Langmuir 1990, 6, 1265-1269
-
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P L
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.
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M NaCl
500 408
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388
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NaCl
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RESOLUTE S E P R R R T I O N / n m
Figure 9. Evanescent wave scattering intensity, measured at maximum Ar+ laser radiation pressure power (40 mW), as a
function of absolute separation. roreflection from both the spectrophotometer cell and coupling prism, though we suspect that these effects are
1265
small. The work described here is, of course, the first experiment that characterizes the distance scales over which the scattering from a particle near a flat surface in an evanescent wave deviates from t h a t predicted theoretically. It is our intention in future work to modulate the Ar+ laser using an electro-optic modulator. This will allow us to accurately control both the modulation rate and depth of the Ar+ laser. It will also permit rapid modulation of the laser such that a t sufficiently high frequency the particle will adopt a mean separation between the two limiting separations defined by the radiation pressure force limits. The frictional force at this mean separation may be obtained from the transfer function.s Finally, we believe that modification of particlesurface interactions by adsorption of surfactants or polymers can be studied, and the direct measurement of subtle features such as depletion forces should be feasible. (6) Randall, R. B. Frequency Anulysk; Bruel and Kjaer:
N b ,1987.
Synthesis, Structure, and Excimer Formation of a Vesicular Assembly Carrying Chiral 9-Anthryl Chromophores Hiroki Sasaki,? Masahiko Sisido,*J and Yukio Imanishit Department of Polymer Chemistry and Research Center for Medical Polymers and Biomaterials, Kyoto University, Sakyo, Kyoto 606, Japan Received September 5,1989. In Final Form: February 7, 1990 A new chiral amphiphile carrying two octadecyl chains and a 9-anthryl group was synthesized from D-9anthrylalanine: (CH3)3N+(CH2)&ONHCH(CH2-9Ant)CON(Cl~3,)2. The amphiphile formed a vesicular assembly in aqueous dispersion, which shows a gel-liquid crystalline transition at 26.5 "C. Absorption and CD spectra indicated no strong ground-state interaction between the anthryl groups. Fluorescence spectra showed a strong excimer emission around 548 nm. The excimer to monomer emission intensity ratio increased monotonically with decreasing temperature. The excimer was found to form within ca. 1 ns after photoexcitation. The excimer emission showed a right-handed circular polarization, but the magnitude of the circular polarization was insensitive to the temperature. These results are consistent with an efficient energy migration along the two-dimensional array of anthryl chromophores followed by trapping into a relatively small number of excimer-forming sites.
The bilayer aggregates of chromophoric amphiphiles have been considered as an effective photon-harvesting system.' The two-dimensional (2-D) structure of the bilayer is suited to collect solar energy that is spread in space. For effective photon harvesting, the energy transfer or migration along the 2-D chromophoric assembly should be so efficient that the photon energy absorbed by one of the chromophores in the 2-D array could effectively be transferred to a photoreaction center. For efficient energy transfer or migration by a dipole-dipole mechanism, the
* Correspondence should be addressed to this author. Present address: Research Laboratory of Resources Utilization, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 227, Japan. + Department of Polymer Chemistry. Research Center for Medical Polymers and Biomaterials. (1) For reviews, see: (a) Fendler, J. H. Membrane Mimetic Chemistry; Wiley: New York, 1982. (b) Calvin, M. Acc. Chem. Res. 1978,11, 369. (c) Matsuo, T. J. Photochem. 1982,31,788. (d) GrBtzel, M. Pure Appl.
*
Chem. 1982,54, 2369.
0743-7463190f 2406-l265$02.50/0
chromophores should have a strong transition dipole moment. In other words, they should show allowed fluorescence or absorption transition between the ground state (SO)and the lowest excited state (SI).In this sense, anthracene is one of the most favorable chromophores.For example, the critical distance ROfor the energy migration between the same kind of aromatic molecules is 8.25 A for 1-methylnaphthalene, 10.0 A for pyrene, and 22.0 A for 9-methylanthracene.2 The efficiency of a single-step energy transfer between a pair of chromophores separated by a distance R can be calculated by Ros/(Rs + R$). The efficiency between two 9-anthryl groups separated by 6 8, is 99.96%, whereas that of the 1-naphthyl groups is 87.1176. These values do not seem much different, but the probability for an excited state to survive after 50 consecutive steps of energy migration is (0.9996)60= 0.98 for the 9-anthryl group but 0.001 for the 1-naphthyl group. (2) Berlman, I. B. Energy Transfer Parameters of Aromatic Compounds;Academic Press: New York, 1973.
