Langmuir 1991, 7, 1765-1769
1765
Effects of Stereochemistry and Alternation on the Adsorption Properties of Surface-Active cis- and trans-2-n-Alkyl-5-hydroxy1,3-dioxanes Klaus Lunkenheimer,’*+Bogdan Burczyk,t Andrzej Piasecki,t and Rolf Hirteg Central Institute of Organic Chemistry, 0-1 199 Berlin- Adlershof, Germany, Institute of Organic and Polymer Technology, Technical University of Wroclaw, 50-370 Wroclaw, Poland, and Institute of Polymeric Chemistry, 0-1503 Teltow-Seehof, Germany Received April 3,1990. In Final Form: February 15, 1991
A systematic study of adsor tion properties of cis- and trans-2-n-alkyl-5-hydroxy-l,3-dioxanes (alkyl: cis, C2H5, n-CsH7, n-CIHs, n- f&, n-C&s, n-C7H15;trans, n-CsH7, n-cd-h, n-C~H15)at the air/water surface at 295 K has been undertaken by using surface tension measurements of their aqueous surface chemically pure solutions. The surface parameters, i.e. standard free energy of adsorption, minimum surface area demand per molecule, and surface interaction parameter, reveal distinct differences due not only to the stereochemicalconfigurationof the molecules but also to even-and odd-numbered characteristics of the n-alkyl chain. The said differences become greater as the alkyl chain becomesshorter. It is concluded that surface tension is a very sensitive physical quantity reflecting even the slightest deviations in the structure of the molecules adsorbed provided that special requirements on the purity of the solutions are followed.
8
Introduction Glycerol acetals obtained from long-chain aldehydes or ketones and glycerol are convenient hydrophobic intermediates in syntheses of chemically degradable surfactants.’” The reaction products of aldehydes with glycerol constitute a four-component mixture of cis- and trans-2-alkyl-4-hydroxymethyl-l,3-dioxolanes and cis and trans-2-alkyl-5-hydroxy-l,3-dioxanes. These compounds may provide a convenient object for the investigation of the effect of substituent configuration in 1,3-dioxacyclane rings on their surface activity. A preliminary investigation of the adsorption at the water solution-air interface of some derivates of 1,3-dioxolane and 1,3-dioxane has been reported earlier.6 This paper focuses upon the adsorption isotherms of cis- and trans-2-n-alkyl-5-hydroxy-l,3-dioxanes obtained from surface tension measurements of their surface chemically pure aqueous solutions. To evaluate surfactant adsorption isotherms in terms of molecular data such as standard free energy of adsorption, AG*,d (and/or surface activity, a ) , minimum surface area demand per molecule adsorbed, Amin, and surface interaction parameter, Ha,one has to comply with special requirements concerning the method of surface tension measurements and the purity of the surfactants used.’+ Thus, a new insight into the dependence of the ~~
~~
* To whom correspondence should be sent: Dr. sc. Klaus Lunken-
heimer, CentralInstituteof OrganicChemistry, 0-1199 Berlin-Adlershof, Rudower Chaussee 6, Germany. + Central Institute of Organic Chemistry. Imtitute of Organic and Polymer Technology. Institute of Polymeric Chemistry. (1) McCoy, D. R. US.Patent 3,909,460,1975; U.S.Patent 3,948,953,
t
1976. (2) Rutzen, H.; GBtte, E.German Patent 1,542,671, 1969. (3) Burczyk, B.; Weclas, L. Tenside Deterg. 1980, 17, 21. (4) Weclas, L.; Burczyk, B. Tenside Deterg. 1981, 18, 19. (5) Burczyk, B.; Piaaecki, A.; Weclas,L. J. Phys. Chem. 1986,89,1032. (6) Lunkenheimer, K.; Wantke, K.-D. Colloid and Polym. Sci. 1981, 259, 354. (7) Lunkenheimer, K.; Miller, R. Tenside Deterg. 1979, 16, 312. (8) Lunkenheimer, K.; Miller, R. J. Colloid Interface Sci. 1987,120, 176.
surfactant parameters on the chemical structure, i.e. on cis/trans configuration and on carbon chain length, could be gained.
