Salt effect on the complex formation between 1-dodecyl-3-methylimidazolium bromide and sodium carboxymethylcellulose in aqueous solution

https://doi.org/10.1016/j.colsurfa.2010.01.034Get rights and content

Abstract

Complex formation between sodium carboxymethylcellulose (NaCMC) and 1-dodecyl-3-methylimidazolium bromide (C12mimBr) has been studied by isothermal titration microcalorimetry (ITC), turbidimetric titration, and the surface tension measurements at varied concentrations of NaBr. The addition of salt is found to influence the formation of C12mimBr/NaCMC complexes markedly. At CNaBr < 0.20 M, C12mimBr forms micelle-like aggregates with the NaCMC polymer chains to form C12mimBr/NaCMC complexes above the critical aggregate surfactant concentration (C1). Free C12mimBr micelles are formed at Cm before the saturation concentration of surfactant on the NaCMC chains (C2). However, when CNaBr > 0.20 M, there is no polyelectrolyte/surfactant complex formation because of complete salt screening of the electrostatic attraction between C12mimBr micelles and the NaCMC chains. Additionally, the addition of salt to a system of 0.01 g/L NaCMC and fixed C12mimBr concentration induces the formation of C12mimBr/NaCMC complexes when CNaBr < 0.20 M, and dissociates the complexes at CNaBr > 0.20 M. The salt effect on complex formation is explained as the result of a competition between the screening interaction (the addition of NaBr screens the electrostatic attraction between surfactant headgroups and stabilizes surfactant micelles) and the increasing interaction (the addition of NaBr compresses the diffusive electric double layers and hence reduces the repulsion between the surfactant heads and carboxylate groups).

Introduction

The interactions of polyelectrolytes with oppositely charged ionic surfactants are commercially important in a number of industrial and commercial applications (e.g., pharmaceutical formulations, food additives, cosmetic products, environmentally friendly paints, etc.). A considerable number of research reports dealing with different aspects of these interactions and methods of characterization have been published [1], [2]. The interaction within the mixtures containing polyelectrolytes and charged ionic surfactants is mainly driven by electrostatic and hydrophobic forces. Key electrostatic factors, such as charge density on micelle surface, linear charge density of the polyelectrolyte polymer, pH value and ionic strength of the solution, have been demonstrated to have a dominant effect on the formation of complexes between polyelectrolytes and oppositely charged surfactants. As a result of the predominant electrostatics in such systems, the addition of salt has a significant effect on the formation of these polyelectrolyte/surfactant complexes.

There are two important effects arising from added salt in polyelectrolyte/surfactant solutions: (1) a screening interaction, where the added ions reduce the electrostatic interaction between surfactant headgroups and stabilizes surfactant micelles, and (2) the increasing interaction, where the addition of NaBr compresses the diffusive electric double layers and hence reduces the repulsion between the surfactant heads and carboxylate groups [3], [4], [5], [6]. As a result, critical concentrations, which generally describe the interactions between surfactants and polyelectrolytes, are dramatically influenced by the added salt. In these solutions, the first critical concentration is critical aggregation concentration (cac or C1), corresponding to the surfactant concentration at which surfactant molecules start to bind to polyelectrolyte molecules in the form of micelles. It has been found that there are different influences on C1 with addition of salt in different systems because of the competition between the two salt effects mentioned above. If the screening effect of the interaction dominates, C1 increases with increasing salt concentration [7], [8], on the contrary, the addition of salt will result in a smaller C1 [4], [5], [6], [9]. If the two salt effects are equal, for instance in the C12C6C12Br2/NaCMC system [3], the result is a constant C1 value over the range of salt concentration. The second critical concentration, C2, is commonly used to represent the surfactant concentration when the polyelectrolyte becomes saturated with bound surfactant. There are few reports [8], [10] describing the influence of added salt on C2, but one can still infer that C2 should have the same trend as C1 because of the influence of the same salt effects. The third critical concentration (Cm), the formation of free surfactant micelles in polyelectrolyte solutions, decreases with the increase of salt concentration in previously reported systems, indicating the expected reduction of electrostatic repulsion among surfactant headgroups [6], [7], [8], [10].

