Synthesis and characterization of self-assembled hydrophobic

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Langmuir 1993,9, 1232-1240

1232

Synthesis and Characterization of Self-Assembled Hydrophobic Monolayer Coatings on Silica Colloids Sergio Brandriss and Shlomo Margel' Department of Chemistry, Bar-Ilan University, Ramat-Gan 52900, Israel Received September 17, 1992. In Final Form: February 16, 1993

Monodispersedsilica colloidsof various diameters (0.25-1.4km) have been coated by the self-assembled technique with a monolayer of hydrophobic surfactants, such as SiCb(CH2),CH3, Si(OMe)3(CH2),,CH3, SiMe2Cl(CH2),,CH3(n= 3,7,17),and HO(CH2)&H3. Methods,suchaselementalanalysis,Fouriertransform infrared spectroscopy,and thermogravimetric analysis have been used for quantitative characterization of these coatings. Other methods, such as advancing contact angle, complete spreading surface tension, and floatability measurements have been used for studying the wettability properties of these coatings. Conditionsto obtain reproduciblecoatingshave been established. The effect of heating of these particles on coating quality has been demonstrated. The range of critical surface tensions of these coatings has also been estimated. This research has demonstrated that the coatings prepared from the longer chain alkylsilane compounds are superior to coatings prepared from the short ones and that the superiority of the coatings is in the rank order of SiCldCH2),,CH3> HO(CH2),,CH3> SiMe2Cl(CH2)nCH3> Si(0Me)s(CHz),,CH3. These studies have led to the synthesisof optimal hydrophobicmonolayer coatingsonto silica colloids.

Introduction Monolayer coatings are often deposited onto colloidal particles (alternativenames: microspheres,nanoparticles, etc.) by physical adsorption and/or covalent binding of appropriate surfactants on the colloid surfaces.13 These coatings are useful for many purposes, i.e. stabilization of microspheres in aqueous and/or organic solvents,2biomedical studies (controlledrelease, drug delivery, models for studying biomolecules adsorption onto microsphere surfaces, et^.),"^ and industrial uses (paints, viscosity reducers, etc.).819 Few of the routine methods to characterize monolayer coatings on flat surfaces, e.g. ellipsometry,lO ESCA at variable angles," polarized FTIR/ATR spectroscopy,12 and contact angle measurements12on a single microsphere, are not useful for colloidal particles. On the other hand, because of the spherical shape and high surface area of polymeric microspheres, other characterization methods which are not efficient,or commonly used, for flat surfaces, i.e. elemental analysis and floatability measurements,13-16may be efficient for characterization of monolayer coatings on colloid surfaces. In this article the synthesis, characterization, and comparison of various hydrophobic monolayer coatings on silicacolloidsof differentdiametersare described.These

* Author to whom correspondence should be addressed.

(1) Candau, F.; Ottewill, R. H. An Introduction t o Polymer Colloids; Kluwer Academic Publishers: Dordrecht, 1990, pp 1-34. (2) Badley, R. D.; Ford, W. T.; McEnroe, F. J.; Assink, R. A. Langmuir 1990, 6, 792. (3) Margel, 5. J. Polym. Sci., Chem. Ed. 1984,22, 3521. (4) Charmot, D. Prog. Colloid Polym. Sci. 1989, 79, 94. (5) Kreuter, J. J.Microencapsulation 1988, 5 (2), 115. (6) Couvreur, L.; Treupel, L. R.; Poupon, M. F.; Brasseur, F.; Puisieux, P. Adv. Drug Delivery Rev. 1990,5, 209. (7) Omullane, J. E.; Davidson, C. J.; Petrak, K.; Tomlinson, E. Biomatertals 1988, 9, 203. (8)McGrath, J. E. J. Chem. Educ. 1981, (Nov), 844. (9) Kim, J. H.; El-Aasser, M. S.; Klein, A.; Vanderhoff, J. W. J.Appl. Polym. Sci. 1988, 35, 2117. (10) McCrackin, F. L.; Passaglia, E.; Stromberg, R. R.; Steinberg, H. L. J. Res. Natl. Bur. Stand., Sect. A 1963, 67, 363. (11) Hazel], L. B.; Brown, I. S.; Freisinger, F. Surf. Interface Anal. 1986, 8, 25. (12) (a) Ulman, A. An Introdution to Ultrathin Organic Films, from Langmuir-Blodgett t o Self-Assembly; Academic Prerrs, Inc.: New York, 1992; pp 48-58. (b) Tillman, N.; Ulman,A.; Schildkraut, S.; Penner, T. L. J. Am. Chem. SOC. 1988,110, 6136.

coatings have been prepared by the self-assembled t e c h n i q ~ e , through ~ ~ - ~ ~ the interaction of octano12 and alkylsilane compounds, such as SiC13(CHdnCH3,Si(OMe)s(CHz),CHa, and SiMe&I(CH2),CHS ( n = 3,7,17), with silica microsphere surfaces. Quantitative characterization of these coatings has been performed by methods, such as elemental analysis, Fourier transform infrared (FTIR) spectroscopy, and thermogravimetric analysis (TGA). Characterization of the wettability of the coatings has been performed by methods such as advancing contact angle measurements on coated colloid pellets, completespreadingsurfacetension, and floatability measurements.

Experimental Section Materials. All solvents used were HPLC grade,and all other reagents were analytical grade. Bicyclohexyl (BCH) was passed through basic alumina (Woelm-Pharma, W200 super) before use. Toluene was distilled over sodium. Pure water was obtained b y passing deionized water through an Elgastat Spectrum reverse osmosis system (Elga, Ltd., High Wycombe, U.K.). SilicaColloids. Two sourcesof monodispersedsilicacolloids have been used in these studies: (a) Monodispersed particles of 0.25,0.7,and 1.4 p m average diameter obtained b y courtesy of E. Merck, Darmstadt, Germany.20 These particles were washed free of solvent (ethylene glycol) by repeated centrifugations with water until HPLC studies indicated the complete removal of ethylene glycol. The particles were then freeze-dried. (b) Monodispersed silica colloids of similar sizes (0.25p m average diameter and a standard deviation of 0.02 pm; 0.7 p m average diameter and a standard deviation of 0.03 pm; 1.4 p m average (13) (a) Marmur, A.; Dodiuk, H.; Pesach, D. J. Adhes. 1987,24, 139. (b) Marmur, A.; Chen, W.; Zogrdi, G. J. Colloid Interface Sci. 1986,113 (l), 114. (14) Garsva, S.; Contreras, S.; Goldfarb, J. Colloid Polym. Sci. 1978,

256, 241.

