A review is given of current areas of interest in the field of colloidal dispersions. It is related to topics discussed at the meeting and directed, in particular, to the properties of concentrated dispersions. Attention is also directed to some unsolved problems in the field.
{"title":"The Rideal Lecture. Colloidal dispersions","authors":"R. Ottewill","doi":"10.1039/DC9909000001","DOIUrl":"https://doi.org/10.1039/DC9909000001","url":null,"abstract":"A review is given of current areas of interest in the field of colloidal dispersions. It is related to topics discussed at the meeting and directed, in particular, to the properties of concentrated dispersions. Attention is also directed to some unsolved problems in the field.","PeriodicalId":12210,"journal":{"name":"Faraday Discussions of The Chemical Society","volume":"40 5","pages":"1-15"},"PeriodicalIF":0.0,"publicationDate":"1990-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"72615624","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Recent experimental work has shown that total internal reflection microscopy (TIRM) can be employed to monitor the position of a particle undergoing diffusion near an interface and thereby characterise weak potential-energy profiles between a particle and a surface. We have previously demonstrated an extension of this technique, which involves the manipulation of colloid particles by the use of radiation pressure forces.This paper describes how we have extended the technique to allow measurement of the absolute separation of a particle from a flat surface as a function of NaCl concentration and how we can monitor the modification, by O-n-dodecylhexaethylene glycol (C12EO6), of the interactions between a quartz plate and a polystyrene particle.
{"title":"Characterisation of the weak interactions between a particle and a plane surface using total internal reflection microscopy and radiation pressure forces","authors":"Murray A. Brown, E. Staples","doi":"10.1039/DC9909000193","DOIUrl":"https://doi.org/10.1039/DC9909000193","url":null,"abstract":"Recent experimental work has shown that total internal reflection microscopy (TIRM) can be employed to monitor the position of a particle undergoing diffusion near an interface and thereby characterise weak potential-energy profiles between a particle and a surface. We have previously demonstrated an extension of this technique, which involves the manipulation of colloid particles by the use of radiation pressure forces.This paper describes how we have extended the technique to allow measurement of the absolute separation of a particle from a flat surface as a function of NaCl concentration and how we can monitor the modification, by O-n-dodecylhexaethylene glycol (C12EO6), of the interactions between a quartz plate and a polystyrene particle.","PeriodicalId":12210,"journal":{"name":"Faraday Discussions of The Chemical Society","volume":"1 2 1","pages":"193-208"},"PeriodicalIF":0.0,"publicationDate":"1990-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"89507325","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
We present a novel approach to the stability of lyophobic colloids by considering transient double-layer disequilibration of interacting particles. Deviations from equilibrium are accounted for via perturbation theory. Incomplete equilibration is related to a retarded desorption of charge-determining ions, which, in turn, leads to an increment in the disjoining pressure. The theory is elaborated for two interacting spheres and it is shown that under realistic conditions the relaxation retardation may become substantial. Under conditions where the relaxation retardation dominates, the stability ratio becomes independent of particle size.
{"title":"Dynamics of colloid particle interaction. Incomplete desorption relaxation","authors":"S. Dukhin, J. Lyklema","doi":"10.1039/DC9909000261","DOIUrl":"https://doi.org/10.1039/DC9909000261","url":null,"abstract":"We present a novel approach to the stability of lyophobic colloids by considering transient double-layer disequilibration of interacting particles. Deviations from equilibrium are accounted for via perturbation theory. Incomplete equilibration is related to a retarded desorption of charge-determining ions, which, in turn, leads to an increment in the disjoining pressure. The theory is elaborated for two interacting spheres and it is shown that under realistic conditions the relaxation retardation may become substantial. Under conditions where the relaxation retardation dominates, the stability ratio becomes independent of particle size.","PeriodicalId":12210,"journal":{"name":"Faraday Discussions of The Chemical Society","volume":"4 1","pages":"261-269"},"PeriodicalIF":0.0,"publicationDate":"1990-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"81697767","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Angle-resolved photoemission spectroscopy has been used to identify four adsorbate-induced features from the surface methoxy species on Cu{111}. In agreement with IR data, the application of dipole selection rules suggests that the O–C axis is oriented perpendicular to the surface. The ordering of the 5a1 and 1e emission bands is reversed compared with the methoxy species adsorbed on the Cu{100} and Cu{110} surfaces. The interpretation of the X-ray absorption data is more difficult, owing to the occurrence of overlapping resonances. A clear polarisation dependence is observed at the oxygen edge, but indicates that the O–C axis could be tilted by up to 30° from the surface normal. This apparent conflict with the photoemission data is attributed to the somewhat arbitrary nature of the standard curve-fitting procedures.
