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Langmuir 2006, 22, 5636-5644
Sorption of Poly(hexamethylenebiguanide) on Cellulose: Mechanism of Binding and Molecular Recognition† Richard S. Blackburn,*,‡ Anna Harvey,‡ Lorna L. Kettle,§ John D. Payne,| and Stephen J. Russell‡ Green Chemistry Group, Centre for Technical Textiles, UniVersity of Leeds, Leeds, LS2 9JT, U.K., Computational Chemistry Group, Intertek, Hexagon House, Blackley, Manchester, M9 8ZS, U.K., and Arch Chemicals (U.K.) Ltd, Hexagon House, Blackley, Manchester, M9 8ZS, U.K. ReceiVed NoVember 8, 2005. In Final Form: May 2, 2006 Antimicrobial agents such as poly(hexamethylene biguanide) (PHMB) find application in medical, apparel, and household textile sectors; although it is understood that certain concentrations need to be applied to achieve suitable performance, there has been very little work published concerning the interactions of the polymer and its adsorption mechanism on cellulose. In this paper, such physical chemistry parameters are examined and related to computational chemistry studies. Adsorption isotherms were constructed: at low concentrations, these were typical Langmuir isotherms; at higher concentrations, they were more indicative of Freundlich isotherms, attributed to a combination of electrostatic and hydrogen-bonding forces, which endorsed computational chemistry proposals. At lower concentrations, electrostatic interactions between PHMB and carboxylic acid groups in the cellulose dominate with a contribution to binding through hydrogen bonding; as the concentration of PHMB increases, hydrogen bonding with cellulose becomes increasingly dominant. At high PHMB concentrations, observations of increasing PHMB adsorption are attributed to monolayer aggregation and multilayer stacking of PHMB through electrostatic interactions with counterions and hydrogen bonding of biguanide groups.
Introduction Antimicrobial agents such as poly(hexamethylenebiguanide hydrochloride) (PHMB, 1) find application in medical, apparel, and household textile sectors.3,4 The effectiveness of PHMB has been studied5 in terms of the ability to “kill” or inhibit the growth of a wide range of microbes, and its use in textile finishing provides effective performance in the end product. Life cycle analysis work that has recently been carried out suggests that the active species can potentially reduce the water, energy, and chemical consumption in the life cycle of a treated product.6
Although it is understood that certain concentrations need to be applied to achieve suitable performance, there has been very little work published concerning the interactions of the polymer and its adsorption mechanism on cellulose. Understanding the mode of action fully, in particular the physical attractions that † Some preliminary findings discussed in this paper were presented at the 228th American Chemical Society National Meeting, August 22-27, 2004, Philadelphia, PA.1,2. * Corresponding author. E-mail:
[email protected]. ‡ Green Chemistry Group, Centre for Technical Textiles, University of Leeds. § Computational Chemistry Group, Intertek. | Arch Chemicals (UK) Ltd.
(1) Blackburn, R. S.; Harvey, A.; Payne, J.; Kettle, L. L. Abstracts of Papers, 228th National Meeting of the American Chemical Society, Philadelphia, PA, August 23, 2004; American Chemical Society: Washington, DC, 2004; 211POLY. (2) Blackburn, R. S.; Harvey, A.; Kettle, L. L.; Payne, J. D.; Russell, S. J. Polym. Prepr. (Am. Chem. Soc., DiV. Polym. Chem.) 2004, 45, 606. (3) Payne, J. J. Soc. Dyers Colour. 1997, 113, 48. (4) Miller, W. M.; White, C. W. NonwoVens World 1986, 129. (5) Wallace, M. L. AATCC ReV. 2001, 1, 18. (6) Blackburn, R. S.; Payne, J. D. Green Chem. 2004, 6, G59.
exist between the polymer and the substrate, could lead to developments in durability during washing and to more efficient application processes. Furthermore, understanding the sorption mechanisms involved and molecular recognition for PHMB would allow future investigation of the application to other fibers such as polyester (PET). Cotton is mainly composed of cellulose, which is essentially a polymer of R-D-glucose units in the 4C1 conformation with β-(1 f 4) linkages. Carboxylic acid groups are formed in the cellulose through oxidation of glucose rings during processing operations such as bleaching or mercerizing, 7 and these provide anionic sites for adsorption.8 Adsorption of cationic polyelectrolytes (CPE) on cellulose has been described in theory as an ion-exchange process with a close 1:1 stoichiometry between charges on polymer and charges on fiber; hence, one cationic center in the CPE associates with one carboxylic acid group in the cellulose.9-11 Additionally, cellulosic fibers have a porous structure, with micropores ranging from 10 to 300 Å in diameter and a rough surface, hence molecular mass and size are important factors in adsorption.12,13 Adsorption occurs by the following processes: transport of CPE from solution to the surface of the substrate, attachment of the CPE on the surface of the substrate, reconformation of the CPE on the surface of the substrate, and detachment of the CPE from the surface (desorption).14 (7) Nevell, T. P. In The Dyeing of Cellulosic Fibres; Preston, C., Ed.; Dyers Co. Publications Trust: Bradford, U.K., 1986. (8) Stana-Kleinschek, K.; Ribitsch, V.; Kreze, T.; Fras, L. Mater. Res. InnoVations 2002, 6, 13. (9) Winter, L.; Wågberg, L.; O ¨ dberg, L.; Lindstro¨m, T. J. Colloid Interface Sci. 1986, 111, 537. (10) Wågberg, L.; Winter, L.; O ¨ dberg, L.; Lindstro¨m, T. Colloids Surf. 1987, 27, 163. (11) Wågberg, L.; O ¨ dberg, L.; Lindstro¨m, T.; Aksberg, R. J. Colloid Interface Sci. 1988, 123, 287. (12) Ha¨ggkvist, M.; Li, T.-Q.; O ¨ dberg, L. Cellulose 1998, 5, 33. (13) Alince, B. J. Appl. Polym. Sci. 1990, 39, 355. (14) van de Ven, T. G. M. AdV. Colloid Interface Sci. 1994, 48, 121.
