TOURNAL
J
O F T H E AMERICAN CHEMICAL SOCIETY K r ~ ~ ~ I r ir ne Ld S Purenr OJ&r
0 Copjrighr lY79 hi rhe 4,tteri~anChemrcalSocrrrj
V O L L M E 101, N U M B E R 1
JANLARY
3, 1979
Chemical Reactions in Monolayer Films. Chromatography, a Multicompartment Trough, and the Hydrolysis of Surfactant Ester Derivatives of Tr is( 2,2'- bi pyr id in e )rut hen ium( I I ) 2 + Steven J. Valenty Cotitribittiotifrom the Gtwrral Elrrtric. Cotiipunj, Corporate Rrsrarrli and Dereloptiirrit. Schetlrc~tadr.,\:e~. l'ork 12301. KrrtJirrd Maj. I O , I978
Abstract: There is renewed interest in studqing chemical reactions of surfactant molecules in monolayer arrays a t the air-water interface. [High-performance liquid chromatography and vapor-phaw chromatography have been used for the first time to detect, quaniitate. and hclp charactcriie the siiiiill aniounts of material contained in auch surface films. c;t. 1 0 - y ~ - l O - ~ Omol/cm'. A multicompartnient trough has been designed 'ind used to ~ I l o ua series o l chemical reactions to be performed upon a welldefined rnonolaqer film. The alkaline hydrolqsis of I(bpy)2Ru"[bpy(COOR)~]12+i n monolayer filnis ( R = CIKH37) is found to proceed inore rapidlq than in homogeneous. aqueous solution ( R = C2H;) a t the same bulk p H . Monitoring the stabilitb of surface films and their h>drol!iic reactions by chromatograph! has proven jupcrior io observing ;\rea changes a t constant surface pressure.
There is renewed interest in studying chemical reactions of surfactant molecules in monolayer assemblies. Use has been made of the high concentration of reactive functionalities constrained in a single plane to produce reaction products and kinetics different from what are observed in homogeneous solution'.' or to achieve ultrathin. oriented polymer fi1ms.j The detection and measurement of chemical reaction in monolayers has been generally carried out with the same tools that are used to study the properties of the monolayer itself e.g., surface area-pressure isotherms, surface potential, and surface viscosity.x However, the variation of monolayer composition and identification of new products cannot be directly obtained from such measurements. When the surfactant reactant and/or product possess characteristic chromophores then visible and infrared spectrometry have provided information concerning the extent of reaction or the nature of the final products. In general. spectroscopic studies alone are not likely to provide either a unique identification or a measure of thc absolute amount of material taking part in a chemical reaction. This is especially true when the reactants and products are structurally similar. This report will demonstrate the use of a multicompartment monolayer trough designed to allow a series of chemical reactions to be perfornied upon a well-defined monolayer and the application of high-performance liquid chromatography (LC) and vapor-phase chromatography (VPC) to separate and quantitate the resulting products. These techniques will be used to compare the hydrolysis kinetics of the surfactant diester derivative of ruthenium(l1) tris(bipyridyl), I , to that of the water-soluble analogue, I I . 0002-7863/79/ I501 -0001 $01 .00/0
Experimental Section Materials. The synthesis and characreri7ation of compounds I~ I\' have been described' with the exception of V \+hose structure was assumed on the similaritq of its abaorption spectrum to IV and i t a intermediacq in the hbdrollsis of I I to 111. The specific preparation of IV used for t h i b report was sqnthcsi7ed a s the perchlorate salt and contained 5 mol Oh I and 9 mol 96 ti-octadecanol b) 1.C analqcis. 11Octadecanol (Sigma. containing 1.3 mol "6 ti-hcxadecanol and 0.9 mol YO n-eicosanol), a11 organic solvents (Burdick and Jackson). and ail inorganic compounds (reagent grade) \+ere used a \ obtained. Tripl) distilled nater \cas used in the preparation o f t h e monolayer subphase solutions. Multieornpartrnent %lonolayer Trough Design. The monolayer trough shonn in Figure I has been fabricdted rrom clear. acrylic . plastic sheet by solvent welding trough side \rails ( I .91 cni ~ l i d e 0.48 c m high) onto a single piece base (244 cm X 61 cin X I .91 e m : h i n Plastics. Inc.. Mount Vernon, X . Y . ) supported on a stone table top o f similar dinienaions. The top and inncr surfaces of the side ~ ~ 1 arc 1 s made hydrophobic by rubbing with paraffin ( m p 60-63 "C). When the trough is filled with distilled water to give a convex meniscus. the continuous surface area is I I 900 cni? (233.7 cni X 50.9 c m ) and the depth is ca. 0.55 cm. The subphase voluine is divided into five unequal compartments by four glass subsurface ~ i i l l s( I .26 cm wide X 0.45 cm high) which extend the inner width of the irough and are fixed in
0 1979 American Chemical Society
1
*[Fl;' a
2
Journal of the American Chemical Society
/ 101:l /
TROUGH SIDE WPLLS
~
BARRIER DISTANCE POTENTIOMETER ASSEYBLI
-
M O ~ 4 8 L ESURFbtEBPRRIERS 16 om COYPARTYE NT 1
--
509tm
~
_
_
~-
SUBSURFACE COYPPRTYENT WALLS Ism.6lcm l5im WPSH COYPPRTYENT 2 WASH
--
wiLnELYY PLATE-LVDT ASSEYBLY
January 3, 1979
--
TROUGH SIDE WALLS
--+
~
B P R R I ~ D R I V EC E L E
61 m
COYPPRTYENT
3
DIPPING WELL BARRIER DRIVE
. CABLE-YOTOR
-
PSSEYBLY
DIPPING WELL
ACRYLIC TROUGH BPSE
SUPPORT TPBLE
BPRRIER DRIVE CPBLE
CPBLE TENSION SPRING
10 20 SCPLE kml
0
Figure 1. Multicompartment monolayer trough r R E E D SPRING ASSEMBLY CABLE
SURFACE PRESSURE
1 r
----LVDT S I G N A L CONDITIONING
TOP VIEW
LVDT EFFECTIVE LENGTH ADJUSTMENT
F L E X I B L E CONNECTION
PHOSPHOR-BRONZE REED SPRING SURFACE A R E A
[LVOT E L E C T R I C A L CONNECTORS
IOOK
-LVOT
X - Y RECORDER
FRONT VIEW
BARRIER CABLE
I T E F L O N COVERED BRASS BARRIER
FILTER PAPER WILHELMY P L A T E
MOTOR S P E E D BARRIER CABLE DRIVE
CONTROLLER
Figure 3. Schematic of electrical system for surface pressure. surface area. and barrier drive cable functions.
