J. Chern. Soc., Faraday Trans. I, 1987, 83 (lo), 3093-3106 The Crystal Structure of Sodium Diheptylsulphosuccinate Dihydrate and Comparison with Phospholipids Jacob Lucassen*? Unilever Research, Port Sunlight Laboratory, Wirral, Merseyside L63 3JW Michael G. B. Drew Department of Chemistry, The University, Whiteknights, P. 0. Box 224, Reading RG6 2AD Sodium diheptylsulphosuccinate can be crystallised in two polymorphic modifications, a monohydrate and a dihydrate. The dihydrate gave crystals of sufficient quality for a single-crystal structure analysis and its full structure has been elucidated. Th? crystal is monooclinic, spacegrpp P2,/a, with cell dimensions a = 7.79 (1) A, b = 9.80 (1) A, c = 32.63 (3) A, p = 92.5 (1)". There are 4 surfactant and 8 water molecules per unit cell.The crystal is racemic, i.e. there are two molecules of each optical enantiomer of the sulphosuccinate per cell. The water molecules form hydrogen-bond links between sulphonate oxygens of adjacent molecules. The discrimination in the crystal between the two optical enantiomers is through these hydrogen bonds. One of the water molecules causes enantiomer pairs to form, straddling two monolayers. The other water molecule is involved in the formation of infinitely long ribbons of alternating enantiomers in one monolayer in the direction of the a axis of the crystal. In the dihydrate, the aliphatic chains have a virtually zero angle of tilt with respect to the normal on the monolayer plane. From X-ray long spacings it is deduced that the monohydrate crystal has a tilt angle of ca.38". The molecular conformation, the packing in the crystal, the array of hydrogen bonds and the tendency to crystal polymorphism show a great similarity with recently elucidated crystal structures of phospholipids. It is suggested that the polymorphic transition between crystal types finds its parallel in monolayer-covered surfaces of sulphosuccinate solutions. Here, slow surface-tension changes could be ascribed to cooperative changes in tilt angle for rows of molecules, as found for phospholipid monolayers. Subsequent rapid formation of intermolecular hydrogen bonds would then consolidate this new con- formation. Sulphosuccinates, mainly in the form of Aerosol OT, the diethylhexyl derivative, find a wide application as wetting and emulsifying agents. AOT itself cannot be obtained in crystalline form; this can be attributed to the branched alkyl chains and to the presence of three asymmetric carbon atoms, which means that the compound is a mixture of diastereoisomers.While there is a vast amount of literature on AOT, sulphosuccinates with straight alkyl chains have received far less attention. Williams et a2.l showed that di-n-alkyl sulpho- succinates could be adequately recrystallised from methanol-water mixtures. Samples obtained by crystallisation had a high degree of surface chemical purity, as indicated by the absence of a surface-tension minimum near the critical micellar concentration. t Present address: Unilever Research Laboratorium, Postbus 1 14,3130 AC Vlaardingen, The Netherlands.3093 102-23094 Sodium Diheptylsulphosuccinate Dihydrate Table 1. Atomic coordinates ( x lo4) and equivalent isotropic thermal parameters (A2 x lo3) with estimated standard deviations in parentheses atom X Y Z Ua 3330 (3) 5098 (9) 2586 (9) 3041 (10) 354 (15) 3074 (14) 2274 (16) -30 (5) 2202 (12) -652 (10) -67 (9) - 1857 (17) - 2249 (21) - 1956 (22) -2481 (22) -2031 (29) -2615 (34) -2072 (46) 1307 (14) 2820 (1 2) 2170 (17) 3047 (23) 2287 (22) 3065 (31) 2312 (32) 3134 (57) 2446 (61) - 2736 (1 0) -121 (13) 357 (4) 2277 (5) 716 (9) 977 (8) 664 (1 2) 1095 (12) 732 (1 2) 1432 (14) 1059 (9) - 1087 (8) -280 (9) -711 (15) - 1526 (19) -890 (19) - 1552 (24) -969 (21) - 1634 (31) - 1142 (34) 2371 (11) 916 (9) 1551 (18) 929 (19) 1557 (22) 846 (22) 1403 (28) 1353 (62) 916 (9) 2191 (13) 735 (45) 857 (1) 307 (1) 926 (2) 484 (2) 879 (2) 1231 (3) 1270 (3) 1675 (3) 2028 (4) 975 (2) 