Elucidating molecular mechanisms of two-dimensional chemical reactions

Abstract

2D reactions on surfaces/interfaces are important but difficult to study. In this issue of Chem, Seki et al. report the use of sum-frequency generation vibrational spectroscopy to elucidate that molecular mechanisms of surfactants facilitated “on-water” 2D polymerization reactions by identifying the key reaction intermediate. 2D reactions on surfaces/interfaces are important but difficult to study. In this issue of Chem, Seki et al. report the use of sum-frequency generation vibrational spectroscopy to elucidate that molecular mechanisms of surfactants facilitated “on-water” 2D polymerization reactions by identifying the key reaction intermediate. Main textChemical reactions are widely encountered everywhere: from stellar explosions and the expansion of the universe to industrial manufacturing of commercial products, agricultural production of food, and biological processes including digestion and metabolism. Extensive research has been performed on bulk chemical reactions by using many different analytical tools to understand their molecular mechanisms; such research has aided in the development of new compounds and novel reactions, optimizing conditions for important reactions and increasing the yields for preferred products.Two-dimensional (2D) reactions on surfaces/interfaces are also important but difficult to study, because such studies require surface/interfacial sensitive techniques. High-vacuum techniques have been developed to investigate mechanisms of various reactions (e.g., catalytic reactions) on surfaces, providing significant understanding regarding these reactions.1Somorjai G.A. Li Y. Introduction to Surface Chemistry and Catalysis.Second Edition. Wiley, 2010Google Scholar However, for reactions occurring on ambient surfaces or at buried (e.g., solid/liquid or solid/solid) interfaces, it is challenging to study in situ in real time at the molecular level due to the lack of appropriate analytical tools.With sum-frequency generation (SFG) vibrational spectroscopy, in this issue of Chem Seki et al. reported exciting research results on investigations of 2D polymerization reactions at liquid/air interfaces facilitated by surfactants.2Seki T. Yu X. Zhang P. Yu C. Liu K. Gunkel L. Dong R. Nagata Y. Feng X. Bonn M. Real-time Study of On-Water Chemistry: Surfactant Monolayer-Assisted Growth of a Crystalline Quasi-2D Polymer.Chem. 2021; 7: 2758-2770Abstract Full Text Full Text PDF Scopus (6) Google Scholar The 2D reactions were followed as a function of time in situ to deduce the molecular reaction mechanisms and identify key reaction intermediates. SFG is a second-order nonlinear optical spectroscopy, which provides vibrational spectra of surfaces/interfaces in situ in real time.3Shen Y.R. Surface Properties Probed by Second-Harmonic and Sum-Frequency Generation.Nature. 1989; 337: 519-525Crossref Scopus (2043) Google Scholar Vibrational spectra are the fingerprints of molecules; therefore, molecular-level interfacial information can be obtained from SFG studies. According to the selection rule of a second-order nonlinear optical process, SFG signal can only be generated from a system with no inversion symmetry (under the electric dipole approximation), leading to the sub-monolayer surface/interface specificity. Most bulk materials have inversion symmetry and so cannot produce SFG signal. In a typical SFG experiment, where a fixed-frequency visible laser beam and a frequency-tunable mid-infrared (IR) beam overlap on a surface/interface, the output beam with the sum frequency of the two input beams is detected, and its intensity is plotted versus the input IR beam wavenumber, producing a vibrational spectrum. According to the vibrationally resonating SFG signal peak center, intensity, and polarization dependence, the coverage, orientation, and orientation distribution of surface/interfacial functional groups can be determined.3Shen Y.R. Surface Properties Probed by Second-Harmonic and Sum-Frequency Generation.Nature. 