(15) Yarar, B.; Kaoma, J. Colloids Surf. 1984, 11, 429. (16) Mutchler, J. P.; Menkart, J.; Schwartz, M. Adu. Chem. Ser. 1967, No. 86, 7. (17) Ulman,A. An Introdution to Ultrathin Organic Films, from Langmuir-Blodgett to Self-Assembly; Academic Press, Inc.: New York, 1992; pp 237-304. (18) (a) Maoz, R.; Sagiv, J. J.Colloid Interface Sci. 1984,100 (2), 465. (b) Pomerantz, M.; Segmuller, A.; Netzer, L.; Sagiv, J. Thin Solid Films 1985,132, 153. (19) Waeserman, S. R.; Tao, Y. T.; Whitesides, G. M. Langmuir 1989, 5, 1074. (20) Unger, K.; Giesche,H.;Kinkel, J. Ger. Pat.Appl.l985,DE3534143.

0743-7463/93/2409-1232$04.00/0 0 1993 American Chemical Society

Langmuir, Vol. 9, No. 5,1993 1233

Monolayer Coatings on Silica Colloids diameter and a standard deviation of 0.07 pm) were prepared by polymerizationof tetraethylorthosilicate, accordingto the Stober method.21 Briefly, particles of 0.25 pm average diameter were synthesizedby adding tetraethylorthosilicate to ethanol solution containing water and ammonium hydroxide. The final solution concentrations of tetraethylorthosilicate,water, and ammonium hydroxide were 0.14,1.2, and 0.87 M, respectively. The resulting solution was then shaken at room temperature (ca. 25 “C)for 12 h. The formed microspheres were washed by evaporation of the unreacted monomer and ethanol and then by repeated centrifugationswith water. The particles were than freeze-dried. Silica particles of larger sizes were formed in a similar procedure by increasing the molar ratio of water/ammonium hydroxide. Characterization of t h e SilicaColloids. The diameter and standard deviation of the particles were determined by scanning electron microscopy (SEM, JEOL, JSM-840 instr~ment).~ Density of the microspheres was measured by density gradient, using standards based on mixtures of methylene bromide and carbon tetrachloride.22 Porosity and surface area were measured by the BET method.23 CoatingsProcedure. (1) Coatings Prepared from Octanol and Alkylsilane Surfactants. Dried silicacolloids were added to the appropriate solvent. The resulting mixture was then sonicated for 30 min. The coating of the dispersed particles was then accomplished by adding the appropriate surfactant to the stirred air-free microspheressuspension. The reaction continued at the desired temperature for the determined time period. Unreacted surfactant was removed by successive centrifugations with the solvent. The washed microspheres were then boiled with methylene chloride, ethanol, and acetone, respectively,and then oven-dried at 60 OC. (2) Coatings Prepared from Polyacrylonitrile. Dried silica microspheresprepared accordingto Stober method (50mg) were coated with polyacrylonitrile (Aldrich) in ethanol (1% solution) at room temperature for 5 h. The microspheres were washed free of unreacted polyacrylonitrile by successive centrifugations with ethanol and then with acetone. Then, the microspheres were oven-dried at 60 “C. The adsorption of polyacrylonitrile on the particles was confirmed by IR absorption peaks at ca. 2240 cm-1 (-CNband) and 2865 and 2940 cm-’ (-CHz- bands). Characterization of the Coatings. (1) Fourier Transform Infrared (FTIR). FTIR spectra were collected on a Bomem MB-100 spectrometer with a MCT detector. These measurements were performed at room temperature with KBr pellets. Each pellet contained 2 mg of coated microspheres and 200 mg of KBr (FTIR grade). These pellets were prepared under a pressure of 40 OOO lb/in.2 for 10min and then kept in a desiccator. (2) Thermogravimetric Analysis (TGA).TGA meaaurementa were done on a Perkin-Elmer TGS-2, Model 3700. Dried samples of about 10 mg in nitrogen atmosphere were held at 100 OC for 15 min, and the temperature then increased at a rate of 20 “C/min up to 500 O C . The reported values are an average of measurementsperformed on at least three samplesof each coating and have a maximum error of about 6%. (3) Elemental Analysis. The percent carbon (% C) of the coatings was obtained through servicesof the analyticallaboratory of the Hebrew University, Jerusalem. The reported values are an average of measurements performed on at least three samples of each coating and have a maximum error of about 5% (4) Wettability Measurements. Wettability studies of the coatings were accomplished in the following ways: (4A) Contact Angle Measurements. Advancing contact angle measurements (and complete spreading surface tension measurements) were performed on pellets of 6 mm diameter prepared from dried-coated microsphere powders. These pellets were formed under a pressure of 6000lb/in.2 for 10 min and then kept in a desiccator. SEM photomicrographs showed that under these conditions the silica microspheres of the formed pellets did not fracture and kept the originalshape and morphology. Contact angles were measured with a Rame-Hart Model 100contact angle

.

