Langmuir 1994,10,3797-3804
3797
Spreading of Clay Organocomplexes on Aqueous Solutions: Construction of Langmuir-Blodgett Clay Organocomplex Multilayer Films N. A. Kotov, F. C. Meldrum, and J. H. Fendler* Department of Chemistry, Syracuse University, Syracuse, New York 13244-4100
E. Tombhcz and I. DBkany Department of Colloid Chemistry, Attila Jdzsef University, Aradi Vt., t. l., H-6720 Szeged, Hungary Received April 1, 1994. I n Final Form: June 23, 1994@ Dilute dispersions of hexadecylammonium (1) and dioctadecylammonium hectorite (2) clay organocomplexes, dispersed in butanolhenzene mixtures, have been spread on the surfaces of water and aqueous electrolyte solutions in a Langmuir trough. Surface pressure (n)us reduced surface area (AR)and surface potential us AR isotherms, in situ reflectivity and Brewster-angle microscopic measurements, ex situ transmission electron microscopy, and small-angle X-ray scattering (SAXS) measurements established the formation,at the aqueous solution-air interface, of a layer of overlapping clay platelets with thicknesses ranging from 50 to 130 A. Langmuir-Blodgett clay organocomplex multilayer films have been prepared from 1 and 2 by Z-type deposition onto solid substrates.
Introduction Clay mineral complexes, sometimes referred to as pillared clays, and clay organocomplexes' have been extensively investigated as suspensions in liquids.2-11 Particular interest in clay organocomplexes has been prompted by their controllable interlayer (basal) distances and surface hydrophobicities,chemical and photochemical stabilities, and relative ease of preparation. No attempt has been made, however, to spread clay organocomplexes on aqueous solution surfaces and to transfer them, layer by layer, to solid substrates. Encouraged by the reported formation of mono- and multiparticulate layers of polystyrene mi~rospheres,'~-~* silylated glass beads,15 and surfactant-coated semiconductor and magnetic nanopartic1es,l6J7we have initiated studies on the spreading of clay organocomplexes in a Langmuir trough. Our inspiAbstract published inAdvance ACSAbstracts, August 15,1994. (1)The term clay organocomplexis used here to describe clay platelets which are interconnected by flexible surfactant chains (see Figure 1). @
Importantly, clay organocomplexes undergo reversible swelling in organic solvents (Le., their interlayer distances,the basal spacing (dL), change to a different extent in different solvents). In contrast,in pillared clays (Le., in clay mineral complexes) the interlayer distances between the platelets are fixed. (2)Thomas, J. M. In Intercalation Chemistry; Whittingham,M. S., Jacobson, A. J., Eds.; Academic Press: New York, 1982;p 55. (3)Thomas, J.M.; Theocharis, C. R. In Modern Synthetic Methods; Sheffold, R., Ed.; Springer Verlag: Berlin, 1989;Vol. 5, p 249. (4)Dekany, I.; Nagy, L. G.; Schay, G. J. Colloid Interface Sci. 1978,
-,
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GR 197 I
