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Adsorption of Alkyltrimethylammonium Bromide and Alkylpyridinium Chloride Surfactant Series on Polytetrafluoroethylene Powder
JOURNAL OF COLLOID AND INTERFACE SCIENCE ARTICLE NO.
177, 359–363 (1996)
0044
Adsorption of Alkyltrimethylammonium Bromide and Alkylpyridinium Chloride Surfactant Series on Polytetrafluoroethylene Powder AJAY K. VANJARA AND SHARAD G. DIXIT 1 Department of Chemical Technology, University of Bombay, Matunga, Bombay 400019, India Received March 29, 1995; accepted June 14, 1995
The adsorption and zeta potential of three different alkyltrimethylammonium bromides, CTAB, TTAB, and DTAB, and two alkylpyridinium chlorides, CPC and DPC, on a PTFE surface were studied. Adsorption isotherms having characteristics similar to those of two plateau regions were observed for all these surfactants. In the case of alkyltrimethylammonium bromide, there is a considerable effect of chain length which could be attributed to the arrangement of surfactant molecules being tilted away from the perpendicular. In the case of alkylpyridinium surfactants, an insignificant effect of chain length is due to a predominantly perpendicular arrangement. The thermodynamics of hemimicellization has been described by evaluating nhm as well as free energy of hemimicellization. q 1996 Academic Press, Inc. Key Words: alkyltrimethylammonium bromide; alkylpyridiniumchloride; adsorption on PTFE.
INTRODUCTION
Polymer surfaces display various kinds of surface chemistry due to their low surface energy and inertness. Among them, polytetrafluoroethylene (PTFE) shows a large degree of hydrophobicity and oleophobicity. PTFE is extensively used as a biomaterial and in several other applications where chemical inertness is desirable. The initial interest in adsorption of surfactants on PTFE and other polymers was in relation to polymer latex particles (1). It was shown that the PTFE surface acquired chemical functional groups during the process of polymerization and they could not be removed completely even with extensive cleaning by dialysis. As a result, characterization of the latex surface became somewhat difficult. Adsorption of cationic, anionic, and nonionic surfactants on PTFE latex particles has been investigated. In particular, decyltrimethylammonium bromide adsorption on PTFE was studied. Direct electrostatic interaction with residual acquired carboxylic groups on the PTFE surface was found to occur. However, interaction between hydrocarbon chains of the surfactant and fluorocarbon chains of PTFE was found to be rather weak due to conformational mismatch (1). The formation of aggregate and other organized structures on the polymer surface has also been reported (2, 3). 1
To whom correspondence should be addressed.
Yao and Strauss (4) studied adsorption of quarternary ammonium surfactants onto chromatographic beads of PTFE from methanol solution. It was found that whereas one- and two-chain alkyl surfactants exhibit saturation, the triplechain surfactant does not. This was attributed to the formation of a monolayer in the cases of single- and double-chain alkyl surfactants. In the absence of hydration, the adsorbate was immobile at all surface densities as it lay flat on the PTFE surface. The hydrated adsorbate exhibits mobility at full surface coverage because surfactant chains are packed normal to the surface as in the case of a triple-chain surfactant. The above authors (5) found that only the vertically oriented mobile surface layer could accommodate secondary lipophilic adsorbate which results in its binding. Thus the orientation of adsorbed surfactant molecules is of interest for the binding of antibiotics and other drugs to surface modified surgical grafts. No studies of adsorption from aqueous solution were reported by the above authors. In the present investigation adsorption of three different alkyltrimethylammonium surfactants and two alkylpyridinium surfactants from aqueous solution onto commercially available PTFE powder has been studied. These cationic surfactants vary in hydrocarbon chain length only. An investigation was planned to study (i) the role of hydrocarbon chain length, (ii) the role of acquired surface functional groups, (iii) electrostatic interaction as determined by zeta potential measurements, (iv) the nature of adsorbed species, and (v) aggregation taking place at the surface. MATERIALS AND METHODS
Materials Polytetrafluoroethylene (PTFE). PTFE powder was procured from Wilson Laboratories (Bombay). The average particle size of PTFE powder was determined on a GALAICIS-1 computerized inspection system. The surface area of PTFE powder was measured by BET (N2 adsorption). Characterization of PTFE powder was carried out by conductometric and FTIR measurements. Surface active agents. Cetyltrimethylammonium bromide (CTAB) was obtained from Bombay Oil Ltd. (Bom-
