Comparison between free and Immobilized Ion Effects on Hydrophobic Interaction:A Molecular Dynamics Study

Insert name
Professor’s name
Course/class
Date
Article Review on Hydrophobic Interactions (HI)
In “Comparison between Free and Immobilized Ion Effects on Hydrophobic Interactions: A molecular dynamics study,” Huang, Kai, Sebastian Gast, Derek Ma, Nicholas Abbott and Izabela Szlufarska discuss the chemistry of hydrophobic interactions (HI) and how they are affected by both free and immobilized ions. Applying an experimental approach and modeling techniques, the authors note that free ions (in the form of free salts) cause changes in water density, particularly near the nonpolar molecular surface thereby affecting HI. In addition, it is noted that immobilized ions affect HI through changes in spatial arrangements and ionic sizes. In essence, the article illustrated that while immobilized ions (particularly univalent proximal anions) weaken HI, free ions strengths the interactions. Other than the effect on the interactions, it is noted that while free ions observe the Hofmeister-like ranking system in order effect on HI in relation to ionic size, immobilized ions do not observe the same system (Huang et al. 13153-1357). Therefore, the article presents the result of a study on HI in which it is reported that free ions and immobilized ions have contrasting effects on the interactions.
HI is at the heart of a lot of renowned chemical interactions characterized as biophysical, colloidal, and interfacial phenomena. These phenomena are seen in everyday life to include nonpolar surfaces forming water beads, proteins and biomembranes folding and assembly, mineral particles’ air flotation, and coalescing of petroleum particles on water surfaces.

They are also seen in water-based solvents and oil-based solvents forming two distinct layers when mixed (as seen in the case of water and oil immiscibility) to name a few of these phenomena and processes. The real nature of HI is observed in water molecules structuring when near non-polar surfaces. This is a simplistic understanding of HI, which fails to acknowledge its relevance within more complex systems of technological and biological structures. These complex systems are described by complex configurations of charged, polar and non-polar domains that exist in close proximity (Garde 277-278). For instance, positively charged ions have been shown to strongly moderate HI when the ions are placed in close proximity to non-polar domains. In fact, the ions can eliminate the interactions or even double them. Huang et al. (13153-1357) add to this understanding of ions interaction with HI, to show that it is possible to optimize systems using HI by aiding self-assembly and molecular recognition. Besides that, it shows that prudently inserting charged ions and groups near hydrophobic domains can assist in tuning the hydrophobic forces that drive the self-assembly and molecular recognition processes.
The results presented in the article by Huang et al. (13153-1357) are not unique. In fact, they are supported by other studies. Firstly, Zeng et al. report on recent experimental advances made in studying HI, focusing on the solid-water and fluid-water interfaces. Of import is the fact that HI decay varied as a function of the surface interface that can either be fluid or solid. In essence, the article concedes that there is a mismatch of HI, to imply that the physical effects of hydrophobicity are controlled and modulated by other factors to include the nature of the associated ions and their interactions. In its conclusion, the article mentions that there is a need to understand how other factors affect HI so as to apply the understanding in chemical and biological systems with the intention of improving performance (Zeng et al.).
Secondly, Huang (2-3) conducted molecular studies on the forces acting on liquid-solid interfaces. Using simulations and theory, the author noted that free ions have an effect on HI, with the effect moderated by water density variations that occur near the non-polar molecular surface. Given that this effect is unique to free ions, the author recommended that it be used as evidence of HI occurrence when soluble additives were used. With regards to immobilized ions, the results showed that HI could be controlled through either turning off or on if the proximal ions were influenced to change their spatial arrangement. In addition, the author noted that changes in ion size affected immobilized ions activity in as far as HI was concerned. Overall, the dissertation makes it clear that free ions strengthened HI while immobilized ions weakened the interactions through changes in ion size and spatial arrangement of the proximal ions (Huang 2-3).
Finally, Ma et al. (347) postulated that proximally immobilized ions had a modulating effect on HI. The article concedes that water surfaces have a unique structure that can mediate HI through biophysical, colloidal and interfacial phenomenon. It also adds that while learning about the effect of charged, polar and non-polar groups effects on HI has academic and scientific value, that value is minimized by the fact that technological and biological systems that make use of HI, do so in domains that include elements from charged functional, polar and non-polar groups in close proximity. With this background, the article contends that studies on HI should be based on chemical heterogeneity to gain a better appreciation of how different ions affect HI. To further advance its agenda, the article presents findings in which surfaces immobilized by alkaline functionalized groups (such as guanidine and amine groups) use co-immobilization to change HI strengths. In this case, HI strength is shown to double when the amine group is protonated while guanidine has the opposite effect. For that matter, functionalized groups are revealed to have different effects on HI. In addition, the article makes it clear that hydrophobicity is not an intrinsic property of any domain. Rather, functionalized groups modulate hydrophobicity. In its conclusion, the article notes that the modulating effect of functionalized groups on hydrophobicity can be used to optimize self-assembly and molecular recognition processes by using functionalized groups to tune the forces driving HI (Ma et al. 347-349).
In conclusion, one must accept that Huang et al. (13153-1357) adds to chemistry knowledge and improves understanding of HI by offering new insights free and immobilized ions effects on HI. In fact, it shows that while free ions strengthen HI, immobilized ions weaken these interactions. In addition, one must acknowledge that other literature sources express similar sentiments by showing that HI interactions are managed by hydrophobicity but moderated by other factors to include the ions sizes, distance, nature, and charge. Overall, the article acts as a vital resource for improving the efficiency of biological and chemical processes that use self-assembly and molecular recognition. Therefore, Huang et al. (13153-1357) add to the understanding of ions interaction with HI, by showing that prudently inserting ions and groups near hydrophobic domains can aid in tuning the hydrophobic forces that drive the self-assembly and molecular recognition processes.

Works Cited
Garde, Shekhar. “Physical Chemistry: Hydrophobic interactions in context.” Nature, 517.7534(2015), 277–279.
Huang, Kai, Sebastian Gast, Derek Ma, Nicholas Abbott and Izabela Szlufarska. “Comparison between Free and Immobilized Ion Effects on Hydrophobic Interactions: A molecular dynamics study.” J. Phys. Chem. B, 119.41(2015), 13152–13159.
Huang, Kai. Molecular Studies of Forces at Liquid-Solid Interfaces. Dissertation, The University of Wisconsin – Madison, 2015. UMI, 2006. AAT 3723141.
Ma, Derek, Chenxuan Wang, Claribel Acevedo-Vélez, Samuel Gellman and Nicholas Abbott. “Modulation of hydrophobic interactions by proximally immobilized ions.” Nature, 517.7534(2015), 347-350.
Zeng, Hongbo, Chen Shi, Jun Huang, Lin Li, Guangyi Liu and Hong Zhong. “Recent experimental advances on hydrophobic interactions at solid/water and fluid/water interfaces.” Biointerphases, 11.018903(2016). http://dx.doi.org/10.1116/1.4937465