Langmuir 1992,8, 2274-2278
2214
Adsorption of Cationic Surfactants on Medical Polymers: Effects of Surfactant and Substrate Structures Jihu Yao and George Strauss' Department of Chemistry, Rutgers, The State University of New Jersey, New Brunawick, New Jersey 08903 Received March 16,1992. I n Final Form: June 15,1992 Cationic surfactantswith two and three alkyl chains were adsorbed on the surfaces of expanded Teflon and knitted Dacron surgical grafts. Didodecyldimethylammoniumbromide (DDAB) showed saturation adsorption, forming monolayers. Tridodecylmethylammonium chloride (TDMAC) formed vertically oriented multilayers, without a saturation limit, on both these polymers. Electron spin resonance spectra of coadsorbed 12-DOXYL-stearicacid methyl ester showed that the adsorbateswere flat-lyingand immobile at low surface densities but vertically close-packedand mobile at high densities. At intermediate binding levels they existed in separate immobile and mobile phases, without any intermediate states. TDMAC formed multilayers even before completion of a saturated monolayer, evidently due to the preferential filling of narrow channels in these polymers. Equilibration of TEMPO between an aqueous phase and surfactant-coated polymers showed that only vertically oriented mobile surface layers could accommodate a secondary lipophilic adsorbate. This finding is important for the binding of antibiotics and other drugs to surface-modified surgical grafts. vertically packed as mono- or multilayers. The PTFE beads had smoothsurfaces,as inferred from the agreement The adsorption of surfactantson solid polymer surfaces of the specific surface area as determined by nitrogen is of major interest, both scientificallyand technologically, adsorption and the area calculated from the bead size and has been studied mainly by measuring adsorption observed by scanning electron microscopy. isotherms. Hydrophobic interactions were found to be The present work was undertaken to study the adsorpimportant in the adsorption of both anionic and cationic tion of surfactants on vascular grafts made of PTFE in N-alkyl amphiphiles on polystyrene surfaces.' Electrothe form of expanded Teflon and poly(ethy1eneterephstatic interactions may also contribute if the polymer thalate)in the form of knitted Dacron, 5-8 mm in diameter, surface contains low concentrations of carboxyl or hyas used in surgical practice. These materials had highly droxyl groups originating from the initiator used in the structured surface topographies as seen in the electron manufacture of the polymer.2 The fluorocarbon surface micrographs in Figures 1 and 2. Of particular interest of poly(tetrafluoroethy1ene)(PTFE) was found to interact was (1) hydrophobic and oleophobic interactions of relatively weakly with hydrocarbonchains of ~urfactants.~ adsorbates with the polymer substrate and their effect on This was ascribed to a poor conformational match between the orientation and mobility of the adsorbate film, (2) the these dissimilar chain types, termed oleophobicity. Other effect of narrow pores and channels in the polymer surface, workers, however, found no difference in affinity of and (3)the abiity of the adsorbed surfactant layer to bind hydrocarbon surfactants for PTFE and p~lyethylene.~ a secondary adsorbate. The aggregation in solution of ionic amphiphileshaving The retention of such a further adsorbate in the form one or several alkyl chains, resulting in micelles and biof antibiotic and antithrombotic agents is essential for layers, finds a parallel in the organization of adsorbate successful use of vascular grafts and protheses. Methods layers of such molecules, due to cooperative e f f e ~ t s . ~ ~ ~ of achieving this have included various methodsof coating In previous work7we studied the adsorption of ammoan existing polymer surface.8 An early method involved nium salts with one, two, and three alkyl chains on PTFE the physical adsorption of benzalkonium (an alkyldimechromatographicbeads. The one- and two-chain surfacthylbenzylammonium chloride) as an anchor layer on tants saturated the surface when a monolayer had formed. graphite which then was able to bind heparin, a mucoThe three-chain surfactanthowever was adsorbed without polysa~charide.~ Another cationic surfactant, tridodecyllimit, indicating multilayer formation. Coadsorbed spin methylammonium chloride, was found to adhere to probes, used as indicators of mobility, showed that the polymer surfaces, enabling the binding of other, anionic, surfactantswere lying flat on the surface at low adsorption bioagents.l0 levels. With increasing surface density they became Materials and Methods * To whom correspondenceshould be addressed.
