Langmuir 1990,6, 1098-1101
1098
and thiocyanate, both with strong complexing tendenother monolayer properties will be necessary to test these cies with Pb2+ and Cd2+ in aqueous ~ o l u t i o ncaused , ~ ~ ~ ~ ~ ideas further. an increase in AV. On the other hand, these anions (which Summary have no tendency to complex with Ba2+ correspondingly appear to produce no change in AV. This, thereThe effect of various metal ions with stearic acid and fore, seems to support the model wherein ions like Pb2+, arachidic acid monolayers at various pH values and conCd2+, and Co2+ interact directly with the fatty acid in a centrations of divalent metal ions was studied by using fairly specific manner, rather than just nonspecifically surface pressure and surface potential measurements. In screening electrostatic charges, as in the case of Ba2+, the pH range 5.0-6.0, the extent of ionization of the fatty Ca2+,and Mg2+. acids studied is sufficient for all the divalent cations to One of the interesting issues associated with divalent interact with the monolayer and to alter its.properties cation effects on fatty acid L-B films, and perhaps monosignificantly. Such effects with Pb2+ were noted a t pH layers a t the air/water interface, is whether or not a 1:l values as low as 4.0. Alkaline earth metals, magnesium, relationship exists between each carboxylic acid group calcium, and barium, appear to interact with fatty acids within a layer and ions like Pb2+ and Cd2+, as opposed electrostatically by screening the negative charges, as demto a more nonspecific 1:2 relationship with alkaline earth onstrated by an increase in the surface potential as the ions.9J9i26 For monolayers and multilayers transferred concentration of these ions in the subphase is increased. to solid substrates, as mentioned above, there is firm eviCadmium, cobalt, and lead significantly decrease surdence to support this m0del.gJ9.~6 For monolayers a t the face potential and appear to interact more strongly via air/water interface, no such evidence has been estabcovalent bonding, thus ordering the molecules in the monolished hitherto, but we can now provide evidence with layer more effectively. Changing the type of anion present this study that the major differences between these ions and possibly causing complexes to form with these ions are consistent with such a model. The much greater degree appears to reduce the interaction between these ions and of condensation for Pb2+ and Cd2+ and a very signifithe fatty acid monolayer, as demonstrated by a positive cant change in AV a t low concentrations, for example, shift in the surface potential. In contrast, in the pressuggest a much tighter packing within the monolayer which ence of alkaline earth metal ions no counterion effects could be facilitated by having each fatty acid associated were noted. with one Pb2+ or Cd2+ ion. Whether the counterion effects noted in this study are in any way reflecting this process Acknowledgment is made to the donors of the Petrois not clear, and certainly more direct observations of leum Research Fund, administered by the American Chemical Society, for support of this research. (40)Sillen, L. G.; Martell, A. E. In Stability Constants of Metal-Ion Registry No. Mg, 7439-95-4;Ca, 7440-70-2;Ba, 7440-39-3; Complexes; Supplement no. 1. Special Pub. No. 25, The Chemical SocCo, 7440-48-4;Cd, 7440-43-9;Pb, 7439-92-1;stearic acid, 57-11-4; iety: London, 1971. arachidic acid, 506-30-9;fluoride, 16984-48-8;chloride, 16887(41)Hogfeldt, E. In Stability Constants of Metal-Ion Complexes, 00-6; iodide, 20461-54-5; thiocyanate, 302-04-5; methanePart A: Inorganic Ligands; IUPAC Chemical Data Series, No. 21; Persulfonate, 75-75-2. gamon Press- Oxford 1982. 40341
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Reaction of a Monolayer with Subphase: Dehydration of a Secondary Alcohol over Sulfuric Acid Jamil Ahmad' and K. Brian Astin* Department of Chemistry, University of Bahrain, P.O. Box 32038, Isa Town, Bahrain Received July 26, 1989 The kinetics of the acid-catalyzed dehydration of 1-phenyl-1-hexadecanol to (2)-1-phenyl-1hexadecene have been studied on aqueous sulfuric acid-air interface for expanded ( 7 ~< 0.1 mN m-l) and compressed (T = 25 mN m-l, kept constant) monolayers. Below a threshold arealmolecule region, the initial rate of the reaction is lower than above it, but as the reaction proceeds, affording more areal molecule, the rate increases till a stage is reached where the effect of the reverse reaction (itself sensitive to the state of expansion) begins to outweigh this, resulting in the novel effect of the reversal in the net direction of the reaction during its course. For compressed film at constant surface pressure, contrived zero-order kinetics are followed, and the reaction is dramatically slower than for the expanded state. But the surface compression while decreasing the rate at the same time increases the yield of the reaction. This has been explained in terms of the product molecules being removed from contact with the subphase at high surface pressures.