Experimental Section Methods. Surfacetension measurementson aqueoussolutions were performed by the ring tensiometer method at 295 K (22 O C ) . The necessary modifications for surfactant solutions given in ref 6 were applied to obtain maximum reliability. The correction factors derived by Huh and Mason were ~ e e d . ~ O The water used was bidistilled from an alkaline permanganate solution. Before use the solutions of the individual surfactants were judged with respect to adulterating surface-active trace contaminants by applying the criteria derived in ref 8. Although GLC analyses of the substances did not reveal any impurities, the original solutions of the surfactants did not entirely fulfill the requirements on surface chemical purity. Hence, stock surfactant solutionswere repurified by using an elaborate procedure described in ref 9 until the grade of purity required for surface chemical investigations was achieved. To prevent acidic hydrolysis of the lJ3-dioxanederivatives, their aqueous solutions always contained 10-9 mol-dm” sodium bicarbonate. The equilibrium surface tension-concentration isotherms of these compounds (Figures 1 and 2), measured at 295 K, were evaluated by applying Frumkin’s well-known surface equation of state,ll which can be derived from basic thermodynamic considerations12 (1- r/r=)+s / ~ ~ ( r / r , )(1) ~] uo and ue denote surface tension of pure water and equilibrium surface tension of surfactant solution, l’ and.’l refer to excess surface concentration at bulk concentration (co) and at surface saturation,respectively. Ha denotesthe partial molar free energy of surface mixing of surfactant and solvent that can be related to Frumkin’s attraction constant a‘ by a’ = HT.. To exploit eq 1 in terms of the experimentally accessible quantities u. and CO, another relationship has to be used connecting~0 and r given by no- ue = -RTT,[ln
eq 2 (9) Lunkenheimer, K.; Pergande, H.-J.;Kriiger, H.Reo. Sci. Instrum. 1987,58,2313. (10) Huh,C.; Mason, S . G.Colloid Polym. Sci. 1976,269,566. (11) Frumkin, A. Z . Phys. Chem. 1926, 116, 466. (12) Lucaeeen-Renders, E. H.Prog. Surf.Membr. Sci. 1976,10,253.
0743-7463/91/2407-1765~02.50/0 0 1991 American Chemical Society
1766 Langmuir, Vol. 7, No. 8, 1991
Lunkenheimer et a1. ,
Chart I
cis 2-n-Alkyl-5 -Hydroxy-lJ Dioxanes
H
H
Ib
la
H
? 1
Id
IC
- r / r p exp(-W/RT rp-1
c0 = a r p . (1
co
(2)
where a stands for the surface activity parameter of the surfactant that is related to its standard free energy of adsorption AGO by
-
Figure 1. Equilibrium surface teneion-concentration isotherms at 295 K. of cis-2-n-alkyl-5-hydroxy-l,3-dioxanes
I
trons 2-n-Alkyl-5-Hydroxy-33 Dioxones
a = exp(ACo/RT) (3) For the regression analysis of experimental isotherms the Marquardt procedure was used.13 To minimize computing time, all function derivations were given in analytical form. For the solution of eqs 1and 2 the explicit relationship between coj and u.j has to be found by numerical means. Accomplishingthe Marquardt iteration procedure, we determined for each function call the solution for I'/I'. from eq 2 by means of a separate search for the zero point using the Newton iteration. By checking the adaption quality, we calculated the standard deviation
(4) in the given case of deterministic concentration values, with Aumj and AU,J being measured and evaluated surface tensions (Au = 00 - u8),m the number of measuring points, and n the number of regression coefficients. The mean errors of regression coefficients i.e. those of the evaluated values of r., a, and He,were determined by using the inverse of the normal matrix." Materials. The investigated derivatives of l,3-dioxane (Ia, Ib) were isolated from a four-component mixture of glycerol ac(Ia), trans-2-alkyl-5-hyetals: cis-2-alkyl-5-hydroxy-l,3-dioxane ,3-dioxodroxyl-l,&dioxane (Ib), cis-2-alkyl-4-hydroxymethyl-1 (Id) lane (IC),and trans-2-alkyl-4-hydroxymethyl-1,3-dioxolane (R= C Z H ~ - ~ - C ~ Hwhich I ~ ) ,were obtained by the direct reaction of an appropriate aliphatic aldehyde (from 1-propanal to l-octanal) with glycerol in the presence of p-toluenesulfonic acid @-TsA) aa catalyst according to the method presented in refs 5 H ~obtained ) and 15. Pure Ia compounds (R= C Z H ~ - ~ - C ~were by slow distillation of the mixtures Ia-Id containing a catalytic amount of p-sTsA. In other cases the four-component mixtures of Ia-Id have been maintained for 2 days at a temperature of about 0 "C in the presence of p-TsA to enrich cis-la and the trans derivative of l,&dioxane (Ib), which, after neutralization, were distilled off aa low- and high-boiling fractions, respectively. In this way pure Ia (R= n-C6Hl1-n-C7Hl6) and all Ib were obtained. Prior to use all compounds were distilled several times and checked for purity by GLC analyses (Giede 18.3 chromatograph with flame ionization detector, metallic column 2 X 0.004 m packed with 10% Carbowax 20 M on Chromosorb W/AW DMCS 80/100meshmadealkali withO.5% KOH,andnitrogenascarrier gas). Physical constants of the substances under study are reported in the literature.
Figure 2. Equilibrium surface tension-concentration isotherms at 295 K. of trans-2-n-alkyl-5-hydroxy-l,3-dioxanes
Deutscher V e r b der Wbnechafbn: Berlin, 1971. (15)Pieeecki, A,; Burczyk, B.Pol. J. Chem. 1980,54,367.
(16) Adameon, A. W. Physical Chemistry of Surfaces; inbmience Publishers: New York, London, Sydney, 1967; p 101.
Table I. Surface Equation of State Parameters compound -ethyl-propyl-butyl-pentyl-hexyl-heptyl-propyl-pentyl-heptyl-
cis-2-n-Alkyl-5-hydroxy-1,3-dioxanes -6.78 f 0.02 1.40 f 0.26 2.70 f 0.08 2.99f 0.03 -6.20i 0.13 4.01 0.15 3.88 f 0.07 -10.20 f 0.05 1.88f 0.12 3.96 0.05 -11.86 f 0.14 3.62 f 0.17 4.11 f 0.02 -16.73 f 0.10 2.25 f 0.10 4.27 f 0.12 -18.61 f 0.16 3.42 f 0.24 trans-2-n-Alkyl-5-hydroxy-l,3-dioxanes 3.21 f 0.04 -11.79 f 0.17 2.67 f 0.25 4.73 f 0.03 -14.27 0.05 2.59 f 0.08 4.83 f 0.04 -21.11 f 0.10 2.93 f 0.12
*
f0.17 AO.09
fO.10 f0.23 fO.10 a0.25 fO.18 f0.16 f0.16
surface equation of statehf these compoundsare compiled in Table I together with the standard deviation of the best fit procedure and the mean errors of the surface equation
of state parameters. The standard free energy of adsorption AGO is plotted against t h e carbon number nc of t h e n-alkyl chain of the dioxane molecules in Figure 3. Evidence has been provided for distinct differences in AGO not only between trans and cis diastereoisomers but also between even- and oddnumbered members of the cis homologues of these series. I n addition, there is a linear relationship between the standard free energy of adsorption and the n-alkyl chain Results length for the longer chain compounds (nc 1 5) only. The equilibrium surface tension-concentration isoFor t h e shorter ones a clear deviation from t h e extraptherms of cis-and trans-2-n-alkyl-5-hydroxy-1,3-dioxanes olated linear relationship has been observed, indicating investigated are given in Figures 1and 2. The data of t h e deviations from Traube's rule.16 The amounta of AGO are more negative for the trans compounds, whereas AGO is (13) Marquardt, D. W. J. SOC.Ind. Appl. Math. l W , l l ,431. (14) Ludwig, R.Methoden der Fehler- und Auagleichsrechnung;VEB
Langmuir, Vol. 7, No.8, 1991 1767
Adsorption Properties of Dioxanes 2-17-Alkyl-5 - Hydroxy23 Dioxanes
2-n - A / k y / - 5 + y d r o x y -
0.60
20
I
m
1
1,3 Dioxanes
n ma K
0.50
I A mrn
l5
- A G O
0.L0
10
030
5
3
5
4
4
2
3
standard 6
5
6
7
nc
1 \
7
Figure 5. Minimum surfacearea demand per molecule adsorbed, Amh, of cis- and trans-2-n-alkyl-5-hydroxy-l,3-diouures aa a function of the number of carbon atoms, nc, of the n-alkyl chain (odd-numbered homologues).
