Xanthophyll films

Xanthophyll Films II. Two-Component Monolayers of Some Xanthophylls and Egg Lecithin at the Air/Water Interface MARIA TOMOAIA-COTI~;EL AND EMIL CHIFU 1 Department of Physical Chemistry, "Babe#-Bolyai'" University, Arany J. Str., No. 11, 3400 Cluj-Napoca, Romania Received September 2, 1982; accepted January 31, 1983 The surface pressure of the binary monolayers of some all-trans xanthophylls 03-cryptoxanthin, zeaxanthin, or lutein) with egg lecithin has been measured by the Wilhelmy method, as a function of the mean area at various compositions. The surface properties of the mixed films at the air]water interface, obtained by compression isotherms, are interpreted in terms of both the additivity rule of the molecular areas and the two-dimensional phase rule applied to the collapse pressures taking into account the excess free energies of mixing of the two components. The findings indicate that each of the xanthophylls under consideration is miscible with egg lecithin in monolayers at all the molar ratios considered. The results are discussed both in terms of the packing of the two components in two-dimensional solutions and according to the molecular structures of the three xanthophylls studied. INTRODUCTION

In the first part of this series (1) we reported the behavior of some all-trans xanthophyll derivatives in monomolecular films at the air/water interface. The surface properties of xanthophylls in single-component monolayers, obtained by compression isotherms: surface pressure-molecular area, are discussed in terms of the molecular structures of carotenoids, being expounded by means of a molecular geometric model we have suggested. Hereby we deal with the binary mixtures of some all-trans xanthophylls and egg lecithin (EL) at the air/water interface. The systems employed consist of mixed monolayers of/3-cryptoxanthin:EL, zeaxanthin:EL, and lutein:EL. The three xanthophylls used for this study are carotenoid pigments extremely common in plants, differing among themselves by the number of hydroxil groups, as well as by the nature of the ionone ring (1, 2). l To whom all correspondence should be addressed.

In view of characterizing the above mentioned mixed systems, the relations surface pressure-mean molecular area are looked through. Thus, some surface characteristics derivable from the compression isotherms (collapse surface pressure, the free molar energy of mixing in the mixed monolayers, and the mean molecular area at given values of the surface pressure) are explored and the dependence on monolayer composition of these parameters is also interpreted. The results are discussed both in terms of the packing of the two components and according to the different theories on molecular interactions in mixed monolayers. The interest in such studies consists in that both xanthophylls and lecithins are structural and functional components of some biological membranes, as the chloroplast membrane (2). On the other hand, taking into account the analogy of the natural membrane with the monomolecular film, a first step in understanding the interactions between the compounds forming the membrane is the study itself of the mixed films. 355 0021-9797/83 $3.00

Journal of Colloid and Interface Science, Vol. 95, No. 2, October 1983

Copyright © 1983 by Academic Press, Inc. All rights of reproduction in any form reserved.

356

TOMOAIA-COTISEL AND CHIFU

In a forthcoming paper two c o m p o n e n t monolayers of these xanthophyUs and dipalmitoyl lecithin will be studied (3). EXPERIMENTAL