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bay). It was 98% pure and was recrystallized twice from an acetone–methanol (3:1) mixture. Tetradecyltrimethylammonium bromide (TTAB) was obtained from Sisco Research Lab. (Bombay). It was 99% pure and was used as received. Dodecyltrimethylammonium bromide (DTAB) was received from Sigma Chemicals Co. It was 99% pure and was used as received. Cetylpyridinium chloride monohydrate (CPC) was procured from Sisco Research Lab. It was 98% pure and was recrystallized twice from acetone. Dodecylpyridinium chloride monohydrate (DPC) was obtained from Aldrich Chemical Co. Inc. It was 98% pure and was used as received. Other chemicals were of analytical grade. Double distilled water was used in all the experiments. Methods Critical micelle concentration. The critical micelle concentrations (CMC) of the surface active agents used were obtained from conductometric measurements as a function of concentration at 307C. The values obtained in 10 03 M potassium chloride solution are given in Table 1. Determination of adsorbed surfactants. In an Erlenmeyer flask, 10 ml of surfactant solution in 10 03 M KCl was equilibrated with 200 mg of PTFE powder for 24 h at 307C and pH 7 { 0.05 on a flask shaker. The solid particles were separated from the supernatant liquid by centrifugation for 15 min. The clear supernatant was removed for analysis of surface active agents. The concentration of cationic surfactants was determined by using the dye transfer technique described by Few and Ottewill (6). The average error of the adsorption experiments was {5%. Measurement of zeta potential. The zeta potential of PTFE particles (5 mg) suspended in surfactant solution (50 ml) was measured with a zeta meter (Rank Brothers Mark II, UK). The suspension of PTFE in surfactant solution was kept on a flask shaker for 24 h at a constant temperature of 307C before the zeta potential measurement began. At least 10 particles were tracked to determine electrophoretic mobility. The zeta potential was calculated from electrophoretic mobility using Smoluchowski’s equation (7). Conductometric titration. Determination of surface carboxylic groups on PTFE powder was carried out by conductometric titration with 0.001 N NaOH solution. Five grams of PTFE powder was suspended in 125 ml of water containing absolute ethanol on flask shaker for 24 h before conductometric titration began. An inert atmosphere was created by passing N2 gas through the solution. FTIR spectroscopy. The infrared spectra were obtained on a JASCO FT/IR300 E Fourier transform infrared spectrophotometer. Each spectrum was recorded at a resolution of 4 cm01 with auto scans. Measurements were made in the spectral region 400–4000 cm01 . The spectrum of PTFE was recorded in the KBr pellet.
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FIG. 1. Zeta potential of PTFE in 0.001 M KCl at various pH.
RESULTS AND DISCUSSION
Characterization of PTFE Surface First, the surface of the PTFE powder used in the present investigation was characterized. The average particle size of PTFE was determined to be 1.65 mm and the surface area was found to be 5.4 m2 /g. The zeta potential of PTFE powder in 10 03 M KCl was measured; the results are shown in Fig. 1. It can be seen that PTFE particles are negatively charged and the charge remains more or less constant over the pH range 5 to 9.5 at 6 { 1 mV. It decreases below pH 5. IR Spectral Study It has been mentioned in the literature (1) that the surface of PTFE acquires carboxylic functional groups during polymerization. Therefore, it was decided to look for any functional groups using FTIR. The spectra were recorded after the powder was brought in contact with water having different pH values for 24 h. Expanded spectra between 1500 and 1800 cm01 are shown in Fig. 2 for pH 4.1, 6.4, and 9.1. It will be seen that the spectra corresponding to pH 4.1 show well-defined peaks at 1710 and 1745 cm01 (Fig. 2a). The peak at 1710 cm01 may be ascribed to the carboxylic ( –COOH) groups in the dimer, whereas the peak at 1745 cm01 may be ascribed to monomeric –COOH groups. As the pH increases to 6.4, peaks at 1724 and 1743 cm01 are seen (Fig. 2b). This again represents the peak due to antisymmetric stretching vibrations of {C|O in the –COOH groups. The spectra corresponding to pH 9.1 show strong peaks at 1710 and 1529 cm01 (Fig. 2c), whereas the spectrum around 1725 cm01 appears to have merged with the 1710 cm01 peak to give a broad peak. The appearance of a new peak at 1529 cm01 clearly indicates the reaction of the alkali with –COOH groups, since the peak at 1529 cm01 is
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FIG. 4. Adsorption isotherms of alkyltrimethylammonium bromide surfactants.
