Liquid phase formation of alkyl- and perfluoro-phosphonic acid derived monolayers on magnesium alloy AZ31 and their chemical properties

https://doi.org/10.1016/j.jcis.2011.04.039Get rights and content

Abstract

Alkyl- and perfluoro-phosphonic acid derived SAMs were successfully formed on Mg alloy by liquid phase method for the first time. The chemical and anticorrosive properties of the prepared SAMs on magnesium alloys were characterized using contact angle measurements, X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and electrochemical measurements. Water contact angle measurements revealed that the maximum advancing/receding water contact angles of n-octyl (OP: CH3(CH2)7PO(OH)2), n-dodecyl (DP: CH3(CH2)11PO(OH)2), n-octadecyl (ODP: CH3(CH2)17PO(OH)2) phosphonic acid, and 2-(perfluorohexyl)ethyl (PFEP: CF3(CF2)5CH2CH2PO(OH)2) phosphonic acid were 105.1/64.7°, 108.3/69.6°, 111.9/75.2°, and 115.2/67.4° respectively. In the case of alkylphosphonic acid SAMs (OP, DP, and ODP), the advancing and receding water contact angles increased with an increase in the preparation time. The angle-resolved XPS (AR-XPS) data revealed that the film thicknesses of the OP, DP, ODP, PFEP on Mg alloy were estimated to be 0.8, 1.2, 1.7, and 1.1 nm, respectively. The XPS O 1s data support that the phosphonic acid derived SAM is covalently bound to the oxide or hydroxide surface of the Mg alloy in a monodenate or bidenate manner. Chemical stability of the alkyl- and perfluoro-phosphonic acid modified Mg alloy surfaces was investigated using aqueous solutions at pH = 4.0, 7.0, and 10.0. The contact angles of OP, DP, and PFEP modified Mg surface decreased rapidly within the first 5 min after immersion in all the aqueous solutions and were less than 20°. On the other hand, the contact angles of the ODP modified Mg alloy after immersion in aqueous solutions at pH 4, 7 and 10 for 5 min were 45.1°, 89.3,° and 85.5°, respectively. The ODP modified Mg alloy had highest chemical stability in four types of the phosphonic acid derived SAMs used in this study, indicating that the molecular density of ODP on Mg alloy would be higher than those of OP, DP, PFEP on Mg alloy. The corrosion resistance of ODP modified Mg alloy was investigated by potentiodynamic polarization curve measurements. The ODP modified Mg alloy exhibits protective properties in a solution containing Cl ions compared to unmodified Mg alloy.

Graphical abstract

This study demonstrates preparation and estimation of chemical properties of phosphonic acid derived self-assembled monolayer prepared on magnesium alloy by liquid phase.

  1. Download : Download high-res image (96KB)
  2. Download : Download full-size image

Highlights

► Phosphonic acid derived self-assembled monolayers (SAMs) were formed on magnesium alloy by liquid phase method. ► Chemical bonding states between the SAMs and magnesium alloy were investigated using X-ray photoelectron spectroscopy. ► Chemical stability and corrosion resistance of the SAMs on magnesium alloy increased with an increase in the alkyl chain length.

Introduction

Magnesium is one of the light metals and has excellent physical and mechanical properties such as low density, good electromagnetic shielding, and high strength/weight ratio [1], [2]. In addition, it is the eighth most abundant element in the Earth’s crust by mass. Thus, magnesium and its alloys are expected to be applied to various transportation equipment industries such as automobiles, railways, and aerospace [3], [4], [5]. Magnesium is the eleventh most abundant element by mass in the human body, so its ions are essential to all living cells, where they play a major role in manipulating important biological polyphosphate compounds like ATP, DNA, and RNA. Thus, magnesium alloys are also expected to be applied to potential biodegradable bone implant materials due to their biodegradability in the bioenvironment [6], [7], [8], [9]. However, a great issue to apply magnesium alloys to these applications is the rapid corrosion of magnesium alloys in chloride containing solutions including environmental humidity and human body. Therefore, it is very important to improve the corrosion resistance of magnesium alloys. An effective mean to achieve this is appropriately to control interface regions between the magnesium alloys and environment by surface modification using ultrathin organic layer. Self-assembled monolayer (SAM) is a potential candidate for controlling the interfaces because they have their ease of manufacture without external aid and strong chemical bonding to the substrate. Typical SAMs, such as alkanethiols reacted with gold and alkylsilanes with silica, have been studied extensively as demonstrated by numerous reports in the literature [10]. In addition, the studies of the properties of n-alkanoic acids reacted with oxidized aluminum [11], [12] and magnesium alloys have been reported [13].