1506 (2) 1486 (4) 1851 (5) 2248 (5) 2625 (5) 3035 (6) 3406 (7) 3797 (8) 2004 (3) 2382 (2) 2738 (4) 3110 (4) 3506 (4) 3882 (5) 4282 (5) 4658 (7) 5024 (1 0) 210 (2) -399 (3) 51 (3) 68 (5) 76 (9) 58 (8) 64 (8) 46 (1 1) 48 (10) 52 (12) 55 (14) 68 (9) 60 (8) 89 (16) 85 (19) 100 (20) 106 (24) 135 (26) 160 (33) 204 (43) 94 (12) 78 (10) 97 (18) 108 (20) 103 (20) 139 (28) 164 (34) 281 (58) 358 (90) 64 (10) 91 (18) In recrystallising a sample of sodium-di-n-heptyl sulphosuccinate from aqueous sol- utions we found evidence for crystal polymorphism. Thin leaflets, formed initially, were slowly replaced by fairly large, well shaped, elongated hexagons.X-Ray powder analysis showed a significant difference in long spoacing between the two types of crystals.The initial precipitate had a spacing of 25.6 A, while for the large crystals 32.6 A was found. Such differences in long spacings for amphoteric molecules are usually ascribed to different angles of tilt of the aliphatic chains with respect to the normal to the plane of the monolayers. Both types of crystal were hydrates. From n.m.r. measurements it appeared that the initial precipitate had just one molecule of hydration water per surfactant molecule, while the final precipitate was a dihydrate. In view of the lack of information on crystal structures of surfactants in general, especially on those crystallised from aqueous solution, we decided to attempt a single- crystal structure analysis.Experimental Only the relatively large, slow-forming crystals were found suitable for a single-crystal analysis. The crystal used was selected out of two precipitate samples which had been leftJ. Lucassen and M. G. B. Drew 3095 Table 2. Anion dimensions atoms length/A atoms angle/" s-C(2) s-O(2) s-O( 1) S-0(3) W - C ( 1) C(2)-C(3) C( 1)-O(22) C( 1)-O(2 1) 0(22)-C(21) C(21)-C(22) C(22)-C( 2 3) C(2 3)-C( 24) C(24)-C( 25) C(2 5)-C(26) C(26)-C(27) C(3)-C(4) 0(32)-C(4) 0(32)-C(3 1) O(3 1)-C(4) C(3 1)-C(32) C(32)-C(33) C( 3 3)-C( 34) c (3 4)-C( 3 5) C( 3 5)-C(36) C( 3 6)-C(3 7) 1.792 (10) 1.458 (8) 1.430 (8) 1.434 (8) 1.500 (15) 1.501 (14) 1.339 (1 3) 1.186 (12) 1.455 (14) 1.477 (1 9) 1.448 (21) 1.464 (21) 1.481 (23) 1.466 (24) 1.41 1 (28) 1.499 (1 7) 1.315 (13) 1.429 (15) 1.189 (14) 1.497 (19) 1.568 (20) 1.5 13 (23) 1.554 (26) 1.51 (3) 1.46 (4) C(2)-S-O( 2) C(2)-S-O( 1) O(2)-s-O( 1) C(2)-S-O( 3) 0(2)-S-O( 3) O( 1)-S-0(3) S-C(2)-C( 1) S-C(2)-C( 3) C( 1)-C(2)-C(3) C(2)-C( 1)-O(22) C(2)-C( 1)-O(2 1) O(22)-C( 1 )-O(2 1 ) C( 1)-0(22)-C(2 1) 0(22)<(21)-C(22) C(22)-C(23)-C(24) C(23)-C(24)-C(25) C(2)-C( 3)-C(4) C(2 l)-C(22)-C(23) C(24)-C(25)-C(26) C(25)-C(26)-C(27) C(4)-0(32)-C(3 1) C( 3)-C (4)- O( 3 2) C(3)-C(4)-0(3 1) 0(32)-C(3 1)-C(32) C(3 1)-C(32)-C(33) C(32)-C(33)-C(34) C(33)-C(34)-C(35) C(34)-C(3 5)-C(36) C(35)-C(36)-C(37) 105.7 (4) 106.4 (4) 112.0 (4) 106.0 (5) 113.3 (4) 112.8 (5) 108.8 (7) 110.4 (7) 113.8 (9) 113.5 (9) 124.8 (10) 121.6 (10) 115.5 (8) 110.5 (10) 117.4 (14) 121.5 (15) 121.6 (18) 120.4 (21) 120.3 (26) 112.4 (9) 115.8 (10) 111.8 (11) 125.8 (11) 108.5 (11) 109.5 (13) 109.5 (16) 11 1.5 (19) 11 1.6 (25) 109.0 (30) to crystallise without induction at room temperature for ca.6 months. The samples had an initial sulphosuccinate concentration of 5 x lop3 mol dmP3 and added NaCl concentrations of 2 x and mol dmP3, respectively. Crystal Data NaSO,C,,H,,, .A4 = 452.3, 2 = 4. The crystal wa? monoclinic, spacegroup P2,La. d, = 1.19 g ~ r n - ~ , d($ = 1.21 g cm-3, F(OO0) = 976, II (Mo K,) = 0.7107 A, ,u = 1.92 ern-'. A crystal of approximate dimensions 1.0 x 0.4 x 0.1 mm was set to rotate about the a axis on a Stoe Stadi 2 diffractometer. Data were taken via w scan with a variable width of (1.5 + sin ,u/tan 8) and a speed of 0.0 166" s-l. A total of 3549 independent reflections were measured with 28 < 50°, of which 1570 with I > 30 ( I ) were used in subsequent refinement.The structure was solved by direct methods. Atoms other than hydrogen were refined using anisotropic thermal