- Email: [email protected]
Interaction of hydroxypropylcellulose with hexadecylbenzyldimethylammonium chloride in the absence and presence of hydrophobic salts
Journal of Molecular Liquids 150 (2009) 86–91
Contents lists available at ScienceDirect
Journal of Molecular Liquids j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m o l l i q
Interaction of hydroxypropylcellulose with hexadecylbenzyldimethylammonium chloride in the absence and presence of hydrophobic salts Mohammad Amin Mir, Aijaz Ahmad Dar, Adil Amin, Ghulam Mohammad Rather ⁎ Department of Chemistry, University of Kashmir, Hazratbal, Srinagar-190 0006, J&K, India
a r t i c l e
i n f o
Article history: Received 24 July 2008 Received in revised form 25 September 2009 Accepted 1 October 2009 Available online 12 October 2009 Keywords: Hyrdoxypropyl cellulose Polymer–surfactant interaction Hydrophobic salts Cloud point
a b s t r a c t The interaction between nonionic semi-flexible polymer, hydroxypropylcellulose (HPC) and cationic surfactant hexadecylbenzyldimethylammonium chloride (C16BzCl) has been studied in aqueous sodium chloride (NaCl), sodium hexanoate (NaHx) and propylammonium chloride (PrACl) solutions employing conductivity, viscosity and cloud point measurements. The salts selected allowed us to investigate the effect of hydrophobic co- and counter-ions compared with simple ions upon polymer–surfactant interaction. Cloud point and viscometric results indicate interaction between C16BzCl and HPC although not reflected from condutometric data. Addition of different levels of salts to the polymer–surfactant system reveal that the charge compensation of micelles bound to the polymer chains is less in the presence of NaHx than in NaCl or PrACl, both showing similar effect. Also the effect of increase in surfactant concentration at a particular polymer plus additive concentration is equivalent to the increase in hydrophobic modification of the polymer as reflected in cloud point and viscosity decrease. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Aqueous polymer–surfactant solutions are interesting both in view of their wide spread practical and technological applications, and in fundamental scientific research for understanding the physicochemical reasons that determine their high performance in paints and coatings, food products, pharmaceuticals, cosmetics, detergent processing and tertiary oil recovery [1,2]. The interaction of nonionic water-soluble polymers with cationic surfactants contrary to that with anionic surfactants [3–5] has been observed to be very weak, a behavior ascribed to bulkiness of the cationic head group, electrostatic repulsion between protonated polymer and surfactant and to hydration shell of the polymer which does not favour interaction with cationic surfactants [6–8]. However, there is evidence of complex formation between cationic tetra- and hexadecyltrimethylammonium chloride and bromide surfactants and cellulose ethers like hydroxypropylcellulose (HPC) and ethylhydroxyethylcellulose [9–12]. Winnik et al. [9] have observed strong interaction between HPC and hexadecyltrimethylammonium chloride leading to reduction of cmc of HTAC to about 1/5th of its value in the absence of HPC. The binding of charged surfactant micelles to a flexible/semi-flexible nonionic polymer has been demonstrated to cause characteristic changes in the hydrodynamic properties of the polymer solution [13]. Bakshi et al. [14,15] studied the interaction of
⁎ Corresponding author. Tel.: + 91 194 2424900; fax: + 91 194 2421357. E-mail address: [email protected] (G.M. Rather). 0167-7322/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.molliq.2009.10.006
polyelectrolytes with the surfactants having aromatic moiety in their head groups and observed that the head group controls the polymer surfactant interactions. Though the effect of surfactant chain length on the interaction with hydroxypropylmethylcellulose (HPMC) has been demonstrated [16], it might be fruitful to find out the effect of an aromatic moiety present in the surfactant head group on its interaction with HPC. In this endeavor, we have attempted to study the interaction between hexadecylbenzyldimethylammonium chloride (C16BzCl) and HPC so that a direct comparison with already reported results of the HTAC–HPC polymer surfactant system [9] can be made. Changes in hydrodynamic properties of polymer–surfactant solutions due to polyelectrolyte character of originally nonionic polymer endowed by interaction with ionic surfactants have been tuned in different ways to optimize the efficiency of formulations like pharmaceuticals, cosmetics etc. Salts can affect properties of polymer–surfactant systems by (i) reduction of degree of electrostatic interaction and (ii) stabilization of surfactant aggregates [17]. Martins et al. [18] studied the aggregation of natural anionic bile salts with HPC in the presence of 0.1 M NaCl. Enhancement of solubility of nonionic polymers on interaction with ionic surfactants has been reported [19–22] to decrease in the presence of salts like NaCl. Dubin et al. [23] showed that surfactant counter-ions play a direct role in the stabilization of PEO/SDS complexes. Benraou [24] reported that interaction between cesium- and tetraalkylammonium-dodecylsulfates and PEO or PVP becomes weaker as the hydrophobicity of the surfactant counter-ion is increased. Hydrophobic salts affect the micellization and adsorption properties [25,26] of surfactants by
M.A. Mir et al. / Journal of Molecular Liquids 150 (2009) 86–91
inducing specific interactions in addition to electrostatic effects. Effect of hydrophobic salts on polymer–surfactant interaction has not received enough attention in spite of being better candidates for understanding and revealing mechanism of such interactions. In this endeavor, the present study focuses on the effect of varying concentration of propylammonium chloride (PrACl) and sodium hexanoate (NaHx) on the hydrodynamic and aggregation properties of HPC/C16BzCl/H2O system employing conductivity, cloud point and viscosity measurements. These salts are capable of furnishing hydrophobic co- and counter-ions respectively relative to the cationic surfactant used in the study and hence will allow us to investigate their effect relative to simple ions furnished by NaCl. 2. Materials and methods 2.1. Materials Hydroxypropylcellulose (HPC, MW 100000) and hexadecylbenzyldimethylammonium chloride (C16BzCl) both from Fluka (ca. 99%) were used as received. Sodium chloride (NaCl) was Qualigens (India) product. Sodium hexanoate (NaHx) and propylammonium chloride (PrACl) were synthesized by neutralizing the corresponding acid and base with concentrated sodium hydroxide and hydrochloric acid (Merck Products, India) solutions respectively. The salts were precipitated by redistilled acetone and purified by recrystallization from water with acetone (Merck) followed by drying under vacuum. 2.2. Methods 2.2.1. Conductivity measurements Conductivity of solutions was recorded at 25 °C by a digital microprocessor based conductivity meter (CyberScan CON500) from Eutech Instruments, having a sensitivity of 0.1 μS cm− 1 and an accuracy of 0.5%. The procedure for calibration has already been reported [27]. Temperature was maintained constant to 25 ± 0.1 °C using a constant temperature bath. 2.2.2. Cloud point determination For determination of the cloud point of solutions of polymer– surfactant and polymer–surfactant–additive systems, both visual observation and spectrophotometric determination were employed. For visual observation a glass tube containing the experimental solution and a precision thermometer was immersed in a water bath the temperature of which could be electronically controlled to within ±0.1 °C. The temperature was increased at the rate ~1 °C/min and the temperature at which the solution became cloudy was recorded. Similarly the temperature at which the solution becomes clear upon cooling was also recorded. The mean of three such measurements, which were reproducible to within ±0.2 °C, was taken as the cloud point of the solution. For spectrophotometric determination a Shimadzu UV-1650PC instrument equipped with water circulating cell holder was used and absorbance of the solution at 400 nm was recorded. The temperature was changed manually in steps of 0.2 °C near the cloud point. The cloud point was determined as the mean of heating and cooling temperatures at which there was an abrupt change in absorbance taken from differential plots of absorbance vs temperature. 2.2.3. Viscosity measurements Viscosity measurements were carried out with an Ubbelohde viscometer under Newtonian flow conditions using the method described in the literature [28]. The solvent flow time in the viscometer was always longer than 200 s, and therefore no kinematic corrections were introduced [29]. All the measurements were done at fixed temperature of 25 ± 0.1 °C. The accuracy of the measured viscosity data was around 0.3%.