LVDT SIDE V I E W AIR
LUNNtLlll
Figure 2. Detail of LVDT-Wilhelmy
plate assemblq.
position with paraffin. While a thin film of water, ca 0.10 c m deep, covers the hydrophilic top surface o f the submerged dividing walls, the subphase solutions within each compartment may be filled and emptied independently. I n normal use. a removable hydrophilic glass strip (50.9 cm X 1.26 cni high X 0.45 cm Icide) is positioned vertically on top o f the subsurface barrier to minimize any interconipartment subphase diffusion. While not used in the present report. the trough is equipped with a dipping well (10.2 c m X 10.2 cm X 5.1 cni deep) at each end to allow for transfer of the monolayer off the surface onto a solid support. e.g.. glass slides. Surfactant films were constrained between the hydrophobic long sides of the trough and two movable surface barriers, defined a s the "LVDT barrier" and the"compression barrier" [27.7 c m X I .91 cm wide X 0.95 cm high, brass covered with Teflon (Temp-R-Tape Type T H . T h e Connecticut Hard Rubber Company. New Haven. Conn.)]. These barriers may be driven independently or together by securing them via cable clamps (see Figure 2) to two continuous loops of plastic jacketed aircraft cable (0.159 cm O.D.. No. AB046-N-7-7; W . M. Berg. Inc., East Rockaway, L. I.,N.Y.) running along each long side of the trough betNeen pulleys (7.1 I cm diameter) mounted on axles ( 0 . 9 5 c m diameter hardened stainless steel) a t the ends. T h e cables are driven by an electric motor (Model 548. Type N S H IZR, 1 /50 hp: Bodine Electric Company, New York. N . Y . ) coupled to one end axle via a timing belt (0.0816 in. pitch, 0.3 I3 in. wide) and pulleys (40 DP: motor, 5.0 c m diameter; cable axle, 7. I O e m diameter). T h e range of linear barrier travel speeds afforded by the motor's variable speed controller (Model 901, Bodine Electric Co.) is 0.75 to ca. 100 cm/min. T h e position of the cable clamped barrier(s) is determined by a pre-
cision tenturn potentionieter (100 KQ. Helipot) coupled to second end axle via a timing belt (0.08I6 in. pitch. 0.3 I3 in. wide) and pulleys (40 DP; potentiometer, 0.95 cni diameter: cable axle. 7.95 cm diameter) which can be disengaged when not required. As sho\\n in detail (Figure 2). one of the niovablc barriers (the "LVDT barrier") carries a Wilhelmy balance to monitor surface pressure. The design is similar to that previously described by Kuhnl" and Fromherz." A phosphor-bronze metal strip (14.0 cm X 0.60 c m X 0.038 I c m ) is fixed at one end and the effective length can be varied by moving a second clamping bracket (effective length used herein is 9.0 c m ) . The free end of the strip has ;I hook on i t 5 underside to suspend the Wilhelrny plate and a n iron core (0. I6 ciii diameter X 0.30 cm length) soldered to the t o p o f a brass s c r w (0-80) attached to its upper side. T h e core/screh, moves vertically inside a linear variable differential transformer ( L V D T Type 050 M H R ; Schaevitr Engineering, Camden. N.J.) mounted onto the barrier with an acrylic holder. A piece of filter paper [ I .Z cni (vertical) X 2.3 cm (hori7ontal): Schleicher and Schull. h o . 589 Black]. just barely dipped (ca. 0.2 c m ) into the subphase. is used a s the hydrophilic plate."' I z As shown in Figure 3. the L V D T iaconnectcd to a n A C excitation source and detector module (Signal Conditioning Unit Vfodel CAS0 2 5 R , Schaeviti Engineering) via two two-wire cables which can be disconnected from the barrier when not required. T h e DC output is fed to an electrometer (Keithley 610B) and plotted on the Y axis of an X-Y recorder (Hewlett Packard Model ZFAM). Whcn measuring surface pressure-molecular area (11-A) isotherms, the voltage from the barrier position sensing potentiometer~batterycombination (see Figure 3) is recorded on the X axis. The trough and barrier drive assembly are protected by four acrylic sheet covers (not shown in the figures) which slide along on Teflon glides and provide support, \ + i t h access to the surface. for a variety of accessories. The trough and its covers are not thermostated but arc
