Molecular reorientation in monolayers at the paraffin-water interface

Molecular Reorientation in Monolayers at the Paraffin-Water Interface 1 RONALD D. N E U M A N Department o f Forest Products, College o f Forestry, University o f Minnesota, St. Paul, Minnesota 55108 Received February 7, 1977; accepted May 26, 1977 Stearic acid monomolecular films on 10-4 M CaClz subsolutions were deposited on paraffin substrates by the Langmuir-Blodgett method over the pH range 2 - 9 . The withdrawal contact angle developed during the deposition of the second monolayer was examined by a cinematographic technique. The contact angle behavior appears to be associated with changes in the molecular orientation of the immersed film molecules at the paraffin-water interface. Evidence for molecular reorientation is presented and discussed. INTRODUCTION

min) through the surface films maintained at 31 mN/m. The withdrawal contact angle, 0~, developed between the film-covered subsolution surface and the emerging solid slide, was photographed with a CinrKodak special 16 mm movie camera during the deposition of the second monolayer. Representative frames were selected and enlarged onto Ortho film, and the 0~ on the compression barrier side of the solid substrate was measured with a contact angle tangentometer. The 95% confidence limits of the reported 0~ values are _+0.5°. The materials, apparatus, and procedures are described in greater detail elsewhere (5-7).

The amphipathic lipid components of biological membranes generally are accepted to be preferentially oriented with their polar groups in the outer bilayer surfaces (1, 2). There have been several suggestions, however, that under certain conditions a "reversal of orientation" may occur with the hydrocarbon tails facing the aqueous phase (3, 4). Reported herein are recent monolayer studies which support the concept of molecular reorientation. E X P E R I M E N T A L METHODS

Stearic acid was deposited from n-hexane spreading solutions onto either calcium-flee or 10-4M CaC12 subsolutions (20-21°C) prepared from triply distilled water to which HC1, KHCOa, or KHCO3 (10 .3 M) plus KOH were added to adjust the subsolution pH. The monolayers were compressed with a Langmuir-type film balance capable of constant surface pressure operation (___0.2 mN/m) and were deposited as shown in Fig. 1 on smooth paraffin-coated microscope slides lowered and raised (7.6 mm/ 1 Published as Scientific Journal Series Paper No. 9531 of the University of Minnesota Agricultural Experiment Station.

RESULTS AND DISCUSSION

Figures 2 and 3 show the 0~ developed during the bilayer deposition of stearic acid and C a - H - S t (stearic acid films on subsolutions containing calcium) monolayers, respectively, as a function of the subsolution pH. The wettability behavior, observed in Fig. 4, does not correspond to the classical view of monolayer deposition in which the polar groups remain oriented toward the aqueous phase (see Fig. 1) because the acute withdrawal contact angles 106

0021-9797/78/0631-0106502.00/0 Copyright © 1978by AcademicPress, Inc. All rightsof reproductionin any formreser'/ed.

Journalof Colloidand InterfaceScience, Vol. 63, No. 1, January 1978

MOLECULAR REORIENTATION SOLID

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indicate that the immersed monolayercovered paraffin substrates have critical surface tensions corresponding to lowenergy surfaces (6). A significant number of the transferred molecules at the paraffin-water interface apparently overturn with their hydrophobic hydrocarbon tails, instead of wettable carboxyl (carboxylate) groups (8), being oriented toward the aqueous subsolution. Any case for molecular reorientation, however, must begin with a consideration of the withdrawal contact angle. The 0o, is a good approximation of the static receding contact angle because deposition was performed in the withdrawal rate range where 0~ (and the transfer ratio, P) is essentially independent of the relative motion between the solid and the subsolution (5). I00

