room temperature, (iv) water adsorption. 293-K isotherms were measured up to ca. PIP0 = 0.15 and Higher outgassing temperatures could not be employed the BET plots, therefore, extended only up to this value. because of the volatility of NH41 in vacuo. Even a t 373 K there was evidence of evaporation of material from the The 273-K isotherm was followed up to ca. PIPo = 0.6. After completion of the water adsorption runs the sample sample cell. The krypton isotherm is shown in Figure 1. The abwas removed and found to have completely caked. Some adsorption points were obtained a t 248 and 253 sence of a well-defined “knee” indicates a low interaction K, below the transition temperature of NH41 (255.4 K). energy between Kr and the surface, and means that only Only four points for each isotherm were obtained because an approximate estimate of the specific surface area (ca. of difficulty with thermostat control a t this temperature. 0.2 m2 g-l) is possible. I
-
- - -
-+
The Journal of Physical Chemistry, Vol. 82, No. 13, 1978 1523
Water Vapor Adsorption 00 NH,I and NH4CI
TABLE I: Theoretical Adsorption Energies (Urnand qOSt)/kJmol-' and Equilibrium Distances Z,/nm for Water Adsorption above Site C (Cation) (100) Face NaF z,
-U -q?
NaCl z,
-Urn -qoSt
NaBr z, -Urn
's
3.21
- 4 0 st
NaI
Z,
-Urn - 40 st
KC1 zrn
-Urn -qo'
KBr
Z,
-urn -qost
KI z, -
urn
-4 0
'
NH,I z,
-urn -qOSt
Flgure 4. Adsorption isosteres for H,O-NH,I.
Lor
, '
8(bos?d on H20 area)
016
'
I
' 024
06
6(W on Kr a m )
'
(ii) 0.2537 15.38 24.04 0.2429 30.88 38.08 0.2412 34.72 41.86 0.2384 43.54 50.56 0.2969 16.98 25.11 0.2960 17.92 25.98 0.2944 20.03 28.37 0.2378 48.49 56.74
(iii) 0.2521 19.72 28.22 0.2438 36.85 43.93 0.2430 42.53 49.51 0.2418 49.29 56.04 0.2993 18.75 26.89 0.2972 21.97 30.03 0.2952 26.14 34.34 0.2413 52.76 60.86
(iv) 0.2555 15.11 23.71 0.2466 30.07 37.2 0.2450 35.20 42.15 0.2435 41.87 48.60 0.3015 16.03 24.17 0.2994 18.80 27.18 0.2971 22.65 30.95 0.2422 47.55 55.70
a The column headings refer to the four functions described in the Introduction.
201-
04
(i)* 0.2810 9.60 18.41 0.2503 26.53 34.15 0.2454 33.05 40.56 0.2425 40.06 47.34 0.2945 17.70 25.93 0.2908 21.26 29.42 0.2877 25.74 33.80 0.3023 22.37 31.16
I
a' 2
08
'#
I0 04
10 '
'
Figure 5. Isosteric heats of adsorption for H,0-NH41.