87
3. Results and discussion 3.1. Interaction between HPC and C16BzCl in the absence of salts Fig. 1 shows the variation of conductivity of C16BzCl solution with concentration in water (inset) and in aqueous 0.3 g/dL HPC solution. Although a single break is observed at the cmc (0.5 mM) of the pure surfactant in water, two breaks are prominent in the presence of polymer. The lower concentration break, C1, corresponds to critical aggregation concentration, cac, marking the beginning of formation of polymer-bound micelles while the higher concentration break, C2, corresponds to the concentration at which saturation of polymer occurs (also known as polymer saturation point, psp) and hence marks the beginning of polymer-free micellization. The values of cac and C2 of C16BzCl with polymer concentrations are presented in Table 1. From the table it is clear that the cac is relatively independent of polymer concentration but there occurs an initial abrupt increase in C2 followed by a decrease and then a linear continuous rise at higher polymer concentrations. The initial increase in C2 may be attributed to increase in the number of available binding sites as the polymer concentration increases while as the drop may be due to decrease in the number of these sites possibly due to formation of polymer aggregates. Evidence for the existence of small polymeric aggregates of HPC has been provided by fluorescence spectroscopy [30,31]. Formation of polymer aggregates is also evident from the initial decrease in reduced viscosity of aqueous solution with polymer concentration (Fig. 2), the subsequent increase being due to increase in the number of such aggregates. It is interesting to note that the dip in the C2-[polymer] curve occurs in the same polymer concentration range as in the ηred–[polymer] curve. This not only excludes the possibility that the dip in the C2–[HPC] curve was due to experimental errors, but also adds strength to the suggestion that HPC exists in aqueous solution in the free form in low concentration range while forming aggregates at high concentration. Increase in C2 of surfactant at higher polymer concentration might be due to increase in number of binding sites on the aggregated form of polymer. In the present study cac is almost equal to cmc of pure surfactant; it seems as if there is no interaction between C16BzCl and HPC. To confirm this, viscosity and cloud point measurements of aqueous HPC solutions were carried out in the presence of varying concentration of C16BzCl, the results of which indicate that interaction does exist between the two.
Fig. 1. Variation of conductivity with aqueous C16BzCl concentration in the presence of 0.3 g/dL of polymer concentration at 25 °C. The inset shows the same in the absence of polymer.
88
M.A. Mir et al. / Journal of Molecular Liquids 150 (2009) 86–91
Table 1 Critical aggregation concentration (cac) and polymer saturation point (C2) values of C16BzCl as a function of [HPC] at 25 °C from conductivity measurement. [HPC](g/dL)
cac (mM)
C2 (mM)
0.008 0.01 0.03 0.05 0.08 0.10 0.13 0.15 0.18 0.20
0.45 0.50 0.54 0.52 0.52 0.48 0.50 0.47 0.52 0.55
1.25 1.29 1.67 1.37 1.21 1.19 1.54 1.61 1.75 1.88
Fig. 3 (inset) depicts the abrupt change in the absorbance of 0.3 g/dL HPC aqueous solution at the cloud point both on the heating as well as cooling curves. The figure also shows the phase separation curve of HPC/water system. The phase separation temperature decreases steeply initially with increasing polymer concentration but flattens out as the polymer concentration becomes large like in other cellulose ethers [20,32]. The effect of added amphiphile, C16BzCl, on the CP of solutions of different polymer concentrations (0.2, 0.3, 0.4, 0.5 g/dL) shows (Fig. 4) that with increase in [C16BzCl] the CP increases rapidly only above the critical aggregation concentration. Below the cac, there is in fact a slight decrease in CP as evident from absorbance data, independent of polymer concentration and attributable to the screening of electrostatic repulsions present in the protonated polymer by Cl− ions from the surfactant [21]. An important conclusion that can be drawn from the CP depression of polymer solutions, in the presence of C16BzCl below its cac, is that surfactant monomers do not bind to polymer chains. Above cac the surfactant micelles formed on the polymer chains increase the CP by introducing polyelectrolyte character into the polymer chains. EMF measurements using sodium ion selective electrode have shown that counter-ion binding in SDS/PPO system starts at cac confirming that bound surfactant exists only in the form of micellar aggregates [33]. As the polymer concentration increases, higher surfactant concentration is required to maintain the CP at a particular value. The CPs of 0.5 as well as 0.6 g/dL polymer solutions increase with increase in surfactant concentration followed by a plateau, a feature that has already been reported for the HPC (0.25 g/dL)/bile salt systems [18,34,35], and HPC/dodecyltrimethylammonium halide systems, dependent on the
Fig. 2. Variation of reduced viscosity of pure HPC solution with concentration at 25 °C.
Fig. 3. Variation of cloud point of pure HPC solution with concentration. The inset shows variation of absorbance of 0.3 g/dL HPC with temperature.
type of halide, the overall effect being attributed to small size and spherical shape of their micelles [36]. The plateau has been found only with certain surfactants, which form micelles of spherical shape, with relatively small charge density. Consequently the repulsion is not strong enough to prevent the approach of a polymer chain and as a result solvent is expelled leading to phase separation [37]. The need for higher surfactant concentration to maintain the CP at a particular level as the polymer concentration increases reflects a balance between hydrophobic and hydrophilic interactions in inducing phase separation. It seems that below 0.5 g/dL HPC, hydrophilicity introduced by the micelles bound to polymer chains is sufficient to prevent phase separation. Relative viscosity of HPC/C16BzCl/water system as a function of surfactant concentration at different polymer concentrations (0.008, 0.03, 0.05, 0.1, 0.3, 0.5 g/dL) is displayed in Fig. 5. The general trend, apparent in the figure, similar to that of the HTAC–HPC system, is that the relative viscosity increases initially at low surfactant concentrations to a certain maximum value, beyond which there is a gradual
Fig. 4. Variation of cloud point of solutions of different HPC concentrations with surfactant concentrations.
M.A. Mir et al. / Journal of Molecular Liquids 150 (2009) 86–91
89
We, therefore, assume the interaction between C16BzCl micelles and HPC is good enough to raise the CP and viscosity of the polymer but is too weak to be reflected as a decrease in C2 of C16BzCl or, may be, the decrease in cmc due to the hydrophobic interactions is compensated by the effect of increase in hydrophobicity of the solvent by polymer. The lack of viscosity maximum below 0.03 g/dL polymer indicates that the interaction between C16BzCl micelles and HPC occurs only above concentration where polymer undergoes aggregation, indicating some relation between polymeric aggregation and its interaction with surfactants. 3.2. Interaction between HPC and C16BzCl in the presence of salts It is imperative to state that environmental variations may have a striking influence on the cac, cloud point and viscosity of polymer– surfactant systems. Thus, altered solvent medium as well as added salts have the ability to change the cac, cloud point and viscosity, by way of modification of solvent structure, micellar charge and/or extent of solvation of clouding species.
Fig. 5. Variation of relative viscosity as a function of surfactant concentration in solutions of different HPC concentrations at 25 °C.
decrease. We note that the amount of surfactant needed for maximum binding increases as the polymer concentration increases. The initial increase in relative viscosity can be attributed to the stretching out of HPC chains due to electrostatic repulsion between micelles bound to the polymer chains. The chain expansion continues until the number of bound micelles reaches its maximum value which, we assume, occurs where the relative viscosity also exhibits a maximum. The subsequent reduction in viscosity, as more surfactant is added, may be due to electrostatic screening of the charge on polymer-bound micelles by excess Cl− ions [13]. The shift of viscosity maximum at increasing polymer concentrations to higher surfactant concentration reflects that as more HPC chains are available, a larger amount of C16BzCl is required for maximum binding. Since in our study, cac is almost equal to cmc of pure surfactant, and there is no reduction in hydrodynamic volume as indicated by no initial decrease in viscosity, folding of polymer chains around the micelle is less probable, possibly due to semi-rigid nature of the HPC.
