1. Introduction

The determination of the structures of self-assembled surfactants, particularly in aqueous solu-tions has been exercised by various physical andchemical techniques as well as by molecular simula-tions during the last ten years [1, 2]. A compre-hensive review of the advancement of surfactantarchitecture, applications and the physical tools tostudy shape, size, interaction between micelles, coun-terion binding is given in detail very recently byDegiorgio & Corti in “Physics of Amphiphiles,

Micelles, Vesicles and Microemulsions” [3], whereasvery recently Magid reviewed the physical behaviorof ionic surfactants in solution [4]. –In contrast tononionic surfactants, i.e. Triton series, C12Ex as wellas nonionic detergents chemically based on sugar, e.g. N-n-undecyl-D-glucoamide [5], cationic surfactants,e.g. single chained of the hexadecyl(cetyl)trimethyl-ammonium bromide (CTAB) or cetylpyridiniumchloride (CPC), or double chained cationic sur-factants of the type of dioctadecyldimethyl-ammon-ium (DSDMA) chloride, reveal antimicrobial, anti-viral and antifungal activities [6-9]. In addition

20 The Rigaku Journal

The Rigaku Journal

Vol. 17/ No. 2/ 2000

CONTRIBUTED PAPERS

CRYSTALLINE POLYMORPHISM OF CETYLTRIMETHYLAMMO-NIUM BROMIDE AND DISTEARYLDIMETHYLAMMONIUM(DSDMA) COMPOUNDS. A COMPARISON OF THE HYDRATEDDSDMA-CHLORIDE, DSDMA-S-(+)-LACTATE AND DSDMA-PYRUVATE SYSTEMS

H. HENRICH PARADIES* AND SHAUN F. CLANCY+*University of Applied Sciences, Biotechnology & Physical Chemistry, Frauenstuhlweg 31, D-58690 Iserlohn, also at University Paderborn, Chemistry &Chemical Engineering, Warburger Strasse 100, D-33095 Paderborn, Germany. E-mail: HParadies@aol.com+Crompton Inc., Research and Development, One American Lane, Greenwich, CT 06831-2559, USA, E-mail: clancsh @cromptoncorp.com

The crystal structures of three forms of hexadecyltrimethly(cetyl)ammonium bromide (CTAB; hexadecyl =cetyl) have been determined revealing not only differences in crystal morphology but also show differentconformations which depend entirely on the growth conditions within a narrow range of physical parametersincluding the application of organic solvent systems and CTAB concentrations. The first CTAB polymorph hascell dimensions of a=0.5634 nm, b=0.7254 nm, c=2.601 nm, α=γ= 90°, β=93.73°. The second one has unit celldimensions of a=0.5596 nm, b=0.7162 nm, c=2.5899 nm and β = 96.64°. Both CTAB polymorph belong to themonoclinic space group P21(#4) so does the third polymorph of CTAB having cell dimensions of a=0.5631 nm,b=0.7259 nm, c=5.1956 nm and β=92.40°, space group P21/n (#14). All crystalline polymorph forms of CTAB do not contain any water in interstitial spaces of the crystal lattice in contrast to the recent determined structure ofhexadecylpyridinium bromide or chloride, which contains one water molecule in the triclinic lattice (Paradies &Habben, Acta Cryst., 1993, C49, 744-747). The molecules of CTAB in the different polymorphs although packeddifferently show that the aliphatic chains are in extended conformation and inclined to the b-axis by 20-24°. TheCTAB molecules are packed in a bilayer running parallel to the 010 plane or 001 plane, respectively. The Br-

anions are located at different positions in the three CTAB polymorphs with respect to the extended n-alkylchains, however, in all polymorphs they are more then 0.4 nm apart from the quaternary nitrogen.

Preliminary crystallographic data are available for the dioctadecyldimethlyammonium chloride (DSDMA ACl-H2O), and for the same double chained cationic surfactant in the presence of S-(+)-lactate or pyruvate,respectively. The crystalline specimens were obtained from their solutions in 2-propanol/hexane/water(80/18/2% v/v) at 20°C. DSDMA - Cl - H2O in a monoclinic unit cell (P21/c), Z=2, a-2.9837 nm, b=1.23101 nm,c=1.4065 nm, β=l 10.06°. DSDMAACl - H2O crystals undergo an irreversible transition to a triclinic unit cellhaving dimensions of a=5.96741 nm, b=2.4620 nm, c=2.813 nm, α= 10.433°, β= 103.06° and γ=74.93°, Z=2 ( P1) when crystallized from solutions containing 80% hexane, 15% 2-propanol and 5% water (v/v) at 10°-15°C.DSDMA in the presence of pyruvate or S-(+)-lactate crystals belong to the monoclinic space group havingdifferent cell dimensions than the DSDMACl-H2O crystals but almost similar dimensions with respect to the c-axis.

