J. Phys. Chem. 1980, 8 4 , 2297-2300
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Solid Monolayers at the Water/Gas Interface. The Polyshell Network F. J. Garfias Faculty of Chemistty, Universidad Nacional Aut6noma de Mhxico, Mexico (Received: October 1, 1979)
The polyshell network is postulated to describe some surface properties of pure water with a 118.8 erg/cm2 surface energy. The following temperature dependence for the surface area per first-plane oxygen is found: Awaterpolyshell = 20.79 + 0.0651t where t is given in "C. To form a new surface, two hydrogen bonds have to be broken per outermost oxygen in the polyshell network, whereas in the previously described polyboat and polychair networks only one has to be. At 25 "C, the surface tension values calculated are 21.8 dyn/cm for the water polychair network and 25.9 dyn/cm for the water polyboat network. When the outermost layer of water molecules is removed to be replaced by fatty alcohol molecules, a saturated polyshell network is obtained. In a T-A diagram, the phase transition at the lower discontinuity can be represented by the saturated polyshell network. The virial theorem is applied to the saturated polyshell network, and an excellent surface-pressure prediction is made when a 9-6 Lennard-Jones potential is used and when the midpoint of the C-C bonds of the hydrocarbon chain is considered as the center of interaction.
Introduction In the determination of the T-A diagram of monolayers of fatty alcohols in water, Adam1 observed that the film behavior was mostly described in terms of three linear segments with two fairly sharp discontinuities. For octadecanol films a t 24.90 "C, the upper discontinuity appeared at ca. 13 dyn/cm, and the lower one at ca. 1dyn/cm. The upper line was steeper than the intermediate line, with the bottom line having a very small slope (see Figure 1). The upper limit of the upper line was the collapse pressure to which a monolayer was compressed without detectable expulsion of molecules to form a new phase. It has been shown in a previous paper2 that the a-A behavior of fatty alcohol monolayers represented by the upper line, from collapse to the upper discontinuity, could be explained in terms of two proposed structures: the saturated polyboat and polychair networks. The aim of the present paper is to describe an additional model of structure, t h e polysh(el1 network, that is essential to understand the PA behavior represented by the intermediate and lower line. The model is presented in two stages. First, a model for the pure water/gas interface is proposed, and then the outermost water molecules are removed to be replaced by fatty alcohol molecules to produce a condensed monolayer.
with respect to the interface. As the back and legs of a chair are in a perpendicular plane to the surface, the projected shape of a hexamer ring into the surface plane corresponds to a rectangle, as can be seen in Figure 2. Two vicinal chairs can have in common two contiguous 0-0 hydrogen bonds when they are joined through the backs or legs or can have in common only one 0-0 bond when they are joined through the seats. Consequentlyeach chair in the polyshell network shares its six 0-0 hydrogen bonds with only four vicinal chairs (see Figure 2). The oxygen atoms in the polyshell surface model are located in distinct parallel planes to the surface. The outermost oxygen plane hereafter will be referred to as the first oxygen plane 01, the one next to the outermost as the second oxygen plane 02,and so forth. Every oxygen plane contains the same number of oxygen atoms. In the polyshell surface network, each first-plane oxygen is bonded to two hydrogens that protude into the gas phase and hydrogen bonded to two second-plane oxygens. The O-H bond is tilted, making an angle of 35.265O with respect to the surface plane. The first-plane oxygen atoms have a lattice with rectangular symmetry. An isometric drawing of the polyshell surface network is illustrated in Figure 3, and the top and plant views are shown in Figure 4.