parameters. Hydrogen atoms bonded to carbon were positioned tetrahedrally and refined ; those bonded to the same carbon atom were given a common thermal parameter. After refinement, a difference Fourier map was calculated in order to locate the hydrogen atoms bonded to the oxygens of the two hydrating water molecules, W(2) and W(1) (see below). Two peaks were located around W(2) but there were three peaks around W(1), u = 7.79 ( I ) A, b = 9.80 (1) A, c = 32.63 (3) A, p = 92.5 I/= 2488.7A3,3096 Sodium Diheptylsulphosuccinate Dihydrate Table 3.The sodium ion environment atoms bond length/A N a a ( 2 ) 2.450 (8) Na-W( 1) 2.501 (9) Na-W(2) 2.303 (10) Na-0(21) 2.548 (9) Na-0(2”) 2.606 (9) Na-O( 1 ji) 2.817 (10) Na-W( 1 i, 2.544 (9) Table 4. Intermolecular hydrogen bonding atoms length/A atoms angle/’ W( 1) - * * O( Pi) 2.95 W(1)-H(W1 1) * . *O( liii) 154 W(1)-0(2”) 2.93 W( 1)-H(W 12) * - O(2”) 93 W(1). .0(21) 2.92 W( 1)-H(W 13) * * eO(21) 148 W(2) * O(3’) 2.85 W(2)-H(W22) * . * O(3”) 166 W(2)..*0(3”) 2.9 1 W(2)-H(W21)..*0(3”) 152 Symmetry elements : i 0.5+x, 0.5-y, z ii -0.5+x, 0.5-y, z 111 -1 +x, y, z iv -x,-y,-z vi - l + x , y, z ... v 0 . 5 - ~ , 0.5+y, - 2 viii 0.5-x, -OS+y, -z indicating some positional disorder. These three positions were all included in the refinement with occupancies of 0.66.The parameters of all these five hydrogen positions were allowed to refine independently and converged successfully. The structure was refined by full-matrix least-squares to R = 0.075 (R, = 0.079). The weighting scheme was w = [a2F+ 0.003F2]-’, a(F) being taken from counting statistics. Calculations were carried out using SHELX 762 and some home-made programs on the Amdahl V7 computer at the University of Reading. Final atomic coordinates are given in table 1 and anion dimensions in table 2. Details of close contacts between oxygen and the sodium ion are given in table 3 and the hydrogen bonds involving the two water molecules in table 4. Torsion angles, thermal parameters, structure factors and hydrogen atomic coord- inates are available as Supplementary Publication No.56672. t Results and Discussion Molecular Conformation in the Crystal The unit cell contains four surfactant molecules (four sodium ions and four sulpho- succinate ions) and eight water molecules. The crystal is racemic, i.e. there are two molecules of each optical enantiomer of the sulphosuccinate per cell. Fig. 1 shows the conformation of the sulphosuccinate ion in the crystal together with -f See Notice to Authors, J. Chem. SOC., Faraday Trans. I, 1987, 83, Part 1.J. Lucassen and M. G. B. Drew y chain 3097 Fig. 1. Conformation of the diheptylsulphosuccinate anion in the crystal together with the atomic numbering scheme. The numbering scheme is based on the convention for phospholipids (see text and fig. 2). y chain O W ) p chain Fig.2. Conformation of dilauroylphosphatidylethanolamine together with the atomic numbering scheme. Coordinates are taken from those of ref. ( 5 ) with the addition of some hydrogen atoms in calculated tetrahedral positions. the atomic numbering scheme. Because of similarities with other dialkyl amphiphiles, notably phospholipids, we used the system suggested by S~ndaralingam~. * for this class of compounds. Thus in fig. 1 the more and the less protruding aliphatic chains are indicated by y (or 3) and /3 (or 2), respectively. In fig. 2 the conformation for dilauroylphosphatidylethanolamine5 is shown for comparison. As is to be expected, the two hydrophobic chains are parallel to each other and approximately parallel to the c direction, but the heptyl group closest to the sulphonate only extends to the fourth carbon of the other chain.This leads to a staggered arrangement of the chain ends in the crystal. Both chains have an all-trans configuration, and the planes through their carbon atoms intersect at an angle of 74.3". By comparison, the same angle for the phospholipid (fig. 2) is only 7.0". As is apparent