{"title":"Structure of the surface methoxy species on Cu{111}","authors":"D. Ricken, J. Somers, A. W. Robinson, A. Bradshaw","doi":"10.1039/DC9908900291","DOIUrl":"https://doi.org/10.1039/DC9908900291","url":null,"abstract":"Angle-resolved photoemission spectroscopy has been used to identify four adsorbate-induced features from the surface methoxy species on Cu{111}. In agreement with IR data, the application of dipole selection rules suggests that the O–C axis is oriented perpendicular to the surface. The ordering of the 5a1 and 1e emission bands is reversed compared with the methoxy species adsorbed on the Cu{100} and Cu{110} surfaces. The interpretation of the X-ray absorption data is more difficult, owing to the occurrence of overlapping resonances. A clear polarisation dependence is observed at the oxygen edge, but indicates that the O–C axis could be tilted by up to 30° from the surface normal. This apparent conflict with the photoemission data is attributed to the somewhat arbitrary nature of the standard curve-fitting procedures.","PeriodicalId":12210,"journal":{"name":"Faraday Discussions of The Chemical Society","volume":"4 1","pages":"291-300"},"PeriodicalIF":0.0,"publicationDate":"1990-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"82197356","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
The force between two parallel planar zwitterionic surfaces has been calculated using Monte Carlo computer simulations. The zwitterions are modelled as two oppositely charged hard spheres joined by a string of length L with parameters chosen to mimic a phospholipid system. All centres interact by a homogeneous Coulomb interaction and by a hard-sphere exclusion. The negative centres are anchored to the surface by a parabolic potential.For distances D between the surfaces, as defined by the location of the negative centres, that are larger than 2L there is an attractive force of the classical van der Waals type. When, on the other hand, D < 2L a strong repulsive force appears, which in the limit D≪ 2L is analogous to a double-layer force.Recently it was suggested (Israelachvili and Wennerstrom, Langmuir, 1990, 6, 873) that the repulsive so-called hydration force observed for biological lipid systems had its origin in confinements on surface excitations induced by a second surface. Here we demonstrate how this mechanism works in a particular microscopic model of the surface. Although still simplistic, several qualitative features of the force in the phospholipid systems are reproduced in the calculations. For example, a reduction of the size of the charged centres leads to a decrease in both the attractive and more significantly the repulsive forces. This mimics the observed difference between phosphatidyl choline and phosphatidyl ethanolamine.
{"title":"Hydration force, steric force or double-layer force between zwitterionic surfaces?","authors":"U. Nilsson, B. Jönsson, H. Wennerström","doi":"10.1039/DC9909000107","DOIUrl":"https://doi.org/10.1039/DC9909000107","url":null,"abstract":"The force between two parallel planar zwitterionic surfaces has been calculated using Monte Carlo computer simulations. The zwitterions are modelled as two oppositely charged hard spheres joined by a string of length L with parameters chosen to mimic a phospholipid system. All centres interact by a homogeneous Coulomb interaction and by a hard-sphere exclusion. The negative centres are anchored to the surface by a parabolic potential.For distances D between the surfaces, as defined by the location of the negative centres, that are larger than 2L there is an attractive force of the classical van der Waals type. When, on the other hand, D < 2L a strong repulsive force appears, which in the limit D≪ 2L is analogous to a double-layer force.Recently it was suggested (Israelachvili and Wennerstrom, Langmuir, 1990, 6, 873) that the repulsive so-called hydration force observed for biological lipid systems had its origin in confinements on surface excitations induced by a second surface. Here we demonstrate how this mechanism works in a particular microscopic model of the surface. Although still simplistic, several qualitative features of the force in the phospholipid systems are reproduced in the calculations. For example, a reduction of the size of the charged centres leads to a decrease in both the attractive and more significantly the repulsive forces. This mimics the observed difference between phosphatidyl choline and phosphatidyl ethanolamine.","PeriodicalId":12210,"journal":{"name":"Faraday Discussions of The Chemical Society","volume":"46 1","pages":"107-114"},"PeriodicalIF":0.0,"publicationDate":"1990-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"73495382","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
By using video imagery and an image data analyser, the ordering phenomena of polymer latex particles in dilute suspensions have been studied. The micrographs clearly showed the existence of localised ordered structure coexisting with disordered particles, and furthermore, fairly long-lived void structures were noted. The lattice vibrations of the particles on two adjoining lattice planes were shown to be coupled. Lattice defects were also found and the lattice vibration of the particle around the defect was anisotropic. The growth of the localised structure was found to obey the Ostwald ripening mechanism. All these findings validate the existence of an interparticle long-range attraction in addition to the short-range repulsion.