˚ 2 for close sectional area per –COOH group to be 25 A packing, it has been calculated that the –COOH groups occupy 29% of the total surface area. Thus, the presence of –COOH groups on the surface is quite significant and is expected to play a major role as far as adsorption on the PTFE surface is concerned. Adsorption and Zeta Potential Measurement
FIG. 2. IR spectra of PTFE at various pH: (a) pH 4.1, (b) pH 6.4, and (c) pH 9.1.
Adsorption of three different alkyltrimethylammonium bromides, CTAB (C16 ), TTAB (C14 ), and DTAB (C12 ), was studied. They vary in hydrocarbon chain length only. The adsorption isotherms are shown in Fig. 4 and the corresponding zeta potential versus concentration curves are shown in Fig. 5. Adsorption isotherms of CTAB, TTAB,
characteristic of –COO 0 groups. The FTIR study clearly establishes the presence of –COOH groups on the surface of PTFE. This was further confirmed by conductometric titration of PTFE powder suspended in a water–ethanol mixture (Fig. 3) against 0.001 N NaOH solution. A clear neutralization point was obtained, as seen from Fig. 3. Assuming the cross-
FIG. 3. Conductometric titration of PTFE with 0.001 M NaOH.
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FIG. 5. Zeta potential of PTFE in alkyltrimethylammonium bromide surfactants.
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and DTAB are similar, showing two plateau regions each. The corresponding zeta potential curves (Fig. 5) show similar variation. The adsorption of CTAB over the entire concentration is higher than that of TTAB, which is higher than that of DTAB. Thus increasing chain length results in increased adsorption. A significant effect of chain length in the lower concentration range indicates the effect of the interfacial water, which has a reduced dielectric constant. Obviously a greater decrease in free energy results in increased adsorption for longer chain length compounds. Also, a longer chain length gives more hydrophobicity to the surfactant, thus facilitating increased adsorption due to associative interaction resulting in the formation of hemimicelles. The sudden increase in adsorption at the first plateau region causes charge reversal of the PTFE surface (Fig. 5). It can also be seen from Fig. 4 that the second plateau region is located nearer to the CMC of the surfactant, above which adsorption remains more or less constant. The concentration of monomer remains constant above the CMC; thus one may conclude that the observed constancy of adsorption above the CMC is due to the adsorption of monomers only and not micelles (9, 10). The concentration at which charge reversal takes place depends upon the chain length of the surfactant, indicating that apart from electrostatic interaction, the effect of solvent in the interfacial region, as well as of the hydrophobicity, plays an important role in the adsorption of these surfactants. The adsorption isotherms of CPC (C16 ) and DPC (C12 ) are shown in Fig. 6; the corresponding zeta potential curves are shown in Fig. 7. In the case of alkylpyridinium chloride there is no significant effect of chain length variation on adsorption and zeta potential (Figs. 6 and 7). Hence the hydrophobic contribution is negligible during the adsorption of these surfactants onto the PTFE surface. In view of the known inertness of PTFE, there is unlikely to be any significant interaction between any of the head groups and the –CF2 groups on the PTFE surface. Therefore, the most likely interaction appears to be that between the positively charged head groups of cationic surfactants and
FIG. 7. Zeta potential of PTFE in alkylpyridinium chloride surfactants.
the negatively charged carboxylate groups ( –COO 0 ) on the PTFE surface. The –CH 20 groups cannot fit on the PTFE surface that has –CF 20 groups, due to conformatory mismatch (11). Thus, a perpendicular arrangement is entropically favorable (1). The considerable effect of the chain length on the adsorption of alkyltrimethylammonium surfactant may be ascribed to surfactant molecules tilted away from the perpendicular (Figs. 4 and 5). In this case considerable hydrophobic interaction can take place and the effect of chain length is significant. In alkylpyridinium surfactants, however, a predominantly perpendicular arrangement appears to be responsible for the insignificant effect of chain length variation on adsorption (Figs. 6 and 7). Thermodynamics of Hemimicellization Gao et al. (12) considered the thermodynamics of adsorption and hemimicellization of surfactant ions and developed an equation based on the assumption that each adsorbed surfactant molecule in the first plateau region is an active site for surface aggregation. At equilibrium, considering the aggregation of nhm monomers of surfactant in solution onto an adsorbed monomer to form a hemimicelle, we may write (nhm 0 1)(Monomer in solution) / (Adsorbed monomer) ` (Hemimicelle)
ahm Å K, ahm01as
FIG. 6. Adsorption isotherms of alkylpyridinium chloride surfactants.