10.1021/la053002b CCC: $33.50 © 2006 American Chemical Society Published on Web 06/06/2006
Sorption of Poly(hexamethylenebiguanide) on Cellulose
Much work has been done by Wågberg on adsorption of CPE on cellulose. Adsorption of low-molecular-mass 3,6-ionene (2, Mr 6000 g mol-1) in electrolyte concentrations less than 10-5 M was rapid, and a 1:1 stoichiometry between the charges on the electrolyte and the carboxylic groups on the fiber was observed, indicating electrostatic attraction. For all cellulose fibers with varying degrees of substitution (DS, % hydroxyl groups oxidized to carboxylates), the adsorption isotherm leveled with increasing application concentration, typical of a Langmuir isotherm. Maximum adsorption concentrations up to 75 mg g-1 for cellulose with DS of 6.9% were observed.9 Adsorption stoichiometry increased to 90% up to 20 mg g-1 adsorbed, but then rapidly reduced to below 70% at higher application concentrations.10 For the higher-molecular-mass poly(diallyldimethylammonium chloride) (DADMAC) (3, Mr 150,000 g mol-1), adsorption was lower and was not stoichiometrically proportional to the charges on the fiber. Again, the adsorption isotherm leveled with increasing application concentration, with a maximum adsorption of adsorption at 12 mg g-1. The DADMAC polymer is about 10 times the length of 3,6-ionene, which indicates that the carboxylic acid groups in the smaller pores of the cellulose were inaccessible and the large polymer mainly adsorbs on the surface. Most pores in cellulosic fibers have a range up to 50100 Å, so the radius of gyration (Rg) of the DADMAC polymer must be greater than 50-100 Å. Further work on the adsorption of three DADMAC polymers with varying molecular weight and Rg (low Mr 8750 g mol-1, Rg 86 Å; medium Mr 48 000 g mol-1, Rg 166 Å; high Mr 1 200 000 g mol-1, Rg 722 Å) on bleached cellulose compounded this observation.15 A Langmuirtype adsorption isotherm was observed with a plateau region for all polymers: low Mr polymer showed maximum adsorption at 11 mg g-1, as it was able to reach all charges in the fiber wall, but high and medium Mr polymers displayed maximum adsorption at 2.5 mg g-1 and were restricted to the external surfaces of fibers. It was concluded that the polymer needed to be smaller than 80 Å to have accessibility to all charges in the fiber wall. There are two proposals for the migration of polymers into the pores of the fibers: they may penetrate the fiber wall through a reptation process11 or through a simple diffusion process.15 The thickness of the cellulose fiber wall is approximately 10 µm in the swollen state, which is a considerable distance, but evidence suggests that CPE diffuse into the fiber from the surface as long as there is an electrostatic driving force for further adsorption.9 Other studies13,16,17 have demonstrated that different CPE reach different levels of the fiber wall and different structural levels of the microcrystalline cellulose substrate depending partly on their molecular mass, but mainly on their size (Rg). In terms of adsorption, it can be concluded that there exists a relationship between the size of CPE molecules and the pore size of the fiber wall. In examining the adsorption of high Mr (8 × 106 g mol-1), medium Mr (4 × 105 g mol-1), and low Mr (2 × 104 g mol-1) cationic polyacrylamides (C-PAM) with the same charge density on cellulose, it was observed that desorption was proportional to molecular weight only, high Mr C-PAM demonstrating lowest desorption as a result of a greater number of ionic association points per polymer molecule.17 Wåbgerg10 found that adsorption stoichiometry decreased with increase in concentration of polyelectrolyte applied and postulated that the polyelectrolyte was adsorbed with a larger fraction of the segments in loops and tails as the surfaces were completely saturated, indicating that (15) Wågberg, L.; Ha¨gglund, R. Langmuir 2001, 17, 1096. (16) Lindstro¨m, T.; So¨remark, K. J. Colloid Interface Sci. 1976, 55, 305. (17) Tanaka, H.; O ¨ dberg, L.; Wågberg, L.; Lindstro¨m, T. J. Colloid Interface Sci. 1990, 134, 229.