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I.; Szlnt6, F.; Nagy, L. G. Progr. Colloid Polym. Sci. 1978.65.125. (6jDekany, I.; Szanto, F.; Weiss, A.; Lagaly, G. Ber. Bunsen-Ges. Phys. Chem. 1986,89,62. (7)DBkany, I.; SzBnt6, F.; Weiss, A.; Lagaly, G. Ber. Bunsen-Ges. Phys. Chem. 1986,90,422. (8) DBkBnv, I.: Szant6. F.: Weiss. A.: Laealv. G. Ber. Bunsen-Ges. Phys. Chem.-l986, 90,427. (9)Dgklny, I.; Nagy, L. G. Colloids Surf. 1991,58, 151. (10)Pratum, T. K. J . Phys. Chem. 1992,96,4567. (11)Carminati,S.;Carniani, C.; Miano, F. Colloids Surf. 1990,48, 209. (12)Sheppard, E.; Tcheuredjian, N. Kolloid 2.2.Polym. 1968,255, 162. (13)Garvey, M.J.; Mitchell, D.; Smith, A. L. Colloid Polym. Sci. 1979,257,70. (14)Schuller, H. Kolloid 2.2.Polym. 1967,216-217, 380. (15)H6rvolgyi,Z.; NBmeth, S.;Fendler, J. H. Colloids Surf A 1993, 71,327. ( 5 ) Dekany,
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ration has been derived from nature's method of manufacturing laminated inorganic composites, such as shells and bones, by biomineralization, these materials possessing exceptional structural and mechanical properties.l8Jg The spreading of hexadecylammonium- and dioctadecylammonium-ion-exchanged hectorites from waterimmiscible organic liquid mixtures on a variety of aqueous solutions and the investigation of the behavior of the clay organocomplexfilms formed are the subject of the present report. Ultrathin clay organocomplex films have been characterized by surface pressure us reduced surface area and surface potential us reduced surface area isotherms, in situ reflectivity measurements, Brewster-angle microscopy, ex situ transmission electron microscopy, and small-angle X-ray scattering (SAXS) measurements. Quantitative, layer-by-layer transfer of the clay organocomplex films generated on aqueous solutions to solid substrates has also been demonstrated.
Experimental Section The preparation, purification, and characterization of hexadecylammonium (1) and dioctadecylammonium (2) hectorite organocomplexes have been described elsewhere.20Water was purified by a Millipore Milli-Qsystem. Cadmiumnitrate (Fisher), sodium nitrate (Fisher), and potassium nitrate (Fisher) were used as received. Clay organocomplexsuspensionswere prepared by sonication inmixturesofbutanol-benzene. Typically, a 0.05-0.1 gsample of 1 was sonicated in 20-25 mL of butanol-benzene (l:l, v/v), and 0.05g of 2 was sonicated in 20 mL of butanol-benzene (3:7, v/v) either in a Branson-2200 ultrasonic cleaner for 24 h or by a Bransonic 1150 sonicator (150 W) for five sessions of 20 min, allowing the dispersion to swell for 24 h between sonications. Dispersions of 1 remained stable for weeks, whereas 2 remained (16)Kotov, N.A.;Meldrum, F. C.; Wu, C.; Fendler, J. H. J. Phys. Chem. 1994,98,2735. (17)Meldrum, F. C.;Kotov, N. A,; Fendler, J. H. J . Phys. Chem. 1994,98,4506. (18)Heywood, B. R.; Mann, S. Adu. Mater. 1994,6 , 9. (19)Lowenstam, H. A.: Weiner, S. On Biomineralization: Oxford University Press: Oxford; 1989. (20)Kotov, N.A.;Putyera, K.; Fendler, J. H.; Tombacz, E.; Dgkany, I. Colloids Surf. 1993,71,317.