from fig. 3 (the c projection), this is caused by a rotation in the ab plane of the chains relative to each other. Both chains are aligned approximately parallel to the c direction, as can be seen in fig. 4. This can only be realised when at least one of the two carboxy groups makes an angle with the adjoining carbon chain. In the event this is the C(2)0(22)0(21)C(l) group, which has an angle of intersection with the /3 chain of 52.9' (see fig.1). The other carboxy group is virtually coplanar with the y chain (angle of 4.3").3098 Sodium Diheptylsulphosuccinate Dihydrate Fig. 3. The c projection of the unit cell. The intermolecular hydrogen bonds are shown as dotted lines. For clarity these are not superimposed on the diagram but shown separately. In the top half of the figure the continuous - ' O(3). . - W(2) - - O(3) ribbons are shown, while in the bottom half of the bonding around a centre of symmetry involving W(1), 0(2), 0(1) and O(21) is shown. The lower right-hand part qf the same figure shows the environment of the sodium ion. There are seven close contacts < 3.00 A (listed in table 3). Six are shown as arrows. The seventh interaction is with W(2), which is immediately below the sodium cation.0 d Crystal Packing and Intermolecular Interactions As is expected for this sort of compound, the crystal consists of a 'stack' of monolayers, arranged back-to-back and face-to-face, i.e. with close mutual contact between hydro- philic and hydrophobic groups, respectively, in adjacent layers. The b-projection (fig. 4) gives the best impression of the molecular packing. In this figure it can be seen that the hydrating water molecules, indicated by W( 1) and W(2), participate in creating an intermolecular network in the hydrophilic layers. They form hydrogen-bond links be- tween neighbouring molecules involving all three oxygens of the sulphonate groups. This is in addition to the interactions between the sodium ion and surrounding sulphonate and water oxygens.As illustrated by table 3, there are seven sodium-oxyg%n distances c 3.0 A, of which six fall yithin the range 2.30-2.61 A. The seventh is 2.82 A, but the next largest distance is 3.75 A. The sodium is bonded to three water molecules, W( l), W(2) and W( li), one monodentate sulphonate oxygen, 0(2), one bidentate sulphonate group O( li) and 0(2i)'f and one carboxyl oxygen, O(21). The geometry around the sodium ions is irregular. The geometry around the water molecules is given in table 2. As stated in the t Roman numerals as superscripts refer to atoms in symmetry-related positions. A full list is given at the bottom of table 4.J. Lucassen and M. G. B. Drew 3099 0 1 B3100 Sodium Diheptylsulphosuccinate Dihydrate bL c sinp Fig.5. Enantiomeric pairs, connected by water W( 1) : (a) in the b projection, (b) in the a projection. Experimental section, three hydrogen atoms were refined for W(1), but only two for W(2). The arrangement around W(2) is straightforward, with hydrogen atoms pointing towacds O(3") and O(3") to form hydrogen bonds with bond lengths of 2.91 and 2.85 A, respectively. This water molecule connects opposite enantiomers in the a direction, giving rise to infinitely long ribbons. The c projection (fig. 3) clearly illustrates the zig-zagging nature of the hydrogen-bonded chains, whereby it connects adjoining rows of molecules in the b direction (see fig. 4). The arrangement around the other water qolecule, W( l), isomore complicated. It js close to three oxygen atoms, O(2'"l at 2.93 A, O(21) ,at 2.92 A and 0(liii) at 2.95 A, and to two sodium ions, Na at 2.50 A and Na" at 2.54 A.A study of the bond angles for W( 1) shows that the hydrogen bonds with the latter two oxygens are complicated by the close proximity of the sodium ions.? Fig. 5 illustrates the hydrogen bonds in which W(1) participates. As can be seen from this figure, it connects pairs of opposite enantiomers located in adjoining crystalline monolayers through the sulphonate oxygen