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the conformation of the adsorbed polyelectrolytes changed with increasing application concentration. As yet, there has been no published work that measures the conformation of CPE of cellulosic surfaces. Initial work on the effect of salt concentration suggested that, at low salt concentrations, CPE adsorb in a flat conformation and that adsorption increases with increase in salt concentration; however, work by van de Steeg et. al. demonstrated that this is not the absolute case and considered the effects of screening.18 In screening-enhanced adsorption, adsorption increases with increasing salt concentration because the salt screens repulsion between polymer segments and this is the dominating effect. In screening-reduced adsorption, adsorption decreases with increasing salt concentration because the dominating effect is the screening of attraction between CPE and the substrate. If attraction is purely electrostatic, which operates over long ranges, the adsorption regime is always screening reduced; over short ranges, nonelectrostatic interactions exist, and in this case, both screening-enhanced and screening-reduced adsorption are found. For polyelectrolytes with a high charge density, the only driving force for adsorption is the charge interaction between polymer segments and the charge on the substrate. As salt concentration is increased, the interaction between the segments and the surface is decreased and the adsorption is hence decreased. When there is a nonelectrostatic contribution to adsorption, as electrolyte concentration is increased, repulsion between polymer segments on the surface of the substrate is decreased and adsorption is increased. However, if there is a specific competition reaction between electrolyte counterions and the surface, then with increasing electrolyte concentration the adsorption will decrease. When polyelectrolytes have a low charge density, adsorption always tends to be higher than that for polymers with high charge density, but in general, adsorption decreases with increase in salt concentration, as adsorption relies upon both electrostatic and nonelectrostatic interactions.18 The authors specifically examined the adsorption of cationic amylopectin (Mr 1 × 107 to 6.5 × 107 g mol-1) on cellulose as a function of NaCl concentration and observed an optimum concentration of electrolyte.19 This was explained by the adsorbed amount increasing with increasing electrolyte concentration due to screening of the charges on the CPE, which was only possible when nonelectrostatic efficiency was sufficiently high. At higher electrolyte concentration, salt cations displaced the CPE from adsorption sites resulting from nonelectrostatic interaction of the salt cations with the cellulose, effecting a reduction in total adsorption concentration. O ¨ dberg et al. examined the thickness of adsorbed layers of high-molecular-weight C-PAM and confirmed that, at low ionic strength, the polymer is essentially adsorbed through an ionexchange process. Upon increasing the ionic strength to 0.5 M NaCl, there was a 4-fold increase in the thickness in the adsorbed layer, which was attributed to flocculation of the polymer at the fiber surface.20 In the adsorption of cationic surfactants on oxides (silica and alumina),21-25 the common feature of experimental isotherms (18) van de Steeg, H. G. M.; Cohen Stuart, M. A.; de Keizer, A.; Bijsterbosch, B. H. Langmuir 1992, 8, 2538. (19) van de Steeg, H. G. M.; Cohen Stuart, M. A.; de Keizer, A.; Bijsterbosch, B. H. Colloids. Surf., A: 1993, 70, 77. (20) O ¨ dberg, L.; Sandberg, S.; Welin-Klintstro¨m, S.; Arwin, H. Langmuir 1995, 11, 2621. (21) Fuerstenau, D. W. J. Colloid Interface Sci. 2002, 256, 79. (22) Koopal, L. K.; Lee, M. E.; Bo¨hmer, M. R. J. Colloid Interface Sci. 1995, 170, 85. (23) Harwell, J. H.; Hoskin, J.; Schechter, R. S.; Wade, W. H. Langmuir 1985, 1, 251.
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with surface aggregation is their shape, which can be divided into three distinct phases in the theoretical model: (1) ionic surfactant molecules are adsorbed individually through an ionexchange mechanism with no interaction between adjacent surfactant molecules; (2) when the surfactants begin to interact, monolayered (hemimicelle) and bilayered (admicelle) aggregates form, which boost adsorption and gives a rise in adsorption; (3) the attractive interaction is counteracted by electrostatic repulsion of the charged headgroups of the surfactants, and the saturation plateau is attained. Some authors describe two additional phases, where further hemimicelle and admicelle formation can allow a surface-stacking effect.26,27 Recent work by Alila et al.28 on the adsorption of cationic surfactants onto cellulose demonstrated that adsorption proceeded by electrostatic and dispersive interactions. Increase in the negative charge density of cellulose improved the electrostatic forces, promoting surfactant adsorption and selfassembly, thus allowing dense packing of adsorbed molecules into hemimicelles and admicelles aggregates. In this paper, physical chemistry parameters that describe the adsorption of PHMB on cotton are examined through various experimental and analytical techniques and related to computational chemistry studies and sorption isotherm theory. Experimental Section After application, analytical techniques were used to quantify the concentration of active species adsorbed onto the fiber: UVvisible spectrophotometry of the residual solution and staining of PHMB treated fabric with eosin (PHMB and eosin form a colored complex). Materials. PHMB was kindly supplied by Arch Chemicals; this was in the commercial form of Reputex 20, a 20% aqueous solution, with an average Mr of 4600 g mol-1 (1, n ) 21). The fabric used was bleached, mercerized, 100% cotton, plain weave, 150 g m-2, supplied by Whaley’s, Bradford, which had been washed five times using ECE detergent in a standard wash cycle. Eosin Y staining indicator (C. I. Acid Red 87, 2′,4′,5′,7′-tetrabromofluorescein) and all other general chemicals were obtained from Aldrich. Application of PHMB. Cotton fabric (5 g) was immersed in an aqueous solution of PHMB, adjusted to pH 7 by using 0.1% NaOH, then placed in stainless steel, sealed dye pots, housed in a laboratory scale Roaches Pyrotec 2000 dyeing machine, using liquor:fiber ratio (LR) of 20:1. The pots were heated to 40 °C over 10 min and then held for 30 min at 40 °C. After the application, the liquor was retained, and the fabric washed for one minute under running cold tap water and dried at room temperature. Eosin Staining. PHMB-treated cotton (0.5 g) was immersed in 50 cm3 of an aqueous solution containing 0.6 g dm-3 Eosin Y and 102 g dm-3 sodium citrate in stainless steel, sealed dye pots, housed in a laboratory scale Roaches Pyrotec 2000 dyeing machine. This was then rotated at ambient temperature for 5 min. The sample was removed and rinsed in warm tap water for approximately 20 min or continued rinsing until no trace of color could be observed in the rinse water. The sample was dried at 50 °C. Color Measurement. After drying, eosin-stained samples were measured using a Datacolor SF600 Spectraflash color spectrophotometer connected to a PC using DCI Color Tools software. From reflectance values (R) at a specified wavelength (λ) of the dyeings, the absorption-scattering coefficient (K/S) of the sample was (24) Adler, J. J.; Singh, P. K.; Patist, A.; Rabinovich, Y. I.; Shah, D. O.; Moudgil, B. M. Langmuir 2000, 16, 7255. (25) Gouloub, T. P.; Koopal, L. K.; Bijsterbosch, B. H.; Sidorova, M. P. Langmuir 1996, 12, 3188. (26) Łajtar, L.; Narkiewicz-Michałek, J.; Rudzinski, W. Langmuir 1994, 10, 3764. (27) Li, B.; Ruckenstein, E. Langmuir 1996, 12, 5052. (28) Alila, S.; Boufi, S.; Belgacem, M. N.; Beneventi, D. Langmuir 2005, 21, 8106.