In addition, it is important to compare the values of Cm and C2; if Cm is larger than C2, surfactant molecules will first saturate the polyelectrolyte, then form the free surfactant micelles. In contrast, when Cm smaller than C2, there is a competition between the formation of free surfactant micelles and the formation of the polyelectrolyte/surfactant aggregation complex at surfactant concentrations between Cm and C2. The last critical concentration, Ce, is the onset of surfactant monomers binding to polyelectrolyte chains through electrostatic attraction, and increases with the increase of the salt concentration, because the further addition of salt screens the electrostatic attraction between the polyelectrolyte/surfactant pair via an ionic atmosphere around the charged sites on the polymer chains. Few reports have mentioned Ce because of the difficulty of determining this value [7]. Moreover, the addition of a sufficiently large amount of salt can completely screen electrostatic interactions between polyelectrolyte and surfactant and prevent the formation of polyelectrolyte/surfactant complexes. In any case, the effect of added salt on the polyelectrolyte/surfactant solutions is rather complicated; more precise studies in this area are needed to reveal the salt effects on complex formation between different polyelectrolytes and charged surfactants.

Surface tension [10], [11], [12], [13], conductivity [14], [15], microcalorimetry [15], [16], [17], [18], [19], [20], [21], turbidity [21], [22], light scattering [23], [24], [25], and electromotive force [25], [26], [27] methods have been extensively used to probe the interactions between polyelectrolytes and surfactants. Among these methods, isothermal titration microcalorimetry (ITC) is a powerful technique which has recently been used to quantify the binding interactions between polyelectrolytes and surfactants [15], [16], [17], [18], [19], [20], [21]. From an ITC thermograph, C1, C2, and Cm can be determined, and the thermodynamic parameters of the system, as well as the association mechanisms can be derived. However, full interpretation of ITC thermogram thermography is still not a completely developed field and there is significant need for further research in the theory and application of this technique.

In the present study, complex formation between sodium carboxymethylcellulose (NaCMC) and dodecyl-3-methylimidazolium bromide (C12mimBr) with the addition of NaBr has been investigated by using ITC, turbidimetric titration, and surface tension measurements. NaCMC was chosen because it is an extensively studied anionic polysaccharide and it is widely used in food and pharmaceutical applications. C12mimBr is a commonly used novel long-chain ionic liquid surfactant with many preferred characteristics (lower critical micelle concentration, higher adsorption efficiency, and better solubilizing, wetting, foaming, and lime-soap dispersing properties) [28], [29], [30], [31] and applications (medicine, construction of porous materials, catalysts, batteries, photoelectrical cells and other electrochemical devices, etc.) [32], [33], [34], [35]. The critical concentrations and thermodynamic parameters were determined by analyzing the ITC data. Finally we construct the model for the complexes in the C12mimBr and NaCMC system in its different solution regions.

Section snippets

Materials

Sodium carboxymethylcellulose (NaCMC; CR grade) was obtained from Sigma Chemical Regent Co. Ltd., China. The average molecular weight (Mw) of NaCMC was 2.5 × 105 g/mol with a degree of substitution of 0.7 (DS, this is the average number of carboxymethyl groups per glucose unit). 1-Dodecyl-3-methylimidazolium bromide (C12mimBr) was prepared and purified as described previously [28]. Sodium bromide (NaBr, >99%) was purchased from Tianjing Chemical Reagents Co. Reagents were used as received, and

Micellization of C12mimBr in the polymer-free system

The micellization of C12mimBr was studied at various NaBr concentrations using isothermal titration microcalorimetry. Calorimetric titration curves of the observed enthalpy (ΔHobs) as a variation of the final C12mimBr concentration are presented in Fig. 1. All the titration curves have sigmoidal shapes with an abrupt decrease at a threshold concentration that corresponds to micelle formation. The critical micelle concentration (cmc) may be obtained by extrapolation of the initial and the