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[1]
where a is the activity of a surfactant monomer in solution, and for a dilute solution a Å c, the concentration of surfac-
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TABLE 1 Mean Aggregation Number of Hemimicelles (nhm), Equilibrium Constant (K), and Free Energy of Hemimicellization of Surfactants on PTFE Surface Surfactant in 0.001 M KCl medium CTAB (16) TTAB (14) DTAB (12) CPC (16) DPC (12)
` t1st
CMC (M) 5 2 1.2 4.8 9.6
1 1 1 1 1
1004 1003 1002 1004 1003
2.16 2.31 2.7 5.8 5.8
1 1 1 1 1
nhm 1004 1004 1004 1004 1004
7 6 5 4 4
tant solution, if C õ CMC, as and ahm are the activities of adsorbed monomer and hemimicelle, respectively, and K is the equilibrium constant. For a PTFE surface with a low charge, the adsorbed amount of monomer is ts and that of hemimicelle is thm . Thus Eq. [1] becomes thm Å K. Chm01ts
[2]
K 1.6 1.82 2.99 2.73 1.48
1 1 1 1 1
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128.8 103.07 76.47 59.22 57.71
18.4 17.1 15.1 14.80 14.43
ACKNOWLEDGMENT One of the authors, Ajay K. Vanjara, acknowledges Reliance Induries Ltd. (India) for financial support of this work.
[3] REFERENCES
where t is the amount of surfactant adsorbed at concentration C and t `1st is the amount adsorbed at the first plateau in the isotherm. The above equation can be used to calculate the optimum values of nhm and the equilibrium constant (K) by trial and error. Subsequently, the standard free energy change for hemimicellization ( DG 7 ) can be calculated from the value of the equilibrium constant ( DG 7 Å 0 RT ln K). The calculated values of nhm , DG 7, and DG 7 per mole of hemicelle for alkyltrimethylammonium surfactant and alkylpyridinium surfactant adsorption onto PTFE surface are given in Table 1. The magnitude of DG 7 for hemimicellization is reasonable when compared with earlier work by Gao et al. (12, 13). It will be seen that for alkyltrimethylammonium surfactants, as the chain length of the surfactant increases from DTAB (C12 ) to CTAB (C16 ), nhm also increases, indicating the significant hydrophobic interaction between adsorbed surfactant ions in the formation of hemimicelle. In the case of alkylpyridinium surfactants this effect is not observed, due to their predominantly perpendicular arrangement, and hence nhm remains constant. It has been reported that micelles and hemimicelles behave in analogous manners (14, 15). When we compare the CMCs of surfactants (Table 1), we can see that the chain length of the surfactant plays an important role in the micel-
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0DG7/nhm for 1 mol of surfactant (kJ)
lar behavior of these surfactants. The tendency of alkyltrimethylammonium surfactants to form hemimicelles is analogous to that of micelle formation. In alkylpyridinium surfactants hemimicelles and micelles behave in different manners. This can be attributed to the predominantly perpendicular arrangement of these surfactants, leading to negligible hydrophobic interaction between adsorbed surfactant molecules and hence negligible effect of chain length.
Gao et al. extended the above equation as follows, ( t 0 t `1st ) Å K, C nhm01 (nhmrt `1st 0 t )
0DG7 for 1 mol of hemicelle (kJ)
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1. Bee, H. A., Ottewill, R. H., and Rance, D. G., in ‘‘Adsorption from Solution’’ (R. H. Ottewill and C. H. Rochester, Eds.), p. 155. Academic Press, New York, 1982. 2. Clunie, J. S., and Ingram, B. T., in ‘‘Adsorption from Solution’’ (G. D. Parfitt and C. H. Rochester, Eds.), p. 105. Academic Press, London, 1983. 3. Bisco, P. D., Cartledge, J. G., Keesom, W. H., and Radke, C. J., J. Colloid Interface Sci. 78, 225 (1980). 4. Yao, J., and Strauss, G., Langmuir 7, 2353 (1991). 5. Yao, J., and Strauss, G., Langmuir 8, 2274 (1992). 6. Few, A. V., and Ottewill, R. H., J. Colloid Interface Sci. 11, 34 (1956). 7. Von Somluchowski, M., in ‘‘Handbuch der Elektrizital und des Magnetismus,’’ (C. L. Graetz, Eds.), Vol. II, p. 336. Barth, 1921. 8. Somasundaran, P., Middleton, R., and Viswanathan, K. V., in ‘‘Structure/Performance Relationship in Surfactants’’ (M. J. Rosen, Ed.), ACS Symposium Series, p. 267. Amer. Chem. Soc., New York, 1984. 9. Gao, Y., Yue, C., Lu, S., and Gu, W., J. Colloid Interface Sci. 100, 581 (1984). 10. Scamehorn, J. F., Schechter, R. S., and Wade, W. H., J. Colloid Interface Sci. 85, 463 (1982). 11. Fowkes, F. M., Ind. Eng. Chem. 56, 40 (1964). 12. Gao, Y., Du, J., and Gu, T., J. Chem. Soc. Faraday Trans. 1 83, 2671 (1987). 13. Gu, T., Gao, Y., and He, L., J. Chem. Soc. Faraday Trans. 1 84, 4471 (1988). 14. Wakamtsu, T., and Fuerstenau, D. W., Adv. Chem. Ser. 79, 161 (1968). 15. Bitting, D., and Harwell, J. H., Langmuir 3, 500 (1987).