0 1990 American Chemical Society
1266 Langmuir, Vol. 6, No. 7, 1990 If one takes 7 A as the interchromophore distance, the survival probability is 0.95 for 9-anthryl group and 1.3 X lo-' for the 1-naphthyl group. This consideration demonstrates t h a t a 2-D assembly of anthryl chromophores is very suitable for the photoenergy-harvesting media on which the multistep and long-range energy migration is possible. The other prerequisite condition for the long-range energy migration is the absence of an energy-trapping site in the assembly. The J-aggregate often occurring in 2-D chromophoric assemblies may work as the energytrapping site, because of its low energy level.3 A disk-like assembly of amphiphiles carrying anthryl groups has been reported by Shimomura et aL4 However, in their system a ground-state dimer of the anthryl groups predominates in absorption and fluorescence spectra, suggesting that the energy migration may not be very efficient. A strong Davydov splitting has been observed in the Langmuir-Blodgett film of 7-(2-anthryl)-lheptanoic acid? but ita fluorescencespectrum has not been reported. In this study, a new anthryl amphiphile carrying two octadecyl groups (9A18) was synthesized from an optically active artificial amino acid, D-9-anthrylalanine6v7
I
CHZ
I
AI a
The same type of amphiphilic amino acids have been reported by Murakami and co-workers using alaninea or histidine9 as the amino acid unit. Similar amphiphiles carrying 1-or 2-naphthyl group have been reported by the present authors.1° T h e chiral structure of the amphiphile is advantageous for obtaining structural information by chiroptical spectroscopy, i.e., circular dichroism (CD) for the gound-state structure and circularly polarized fluorescence (CPF)" for the excited-state structure.
Experimental Section Materials. The amphiphilic amino acid derivative was prepared from D-9-anthrylalanine. The synthesis and the optical resolution of the latter amino acid have been reported.6~7 No L isomer was found in the 1H NMR spectrum of methyl D-9anthrylalaninate measured in the presence of a chiral shift reagent.6 The amphiphile was prepared by a similar proced u r e a s r e p o r t e d f o r t h e a m p h i p h i l i c L - 1 - or 2-naphthylalanines.10 All the intermediates were checked by IR, 1H NMR (90 MHz), and TLC. (3) Shimomura, M.; Ando, R.; Kunitake, T. Ber. Bunsen-Ges. Phys. Chem. 1983,87, 1134. (4)(a) Shimomura, M.; Hashimoto, H.; Kunitake, T. Chem. Lett. 1982, 1285. (b)Vincett, P.; Barlow, W. Thin Solid Films 1980, 71, 305. (5) Durfee, W. S.;Storck, W.; Willig, F.; von Frieling, M. J.Am. Chem. SOC.1987,109, 1297. (6)Egusa, S.;Sisido, M.; Imanishi, Y. Bull. Chem. SOC.Jpn. 1986,59, 3175.
(7) Kawai,M.; Matauura, T.; Butsugan, Y.; Egusa, S.; Sisido, M.; Imanishi, Y. Bull. Chem. SOC.Jpn. 1985,58, 3047. (8)Murakami, Y.; Nakano, A.; Fukuya, K. J. Am. Chem. SOC.1980, 102,4253. (9) Murakami, Y.; Nakano, A.; Yoshimatsu, A.; Mataumoto, K. J.Am. Chem. Soc. 1981,103, 2750. (10)Sisido, M.;Sato, Y.; Sasaki, H.; Imanishi, Y. Langmuir 1990,6, 177-182. (11) For a review, see: Riehl, J. P.; Richardson, F. S. Chem. Reu. 1986, 86, 1.