Surface pressure (Tr, m N . m - i) versus mean area (4,/~2. molecule-i) measurements were p e r f o r m e d at air/water interface for the monolayers of pure components and of their mixtures. The surface pressure on the m o n o layers was determined by the Wilhelmy method, with a precision o f +0.5 m N . m -i, while the molecular areas were measured to within +_2 A2. molecule-i, as previously shown (4). The monolayer-forming substances were obtained from the same sources (1, 5). During the present series o f experiments, the pigment purity was checked by thin-layer chromatography and spectrophotometrically. No degradation o f x a n t h o p h y l l s was f o u n d throughout the experiments. Details on the experimental conditions have been presented already (1, 5). Known amounts of substances in spreading solvents were delivered onto the surface with a micropipet and generally a 2- to 10-min period was allowed for solvent evaporation prior to commencing of the compression. Higher waiting times, up to 1 hr, have been used, too, but they had no influence upon the shape of the compression isotherms. Benzene was used as solvent, since all the substances studied are soluble in it, except zeaxanthin and lutein. For the latter pigments mixtures of benzene and ethanol, containing 2-4% absolute ethanol were used. Special care was taken so that all the monolayers be spread at the same initial area (about 130 ,3,2. molecule- i), avoiding thus the artefacts of spreading kinetics (6). Doubly distilled water was used as subphase. The time required to obtain a complete surface p r e s s u r e - a r e a curve, after starting the c o m p r e s s i o n o f the spread film, ranges from 10-30 rain, the compression speed being o f 2-9 .~=.molecule -1. min -i. The results are not dependent on the Journal of Colloid and Interface Science, Vol. 95, No. 2, October 1983

compression speed in this range. The measurements were performed at ambient temperature (22 _+ 2°C). Each curve shown in this paper represents a mean result of at least 10 different measurements. RESOLTS AND DISCUSSION

Surface Pressure-4rea Curves The experimental results are plotted in Figs. 1-3 as surface pressure versus mean molecular area for the two c o m p o n e n t films of xanthophyU and EL at the air/water interface for some compositions. For the sake of clarity, the isotherms of additional concentration studied are not drafted. The compression isotherms for pure xanthophyUs as well as the EL films are similar to those previously reported by us (1, 5). The values o f collapse surface pressures marked in Figs. 1-3 by arrows show that the xanthophylls are easier collapsable in the m o n o c o m p o n e n t film as compared to the film of pure EL.

The Mean Molecular Area and the Additivity Rule In Figs. 4-6 the mean molecular areas are plotted at indicated surface pressures as a

3O

10

o'

I

20

I

40

60

80

100

120

A, ~2/molecule FIG. l. Surface pressure versus area curves of EL and ~-eryptoxanthin monolayers at the air/water interface at various mole fractions of EL. XEL: (O) 0.0; (~) 0.25;

(o) 0.5; (o) 0.75; (~) 1.0.

357

X A N T H O P H Y L L FILMS, II

m-l). The additivity rule is given by: 4(

A = xlA1 + x2A2,

[1]

where A is average molecular area in the twocomponent film, A ~and Az are the molecular areas in the two single-component films at the same given surface pressure, while x~ and xz are the mole fractions of the components in the monolayer. The mean molecular areas of zeaxanthin:EL-mixed films show negative deviaIC- 3O 50 tions from ideality (dashed line, Fig. 5), especially at small mole fractions of EL and at A,~.S,/rnolecule low surface pressures, i.e., 5 m N . m -~. The FIG. 2. Surface pressure-area curves of EL and zeadeviations decrease slightly by increasing the xanthin monolayers (symbols as in Fig. 1). surface pressures (Fig. 5, full line), yet they are not zero even at high surface pressure of 35 mN- m -1. For the lutein:EL system, it is noticed that function of the molar fraction of EL. The the experimental curves of the mean molecmean molecular areas were obtained from ular area present negative deviations of about the diagrams presented above (Figs. 1-3) by the same magnitude (Fig. 6, full line) from graphic interpolation at the shown surface ideality at compositions rich in xanthophyll pressures. at all the surface pressures considered. It is noted that in the #-cryptoxanthin:EL The condensing effect observed in the system (Fig. 4) the mean area values follow mixed films has sometimes been interpreted the additivity rule (dashed line) in the conin terms of special mechanisms for the densed monolayers (at high surface prespacking of the compounds (7), taking into sures, i.e., 20 and 35 m N . m-~), and the negaccount the geometric accommodation or ative deviations are found at low surface pres"space-filling" possibilities (8), in relation to sures only (in the expanded state, i.e., 10 raN. the molecular associations (9), or in terms of specific interactions in the monolayers (10). The condensing effect of xanthophylls on EL monolayer at low surface pressures, gen40 erally noted in the above binary systems, could be explained on the basis of the intermolecular cavities caused by the presence of unsaturated 30 fatty acyl chains in the EL molecules. The reduction in the mean molecular area for the t~ zeaxanthin:EL and lutein:EL systems, even at high surface pressures, can be accounted for, 10 at least in part, by the geometrical fit between the components in the mixed monolayers. It seems likely that this process will always be 0 30 50 70 90 110 130 accompanied by altered molecular interacA,.~=/molecule tions resulting in enhanced intermolecular hydrophobic interactions (8) or even in specific FIG. 3. Surface pressure-area curves of mixed monointeractions between the polar groups (6, 10). layers of EL and lutein (symbols as in Fig. 1). Journal of Colloid and Interface Science. Vol. 95, No. 2, October 1983