Figure 3. Dependence of free energy of adsorption, A G O , of cis- and trans-2-n-alkyl-5-hydroxy-1,3-dioxanes on the number of carbon atoms, nc, of the alkyl chain. C I S -2-n-Alkyl-5-Hydroxy
6
"c
2-n- Alkyl -5-Hydroxy-13 Dioxanes
- 1,3 Dioxanes
0.601
,
nm' mTc
H* 31
\ k
trons
I
2 2
3
4
5 "c
6
-
7
Figure 4. Dependence of minimum surface area demand per molecule adsorbed, A h , of cis-2-n-alkyl-5-hydroxy1,3-dioxanes on the number of carbon atoms, nc, of the n-alkyl chain (0.10 nm2 = 10 AS).
smaller (more negative) for the even-numbered members than for the odd-numbered ones within the series of the cis isomers. More negative AGO means stronger surface activity. It is interesting to note that for the longer carbon chain compounds (nc 1 5) about 60% of the free energy difference referred to the configurational changes between cis and trans diastereoisomers is brought about by some structural differences existing between even- and oddnumbered members of the cis homologous series. The strongersurface activity of the trans isomers in comparison with the analogous cis isomers was already observed earlier.sJ7 The minimum surface area demand per molecule adsorbed, Amin, resulting from Adn = where NL denotes Lochschmidt's number, is illustrated in dependence on nc in Figure 4 and 5. The said values distinctly differ from each other, again reflecting structural peculiarities characteristic of cis and trans isomers as well aa (17) The numerical differenced between the AGO values given in ref 6 and thmobtainedin this inventtigation are mainlydue tothe application
of different units for the standard state of the chemical potential. In addition, the problem of surface chemical purity was dbregarded and another adrorption isotherm was uaed in ref 6.
3
6
-
5 nc
6
7
Figure 6. Dependence of surface interaction parameter HI of cis and trans 2-n-alkyl-bhydroxy-1,3-dioxanee on the number of carbon atoms n, of the n-alkyl chain.
of even- and odd-numbered alkyl chain members. It is obvious that comparable cis dioxane molecules occupy a considerably larger area in the adsorption layer than the trans molecules (Figure 5). The two curves seem to approach limiting A- values at longer carbon chains (nc 1 7) characteristic of the odd-numbered cis and oddnumbered trans coppounds. The former is about 4 A2 larger than the latter. Within the homologous series of the cis compounds the odd-numbered representatives occupy a comparativelylarger surface area than the evennumbered ones, as long as the n-alkyl chain has lese than five carbon atoms. However, the A- values obtained for longer carbon chain lengths (nc 1 5) are slightly smaller for the odd-numbered than for the even-numbered homologues. The most striking distinction between even- and oddnumbered chain homologues is obtained for the surface interaction parameter I+ (Figure 6). Similar to the course of the parameters AGO and A h , the differences in W between the even- and odd-numbered cis members are greater as the chain length becomes shorter. It is worthwhile drawing attention to the fact that the slopes of the two curves H ' (nc) have opposite signs: a positive one for the even-numbered members with lower values and a negative one for the odd-numbered members with higher values. A negative slope means that the interaction
1768 Langmuir, Vol. 7, No. 8, 1991 between the dioxane molecules in the surface layer will decrease with increasing chain length-a result that seems to conflict with common trends in surfactant adsorption properties. The said differences in the amount of surface interaction parameters seem to approach a constant but small value with increasing chain length. For the trans odd-numbered compounds,there is almost no dependence of H’on the carbon chain length.