Sasaki et al. The following abbreviations will be used: antAla = 9-anthrylalanine, Boc = (tert-butyloxy)carbonyl,2Cla = dioctadecyl, C5 = (CH2)5,(Bo420 = di-tert-butyl dicarbonate, EDC = l-ethyl3-(3-(dimethylamino)propyl)carbodiimide, HOBt = l-hydroxybenzotriazole, T H F = t e t r a h y d r o f u r a n , a n d D M F = dimethylformamide. Box-D-antAla. HCh-antAla (728 mg) was suspended in a water/THF (1/3 v/v) mixture, and the mixture was cooled with ice. NaHC03 (500 mg) and (Bo420 (632 mg) were added, and the mixture was stirred for 3 h a t 0 "C and for 12 h at room temperature. The THF was evaporated, and the aqueous layer was acidified with 5 % KHS04 solution to pH 2. The oil was extracted with ethyl acetate. The extract was washed with 10% NaCl solution and dried on Na2S04. The ethyl acetate was removed, and the residue was crystallized by adding hexane: yield 690 mg; mp 160-162 OC. Anal. Calcd for C22H23N04: C, 72.31; H, 6.34; N, 3.83. Found: C, 72.29; H, 6.44; N, 3.57. Boc-~-antAla-2Cl~. Boc-D-antAla (675 mg) and HN-2Cla (1.6 g, Eastman Kodak) were dissolved in a DMF/CH&12 (1/4 v/v) mixture and cooled to 5 "C. EDC (357 mg) and HOBt (250 mg) were added, and the mixture was stirred for 3 h at 5 "C and for 24 h a t room temperature. The solvent was evaporated, and the residue was dissolved in chloroform. The solution was washed with aqueous solutions of KHS04, NaC1, NaHC03, and NaCl, respectively, and dried on Na2S04. The solvent was evaporated, and the residual oil was fractionated with a Sephadex gel (LH-SO/chloroform): yield 550 mg. Anal. Calcd for C58H=N2O3: C, 80.13; H, 11.13; N, 3.22. Found: C, 79.85; H, 11.17; N, 3.31. Br-C~,-~-AntAla-2C18. Boc-~-antAla-2Cls(520 mg) was dissolved in THF (5 mL), and 5 N HC1 in dioxane (10 mL) was added at room temperature. After 30 min, the removal of the Boc group was confirmed by TLC. The solution was neutralized with aqueous NaOH and cooled with ice. 6-Bromohexanoyl chloride (255 mg) dissolved in benzene (1 mL) was added dropwise to the solution. A t the same time, triethylamine was added with vigorous stirring to keep the pH of the solution neutral or weakly basic. After 1h, the solvent was removed and the residue was redissolved in chloroform. The solution was washed with 4 % NaHC03, NaC1, 10% citric acid, and NaCl solutions and dried on Na2S04. The solvent was evaporated, and the oily residue was fractionated with a Sephadex gel (LH-2O/chloroform): yield 300 mg. Anal. Calcd for C59H97N202Br:C, 74.88; H, 10.33; N, 2.96; Br, 8.45. Found: C, 74.29; H, 10.39; N, 3.14; Br, 8.83. Me3N+-C5-antAla-2Cls (9A18). Chloroform solution (2 mL) of Br-Cs-antAla-2Cls (280 mg) was added to an ethanol solution of trimethylamine (ca. 2076, 10 mL). The mixture was left standing at room temperature for 8 days in the dark. The solvent and the amine were evaporated off, and the residual oil was fractionated with a Sephadex gel (LH-20/chloroform). The product was an oil: yield 100 mg. The TLC and HPLC analyses showed a single peak when monitored by the absorbance of the anthryl group. The elemental analysis indicated the presence of water. Calcd for C62H1~N302Br:C/N, 17.17; Br/N, 1.90. Found: C/N, 17.67; Br/N, 2.05. Preparation of Vesicular Assembly. The vesicular assembly was prepared in the same method as reported previously.'o 9A18 (6 mg) was dissolved in chloroform (1 mL), and the solvent was flushed off by a stream of nitrogen. The flask was then evacuated. Distilled water (3 mL) was added to the flask, and the flask was shaken with a vortex mixer to obtain a homogeneous dispersion. The dispersion was sonicated with a sonicator (Tomy Seiko, UR-POOP 40-6OW) for 5-10 min at 40 "C. A t this stage, the dispersion was almost transparent. The spectroscopic measurements were made after diluting the dispersion with about a 10-fold amount of distilled water. Measurements. The following instruments were used: TEM, Hitachi H-6OOS; DSC, Daini-Seikosha SSC-580; absorption, Hitachi 200-20; CD, Jasco 5-20; CPF, Jasco FCD-1.12 Fluorescence decay curves were measured on a home-made single(12) Sisido, M.; Egusa, S.; Okamoto, A.; Imanishi, Y. J. Am. Chem. SOC. 1983,105,3351. (13) Castellan, A.; Desvergne, J.-P.; Bouas-Laurent, H. Chem. Phys. Lett. 1980, 76, 390.