3.2.1. Effect of added salts on polymer solution In case of pure polymer solution the CP is practically independent of the presence of NaHx, decreases slightly initially in the presence of NaCl, PrACl (Table 2) and C16BzCl below its cmc (Fig. 4), whereas relative viscosity remains constant in NaCl and PrACl but increases with [NaHx] (Table 3) and [C16BzCl] below its cmc (Fig. 5). The small depression in CP of the polymer solution in the presence of NaCl, PrACl and C16BzCl (below its cmc) is probably a result of screening of electrostatic interactions in the protonated polymer chains by the Cl− ions, common to the above salts, the effect being practically the same and thus depicting dominant character of Cl− in this respect. The inability of NaHx to depress the CP of pure polymer solution may be attributed to large size of the Hx− ion and hence small charge density and also its hydrophobic character compared to Cl−. The increase in relative viscosity of polymer solution in the presence of NaHx and C16BzCl contrary to that of NaCl, is probably a result of decrease in the polar character of the medium [38–41], making the solvent good in the sense that polymer chains prefer solvent molecules in their surroundings to their own segments and thus become unfolded with a consequent increase in viscosity. Significant increase in viscosity of polymer solution with NaHx may
Table 2 Cloud point of 0.3 g/dL HPC solution at different surfactant and salt concentrations. [C16BzCl] (mM)
0.0
0.2
0.5
1.6
CP (°C)
CP (°C)
CP (°C)
CP (°C)
Visual [Salt] (mM)
NaCl
0.0 8.4 14.3 18.8 50.0
47.0 46.5 46.5 46.5 46.0
Absorbance
Visual
Absorbance
Visual
Absorbance
Visual
Absorbance
48.8 46.5 47.5 47.3 47.5
46.5 41.0 41.5 42.0 42.5
47.6 42.5 43.2 44.0 44.5
49.0 39.0 40.0 41.0 41.0
50.6 41.5 42.0 43.0 43.2
57.0 37.0 38.0 38.0 39.0
58.4 39.7 40.0 40.5 40.5
48.8 48.5 48.5 49.0
46.5 46.2 46.5 47.5
47.6 46.5 47.0 48.2
49.0 46.0 46.5 47.5
50.6 47.5 48.3 49.0
57.0 45.5 46.0 46.5
58.4 47.5 47.5 48.0
48.8 46.5 47.5 47.7
46.5 40.5 42.0 42.5
47.6 41.5 43.0 44.0
49.0 38.5 40.5 42.5
50.6 40.3 42.0 44.0
57.0 38.0 39.5 40.5
58.4 40.0 41.5 43.0
NaHx 0.0 8.4 14.3 50.0
47.0 47.0 47.0 47.5 PACl
0.0 8.4 18.8 50.0
47.0 46.5 46.5 46.5
Error limit in CP determination by visual and absorbance methods = ± 0.2 °C.
90
M.A. Mir et al. / Journal of Molecular Liquids 150 (2009) 86–91
Table 3 Relative viscosity of 0.3 g/dL HPC solution at different surfactant and salt concentrations at 25 °C. [C16BzCl] (mM)
0
0.2
0.5
1.6
ηrel
ηrel
ηrel
ηrel
3.19 2.87 2.84 2.83
3.45 2.83 2.81 2.83
3.7 2.89 2.84 2.86
3.19 2.97 3 3.13
3.45 3.03 3.07 3.19
3.7 3.16 3.22 3.33
3.19 2.74 2.81 3.11
3.45 2.91 2.88 2.87
3.7 2.9 2.88 2.88
[Salt] (mM)
NaCl
0 10 20 40
2.89 2.80 2.82 2.82 NaHx
0 10 20 40
2.89 3.23 3.27 3.34 PrACl
0 10 20 40
2.89 2.87 2.91 2.90
be due to predominance of hydrophobic effect of Hx− over its weak electrostatic effect. However, measurements at 42.5 °C have revealed that NaHx and C16BzCl (below its cmc) have no effect on viscosity, indicating that difference in the behavior of viscosity and CP of pure polymer solution in the presence of salts is a result of the difference in temperature in viscosity (25 °C) and CP measurements. Increasing temperature leads to phase separation [42,43] as well as weakening of hydrophobic interactions [44] due to partial breakdown of 3D structure of water. Lowered polar nature of the medium has similar consequences on hydrophobic interactions as that of high temperature. Lack of effect of hydrophobic additives on the CP may, therefore, be attributed to the dominance of former effect of temperature. 3.2.2. Effect of salts on HPC–C16BzCl interaction Both cac and cmc of C16BzCl in pure aqueous as well as in 0.3 g/dL HPC show large decrease (Fig. 6) with increase in salt concentration. The figure indicates that cac is almost equal to cmc in the absence as well as in the presence of salts. The large decrease in low NaCl concentration compared to hydrophobic salts is due to its large salt effect, but at higher concentrations salt as well as hydrophobic effect
decrease cac and cmc thereby leveling the effect of salts to the same value. Salts like NaHx significantly decrease the cmc [26] and cac of surfactants initially, the decrease slowing down to a constant value or sometimes followed by a small increase with increasing salt concentration. It can be concluded that presence of salts results in an overall decrease in both cmc and cac which is expected to be reflected in CP and viscosity changes of the system. The influence of the nature and increasing concentration of salt on the CP and relative viscosity of polymer solution (0.3 g/dL) at different levels of surfactant addition is depicted in Table 2 and Table 3 respectively. Addition of salts to HPC/C16BzCl/water system results in a decrease in CP as well as viscosity, due to screening of electrostatic interactions, a behavior well known both from CP [19,20,22] and viscosity [13] data. This decrease becomes more prominent at higher surfactant levels for a particular salt concentration, irrespective of its nature. A significant decrease in CP and viscosity of the HPC/0.2 mM C16BzCl system in the presence of salts indicates that in such systems micelles are formed on the polymer chains at or below 0.2 mM surfactant as seen from Fig. 6 which shows that above 2 mM salt cac is below 0.2 mM. Micelle bound polymer chains get a hydrophobic coating in the presence of salts in contrast to a hydrophilic one in their absence. This will be equivalent to the hydrophobic modification of the polymer. A low cac of pyrene-labeled PEO [45,46], compared to unlabeled polymer, has been taken as a sign that pyrene-labeled PEO is in fact a model of hydrophobically modified polymer, rather than a true representation of PEO. Since phase separation is strongly dependent on the hydrophobic modification of the polymer, increase in hydrophobicity decreases the temperature for phase separation. Increase in surfactant level, keeping the concentration of salt constant, makes the polymer increasingly hydrophobically modified and depresses the CP accordingly. Since increase in CP and viscosity of polymer solution is due to the polyelectrolyte effect induced by ionic micelles bound to the polymer chains, screening of electrostatic interactions by the salts are expected to depress both the CP and viscosity. The CP and viscosity of polymer solution at a particular surfactant level are decreased by the additives in the following order: NaCl ≈ PrACl > NaHx. This order can be explained in the light of influence of co- and counter-ions on the effective charge of micelles. The increase in effective micellar charge and counter-ion dissociation on increasing the hydrophobocity of sodium carboxylates studied by Anacker and Underwood [47] is consistent with our observation of a small decrease in CP and viscosity of polymer solution at a particular surfactant level in the presence of NaHx than in the presence of NaCl. A low depression by the former may be attributed to a higher effective micellar charge of C16BzCl or lower counter-ion binding in its presence. PrACl furnishes PrA+ co-ions which can affect the micellization parameters [48–50]. The results, however, reveal that the co-ion has no effect on C16BzCl–HPC system as the CP and viscosity decrease is almost similar to that in the presence of NaCl. We may, therefore, conclude that the screening of the electrostatic repulsions by Cl− ions between micelles bound to the polymer chains is strong enough to be affected by the presence of PrA+ ions in place of Na+ ion. 4 . Conclusion
Fig. 6. Variation of cac/cmc of C16BzCl/C16BzCl + HPC (0.3 g/dL) aqueous solutions with salt concentration at 25 °C.
