236
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surface. Besides special losses which take place (at a molecular level) in the vicinity of the film front, there are dissipative processes due to liquid flow in the film and meniscus. The former contribution is largely unknown5 and will not be discussed here. The latter are significant and can be evaluated by hydrodynamic considerations. As is emphasized in ref 5, viscous dissipation in the film region is strong and dominates dissipation in the meniscus. Considering the steady laminar flow in a liquid layer of uniform thickness t, it can be shom7v8that the frictional shear stress on the solid wall in contact with the spreading film is given by 7,= 3p(U/~),where p is the liquid viscosity and U is the steady velocity of the film front. As a simple calculation can show, the weight of the advancing film and the viscous force on the solid surface due to flow in the meniscus region8p9 are negligible, compared to the total force due to liquid flow in the film. By equating the power of the stretching force f to the power of the friction force 7, we get the following relation between the velocity U of (7) Batchelor, G. K. A n Introduction to Fluid Dynamics; Cambridge University Press: Cambridge, England, 1967;pp 180-183. ( 8 ) Teschke, 0.; Tenan, M. A.; Galembeck, F., to be published. (9) Huh,C.; Sriven, L. E. J.Colloid Interface Sci. 1971,35,85(see eq 27 and Figure 6).
the film front and the solid wettability difference represented by A cos e?
where i stands for the time derivative of the instantaneous film length 1. Let us approximate the spreading ratio 1/1 by l/At and the velocity U by (h + h)/At, where At is the current pulse width corresponding to film formation and h and h are the maximum values of the film length and wedge length, respectively. With these approximations, we get, from eq 1, U and h expreszed in terms of the measurable quantities: t, A cos e, At, h, YLG, and p. Figure 2 compares the experimental values for h and U with those from eq 1. It becomes clear from the figure that wetting effects play a fundamental role in the hydrodynamics of film spreading. In conclusion, ascending sulfuric acid solution films are formed on vertical iron electrodes polarized a t a voltage corresponding to the passive state. The model presented in this paper agrees with the experimental results and shows that the driving force on the advancing film is due to changes in the solid-liquid contact angle produced by chemical reactions a t the electrode-solution interface.
Thermal Properties of n -Hexadecane Solubilized in an Aqueous Lamellar Liquid Crystal Kilian O'Neill and Anthony J. I. Ward* Chemistry Department, University College, Belfield, Dublin 4, Ireland Received July 30, 1987. I n Final Form: October 16, 1987 The aqueous lamellar phase of the nonionic surfactant n-dodecyl tetraoxyethylene glycol ether solubilizes n-hexadecane to the extent of ca. 55% w/w (Moucharafieh, N.; Friberg, S. E.; Larsen, D. W. Mol. Cryst. Liq. Cryst. 1979,53, 189). Measurements of the enthalpy of fusion, Hf, of the solubilized oil indicate it to exist in two identifiably different thermal states. One state is characterized by an Hfof ca. 18 cal/g and the other by a value of 56.3 cal/g consistent with that of the isotropic liquid oil. Fractions of each component derived from the thermal measurements are shown to be consistent with those derived from the observed order parameters of the solubilized oil.
Introduction Recent NMR and small-angle X-ray (SAXS)studies'-" of the aqueous lamellar phase of the nonionic surfactant n-dodecyl tetraoxyethylene glycol ether (C12E04)have indicated the presence of oil in an essentially isotropic state. A similar conclusion was also reached5 about the state of alkane solubilized in lipid bilayers. All of these studies indicated the degree of partitioning of the solubilized oil between a state where it penetrates between the surfactant molecules and a state where the oil resides unmixed with surfactant a t the bilayer center was dependent upon the chain length of the alkane; thus, the (1)Mouchardieh, N.;Friberg, S. E.; Larsen, D. W. Mol. Cryst. Liq. Crvst. 1979. - , 53. - - , 189. ~ - -
"(2)Ward, A.J. I.; Friberg, S. E.; Larsen, D. W.; Rananavare, S.B. J. Phys. Chem. 88, 826. (3)Ward A.J. I.; Friberg, S. E.; Larsen, D. W. ACS Symp. Ser. 1985, 272, 185. (4)Ward, A. J. I.; Friberg, S. E., Larsen, D. W.; Rananavare, S. B. Langmuir 1985,1 , 24.