Polyshell Network for the Water/Gas Interface A continuum model for liquid water has been adopted in the present work to describe the interface. Water molecules can form two different types of hexamer rings, the boat and the chair conformations. Because of the assymetry of the surface fields, it is here proposed that the outermost layer of the water molecules becomes highly structured, to the extent that it can consist solely of chairs or boats. Two structures have been previously described,2 the polychair and polyhoat surface networks, which have the property that any 0-0 hydrogen bond is common to two different rings, and consequently each hexamer ring is surrounded by six vicinal rings (see Figure 2). TOexplain adequately the polyshell structure, I will refer to three parts of a chair: back, seat, and legs. In the polyshell network, the back and legs of each superficial chair are contained in parallel planes that are perpendicular to the water/air interface. A plane containing the back of one chair is at 0.81651, 8, apart from the plane containing its legs, where L is the length of the 0-0 hydrogen bond. 'The plane containing the 0-0 hydrogen bonds corresponding to the seats is at an angle of 35.26O
Saturated Polyshell Model The maximum capacity of a polyshell network for saturation, with a normal fatty alcohol, occurs when the outermost layer of water molecules corresponding to the first plane of oxygen atoms is replaced by ROH molecules to give a particular state of solid films that will hereafter be described as the "saturated polyshell network". The oxygen atoms of the first plane are fatty alcohol oxygens; the 0-C bond takes the same orientation as one of the two O-H surface bonds, and thus the hydrocarbon chain protudes into the gas phase. The oxygen atom of the ROH molecules forms two hydrogen bonds with vicinal atoms through two long-pair electrons. It is here proposed that at the lower discontinuity which occurs at about 1dyn/cm, the structure of the monolayer corresponds to a saturated polyshell network with fatty alcohol molecules; on compression above 1 dyn/cm, a phase transformation takes place and the saturated polyshell network is gradually transformed into a saturated polychair network; at the upper discontinuity present at around 13 dyn/cm, for octadecanol at 25 OC,the monolayer consists solely of a saturated polychair network. When the
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The Journal of Physical Chemistry, Vol. 84, No. 18, 1980
E 40
Y 5 30 n
Q
K
POLYBOAT
1
w W
Garfias
POLYCHAIR
10 SAT, POLYSHELL
f9 20 21 22 23 AREA PER MOLECULE, s q . 8
Figure 1. n-A behavior of fatty alcohol films.
TOP VIEW gas phase
.
.
WCYBOAT or POLYCHAIR
-
SUPERFICIAL H c. 1st. 0 PLANE
u POLYSHELL
Figure 2. Vicinal rings in polyboat, polychair, and polyshell networks.
water phase SIDE VIEW Figure 4. Top and side views for the polyshell network at the water/gas interface. I-
Flgure 3. Isometric for the polyshell network at the water/gas surface.
saturated polyshell network is expanded to lower pressures than 1 dyn/cm, the polyshell network becomes unsatu-
rated. As there are two 0-H surface bonds in the polyshell network, the hydrocarbon chains could in principle assume either orientation. When the polyshell network is saturated with fatty alcohol molecules, all of the hydrocarbon chains take the same orientation to minimize repulsion. The hydrocarbon chains are contained in parallel planes that are perpendicular to the surface, as illustrated in Figure 5. The position of the carbon atoms of the hydrocarbon chains was calculated relative to the first oxygen plane. The bond lengths and bond angles here considered can be found elsewhere.2 If in the saturated polyshell network the 0-C bond is assumed to be at an angle of 35.265' to the surface, then the hydrocarbon chain has a tilt of 70.53'. The saturated polyshell and the saturated polychair networks are assumed to mix ideally. It then follows that for each point in an isotherm, between the two discontinuities, the surface pressure is given by (1) 7 = XTsat. polyshell + (1 - x ) n s a t . polychair where x is the fraction of the surface covered by a saturated polyshell network, rsat. poly&.u is the surface pressure of a saturated polyshell network, and TSat. polychdr is the surface pressure of a saturated polychair network. The T-A behavior of fatty alcohol films represented by the lower line in Figure 1 corresponds to an unsaturated polyshell network. Discussion The surface area per fatty alcohol molecule for saturated polychair networks is a function of chain length and temperature.2 It was also found that below 30 "C the 02-01-C angle in the film molecule is 102.68', and consequentlythe 0-0 hydrogen bond length L can be related to surface
- -- - - --
7-------
I I f
i
- -r
I d
-
I
I
I
d
4
!
