Michael Graupe

University of Houston, Houston, Texas, United States

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Publications (24)84.33 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: The solution-phase thermal desorption of three series of self-assembled monolayers (SAMs) on gold generated from terminally perfluorinated alkanethiols was examined. Series 1 SAMs, F(CF2)x(CH2)11SH, where x = 1–10, consisted of a constant hydrocarbon segment length with an increasing fluorocarbon segment length. Series 2 SAMs, F(CF2)10(CH2)ySH, where y = 2–6, 11, consisted of a constant fluorocarbon segment length with an increasing hydrocarbon segment length. Series 3 SAMs, F(CF2)x(CH2)ySH, where x = 1–10 and y = 16 – x, consisted of both hydrocarbon and fluorocarbon segments in which the segment lengths were varied while holding the total chain length constant at 16 carbon atoms. SAMs from these three series were prepared and characterized using both ellipsometry and contact-angle measurements. The resultant SAMs were shown to be highly hydrophobic and oleophobic. The SAMs were heated in decalin (DC) and perfluorodecalin (PFD) at 80 °C for various periods of time to monitor their thermal stability when exposed to hydrocarbon versus fluorocarbon solvents. In general, SAMs derived from n-alkanethiols and terminally perfluorinated alkanethiols exhibited diminished thermal stabilities upon heating in a hydrocarbon solvent (DC) versus heating in a perfluorocarbon solvent (PFD). The thermal stability of the SAMs increased with increasing lengths of the CF2 or CH2 segments. We also examined the kinetics of thermal desorption of these SAMs. From these studies, SAMs composed of higher degrees of terminal perfluorination exhibited smaller rate constants for the initial stage (fast regime) of desorption. When compared with analogous alkanethiol SAMs, the terminally perfluorinated SAMs exhibited greater thermal stabilities in both DC and PFD. In addition, values of the rate constants for desorption of the alkanethiol SAMs were approximately double those of the terminally perfluorinated SAMs having similar chain lengths.
    The Journal of Physical Chemistry C 09/2011; 115(40):19749–19760. DOI:10.1021/jp204970b · 4.77 Impact Factor
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    ABSTRACT: ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.
    ChemInform 06/2010; 30(22). DOI:10.1002/chin.199922081
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    ABSTRACT: Self-assembled monolayers (SAMs) formed from semifluorinated alkanethiols (SFATs) CF3(CF2)9(CH2)nSH (F10HnSH: n = 2, 11, and 17) on poly-crystalline Au and Ag were characterized by X-ray photoelectron spectroscopy, infrared reflection absorption spectroscopy, and near edge X-ray absorption fine structure spectroscopy. SFATs were found to form highly ordered and densely packed SAMs on both substrates. The molecules are strongly bonded to the substrates via their sulfur head groups, in the same manner as conventional alkanethiol (AT) SAMs. The hydrocarbon (except for n = 2) and fluorocarbon parts of the adsorbed SFATs retain the expected planar zigzag and helical conformations of the respective bulk materials. The orientation of the fluorocarbon chains does not depend on the substrate. These entities are almost perpendicular to the substrate in F10H2S/Au and F10H2S/Ag and become slightly more tilted in SFAT SAMs with longer hydrocarbon moieties. However, the alkyl parts of the SFAT films exhibit tilt and twist angles that are similar to those of normal alkanethiol films on Ag and Au substrates despite the reduced packing density in the SFAT films as compared to normal AT SAMs. We suggest that the substrate-related differences in tilt and twist angles for both systems are associated with the different character of the head-group-substrate bonding on Au and Ag.
    Israel Journal of Chemistry (Online) 03/2010; 40(2):81 - 97. DOI:10.1560/0K18-2RLA-GAQD-NHRU · 2.22 Impact Factor
  • Mitsuru Takenaga · Sadaharu Jo · Michael Graupe · T Randall Lee
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    ABSTRACT: The systematic variation of the van der Waals surface energy with fluorination for a series of self-assembled monolayers (SAMs) generated by the adsorption of partially fluorinated alkanethiols onto the surface of gold is examined experimentally and theoretically. The surface energy is elucidated on the basis of an effective Hamaker constant, which is obtained as a combination of the respective Hamaker constants of fluorocarbons and hydrocarbons; the fraction depends on the degree of fluorination. The good agreement between experiment and theory is discussed. In addition, the Hamaker constants of various liquids contacted on the well-defined hydrophobic surfaces are interpreted using modified Lifshitz theory.
