CLAY- ORGANIC STUDIES

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1 Clay Minerals (1965) 6, 91. CLAY- ORGANIC STUDIES X. COMPLEXES OF PRIMARY AMINES WITH MONTMORILLONITE AND VERMICULITE* G. W. BRINDLEY Materials Research Laboratory, and Department of Geochemistry and Mineralogy, The Pennsylvania State University, University Park, Pa., U.S.A. (Received 25 May 1965) ABSTRACT: Basal spacing data for neutral amine complexes with montmorillonite and vermiculite are explained on the basis of two layers of fully extended amine molecules inclined at about 65 ~ to (001) with the NH 2 groups closely associated with pairs of silicate oxygen atoms. The molecular orientation is shown to be favourable for good intermolecular packing and for possible NH.. O bonding, but the results do not constitute proof of such bonding. Packing of the molecules on the silicate surfaces and with respect to each other may explain the formation of double layer complexes. The amine data are correlated with corresponding data for the low-temperature (or below melting point) series of alcohol-montmorillonite complexes and it is suggested that the molecular layers may have some solid-like characteristics. Primary neutral amines form long-spacing complexes with montmorillonite and with graphitic acid (Arag6n, Cano Ruiz & MacEwan, 1959) and with vermiculite (Sutherland & MacEwan, 1961; Sutherland, 1962) in which the separation of the sheets corresponds to two layers of molecules oriented more or less normally to the sorbate layers. For graphitic acid (and montmorillonite is said to give 'completely analogous results') the 'actual value of the layer separation is too low to accommodate two fully extended molecules'. For vermiculite 'the slope of the graph of spacing against chain-length is similar to that found with montmorillonite (about 2.3 A/C atom) suggesting that here also the orientation of the chains may be perpendicular to the layers'. The present note shows that the increase of spacing by 2.3 A/C atom and the overall spacings can be interpreted in terms of two layers fully extended molecules inclined at about 65 ~ to (001), and the resulting model is very similar to that obtained for alcohol-montmorillonite complexes by Brindley & Ray (1964). *Contribution No from the College of Minerals Industries, The Pennsylvania State University, University Park, Pa.

2 92 G. W. Brindley ANALYSIS OF THE DATA The basal spacings of the amine complexes with montmoriuonite and with two vermiculites obtained by MacEwan and his colleagues (see Fig. 1) show a nearly linear variation of spacing d(001) with n number of carbon atoms in the molecule. If the molecular orientation with respect to (001) remains the same when the chain length is increased, then the gradient ad/an provides an estimate of the chain inclination, ~, to (001). sin q~ = (observed ad/an)/2.54 where 2.54 A is the increase in d(001) per carbon atom for a double layer of molecules standing normal to (001) o 20 ~o L I I I I I! I I I I n, carbon atoms FIG. 1. Basal spacings, d, in A versus, n, number of carbon atoms in alcohol and amine complexes. Alcohol-montmorillonites (Brindley & Ray, 1964). Amine-montmorillonites (Arag6n et al., 1959). Amine-vermiculites (Irish) (Sutherland & MacEwan, 1961). Amine-vermiculites (West Chester) (Sutherland & MacEwan, 1961). Line 1, d= 14"20+ 2"48n (n even number); Line 2, d= 13"63+2"288n (n even number); and Line 3, d= 13"33 + 2,288n (n odd number).