Langmuir, Vol. 6, No. 7,1990 1267
Chiral Vesicular Assembly of Anthryl Chromophores
.. -
0.2 I.Lm
200
T I T I
30
LO
Figure 2. DSC thermogram of the sonicated aqueous dispersion of 9A18. Heating rate = 2 "C/min. photon counting apparatus equipped with an air discharge lamp (fwhm = 3 ns). Fast electronics were obtained from Ortec. Results a n d Discussion Observation of t h e V e s i c u l a r S t r u c t u r e . The structure of the aggregates of 9A18 was observed by a transmission electron microscopy (TEM) using uranium acetate as a negative stain. Figure 1 shows the electron micrograph of the aqueous dispersion of 9A18 after sonication. Small vesicles with diameters between 200 and 400 A are formed. DSC Thermonram. A Dhase chanee of the 9A18 vesicular assemb6 was followed by DSE thermography (heating rate +2 'C/min). The thermogram is shown in Figure 2. An endothermic peak at 26.5 "C may indicate a gel (low temperature)-liquid crystalline (LC) (high temperature) transition. However, the peak is very broad and has a shoulder a t the low-temperature side. This may be explained in terms of the distribution of the size and shape of the vesicles as observed in the electron micrograph (Figure 1). Absorption a n d C D Spectra. Figure 3 (bottom) compares absorption spectra of 9A18 in methanol and in the vesicular state below and above the gel-LC transition temperature. Both the 'La band (340-400 nm) and the 'Bb band (24&270 nm) shifted to longer wavelengths in the vesicular state. However, no difference was observed in the absorption spectra below and above the transition temperature. Therefore, the shift may be accounted for ~~
300
',\ 350
I
LW
hlnm)
Figure 1. Electron micrograph of the aqueoos dispersion of9A18 after sonication, The sample was negatively stained with uranyl acetate. [9Al8] = 2 x lo-' M.
20
250
Figure 3. Absorption (lower curves) and CD (upper curves) spectra of 9A18 at 10 O C (-) and 40 O C (-. -) in the aqueous dispersion ([9A18] = 1 X IO-' M). The spectra in methanol M. solution at 25 O C (- - -) are also shown, [9A181 = 5 X not by a specific electronic interaction, such as an excit o n in t h e vesicular assembly, b u t by a simple environmental effect. In the disk-like bilayer of anthryl amphiphiles carrying a single long alkyl chain, a groundstate dimer was detected as the absorption peak at 400 n m 4 No ground-state dimer is observed in the 9A18 assembly, either in the gel phase or in the LC phase. The difference between the amphiphile having single alkyl chain and the present case may be interpreted in terms of a longer interchromophore distance in the present bisalkyl amphiphile. No marked change was observed in the CD spectra on going from the methanol solution to the vesicular state in the aqueous solution (Figure 3, top). The CD spectra in the vesicular state below and above the gel-LC transition temperature are virtually identical. The results of CD spectroscopy contrast the CD spectra observed for the vesicular assembly of the corresponding amphiphilic derivative of 2-naphthylalanine (2N18). In the latter case, an enhancement of CD intensity was observed in the vesicular state, especially in the gel phase.l0 The CD spectra of 9A18 indicate that no particular orientation dominates in the vesicular assembly and that no specific interaction, such as the formation of the dimer and the higher aggregates, occurs in the assembly. Fluorescence S p e c t r a a n d Fluorescence Decay Analysis. Figure 4 shows fluorescence spectra of 9A18 in the vesicular assembly. The fluorescence spectra consist of monomer and excimer emissions. The latter was assigned not to an excited dimer but t o the excimer, because no ground-state dimer has been observed in the absorption spectrum. The absence of the ground-state interaction is further confirmed by the coincidence of the absorption spectrum with fluorescence excitation spectra monitored at 400 nm (monomer fluorescence) and a t 550 nm (excimer). The fluorescence spectrum shows strong excimer fluorescence. The intensity of the excimer increases with lowering the temperature, but that of the monomer fluorescence was insensitive to the temperature. The temperature dependence was reversible as shown by an Arrhenius-type plot of the excimer/monomer intensity ratio inserted in Figure 4. The slopes in the figure do not show any change a t the gel-LC transition temperature. This is consistent with the absence of spectroscopic change in the UV and CD spectra of the vesicular amphiphiles a t that temperature.
