SFG study of two-dimensional orientation of surface methyl groups on cadmium arachidate Langmuir-Blodgett films

Akamatsu, Naotoshi; Domen, Kazunari; Hirose, Chiaki

doi:10.1021/j100141a029

Cited by 73 publications

(58 citation statements)

References 17 publications

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“…For an isotropic surface, p-polarized infrared and s-polarized visible beams necessarily produce an s-polarized signal. 5,[25][26][27][28] With these polarizations, Eq. ͑1͒ becomes…”

Section: ͑1͒mentioning

confidence: 99%

Multiplexed polarization spectroscopy: Measuring surface hyperpolarizability orientation

Shultz

Bisson

Groenzin³

et al. 2010

The Journal of Chemical Physics

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Infrared-visible sum frequency generation (SFG) has seen increasing usage as a surface probe, particularly for liquid interfaces since they are amenable to few alternate probes. Interpreting the SFG data to arrive at a molecular-level configuration on the surface, however, remains a challenge. This paper reports a technique for analyzing and interpreting SFG data--called polarization-angle null or PAN-SFG. PAN-SFG enables ready identification of the ratio of the surface tangential and longitudinal hyperpolarizabilities--the hyperpolarizability direction--as well as the phase relationship between these components separated from the optical factors due to the substrate and experimental geometry. Separation of the surface optical factors results in an immediate connection between the null angle and the surface species polarization. If the Raman polarizability is also known, then PAN-SFG analysis, like the previously reported null techniques, provides a very accurate orientation. In addition, the reported polarization-angle, phase-shift analysis enables facile separation of the nonresonant background polarization from that of the resonant signal. Beyond orientation, PAN-SFG can be used to deconvolute overlapping resonances and identify components beyond a dipole response. This paper reports PAN-SFG for two systems providing deeper insight into both. An acetonitrile-water mixture was previously reported to undergo a phase transition at 7 mol %, attributed to a sudden change in orientation. PAN-SFG demonstrates that acetonitrile generates a classic dipole response and provides compelling evidence that the acetonitrile configuration remains constant as a function of concentration. An alternate model for the phase transition is presented. Like many aqueous systems, the SFG spectrum of the hydrogen-bonded region of ice consists of broad and overlapping features; features previously identified with PAN-SFG. Here PAN-SFG analysis is used to show that the reddest of these, the feature at 3098 cm(-1), contains a significant quadrupole contribution that grows as the temperature is lowered. The quadrupole and its temperature dependence are used to assign the 3098 cm(-1) feature to bilayer-stitching-hydrogen bonds. This is the first definitive assignment in the hydrogen-bonded region of water.

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“…For an isotropic surface, p-polarized infrared and s-polarized visible beams necessarily produce an s-polarized signal. 5,[25][26][27][28] With these polarizations, Eq. ͑1͒ becomes…”

Section: ͑1͒mentioning

confidence: 99%

Multiplexed polarization spectroscopy: Measuring surface hyperpolarizability orientation

Shultz

Bisson

Groenzin³

et al. 2010

The Journal of Chemical Physics

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“…The theory behind SFG as a surface-sensitive spectroscopic technique and the analysis to obtain orientation of methyl and methylene groups has been discussed previously. [12][13][14][15][16][17][18][19] Here we summarize the main points to understand the work reported in this paper. The second-order nonlinear susceptibility can be expressed using a Lorentzian function with a resonance peak at ω q and width Γ q corresponding to a resonance mode q.…”

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confidence: 96%

Molecular Structure of Hydrophobic Alkyl Side Chains at Comb Polymer-Air Interface