Valentj- / Chemical Hractions in M o n o l a j w Films i n ;I room where the ambient temperature (22 f I "C) is maintained bq a n air conditioner. Prior to attempting any monolayer work, the trough w a s filled with distilled water uhich u;is exchanged daily for ;i ?-week period in an attempt to remove any readily soluble cont a m i n a n t s in the plastic. L V D I U ilhelmy Balance Calibration. The surface pressure ciilibratioii of t h e barrier mounted LVDT Wilhelmy balance combination \+;is done by measuring the 11- .4 isotherm ofwoctadccanol on 10-3 M UaCI iit 2 2 " C and comparing it direct11 to that obtained under identical conditions with ;I conventioncil quart7 spiral Wilhclmy balance ( I l / A h = 2 l .7 dyn/cm/cm). I n both cases. the filter paper plate uiis totall! immersed in the subphase and alloucd to drain to ii constant \+eight while partiall) submerged and the resulting clean surf;icc reference position w a s noted. The film area was varied b) moving the "coniprc\sion barrier" Nhilc keeping the "LVDT barrier" fixed. Using the quart7 spiral. tlie surface pressure ;it 1%hich the change of slope occurs for octadccanol w a s found to be 13.8 d l n / c n i (lit.'? -13.5 d j n / c m ) . Bq referencing the LVDT balance generated surface pressure sc;ilc to this v:iIue a n overall I I - A isotherm of octadecanol identical to t11;it using t h e quart7 spiral was obtained. The molecular ;irc;is o f t h e l o ~ e and r upper branches of the I I - A curve extrapolated to I I = 0 are 2 I . 5 f 0.2 A2 and 20. I & 0. I A2, respectively. After obtaining ;I reference reading for it clean surface. day-to-day calibration ofthe recorded I 1 scale nits done b) adding standard weights to the hook suspending the pnrtiallj immersed paper plate (11. weight): 3-5.7 dqn/cm. 0. I628 g. and 15.4 d!n/cni. 0.07 I 4 g. Hydrolysis Procedure llsing the lb.lulticompartment Trough. Solutions of l .0X lo-' M \aCI ;ire added to compartment l a n d both of the \\ash cornp;irtnients. Aqueous N;iHCO?/NaCI (both I .O X 10-3 M, pH 8.2) and HClO., (1.0 X IO-' M ) a r c added tocompurtiiients 2 and 3. respectively. The surfiice of the subphase i n compartnient I is cleaned by repetitive compression barrier sueepings and contamination removed by aspiration before obtaining the reference surface pressure reading (11 = 0). With the t u o barriers separated by 70 c i i i ( L V D T barrier immobile. the compression barrier claniped to cable), 75 p L of a solutiori of I i n CHCI? (3.96 X 10-j M . sqnthesi7ed LIS the dichloride s;iItl4) is applied dropwise to the top o f a hydrophilic, rounded glass rod (0.4c m diameter) immersed in the subphase which extends c;i. I cni above the surface. After the addition is complete (ca. I inin). 5 0 p L of pure CHCI? arc used to rinse the glass rod before the latter is removed from the subphase. When cii. 3 niin had elapsed from t h e start of the addition. t h e film ~ i i compressed s ( I I 2 cm'/niin) to either I I = 10 or 33.3 d!n/ciii. The L V D T barrier u a s then clamped to the cable and the film moved ( 5 9 cm/niin) a t constant area iicross the first wash compartment onto the h a H C 0 3 / N a C I subphase. The reaction t i m e starts when the front edge of the film crosses onto the alkaline solution. The LVDT barrier w ~ i sthen released from the cublc and the compression barrier driven (human servo control) to keep 11 constant. After a time interval. the L V D T barrier was once again claniped to the cable. the film transported (59 cin/min) a t constant area across the second wash compartment onto the HCIOJ subphase. and the LVDT barrier released from the cable in order to maintain constant I I by driving the compression barrier. When the area became constant (usunllq within five m i n ) , both barriers u c r c moved apart to ca. 58-cni separation (still on compartment 3. I I observed to f ~ i l l to 7ero) and t h e "skim" surface barrier (27.7 c m X 0.95 cin X 0.95 cni: Teflon coated brass) placed on the surface directl! in front o f t h e LVDT barrier (-2 cm distance). Subphase solution w a s carefull\ removed (without disturbing the monolayer film) to lower its surface to the same level as the top of the trough side walls. T h e clamped compression barrier was then used to compress and collapse t h e film against the skim bllrrier until c;~.0.5 cm separation distance remained. After the t M o barriers a c r e c;ircfull! arranged intoan elongated "V", the "skim" spatula (btainlcss steel. I .0cni wide) was immersed into the subphase ;it the open end of that "V" ;it tlie trough side uaII sandwiched tightly between the tuo barriers. Thus. it can skim off the surface film while being d r a u n toward the closed end of the "V". T h e skimmed film \%ast h e n dissolved in ca. 2 niL of CHCI3. T h e area between the barriers uiis skimmed once again and the skim dissolved i n the same C H C l ? solution. A cotton tipped stick was then used to perform a final skim between the tightly closed barriers. I f a colored residue was visible on the swab, it was washed off into the original C H C I ? solution with a sniall volume ( < I mL) of fresh CHCI3.T h e few drops of supernatant aqueous subphase which was carried along into the vial were carefull) renioved with a syringe and the C H C I ? solution u a s evaporated to dryness using a i X 2 stream a t 45 "C. T h e
