N-Substituted aldonamides

N-Substituted Aldonamides III. Relationship between Chemical Structure and the Formation of Lyotropic Mesophases M. LOOS, D. BAEYENS-VOLANT, 1 C. DAVID, G. SIGAUD, AND M. F. ACHARD Universitd Libre de Bruxelles, Campus Plaine CP 206/1, Facultd des Sciences, Boulevard du Triomphe, 1050 Brussels, Belgium; and Centre de Recherche Paul Pascal, Universit~ de Bordeaux I, 33405 Talence, France Received April 18, 1989; accepted August 11, 1989 The lyotropic behavior of a series of N-substituted aldonamides which have both lyotropic and thermotropic characteristics, is systematically investigated as a function of the length, branching, and cyclization of the aldonic residue and of the alkyl chain. The ability of these c o m p o u n d s to form lyotropic phases is related to their chemical structure. The structures of the lamellar thermotropic and lyotropic phases are compared, and the continuity between both phases is presented. © 1990AcademicPress,Inc.

INTRODUCTION

A series of N-substituted aldonamides of the general formula R-NH-CO-R', where R is an alkyl chain and R' a linear carbohydrate moiety, has been synthetized in our laboratory ( 1, 2). Some of these compounds form a thermotropic lamellar mesophase of the smectic A type in which the molecules in the transconfiguration are arranged in monolayers stabilized by hydrogen bonding and hydrophobic interactions. The stability of the lamellar mesophase depends on the length and branching of the alkyl and aldonic groups as previously discussed. All these aldonamides are amphiphilic molecules. The carbohydrate part is soluble in water and the hydrocarbon tail is soluble in nonpolar solvents. This dichotomy results in the formation of lyotropic liquid crystals in convenient solvents, at appropriate temperatures and concentrations. The present work is concerned with a systematic investigation of the relationship between the ehem-

ical structure of these compounds and their ability to form lyotropic phases. Length, branching, and cyclization of the aldonic residue and of the hydrocarbon chain is investigated. The continuity observed between the smectic phase of the neat compound and the lyotropic lamellar phase is considered and interpreted.

1 TO w h o m correspondence should be addressed.

EXPERIMENTAL

The amines were added in stoichiometric proportion to the aldonic acid or to the aldonolactone and allowed to react according to the procedure described in Ref (1). The aldonamides obtained were characterized by the frequency of the amide I absorption in IR and their 13C NMR spectra. The existence oflyotropic phases is first observed on contact preparations. Further investigations are then made in test tubes. The aldonamides-water mixtures are prepared by weight in small tubes with distilled water. The tubes are sealed and rapidly warmed to the isotropic phase in an oil bath. They are vig-

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Journalof ColloidandInterfaceScience,Vol. 138,No. 1, August1990

FORMATION

OF LYOTROPIC

orously shaken and allowed to cool. After 12 h at rest, the phase transition temperatures ( L I) are observed in a first step, by the disappearance of the birefringence with polarizing sheets. In a second step, the birefringent gel is observed and the textures are identified with a Laboval (ZEISS) polarizing microscope equipped with a Mettler FP 52 hot stage using a heating rate of 3°C/min. The C-L and CH transition temperatures are also observed by polarizing microscopy. X-ray diffraction experiments were made at the Centre Paul Pascal of Talence, Bordeaux. The samples were contained in Lindeman glass tubes. The X-ray patterns were recorded in the thermotropic mesophase in a Guinier chamber using the CoK~ radiation. RESULTS

AND

DISCUSSION

1. Relation Between Chemical Structure and Lyotropy The different amides listed in Table I have been examined by hot stage polarizing microscopy in the neat phase and in water solution. The textures observed are, depending on the nature of the amide and its concentration in water, those reported by Rosevear (3) for the lamellar and for the hexagonal phases. For the lamellar phases, spherulites, oily streaks, and homeotropic areas are observed on heating; bfitonnets precipitate and coalesce on rapid cooling from the isotropic phase. For the hexagonal phase, a colored fan-like texture is obtained. The results are summarized in Table I. Comparison of the thermotropic and lyotropic behavior of compounds I to V shows that the formation ofmesophases requires a minimum length of the alkyl chain (7 carbons) in both cases. Branching of the alkyl chain (compound VI) inhibits the formation of thermo and lyotropic phases. Cyclization of the alkyl chain (compound VII) is not a restriction to the formation of a lyotropic lamellar phase as it is for the neat compound. If the alkyl chain is long enough, (compounds VIII to XIII) an aromatic group does not have to be situated next to the amide bond to allow the formation