Also, the number of adsorption points was restricted to a few to try to minimize any change in the surface area of the CY phase caused by passing the sample through its transition temperature. Comparison between the krypton isotherm and the monolayer amount from the water BET plots indicates an effective cross-sectional area of an adsorbed water molecule of about 0.35 nm2,certainly not less than 0.25 nm2. This compares with a value of 0.105 nm2 for a close-packed monolayer. Isosteric Heats of Adsorption. Adsorption isosteres, i.e., plots of In P vs. 1/T, calculated from H,0-NH41 isotherms of 273,283, and 293 K are shown in Figure 4. The straight lines obtained indicate the constancy of qst in the temperature range considered. The values of the slopes of the isosteres were obtained from a linear least-squares technique. The estimated errors in the values of qat (Figure 5 ) were *1.3 kJ mol-1 (ca. *3%) a t low coverages, while from ca. 8Kr = 0.09-0.29 the errors were, on average, 3t0.7 to k J mol-l (ca. *1,5%). At higher coverages the error increased until a t OKr = 0.41 the error was k2.6 k J mol-l (ca. *4.6%). Ross and Oliver14 estimated that the minimum error inherent in the calculated qst values was f l % from the process of plotting, curve-drawing, and interpolation of the n vs. P curves. The increasing errors a t higher coverages reflect the increasing uncertainty involved
in interpolation as the isotherms tend to become parallel to the pressure axis. Some values of qst for water adsorption on the /3 phase (CsC1 structure) of NH41were obtained from adsorption isotherms a t 248 and 253 K. The values of qst obtained from the slopes of the isosteres were of the same order as those of the cy phase, e.g., ca. 45 kJ mol-' ( 8 =~ 0.24) ~ increasing to ca. 47 kJ mol-l (OKr = 0.38). However, these particular values of qst are uncertain because of poor temperature control a t these low temperatures. Theoretical Adsorption Energies. The calculations showed that, with one exception (KF), the cation site (C) was the most favored energetically by all four functions. For KF, the anion site was apparently preferred, but the resulting equilibrium distance was doubtful, i.e., smaller than the sum of the F- radius (0.136 nm) and the effective radius of the water molecule in ice (0.138 nm). The results for the cation site obtained with NH41 and the halides of Na and K (except KF) are summarized in Table I. The equilibrium distances z, may be compared with values of the sum of the cation radius and the water molecule radius (0.138 nm); these are 0.233 nm for Na+ -H,O, 0.271 nm for K+ -HzO, and 0.286 nm for NH4+-H20. The values of qOStgiven in Table I were obtained using eq 4; the calculation of E v i b is discussed elsewhere.13J5 In general, the calculated values of z, are very reasonable, and compare favorably with the minimum distances quoted above. NH41 is an exception in that functions (ii), (iii), and (iv) give values of z, which are too low. However, a rigorous interpretation of z, for site C (and D) is very difficult with water as the adsorbed molecule. In the expressions for UDand UR,z is taken as the distance between a plane joining the mass centers of the surface ions and the mass center of the adsorbate molecule, whereas in the expressions for U, and VI, z is taken as the distance between the mass center of the surface ion and the electronic center of the dipole. Clearly for water the center of mass does not coincide with the electronic center of the resultant dipole. Comparison between the theoretical values of qOBt and experimental heats is inhibited by the lack of experimental
1524
P. G. Hall and M. A. Rose
The Journal of Physical Chemistry, Vol. 82, No. 13, 1978 n
n
ld6mol
P 500-
4001
/
20
Figure 6. Adsorption isotherm for Kr-NH4CI at 77.5 K. Sample (41.74 g) outgassed at 383 K.
NaCl(-44 kJ data. Thus, only with NaF (-46 kJ mol-1),2KC1 (ca. -13 to -53 kJ mol-1),26and NH41 (40 kJ mol-l, present work) are experimental values available. A further difficulty arises in that experimental heats can be substantially higher than theoretical values, since the latter do not take into account surface heterogeneities, including more than one type of crystal face exposed, and lateral interactions. Other factors not taken into account include possible hydration interaction, water dipole orientation, and relaxation of surface ions. Hydrogen bonding may also make a significant contribution, particularly in the case of the fluorides, as seems evident from the present results for NaF. It would be expected that adsorption of a polar molecule on an ionic surface would be greatly influenced by specific interactions, Le., electrostatic and induced interactions. This is evident from Table I. For example, with adsorption on site C using the “6-exp” function the specific interactions contributed 75.5% of the attractive interaction. The effect of these specific interactions is to make site C so much more preferable, energetically, for adsorption in comparison with the other sites. With such an imbalance the adsorbed water molecule is unlikely to show any mobility on the surface, as the energy barriers to translation are very high. If the adsorbed molecule is considered as a one-dimensional oscillator, located a t the potential energy minimum, the ratio, R,, of adsorbed atoms translating, Le., mobile, to those nontranslating, i.e., immobile, can be obtained. Calculation of R, from the “6exp” function, using the procedure of House and Jaycock? gave a value of 0.0087, i.e., 99.2% localized adsorption, for position C. Ammonium Chloride. The sample was outgassed for 6 h a t 383 K and krypton adsorption was measured at 77.5 K. The isotherm obtained is shown in Figure 6. A BET plot was linear within the range PIP, = 0.1-0.28, and from
80
240 PIN 6 2 403
Flgure 7. Adsorption isotherm for H,O-NH,CI
at 273 K.