{"title":"Inhomogeneity of solute distribution in ionic systems","authors":"N. Ise, H. Matsuoka, Kensaku Ito, H. Yoshida","doi":"10.1039/DC9909000153","DOIUrl":"https://doi.org/10.1039/DC9909000153","url":null,"abstract":"By using video imagery and an image data analyser, the ordering phenomena of polymer latex particles in dilute suspensions have been studied. The micrographs clearly showed the existence of localised ordered structure coexisting with disordered particles, and furthermore, fairly long-lived void structures were noted. The lattice vibrations of the particles on two adjoining lattice planes were shown to be coupled. Lattice defects were also found and the lattice vibration of the particle around the defect was anisotropic. The growth of the localised structure was found to obey the Ostwald ripening mechanism. All these findings validate the existence of an interparticle long-range attraction in addition to the short-range repulsion.","PeriodicalId":12210,"journal":{"name":"Faraday Discussions of The Chemical Society","volume":"408 1","pages":"153-162"},"PeriodicalIF":0.0,"publicationDate":"1990-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"76490723","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
H. Hanley, J. Pieper, G. C. Straty, R. Hjelm, P. Seeger
Neutron scattering intensities from an aqueous mixture suspension of 91 nm polystyrene latex particles and 54 nm silica particles are reported in the range 0.02 < Q/nm–1 < 0.2, where Q is the momentum transfer. The suspension was dense at a mixture volume fraction of 0.15, and the polystyrene/silica particle ratio was ca. 1.7 : 1. Results are given for the suspension at rest and under shear. The sheared data were obtained with a concentric cylinder shearing apparatus constructed and tested at the SANS facility of the National Institute of Standards and Technology and the pulsed neutron facility, LANSCE, of the Los Alamos National Laboratory. The design and operation of the cell is described. The shear-influenced behaviour of the mixture is compared with and contrasted to that of a pure polystyrene suspension that can form a crystal lattice in equilibrium, but which melts to a liquid-like structure under shear.A method is proposed to measure, by contrast matching or variation, the polystyrene and silica partial scattered intensities from the mixture suspension in H2O–D2O solvents of different scattering-length densities. Estimates of the partial structure factors are given.
{"title":"Structure of a binary colloidal suspension under shear","authors":"H. Hanley, J. Pieper, G. C. Straty, R. Hjelm, P. Seeger","doi":"10.1039/DC9909000091","DOIUrl":"https://doi.org/10.1039/DC9909000091","url":null,"abstract":"Neutron scattering intensities from an aqueous mixture suspension of 91 nm polystyrene latex particles and 54 nm silica particles are reported in the range 0.02 < Q/nm–1 < 0.2, where Q is the momentum transfer. The suspension was dense at a mixture volume fraction of 0.15, and the polystyrene/silica particle ratio was ca. 1.7 : 1. Results are given for the suspension at rest and under shear. The sheared data were obtained with a concentric cylinder shearing apparatus constructed and tested at the SANS facility of the National Institute of Standards and Technology and the pulsed neutron facility, LANSCE, of the Los Alamos National Laboratory. The design and operation of the cell is described. The shear-influenced behaviour of the mixture is compared with and contrasted to that of a pure polystyrene suspension that can form a crystal lattice in equilibrium, but which melts to a liquid-like structure under shear.A method is proposed to measure, by contrast matching or variation, the polystyrene and silica partial scattered intensities from the mixture suspension in H2O–D2O solvents of different scattering-length densities. Estimates of the partial structure factors are given.","PeriodicalId":12210,"journal":{"name":"Faraday Discussions of The Chemical Society","volume":"80 1","pages":"91-106"},"PeriodicalIF":0.0,"publicationDate":"1990-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"75745744","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
The paper describes an experimental study of the rheology of concentrated latex under conditions where a secondary miminum in the interparticle pair potential is expected. Particular attention is given to the Bingham stress σB derived from steady-shear-flow data, which is taken as a measure of the additional or extra stress arising from the interparticle attraction. The data are used to deduce a correlation describing the dependence of the Bingham stress on the strength of attraction, S, the latter being taken a proportional to the calculated depth of the secondary minimum, with the result σB=kf(ϕ)S1.9d–2, where k is a constant and d is the particle diameter. A weak attraction between colloidal particles can also be induced by the addition of non-adsorbing polymer. Data for systems of this type are analysed similarly and shown to support this dependence. Earlier data illustrating the effect of particle size and concentration on the storage modulus are also discussed. The dependence upon volume fraction is shown to be very similar to that of the Bingham stress and the variation of both properties is described well by an equation deduced from liquid-state theory on the assumption that the attractive forces do not greatly perturb the structure, this reads G∞, σB∝(4ϕ2+ 2ϕ3–ϕ4)/(1 –ϕ)3. It is also shown, however, that for volume fractions ϕ > 0.2 or so this is virtually indistinguishable from the concentration dependence of the modulus observed for particulate gels formed by coagulation in the primary minimum. It would thus appear that the concentration dependence of the modulus is very insensitive to local structure and the strength of the attraction. In contrast, this is not the case for the particle-size dependence. The effect of attractive forces on the low-shear and high-shear viscosity coefficients is also described.