Conclusions

The salt effect of NaBr on complex formation of C12mimBr and NaCMC was investigated by microcalorimetry, turbidimetric titration and surface tension measurements. The mechanism for C12mimBr/NaCMC complex formation was found to be different at various NaBr concentrations. At CNaBr < 0.20 M, C12mimBr monomers may bind on the polymer chain above Ce and an endothermic peak is detected in the enthalpy curves obtained from the ITC. Second, micelle-like aggregates associate with NaCMC chains to form C12

Acknowledgments

The work was supported by the National Natural Science Foundation of China (Nos. 20773081, 20873074), National Basic Research Program (2007CB808004, 2009CB930101), and Laboratory of Organic Optoelectronic Functional Materials and Molecular Engineering, TIPC, CAS. We also thank Dr. J. David Van Horn (Visiting Professor, Shandong University) for editorial assistance.

References (44)

  • T. Matsuda et al.

    Salt effect on complex formation of neutral/polyelectrolyte block copolymers and oppositely charged surfactants

    Langmuir

    (2008)
  • X.Y. Wang et al.

    Salt effect on the complex formation between polyelectrolyte and oppositely charged surfactant in aqueous solution

    J. Phys. Chem. B

    (2005)
  • C. Wang et al.

    Interactions between methacrylic acid/ethyl acrylate copolymers and dodecyltrimethylammonium bromide

    J. Phys. Chem. B

    (2003)
  • C. Wang et al.

    New insights on the interaction mechanism within oppositely charged polymer/surfactant systems

    Langmuir

    (2002)
  • S. Dai et al.

    Isothermal titration calorimetry studies of binding interactions between polyethylene glycol and ionic surfactants

    J. Phys. Chem. B

    (2001)
  • C. Monteux et al.

    Adsorption of oppositely charged polyelectrolyte/surfactant complexes at the air/water interface: formation of interfacial gels

    Langmuir

    (2004)
  • S. Trabelsi et al.

    Co-adsorption of carboxymethyl-cellulose and cationic surfactants at the air–water interface

    Langmuir

    (2007)
  • C.G. Bell et al.

    Macroscopic modeling of the surface tension of polymer–surfactant systems

    Langmuir

    (2007)
  • D. Mitra et al.

    Physicochemical studies on the interaction of gelatin with cationic surfactants alkyltrimethylammonium bromides (ATABs) with special focus on the behavior of the hexadecyl homologue

    J. Phys. Chem. B

    (2008)
  • G.Y. Bai et al.

    Thermodynamics of the interaction between a hydrophobically modified polyelectrolyte and sodium dodecyl sulfate in aqueous solution

    J. Phys. Chem. B

    (2004)
  • S. Dai et al.

    Binding characteristics of hydrophobic ethoxylated urethane (HEUR) and an anionic surfactant: microcalorimetry and laser light scattering studies

    J. Phys. Chem. B

    (2001)
  • G. Wang et al.

    Titration calorimetric study of the interaction between ionic surfactants and uncharged polymers in aqueous solution

    J. Phys. Chem. B

    (1998)
  • Cited by (30)

    • Rapid photo-reductive destruction of hexafluoropropylene oxide trimer acid (HFPO-TA) by a stable self-assembled micelle system of producing hydrated electrons

      2021, Chemical Engineering Journal
      Citation Excerpt :

      Based on the above discussion, it can be concluded that the HFPO-TA degradation in HPA/CTAB/HFPO-TA ternary systems are closely related to the hydrated electron yield of HPAs and the structure of the self-assembled micelle. The structural difference of the self-assembled micelle may also show different responses to pH [41,42]. As shown in Fig. 6a-f, the decomposition and defluorination of HFPO-TA in the HPA/CTAB/HFPO-TA systems were evaluated at pH 4, 6, 8, and 10, respectively, which was initially adjusted without buffer but kept stable within 0.5 pH unit during the reaction.