MESOPHASES

129

of lyotropic phases. The requirements for formation oflyotropic phases are thus less severe than those for the thermotropic ones. The behavior of a few characteristic systems (compounds II to V, XIV and XV) will now be quantitatively compared using the phase diagrams given in Figs. 1, 2, and 3. The diagrams of Figs. 1 and 2 show that if the length of the aldonic group is constant (5 carbons) and if the length of the linear hydrocarbon chain is increased from 6 to 12 carbons: - - n o mesophase is formed for 6 carbons; - - a small hexagonal and a small lamellar phase are observed for 7 carbons; and --broader hexagonal and lamellar phases are formed for 8 carbons. If the alkyl chain still grows, ( 10 to 12 carbons) (Figs. 2B and 2C) the hexagonal phase disappears and a broad lamellar phase is observed. Indeed, an increase of the alkyl tail increases the hydrophobic character and hence favors the formation of the lamellar phase, in agreement with the R theory (4). Comparison of Figs. 2B and 3A shows that a hexagonal phase is observed in addition to the lamellar one, if the aldonic group changes from 5 to 7 carbons for a given chain length of the amine ( 10 carbons). Thus, an increase in the hydrophilic characteristic of the molecule increases the stability of the hexagonal phase which requires more important interactions between water and the amphiphile. If, for a given chain length of the amine ( 12 carbons), the aldonic group (6 carbons) is branched (Fig. 3B), the available area per polar head increases. As a consequence, the hydrophilic properties are enhanced and a hexagonal phase appears in addition to the lamellar one, in agreement with the previous observations. In this series of compounds, no cubic phase is observed between the lamellar and the hexagonal phases; continuous curves are obtained for the transition temperatures from the lamellar to the isotropic phase in the whole range of composition. This continuity between the lamellar thermotropic and lyotropic phases will be discussed in the next sections. Journal of Colloid and Interface Science, Vol. 138, No. I, August 1990

TABLE I Thermotropic and Lyotropic Properties of N-Substituted Aldonamides Th~ot~c

c~poends Experiments Tc-4*C

Aldonamlde

PD

L CHzOH(CHOH)s--C--Ntt--(CH2)s--CH s

89

Lyotropic Liquid cc~pounds TC--L°C TL-!°C Hexagonal Lanmll~r .

.

.

.

II O PD

94

IIL CHzOH(CHOH)3--C - - N H - - (CHz~--CH 3 II O

PD

IV. CHzOH(CHOH)3--~ - - N H - - (CHz)9--CH 3

IL CH zOH(CHOH)3--~ - - N H - - (CH~)6--CH3

95(96)

Yes

Yes

96(97)

120(123)

Yes

Yes

PD

100(99)

147(151)

--

Yes

PD

106(105) 158(160)

--

Yes

O

O V. CH2OH(CHOH)s--~ - - N H - - (CH2)n--CH s O H)

~H3

VI. CH zOH(CHOHh-- C--NH--C--CH z--C--CH 3 II I I O CH3 CH5

128

m

m

VII. CH 2OH(CHOH)s--~ - - N H - - ? H - - (?Hz)n

137

--

--

Yes

0 123(126)

VHL CH20H(CHOH),-- ~ - - N H - - (CH2)2===~ O

117(119)

IX. CH zOH(CHOH),-- ~ --NH - - (CH2),t===~ O X. CH zOH(CHOH),-- ~ - - N H ~ ( C H z ) , - -

Yes

146(148) 166(169)

CH,

0 (149B157-172)

XI. CHzOH(CHOH),-- ~ - - N H - - ~ O C H , O XII. CHzOH(CHOH)3--C - - N H - - ( C H ~ 2 ~ II O

(135B-

-" ~

136B-

O

138.3)

/ : k XIII. C H z O H ( C H O H ) , - - C - - N I t - - ( C H z ) z ~ ICI - (CHz),-'-CH, ii ii O O

C

129.8(131)

PD

157(159) 188(190)

Yes

Yes

OH O I II XV. CH zOH--CHOH --CH 2~ C - - C - - N H ~ (CH2)n--CH s I CHzOH

PD

90(92)

Yes

Yes

OH 0 I II XVI. CHzOH--CHOH--CH z ~ C - - C - - N I t - - (CHz)x~--CHs I CHzOH

C

XIV, CH zDH(CHOH)5--C - - N H - - (CHz)9-- CH3 II O

Note. C, Contact preparation; PD, phase diagram from different solutions.