1268 Langmuir, Vol. 6, No. 7, 1990
Sasaki et al.
(T'C)
40
20
0 LdrJ
i
1
400
500
600 h(nm)
Figure 4. Temperature dependence of fluorescencespectra of the vesicular assembly of 9A18 ([9A18] = 1 X l W M). The insert is an Arrhenius plot of the excimer (550 nm) to monomer (397 nm) intensity ratio. The negative temperature dependence of the excimer formation indicates that the process is not governed by any thermal processes, such as the fluctuation of the anthryl groups. In the vesicular assemblies of the naphthylalanine amphiphiles, an optimum temperature for the excimer/monomer ratio was found at the gel-LC transition temperature.1° A positive temperature dependence was observed below the transition point, and a negative dependence was seen above the point. In the naphthylalanine vesicles, the negative temperature dependence has been interpreted in terms of the thermal dissociation of relatively unstable naphthalene excimers. However, it is difficult to adopt the same interpretation to the present case, since the stabilization energy of the anthracene excimer may be higher than the naphthalene excimer. In the present case, the negative temperature dependence is observed even below the transition temperature. Moreover, if the excimer was very unstable, it would not be formed in such a large amount as experimentally observed. Incidentally, in a series of a,w-bis(anthryl)alkanes,An(CH2),-An, the excimer intensity is reported to increase with the temperature.'* A tentative explanation for the unusual behaviors of 9-anthryl amphiphiles is that the excimer formation is preceded by an efficient energy migration along the 2-D assembly of anthryl chromophores. The energy migration can be more efficient at low temperatures due to the more dense arrangement of the anthryl chromophores. The excimer formation process can be directly followed by a timecorrelated single-photoncounting method. Figure 5 shows the decay curve of the excimer fluorescence at 10 "C,after exciting the vesicle by a pulsed light. Three exponential components were needed to simulate the decay curve, but no rise component with a rise time longer than the instrumental limit of time resolution (about 1 ns) was found. T h e rise was not found a t 40 " C , either. Incidentally, in the naphthyl amphiphiles an excimer rise time of 5-10 ns has been observed. A rise time of 3-6 ns has been reported for intramolecular excimers of An(CHz),-An compounds with n = 2-10 in methylcyclohexane and ethanol s01utions.l~The rise time of the present anthryl vesicles, if it exists, is much shorter than the naph(14)Ferguson, J. Chem. Phys. Lett. 1980, 76, 398.
0 0 (0)
50 (14)
loo 1271
150 (411
2w
(54)
250
(681
chmmls (ESSC)
Figure 5. Decay curve of the excimer emission of 9A18 in the vesicular assembly at 10 "C. [9A18] = 5 X 10-6 M. The decay curve was fitted to a three-component exponential function,I ( t ) = 0.33 exp(-t/3.1 ns) + 0.36 exp(-t/22 ns) + 0.26 exp(-t/79 ns).