Blackburn et al. Table 1. pKa Values for Tautomers of PHMB
calculated using the Kubelka-Munk equation (eq 1).29 K/S provides a representation of color strength. K/S )
(1 - Rλ)2 2Rλ
(1)
UV-Visible Spectrophotometry. The exhausted application baths were measured using a Jasko V-530 UV-visible-NIR spectrophotometer at 236 nm, the wavelength of maximum absorption (λmax) for the residual PHMB solution. Concentrations were calculated from calibration graphs. Streaming Potential. The method used was similar to that described in detail in previous work;30,31 an electrolyte solution was forced through the capillary system in which the solid phase (cotton sample) was stationary and the liquid phase (aqueous KCl solution) was mobile. The cotton sample was separated into yarns and soaked overnight in 10-3 M KCl then “sandwiched” in a glass tube by two electrodes to form a plug, the electrodes were connected, and electrical resistance recorded. Different pressures of the electrolyte were pumped around the system; for each pressure interval, the average electrical resistance was recorded. Conductivity of 10-3 M and 10-1 M KCl across the plug were recorded. A linear relationship was obtained between the measured streaming potential E and applied pressure P, the slope of the line (E/P) was used to calculate zeta potential (ζ) using the Helmholtz-Smoluchowski equation (eq 2):32 E DζR ) P 4πηK
(2)
where D and η are the dielectric constant and viscosity, respectively, of the electrolyte solution (10-1 M KCl), R is the resistance of the plug with the experimental solution (10-3 M KCl), and K is the product of the specific conductivity and resistance of the plug of the streaming potential cell (10-1 M KCl).
Computational Chemistry Physical Properties and Structure of PHMB. The pKa values for the monomer units of the PHMB oligomer were estimated by using ACD Labs LogD software (V7.05) (Table 1). From these estimations, at pH 7, these three species (L, HL, H2L) will most likely exist in their +1 ionized form (HL). For all three types of oligomer, the different tautomers have slightly different pKa values. Calculations were carried out to determine which +1 tautomer is of the lowest energy for these three PHMB derivatives, and it was found by modeling studies that the most (29) McDonald, R. J. Soc. Dyers Colour. 1980, 96, 486. (30) Pusˇic´, T.; Grancaric´, A. M.; Soljacˇic´, I.; Ribitsch, V. J. Soc. Dyers Colour. 1999, 115, 121. (31) Grancaric´, A. M.; Tarbuk, A.; Pusˇic´, T. Color. Technol. 2005, 121, 221. (32) Jacobasch, H. J.; Baubo¨ck, G.; Schurz, J. Colloid Polym. Sci. 1985, 263, 3.
Sorption of Poly(hexamethylenebiguanide) on Cellulose
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Figure 1. (a) Electrostatic potential plotted onto a total electron density isosurface for PHMB. Blue is used to represent the strongest negative electrostatic potential on the molecule, and red the strongest positive electrostatic potential. The other colors are the values between, green is neutral. (b) PCF charges for PHMB.
Figure 2. Interactions of cellulose and PHMB showing evidence of electrostatic forces (blue arrows) and hydrogen bonding (red arrow).
stable form is one that has the +1 charge delocalized over the whole biguanide (BG) section. The geometry of PHMB was optimized by using a DFT method (B88PW91/DZVP), and the electrostatic potential was plotted onto a total electron density isosurface (Figure 1a). The charge distribution was calculated by using point charge fitting (PCF) charges; these point charges should give a reliable indication of the relative charge at the atomic positions (Figure 1b). These computational models indicated that there was extensive delocalization of the positive charge over the polymer repeat unit. Counterions were introduced to the region near each biguanide and the system minimized again (this model was used for all subsequent molecular dynamics calculations). A flat dynamic trajectory was monitored over time and radius of gyration calculated at equilibrium. The structure was in a folded form wrapped around itself in a roughly globular spherical shape with Rg of 22 Å. Cotton Surface Model. The cellulose content of cotton is 95-99%; cellulose is composed of linear chains of covalently linked glucose residues. In the primary plant cell wall, one cellulose polymer consists of roughly 6000 glucose units, and these cellulose polymer chains form alternating crystalline and amorphous structures called microfibrils, each with a diameter of 20-30 nm containing about 2000 molecules; in crystalline sections, cellulose forms three-dimensional lattices due to the formation of the highest possible number of hydrogen bonds.
There are 4 types of cellulose (cellulose I-IV); however, it is known that cellulose I is found in native cellulose existing in two crystal phases namely, IR and Iβ, which can be found not only within the same sample but also along a given microfibril.33 Both coexist in natural cellulose. Iβ-rich specimens have been found in cotton, wood, and ramie fibers (whereas IR-rich samples have been found in bacterial and algal cellulose). Consequently, cellulose Iβ has been used as a model for a cotton surface. A model of the crystalline part of a native microfibril was built from previously published coordinates of the Iβ allomorph; the unit cell comprises three layers where the top and bottom are identical. The separation of layers is approximately 3.8 Å, and there are no intersheet O-H‚‚‚O bonds, therefore, the sheets are held together by only hydrophobic interactions and weak C-H‚‚‚O bonds. Within each sheet, the chains are involved in a network of O-H‚‚‚O hydrogen-bonding. The approximate dimensions of one glucose residue determined experimentally are 4 Å × 6.5 Å. Using the cell parameters (crystal axis length and angles) from experimental data,34 the morphology of the crystal was predicted and the relative surface area of the faces estimated. This model presents two important faces, namely (100) (33) Nishiyama, Y.; Sugiyama, J.; Chanzy, H.; Langan, P. J. Am. Chem. Soc. 2003, 125, 14300. (34) Nishiyama, Y.; Langan, P.; Chanzy, H. J. Am. Chem. Soc. 2002, 124, 9074.