DSDMAX molecules, known also as cationic lipids,are capable of forming unilamella or multilameliavesicles or phases, that can swell in a solvent. Forionic systems the swelling in water and in oil-likeenvironments is caused by electrostatic double layerforces [10]. Moreover, these fully synthetic vesicleshas been used as model systems to study ultra fastproton transfers of biologically active molecules ex-perimentally [11-13]. Furthermore, cationic lipids areknown as efficient non-viral reagents to transfectouter DNA into animal cells in vitro [14] throughliposome like DNA complexes. Anti-viral and anti-microbial activities in-vitro & in-vivo of single anddouble chained N-cationic surfactants have beenreported and are being used in medical applications[15], where the antiviral activities can be considerably enhanced through complexes of the cationic N-sur-factant and Zn-D-gluconate [16]. In addition, theenvironmental concerns of double chained N cationicsurfactants of the DSDMA X type including theirbiodegradability [17] which is related to the inter-actions between DNA or RNA & proteins and thesurfactant at very low surfactant concentrations prom-pted us to study the structures of DSDMA X withX=Cl -, S-(+)-lactate or pyruvate, respectively, in thecrystalline state, since the “monomers” of thesematerials are the biologically active entities rather themicellar or vesicular state of these surfactants with re-spect to their critical micelle concentration (CMC)[18].

Calculations of the energy changes due toconformational changes of the packing of the DNA,performed by Riemer & Bloomfield [19] using theencapsulation of DNA by bacteriophage T4 as amodel system, it became obvious that electrostaticrepulsions dominate the other forces which oppose the collapse of DNA into an extended conformation. Theprocess of collapsing can be facilitated by neutrali-zation of the high charge density within the condensed DNA domain. The purely electrostatic counterioncondensation theory of polyelectrolytes [20] andneutralization depend on the condensation of counter-ions close to the backbone of the DNA and/or thecationic double chained micelles, which are inequilibrium with their monomers in the outer layer in a process which is driven by the charge density of thebiopolymer, the character and nature of the counter-ions, including their valence, structure and chirality.The extent of neutralization under specified ionic andDSDMA X conditions can be estimated from theorywhich provides the means to determine critical

conditions beyond which the collapse of T4 DNAbecomes spontaneous. However, these conditionsconceivably depend not only on the charge of thecounterion, its structure, or the N-ionic surfactantconcentration (concentrations of charged species vs.total surfactant concentration where the extended T4DNA is the polyelectrolyte), but also on the geometryof the collapse and the geometry of DSDMA inducedmechanism leading to a collapse, which is balancedbetween DNA-DNA contacts and DNA.DSDMAcontacts including solvent. The possible role of thecounterions, including those having a chiral center, instabilizing various DSDMA X forms in aqueoussolutions (e.g. unilamellar vesicles, threadlike mi-celles & spherical micelles [21]) is an additionalfactor in stabilizing the compact (globule) form of theDNA. The counterion dependency influences consi-derably the micelle size, shape and micellar inter-actions as well as the transitions from spherical torod-like structures, or spherical-entanglement-rod-like assemblies [22]. In addition the present analysis,where we discovered various crystalline forms ofCTAB and DSDMACl-water or their biochemicalderivatives, S-(+)-lactate and pyruvate, should yieldinformation about counterion location, hydration, and n-alkyl-chain packing taken recent reports on thestructures of dodecyldimethylpropylammonium bro-mide monohydrate and hemihydrate [23, 24], as wellas the CTAB structure [25], N-n-butylpyridiniumchloride [26], and of N-hexadecylpyridinium chlo-ride monohydrate into account [27].

Polymorphism is an important aspect of thethermodynamics of molecular crystals, even littleattention has been devoted to this specific problem sofar, particularly for N-cationic surfactants. However,it frequently happens that different polymorphs areobtained by crystallization from different solventsystems, in particular when the molecule can adoptdifferent possible conformations, and the number ofpolymorphs would certainly increase considerably ifa larger number of solvents and other physical para-meters were investigated, especially for such flexiblemolecules like CTAB or DSDMAX. The results ofsuch experiments will be described in this contribu-tion, where we are able to show that differentcrystalline forms of CTAB can be obtained revealingdifferent conformations and packing density of the n-alkyl chains within the lattices. Furthermore, the in-fluence of organic counterions on the structure ofDSMAX (X=Cl- , S-(+)-lactate or pyruvate) and theirinfluence on the packing of the n-alkyl chains will be

Vol. 17 No. 2 2000 21

shown. Moreover, DSDMA is chemically relativesimple as a lipid-model it would be interesting to seethe changes upon binding of biochemical activemolecules, e.g. pyruvate or S-(+)-lactate, on the mo-lecular level, especially in comparison to the chlorideform, as well as to the recently determined structure of the DSDMA-Br-H2O by Okuyama et al. [28].

II. Experimental and Structure Determination

CTAB as a crude product was recrystallized twice from concentrated aqueous NaBr solutions, and sub-sequently from ethylacetate/ chloroform/acetone mix-tures (90/5/5%v/v) until CTAB showed a constantmelting point of 185.5°C, a clearing point of 290.7°Cand a transition temperature of 175-182.6°C [29]. The chemical composition of the crystalline material re-vealed a molecular weight of 364.5, and a elementaryanalysis equivalent to a compound of C20H44NBr,containing no water.