bonds. Finally, the arrows in fig. 3 illustrate the environment of the sodium ion in the c projection. Comparison with known Crystal Structures Remarkably few surfactant crystal structures have been elucidated. In some instances, e.g. for ethylene oxide adducts, dodecylbenzenesulphonate or AOT, impediments pre- t This steric hindrance between sodium ions and hydrogen atoms could give rise to acid hydrolysis6 once the constraint of strict stoichiometry has been removed, as it is in a monolayer at the air-water surface.If for such a monolayer intermolecular distances were comparable to those in the crystal, sodium ions could be ejected from the surface while the combination of water molecule and sodium ion would be replaced by a hydrated proton.'J. Lucassen and M. G. B. Drew 3101 venting the formation of suitable crystals are obvious. A wide distribution of molecular weights, chain branching, ortho-para isomerism and diastereoisomerism are all detri- mental to controlled growth of single crystals. Many of the remaining surfactants, even if they are pure, crystallise in unmanageably thin leaflets or long needles, unsuitable for single-crystal X-ray crystallography.This seems to be the case especially for crystals grown from aqueous solutions and, as far as we could establish, all structure deter- minations have been carried out on crystals obtained from organic solvents. Thus the sulphosuccinate crystal grown from aqueous solution seems to be exceptional.? The reason for this may be related to the need to form an intricate network of hydrogen bonds in the crystal. For thin leaflets, crystal growth in the direction perpendicular to the monolayers is relatively slow, while growth in the plane of the monolayers, i.e. equivalent to our a and b axes, is fast. We suggest that the need for forming hydrogen bonds in these directions, and to manoeuvre the right enantiomer in the appropriate position, will slow down the growth rate relative to that in the c direction, thus resulting in crystals with a more ‘ balanced ’ size.It has been found for a number of crystal structures that water molecules play an essential role in the formation of a three-dimensional network of hydrogen bonds. Early examples include mesotartaric acid monohydrate, a oxalic acidg and acetylenedicar- boxylic acid dih~drate.~ Another group of related molecules for which various aspects of conformation and packing are being thoroughly studied are the phospholipids. As late as 1972, Sundar- alingam3 stated : ‘The phospholipids form a major fraction of biological membranes but the X-ray structure of none has yet been determined’.During recent years, however, considerable progress has been made in elucidating their crystal structures, and they show features which are remarkably similar to those of the sulphosuccinate crystal reported here. The similarities are in the general conformation of the molecules (see fig. 1 and 2) as well as in the characteristic hydrogen-bonded organisation of their polar groups. In lecithin dihydrate,’O which (in spite of having been crystallised from an ethanol-ether-water mixture) contained two hydration molecules per lipid molecule, the analogy is nearly perfect. One of the water molecules is located between phosphate oxygens of adjoining lecithin molecules in a monolayer, thus forming an infinite hydrogen-bonded phosphate-water-phosphate ribbon and providing stability in the a direction.The other water molecule establishes a link across the bilayer interface, in this case between one phosphate oxygen and the corresponding symmetry-related water molecule in the adjacent monolayer. In D,L-dilauroylphosphatidylethanolamine, 5* l1 the conformation of the aliphatic chains and the arrangement of optical enantiomers in the crystal is virtually the same as for the sulphosuccinate. For this compound, which was crystallised from glacial acetic acid, the arrangement of the headgroups is compared with that of sulphosuccinate