Molecular level investigations of specific ion effects at air/aqueous interface

Abstract

Specific ion effects at air/aqueous interfaces are widely involved in physical, chemical, environmental, and biological processes imperative in modern surface science and technology. The kinetics and thermodynamics of these processes are determined by the ion speciation, and the influence of ions on the structure, conformation, and bonding environment of the molecular groups present at the air/aqueous interface. Several research groups have put a lot of effort into this research with various experimental and theoretical approaches. However, a detailed molecular level understanding of the specific ion phenomena is lacked to date. In the present thesis, the focus is positioned on providing the molecular level details into the intermolecular interactions as well as the structure and conformations of interfacial molecules to better comprehend the specific ion effects at the air/aqueous interface. In this direction, the present thesis investigates the water evaporation process, laser induced liquid microjets, and ordering/disordering of macromolecule structure in the presence of ions at the air/aqueous interface in unprecedented detail. To achieve the targeted research objectives, in the experimental scheme, we have developed experimental setups: a) Time-resolved Newton’s ring interferometry to investigate the evaporation dynamics of sessile aqueous droplets. b) Laser-induced microjets to study radiation-induced deformation of air/aqueous interface. We have utilized a state-of-the-art surface-specific sum frequency generation (SFG) vibrational spectroscopic tool to probe the molecular level structural details governing the specific ion effects at the air/aqueous interface. The first research work of the thesis intends to investigate specific ion effects in the water evaporation process at the air/water interface. During evaporation, a water molecule breaks the intermolecular hydrogen (H-) bonds with its neighboring molecules at the air/water interface and enters the vapor phase. The phenomenon of water evaporation has been studied meticulously owing to its great importance in diverse fields of science and technology, specifically in Earth’s water cycle. Most of the reported work primarily focuses on the macroscopic approach, which leads to a gap in the mechanistic interpretation of the process. This motivates the present research work to provide fundamental insights into the water evaporation process through molecular level details into the complex bonding environment of the surface water molecules evaporating from the air/water interface. For this, we have utilized Hofmeister ions (kosmotropes: HPO4 2−,SO4 2−, and CO3 2−and chaotropes: NO3 −, and I−) purposefully to perturb the H-bonding environment at the air/water interface to excerpt their influence on the evaporation process. In our experimental scheme, we have developed in-house time-resolved interferometry to study the evaporation dynamics of sessile aqueous droplets. It is found that the kosmotropes reduce evaporation, whereas chaotropes accelerate the evaporation process of sessile salt droplets that follows the Hofmeister series: HPO4 2− < SO4 2− < CO3 2− < Cl− ≈ water < NO3 − < I−.To extract deeper molecular level insights into the observed Hofmeister trend in the evaporation rates, we have employed surface-specific SFG vibrational spectroscopy to investigate the air/water interface in the presence of Hofmeister ions. The SFG spectra reveal that the presence of ions significantly impacts the strength of the H-bonding environment and the orientation of free OH oscillators from ~ 36.2º to 48.4º at the air/water interface that follows the Hofmeister series, which in turn govern the evaporation process. It is established that the slow evaporating water molecules experience a strong H-bonding environment with free OH oscillators tilted away from the surface normal in the presence of kosmotropes. In contrast, the fast evaporating water molecules experience a weak H-bonding environment with free OH oscillators tilted towards the surface normal in the presence of chaotropes at the air/water interface. The present research work is this chapter demonstrates a comprehensive understanding of the role of hydrogen bonding environment and the orientation of the free OH moieties in governing the evaporation process at the air/water interface. The outcomes of this work would be helpful in designing interfacial water structures to manipulate the evaporation processes for future needs. In the second research problem, we investigated specific ion effects in the generation of laser-induced liquid microjets at the air/rhodamine 6G (Rh6G) dye solution interface. Highly focused, fine diameter, and fast liquid microjets are generated when the air/aqueous interface is deformed upon absorbance of a laser beam in the aqueous media. They find potential applications as painless needle-free drug injection systems in the medical industry and especially in vaccination drives during Covid-endemic to mitigate the risk of disease spread owing to the contaminated needle waste worldwide. An efficient drug delivery system demands fine diameter, fast liquid jets with controlled speed and penetration depths. In the past ten years, exciting research works are reported employing the ideology of variation in laser energy, laser