130 Journal of Colloid and Interface Science, Vol. 138,No. 1, August1990

137(139)

98(100) 155(157)

Yes

FORMATION OF LYOTROPIC MESOPHASES A

B

CH3 (Ctl2) 5 NII-CO-(CliOII) ]-Cllxtlll

CII3- (CiI2)6-NH C0- (ClIOH)3-CH2OH

?oo

200 •r"c I

IB8

180 !

160

160.

140

14o

120

120

I

'

I

ioo

100 80 60

60 K+I

40

~o

20

20

0

o

20

40

60

80 I00

K÷I

20

ALOONAMIDE , %W

FIG. 1. Binaryphasediagramsfor aqueousN-hexyland N-heptylribonamidesystems.

2. Rearrangement of the Molecules at the Transition from the Crystal to the Lamellar Thermotropic Phase Single crystal X-ray study of N-decylribonamide (5) has shown that due to the strong cohesion by hydrogen bonds, the molecules are arranged in a monolayer with a layer spacing of 16.1 A (Fig. 4). The alkyl chains are in extended conformation and the molecules are tilted in the layers. The carbohydrate moieties are hydrogen bonded to form monolayers and the alkyl chains do not interdigitate. Each hydroxyl oxygen atom is involved in two hydrogen bonds. There are thus 8 hydrogen bonds of this type per molecule. The carbonyl oxygen atom and the nitrogen atom are also each involved in one hydrogen bond. This gives a total of 10 hydrogen bonds per molecule (2). The structure of the lamellar thermotropic phase has also been obtained by X-ray diffraction on the neat compound. The lamellar spacing is 29.6 A for the N-decylribonamide, and 31.0 and 36.8 A for the dodecyl and the hexadecylisosaccharinicamide, respectively. The distance of 29.6 A obtained in the lamellar thermotropic phase of N-decylribonamide is intermediate between one and two molecular lengths ( 19.6 and 39.2 A). This indicates that the lamellar thermotropic phase is of the SA type and could probably be interdigitated; the molecules are indeed perpendicular to the lay-

131

ers as indicated by the presence of homeotropic areas in the textures (2). The layer spacing of the thermotropic mesophase decreases as the temperature increases (Table II). This could be due to the appearance of t-g conformations in the alkyl chains with the subsequent decrease of their length. The conformation changes of the segments significantly increase with increasing the length of the hydrocarbon chain. These observations and model are in agreement with those reported by Goodby et al. for N-octylgluconamide (6-7). The formation of the thermotropic SA phase by the melting of the crystal results from transformations which have not been identified in the present case. Melting of the alkyl chain has often been proposed to occur in other systems. Another interesting suggestion

A

m

CH3-~CH2) 7 NH-C0-(CHOH) 3 CH20H CH3-(C/12)9-NH-CI}-(CHOtt)3-CII2~}II

200

200

T°C

T°C

160

]60

180

180

140

I~0

/20

~

I

120

100

L

IOO

_K B0

80

60

60

KL

40

~0 K+I

2X

20.

20

40

60

80

100

20

40

60

80

I00

c cII f (c112) I I -NH CO(CilOH) }-CH20tl

To:2¢0, 18o

14o 120 L

I I)O

80 ~0 40

K+l

0 20

40

60

80

100

ALDONAMIDE , % W

FIG. 2. Binaryphase diagramsfor aqueousN-octyl,Ndecyl,and N-dodecylribonamidesystems. Journal of Colloid and Interface Science,

Vol. 138, No,

1, A u g u s t

1990

132

LOOS ET AL. A

B

CH3- (C112)9- till- CO- ( CHOH) 5 -CEt2OIJ

TABLE II

CH$-(C112) I I -N}I-CO-C-CiI2-CHOH-CH 2Off

CHzOH 200 180

I

"'"~

160

180

T°C 160t

/L

140

140

X-ray Lamellar Spacing(d) as a Function of the Temperature for the SAPhase of Different Aldonamides

K*L

N-decylribonamide

I

I?0

120 100

I00

80

K+L

80~ K+I

60

601

40

40t

20,

2UI 01 20

40

60

80 ALDO

100 NAMIDE

.... ::----]

N-dodecylisosaccharinicamide

20 .% W

FIG. 3. Binaryphase diagrams for aqueousgluconamide and isosaccharinicamide systems.

