3122
Langmuir 1994,10, 3122-3133
Nucleophilic Displacement Reactions at Benzyl Halide Self-AssembledMonolayer Film Surfaces Timothy S. Koloski,*$tCharles S. Dulcey, Quillian J. Haralson,* and JeffreyM. Calvert* Center for Bio I Molecular Science and Engineering, Code 6900 Naval Research Laboratory, Washington, D. C. 20375-5348 Received February 2, 1994. I n Final Form: June 13, 1994@ Surface chemicalreactions at self-assembledmonolayer (SAM)films containing benzyl halide functional groups have been investigated. The benzyl halide S A M films were chemisorbed onto silicon or silicon dioxide substrates. Nucleophilic substitution at the benzyl chloride group, employing NaI, results in exchangeofchlorine for iodine. The rate for surface halogen exchangewas compared to the rate in solution by 'H NMR kinetics on a model compound. The rates and yields for reactions in which the benzyl halide is converted to ethylenediamine or 3-pyridine groups were studied by W absorption spectroscopy and X-ray photoelectron spectroscopy. The utility of the benzyl halide S A M films as surface-imaging resists and the importance of the surface modifications in a lithographic process employing ligand-based selective metalization are also discussed.
Introduction The study of self-assembled organic thin film materials has been of considerable interest over the last decade.l The use of these materials for a range of applications, including biological interfaces, model surfaces for studies of wetting and adhesion, corrosion protection, electrochemistry, and the fabrication of microelectronic circuitry, has recently been reviewed.2 In addition, S A M films have been used to provide reactive surfaces for the buildup of three-dimensional structures normal to the substrate surface, particularly for nonlinear optical application^.^^^ Our studies were initiated on the basis of our interest in these materials as ultrathin imaging layers for lithographic ~ a t t e r n i n g . ~We - ~ have recently reported processes in which benzyl halide S A M films are patterned by UV,9J0X-ray,l0J1 or e-beam (scanning tunneling microscope)12radiation sources and subsequently modified to produce selective metal buildup from the surface. We have also demonstrated that patterned S A M films are useful reaction templates for the selective attachment of a wide variety of other material^,',^ especially biological moieties such as cells,13proteins,14 and nucleic acids15in
* To whom correspondence should be addressed. t
ONT-ASEE Postdoctoral Fellow. Current address: Dept. of Chemistry, Morehouse College,
Atlanta, GA 30314. Abstract published inAdvanceACSAbstracts, August 1,1994. (1)Ulman, A. A n Introduction to Ultrathin Organic Films: From Langmuir-Blodgett to Self-Assembly;Academic Press: San Diego, CA, @
1991. (2)Swalen, J. D.; et al. Langmuir 1987,3,932. (3)Marks, T. J.; D. Li U.S. Patent 5,156,918(1992). (4)Kakkar, A. K.;et al. Langmuir 1993,9,388. (5)Dulcey, C. S.;Georger, J. H.; Krauthamer,V.; Fare, T. L.; Stenger, D. A.; Calvert, J. M. Science 1991,252,551. (6) Calvert, J. M.; et al. Solid State Technol. 1991,34(10), 77. (7)Calvert, J. M. In Organic Thin Films and Surfaces, Volume One; Ulman, A., Ed.; Academic Press: Boston, 1994. (8) Calvert, J. M. J . Vac. Sci. Technol. B 1993,11(6), 2155. (9)Dulcey, C. S.;Koloski, T. S.;Dressick, W. J.;Chen, M. S.;Georger, J. H.; Calvert, J. M. Proc. SOC.Photo-Opt. Instrum. Eng. 1993,1925, 657. (10)Calvert, J. M.; et al. Opt. Eng. 1993,32(10), 2437. (11)Suh, D.; Simons, J. K.; Taylor, J. W.; Calvert, J. M.; Koloski, T. S . J . Vac. Sci. Technol. B 1993,11(6), 2850. (12)Marrian, C. R.K.; Perkins, F. K.; Brandow, S. L.; Koloski, T. S.; Dobisz, E. A.; Calvert, J. M. Appl. Phys. Let. 1994,64, 1. (13)Stenger, D. A.; et al. J . Am. Chem. SOC.1992,114, 8435. (14)Bhatia, S. K.;et al. Anal. Biochem. 1993,208,197. (15)Chrisey, L. A.; Roberts, P. M.; Benezra, V. I.; Dressick, W. J.; Dulcey, C. S.; Calvert, J. M. Proc. Mater. Res. Soc., in press. ~~
well-defined geometries on the substrate surface. Therefore, we are particularly interested in understanding and quantifying surface chemical reactions on these SAM films. Substitution reactions at S A M film surfaces have been investigated by other researchers.16-21 Marks and coworkers reported results on several substitution reactions at benzyl halide S A M film^.^,^ Benzyl halides are known to be quite reactive groups for nucleophilic displacement reactions due to the electrophilic nature of the benzylic carbon atom and the ability of the halogen to act as a good leavinggroup.22 Therefore, benzyl halide SAM films (BzC1 and Bz-I) were chosen as ideal candidates for the study of nucleophilic displacement reactions which occur at surfaces. In this paper, we present a detailed investigation of several nucleophilic substitution reactions that occur at benzyl halide S A M surfaces. The rates and extent of surface reaction are quantitated by X-ray photoelectron spectroscopy (XPS) surface analysis and UV absorption spectroscopy. The results of these analyses are discussed in terms ofthe reaction mechanisms and packing densities of molecules in the S A M films.