in fig. 6. Hydrogen-bonded ribbons are formed linking up amine and phosphate groups in adjacent molecules. The chemical bond between phosphate and amine in the same molecule will provide a ‘ cross-link ’ between the abovementioned ribbons and will cause additional rigidity in the crystal.Hydrogen-bonded links between the monolayers are formed via an acetic acid molecule of crystallisation. Both in the phospholipid and in the sulphosuccinate, the p and y chains, i.e. the less and the more protruding chains, respectively, form a triangular lattice. The inserts in fig. 6 show that the lattices are slightly more complex than those based on simple translation, suggested by Albrecht et aZ.12 It can be seen that while in the phospholipid the a and /? chains are arranged in single rows, in the sulphosuccinate they form double rows. f It is noteworthy that the mean thermal parameters of the atoms in the anion (table 1) increase markedly towards the ends of the chains.This reflects considerable freedom of movement in this region and indicates the difficulty in obtaining crystalline order for long-chain surfactants.3102 Sodium Diheptylsulphosuccinate Dihydrate P Fig. 6. The disposition of head groups for (a) a phospholipid and (b) a sulphosuccinate. (a) Dilauroylphosphatidylethanolamine [coordinates from ref. (5)] is here plotted out down the long axis. Hydrogen bonds [of the type N-H.--O(PO,)] that hold the molecules together are illustrated as dotted lines. (b) The diheptylsulphosuccinate anion plotted down the long axis. The - - - W(2) - - - O(3). . - W(2) continuous hydrogen bonding is shown. The inserts in this figure illustrate the triangular lattice formed by the protruding aliphatic chains.The shorter (B) chains are shown as open circles and the longer (7) chains as closed circles.J . Lucassen and M. G. B. Drew 3103 Fig. 7. Suggested packing (b projection) for crystal with tilted chains. Angle of tilt is 38". Ref. (4) describes some other phospholipid crystal structures, all showing similar conformation, packing and hydrogen-bond arrangements. Alternative Crystal Structure of Sodium Diheptylsulphosuccinate We mentioned before that sodium diheptylsulphosuccinate crystallises in two modifi- cations, distinguishable by their X-ray long spacings. The alternative crystal form, which precipitates first, was shown by n.m.r. to contain only one molecule of hydration water per surfactant molecule. Obviously, the hydrogen-bond network will in that case be less intricate and more readily formed, resulting i? strongly asymmetric crystals.The reduction of the long spacing from 32.6 to 25.6 A must increase the intermolecular distances in the ab plane; hydrogen-bond links in this plane are the most likely to have disappeared. Fig. 7 shows a possible structure for this metastable crystal in which W(2) has disappeared and in which an angle of tilt of arccos(25.6/32.6) = 38" between the chains and the normal to the monolayer planes has been introduced. This type of crystal could then grow by simple juxtaposition of doublets of enantiomers connected by W( 1). Molecular arrangements with similar tilt angles (41 ") have been found for crystals of lysophosphatidylcholine l3 and of cerebroside. l4 We can compare the postulated tilted structure in fig. 7 with the equivalent projection of the dihydrate in fig.4. The correct arrangement of enantiomers in the dihydrate crystal depends upon the ability to form the proper intermolecular hydrogen bonds with W(l) and W(2). In the tilted structure the hydrogen bonds in the a direction, which primarily involve W(2), are absent and the packing conditions in that direction may well be relaxed. In the dihydrate W(l) also participates in hydrogen bonding with 0(21), but it is impossible to tell if such a bond would persist in the tilted structure. However, it does seem clear that there is less likelihood of a strict ordering of optical enantiomers in the monohydrate than in the dihydrate.3 