0743-7463f94f2410-3797$04.50~0 0 1994 American Chemical Society
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3798 Langmuir, Vol. 10,No. 10,1994 homogeneous for only 2 h; shaking the flask resulted, however, in the redispersion of 2. Trace amounts of water adversely affected the stability of colloidal dispersions of 1 and 2. An appropriate amount (20-750 pL) of the suspension of 1 or 2 was spread on the surface of water or aqueous electrolytes in the Langmuir balance. Surface pressure us reduced surface area (or surface potential us reduced surface area) isotherms were taken subsequentto 30 min of incubation to allow the spreading solvent to evaporate. Note that n-butanol may dissolveinto the bulk subphase and adsorb to some extent at the interface. A homemade Brewster-angle microscope (BAM) was used to obtain images of the spread clay organocomplex films on the aqueous surfaces in the Langmuir film balance. Illumination was provided by a Hughes (3235 HP-PC) He-Ne (632.8nm, 20 mW) CW laser, focused to a 1.3 mm spot. After being passed through a tunable foil polarizer (NRC RSA-2) to ensure ppolarization, the laser beam impinged upon the aidwater interface at the Brewster angle (ca. 53"). The refracted beam was absorbed by a piece of black Teflon, placed at the bottom of the trough. The beam reflected from the absorbed water surface coincided with the optical axis of the detection system [a lens of focal length f = 24 mm and a small CCD camera (MTI CCD 72, sensitivityca.0.002lux)]. The images were viewed on a monitor and videotaped on a VHS VCR (JVC HR-S6700U). A similar setup was used for in situ reflectivity measurements. A 5 mW HughesHe-Ne laser was mounted on a rotator. The CCDcamera was replaced by a Spectra Physics 404 laser power meter, connected to a digital voltmeter. The intensity of the light reflected from the water surface was measured in 0.5-1.0" intervals. The intensity of the incident laser beam was taken at 10-20 points prior and subsequent to the measurements. The laser stability was within f3%. The obtained reflectivity curve was fitted by SigmaPlot 5.0software and the model based on the classicalFresnel equations.21The refraction indexand thickness of the clay particulates served as fitting parameters. The error of the latter was assessed by assuming a 5% deviation of the from the mean values. experimental data (Mo) Surface pressure (n)us reduced surface area (AR)isotherms were determined by using a Lauda film balance at 22 "C. The cm2/gscale (instead of the customaryA2/particle)was used in all ll us AR isotherms to circumvent the inaccuracy in determining the number of clay particles in a given volume of dispersion. Surface potentials at the aqueous solution-air interface were measuredby the vibrating plate method.22The surfacepotentials of the spreading solution were subtracted from the reported surface potentials of the clay organocomplex dispersions in the n us AR plots. Hydrodynamic diameters of dispersed 1 and 2 particles were determined by dynamic light scattering using a Brookhaven BI 2030 autocorrelator and a Spectra Physics 200 Argon ion laser as the light source at different sampling times (10-50 s). The random deviation of each measurement around the mean value did not exceed 10%. X-ray diffraction measurements were taken on a Phillips PW 1830diffractometer (Cu Ka, A = 1.54A). The periodicities were calculated from the peak positions by the Bragg equation, using the PW 1877automated powder diffraction program which has an accuracy of f0.1A. Small-angle X-ray scattering (SAXS) measurements were performed by using the compact Kratky Camera (Model KKK 1129; Anton Parr Co.).
Results and Discussion Properties of Clay Organocomplexes. Hectorites are layered aluminosilicates with exchangeable cations distributed over their external and internal surfaces. The layers, composed of sheets (each having edges of ca. 100 (21) Born, M.; Wolf, E. PrincipZes of Optics; Perganon Press: New York, 1965. (22) We are grateful to Professor Roger Leblanc for allowing us to use his facilities at the Centre de Recherche en Photobiophysique (Universitedu Quebeca Trois-Rivieres, Trois-Rivieres, Quebec, Candada G9A 5H7)and to Ms. Lei Shao for practical assistance.