Experimental Section General Procedures. Solvents and reagents were used as
received, withoutfurther purification,unless otherwiseindicated. Organosilanetreatment solutions,used for SAM film formation, were prepared inside a He-filled Vacuum Atmospheres drybox. The procedures and reagentsfor surfacecatalysisz3and electroless s ~ ~ been ~~~ metalization of ligand-bearing S A M s ~ r f a c e have previously described in detail elsewhere. Briefly, an aqueous, colloidal Pd catalyst is puddled onto the ligand-bearing S A M surface for 20 min. The catalyzed surface is then rinsed with (16)Tillman, N.; Ulman, A,; Penner, T. L. Langmuir 1989,5,101. (17)Tillman, N.; Ulman, A.; Elman, J. F. Langmuir 1989,5,1020. (18)Wasserman, S. R.;Tao, Y.; Whitesides, G. M. Langmuir 1989, 5,1074. (19)Balachander, N.; Sukenik, C. N.Langmuir 1990,6 , 1621. (20)Kurth, D. G.; Bein, T. Langmuir 1993,9,2965. (21)Lee,Y. W.;Fteed-Mundell, J.;Sukenik, C. N.;Zull, J. E.Langmuir 1993,9, 3009. (22)(a) Streitwieser, Chem. Rev. 1966,56, 571. (b) Streitweiser, Solvolytic Displacement Reactions;McGraw-Hill Book Co.: New York, 1962. (23)Dressick, W. J.; Dulcey, C. S.; Georger, J. H.; Calabrese, G. S.; Gulla, M.; Calvert, J. M. J. Electrochem. SOC.1994,141,210. (24)Dressick, W. J.; Dulcey, C. S.; Georger, J. H.; Calvert, J. M. Chem. Mater. 1993,5, 148. (25)Calvert, J.M.; et al. InPolymersforMicroelectronics;Thompson, L. F., Wilson, C. G., Tagawa, S., Eds.; ACS Symposium Series 537; American Chemical Society; Washington, DC, 1993;p 210.