this the following values were obtained; n,(Kr), 31.3 pmol; A K ~0.087 , m2 g-l; BET c value of 53. The adsorption of water on NH4C1 a t 273 K is shown in Figure 7. After the initial up take of water following a dosage, the NH4C1continued to take up water for 24 h, after which differentiation between adsorption on the sample and adsorption on the glass walls, etc., became difficult. Point A is where the temperature control unit failed and the temperature of the sample rose to 278 K for 3 h. The first six points were obtained after 24-h equilibration, followed by overnight outgassing. The rest of the points were obtained from consecutive doses after 24-h equilibration. Multitemperature isotherms were not obtained because the adsorption was not reversible. It was found that on increasing the temperature of the sample from 273 to 283 or 293 K, followed by decreasing the temperature to 273 K, the sample adsorbed all the water vapor available in the doser. This could reflect the incorporation of some of the adsorbed water into the crystal lattice as water of crystallization. This transfer of adsorbed water from the surface of the crystals into the crystals was aided by the temperature changes such that on returning to 273 K more adsorption occurred on the now unoccupied sites. A t the end of the water adsorption run the sample was completely caked. The mechanism whereby solid particles coagulate together to form a caked solid is thought to be due to (a) crystal bridging or (b) plastic deformation and cohesive forces. The probability of crystal bridging increases with increasing moisture content. The water adsorbed on the particles’ surfaces dissolves part of the soluble salt. tam ma^^^^ showed that in the contact area of two granules, the stress of the sold increases the solubility of the solid. The movement of the water from the solution would cause the salt to be deposited between the particles forming a
Water Vapor Adsorption on ",,I and NH,CI
crystal bridge which would bind the particles together. The formation of these bridges is aided by temperature fluctuations, hydration, and unequally distributed stresses. For NH4N03 prills, Thompson28 supports a caking mechanism involving a plastic deformation analogous to metallic creep. This mechanism involves the formation of an area of close contact between particles and the holding of the particles together by cohesive forces. Most crystal materials do not flow appreciably a t ordinary temperatures, but with increasing moisture content their plasticity increases, allowing areas of close contact to be formed between particles. The cohesive forces likely to be involved in the caking mechanism are: (a) ionic and covalent forces, (b) van der Waals forces, (c) crystal interlocking, (d) capillary adhesion. Capillary adhesion occurs when adsorbed water condenses around the granules and coalesces in the gap between the two bodies. The meniscus so formed first draws on the particles by means of surface tension, and, secondly, reduced the pressure of the liquid by virtue of its concave shape. Bookey and R a i ~ t r i c kwhen ~ ~ dealing with water adsorbed on fertilizer materials noticed that caking became noticeable in the range PIP, = 0.2-0.3. They thought that at this relative pressure, with a monolayer formed by PIP, = 0.18, the water became sufficiently active to bring ions from the surfaces into some form of solution, and recrystallization from this solution caused the caking. With the H20-NH4C1 system, adsorption was continued up to P/Po = 0.7. It is likely that crystal bridging is the mechanism for the caking of NH4C1, being aided by the incorporation of some of the adsorbed water into water of crystallization. At high relative pressures the mechanism of crystal bridging would be reinforced by capilary adhesion between the NH4C1 particles. Solution Effects. Although NH4C1and NH41are both water soluble salts, their behavior to adsorbed water is different. The H20-NH41 isotherms are type I1 in shape showing the formation of a reasonably well-defined monolayer. At relative pressures of ca. 0.6 the NH41takes up water a t constant pressure indicating that the salt is beginning to dissolve in the adsorbed water. NH4C1differs considerably; it adsorbs more water than NH41but shows no inflection point corresponding to completion of a monolayer. In fact the isotherm is similar in shape to a type I11 isotherm. It seems as if part of the adsorbed water is incorporated into the NH4C1, possibly as water of
The Journal of Physical Chemistry, Voi. 82, No. 13, 1978
1525
crystallization, or as NH4C1 solution. This equilibrium between adsorbed water and the incorporated water is disturbed by any temperature variation, more adsorbed water being incorporated, leaving adsorption sites available for further adsorption from the vapor phase. This difference in behavior is not simply related to solubility. NH41is hygroscopic and five times as soluble as NH4C1and so it would be expected to show any anomolous behavior to adsorbed water. It does take up water rapidly a t relative pressures of ca. 0.6, but at lower relative pressures the adsorption is reversible. NH4C1,on the other hand, which is much less soluble, shows no reversibility and adsorbs a large amount of water in comparison with NH41.