Gautam

Dhinojwala

2001

Macromolecules

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Introduction. Polymers with long side chains (comb polymers) provide unique opportunities to tailor surface properties of polymer films for applications in the areas of adhesion, friction, membrane transport, and biocompatibility. [1][2][3][4][5][6][7][8] In particular, polymers containing long alkyl or fluorinated alkyl side chains have been used to generate hydrophobic surfaces with surface energies that are comparable to those created by well-ordered self-assembled monolayers (SAM). 1-2,9-10 Comb copolymers with alkyl side chains typically show two thermal transitions: one associated with smectic liquid crystalline order where the d spacing is proportional to the length of the side groups 1,2 and the other associated with crystallization of the alkyl side chains. The surface energy of alkyl side chain comb polymers decreases with increase in the length of the alkyl side groups until a certain critical chain length, after which it asymptotes to a constant value. 1 These observations are reminiscent of results for SAM's where the length of the alkyl chains, packing, and mobility of the chains control the surface energies. 9,10 Although, considerable work has been done in the past in understanding the molecular ordering of SAM, very little is known about the molecular structure of alkyl chains at the comb polymer-air interface. 11 In this Communication, we report the first spectroscopic measurement of the molecular structure of octadecyl alkyl side chains at poly(vinyl octadecyl carbamatesco-vinyl acetate)-air interface (the polymer will be referred to as octadecyl polymer for simplicity) using the surface sensitive sum-frequency generation (SFG) technique. SFG involves mixing a visible high-intensity laser beam of frequency ω 1 , with a tunable infrared wavelength source of frequency ω 2 . According to the dipole approximation, generation of a SFG photon at (ω 1 + ω 2 ) is forbidden in the centrosymmetric bulk and is nonzero at interfaces where the inversion symmetry is broken. 12-16 SFG intensities are resonantly enhanced when ω 2 overlaps with the resonant frequency of a molecular vibrational mode that is both infrared and Raman active. The intensity, polarization, and resonance frequency of the SFG signal yield chemical and orientational information on molecules at interfaces. [12][13][14][15][16][17][18] Here, we compare the differences in the surface structure of 65 nm thick octadecyl polymer films prepared by spin-coating to a 7 nm polymer layer prepared by solution dipping on a mica substrate. In both cases, the SFG signals show strong methyl asymmetric and symmetric stretching vibrations indicating ordered methyl terminal groups. For the adsorbed octadecyl films, the SFG signal associated with methylene vibrations is higher indicating increased gauche defects in

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“…There have been concerns that the possible rotation around sp 3 hybridized C-C sigma bonds would invalidate the analysis model being proposed. It is worth noting that the tight packing of the non-ionic MIBC molecules at the air/water interface would minimize such rotation, which has been observed with dodecanol and phospholipids [6,[19][20][21]28]. Furthermore, molecular mechanics calculations suggested that the probability of gauche conformation was only 1.5% (see further details in SI); it is thus a valid assumption that the C-C rotation is minimal.…”

Section: Theoretical Modellingmentioning

confidence: 89%

“…(5) can be probed by the symmetric C-H stretching mode t s while the anti-symmetric (degenerate) CH stretching mode can be described by the elements in Eqs. (6, 7) [19,20]. Furthermore, Eq.…”

Section: Theoretical Modellingmentioning

confidence: 98%

Probing the Molecular Orientation of Methyl Isobutyl Carbinol at the Air–Water Interface

Nguyen

Shahir

Nguyen

2017

J Surfact & Detergents

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The molecular orientation of a branched and short‐chained methyl isobutyl carbinol (MIBC) at the air–water interface was examined by sum frequency generation vibrational spectroscopy. The weak vibrational coupling model was spectroscopically validated and employed in developing the data analysis model of this multiple terminal methyl groups‐containing structure. The exceptional frothability of MIBC was explained by its molecular orientation at the interface; MIBC was found to tilt at an average angle of 39o to the surface normal, exposing its hydrophobic region to the air medium. Interestingly, this orientation scheme is similar to the hydrogen bonded OH bonds of the interfacial water molecules.

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SFG study of two-dimensional orientation of surface methyl groups on cadmium arachidate Langmuir-Blodgett films

Cited by 73 publications

References 17 publications

Multiplexed polarization spectroscopy: Measuring surface hyperpolarizability orientation

Multiplexed polarization spectroscopy: Measuring surface hyperpolarizability orientation

Molecular Structure of Hydrophobic Alkyl Side Chains at Comb Polymer-Air Interface

Probing the Molecular Orientation of Methyl Isobutyl Carbinol at the Air–Water Interface

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