Discussion The 2-n-alkyl-5-hydroxy-l,&dioxanesare surface active substances, the equilibrium surface tension-concentration isotherms of which can be described definitely by Frumkin’s surface equation of state of regular surface behavior.11 The adsorption properties following from it exhibit pronounced peculiarities, part of which has never been observed before with ordinary surfactants. In general, adsorption properties of a homologous series of surfactants can be described as the sum of nearly independent contributions from the polar “head group” and from the nonpolar “tail”. The contribution of the latter can then be described in terms of equal increments per carbon atom of a straight chain.18 The compounds under discussion having chain lengths shorter than or equal to n-pentyl basically deviate from this idea, indicating that the mechanism of revealing surface activity must be more complex than usually met with simple surfactants owing to certain peculiarities in the structure of the dioxane head group. Let us consider the course of the standard free energy of adsorption within the homologous series together with the correspondinglimiting surface areas in relation to the configuration of the molecules concerned. And as there are clear effects exclusively due to differences in even- or odd-numbered chain characteristics, we shall confine discussion to one of the characteristics, i.e. to the odd numbered, when discussing the influence of configuration. The standard free energy of adsorption, AGO, is a measure of the solute’s surface activity in terms of the free energy of transfer of solute molecules from water to the surface. It can at least approximately be attributed to the number of water molecules in contact with the solvent or to the correspondingsurface area of the cavity created by the solute in the bulk phase of water.18J9 For amphiphiles the main contributions to AGO arises from the gain in free energy of interactions between hydrocarbon tail and water associated with the “stripping off“ of the water molecules in the adsorption process.lg Hence, for straight-chain amphiphiles, AGO is usually a linear function of the number of CH2 groups in the chain.ID This fact has been observed with the 2-n-alkyl-bhydroxy-l,&dioxanes for chain lengths nc 1 4 (cis, evennumbered) and nc 16 (cis and trans, odd-numbered)only. For shorter chain lengths the increment of AGO per CH2 group is smaller than that characteristic of ordinary straight chain surfactants. This means that the shorter hydrophobic chain will not be completely extruded from the bulk water phase. Part of it will still interact with water molecules in the bulk. According to the findings in Figure 3 this interaction is more favorable for the cis isomers compared to the correspondingtrans isomer. This behavior should also be reflected in the limiting surface areas per molecule adsorbed of the two isomers. (18) Stauff, J. 2. Electrochem. 1966,59, 246. (19) Tanford, Ch. The Hydrophobic Effect;Wiley: New York, Chichester, Brisbane, Toronto, 1980, p 5.
Lunkenheimer et al. Let us now consider, how the course of the limiting surface area per molecule adsorbed may reasonably be explained in dependence on the n-alkyl chain length. According to stereochemistry, the six-membered 1,3-dioxane ring has a well-defined chair conformationwith the substituents assuming either equatorial (e) or axial (a) positions being energetically unequal.20 As the substituent at the C2 atom occupies the energetically more favorable equatorial position, the hydroxyl group at position 5 is in axial position in the disubstituted cis-2n-alkyl-5-hydroxy-l,3-dioxanes, whereas both substituents occupy equatorial positions in the trans compound^.^ Recently it has been shown that the hydrophobic character of the l,&dioxane ring is much weaker than that of the hydroxy group.21 Thus, in the adsorption layer the 5-hydroxy group of the said dioxane molecules will extend toward the bulk water phase to render its maximum interaction with surrounding water molecules. This can easily be realized for the cis configuration with ita axial 5-hydroxy group which is directed into the adjacent aqueous bulk phase and the 1,3-dioxane ring remaining flat in the surface. The two oxygen atoms of the dioxane ring may then also interact with the aqueous phase, however, at a lower degree. When we come to compare this supposed arrangement of the (short chain) cis stereo isomers with that of the trans isomers, we can see, however, that the positions of the trans molecules must necessarily settle in a somewhat different state. From the stereochemical restrictions of the trans configuration, i.e. from the equatorial position of the 5-hydroxy group, one can expect that the strong hydrophilicity of the hydroxy group aiming at its “deeper immersing”into the aqueous bulk phase will result in tilting the adsorbed molecules from a flat position to one somewhat sloping toward the gaseous phase. From this consideration it necessarily follows that the cross-sectional areas of comparable cis compounds are larger than those of the trans ones. This conclusion is