Langmuir 1988,4, 1049-1054
1049
Intermolecular Effects of Stereochemistry. Mixing Thermodynamics in Monolayers of Diastereomeric Surfactants Edward M. Arnett,” Noel Harvey, and Philip L. Rose Department of Chemistry, Duke University, Durham, North Carolina 27706 Received March 29, 1988 This article continues an unprecedented investigation of the effects of stereochemistry on packing in monolayers at the air-water interface. Force-area isotherms are compared for a series of diastereomeric two-chain diacids whose chains are joined at various points by a carbonyl bridge. The surface properties of the diacids are dependent on the lengths of the chains, the position of the carbonyl bridge, and the stereochemistry (meso or dl) at the point of juncture on the chains. Several sets of diastereomeric pairs of varying chain length and position of juncture provide a unique opportunity to study monolayer mixing by the Goodrich excess free energy analysis. Despite the structural similarity of the meso and d l components of the monolayers, nonideal mixing is observed for several systems. Deviations from ideality depend on the lengths of the chains, position of the carbonyl bridges, and changes in molecular stereochemistry at these bridges. Although the overall behavior clearly depends on different balances between inter- and intramolecular forces as molecular structure and film pressure are varied, we are unable to provide a rigorous quantitative separation of these effects. Direct comparison of the thermodynamics of mixing for all four sets of diastereomers allows qualitative description of structural effects in film compression and mixing.
Introduction During the past decade we have pursued a systematic investigation of molecular recognition through stereoselective interactions in monolayers. With the notable exception of phosphatidylcholines,’ all of the surface properties of all chiral surfactants which we have studied so far are strongly stereoselective in discriminating between the properties of films spread from enantiomers and those cast from the corresponding racemic mixture. These results are of biophysical relevance because virtually all of the compounds in cell membranes, except water, are chiral. However, very few of the many studies of lipids and other surfactants at interfaces have explicitly considered the effects of stereochemistry. More fundamentally, such stereochemical comparisons are a powerful tool for elucidating intermolecular forces because enantiomers are perfect models for each other so that any differences between their monolayer properties and those of their racemic mixture are the result of symmetry effects on intermolecular forces. More recently, we reported a comparison of the forcearea isotherms, equilibrium spreading pressures, and free energies of activation to viscous flow for four pairs of diastereomeric surfactants made by joining two pentadecanoic acid units through a carbonyl group at the 3,3‘, 6,6’, 9,9’, and 12,12’ positions.2 The eight compounds comprise four pairs of diastereomers (meso or dl) which differ only by the position of the carbonyl cross-link. Since they are stereoisomers, any differences between their surface properties must have a stereochemical origin. Clearly defined differences in their force-area isotherms classified the diacids in terms of the distance between the carboxylate head groups and the stereochemistry at the point of attachment of the carbonyl group. The behavior was readily interpretable through a molecular mechanics (1) (a) Arnett, E. M.; Gold, J. M. J. A m . Chem. SOC.1982,104, 636. (b) Arnett, E.M.; Gold, J. M.; Harvey, N. G.; Johnson, E. A.; Whitesell, L. G. In New Applications of Phospholipid Bilayers, Thin Films and Vesicles; Plenum: New York, in press. (2) Harvey, N.; Rose, P.; Porter, N. A.; Huff, J. B.; Arnett, E. M. “Monolayer Properties of Eight Diastereomeric Two-Chain Surfactants at the Air Water Interface-A Resultant of Intramolecular Forces”; J . Am. Chem. Soc., in press.