A
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Figure 1. Schematic illustration of a stack of clay organocomplexes,swollen in an organic solvent (e.g. a 1:l v/v butanol: benzene mixture, the spreading solvent). d~ = basal spacing, and dfilm = the thickness of a stack of clay organocomplexes spread on an aqueous electrolyte. Depending on the extent of swelling and on the surface pressure, dfilm can contain 3-5 silicate layers (seealso Figure 14). Thecationicalkylammonium surfactants (indicated by the squiggly lines) replaced the exchangeable cations on the surfaces of the silicate layers. nm x 100 nm x 1nm), are stacked parallel to one another to form plate^.^^*^^ In water, the hydrophilic hectorites (prior to exchanging their cations by surfactants) used in the present work were found to give stable and optically transparent dispersions at a wide range of concentrations. Mean hydrodynamic diameters of aqueous hectorite dispersions were determined to be 165 f 10 nm by dynamic light scattering. Considering the intrinsic deviations from spherical symmetry and polydispersities of the clay particles, these values are in reasonable agreement with those cited above. Hectorite organocomplexes are formed by exchanging the cations on the clay platelets by hexadecylammonium (1)and dioctadecylammonium (2) cations.20 Schematics of a stack of swollen clay organocomplexes are shown in Figure 1. In contrast to untreated hectorites, the hydrophobic clay organocomplexes1 and 2 could not be dispersed in water. A 1:l (v/v)mixture of butanol and benzene was found to be the best dispersing medium for 1. The mean hydrodynamic diameter of 1 in this liquid mixture was determined to be 190f 10 nm at nearly infinite dilutions. 2 could be best dispersed in a 3:7 butano1:benzene mixture. The mean hydrodynamic diameter of 2 was determined to be 650 f30 nm, also at nearly infinite dilutions. These measurements indicated negligible association of 1 in the butanol-benzene dispersing medium, in contrast to 2, which is likely to be present as agglomerates of several primary platelets. (23)Van Olphen, H.AnIntroduction to CZuy Colloid Chemistry; WileyInterscience: New York, 1977; 2nd ed. (24)Newman, A. C. D. Chemistry of CZays and Clay Minerals; Mineralogical Society: London, 1987.
Spreading of Clay Organocomplexes
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Figure 3. Incident angle-dependentreflectivitiesof 1 (2.0 g/L M Cd(NOd2 in Bu0H:benzene= 1:1,v/v) on aqueous5.0 x at ll = 40 mN/m. Black dots = experimental points; solid line = obtained by curve fitting using nfilm = 1.51 and dfilm = 118 A as adjustable parameters; and broken line = bare water.
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Figure 2. Surface pressure us reduced surface area isotherms for the spreading of 1 (5.0 g L in Bu0H:benzene = 1:1, v/v) on a water subphase. Stopping the compression at surface pressures below the collapse of the film permitted the observation of repeated reversible expansion-compression cycles; expansion = broken line (A). Conversely,no reversibility was observed if the films were compressed fullyt o collapse pressure (and beyond). Repeated compression isotherms shifted progressively to smaller surface areas (B-G). A Brewster-angle microscopic image of 1 (curveA, at II = 5 mN/m) is illustrated in the insert.
Spreading of Hydrophobic Clay Organocomplexes on Water Surfaces. Spreading of 1 and 2 resulted in the formation of whitish patches on the water surface. These patches were much thicker than those observed in the formation of surfactant monolayers. Applying increasing surface pressure of 1,up to 25 mN/m, caused the patches to coalescceinto a denser, rather contiguous film (curveA in Figure 2). As the pressure was decreased, the compressed layer of 1 did not return to its initial state, but expanded slightly, remained intact, and covered approximately30-50% of the water surface. Compression of 1 on water surfaces to a pressure of 25 mN/m resulted in almost reversible isotherms (broken line, curve A in Figure 2). The continued exponential rise of the surface pressure with very little change in the surface area was observed for greater degrees of compression until IT = 40 mN/m was reached. At this point, a marked decrease in the slope of IT us AR curve occurred (see isotherm B in Figure 2),which signaled the collapse of the film. Visually, some creasing of the clay organocomplexfilm was observable in the IT = 28-40 mN/m surface pressure range, and the collapse manifested in the formation of a band of crumpled film next to the moving barrier in the Langmuir trough. Crumpling only occurred in the vicinity of the moving barrier; the rest of the area retained precollapse appearance. Recompression shifted the ll us AR isotherm by an area equal to that reduced during the compression. Similar behavior was observed upon repeating the compression (to collapse pressure), expansion, and recompression cycle several times (B-G in Figure 2). Apparently compression beyond IT = 40 mN/m resulted in irreversible changes of the clay organocomplex films. Compression of 2 on water to 25 mN/m caused a reversible thickening of the film, whereas irreversible film collapse occurred if the surface pressure was increased to