    Journal of Colloid and Interface Science 05/2008; 320(1):264-7. DOI:10.1016/j.jcis.2007.12.048 · 3.37 Impact Factor
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    ABSTRACT: We demonstrate that it is possible to achieve exceptionally fine control over the anchoring of liquid crystals via the use of semi-fluorinated self-assembled monolayers of varying spacer length. We use this approach to map the detailed shape of an anchoring transition surface in thermodynamic phase space and to explore the links between anchoring and orientational wetting phenomena. These results allow one to design a substrate that will place a standard liquid-crystal film arbitrarily close to an anchoring transition between homeotropic and planar anchoring.
    EPL (Europhysics Letters) 01/2007; 59(3):410. DOI:10.1209/epl/i2002-00210-x · 2.10 Impact Factor
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    ABSTRACT: Compositionally mixed, self-assembled monolayers (SAMs) derived from 16,16,16-trifluorohexadecanethiol and a normal alkanethiol, either hexadecanethiol or pentadecanethiol, were formed on Au(111) substrates. The relative composition of the films was determined using X-ray photoelectron spectroscopy and was found to approximately equal the equimolar composition of the isooctane solution from which they were formed. The frictional properties of the mixed films were measured on the nanometer scale using atomic force microscopy and were observed to decrease when the chain length of the CH(3)-terminated component was shortened by one methylene unit (i.e., when hexadecanethiol was replaced by pentadecanethiol). For comparison, the frictional properties of a mixed-chain-length CH(3)-terminated SAM derived from hexadecanethiol and pentadecanethiol in a 1:1 ratio was also examined. In contrast to the mixed CF(3)/CH(3) system, the latter mixed-chain-length system exhibited relatively higher friction when compared to single-component SAMs derived solely from either hexadecanethiol or pentadecanethiol. For both types of mixed films, the change in frictional properties that occurs as a result of modifying the position of neighboring terminal groups with respect to the surface plane is discussed in terms of the influence of local packing environments on interfacial energy dissipation (friction).
    Langmuir 03/2005; 21(3):933-6. DOI:10.1021/la0488607 · 4.46 Impact Factor
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    ABSTRACT: Self-assembled monolayers (SAMs) formed from alkyl-terminated, CF3-terminated, CF3CF2-terminated, and C10F21-terminated alkanethiolates on gold substrates were examined by low-energy (eV) ion−surface collisions to determine the impact of varying the degree of fluorination of the thiolate tail group. The fluorine-terminated SAMs are compared and contrasted to previously examined CF3-terminated Langmuir−Blodgett films. Polyatomic (M+• for benzene and pyrazine) and atomic (Mo+• and Cr+•) ions were collided with the different SAM films at various collision energies (20−70 eV). Data indicate that substitution of CH3 with CF3 as the terminal group has a substantial influence on the ion−surface interactions, including energy transfer (fragmentation), electron transfer (neutralization), and atom/group transfer (reaction). However, slight penetration into a depth of the film is apparent and illustrated with the formation of certain ion−surface reaction products and the intensities of those products. This penetration has a noticeable effect on low-energy ion−surface collisions. Also, results for a series of CF3(CH2)nS−Au films (where n = 12, 13, 14, 15) illustrate that ion−surface reactions vary with the orientation of the fluorine atoms that are present in the terminal group. The results from collisions of low-energy ions with these relatively well-defined surfaces allow further characterization of the collision events that have the potential to become valuable tools in surface analysis.