1989; 337: 519-525Crossref Scopus (2043) Google ScholarIn the research reported by Seki and colleagues,2Seki T. Yu X. Zhang P. Yu C. Liu K. Gunkel L. Dong R. Nagata Y. Feng X. Bonn M. Real-time Study of On-Water Chemistry: Surfactant Monolayer-Assisted Growth of a Crystalline Quasi-2D Polymer.Chem. 2021; 7: 2758-2770Abstract Full Text Full Text PDF Scopus (6) Google Scholar SFG was used to investigate molecular reaction mechanisms of an “on-water” reaction: surfactant-mediated polymerization to prepare quasi-2D polyaniline (PANI) film at the water/air interface. On-water chemical reactions go against the traditional belief that substances do not react unless dissolved. Such reactions play significant roles in improving Diels-Alder reactions, Claisen rearrangements, and Grignard reactions.2Seki T. Yu X. Zhang P. Yu C. Liu K. Gunkel L. Dong R. Nagata Y. Feng X. Bonn M. Real-time Study of On-Water Chemistry: Surfactant Monolayer-Assisted Growth of a Crystalline Quasi-2D Polymer.Chem. 2021; 7: 2758-2770Abstract Full Text Full Text PDF Scopus (6) Google Scholar These reactions are also applied in microdroplet chemistry, liquid-water interfacial synthesis, and surfactant-assisted 2D reactions.2Seki T. Yu X. Zhang P. Yu C. Liu K. Gunkel L. Dong R. Nagata Y. Feng X. Bonn M. Real-time Study of On-Water Chemistry: Surfactant Monolayer-Assisted Growth of a Crystalline Quasi-2D Polymer.Chem. 2021; 7: 2758-2770Abstract Full Text Full Text PDF Scopus (6) Google Scholar However, molecular mechanisms of on-water reactions have not been elucidated. Here, the polymerization reaction to prepare polyaniline was investigated by using SFG at the water/air interface without and with three different types of surfactants: sodium oleyl sulfate (SOS), sodium dodecylbenzene sulfonate (SDBS), and sodium dodecyl sulfate (SDS). It was found that the conductivity and crystallinity of the prepared PANI films increases following this trend: SOS-water > SDBS-water > SDS-water > air-water. With the help of SFG spectra calculated using ab initio molecular-dynamics simulations, time-dependent SFG experiments on these on-water chemical reactions show conditions to prepare PANI films with high quality (high crystallinity and conductivity). To achieve high-quality PANI films, a key reaction intermediate, a positively charged aniline derivative with = NH2 end, is required to be present and ordered at the water/air interface (Figure 1). Such intermediate behavior can be facilitated by the favorable interactions between the intermediate molecules and negatively charged surfactant headgroups, leading to the formation of high-quality PANI films. The varied qualities of various PANI films prepared with the use of different surfactants result from varied surface charge densities at the water/air interface with different surfactants present. The surface charge densities were deduced from the phase-sensitive SFG studies on interfacial water molecules. With the presence of SOS, the water/air interface has the highest negative charge density, inducing the best accumulation and ordering of the positively charged aniline intermediates at the interface, leading to the PANI films with the best quality.2Seki T. Yu X. Zhang P. Yu C. Liu K. Gunkel L. Dong R. Nagata Y. Feng X. Bonn M. Real-time Study of On-Water Chemistry: Surfactant Monolayer-Assisted Growth of a Crystalline Quasi-2D Polymer.Chem. 2021; 7: 2758-2770Abstract Full Text Full Text PDF Scopus (6) Google ScholarIn this research, SFG studies elucidated the molecular mechanisms of the on-water reactions in situ in real time along the entire reaction process. SFG results demonstrated the significance of the interfacial electric field and electrostatic interactions on interfacial polymerization. Such knowledge is important for designing better surfactants to facilitate polymer thin films synthesis.The approach of using SFG to probe surface/interfacial chemical reactions is generally applicable. In addition to on-water reactions, SFG has been applied to study chemical reactions at buried interfaces in complex systems including polymer materials. For example, SFG was successfully used to study interfacial chemical reactions between nylon amine4Li B. Andre J.S. Chen X. Walther B. Paradkar R. Feng C. Tucker C. Mohler C. Chen Z. Observing a Chemical Reaction at a Buried Solid/Solid Interface in Situ.Anal. Chem. 2020; 92: 14145-14152Crossref PubMed Scopus (11) Google Scholar or poly(ethylene vinyl alcohol) hydroxyl5Andre J.S. Li B. Chen X. Paradkar R. Walther B. Feng C. Tucker C. Mohler C. Chen Z. Interfacial Reaction of a Maleic Anhydride Grafted Polyolefins with Ethylene Vinyl Alcohol Copolymer at the Buried Solid/Solid Interface.Polymer (Guildf.). 2020; 212: 123141Crossref Scopus (8) Google Scholar groups and maleic anhydride (MAH) grafted polyethylene at buried solid/solid interfaces. The detailed reaction mechanisms, temperature-dependent reaction kinetics, and activation energy of interfacial reactions were determined.4Li B. Andre J.S. Chen X. Walther B. Paradkar R. Feng C. Tucker C. Mohler C. Chen Z. Observing a Chemical Reaction at a Buried Solid/Solid Interface in Situ.Anal. Chem. 2020; 92: 14145-14152Crossref PubMed Scopus (11) Google Scholar,5Andre J.S. Li B. Chen X. Paradkar R. Walther B. Feng C. Tucker C. Mohler C. Chen Z. Interfacial Reaction of a Maleic Anhydride Grafted Polyolefins with Ethylene Vinyl Alcohol Copolymer at the Buried Solid/Solid Interface.Polymer (Guildf.). 