(21) Stober, W.;Fink, A,; Bohn,E. J.Colloid Interface Sci. 1968,26, 62. (22) Unger, K. K.Porous Silica; Eleevier Scientific Publishing Co.: Oxord-New York; 1979; pp 15-56. (23) Low, B. W.; Richards, F. M. J. Am. Chem. SOC.1962, 74,1660.

goniometer equipped with an environmental chamber, as previouslydescribed.12 The humidity in the environmental chamber was 100%. Water advancing contact angle measurements were performed with 10-pLwater drops a t three different locations of each pellet. The reported values are an averageof measurements performed on at least three pellets of each coating and have a maximum error of about 4 % (4B)Complete Spreading Surface Tension Measurements.l3 Ethanol/water mixtures of various surface tension values were prepared and carefully stored, to avoid significant c k g e due to 6vaporation. Surface tensions of these liquids were measured at room temperature by the duNuoy method. A 10-pL drop of each of these liquids was placed on each tested pellet (one drop only on each pellet) in a monotonic order of decreasing surface tension. The highest surface tension of the liquid which led to complete spreading of the drop on the pellet surface was then noted. The reported values are an average of at least three experiments carried out with each coating and have a maximum error of about 3 % . (4C) Floatability (Total Floating a n d Total Sinking) Measurements.l”l6 Dried coated silica colloids were placed on amicroscopeglass slide and then smashed carefullywith a spatula in order to obtain a nicely dispersed powder. The dispersed powder was then placed on the surface of a series of liquids of progressively decreasing surface tension (prepared as previously described) maintained at room temperature in a 30-mL beaker (approximately5 mg of microsphereson each liquid). The lowest surface tension of the liquid which led to total floating of the particles and the highest surface tension of the liquid which led to total sinking of the particles were then noted. The reported values are an average of at least three experiments carried out with each coating and have a maximum error of about 3%. The complete spreading surface tension measurements and the floatability measurements are based entirely on visual inspection,since it is easy and accurate to determine the exiatence of complete spreading and/or total floating and total sinking.

.

Results and Discussion Typical SEM photomicrographs of silica colloids obtained from Merck and prepared according to the Stober

methodz1are shown in Figure 1. Both species of microspheres have a relatively narrow size distribution, approximately 3% standard deviation for “Merck”particles and about 8% standard deviation for the particles prepared by the Stober method. The measured density of both species of silica microspheres was approximately 2.0 f 5 % g/mL. The observed porosity was between 8 and 15 A, and the measured surface area was similar to the calculated one, demonstratingthe nonporousstructure of these microspheres. Both the extremely low porosity of these particles and their highly hydrophiliccharacter may assure that the interaction between the studied hydrophobic surfactants and the silica microspheres should preferably be on the particle surfaces. IR Absorbance of Bydroryl Groups on Silica Colloids. The IR absorbances of SiOH groups and water on silica particles have been extensively s t ~ d i e d . The ~~*~~ hydration/dehydration chemistry of the silica surfaces is demonstrated in Figure 2. The absorbances at approximately 3740cm-l and at approximately3660cm-l of dried silicaparticles are due to free silanols and hydrogen-bonded silanols, respectively (Figure 2A). Water molecules are physically adsorbed onto silica surfacesthrough ita silanol groups (Figure 2B). The water physisorption is uncontrolled and can reach several layers thick. The physisorption of water molecules onto silica is reversible (Figure 2B), and the multilayer physisorbed water molecules can easily be removed by degassing,as demonstrated in Figure (24) Hair, M. L.Infrared Spectroscopy in Surface Chemistry; Marcel Dekker: New York, 1967. (25) Angst, D. L.; Simmons, G. W. Langmuir 1991, 7, 2236.

1234 Langmuir, Vol. 9, No. 5, 1993

Figure 1. SEM photomicrographs of silica colloids of 0.7 pm average diameter obtained by Merck, (A, X25000; B, X5000) and prepared according to the Stober method (C, X5000).

3A,B. Upon heating, the surface silanol groups start to condense and eliminate water. However, the hydration/ dehydration equilibrium is completely reversible up to about 400 "C (Figure 2C). Upon further heating of silica, above approximately 850 OC, sintering starts to occur (Figure 2C). The silica therefore becomes hydrophobic and physisorption of water moleculesdoes not occur,since the hydration/dehydrationequilibrium becomes irreversible. Indeed,the hydroxyl groupsabsorbances completely disappear by heating the silica colloids at 900 "C, as demonstrated in Figure 3C. Coating Conditionsand Reproducibility. In water,

Brandriss and Margel

dried noncoated silica colloids can easily be dispersed to separatedparticles. However,in organicsolvents,e.g. BCH and toluene, these particles, because of their hydrophilic character, form small and large size aggregates,which can be easily examined by light microscopy. Our studies have demonstrated that during coating of silica colloids with hydrophobic alkylsilane surfactants in organic solvents, the microsphere aggregates break stepwise, and by the completionof the coatingprocess,fully dispersedseparated microsphere suspensions are usually observed. FTIR spectra of noncoated and OTS (octadecyltrichlorosi1ane)-coateddegassed silica colloids (0.25 pm average diameter) prepared according to the Stober method are shown in Figure 4A. The noncoated microspheres do not possess any significant absorption peaks in the range of frequenciesshown. The absorption peaks of OTS-coated colloids at ca. 2850 cm-l and at ca. 2920 cm-l correspond to -CH2- stretching bands. The absorption peak at ca. 2960 cm-l corresponds to the -CH3 stretching band. Previous studies have demonstrated that the asymmetric -CH2- stretching band at ca. 2920 cm-l can be selected as best suited for quantitative correlation of similar coatings prepared on solid The absorption peaks at ca. 2920 cm-l of OTS-coated degassed silica colloids (0.25 pm averagediameter) prepared by the Stober method isO.10-0.11, as shown in Table I. These absorption values are related to a measured percent C for these coatings of 2.1 % (Table 111),similarto the theoretical percent C (2.16) calculated for a close-packed OTS monolayer coating on silica colloids of 0.25 pm average diameter (Table 11).The absorption peak at ca. 2920 cm-l of OTS-coated nondegassed silica colloids prepared by the Stober method is 0.12-0.2 (Table I), indicating OTS coverage higher than a monolayer. These results may be explained by the multilayer physisorbed water molecules which were not removed before coating by degassing. The uncontrolled amount of physisorbed water molecules is probably the main cause for the broad range of absorbance values at ca. 2920 cm-l (0.12-0.2) measured for coatings of OTS onto nondegassed silica colloids prepared under similar experimental conditions. Indeed, previous studies have demonstrated that too much water in the silanation solution should be avoided during coating, in order to prevent self-polymerization of trichloroalkylsilane surf a ~ t a n t s . ~ ~This * ~ polymerization ~*~* process may happen through the formation of Si-&Si bonds before or after the binding of OTS to silica surfaces. FTIR spectra of noncoated and OTS-coated degassed silica colloids (0.25 pm average diameter) obtained from Merck are shown in Figure 4B. These noncoated colloids possess absorption peaks, at ca. 2875 cm-l and at ca. 2950 cm-l (Figure 4B(1)), respectively. These peaks may indicate that the silica colloids of Merck are stabilized by a hydrocarbon type surfactant/s, in contrast to the colloids prepared by the Stober method which are formed by a free surfactant dispersionpolymerizationprocess.' FTIR spectra of OTS coatings of Merck silica colloids showed inconsistent behavior (Table I), since under similar preparation conditions each coating experiment resulted in different FTIR absorption peak intensities. Figure 4B(2) demonstrates an FTIR spectrum of a specific experiment of OTS coating on these colloids. The absorption peak at ca. 2920 cm-l cannot be measured since this peak is overlapped with the absorption peak at ca. 2950 cm-l of the noncoated microspheres. The inconsistentbehavior (26) Nezer, L.; Iscovici, R.; Sagiv, J. Thin Solid Films 1983,99,235. (27) Margel, S.; Sivan, 0.; Dolitzky, Y. Langmuir 1991, 7, 2317. (28) Silberzan, P.; Leger, L.; Ausaerre, D.; Benattar, J. J. Langmuir 1991, 7, 1647.