0743-7463/94/2410-3122$04.50/00 1994 American Chemical Society
Nucleophilic Displacement Reactions deionized (DI) water and then immersed in an electroless Ni plating bath (NIPOSIT 468, Shipley Co.) for a typical time of -30 min. Substrate Cleaning Method. Substrates were silicon wafers, native oxide or thermal oxide containing -300-400 of thermally grown oxide, or fused silica slides. The standard method for cleaning substrates prior to film formation consisted of immersion in 1:l methanoMC1 (concentrated)for a minimum of 30 min at room temperature. After being rinsed throughly in DI water, the substrates were immersed in concentrated HzS04 for at least 30 min and rinsed throughly with DI water. The substrates were then immersed in boiling water and dried under a stream of filtered Nz prior to immersion in the silane treatment solution. Contact Angle Measurements. Contact angle measurements were made using a Zisman-type goniometer.26 Static, sessile drops (15-2OpL) ofDI water were applied to the modified substrate surfaces with a micropipette. Measurements were made on both sides of the drops, and a minimum of three measurements were taken on each region of a given substrate. Measurements made across the modified substrate surfaces were within 3", indicating uniformity of film coverage across the substrate. Solvents. High purity acetonitrile (Burdick & Jackson), toluene, benzene, dimethylformamide (DMF), chloroform, and methanol (Aldrich Sure Seal) were anhydrous. Benzene was distilled from Nahenzophenone ketyl and was degassed prior to use. Deuterated acetonitrile (CambridgeIsotope Laboratories) was dried over CaHz prior to use. Reagents. @-Chloromethylphenyl)trimethoxysilane ((C&0)3S ~ - C ~ H ~ - C HCMPTMS) ZC~; and (p-chloromethylpheny1)trichlorosilane (C~~S~-C&-CHZC~; CMPTCS)were obtained from Hiils America Inc. Li metal (sand) and butyllithium (1.0 M solution in hexanes) were purchased from Aldrich and were handled under an inert atmosphere ofAr or He. Ethylenediamine was obtained from Alfa Products and was distilled from CaHz under dry Ar prior to use. 3-Bromopyridine, sulfuric acid, and hydrochloric acid were obtained from Aldrich. The lithium salts of ethylenediamine (Li-EDA)27and3-pyridine (Li-PYR)28 were prepared according to literature procedures. After preparation, Li-EDA was sealed under vacuum in glass ampules and was used within 30 days after being opened. Ly-PYR was stored and handled under an inert atmosphere. Instrumentation. Uv-vis spectra were recorded on aVarian CARY 2400 UV-vis spectrophotometer. Film thicknesses were measured using a Gaertner Model 115C scanning ellipsometer as described e1~ewhere.l~ NMR spectra were recorded on a Bruker AM-250 spectrometer equipped with a dual IWbroad band probe. Deep UV exposures were performed on an ArF excimer laser operating at 193 nm. XPS surface analyses were performed on a Surface Science Instruments Model SSX-100-03 XPS spectrometer using a 30" take off angle and -600 pm spot size. Halide Exchange Solution Kinetics Monitored by 'H NMR. In a typical kinetics experiment, 9 pL (0.05 mmol) of CMPTMS and 0.5 g of CD3CN (0.65 mL) were added to an NMR tube inside a He-filled glovebox. A small amount (-10 pL) of toluene, as an internal integration standard, was also added. The tube was removed from the glovebox and an initial (t = 0 ) spectrum taken. The contents of the tube were then transferred under an inert atmosphere to an NMR tube containing a known mass of NaI (7.5 mg for 1:l and 75 mg for 1O:l runs). NMR spectra were taken and stored at intervals during the exchange reaction. Integrations of the peak areas for the benzylic protons were then normalized against the methyl peak of the toluene internal standard. Preparation of Benzyl Chloride (Bz-C1)Functionalized S A M Surfaces. A 1%(v/v)solution of CMPTCS in toluene was prepared under He and transferred to an Ar-filled glovebag. Cleaned substrates were immersed in this solution for 5-7 min, (26) Zisman, W. In Advances in Chemistry;Fowkes, F. M., Ed.; ACS Press: Washington, DC, 1964; Vol. 43, Chapter 1. (27) Beumel, 0. F.; Harris, R. F. J. Org. Chem. 1963,28,2775. (28) Parham, W. E.; Piccirilli, R. M. J. Org. Chem. 1977, 42, 257.