2020; 212: 123141Crossref Scopus (8) Google Scholar SFG was also applied to study other interfacial chemical reactions at polymer interfaces such as those involving isocyanates at the primer/sealant interface6Zhang S. Andre J.S. Hsu L. Toolis A. Esarey S.L. Li B. Chen Z. Nondestructive in Situ Detection of Chemical Reactions at the Buried Interface between Polyurethane and Isocyanate-Based Primer.Macromolecules. 2020; 53: 10189-10197Crossref Scopus (12) Google Scholar and those involving methoxy/epoxy functionalities of adhesion promoters at buried polymer/silicone interfaces.7Lin T. Wu Y. Santos E. Chen X. Ahn D. Mohler C. Chen Z. Molecular Insights into Adhesion at a Buried Silica-Filled Silicone/Polyethylene Terephthalate Interface.Langmuir. 2020; 36: 15128-15140Crossref PubMed Scopus (9) Google Scholar These in situ SFG studies on polymer interfaces elucidated molecular mechanisms of polymer adhesion important for coatings, packaging, etc. Various catalytic reactions, not only under high vacuum but also under various pressures, were studied by SFG,8Chen Z. Gracias D.H. Somorjai G.A. Sum Frequency Generation (SFG) – Surface Vibrational Spectroscopy Studies of Buried Interfaces: Catalytic Reaction Intermediates on Transition Metal Crystal Surfaces at High Reactant Pressures; Polymer Surface Structures at the Solid-Gas and Solid-Liquid Interfaces.Appl. Phys. B. 1999; 68: 549-557Google Scholar which has provided valuable insight into important molecular mechanisms regarding such catalytic reactions such as the interfacial behavior of catalysts or reaction intermediates.8Chen Z. Gracias D.H. Somorjai G.A. Sum Frequency Generation (SFG) – Surface Vibrational Spectroscopy Studies of Buried Interfaces: Catalytic Reaction Intermediates on Transition Metal Crystal Surfaces at High Reactant Pressures; Polymer Surface Structures at the Solid-Gas and Solid-Liquid Interfaces.Appl. Phys. B. 1999; 68: 549-557Google Scholar,9Ge A. Rudshteyn B. Videla P.E. Miller C.J. Kubiak C.P. Batista V.S. Lian T. Heterogenized Molecular Catalysts: Vibrational Sum-Frequency Spectroscopic, Electrochemical, and Theoretical Investigations.Acc. Chem. Res. 2019; 52: 1289-1300Crossref PubMed Scopus (40) Google Scholar Advanced SFG techniques with ultrafast phase-sensitive measurements were utilized to study ultrafast kinetics of chemical reactions at the water/air interface. It was found that the reaction kinetics of the photoionization of phenol at the air/water interface are 104 times faster than those in bulk water, demonstrating the effects of the interfacial environment on chemical reaction kinetics.10Kusaka R. Nihonyanagi S. Tahara T. The Photochemical Reaction of Phenol Becomes Ultrafast at the Air-Water Interface.Nat. Chem. 2021; 13: 306-311Crossref PubMed Scopus (34) Google ScholarIn summary, SFG has been developed into a powerful and unique interfacial sensitive tool to study chemical reactions on surfaces and at buried interfaces in situ. Molecular mechanisms regarding such reactions can be obtained, which not only provides fundamental understanding of the effects of chemical interfacial environments on chemical reactions but also elucidates important knowledge on how interfacial reactions can be manipulated and how to design better interfacial reactions to improve key interfacial properties of materials/devices for important applications. Main textChemical reactions are widely encountered everywhere: from stellar explosions and the expansion of the universe to industrial manufacturing of commercial products, agricultural production of food, and biological processes including digestion and metabolism. Extensive research has been performed on bulk chemical reactions by using many different analytical tools to understand their molecular mechanisms; such research has aided in the development of new compounds and novel reactions, optimizing conditions for important reactions and increasing the yields for preferred products.Two-dimensional (2D) reactions on surfaces/interfaces are also important but difficult to study, because such studies require surface/interfacial sensitive techniques. High-vacuum techniques have been developed to investigate mechanisms of various reactions (e.g., catalytic reactions) on surfaces, providing significant understanding regarding these reactions.1Somorjai G.A. Li Y. Introduction to Surface Chemistry and Catalysis.Second Edition. Wiley, 2010Google Scholar However, for reactions occurring on ambient surfaces or at buried (e.g., solid/liquid or solid/solid) interfaces, it is challenging to study in situ in real time at the molecular level due to the lack of appropriate analytical tools.With sum-frequency generation (SFG) vibrational spectroscopy, in