{"title":"Rheology of weakly interacting colloidal particles at high concentration","authors":"R. Buscall, Ian J. McGowan, C. A. Mumme-Young","doi":"10.1039/DC9909000115","DOIUrl":"https://doi.org/10.1039/DC9909000115","url":null,"abstract":"The paper describes an experimental study of the rheology of concentrated latex under conditions where a secondary miminum in the interparticle pair potential is expected. Particular attention is given to the Bingham stress σB derived from steady-shear-flow data, which is taken as a measure of the additional or extra stress arising from the interparticle attraction. The data are used to deduce a correlation describing the dependence of the Bingham stress on the strength of attraction, S, the latter being taken a proportional to the calculated depth of the secondary minimum, with the result σB=kf(ϕ)S1.9d–2, where k is a constant and d is the particle diameter. A weak attraction between colloidal particles can also be induced by the addition of non-adsorbing polymer. Data for systems of this type are analysed similarly and shown to support this dependence. Earlier data illustrating the effect of particle size and concentration on the storage modulus are also discussed. The dependence upon volume fraction is shown to be very similar to that of the Bingham stress and the variation of both properties is described well by an equation deduced from liquid-state theory on the assumption that the attractive forces do not greatly perturb the structure, this reads G∞, σB∝(4ϕ2+ 2ϕ3–ϕ4)/(1 –ϕ)3. It is also shown, however, that for volume fractions ϕ > 0.2 or so this is virtually indistinguishable from the concentration dependence of the modulus observed for particulate gels formed by coagulation in the primary minimum. It would thus appear that the concentration dependence of the modulus is very insensitive to local structure and the strength of the attraction. In contrast, this is not the case for the particle-size dependence. The effect of attractive forces on the low-shear and high-shear viscosity coefficients is also described.","PeriodicalId":12210,"journal":{"name":"Faraday Discussions of The Chemical Society","volume":"110 1","pages":"115-127"},"PeriodicalIF":0.0,"publicationDate":"1990-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"82245635","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
The DLVO theory of colloid stability provided an explanation of the long-standing Schulze–Hardy rule, i.e. that the critical coagulation concentration (c.c.c.) is proportional to some inverse power of the counter-ion valency (z). For particular limiting cases the inverse power exponent was calculated to be either 2 or 6. By using the charge-regulation approach to double-layer interaction for real surfaces with identifiable surface sites, dissociation equilibria, new insights into the link between physiochemical parameters and the Schulze–Hardy exponent are obtained. The model also produces an important result, viz. that the total double-layer potential for the isolated particle at the c.c.c. solution condition, while different for each of a series of real materials, is independent of counter-ion valency (z).
