Langmuir 1987, 3, 132-133
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Acknowledgment. We gratefully acknowledge financia1 support of this work by the National Science Foundation (CHE-8309454). A.L.G. acknowledges financial support in the form of a summer graduate fellowship sponsored by the Analytical Division of the American
Chemical Society and funded by the Society of Analytical Chemists of Pittsburgh. Registry No. DHPZN,123-33-1;Ag,7440-22-4; Pb, 7439-92-1; pyridine, 110-86-1.
Letters Dipalmitoyllecithin Monolayers at the Air/Water Interface K. S. Birdi Fysisk-Kemisk Institut, Technical University, Lyngby-2800, Denmark Received June 11, 1986. I n Final Form: November 20, 1986 The surface pressure, II,vs. area, A, isotherms (obtained by using the conventional Langmuir balance) of the monolayers of dipalmitoyllecithin (DPPC) have been measured as a function of pH of the subphase. The substrate was buffered to the pH values of 4.5,6.0, and 7.5, with sodium/potassium phosphate. These II vs. A isotherms are compared with the data reported in the literature, at the same subphase pH, where a different compression procedure was used. Introduction The understanding of the physical properties (especially the surface activity) of the lecithin molecule is of much importance, due to the fact that it is the principal constituent of all cell membranes. This has given rise to much investigation of the lecithin molecule, both in membranes and in other model systems, such as monolayer^.'-^ In a recent s t ~ d ythe , ~ time-dependent response to a shearing stress of a monolayer of dipalmitoyllecithin (DPPC), which had been adsorbed on the surface of water, was measured as a function of both pH and the surface concentration. In these investigations a new method was used for obtaining surface pressure (II) vs. area (per molecule) isotherms. The procedure used by Abraham and Ketterson3 was different than as described by the conventional Langmuir trough m e t h ~ d . ~ ~ ~ ~ ~ In order to distinguish these two methods we will designate the procedure used by A-K3 as II vs. A d , while the Langmuir isotherms we shall designate as II vs. A. This is essential, since we will show that the two methods do not provide equivalent results. The II vs. Aak data reported by A-K were not easily explained, and therefore they mentioned, "it is important for this reason that II vs. A diagrams from conventional Langmuir troughs be available for comparison", to their method. This remark prompted us to undertake the studies of II vs. A measurements, as reported herein. The data obtained by the II vs. Aakprocedure showed no feature that could be attributed to a phase transition, as compared to the II vs. A isotherms from the Langmuir balance m e t h ~ d . ~The ? ~conventional ~ Langmuir ll vs. (1)Israelachivili, J. N.; Marcelja, S.; Hom, R. G. Q.Rev. Biophys. 1980, 13, 121. (2) Chattoraj, D. K.; Birdi, K. S. Adsorption and the Gibbs Surface Excess; Plenium: New York, 1984. (3) Abraham, B. M.; Ketterson, J. B. Langmuir 1985, I, 708. (4) Gaines, G.L. Insoluble Monolayers a t the Liquid-Gas Interfaces; Wiley-Interscience: New York, 1966. (5) Adamson, A. W. Physics and Chemistry of Surfaces, 4th ed. Wiley: New York, 1982.
0743-7463/87/2403-0l32$01.50/0
A isotherms have been found to exhibit phase transition as a function of temperature, T, which are reported to be in agreement with the data as reported from lipid-bilayer membrane studiesa2 Materials and Methods The lecithin, DPPC, was used as supplied by Sigma (USA) (highest purity synthesized product) (same quality and supplier as used by A-K). DPPC solutions in chloroform used were of concentrations 0.5 mg/mL for spreading the monolayers on water (as was also used by A-K) . The buffer was prepared from sodium/potassium phosphate (Sorensen), and pH was adjusted to the desired value. The spread DPPC monolayer was compressed from 743 to 118 cm2 over a period of 4 or 40 min, as described by us elsewhere.2 Since no difference was found from these compression rates, all the data reported here are based on the 4-min compression rates. Results and Discussion Even though extensive data have been reported on the monolayers of DPPC, there still remains much to be investigated, especially as regards the effect of subphase composition and t e m p e r a t ~ r e ~ on? the ~ . ~II vs. A isotherms. The II vs. A isotherms of different subphase pH (4.6, 6.0, and 7.5) values are given in Figure 1. The isotherms are in agreement with the data reported in literature, where the subphase was water.6 These II vs. A data on buffer solutions of varying pH clearly show that the monolayers are independent of pH. The inflection point at ca. 45 A2/molecule is found to be in agreement with the literature data.2*3-6-8 (6) Villalonga, E. Biochim. Biophys. Acta 1968, 163, 290. (7) Munden, J. W.; Swarbrick, J. J. Colloid Interface Sci. 1973, 42, 657. ..
(8) Albrecht, 0.; Gruler, H.; Sackmann, E. J. Colloid Interface Sci. 1981, 79, 319.
0 1987 American Chemical Society
Langmuir, Vol. 3, No. 1, 1987 133
Book Reviews 'n
II vs. Aakisotherms were obtained by the movement of the 1 subphase liquid level, up or down, in a conical container.