Introduction
(1) Piirma, I.;
Chen, S. J. Colloid Interface Sci. 1980, 74,90. (2) Connor, P.; Ottewill, R. H. J. Colloid Interface Sci. 1971,37,842. (3) Bee, H. E.; Ottewill, R. H.;Rance, D. G.; Richardson, R. A. In Adsorption from Solution;Ottewill, R. H., Rochester, C. H., Smith, A. L., E&.; Academic Prees: London, 1983; p 155. (4) Owens, N. F.; Richmond,P.; Gregory,D.; Mingins, J.; Chan, D. In
Wetting, Spreading, and Adhesion; Padday, J. F., Ed.; Academic Press: London, 1978; p 127. (5) Clunie, J. S.; Ingram, B. T. In Adsorption From Solution at the SolidlLiwid Interface; Parftt, G. D., Rochester, C. H., Eds.: Academic Press: London, 1983; p 105. (6) Bieio, P. D.; Cartledge, J. G.; Keesom, W. H.; Radke, C. J. J. Colloid Interface Sci. 1980, 78, 225. (7) Yao, J.; Strauss, G. Langmuir 1991, 7, 2353.
0743-7463/92/2408-2274$03.00/0
Materials. Tridodecylmethylammonim chloride (TDMAC), from Polysciences Inc., was recrystallized twice from hexane. Didodecyldimethylammoniumbromide (DDAB),from Eastman Organic Chemicals, was 99% pure and wed as received. The spin probes 12-DOXYL-stearic acid methyl ester (12DOXYL-ME),16-DOXYL-stearicacidmethyl ester (16-DOXYLME), and TEMPO were obtained from Sigma Chemical Co. (8) Hoffman, A. S. J. Appl. Polym. Sei.: Appl. Polym. Symp. 1988, 42, 251. (9) Whiffen, J. D.; Gott, V. L. Surg., Gynecol. Obstet. 1965,121,287. (10) Harvey, R. A.; Greco, R. S. Ann. Surg. 1981,194,842.
0 1992 American Chemical Society
Surfactants Adsorbed on Polymers
Figure 1. Scanning electron micrographs of expanded Teflon grafts: A, 200X; B, 2000X; C, 5000~. Expanded Teflon grafts, from W.L. Gore, Inc., and knitted Dacron grafts, from Bard Cardiosurgery,Inc., were obtained by courtesy of Dr. Richard Harvey of the R. W. Johnson Medical School. The grafts were freed of any adhering material by suspending them successivelyin chloroform, methanol, and water (three washings in each solvent), then drying them in air. The scanningelectron micrographs (Figures 1and 2) show that both polymers had a fine structure of closely spaced threads that formed narrow channels. The appearanceof finer and finer detail with increasing magnification suggests that these surface structures have a fractal geometry. Specific surface areas were measured with a Monosorb analyzer (Quantachrome Corp.) by adsorption of nitrogen, and using the BET equation." Specific areas were 1.13 f 0.05 m2/gfor expanded Teflon and 0.52 i 0.06 m2/g for knitted Dacron. Preparation of Surfactant-Coated Polymers. The method was as described previously? Typically,l-cm lengths of polymer tubing weighing ca. 100 mg were equilibrated for 48 h with 2 mL of methanol solutions of TDMAC or DDAB of different concentrations ranging from zero to about 7 X M. In some cases the solutions also contained 12-DOXYEME a t 1.25 X lo4 M (for Teflon) or 2.5 X lo4 M (for Dacron). Subsequently the samples were withdrawn, thoroughly washed with water, and air-dried. Adsorption Isotherms. Surfactanbcoated polymer samples were extracted twice with chloroform. This recovered all surfactant quantitatively. The extracts were analyzed by a dyetransfer method, using Orange II.12 The method depends on the formation of a complex between the anionic dye and a cationic surfactant which collects in the chloroform phase in which the (11) Brunauer, S.; Emmett, P. H.; Teller, E. J. Am. Chem. SOC.1938, So,309. (12) Few, A. V.; Ottewill, R. H. J. Colloid Sci. 1956,II, 34.