0743-7463/ 8812404-1049$01.50/0
model, which leads to the conclusion that the dl compounds, in contrast to their meso isomers, have a preferred conformation that resists compression, thereby adding an intramolecular component to the force-area compression curve in addition to the intermolecular interactions which it has in common with the meso isomers (Figure 1). A parallel investigation of surface viscosities and free energies of activation for viscous flow gave further evidence of a difference in intermolecular interactions between the sets of diastereomers. The dl isomers indicated a greater resistance to Newtonian flow than their meso cognates. To the best of our knowledge, that report was the first systematic investigation of diastereomeric effects in monolayers and demonstrated very clearly their promise for clarifying the role of molecular shape as a contribution to intermolecular versus intramolecular interactions. The problem of separating stereochemically dependent inter- and intramolecular interactions in diastereomeric compounds has been approached previously by using ab initio molecular orbital calculations but has received little experimental a t t e n t i ~ n .These ~ calculations have shown that diastereomeric compounds such as 2,3-dicyanobutane exhibit significant energetic dependence on intramolecular configuration about their chiral centers. More difficult to elucidate are the effects of stereochemistry on intermolecular interactions, especially since there are two differentiable chiral centers on each individual molecule and considerable rotational freedom between chiral centers as the molecule exists in free space. The question may now be raised as to whether the effect of intramolecular stereochemistry on intermolecular interactions is detectable experimentallyin mixtures of these diastereomeric surfactants in highly oriented monolayer films. Considering the structural similarity of the diastereomers, one might assume that ideal mixing should occur in every case. However, the sharp differences between their surface viscosities and force-area isotherms suggest that sufficient differences in intermolecular interactions may be at work so that nonideal mixing or even segregation might occur. Accordingly, we report here our (3) Craig, D.P.; Radom, L.; Stiles, P. J. h o c . R. SOC.London, A 1971, A343. 11.
0 1988 American Chemical Society
1050 Langmuir, Vol. 4, No. 4, 1988
Arnett et al. purified hexane/ethanol have also been described in detail.5 The subphase water was triply distilled. All experiments were carried out a t subphase p H 5.50. Spreading Solutions. The proper amount of each diastereomer to be delivered to the surface was determined in “scouting” runs performed by delivering 4.403 X 10l6molecules to a typical area of 6.84 X lo’* A2. Solutions that formed highly expanded films at greater than 100 A2/moleculewere spread by using smaller aliquots of spreading solution until the entire isotherm from n 0 to approximately 40 dyn/cm could be recorded. Solutions of dllmeso mixtures for each of the diastereomeric pairs were prepared by delivering an aliquot of each dl or meso hexane-ethanol solution to a stoppered, 2-mL test tube via an Agla microliter syringe. Special care was taken to ensure that each droplet of solution was delivered directly to the bottom of the tube and not to the tube wall in order to minimize concentration errors due to hexanes evaporation. Each mixture was generally made twice, and the n / A data obtained from these two sets of mixtures agreed to within h1.3 A2/molecule (95% confidence) at a given n. Langmuir Film Balance Techniques. The automated Langmuir balance employed a torsion head/ floating barrier assembly sensitive to film pressures of 0.005 dyn/cm. Its construction, cleaning, preparation, calibration, and technical specifications have been described in detail.5 The rate of compression for each monolayer film was varied from 7.1 to 29.8 (A2/molecule)/min. No difference was seen in the II/A isotherms within this compression range. For all isotherms reported here the rate of compression was kept constant at 19.2 (A2/molecule)/min. Each film of every dl and meso diastereomer and their mixtures was compressed to as high a surface pressure as possible without breaking the positive meniscus of the water subphase (=40 dyn/cm) and was reexpanded immediately at the same rate. Each diastereomer and mixed diastereomer isotherm was obtained at 23.8 f 0.3 “C and reproduced 5-12 times. The stability of the films was checked once every six replications by compressing to a fixed surface pressure and halting compression for a minimum of 3 min. The validity of the film balance calibration was checked every 12 h of working time with the well-known stearic acid isotherm.’l Equilibrium Spreading Pressures (ESPs). ESPs of the surfactants were determined on pure water at 24.0 “C by Du Nouy ring tensiometry using either a Fisher