Acknowledgment. The authors acknowledge the award of an S.R.C. Research Studentship to M.A.R.
References and Notes B. W. Davis and R. A. Magee, J. colloid Interface Sci., 45, 487 (1973). P. B. Barraclough and P. G. Hail, Surf. Sci., 46, 393 (1974). P. G. Hall and F. C. Tompkins, Trans. Faraday Soc., 58, 1734 (1962). Y.-F. Y. Yao, J . Colloid Interface Sci., 28, 376 (1968). M. J. Jaycock and J. C. R. Waldsax, J. Chem. Soc., Faraday Trans. 1 , 70, 1501 (1974). N. Fukuta and Y. Paik, J . Appl. fhys., 44, 1092 (1973). W. C. J. Orr, Trans. Faraday Soc., 35, 1247 (1939). R. H. Van Dongen, Ph.D. Thesis, Vitgevrij Waltman-Delft 1972. W. A. House and M. J. Jaycock, J . Chem. Soc., 1710 (1974). A. Muller, R o c . R. Soc. London, Ser. A , 154, 624 (1936). E. A. Mason and W. E. Rice, J . Chem. fhys., 22, 843 (1951). G. A. Cook, "Argon, Helium and Rare Gases", Vol. 1, Wiley-Interscience, New York, N.Y., 1961, p 316. M. A. Rose, Ph.D. Thesis, University of Exeter, 1976. S. Ross and J. P. Olivier, "On Physical Adsorption", Interscience, New York, N.Y., 1964. P. G. Hall and M. A. Rose, Surf. Sci., in press. G. W. Brindley and F. E. Hoare, froc. R. Soc. London, Ser. A , 159, 395 (1927); 152, 342 (1935); 159, 395 (1937); Nature(London), 135, 474 (1935); Trans. Faraday Soc., 33, 268 (1937). W. Shockley, fhys. Rev. A , 70, 105 (1946). G. W. Brindley, fhys. Rev., 43, 1030 (1933). A. J. Michael, J . Chem. fhys., 51, 5730, (1969). V. Trew and S. Hussain, Trans. Faraday Soc., 57, 223 (1961). E. Albasiny and J. Cooper, froc. phys. Soc. (London),82, 801 (1963). L. Pauling, f r o c . R. Soc. London, Ser. A , 114, 81 (1927). F. London, Z. fhys. Chem., B11, 222 (1930). M. Born and J. E. Mayer, Z. fhys., 75, 1 (1932). E. A. Moelwyn-Hughes, "Physical Chemistry", 2nd ed,Pergamon Press, New York, N.Y., 1957, p 337. P. G. Hall and F. C. Tompkins, J . fhys. Chem., 66, 2260 (1962). G. Tamman, Z . Anorg. Chem., 107, 214 (1919). D. C. Thompson, f r o c . Fert. Soc., 125 (1972). J. B. Bookey and R. Raistrick, "Chemistry and Technology of Fertilizers", V. Sauchelli, Ed., Reinhold, New York, N.Y., 1960.
