Chinese Chemical Letters  2015, Vol.26 Issue (03):285-288   PDF    
Impact of an ionic surfactant on the ion transfer behaviors at meso-liquid/liquid interface arrays
Kui Gao, Xu-Heng Jiang, Dao-Pan Hu, Shu-Juan Bian, Meng Wang, Yong Chen     
* Corresponding authors at:School of Chemical and Environmental Engineering, Shanghai Institute of Technology, Shanghai 201418, China
Abstract: The influence of ionic surfactants, cetyltrimethylammonium bromide (CTAB), self-assembled within silica-nanochannels of a hybrid mesoporous silica membrane (HMSM) on simple ion transfer (IT) behaviors at the meso-water/1,2-dichloroethane (W/DCE) interface arrays supported by such a HMSM was investigated by voltammetry for the first time. Significantly, it is found that the CTAB in HMSM can dramatically enhance the peak-current responses corresponding to ITs of some anions and even lower their Gibbs transfer energies from W to DCE, which could be ascribed to an anion-exchange process between anions and the bromide of CTAB associated with partial ion-dehydration induced by the CTAB. This work will provide a new strategy to study anion transfer processes and improve the electroanalytical performance for anion detection at the liquid/liquid interface.
Key words: Mesoporous membrane     Surfactant     Liquid/liquid interface     Ion transfer     Voltammetry    
1. Introduction

Electrochemistry at the membrane-supported liquid/liquid (L/L) interface has recently attracted much attention owing to its combining the advantages of micro-L/L interface electrochemistry [1] with the diversity of membrane materials [2, 3, 4, 5, 6, 7] and its widely electrochemical applications especially in the electrochemical detection of various ions [4, 5, 6, 7]. So far,all those membranes employed to support the L/L interface have mainly focused on some freestanding homogeneous porous membranes with pore sizes ranging from micrometer (<2.0 nm) to macrometer (>50.0 nm),including zeolite [2],porous anodic alumina [3],track-etched polymer [4, 5] membranes and silicon chips [6, 7]. However,mesoporous materials have rarely been applied in this field until now mainly due to that it is a challenge to obtain homogeneous mesoporous membranes with vertical alignment of mesopores [8].

>In the recent decade,a kind of free-standing hybrid mesoporous silica membranes (HMSMs) with unique structure of pores-in-pores have been developed [8] and applied in the fields of electrochemistry [9, 10, 11] and membrane science [12, 13, 14, 15, 16]. For example,some HMSMs synthesized by using CTAB as structure-directing agent (SDA), namely CTAB-templated HMSMs,can be applied as a hard template for the electro-deposition of nanowire arrays [9],or an attractive solid-extraction membrane [12] and nanofiltration membrane for the separation of molecules in aqueous solution [13, 14, 15]. However, there have been few reports on their applications in the L/L interface electrochemistry [16, 17]. In our recent work,a new CTAB-templated HMSM with structural integrity andmeso-sized selectivity had successfully been synthesized and applied to support themeso-W/ DCE interface arrays for the size-selective ion transfer or extraction at the HMSM-supported W/DCE interface [16],which can be further applied to electrochemically fabricate a membrane electric-device by Huanget al. [17].

Nevertheless,it should be pointed out that our previous work only explored the physical volume-exclusion of silica-nanochannels to ion transfer because CTAB was removed from HMSM by solventextraction [16]. The possible influence derived from the chemical property of CTAB on ion transfer has still been unknown. As far as the transports of molecules or ions through CTAB-templated HMSMs in aqueous solution are concerned,it has been found that CTAB selforganized inside HMSMs can affect their diffusion behaviors through HMSMs in aqueous solution [13, 15]. Moreover,some previous studies had demonstrated the significant influences of ionic surfactants on the adsorption or transfer behaviors of ions [7, 19, 20, 21] and the electron transfer at the L/L interface [22],such as the improvement of protein detection at the L/L interface owing to the formation of protein-surfactant deposit [7] or micelles [21]. Therefore,it is indeed necessary to further investigate the possible influences of CTAB self-assembled inside silica-nanochannels on the ion transfer behaviors at the HMSM-supported L/L interface.