supported by the results of the free energies of adsorption of these molecules as the trans isomers possess stronger surface activity. On the other hand, stronger surface activity means less hydrophobic contacts with water.lg According to the findings of A- and AGO, it is obvious that the position of dioxane molecules in the adsorption layer will change with increasing alkyl chain length resulting in some bending of the end of the chain out of the solution as soon as the homologues following Traube’s rule are concerned. This bending obviously starts with the homologues of carbon number 4 (even-numbered members) and/or 5 (odd-numbered members). It is at maximum for the trans isomers. Thus, for example, the difference in the A- values of the trans-n-propyl and the trans-n-pentyl compound amounts to = 0.17 nm2/molecule compared with (AAA-)ch = 0.14 nm2/ molecule for the corresponding cis compounds. In this context, it is interesting to compare the corresponding differences of the free energy of adsorption. For the trans isomers (AAGo)t- = -2.48 kJ/mol only, whereas (AAG0)& amounts already to-5.66 kJ/mol for the cis isomer. These results suggest that the dioxane group of the trans-2-npentyl homologue is assumed to be deeper immersed into the bulk water phase than the dioxane group of the 2-npropyl homologue. Otherwise the difference (AAGo)*w should not deviate so distinctly from the value AAGO = (20)Gittine,V.M.;WynJonea,E.;Whita,R.F.M.InlnternalRotation in Molecules; Orville-Thomas, W. J., Ed.;Wiley: London, New York, Sydney, Toronto, 1974; p 425. (21) Sokolowski,A,;Burczyk, B.; Olea, J. J . Phys. Chem. 1984,88,807.
Adsorption Properties of Dioxanes
-6.80 kJ/mol characteristic of the gain in AGO per two CH2 groups in the Traube region of the said homologues. Thus a maximum interaction of the equatorial OH group with bulk water molecules can be realized also for the longer chain trans isomers. The most striking feature of this investigation is the phenomenon of alternation. Such alternation of physical properties is of common occurrence in homologous series containing long carbon chains.22 Oscillation in melting points and solubilities of mono- and dicarboxylic acids is a well-known fact. However, there are many other physical properties observed in liquids or solutions of organic substances which also reveal to be phenomena of alternation, such as dipole and magnetic moments, complex formation and association constants as well as viscosities. These facts furnish evidence that phenomena of alternation occurring in solvents and solutions cannot be explained on the basis of the theory of packing by, which this phenomenon is generally explained.*% Although so far there has been no satisfying theory that could explain phenomenon of alternation quantitatively, a more promising concept is that of "alternating polarity", going back to Cuy.28 According to this theory the carbon atoms along the n-alkyl chain are charged positively and negatively in alternation, dependingon the (partial)charge of the end group. Salukaev modified this hypothesis postulating an induced sort of double bonding along the n-alkyl chain at the expense of an alternatingly occurring weakening and strengthening of the UC-C bonds. Following this hypothesis, the manifold experimental phenomena can be explained.14*= A more substantiated hypothesis stems from Gutmarnn According to his first rule of donor-acceptor interaction, there will always be a lengthening of the bond between the donor (acceptor) atom and the neighboring atom provided that a donor-acceptor bond is formed. Alternatingly lengthening and shortening of bonds are induced by it spreading across the entire molecule and resulting in different net charges at the terminal atoms for the evenand odd-membered chain. Introducing this concept to the surface phenomena of this investigation means that donor-acceptor bonding must occur at the functional end group. As we have already discussed above, hydrogen bonding between the 5-hydroxy group and water is very plausible. There is, however, good reason to believe that also the l,&oxygen atoms are involved in donor-acceptor interaction with an additional number of water molecules. This, in turn, should reinforce the effect of "homologiz a t i o r ~ "along ~ ~ the adjacent n-alkyl chain. Thus, differences between the inherent properties of the even and the odd members of the homologous series can be established, which are to be reflected in the adsorption properties. (22) Aneell, M. F.; Gigg, R. H.In Rodd's Chemistry of Carbon Compounds;Goffey, S., Ed.;Ebevier Publiehig Company: Amsterdam, London, New York, 196s; Vol. I Part C, p 124. (23) Reid, K. F. Properties and Reactions of Bonds in Organic Molecules; Longmane: London, 1968; p 109. (24) sdukaev, L.P.Homologkacija organitscheskich molekul; Izdatelstvo Woroneskogo Univereiteta: Woronesh, 1968. (25) Salukaev, L. P. Z. Org. Chim. 1976, 12, 1600. (26) Cuy, E.J. J. Am. Chem. SOC.1920,42,W3. (27)Gutmann,V.Private communication. cf. Gutmann,V. The DonorAcceptor Approach to Molecular Interactione; Plenum Prese: New York and London, 1978.