1. The study reveals interaction between the surfactant and the polymer as evident from change in CP and viscosity of HPC with the surfactant concentration. 2. cac is independent of the polymer concentration while cmc increases linearly with [HPC] only above at which polymeric aggregates are formed. 3. At a given HPC concentration, the CP increases with surfactant concentration above cmc. However, the rate of this increase slows
M.A. Mir et al. / Journal of Molecular Liquids 150 (2009) 86–91
down at higher polymer concentration and eventually becomes zero at high surfactant level. 4. The behavior of polymer viscosity with surfactant concentration is similar to that reported for HTAC/HPC system indicating negligible effect of aromatic substituent in the surfactant head group on the polymer–surfactant interaction. 5. Salts decrease CP and viscosity of the polymer–surfactant solutions, the decrease at a particular salt concentration being more at higher surfactant level. However, the decrease is less in the presence of NaHx than NaCl and PrACl, revealing that hydrophobicity of the coion has very little effect compared to that of counter-ion. Acknowledgements We are thankful to the head of the Department of Chemistry, University of Kashmir, for providing the laboratory facilities and his constant encouragement and inspiration. MAM (JRF) acknowledges the financial support [file no.: 09/251(0021)/2008-EMR-I], from the Council of Scientific and Industrial Research, India. References [1] J.C.T. Kwak, Polymer Surfactant Systems: Surfactant Science series 77, Marcel Dekker, New York, 1998. [2] B. Jonson, B. Lindman, K. Holmberg, B. Kornberg, Surfactants and Polymers in Aqueous Solutions, John Wiley, 1998. [3] S. Saito, in: M.J. Schick (Ed.), Nonionic Surfactant Physical Chemistry, Dekker, New York, 1987, chapter 15. [4] E.D. Goddard, K.P. Ananthapadmanabhan (Eds.), Interaction of Surfactants with Polymers and Proteins, CRC Press, USA, 1993. [5] G. Perron, J. Francoeur, J. Desnoyers, J. Kwak, Can. J. Chem. 65 (1987) 990. [6] K. Hayakana, J.C.T. Kwak, in: D.N. Rubingh, P.M. Holland (Eds.), Cationic Surfactants: Physical Chemistry, Marcel Dekker, New York, 1991. [7] R. Nagarajan, Colloids Surf. 13 (1985) 1. [8] K. Shirihama, M. Tohdo, M. Murahashi, J. Colloid Interface Sci. 86 (1982) 282. [9] F.M. Winnik, M.A. Winnik, S. Tazuke, J. Phys. Chem. 91 (1987) 594. [10] R. Zana, W. Binana-Limbel'e, N. Kamenka, B. Lindman, J. Phys. Chem. 96 (1992) 5461. [11] G. Wang, G. Olofsson, J. Phys. Chem. 99 (1995) 5588. [12] A. Goldszal, S. Costeux, M. Djabourov, Colloids Surf. A 112 (1996) 141. [13] P. Hormnirun, A. Sirivat, A.M. Jamieson, Polymer 41 (2000) 2127. [14] M.S. Bakshi, I. Kaur, Progr. Colloid. Polym. Sci. 122 (2003) 37. [15] M.S. Bakshi, I. Kaur, Colloid Polym. Sci. 282 (2004) 476.
91
[16] P.C. Griffiths, P. Stilbs, A.M. Howe, T.H. Whitesides, Langmuir 12 (1998) 5302. [17] B. Lindman, K. Thalberg, in: E.D. Goddard, K.P. Ananthapadmanabhan (Eds.), Interaction of Surfactants with Polymers and Proteins, CRC Press, 1993, Chapter 5. [18] R.M. Martins, C.A. Silva, C. Becker, D. Samios, M. Christoff, C.I.D. Bica, J. Braz. Chem. Soc. 17 (2006) 944. [19] B. Lindman, A. Carlsson, G. Karlström, M. Malmsten, Adv. Colloid Interface Sci. 32 (1990) 183. [20] A. Carlsson, Nonionic Cellulose Ethers—Interactions with Surfactants, Dissertation, University of Lund, Solubility and other Aspects, 1989. [21] G. Karlstrom, A. Carlsson, B. Lindman, J. Phys. Chem. 94 (1990) 5005. [22] A.L. Kjoniksen, B. Nystrom, B. Lindman, Langmuir 14 (1998) 5039. [23] P.L. Dubin, J.H. Gruber, J. Xia, H. Zhang, J. Colloid Interface Sci. 148 (1991) 35. [24] M. Benrraou, B. Bales, R. Zana, J. Colloid Interface Sci. 267 (2003) 519. [25] M. Jan, A.A. Dar, A. Amin, N. Rehman, G.M. Rather, Colloid Polym. Sci. 285 (2007) 631. [26] A. Amin, A.A. Dar, M.A. Bhat, M. Jan, N. Rehman, M.A. Mir, G.M. Rather, in press. J. Dispers. Sci. Technol., 29 (2008) 406. [27] G. Jones, B.C. Bradshaw, J. Am. Chem. Soc. 55 (1933) 1780. [28] C. Gamboa, L. Sepulveda, J. Colloid Interface Sci. 113 (1986) 566. [29] S. Ozeki, S. Ikeda, J. Colloid Interface Sci. 77 (1980) 219. [30] F.M. Winnik, Macromolecules 20 (1987) 2745. [31] F.M. Winnik, M.A. Winnik, S. Tazuke, C.K. Ober, Macromolecules 20 (1987) 38. [32] C. Holmberg, L.O. Sundelof, Langmuir 12 (1996) 883. [33] D.M. Bloor, W.M.L. Wan-Yunus, W.A. Wan-Badhi, Y. Li, J.F. Holzwarth, E. Wyn-Jones, Langmuir 11 (1995) 3395. [34] R.M. Martins, C.A. Silva, C. Becker, D. Samios, M. Christoff, C.I.D. Bica, Colloid Polm. Sci. 284 (2006) 1353. [35] K. Holmberg, B. Jönsson, B. Kronberg, B. Lindman, Surfactant and Polymers in Aqueous Solution, 2nd Edn, John Wiley, 2003, p. 197. [36] C.J. Drummond, S. Albers, D.N. Furlong, Colloids Surf. A 62 (1992) 75. [37] F.E. Antunes, E.F. Marques, R. Gomes, K. Thuresson, B. Lindman, M.G. Miguel, Langmuir 20 (2004) 4647. [38] Z.S. Bezo, E. Berecz, I. Lakotas, J. Lakotas-Szabo, Prog. Colloid Polym. Sci. 82 (1990) 229. [39] I.V. Rao, E. Ruckenstein, J. Colloid Interface Sci. 113 (1986) 375. [40] N. Nischicodo, Y. Moroi, H. Uehara, R. Matuura, Bull. Chem. Soc. Jpn 47 (1974) 2634. [41] K.S. Birdi, S. Backlund, K. Sorensen, T. Trag, S. Dalsager, J. Colloid Interface Sci. 66 (1978) 118. [42] N. Kamenka, I. Burgaud, R. Zana, B. Lindman, J. Phys. Chem. 98 (1994) 6785. [43] S.K. Singh, S. Nilsson, J. Colloid Interface Sci. 213 (1999) 133. [44] N. Muller, Acc. Chem. Res. 23 (1990) 23. [45] F. Quina, E. Abuin, E.A. Lissi, Macromolecules 23 (1990) 5173. [46] Y. Hu, C. Zhao, M.A. Winnik, P.R. Sundararajan, Langmuir 6 (1990) 880. [47] E.W. Anacker, A.L. Underwood, J. Phys. Chem. 85 (1981) 2463. [48] N. Muller, R.H. Birkhahn, J. Phys. Chem. 72 (1968) 583. [49] B.C. Paul, S.S. Islam, K. Ismail, J. Phys. Chem. B 102 (1998) 7807. [50] L. Marszal, Langmuir 4 (1988) 90.