Herein,a CTAB-containing HMSM is employed to construct the meso-W/DCE interface arrays surrounded by CTAB. The simple ion transfer (IT) behaviors occurring at the meso-W/DCE interface arrays with or without CTAB are investigated by cyclic voltammetry (CV) with comparison to the previous work on the IT at the meso-W/DCE interface arrays supported by CTAB-removed HMSM [16]. The aim of this work is to further examine the possible impact of CTAB in HMSM on the IT behaviors at the HMSM-supported L/L interface and exploit more potential applications of such CTABtemplated HMSMs in the L/L interface electrochemistry based on their well-known size-selective nanofiltration of molecules and ions in aqueous solution or at the L/L interface [14, 15, 16]. 2. Experimental

Bis(triphenylphosphoranylidene)ammonium tetraphenylborate (BTPPATPB) was prepared using bis(triphenylphosphoranylidene) ammonium chloride (BTPPACl,97%) (Sigma-Aldrich) and sodium tetraphenylborate (NaTPB,98%) (Acros) [2, 16]. All other chemicals were purchased from Sinopharm,China. PET membranes were obtained from Haoxia Nuclepore membrane Ltd., China (pore diameter~0.5mm,thickness~5mm and porosity (5±1.4)%). The CV experiments were performed using a potentiostat (CHI760D,CHI,USA) and a four-electrode electrochemical cell [16]. The CTAB-containing HMSM and the CTAB-removed HMSM were prepared by the aspiration-induced infiltration method with or without solvent-extraction according to the previous reports [14, 16] and respectively denoted as CTAB-HMSM and extracted-HMSM. The electrochemical cells used in this work are shown as below: AgjAgCljaqueous solutionjj20 mmol L-1 BTPPATPB (DCE)j1 mmol L-1-1 BTPPACl + 10 mmol L-1 KCljAgCljAg. Aqueous solution only containing 0.1 mol L-1 KCl is for the background. Aqueous solutions (xmmol L-1 Y + 0.1 mol L-1 KCl) are for IT,where Y is TEACl (x=1) and NaClO4,KSCN,NaBF4 (x= 10). The double bar represents the membrane-supported W/DCE interface. 3. Results and discussion

Fig. 1a shows the CV curves of background (curve A) and ITs of TEA+ obtained by using extracted-HMSM (curve B) and CTABHMSM (curve C). Firstly,it is obvious that no faradaic current response appears and no electrochemical instability happens within the background window corresponding to the transfer of ionic surfactant across W/DCE interface as reported previously [23],which indicates that the component ions of CTAB,namely CTA+ and Br- ,cannot transfer across the CTAB-HMSM-supported W/DCE interface because CTAB are closely attached on the inner surface of silica-nanochannels [15, 18]. Additionally,symmetrically peak-shaped CV curve (Fig. 1a,curve C) are always obtained for the IT of TEA+ across the W/DCE interface supported by CTAB-HMSM, which are completely different from the asymmetrical wave (Fig. 1a,curve B) obtained by using extracted-HMSM. It has been found that the W/DCE interface supported by extracted-HMSM forms at the membrane surface and presents asymmetric diffusion field,which results in asymmetric CV responses for IT [16]. All those symmetrically peak-shaped CV curves withmA-level current response (Fig. 1a,b) illuminate that the W/DCE interface supported by CTAB-HMSM should exist inside the silica-CTAB nanochannels due to their relatively hydrophobic inner environment caused by the alkyl chain (cetyltrimethyl group) of CTAB [12, 18]. As for the IT of ClO4- across the W/DCE interface supported by using different HMSMs,asymmetric CV responses are obtained by using extracted-HMSM (Fig. 1c),which completely accords with the characteristics ofmeso-W/DCE interface arrays without CTAB [16]. However,symmetrically peak-shaped CV responses for the IT of ClO4- are also obtained by employing CTAB-HMSM (Fig. 1d). Therefore,as shown as Fig. 1e,themeso-W/DCE interface arrays surrounded by CTAB with symmetrically linear diffusion field could form in CTAB-HMSM,where a relatively hydrophobic zone (abbreviated herein as RHZ) with micrometer-scaled thickness caused by CTAB should exist between W and DCE.