3 Clay-amine complexes 93 From the mean straight line drawn visually through the experimental points in Fig. 1, a value of ~d/an = 2"30 A/C atom is obtained, giving a value of ~ = 65 ~ with an estimated accuracy of +5 ~. Calculations of the best straight lines through the vermiculite data for the amines C6-C16 by the method of least squares give the following: Irish vermiculite, d = n, q~ = 66 ~ West Chester vermiculite*, d = n, ~ = 68 ~ In order to calculate the overall basal spacings, the contacts between the molecules and the silicate surfaces must be considered in addition to the molecular dimensions and inclination angle ~. For primary alcohol complexes with montmorillonite, Brindley & Ray (1964) showed that the chain inclination and the overall spacings were consistent with OH.. O bonds between the OH terminations and the silicate oxygens. Similar considerations arise with the NH2 terminations of amine molecules and with the NH3 terminations of alkylammonium ions (see discussion by Weiss et al., 1959, 1963a, b). Weiss, Michel & Weiss (1959) gave for the NH.. O distances in normal hexylammonium montmorillonite the value '2.85 A at the most' and a calculated chain inclination of 55 ~ to (001). For the neutral amine complexes, the basal spacings are calculated as follows: taking the O--O separation across the hexagonal (or ditrigonal) group in the silicate surface as 5.18 A (the a parameter of the structure) and the NH.. O distance as 2.90 A, one finds (see Fig. 2) the N atom lying 1.30 A from the line of oxygen centres, and the bond angle H-N-H = ~ which is near the expected value of 120 ~ The first chain link NC1 is normal to the line of oxygen centres and therefore normal to (001). The chain extends in accordance with C--C bonds 1.54 A and bond angles of 109 ~ 28', until the final CH3 group is reached with an effective size given by the van der Waals radius rr~ = 1"20 A. The chain inclination depends on the angle NC~C2 and if this angle is 109 ~ 28', the chain inclination is ~ = 55 ~ in agreement with Weiss, Michel & Weiss (1959), but if NC~Cz = 120 ~ then q~ = 65 ~ in agreement with the value derived from Ad/An. If the terminal CH3 groups of the two layers of amine molecules make van der Waals contacts at the mid-plane, then taking angle NC~C2 = 120 ~ and the thickness of th~ silicate sheet as 6"60 A between centres of oxygen planes, one calculates the following equations: (for chains with n even) d(001) = n (for chains with n odd) d(001) = n The corresponding lines, shown in Fig. 1, agree closely with the experimental data apart from the two low values for the C-18 and C-20 amines, which in fact were not pure amines. The values of d(001) calculated from these equations and the experimental values for vermiculites (see Table 1) show generally a good agreement. The experimental * The writer is indebted to an unknown reviewer and to Dr R. Greene-Kelly for making data available from the Ph.D. thesis of H. H. Sutherland. See also Table i. The present writer has not seen rite thesis itself.

4 94 G. W. Brindley data for montmoriuonite, shown in Fig. 1, are not included in Table 1 because they were taken from a small scale published figure (Arag6n, Carlo Ruiz & MacEwaa, 1959, Fig. 1). TABLE 1. Observed and calculated basal spacings, d, in A for amine vermiculites n d(calculated) d(observed) ' " "93 31 " " "51 36'6 36"2 36' " "0 38 ' "4 41 "5 41 " '66 44"9 45 "4 45 " "24 49"5 50"3 50" "81 52"4* "39 56"3t d(calculated) = t n (n even), = n (n odd). n = number of carbon atoms in chain. 1. Irish vermiculite (Sutherland & MacEwan, 1961). 2. West Chester vermiculite (Sutherland & MacEwan, 1961). 3. West Chester vermiculite (Sutherland, 1962); see footnote, p. 93. * Not pure sample: Armeen '18D'. t Not pure sample: Liljeholmens Stearinfabriks AB 'C2o-C2~'. DISCUSSION The constant terms in the d versus n equations The constant terms in the calculated equations, (n even) and (n odd), are noticeably gt~eater than those in the equations derived by the least squares method, namely and The latter, however, are extremely sensitive to the gradient of the line, being in effect the result of an extrapolation to n = 0, from the observed range of data, n to n In Table 1, the observed values are seen to fall below the calculated values for n = 6 and 7. Possibly these short molecules are somewhat less steeply inclined to (001). For n~6, complexes of the type under consideration apparently are not formed, and this also was found by Brindley & Ray (1964) for the alcohol complexes. It may well be that the constant chain inclination with ~b ~ 65 ~ is established only for molecules with n > 8. It seems clear that the constant term in the d versus n equation can be estabfished accurately from experimental data only if the range of observed values can be extended to smaller values of n, and/or if the accuracy of values in the higher ranges of n can be substantially improved. Neither of these is likely to be feasible.