residue was dissolved in 50?6 aqueous T H F (0.015 M M e S O j H . 0.5'?6 H O A c ) just prior to analysis by HPL.C. The monolayer hydrolqsis of I V was done in the same manner having waited I 5 niin after applying 480 pL of a I .O m g / m L solution in C H C I ? to the surfiicc of I .O X IO-' .% NaClIi n compartment I . Bulk Aqueous Solution Hydrolysis. The appropriate volume of ;I 1.06 X IO-' M solution of I I I ' i n aqueous NaCl ( I O p 3 M ) toachieve a concentration of 1 X 10-5 to 5 X IO-' M was transferred to a 50-rnl. volumetric flask. diluted to the mark with aqueous NaHCO?/haCI (both 1.0 X IO-' M , p H 8.2). mixed. and maintained in t h c d a r k a t 22 f 1°C with agitation prior to every 150-pL aliquot removed for LC analysis. The hydrolysis was stopped upon injection into the acidic c h ronia t ogra p h y mob i I e p ha se . I Chromatographic .Analyses. High-performance liquid chroniatographic ( L C ) of I-V utilized a Waters Associates Model 244 liquid chromatograph, pBondapak/Clxcolumn (30 c m X 0.39 cm I.D.), and a n aqueouh tetrahydrofuran ( T H F ) solvent system with added alkyl sulfonate anions according to procedures previousl) described.'" n0ctadec;rnol \\as detected by both LC (pBond;ipak/CIx. ixicratic 25% aqueous T H F with no added alkyl sulfonates. 2.0 niL/niin, AKI detector, retention time = 2.0 min, minimum amount detectable = I .5 X IO-'] mol) and vapor phase chromatography (VPC). The VPC :inalysis eniploqed a Varian Model I740 gas chromatograph ( I83 cni X 0.2 cm I.D. OV-101 on 100/120 Var .Aport 30 glass column. 200 O C , 20 psi He, FID, retention time = 26.0 inin, minimum amount detectable = I X IO-" mol).
Results Trough Operations. Film Transfers. With 1 .O X I OP3 M NaCl in all compartments a film of I was formed on the surface of compartment 1 and compressed to II = 10.0 dyn/cm. After the film is shifted (59 cm/min) at constant area across compartment 2 onto compartment 3, the surface pressure is observed to decrease by 0.5 dyn/cm. Upon return to compartment 1 an additional decrease of ca. 0.3 dyn/cm has occurred. Variations of 50.5 dyn/cm, which are apparently due to slight distortion of the trough dimensions, are also observed as the LVDT barrier is moved along the length of the trough (stopped for measurement of n) in the absence of a surface film. With 1 .O X IO-' M NaCl in compartments 1 and 2 and the intervening wash compartment, a film of n-octadecanol was formed on compartment 1 and compressed to II = I O dyn/cni leaving a film-free surface area between the first subphase wall and the compression barrier (ca. 30 cni separation). With the vertical barrier in place on top of this first subphase wall, a concentrated, deep-red solution of methyl orange in distilled HzO was carefully pipetted into the subphase of Compartment 1 forming a band ca. 3--4 cm wide across the entire width of the trough adjacent to the subphase wall. The vertical barrier was slowly removed allowing a continuous Mater surface to connect Compartments 1 and 2. After standing ca. 5 min, it was clear that the dye was diffusing down the length of compartment 1 but was not diffusing sufficiently rapidly (if at all) across the subsurface wall to be observed visually in the wash compartment (a length of white paper had been placed between the transparent trough base and the supporting table for this experiment). The film of octadecanol was moved (59 cm/min) at constant area from compartment 1 to compartment 2. While there was no apparent dye diffusion into this latter compartment, a yellow coloration of decreasing intensity (going toward 2) was observed in the wash compartment. After several film transfers back and forth between 1 and 2, dye is observed in 2. Hence, there is little or no subphase mixing between the major compartments accompanying a single, one-way transfer of a surface film by this qualitative observation. Trough Operations. II-A Isotherms. A film of I (3.05 X IO-' mol) was formed on 1 .O X IO--7 M NaCI in compartment 1 and its I I - A isotherm was measured. This & A curve is shown (Figure 4) in comparison to that obtained with a calibrated quartz spiral Wilhelmy balance. The surface pressure was then reduced to ca. 2 dyn/cm and the film moved at fixed area
4
Journal of the American Chemical Society 40
'7-
30
-
101:l
/ January 3, 1979
""t
5 2
/
20-
-c 0 t
-1
20
I
i
lot I
0; 10
-
50
! 100 150 HYDROLYSIS TIME ( M I N I I
L--+ 200
a
Figure 5. Plot of percent area vs. hydrolysis time for I as ii function of pH at constant surface pressure (22-3 "C). All percentages expressed relative to original area ( A 0 = lOO%) observed for I on 1 .O X I O v 3 pI.l h a C I in compartment I . Key:++, 1.0 X V iiaHC03/NaCI. pH 8.2. I O dyn/cm; a,1.0 X M boratc/KCl. pH 9.0. IOdyn/cni: m3 1.0 X IO-? M borate/KCI, pH 10.0:---8--. 1.0 X IO-) M NaHC03/NaCI, pH 8.2.