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Fig. 1. (a) CVs for the background of CTAB-HMSM-supported W/DCE interface (curve A) and the IT of TEA+ across the W/DCE interface supported by extracted-HMSM (curve B) and CTAB-HMSM (curve C) at 10 mV s-1 ; (b) CVs for the IT of TEA+ across the CTAB-HMSM-supported W/DCE interface under different scan rates; CVs for the IT of ClO4- across the W/DCE interface supported by extracted-HMSM (c) and CTAB-HMSM (d) under different scan rates. The insets in (b)-(d) are corresponding plots of peak current for IT from W to DCEvs.v1/2 . (e) Scheme of the W/DCE interface with symmetric linear diffusion field formed inside the silica-CTAB nanochannels.

Since the IT processes from W to DCE by using both CTABHMSM and extracted-HMSM are dominated by linear diffusion due to the confinement of silica-nanochannels in HMSM,it is possible to further investigate the influence of CTAB on the ion diffusion behaviors inside the silica-nanochannels. Fig. 1b-d respectively show that the peak currents (Ip)ofCVcurvesforthe IT of TEA+ and ClO4- from W to DCE increase with the scan rate (n) and display a linear dependence on n 1/2 .Accordingtothe Randles-Sevcik equation [1],the diffusion coefficient of ClO4- in silica-nanochannels of CTAB-HMSM is calculated as about (1.2±0.1)×10-5 cm2 s-1 . This value is smaller than that obtained by using extracted-HMSM ((1.9×0.1)×10-5 cm2 s-1 ),which can be ascribed to the hindrance effect of CTAB in HMSM on the transport of molecules or ions inside the silica-CTAB nanochannels as reported previously [15, 18]. Indeed,the diffusion coefficient of TEA+ in silica-nanochannels ((4.0±1.5)×10-6 cm2 s-1 )also becomes smaller when the W/DCE interface is supported by CTAB-HMSM instead of extracted-HMSM [16]. While,it is noteworthy that the Ip corresponding to the IT of ClO4- occurring at the CTAB-HMSM-supported W/DCE interface are~200% larger than that obtained by using extracted-HMSM,whereas this value for TEA+ transfer is almost ignorable. As pointed out by Yamaguchi et al. [12],CTAB-HMSM can efficiently extract anions from aqueous solution into silica-CTAB nanochannels based on an anionexchange process between some anions and the bromide of CTAB. Thus,it is possible that ClO4- could be extracted into the silicaCTAB nanochannels before IT process,which leads to the preconcentration of ClO4- in the RHZ and the corresponding currentenhancement phenomenon.