Contents lists available at ScienceDirect
Journal of Molecular Liquids j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m o l l i q
Interaction of hydroxypropylcellulose with hexadecylbenzyldimethylammonium chloride in the absence and presence of hydrophobic salts Mohammad Amin Mir, Aijaz Ahmad Dar, Adil Amin, Ghulam Mohammad Rather ⁎ Department of Chemistry, University of Kashmir, Hazratbal, Srinagar-190 0006, J&K, India
a r t i c l e
i n f o
Article history: Received 24 July 2008 Received in revised form 25 September 2009 Accepted 1 October 2009 Available online 12 October 2009 Keywords: Hyrdoxypropyl cellulose Polymer–surfactant interaction Hydrophobic salts Cloud point
a b s t r a c t The interaction between nonionic semi-flexible polymer, hydroxypropylcellulose (HPC) and cationic surfactant hexadecylbenzyldimethylammonium chloride (C16BzCl) has been studied in aqueous sodium chloride (NaCl), sodium hexanoate (NaHx) and propylammonium chloride (PrACl) solutions employing conductivity, viscosity and cloud point measurements. The salts selected allowed us to investigate the effect of hydrophobic co- and counter-ions compared with simple ions upon polymer–surfactant interaction. Cloud point and viscometric results indicate interaction between C16BzCl and HPC although not reflected from condutometric data. Addition of different levels of salts to the polymer–surfactant system reveal that the charge compensation of micelles bound to the polymer chains is less in the presence of NaHx than in NaCl or PrACl, both showing similar effect. Also the effect of increase in surfactant concentration at a particular polymer plus additive concentration is equivalent to the increase in hydrophobic modification of the polymer as reflected in cloud point and viscosity decrease. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Aqueous polymer–surfactant solutions are interesting both in view of their wide spread practical and technological applications, and in fundamental scientific research for understanding the physicochemical reasons that determine their high performance in paints and coatings, food products, pharmaceuticals, cosmetics, detergent processing and tertiary oil recovery [1,2]. The interaction of nonionic water-soluble polymers with cationic surfactants contrary to that with anionic surfactants [3–5] has been observed to be very weak, a behavior ascribed to bulkiness of the cationic head group, electrostatic repulsion between protonated polymer and surfactant and to hydration shell of the polymer which does not favour interaction with cationic surfactants [6–8]. However, there is evidence of complex formation between cationic tetra- and hexadecyltrimethylammonium chloride and bromide surfactants and cellulose ethers like hydroxypropylcellulose (HPC) and ethylhydroxyethylcellulose [9–12]. Winnik et al. [9] have observed strong interaction between HPC and hexadecyltrimethylammonium chloride leading to reduction of cmc of HTAC to about 1/5th of its value in the absence of HPC. The binding of charged surfactant micelles to a flexible/semi-flexible nonionic polymer has been demonstrated to cause characteristic changes in the hydrodynamic properties of the polymer solution [13]. Bakshi et al. [14,15] studied the interaction of
⁎ Corresponding author. Tel.: + 91 194 2424900; fax: + 91 194 2421357. E-mail address: [email protected] (G.M. Rather). 0167-7322/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.molliq.2009.10.006
polyelectrolytes with the surfactants having aromatic moiety in their head groups and observed that the head group controls the polymer surfactant interactions. Though the effect of surfactant chain length on the interaction with hydroxypropylmethylcellulose (HPMC) has been demonstrated [16], it might be fruitful to find out the effect of an aromatic moiety present in the surfactant head group on its interaction with HPC. In this endeavor, we have attempted to study the interaction between hexadecylbenzyldimethylammonium chloride (C16BzCl) and HPC so that a direct comparison with already reported results of the HTAC–HPC polymer surfactant system [9] can be made. Changes in hydrodynamic properties of polymer–surfactant solutions due to polyelectrolyte character of originally nonionic polymer endowed by interaction with ionic surfactants have been tuned in different ways to optimize the efficiency of formulations like pharmaceuticals, cosmetics etc. Salts can affect properties of polymer–surfactant systems by (i) reduction of degree of electrostatic interaction and (ii) stabilization of surfactant aggregates [17]. Martins et al. [18] studied the aggregation of natural anionic bile salts with HPC in the presence of 0.1 M NaCl. Enhancement of solubility of nonionic polymers on interaction with ionic surfactants has been reported [19–22] to decrease in the presence of salts like NaCl. Dubin et al. [23] showed that surfactant counter-ions play a direct role in the stabilization of PEO/SDS complexes. Benraou [24] reported that interaction between cesium- and tetraalkylammonium-dodecylsulfates and PEO or PVP becomes weaker as the hydrophobicity of the surfactant counter-ion is increased. Hydrophobic salts affect the micellization and adsorption properties [25,26] of surfactants by
M.A. Mir et al. / Journal of Molecular Liquids 150 (2009) 86–91
inducing specific interactions in addition to electrostatic effects. Effect of hydrophobic salts on polymer–surfactant interaction has not received enough attention in spite of being better candidates for understanding and revealing mechanism of such interactions. In this endeavor, the present study focuses on the effect of varying concentration of propylammonium chloride (PrACl) and sodium hexanoate (NaHx) on the hydrodynamic and aggregation properties of HPC/C16BzCl/H2O system employing conductivity, cloud point and viscosity measurements. These salts are capable of furnishing hydrophobic co- and counter-ions respectively relative to the cationic surfactant used in the study and hence will allow us to investigate their effect relative to simple ions furnished by NaCl. 2. Materials and methods 2.1. Materials Hydroxypropylcellulose (HPC, MW 100000) and hexadecylbenzyldimethylammonium chloride (C16BzCl) both from Fluka (ca. 99%) were used as received. Sodium chloride (NaCl) was Qualigens (India) product. Sodium hexanoate (NaHx) and propylammonium chloride (PrACl) were synthesized by neutralizing the corresponding acid and base with concentrated sodium hydroxide and hydrochloric acid (Merck Products, India) solutions respectively. The salts were precipitated by redistilled acetone and purified by recrystallization from water with acetone (Merck) followed by drying under vacuum. 2.2. Methods 2.2.1. Conductivity measurements Conductivity of solutions was recorded at 25 °C by a digital microprocessor based conductivity meter (CyberScan CON500) from Eutech Instruments, having a sensitivity of 0.1 μS cm− 1 and an accuracy of 0.5%. The procedure for calibration has already been reported [27]. Temperature was maintained constant to 25 ± 0.1 °C using a constant temperature bath. 2.2.2. Cloud point determination For determination of the cloud point of solutions of polymer– surfactant and polymer–surfactant–additive systems, both visual observation and spectrophotometric determination were employed. For visual observation a glass tube containing the experimental solution and a precision thermometer was immersed in a water bath the temperature of which could be electronically controlled to within ±0.1 °C. The temperature was increased at the rate ~1 °C/min and the temperature at which the solution became cloudy was recorded. Similarly the temperature at which the solution becomes clear upon cooling was also recorded. The mean of three such measurements, which were reproducible to within ±0.2 °C, was taken as the cloud point of the solution. For spectrophotometric determination a Shimadzu UV-1650PC instrument equipped with water circulating cell holder was used and absorbance of the solution at 400 nm was recorded. The temperature was changed manually in steps of 0.2 °C near the cloud point. The cloud point was determined as the mean of heating and cooling temperatures at which there was an abrupt change in absorbance taken from differential plots of absorbance vs temperature. 2.2.3. Viscosity measurements Viscosity measurements were carried out with an Ubbelohde viscometer under Newtonian flow conditions using the method described in the literature [28]. The solvent flow time in the viscometer was always longer than 200 s, and therefore no kinematic corrections were introduced [29]. All the measurements were done at fixed temperature of 25 ± 0.1 °C. The accuracy of the measured viscosity data was around 0.3%.