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Assuming Young's equation (9) to be applicable, the 0~ behavior then can be considered to be governed by either individual or collective changes in the specific surface free energies of the solid-vapor (Ysv), solid-liquid (YsL), and liquid-vapor (yLv) interfaces. The deposition process was carried out at constant surface pressure, hence yLv remains constant. In addition, Ysv is nearly constant because contact angle measurements of sessile drops of water (102 °) and glycerol (92 °) on the bilayers immediately after deposition indicate little, if any, differences due to molecular packing or occluded water molecules even though in C a - H - S t deposition the local transfer ratio of the second layer, initially constant with increasing subsolution pH, decreases slightly above pH 6.8 from unity to about 0.87 at pH 9.0. To a first approximation, therefore, the 0~ behavior appears to be determined primarily by changes in the solid-liquid interfacial free energy. Examination of the film behavior sheds further insight into possible explanations for the 0o~ changes. It is imperative that the transfer ratio behavior during monolayer deposition on the paraffin substrates be characterized and well understood. Film balance measurements, complemented with radiochemical measurements on deposited bilayers, indicate the local transfer ratio Journal of Colloid and Interface Science, Vol, 63, No. 1, January 1978

108

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FIG. 3. The O~of Ca-H-St bilayers deposited on paraffinat 31 mN/m. of the first monolayer is unity (pl = 1.00 ___ 0.02) irrespective of the subsolution pH, thereby demonstrating a close-packed surface film, indeed, is transferred onto the paraffin surface (5). The transferred monolayer, furthermore, is stable and does not dissolve in the subsolution to any measureable extent. Differences in the transfer ratio or film dissolution after deposition can thus be ruled out. On the other hand, the acute 0~ suggest that orientative changes may have occurred in the immersed monolayer as discussed previously. Molecular reorientation must happen very rapidly either during the transfer process or after deposition because systematic 0~ changes along the length of the emerging solid substrate were not observed and 0~ was independent of the time the monolayer-covered slide was held under the subsolution prior to the withdrawal operation. The assumption of molecular reorientation receives additional support from deposition experiments on Teflon (TFE) substrates. The 0~ developed during C a H - S t bilayer deposition on smooth polytetrafluoroethylene surfaces, prepared using the procedures developed by Fox and Journal of Colloid and Interface Science,

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1, J a n u a r y

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Zisman (10), would be expected to be identical to the 0,o observed with paraffin substrates because the molecular packing in the immersed monolayers on TFE appears to be the same as that on paraffin (see Table I). The angles, however, are not equal. The 0~ is at most only very slightly influenced by the intermolecular forces arising from the solid surfaces below the long-chain monolayer (11). It is difficult to account for the difference in 0~ except by inferring, analogous to the early studies of Langmuir (12), that it arises from different orientations in the immersed monomolecular films, the amount of reorientation being influenced by the molecular groups in the underlying solid surface. When a paraffin substrate having been immersed prior to film spreading is withdrawn through a stearic acid monolayer on distilled water, a 0,o of 74° develops even though no film deposits. The work of adhesion (Wa) of water to paraffin then can be calculated using the combined YoungDupr6 equation (9) to be 54 mJ/m z. Phillips and Riddiford (13) have shown that there are grounds for assigning a value of 33 mJ/m 2 to the specific surface free energy of paraffin. Combining Fowkes' expression

MOLECULAR REORIENTATION

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Journal of Colloid and Interface Science, Vol. 63, N o . 1, January 1978

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(14) for the interfacial tension with D u p r e ' s equation gives the dispersion force contribution to the work of adhesion of the liquid to a film-free solid (WaA0) as 2(yLaysa)112 where yL~ and ys ~1are the dispersion force contributions to the specific surface free energy of the liquid and solid, respectively. Assuming the film pressure of adsorbed water v a p o r on paraffin is negligible and the intermolecular forces are entirely dispersion forces, WA can be shown to be equal to W~0. It m a y be fortuitous, but the work of adhesion is 54 mJ/m 2 when ys a equals 33 mN/m. Thus, the w o r k of adhesion of w a t e r to paraffin determined by 0~ m e a s u r e m e n t s is not only in excellent agreement with early experimental determinations (15) but also with more recent theoretical considerations. The larger 0o~ of 78 ° o b s e r v e d during bilayer d e p o s i t i o n - - u n d e r those subsolution conditions where the m o n o l a y e r composition is principally un-ionized stearic acid m o l e c u l e s - - i n d i c a t e s a lower w o r k of adhesion and thus implies that the i m m e r s e d m o n o l a y e r - c o v e r e d paraffin surface m a y be s o m e w h a t more hydrophobic than the original (monolayer-free) paraffin substrate. The paraffin was purified in the molten state by passage through silica gel to rem o v e polar impurities. Paraffin surfaces, in addition to containing methyl groups, have been suggested to possess some methylene groups (13). Such a surface would be more wettable than one c o m p o s e d solely of methyl groups because methylene groups have a greater attraction for w a t e r (16). A reorientation of the un-ionized stearic acid molecules at the p a r a f f i n - w a t e r interface to the extent where the o u t e r m o s t surface primarily consists of methyl groups would be consistent with the wettability results. The 0o, b e h a v i o r o b s e r v e d in Figs. 2 and 3 suggests the following view of molecular orientation. U n d e r acidic subsolution conditions, the un-ionized stearic acid molecules apparently overturn (Fig. 5a), and the

