Synthesis and characterization of soluble chitosan/sodium carboxymethyl cellulose polyelectrolyte complexes and the pervaporation dehydration of their homogeneous membranes

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Abstract

Soluble polyelectrolyte complexes (PECs) between chitosan (CS) and sodium carboxymethyl cellulose (CMCNa) were synthesized in aqueous hydrochloric acid (HCl) and obtained in their solid form. FT-IR, elemental analysis (EA), thermal gravity analysis (TGA), wide-angle X-ray diffraction (WAXD), and viscometry were used to characterize the chemical structure, composition, thermal stability, crystallinity and solution property of PECs, respectively. These PECs could be dissolved in aqueous NaOH and their homogeneous polyelectrolyte complex membranes (HPECMs) were made by solution casting method. Both the surface morphology of HPECMs and the morphology of single PEC aggregate were examined by field emission scanning electron microscope (FESEM), and atomic force microscopy (AFM). Effects of the water content in aqueous ethanol and temperature on the swelling behavior and pervaporation dehydration of HPECM were examined. A good performance of J = 1.14 kg/m2 h, α = 1062 was obtained with HPECM0.25 in dehydrating 10 wt.% water–ethanol at 70 °C. The swelling behavior and pervaporation performance were interpreted in terms of structure characteristics of both PECs and HPECMs.

Introduction

Chitosan (CS) is one of the most widely studied natural polymers due to its extensive use in many applications, which include food processing, biomedical, controlled release, gel, capsules and pharmaceutical applications [1], [2]. Chitosan has also been known for its good membrane forming ability and has manifold use in membrane separation technology including reverse osmosis [3], gas separation [4], [5], [6] and pervaporation [7], [8], [9], [10], [11], [12] due to its high hydrophilicity and excellent chemical-resistant properties [13]. Pervaporation is efficient and energy saving for separating azeotropic mixtures and the past decades had witnessed substantial progress and exciting breakthroughs in both the fundamental and application aspects of pervaporation [14], [15], [16]. Organics dehydration, separation of organics from water and binary organics separation constitute the application scope of pervaporation, among which organics dehydration is presently the most studied from the applied point of view [17], [18].

For pervaporation dehydration, the membrane should be hydrophilic to achieve high permeation flux. However, on the other hand, this hydrophilicity always results in swelling of the membrane in aqueous feed solution and its selectivity fall if the swelling degree is too large. So, a crucial issue for good pervaporation performance in dehydration is to make a balance between hydrophilicity of a membrane and its swelling degree in feed solution under pervaporation operation [19]. From this regard, a lot of modifications have been done with chitosan, most of which are polymer blending [20], [21], [22], [23], [24], cross-linking [25], [26], [27], [28], [29], and organic–inorganic hybrid [30], [31], [32]. These modifications were capable of improving the selectivity of chitosan membranes while unfortunately they are not capable of considerably improving the permeation flux of chitosan membrane, and in some cases always reduced the permeation flux compared with pristine chitosan. Liu et al. [33], [34], [35], [36] recently introduced a type of novel organic–inorganic chitosan membrane, in which charged nano-sized silica particles functioned both as a cross-linking agent and as a spacer between chitosan chains. It was reported that this type of organic–inorganic chitosan membranes are capable of improving selectivity while maintaining the permeation flux of chitosan membranes. Liu's result is quite encouraging because improving the selectivity of chitosan membranes always sacrifices their permeation flux. However, till to now no chitosan membrane has been reported to be able to greatly improve permeation flux while maintaining its good selectivity in pervaporation dehydration. It is considered that that hydrogen bonding inside chitosan membrane should be broken, so that the mass transfer resistance toward water could be reduced.

Sodium carboxymethyl cellulose (CMCNa) is another important artificial-nature polymer derived from cellulose. As the starting material of CMCNa, cellulose has already found various applications in the field of membrane separation [37], [38]. However, applications of CMCNa in the field of membrane separation are rarely reported. In this study, soluble polyelectrolyte complexes (PECs) between CS and CMCNa were synthesized in aqueous HCl and obtained in their solid form, which were then dissolved in aqueous NaOH to prepare their novel homogeneous polyelectrolyte complex membranes (HPECMs). These HPECMs have both inherently ionic cross-linking structure and ionized carboxylic acid groups, which could provide HPECMs with good selectivity and hydrophilicity, respectively. So, it is expected that HPECMs made of PECs between CS and CMCNa may be able to greatly improve its permeation flux in pervaporation dehydration while maintaining its selectivity.

Section snippets

Materials

Chitosan with number averaged molecular weight 200,000 and degree of deacetylation 90% was obtained from Yuhuan Chemical Company, Zhejiang, China. Sodium carboxymethyl cellulose was purchased from Sinopharm Chemical Reagent Co., Ltd., Shanghai, China. The intrinsic viscosity of CMCNa in 0.1 M sodium hydroxide (NaOH) aqueous solution at 30 °C is 625.1 mL/g and its degree of substitution was 0.85. Polysulfone ultra-filtration membrane was obtained from Development Centre of Water Treatment

Synthesis and characterization of PECs

The theoretical principles of synthesizing soluble CS/CMCNa PECs was given and discussed in Fig. 1 in Section 2.2. Fig. 2 gives the optical photographs of solutions of 0.01 M CMCNa and 0.01 M CS in 0.007 M HCl, the CMCNa solutions added with different amount of 0.01 M CS solution. The optical photographs of solid PECs obtained and its casting solution are also given in Fig. 2(k) and (l), respectively. It can be seen from Fig. 2 that turbidity occurred immediately in the CMCNa solution upon the

Conclusion

A series of soluble polyelectrolyte complexes between CS and CMCNa with the composition (MCS:MCMCNa) or ionic cross-linking degree ranged from 0.25 to 0.55 were synthesized by tuning [HCl] in CS and CMCNa parent polyelectrolyte solutions. Both EA and FT-IR show that the ICD of PECs decreases with increasing [HCl]. WAXD shows that the crystallinity of PECs is reduced greatly due to the ionic complexation between CS and CMCNa, which destroys the ordered packing of chitosan chains. TGA result

Acknowledgement

Financially support from the NNSFC (50633030, 20876134 and 20574059) is greatly appreciated.

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