40
80
120
*
Figure 1. Surface pressure (II,mN/m, dyn/cm) vs. area/molecule (A = m2 = A9 for DPPC monolayers spread on aqueous subphase with various pHs (at 25 "C): (- - -) 4.5; (--) 6.0; (-) 7.5.
It is not easy to explain why the surface pressure (II) as measured by the two different methods (e.g., II vs. Aak and II vs. A ) gives different values for the same surface concentration. Furthermore, since DPPC chloroform solutions were used in the both methods, this rules out any solvent effeds. In the conventional Langmuir method, the monomolecular film is compressed by the barrier.2*4*6 On the other hand, in the method used by A-K, the liquid in the conical container was used to change the surface area. In other words, while the film-forming molecules are compressed by the barriers'in the II vs. A isotherms, the
The latter movement of the subphase liquid level could give rise to the possibility of lecithin, DPPC, adsorption on the surface of the container, and thereby the value of II measured would be different (lower at pH 7.5) than those measured by the Langmuir method. If the II vs. A isotherms are independent of pH, then one should expect that the dependence of surface viscosity on II will also be independent of pH. The latter was also observed by A-K.3 Before these factors can be resolved, it is safe to conclude that the II vs. A isotherms of DPPC are independent of the subphase pH between 4.5 and 7.5. As regards the equilibrium state of II vs. A isotherms, we would like to mention that the phase-transition region, Figure 1, generally designated as liquid-expanded and liquid-~ondensed,2.~~~ has been found to disappear as the temperature approaches the phase-transition temperature, T,,, for different lipids.2i8 For example, the values of Tt, as found from II vs. A isotherms are 22.3 and 41.3 OC for dimyristoyllecithin (DML) and DPPC, respectively. These values of T,, agree with the data reported for the phasetransition in lipid-bilayer system^.^^^
Acknowledgment. The excellent technical help of J. Klausen is acknowledged. (9)Tanford, C . The Hydrophobic Effect; Wiley: New York, 1980.
Book Reviews Thin Films from Free Atoms and Particles. Edited by Kenneth J. Klabunde. Academic Press, Inc., New York. 1985.
It is fair to say that the area of production of thin films has a profound effect on a wide variety of established and emerging technologies throughout the world and that, at least in the US, the process of thin-film formation is not well studied or understood by chemists. Thus the editor's hope of beginning to acquaint the chemical community with the basics of (primarily inorganic) t h i n - f i i formation is laudable. This book falls short of its stated objective in several key thin-film processes. Thin-film materials which have been formed from some sort of gas-phase procesa may or may not possess the properties of the bulk material. The formation of the thin film from two or more reactive atomic or molecular species may be the only way of obtaining a material with a desired mechanical, electrical, or chemical property-and for this reason alone this area is bound to see tremendous growth in the next several years. The first chapter of this review book, written by the editor, gives an introduction and an overview to the types of free atoms and particles (small molecular clusters) which are commonly found as intermediates in inorganic f i formation processes. Although mention is made of molecular beam epitaxy (MBE) and chemical vapor deposition (CVD) methods, it is clear that the focus of this book is on sputtering/glow discharge methods (to the extent that the method is competitive with MBE and CVD, ion beam deposition in high vacuum is dealt with in Chapter 5, but no equivalent chapter is included for MBE or CVD methods). Two useful tables appear in Chapter 1 which summarize (from a previous book by the editor) the properties of atomic and reactive molecular species useful for the formation of thin films. Chapter 2 deals with the clustering of free atoms and particles, which occurs immediately prior to incorporation into the growing
thin film. The properties of these clusters are detailed from a chemist's perspective. It is interesting to see the extrapolation of cluster chemistry and physics to a hypothetical explanation of how the properties of the thin film are determined at the microscopic level. Chapter 3 (Venugoplan and Avni) gives a detailed review of the current understanding of glow discharge plasma deposition procgsses-from the perspective of what is happening at the atomic and molecular level within the plasma. The authors do not deal with RF plasmas and refer the reader instead to an earlier book edited by one of the authors. A good overview is given for the atomistics of glow discharge plasmas and how these plasmas have been probed with a variety of spectroscopic techniques. Chapter 5 (Gautherin, Bouchier, and Schwebel) gives a complete overview of ion beam deposition methods, generally carried out in very good vacuum environments. These methods hold the promise for producing films of extremely high purity because of the ability to m a s select the fi-forming particle from most other competitors. Chapter 6 (Webb) gives an introduction overview to magnetron sputtering processes, but without the mechanistic detail given the glow discharge plasma methods in Chapter 3. This type of sputtering continues to be the workhorse for a number of semiconductor device fabrication processes, as well as other high-technology thin-film applications. An empirically derived compendium is given of parameters which affect film properties (sputtering geometries, sputtering rates, sputtering gas, etc.) as they apply to the formation of many compound semiconductors, oxide films, piezoelectric films, and amorphous hydrogenated silicon. Chapters 4 and 7 detail the formation of diamondlike and boron nitride f i i s (Weissmantel)and silicon carbide films (Matsunami), respectively. These films are prized because of their mechanical hardness and insulating characteristics. Chapter 7 does detail
