Structure and swelling of halogenated mesophase pitches

Structure and swelling mesophase pitches H. Preiss,

H. Miessner,

Central Institute of Physical (Received 12 March 7991)

K.-H.

Richter

Chemistry,

Rudower

of halogenated

and K. Szulzewsky Chaussee

5, 1199 Berlin,

Germany

An attempt has been made to characterize the structure of the reaction products resulting from the halogenation of mesophase pitch. FT-i.r. spectroscopy, X-ray photoelectron spectroscopy and X-ray diffraction show the presence of halogenated aromatics and hydroaromatics which are of a lower lamellar orientation than in non-halogenated mesophase pitches. The halogenated pitches swell in contact with solvents acting as electron donors or acceptors. Basic organic solvents are the most active swelling agents. A crosslinked structural model is proposed which comprises ordered domains of lamellar aligned aromatics and a disordered network. The crosslinks are bonding regions to join the ordered domains and the disordered material. It is assumed that halogenated hydroaromatic molecules assist in producing the crosslinks. (Keywords:

structural

analysis;

swelling; mesophasc

pitch)

of aromatics to polynuclear molecules is the main chemical reaction during the thermal transformation of pitches to mesophase pitches containing mesophase spheres. The alignment of the polyaromatics favours their lamellar orientation. In the mesophase spheres, the polyaromatic molecules are approximately parallel to each other, forming a liquid crystal structure with a more perfect orientation than in the coexisting isotropic pitch matrix. In structural terms, the chemistry of mesophase pitch may be regarded as governed both by the graphite structure and the structure of aromatic hydrocarbons. In fact, in the reaction of mesophase pitch with halogens or halides, evidence was found for an intercalation-like absorption as well as a chemisorption of halogens’. Upon heat treatment up to 400 K as well as solvent extraction, only the absorbed loosely held halogen can be desorbed, whereas a strongly held portion of halogen remains chemisorbed, forming halogenated mesophase pitches. The question arises as to the structure of the halogenated mesophase pitches. It is assumed that halogen atoms are fixed to the carbon atoms of the polyaromatics by substitution of hydrogen atoms, and that the liquid crystal character of the mesophase pitch will be preserved throughout halogenation. In contrast to mesophase pitches, however, their halogenation products do not fuse during heat treatment. Thus, a macromolecular crosslinked network is more likely. In this study, X-ray diffraction, i.r. spectroscopy and X-ray photoelectron spectroscopy (X.P.S.) were used to study the structure of halogenated mesophase pitches. The swelling technique is a convenient method for studying a macromolecular network structure, especially in the cases of polymers and coals, where the size of the macromolecules and the forces holding them together can be probed by swelling experiments. Swelling is a penetration mechanism of solvent molecules through the solid matrix involving breakage of weak chemical bonds and rearrangement of macromolecular parts of the Condensation

0016-2361/91/09103946 :(;: 1991 Butterworth-Heinemann

Ltd.

structure (structural relaxation). The driving force for penetration is the chemical interaction of the solvent molecules with special sites in the macromolecular structure. It is envisaged that covalent crosslinks, strong polar bonds and a high degree of entanglements prevent the macromolecules from dissolving in the solvents. In this work the dynamics of solvent swelling as well as equilibrium swelling are used to provide information on the structure of the halogenated pitch and the solventmesophase pitch interaction. EXPERIMENTAL Materials

Coal tar pitch was converted to mesophase pitch by heat treatment up to 750K under flowing nitrogen at a heating rate of 8 K min- ’ without subsequent annealing at the maximum temperature. After cooling to room temperature under nitrogen flow, the carbonization product was ground into particles with diameters of <90pm. Using standard organic chemical analysis, a hydrogen content of 3.6 wt% and a carbon content of 93.8 wt% were determined. Bromination of the ground mesophase pitch was carried out according to Ref. 1 by bringing it into contact with liquid bromine (weight ratio of 15). After the reaction had finished, surplus bromine was removed under flowing air at room temperature and subsequently at 400K. The bromine content in the reaction product, determined analytically according to the method devised by Schoeniger’, was 48.5 wt%. Chlorinated mesophase pitch was produced with iodine chloride and chlorine gas. The reaction was performed by addition of 50g liquid ICl to log mesophase pitch at 273 K. After all the ICI had been added, the reaction mixture was gently heated to 373 K under reflux, and then dried chlorine gas was introduced. When ICI, condensed (after - 2 h), the introduction of chlorine gas was stopped and the iodine chlorides were