33.3 dyn/cm.
Figure 4. LVDT-Wilhelmy balance measured P A isotherm5 (22 "C) of I on I .O X M NaCl in compartment 1 and after being shifted onto I .O X IO-? M HC104 in compartment 3 (compression rate i n both cases I08 cm2/min), solid lines. Data obtained using quartz spiral Wilhelmq balance for I on M NaCI. 0 .
Table 1. Hydrolysis of I in Monolayers, I1 = I O d y n / c m u time. min
I mol x IO9
0 2 2 8 h I5 3.5 213 5.0 196 7.0 186 161 10.0 12.5 133 15.5 102 22.5 78 30.5 45 37.6 35 37.6 32 44.8 19 60.8 17 75.2 IO 90.3 IO 120.5 6 150.5 6 228 5
IV re1 re1 mol % mol x io9 mol %
[ I l l ] = 100 -([I] + [IV]] re1 mol oh
0 20 47 52 65 84 91
5 15
100h 93 86 81 71 58 45 34 20 15 14 8 8 5 5 3 3 2
100 1 I8 1 I5 I21 103 101 72 82 42 47 38
0 9 20 23 29 37 40 44 52 51 53 45 44 32 36 18
21 17
-77
28 34 33 47 48 63 59 79 76 81
Monolayer Il = 10.0 f 0.5 dyn/cm. I .O X IO-' M aqueous N a H C 0 3 / N a C I , pH 8.2, 22--23 "C. Absolute recovery of I at t = 0 is 78 f 5 mol 96 (average of seven experiments): 292 X 1 OWy mol of I originally applied t o surface.
across M NaCl in compartment 2 onto IO-' M HC104 in compartment 3. After standing 5 min, a second &A curve, also shown in Figure 4, was recorded. Trough Operations. Hydrolysis of I at Constant n. Films of I (3.05 X IO-' mol) were formed on 1.0 X M NaCl in compartment I , compressed to II = I O dyn/cm, and moved across the wash surface onto various alkaline solutions ( 1 .O X M NaHC03/NaCI, pH 8.2; 1.0 X IO-3 M borate/KCl, pH 9.0; 1.0 X IO-' M borate/KCl, pH 10.0) contained i n
compartment 2. Keeping ll constant ( 1 0.0 f 0.5 dyn/cm), the film shrinkage relative to the original area observed on IO-' M NaCl was monitored as a function of time (Figure 5 ) . After an initial rapid decrease, the shrinkage slowly approaches the area expected for a completely hydrolyzed film (two octadecanol molecules per molecule of I ) . In one long-term experiment utilizing 1.0 X IO-' M N a H C 0 3 / N a C I , aliquots ( I O mL) of this subphase were withdrawn periodically from under the film up to a hydrolysis time of 150 min and shown to have a pH (8.1 f 0.1) invariant with time. Trough Operations. Recovery of Films for Chromatographic Analysis. Films of I (3.05 X IO-' mol) were formed on I .O X 1 O-' M NaCl i n compartment 1, compressed to Il = I O dyn/ cm, and transferred at fixed area across IO-' M NaCl containing subphases onto 1 .O X I OW2 M HClO4 in compartment 3. After moving the barriers to their maximum separation (ca. 58 cm; TI = 0 a t barrier separations 2 3 5 cm), the "skim barrier" was placed ca. 2 cm in front of the LVDT barrier such that the Wilhelmy balance assembly would not obstruct film recovery. It has been demonstrated by sprinkling talc onto the surface that the film (at n = 0 dyn/cm) remains stationary while moving the LVDT barrier back ca. I O cm. Hence, no film is trapped between the LVDT and skim barriers. The film is compressed and collapsed into visibly reddished threads between the skim and compression barriers. The film is then skimmed off and analyzed quantitatively by LC. The average absolute recovery of 1 observed for seven separate trials was 78 f 5 mol %with no hydrolysis product (IV) present. Films of n-octadecanol (6.00 X lo-' mol) manipulated i n the same fashion showed absolute recoverys of 80, 96 (two experiments only), and 90 f 3 mol % a s analyzed by LC and VPC, respectively. The recovery efficiency of an n-octadecanol film spread, collapsed, and recovered from IO-' M HC104 was the same (90 mol %) as that observed for films transferred from other compartments. Hydrolysis of I in Monolayer Films. Stable films of I could be formed on M NaCl and hydrolyzed according to the procedure given i n the Experimental Section. The results are given i n Tables I and I I and plotted for 11 = I O dyn/cm in Figure 6. The kinetic rate constants are summarized in Table IV. Under the LC conditions used for analysis of the surfactant ruthenium compounds, I I I elutes at the column void volume
Valentjs
/
5
Cheniical Reactions in Monolayer Films
Table I I . Hqdrolysis of I in Monolabers. I I = 33.3 dyn/cni"
I ti iiic. iiiin
I
I
I
I
I
1
I
I
1
Y
I
I
100
IV
mol x 10"
re1 iiiol YO
0.0 3.2 5.2 7. I 10.2
234 210 161 I42
I 00h 90 69 60
I 55
66
15.2
I05 93 57
45 40 24
11101
x
IO"
90
re1 mol 91
~
20.0 30.2
0
0
II 19 33
12
80
5 I4 7
18 24 32 23
10 14 10
Monolayer: II = 33.3 f 0.5 dyn/cm. I .0X I O - ' M N a H C O j / NirCI. pH 8.2. 22-23 OC. Absolute recover) of I at r = 0 is 77 mol YO. 305 X I O - " niol of I originall! applied to surl'acc.