I
I
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-
TOP VIEW HYDROCARBON CHAIN
OXYGEN
Flgure 5. Orientation of hydrocarbon chains in a polyshell network.
area. In the transformation from saturated polychair to saturated polyboat network by film compression, it was also shown that the length of the 0-0 superficial bonds remains constant. The surface area reported by Deo et al.4 for ClaOH, C,OH, C,,OH, and C2,0H, at the lower discontinuity, has the following linear dependence on temperature: Asa~,polyshell = 19.788 -k 0.0651t (2) where t is the temperature in OC. Although for higher fatty alcohols the surface area of saturated polyshell networks seems to be independent of chain length, the experimental evidence4shows slightly larger areas for C140Hthan those predicted by eq 2. For octadecanol an 02-01-02 bond angle of 109.47' is considered, giving the following relationship between surface area and bond length: A s a t . polyshell = 2*666L2 (3)
Solid Monolayers at the Water/Gas Interface
L can be estimated by use of eq 2 and 4 from ref 2. Agreement between predicted areas by use of eq 3 and experimental surface areas for octadecano14is extremely ood. For example, at 24.90 "C the estimated L is 2.8285 %,which gives a predicted surface area of 21.33 A2, very close to the experimental value of 21.35 A'. For higher alcohols than octadecanol, a phase transformation from saturated polychair to saturated polyshell at constant 0-0 bond length demands certain flattening of the polyshell network. For CzoOHthe 02-01-02bond angles becomes 110.67", and for Cz28Hit is 111,87". Surface pressure at, the lower discontinuity is calculated by applying the virial theorem to the saturated polyshell network,2 Interaction forces are projected on the surface plane (x-y) to apply the virial theorem in two dimensions. Thus, if fi is the ratio of the distance r projected on the surface plane to the actual distance r, then the virial equation taker3 the form5 nA := kT t- 4.5ACf:rc9 - 3BCf:ry6 (4) With the A and B values determined in the previous work? the polyshell model is tested by predicting the surface pressure at the lower discontinuity. If one applies eq 4 to octadecanol films at 24.90 "C, with an 0-0 bond length of 2.8286 A, a surface pressure of 0.63 dyn/cm is obtained, which is in excellent agreement with the experimental value of ca. 0.6 dyn/cm. One of the most relevant differences among polyboat, polychair, and polyshell structures is that in the saturated polyshell each O1 oxygen atom of the fatty alcohol molecules is hydrogen bonded to two vicinal water molecules, whereas in the saturated polyboat or polychair networks the O1 oxygen atoms are bonded to three vicinal water molecules. Consequently, it is to be expected that the polyshell structures are not as strong as either the polyboat or the polychair ones. Surface viscosity for a saturated polyboat network should be similar in magnitude to the surface viscosity of a saturated polychair network and should fall off very steeply in the transformation from polychair to polyshell. At 25 OC, the surface viscosity for an octadecanol film at a surface pressure of 25 dyn/cm is 0.206 surface poise; at 18 dyn/cm it is 0.249 surface poise; and it falls off to 0.0192 surface poise for a film showing a surface pressure of 1 dyn/ cma6 The specific resistance to evaporation is enhanced by the presence of a monolayer. Here again the polyboat and the polychair networks should provide a considerably higher resistance to evaporation than the polyshell one. A CzzOHfilm at 40 and 20 dyn/cm gives specific resistances of 18 and 10 seg/cm, respectively, which are far greater than the specific resistance of 2 seg/cm for a film having a surface pressure of 5 d ~ n / c r n . ~ Fatty acid films shatw larger polyshell areas than fatty alcohols.* It can be shown that the greater area is due to a steric effect due to hlydrogen bonding between the carboxylic oxygen of one molecule and the hydroxilic proton of the contiguous one. A H-O-C angle of 106.3", a 0C 4 angle of 1%4.9",a 0 4 4 angle of 124.1", and bond lengths of 0.972 A for (3-H, 1.343 A for 0-C, and 1.202 A for C=O were ~onsidered.~ If it is assumed that the hydroxilic proton is sticking out at an angle of 53.15', then an area of 22 A2 per acid molecule would not keep the carboxylic oxygen and its neighboring hydroxyl proton sufficiently apart, and the polyshell surface area has to be extended to 24 Azto alllow for a 2-A separating distance. Polyshell, polychair, and polyboat structures for pure water should account for the surface properties of pure water, and in particular for surface tension. The polyshell
The Journal of Physical Chemistry, Vol. 84,
No. 78, 1980 2299