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Structure

and swelling

of mesophase pitches:

H. Preiss et al.

removed by distillation. After drying at 400 K, a chlorine content of 38 wt% was determined. Characterization

methods

X-ray powder diffraction was carried out using a two-circle diffractometer with Cu Ka radiation. 1.r. spectroscopy was carried out on samples diluted in KBr pressed pellets with a double beam FT-i.r. spectrometer. The spectra were registered in the frequency range 4000-400 cm ’ at a resolution of 2 cm- ‘. X.p.s. spectra were obtained using a spectrometer operated with Mg Krx radiation. Swelling technique Samples of - 1 g were dried and weighed in closed gravity beakers to within +O.O005g. The open beakers were then placed in a dessicator over solvent. The solvents used were pyridine, n-butylamine, AsCl,, benzene, acetone, n-hexane, carbon tetrachloride, methanol and n-amyl alcohol, The dessicators were sealed and maintained at a constant temperature of 295K. At set time intervals, the beakers were removed, sealed, and weighed, and then they were opened and returned to the dessicators. The mass of solvent uptake per mass of sample was calculated as a function of time.

RESULTS

AND DISCUSSION

Ix. spectroscopy The i.r. spectra of mesophase pitch, and its bromination and chlorination products, respectively, are characterized by bands at - 1600 cm- ’ and in the region of aromatic C-H deformation vibrations between 700 cm - I and 900cm-‘. The band at 1600 cm- ’ was observed by many authors studying various carbonaceous samples, and up to now it could not be interpreted unequivocally. According to some authors, this band is caused by aromatic C-C vibrations as well as by the vibration of conjugated carbonyl groups ‘v4. There are two reasons why this band may be associated with aromatic CC vibrations: the low oxygen content of the pitches investigated; and the band changes only insignificantly in intensity and position with halogenation. In the spectra of the halogenated pitches, no bands could be unambiguously associated with C-Cl or C-Br vibrations. The i.r. spectral changes of the aromatic C-H vibrations, however, indicate the influence of the halogenation and dehydrohalogenation effect. Figure 1 shows the 700_1000cm-’ range of the spectra with aromatic bands at 748, 835 and 873 cm-‘. In general, the 70&900cm-’ range can be divided into the out-of-plane C-H vibrations of highly substituted carbon six-membered rings in the higher range (80&900 cm- ‘) and into the less substituted six-membered rings at - 70&800 cm- ‘. Therefore, as H atoms are substituted by chlorine or bromine, the intensity of the less substituted C-H vibrations decreases. The predominance of the 875 and 835 cm- ’ bands over the 748cm-’ band in halogenated pitches suggests a high degree of substitution. The peaks of the halogenated pitches in the 70&900cm-’ range can be attributed to aromatic protons and olefinic protons. Thus, the presence of chloro hydroaromatics resulting from addition reactions also has to be taken into consideration.

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1

IQ

wavenumber

cm4

700

Figure 1 1.r. spectral range of C-H out-of-plane vibrations of: (a) mesophase pitch; (b) brominated mesophase pitch; (c) chlorinated mesophase pitch

X-ray photoelectron spectroscopy The overall spectrum of the chlorinated mesophase pitch reveals intense C 1s and Cl 2p signals. The binding energy determined at the peak maxima in the individual regions is 286.7 eV for the C 1s and 202.0 eV for the Cl 2p line. A characteristic of the C 1s line is its high full width at half maximum (2.8 eV). Because this line width than for aromatic hydrocarbons is -0.5 eV broader measured in similar experimental conditions, the presence of two aromatic carbon species is likely; that at the higher binding energy may be associated with chlorine-substituted carbon atoms and that at the lower binding energy with non-substituted carbon atoms. X-ray diflraction The X-ray diffraction patterns of mesophase pitch and its bromination and chlorination products are shown in Figure 2. The patterns of the halogenated pitches are characterized by a high background which indicates the presence of a larger proportion of disordered material. The patterns reveal three diffraction lines for each sample, and it is to be assumed that ordered domains are distributed in an amorphous material. The d spacings calculated from the position of the diffraction lines are listed in Table I. In the case of mesophase pitch a d spacing of 3.45 A is determined from the strongest diffraction line, which corresponds to the (002) graphite reflection. Thus, it may be caused by the interlayer spacing of the aromatic sheets in the mesophase pitch. After chlorination and bromination these interlayer spacings increase to 3.65 and 3.7OA, respectively, which are close to double the van der Waals radii of chlorine and bromine. Moreover, these main diffraction Table 1