Colorless plate-shaped crystals were obtainedfrom ether/ethanol (80/20%v/v) solution at 10°C, orfrom ethylacetate/water (70/30%v/v) at 20°C. Thecrystals grown at 20°C or at 10°C have the same crys-tal habit and display the same morphology includingdissolution kinetics in physiological buffers at pH 7.0. According to the chemical analysis these crystals alsocontain no stoichiometric amounts of water (poly-morph 1). –Crystal survey, unit cell determination,and data collection were performed using CuKαradiation at -120°C for these particular CTABcrystals. A single crystal having approximate dimen-sions of 0.30 x 0.20 x 0.025mm was mounted on aglass fiber. All subsequent measurements were madeon a Rigaku AFC-5R Diffractometer with graphitemonochromatized CuKα radiation, and a 12 kW rota-ting anode generator. Experimental conditions andrefinement parameters for the three CTAB poly-morphs are listed in Table 1.

Another crystalline form of CTAB was obtainedfrom purely organic solvents comprising of ether/ethylacetate (90/10%v/v) at 20-25°C. A colorlessplate-shaped, however elongated in one directioncrystals with approximate dimensions of 0.4 x 0.3 x0.0l mm were obtained, and one single crystal wasmounted on a glass fiber at 20°C and processed fordata collection etc. (polymorph III).

The third polymorph of CTAB (polymorph II)consists of also of plate-shaped single crystals grownfrom solution containing ethanol-water (80/20%v/v)at 20°C. These crystals were extremely thin plates

partly grown together and did not diffract very well ascompared to to the other CTAB polymorphs. Anumber of crystals were measured and one of thetabular crystal of CTAB was selected. A monoclinicunit cell was found (Table 2), and β was initiallyobtained from a least squares fit based on the settingangle of twenty reflections. A routine search formissing zones located a few weak reflections alongthe c-axis which indicated a possible larger cell withthe c-axis tripled. The data were collected for thelarger cell and the structure was solved by directmethods-as the other ones in this contribution-to givethree parallel molecules related by z, z+¼, and z+¾,respectively, while the corresponding x and y coor-dinates were the same. The magnitude of the c-axiswas therefore reduced to the original value, and theindices and the coordinates were transformed to thereduced unit cell.–The structure was refined by full-matrix least squares and difference Fourier methods.Although a data set was suitable for structure solutionand refinement was obtained using a rotating anodesource, not all hydrogen atoms could be refined aniso-tropically, because of the relatively low reflec-tion toparameter ratio. The bromine atom was refined aniso-tropically, and the remaining non-hydrogen atoms,hydrogen positions were calculated assuming idealgeometries. The maximum residual electron densityfrom the final difference Fourier map is 2.05 x 10-2 e-/nm3, which is 0.22 nm from Br(l). This peak is tooclose to the bromine atom to be a water molecule andis probably caused by errors in the data which are theresult of the poor crystal quality.–Anomalous scat-tering terms were included to determine the absolutestructure of this CTAB molecule. Structure “A” wasrefined to convergence with a resulting R factor of0.117 and a weighed R factor of 0.127, respectively(Table 1). Structure “B” was generated and refined toconvergence with an R factor of 0.118 and a weighedR factor of 0.128. Structure “A" was used in the finalmodel.

As expected, this CTAB structure (polymorph II)is a polymorphic form of the other two samples ofCTAB. This sample crystallizes in P21/n, while theprevious CTAB polymorphs crystallize in P21 (Table2).

DSDMAACl-water crystals were prepared bydissolving the material in 2-propanol at 60°C,centrifuging the non-crystalline precipitate in a J2-21(Beckmann) centrifuge at 25,000 rpm (5°C) for 10minutes. To the supernatant n-hexane has been addedat 20°C, and the occurring precipitate (non-

22 The Rigaku Journal

crystalline) was centrifuged off, and the remainingclear solution was kept at 20°C for several weeks. The crystals, which develop after two weeks look likesabers, they are transparent and have normally sizes of 2.0 x 0.1 x 0.05 mm. Single crystals suitable for X-raydiffraction, which have been developed over a periodof time of four weeks have sizes of 1.5 x 0. 2x 0.l mm(20°C). Many of them were usually multiple crystalswith the common axis normal to the long surface.After many trials of X-ray diffraction investigations of such crystals, only a few were found to be suitable forX-ray diffraction analysis. Approximate dimensionsof these crystals used in this study were of sizes of 1.5x 0.25 x 0.l mm (20°C). –Plate like crystals ofDSDMAACl-water were obtained at 25°C, normallywith dimensionsof 0.15 x 0.15 x 0.0l mm.

The micro-crystalline material specimens ofDSDMAACl-water were examined by powder x-raydiffraction similar as described in ref. 30. Indexing ofthe diffraction lines was performed using the software program TREOR 90 from Cerius 2.4 obtained fromMolecular Simulations Inc., San Diego (USA).