fraction of oil which penetrates increases as the alkane chain length decreases. The presence of such a component of the oil fraction located a t the bilayer center has consequences in the understanding of their Rates at which molecules are either lostg or within associated lipid structures may be a reflection of their distribution within the host structure. These processes are important in areas as diverse as percutaneous absorption of drugs and tertiary oil recovery. This paper (5)Gruen, D.W. R.; Haydon, D. A. Biophys. J . 1981,33,167. (6)White, S. H. Ann. N . Y. Acad. Sci. 1977,303,273. (7)Gruen, D.W. R. Biochim. Biophys. Acta 1980,595,161. (8)Evans, D.F.; Mitchell, D. J.; Ninham, B. W. J . Phys. Chem. 1986, 90,2817. (9) Imokawa, G.; Hattori, M. J. Invest. Dermatol. 1985,84, 282. (10)Carroll, B. J.; O'Rourke, B. G. C.; Ward, A. J. I. J . Pharm. Phurmacol. 1982,34, 287. (11)Friberg, S. E.;Mortenson, M.; Neogi, P. Sep. Sci. Technol. 1985, 20,285. (12)Neogi, P.; Kim,M.; Friberg, S. E. Sep. Sci. Technol. 1985,20,631. (13) Friberg, S. E.; Kayali, I.; Rhein, L.; Hill, R. Cosmet. Toiletries 1987,102, 135.
0 1988 American Chemical Society
Langmuir, Vol. 4, No. 1, 1988 237
Letters
60
0 O.
-
h
0
B \ r
m
U
"
0 40
0
Q B 5
m
u
h
15
20
25
0
0
Q 10
0
-
c
r
m
L
TEMPERATURE,('C)
Figure 1. Thermograms obtained on heating at various values of n-hexadecane content (values in parentheses = wofl/(wog + wma,)) in a lamellar phase of C12E0,/water (3:2 w/w).
4J 8 W
20
presents some preliminary differential thermal analysis (DTA) measurements of this system with a view to testing the consistency of a simple two-state model of the solubilizate.
Experimental Section Endotherms were recorded with a differential thermal analyzer (Stanton & Redcroft, Model 610) at a heating rate of 1OC/min using samples of ca.5 x lO-9 g encapsulated in aluminum crucibles. All heating curves were obtained after the samples were cooled to ca. 273 K and after it had been established, from deuterium NMR observations, that the phase integrity was maintained over this temperature range. Results and Discussion The endotherm associated with the melting of solubilized n-hexadecane occurs a t 18-19 "C (Figure 1). It is well-defined a t high oil contents but becomes much broader a t low concentrations. Values for the apparent enthalpy of fusion, m f , o b d were derived from the experimental endotherms after calibration with several materials with known heats of fusion in this temperature range. The value of AHf,obsd increases with increasing oil: surfactant molar ratio (R, a t a fixed surfactankwater ratio (Figure 2). At high oii contents, AHf,obsd, approaches a value of 56.3 cal/g, which is similar to that expected for the melting of pure n-hexadecane.14 No endotherm was observed for values of R,,, < 0.2 mol of oil/mol of surfactant under the experimental conditions used. An approximately linear increase in AHf,,,,,, with increasing fraction of oil was found a t intermediate oil contents. Assuming that the system is essentially a t equilibrium, the chemical potentials of the n-hexadecane molecules in the solid, liquid, and the state mixed with the surfactant chains of the bilayer will be equal. The Gibbs free energy changes for the tansitions between these states will be zero in the absence of any significant contributions from heats of mixing, and the observed enthalpy change may be written as mf,obsd = Plml PZm2 (1) e-*
where AHl is the enthalpy associated with the fusion of (14) Boned, C.; Peyrelasse, J.; Moha-Ouchane, M. 1986,90,634.
J. Phys. Chem.
0 0
0.5
OIL
FRACTION
Figure 2. Variation of A H f , , ~with n-hexadecane content.