Journal o f Colloid a n d Interface Science, Vol. 63, No. 1, January 1978

TABLE I Ca-H-St Bilayer Deposition on Teflon Withdrawal contact angle (°)

Apparent transfer ratio

pH

T FE

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p, b

Pz

2.1 5.5 8.4 6.0 a

63.2 _+ 0.7C 39.7 _+ 0.8 74.7 +_ 2.3 48.2 _+ 1.5

78.3 49.7 58.0 62.3

0.99 1.02 -1.03

0.86 0.96 ---

a Determined from 0,o curve in Fig. 3. Preliminary measurements indicated Pl was unity throughout pH range. c 95% confidence limits. a CaC1.2not added to subsolution. 0o~ remains constant with increasing subsolution p H until p H 5.5 and 4.2 for stearic acid and C a - H - S t films, respectively. At about these bulk p H values, ionization b e c o m e s significant in the two m o n o m o l e c ular films (17, 18). The ionized species p r e s u m a b l y remain oriented with their head groups toward the w a t e r (Fig. 5b), and the reduction in the interfacial free energy is reflected by a decrease in 0~. Although 0,o eventually b e c o m e s zero in the case of stearic acid monolayers, 0o~ does not continue to decrease with C a - H - S t monolayers, but begins to increase at p H 6.4. It is significant that this abrupt wettability change occurs at the subsolution p H where the ionized film species in the C a - H - S t m o n o l a y e r are believed to associate into two-dimensional or surface micelles (7). Although the calcium stearate surface micelles 2 also a p p e a r to reorient (Fig. 5c), overturning at the p a r a f f i n - w a t e r interface m a y not be complete. The decrease in 0o~ a b o v e p H 8.0 is attributed to the in2 Properly interpreted, there are striking similarities in the interfacial behavior of fatty acid monolayers on subsolutions of alkali metal and calcium salts. The sodium stearate molecules also are believed to associate into surface micelle8 on alkaline subsolutions, but at higher pH values, presumably beginning at about pH 9.0.

MOLECULAR REORIENTATION

111

SUBSOLUTION

PARAFFIN

STEARIC ACID

CALCIUM DISTEARATE

CALCIUM STEARATE SURFACE MICELLE

FIG. 5. Schematic representations of close-packed C a - H - S t monolayers at the paraffin-water interface at (a) pH 2.0-4.2, (b) pH 4.2-6.4, and (c) pH 6.4-8.0.

creasing concentration of water molecules in the immersed monolayer due to "voids" in the original C a - H - S t surface film (7). The driving force for molecular reorientation is suggested from a consideration of the overturning process. The origin of the greater enthalpy decrease or entropy increase necessary to offset the enthalpy increases on reorientation of un-ionized stearic acid molecules due to the formation of methyl-water and carboxyl-paraffin interfaces, along with the desolvation of the polar groups, must arise in the aqueous phase and is attributed to a change in the vicinal water structure. Surface films are believed to enhance the structure of the underlying water molecules (19, 20). While it is not possible at the present time to evaluate the thermodynamic contributions, overturning would appear to be most likely an entropy-driven process, the water structure adjacent to the head groups of un-ionized stearic acid monolayers (and calcium stearate surface micelles) being more highly structured than the water at a surface comprised of methyl groups. Calcium distearate molecules do not

reorient because the hydrophilic-solvent affinity of the carboxylate ion is much greater than that of the carboxyl group (21). Any favorable free energy change due to the decreased ordering in the vicinal water structure on reorientation of the distearate molecules apparently is not sufficient to overcome the large enthalpy increase, and consequently the calcium distearate molecules do not overturn, but remain with their head groups oriented toward the water. It should be emphasized that the proposed orientations may be peculiar to the closepacked film and may not necessarily occur at other states where the associated water structure may be different (22). Other interpretations of the nonzero contact angle observed during monolayer deposition have been advanced. Langmuir (3) suggested that the so-called "zipper angles" arise from strong attractive forces exerted by the head groups leading to expulsion of the water layer. In certain cases, 0~ > 90°, and explanation s for the deposition of these films (X-type) only during immersion usually involve an overturning or molecular rearrangement of mole-