Monolayer Coatings on Silica Colloids

Langmuir, Vol. 9,No. 5,1993 1235

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of OTS coatings of Merck colloids may be explained by the original coating of these particles which cover, at least partially, the silica surfaces. Indeed, a previous publication by Seliger et al.,29which described the use of these two silica colloid species (1.5pm average diameter) as solid supports for oligonucleotidesynthesis, concluded that the silicacolloidsof Merck were more efficient for this purpose, since they were significantly more stable toward aggregation than the colloids prepared according to the Stober method. This conclusion probably could be explained by the original surfactant protection coating of the silica colloids of Merck. Upon heating the noncoated colloids of Merck at 380 O C for approximately5 h, the absorption peaks at ca. 2875 cm-I and at ca. 2950 cm-' disappeared (Figure 4C(1)),indicating the removal by this process of the original organic coating belonging to the colloids of Merck. OTScoatings (aswell as other alkylsilanecoatings) (29) Seliger, H.; Kotachi, U.; Schrpf, C.; Martin, R.; Eisenbeiss, F.; Kinkel, J. J. Chromatogr. 1989, 476, 49.

of these degassed particles resulted in coatings equivalent to those obtained by using degassed silica colloids formed by the Stober method, as demonstrated in Figure 4C(2), and by the measured absorption peak at ca. 2920 cm-l (0.104.11)of these coatings (Table I). In fact, it has also been demonstrated that irreproducibleOTS coatings onto silica colloids prepared according to the Stober method can be initiated by precoating these particles with appropriate surfactants, e.g. polyacrylonitrile (see experimental procedure). However, it is possible to reobtain reproducible OTS monolayer coatings by burning these particles (380"Cfor 5 h), thereby removing the surfactant from the particles surface, and then coating the degassed particles. On the other hand, "poor" OTS coatings were formed if the particles, before coating, were preheated to 900 "C for 2 h instead of 380 "Cfor 5 h, as demonstrated in Figure 4D and by the measured absorption peaks at ca. 2920cm-l, i.e. 0.02-0.03for particles heated at 900 "C compared to 0.104.11for particles heated at 380 O C (Table I). These results may be explained by the irreversible hydrophobic nature (lack of surface hydroxyl groups) of silica colloids obtained after heating at 900 "Cfor 2 h (Figures 2 and 3). A relatively lower OTS coverage on silica colloids was also obtained if the particles which were heated at 380 O C for 5 h were stored before coating in a closed flask under Na atmosphere instead of air, as demonstrated by the measured absorption peak at ca. 2920 cm-l (Table I), i.e. 0.06-0.08for microspheres stored under N2 compared to 0.104.11for microspheres stored under air. These results are in agreement with previous studies,25showing that some traces of water, i.e. amonolayer,adsorbed onto silica flat surfaces are essential in order to obtain close-packed monolayer coatings of appropriate trichloroalkylsilane surfactants. The water physisorbed monolayer provides enough hydroxyl groups for anchoring the trichloroalkylsilane Surfactants onto the silica surfaces and obtaining

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Brandriss and Margel

1236 Langmuir, Vol. 9,No. 5, 1993 I

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Table I. Characterization by FTIR of OTS Coatings onto Silica Colloidsa treatment FTIR absorption colloid type (before coating) (ca. 2920 cm-l) irreproduciblevalues Merck b irreproduciblevalues Merck degassing' 0.12-0.20 Stober b 0.10-0.11 Stober degassing' heating at 380 "C 0.10-0.11 Merck, Stober degassind Merck, Stuber heating at 900 "C 0.02-0.03 degassinge Merck, Stober heating at 380 "C 0.06-0.08 storing under Nz degassingf

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a Each 50 mg of silica colloids of 0.25 pm average diameter (treated as described in the table) was coated with 42 pL of OTS in 4 mL of BCH at room temperature for approximately 20 h. The measured range of absorption peaks at ca. 2920 cm-1 repreeenta at least five different experimentsof each coating. * A stock of driedmicrospheres was stored at room temperature in a closed flask under air. For each coating experiment, 50 mg of silica colloids was taken from the flask and coated with OTS. For each coating experiment, 50 mg of silica colloids was taken from the stock described in (b),degassed (ca. 20 mmHg, 60 "C for 2 h), and coated immediatelywith OTS. d A stock of dried microsphereswas heated at 380 "C for 5 h and stored then at room temperature in a closed flask under air. For each coating experiment,50 mg of silica colloidswas taken from the flask,degassed (ca. 20 mmHg, 60 "C for 2 h), and coated immediately with OTS. e Like (d), substituting heating of the silica colloids at 380 O C for 5 h by heating at 900 "C for 2 h. f A stock of dried microspheres was heated at 380 "C for 5 h, cooled under NZto room temperature, and stored at room temperature in a closed flask under Nz. For each coating, 50 mg of silica colloids was taken from the flask, degassed (ca. 20 mmHg, 60 "C for 2 h), and coated immediately with OTS.