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Blackburn et al.
and (011), of about the same surface area (A detailed structure is given by Supporting Information, Figure S.1). Experimental data35 has shown that 1 kg of cotton contains typically 30 mmol acid groups, which is equivalent to 1.8 × 1022 acid groups. The number of glucose rings can be roughly approximated by dividing the surface area (135 000 m2 kg-1 from water sorption experiments36) by the area of the unit cell for cellulose Iβ: 135 000 m2 kg-1/4.9 × 10-19 m2 ) 2.8 × 1023 glucose rings kg-1. The surface area of one glucose ring in the cotton surface was estimated by calculating the Connolly surface (at the van der Waals radii) for a periodic cellulose system containing 52 glucose units and dividing by the number of monomer units; this suggests that approximately 6.4% of glucose rings have been oxidized to carboxylates. A cellulose surface was modeled by cleaving a layer of glucose chains from the crystal structure, and then the carboxylic acid group was added and the central section of this surface was minimized. A proton was removed and a counterion introduced: the Na+ counterion appears to preferentially be located to have interactions with the COO- group and also the hydroxyl groups on the neighboring chain (A detailed structure is given by Supporting Information, Figure S.2). Further computational studies of the interactions of cellulose and PHMB using molecular dynamics calculations showed evidence of electrostatic forces (with carboxylate groups) and hydrogen bonding (with hydroxyl groups) with this modeled cellulose surface (Figure 2).
Theory
KLCe 1 + aLCe
(3)
where qe is the equilibrium concentration of sorbate on the sorbent (solid-phase) (mg g-1), Ce is the equilibrium sorbate concentration in solution (mg dm-3), KL (dm3 g-1) and aL (dm3 mg-1) are Langmuir constants. The constants KL and aL are evaluated through linearization of eq 3 (eq 4). (35) Schleicher, H.; Lang, H. Papier 1994, 48, 765. (36) Kra¨ssig, H. A. Cellulose: Structure, Accessibility, and ReactiVity; Gordon and Breach Science: Philadelphia, 1993. (37) Langmuir, I. J. Am. Chem. Soc. 1916, 38, 2221. (38) Langmuir, I. J. Am. Chem. Soc. 1918, 40, 1361.
(4)
Therefore, a plot of Ce/qe versus Ce should yield a straight line of intercept value 1/KL and slope aL/KL if the isotherm obtained through experiment observes the Langmuir expression. The theoretical monolayer capacity is q0 and is numerically equal to KL/aL. However, the linearity of eq 4 is only respected at low solution concentrations, where the model follows Henry’s law: as Ce becomes lower, aLCe is much less than unity and qe ) KLCe. Freundlich Isotherm. The Freundlich isotherm39 suggests that sorption energy exponentially decreases on completion of the sorptional centers of an adsorbent and describes heterogeneous systems, which are characterized by the heterogeneity factor 1/nF. When n ) 1/n, the Freundlich equation reduces to Henry’s law. Hence, the empirical equation (eq 5) can be written:
qe ) KFCe1/nF
(5)
where qe is the equilibrium concentration of sorbate on the sorbent (solid-phase) (mg g-1), Ce is the equilibrium sorbate concentration in solution (mg dm-3), KF is the Freundlich constant (dm3 g-1), and 1/nF is the heterogeneity factor. The capacity constant KF and the affinity constant nF are empirical constants dependent on several environmental factors. A linear form of the Freundlich isotherm can be obtained by taking logarithms of eq 5 (eq 6).
ln qe ) ln KF +
Equilibrium Model. To fully understand the sorption system involved between PHMB and cellulose, it is important to establish the most appropriate correlation for the equilibrium curves, which can be obtained by measuring the sorption isotherm of PHMB onto cellulose using experimental data. Langmuir Isotherm. The Langmuir isotherm describes sorption onto specific homogeneous sites within an adsorbent.37,38 Langmuir’s model of adsorption depends on the assumption that intermolecular forces decrease rapidly with distance and consequently predicts the existence of monolayer coverage of the adsorbate (PHMB) at the outer surface of the adsorbent (cellulose). It is then assumed that once a sorbate molecule occupies a site, no further adsorption can take place at that site. Moreover, the Langmuir equation is based on the assumption of a structurally homogeneous adsorbent where all sorption sites are identical and energetically equivalent and there is no interaction between molecules adsorbed on neighboring sites. Theoretically, the sorbent has a finite capacity for the sorbate. Therefore, a saturation value is reached beyond which no further sorption can take place. The saturated or monolayer (as Ct f ∞) capacity can be represented by the expression represented in eq 3:
qe )
Ce aL 1 ) + Ce qe KL KL
1 ln Ce nF
(6)
Therefore, a plot of ln qe versus ln Ce should yield a straight line of intercept value ln KF and slope 1/nF if the isotherm obtained experimentally observes the Freundlich expression; if n > 1, then the adsorption is favorable. The Freundlich isotherm is another form of the Langmuir approach for adsorption on an “amorphous” surface where the amount of adsorbed material is the summation of adsorption on all sites. The Freundlich isotherm is derived by assuming an exponential decay energy distribution function inserted into the Langmuir equation. It describes reversible adsorption and is not restricted to the formation of the monolayer. Thermodynamic data such as adsorption energy can be obtained from KL and KF (eq 7), where K is constant in terms of dm3 mol-1.40
-∆G ) RT ln K
(7)
Results and Discussion Previous studies3 of the adsorption of PHMB onto cellulose had proposed that adsorption was exclusively through electrostatic interaction between the cationic PHMB polymer and anionic carboxylate groups in the fiber, however, these postulations were not based on detailed experimental studies. To understand the mechanism of adsorption of PHMB onto cellulose, computational chemistry was used to propose interactions that could potentially occur between the two species. This was done by constructing an idealized unit cell of cellulose that could adsorb PHMB. Results from UV-visible spectrophotometric analysis of residual PHMB solution in the exhausted application bath (Figure 3) demonstrated that, as initial concentration of PHMB applied (C0) increases, the concentration of PHMB adsorbed also increases. The overall shape of this isotherm is interesting as it (39) Freundlich, H. M. F. Z. Phys. Chem. 1906, 57, 385. (40) Kim, Y.; Kim, C.; Choi, I.; Rengraj, S.; Yi, J. EnViron. Sci. Technol. 2004, 38, 924.