    Langmuir 04/2002; 18(10). DOI:10.1021/la010998r · 4.46 Impact Factor
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    ABSTRACT: Alkanethiols possessing terminal phenyl groups (C6H5(CH2)nSH, n) 12-15) were adsorbed onto the surface of gold to afford phenyl-terminated self-assembled monolayers (SAMs). The SAMs were characterized by optical ellipsometry, polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS), atomic force microscopy (AFM), and contact angle goniometry. The films generated from the phenyl-terminated alkanethiols exhibited greater thicknesses but similar crystallinities and well-ordered lattice structures when compared to analogous SAMs generated by the adsorption of normal alkanethiols onto gold. Advancing contact angle measurements using water as the test liquid supported the presence of interfacial phenyl moieties. Furthermore, contact angle measurements using the test liquids methylene iodide (MI), dimethyl formamide (DMF), and nitrobenzene (NB) revealed an odd-even effect as a function of the number of methylene units underneath the terminal phenyl groups. The tribological properties of the phenyl-terminated films were characterized by AFM and compared to those of films derived from normal alkanethiols and other materials presenting aromatic hydrocarbon moieties at the interface (i.e., graphite and C60). The phenyl-terminated SAMs exhibited a substantially higher frictional response than graphite, a slightly higher frictional response than normal alkanethiol SAMs, but a much lower frictional response than C60-terminated SAMs.
    Langmuir 11/2001; 17(23):7364-7370. DOI:10.1021/la0111497 · 4.46 Impact Factor
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    ABSTRACT: We report the formation and subsequent topochemical polymerization of semi-fluorinated self-assembled monolayers (SAMs) containing diacetylene groups. The SAMs used were formed via the adsorption of the following derivatives:  CF3(CF2)n(CH2)16CCCC(CH2)10SH (n = 2, 3) and CF3(CF2)n(CH2)11CCCC(CH2)10SH (n = 9, 12) onto the surface of gold. The polymerization of the monolayers was initiated by UV irradiation and was monitored using resonance Raman spectroscopy. SAMs with short semi-fluorinated portions were found to polymerize more effectively than those with longer portions, indicating that steric factors become important with increasing perfluorinated chain length.
    Langmuir 09/2001; 17(21). DOI:10.1021/la010828e · 4.46 Impact Factor
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    ABSTRACT: Long-range interactions between self-assembled monolayers (SAMs) of semifluorinated alkanethiols have been studied by direct force measurements in water and aqueous NaCl solutions. SAMs prepared from three different thiols, with identical fluorinated head groups but varying hydrocarbon spacer lengths, were investigated: CF(3)(CF(2))(9)(CH(2))(x)SH, where x=2, 11, or 17. Force measurements show that the interactions in water and electrolyte solutions are composed of both double-layer interactions emerging from what appears to be charges adsorbed onto the surfaces and long-range "hydrophobic" attractions, in excess of the expected van der Waals forces. The three investigated thiols produce similar results in force measurements, though the contact angles with water are slightly different. The "hydrophobic" attraction has the form of step-like attractive discontinuities in the force profiles at separations ranging from 20 to 40 nm, caused by bridging of microscopic bubbles residing at the surfaces. The shape or range of these discontinuities are not significantly affected by replacement of the water with either 1 mM or 1 M NaCl solutions. The origin of the charges causing the electrostatic double-layer interaction is unclear, but some possible causes are discussed. Copyright 2001 Academic Press.
    Journal of Colloid and Interface Science 03/2001; 235(2):391-397. DOI:10.1006/jcis.2000.7318 · 3.37 Impact Factor
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    ABSTRACT: The structures of semifluorinated alkanethiol self-assembled monolayers (SAMs) generated by the adsorption of CF3(CF2)(9)(CH2)(n)SH (F10HnSH, n = 2, 6, 11, 17, 33) onto gold were investigated with atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and surface plasmon resonance spectroscopy. Images obtained by AFM of the F10H2SH SAM showed a remarkably ordered 2D hexagonal lattice with a lattice constant, alpha = 5.9 +/- 0.1 Angstrom, on Au(111)/mica. As the total number of carbon atoms in the alkyl spacer groups (n) was increased, the fluorocarbon adlayer structure appeared more disordered in the AFM images; however, the thicknesses of the fluorocarbon layers estimated from the C 1s (CF3), C 1s (CF2), and F 1s XPS signal intensities were indistinguishable in all of the SAMs. In contrast, the C ls (CH2) signals revealed that the tilt angles of the alkyl spacer groups depended strongly on n. We utilized a contrast variation SPR technique with various contacting media for an independent determination of the refractive indices and the film thicknesses of the semifluorinated SAMs. The obtained data were consistent with our AFM and XPS results, which show that the longer alkyl chains pack more densely than the shorter ones in these SAMs.