358

TOMOAIA-COTISEL A N D CHIFU

1

~oo

8

o~ -6 E 60

40

"~"

~')'"

• 1"'If"

20

0.25

0.5

0.75

1

XEL

FIG, 4. Mean molecular area of monolayers of EL and /3-cryptoxanthin as a function of composition at three values of the surface pressure. ~r: 10 r a N . m -~ (®); 20 m N . m -~ (~); 35 r a N . m -~ (e).

In order to decide on the behavior of the two-dimensional solutions, the properties of the xanthophyll:EL-mixed films are interpreted in terms of the surface phase rule applied to the collapse pressure (4, 11-13).

Miscibility and the Two-Dimensional Phase Rule The surface phase rule in the case of plane interfaces (14) in absence of the chemical reactions and at constant temperature and external pressure becomes: w = (c - ~o) - (ff - s)

[21

w being the variance of the system, c the

number of components, ~0 the number of bulk phases, ~ the number of surface phases in equilibrium with one another, and s the number of types of surface. Further on, we applied the two-dimensional phase rule particularly for discussion on the equilibrium between the surface film and the bulk-collapsed phase. If the components are miscible in the film, then any of all the types of surface (air/water, collapsed phase/water, collapsed phase/air) bears only one surface phase. It results that s = 3 and ~ = 3. Taking into account that c = 4 (water, xanthophyll, EL, air--assumed to be one component for brevity (12)) and ~o = 3 (water, air, collapsed phase) according to Eq. [2], the variance of the system is w = 1, and in consequence the surface collapse pressure varies as function of the monolayer composition: 7re 7ro(xi). [3] =

Studying again Figs. 1-3, it may be noted that the continuous variation of the collapse pressure over the entire range of the film composition reveals, in accord with the phase rule, complete miscibility of the EL with the xanthophylls mentioned above. In order to have a better information on the behavior of xanthophyll:EL films, the collapse surface pressure a-c of the mixed films versus composition plotted in Figs. 79 was put in question. The concentration

lOG

10C

5

-5 80

f

80

///

g< 6( .g

,£

4¢

40

I

0

0.25

0.5

0.75

XEL

FIG. 5. Mean molecular area-mole fraction of EL, plots for EL and zeaxanthin, rr: 5 m N . m -1 (©); 20 r a N . m -~ (Q); 35 m N . m -~ (O). Journal of Colloid and Interface Science, Vol. 95, No. 2, October 1983

0

0.25

I

I

(15

0,75

XEL

FIG. 6. Mean molecular area of mixed monolayers of EL and lutein as a function of composition (symbols as in Fig. 5).