network has two hydrogen bonds broken per O1 oxygen, whereas the polyboat and polychair have only one. If a polyshell surface structure is assumed for water with a 118.8 erg/cm2 surface energy, then the surface energy per hydrogen bond broken is 59.4 erg/cm2. The polyshell, polychair, and polyboat networks at the same temperature have the same 0-0 bond length, and then the surface area ratio is 1.15 for polyshell to polychair and 1.22 for polyshell to polyboat. When the 0-0-0 bond angles are 109.47", the surface energy of a polychair network becomes 68.3 erg/cm2 and the surface energy of a polyboat network is 72.4 erg/cm2. The surface energy of 68.3 erg/cm2 for a water polychair network that has been inferred from surface tension data of water compares very well with the value of 62 erg/cm2 that was calculated in a previous paper2 for a polychair network that obeys the Stockmayer potential. To estimate the surface tension of a water polychair and polyboat network, it is necessary to know the temperature coefficient of the surface tension; Gittensgreports a value of -0.1561 dyn/(cm K). As the main contribution to the entropy of surface formation is due to the fact that a molecule may occupy a position in the immediately subjacent bulk phase or in the surface,l0 it is expected that anyone of the polyboat, polychair, or polyshell networks should have the same temperature coefficient of surface tension. Surface tensions can be calculated from surface energies by eq 5, where Uais the surface energy, y is the = + T(dr/dT),,v (5) surface tension, and T i s the absolute temperature. At 25 "C, the surface tension values calculated are 21.8 dyn/cm for the water polychair network and 25.9 dyn/cm for the water polyboat network. The evidence presented here suggests that when water has a surface tension of 72.3 dyn/cm at 25 "C, ik can be represented by a polyshell network. Sobol et al.ll demonstrated that when a small bubble of air is placed in contact with deaerated water, surface tension decreases to values even smaller than 44 dyn/cm. Unfortunately, bubble collapse at 44 dyn/cm did not allow us to follow the experiment to completion. However, the experimentdl demonstrate that in a nonequilibrium system the water surface is gradually transformed from a high-energy surface structure, the polyshell, into a much lower energy surface structure, the polyboat, when a small amount of air diffuses into deaerated water. It is possible that, if further air diffusion occurs, the polyboat may even transform into the polychair. In a previous paper,2 it was shown that there are two contributions to surface pressure: kinetic and chain interactions. An ideal film shows only the kinetic effect, as chain-to-chain attraction forces are of the same magnitude as repulsion forces. It was found2that the existence of a saturated polychair configuration at around 21 dyn/cm, with a collapse to upper discontinuity surface area ratio of 1.06, implied an undistorted surface network with no chain interaction effect and with structural dimensions similar to those of water. One experiment reported by Deo et al.4carried out with CzoOHat 33.41 "C is quite close to fulfilling conditions of ideality, and consequently it was estimated that the 0-0 bond length for water is 2.935 A at 33.41 "CS2 As surface pressures at the lower discontinuity are all of the same order of magnitude (1 dyn/cm), it is to be expected that a pure polyshell network for water has the same temperature dependence as a saturated polyshell structure. Consequently, the following temperature dependence for the surface area per first-plane oxygen in the
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J. Phys. Chem. 1980, 84, 2300-2305
water polyshell network is proposed: Awaterpolyshell =
20.79
+ 0.0651t
(6)
where t is given in "C. Acknowledgment. The present work was reglized while the author was in commission at the Secretaria Ejecutiva del Consejo de Estudios de Posgrado.
References and Notes (1) N. K. Adam, "The Physics and Chemistry of Surfaces", Dover Publications, New York, 1968, p 47.
(2) F. J. Garfias, J . Phys. Chem., 83, 3126 (1979). (3) M. Karplus and R. N. Porter, "Atoms and Molecules", W. A. Benjamin, New York, 1970, p 505. (4) A. V. Deo, S. E. Kulkarni, M. K. Ghapurey, and A. E. Biswas, Indian J . Chem., 2, 43 (1964). (5) In eq 6 of ref 2, f , should appear raised to the second power. (6) W. D.Harkins, "Physical Chemistry of Surface Films", Reinhold, New York, 1952, p 143. (7) V. K. Lamer, T. W. Healy, and L. A. G. Aylmore, J. Co//o/dSd.,19, 673 (1964). (8) H. D. Cook and H. E. Ries, Jr., J . Phys. Chem., 60, 1533 (1956). (9) G. J. Gittens, J. Colloid Interface Sei.,30, 406 (1969). (10) J. T. Davies and E. K. Rideal, "Interfacial Phenomena", Academlc Press, London, 1961, p 12. (11) H. Sobol, F. J. Garfias, and J. Keller, J. Phys. Chem., 80, 1941 (1976).