87
3. Results and discussion 3.1. Interaction between HPC and C16BzCl in the absence of salts Fig. 1 shows the variation of conductivity of C16BzCl solution with concentration in water (inset) and in aqueous 0.3 g/dL HPC solution. Although a single break is observed at the cmc (0.5 mM) of the pure surfactant in water, two breaks are prominent in the presence of polymer. The lower concentration break, C1, corresponds to critical aggregation concentration, cac, marking the beginning of formation of polymer-bound micelles while the higher concentration break, C2, corresponds to the concentration at which saturation of polymer occurs (also known as polymer saturation point, psp) and hence marks the beginning of polymer-free micellization. The values of cac and C2 of C16BzCl with polymer concentrations are presented in Table 1. From the table it is clear that the cac is relatively independent of polymer concentration but there occurs an initial abrupt increase in C2 followed by a decrease and then a linear continuous rise at higher polymer concentrations. The initial increase in C2 may be attributed to increase in the number of available binding sites as the polymer concentration increases while as the drop may be due to decrease in the number of these sites possibly due to formation of polymer aggregates. Evidence for the existence of small polymeric aggregates of HPC has been provided by fluorescence spectroscopy [30,31]. Formation of polymer aggregates is also evident from the initial decrease in reduced viscosity of aqueous solution with polymer concentration (Fig. 2), the subsequent increase being due to increase in the number of such aggregates. It is interesting to note that the dip in the C2-[polymer] curve occurs in the same polymer concentration range as in the ηred–[polymer] curve. This not only excludes the possibility that the dip in the C2–[HPC] curve was due to experimental errors, but also adds strength to the suggestion that HPC exists in aqueous solution in the free form in low concentration range while forming aggregates at high concentration. Increase in C2 of surfactant at higher polymer concentration might be due to increase in number of binding sites on the aggregated form of polymer. In the present study cac is almost equal to cmc of pure surfactant; it seems as if there is no interaction between C16BzCl and HPC. To confirm this, viscosity and cloud point measurements of aqueous HPC solutions were carried out in the presence of varying concentration of C16BzCl, the results of which indicate that interaction does exist between the two.
Fig. 1. Variation of conductivity with aqueous C16BzCl concentration in the presence of 0.3 g/dL of polymer concentration at 25 °C. The inset shows the same in the absence of polymer.
88
M.A. Mir et al. / Journal of Molecular Liquids 150 (2009) 86–91
Table 1 Critical aggregation concentration (cac) and polymer saturation point (C2) values of C16BzCl as a function of [HPC] at 25 °C from conductivity measurement. [HPC](g/dL)
cac (mM)
C2 (mM)
0.008 0.01 0.03 0.05 0.08 0.10 0.13 0.15 0.18 0.20
0.45 0.50 0.54 0.52 0.52 0.48 0.50 0.47 0.52 0.55
1.25 1.29 1.67 1.37 1.21 1.19 1.54 1.61 1.75 1.88
Fig. 3 (inset) depicts the abrupt change in the absorbance of 0.3 g/dL HPC aqueous solution at the cloud point both on the heating as well as cooling curves. The figure also shows the phase separation curve of HPC/water system. The phase separation temperature decreases steeply initially with increasing polymer concentration but flattens out as the polymer concentration becomes large like in other cellulose ethers [20,32]. The effect of added amphiphile, C16BzCl, on the CP of solutions of different polymer concentrations (0.2, 0.3, 0.4, 0.5 g/dL) shows (Fig. 4) that with increase in [C16BzCl] the CP increases rapidly only above the critical aggregation concentration. Below the cac, there is in fact a slight decrease in CP as evident from absorbance data, independent of polymer concentration and attributable to the screening of electrostatic repulsions present in the protonated polymer by Cl− ions from the surfactant [21]. An important conclusion that can be drawn from the CP depression of polymer solutions, in the presence of C16BzCl below its cac, is that surfactant monomers do not bind to polymer chains. Above cac the surfactant micelles formed on the polymer chains increase the CP by introducing polyelectrolyte character into the polymer chains. EMF measurements using sodium ion selective electrode have shown that counter-ion binding in SDS/PPO system starts at cac confirming that bound surfactant exists only in the form of micellar aggregates [33]. As the polymer concentration increases, higher surfactant concentration is required to maintain the CP at a particular value. The CPs of 0.5 as well as 0.6 g/dL polymer solutions increase with increase in surfactant concentration followed by a plateau, a feature that has already been reported for the HPC (0.25 g/dL)/bile salt systems [18,34,35], and HPC/dodecyltrimethylammonium halide systems, dependent on the
Fig. 2. Variation of reduced viscosity of pure HPC solution with concentration at 25 °C.
Fig. 3. Variation of cloud point of pure HPC solution with concentration. The inset shows variation of absorbance of 0.3 g/dL HPC with temperature.
type of halide, the overall effect being attributed to small size and spherical shape of their micelles [36]. The plateau has been found only with certain surfactants, which form micelles of spherical shape, with relatively small charge density. Consequently the repulsion is not strong enough to prevent the approach of a polymer chain and as a result solvent is expelled leading to phase separation [37]. The need for higher surfactant concentration to maintain the CP at a particular level as the polymer concentration increases reflects a balance between hydrophobic and hydrophilic interactions in inducing phase separation. It seems that below 0.5 g/dL HPC, hydrophilicity introduced by the micelles bound to polymer chains is sufficient to prevent phase separation. Relative viscosity of HPC/C16BzCl/water system as a function of surfactant concentration at different polymer concentrations (0.008, 0.03, 0.05, 0.1, 0.3, 0.5 g/dL) is displayed in Fig. 5. The general trend, apparent in the figure, similar to that of the HTAC–HPC system, is that the relative viscosity increases initially at low surfactant concentrations to a certain maximum value, beyond which there is a gradual
Fig. 4. Variation of cloud point of solutions of different HPC concentrations with surfactant concentrations.
M.A. Mir et al. / Journal of Molecular Liquids 150 (2009) 86–91
89
We, therefore, assume the interaction between C16BzCl micelles and HPC is good enough to raise the CP and viscosity of the polymer but is too weak to be reflected as a decrease in C2 of C16BzCl or, may be, the decrease in cmc due to the hydrophobic interactions is compensated by the effect of increase in hydrophobicity of the solvent by polymer. The lack of viscosity maximum below 0.03 g/dL polymer indicates that the interaction between C16BzCl micelles and HPC occurs only above concentration where polymer undergoes aggregation, indicating some relation between polymeric aggregation and its interaction with surfactants. 3.2. Interaction between HPC and C16BzCl in the presence of salts It is imperative to state that environmental variations may have a striking influence on the cac, cloud point and viscosity of polymer– surfactant systems. Thus, altered solvent medium as well as added salts have the ability to change the cac, cloud point and viscosity, by way of modification of solvent structure, micellar charge and/or extent of solvation of clouding species.
Fig. 5. Variation of relative viscosity as a function of surfactant concentration in solutions of different HPC concentrations at 25 °C.
decrease. We note that the amount of surfactant needed for maximum binding increases as the polymer concentration increases. The initial increase in relative viscosity can be attributed to the stretching out of HPC chains due to electrostatic repulsion between micelles bound to the polymer chains. The chain expansion continues until the number of bound micelles reaches its maximum value which, we assume, occurs where the relative viscosity also exhibits a maximum. The subsequent reduction in viscosity, as more surfactant is added, may be due to electrostatic screening of the charge on polymer-bound micelles by excess Cl− ions [13]. The shift of viscosity maximum at increasing polymer concentrations to higher surfactant concentration reflects that as more HPC chains are available, a larger amount of C16BzCl is required for maximum binding. Since in our study, cac is almost equal to cmc of pure surfactant, and there is no reduction in hydrodynamic volume as indicated by no initial decrease in viscosity, folding of polymer chains around the micelle is less probable, possibly due to semi-rigid nature of the HPC.