Langmuir, Vol. 10, No. 9, 1994 3123 rinsed with fresh toluene, and then baked on the surface of a VWR Series 400HPS programmable hotplatektirrer for 5 min at 120 "C. Bz-Cl films exhibited water contact angles of 65-70", which are comparable to other benzyl chloride functionalized surfaces reported in the l i t e r a t ~ r e . ~ Average film thicknesses were measured by ellipsometry to be 9 A, with a 3a variation of 3 A. These data are consistent with homogeneous surface coverage of Bz-Cl functional groups of monolayer thickness. Preparationof Benzyl Iodide (Bz-I)FunctionalizedS A M Surfaces. CMPTCS-treated substrates were treated in a 0.67 M solution ofNaI in acetonitrile (100 g L ) at 25 "C. On the basis of kinetics studies a reaction time of 24 h was found to be sufficient for exchange ofchlorine for iodine. The substrates were removed from the treatment solution, rinsed with fresh acetonitrile, sonicated 5 min in a methanokhloroform (1:l) solution, rinsed three times with DI water, and dried under a stream of filtered Nz. Reaction of Bz-I Surfaces with Li-EDk Bz-I functionalized substrates were immersed in a 9.0 mM solution of Li-EDA in DMF (15 mg/25 mL) inside a He-filled glovebox (or Ar-filled glovebag) at room temperature. On the basis of kinetic studies reaction times of 5 min were found to be sufficient for the exchange. The substrates were removed from the inert atmosphere, rinsed with fresh DMF, and then rinsed three times with DI water. The rinsed substrates were then dried with a stream of filtered Nz. Reaction of Bz-Cl Surfaces with Li-EDA. The procedure was identical to that described above for modification of Bz-I surfaces with Li-EDA, except the immersion times were extended to 275 min for Bz-C1 surfaces. Reaction of Bz-ISurfaceswith Li-PYR. Bz-I functionalized substrates were treated inside a He-filled drybox at 25 "C with a 3.0 mM solution of Li-PYR in DMF (6 mg/25 mL) for > 30 min. The substrates were removed from the drybox, rinsed with fresh DMF, rinsed three times with DI water, and then dried under a stream of filtered Nz. Reaction of Bz-E1 and Bz-EC1 Surfaces with Li-PYR. The procedure for the treatment of these surfaces is identical to that described above for treatment of Bz-I surfaces with Li-PYR. After the treatment and rinse steps, both surfaces were metalized as described above using an electroless Ni bath and produced smooth, homogeneous metal deposits. W-vis Kinetics of Halide Exchange. A CMPTCS-treated fused silica slide was treated in a 0.33 M solution of NaI in acetonitrile (50 g L ) for 8 h. At 30 min intervals during the treatment the slide was removed, sonicated in methanoY chloroform (l:l), and rinsed with acetonitrile and water, and a W spectrum was recorded prior to re-immersion in the NaI treatment solution. The increase in absorbance at 250 nm was monitored us treatment time. The natural logarithm of the increase at 250 nm (-ln(A, - AJA,)) was plotted us time during the first half-life of the reaction. The slope of this line yielded the rate constant for the surface halide exchange.
Results and Discussion The ability to change the functionalityat existing S A M films is of importance for a rapidly growing number of applications. A basic understanding of the reactions which occur on S A M surfaces,with an emphasis on quantitation of the reaction rates and determination of the extent of reaction,has been the impetus for this work. Our studies have focused on the surface exchange of chloride for iodide at Bz-Cl S A M films. We also have investigated several nucleophilic substitution reactions that occur at surfacebound Bz-Cl and Bz-I groups,which result in replacement of the halide with an EDA or PYR group. Benzyl Chloride to Benzyl Iodide Exchange. A common method for the formation of alkyl or aryl iodides from the corresponding alkyl or aryl chlorides is the Finkelstein reactionz9(eq 1). In this halogen exchange, a (29) March, J. Advanced Organic Chemistry, 2nd ed.; McGraw-Hill, Inc.: New York, 1977; p 391.
Koloski et al.
3124 Langmuir, Vol. 10, No. 9, 1994 R-X
+ NaI - R-I + NaX
(1)
solution of NaI is used to effect nucleophilic displacement of the chloride, resulting in the formation of alkyl or aryl iodides. The high solubility of NaI in acetone or acetonitrile, together with the relative insolubility of NaCl in these solvents, results in essentially quantitative conversion to the iodinated product. Reaction ofbenzyl chloride functionalized organosilanes with NaI solution has previously been used3 to prepare benzyl iodide functionalized organosilanes (eq 2). Unlike
+
-
C13Si(p-C6H4)CH,C1 NaI I,Si(p-C,H,)CH,I
+ NaCl (2)