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Adsorption of Alkyltrimethylammonium Bromide and Alkylpyridinium Chloride Surfactant Series on Polytetrafluoroethylene Powder AJAY K. VANJARA AND SHARAD G. DIXIT 1 Department of Chemical Technology, University of Bombay, Matunga, Bombay 400019, India Received March 29, 1995; accepted June 14, 1995
The adsorption and zeta potential of three different alkyltrimethylammonium bromides, CTAB, TTAB, and DTAB, and two alkylpyridinium chlorides, CPC and DPC, on a PTFE surface were studied. Adsorption isotherms having characteristics similar to those of two plateau regions were observed for all these surfactants. In the case of alkyltrimethylammonium bromide, there is a considerable effect of chain length which could be attributed to the arrangement of surfactant molecules being tilted away from the perpendicular. In the case of alkylpyridinium surfactants, an insignificant effect of chain length is due to a predominantly perpendicular arrangement. The thermodynamics of hemimicellization has been described by evaluating nhm as well as free energy of hemimicellization. q 1996 Academic Press, Inc. Key Words: alkyltrimethylammonium bromide; alkylpyridiniumchloride; adsorption on PTFE.
INTRODUCTION
Polymer surfaces display various kinds of surface chemistry due to their low surface energy and inertness. Among them, polytetrafluoroethylene (PTFE) shows a large degree of hydrophobicity and oleophobicity. PTFE is extensively used as a biomaterial and in several other applications where chemical inertness is desirable. The initial interest in adsorption of surfactants on PTFE and other polymers was in relation to polymer latex particles (1). It was shown that the PTFE surface acquired chemical functional groups during the process of polymerization and they could not be removed completely even with extensive cleaning by dialysis. As a result, characterization of the latex surface became somewhat difficult. Adsorption of cationic, anionic, and nonionic surfactants on PTFE latex particles has been investigated. In particular, decyltrimethylammonium bromide adsorption on PTFE was studied. Direct electrostatic interaction with residual acquired carboxylic groups on the PTFE surface was found to occur. However, interaction between hydrocarbon chains of the surfactant and fluorocarbon chains of PTFE was found to be rather weak due to conformational mismatch (1). The formation of aggregate and other organized structures on the polymer surface has also been reported (2, 3). 1
To whom correspondence should be addressed.