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thereby high-quality coatings. On the other hand, the presence of too much water, i.e. multilayers, adsorbed onto silica surfaces results in irreproducible coatings which usually consist of more than a monomer coverage, as was previously demonstrated (Table I). Figures 5 and 6 describe the kinetic behavior of OTS coatings prepared under different conditions onto silica colloids of 0.25 and 0.7 pm average diameters, respectively. It is redemonstrated that OTS monolayer coverage can be obtained by heating the microspheres at 380 "C for 5 h, storing the particles then at room temperature in a closed flask under air and degassing (ca.20 mmHg, 60 "C for 2 h) the particles before OTS coating; i.e. under these conditions the measured absorption peak at ca. 2920 cm-l was ca. 0.1 for particles of 0.25 pm average diameter and ca. 0.038 for particles of 0.7 pm average diameter. A relatively lower OTS coverage and slower coating kinetics were observed when the particles were treated in a similar way, substituting storing the particles in a closed flask under air with storing under N2 atmosphere. On the other hand, irreproducible OTS coatings and relatively faster coating kinetics were observed if the particles were treated in a similar way (380"C for 5 h and stored then in a closed

TIME ( h )

Figure 5. Kinetics of OTS coatings onto silica colloids. Silica

colloids (0.25 pm average diameter), before coating with OTS, were heated a t 380 "C for 5 h and treated then in various ways: (A) stored at room temperature in a closed flask under Nz and degassed (20 mmHg, 60 O C for 2 h) before coating; (B) stored a t room temperature in a closed flask under air and degassed before coating; (C, 1and 2) stored a t room temperature in a closed flask under air and coated without degassing. Each coating was performed with 50-mgmicrospheres in 4 mL of BCH with 42 p L of OTS at room temperature for 20 h.

flask under air) but were not degassed before OTS coating (Figures 5 and 6). The previously described results established conditions for preparing optimal reproducible OTS (aswell as other alkylsilane) coatings on silica colloids. This includes, removing organic contaminants (e.g. surfactants) from the silica surfaces (380"Cfor 5 h) and degassing (ca. 20 mmHg, 60 "Cfor 2 h) the particles before coating. The proceeding coating studies are all based on these optimal coating conditions. Figure 7 describesthe capacity of silicacolloids of various diameters, 0.25, 0.7, and 1.4 Mm, toward OTS. It is

Langmuir, Vol. 9, No.5, 1993 1237

Monolayer Coatings on Silica Colloids

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SURFACE SURPLUS (moles/moles 1

Figure 7. FTIR absorption intensities at ca. 2920 cm-l of silica colloids of various diameters as a function of OTS concentration in the silanation solution. OTS concentration in solution was measured relative to the calculated concentration required in order to obtain a close-packed OTS monolayer coating onto the silica colloids (Table 11),assuming a cross-sectionalarea for OTS of 20 &.I9 Silica microspheres (50 mg) of the desired diameter in 4 mL of BCH were coated with increasing OTS concentrations at room temperature for 20 h.

demonstrated that, under the experimental conditions, in order to obtain an optimal OTS coverageon these surfaces, it is essential to have in the silanation solution an initial OTS concentration that is at least 5 times higher than that required for formation of close-packedOTS monolayer coating on the particle surfaces. Figure 7also demonstrates that the OTS coating on the different size particles is in relatively good agreement with the surface area ratio of these particles, i.e. the measured optimalabsorption peaks at ca. 2920 cm-l for 0.25,0.7,and 1.4 pm average size particles were 0.10,0.038,and 0.027,respectively. A Comparison between Coatings Prepared from Different Surfactants. Quantitative characterization of the surfactant coatings on silica colloids was performed by different techniques, e.g. FTIR, TGA, and percent C measurements. Figure 8 demonstrates a typical TGA behavior of noncoated (A) and coated (B)silica colloids. The difference in the weight loss between the coated particles and the noncoated ones in temperatures ranging from approximately300to 430OC were used for estimating the percent hydrocarbon of the coated surfactants. Table I1 illustrates the percent C calculated for close-packed monolayer coatings of silica colloids of various diameters with alkylsilane surfactants of different Iengths (SiX3-

920

I

000

19000

27000

35000

,

1

I

43000

TEMPERATURE ('C)

Figure 8. TGA behavior of noncoated (A) and OTS-coated (B) silica colloids (0.25 pm average diameter). Table 11. Percent C Calculated for a Close-Packed Monolayer Coating of Silica Colloids with Alkylsilane Surfactants of Different Lengths. alkylsilane length colloid diameter (no. of carbons) c (%I 0.25 ClS 2.16 CS 0.96 c 4 0.48 0.7 ClS 0.77 CS 0.34 c4 0.17 1.4 ClS 0.39 CS 0.17 c4 0.09

The area per each bonded alkylsilane molecule was estimated to be 20 A2, as was demonstrated by previous studies.lg Table 111. Characterization by FTIR,TGA, and Elemental Analysis of Coatings Prepared from SiCla(CH2)1&Haand from Si(OMe)3(CH2)1&Hsonto Silica Colloids under Various Conditionsa

FTIR

absorption elemental TGA

(ca.2920

analysb

(% w t

coatings conditions cm-1) (% C) lossb) 0.0 0.0 0.0 Si(OMe)&H2)1,CH3 toluene, R F 0.03 1.2 1.3 Si(OMe)3(CH2)1,CH3 toluene, 110 "C 0.01 0.9 0.9 Si(OMe)3(CH2)17CH3 BCH, RT toluene, RT 0.09 1.9 1.9 SiCl&H2)1&H3 0.10 2.0 2.0 SiC13(CH2)1,CH3 toluene, 110 "C 0.11 2.1 2.2 SiC13(CH2)1,CH3 BCH, RT a 50mg of microspheres (0.25pm average diameter)in 4 mL of the desired solvent was coated with 42 pL of each of the alkylsilane surfactants for approximately 20 h. *The reported valuen were obtained after subtraction of the percent weight loss of the noncoated microspheres (ca.0.3%). Room temperature.