In order to further verify the current-enhancement phenomenon observed above for anion transfer,IT behaviors of SCN- and BF4- at the W/DCE interface supported respectively by CTABHMSM and extracted-HMSM are investigated. Fig. 2a-d presents stable CV curves without any electrochemical instability,which can further confirm that CTAB are closely attached on the inner surface of silica-nanochannels as mentionedabove.Theshape and the current responses of CV curves (Fig. 2a,c) corresponding to the ITs of SCN- and BF4- across the extracted-HMSMsupported W/DCE interface are also different from those CVs obtained by using CTAB-HMSM. According to Fig. 2b,d,the diffusion coefficients of SCN- and BF4- in silica-CTAB nanochannels are respectively calculated as about (1.4±0.1)×10-5 and (1.8±0.1)×10-5 cm2 s-1 based on the Randles-Sevcik equation, which are smaller than the values obtained by using extracted-HMSM ((2.4±0.1)×10-5 for SCN- and (4.0±0.2)×10-5 cm2 s-1 for BF4- ). However,the current responses for the ITs of SCN- and BF4- at the CTAB-HMSM-supported W/DCE interface are also~180%-220% larger than that obtained by employing extracted-HMSM,indicating that the pre-concentration of ions in the RHZ through an anion-exchange process between them and the bromide of CTAB should be possible and reasonable as shown as Fig. 2e. Moreover,there is another significant phenomenon for anion transfer that all peak potentials of CVs corresponding to ITs of ClO4- ,SCN- and BF4- from W to DCE shift positively~50-80 mV when the W/DCE interface is supported by CTAB-HMSM instead of extracted-HMSM. It is well-known that the positive potential-shift for anions transfer means the facilitated anion transfer (FAT),which is normally fulfilled by the addition of ionophore into organic phase to lower the Gibbs transfer energy of anion at the L/L interface [24]. According to the previous reports [12, 18],there is a partial-dehydration process during the extraction or transport of molecules or ions from aqueous solution into CTAB-HMSM. Therefore, the positive potential-shift observed herein indicate that the RHZ in CTAB-HMSM can play a role in the partial dehydration of anions before IT processes and thus lowering their corresponding Gibbs transfer energies from W to DCE byca.4.8-7.7 kJ (ΔGW→O =nFDEW→O ),as shown in Fig. 2e. However,we found it is still impossible for some strongly hydrophilic anions,such as SO42- ,Cl- and NO3- ,totransfer across such CTAB-HMSM-supported W/DCE interface,which could be owing to the weak abilities of anion-exchange and dehydration of CTAB-templated HMSM because the extraction order of anions into CTAB-HMSM is ClO4- >>NO3- >Br- >Cl- [12]. Maybe,other anionexchange membranes with stronger abilities of anion-exchange and dehydration than CTAB-templated HMSM can solve that problem. Also, more HMSMs synthesized by using different surfactants should be developed for the ion-transfer voltammetric studies in the future.

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Fig. 2. CVs for ITs of (a) SCN- and (c) BF4- across the W/DCE interface supported by extracted-HMSM (curve A) and CTAB-HMSM (curve B) at 10 mV s-1 ; CVs for ITs of (b) SCN- and (d) BF4- across CTAB-HMSM-supported W/DCE interface under different scan rates. The insets in (b) and (d) are the corresponding plots ofIpvs.v1/2 . (e) Scheme of the anion-exchange process associated with partial ion-dehydration occurring between the bulk aqueous solution and a relatively hydrophobic zone (RHZ) caused by CTAB.
4. Conclusion

In summary,the meso-W/DCE interface arrays surrounded by ionic surfactants,CTAB,are built by using a HMSM composed of silica-CTAB nanochannels in PET membrane. It is found that the CV responses for ITs at suchmeso-W/DCE interface arrays are closely related to CTAB. On the one hand,the W/DCE interface with symmetric linear diffusion field should form inside the silica-CTAB nanochannels of HMSM due to their relatively hydrophobic inner environment caused by CTAB,which results in symmetrically peak-shaped CV curves. On the other hand,CTAB can dramatically enhance the peak current responses corresponding to the ITs of some anions and even lower their Gibbs transfer energy from W to DCE due to an anion-exchange process between anions and the bromide of CTAB associated with partial ion-dehydration,which is expected to provide a new strategy to study anion transfer processes and improve the electroanalytical performance for anion detection at the liquid/liquid interface,as well as offer new insight into the transport processes of hydrated anions across biomembranes in bioscience.

Acknowledgments

This work was supported by the National Science Foundation of China (No. 21005049) and the Natural Science Foundation of Shanghai,China (No. 14ZR1440900)

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