this issue of Chem Seki et al. reported exciting research results on investigations of 2D polymerization reactions at liquid/air interfaces facilitated by surfactants.2Seki T. Yu X. Zhang P. Yu C. Liu K. Gunkel L. Dong R. Nagata Y. Feng X. Bonn M. Real-time Study of On-Water Chemistry: Surfactant Monolayer-Assisted Growth of a Crystalline Quasi-2D Polymer.Chem. 2021; 7: 2758-2770Abstract Full Text Full Text PDF Scopus (6) Google Scholar The 2D reactions were followed as a function of time in situ to deduce the molecular reaction mechanisms and identify key reaction intermediates. SFG is a second-order nonlinear optical spectroscopy, which provides vibrational spectra of surfaces/interfaces in situ in real time.3Shen Y.R. Surface Properties Probed by Second-Harmonic and Sum-Frequency Generation.Nature. 1989; 337: 519-525Crossref Scopus (2043) Google Scholar Vibrational spectra are the fingerprints of molecules; therefore, molecular-level interfacial information can be obtained from SFG studies. According to the selection rule of a second-order nonlinear optical process, SFG signal can only be generated from a system with no inversion symmetry (under the electric dipole approximation), leading to the sub-monolayer surface/interface specificity. Most bulk materials have inversion symmetry and so cannot produce SFG signal. In a typical SFG experiment, where a fixed-frequency visible laser beam and a frequency-tunable mid-infrared (IR) beam overlap on a surface/interface, the output beam with the sum frequency of the two input beams is detected, and its intensity is plotted versus the input IR beam wavenumber, producing a vibrational spectrum. According to the vibrationally resonating SFG signal peak center, intensity, and polarization dependence, the coverage, orientation, and orientation distribution of surface/interfacial functional groups can be determined.3Shen Y.R. Surface Properties Probed by Second-Harmonic and Sum-Frequency Generation.Nature. 1989; 337: 519-525Crossref Scopus (2043) Google ScholarIn the research reported by Seki and colleagues,2Seki T. Yu X. Zhang P. Yu C. Liu K. Gunkel L. Dong R. Nagata Y. Feng X. Bonn M. Real-time Study of On-Water Chemistry: Surfactant Monolayer-Assisted Growth of a Crystalline Quasi-2D Polymer.Chem. 2021; 7: 2758-2770Abstract Full Text Full Text PDF Scopus (6) Google Scholar SFG was used to investigate molecular reaction mechanisms of an “on-water” reaction: surfactant-mediated polymerization to prepare quasi-2D polyaniline (PANI) film at the water/air interface. On-water chemical reactions go against the traditional belief that substances do not react unless dissolved. Such reactions play significant roles in improving Diels-Alder reactions, Claisen rearrangements, and Grignard reactions.2Seki T. Yu X. Zhang P. Yu C. Liu K. Gunkel L. Dong R. Nagata Y. Feng X. Bonn M. Real-time Study of On-Water Chemistry: Surfactant Monolayer-Assisted Growth of a Crystalline Quasi-2D Polymer.Chem. 2021; 7: 2758-2770Abstract Full Text Full Text PDF Scopus (6) Google Scholar These reactions are also applied in microdroplet chemistry, liquid-water interfacial synthesis, and surfactant-assisted 2D reactions.2Seki T. Yu X. Zhang P. Yu C. Liu K. Gunkel L. Dong R. Nagata Y. Feng X. Bonn M. Real-time Study of On-Water Chemistry: Surfactant Monolayer-Assisted Growth of a Crystalline Quasi-2D Polymer.Chem. 2021; 7: 2758-2770Abstract Full Text Full Text PDF Scopus (6) Google Scholar However, molecular mechanisms of on-water reactions have not been elucidated. Here, the polymerization reaction to prepare polyaniline was investigated by using SFG at the water/air interface without and with three different types of surfactants: sodium oleyl sulfate (SOS), sodium dodecylbenzene sulfonate (SDBS), and sodium dodecyl sulfate (SDS). It was found that the conductivity and crystallinity of the prepared PANI films increases following this trend: SOS-water > SDBS-water > SDS-water > air-water. With the help of SFG spectra calculated using ab initio molecular-dynamics simulations, time-dependent SFG experiments on these on-water chemical reactions show conditions to prepare PANI films with high quality (high crystallinity and conductivity). To achieve high-quality PANI films, a key reaction intermediate, a positively charged aniline derivative with = NH2 end, is required to be present and ordered at the water/air interface (Figure 1). Such intermediate behavior can be facilitated by the favorable interactions between the intermediate molecules and negatively charged surfactant headgroups, leading to the formation of high-quality PANI films. The varied qualities of various PANI films prepared with the use of different surfactants result from varied surface charge densities