104 Sodium Diheptylsulphosuccinate Dihydrate Table 5.X-Ray long spacings for various sodium dialkylsulphosuccinates long spacingjA alkyl group untilted tilted - hexyl 23.5 heptyl 32.6 25.6 27.5 hexadecyl 55.6 - octadecyl 60.4 octyl - nonyl 38.4 - - We attempted to prepare single crystals of the monohydrate to test these tentative conclusions. This proved impossible, but for the sodium dihexyl sulphosuccinate homologue it was found that a tilted structure (c = 23.50 A) could be grown from concentrated aqueous solution in fairly large, diamond-shaped crystals. However, these crystals were not suitable for single-crystal analysis as the diffraction pattern was weak and the spots were very broad. This is not inconsistent with our speculation that in the tilted structure the constraining lateral hydrogen bonds and the strict enantiomer ordering are absent. Table 5 shows X-ray powder diagram long spacings for a number of pure dialkyl- sulphosuccinates.Both for untilted and tilted structures there is a linear relationship between long spacing and chain length. Least-sqtares fits for both structures gave increments per methylene group of 10.255 and 1.00 A, respectively. This compares well with the increments of 1.25 and 0.98 A expected from carbonxarbon distances in an all- trans chain. The extrapolated long spacing for zero aliphatic chain length could be thought to represent the effective thickness of the hydrophilic layers in the crystal. The fact that it changes by roughly the same factor as the abovementioned increment in going from untilted to tilted (0.748 compared to 0.797) provides another argument in favour of the crystal structure suggested in fig.7. Once again, there is a striking analogy between sulphosuccinates and phospholipids, this time with regard to crystal polymorphism, reorganisation of hydrocarbon chains and changes in hydration. For n-palmitoylgalactosylsphingosine15 an untilted anhydrous crystal transforms into a tilted hydrated modification. Dilauroylphosphatidyl ethan- olamine," on the other hand, dehydrates upon conversion into a more stable form. Thus, conversion into more stable forms can involve either hydration or dehydration. De- pending on molecular geometry and available hydrogen- bond donors and acceptors, inclusion of an extra water molecule can apparently either increase or decrease the internal energy of a crystal.While for the sulphosuccinates, transformation from metastable to stable appears to proceed through a nucleation-and-growth mechanism, for similar transitions in the phospholipids rearrangements occur in existing crystals through cooperative processes. Such processes, involving changes in state of hydration and in hydrophobic chain orientation, are of potential importance in determining the biological function of lipid membranes. Relation between Ordering Processes in Crystals, Membranes and Surfaces It had been observed'' that submicellar solutions of sulphosuccinates showed a slow surface-tension equilibration which could not be explained on the basis of diffusion toJ. Lucassen and M. G. B. Drew 3105 the surface only. Our very pure samples of diheptylsulphosuccinate showed a similar slow surface-tension change and a simultaneous increase in the surface dilational mod- ulus. It has been argued'' that slow changes in surface tension and increased modulus values are likely to be caused by cooperative ordering in the surface rather than by adsorption barriers between surface and solution.The question arises whether polymorphic transitions in crystals and restructuring of monolayers in single surfaces have in fact a common cause. Certainly, those forces which are operative in determining crystalline order in three dimensions will make their influence felt in one or two directions only, before crystallisation sets in. One possibility is that after sulphosuccinate ions arrive at the surface in an on-average tilted position, they move into a more upright