Langmuir, Vol. 8, No. 9, 1992 2275
Figure 2. Scanning electron micrographs oi knitted Uacron grafts: A, 20.2X; B, 200X; C, 5000X. free dye is insoluble. Surface densities of adsorbates were calculated from these analyses and the specific surface areas of the polymers, as obtained by nitrogen adsorption. Electron Spin Resonance (ESR)Measurements. A Varian E 1 2 spectrometer interfaced with an IBM PC computer was used to record room temperature ESR spectra of solid polymer samples having adsorbed spin probes. Samples were cut into small pieces and placed into 5-mm or 8-mm sample tubes. Singlecomponent spectra were characterized by their maximum hyperfine splitting (2A,). Order parameters, S, which can range from 0 (completely mobile) to 1.0 (fully immobile) were calculated from the relation13
S = 1.5
&-A0 A, - 0.5(A,,
+ A),
where A0 = (1/3) (A,, + A, + A,) and A,, A, and A,, are the hyperfine crystal tensors. For the DOXYL group these were taken as 5.9, 5.4, and 32.9 G.I4 Two-component spectra were analyzed for the relative contribution of a mobile and an immobile component by comparing them with trial sums of mobile and immobilereference spectra in different ratios until an match was obtained? Results were expressed as f m , the mobile fraction of the adsorbate.
Results and Discussion Adsorption Isotherms. Surfacedensities of adsorbed surfactants were plotted as a function of their concentrations in solution. The two-chain surfactant DDAB (13) Griffith, 0. H.; Jost, P. C. In Spin Lubeling-Theory und Applzcutzons; Berliner, L. J., Ed.;Academic Press: New York, 1976; p 453. (14) Jost, P. C.; Libertini, L. J.; Hebert, V. C.; Grifith, 0. H. J. Mol. Biol. 1971,59, 77.
2276 Langmuir, Vol. 8, No. 9, 1992 5 . 0 ~ 7 --. - - - a - - r I
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Solution Concentration/l O-z M Figure 4. Adsorption isotherms of TDMAC on Teflon and Dacron grafts. The numbers refer to the data points listed in Table I. showed adsorption liiita on both Teflon and Dacron (Figure 3),whereas TDMAC, with three alkyl chains, was found to adsorb without a saturation limit, on both these polymers (Figure 4). This indicated that DDAB was adsorbed as a monolayer, but that TDMAC formed multilayers. These differences seem to arise from the presence of a third alkyl chain in TDMAC. When TDMAC is in a closely-packed state, some of the chains may extend upward away from the substrate surface and enable additional surfactant layers to be adsorbed by hydrophobic interaction. This may occur even before all of the surface is covered, resulting in irregular clusters. Such complex aggregates of three-chain surfactanta have been observed in other systems.15 Teflon in the form of a latex has been shown to contain free carboxyl groups that cover 2 %5 of the surface and give rise to a "knee" in the adsorption isotherms of cationic ~urfactants.~The Dacron surface can also be expected to contain residual anionic groups. The isotherms for both types of polymeric grafta, however,showed no observable discontinuities as might result from such surface charges. Any inflection pointa due to charged groups covering a few percent of the surface would occur at adsorbate densities of the order of 1 X 10-11 mol cm-2 and would not affect the overall shapes of the isotherms. Mobility Studies. The mobility of adsorbates was characterized by taking ESR spectra of the spin probe
-
(15) Kunitake, T.; Kimizuka, N.; Higaehi, N.; Nakaehima, N. J. Am. Chem. SOC.1984,106,1978.
Figure 6. ESR spectra of 12-DOXYL-MEcoadsorbed with diEferent surface densities of TDMAC on Teflon and Dacron grafts. 0 = probe adsorbed alone. Other numbers refer to the points in Figure 4. Table I. Mobile and Immobile Adsorbate Denritier adsorbate density/ 1O-g mol c m - 2 ~ _ _ _ _ _ system datapt f,,, total mobile immobile _______ DDAB/Teflon 1 0.25 0.125 0.031 0.094 2 0.75 0.292 0.219 0.073 -1.0 0.323 0.323 -0 3 4 -1.0 0.323 0.323 -0 DDAB/Dacron 5 0 0.034 0 0.034 6 0 0.118 0 0.118 7 0 0.126 0 0.126 TDMAC/Teflon 8 0.35 0.170 0.060 0.110 9 0.70 0.353 0.247 0.106 ~
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12-DOXYL-ME coadsorbed with the surfactants. Airdried polymer samples with surface layers of surfactant and spin probe gave spectra indicating almost complete immobilization at all binding levels and were not very informative. Samples that had been hydrated by equilibrating them at 100%relative humidity, however, showed characteristic changes in mobility, depending on the binding level of surfactant. This showedthat theadsorbate can be mobile only when hydrated. ESR spectra of the 12-DOXYL-ME spin probe in the presence of various surface densities of DDAB on Teflon and on Dacron are shown in Figure 5. The numbers for each spectrum correspond to the numbered data pointa in Figure 3 and Table I. ESR spectra of this probe in the presence of TDMAC on Teflon and on Dacron are given in Figure 6, where now the numbers refer to the pointa in Figure 4 and Table I. Spectra labeled 0 are for the spin probe alone on each of the polymers.