at the water/air interface with different surfactants present. The surface charge densities were deduced from the phase-sensitive SFG studies on interfacial water molecules. With the presence of SOS, the water/air interface has the highest negative charge density, inducing the best accumulation and ordering of the positively charged aniline intermediates at the interface, leading to the PANI films with the best quality.2Seki T. Yu X. Zhang P. Yu C. Liu K. Gunkel L. Dong R. Nagata Y. Feng X. Bonn M. Real-time Study of On-Water Chemistry: Surfactant Monolayer-Assisted Growth of a Crystalline Quasi-2D Polymer.Chem. 2021; 7: 2758-2770Abstract Full Text Full Text PDF Scopus (6) Google ScholarIn this research, SFG studies elucidated the molecular mechanisms of the on-water reactions in situ in real time along the entire reaction process. SFG results demonstrated the significance of the interfacial electric field and electrostatic interactions on interfacial polymerization. Such knowledge is important for designing better surfactants to facilitate polymer thin films synthesis.The approach of using SFG to probe surface/interfacial chemical reactions is generally applicable. In addition to on-water reactions, SFG has been applied to study chemical reactions at buried interfaces in complex systems including polymer materials. For example, SFG was successfully used to study interfacial chemical reactions between nylon amine4Li B. Andre J.S. Chen X. Walther B. Paradkar R. Feng C. Tucker C. Mohler C. Chen Z. Observing a Chemical Reaction at a Buried Solid/Solid Interface in Situ.Anal. Chem. 2020; 92: 14145-14152Crossref PubMed Scopus (11) Google Scholar or poly(ethylene vinyl alcohol) hydroxyl5Andre J.S. Li B. Chen X. Paradkar R. Walther B. Feng C. Tucker C. Mohler C. Chen Z. Interfacial Reaction of a Maleic Anhydride Grafted Polyolefins with Ethylene Vinyl Alcohol Copolymer at the Buried Solid/Solid Interface.Polymer (Guildf.). 2020; 212: 123141Crossref Scopus (8) Google Scholar groups and maleic anhydride (MAH) grafted polyethylene at buried solid/solid interfaces. The detailed reaction mechanisms, temperature-dependent reaction kinetics, and activation energy of interfacial reactions were determined.4Li B. Andre J.S. Chen X. Walther B. Paradkar R. Feng C. Tucker C. Mohler C. Chen Z. Observing a Chemical Reaction at a Buried Solid/Solid Interface in Situ.Anal. Chem. 2020; 92: 14145-14152Crossref PubMed Scopus (11) Google Scholar,5Andre J.S. Li B. Chen X. Paradkar R. Walther B. Feng C. Tucker C. Mohler C. Chen Z. Interfacial Reaction of a Maleic Anhydride Grafted Polyolefins with Ethylene Vinyl Alcohol Copolymer at the Buried Solid/Solid Interface.Polymer (Guildf.). 2020; 212: 123141Crossref Scopus (8) Google Scholar SFG was also applied to study other interfacial chemical reactions at polymer interfaces such as those involving isocyanates at the primer/sealant interface6Zhang S. Andre J.S. Hsu L. Toolis A. Esarey S.L. Li B. Chen Z. Nondestructive in Situ Detection of Chemical Reactions at the Buried Interface between Polyurethane and Isocyanate-Based Primer.Macromolecules. 2020; 53: 10189-10197Crossref Scopus (12) Google Scholar and those involving methoxy/epoxy functionalities of adhesion promoters at buried polymer/silicone interfaces.7Lin T. Wu Y. Santos E. Chen X. Ahn D. Mohler C. Chen Z. Molecular Insights into Adhesion at a Buried Silica-Filled Silicone/Polyethylene Terephthalate Interface.Langmuir. 2020; 36: 15128-15140Crossref PubMed Scopus (9) Google Scholar These in situ SFG studies on polymer interfaces elucidated molecular mechanisms of polymer adhesion important for coatings, packaging, etc. Various catalytic reactions, not only under high vacuum but also under various pressures, were studied by SFG,8Chen Z. Gracias D.H. Somorjai G.A. Sum Frequency Generation (SFG) – Surface Vibrational Spectroscopy Studies of Buried Interfaces: Catalytic Reaction Intermediates on Transition Metal Crystal Surfaces at High Reactant Pressures; Polymer Surface Structures at the Solid-Gas and Solid-Liquid Interfaces.Appl. Phys. B. 1999; 68: 549-557Google Scholar which has provided valuable insight into important molecular mechanisms regarding such catalytic reactions such as the interfacial behavior of catalysts or reaction intermediates.8Chen Z. Gracias D.H. Somorjai G.A. Sum Frequency Generation (SFG) – Surface Vibrational Spectroscopy Studies of Buried Interfaces: Catalytic Reaction Intermediates on Transition Metal Crystal Surfaces at High Reactant Pressures; Polymer Surface Structures at the Solid-Gas and Solid-Liquid Interfaces.Appl. Phys. B. 1999; 68: 549-557Google Scholar,9Ge A. Rudshteyn B. Videla P.E. Miller C.J. Kubiak C.P. Batista V.S. Lian T. Heterogenized Molecular Catalysts: Vibrational Sum-Frequency Spectroscopic, Electrochemical, and Theoretical Investigations.Acc. Chem. Res. 2019; 52: 1289-1300Crossref PubMed Scopus (40) Google Scholar Advanced SFG techniques with ultrafast phase-sensitive measurements were utilized to study ultrafast kinetics of chemical reactions at the water/air interface. It was found that the reaction kinetics of the photoionization of phenol at the air/water interface are 104 times faster than those in bulk water, demonstrating the effects of the interfacial environment on chemical reaction kinetics.10Kusaka R. Nihonyanagi S. Tahara T. The Photochemical Reaction of Phenol Becomes Ultrafast at the Air-Water Interface.Nat. Chem. 2021; 13: 306-311Crossref PubMed Scopus (34) Google ScholarIn summary, SFG has been developed into a powerful and unique interfacial sensitive tool to study chemical reactions on surfaces and at buried interfaces in situ. Molecular mechanisms regarding such reactions can be obtained, which not only provides fundamental understanding of the effects of chemical interfacial environments on chemical reactions but also elucidates important knowledge on how interfacial reactions can be manipulated and how to design better interfacial reactions to improve key interfacial properties of materials/devices for important applications. Chemical reactions are widely encountered everywhere: from stellar explosions and the expansion of the universe to industrial manufacturing of commercial products, agricultural production of food, and biological processes including digestion and metabolism. Extensive research has been performed on bulk chemical reactions by using many different analytical tools to understand their molecular mechanisms; such research has aided in the development of new compounds and novel reactions, optimizing conditions for important reactions and increasing the yields for preferred products. Two-dimensional (2D) reactions on surfaces/interfaces are also important but difficult to study, because such studies require surface/interfacial sensitive techniques. High-vacuum techniques have been developed to investigate mechanisms of various reactions (e.g., catalytic reactions) on surfaces, providing significant understanding regarding these reactions.1Somorjai G.A. Li Y. Introduction to Surface Chemistry and Catalysis.Second Edition. Wiley, 2010Google Scholar However, for reactions occurring on ambient surfaces or at buried (e.g., solid/liquid or solid/solid) interfaces, it is challenging to study in situ in real time at the molecular level due to the lack of appropriate analytical tools. With sum-frequency generation (SFG) vibrational spectroscopy, in this issue of Chem Seki et al. reported exciting research results on investigations of 2D polymerization reactions at liquid/air interfaces facilitated by surfactants.2Seki T. Yu X. Zhang P. Yu C. Liu K. Gunkel L. Dong R. Nagata Y. Feng X. Bonn M. Real-time Study of On-Water Chemistry: Surfactant Monolayer-Assisted Growth of a Crystalline Quasi-2D Polymer.Chem. 2021; 7: 2758-2770Abstract Full Text Full Text PDF Scopus (6) Google Scholar The 2D reactions were followed as a function of time in situ to deduce the molecular reaction mechanisms and identify key reaction intermediates. SFG is a second-order nonlinear optical spectroscopy, which provides vibrational spectra of surfaces/interfaces in situ in real time.3Shen Y.R. Surface Properties Probed by Second-Harmonic and Sum-Frequency Generation.Nature. 1989; 337: 519-525Crossref Scopus (2043) Google Scholar Vibrational spectra are the fingerprints of molecules; therefore, molecular-level interfacial information can be obtained from SFG studies. According to the selection rule of a second-order nonlinear optical process, SFG signal can only be generated from a system with no inversion symmetry (under the electric dipole approximation), leading to the sub-monolayer surface/interface specificity. Most bulk materials have inversion symmetry and so cannot produce SFG signal. In a typical SFG experiment, where a fixed-frequency visible laser beam and a frequency-tunable mid-infrared (IR) beam overlap on a surface/interface, the output beam with the sum frequency of the two input beams is detected, and its intensity is plotted versus the input IR beam wavenumber, producing a vibrational spectrum. According to the vibrationally resonating SFG signal peak center, intensity, and polarization dependence, the coverage, orientation, and orientation distribution of surface/interfacial functional groups can be determined.3Shen Y.R. Surface Properties Probed by Second-Harmonic and Sum-Frequency Generation.Nature. 