Autotensiomat or a manual Cenco Du Nouy tensiometer (no. 70535). Before each measurement, the ring was cleaned by flaming and the surface tension of the freshly aspirated water surface taken. In each experiment, 0.5-1.0 mg of surfactant crystals, far in excess of the amount needed to form a monolayer, was delivered carefully to the subphase surface of a 6.8-cm-diameter Teflon dish (Autotensiomat) or a 6.5-cm-diameter Pyrex T-cup (Cenco). The crystals were allowed to equilibrate with their monolayers for a minimum of 18 h, with successive experiments being performed at 24 and 36 h. Equilibration was assumed when the readings changed by no more than 0.2 dyn/cm in a 4-h period. Each experiment was repeated at least 3 times with fresh subphase and crystals. Data Analysis. All data reported here are analyzed at the 95% confidence limit by using the ‘student t” for 5-12 repetitions for II/A isotherms and 3-6 repetitions for ESPs. The free energy changes of compression for the C-12, C-15, and C-18 diastereomers linked a t the 6,6’ position and for C-15 diastereomers linked a t the 9,9’ position by a carbonyl group and their mixtures were determined by graphical integration from n 0 to the surface pressure in question. Surface pressures were measured out to 400 A2/molecule for the C-156,6’, C-18:6,6’, and C-15:9,9’ compounds and their mixtures and remained in the neighborhood of 10 mdyn/cm until around 120 A2/molecule. Above 400 A2/molecule, surface pressure measurements were not reproducible enough to use in this treatment. The C-12:6,6’ compounds were measured out to 500 A2/molecule for the dl diastereomer and 300 A2/molecule for the meso diastereomer, where no surface pressure was detected. Errors reported in these measurements are based on the propagation of errors at the 95%
=
meso Figure 1. Top view of the preferred orientation of dl and meso diastereomers a t the air/water interface. T h e meso isomer sits a t the air/water interface in a fashion analogous to two straight-chain carboxylic acids joined a t some point along the hydrocarbon chains. The d l isomer has its lowest energy conformation where the carboxylate head groups are extended from one another, requiring an additional amount of work to force them into a colinear position upon compression of the film in addition to the work required to raise the hydrocarbon chains out of the surface.
study of mixtures of two dl and meso pairs of pentadecanoic acid monomers joined at the 6,6’and 9,9’ positions by a carbonyl linkage, 6,8-dinonyl-7-oxotridecanedioic (C15:6,6’)and 9,ll-dihexyl-10-oxononadecanedioic acids (C15:9,9’), and two diacid diastereomeric pairs with chains of 1 2 and 18 carbon units in length that are joined at the 6,6’positions, 6,8-dihexyl-7-oxotridecanedioic (C-12:6,6’) and 6,8-didodecyl-7-oxotridecanedioic acids (C-18:6,6’). This series of structural isomers offers a novel opportunity to compare systematically the effects of (a) carbonyl linkage position, (b) fatty acid chain length, and (c) stereochemistry on intermolecular interactions. It is reasonable to assume that chiral discrimination exists between diastereomers in these monolayers as has been shown for several enantiomeric sy~tems;~J the problem is to separate these effects from the intramolecular contributions to film compression energetics. With this problem in mind, we have undertaken a Goodrich6 excess free energy analysis for mixtures of the dl and meso isomers for all four sets of diastereomers. Since the components of each film differ only in the stereochemistry about one chiral center, large differences in molecular structure, size, charge, and interaction with subphase water are not complicating factor~.’-~
Experimental Section Every precaution was taken to ensure that all materials and instruments used in this study were clean. Purification and cleansing routines have been described e l ~ e w h e r e . ~ Preparation and Purification of Materials. The four two-chain diastereomeric dicarboxylic acid pairs were prepared, purified, separated, and stored as described previously.10 The preparation and handling of solutions of the diastereomers in ( 4 ) Stewart,M.; Arnett, E. M. In Topics in Stereochemistry; Eliel, E. L., Allinger, N. L., Eds.; Wiley: New York, 1982; Vol. 13. (5) Arnett, E. M.; Chao, J.; Kinzig, B.; Stewart, M.; Thompson, 0.; Verbiar, R. J. Am. Chem. SOC.1982, 104, 389. (6)Goodrich, F. C. R o c . Int. Congr. Surf. Act., 2nd 1957, I , 85. (7) Shah, D. 0.;Capps, R. W. J. Colloid Interface Sci. 1968, 27, 319. (8) (a) Guay, D.; Leblanc, R. M. Langmuir 1987, 3, 575. (b) Gaines, G. L.; Tweet, A. G.; Bellamy, W. D. J. Chem. Phys. 1965, 42, 2193. (9) Lucassen-Reynders, E. H. J. Colloid Interface Sci. 1973, 42, 554. (10) (a) Porter, N. A.; Ok,D.; Adams, C. M.; Huff, J. B. J. Am. Chem. Soc. 1986,108,5025. (b) Porter, N. A.; Ok,D.; Huff, J. B.; Adams, C. M.; McPhail, A. T.; Kim, K. J . Am. Chem. SOC.1988, 110, 1896.