. - O r
Q" c O r
4J 0
m
I:
0 . 5
0
r
0
0
U Q X
E 0 0
0 1 0
I
I
1
I
I
I
I
0.5
OIL
FRACTION
Figure 3. Variation of the fraction of oil in the mixed state with oil fraction: 0 ,calculated from eq 1;0,calculated from eq 2 with data from ref 4.
a fraction p1of essentially bulk hexadecane and AH2is that associated with the fraction p 2 ,which is mixed with the
238
Langmuir 1988,4, 238
amphiphile chains. A value of 53.3 cal/g is inferred for AH1 from the observed limiting value of AHf,obsd a t high oil contents. Extrapolation of the data to infinite dilution gives a value of 18 cal/g for AH2. This value is in accord, within the context of the assumptions used, with the entropy change upon melting of this "mixed" oil fraction being smaller than that of the isotropic fraction. Values of p2 were calculated by using eq 1 and are presented as a function of oil content a t a fixed surfactant:water ratio (Figure 3). A comparison between these values derived from the thermal measurements can made with those from previously observed4 quadrupolar splittings of the solubilizate, i.e. Avobad
PlAvl
+ P ~ A v...~
(2)
where Av, and Avz refer to the quadrupolar splittings in the "mixed" and "unmixed" states, respectively (hence Au2 = 0). Good agreement between the two sets of values is seen in Figure 3, taking into account the fact that the NMR measurements were made4 a t 298 K.
This preliminary study has demonstrated the utility of differential analysis to investigate the state of solubilized n-hexadecane directly. In this respect, this study reveals similarities to a recent study14 of microemulsions using differential scanning calorimetry that provided information about the degree of oil penetration in the associated surfactant structure. The results are consistent with the previously adopted simple two-state model for the solubilizate system. A more disordered state than the pure n-hexadecane solid is indicated from the lower value of AH2 for the oil component, which is mixed with the amphiphile as expected; i.e., there will be a smaller entropy difference between this state and the melted isotropic state. The upper limit to this portion of the solubilized oil appears to be a t approximately 1 mol of oil to 5 mol of surfactant. This oil content also corresponds to that which gave the largest ordering effect in the bilayer as shown by the compositional dependence of the solubilizate order parameters. Registry No. CI2EO4,5274-68-0; n-hexadecane, 544-76-3.
Book Reviews Surface and Interfacial Aspects of Biomedical Polymers. Vol. 1: Surface Chemistry and Physics. Joseph D. Andrade, Editor. Plenum, New York, 1985. As stated in the Preface, this book is intended to provide a fundamental basis for the study of interaction of polymers with living systems, biochemicals, and aqueous solutions. The editor has met his goals in a significant and in-depth fashion. Volume 1 of Surface and Interfacial Aspects of Biomedical Polymers containschapters dealing with the fundamental aspecta of surface chemistry and the physics of polymers, polymer surface dynamics, model polymers for probing surface and interfacial phenomena, polymer-oriented monolayers and multilayers as model surfaces, X-ray photoelectron spectroscopy (XPS), surface infrared spectroscopy, contact angle and interface energetics, interfacial electrochemistryof surfaces with biomedical relevance, interface acid-base/charge-transferproperties, graft copolymer and block copolymer surfaces,and interfacial tensions at amorphous-water interfaces: theory, surface Raman spectroscopy, and polymer surface analysis. This volume provides useful information on the theory and techniques utilized in the characterization of polymer surfaces and interfaces. Specifically, the volume provides information on elemental surface composition, organic functionalgroup,present, the variation in composition and molecular character with depth,
and to some extent, the lateral distribution of the elemental and molecular character of the surface. While comprehensive in its approach, this volume points out limitations of the various techniques utilized to characterize surfaces and interfaces. Moreover, gap areas in theories and experimental techniques are presented. For the student, teacher, or scientist involved in the study of surfaces and their interfacial properties, this volume serves as both a useful textbook and a comprehensive reference. The editor and his contributors have gone to painstaking effort in constructing chapters which present their topic material in a clear, concise, and highly readable fashion. The use of tables and figures in the volume is excellent. Of special importance to potential users of thisvolume is the detailed and comprehensivelist of references provided at the end of each chapter. These will be especially useful to those requiring the original presentation of theory, techniques, perspectives, and data and their interpretations. Surface and Interfacial Aspects of Biomedical Polymers. Vol. I: Surface Chemistry and Physics is recommended to individuals with a commitment to the study of polymer surfaces and interfaces. It will be useful as a textbook and a reference text. Its value is enhanced by the fact that Volume 2, Protein Adsorption, is also available. It is hoped that the editor, Dr. Joseph D. Andrade, will continue this excellent series. James M. Anderson, Case Western Reserve University
Additions and Corrections William D. Machin* and Peter D. Golding: Adsorption of n-Butane on Silica Gel. 1987, 3, 346-349. The correct expression for eq 13 is cyp - aPo= In ( A / A o ) / ( T- 468.6) The correction amounts to ca. a 0.5% increase in ap and does not have a significant effect on the values of cyp plotted in Figure 3.
