###### Abstract  
Membrane capacitive deionization (MCDI) is a modification of capacitive deionization (CDI) using ion-exchange membranes (IEM) in front of the electrodes. Electrode properties, especially the specific surface area, are known to be strongly related with desalination performance in CDI, but the effects of other properties in MCDI are not fully understood. The objective of this study was to investigate the effect of hydrophilicity in activated carbon electrodes on desalination performance in MCDI. Two types of activated carbon (P60 and YS-2) whose specific surface areas were similar were used as electrode materials, but they had different hydrophilicity (i.e., P60 was originally hydrophobic and YS-2 was relatively hydrophilic due to its nitrogen-containing surface chemistry). These hydrophilic electrodes (either the electrode itself or modified with polydopamine (PDA)) led to an increase in the salt adsorption capacity (SAC) in MCDI because they facilitated the access of both ions and water molecules into the electrode pores. In particular, the SAC of the P60 electrode displayed a large increase to almost reach that of the YS-2 electrode due to the improved hydrophilicity with PDA modification and the insignificant effects of PDA modification on an already hydrophilic YS-2 electrode. Additionally, PDA-modified IEM in MCDI reduced the SAC as a result of the additional insulating PDA layer with little changes in hydrophilicity. m pacs: 12.30.  
an electrode and a commercial IEM. [PERSON] et al. [19] demonstrated an improved salt removal efficiency of approximately 37% in MCDI when using graphite-based electrodes incorporated with single-walled carbon nanotubes and IEM. An additional 5% salt removal efficiency was achieved by applying higher voltages of up to 1.6 V. Placing the IEM in front of the electrode also significantly increased the charge efficiency of the salt ion adsorption/desorption [20; 21] due to the inhibition of co-ion repulsion affected by the IEM acting as a charge barrier and the additional ion storage in the interparticle spaces of the electrode [22; 23; 24]. Electrode properties are strongly correlated with the desalination performance in CDI and MCDI [25]. It is well known that the specific surface area of the electrode is linearly correlated with the salt adsorption capacity (SAC) [1]. Besides the specific surface area, which is a physical property, hydrophilicity is also considered to improve the desalination performance in CDI because a hydrophilic electrode surface allows the ions to be transported more easily within the porous electrode. Although the electrode modification in CDI has been studied in the literature [26; 27; 28], studies focusing on the hydrophilicity of electrode have been limitedly reported. For example, [PERSON] et al. [29] reported approximately twice the desalination performance using an AC electrode modified with TiO\({}_{2}\) using the sol-gel spray method. The TiO\({}_{2}\)-modified electrode significantly increased the hydrophilicity from 132\({}^{\circ}\) to 15\({}^{\circ}\) of contact angle. [PERSON] et al. [30] exhibited that polydopamine (PDA)-modified AC reduced the contact angle from 120\({}^{\circ}\) to 65\({}^{\circ}\), resulting in more than twice the desalination performance in CDI Despite these previous works on the hydrophilic modification of electrodes in CDI [29; 30], there is limitedly published data for the effects of hydrophilic electrodes on the desalination performance in the presence of IEM, that is MCDI. The objective of this study was to investigate the effect of hydrophilicity in AC electrodes on the maximum capacity utilization in MCDI. Two ACs (hydrophobic P60 and hydrophilic YS-2) were chosen as electrode materials and compared with each other as they have similar specific surface areas but different hydrophilicities. SACs were obtained from single-pass desalination experiments in MCDI. Furthermore, the AC electrodes and/or IEMs were modified with PDA to enhance their hydrophilicity.