(CHz),,CH3, n = 3,7,17),assuming a cross-sectional area of 20 A2 for each bonded alkylsilane m01ecule.l~ A comparison between coatings prepared from SiC13(CHz)&H3 (OTS) and from Si(OMe)3(CHz)&H3 (OTMS) on silica colloids of 0.25 pm average diameter under different conditions is shown in Table 111. Optimal coverage of OTS and OTMS were obtained in BCH at room temperature and in toluene at 110 "C, respectively. However, OTS coveragewas significantly higher than that of OTMS, approximately by a factor of 2-3 as demonstrated in Table III. Similar differences were observed for OTS and OTMS coatings on silica colloids of 0.7 and 1.4 pm average diameters. A comparison between coatings prepared from octano12 and alkylsilane compounds, such as SiC13(CH2),CH3, Si-

1238 Langmuir, Vol. 9, No. 5, 1993 Table IV. Characterization by FTIR, TGA, and Elemental Analysis of Coatings Prepared from Octanol and from Different Alkylsilane Surfactants onto Silica Colloidsa elemental FTIR absorption analysis TGA coatings (ca. 2920 cm-') (9% C) (% w t lossb) SiC13(CH2)17CH3 0.11 2.1 2.2 S ~ ( O M ~ ) ~ ( C H Z ) ~ ~ 0.03 CH~ 1.2 1.3 0.04 1.1 1.2 SiC13(CH2)7CH3 Si(OMe)3(CH2)7CH3 0.008 0.5 0.5 SiMe2Cl(CH2)7CH3 0.01 0.55 0.55 HO(CHd7CH3 0.014 0.8 0.6 SiCl&H2)3CH3 0.017 0.6 Si(OMe)3(CH2)3CH3 0.001c 0 The silica colloids (0.25 pm average diameter) were coated with the trichloroalkylsilane surfactants and with dimethylchlorooctylsilane in BCH at room temperature, with the trimethoxyalkylsilane surfactants in toluene at 110 OC, and with octanol in octanol as a solvent at 180 OC. All coatings were carried out for approximately 20 h and surfactant mole concentrations (except octanol) 20 times higher than that required for a close-packed monolayer coverage. b The reported values were obtained after subtraction of the percent weightloss of the noncoated microspheres (ca. 0.3%). Approximate value, due to the sensitivity limitation of the equipment. Table V. Characterization by FTIR of Coatings Prepared from SiCl3(CH2),CH3(a = 3,7,17) onto Silica Colloidsa FTIR FTIR coatings absorption (cm-l) absorption width (cm-l) SiC13(CH2)&H3 2919.5 18.0 SiC13(CH2)7CH3 2924.0 27.0 SiC13(CH2)3CH3 2924.0 27.0-31.5 0 50.0 mg of microspheres (0.25 pm average diameter) in 4 mL of BCH was coated with 42 p L of each of the trichloroalkylsilane surfactants at room temperature for approximately 20 h.

( O M ~ ) ~ ( C H Z ) ~and C HSiMezCl(CHzhCH3 ~, ( n = 3,7,17) on silica colloids of 0.25 pm average diameter is demonstrated in Table IV. Similar differences were observed for the same coatings on silica colloids of 0.7 pm average diameter. These results demonstrate that the coverage of these surfaces by the surfactants is in the following rank order: SiC13(CHz),CH3 > HO(CHz),CH3 > SiMeZCl(CHdnCH3 > Si(OMe)&Hz),CH3. FTIR peak absorption frequencies and widths of (CH3 band at ca. 2920 cm-l can also be used for evaluation of the degree of order of the different coatings.lZbEach of the trichloroalkylsilane Surfactants (SiCL(CHz),CH3, n = 3, 7, 17) used for the coatings of silica microspheres showed a liquid phase absorption (in CC4 solution) in the range of 2980-2930 cm-'. A shift toward lower frequencies was obtained for all coatings prepared from these surfactants, as demonstrated in Table V. The shift in the absorption peak frequency of SiCb(CH2)17CH3coatings is -10 cm-l, and for SiCl&Hz)7CH3 and SiCl&Hz)&Hs coatings is -5 cm-l. The effect of shortening the alkyl chain length is to reduce the frequency shift by 5 cm-l. This kind of difference in the shift was earlier interpreted as a result of a more liquidlike, desordered environment of the methylene units of the shorter chain length coated surfactants compared to OTS coating.lZ In addition to the frequency shift, a distinct bandwidth broadening of the shorter chain length surfactant coatings compared to OTS was also observed,i.e. bandwidths of 18,27, and 2731.5 cm-' were obtained for coatings prepared from SiC13(CHz)17CH3,SiC13(CHzhCH3,and SiC13(CHz)&H3, respectively (see Table V). Like the frequency shift, the increased widths is an effect of increasing liauidity and disorder of the coatings.1zb Both, FTfR absorption frequency and width of OTS coatings onto silica colloids (2919 cm-l and 18.0 cm-l, respectively) are only slightly

Brandriss and Margel Table VI. Characterization of Coatings Wettability As Studied by Contact Angle and by Complete Spreading Surface Tension Measurements. adv. contact complete spreading coatings angle (deg) surface tension (dyn/cm) SiCl3(CH2)17CH3 >170 27.7 Si(OMe)3(CH2)1,CH3 spread SiC13(CH2),CH3 150-165 27.7 Si(OMe)3(CH2),CH3 spread 41.0 SiMe2Cl(CH&CH3 spread 38.0 HO(CHd7CH3 143-158 38.0 SiCl3(CH2)3CH3 150-165 27.5-29.0 noncoated colloids spread >72.5b =These measurements were performed with 0.25 pm average diameter microspheres, according to the description in the experimental part. Water drops (surfacetension 72.5 dyn/cm) completely spread on the noncoated colloid pellets.