I
I
1
I
400
500
600
h (nm)
Figure 6. CPF spectra of 9A18 in the vesicular assembly at 15 O C (-) and at 40 O C (- - -), [9A18] = 1 X 10-4 M. thy1 case and the intramolecular case. The failure to detect the rise is consistent with an explanation that the excited energy migrates very efficiently along the 2-D surface. T o conclude, efficient energy migrations should be involved to interpret the strong excimer fluorescence and its negative temperature dependence. Since no groundstate excitonic interaction was found, the energy migration should be occurring through a random-hoppingmechanism by the dipole-dipole interaction. Circularly Polarized Fluorescence Spectra. Figure 6 shows CPF spectra of the anthryl vesicles below and above the transition temperature. The CPF spectra are shown with the Kuhn's circular dissymmetry factor, gem, as the ordinate. The ,g, is defined as the difference of the intensities of left- and right-handed circularly polarized fluorescence divided by the total fluorescence intensity. No CPF signal is detected at the monomer emission region. This may be explained in terms of the random orientation of the anthryl chromophores, as has been suggested from the CD spectroscopy. The CPF spectra at the excimer region show a negative signal, indicating a chiral structure of the excimer. The CPF spectra are virtually identical below and above the transition temperature, indicating that the excimer configuration is independent of the phase structure. The anthryl excimer should be stabilized enough not to alter its configuration by the change of the vesicular structure. In conclusion, the vesicular assembly of the anthryl amphiphile is a 2-D chromophoric assembly in which the ground-state interaction (dimer formation) is effectively
Langmuir 1990,6, 1269-1212 suppressed, but the energy migration is efficient. Therefore, the high potential of the anthryl vesicle as an efficient photo-harvesting system was shown. However, the problem of the excimer formation which may work as an energy trap remained unresolved, although the number of the excimer site may not be very large.
1269
Acknowledgment. We thank Dr. H. Hasegawa of the Department of Polymer Chemistry, Kyoto University, for measuring the TEM photographs. M.S. also acknowledges the financial support from the Grant-in-Aid for Scientific Research No. 63470095 from the Ministry of Education, Science, and Culture, Japan.
Effect of a Soap Film on the Catenary P. Mohazzabi,"J. P. McCrickard, and F. Behroozi Department of Physics, University of Wisconsin-Parkside, Kenosha, Wisconsin 53141 Received November 20,1989 The shape of a string, suspended from both ends, under the combined effect of gravity and surface tension is studied. Exact equations of the system are derived which produce results that are in good agreement with the experimental observations. The shape of the string, to a good approximation,turns out to be an arc of a circle.
Introduction The determination of the shape acquired by a uniform chain which is suspended from two points, known as the catenary (from Latin catena for chain), has a long and venerable history.' Galileo believed the shape to be a parabola. However, Huygens showed that the curve is nonalgebraic and therefore not parabolic. In 1691 and again in 1692,Leibnitz published the correct equation of the catenary but did not give a proof. It was Jacob (James) Bernoulli (1654-1705)who furnished the proof establishing that Leibnitz's equation for the catenary was correct. He further studied the shape of strings with variable density and under action of a central force.2 Nowadays, the catenary is usually treated as an example of the application of calculus of variations in modern textbook^.^ We became intrigued with the effect of a soap film on the shape of a hanging string after observing our children at play. They were making large soap bubbles using a meter-long piece of knitting yarn tied to a horizontal rod. Each end of the yarn was tied in a loop which could slide along the rod. With the two ends together, one dipped the rod and yarn into a soap solution and removed it. While the rod was held in a horizontal position, the ends of the yarn were pulled apart, and a film was formed as illustrated in Figure 1. When the rod was jerked rapidly normal to the film, air pressure forced the film to bulge and shed a large bubble. Interesting and amusing as the bubbles were, we were more intrigued by the shape of the string under the combined action of gravity and surface tension. Upon closer examination, we noticed several novel features. In describing these features, it is convenient to refer to Figure (1) See, for example: Rouse Ball, W. W .A Short Account of the History Mathematics; Dover: New York, 1960. (2) Eves, H. Great Moments in Mathematics. The Mathematical Association of America, 1981. ( 3 )Boas,M. L.Mathematical Methods in the Physical Sciences, 2nd ed.; Wiley: New York, 1983. of
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ili D
Figure 1. String suspended from a rod under the combined action of gravity and surface tension of a soap film. The soap film fills the region enclosed by AOBA.
1. When the ends of the string are pulled apart by a distance 2x0, the lower end of the string remains clumped together for a total length 21, 1 on each side. For small values of 2x0, the angle of the string with the rod, 60, and the length 1 are relatively large. As the ends are pulled apart, 1 and 60 both decrease, until 90 reaches zero. Pulling the ends further apart causes the string to contact the rod as shown in Figure 2. Under this condition, 60 remain zero, but the clumped length and the distance between the two contact points decrease as the ends are separated further. In this paper, we derive the equation of the string by a straightforward application of Newtonian mechanics and 0 1990 American Chemical Society