70 I-
z 60
W V W rT
50 2
I 0
40 30
Table 111. Hvdrolysis of IV in Monolayers" Iv
20
tiiiie. mol
iiiin
x
10'
161 k 7 IO4 89 64 27
0 39.4 61
90 I65
re1 mol
"41
I 00h 65 55 39 17
*
Mono1a)cr: 11 = 10.0 0.5 d\n/cni. I .0X 10-3 44 Y a H C 0 3 / 3aCI. pH 8.2. 21-22 OC. Absolute recover! of IV at r = 0 is 5 I mol (I
?O.
with other components ( T H F oxidation products) and cannot be routinely measured, In the expectation that this watersoluble product would diffuse away from the surface film during the course of the reaction, the entire bicarbonate and wash (separating compartments 2 and 3) subphase volume (ca. 2.5 L) was recovered from two of the experiments, concentrated to dryness with the rotary evaporator, and analyzed by LC using conditionsIh for water-soluble ruthenium derivatives. The absolute recoveries of I I I for hydrolysis times of 75.2 and 150.5 min are 37 and 3 1 mol %, respectively. N o control experiment was done to determine how much 111 was lost during the recovery, concentration, and other experimental manipulations. Due to the lack of precision in analyzing for n-octadecanol in the presence of ruthenium compounds by LC (differential refractive index detector) at this sample level and the apparent thermal degradation of I to n-octadecanol in the hot injection port of the gas chromatograph, VPC analysis for the octadecanol was done only after a single long-term hydrolysis to confirm its presence as the major product. A film of I was formed on IO-? M NaC1. hydrolyzed on M NaHC03/ NaCl and then on ca. IO-' M N a O H until no further area shrinkage was noted, and recovered from IO-' M HCIOJ. The n-octadecanol reaction yield was found to be 59 mol %. Hydrolysis and Stability of IV in Monolayer Films. A film of I V (3.1 X IO-' mol) formed on IO-' M HC104 and compressed to n = I O dyn/cm did not exhibit any decrease in area with time (30 min) and gave an absolute recovery for IV of 90 mol %. I n contrast, a second film, identical i n composition, showed a slow shrinkage a t II = I O dyn/cm on IO-' M NaCl over 30 min (the least-squares determined first-order rate constant for shrinkage is 3.6 X IO-3 min-I, r2 = 0.98) and gave a 43 mol % recovery of IV after shifting the film onto aqueous HCIOJ. Upon compression to II = 33.3 dyn/cm on IO-' M NaCI. a third film showed an even faster rate of shrinkage (estimated first-order rate constant 1 2 min-I) and allowed only a 4 mol % absolute recovery after 30 min (as usual the film was shifted to aqueous H C I 0 4 for skimming).
IO
0
0
I
I
20
40
60 80 100 120 H Y D R O L Y S I S TIME ( M I N . 1
",
I 140
228
Figure 6. Relative concentrations of reactant ( I . 0 )and products ( I V . A ; 111. m) during hydrolysis on 1.0 X IO-? M NaHC03/NaCI (10.0 f 0.5 dqn/cm, 22-23 "C. initial amount of I is 3.0 X IO-' niol at a surface concentration of 6.7 X I O i 3 molecules/cm2) as determined by LC. The amounts of I l l were calculated from 100 - I[I] t [ I V l l .