focusing inside the liquid and capillary diameters to produce liquid jets with desired characteristics. In the present research work, we have utilized sodium salts of Hofmeister series anions (SO4 2−,I−and SCN−), which are the imperative constituents of drugs/vaccines, to showcase the contribution of ion-specific optical absorbance in determining the liquid jet characteristics, which has not been explored to date. In this direction, we have developed an in-house experimental setup using a picosecond laser to generate liquid microjets. We are able to generate liquid microjets of diameter ~ 40 m, speed ~ 252 m/s with Rh6G at laser (532 nm) pulse energy of 400 J within a microcapillary of inner diameter ~ 400 m. It is witnessed that the presence of ions do influence the velocity and power of liquid jets in Rh6G, which obeys the Hofmeister order as: SO4 2− ≈ Rh6G > I− > SCN−. The observed ion-specific liquid jet velocity is attributed to the ion-induced variation in the optical absorbance of Rh6G that also follows the Hofmeister series. This ion-specific optical absorbance is ascribed to the relative adsorption of ions toward the Rh6G molecule. It is suggested that the hydrophilic kosmotropic anion (SO4 2−) prefers to bind to the water molecules, whereas the hydrophobic chaotropic anions (I−,and SCN−) are prone to bind the NH- and hydrophobic CH-moieties of Rh6G. Therefore, this ion-specific interaction with the Rh6G in the solution determines the liquid jet velocities, which is then utilized to showcase the ion-specific penetration depths of liquid jets simulated for model soft tissues. The maximum penetration depth is observed with SO4 2−, however, a three-fold reduction in penetration depth is reported for I−,and SCN−. It is evident from the results that the present research work demonstrates the contribution of ion-specific interactions in governing the jet velocity and jet power that is crucial in the development of needle-free drug delivery systems with controlled penetration depths. The third research work of the thesis provides molecular level insights into the specific ion effect in ordering/disordering of macromolecule structure at the air/aqueous interface. Since 1888, when Franz Hofmeister first proposed the series of anions according to their efficacy to precipitate macromolecule in their aqueous solution, the series spans its horizons in various fields of science and technology like biophysics, chemistry, colloids, and environmental sciences. However, a detailed molecular level understanding of the Hofmeister phenomenon in a ternary system of ion, macromolecule, and water is still elusive. The current understanding involves the contribution of ion-water and ion-macromolecule interactions. However, some recent experimental reports have professed the undeniable contribution of ion-specific water-macromolecule interactions in the Hofmeister phenomenon, the exact molecular level mechanism of which has remained unknown to date. In this direction, we investigated the Hofmeister effect at the air/ polyvinylpyrrolidone (PVP) aqueous interface using surface-specific SFG vibrational spectroscopy in different polarization schemes. The spectral signature observed from the ssp polarisation scheme reveals ion-specific ordering of water molecules following the Hofmeister series. However, it does not reflect any impact on the structure of the PVP macromolecule. Interestingly, the ppp-SFG spectra in the CH-stretch region reveal that ions significantly impact the structure of PVP macromolecule at the air/aqueous interface This is evidenced by the ion-induced changes in the orientation angle of vinyl chain CH2-groups from 62.5 to 33.6 that follows the Hofmeister series: SO4 2− > Cl− > NO3− > Br− > ClO4 − > SCN− . The minimal orientation angle of CH2-groups in the presence of chaotropic anions ClO4 −,and SCN− indicates the ion-induced significant reordering in the PVP vinyl chains, which finds an intriguing correlation with the ion-specific water structure at the air/aqueous interface. From ppp SFG spectra in OH-stretch region, it is compelling to observe that the presence of chaotropic anions brings significant spectral blue shift of ~ 40 cm-1 in the OH-stretch band at 3540 cm-1. The blue shift in the OH-feature has been attributed to the weaker interactions between the interfacial water molecules and the hydrophobic moieties of the PVP macromolecules at the air/aqueous interface. This enables us to comprehend the molecular level mechanism of the Hofmeister effect as follows. The weakly hydrated chaotropic anions (ClO4 −,and SCN−), which are prone to adsorb towards the hydrophobic surface (-CH, -CH2 of polymer backbone), offer binding sites to the surrounding water molecules to form weak interactions with the hydrophobic moieties of the macromolecule, that, in turn, reorders the macromolecule structure at the interface. The current research provides the first spectroscopic evidence of cooperative participation of ion-specific water-macromolecule interactions in the molecular level mechanism of the Hofmeister effect, along with the well-known ion-water and ion macromolecule interactions. The molecular level approach developed in this thesis could potentially motivate new experiments and theoretical studies to better comprehend the fundamentals of specific ion phenomena that widely prevail in various scientific and technological applications.