    Langmuir 03/2001; 17(6):1913-1921. DOI:10.1021/la001212c · 4.46 Impact Factor
  • Interfacial properties on the submicron scale: ACS Symposium Series 781, Edited by J.E. Frommer, R. Overney, 01/2001: pages 58-75; American Chemical Society.
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    ABSTRACT: The surfaces of glass coverslips of the type typically used for protein crystallization were modified with four types of transparent, chemically distinct self-assembled monolayers (SAMs). The SAM-functionalized surfaces exhibit a much higher degree of order and chemical uniformity than silanized glass, as judged by contact angle measurements. These characteristics lead to a marked increase in the range of solution conditions under which large crystals of lysozyme, α-lactalbumin, ribonuclease, hemoglobin, thaumatin, and catalase are observed to form. The results are rationalized in terms of a marked reduction in the rate of non-productive nucleation relative to the rate of crystal growth.
    Journal of Crystal Growth 09/2000; 218(2-4-218):390-398. DOI:10.1016/S0022-0248(00)00541-8 · 1.70 Impact Factor
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    ABSTRACT: Near-edge absorption fine structure (NEXAFS) measurements are used to probe the molecular orientation of semifluorinated (SF) mesogens, −(CH2)x(CF2)yF, which are attached to (i) the isoprene backbone of polyisoprene or a styrene-isoprene diblock copolymer (“soft” substrate), and (ii) a Au-covered solid substrate via a thiol link (“hard” substrate). The SF groups on both surfaces are oriented and on average are tilted from the sample normal. The tilt angle, τF-helix, of the fluorinated part of the SF group on each substrate is determined exclusively by the combination of x and y, increasing with increasing x and with decreasing y. Moreover, τF-helix is found to be independent of the surface topology (flat surfaces vs surfaces covered with holes or islands of the copolymer), casting solvent, and the architecture of the SF group (single vs 2-armed monodendron). Comparing the orientation of the SF groups on both substrates reveals that τF-helix is approximately 14° higher on the “soft” substrate.
    Macromolecules 08/2000; 33(16):6068--6077. DOI:10.1021/ma991710w · 5.80 Impact Factor
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    ABSTRACT: The microstructure, wettability, and thermal stability of self-assembled monolayers (SAMs) on gold generated from semifluorinated alkanethiols F(CF2)(10)(CH2)(n)SH, where n = 2, 6, 11, 17, and 33 (F10HnSH), were examined by polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) and dynamic contact angle measurements. Analysis by PM-IRRAS revealed that the length of the methylene spacer (Hn) influenced the tilt angle of the fluorocarbon segments in the semifluorinated SAMs. As the length of the methylene spacer was increased, the tilt angle of the perfluorocarbon moiety increased with respect to the surface nomlal. The longer methylene spacers (HIE, n = 11, 17, and 33) exhibited well-ordered trans-extended conformations as indicated by the position of the antisymmetric methylene band (nu(a)(CH2) = 2919 cm(-1)). Shortening the length of the methylene spacer to It = 6, however, led to a decrease in conformational order (nu(a)(CH2) = 2925 cm(-1)). Dynamic contact angle measurements using the Wilhelmy plate method showed that the semifluorinated SAMs were poorly wet by both water (average theta(a) = 120 degrees) and hexadecane (average theta(a) = 81 degrees). The wettability varied with the length of the methylene spacer; in particular, both the thinnest and the thickest semifluorinated SAMs (derived from F10H2SH and F10H33SH, respectively) exhibited relatively low dynamic contact angle values. In addition, the thermal stability of the semifluorinated SAMs was found to increase as the length of the methylene spacer was increased. Overall, these films exhibited remarkable resistance to thermal degradation (e.g., SAMs derived from F10H33SH sustained a relatively high contact angles after incubation at 150 degrees C for 1 h in air).