Calculated

d spacings

(A) ~ Reflection

Pitch type Mesophase Chlorinated Brominated

pitch mesophase mesophase

pitch pitch

(002)

(10)

(11)

3.45 3.65 3.70

2.10 2.11 2.08

1.21 1.21 1.20

Structure

a

I 1

0%

015

I

I

OS26

0,37 sin 8 I h I A-’

I

0,443

Figure 2 X-ray diffraction patterns of: (a) mesophase pitch; brominated mesophase pitch; (c) chlorinated mesophase pitch

1

0,59 (b)

lines are characterized by a decrease in intensity and an increse in line width after halogenation (Figure 2). ‘the weak bands centred at d = - 1.20 and N 2.10 A do not change significantly in position and intensity (Table 1) on halogenation. These bands can be assigned to a 11 line and a 10 line of a biperiodic pre-graphite structure. Such results strongly suggest that the C-C bonds in the polyaromatic molecules have remained unchanged during halogenation. Consequently, it is to be assumed that the size as well as the planarity of the polyaromatics are mainly preserved in the ordered domains. From the X-ray results, it appears that the ordered portion of the halogenated pitches has a lamellar structure in which polyaromatics are stacked in parallel. The halogen atoms are bound to the periphery of the planar aromatic molecules. The increase of the line width of the main diffraction peak of the halogenated pitches in comparison with the non-halogenated pitch indicates a decrease in the extension of the ordered domains in the direction perpendicular to the aromatic sheets.

and swelling

of mesophase pitches:

H. Preiss et al.

pair acceptors, e.g. AsCl,, and electron pair donors, e.g. pyridine. Pyridine is well known as a very active swelling agent for coals, showing a similar solubility parameter to that of coa15*6. Thermodynamic swelling theory predicts for a crosslinked network that maximum swelling occurs when the solubility parameter of the solvent is equal to that of the macromolecular network7s. When a powder of chlorinated mesophase pitch was in contact with pyridine, n-butylamine, quinoline, oleum or AsCl, in a weight ratio of 1:l, heating of the mixture occurred. A macroscopic swelling of the powder was observed, and a dry powder was formed showing that the solvent was imbibed by the solid in each case. Subsequent exposure of the interaction product to air caused an expulsion of the imbibed penetrants. The reaction product with pyridine rapidly reduced in weight. Part of the pyridine, however, remained imbibed at room temperature even under vacuum. Expulsion of this residual solvent is accomplished under vacuum only at 550K. Additionally, gas-induced swelling was used for aiding in the understanding of the swelling mechanism. Thus, a powder of the chlorinated pitch was subjected to AsCl, vapour in a closed vessel at room temperature. The mass of solvent uptake per mass of sample as well as the volume increase were determined as a function of exposure time, t,. Results from these studies are shown in Figure 3. It is evident that the volume swelling increases with the mass uptake over a long time scale; even after 200 days there is no indication of an equilibrium state. Since almost no pre-existing pore system is present in the chlorinated pitch, it is to be assumed that the solvent molecules enter the solid matrix. X-ray investigations of the swelling products with AsCl, and pyridine, respectively, did not show any reflections. In spite of a broad alignment of the aromatic molecules in the halogenated pitches, no intercalationlike reaction appears to occur.

Swelling by liquid solvents

Preliminary solubility experiments with some liquid halides and amines revealed that a portion of mobile material can be extracted from the halogenated mesophase pitches by those solvents. Immersion of the pitches in alcohols and non-polar solvents, however, gave no indication of solubility. These results prompted us to investigate their ability to swell in contact with electron

v. OO

1

LO

I

I

30

II

I

120

tEld

I

I,

160

Figure 3 Arsenic(II1) chloride uptake per mass of sample (a) and volume increase (b) of chlorinated mesophase pitch as a function of exposure time, t,

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Structure and swelling of mesophase pitches: H. Preiss et al. Dynamic swelling by vapours of organic solvents