the trimethoxysilyl- or trichlorosilylbenzyl chloride organosilanes, the isolated iodinated products prepared according to eq 2 are reported to be u n ~ t a b l e .Therefore, ~ the iodinated products must be used for surface film formation immediately after preparation. Due to the instability of the iodinated organosilanes, it would be advantageous to effect the halogen exchange a t the benzyl chloride group of a surface-bound S A M film. The relative order of reactivity of halide exchange a t surface-bound a-haloacetyl, benzyl, and alkyl halides has recently been reported.21 However, the extent and rates of these exchanges on the surface have not been quantified. Furthermore, the reactivity observed in solution may be inhibited by factors such as steric constraints and solvation effects in the surface bound analogues. We have therefore performed a detailed investigation of the solution and surface exchange of chloride for iodide in benzyl chloride functionalized organosilanes. Solution Halogen Exchange. Treatment of commercially available ( C H ~ O ) ~ S ~ ( ~ - C G H(CMPTMS) ~)CH~C~ with NaI in acetonitriled3 solution results in nucleophilic displacement of chloride by iodide and the formation of (CH3013Si@-C6H4)CH2I(IMPTMS)(eq 3). The reaction was moni-
+
-
(CH30)3Si(p-C,H,)CH,C1 NaI (CH,0),Si(p-C6H,)CH,I
+ NaCl
(3)
tored by lH NMR, which showed essentially complete conversion to IMPTMS. The trimethoxysilyl derivative, CMPTMS, was chosen for the solution study rather than the trichlorosilyl version, CMPTCS, to avoid reaction of NaI with the Si-C1 groups. The methoxy groups of CMPTMS are unreactive toward NaI, as determined by 'H NMR, and do not interfere with the determination of reaction rates for benzyl halide exchange. The rate of reaction was found to be dependent on the concentration ofNaI, and the required reaction time was -40 h to obtain =-95%conversion at equimolar concentrations of reactants. After complete reaction, as determined by 'H NMR, the originally colorless solution was light-yellow in color, and a white precipitate, presumably NaC1, had formed. No line broadening due to NaCl precipitation was observed. The 'H NMR spectrum of IMPTMS in CDBCN exhibits doublets centered a t 6 7.48 (d, 3J= 8.2 Hz) and 7.40 (d, 3J= 8.1 Hz) for the aromatic ring protons, and singlets a t 6 4.50 and 3.52 for the benzylic and methoxy protons. In comparison, the lH NMR spectrum of CMPTMS in CD3CN exhibited resonances a t 6 7.63 (d, 3J= 5.4 Hz), 7.46 (d, 3J= 5.4 Hz), 4.66 (s), and 3.60 (s) for the aromatic ring, benzylic, and methoxy protons, respectively. 'H NMR spectra, in the region containing the benzylic and methoxy protons, are shown in Figure 1. The bottom spectrum is CMPTMS starting material. The top spectrum is IMPTMS, acquired after reaction with NaI. The effect of exchange of chloride for iodide was to shift all of the
resonances to higher field. This is consistent with the replacement of the chloride with the less electronegative iodine atom.30The largest shift in resonance was observed for the benzylic protons that are in closest proximity to the substituted halide. The methoxy proton resonances were least affected by the halogen exchange. As mentioned above, the rate of reaction is dependent on the concentration of NaI. 'H NMR was used to obtain a rate constant for the solution exchange reaction. CMPTMS, 10 pL of toluene, and CD3CN were combined in a n NMR tube, and a n initial ( t = 0) NMR spectrum was taken. The added toluene served as a chemically inert internal standard for integration purposes. The contents of the NMR tube were then transferred under an inert atmosphere to a n NMR tube containing a known mass of NaI. NMR spectra were then taken a t intervals during the subsequent exchange reaction. The chemical shifts of the benzylic protons (6 = 4.66 and 4.50; Figure 1)were sufficiently different in CMPTMS and IMPTMS and could be accurately integrated and then individually ratioed to the integrated area of the toluene internal standard. To obtain reaction rates, the disappearance of CMPTMS or the appearance of IMPTMS was plotted us reaction time. The dependence ofthe reaction rate on the concentration ofNaI was determined by kinetics experiments performed a t equimolar concentrations of reactants and a t a 10-fold molar excess of NaI. The resulting data are plotted in ~ mechanism, Figure 2A and 2B. For an S Nexchange equimolar concentrations of reactants would be expected to show a second-order dependence on the reaction rate. A plot of LCMPTMS1-l us time yielded a straight line with zero intercept and a slope of 2.7 x ssl (Figure 2A). A 10-fold excess of NaI would be expected to result