45 mN/m or above. The behaviors of 1 and 2, in this respect, were quite similar. Brewster-angle microscopic images of 1 on water a t IT = 1-2 mN/m revealed the presence of irregular 100-300 pm aggregates (see insert in Figure 2). Attempts to determine the thicknesses of the films formed from 1 and 2 on water surfaces by reflectivity measurements were unsuccessful due to the lack of uniformity a t all surface pressures. In summary, although reproducible ll us AR isotherms were obtained on spreading 1 and 2 on water surfaces, lack of uniformity and the formation of films which were thicker than desired encouraged us to select a better spreading condition. Spreading 1 on aqueous electrolyte solutions proved to be satisfactory. Results of these experiments are described in the next section. SpreadingClay Organocomplexeson Electrolyte Solutions. The appearance and mechanical properties of spread clay organocomplexes (prepared from 1) were found to be profoundly affected by the introduction of electrolytesintothe subphase. The presence of electrolytes in the subphase had no marked effect on the IT us AR isotherms of the more hydrophobic 2. The presence of 1 over an aqueous electrolytesolution,in the expanded state, was not observable by the naked eye. This implied, of course, that the thickness of organoclay films, spread on electrolyte solutions, had to be in the nanometer range. The measured in situ incident angle-dependent reflectivities were in accord with this expectation. Thickness and refractive indices of the clay organocomplexes were evaluated by fitting the measured reflectivities,R, into eq 1using nfilm and dfilm as adjustable parameters:21
21r01121r112cos2p> (1)
where 18 = 2;~dltfil~dfil~~1/3,,T I = (nfilm COS 8 - Cl)/(nfilm COS 8 + d, r 2 = ( n w a t e g l - n f i l m c d / ( n w a t e g l + n f i l m c d , ~1 = (1 -(sin2 8)/nfi1m2))1’2, c2 = (1- (sin2 8)/nwak?))1’2,nfilm and d m m are the refractive index and thickness of the particulate film, 8 is the incident angle, and 3, is the wavelength of incident light (632.8 nm). The pair of ltfilm and dmm,correspondingto the minimal sum of normalized deviations of experimental points from the theoretical curve, was taken as characteristic of the film. The experimentwith a clean water surfaceyieldedthe expected values for the best fit ( n = 1.33 f 0.003 and d x 0). Similarly, experimentally determined, incident angledependent reflectivities of surface-coated CdS particles spread on water could be best fitted with values of n =
3800 Langmuir, Vol. 10,No. 10, 1994
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M Cd(N0312 at II Figure 4. Brewster-angle microscopic images of 1 (2.0 g/L in Bu0H:benzene = 1:1,v/v) on aqueous 1.0 x Cd(N03)2at II = 50 mN/m (D). The dimensions of the = 2.0 mN/m (A), ll = 10 mN/m (B),and n = 20 m/m (C) and on 5.0 x rectangles on the images are 2.0 mm by 1.5 mm.
Table 1. Reflectivity of Hexadecylammonium Hectorite Films (1) on Electrolyte Surfaces AR = 14 x lo5 cm2/g AR = 7 x lo5 cm2/g
dfilm
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(A>
electrolyte
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nfiim
Cd(N03)210 mM Cd(N03)250 mM Cd(N03)2100 mM Na(N03) 10 mM Na(N03)50 mM
1.51 1.50 1.50 1.55 1.62
50
1.53
128
48 42 98 88
1.51
118
1.58 1.50 1.64
60 134 127 0
1.78 f0.02 and d = 3.8 f 0.4nm, which agreed well with those determined by independent methods (TEM). Furthermore, a series ofnumerical modulations has shown that values for nfilm and dfilm can be determined from a single reflectivity curve with a low risk of degenerate pairs (secondary minima), provided that the precision of the experiment is within 5%. The results of the reflectivity measurements (Figure 3) are presented in Table 1. The error margins for both ltfilm and dfilm are &lo%. Thicknesses in the 50-130 range clearly imply that spreading of 1 results in the formation of 5- 13intercalated sheets of organoclay particles which are lying on their faces parallel to the electrolyte solution surface. Brewster-anglemicroscopic images have indicated that clay organocomplex films are quite uniform if they are spread over aqueous Cd(N03)2 (Figure 41, even a t high surface pressures (Figure 4D). Surface pressure us surface concentration isotherms also reflected the differences in the spreading behavior of 1 over electrolyte^^^^^^ (see Figures 5-7). In general, increasing electrolyte concentrations are expected to promote electrostatic repulsions and, hence, to increase the surface pressure a t a given surface concentration.This (25)Gaines, G.L.,Jr.Insoluble Monolayers at Liquid-Gas Interfaces; Interscience Publishers: New York, 1966. (26)Ulman, A. An Introduction to Ultrathin Organic Films from Langmuir-Blodgett to Self-Assembly;Academic Press: San Diego, CA, 1991.