position, allowing them to form intermolecular hydrogen bonds, similar to those observed in the a direction of the crystal [involving W(2)].Such an ordering in one direction would result in polymer-like hydrogen-bonded rows of molecules. In the liquid-air surface such rows will, at least originally, consist of random mixtures of both optical enantiomers. If there were an additional ordering in alternate optical enantiomers, such polymeric units in the surface could serve as nuclei for crystallisation. Evidence for any enantiomer ordering in surfaces is hard to obtain.lg We did observe, however, that above the solubility limit rather well shaped crystals do form in the air-water surface in such a position that their ab planes are parallel to the surface.Thus a change in angle of tilt of adsorbed molecules, giving then a closer packing and a more perpendicular arrangement of their chains, could be the cause of measured slow changes of surface tension with time. The slowness of the rearrangement should be ascribed to its cooperative character, to the necessity of moving molecules into another conformation and possibly to the need for a limited enantiomer ordering. In itself, formation of hydrogen bonds, once the intermolecular distance is as required, should be a very fast process. Thus the knowledge of the detailed crystal structure of a surfactant may give a better idea of its behaviour in surfaces. It should also give useful information on the ordering in thin films and membranes.Especially interesting is the tendency to form intermolecular hydrogen bonds, either with or without direct involvement of water. If such bonds are formed in the lateral direction, i.e. in the monolayer plane, they can be expected to lead to an increased rigidity, or elastic modulus for surfaces and thin films. The measured slow increase' of surface elastic modulus during equilibration does, therefore, fit in with the general picture. For lipid membranes, with hydrophilic groups, on the outside of the membrane, these lateral bonds will provide the required stability and rigidity. All lipid crystal structures solved so far show lateral hydrogen bonding. For phospholipid monolayers there is considerable evidence2O. 21 for polymorphism which is related to changes in tilt angle.Thus for phospholipids there is a clear relation between rearrangement in monolayers and the change of crystal structure, and the case for a similar relation for the sodium diheptylsulphosuccinate appears to be quite convincing. There is possibly a structural similarity between isolated bimolecular layers in crystals and free Newton black films. Such films consist of bimolecular layers as well, with only a few more water molecules associated per surfactant molecule than found in the sulphosuccinate dihydrate. For sodium dodecylsulphate films, for example, de Feyter and Vrij22 estimated the number of associated waters as 5-8. They also found evidence that the dodecyl sulphate and sodium ions in Newton black films form a highly ordered two-dimensional lattice.Thus, just as in the crystal, water-mediated hydrogen bonds could well have a struc- turing function for Newton black films. Indirect evidence supporting the presence of such bonds is present in ref. (23). Both urea and sucrose had definite but opposite effects on the free energy of film formation.3 106 Sodium Diheptylsulphosuccinate Dihydrate Conclusion The work reported here represents the first full crystal-structure determination of a surfactant crystallised from aqueous solution. Some hitherto little-known aspects of intermolecular interaction forces between the ionic headgroups in crystals have been revealed. Hydrogen bonding through hydrating water molecules plays an important role just as it does for phospholipids, and this could be a more general feature in surfactant crystal structures than has been realised so far.The authors thank Dr C . D. 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