concerns the hydrogen bonds which would become dynamic at the phase transition (8). Interdigitated lametlar mesophases obtained from monolayer crystal has also been observed for esters like octyl-D-gluconate, which possess linear carbohydrate moieties (9). On the contrary, with cyclic carbohydrates like 1-O-a-Dglucopyranosides, the crystal and the lamellar SA phase both have an interdigitated arrangement (10). It must be emphasized that a discontinuity is observed in contact preparations between

N-hexadecylisosaccharinicamide

T °C

d (A)

105 111 117 123 129.5 135 93 102.5 112.5 122 101.5 105 109 112 116 119.5 123 127 130.5 134 137.5 141 145

29.60 29.50 29.39 29.28 29.24 29.23 31.00 30.83 30.54 30.32 36.77 36.61 36.45 36.28 36.12 35.89 35.73 35.50 35.27 34.97 34.67 34.46 34.17

the SA thermotropic phases of N-decylribonamide and dodecylcyanobiphenyl. Nonmiscibility of these molecules in the SA phase as well as in the isotropic phase is the consequence of their very different chemical structures which give rise to intermolecular interactions of a different nature in the organized phases.

3. Continuity of the Larnellar Thermotropic and Lyotropic Phases

FIG. 4. Packing of N-decylribonamidein the unit cell: monolayer arrangement. Journal of Colloid and Interface Science, Vol. 138, No. 1, August 1990

If contact preparations are made between the lyotropic (lamellar) phase of N-decylribonamide (80 wt%) and, either the lamellar thermotropic phase of the same c o m p o u n d or the classical lyotropic lamellar phase of A O T (20 wt%), the absence of a sharp boundary confirms the mutual miscibility of the phases and allows us to conclude their similitude.

FORMATION OF LYOTROPIC MESOPHASES CONCLUSION T h e N-substituted aldonamides have both lyotropic a n d t h e r m o t r o p i c characteristics. The rigidity o f the amide b o n d and the hyd r o g e n - b o n d e d carbohydrate core are responsible for t h e r m o t r o p i c behavior, and the well separated hydrophilic a n d h y d r o p h o b i c parts, are responsible for lyotropic behavior. The formation o f l y o t r o p i c phases in the Nsubstituted aldonamides requires a m i n i m u m length o f 7 carbons for the amine residue. Hexagonal a n d lamellar phases are observed as a function o f the h y d r o p h i l i c - h y d r o p h o b i c balance in agreement with the R theory. The requirements to form lyotropic phases are, however, less severe than to form thermotropic phases. A lamellar t h e r m o t r o p i c phase o f the SA type is obtained from a monolayer crystal after not yet identified transformations. T h e lamellar phase remains stabilized by intermolecular O H hydrogen b o n d s between the carbohydrate moieties. The progressive penetration o f water in this 5'1 smectic layer is responsible for the continuity between the t h e r m o t r o p i c and lyotropic lamellar meso-

133

phases. The lyotropic phases o f the N-alkylribonamides are o f the same type as those for the classical lyotropic lamellar phases. ACKNOWLEDGMENTS We thank the Commission of the European Communities for financial support of this research and the Institut pour l'Encouragement de la Recherche Scientifique dans l'Industrie et l'Agriculturefor the grant to one of us (M.L.). REFERENCES 1. Baeyens-Volant, D., Cuvelier, P., Fornasier, R., Szalai, E., and David, C., Mol. Cryst. Liq. Cryst. 128, 277 (1985). 2. Baeyens-Volant, D., Fornasier, R., Szalai, E., and David, C., Mol. Cryst. Liq. Cryst. 135, 93 (1986). 3. Rosevear,F. B., J, Am. OilChem. Soc. 31,628 (1954), 4. Winsor, P. A., Chem. Rev. 68, 2 (1968). 5. Tinant, B,, Declercq, J. P., and Van Meerssche, M., Acta Crystallogr. Sect. C 42, 579 ( 1986). 6. Pfannemtiller, B., Welte, W., Chin, E., and Goodby, J. W., Liq. Cryst. 1, 357 (1986). 7. Goodby, J. W., Marcus, M. A., Chin, E., Finn, P. L., and Pfannemtiller, B., Liq. Cryst. 3, 1569 (1988). 8. This suggestionhas been made by one of the referees. 9. Bhattacharjee, S., Jeffrey, G. A., and Goodby, J. W., Mol. Cryst. Liq. Cryst. 131, 245 (1985). 10. Moews, P. C., and Knox, J. R., J. Amer. Chem. Soc. 98, 6628 (1976).

Journal of Colloid and Interface Science, Vol. 138, No. 1, August 1990