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1 O s AR, cm2/g Figure 5. Surface pressure us reduced surface area isotherms for the spreading of 1 (5.0 g/L in Bu0H:benzene = 1:1,v/v) on (B), 1.0 x (C), pure water (A) and on aqueous 2.0 x M NaN03 (D)solutions. and 5.0 x
behavior is likely to arise from cation adsorption from the subphase onto unexchanged sites on both the external and internal surfaces of the organoclay platelets, thereby increasing the surface charge density. Increased charge repulsion induced a partial disaggregation of the clay agglomerates and, thus, increased the number of particles floating on the aqueous solution surface, and promoted the formation of a better ordered film. The order of effectiveness (Na+ < K+ < Cd2+;Figure 7) followed the well-known order of affinity of these cations to clay particle^.^^^^^ The inflection points in the II us AR isotherms are determined by a delicate balance between the interparticle attraction and repulsion forces. The rise of I3 for 0.05 M NaN03 (Figure 5 ) is much more gradual than that for 0.05 M Cd(NO& (Figure 6). This can be attributed to a more substantial contribution of shortrange attraction forces for advanced stages of compression over an aqueous NaN03 subphase. (27)Matijevic, E.Surface and Colloid Science; Plenum Press: New York, 1982;Vol. 12.
Spreading of Clay Organocomplexes
Langmuir, Vol. 10,No. 10, 1994 3801
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1 0 ' AR. cm2/g Figure 9. Surface pressure us reduced surface area (A) and surface potential us reduced surface area (B) isotherms for 1 (5.0 g/L in butano1:benzene = 1:l v/v) on aqueous 1.0 x M Cd(N03)2. Zero potential corresponds to the interfacial potential drop at the 1.0 x los2 M Cd(NO3)dair interface.
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Figure 6. Surface pressure us reduced surface area isotherms for the spreadingof 1 (2.0 g/Lin Bu0H:benzene = 1:1, v/v) on 1.0 x 2.0 x pure water and on aqueous 5.0 x 5.0 x los2, 0.1, and 0.5 M Cd(NO& solutions(from left to right).
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10' AR, cm'/g Figure 10. Surfacepressure us reduced surface area isotherms for the spreadingof 1 (5.0 g/Lin Bu0H:benzene = 1:1, v/v) at initial surface areas (AR) of 11.0 x lo6 cmz/g (A), 22.0 x lo5 cm2/g(B),and 73.5 x lo6 cm2/g(C).
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l o 5 AR. cm'/g Figure 7. Surface pressure us reduced surface area isotherms for the spreadingof 1 (5.0 g/Lin Bu0H:benzene = 1:1, v/v) on M N d 0 3 (B), 1.0 pure water (A) and on aqueous 1.0 x x 10-2MKN03M(C),and 1.0 x 10-2M(D) Cd(N03)~ solutions.
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Figure 8. Surface potential us reduced surface area isotherms for 1 (5.0 g/L in butano1:benzene = 1:lv/v) on aqueous 8 x M KNo3 (B),and 8 x M NaN03 M Cd(N03)~ (A), 8 x
(0.