Yao and Strauss (4) studied adsorption of quarternary ammonium surfactants onto chromatographic beads of PTFE from methanol solution. It was found that whereas one- and two-chain alkyl surfactants exhibit saturation, the triplechain surfactant does not. This was attributed to the formation of a monolayer in the cases of single- and double-chain alkyl surfactants. In the absence of hydration, the adsorbate was immobile at all surface densities as it lay flat on the PTFE surface. The hydrated adsorbate exhibits mobility at full surface coverage because surfactant chains are packed normal to the surface as in the case of a triple-chain surfactant. The above authors (5) found that only the vertically oriented mobile surface layer could accommodate secondary lipophilic adsorbate which results in its binding. Thus the orientation of adsorbed surfactant molecules is of interest for the binding of antibiotics and other drugs to surface modified surgical grafts. No studies of adsorption from aqueous solution were reported by the above authors. In the present investigation adsorption of three different alkyltrimethylammonium surfactants and two alkylpyridinium surfactants from aqueous solution onto commercially available PTFE powder has been studied. These cationic surfactants vary in hydrocarbon chain length only. An investigation was planned to study (i) the role of hydrocarbon chain length, (ii) the role of acquired surface functional groups, (iii) electrostatic interaction as determined by zeta potential measurements, (iv) the nature of adsorbed species, and (v) aggregation taking place at the surface. MATERIALS AND METHODS
Materials Polytetrafluoroethylene (PTFE). PTFE powder was procured from Wilson Laboratories (Bombay). The average particle size of PTFE powder was determined on a GALAICIS-1 computerized inspection system. The surface area of PTFE powder was measured by BET (N2 adsorption). Characterization of PTFE powder was carried out by conductometric and FTIR measurements. Surface active agents. Cetyltrimethylammonium bromide (CTAB) was obtained from Bombay Oil Ltd. (Bom-
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bay). It was 98% pure and was recrystallized twice from an acetone–methanol (3:1) mixture. Tetradecyltrimethylammonium bromide (TTAB) was obtained from Sisco Research Lab. (Bombay). It was 99% pure and was used as received. Dodecyltrimethylammonium bromide (DTAB) was received from Sigma Chemicals Co. It was 99% pure and was used as received. Cetylpyridinium chloride monohydrate (CPC) was procured from Sisco Research Lab. It was 98% pure and was recrystallized twice from acetone. Dodecylpyridinium chloride monohydrate (DPC) was obtained from Aldrich Chemical Co. Inc. It was 98% pure and was used as received. Other chemicals were of analytical grade. Double distilled water was used in all the experiments. Methods Critical micelle concentration. The critical micelle concentrations (CMC) of the surface active agents used were obtained from conductometric measurements as a function of concentration at 307C. The values obtained in 10 03 M potassium chloride solution are given in Table 1. Determination of adsorbed surfactants. In an Erlenmeyer flask, 10 ml of surfactant solution in 10 03 M KCl was equilibrated with 200 mg of PTFE powder for 24 h at 307C and pH 7 { 0.05 on a flask shaker. The solid particles were separated from the supernatant liquid by centrifugation for 15 min. The clear supernatant was removed for analysis of surface active agents. The concentration of cationic surfactants was determined by using the dye transfer technique described by Few and Ottewill (6). The average error of the adsorption experiments was {5%. Measurement of zeta potential. The zeta potential of PTFE particles (5 mg) suspended in surfactant solution (50 ml) was measured with a zeta meter (Rank Brothers Mark II, UK). The suspension of PTFE in surfactant solution was kept on a flask shaker for 24 h at a constant temperature of 307C before the zeta potential measurement began. At least 10 particles were tracked to determine electrophoretic mobility. The zeta potential was calculated from electrophoretic mobility using Smoluchowski’s equation (7). Conductometric titration. Determination of surface carboxylic groups on PTFE powder was carried out by conductometric titration with 0.001 N NaOH solution. Five grams of PTFE powder was suspended in 125 ml of water containing absolute ethanol on flask shaker for 24 h before conductometric titration began. An inert atmosphere was created by passing N2 gas through the solution. FTIR spectroscopy. The infrared spectra were obtained on a JASCO FT/IR300 E Fourier transform infrared spectrophotometer. Each spectrum was recorded at a resolution of 4 cm01 with auto scans. Measurements were made in the spectral region 400–4000 cm01 . The spectrum of PTFE was recorded in the KBr pellet.
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FIG. 1. Zeta potential of PTFE in 0.001 M KCl at various pH.
RESULTS AND DISCUSSION
Characterization of PTFE Surface First, the surface of the PTFE powder used in the present investigation was characterized. The average particle size of PTFE was determined to be 1.65 mm and the surface area was found to be 5.4 m2 /g. The zeta potential of PTFE powder in 10 03 M KCl was measured; the results are shown in Fig. 1. It can be seen that PTFE particles are negatively charged and the charge remains more or less constant over the pH range 5 to 9.5 at 6 { 1 mV. It decreases below pH 5. IR Spectral Study It has been mentioned in the literature (1) that the surface of PTFE acquires carboxylic functional groups during polymerization. Therefore, it was decided to look for any functional groups using FTIR. The spectra were recorded after the powder was brought in contact with water having different pH values for 24 h. Expanded spectra between 1500 and 1800 cm01 are shown in Fig. 2 for pH 4.1, 6.4, and 9.1. It will be seen that the spectra corresponding to pH 4.1 show well-defined peaks at 1710 and 1745 cm01 (Fig. 2a). The peak at 1710 cm01 may be ascribed to the carboxylic ( –COOH) groups in the dimer, whereas the peak at 1745 cm01 may be ascribed to monomeric –COOH groups. As the pH increases to 6.4, peaks at 1724 and 1743 cm01 are seen (Fig. 2b). This again represents the peak due to antisymmetric stretching vibrations of {C|O in the –COOH groups. The spectra corresponding to pH 9.1 show strong peaks at 1710 and 1529 cm01 (Fig. 2c), whereas the spectrum around 1725 cm01 appears to have merged with the 1710 cm01 peak to give a broad peak. The appearance of a new peak at 1529 cm01 clearly indicates the reaction of the alkali with –COOH groups, since the peak at 1529 cm01 is
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FIG. 4. Adsorption isotherms of alkyltrimethylammonium bromide surfactants.