Phosphonic acid derived SAMs are believed to create a more robust modification layer compared to those created from chemisorbed carboxylic acids, especially after annealing to maximize the number of P–O bonds to the metal oxide lattice [14], [15]. Thus, the interactions between phosphonic acid and metal oxides have been studied on engineering materials surfaces such as steel [16], titanium [17], [18], [19], [20], [21], tantalum[22], [23], [24], copper [25], zirconium [19], [26], iron [21], [27], aluminum [21], [25], [28], [29], and silicon [26], [30], [31]. Giza et al. investigated tailoring of oxide chemistry on aluminum by means of low-pressure water and argon plasma surface modification to influence the kinetics of the self-assembly process of octadecylphosphonic acid monolayers [32]. They concluded that the hydroxyl-rich surface led to significantly accelerated adsorption kinetics of the phosphonic acid on the aluminum oxides. Hoque et al. performed the comparison studies of perfluorodecyldimethylchlorosilane (PFMS), octadecylphosphonic acid (ODP), decylphosphonic acid (DP), octylphosphonic acid (OP), and perfluorodecylphosphonic acid (PFDP) from view points of chemical stability and robustness [33]. They showed the stability against warm nitric acid of ODP/Al SAM to be the most stable followed by PFDP/AS, DP/Al, PFMS/Al, and OP/Al SAM. Ito et al. prepared octadecylphosphonic acid (ODPA) on GaN substrates and characterized them from the viewpoint of chemical stability [34]. They showed that as-prepared ODPA monolayers were desorbed from the GaN substrate by soaking in an aqueous solution, whereas ODPA monolayers heated at 160o exhibited suppressed desorption in acidic and neutral aqueous solutions. It is known that the interaction or chemical bonding states change as being monodenate [22], [26], [29], bidenate [17], [24], or tridenate [35], [36], depending on the materials surfaces. Various interactions between phosphonic acid and metal oxides have been widely investigated; however, the preparation and chemical properties of phophonic acid on magnesium alloys have been not yet investigated.

In this paper, we report the preparation of alkyl- and perfluoro-phosphonic acid SAMs on magnesium alloy. The chemical and anticorrosive properties of the prepared SAMs on magnesium alloys were characterized using X-ray photoelectron spectroscopy (XPS), contact angle measurements, atomic force microscopy (AFM), and electrochemical measurements.

Section snippets

Experimental procedures

Magnesium alloy AZ31 (composition: 2.98% Al, 0.88% Zn, 0.38% Mn, 0.0135% Si, 0.001% Cu, 0.002% Ni, 0.0027% Fe, and the rest is Mg) with a thickness of 1.5 mm was used as the substrate. Prior to the preparation of phosphonic acid SAM, all the magnesium alloy substrates were polished using some SiC emery papers and alumina suspension containing alumina nanoparticles with a diameter of 50 nm by buffing machine, resulting in the mirror finished surface. The substrates were then ultrasonically cleaned

Preparation and characterization of alkylphosphonic acid derived monolayers

Phosphonic acids are known to interact strongly with transition metal oxides such as tantalum oxide (Ta2O5) [22] and titanium oxide (TiO2) [18], [43]. The phosphonic acids attach to the metal oxides (which are basic hydroxyl surfaces) via acid–base interactions that anchor the molecules to the surface. Thus, it is expected that the phophonic acid would attach to the MgO or Mg(OH)2 surface.

The wetting properties of the SAM modified magnesium alloy provide a simple assay of the quality of the

Conclusions

Alkyl- and perfluoro-phosphonic acid derived SAMs were successfully formed on Mg alloy by liquid phase method. The chemical and anticorrosive properties of the prepared SAMs on magnesium alloys were characterized using contact angle measurements, XPS, AFM, and electrochemical measurements. In the case of alkylphosphonic acid SAMs (OP, DP, and ODP), the advancing and receding water contact angles increased with an increase in the preparation time. The maximum advancing and receding water contact

Acknowledgment

This research was supported by the Adaptable and Seamless Technology Transfer Program through Target-driven R&D (A-STEP) from Japan Science and Technology Agency and was carried out by the joint research program of the EcoTopia Science Institute, Nagoya University.

References (56)

  • J.E. Gray et al.

    J. Alloy. Compd.

    (2002)
  • Z. Yong et al.

    Appl. Surf. Sci.

    (2008)
  • A.R. Phani et al.

    Surf. Coat. Technol.

    (2006)
  • B.L. Mordike et al.

    Mater. Sci. Eng. A.

    (2001)
  • L. Xu et al.

    Biomaterials

    (2009)
  • F. Witte et al.

    Biomaterials

    (2005)
  • F. Witte et al.

    Biomaterials

    (2006)
  • Y. Liu et al.

    Appl. Surf. Sci.

    (2006)
  • J. Schwartz et al.

    Mater. Sci. Eng. C

    (2003)
  • I. Gouzman et al.

    Surf. Sci.

    (2006)
  • T. Ishizaki et al.

    Electrochim. Acta

    (2010)
  • A. Paszternák et al.

    Electrochim. Acta

    (2010)
  • A. Pardo et al.

    Corr. Sci.

    (2008)
  • H. Won et al.

    J. Alloy Compd.

    (2006)
  • L. Xu et al.

    J. Biomed. Mater. Res.

    (2007)
  • A. Ulman

    An Introduction to Ultrathin Organic Films: Fro, Langmuir-Blodgett to Self-Assembly

    (1991)
  • D.L. Allara et al.

    Langmuir

    (1985)
  • D.L. Allara et al.

    Langmuir

    (1985)
  • T.J. Gardner et al.

    J. Am. Chem. Soc.

    (1995)
  • B. Vercelli et al.

    Langmuir

    (2003)
  • A. Raman et al.

    Langmuir

    (2006)
  • E.S. Gawalt et al.

    Langmuir

    (2001)
  • S. Marcinko et al.

    Langmuir

    (2004)
  • G. Hahner et al.

    Langmuir

    (2001)
  • E.S. Gawalt et al.

    Langmuir

    (1999)
  • R. Hofer et al.

    Langmuir

    (2001)
  • B. Adolphi et al.

    Anal. Bioanal. Chem.

    (2004)
  • M. Textor et al.

    Langmuir

    (2000)
  • Cited by (0)

    View full text