3.2.1. Effect of added salts on polymer solution In case of pure polymer solution the CP is practically independent of the presence of NaHx, decreases slightly initially in the presence of NaCl, PrACl (Table 2) and C16BzCl below its cmc (Fig. 4), whereas relative viscosity remains constant in NaCl and PrACl but increases with [NaHx] (Table 3) and [C16BzCl] below its cmc (Fig. 5). The small depression in CP of the polymer solution in the presence of NaCl, PrACl and C16BzCl (below its cmc) is probably a result of screening of electrostatic interactions in the protonated polymer chains by the Cl− ions, common to the above salts, the effect being practically the same and thus depicting dominant character of Cl− in this respect. The inability of NaHx to depress the CP of pure polymer solution may be attributed to large size of the Hx− ion and hence small charge density and also its hydrophobic character compared to Cl−. The increase in relative viscosity of polymer solution in the presence of NaHx and C16BzCl contrary to that of NaCl, is probably a result of decrease in the polar character of the medium [38–41], making the solvent good in the sense that polymer chains prefer solvent molecules in their surroundings to their own segments and thus become unfolded with a consequent increase in viscosity. Significant increase in viscosity of polymer solution with NaHx may
Table 2 Cloud point of 0.3 g/dL HPC solution at different surfactant and salt concentrations. [C16BzCl] (mM)
0.0
0.2
0.5
1.6
CP (°C)
CP (°C)
CP (°C)
CP (°C)
Visual [Salt] (mM)
NaCl
0.0 8.4 14.3 18.8 50.0
47.0 46.5 46.5 46.5 46.0
Absorbance
Visual
Absorbance
Visual
Absorbance
Visual
Absorbance
48.8 46.5 47.5 47.3 47.5
46.5 41.0 41.5 42.0 42.5
47.6 42.5 43.2 44.0 44.5
49.0 39.0 40.0 41.0 41.0
50.6 41.5 42.0 43.0 43.2
57.0 37.0 38.0 38.0 39.0
58.4 39.7 40.0 40.5 40.5
48.8 48.5 48.5 49.0
46.5 46.2 46.5 47.5
47.6 46.5 47.0 48.2
49.0 46.0 46.5 47.5
50.6 47.5 48.3 49.0
57.0 45.5 46.0 46.5
58.4 47.5 47.5 48.0
48.8 46.5 47.5 47.7
46.5 40.5 42.0 42.5
47.6 41.5 43.0 44.0
49.0 38.5 40.5 42.5
50.6 40.3 42.0 44.0
57.0 38.0 39.5 40.5
58.4 40.0 41.5 43.0
NaHx 0.0 8.4 14.3 50.0
47.0 47.0 47.0 47.5 PACl
0.0 8.4 18.8 50.0
47.0 46.5 46.5 46.5
Error limit in CP determination by visual and absorbance methods = ± 0.2 °C.
90
M.A. Mir et al. / Journal of Molecular Liquids 150 (2009) 86–91
Table 3 Relative viscosity of 0.3 g/dL HPC solution at different surfactant and salt concentrations at 25 °C. [C16BzCl] (mM)
0
0.2
0.5
1.6
ηrel
ηrel
ηrel
ηrel
3.19 2.87 2.84 2.83
3.45 2.83 2.81 2.83
3.7 2.89 2.84 2.86
3.19 2.97 3 3.13
3.45 3.03 3.07 3.19
3.7 3.16 3.22 3.33
3.19 2.74 2.81 3.11
3.45 2.91 2.88 2.87
3.7 2.9 2.88 2.88
[Salt] (mM)
NaCl
0 10 20 40
2.89 2.80 2.82 2.82 NaHx
0 10 20 40
2.89 3.23 3.27 3.34 PrACl
0 10 20 40
2.89 2.87 2.91 2.90
be due to predominance of hydrophobic effect of Hx− over its weak electrostatic effect. However, measurements at 42.5 °C have revealed that NaHx and C16BzCl (below its cmc) have no effect on viscosity, indicating that difference in the behavior of viscosity and CP of pure polymer solution in the presence of salts is a result of the difference in temperature in viscosity (25 °C) and CP measurements. Increasing temperature leads to phase separation [42,43] as well as weakening of hydrophobic interactions [44] due to partial breakdown of 3D structure of water. Lowered polar nature of the medium has similar consequences on hydrophobic interactions as that of high temperature. Lack of effect of hydrophobic additives on the CP may, therefore, be attributed to the dominance of former effect of temperature. 3.2.2. Effect of salts on HPC–C16BzCl interaction Both cac and cmc of C16BzCl in pure aqueous as well as in 0.3 g/dL HPC show large decrease (Fig. 6) with increase in salt concentration. The figure indicates that cac is almost equal to cmc in the absence as well as in the presence of salts. The large decrease in low NaCl concentration compared to hydrophobic salts is due to its large salt effect, but at higher concentrations salt as well as hydrophobic effect
decrease cac and cmc thereby leveling the effect of salts to the same value. Salts like NaHx significantly decrease the cmc [26] and cac of surfactants initially, the decrease slowing down to a constant value or sometimes followed by a small increase with increasing salt concentration. It can be concluded that presence of salts results in an overall decrease in both cmc and cac which is expected to be reflected in CP and viscosity changes of the system. The influence of the nature and increasing concentration of salt on the CP and relative viscosity of polymer solution (0.3 g/dL) at different levels of surfactant addition is depicted in Table 2 and Table 3 respectively. Addition of salts to HPC/C16BzCl/water system results in a decrease in CP as well as viscosity, due to screening of electrostatic interactions, a behavior well known both from CP [19,20,22] and viscosity [13] data. This decrease becomes more prominent at higher surfactant levels for a particular salt concentration, irrespective of its nature. A significant decrease in CP and viscosity of the HPC/0.2 mM C16BzCl system in the presence of salts indicates that in such systems micelles are formed on the polymer chains at or below 0.2 mM surfactant as seen from Fig. 6 which shows that above 2 mM salt cac is below 0.2 mM. Micelle bound polymer chains get a hydrophobic coating in the presence of salts in contrast to a hydrophilic one in their absence. This will be equivalent to the hydrophobic modification of the polymer. A low cac of pyrene-labeled PEO [45,46], compared to unlabeled polymer, has been taken as a sign that pyrene-labeled PEO is in fact a model of hydrophobically modified polymer, rather than a true representation of PEO. Since phase separation is strongly dependent on the hydrophobic modification of the polymer, increase in hydrophobicity decreases the temperature for phase separation. Increase in surfactant level, keeping the concentration of salt constant, makes the polymer increasingly hydrophobically modified and depresses the CP accordingly. Since increase in CP and viscosity of polymer solution is due to the polyelectrolyte effect induced by ionic micelles bound to the polymer chains, screening of electrostatic interactions by the salts are expected to depress both the CP and viscosity. The CP and viscosity of polymer solution at a particular surfactant level are decreased by the additives in the following order: NaCl ≈ PrACl > NaHx. This order can be explained in the light of influence of co- and counter-ions on the effective charge of micelles. The increase in effective micellar charge and counter-ion dissociation on increasing the hydrophobocity of sodium carboxylates studied by Anacker and Underwood [47] is consistent with our observation of a small decrease in CP and viscosity of polymer solution at a particular surfactant level in the presence of NaHx than in the presence of NaCl. A low depression by the former may be attributed to a higher effective micellar charge of C16BzCl or lower counter-ion binding in its presence. PrACl furnishes PrA+ co-ions which can affect the micellization parameters [48–50]. The results, however, reveal that the co-ion has no effect on C16BzCl–HPC system as the CP and viscosity decrease is almost similar to that in the presence of NaCl. We may, therefore, conclude that the screening of the electrostatic repulsions by Cl− ions between micelles bound to the polymer chains is strong enough to be affected by the presence of PrA+ ions in place of Na+ ion. 4 . Conclusion
Fig. 6. Variation of cac/cmc of C16BzCl/C16BzCl + HPC (0.3 g/dL) aqueous solutions with salt concentration at 25 °C.