Langmuir, Vol. 7, No.8, 1991 1769 Consequently, it is the peculiar chemical structure of the 5-hydroxy-1,3-dioxane head group that gives rise to the marked effects of alternation within the 2-n-alkyl substitutes. The cooperative donor-acceptor interaction is probably responsible for the remarkable differences in the surface interaction parameter Ha. As a whole, the interactions between the adsorbed molecules are only moderate. Within the cis series the resulting Havalues are even maximal for the shortest odd-membered homologue, where the concerning cross-sectional areas are the largest. In contrast to that, the Ha values are minimal for the corresponding even-membered homologue. This quite significant discrepancy cannot be explained in terms of interactions between hydrocarbon chains, which usually increase with increasing n-alkyl chain length. This appears to be a sure indication that the effects of alternation may be as strong as the effects due to hydrocarbon interaction and that they may obviously have an opposite sign. Although for the first time Ueno et al. reported on alternatingly occurring differences in the saturation adsorption values of octaethylene glycol n-alkyl ethers,= such a complex set of findings has never been observed so far. Summarizing, the adsorption properties of the compounds under discussion are mainly determined by two structural effects. The first is based on the geometry of the molecules resulting from stereochemical requirements. The second effect is connected with the phenomenon of alternation, which is still lacking a satisfactory theoretical explanation in terms of appropriate molecular models.
Conclusions According to the findings of the adsorption properties of the homologous series of some cis- and trans-2-n-alkyl5-hydroxy-1,3-dioxanes,the measurement of surface tension represents a convenient method for scrutinizing structural effects on the adsorption behavior of the molecules adsorbed. The suitability of surface tension as the " m a t exact and most sensitive physical characteristic" was already mentioned by one of the pioneers of surface chemistry investigating fatty acids 80 years ago.B The fact that the pronounced effects of alternation found in the surface chemical properties of surfactants have almost escaped observation is assumed to be mainly due to an insufficient grade of purity. There is no doubt that the even-odd characteristics detected reflect a real property of the molecules investigated. Attention should be drawn to the aspect of interactions between the "localized" u bonds in saturated molecules, which has been known in principle for many years but overlooked for lack of explanation. The important role of "a-conjugative interactions" in explaining apparent anomalies of some properties of molecules has convincingly been demonstrated by Dewar.30 We think it worthwhile to refer also to the neglected phenomenon of alternation in terms of current chemical theories. The results of the 2-n-alkyl-5-hydroxy-1,3-dioxanes presented here might serve as a stimulating argument. (28) Ueno, M.; Takasawa, Y.; Miyaehige, H.; Tabata, Y.; Meguro, K, Colloid Polym. Sci. 1981,259, 761. (29) Szyszkoweki, B. von 2.Phys. Chem. 1908,64,386. (30) Dewar, M. J. S. J. Am. Chem. SOC. 1984,106,889.