(11) Gaines, G. L. Insoluble Monolayers at Liquid-Gas Interfaces; Wiley: New York, 1966; p 220.
Intermolecular Effects of Stereochemistry
Langmuir, Vol. 4, No. 4,1988 1051
=
nmz2iz-2 HOOC
C-18 6,6’
C - 1 2 6.6’
,
I
0
I
20
I
,
120
140
,
40
00
60
100
160
100
200
A’ /molecule
Figure 2. Surface pressure-area isotherms for the compression cycle of dl and meso C-126,6’diacids and their mixturea on a pure water subphase at 24 O C compressed at a rate of 19.2 (Az/molecule)/min. A = 0% d l (or 100% meso), B = 25% dl, C = 50% d l , D = 75% d l , and E = 100% dl.
0
I
I
I
I
I
I
I
I
I
I
20
40
60
80
100
120
140
160
180
200
A’ /molecule Figure 4. Surface pressure-area isotherms for the compression cycle of dl and meso C-18:6,6’diacids and their mixtures on a pure water subphase at 24 “C compressed at a rate of 19.2(A2/molecule)/min. A = 0% d l (or 100% meso),B = 25% dl, C = 50% d l , D = 75% d l , and E = 100% dl. 40 r
-E . -c E
20
“I
”.E 10
0
10
20
30
40
50
60
70
80
90
IO0
I10
I20
I30
I
0
A’/
molecule
20
40
10
80
100
120
140
160
180
200
.i2/molecule
Figure 3. Surface pressure-area isotherms for the compression cycle of dl and meso C-156,6’diacids and their mixtures on a pure water subphase at 24 “C compressed at a rate of 19.2(A2/molecule)/min. A = 0% d l (or 100% meso), B = 25% d l , C = 50% dl, D = 75% d l , and E = 100% dl.
Figure 5. Surface pressure-area isotherms for the compression cycle of dl and mea0 C-159,Ydiacids and their mixtures on a pure water subphase at 24 “C compressed at a rate of 19.2 (A2/molecule)/min. A = 0% d l (or 100% meso), B = 25% d l , C = 50% dl, D = 75% dl, and E = 100% dl.
confidence involved in analysis from both experimental and graphical integration sources.
Figure 5 gives the force/area curves for the C-15 diastereomers linked at the 9,9’ positions and their mixtures. It is immediately obvious that these films and their mixtures are more highly expanded than the same fatty acid chain length compound linked at the 6,6’ positions. It is interesting that at higher surface pressures chain length does not drastically alter the II/A properties of any of these films. It appears that in the more condensed states the carbonyl linkage serves to keep the cross sectional area of each “fatty-acid dimer” nearly the same, allowing very efficient packing. This is especially true in the meso surfactants, where intramolecular stereochemistry favors the orderly side-by-side packing of diacid units.2J0 Phase Transitions and Area/Composition Relationships. It is readily apparent that the pure dl diastereomers show a collapse to some three-dimensional state, as has been observed for most overcompressed lipid films.13 The resulting plateau region is extraordinarily stable, decaying in surface pressure no more than 0.1 (dyn/cm)/h when compression is halted. In accordance with the “two-dimensional” phase rule of Defay and
Results and Discussion II/A Isotherms. Figures 2, 3, and 4 compare the force/area properties of the 6,6’-linked surfactants and their dllmeso mixtures. It is apparent that the properties of the dl films (highly expanded, collapse pressure between 28 and 35 dyn/cm) are similar despite the increase of hydrocarbon chain length. Correspondingly, the properties of the films cast from meso diastereomers show roughly the same behavior despite chain length (more condensed, limiting molecular area of about 60 A2/molecule). Increasing the chain length of the fatty acid units in each diastereomeric pair has the effect of condensing both the pure dl and meso films and their mixtures, especially at lower surface pressures. For example, the “lift-off“ area for the meso and dl diastereomers of the C-12surfactants was approximately 300 and 500 A2/molecule,respectively, while that for the C-18 diastereomers was 76 and 104 A2/molecule, respectively. This is directly analogous to the condensationof the single-chain fatty acid series, where longer chain fatty acids (e.g., stearic acid) form more condensed monolayer films than those with shorter hydrocarbon chains (e.g., dodecanoic acid).12
(12)Nutting, G.C.;Harkins, W. P. J. Am. Chem. SOC.1939,61,1180. (13) Handa, T.;Nakagaki, M. Colloid Polym. Sci. 1979,257,374.