Preliminary crystallographic data of DSDMAS-(+)-lactate and DSDMA pyruvate, respectively, wereobtained from crystals grown form solutionscontaining 80% (v/v) n-hexane, 15% (v/v) 2-propanol and 15% (v/v) water at temperatures between 10-15°C by means of vapor diffusion as described in ref.31. The preliminary crystallographic data includingunit cell dimensions and density obtained from small

Vol. 17 No. 2 2000 23

P I P II P III

Scan type ω-2θ w-2θ w-2θ

Scan rate in ω (°/min) 8 32 8

Scan width (°) 0.27 0.33 0.53

2θmax (°) 120.2 120.1 120.1

Range h 0 → 6 0 → 6 0 → 6

k 0 → 8 0 → 8 0 → 8

l -29 → 28 -56 → 54 -28 → 24

No. of reflections measured 1878 3856 1903

No. of unique reflections 1686 3459 1787

Rint 0.087 0.147 0.062

Corrections:

Lorentz-polarization Yes Yes Yes

Absorption, transmission facors (x 10-6) 0.80-1.41 0.78-1.0 0.51-1.38

Secondary extinction coefficient 0.39810 0.1983 0.1983

H atoms included in calculated positions: C-H (Å) 0.95 0.95 0.95

Function minimized in full-matrix least-squares refinement Σw(|Fo|-|Fc|)2 Σw(|Fo|-|Fc|)2 Σw(|Fo|-|Fc|)2

Least-squares weights w 4Fo2/σ2(Fo)2 4Fo2/σ2(Fo)2 4Fo2/σ2(Fo)2

p-Factor weighting intense reflections 0.03 0.03 0.03

Anomalous-dispersion correction All non-H atoms All non-H atoms All non-H atoms

No. of observations [I>3.00σ(I)] 1351 1452 797

No. of variables 89 190 89

Reflection parameter ratio 15.18 7.64 8.96

Residuals R, wR 0.054; 0.065 0.060; 0.072 0.117; 0.127

Goodness-of-fit indicator S 2.02 2.04 2.97

Maximum shift/e.s.d. in final cycle 0 0 0

Maximum peak in final difference map (e nm-3) 0.68 x 10-2 0.84 x 10-2 2.05 x 10-2

Minimum peak in final difference map (e nm-3) -0.50 x 10-2 -0.48 x 10-2 -1.12 x 10-2

Linear absorption coefficient (cm-1) 26.6 26.0 25.97

Temperature for data collection (°C) -120 20 23

Table 1 Experimental conditions and structure-refinement parameters for CTAB Polymorphs (PI-III)

single crystals or of the micro-crystalline material ofDSDMA X class are listed in Table 3.

III. Results and Discussion

The different cell dimensions of the polymorphsof CTAB are listed in Table 2. In none of the crystalforms of CTAB we observed any water moleculesbonded to either the bromine as seen in various otherforms, e.g. cetylpyridinium chloride, dodecylpyridin-ium bromide (Paradies unpublished or dodecyl-dimethylpropylammonium bromide monohydrateand hemihydrate [23, 24, 27]. Furthermore, all CTABmodifications crystallize in a monoclinic lattice quitedifferent form the other structures which reveal atriclinic unit cell. In general, all the bond distancesand C-C-C angles of the n-alkyl chain are similar tothose found for n-octadecane by Nyburg & Lhth [32].In addition, the results obtained show a slightshortening of the C-C bonds and a widening of the C-C-C angles compared to n-pentane [33], which couldarise from the vibrational motion of the C atoms inplanes normal to the C16 chain axes. This is to be seenin all CTAB polymorphs (Figs. 1-5), which reveals asmall increase in the size of the thermal ellipsoids inthe C16 chains as one moves away from the center ofthe CTAB molecule. This has the known effect ofbringing the mean positions of the C atoms inwardstowards the chain axes by shortening the C-C bondswhile widening the C-C-angles. This is particular

24 The Rigaku Journal

CTAB Unit Cell Space Group and Density

Polymorph IF.W. = 364.45

a=0.5596(5) nm P21 (#4)

b=0.7162(4) nm

c=2.5899(3) nm dexp=1.165 g cm-3

β = 96.64(3)° dtheo=1.170 g cm-3

V=1.035 nm3 Z=2

Polymorph IIF.W. = 364.45

a=0.5631(4) nm P21/n (#14)

b=0.7259(5) nm

c=5.1956(4) nm dexp=1.138 g cm-3

β = 92.40(4)° dtheo=1.141 g cm-3

V=2.12 nm3 Z=4

Polymorph IIIF.W. = 364.45

a=0.5634(2) nm P21 (#4)

b=0.7254(1) nm

c=2.601(2) nm dexp=1.392 g cm-3

β = 93.78(4)° dtheo=1.142 g cm-3

V=1.0607 nm3 Z=2

Table 2. Cell dimensions of CTAB-polymorphs crystallized from organic or organic/water solutions.

Table 3 Preliminary cell dimensions obtained from crystallinespecimens of DSDMAX Awater.