different than that reported for highly close-packed OTS monolayer coatings on flat silica surfaces (2918 and 16.0 cm-1, respectively),12J8 indicating the relatively high quality OTS coatings on these particles. Characterization of Coatings Wettability. The previously described characterization methods, i.e. FTIR, TGA, and percent C, provide only quantitative information on the surfactant coatings. Additional knowledge on the packing and order of these coatings should be obtained by studying the wettability properties of these coatings:12,18,19.30 (1) Advancing Contact Angle Measurements. Direct contact angle measurements on a single coated microsphere cannot be accomplished because of the very smallsurfaceof the particle. However,Table VI illustrates that measurements of advancing contact angles on uniform pellets composed of the coated microspheres may provide useful details on the coating quality. It should however be taken into account that the expected water contact angles of the coatings may significantly be influenced by the unique round bumpy shape of the pellets used for these studies. Indeed, Table VI illustrates very high water advancing contact angles for some coatings, e.g. OTS coatings on silica colloids of 0.25 pm average diameter resulted in water advancing contact angles higher than 170°, compared to approximately llOo on flat silica surfaces.18 However, these studies are still useful for comparison between different coatings, since all of them were performed under similar experimental conditions. The results shown in Table VI clearly demonstrate the wetting superiority of (1)coatings prepared from trichloroalkylsilane surfactants to coatings prepared from the other surfactants, (2) coatingsprepared from SiC&(CH2)17CH3 to coatings prepared from SiCl3(CHz),CH3, n = 3 or 7, and (3) coatings prepared from HO(CHZ)~CH~ to coatings prepared from Si(OMe)&H2)7CH3 or from SiMezCl(CH2)7CH3. On the other hand, these measurements were not sensitive enough to show differences between coatings prepared from (1) SiCl3(CH&CH3 and SiC13(CH2)3CHa, (2) Si(OMe)3(CH2)17CHa and Si(OMe)&H2)7CH3, and (3) SiMezCl(CH2)7CH3 and Si(OMe)3(CHzhCH3. (2) Complete Spreading Surface Tension Measurements. It is common to characterize solid surfaces by their criticalsurfacetension (CST)values. The classical procedure for determining the CST of a solid surface was developed by Zisman et al.31 This method involves measurements of contact angles of a homologous series of (30)Riedo, F.; Czencz, M.; Liardon, 0.; Kovata,E. Helu. Chim. Acta 1978, 61 (5), 1912. (31) Zisman, W. A. Adu. Chem. Ser. 1964, No, 43, 12.

Monolayer Coatings on Silica Colloids

liquids on the solid to be characterized. The results are plotted in the form of cos B vs y, where B is the measured contact angle and y is the surface tension of the liquid. Extrapolation of the curve to cos B = 1 defined a corresponding surface tension, which is termed the CST. The latter is used to characterize the surface of the solid, in the sense that every liquid of y smaller than the critical surface tension will completely wet the solid. However, this method for determing the CST of solid surfaces is not applicable for particles, particularly for colloidal size particles, due to the uncertainty in contact angle measurements on these surfaces. A more recent publication by Marmur et al.13adescribed a relatively simple empirical method to determine the CST of solid surfaces without the need for contact angle measurements. This method is based on placing a liquid drop prepared from a series of liquids with progressively decreasing surface tension onto the tested solid surface and observing whether the drop led to complete spreading or whether the drop led to a nonzero contact angle. The measured complete spreading surface tension is related to the highest surface tension of the liquid which led to complete spreading. For a more detailed description of this method, see ref 13a. According to this method the CST of a solid surface or a coating on a solid surface should be equal to the measured complete spreading surface tension of this surface. This method is similar to Zisman's method since, after all, both methods determine the highest surface tension of liquid which will just spread on the surface. However, the complete spreading surface tension method in contrast to Zisman's method determines the point of B = 0 experimentally rather than by graphical extrapolation and there is no need for instrumental measurements of contact angles. Therefore, this method is simpler and more appropriate for studyingwettabilityproperties on colloidal particles. It should, however, again be taken into account, as previously discussed, that the measured complete spreading surfacetensions of the coated silicacolloid pellets (Table VI) may not describe precisely the CST of single coated microspheres. However, these measured values should at least be useful for a relative comparison between the different coatings on silica colloids. Indeed, Table VI clearly demonstrates the wetting superiority of coatings prepared from trichloroalkylsilane surfactants over coatings prepared from the other surfactants. For example, the complete spreading surface tension of coatings prepared from SiC13(CH2)7CH3is 27.7 dyn/cm (all liquids with surface tensions lower than 27.7 dyn/cm will completely spread on pellets of these surfaces) compared to complete spreading surface tension of 38 dyn/cm for coatings prepared from HO(CH2)7CH3. On the other hand, the completespreading surfacetension measurementa were not sensitive enough to show the effect of chain length on the coating, i.e. all coatings based on SiC13(CH2),CH3, n = 3,7, and 17, showed similar complete spreading surface tension values (27-29 dyn/cm). These measurementsalso demonstratedthe wetting superiority of coatings prepared from HO(CH2)7CH3 and from SiMe2Cl(CH2)7CH3 to coatings prepared from Si(OMe)3(CH2)7CH3,but did not show the difference between coatings prepared from HO(CH2)7CH3to coatings prepared from SiMe2Cl(CH2)7CH3. (3) Floatability (TotalFloating and Total Sinking) Measurements. Another simple, empirical method for rapid determination of CST of solid surfaces, particularly fibers, has been reported by Mutchler et al.16 Later on, Marmur et aL13b demonstrated the applicability of this method for determiningthe CST of polymeric beads (73-

Langmuir, Vol. 9, No. 5, 1993 1239 Table VII. Characterization of Coatings Wettability As Studied by Floatability Measurements.

coatings

surface tension range (dyn/cm) float sink 27.5 72.5 30.2 >72.5b 44.5 40.0 35.0 >72.5c

24.0 30.0 24.2 30.0 27.5 27.0 26.3 >72.5c

transition region (dyn/cm) 3.5 42.5 6.0 >42.5 17.0 13.0 8.7

OThese measurements were performed with 0.25 pm average diameter microspheres, according to the description in the experimental part. * In water (surface tension 72.5 dyn/cm), part of the S ~ ( O M ~ ) ~ ( C H Zcoated ) ~ C Hmicrospheres ~ sinks and the other part floats. The noncoated silicamicrospherescompletelysink in water.