Measurements of ll us AR isotherms confirmed the differential adsorptions of cations on 1 floating over electrolyte solutions. Surface potentials at a low surface area increased in the order of Na+ < K+ < Cd2+ (see Figure 8). The crossover of the curves occurs when the surface pressure increases due to the change of adsorption/ desorption equilibrium upon the agglomeration of the particles caused by the short-range attraction of the
hydrocarbon tails. The surface potential was significantly more sensitive than the surface pressure to the transformation of the film structure. Particularly, at large surface areas, the increase of the surface potential was found to be much more pronounced than that of the surface pressure (Figure 9). The position of the ll us AR isotherms a t a given electrolyte type was found to depend markedly on the initial concentration of 1 introduced onto the film balance (Figure 10). Apparently, there is no preferred stacking number; the total available area influenced the position of the surface pressure isotherms and the morphology of the organoclay platelets. Transmission electron microscopy (TEM) provided information on the structure of 1 on aqueous 1.0 x M Cd(N0& a t different surface pressures (Figure 11).At relatively low surface pressures (ll = 2 mN/m; AR = 23 x lo5cm2/g),organoclay platelets appeared to be oriented entirely parallel to the water surface (A and B in Figure 11)and to cover the surface by a contiguous film. In this range of surface pressure, it was difficult to identify the borders between separate organoclay particles. Distinct changes in coloring and the presence of elongated dark zones indicated the presence of stacks of overlapping organoclay platelets of different heights or the folding of particles (Figure 11B). The sizes ofthe grains constituting the organoclay film, determined from TEM images (Figure 11A,B),were on the order of 150-200 nm, as is consistent with values determined by dynamic light scattering (vide supra). During the first stages of compression (n = 10 mN/m), multilayer organoclay aggregates formed which were thicker than the surrounding film (see Figure 11C). Upon further compression (n = 20 mN/m), the number
3802 Langmuir, Vol. 10,No.10,1994
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Figure 11. Transmission electron microscopic images of 1, transferred to carbon-coated copper grids, subsequent to spreading M Cd(NO& solutions, at ll = 2 mN/m, AR = 23 x lo5 cm2/g(A (3.0 g/L of 1 in Bu0H:benzene = 3:7, v/v) on aqueous 5.0 x and B, top left and right, respectively), ll = 10 mN/m, AR = 14 x lo5 cm2/g(C, middle left), ll = 20 mN/m, AR = 11 x lo5 cm2/g (D, middle right), ll = 50 mN/m, AR = 6.5 x lo5 cm2/g(E, bottom left), and ll = 70 mN/m, AR = 1.3 x lo5 cm2/g(F, bottom right). The bars are 500 nm (A), 100 nm (B), 1pm (C), 500 nm (D), 500 nm (E), and 500 nm (F).
of thick multilayer aggregates significantly multiplied (Figure 11D) and crumpled. At higher pressures (n= 50 mN/m), some reorientation of the platelets occurred due to turnover, elastic deformation, and crumpling (Figure 11E),leading to an almost disordered film as collapse took place (n = 70 mN/m; Figure 11F). It is essential to note that, at any stage of compression, no light domains were observed from which clay particles would have been excluded. Such domains were observed for many other solid particle monolayers.16 Consequently, compression of free surfactant floating on the surface has a negligible contribution to n. However, a t high AR and electrolyte concentrations, some surfactant molecules may become desorbed from the clay platelets, but the adsorptiondesorption equilibrium shifts toward the original clay organocomplex upon compression of the film.