1989; 337: 519-525Crossref Scopus (2043) Google Scholar In the research reported by Seki and colleagues,2Seki T. Yu X. Zhang P. Yu C. Liu K. Gunkel L. Dong R. Nagata Y. Feng X. Bonn M. Real-time Study of On-Water Chemistry: Surfactant Monolayer-Assisted Growth of a Crystalline Quasi-2D Polymer.Chem. 2021; 7: 2758-2770Abstract Full Text Full Text PDF Scopus (6) Google Scholar SFG was used to investigate molecular reaction mechanisms of an “on-water” reaction: surfactant-mediated polymerization to prepare quasi-2D polyaniline (PANI) film at the water/air interface. On-water chemical reactions go against the traditional belief that substances do not react unless dissolved. Such reactions play significant roles in improving Diels-Alder reactions, Claisen rearrangements, and Grignard reactions.2Seki T. Yu X. Zhang P. Yu C. Liu K. Gunkel L. Dong R. Nagata Y. Feng X. Bonn M. Real-time Study of On-Water Chemistry: Surfactant Monolayer-Assisted Growth of a Crystalline Quasi-2D Polymer.Chem. 2021; 7: 2758-2770Abstract Full Text Full Text PDF Scopus (6) Google Scholar These reactions are also applied in microdroplet chemistry, liquid-water interfacial synthesis, and surfactant-assisted 2D reactions.2Seki T. Yu X. Zhang P. Yu C. Liu K. Gunkel L. Dong R. Nagata Y. Feng X. Bonn M. Real-time Study of On-Water Chemistry: Surfactant Monolayer-Assisted Growth of a Crystalline Quasi-2D Polymer.Chem. 2021; 7: 2758-2770Abstract Full Text Full Text PDF Scopus (6) Google Scholar However, molecular mechanisms of on-water reactions have not been elucidated. Here, the polymerization reaction to prepare polyaniline was investigated by using SFG at the water/air interface without and with three different types of surfactants: sodium oleyl sulfate (SOS), sodium dodecylbenzene sulfonate (SDBS), and sodium dodecyl sulfate (SDS). It was found that the conductivity and crystallinity of the prepared PANI films increases following this trend: SOS-water > SDBS-water > SDS-water > air-water. With the help of SFG spectra calculated using ab initio molecular-dynamics simulations, time-dependent SFG experiments on these on-water chemical reactions show conditions to prepare PANI films with high quality (high crystallinity and conductivity). To achieve high-quality PANI films, a key reaction intermediate, a positively charged aniline derivative with = NH2 end, is required to be present and ordered at the water/air interface (Figure 1). Such intermediate behavior can be facilitated by the favorable interactions between the intermediate molecules and negatively charged surfactant headgroups, leading to the formation of high-quality PANI films. The varied qualities of various PANI films prepared with the use of different surfactants result from varied surface charge densities at the water/air interface with different surfactants present. The surface charge densities were deduced from the phase-sensitive SFG studies on interfacial water molecules. With the presence of SOS, the water/air interface has the highest negative charge density, inducing the best accumulation and ordering of the positively charged aniline intermediates at the interface, leading to the PANI films with the best quality.2Seki T. Yu X. Zhang P. Yu C. Liu K. Gunkel L. Dong R. Nagata Y. Feng X. Bonn M. Real-time Study of On-Water Chemistry: Surfactant Monolayer-Assisted Growth of a Crystalline Quasi-2D Polymer.Chem. 2021; 7: 2758-2770Abstract Full Text Full Text PDF Scopus (6) Google Scholar In this research, SFG studies elucidated the molecular mechanisms of the on-water reactions in situ in real time along the entire reaction process. SFG results demonstrated the significance of the interfacial electric field and electrostatic interactions on interfacial polymerization. Such knowledge is important for designing better surfactants to facilitate polymer thin films synthesis. The approach of using SFG to probe surface/interfacial chemical reactions is generally applicable. In addition to on-water reactions, SFG has been applied to study chemical reactions at buried interfaces in complex systems including polymer materials. For example, SFG was successfully used to study interfacial chemical reactions between nylon amine4Li B. Andre J.S. Chen X. Walther B. Paradkar R. Feng C. Tucker C. Mohler C. Chen Z. Observing a Chemical Reaction at a Buried Solid/Solid Interface in Situ.Anal. Chem. 2020; 92: 14145-14152Crossref PubMed Scopus (11) Google Scholar or poly(ethylene vinyl alcohol) hydroxyl5Andre J.S. Li B. Chen X. Paradkar R. Walther B. Feng C. Tucker C. Mohler C. Chen Z. Interfacial Reaction of a Maleic Anhydride Grafted Polyolefins with Ethylene Vinyl Alcohol Copolymer at the Buried Solid/Solid Interface.Polymer (Guildf.). 