1. The study reveals interaction between the surfactant and the polymer as evident from change in CP and viscosity of HPC with the surfactant concentration. 2. cac is independent of the polymer concentration while cmc increases linearly with [HPC] only above at which polymeric aggregates are formed. 3. At a given HPC concentration, the CP increases with surfactant concentration above cmc. However, the rate of this increase slows
M.A. Mir et al. / Journal of Molecular Liquids 150 (2009) 86–91
down at higher polymer concentration and eventually becomes zero at high surfactant level. 4. The behavior of polymer viscosity with surfactant concentration is similar to that reported for HTAC/HPC system indicating negligible effect of aromatic substituent in the surfactant head group on the polymer–surfactant interaction. 5. Salts decrease CP and viscosity of the polymer–surfactant solutions, the decrease at a particular salt concentration being more at higher surfactant level. However, the decrease is less in the presence of NaHx than NaCl and PrACl, revealing that hydrophobicity of the coion has very little effect compared to that of counter-ion. Acknowledgements We are thankful to the head of the Department of Chemistry, University of Kashmir, for providing the laboratory facilities and his constant encouragement and inspiration. MAM (JRF) acknowledges the financial support [file no.: 09/251(0021)/2008-EMR-I], from the Council of Scientific and Industrial Research, India. References [1] J.C.T. Kwak, Polymer Surfactant Systems: Surfactant Science series 77, Marcel Dekker, New York, 1998. [2] B. Jonson, B. Lindman, K. Holmberg, B. Kornberg, Surfactants and Polymers in Aqueous Solutions, John Wiley, 1998. [3] S. Saito, in: M.J. Schick (Ed.), Nonionic Surfactant Physical Chemistry, Dekker, New York, 1987, chapter 15. [4] E.D. Goddard, K.P. Ananthapadmanabhan (Eds.), Interaction of Surfactants with Polymers and Proteins, CRC Press, USA, 1993. [5] G. Perron, J. Francoeur, J. Desnoyers, J. Kwak, Can. J. Chem. 65 (1987) 990. [6] K. Hayakana, J.C.T. Kwak, in: D.N. Rubingh, P.M. Holland (Eds.), Cationic Surfactants: Physical Chemistry, Marcel Dekker, New York, 1991. [7] R. Nagarajan, Colloids Surf. 13 (1985) 1. [8] K. Shirihama, M. Tohdo, M. Murahashi, J. Colloid Interface Sci. 86 (1982) 282. [9] F.M. Winnik, M.A. Winnik, S. Tazuke, J. Phys. Chem. 91 (1987) 594. [10] R. Zana, W. Binana-Limbel'e, N. Kamenka, B. Lindman, J. Phys. Chem. 96 (1992) 5461. [11] G. Wang, G. Olofsson, J. Phys. Chem. 99 (1995) 5588. [12] A. Goldszal, S. Costeux, M. Djabourov, Colloids Surf. A 112 (1996) 141. [13] P. Hormnirun, A. Sirivat, A.M. Jamieson, Polymer 41 (2000) 2127. [14] M.S. Bakshi, I. Kaur, Progr. Colloid. Polym. Sci. 122 (2003) 37. [15] M.S. Bakshi, I. Kaur, Colloid Polym. Sci. 282 (2004) 476.
91
[16] P.C. Griffiths, P. Stilbs, A.M. Howe, T.H. Whitesides, Langmuir 12 (1998) 5302. [17] B. Lindman, K. Thalberg, in: E.D. Goddard, K.P. Ananthapadmanabhan (Eds.), Interaction of Surfactants with Polymers and Proteins, CRC Press, 1993, Chapter 5. [18] R.M. Martins, C.A. Silva, C. Becker, D. Samios, M. Christoff, C.I.D. Bica, J. Braz. Chem. Soc. 17 (2006) 944. [19] B. Lindman, A. Carlsson, G. Karlström, M. Malmsten, Adv. Colloid Interface Sci. 32 (1990) 183. [20] A. Carlsson, Nonionic Cellulose Ethers—Interactions with Surfactants, Dissertation, University of Lund, Solubility and other Aspects, 1989. [21] G. Karlstrom, A. Carlsson, B. Lindman, J. Phys. Chem. 94 (1990) 5005. [22] A.L. Kjoniksen, B. Nystrom, B. Lindman, Langmuir 14 (1998) 5039. [23] P.L. Dubin, J.H. Gruber, J. Xia, H. Zhang, J. Colloid Interface Sci. 148 (1991) 35. [24] M. Benrraou, B. Bales, R. Zana, J. Colloid Interface Sci. 267 (2003) 519. [25] M. Jan, A.A. Dar, A. Amin, N. Rehman, G.M. Rather, Colloid Polym. Sci. 285 (2007) 631. [26] A. Amin, A.A. Dar, M.A. Bhat, M. Jan, N. Rehman, M.A. Mir, G.M. Rather, in press. J. Dispers. Sci. Technol., 29 (2008) 406. [27] G. Jones, B.C. Bradshaw, J. Am. Chem. Soc. 55 (1933) 1780. [28] C. Gamboa, L. Sepulveda, J. Colloid Interface Sci. 113 (1986) 566. [29] S. Ozeki, S. Ikeda, J. Colloid Interface Sci. 77 (1980) 219. [30] F.M. Winnik, Macromolecules 20 (1987) 2745. [31] F.M. Winnik, M.A. Winnik, S. Tazuke, C.K. Ober, Macromolecules 20 (1987) 38. [32] C. Holmberg, L.O. Sundelof, Langmuir 12 (1996) 883. [33] D.M. Bloor, W.M.L. Wan-Yunus, W.A. Wan-Badhi, Y. Li, J.F. Holzwarth, E. Wyn-Jones, Langmuir 11 (1995) 3395. [34] R.M. Martins, C.A. Silva, C. Becker, D. Samios, M. Christoff, C.I.D. Bica, Colloid Polm. Sci. 284 (2006) 1353. [35] K. Holmberg, B. Jönsson, B. Kronberg, B. Lindman, Surfactant and Polymers in Aqueous Solution, 2nd Edn, John Wiley, 2003, p. 197. [36] C.J. Drummond, S. Albers, D.N. Furlong, Colloids Surf. A 62 (1992) 75. [37] F.E. Antunes, E.F. Marques, R. Gomes, K. Thuresson, B. Lindman, M.G. Miguel, Langmuir 20 (2004) 4647. [38] Z.S. Bezo, E. Berecz, I. Lakotas, J. Lakotas-Szabo, Prog. Colloid Polym. Sci. 82 (1990) 229. [39] I.V. Rao, E. Ruckenstein, J. Colloid Interface Sci. 113 (1986) 375. [40] N. Nischicodo, Y. Moroi, H. Uehara, R. Matuura, Bull. Chem. Soc. Jpn 47 (1974) 2634. [41] K.S. Birdi, S. Backlund, K. Sorensen, T. Trag, S. Dalsager, J. Colloid Interface Sci. 66 (1978) 118. [42] N. Kamenka, I. Burgaud, R. Zana, B. Lindman, J. Phys. Chem. 98 (1994) 6785. [43] S.K. Singh, S. Nilsson, J. Colloid Interface Sci. 213 (1999) 133. [44] N. Muller, Acc. Chem. Res. 23 (1990) 23. [45] F. Quina, E. Abuin, E.A. Lissi, Macromolecules 23 (1990) 5173. [46] Y. Hu, C. Zhao, M.A. Winnik, P.R. Sundararajan, Langmuir 6 (1990) 880. [47] E.W. Anacker, A.L. Underwood, J. Phys. Chem. 85 (1981) 2463. [48] N. Muller, R.H. Birkhahn, J. Phys. Chem. 72 (1968) 583. [49] B.C. Paul, S.S. Islam, K. Ismail, J. Phys. Chem. B 102 (1998) 7807. [50] L. Marszal, Langmuir 4 (1988) 90.