Surfactants Adsorbed on Polymers On each polymer,two limitingstates could be recognized from the general shapes of the spectra, and from the a highly mobile maximum hyperfine splittings, 2A,: state, as exemplified by spectrum 4 in Figure 5, with a maximum splitting of 33.6 G, observed at a high binding level of DDAB on Teflon, and an immobilized state, as in the spectra labeled 0 in Figures 5 and 6, with a maximum splitting of 56.7 G on Teflon and 64.5 G on Dacron, seen when the spin probe was adsorbed without any surfactant. From these Am, values, order parameters S of 0.11 for the mobile limit and 0.78 and 0.96 for the immobile extremes on Teflon and Dacron, respectively, were calculated using eq l. States with these order parameters were defined as the ‘immobile” and “mobile” states for the systems studied here. The slightly different order parameters for the spin probe by itself indicate a lower affinity of the DOXYL group for Teflon than for Dacron, or greater flexibility of the Teflon polymer chains. At intermediate binding levels, two-component ESR spectra resulted. These were analyzed by using the reference spectra for the immobile and mobile limits to obtain fm,the fraction of adsorbate in the mobile state, as given in Table I. It is noteworthy that the adsorbate existed in two distinct states, indicating a two-phase equilibrium, rather than being in a uniform state of some intermediate degree of mobility. These data are consistent with a model where the probe, either by itself or together with a low surface density of surfactant, lies flat on the substrate surface, thus keeping the DOXYL group immobile. With increasing surface density, part of the surfactant, and with it part of the probe, becomes close-packed and vertically oriented. These upright chains, in patches on the surface that represent a separate phase, have liquidlike mobility. DDAB on Teflon (Figure 5, spectra 1-4) showed a gradual increase in fm with increasing binding level of the surfactant until all of the surfactant was in the mobile state, with fm= 1.0. This point coincided with the binding level at which the surface density became constant (Figure mol 3), indicating saturation, at approximately 3.2 X cm-2,correspondingto 52 A2molecule-’. This cross section is consistent with the value for a vertically oriented amphiphile with two saturated alkyl chains and an-N(CH& headgroup. The mobile fraction fm therefore can be regarded as a measure of the fractional surface covered by a vertically oriented, closely packed, adsorbate layer. Adsorption of DDAB on Dacron (Figure 5, spectra 5-7) showed a completely different behavior. Here the ESR spectra of the coadsorbed DOXYL probe indicated an immobile state (fm = 0) at all binding levels, up to saturation. Its limiting saturation surface density of 1.2 X 10-lomol cm-2 (Figure 3) was much lower than on Teflon. This density correspondsto a molecular cross section of 138A2. This value suggested that DDAB forms a flatlying monolayer at saturation coverage. To check this possibility, the area occupied by a flat DDAB molecule was estimated from data given by Tanford.16 The chain length, expressed as 1.5 + 1.265nC,with n, = 12 carbons, is 16.7 A. The chain width was estimated at 4.2 A, giving an area of 70 A2 per chain. The area occupied by the headgroup was estimated at 16A2, thus giving a total area of 156A2. This area, only slightly larger than that obtained from the saturation density, is consistent with a flat-lying DDAB layer. It is possible, however, that only the hydrocarbon chains lie on the surface, with the headgroup curving upward. Such a conformation is supported by the observation7 that a long-chain spin probe with the (16) Tanford,C.TheHydrophobicEjfect,2nd ed.;Wiley-Interscience:
New York, 1980; Chapter 5.