2020; 212: 123141Crossref Scopus (8) Google Scholar groups and maleic anhydride (MAH) grafted polyethylene at buried solid/solid interfaces. The detailed reaction mechanisms, temperature-dependent reaction kinetics, and activation energy of interfacial reactions were determined.4Li B. Andre J.S. Chen X. Walther B. Paradkar R. Feng C. Tucker C. Mohler C. Chen Z. Observing a Chemical Reaction at a Buried Solid/Solid Interface in Situ.Anal. Chem. 2020; 92: 14145-14152Crossref PubMed Scopus (11) Google Scholar,5Andre J.S. Li B. Chen X. Paradkar R. Walther B. Feng C. Tucker C. Mohler C. Chen Z. Interfacial Reaction of a Maleic Anhydride Grafted Polyolefins with Ethylene Vinyl Alcohol Copolymer at the Buried Solid/Solid Interface.Polymer (Guildf.). 2020; 212: 123141Crossref Scopus (8) Google Scholar SFG was also applied to study other interfacial chemical reactions at polymer interfaces such as those involving isocyanates at the primer/sealant interface6Zhang S. Andre J.S. Hsu L. Toolis A. Esarey S.L. Li B. Chen Z. Nondestructive in Situ Detection of Chemical Reactions at the Buried Interface between Polyurethane and Isocyanate-Based Primer.Macromolecules. 2020; 53: 10189-10197Crossref Scopus (12) Google Scholar and those involving methoxy/epoxy functionalities of adhesion promoters at buried polymer/silicone interfaces.7Lin T. Wu Y. Santos E. Chen X. Ahn D. Mohler C. Chen Z. Molecular Insights into Adhesion at a Buried Silica-Filled Silicone/Polyethylene Terephthalate Interface.Langmuir. 2020; 36: 15128-15140Crossref PubMed Scopus (9) Google Scholar These in situ SFG studies on polymer interfaces elucidated molecular mechanisms of polymer adhesion important for coatings, packaging, etc. Various catalytic reactions, not only under high vacuum but also under various pressures, were studied by SFG,8Chen Z. Gracias D.H. Somorjai G.A. Sum Frequency Generation (SFG) – Surface Vibrational Spectroscopy Studies of Buried Interfaces: Catalytic Reaction Intermediates on Transition Metal Crystal Surfaces at High Reactant Pressures; Polymer Surface Structures at the Solid-Gas and Solid-Liquid Interfaces.Appl. Phys. B. 1999; 68: 549-557Google Scholar which has provided valuable insight into important molecular mechanisms regarding such catalytic reactions such as the interfacial behavior of catalysts or reaction intermediates.8Chen Z. Gracias D.H. Somorjai G.A. Sum Frequency Generation (SFG) – Surface Vibrational Spectroscopy Studies of Buried Interfaces: Catalytic Reaction Intermediates on Transition Metal Crystal Surfaces at High Reactant Pressures; Polymer Surface Structures at the Solid-Gas and Solid-Liquid Interfaces.Appl. Phys. B. 1999; 68: 549-557Google Scholar,9Ge A. Rudshteyn B. Videla P.E. Miller C.J. Kubiak C.P. Batista V.S. Lian T. Heterogenized Molecular Catalysts: Vibrational Sum-Frequency Spectroscopic, Electrochemical, and Theoretical Investigations.Acc. Chem. Res. 2019; 52: 1289-1300Crossref PubMed Scopus (40) Google Scholar Advanced SFG techniques with ultrafast phase-sensitive measurements were utilized to study ultrafast kinetics of chemical reactions at the water/air interface. It was found that the reaction kinetics of the photoionization of phenol at the air/water interface are 104 times faster than those in bulk water, demonstrating the effects of the interfacial environment on chemical reaction kinetics.10Kusaka R. Nihonyanagi S. Tahara T. The Photochemical Reaction of Phenol Becomes Ultrafast at the Air-Water Interface.Nat. Chem. 2021; 13: 306-311Crossref PubMed Scopus (34) Google Scholar In summary, SFG has been developed into a powerful and unique interfacial sensitive tool to study chemical reactions on surfaces and at buried interfaces in situ. Molecular mechanisms regarding such reactions can be obtained, which not only provides fundamental understanding of the effects of chemical interfacial environments on chemical reactions but also elucidates important knowledge on how interfacial reactions can be manipulated and how to design better interfacial reactions to improve key interfacial properties of materials/devices for important applications. Real-time study of on-water chemistry: Surfactant monolayer-assisted growth of a crystalline quasi-2D polymerSeki et al.ChemAugust 24, 2021In BriefWater-surfactant interfaces provide a unique platform for generating highly crystalline, functional 2D material films through surfactant-monolayer-assisted interfacial synthesis (SMAIS). Sum-frequency generation spectroscopy selectively probes the vibrational response of interfacial species, thereby allowing to monitor the quasi-2D polyaniline film formation in real time. We found that the cationic intermediates are accumulated near the negatively charged surfactant interfaces. The crystallinity and conductivity of the 2D material films are positively correlated with the surface-charge density. Full-Text PDF Open Archive