472 pm average diameter) and/or coatings on these beads. This method is based on placing the dired coated beads on a series of liquids with progressivelydecreasing surface tension and observing whether the particles sink or float. If the CST of the coated beads is less than the surface tension of the tested liquid, they will completely float, owing to surfacetension forces. If the CST of these coated beads is greater than the surface tension of the tested liquid, they will completely be wetted by the liquid and will sink. The tested liquid, of course, must have densities less than that of the tested beads. For more detailed description on the floatability measurements,see refs 13b and 16. This method has been used for studying the floatability properties of the different coatings on silica colloids, as demonstrated in Table VII. All measurements showed similar behavior: above a certain surface tension of the tested liquid all the coated microspheres float; a transition region, where the percent of sinking microspheres increases with the decrease in surface tension of the tested liquid; finally, below a certain surface tension all the microspheres sink. For exmaple, Table VI1 demonstrates that above a surface tension of 27.5 dyn/cm all OTS-coated microspheresfloat (totalfloating),between 27.5 and 24 dyn/cm (a transition region of 3.5 dyn/cm) the percent of sinkingOTS-coatedmicrospheres increases with the decrease in the surface tension, and below 24.0 dyn/ cm all the coated microspheres sink (total sinking). A previous publication'3b has demonstrated that the existence of the transition region of coatings prepared on the same support surface is mainly due to variations in the surface energies of these coatings,i.e. variability in coatings uniformity, otherwise the total floating and total sinking surface tensions of each coating should be the same. The range of the transition region therefore may be used as an indication of the degree of uniformity of the coatings. For example,the transition region surface tensions of coatings prepared from OTS and from octanol are 3.5 and 13.0 dyn/cm, respectively (Table VII), indicating that OTS coatings are significantlymore homogeneous than coatings prepared from octanol. These studies have demonstrated that the floatability measurements are the most sensitive ones for studying the wettability properties of thin films on colloidal particles. Table VI1 clearly demonstrates the wetting superiority of coatings prepared from longer chain alkylsilane surfactante to coatings prepared from the shorter ones. For example, the total floating,total sinking, and the transition region surface tensions are 27.5, 24.0, and 3.5 dyn/cm, respectively, for coatings prepared from SiC13(CH2)17CH3,30.2,24.2,and 6.0 dyn/cm, respectively, for coatings prepared from SiC13(CH2)7CH3,and 35.0,26.3, and 8.7 dyn/cm, respectively, for coatings prepared from

1240 Langmuir, Vol. 9, No. 5, 1993

Brandriss and Margel

SiCL(CH2)3CH3. In a similar way, Table VI1 also clearly various diameters. The conditions to obtain reproducible demonstrates that the wetting superiority of the coatings coatings of these Surfactants on silica colloids have been is in the following rank order: SiC13(CH2),CH3 > established. The feasibility of characterization of wetHO(CHz),CH3 > SiMe&l(CH2),CH3 > S ~ ( O M ~ ) ~ ( C H Z ) tability ~of coatings on colloid surfaces by noncommon CH3. methods, such as advancing contact angle measurements The CST of each coating on the particles should lie on coated colloid pellets, complete spreading surface between its total floating surface tension and its total tension, and floatability measurements, has been demsinking surface tension.16 Marmur et ai., however, demonstrated. Among these methods, the floatability meaonstrated that the total sinking surface tensions of coatings surements were found to be the most sensitive ones. By on polymeric beads, rather than the total floating surface use of this method, the relative variability in coatings tensions, closely resemble the reported CST of these uniformity and the average critical surface tensions of the coatings on flat surfaces of the same composition. OTS studied coatings were estimated. The superiority of coatings on flat silica surfaces were intensivelyinvestigated coatings prepared from the longer chain alkylsilane and considered to be composed of highly organized closesurfactants to coatings prepared from the short ones was packed monolayer c0atings.~~J~3~ The totalsinkingsurface also shown. These studies also demonstrated that the tension of OTS coatings on silica colloids (24.0 dyn/cm) superiority of the coatings is in the rank order of SiCl3is not far from the reported CST of close-packed methyl (CHzhCH3 > HO(CH2),CH3 > SiMezCl(CHz),CH3 > Simonolayer coatings on flat surfaces prepared from long(OMe)dCH2),CH3. chain n-alkanoic acids (22-24 d y n / ~ m and ) ~ ~from OTS This paper presents preliminary studies. Further (20-22 dyn/cm).12t22The relatively narrow transition studies concerningthe synthesis,characterization,and use region of OTS coatings on silica colloids (3.5 dyn/cm) and of various colloidal particles self-assembled coated with the small difference between the measured total sinking different o-functionalized alkylsilane surfactants, e.g. surface tension of OTS coatings on silica colloids and the SiC13(CH2),X (X = -C02Me, -CN, -OH, -Ph, etc.) are CST of OTS coatings on flat silica surfaces may indicate ongoing in our laboratories. the relatively high quality of these coatings. Summary and Conclusions. This paper descrbes the Acknowledgment. Thanks to A. Marmur for sugsynthesis and characterization of self-assembled hydrogesting the use of the total spreading surface tension and phobic monolayer coatings prepared from octanol and floatability measurements for these studies. This grant various alkylsilane surfactants on silica microspheres of was partially supported by the German-Israel Foundation (GIF) and by Minerva (Otto Meyerhoff Center for the (32) Ziaman, W . A. In Adhesion and Cohesion; Weiss, P.,Ed.; study of drug-receptor interactions). Elsevier: New York, 1962.