Sorption of Poly(hexamethylenebiguanide) on Cellulose
Figure 3. Adsorption isotherm of PHMB exhausted onto cellulose to C0 ) 100 000 mg dm-3. Inset: Adsorption isotherm of PHMB exhausted onto cellulose to C0 ) 500 mg dm-3 only.
Figure 4. Plot of Ce/qe vs Ce of data in adsorption isotherm of PHMB exhausted onto cellulose to C0 ) 100 000 mg dm-3. Inset: Plot of Ce/qe vs Ce of data in adsorption isotherm of PHMB exhausted onto cellulose to C0 ) 500 mg dm-3 only.
Figure 5. Log plot of data in adsorption isotherm of PHMB exhausted onto cellulose to C0 ) 100 000 mg dm-3. Inset: Log plot of data in adsorption isotherm of PHMB exhausted onto cellulose to C0 ) 500 mg dm-3 only.
displays characteristics of both Langmuir and Freundlich isotherms, as there appears to be an initial plateau region, typical of Langmuir isotherms, up to C0 ) 500 mg dm-3, but then the curve increases logarithmically up to C0 ) 5000 mg dm-3 and follows the trend to C0 ) 100 000 mg dm-3, which is typical of Freundlich isotherms. Figures 4 and 5 show residual linear plots to demonstrate correlation (R2) to both Langmuir and Freundlich isotherms, respectively. Table 2 shows a summary of the isotherm constants and correlation (R2) for both Freundlich and Langmuir isotherms in analysis of residual PHMB in solution after application to cotton. At low concentrations (C0 < 500 mg dm-3), isotherm correlation
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is closest to a Langmuir type, indicating limited, site-specific adsorption (initial exponential rise and then plateau) as a result of electrostatic bonding forces between the delocalized positive charge in PHMB and carboxylate groups in cellulose. However, from experimental data, this adsorption is clearly not limited, and as application concentration of PHMB increases, isotherm correlation is certainly more typical of a Freundlich isotherm (logarithmic curve). Adsorption by a Freundlich isotherm is indicative of unlimited adsorption of a species at nonspecific sites within a substrate and in aqueous exhaustion of species onto cellulose is often an indication of adsorption via hydrogenbonding forces, and of sorbate interactions creating “layers” on the substrate. In terms of the constants obtained from the plotted data, in the Freundlich expression, if nF > 1, then the adsorption is favorable, and this was observed in all cases. For the Langmuir expression, the theoretical monolayer capacity is q0 and is numerically equal to KL/aL. From the experimental data, it can be seen that the theoretical monolayer capacity of PHMB adsorption onto cellulose is 5.3 mg g-1, depending upon the analysis method; this is observed in Figure 3, where an initial plateau at low application concentrations is observed just above 5 mg g-1. As discussed earlier, a typical bleached, mercerized cotton fiber contains 3 × 10-5 mol carboxylic acid groups per g fiber; hence, if we consider a single BG repeat unit of PHMB (Mr 219.7 g mol-1) binding through an electrostatic mechanism in 1:1 stoicheiometry with a single carboxylic acid group, then the maximum mass of BG units that can bind is 6.59 mg g-1, which is very close to the experimental value for the monolayer capacity of PHMB adsorption on cellulose observed herein of 5.3 mg g-1, giving an adsorption stoichiometry of 80% for the monolayer. This value is not 100% of the sites available because of accessibility, considering the Rg of PHMB at 22 Å. According to Stone et al.41 (see Supporting Information, Figure S.3), approximately 25% of a rayon fiber would be inaccessible to a polymer with molecular diameter of 22 Å; bleached, mercerized cotton would display similar trends, hence, the adsorption stoichiometry of 80%. It is expected that PHMB would have the ability to diffuse into the inner pores of the fiber, which is in agreement with the work of Wågberg,15 which demonstrated that a polymer needed an Rg below 80 Å to reach all charges in the fiber wall in its micropores; other large CPE with an Rg higher than 80 Å, such as poly(DADMAC), achieved a maximum qe of 2.5 mg g-1, and significantly higher exhaustion figures of the smaller 3,6-ionene (Mr 6000 g mol-1, qe,max ) 75 mg g-1) demonstrated that it did indeed diffuse into the inner pores of cellulose. It is expected that electrostatic forces would be the primary attractive force operating between cellulose and PHMB, in agreement with literature on adsorption of CPE,9-11 particularly at low application concentrations. Such electrostatic forces are stronger than hydrogen-bonding forces; this is evidenced by exhaustion figures of PHMB of over 90% at C0 < 100 mg dm-3, which would be mainly through electrostatic attraction. However, the experimental data at higher concentrations suggests that the interactions between the substrate and the adsorbate occur by a combination of both electrostatic and hydrogen-bonding forces with the cellulose and potentially through interactions of the PHMB with itself through stacking, enabling nonspecific unlimited adsorption over the application concentrations used in this study. To further endorse the experimental data, molecular dynamics simulations (Cerius, Dreiding FF, Gasteiger charges, 300 K, EWALD description of Coulombic and van der Waals (41) Stone, J. E.; Treiber, E.; Abrahamson, B. Tappi J. 1969, 52, 109.