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cules beneath the water surface (3, 23). Our findings, however, suggest that molecular reorientation is not restricted just to X-type deposition, but also occurs in Y-type films, and is related to the molecular species present in the monolayer. A preliminary study of the 0o, behavior of phosphatidyl serine (bovine, Applied Science) monolayers on calcium-containing subsolutions was recently undertaken to examine further the question of molecular reorientation. We observed a sudden increase in 0~ above a critical bulk pH (-8.5), a pH value which reportedly corresponds to a net charge per molecule somewhat greater than unity (24). If surface micelle formation in close-packed films is a general physicochemical phenomenon, it would not be unreasonable to expect a change in 0o, once again, when the surface charge density is about 0.62 negative charges/hydrocarbon chain (or 1.25 charges/ phospholipid molecule) (7). The formation of surface micelles of Ca2+-phosphatidyl serine complexes and their subsequent reorientation in the lipid bilayer regions of biological membranes may be closely associated with the structural and electrical changes occurring during membrane excitation.

20.

ACKNOWLEDGMENTS

21.

This work was supported in part by and performed at The Institute of Paper Chemistry and the University of Idaho Engineering Experiment Station. The constructive comments offered by Dr. G. L. Gaines during the preparation of the manuscript are gratefully appreciated.

Journal of Colloid and Interface Science, Vol.63, No. 1. January 1978

REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9.

10. 11. 12. 13. 14. 15. 16. 17. 18. 19.

22. 23. 24.

Hechter, O., Ann. N. Y. Aead. Sci. 195, 506 (1972). Bretscher, M. S., Nature 258, 43 (1975). Langmuir, I., Science 87, 493 (1938). Moos, G., Ann. N.Y. Acad. Sci. 195, 176 (1972). Neuman, R. D., Ph.D. thesis, The Institute of Paper Chemistry, Appleton, Wis., 1973. Neuman, R. D., J. Colloid Interface Sci. 50, 602 (1975). Neuman, R. D., J. Colloid Interface Sci. 53, 161 (1975). Adam, N. K., "The Physics and Chemistry of Surfaces," p. 188. Dover Publ, New York, 1968. Aveyard, R., and Haydon, D. A., "An Introduction to the Principles of Surface Chemistry," p. 195. Cambridge Univ. Press, London, 1973. Fox, H. W., and Zisman, W. A., J. Colloid Sci. 5, 514 (1950). Adam, N. K., Advan. Chem. Ser. 43, 52 (1964). Langmuir, I., Trans. Faraday Soc. 15, 62 (1920). Phillips, M. C., and Riddiford, A. C., J. Colloid Interface Sci. 22, 149 (1966). Fowkes, F. M., Advan. Chem. Ser. 43, 99 (1964). Adam, N. K., and Jessop, G., J. Chem. Soc. 1863 (1925). Adam, N. K., and Elliott, G. E. P., J. Chem. Soc. 2206 (1962). Bagg, J., Haber, M. D., and Gregor, H. P., J. Colloid Interface Sci. 22, 138 (1966). Deamer, D. W., Meek, D. W., and Cornwell, D. G, J. Lipid Res. 8, 255 (1967). Davies, J. T., and Rideal, E. K., "Interfacial Phenomena," 2nd ed., p. 369. Academic Press, New York, 1963. Dreher, K. D., and Sears, D. F., Trans. Faraday Soc. 62, 741 (1966). Kavanau, J. L., "Structure and Function in Biological Membranes," p. 92. Holden-Day, San Francisco, 1965. Gershfeld, N. L., and Pagano, R. E., J. Phys. Chem. 76, 1231 (1972). Honig, E. P. ,J. Colloid lnterface Sci. 43, 66 (1973). Papahadjopoulos, D., and Ohki, S., Science 164, 1075 (1969).