˚ 2 for close sectional area per –COOH group to be 25 A packing, it has been calculated that the –COOH groups occupy 29% of the total surface area. Thus, the presence of –COOH groups on the surface is quite significant and is expected to play a major role as far as adsorption on the PTFE surface is concerned. Adsorption and Zeta Potential Measurement
FIG. 2. IR spectra of PTFE at various pH: (a) pH 4.1, (b) pH 6.4, and (c) pH 9.1.
Adsorption of three different alkyltrimethylammonium bromides, CTAB (C16 ), TTAB (C14 ), and DTAB (C12 ), was studied. They vary in hydrocarbon chain length only. The adsorption isotherms are shown in Fig. 4 and the corresponding zeta potential versus concentration curves are shown in Fig. 5. Adsorption isotherms of CTAB, TTAB,
characteristic of –COO 0 groups. The FTIR study clearly establishes the presence of –COOH groups on the surface of PTFE. This was further confirmed by conductometric titration of PTFE powder suspended in a water–ethanol mixture (Fig. 3) against 0.001 N NaOH solution. A clear neutralization point was obtained, as seen from Fig. 3. Assuming the cross-
FIG. 3. Conductometric titration of PTFE with 0.001 M NaOH.
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FIG. 5. Zeta potential of PTFE in alkyltrimethylammonium bromide surfactants.
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and DTAB are similar, showing two plateau regions each. The corresponding zeta potential curves (Fig. 5) show similar variation. The adsorption of CTAB over the entire concentration is higher than that of TTAB, which is higher than that of DTAB. Thus increasing chain length results in increased adsorption. A significant effect of chain length in the lower concentration range indicates the effect of the interfacial water, which has a reduced dielectric constant. Obviously a greater decrease in free energy results in increased adsorption for longer chain length compounds. Also, a longer chain length gives more hydrophobicity to the surfactant, thus facilitating increased adsorption due to associative interaction resulting in the formation of hemimicelles. The sudden increase in adsorption at the first plateau region causes charge reversal of the PTFE surface (Fig. 5). It can also be seen from Fig. 4 that the second plateau region is located nearer to the CMC of the surfactant, above which adsorption remains more or less constant. The concentration of monomer remains constant above the CMC; thus one may conclude that the observed constancy of adsorption above the CMC is due to the adsorption of monomers only and not micelles (9, 10). The concentration at which charge reversal takes place depends upon the chain length of the surfactant, indicating that apart from electrostatic interaction, the effect of solvent in the interfacial region, as well as of the hydrophobicity, plays an important role in the adsorption of these surfactants. The adsorption isotherms of CPC (C16 ) and DPC (C12 ) are shown in Fig. 6; the corresponding zeta potential curves are shown in Fig. 7. In the case of alkylpyridinium chloride there is no significant effect of chain length variation on adsorption and zeta potential (Figs. 6 and 7). Hence the hydrophobic contribution is negligible during the adsorption of these surfactants onto the PTFE surface. In view of the known inertness of PTFE, there is unlikely to be any significant interaction between any of the head groups and the –CF2 groups on the PTFE surface. Therefore, the most likely interaction appears to be that between the positively charged head groups of cationic surfactants and
FIG. 7. Zeta potential of PTFE in alkylpyridinium chloride surfactants.