X Unit Cell Space Group/Density

ClAH2O* (n)F.W. = 603.55

20°C

a=2.987 nm

b=1.2310 nmc=1.4065 nmβ=110.835°V=2.296 nm3

P21/c (#14)Z=2dm=0.812 g cm-3

dtheo=0.809 g cm-3

ClAH2O* (t)F.W. = 603.55

15°C

a=5.9674nmb=2.4620 nmc=2.8130 nm

α=104.33°

β=110.835°

γ=74.93°

V=5.242 nm3

P1 (#2)Z=2dm=0.675 g cm-3

dtheo=0.613 g cm-3

S-(+)5-LactateF.W. = 600.04

20°C

a=1.38163 nmb=4.402 nmc=0.8685 nmβ=62.54°V=2.212 nm3

P21 (#4)Z=2dm=0.857 g cm-3

dtheo=0.861 g cm-3

PyruvateAH2OF.W. =598.02

20°C

a=1.0579 nmb=4.222 nmc=0.8685 nmβ=88.823°V=2.195 nm3

P21 (#4)Z=2dm=0.912 g cm-3

dtheo=0.909 g cm-3

BrAH2O [28]F.W. = 648

(n-Hexane-CHCl3)

20°C

a=3.811 nmb=0.7890 nmc=0.7418 nma=104.32°

β=110.835Eγ=74.93°

V=2.055 nm3

P1 (#2)Z=2dm=1.04 g cm-3

dtheo=1.047 g cm-3

BrAH2O **F.W. = 648

15-20°C

a=2.5694 nmb=1.254 nmc=1.541 nmβ=111.84°V=2.315 nm3

P21 (#14)Z=2dm=0.997 g cm-3

dtheo=0.987 g cm-3

* DSDMAAClAH2O undergoes a transition to a triclinic form when raising the water content of the crystallization solution at lowertemperatures (10-15EC).

** DSDMAABrAH2O has been crystallized from solutionscontaining 2-isopropanol/n-hexene/water (80/18/2 % v/v).

seen in Fig. 3 of the polymorph form II. The shortestnon-bonded contacts are all H-H. Moreover, thecation moiety is arranged in an anti-parallel fashionwith respect to the long n-alkyl chain axis, and givingrise to interdigitation within the bilayer like formation(see e.g. Figs. 2 & 4). This represents in turn a highcharge density on its unpolar surface area owing to the large bulky hydrocarbon chains. Surprisingly, it hasnot been possible to crystallize CTAB as a mono-hydrate or as hemihydrate, respectively, as it has beenshown for dodecyldimethylpropylammonium bro-mide monohydrate and hemihydrate [23, 24, 27], even when the crystallization conditions are performed inthe presence of a certain amount of water and organicsolvents. The crystal packing and the molecular struc-ture of the three polymorphs of CTAB are shown inFigs. 1-5. Particular interesting distances, bond angles and conformational angles are summarized in Table 4, including the intermolecular distances of the brominein the three different polymorphic forms of CTAB and their close contacts, respectively. In gen-eral, allchains have a zigzag conformation with their zigzagplane parallel to each other. An interesting feature isthe “dent” to be seen in polymorph 1 (C7-C8-C9-C10),which cannot be noticed in the other two polymorphicforms of CTAB (see Table 4). In addition it is quiteremarkable that all the n-alkyl chains pack almostidentical in polymorphs II & III, and only to someextent in polymorph III, although there are enoughother folding possibilities within the N-cationicsurfactant molecule. This folding pattern, togetherwith the dihedral angles of the folding part of theCTAB molecule has been recognized as the shortfolding model on lamellae surfaces of the poly-ethyl-ene single crystal. Another interesting part is the head-to-tail packing of the CTAB molecule in polymorph II

Vol. 17 No. 2 2000 25

Fig. 1 (A) Atomic numbering and molecular con-formation of CTAB; (B) Polymorph I packing of CTAB.

Fig. 2 Bilayer structure of CTAB in polymorph I of CTAB.

by almost doubling the c-axis, resulting finally in atypical n-alkyl chain arrangement with four molecules of CTAB in the unit cell, whereas the a & b axes, and β did not change drastically. Particularly for polymorphII the zigzag planes are arranged in parallel fashion(Fig. 3) with about 0.31 nm separation, whereas theseparation distance in polymorph I is approximately0.35 nm, and in polymorph III 0.351 nm, respectively. So, all structures provide stable packing of hydro-carbon chains in a bilayer like structure, adopting themost simple and compact folding pattern with theexception of the above mentioned deviations in poly-morph I. One zigzag chain in polymorph II is transla-ted by three methylene unit along the chain axis to theother one. The average C-C bond lengths and C-C-C

bond angles in the CTAB molecules are 0.152nm and110-112°, respectively. The distinct differences be-tween the three polymorphs of CTAB are listed inTable 4, including the changes around the quaternarynitrogen. Furthermore, for the polymorphic forms I &II of CTAB the packing cross section per molecule inthee layer plane (0-475 nm2) is determined by thespace group requirements, the bromide coincides with the bc plane of the unit cell. Since the cross-section ofone hydrocarbon chain is approximately 0.18 nm', thetwo hydrocarbon chains (Figs. 2 & 5) are tilted by40.7° =cos-1(0.36/0.475).

The situation is completely different forDSDMA-chloride-water single crystals or theirorganic counterion derivatives, e.g. DSDMA-S-(+)-lactate and DSDMA-pyruvate, respectively. Despitethe fact, that it is extremely difficult to grow singlecrystals of sizes suitable for X-ray diffraction analy-

26 The Rigaku Journal

Fig. 3 Molecular packing of CTAB of polymorph II in thecrystalline lattice.