Introduction The study of a chemical reaction between an insoluble monolayer and the subphase a t varying surface pres0743-7463/90/2406-1098$02.50/0
sure P (or area/molecule) is potentially a valuable tool in investigating the structure of the surface films. We have recently a number of such systems which fall into two categories: those in which the state of corn0 1990 American Chemical Society
Reaction of a Monolayer with Subphase
pression of the monolayer drastically affects the rate of the reaction and the ones where rate is largely unaffected by the variation in surface pressure. A reaction belongs to one type or the other depending on how the accessibility of the reacting functionality in the molecule to the subphase is affected by surface compression. For example, the rate of acid-catalyzed cyclization of the monoterpenoid alcohol nero1,l which takes place easily in the expanded film of nerol, is disfavored at high surface pressures, since under these conditions the molecule does not have the folded conformation required for cyclization. The acid-catalyzed dehydration of the tertiary alcohol 1,l-diphenyl-1-octadecanolis slowed down dramatically as the surface pressure is increased, consistent with the fact that under these conditions the participating @-hydrogenbecomes inaccessible to the subphase.2 On the other hand, reactions where the accessibility of the functionality is not influened by surface compression are not greatly influenced by changes in the surface pressure. Examples are the chromic acid oxidation of 1-phenyl-1-hexadecan01~ and the hydrolysis of an octadecyl ester.4 In the present paper, we report the kinetics of dehydration of 1-phenyl-1-hexadecanol (1) 58% over aqueous sulfuric acid subphase in expanded and compressed films to give (2)-1-phenyl-1-hexadecene (2).s For compressed monolayers, the method has been improved to follow the reaction at a constant surface pressure, allowing us to study the effect of sustained compression throughout the course of the reaction. The reaction is reversible with the return reaction, Le., the hydration of the alkene 2, itself being sensitive to the state of compression of the monolayer.
1
2
Experimental Section Materials. The water used was triply distilled-first through a Pyrex still followed by a two-stage distillation through a Haeracus Destamat quartz still. The subphase acid solution was prepared by mass with Fluka (Puriss) sulfuric acid. l-Phenyl-1hexadecanol was prepared by sodium borohydride reduction of 1-phenylhexadecanone (TCI Chemicals, Tokyo, >97 % by GC), 16 mmol, affording the alcohol (13 mmol): 81% yield; mp 52-54 "C. Anal. Calcd for CzzH3~0:C, 82.95; H, 12.03. Found: C, 82.93; H, 12.30. (2)-1-Phenyl-1-hexadecene(2) was prepared by acidcatalyzed dehydration (60% H2S04) of the alcohol (1) (1.6 mmol) affording the alkene (0.5 mmol): 31% yield; mp 31-33 "C; IR (CSz soln cm-1) 3084,3029,2928,2845,1625,1430,757,694; 1H NMR (270 MHz,CDC13) 6 7.42-7.24 ( m , 6 H),4.95 ( d d , J = 7.5 Hz, 1 H), 2.35-2.05 (m, 2 H), 1.24 (br s, 24 H), 0.88 (t, J = 6.5 Hz, 3 H). Anal. Calcd for C22H36: C, 87.92; H, 12.08. Found: C, 87.61; H, 12.43. (1) Ahmad, J.; Astin, K. B. J.Am. Chem. SOC.1986, 108, 7434. (2) Ahmad, J.; Astin, K. B. J . Am. Chem. SOC.1988,110,8175. (3) Ahmad, J.; Astin, K. B. Langmuir 1988,4,780. 1979, 100, 1. (4) Valenty, S. J. J. Am. Chem. SOC. (5) Fromherz, P. Rev. Sci. Instrum. 1975, 46, 1380.