Langmuir, Vol. 8, No. 9,1992 2277 nitroxide group at the cationic headgroup (hexadecyldimethylammonium-TEMPO) gave a mobile ESR spectrum even when adsorbed at a very low surface density. Possible causes for the different adsorbate orientations on the two polymers include an affinity of the DDAB headgroups for the ester groups of Dacron, and, conversely, a low affinity of DDAB for Teflon which is hydrophobic thus favoring vertical packing, with and also ole~phobic,~ the headgroups facing away from the substrate. Spectra of the DOXYL probe in the presence of TDMAC, on both Teflon and Dacron (Figure 61, also showed a rise in fmwith increasingbinding level. Almost all of the adsorbed TDMAC became mobile at the highest level tested for each polymer, but even then a trace of immobile adsorbate remained (spectra 11and 14). Table I lists total adsorbate densities, as plotted in the adsorption isotherms (Figures 3 and 41, mobile fractions fm, as calculated from the ESR spectra, and the resulting densities of the mobile and immobile phases. The latter data reveal that the immobile densities for TDMAC remained constant with varying total adsorbate densities. Immobile densities were ca. 0.10 X lo+ and 0.35 X 10-9 mol cm-2on Teflon and Dacron, respectively. For DDAB on Teflon, in contrast, the immobile adsorbate density decreased with increasing total density, eventually reaching zero. The persistence of an immobile TDMAC fraction of constant density apparently arises from the surface topography of the expanded Teflon and knitted Dacron substrates. The scanning electron micrographs (Figures 1and 2) show highly structured surfaces,including threads in close proximity. The resulting channels may become filled with multilayers of TDMAC. Such multilayers, immobilizeddue to their attachment to opposing surfaces and possibly also due the inability of water of hydration to penetrate into such filled channels, can be expected to form before all of the surface is saturated by a monolayer. The above model is supported by theoreticalconsiderations of YinI7 who showed that for substrates with a fractal geometry an adsorbate fills the pores sequentially, from small size to large. This channel effect is not observed with DDAB, apparently because it does not form multilayers. Earlier work with TDMAC adsorbed on smooth-surfaced chromatographic Teflon beads also showed multilayer formation but no residual immobile fraction? thus supporting the above model of the channel effect. The lower level of the immobile adsorbate density of TDMAC on Teflon, compared to Dacron,may reflect differencesin the number and sizes of channels. It may also arise from the oleophobic character of Teflon and its greater hydrophobicity. Relative Surface Affinities. In order to characterize the relative hydrophobicities of Teflon and Dacron, ESR spectra were taken of 16-DOXYL-ME,adsorbed on Teflon and on Dacron in the absence of surfactant. As shown by the spectra and their hyperfine splittings in Figure 7, this probe was mobile on Teflon, and immobile on Dacron. This may be due to the somewhat polar 16-DOXYL group, at the end of the chain, becoming mobile by curving away from the hydrophobic Teflon surface but remaining bound to the more hydrophilic Dacron. In earlier work7 such curvingaway of a polar group was indicated,as already mentioned. Alternatively, if all of the 16-DOXYL-ME molecule lies flat on both these polymers,then the mobility difference may report the greater flexibility of Teflon chains, compared to Dacron. The 12-DOXYL-ME probe when present by itself (spectra 0 in Figures 5 and 6) was essentially immobile on (17) Yin, Y.-B. Langmuir 1991, 7, 216.
Yao and Straws
2278 Langmuir, Vol. 8,No. 9, 1992
vertically oriented surface films. For Dacron precoated with DDAB (d), no split peak resulted, showing absence of a mobile organic phase, in accordance with the previous evidencefor an immobileflablying monolayer. From theae results it appeare that the quantity of secondaryadsorbate bound depends on how much mobile primary adsorbate is present.
Conclusions
Figure 7. ESR spectra of 16-DOXYL-ME, in the absence of surfactant, adsorbed on (a)Teflon, with 2A, = 35.2 G, and (b) Dacron, with 2A, = 63.1 G.
Figure8. ESR spectra of TEMPO after equilibration of aqueous
TEMPO solutions with (a) TDMAC-coated Teflon; (b) DDAB-
coated Teflon; (c) TDMAC-coated Dacron; (d) DDAB-coated Dacron. Volumes of 0.013 M aqueous TEMPO,per milligram of polymer, were (a) 0.2 pL, (b) 0.08 pL, and (c and d) 0.25 pL. Surfactant densities, in 10-B mol an-*, and the corresponding data points in Table I and in Figure 3 or 4, were as follows: (a) 3.16, point 11; (b) 0.323, point 4; (c) 7.10, point 14; (d) 0.126, point 7.
both polymers, but with an order parameter of 0.78 on Teflon and 0.96 on Dacron. This probe, therefore, can also report differences in affmity or flexibility, but with much lesa sensitivity.