Fig. 4 Molecular packing of CTAB of polymorph III in thecrystalline lattice.

sis, particularly in the presence of the organiccounterions, it has been found that a variety of crystals of this material can be obtained, which are micro-crystalline and having a mixture of different crystal-line forms in one crystallization batch, which are noteasily being distinguished by the microscopic exami-nation under polarizing light. However, powder dif-fraction pattern of these materials gave some insightsinto packing, hydration, space groups and orientations of the hydrocarbon chains within the crystallinematerials. Together with molecular simulation calcu-lations (Cerius 2, version 4.0. Molecular Simulations,San Diego, USA) it was possible to model thearrangements of the double chained cationic surfac-tants in the presence of the so important counterionse.g. S-(+)-lactate or pyruvate.

For inspection some crystalline specimens ofDSDMAAClAH2O, DSDMAAS-(+)-lactate andDSDMAApyruvateAH2O are shown in Fig. 6a, b, and c, which have been processed for X-ray diffractionstudies and analysis. For single crystal diffractionanalysis normally 3,125 reflections greater than3σ(Fo) were used in the data analysis. On the basis ofthe measured densities by the flotation method andthe unit cell volume, the unit cell contains for allspecimens analyzed two molecules in the unit celltogether with water consistent with the chemical ele-mentary analysis of these materials, all of them areactually monohydrates [34]. All densities of thecrystalline specimens are slightly lower than thereported density for DSDMAABrAH2O by Okuyama et

Vol. 17 No. 2 2000 27

Fig. 5 Close contacts of bromine and the quaternary nitrogen of polymorph II of CTAB.

al. [28]. Depending on the quality of the crystal data,hydrogen atoms, after an anisotropic refinements ofnon-hydrogen atoms, located at their calculatedpositions with isotropic temperature factors wereincluded in the refinement. The quality minimizedwas Σw(|Fo|2-|Fc|2) with w=1.0 for reflections withFo>50.0 or with w = 0.5 for the other reflections.After several cycles some hydrogen atoms with unac-ceptable geometry were excluded from furthercalculations. Some of the observed structure factorsfor the (h00) reflections with h < 10 were constantlysmaller than those of the calculated ones, anobservation noted for the DSDMAABrAH2O structure,too [28]. This discrepancy was particularly noticed for the (200) reflection. One reason for these disagree-ments between observed and calculated structurefactors are most likely due to the different effects ofthe absorption coefficients along their long axis,which is normal to the plate needle like surface of thecrystal, and of course the crystal quality in general.The final R-value was 0.085, 0.090 and 0.10 (Rw =

0.085, 0.092, and 0.098) for all non-hydrogen atomsand 70 hydrogen atoms respectively.

We will summarize the results obtained for thesematerials briefly, and giving an overview for thevarious structures so far elucidated, and were reliableexperimental evidence has been obtained. Themolecular structure of DSDMAACl AH2O is shown inFig. 7a & b, including the numbering scheme (Fig.7b). The conformation of the two n-alkyl chains arevery different form the one found for the DSDMAABrAH2O structure, although another crystallinemodification of DSDMAAClAH2O reveals also P1 sym-metry, when the crystal of DSDMAAClAH2O areprepared under different conditions (see Table 3). The transition of the former (n) to the latter one (t) can also be documented by X-ray powder diffraction (Fig. 8),whereas Fig. 9 shows a successfully indexed (experi-mentally) powder pattern of DSDMAAClAH2O (n) onthe basis of 58 recorded reflections, which is alsoconsistent with the single crystal X-ray diffraction

28 The Rigaku Journal

Fig. 6 Single crystals of DSDMAAClAH2O (top), ofDSDMAAS-(+)-lactate AH2O (middle) and DSDMA-pyru-vateAH2O (bottom) between cross polarizers. The barequals 0.1 mm.

Table 4 Structural differences between CTAB polymorphs.

Intramolecular distance (nm) P1 P2 P3

C9-C10 0.1411 0.1503 0.1512

C10-C11 0.1421 0.1542 0.1465

C9-C8 0.1501 0.1522 0.1502

C8-C7 0.1501 0.1472 0.1641

N1-C17 0.1412 0.1491 0.1505

C1-C18 0.1591 0.1522 0.1414

N1-C19 0.1506 0.1471 0.1577

N1-C16 0.1514 0.1522 0.1453

Intramolecular bond angles (°)

C7-C8-C9 116.7 115.1 112.3

C6-C7-C8 134.4 114.2 112.2

C8-C9-C10 133.7 112.2 110.4

C9-C10-C11 128.7 114.1 116.4

C11-C12-C13 128.4 114.1 121.4

C12-C13-C14 123.9 114.1 111.3

Torsion angles (°)

N1-C16-C15-C14 173.1 160.2 -172.5

C7-C8-C9-C10 -144.0 178.2 174.3

Intermolecular distances (nm)