(6) Adamson, A. W.Physical Chemistry of Surfaces;Wiley: New York, 1982; pp 148-150. (7)Davies, J. T.; Rideal, E. K. Interfacial Phenomena; Academic: New York, 1963; pp 282-288. (8) The alkene was observed as a single peak on both HPLC and capillary GC and cochromatographed with an authentic sample of (2)1-phenyl-1-hexadecene.
Langmuir, Vol. 6, No. 6, 1990 1099 Apparatus. The apparatus used in this study has been fully described earlier.2~5 The reaction was carried out in a circular multicompartment-essentially a modified Langmuir trough with Wilhelmy plate balance, manufactured by Mayer-Feintechnic, Gottingen. The trough is made of PTFE and is fixed onto an aluminum base through which water can be circulated from a thermostated bath. Two P T F E surface barriers are connected through separate shafts to an axle and can be moved independently to vary the enclosed surface area, whose value is displayed digitally, or moved in concert from one part of the trough to another. The surface pressure is digitally displayed and can be read to 0.1 mN m-1. The trough can be operated at constant surface pressure whereby the area enclosed between the barriers automatically varies to maintain the surface pressure constant at a predetermined initial value. Kinetics. The trough was filled with the sulfuric acid solution a t 25 & 0.1 "C, and the surface was swept by moving the compression barrier over the entire surface and applying suction to remove any adsorbed impurities. The barriers were then positioned to give an area of about 300 cm2 between them. A suitable amount of the (1.91 x M) alcohol solution (1) in hexane was spread on the surface by using a Hamilton microliter syringe. For the reaction of the compressed film, the area was then reduced by moving the compression barrier. To minimize any reaction taking place before the desired surface pressure being reached, the compression was carried out rapidly taking less than 30 s. The apparatus was then switched on to the constant pressure mode, allowing the area to be adjusted as necessary. After the reaction time, using the coupled motion of the barriers, the monolayer was swept over a compartment filled with water and into a reservoir in which 0.5 mL of hexane was placed. The hexane solution was withdrawn with a Pasteur pipet and analyzed by high-performance liquid chromatography (HPLC) using an LKB system with LKB 2151 variable-wavelength detector and an LKB 2220 recording integrator. The mixture was resolved by isocratic elution with 85% ethanol-15% water on a 25-cm Supelcosil LC-8 5-wm column. The alcohol (1) and the alkene (2) were detected in the UV at 210 nm at 0.02 absorbance unit full scale (AUFS). The areas under the peaks were measured electronically and the concentrations determined by using the molar response factors calculated from standard solutions of the alcohol and the alkene. Between runs, the trough was cleaned with concentrated sulfuric acid followed by copious amounts of triply distilled water. Blank runs carried out after this cleaning procedure showed no residual material from the previous runs. To assess the amount of the material recovered as a percentage of the material originally spread, a recovery experiment was performed by spreading an amount of the alcohol over water and sweeping it into the reservoir containing 0.5 mL of hexane. HPLC analysis of the hexane solution showed that >90% of the material originally spread was recoverable. To estimate the errors in measurements, for each of the three monolayers, at least three observations were made for one specified reaction time. Repeat injections of a sample in HPLC gave a reproducibility of within 2 % .
Results and Discussion As has been pointed out by Valenty? the only reliable method of following surface reactions is by direct analysis of the reactants and products. Since both the reactant and the product in this reaction absorb strongly in the UV, the amounts involved in the reaction, ca. 10+ g, were readily analyzed by HPLC. The surface pressure-area isotherm of alcohol 1 has been reported previ~usly.~ The rate of reaction over 58% H2S04 was studied at three different areas/molecule: 2.7 and 1.1 nm2/molecule for the expanded film and 0.40 nm2/molecule for the compressed film. Also, the initial rates of the reaction (when