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Table 2. Summary of the Isotherm Constants and the Correlation for Different Isotherms Freundlich isotherm
Langmuir isotherm
experimental analysis (to C0d)
KF (dm3 mol-1)a
nF
R2
∆G (kJ mol-1)
KL (dm3 mol-1)a
aL (dm3 mg-1)
q0 (mg g-1)
500 mg dm-3 5000 mg dm-3 100000 mg dm-3
2852 2300 1935
2.46 1.94 1.76
0.906 0.952 0.980
-20.7 -20.1 -19.7
1656
0.07
5.298
a
R2 0.998 0.670 0.871
∆G (kJ mol-1) -19.3
Mr PHMB ) 4600 g mol-1.
interactions cutoff ) 10 Å, 400+ ps) were carried out by using the same cellulose surface described earlier to observe the behavior of the sequential addition of three PHMB oligomers interacting with the cellulose surface. For the first PHMB oligomer, 8 of the 12 BG units interacted with (of the total 12) COO- groups, the other BG groups were involved in hydrogen bonding and were generally located between two groups interacting with COO-. For the second PHMB oligomer, five of the BG units interacted with COO- groups, all the rest hydrogen-bonded, and for the third PHMB oligomer, only two of the BG groups interacted with COO-, with the rest hydrogen bonding. Hence, computational chemistry predictions demonstrate that interactions of PHMB and cellulose models operate by both electrostatic and hydrogen-bonding mechanisms, electrostatic bonding dominating initially, and then hydrogen bonding dominating at higher concentrations of PHMB. Ionic strength is also a factor in adsorption;18-20 in the initial application systems for PHMB used in this study, no electrolyte was added and the electrolyte concentration would be less than 10-5 M,9 and at such concentrations, adsorption would be at an optimum. With maximum exchange of ionic groups between PHMB and cellulose to the theoretical monolayer capacity (5.3 mg g-1), liberation of counterions could increase the effective electrolyte concentration up to 1.2 × 10-3 M during the application cycle. The effect of increasing initial electrolyte concentration in application of PHMB to cellulose is worthy of further study. Van de Steeg18 noted that, as the charge density (R) of the polymer increases, there is a decrease in adsorption of the polyelectrolyte at low salt concentrations, as adsorption is not solely reliant upon electrostatic interactions and a significant contribution to adsorption arises from nonelectrostatic interactions. This can explain the observation that PHMB has a significantly higher qe,max than 3,6-ioniene (2), which has a similar molecular mass and size, but lower adsorption;9 the cationic charge of PHMB is delocalized over several atoms, corresponding to lower R than 3,6-ioniene, where the cationic charges are not delocalized and focused on one N atom, corresponding to higher R. At all application concentrations of PHMB on cellulose, the pH of the system was pH 6-7. At this pH, the ζ of raw cotton fibers is -10 mV.42 Pretreatment of cellulose fibers improves accessibility of dissociable groups; mercerization produces a pronounced increase in surface charge density and increases ζ in the range pH 6-7 to -12 mV.43 By using streaming potential measurements, ζ of bleached and mercerized cotton in the range pH 6-7 has been determined to be -20 to -25 mV,30,31 which was in agreement with the results of the streaming potential experiments conducted herein, giving an average ζ of -24 mV. Clearly, bleaching and mercerizing of cotton increases the electronegativity of the surface of the substrate and provides significant electrostatic potential for adsorption of CPE such as PHMB. (42) Bellmann, C.; Caspari, A.; Albrecht, V.; Loan Doan, T. T.; Ma¨der, E.; Luxbacher, T.; Kohl, R. Colloids Surf., A 2005, 267, 19. (43) Ribitsch, V.; Stana-Kleinschek, K.; Kreze, T.; Strnad, S. Macromol. Mater. Eng. 2001, 286, 648.
Table 3. CSD Predictions of PHMB Stacking Interactions
It is proposed that exhaustion in excess of the theoretical monolayer capacity (5.3 mg g-1), when no further ionic sites in the substrate were available for electrostatic interaction with the adsorbate, was initially achieved through PHMB binding to the cellulose exclusively through hydrogen bonding. To evidence this, several dynamics runs were carried out to investigate the behavior of PHMB oligomer on the same cellulose model surface as used previously, which had not been modified with COOgroups. It was observed that PHMB interacted with the cellulose surface via a combination of hydrogen bonding with different oxygen types in the surface of the cellulose, including -OH groups, the oxygen in the sugar ring, and the C-O-C link between the sugar units, and it was found that 4 out of 12 BG units had interacted via hydrogen bonding by 500 ps (see Supporting Information, Figure S.4). At high concentrations of PHMB, further increase in adsorption onto cellulose is most probably due to the formation of multilayers of PHMB through separate oligomer molecules interacting with each other and possibly stacking. Computational investigation of the behavior of PHMB oligomers in a high-concentration environment was conducted to predict the likelihood of these interactions. Through consulting the Cambridge Scientific Database (CSD), it was found that the BG group features in four structures; some of these structures show the BG units interacting by hydrogen bonding and electrostatic interactions with counterions, there may also be some possible stacking interactions. These structures are summarized in Table 3. To determine whether PHMB oligomers may take part in favorable stacking interactions in a high-concentration environment, interaction studies were carried out. The PHMB trimer structure was minimized (with and without Cl- ions) using molecular modeling methods (Dreiding FF with Gasteiger
Sorption of Poly(hexamethylenebiguanide) on Cellulose
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Table 4. Calculated PHMB Stacking Interactions in a High Concentration Environment