Br-C1 — 0.3591 —

C1-C2 — 0.1522 0.1595

Br-C18 0.3552 — —

Br-C17 0.3542 — 0.3514

C1-C2 0.1431 — —

C1-C3 — — 0.2615

analysis. Furthermore, all the DSDMAACLAH2Ostructures (n & t) show extensive hydrogen bondingbetween water molecules, and no contacts between Cl-

and water, or Cl- and the quaternary nitrogen (Nl),respectively. In addition only one of the n-alkyl chainis bent to almost right angles at the C21 atom. Thefolding and the intermolecular distances betweenoxygen (water), chloride and the intramolecular dis-tances of the folded hydrocarbon chains is shown inFig. 10a, whereas Fig. 10b reveals the web-likestructure of DSDMAAClAH2O in the a-c plane of twounit cells. Figure 10c shows the triclinic form ofDSDMAAClAH2O (Table 3) revealing a typical longbilayer spacing with close contacts of thehydrocarbon chains and the interstitial spacingbetween the chlor-ine and the hydrogen of the water.Moreover, we observe a much shorter distancebetween N(l) and the chlorine as well as of the oxygen of water to the nitrogen (0.4170 nm). Theintermolecular water to water distance amounts to1.845 nm, whereas the intrachain distances of thehydrocarbon chain amounts to approximately 0.49nm for the shortest contact, and 0.612 nm to thelongest distance, respec-tively. Furthermore, in thismodification of DSDMAA ClAH2O the methyl carbonsof the tail end of the C18 chain are not alignedparallel, they are moved by 35E away from the zigzagchain of the n-alkyl chain. (Figs. 7a & 11).

Changing the organic solvent composition, e.g.using xylene instead of n-hexane which contains asmall amount of water (0.1%v/v), we were able todiscover another crystalline form of DSDMAAClAH2O, having cell dimensions of a = 1.611 nm, b =0.9170 nm, c = 2.936 nm, with cell angles of

Vol. 17 No. 2 2000 29

Fig. 7 (A) Atomic labeling of DSDMAAClAH2O andmolecular conformation of the stretched hydrocarbonchain according to ab initio calculation (Gaussian 94). (B)Molecular structure according to the single crystal X-raydiffraction study of DSDMAAClAH2O.

(A)

Fig. 8 Transition of the (n) type structure of DSDMAAClAH2O to the (t) type structure (P) as observed from small crystals ofDSDMAAClAH2O by X-ray powder diffraction.

α=99.09°, β=114.24°, γ=104.04°, with d exp=0.867gAcm-3, V=3.67 nm3 , having the space group P1 [34].The packing is shown in Fig. 11a, whereas some inter-esting intramolecular & intermolecular distances areshown in Fig. 11b, respectively. These results show,that it is possible to induce different crystalline formsof DSDMAAClAH2O by slightly changing the crystal-lization conditions. Furthermore, the hydrocarbonchain pack in this particular crystal form of DSDMAAClAH2O laterally pursuant to the triclinic modeaccording to Abrahamsson et al. [35]. In this specialcase the subcell dimensions are as = 0.860 nm, bs =0.456 nm, cs =1.410 nm, αs =78.5°, βs = 104.2° and γs

= 99.5°. The packing cross section per chainperpendicular to the long axis is 0.190 nm2 giving adensely packed chain matrix, similar as observed forthe DSDMAABrAH2O structure [28].

It is well known, that phospholipids and otheramphiphiles spontaneously assemble into a variety ofcomplex structures, some of them have already foundindustrial applications, particularly in the field of drug delivery, pharmaceutical technology, car wash indus-try, household, and in the production of nanomaterials [36], although this field is just starting and beginningto flourish. Furthermore, the DSDMA X salts are themain active ingredient in commercial fabric softeners, apart from their structural similarity and use asartificial lipids. The presence of the two long alkylchains and an ionic head group means that thesematerial can be structurally related to the hydrophobic part of the bilayer. When hydrated, as seen for the

DSDMA X materials as very simple membranes theyform structures in which interpenetrating, unconnec-ted water channels (see e.g. Fig. 10c) combined withloose voids are separated by a continuos lipid bilayer.The structure is stabilized by aggregation of thecationic surfactants through hydrogen bonding andvan der Waals forces and Coulombic attractions,which depend also on the nature of anion, e.g. organicvs. inorganic, and possibly also on the chirality of thecounterion as it has been shown very recently for theDSDMA-S-(+)-lactate system in aqueous solutions[21, 37]. Therefore, attempts have been made tocrystallize this material in the presence of S-(+)-lactate or pyruvate, respectively, since these meta-bolic molecules have to pass through the membranebarriers in order to fulfill their metabolic duties. –TheDSDMA-S-(+)-lactate-water or its cor-responding R-enantiomer as well as the racemic compound of lac-tate can provide a unique means not only to measureintermolecular forces quantitatively, but also to mod-ulate them by the chiral architecture of the chiralcompound at will. Arnett et al. [38] and Shinitzki &Haimovitz [39] have shown that chiral discriminationmay greatly enhanced in the 2-dimensional conden-sed phase, this is particularly shown for the enantio-meric and the racemic N-palmitoyl and N-stearoylserine derivatives, especially for the N-stearoyl serine methylesters in anionic micelles. The chiral recogni-tion could be clearly documented and quantitativelycharacterized by surface pressure/area (π/A) dia-grams as well as by circular dichroism (CD) in aque-ous alkaline solutions. The CD spectra indicated

30 The Rigaku Journal

Fig. 9 Indexed powder diffraction pattern of DSDMAAClAH2O (n) type.