    • Roles of solution chemistry and reagent–reagent interaction on carboxymethylcellulose adsorption onto graphite and implications on its floatability

      2021, Minerals Engineering
      Citation Excerpt :

      It is well known that ion–reagent and reagent–reagent interactions are the major phenomena that affect reagent adsorption, mineral floatability and gangue depression (Grzadka, 2011, 2012; Kor et al., 2014; Manono et al., 2018; Somasundaran et al., 1991). For example, CMC can form complexes with 1-dodecyl-3-methyl imidazolium bromide collector in the presence of Na+ with different adsorption mechanisms dependent on the electrolyte concentration (Liu et al., 2010). In addition, sodium dodecyl sulphate can form complexes with hydroxyethyl cellulose and hydrophobically modified cationic cellulose which in turn enhances sodium dodecyl sulphate adsorption onto silica surfaces (Terada et al., 2004).

    • Effects of different variables on photodestruction of perfluorooctanoic acid in self-assembled micelle system

      2020, Science of the Total Environment
      Citation Excerpt :

      As shown in Fig. S9a, the original ternary self-assembled micelle carried a net positively charge with ζ potential of +47.7 mV at pH 6. As the increase of NaCl concentration, the ζ potential and the size of the ternary self-assembled micelle also increased (Fig. S9b), suggesting that the presence of NaCl would make the micelle looser, similar results were also observed in prior studies (Liu et al., 2010; Zhang et al., 2014). Therefore, the ionic strength effect on the degradation of PFOA by the ternary self-assembled micelle system can be proposed and illustrated in Fig. S7c.

    • Effect of mono- and dicationic ionic liquids on the viscosity and thermogelation of methylcellulose in the semi-diluted regime

      2019, Carbohydrate Polymers
      Citation Excerpt :

      The effect of charged surfactants (conventional and gemini) and salts on the properties of natural polymer has been the subject of many reports (Bao, Li, Gan, & Zhang, 2008; Dar, Garai, Das, & Ghosh, 2010; Shah, Chat, Maswal, Rather, & Dar, 2016; Silva, Antunes, Sousa, Valente, & Pais, 2011; Torcello-Gómez & Foster, 2014; Torcello-Gómez et al., 2015; Villetti, Bica et al., 2011). Currently, the role of ILs on the properties of cellulose derivatives is rapidly gaining prominence, due to its application as new mixtures (Das, Ray, & De, 2014; Liu, Zheng, Sun, & Wei, 2010, [Liu et al.,2012]; Pal & Yadav, 2017; Ray, Das, De, & Das, 2015). According to Liu et al.,2012 the complex carboxymethylcellulose/CnMIMBr is formed due to the interplay of electrostatic and hydrophobic interactions, and the addition of NaBr increase the interactions due to compression of the electric double layers (Liu et al., 2010).

    • Effect of anionic polyelectrolyte sodium carboxymethylcellulose on the aggregation behavior of surface active ionic liquids in aqueous solution

      2017, Journal of Molecular Liquids
      Citation Excerpt :

      The investigation of interactions amid SAILs and polyelectrolyte with opposite charges have fascinated attention due to complex formation between SAIL monomers and polyelectrolyte chain and several applications in pharmaceuticals, detergency, rheological control, etc. [11,12] Although, several reviews have been allocated to study the interactions between oppositely charged surfactants and polyelectrolytes while no exhaustive work have been reported regarding the interaction between oppositely charged SAILs and polyelectrolytes are scarce. The effect of different concentrations of salt, sodium bromide on complex formation between 1-dodecyl-3-methylimidazolium bromide [C12mim][Br] and polyelectrolyte NaCMC in aqueous solution have been analyzed by Liu et al. using surface tension, isothermal titration microcalorimetry, and turbidimetric titration [13]. They figured out that at low salt concentration, the salt-enhancing effect is large but at higher concentration the screening interaction becomes more dominant resulting in reducing the interactions between the SAIL and polyelectrolyte.

    View all citing articles on Scopus
    View full text