Table I \ . Kinetic Sumni,irI H\droILu$ of I J n d IV in MonolJ\er\ reaction description [compd. 11 (dbn/cni). reaction time ( m i n ) ]
I . I I = IO. 0-45. h y d r o l y s i ~ " ~ I . I I = IO. 45-1 50. h>drol>sish.[ I . II = 33.3, 0-30. hqdrol>sish IV. I1 = 10. 0-165. hydrolysish IV. II = 10. 0-30. dissolution(/ IV. I1 = 33.3. 0-30, dissolution" I V . I 1 = 10. 0-30. dissolution"
X - 1 ( r? ) 1'
x 10'niin-1 5.3 (0.99) I .I (0.89) 4.6 (0.97 j
k-lI(r+ 10: niin-1
x
0.90 (0.76)
I . I ( I .00) 0.40 (0.95) 2 200 0
Pseudo-first-order rate constants derived for the disappearance of I and IV by fitting the LC concentration (hydrolysis) or surface area (dissolution) data to a n exponential curve by a least-squares linear regression; r 2 is the coefficient o f determination. Subphase: I .0 X M h a H C 0 3 / S a C I . pH 8.2. 22-23 " C . T h e plot of log [ I ] vs. time shows an intersection point at I = 45 min. (/ Subphase: I .O X I O - ? M YaCI. 22 O C . Subphase: I .0 X I O - ? M HCIOJ. 22 O C . ('
The results for the hydrolysis of I V on IO-' M N a H C O j / NaCl are shown in Table 111 and the derived kinetic rate constants summarized in Table IV. Hydrolysis of I1 in Homogeneous Aqueous Solution. The hydrolysis of I 1 (1.0 X IOp4 M ) in aqueous alkaline solution to I l l is shown in Figure 7 . ' " The time dependence of the reaction composition can be calculated for consecutive pseudofirst-order reactions with k l = 8.62 X IO-' min-' and k2 = 2.6 X IO-' min-I. The close fit of theory with the experimental points is evident from Figure 7 . The hydrolyses of several concentrations of I 1 ( 1 .O X 1 0V5 M , 1 .O X 1 OWh M. 5.0 X 1 O-' M) in 1.0 X IO-' M N a H C 0 3 / Y a C I (pH 8.1 & 0.1) follow similar kinetics at a slower rate as shown in Table V.
Discussion Lnromarograpny ana neacrions in ivionoiayer r irrns. I L iiau been previously observed that the area of a monolayer of I
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Journal of the American Chemical Society
/
101:l
/ January 3, I979
Table V. Kinetic S u m m a r v : Hldrollsis o f I 1 i n .Aqueous Solution [Illo.niol/L." 1.0 X I O - 4 h 1.0 x 10-5 1.0 x lo-"
5.0 x IO-'
II k l . min-1
-+
c'
8.6 X IO-? 4.9 x 10-3 5.2 x 10-3 5.0 x 10-7
v
- v Ill k,. rnin-' -f
r21'
0.99 1.00 0.99 0.97
kl/k2
2 . 2 x IO-?
3.9
I .o x 10-2
4.9
2.0 x lo-' -7 x 10-4
2.6 -7 ~~~
(' Rcaction tcmpcraturc = 21 - 2 2 O C .
~~
1.0 X IO-? M boratc/KCI. p H 10.0. 1.0 X 10-3 hl YaHCO3/NaCI, p H 8.2. Data were fit io a n cxponcntial curvc [ I l l = [ll]oe-Al' by a least-squares linear rcgres\ion: r 2 is t h e cocfficient of determination. P k ? was obtaincd by a trial and crror fitting ol' the concentration data of V and the calculated k I to the consccutivc. pseudo-first-urdcr reaction equation: [ V I = [ I l l , , [ h i / ( h ? - h I ) ] [ ~ min-I. A separate first-order kinetics, k - l ~ =~ 9.0 series of experiments starting with I\' gives an almost identical first-order rate constant. k-l\clb\d = I . I X IO-' min-l. These observed rate constants must be correctcd for the dissolution of 1V into the subphase during the hydrolysis as estimated b) ~l) on 1.0 X IO-' the first-order film shrinkage ( k - l ~ \ ~ observed M NaCl (II = 10 dyn/cm): k-l\ = k-l\ilb'd - k.-r\,'"' = ( 1 . 1 - 0.4) X IO-' min-l = 7.0 X IOp3 min-l. LC analysis of films recovered from various subphases clearly confirms dissolution of IV from the monolayer in the case of 1.0 X IO-3 M NaCI. Thus, a comparison of the nionolayer and bulk phase h q drolyses (pOH 5.8) shows qualitative agreement with theory: k-1 = 10k-11and k - 1 1 = 7 k - 1 . I t is clear however that the observed rate constants for the hydrolyses in monolayers are lower than expected from the calculated surface pOH. A second comparison also shows a qualitative agreement with theory: k - l / k - l v = 7.6 (experiment) vs. 4.4 (calculated).
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Journal of the American Chemical Society
An alternative, but similar, comparison is suggested by the rate enhancing effect of incorporating a small amount of quaternary amine surfactant into a film of octadecyl acetate undergoing alkaline hydrolysis by Davies." The disappearance of I according to two rate constants may reflect the higher surface charge density found i n the early portion of the sequential hydrolysis reactions ( t < 20 min) where the doubly charged I predominates than that at longer reaction times ( t > 40 min) where I is only a minor component in the mainly singly charged film. The calculated rate ratio,
is lower but close to the value observed, 4.8. Lastly, the surface change density may be increased substantially bq raising the surface pressure at which I is hydrolyzed. For films of I at 11 = 33.3 dyn/cm pOH is I .6 (i = 2, A = 90 A?, c = 2 X 1 0-3 M) in comparison to pOH I .9 obtained previously for n = I O dyn/cm. As before, the theoretical value.