convincingly for this class of cationic compounds theformation of a unique supramolecular chiral organi-zation at the micellar surface. Moreover, the criticalmicelle concentration (CMC), the degree of ioniza-tion and the apparent aggregational numbers aredifferent for the racemic vs. the enantiomeric com-pound, a phenomenon which has also been observedfor the DSDMAS X salts when measured in thepresence of either S-(+)-lactate or the R-(-) enantio-mer vs. the racemic (R,S)-lactate, or mandelic acid,respectively. Therefore, it might be of interest to lookinto the crystalline structures of the DSDMA-S-(+)-lactate-water system. as well as to its metabolic coun-terpart of DSDMAApyruvateAwater system, respec-tively. In light of the very recent results on themodulation of the human red cell membrane by Ca-pyruvate as well as their changes in the elastic moduliand bending modulus in the presence of either Ca2+

Vol. 17 No. 2 2000 31

Fig. 10 (A) Folding and intermolecular distances ofDSDMAAClAH2O (n) type; (B) Web-like structure ofDSDMAAClAH2O (n) type in the azc plane of two unitcells. (C) Channel like structure of DSDMAAClAH2O (n)type with dimensions of the voids within two unit cells inthe bzc plane.

Fig. 11 (A) Molecular conformation and packing ofanother triclinic form of DSDMAAClAH2O. (B) Intramo-lecular and intermolecular distances of another triclinicform of DSDMAAClAH2O with the cell dimensions of a =1.611 nm, b = 0.9170 nm, c = 2.936 nm, with cell angles of α = 99.09°, β = 114.24°, γ = 104.04° ( P1).

pyruvate or Ca-pyruvate [40], the structure of a simple membrane, or the entity of building a simple mem-brane and perform as a membrane barrier, can bemodeled through the DSDMA-pyruvate. water sys-tem and the DSDMA-S-(+)-lactate. water system,respectively. This may also be interesting to study any changes in the orientation of the hydrocarbon chainswithin the structures, which may occur in the presence of either pyruvate or lactate. Figures 12 & 13 show the packing of DSDMA-S-(+)-lactate-water; the unit celldimensions are listed in Table 3, revealing an verylong axis of b=4.407 nm and a short axis of c=0.8685nm. The observed space group is also P21. With twomolecules per unit cell according to the experimen-tally determined density and the unit cell volume,respectively. A typical double bilayer structure canalso be noticed having an average spacing of 0.45 nm,whereas the S-(+)-lactate is hydrogen bonded to thewater molecule. Figure 13 shows the space fillingmodel of the DSDMA-S-(+)-lactate. water systemalong their b-axis, revealing two hydrocarbon chainsrunning parallel to each other in a zigzag manner.However, the folding of the hydrocarbon chain isfolded in particular way resulting in a S-form wherethe two chains of each DSDMA molecule are grouped head to head rather than head to tail. This is alsonoticed in Fig. 12. Moreover, the lactate is orientatedclose to the head group, quite different fromourstructures of DSDMAAClAH2O.

A similar but slightly different behavior is seen inFig. 14 for DSDMAApyruvateAwater. Interestingly the

long axis is only slightly changed (b=4.222 nm),however, β has changed from 62.4° for the DSDMA-S-(+)-lactate-water system to 88.8° for the pyruvatesystem. The unit cell volume did not change verydrastically from 2.122 nm3 to 2.195 nm3 . Extensivehydrogen bonding between the water molecule andthe pyruvate is also to be seen, where the keto groupof the pyruvate is in a trans position, quite differentfrom the crystal structures of the alkali and Ca-salts of pyruvate, where the keto group is in a cis position(Paradies, Quitschau, unpublished results). Figure 15underlines the preliminary structure determination ofDSDMAApyruvateAwater through a simulated powderdiffraction pattern compared to the experimental data.

These preliminary crystallographic data allowsmolecular simulations calculations in order to provethe dynamics of these simple membrane systems inthen presence of important metabolites. These datacan be used in simulation tests of lipid bilayers,simulations of Langmuir-Blodgett films and for X-Ray reflectivity measurements, respectively. Thissystems also reveal a variation in the tilt with respectto the head group area when compared to resultsobtained by Adolf et al. [41, 42], however, in theirmodel no water was incorporated and no use of a more appropriate representation of the chloride has beenperformed. These experimental investigations mayalso be helpful in estimating transport coefficients,intra- and inter diffusion phenomena including coup-led transport phenomena.

32 The Rigaku Journal

Fig. 12 Packing of DSDMAAS-(+)-lactateAH2O in a monoclinic unit cell.

Acknowledgement

One of the authors (H.H.P.) acknowledges thecontinuous support and encouragement of Mrs.Christa Koppelkamp during the difficult and to someextent unsuccessful attempts of crystallization of theDSDMA X compounds.

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Fig. 13 Space filling model of DSDMAAS-(+)-lactate AH2Oin two monoclinic unit cells, showing two hydrocarbonchains running parallel to each other but are bent to a S-form. A head to tail arrangement is to be seen due to thepresence of S-(+)-lactate and extensive hydrogenbonding between the lactate and water, respectively.

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34 The Rigaku Journal