1052 Langmuir, Vol. 4, No. 4, 1988
42
40
Arnett et al.
\
J
-
36 -
‘,
38
0
\
10
20 3 0 4 0
50
.
60 70 80 9 0 1 0 0
%d,l
Figure 6. Surface pressure at the collapse point versus percent diacid dllmeso mixtures at 24 “C. film composition for C-15:9,9’ Table I. Equilibrium Spreading Pressure (dyn/cm) compd C-126,6‘ C-15:6,6’ C-186,6‘ C-15:9,9’
meso 32.03 f 1.35 17.90 f 0.84 20.47 f 2.27 16.75 f 1.43
dl 21.67 12.93 12.23 20.73
f f f f
1.28 0.76 0.46 3.82
Crisp,14this indicates that there are two phases in equilibrium, most likely consisting of a monolayer and a three-dimensional aggregated state. In addition, overcompression in this plateau region does not result in increased surface pressure even if the moving barrier of the film balance is run all the way up to the floating barrier of the torsion head assembly. The exception to this is the C-18:6,6’ compound, where a small increase in surface pressure is noted upon further compression in the plateau region. The exact nature of the “collapsed” phase in this plateau region is not clear. It is evident from Figures 2-5 that the phase transition for the films spread from dl diastereomers is affected by incorporation of the meso isomer into the monolayer matrix. The general trend for all four diastereomeric mixtures is a gradual increase in the transition pressure (IIt)of the dl component with increasing meso composition. According to the surface phase rule, this is indicative of mixing between the dl and meso components at n I nt.14 A plot of nt vs percent dl for the C-15:6,6’ surfactants is shown in Figure 6 and indicates nonideal miscibility. The same general phenomenon of nonideal miscibility is observed for the other three diastereomeric sets whose plots are not presented here. The mixing of dllmeso surfactants below the phase change was monitored at surface pressures from 2.5 to 15.0 dyn/cm via the additive area re1ation~hip.l~The results in Figure 7a-d indicate nonideal miscibility for all four sets of surfactants at the lower surface pressures, assuming a more ideal (or conversely, segregated) matrix at 15 dyn/cm. Coupled with the lIt phase diagrams we conclude that dllmeso diastereomers form nonideal mixtures for all four sets. Equilibrium Spreading Pressures. ESPs of each isomer are given in Table I. It is apparent that there are large differences in the spontaneous spreading of dl and meso monolayers in equilibrium with their respective bulk phases for all four sets of diastereomers. However, there appears to be no discernible trend in the ESPs as a (14)(a) Defay, R. Doctoral Dissertation, Brussels, 1932. (b) Crisp, D.
J. Surface Chem. (Suppl. Res., London) 1949,17. (15) Gaines, G. L.Insoluble Monolayers at Liquid-Gas Interfaces: Wiley: New York, 1966; p 282.