    The Journal of Physical Chemistry B 08/2000; 104(31):7417-7423. DOI:10.1021/jp0003499 · 3.30 Impact Factor
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    ABSTRACT: The wettability and friction of self-assembled monolayers (SAMs) of terminally fluorinated alkanethiols, CF3(CH2)nSH with n=9–15, and their nonfluorinated analogs, CH3(CH2)nSH with n=9–15, were examined and compared. Surprisingly, the CF3-terminated monolayers were wetted more by water, glycerol and N,N-dimethylformamide than were the CH3-terminated SAMs. The terminally fluorinated films, however, exhibited significantly lower surface energies. The origin of these trends is discussed. The two types of monolayers were also studied by atomic force microscopy (AFM). Compared to the CH3-terminated SAMs, the fluorinated films revealed an approximately 3-fold increase in friction. A new model for the frictional response, which is based on the difference in size between the methyl and trifluoromethyl groups, is proposed.
    Colloids and Surfaces A Physicochemical and Engineering Aspects 08/1999; 154(1-2-154):239-244. DOI:10.1016/S0927-7757(98)00902-9 · 2.75 Impact Factor
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    ABSTRACT: The origin of frictional forces in self-assembled monolayers (SAMs) was investigated through systematic correlation of the frictional properties with the chemical structure/composition of the films. Atomic force microscopy was used to probe the frictional properties of the SAMs formed by the adsorption of methyl-, isopropyl-, and trifluoromethyl-terminated alkanethiols on Au(111) surfaces. The frictional properties of mixed monolayers composed of varying concentrations of the methyl- and trifluoromethyl-terminated thiols were also studied. Polarization modulation infrared reflection adsorption spectroscopy was used to measure the vibrational spectra of each of these monolayers and in turn to determine that each was characterized by a well-packed backbone structure. For these films, which differed only in the nature of the outermost chemical functionality, a substantial enhancement in the frictional response was observed for films with isopropyl- and trifluoromethyl-terminal groups and for mixed monolayers containing small concentrations of the trifluoromethyl-terminated component. These results strongly support the model that the difference in friction in such systems arises predominantly from the difference in the size of the terminal groups. Larger terminal groups in films of the same lattice spacing give rise to increased steric interactions that provide pathways for energy dissipation during sliding.
    Langmuir 04/1999; 15(9):3179-3185. DOI:10.1021/la981497h · 4.46 Impact Factor
  • Journal of the American Chemical Society 03/1999; 121(13). DOI:10.1021/ja983582h · 12.11 Impact Factor
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    ABSTRACT: This paper describes the synthesis of a series of alkanethiols containing perfluorinated terminal segments: F(CF2)m(CH2)nSH, where m=1, n=9–15; m=2, n=11–14; m=3, n=10–13; and m=4, n=9–12. Fluorinated alkyl iodides of the general formula F(CF2)m(CH2)nI, where m=1–4 and n=0 or 1, were added to long-chain ω-olefins that were functionalized at the α-terminus with a thioacetate group. The reactions proceeded in good yields under free radical conditions. Reduction of the resulting secondary iodides gave long-chain alkanethioacetates with perfluoroalkyl terminal segments. These intermediates were readily transformed into the corresponding terminally perfluorinated alkanethiols by acidic deprotection. The product thiols should find use in the generation of well-defined fluorinated interfaces using the self-assembled monolayer (SAM) technique.
    Journal of Fluorine Chemistry 02/1999; 93(2):107-115. DOI:10.1016/S0022-1139(98)00284-X · 1.95 Impact Factor
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    ABSTRACT: Studies of the wettability and friction of self-assembled monolayers (SAMs) generated from the terminally fluorinated alkanethiols (CF3(CH2)nSH, n = 9–15) on gold are reported. These data were compared to those of SAMs generated from the non-fluorinated analogs (CH3(CH2)nSH, n = 9–15). The CF3-terminated SAMs were less wettable by hexadecane and methylene iodide, but surprisingly more wettable by water and glycerol than were the CH3-terminated SAMs. Measurements of friction by atomic force microscopy (AFM) revealed an approximate threefold increase in friction on the CF3-terminated films compared to the CH3-terminated films. A new model for the frictional response of fluorinated SAMs is proposed.
    Materials Research Bulletin 02/1999; 34(3):447-453. DOI:10.1016/S0025-5408(99)00028-8 · 2.29 Impact Factor

Publication Stats

928 Citations
84.33 Total Impact Points


  • 1997–2011
    • University of Houston
      • Department of Chemistry
      Houston, Texas, United States
  • 2000
    • Cornell University
      • Department of Materials Science and Engineering
      Итак, New York, United States