XANTHOPHYLL

359

FILMS, II

show positive deviations (dashed lines in Figs. 8 and 9) from the ideal behavior (full lines). For the latter two pigments, this is consistent with the data presented in Figs. 5 and 6 which have shown a condensing effect of xanthophyll on EL films at about the same mole fractions of EL. FIG. 7. Collapse pressure of mixed EL$-cryptoxanthin monolayers as a function of the mole fraction of EL.

dependence of the collapse pressure in the ideal case is (15): x exp (a, - ~cI)AcI I kT + x2 e-p (rc - ?Tcz)Ad = 1,

kT

[4]

where, rci is the collapse pressure of component i in the pure film, while A,; represents the corresponding collapse molecular area, k and T are Boltzmann’s constant and absolute temperature, respectively. When (P, - R,i)Aci $ kT, asymptotic expansion of the exponentials leads to:

hk1

+ x2&2).

[51

From Fig. 7, it is to be noted that for pcryptoxanthin:EL monolayers, the collapse surface pressure follows Eq. [5] for ideality (full line). This observation is in agreement with the results obtained by applying the additivity rule (Fig. 4) of the molecular areas at high surface pressures. In contrast, for zeaxanthin:EL and 1utein:EL monolayers, the experimental curves of collapse pressures

Excess Free Energy of Mixing In order to have a better understanding of the thermodynamics of the interaction between one molecular species of xanthophyll and EL in the monolayers, we further evaluated the excess free molar energy of mixing, AGL, for the binary mixtures at the air/water interface (6, 16), by: AGk = N

the A values having the known significance, Nis Avogadro’s constant, K* is a surface pressure beyond which the surface binary mixture can be treated as an ideal two-dimensional solution and conventionally a* = 0 (6, 16-18). The upper limit for the integrations may be arbitrarily selected. Equation [6] is applied to any surface pressure ?r, yet smaller than the one corresponding to the collapse of each pure component. Thus, the AGL values were directly calculated graphically using the compression isotherms for the pure and mixed monolayers from Figs. l-3. For the sake of comparison, we have taken as upper integration limits the same values of the surface pressure as in the case of the molecular mean area versus composition relations (Figs. 4-6).

qd7~ 0

vr(A - x,Al - x~~~)Q!T [6] s**

y;m 0.25

0.50

0.75

0

1

XEL

FIG. 8. Collapse pressure of mixed Ekzeaxanthin monolayers as a function of the mole fraction of EL.

0.25

0.5

0.75

1

xEL

FIG. 9. Collapse pressure of mixed EL:lutein monolayers as a function of the mole fraction of EL. hurnai

of Co/bid and Inrerface Science, Vol. 95, No. 2, October 1983

360

TOMOAIA-COTISEL AND CHIFU 0

/ O~

01

,9

/ .o/" tic

~ o2

I

"0

O. 5

0.5

075

I

WZO. 2 I

×EL

FIG. 10. Excess free energy of mixing, AG E, for the fl-cryptoxanthin:EL mixtures, at three values of the surface pressure (symbols as in Fig. 4).

0.3

~", \ x\ t.__qk.

/

,.I AIJ

I

I

I

(125

0.5

0.75

×EL

FIG. 12. Excess free energy of mixing for iutein:EL mixtures (symbols as in Fig. 5).

In Figs. 10-12, we have plotted the free excess energies of mixing for the systems of ~-cryptoxanthin:EL, zeaxanthin:EL as well as for lutein:EL, respectively. It is noticed in the diagrams that the AG E values increase with increasing the surface pressures. The small negative AG E values found in the #-cryptoxanthin:EL monolayers (Fig. 10) are in agreement with the results obtained by the collapse pressure method (Fig. 7) and these observations reveal the quasi-ideal behavior of these binary mixtures at the air/ water interface. Further on, in the zeaxanthin:EL (Fig. 11) and especially for lutein:EL (Fig. 12) monolayers, the negative AG E values are higher than twice the corresponding values for/~cryptoxanthin:EL (Fig. 10) and their experimental curves show a minimum at mole fractions of EL ranging between 0.25 and 0.5. These results are in general accord with the previous ones obtained by the collapse pressure method for zeaxanthin:EL (Fig. 8) and lutein:EL (Fig. 9) monolayers. The enhanced collapse pressures in the mixed monolayers