Secondary Adsorption of Amphiphilic Compound#. The ability of differently oriented adsorbate f h to bind further adsorbates was tested by equilibrating them with aqueous solutions of TEMPO. This compound is soluble in water and in organic solvents. Ita ESR spectrum has a slightly wider maximum hyperfine splitting in an aqueous phase than in an organic phase.18J9 As a result, a split high-field peak is observed if the probe distributes iteelf between these phases. Taking advantage of this spectral difference, it was possible to test adsorbed surfactanta for the presence of an organic phase able to accommodate a secondary adsorbate. In an effort to keep the volumes of the aqueous and potential organic phases roughly equal, the surfactantcoated polymers were equilibrated with volumes of the order of 0.1-0.2 pL/mg of polymer of 0.013 M aqueous TEMPO solutions. These smallvolumes formed aqueous surface films. Figure 8shows ESR spectra of TEMPO on Teflon and Dacron precoated with TDMAC or DDAB at surface densities corresponding to the high end of each of the adsorption isotherms in Figures 3 and 4, as given in the Legend. Split high-field peaks were observed for Teflon precoated with either TDMAC or DDAB (a and b) and also for Dacron precoated with TDMAC (c). This demonstrated the presence of a mobile organic phase in all these cases, supporting the evidence for mobile, (18)Hubbell, W. L.; McConnell, H. M. Proc. Natl. Acad. Sci. U.S.A. 1968, 61, 12. (19) Straw, G.; Schurtenberger, P.; Hauser, H. Biochim. BiOphy8. Acta 1986,858,169.
From the experimental results it is seen that cationic surfactant layers on the surgical grafts studied here undergo different changes in their organization a t progressively higher binding levels. These patterns depend on (1)the number of alkyl chains of the surfactant, (2) the chemical nature of the substrate surface, and (3) its topography. Initially, when the surfactant is present at low surface density, it lies flat on the substrate surface, in all the systems studied. If the affmity of the surfactant for the substrate is larger than competing forces, the surfactant eventually saturates the surface with a flablying monolayer that is immobile, as observed for DDAB on Dacron. When other forces, such as hydrophobic interactions between neighboringsurfactants and/or hydration of headgroups, can compete with weaker surfactantsubstrate interactions, then the initially flablying adsorbate reorients itself as the binding level increases to form a close-packed vertical layer, and will saturate the surface with such a layer having liquidlike mobility, as in the case of DDAB on Teflon. A lower affmity of surfactante and probes for oleophobic Teflon, compared to the somewhat hydrophilic Dacron surface, and/or a greater flexibility of Teflon chains, compared to Dacron, is seen from the slightly higher mobility of 12-DOXYL-MEand the muchgreatermobility of 16-DOXYL-ME on Teflon. At intermediate binding levels the adsorbate exists in two discrete phases, a flat-lying and a vertically closepacked one, whose proportion changes with increasing binding level until only the vertical phase remains. When surfactants with three alkyl chains, such as TDMAC, form close-packed vertical layers, then one or more chains appear to point away from the substrate, enabling subsequent adsorbate layers to bind. This type of interaction has two consequences: (1) adsorption can proceed without a saturation limit as more and more layers are piled on top of each other; (2) a special "channel effect" operates when the polymer substrate has narrow channels of a width corresponding to the thickness of a limited number of adsorbate layers. In such filled channels the adsorbate remains immobile even at high binding levels. Both these effecta were observed with TDMAC on Teflon and Dacron, whereas only multilayer formation but no residual immobile fraction was previously seen with TDMAC on F'TFE particles that had no narrow channels.' Secondary adsorption of a lipid-soluble substance on surfactant-coatedTeflon or Dacron surgicalgrafta requires the presence of a mobile adsorbate layer, i.e. a vertically close-packed one, as observedwithTEMP0 as aprototype solute without surfactant properties. The requirements for optimal binding of antibiotics and drugs to such grafts therefore are (1)a precoating of the polymer surface with a three-chain (or four-chain) cationic surfactant in which the balance between surfactantaubstrate affmity and attractive forces between surfactant molecules is such as to produce a vertically packed and mobile multilayer adsorbate film and (2) use of a polymer substrate with a surface topographysuch as to maximize the area available for such mobile multilayer films. NO. DDAB, 3282-73-3;TDMAC, 7173-54-8;PTFE, 9002-84-0; poly(ethyleneterephthalate), 25038-59-9.