Dynamic experiments were performed to study the effect of the type of the pitch and of the solvent on swelling. Polar basic solvents and non-polar solvents were used. Before presenting the results it is necessary to clarify the influence of pore structure on the data obtained. Mercury porosimetry up to 200MPa yielded pore volumes of -0.02 cm3 g-’ for mesophase pitch and its halogenation products. Using nitrogen adsorption, the BET surfaces determined by the method of Haul and Diimbgen” (single point procedure) were <2m2 g-‘, and also showed low porosity. During solvent uptake a portion of the solvent fills the pores and does not contribute to swelling, but there is doubt that this portion of solvent is equal to the pre-existing pore volume. Because of pore wall elasticity, it may be assumed that the pores are likely to collapse during the solvent transport process. When pores collapse, the amount of solvent uptake which is associated with capillary condensation in pores is lower than that assessed by the pre-existing pore volume before exposure to the vapours. Moreover, in cases of polar basic solvents the amount of solvent able to condense in pores is very low in relation to the total uptake because of the low porosity. Thus, no pore structure correction of the swelling was made. The relatively large experimental error in the determination of the low pore volumes was another reason for not making a correction. The mass uptake of chlorinated as well as brominated mesophase pitch by polar and non-polar solvents was determined as a function of time. In order to yield additional information on the swelling mechanism, nonhalogenated pitch was also subjected to the vapours. The solvent uptake versus exposure time curves are shown in

100

150

Figure 5 Mass uptake of n-butylamine function of time: (0) mesophase pitch; pitch

0

50

la0

per mass

I

150

200

c

Figure 7 Solvent uptake (0) carbon tetrachloride;

50

la0

150

200

250 300 tElh

Figure 4 Pyridine uptake (0) mesophase pitch; (0) inated mesophase pitch

per mass of sample as a function of time: brominated mesophase pitch; (A) chlor-

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1991,

of sample as a mesophase

tE/h

250

300

Figure 6 Mass uptake of alcohols per mass of sample as a function of time: (0) mesophase pitch, methanol; (0) brominated mesophase pitch, n-amyl alcohol; (A)chlorinated mesophase pitch, methanol; (A) chlorinated mesophase pitch, n-amyl alcohol

Oo’ 50

-0

300 tE/h

(A) chlorinated

Figures 4-7.

The first major observation from these studies is that basic solvents are the most active swelling agents. Pyridine and n-butylamine, respectively, by far exceed the other polar and non-polar solvents in mass uptake. The second major observation from the experiments is that chlorinated pitch swelled most upon exposure to pyridine, followed by brominated pitch, and the nonhalogenated mesophase pitch swelled the least. The third major observation is that the mass uptake upon exposure to pyridine and n-butylamine, respectively, continued with time, and there is no evidence of

250

200

I

100

150

200

250 300 tElh

per mass of chlorinated mesophase pitch: (0) n-hexane; (0) acetone; (A)benzene

an equilibrium state even after 900 h. It was also observed that the samples became wet or were in a pool of solvent. This was not the case for alcohols or non-polar solvents; an apparent equilibrium state was attained for these solvents after exposure for - 100 h. These observations are from a chemical interaction between halogenated pitches and amines, which manifests itself not only by a strong swelling but also by the ability of such solvents to dissolve a portion of the solid

Structure and swelling of mesophase pitches: H. Preiss et al.