a~

,c( / ~ x

-~ 0
~, 0.2-

/ j g . / .t"

j

.~. ,,

-

\\ x "s---~--1 -~" .av" \'-*-- - ~ I 025

[ 0.5

_ I 0.75

× EL

FIG. 11. Excess free energy of mixing for zeaxanthin:EL mixtures (symbols as in Fig. 5). Journal of Colloid and Interface Science, Vol. 95, No. 2, October 1983

of zeaxanthin:EL and lutein:EL as well as the negative values of the excess free energies of mixing indicate a high stability of the mixed films, and probably stronger interactions than in an ideal mixture. Therefore, in the first approximation these findings could be interpreted in terms of a regular molecular arrangement in fl-cryptoxanthin:EL, zeaxanthin:EL, and lutein:EL systems, the condensing effect being a result of the molecular packing that provides enhanced dispersive interactions and probably even specific polar interactions are involved, especially for the systems with lutein or zeaxanthin. The differences in mixture behavior of the studied xanthophylls with EL are due to their different molecular structures, i.e., the bipolar nature of zeaxanthin and lutein as well as the presence of an a-ionone ring on the lutein xanthophyll structure. REFERENCES 1. Chifu, E., Zsak6, J., and Tomoaia-Coti~el, M., J. Colloid Interface Sci. 95, 346 (1983). 2. Isler, O., in "Carotenoids." Birkh~iuser Verlag, Basel-Stuttgart, 1971. 3. Chifu, E., and Tomoaia-Coti~el, M., unpublished resuits. 4. Tomoaia-Coti~el, M., and Chifu, E., Gazz. Chim. Ital. 109, 371 (1979); Tomoaia-Coti~el, M., Ph.D. thesis, "Babe~-Bolyai" University of Cluj-Napoca, 1979. 5. Tomoaia-Coti~el, M., Zsak6, J., and Chifu, E., Ann. Chim. (Rome) 71, 189 (1981). 6. Gaines, G. L., Jr., in "Insoluble Monolayers at Liquid-Gas Interfaces." lnterscience, New York, 1966.

XANTHOPHYLL FILMS, II 7. Shah, D. O., J. ColloidlnterfaceSci. 37, 744 (1971). 8. Cadenhead, D. A., and MiiUer-Landau, F., J. Colloid Interface Sci. 78, 269 (1980). 9. Shah, D. O., J. Colloid Interface Sci. 32, 577 (1970). 10. Rods, O. A., and Shah, D. O., J. Colloid Interface Sci. 29, 279 (1969). 11. Chifu, E., and Tomoaia-Coti~el, M., Rev. Roumaine Chim. 27, 27 (1982). 12. Phillips, M. C., and Joos, P., Kolloid-Z. Z. Polym. 238, 499 (1970). 13. Nakagaki, M., and Funasaki, N., Bull. Chem. Soc. Jpn. 47, 2094 (1974). 14. Defay, R., Prigogine, I., Bellemans, A., and Everett,

15.

16.

17.

18.

361

D. H., in "Surface Tension and Adsorption," p. 77. Longmans, Green, London, 1966. Joos, P., Bull. Soc. Chim. Belges 78, 207 (1969); Joos, P., and Demel, R. A., Biochim. Biophys. Acta 183, 447 (1969). Goodrich, F. C., in "Proceedings, International Congress Surface Activity," 2nd, Vol. 1, p. 85. Butterworths, London, 1957. Costin, I. S., and Barnes, G. T., J. Colloid Interface Sci. 51, 106 (1975); Bacon, K. J., and Barnes, G. T., J. Colloid Interface Sci. 67, 70 (1978). Fukuda, K., Kato, T., Machida, S., and Shimizu, Y., J. Colloid lnterface Sci. 68, 82 (1979).

Journal of Colloid and Interface Science, Vol. 95, No. 2, October 1983