the negatively charged carboxylate groups ( –COO 0 ) on the PTFE surface. The –CH 20 groups cannot fit on the PTFE surface that has –CF 20 groups, due to conformatory mismatch (11). Thus, a perpendicular arrangement is entropically favorable (1). The considerable effect of the chain length on the adsorption of alkyltrimethylammonium surfactant may be ascribed to surfactant molecules tilted away from the perpendicular (Figs. 4 and 5). In this case considerable hydrophobic interaction can take place and the effect of chain length is significant. In alkylpyridinium surfactants, however, a predominantly perpendicular arrangement appears to be responsible for the insignificant effect of chain length variation on adsorption (Figs. 6 and 7). Thermodynamics of Hemimicellization Gao et al. (12) considered the thermodynamics of adsorption and hemimicellization of surfactant ions and developed an equation based on the assumption that each adsorbed surfactant molecule in the first plateau region is an active site for surface aggregation. At equilibrium, considering the aggregation of nhm monomers of surfactant in solution onto an adsorbed monomer to form a hemimicelle, we may write (nhm 0 1)(Monomer in solution) / (Adsorbed monomer) ` (Hemimicelle)
ahm Å K, ahm01as
FIG. 6. Adsorption isotherms of alkylpyridinium chloride surfactants.
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coidal
[1]
where a is the activity of a surfactant monomer in solution, and for a dilute solution a Å c, the concentration of surfac-
AP: Colloid
363
SURFACTANT ADSORPTION ON POLYTETRAFLUOROETHYLENE
TABLE 1 Mean Aggregation Number of Hemimicelles (nhm), Equilibrium Constant (K), and Free Energy of Hemimicellization of Surfactants on PTFE Surface Surfactant in 0.001 M KCl medium CTAB (16) TTAB (14) DTAB (12) CPC (16) DPC (12)
` t1st
CMC (M) 5 2 1.2 4.8 9.6
1 1 1 1 1
1004 1003 1002 1004 1003
2.16 2.31 2.7 5.8 5.8
1 1 1 1 1
nhm 1004 1004 1004 1004 1004
7 6 5 4 4
tant solution, if C õ CMC, as and ahm are the activities of adsorbed monomer and hemimicelle, respectively, and K is the equilibrium constant. For a PTFE surface with a low charge, the adsorbed amount of monomer is ts and that of hemimicelle is thm . Thus Eq. [1] becomes thm Å K. Chm01ts
[2]
K 1.6 1.82 2.99 2.73 1.48
1 1 1 1 1
10/22 10/18 10/10 10/10 10/10
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128.8 103.07 76.47 59.22 57.71
18.4 17.1 15.1 14.80 14.43
ACKNOWLEDGMENT One of the authors, Ajay K. Vanjara, acknowledges Reliance Induries Ltd. (India) for financial support of this work.
[3] REFERENCES
where t is the amount of surfactant adsorbed at concentration C and t `1st is the amount adsorbed at the first plateau in the isotherm. The above equation can be used to calculate the optimum values of nhm and the equilibrium constant (K) by trial and error. Subsequently, the standard free energy change for hemimicellization ( DG 7 ) can be calculated from the value of the equilibrium constant ( DG 7 Å 0 RT ln K). The calculated values of nhm , DG 7, and DG 7 per mole of hemicelle for alkyltrimethylammonium surfactant and alkylpyridinium surfactant adsorption onto PTFE surface are given in Table 1. The magnitude of DG 7 for hemimicellization is reasonable when compared with earlier work by Gao et al. (12, 13). It will be seen that for alkyltrimethylammonium surfactants, as the chain length of the surfactant increases from DTAB (C12 ) to CTAB (C16 ), nhm also increases, indicating the significant hydrophobic interaction between adsorbed surfactant ions in the formation of hemimicelle. In the case of alkylpyridinium surfactants this effect is not observed, due to their predominantly perpendicular arrangement, and hence nhm remains constant. It has been reported that micelles and hemimicelles behave in analogous manners (14, 15). When we compare the CMCs of surfactants (Table 1), we can see that the chain length of the surfactant plays an important role in the micel-
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0DG7/nhm for 1 mol of surfactant (kJ)
lar behavior of these surfactants. The tendency of alkyltrimethylammonium surfactants to form hemimicelles is analogous to that of micelle formation. In alkylpyridinium surfactants hemimicelles and micelles behave in different manners. This can be attributed to the predominantly perpendicular arrangement of these surfactants, leading to negligible hydrophobic interaction between adsorbed surfactant molecules and hence negligible effect of chain length.
Gao et al. extended the above equation as follows, ( t 0 t `1st ) Å K, C nhm01 (nhmrt `1st 0 t )
0DG7 for 1 mol of hemicelle (kJ)
coidal
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AP: Colloid