matrix. This soluble portion is far higher for amines and halides (e.g. AsCl,, SbCl,)’ than for alcohols and non-polar solvents. It is convenient to explain the mass uptake of most penetrants in terms of the regular solution theoryg’rO. A solubility maximum occurs when the solubility parameter, 6, of the solvent is close to that of the solute. For swelling experiments with polymers and coals, the swelling maximum is also explained in terms of the 6 values of the matrix and swelling agent6,*. In addition, the strongly enhanced swelling by amines may be correlated with strong intermolecular forces between amines and halogenated pitches. Forces of charge transfer between these nucleophilic (electron donor) solvents and pitches would be assumed as the respective intermolecular forces. Aromatic sites in the structure of the solid may act as electron pair acceptors to the lone pair of the nitrogen atoms forming complexes. The increase in pyridine uptake from mesophase pitch to brominated and to chlorinated mesophase pitch (Figure 4) indicates an increasing acceptor force of the aromatics present in these pitches. This effect is attributed to the electron withdrawing effect of the substituents increasing in the order H to Br to Cl. Since the pioneering work of Menshutkin’ ‘, molecular complexes of pyridine with halogenated hydrocarbons are known to exist. Mesophase pitch and their halogenation products, however, can also act as electron donors, as is evident from the fact that they imbibe arsenic and antimony chlorides’. When samples are exposed to a saturated vapour of a basic polar solvent, the solvent may be adsorbed at the surface or may enter the matrix and condense in pores whereby mobile molecules from the matrix can be extracted. The resulting solution will have a lower vapour pressure than the pure solvent, and more solvent will condense to dilute the solution. Since the vapour pressure of the solution will always be lower than that of the pure solvent, the process continues and no equilibrium will be attained. In the cases of alcohols and non-polar solvents, solubility experiments with these liquid solvents revealed that any extraction of mobile molecules from the solid matrix could almost be ruled out. Therefore, an equilibrium state upon exposure of pitch samples to vapours of these solvents is to be expected from adsorption theory and swelling theoryg*iO, respectively. This is corroborated by our swelling experiments. STRUCTURE

AND SWELLING

a maximum of 50% of the H atoms at the periphery of

the compact units are substituted. In general, the halogenated polyaromatics may be planar and may aggregate to form stacked anisotropic domains. From the main diffraction peak in the halogenated product, an interlayer distance of 3.65 A for the chlorinated product and 3.70A for the brominated product results. The non-fusibility of the halogenated mesophase pitch and the broadening of the main diffraction peak in comparison to its mesophase pitch counterpart, however, oppose a liquid crystal order such as observed in the non-halogenated product. In order to explain this behaviour, additional halogenation pathways also have to be considered. It is well known from organic chemistry that the addition of chlorine and bromine to aromatic systems plays a progressive role with an increase in the size of the polyaromatics. Hydroaromatics are formed as a result of addition reactions. A mechanism of halogen addition to aromatics in pitch, which involved the formation of intermediate aromatic cation radicals was recently proposed I4 . Addition reactions in mesophase pitch seem likely with respect to the formation of arene radicals from polyaromatics with antimony pentahalides’ 5. If chlorination is performed at relatively high temperatures (in our case at 400K), the hydroaromatics are not stable and partly decompose following different pathways. Elimination of HCl should be the main reaction resulting in substitution products. The higher the temperature, however, the more Friedel-Crafts condensation, diene synthesis and destructive ring contraction will compete with dehydrohalogenation. Possible reactions are represented in Figure 8 by a model naphthaleneanthracene system. In some cases, structures are formed with additional H and Cl in reactive tertiary positions. Progressive condensation gives irregularly shaped clusters, which are characterized by non-planar structural elements and additional covalent bonds. In conclusion, the chlorinated mesophase pitches are thought to consist of aromatics and hydroaromatics, which form a crosslinked macromolecular network. The crosslinks could be due to covalent bonds or/and entanglements between the clusters. Embedded in the network are less crosslinked domains of lamellar bundled halogenated aromatics, which are fixed in the network

MECHANISM

The preparation of halogenated mesophase pitches by halogenation of mesophase pitch in the presence of a catalyst (e.g. SbCl,, I*) suggests that halogen atoms are attached to carbon atoms mainly by hydrogen substitution. The i.r. results and the decrease in the elemental H/C ratio from 0.44 to 0.32 during halogenation are consistent with the proposed halogenation pathway. With regard to the black colour and the relatively intense biperiodic X-ray bands of the reaction product, it is assumed that the size of the polyaromatic units should essentially be preserved upon halogenation. Studies on the structure of pyrolysed pitches and model pitches have been made by different authors using i3C n.m.r. and X-ray techniques”Ti3. These authors found that the polyaromatic units are compact, having a size of 2&70 carbon atoms. Our C/H ratios for the mesophase pitch are consistent with this range. Upon chlorination

x3c-

0

&

@?