charges, Ewald Columbic and van der Waals methods), the geometry of these trimers was then constrained as a rigid body and the interaction energy between trimers was calculated. Results are summarized in Table 4. Not surprisingly, the trimers without Cl- ions are not involved in a favorable interaction (as compared to the trimers with counterions); this is illustrated by the positive interaction energy and the large distance of separation and is probably due to the repulsion of the two positively charged trimers. Conversely, the structure with counterions had a large favorable interaction with a small distance of approach. PHMB oligomers may interact in other ways such as sideto-side interactions, for example between the trimer/Cl- systems, which has a favorable interaction energy (-172.3 kJ mol-1), or by sheets of interacting trimers (see Supporting Information, Figure S.5). These may contribute to large-scale aggregation or ordering on the surface of the cellulose. Note that the interaction energy per molecule in the sheets of rigid bodies is similar to that between two molecules. Work by Alila et al. on the adsorption of cationic surfactants onto cellulose demonstrated that adsorption proceeded by electrostatic and dispersive interactions allowing dense packing of adsorbed molecules into hemimicelles and admicelles aggregates, and this was manifest as an s-shaped isotherm for a plot of qe versus log Ce.28 While the experimental and computational work with PHMB does not describe classical micellar formation (although this would be worthy of further study), a residual plot after the work by Alila et al. does manifest a similar s-shaped isotherm (Figure 6). Hemimicelle (monolayered aggregation) formation may be attributed to the side-to-side interactions of PHMB contributing to ordering on the surface, and admicelle (bilayered aggregation) formation attributed to various stacking interactions possible for PHMB, which may actually exceed a bilayer and occur in multilayers. These observations for stacking of PHMB on cellulose are also similar to the sorption of direct dyes onto cellulose, where large, planar, hydrogen-bonding dye molecules adsorb via a Freundlich isotherm and form multilayers of the dye on the fiber surface through dye-dye stacking.44 PHMB has high potential for hydrogen bonding in solution through its BG groups, and it was thought that, although extensive hydrogen bonding with water is observed in solvation, the driving force for exhaustion would be the hydrophobic -(CH2)6moieties in the polymer backbone getting away from water to a solid phase through adsorption. PHMB oligomers were investigated using GROMACS software in solution phase (300 K): conformations in this region were slightly kinked and could be described as having loops in the chains that were due to hydrogen bonds between different BG groups along the chain with typical dimensions of 20 Å × 25 Å for the kinked structure (see Supporting Information, Figure S.6). In the equilibrium region of these solution-phase simulations, a range of 7-15 hydrogen (44) Shore, J. In Cellulosics Dyeing; Shore, J., Ed.; Society of Dyers and Colourists: Bradford, U.K., 1995.
Figure 6. Plot of qe vs log Ce of data in adsorption isotherm of PHMB exhausted onto cellulose to C0 ) 100 000 mg dm-3. Inset: Plot of qe vs log Ce of data in adsorption isotherm of PHMB exhausted onto cellulose to C0 ) 500 mg dm-3 only.
Figure 7. Plot of K/S of eosin-stained cotton vs application concentration of PHMB (C0). Inset: Plot of K/S of eosin-stained cotton vs C0 to C0 ) 500 mg dm-3 only.
bonds were observed; hence, if one BG group can hydrogenbond with itself, then perhaps it is likely that it can hydrogenbond with other chains, which may give rise to a hydrogenbonded network of PHMB polymers when in solution. The hydrophobicity of a molecule is related to the loss in entropy of the solvent system caused by forced ordering of water molecules (optimization of hydrogen bonds) along the surface of a molecule when it offers no electrostatic or polar interactions (i.e., a nonpolar molecule). Hence, the extent of the scaffold of ordered water should scale in a linear fashion with the size of the surface area accessible to water. If a geometric rearrangement of the PHMB polymer allowed an alignment of alkyl chains, this would minimize the surface area of nonpolar sections (hydrophobic surface area) accessible to water and may give rise to multiple layers of polymer chains. However, dynamics calculations revealed little or no evidence for obvious dominant hydrophobic behavior for PHMB other than through hydrogen-bonding interactions. It may be that the alkyl chains have some hydrophobic effect, but this is probably not as dominant as the ability of the charged BG section to interact with water, counterions, and other BG sections. Thus, the driving force for exhaustion must be the stronger electrostatic and hydrogen-bonding interactions between PHMB and cellulose and in PHMB stacking. Figure 7 shows the increasing K/S value associated with eosin staining of PHMB-treated cotton as the concentration of PHMB applied is increased, indicating a higher concentration of PHMB adsorbed on cellulose with increasing initial application concentration (C0). Although this is not a “true” isotherm, the same observations are made, insofar as the adsorption does show an initial plateau at low concentration but over all concentrations
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applied is unlimited, which would indicate evidence of nonsitespecific adsorption (Freundlich).
interactions with counterions and hydrogen bonding of biguanide groups.
Conclusions
Acknowledgment. We would like to thank Arch Chemicals (UK) Ltd. and The Engineering and Physical Sciences Research Council for the provision of a Ph.D. CASE award scholarship to Miss Harvey. Thanks also to Yorkshire Forward Green Chemistry CIC for access to the Jasko V-530 UV-visible-NIR spectrophotometer.
Adsorption isotherms were constructed by analysis of the residual baths following application of PHMB to cellulose. At low concentrations, these were typical of Langmuir adsorption isotherms, but at higher concentrations were more indicative of Freundlich adsorption isotherms. This was attributed to a combination of electrostatic and hydrogen-bonding forces, which endorsed computational chemistry proposals. At lower concentrations, electrostatic interactions between PHMB and carboxylic acid groups in the cellulose dominate with a contribution to binding through hydrogen-bonding; as concentration of PHMB increases, hydrogen-bonding with cellulose becomes increasingly dominant. At high PHMB concentrations, observations of increasing PHMB sorption are attributed to monolayer aggregation and multilayer stacking of PHMB through electrostatic
Supporting Information Available: Experimentally determined structure of cellulose Iβ (S.1), the modified cellulose surface with introduced COO- group (S.2), volume of water inaccessible to molecules of increasing size (S.3), hydrogen-bonding of the PHMB oligomer with different types of oxygen in the cellulose surface (S.4), interacting PHMB trimers (S.5), and intramolecular hydrogen-bonding interactions for PHMB in solution at 300 K (S.6). This material is available free of charge via the Internet at http://pubs.acs.org. LA053002B