Spectroscopy of Polyenes. 5. Absorption and Emission Spectral Properties of Polyene AcCds/Esters Related to Retinoic Acid/Ester as Homologues' Paritosh K. Das" Radiation Laboratory, University of Notre Dame, Notre Dame, Indiana 46556
and Ralph S. Becker Department of Chemistry, University of Houston, Houston, Texas 77004 (Received: January 7, 1980)
A number of all-trans polyene acids and esters with chain lengths varying over a range including two longer and two shorter homologues of retinoic acid/ester have been studied for their spectral and photophysical properties. The polyene acids form intermolecularly hydrogen-bonded dimers in nonpolar solvents (3methylpentane and carbon tetrachloride). Upon increasing the polyene chain length as well as upon dimer formation (in the cases of the polyene acids), pronounced changes occur in the absorption-emission spectral maxima, infrared absorption frequencies (carbonyl and hydroxyl stretching), and fluorescence lifetimes and quantum yields. The photophysical dynamics is discussed in terms of a fluorescing state that is primarily of lA;- character (dipole forbidden) in the longer polyene systems.
Introduction Polyene systems containing the retinyl moiety have been widely ~ t u d i e d ~ for- ~their spectral and photodynamical properties in recent times. This is primarily because the retinyl systems form the chromophores in visual and photosynthetic pigments and in the intermediates that result following light absorption by them. Three low-lying singlet excited states6-12can be involved in the photodynamics of the retinyl polyenes. These are l(n,a*), lA *(dipole forbidden), and lB,* (strongly dipole allowed). Tbe relative locationg13 of these three states has been shown to be dependent on (1)structural factors such as geometric distortion of the polyene chain, (2) the nature of the heteroatom at the end of the chain, and (3) environmental factors such as the nature of the solvent (polar, nonpolar, or H bonding) as well as the presence of H-bonding agents. Another interesting feature that has been revealed through the recent spectral and photophysical investigation of retinals,14retinols,15and retinoic acids14J6is that they form aggregates (dimers) under certain conditions of solvent, temperature, and concentration. The structure of the dimers and the nature of intermolecular interaction in them appear to be different for the three classes of retinyl polyenes, and, consequently, their excited-state properties are also affected in very different fashions. Understandably, the relative order and location of the three low-lying singlet excited states, and the effect of dimer formation on spectral and photophysical behavior, are all sensitive functions of polyene chain length. Re0022-3654/80/2084-2300$0 1.OO/O
cently, we undertook a detailed study of the excited-state properties of polyenes related to the retinyl systems as homologues and analogues. In the previous p a p e r ~ , l ~we -~l have reported on the absorption-emission spectral properties as well as the triplet-state photophysics of a number of polyene aldehydes, ketones, alcohols, Schiff bases, and protonated Schiff bases. The present work is concerned with the results of similar studies on a number of all-trans polyene acids and esters related to retinoic acid and ester as homologues. We have examined how the formation of linear, tail-to-tail dimers in the cases of the polyene acids affects their absorption-emission spectral maxima and radiative and nonradiative lifetimes as the polyene chain length is varied over a range including two shorter and two longer homologues of retinoic acid. Also, we have carried out an infrared spectral study in order to obtain evidence in support of formation of intermolecularly hydrogenbonded dimers in the cases of the polyene acids. The structure of the polyene acids and esters (ethyl) under examination are shown in Figure 1. Nomenclature of the polyene systems by the numbers of carbon atoms is used and does not include the two carbon atoms of the ethyl group in the cases of the esters.
Experimental Section The polyene esters were synthesized17 from the corresponding lower polyenals or polyenones of known structure by using a Wittig reaction with triethyl phosphonoacetate (in tetrahydrofuran in the presence of sodium amide) or 0 1980 American Chemical Society