-d XL

x3c-

0 co

/x )

X’ ‘X

+lp-~

/+ onthracene

lX$gy

-CnXm

(jyjQ x ‘X

\

W-

‘X

aI3 X’ ‘X

Figure 8 Proposed reaction mechanisms of chloro hydroaromatics applied to a naphthalene-anthracene model system (X = H, Cl)

FUEL, 1991, Vol 70, September

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Structure and swelling of mesophase pitches: H. Preiss et al.

by bonding regions (C atoms or short bridges). The extension of the ordered domains normal to the aromatic layers is lower than in non-halogenated pitches. Besides the van der Waals gaps between the aromatic layers in the ordered domains (interlayer distance 3.65 A for Cl and 3.70 A for Br, respectively), there are additional gaps between the irregularly shaped clusters and ordered domains. These gaps seem to be inaccessible to nitrogen because only a BET surface area of 1.5 m2 g-r was determined. Upon exposure to special vapours, however, the halogenated pitches imbibe solvent molecules and swell in volume. Regarding the proposed structural model, a low density of crosslinks and a strong chemical interaction would account for a strong swelling effect. Amines were shown to be the most active swelling agents. The interaction between the solid matrix and the solvent molecules can be described by a two-component model comprising the effects of dispersion forces and polar forces. The latter are connected with the electron lone pair of nitrogen atoms. Thus, polar basic solvents may interact to form electron donor-acceptor complexes. When halogen atoms substitute hydrogen in aromatics, the electron withdrawing effect of the halogen increases the acceptor force of the aromatic system in the pitch. The increase of the pyridine uptake in the order from mesophase pitch to brominated and to chlorinated pitch corresponds to the increase of the withdrawing effect from H to Br to Cl. We assume that the pyridine molecules are imbibed in the solid by penetration into the gaps between the structural domains as well as into the van der Waals gaps between the aromatic layers of the domains whereby the gaps irregularly expand. The intensity decrease of the main X-ray line confirms the loss of three-dimensional ordering of the domains during pyridine uptake. The uptake of methanol, acetone and benzene, respectively, is lower in comparison with pyridine. In terms of chemical reactivity this decrease is due to the decreased nucleophilic properties of these penetrants. According to

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FUEL, 1991, Vol 70, September

the donor number approach of Gutmann to classify the nucleophilic properties of solvents16, pyridine has the highest electron donor number. Swelling experiments of coals also showed that pyridine is the most active swelling agent5v6. A comparison of swelling of coal with that of mesophase pitch, however, is an oversimplification because the breakage of hydrogen bonds is the main mechanism of coal swelling by polar basic solvents. For non-polar solvents, e.g. n-hexane and carbon tetrachloride, we assume that only dispersion interactions would account for the observed swelling mechanism. This swelling is very low. It is thought that dispersion interactions also contribute to the swelling of polar basic solvents which comprises contributions of the heteroatom and those of the rest of the molecule. At present the two contributions cannot be assessed if any separation is at all legitimate. REFERENCES 1 2 3 4 5 6 I 8 9 10 11 12 13 14 15 16 17

Prerss, H. Fuel 1989, 68, 1251 Schoeniger, W. Microchim. Acta 1955, 123; 1956, 670, 869 Mattson, J. S. and Mark, H. B. ‘Activated Carbon’, Marcel Dekker, New York, 1971 Retcofsky, H. L. and Friedel, R. A. Fuel 1968, 47, 487 Larsen, J. W., Green, T. K. and Kovac, J. J. Org. Chem. 1985, SO, 4729 Reucroft, P. J. and Patel, K. B. Fuel 1983, 62, 279 Flory, P. J. ‘Principles of Polymer Chemistry’, Cornell University Press, Ithaca, 1953 Rodriguez, F. ‘Principles of Polymer Systems’, McGraw-Hill, New York, 1982 Hildebrand, J. H. and Scott, R. L. ‘The Solubility of Nonelectrolytes’, Dover Publications Inc., New York, 1964 Reichardt, C. ‘Solvents and Solvent Effects in Organic Chemistrv’. VCH Verlaasaesellschaft, Weinheim, 1988 Mdnshutkin, B. N.-Z. Phys. Chem. 1860,6,41 Ollivier, P. 3. and Gerstein, B. C. Carbon 1986,24, 151 Guet, J. M. and Tchoubar, D. Fuel 1986,65, 107 Greinke, R. A. Fuel 1984, 63, 1374 Forsyth, D. A. and Olah, G. A. J. Am. Chem. Sot. 1976,89,4086 Gutmann, V. ‘The Donor-Acceptor Approach to Molecular Interactions’, Plenum Press, New York, 1978 Haul, R. and Diimbgen, G. Chem.-Ing. Tech. 1963, 35, 586