and the experimentally derived ratio, 0.9, show little quantitative agreement. I t is of import to note that a comparison based only on film shrinkage (Figure 5 ) would suggest a faster reaction proceeded at the higher surface pressure whereas quantitative analysis of the recovered films show little, if any, difference. LC analysis clearly shows that I V has a very high rate of dissolution at 33.3 dyn/cm (Table IV) and a constant, equilibrium concentration of IV in the surface film is attained very rapidly i n the hydrolysis reaction starting with I (Table 11). The deviations of the experimental values from those calculated m a y reflect the interfacial ion concentration uncertainties caused by slow diffusion away from the surface of both I l l and IV. orientation changes at the interface upon going froni I to I V , as well as deficiencies in the theory treating monolayer films of ionized surfactant molecule^.^^ U I evertheless, the rate enhancement obtained by having a positively charged surface film rather than a neutral film is striking. I n order to achieve experimentally reasonable rates for the hydrolysis of ethyl octadecanoate, Alexander and Rideal'-' w r e required to employ I I\; N a O H (pOH O!) in the subphase. The hydrolysis rate constant observed, k = 2.09 X IO-' min-' (21.2 O C . 3 dyn/cm), lies between k-1 and k-11 obtained in this study on a subphase having a much lower hydroxyl ion
/ 101:l / January 3, 1979
concentration, pOH 5.8. Furthermore, while a tenfold difference was noted between the rates of hydrolysis for the positively charged ester contained in the surface film and in homogeneous solution, Alexander and Rideal reported for neutral esters that the reaction velocity and activation energy were approximately the same in both environments.'" Acknowledgment. The author wishes to thank G . L. Gaines, Jr., for his encouragement and helpful discussions leading to the preparation of this report, P. E. Behnken for experimental assistance, and J . E. Girard and T. Takekoshi for allowing him generous use of their high-performance liquid chromatographs. References and Notes (1) B. E. Horseyand D. G. Whitten, J. Am. Chem. SOC.. 100, 1293 (1978). (2) D. G. Whitten, F. R. Hopf, F. H. Quina, G. Sprintschnik, and H. W. Sprintschnik, Pure Appl. Chem., 49, 379 (1977). (3) D. Naegele and H. Ringsdorf, J. Polym. Sci.. Polym. Chem. Ed., 15, 2821 (1977). (4) V. Enkelmann and J. 8.Lando, J. Polym. Sci., Polym. Chem. Ed., 15, 1843 (1977). (5) S. A. Letts, T. A. Fort, Jr.. and J. B. Lando. J. Colloid Sci., 56, 64 (1976). (6) C. Rosilio and A. Raudel-Teixier, J. Po/ym, Sci., Polym. Chem. Ed., 13, 2459 (1975). (7) A . Dubault, C. Casagrande. and M. Veyssie, J. Phys. Chem., 79, 2254 (1975). (8) G. L Gaines, Jr.. "Insoluble Monolayers at Liquid-Gas Interfaces", Interscience, New York, N.Y., 1966. (9) G. L. Gaines, Jr., P. E. Behnken, and S. J. Valenty, J. Am. Chem. SOC.. 100, 6549 (1978). (10) H. Kuhn, D. Mobius, and H. Bucher in "Techniques of Chemistry." Vol I, Part 1118, A. Weissberger, Ed., Interscience, New York, N.Y.. 1972, p 651. (11) P. Frornherz, Rev. Sci. Instrum., 46, 1380 (1975). (12) G. L. Gaines, Jr.. J. Colloid lnterface Sci., 62, 191 (1977). (13) W. D. Harkins and L. E. Copeland, J. Chem. Phys.. 10, 272 (1942). (14) Compound Ib in ref 9. (15) Compound V in ref 9. (16) S. J. Valenty and P. E. Behnken, Anal. Chem., 50, 669 (1978). (17) D. 0. Shah and J H. Schulman, J. Colloid lnterface Sci., 25, 107 11967). (181 I. Langrnuir and D. F. Waugh. J Am. Chem. SOC., 62, 2771 (1940). (19) (a) P Frornherz, Biochim. Biophys. Acta, 225, 382 (1971): (b) P. Fromherz and D. Marcheva. FEES Lett., 49, 329 (1975): (c) J. Peters and P. Frornherz, Biochim. Biophys. Acta, 394, 11 1 (1975). (20) (a) S. J. Valenty, 7th Northeast Regional Meeting of the American Chemical Society. Albany, N.Y., August 8-11, 1976, No. 261: ( b ) S. J. Valenty, Macromolecules, to be published in Nov.-Dec., 1978 issue. (21) A. A. Frost and R. G. Pearson, "Kinetics and Mechanism", Wiley, New York, N.Y., 1953. p 153 ff. (22) J. T. Davies. Adv. Catal., 6, 1 (1954). (231 G. L. Gaines, Jr.. J. Chem. Phys., 69, 2627 (1978). (24) A. E Alexander and E. K . Rideal, Proc. R. SOC.London. Ser. A, 163, 70 (1937). (25) J. E. Charbonneau and G. N. Kowkabany, J. Colloid lnterfac. Sci., 33, 183 (1970). (26) NOTEADDEDIN PROOF: For a recent elegant use of VPC to study a chemical reaction in monolayers adsorbed onto a Pt surface, see M. A. Richard, J. Deutch, and G. M. Whitesides, J. Am. Chem. Soc.. 100, 6613 (1978).