function of either chain length or carbonyl linkage position. Film Stability. The stability of each film was checked by stepwise compression to a given surface pressure while monitoring the decay in II at constant A. The films of the diastereomers were stable at surface pressures below their ESPs (decaying no more than 0.1 (dyn/cm)/min); above their ESP, the dl films were still stable, while the meso films showed greater decreases in n. By this criterion the dl films were metastable, while the meso films were unstable. No significant hysteresis was observed at the compression rates studied when the films were compressed within the stable n / A regime below the dl phase transition. It is noteworthy that the dllmeso mixtures were all stable above the ESPs of the pure components. Gaines has proposed that this behavior is evidence for molecular interaction.16 Thermodynamics of Mixing for Diastereomeric Surfactants in Monolayers. Goodrich‘s original derivation and treatment for obtaining excess free energies of mixing utilize the differences between the free energies of compression of the pure film components and their mixtures from n 0 to some specified surface pressure as expressed by the following formula:
=
where A designates average molecular area in mixed (1,2) and pure (1 and 2) films and Nl and N 2 are the mole fractions of components 1 and 2, respectively. The inherent assumptions of this method have been reviewed and refined, especially at very low surface press~res.~J~J* Our treatment has accounted for these where possible and is based for the most part on Goodrich’s basic assumptions. Since we have shown nonideal mixing to occur in the n = 2.5-15.0 dyn/cm range, the excess free energies of interaction were calculated for compressions of each component and mixture to these surface pressures. In addition, these surface pressures are below the ESPs and/or monolayer stability limit so that dynamic processes arising from reorganization, relaxation, or film loss do not contribute significantly to the work of compression. Parts a-d of Figure 8 give the excess free energies of mixing for all four sets of diastereomers. The highly expanded C-12:6,6’ and C-15:9,9’ films show marked deviation from ideality upon compression to 10.0 and 15.0 dyn/cm, indicating a dependence of packing on stereochemistry as the film is compressed. Conversely, the C-15:6,6’ and C-18:6,6’ systems show an ideal expenditure of energy upon compression at every surface pressure across the entire composition range. Stereochemistry appears to play little or no role in the molecular interactions between dl and meso isomers for these films. These differences in AG” can be interpreted on the basis of molecular properties by using a “hedge-clipper”model for these compounds. If one considers each molecule to be divided into a head group section below the carbonyl group represented as the “blades” and the hydrocarbon tails above the carbonyl group as the “handles”, with the carbonyl group as the ”bolt” that joins the two sections, the usual bent form of the hedge-clippers represent the meso isomer (Figure 9). If one blade and handle section trade positions about the bolt, the hedge-clippers represent (16) Gainea, G. L. J. Colloid Interface Sci. 1966,21, 315. (17) Pagano, R. E.; Gershfeld, N. L. J . Colloid Interface Sci. 1972, 41, 311. (18) Rakshit, A.K.;Zografi, G. J . Colloid Interface Sci. 1980,80, 474.
Intermolecular Effects of Stereochemistry
Langmuir-, Vol. 4 , No. 4, 1988 1053
n = 2.5 dynlcm 120
n = 5.0 dynlcm n = 10.0 dynlcm n = 15.0 dynicm 0
10 20
40
30
50 % d,l
60
70
50
80 90 100
I
,. ,. , . ,. ,
I
10
0
160
140
-.
20
30
.
, , ,
5 0 60 4 h d,i
40
70
. , . , . 80
1
90 100
d) C-15 9,9'
-
n = 10.0 dynlcm
n = 10.0dynlcm n = 15.0dynlcm ,. , . , . , . , .
5 0 ! . 0 10
20
40
30
50
I
. , 70
60
., . 80
, .
Il=15Odynlcm
t
90 100
0
10
20
30
40
% d,l
300
200 -
-: 8
C)
a) C-12 6,6'
300
50
f
z -P X
5
2oo
f
-200-300
-a!
a.-sFX
200 300
0
8 0 I3
8 '
-
I
.
I
1
0;c-
--e---e--.J---, 2.5, 5.0, 10.0, 15.0
-100-
04c -100
- - I
I
I
.
, .
I
.
I
,
,
8
-200-
w"
.
-300
1
.
I
.
I
,
I
,
I
. I
,
I
,
I
,
I
,
I
b) C-15 6,6'
100-
.c
: cn
9 0 100
01
w
A
80
100-
0
-100-
70
C-18 6,6'
L.
8
60
% d,l
---
t--+--f----
-
2 . 5 , 5.0,10.0, 15.0
-200-300
1
.
1
.
1
.
1
.
1
*
I
. I
.
I
.
i
the dl diastereomer. One may vary molecular properties by (a) changing the position of the carbonyl linkage, (b) changing the overall length of the fatty acid units, and (c) changing the stereochemistry at the carbonyl bolt. These compounds present a unique set in which all three variables are represented: (1)The (2-12, C-15, and C-18 diastereomers linked at the 6,6' position comprise a set in which the blade lengths are
kept constant at 6 carbons while the handle length is varied from 6 to 12 carbon units. The more highly expanded (3-12 system is the only one which shows an excess energy of mixing and has the shortest handles. (2) The C-12:6,6' and C-15:9,9' systems comprise a set in which the handles are kept constant at six carbon units while the blades are varied from six to nine carbons. Both of these systems form expanded monolayers, and both have
1054 Langmuir, Vol. 4, No. 4, 1988
‘BLADES’
Arnett et al.