{"title":"Charge-regulation modelling of the Schulze–Hardy rule and related coagulation effects","authors":"I. M. Metcalfe, T. Healy","doi":"10.1039/DC9909000335","DOIUrl":"https://doi.org/10.1039/DC9909000335","url":null,"abstract":"The DLVO theory of colloid stability provided an explanation of the long-standing Schulze–Hardy rule, i.e. that the critical coagulation concentration (c.c.c.) is proportional to some inverse power of the counter-ion valency (z). For particular limiting cases the inverse power exponent was calculated to be either 2 or 6. By using the charge-regulation approach to double-layer interaction for real surfaces with identifiable surface sites, dissociation equilibria, new insights into the link between physiochemical parameters and the Schulze–Hardy exponent are obtained. The model also produces an important result, viz. that the total double-layer potential for the isolated particle at the c.c.c. solution condition, while different for each of a series of real materials, is independent of counter-ion valency (z).","PeriodicalId":12210,"journal":{"name":"Faraday Discussions of The Chemical Society","volume":"14 1","pages":"335-344"},"PeriodicalIF":0.0,"publicationDate":"1990-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"78290396","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
G. Tourillon, D. Guay, A. Fontaine, R. Garrett, Gwyn P. Williams
NEXAFS spectroscopy is a technique well suited to probing metal/organic molecule or metal/polymer interfaces. Two systems, related to the Pt/organic conducting polymer and Ni/polyacrylonitrile interfaces, are discussed in detail.For poly-3-alkylthiophene (and selenophene) thin films electrochemically deposited onto Pt, the C K-edge characteristics reveal that (i) the polymeric chain unit is composed of the same architecture as the monomer and (ii) the doping proceeds via a narrowing of the band gap with the appearance of metallic-like behaviour. The same tendencies are observed by varying the structure of the monomer, the nature of the dopant and the film thickness, which evidences a similar conduction mechanism for these five-membered polyheterocycles. The NEXAFS spectra reveal that the polymeric chains are well ordered on the metallic surface: in its undoped state, the layers lie flat on Pt with strong interactions between the electron states of the metal and the antibonding π* band of the polymer; when doped, the NEXAFS characteristics exhibit changes as a function of thickness. The first layer lies flat on the Pt surface while the other ones are randomly oriented, due to the intercalation of anions during the oxidation process. Finally, the orientation of the polymeric chain switches from a ‘lying-down’ to an ‘on-edge’ configuration when a long alkyl chain is added on the ring, this alkyl chain is oriented perpendicular to the Pt surface.Polyacrylonitrile (PAN) thin films electrochemically deposited on Ni have been studied by NEXAFS as a function of the film thickness and annealing treatment. For 20 A thick films, the polymer chains are oriented perpendicular to the surface with the CN groups parallel to it. Below a few A, no polymerization occurs, but molecules are adsorbed perpendicular to the surface. Annealing at 300 °C results in the loss of the majority of the N content of the film in contrast to the admitted mechanism for bulk PAN.
{"title":"Characterization of metal/organic molecule and metal/polymer interfaces by NEXAFS spectroscopy","authors":"G. Tourillon, D. Guay, A. Fontaine, R. Garrett, Gwyn P. Williams","doi":"10.1039/DC9908900275","DOIUrl":"https://doi.org/10.1039/DC9908900275","url":null,"abstract":"NEXAFS spectroscopy is a technique well suited to probing metal/organic molecule or metal/polymer interfaces. Two systems, related to the Pt/organic conducting polymer and Ni/polyacrylonitrile interfaces, are discussed in detail.For poly-3-alkylthiophene (and selenophene) thin films electrochemically deposited onto Pt, the C K-edge characteristics reveal that (i) the polymeric chain unit is composed of the same architecture as the monomer and (ii) the doping proceeds via a narrowing of the band gap with the appearance of metallic-like behaviour. The same tendencies are observed by varying the structure of the monomer, the nature of the dopant and the film thickness, which evidences a similar conduction mechanism for these five-membered polyheterocycles. The NEXAFS spectra reveal that the polymeric chains are well ordered on the metallic surface: in its undoped state, the layers lie flat on Pt with strong interactions between the electron states of the metal and the antibonding π* band of the polymer; when doped, the NEXAFS characteristics exhibit changes as a function of thickness. The first layer lies flat on the Pt surface while the other ones are randomly oriented, due to the intercalation of anions during the oxidation process. Finally, the orientation of the polymeric chain switches from a ‘lying-down’ to an ‘on-edge’ configuration when a long alkyl chain is added on the ring, this alkyl chain is oriented perpendicular to the Pt surface.Polyacrylonitrile (PAN) thin films electrochemically deposited on Ni have been studied by NEXAFS as a function of the film thickness and annealing treatment. For 20 A thick films, the polymer chains are oriented perpendicular to the surface with the CN groups parallel to it. Below a few A, no polymerization occurs, but molecules are adsorbed perpendicular to the surface. Annealing at 300 °C results in the loss of the majority of the N content of the film in contrast to the admitted mechanism for bulk PAN.","PeriodicalId":12210,"journal":{"name":"Faraday Discussions of The Chemical Society","volume":"10 1","pages":"275-290"},"PeriodicalIF":0.0,"publicationDate":"1990-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"84859961","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
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