in pseudo-first-order reaction kinetics and a n observed rate which is approximately 10times faster than the equimolar reaction. The resulting data revealed a first-order dependence on the reaction rate, and a plot of -ln[CMPTMSl us time yielded a straight line with zero intercept and a slope of 3.6 x s-l (Figure 2B). Surface Halogen Exchange. The chloride to iodide exchange observed in solution was also characterized at surface-bound benzyl chloride (Bz-C1) groups. Bz-C1 surfaces were obtained by immersion of clean, hydroxylated substrates in a toluene solution of CMPTCS. Bz-C1 S A M films were converted to benzyl iodide S A M films (Bz-I) by contacting a Bz-C1 S A M with an acetonitrile solution of NaI. The extent of the surface exchange reaction was studied by XI'S and Rutherford backscattering spectrometry (RBS). The surface exchange reaction kinetics were characterized using XPS and W absorption spectroscopy. a . Extent of Reaction. The X P S spectrum of a Bz-C1 S A M on a Si native oxide surface exhibits a n asymmetric C12p peak centered a t a binding energy of 200.2 eV. This binding energy is ~haracteristic~l of an organic chlorine species, and no evidence of inorganic C1- was observed. Arelatively intense Si 2p singlet centered a t 99.9 eV which is due primarily to the underlying Si substrate is also observed. The areas of these peaks were measured and multiplied by the appropriate sensitivity factors31to obtain relative areas. The ratios of the relative areas of C1 to Si obtained in this manner provided a n average atomic percentage of C1 on a Bz-C1 S A M film on Si of 7.0 & 1.0%. XPS analysis of a Bz-I S A M on Si, obtained by conversion of a Bz-C1 S A M in NaI solution (0.67 M for 24 h), exhibits an 1 3 d doublet centered a t 620.6 eV and the 99.9 eV Si (30)Becker, E. D. High Resolution NMR;AcademicPress: New York, 1969; p 73. (31) Muilenberg, G.E.Handbook ofX-Ray Photoelectron Spectroscopy; Perkin-Elmer Corp: Eden Prairie, MN, 1979.
Nucleophilic Displacement Reactions
i
'
1
4.8
~
1
4.6
Langmuir, Vol. 10, No. 9, 1994 3125
~
1
4.4
'
1
4.2
'
1
4.0
ppw
'
1
3.0
'
1
3.6
'
l
3.4
'
1
3.2
'
Figure 1. IH NMR (250 MHz) spectra of CMPTMS (bottom)and IMPTMS (top) in CD&N showingthe benzylic and methoxy proton resonances.
2p peak. The C12p peak observed in the original Bz-C1 S A M is no longer present after the NaI treatment. The observed atomic percentage of I (us Si)on the Bz-I S A M film prepared in this manner was 3.5 f 0.5%. To determine the extent of surface conversion of Bz-C1 to Bz-I S A M films on Si and approximate rates for the exchange, Bz-C1 S A M films were immersed for varying times in acetonitrile solutions containing two different concentrations of NaI. The lower concentration treatment solution contained 0.1 M NaI, and the treatment times spanned the range 0-48 h. The higher concentration solution was 1.0M in NaI, and treatment times were from 0 to 8 h. After treatment, the samples were analyzed for C1, I, and Si by XPS. As above, the C1 and I relative areas were normalized to the Si area to yield atomic percentages of C1 and I (us Si) on the surface at various times during the exchange process. The rate of I loss from the surface during XPS analyses was measured and found to be minimal. Specifically, a Bz-I SAM film was analyzed by X P S over a period of 6 h. At 30 min intervals during the analysis the I to Si ratio was determined. The loss of I was linear over the 6 h
analysis, and the slope of this line gave a zero-order rate constant of 2.4 x s-l. Because analysis of I requires -30 min to obtain a good signal to noise ratio, and I is always the first element scanned, the loss of I signal due to X-ray desorption during XPS analysis is 55% of the measured signal. Therefore, loss of I due to X-rays during XPS analysis is considered minimal. Figure 3 shows XPS spectra of the C1 and I regions of the Bz-C1 S A M film before and after 48 h treatment with 0.1 M NaI solution. Plots of %C1and %I us time are shown for the 0.1 and 1.0 M experiments in Figure 4. These results show that, as chlorine is removed from the surface, iodine approaches alimitingvalue of 3.5 f0.5%. An equal exchange of C1 for I would result in approximately 7.0% I on the surface after treatment with NaI; therefore, approximately 45-60% of chlorine sites are replaced by iodine. One possible explanation for incomplete exchange is the larger size of I compared to C1. Space-fillingarguments based on the differences in atomic size of I and C1, as well as expected differences in the C-I and C-CI bond lengths, could explain why every chlorine atom in a dense-packed
Koloski et al.
Langmuir, Vol. 10,No.9,1994