4
-&
“HANDLES“
‘BOLT’
SUBPHASE
Figure 9. “Hedge-clipper”representation of the diacids. The “blade”portion extends from the carboxylate head groups immersed in the water surface to the carbonyl “bolt”. The “handle” portion is the hydrocarbon tails above the carbonyljunctions. Real hedge-clippers are made of two enantiomeric pieces held in the conformation that provides minimum overlap of the blades when held colinear.
significant deviation from ideal mixing thermodynamics. It is interesting to note that the C-12:6,6’ films are more highly expanded than the C-15:9,9’. We have shown previously that, within a homologous C-15 series, increasing the carbonyl linkage distance from the carboxylate head groups (from 3,3’ to 12,129 resulted in a concomitant expansion of the dl and meso films as observed in the IIlA isotherms of the pure dl and meso components.2 This indicated that the intramolecular head group separation as the molecule sits on the water surface contributes significantly to the energetics of film compression. Here, C-12:6,6’, having a shorter carboxylate/carbonyl distance, forms a more expanded film, indicating a large handle contribution to film compression energetics. (3) The C-15:6,6’ and C-15:9,9’ systems comprise a set in which the overall chain length is constant while the carbonyl linkage positions is changed. This variation results in a change of both blade and handle length. The 9,9’ compounds form more highly expanded films, presumably for the reasons discussed above, and show large excess free energies of mixing. The Goodrich treatment has essentially subtracted out the differences in the degree of film expansion between diastereomers (eq 1)and hence the primary contribution from intramolecular carboxylate separation and handle interactions in the pure dl and meso films. The results indicate a significant contribution from the handle portion of the molecule to intermolecular interaction. It must be kept in mind that since dl and meso film components do interact, the intramolecular contribution to film compression may be altered. This would arise from conformational perturbations as molecules interact, thereby precluding complete separation of inter- and intramolecular contributions to the thermodynamics of compression. However, the comparison of several sets of data between several molecules of varying structure does point to the importance of certain molecular features. The repeating pattern deduced from these comparisons is that the handle portions of these molecules play an integral role in molecular associations between dl and meso film components. At large areas per molecule, all three polar moieties (carboxylate head groups and carbonyl linkage) should be interacting hydrophilically with the
water subphase, while the hydrocarbon handles are free to interact hydrophobically (Figure 9). With this model, it is not surprising that the shorter handle (i.e., six carbons) systems express stereochemically dependent dllmeso interactions through excess free energies of mixing. These short hydrocarbon chains must have their van der Waals interactions occur at points closer to the chiral bolt than their longer chain counterparts. This steric contribution to packing is preserved as the mixed film is compressed. Conversely, the longer handle systems (i.e., 9 and 1 2 carbons) are freer to interact at points farther away from the chiral bolt at large areas per molecule. Little or no detectable steric influence on packing is observed as the mixed films are compressed, resulting in ideal free energies of mixing. This may also be envisioned as a “burying” of the chiral centers between the hydrocarbon chains, masking any steric influence on hydrocarbon associations and, hence, packing.
Conclusions This investigation provides a unique study of the effects of stereochemistry and chain length for a series of diastereomeric two-chain dicarboxylic acids joined at various points along the chains by a carbonyl group. Force-area curves are highly sensitive to the lengths of the chains, the position of the carbonyl linkage, and the stereochemistry (meso or d l ) a t the point of linkage on the chains. Four sets of diastereomers provide ideal pairs for examination of monolayer mixing by the Defay-Crisp and Goodrich criteria. Again, the ideality of mixing for various combinations of diastereomers is dependent on the chain length and position of the carbonyl junction, which appear to determine the extent of stereochemical influence on intermolecular associations as gauged by excess thermodynamic quantities. Acknowledgment. This work was supported by Grants NIH-R01-GM28757 and NSF-CHE-8412976. We thank Eun Carol Yi, Dr. Kwang-Yoo Kim, and Prof. Ned A. Porter for the synthesis and purification of the C-12:6,6’ and C-18:6,6’ surfactants. Registry No. C-15:6,6’, 115077-71-9; C-15:9,9’, 115077-72-0; C-12:6,6’, 115077-73-1; C-18:6,6’, 115077-74-2.