5 Clay-amine complexes 95 An alternative approach is to consider whether the smaller constant terms are compatible with a reasonable model. If the value of q~ ~ 65 ~ is accepted and if the bond constants for the alkyl chain are regarded as well established, then one must look either to a closer packing of the NH2 group against the silicate surface, or to some degree of close packing of the terminal --CH3 groups at the mid-plane. The former is very unlikely; the latter is possible but at present there is no supporting evidence. Molecular packing, or hydrogen bonding, as the explanation [or ~ ~ 65 ~ Caution is needed in drawing conclusions as to hydrogen bonding on the basis of the results obtained. Admittedly the model was arrived at in this way, but it can be seen from Fig. 2b that an adjacent molecule fits rather well with the one which is fully drawn; in other words, the orientation is also favourable for good packing of the chains. (~ 5* Mid-~ane [lllllljlllll o (a) A, h 5"18 ~. FIG. 2. (a) Idealized arrangement of surface oxygen atoms in montmorillonite and vermiculite. (b) Possible arrangement of amine molecule, with two NH-O contacts to surface oxygens, and 9 = 65 ~ Total basal spacing d = 2h A. The possible position of an adjacent molecule is shown by broken lines. The chain orientation of 65 ~ requires the angle NC1Cz to be 120" rather than the tetrahedral angle 109 ~ 28'. The larger value may be a consequence of the molecular packing (in which case, packing is important), but additionally some steric effects between the nitrogen atom and the CH2 group of the second carbon atom may be involved. In connection with molecular packing, it is important to remember that approximately one-sixth of the hexagonal holes of the silicate oxygen surfaces will be

6 96 G. W. Brindley occupied by monovalent cations in montmoriuonite, and divalent cations in vermiculite. With one neutral amine chain per hexagonal site, five-sixths of the total sites on each oxygen surface may be occupied by chain molecules, which means that the total number of chains stretching into the inter-silicate space exceeds considerably the maximum number which can be accommodated in a single layer. Therefore double layers of organic molecules tend to be formed. If the molecules were less densely associated with the silicate surfaces than this argument suggests, then single rather than double molecular layers might be expected.* Comparison with corresponding data for alcohols, and the nature of the organic layers Brindley & Ray (1964) showed that the alcohols formed two series of longspacing complexes corresponding to temperatures respectively above, and below, the pure alcohol melting points. Data for the low-temperature series, obtained only for C-12 and higher alcohols, are shown in Fig. 1 and these correspond to solid alcohols, Amines above Cz0 are normally solid at room temperatures. However, the data for the lower amines also fit reasonably well the same linear relation, from which it appears that the liquid amines behave in the same way as the solid amines (and the solid alcohols) when adsorbed in montmoriuonite. MacEwan & Arag6n (1959) considered that straight chain amines sorbed by graphitic acid possibly changed from a liquid to a solid state when the number of carbon atoms exceeded ten. Tensmeyer, Hoffmann & Brindley (1960) in infrared studies found that certain ketones sorbed into montmorillonite had some of the characteristics of the solid rather than of the liquid organic material. The suggestion in the previous paragraph that the double layer amine complexes may be close-packed also points to the possibility of the organic layers having solid-like characteristics. REFERENCES ARAG6N F., CANO Rutz J. & MACEWAN D.M.C. (1959) Nature, Lond. 183, 740. BRINDLEY G.W. & RAY S. (1964) Am. Miner. 49, 106. MACEWAN D.M.C. & ARA~6N DE LA CRUZ F. (1959) Nature, Lond. 184, SUTHERLAND H.H. (1962) Ph.D. thesis, Dundee, Scotland. SUTHERLAND H.H. & MAcEWAN D.M.C. (1961) Clay Miner. Bull. 4, 229. TENSMEYER L.G., HOFFMASrN R.W. & BRINDLEY G.W. (1960) J. phys. Chem. 64, WEIss A. (1963a) Angew. Chemic (Int. Edn), 2, 134. WEISS A. (1963b) Clays and Clay Minerals. Proc. loth Con/., p Pergamon Press, Oxford. WEISS A., MICHEL E. & WEISS A. (1959) Hydrogen Bonding, p Pergamon Press, Oxford. * A less dense association may arise when complexes are formed with cationic amines (i.e. alkylammonium ions). These do not form double layers of organic molecules. However, neither do they form simple single layers; the results of Sutherland & MacEwan (1961) indicate that a variety of molecular arrangements can occur with alkylammonium ions.

Ligand Close-Packing Model

Ligand Close-Packing Model! VSEPR is an electronic model for explaining molecular geometry.! The Ligand Close-Packing (LCP) model is a steric model. L The geometry of an AX n molecule is that which allows