Construction of Langmuir-Blodgett Clay Orga-
nocomplexMultilayerFilms. Langmuir-Blodgett clay organocomplexmultilayer films could be readily prepared from 1 and 2 by Z-type deposition onto solid substrates. Z-type deposition is prevalent for monolayers which are prepared from surfactants whose hydrophobic hydrocarbon chains are terminated by a weak polar g r ~ ~ p . ~ ~ Transfer to a substrate was monitored by the absorption spectra of rhodamine 6G, adsorbed onto the clay organocomplexes from the aqueous subphase (which contained ca. M rhodamine 6G; Figure 12). Remarkable linearity was observed in the successive transfer of clay organocomplexes. The clay film transfer efficiency was close to 1.0 at n = 30 d / m . Generally, the clay platelets are much easier to transfer to a solid support by the LB technique than are other solid particles. Clay organocomplex films compressed over 5.0 x lov2 M Cd(NO& and subsequently transferred to quartz
Langmuir, Vol. 10,No. 10,1994 3803
Spreading of Clay Organocomplexes
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NUMBER of LAYERS Figure 12. Dependence of the differential absorption peak height at 532 nm (rhodamine 6G containing LB films-blank LB films) on the number of layers (indicated on the z axis)of 1 (A) and 2 (B)transferred to a quartz plate at ll = 30 mN/m. The insert shows the absorption spectra (absorption us wavelength) of a diluted solution of rhodamine 6G in water (A)and adsorbed onto 5 (B),4 (C),3 (D), 2 (E), and 1 (F)layers of 1 in air. 40000
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Figure 13. SAXS diffraction patterns of 10,14,and 20 layers of clay organocomplexeson a quartz substrate. For clarity, the curves for 14 and 20 layers have been shifted upward along they axis. substrates were examined by small-angleX-ray scattering (SAXS).This method allowed the visualization of spacial periodicity of electron density distribution in transverse direction to the film. SAXS patterns for 10, 14, and 20 layers of clay organocomplexes, deposited a t 35 d i m , are shown in Figure 13. The position of the peak for different numbers of layers varied in a small interval of 28 = 1.9-2.1" which corresponded to a periodicity with a step of 43-45 A in an excellent agreement with the film thickness (see d a m in Figure 1) determined by angledependent reflectivity measurements (48A, Table 1). The observed periodicity could not correspond to the basal spacing (see dL in Figure 1) since dL for alkylammonium clay organocomplexes are in the range of 25-30 A.zoThus, the clay organocomplex layers do not form a continuous stack of platelets upon their Langmuir-Blodgett-type deposition but are interspaced with adsorbed cations
Figure 14. Schematic representation of spreading of 1 on an aqueous electrolyte solution at very low (A), medium (B),and high (>40 mN/m; C) surface pressures. (cadmium in the case of the sample used for SAXS) and their hydration shell.
Conclusion The most significant result of the present work is the demonstration that clay organocomplexes, dispersed in a n appropriate organic solvent, can be spread on aqueous electrolyte solutions. Spreading results in the formation of a film composed of 5-13 intercalated sheets of clay organocomplex platelets. At relatively low surface pressures, the clay organocomplex film consists of individual separate platelets, some of which are in domains. With increasing surface pressure, surface coverage becomes more complete and sheets of neighboring clay organocomplexes intercalate to a greater extent and experience
3804 Langmuir, Vol. 10,No.10,1994 elastic deformation. Compression to pressures greater than 40 mN/m results in the irreversible collapse of organocomplexplatelets and in the subsequent formation of disordered structures. An oversimplified schematic diagram of this process is illustrated in Figure 14. Importantly, it has also been shown that layer-by-layer transfer of the organoclay platelets to solid substrates provides LB-type structures with linearly variable thicknesses. This, in turn, allows the preparation of electrodes,28coated by ultrathin clay organocomplexes in a (28) Bard, A. J.;Mallouk, T.InMoEecularDesignofElectrode Surfaces; Murray, R. W., Ed.; Wiley: New York, 1992; p 270.
Kotov et al. controllable and reproducible manner, and opens the door to the construction of novel sensing and electrochemical devices.
Acknowledgment. Support of this research by grants from the National Science Foundation and the U S Hungarian Joint Fund (Hungary; JF No. 227/92) is gratefully acknowledged. We thank Mr. LBsz16 Turi for his help in the X-ray diffraction measurements. Note Added in Proof. This work was presented a t the Sixth International Conference on Organized Molecular Films (LB6; Trois-Rivieres, Canada; July 4-9, 1993).
