Measurement of the Surface Free Energy of Amorphous Cellulose by Alkane Adsorption: A Critical Evaluation of Inverse Gas Chromatography (IGC)

Surface energies of amorphous cellulose “beads” were measured by IGC at different temperatures (50 to 100°C) using n-alkane probes (pentane to undecane). The equation of Schultz and Lavielle was applied which relates the specific retention volume of the gas probe to the dispersive component of the surface energy of the solid and liquid, γ^d_s and γ^d_l, respectively, and a parameter (“a”) which represents the surface area of the gas probe in contact with the solids. At 50°C, γ^d_s was determined to be 71.5 mJ/m², and its temperature dependence was 0.36 mJ m^-2 K^-1. Compared with measurements obtained by contact angle, IGC results were found to yield higher values, and especially a higher temperature dependence, d(γ^d_s)/dT. Various potential explanations for these elevated values were examined. The surface energy, as determined by the Schultz and Lavielle equation, was found to depend mostly on the parameter “a”. Two experimental conditions are known to affect the values of “a”: the solid surface and the temperature. While the surface effect of the parameter “a” was ignored in this study, the dependence of the surface energy upon temperature and probe phase was demonstrated to be significant. Several optional treatments of the parameter “a” were modeled. It was observed that both experimental imprecision, but mostly the fundamental difference between the liquid-solid vs the gas-solid system (and the associated theoretical weakness of the model used), could explain the differences between γ^d_s and d(γ^d_s)/dT measured by contact angle and IGC. It was concluded that the exaggerated temperature dependence of the IGC results is a consequence of limitations inherent in the definition of parameter “a”.     

Dispersion-Polar Surface Tension Properties of Organic Solids

A new definition for work of adhesion W_a is applied to computationally define the dispersion γ_s ^d and polar γ_s ^d components of the solid surface tension γ_s = γ_s ^d + γ_s ^d for twenty-five low energy substrates. These calculated surface properties are correlated with surface composition and structure. Surface dipole orientation and electron induction effects are respectively distinguished for chlorinated and partially fluorinated hydrocarbons. Published values for critical surface tension of wetting γ_c are correlated with both γ_s ^d and γ_s.     

Contact angle and IGC measurements for probing surface-chemical changes in the recycling of wood pulp fibers

The objective of this study was to use dynamic contact angle (DCA) analysis and inverse gas chromatography (IGC) to probe the surface-chemical changes in wood pulp fibers during recycling. A simplified wet-dry-rewet cycle was performed on hardwood bleached kraft fibers to represent the recycling process. The DCA measurements revealed that the overall effect of recycling was an increase in the non-polar (dispersive) component and a corresponding decrease in the polar component of the surface free energy, hence resulting in a total surface free energy that remained essentially unaltered. The DCA experiment also showed that virgin fibers lost both their electronaccepting (γ_S ⁺) and their electron-donating (γ_S ^-) characteristics when converted to paper. Upon rehydration, the fibers recovered some surface acidity (γ_S ⁺) but surface basicity (γ_S ^-) continued to decrease. The changes in polar surface free energy correlate well with the changes in hydroxyl number determined independently using the acetylation method. IGC could not detect changes in the dispersive component of the surface free energy induced by recycling. The acid-base (KA and KB) changes in the IGC measurements were also indistinguishable between virgin fibers and recycled fibers. This research concludes that DCA analysis is preferable to IGC in better reflecting the surface changes in fiber recycling, and γS-can at least be treated as an empirical parameter to complement the hydroxyl availability data in distinguishing among virgin, paper, and recycled fibers.     

IGC study of filler–filler and filler–rubber interactions in silica‐filled compounds

A modification of the existing methods for evaluating the dispersive and specific components of surface free energy (γ_d and γ_s, respectively) was made to investigate filler–rubber and filler–filler interactions by inverse gas chromatography. Four silicas as fillers and various probes that mimic elastomers were employed in this study. It was shown that the pretreatment of silicas with helium could increase γ_d and decrease γ_s. Modification of the silica surface with silane could enhance the dispersive interaction and weaken the specific interaction. The temperature dependence of the interfacial interaction was also investigated, and it was found that lower temperatures favored filler–rubber interactions and mixing efficiency. Tests on different sizes of agglomerates demonstrated the existence of a filler–rubber and filler–filler network. It was also found that γ_d played a role in agglomeration or filler–filler interaction. Our study showed that the larger the specific surface area was, the stronger the dispersive and specific interactions were. The effectiveness of various fillers and elastomer probes was also compared. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 2517–2530, 2001     

Inverse gas chromatographic observation of thermodynamic interaction between poly (vinylidene fluoride) and organic solvents

Summary Inverse gas chromatography (IGC) has been applied to observe the interaction between poly(vinylidene fluoride) (PVDF) and organic solvent systems under conditions approaching infinite dilution of the volatile component. The specific retention volumes (V_g ⁰) were calculated from the retention volumes of the PVDF/solvent systems. The retention diagrams (RDs) which were plotted as ln V_g ⁰ vs. reciprocal temperature, corresponded with the thermodynamic behavior of PVDF. PVDF dissolves well in carbonyl-containing solvents at high temperatures. In these systems, the solution is converted easily to a thermoreversible gel by standing at room temperature. The Flory-Huggins interaction parameter (χ₁₂) and molar heat of sorption of probe absorbed by the amorphous part of PVDF (Δ H₁ ^s) calculated from the retention values indicated that the characteristics of the interaction between PVDF and the solvent (probe) varied with temperature and probe properties. The results of IGC of PVDF/solvent systems were discussed in relation to the dissolution and gelation behavior of PVDF. Received: 29 September 1999/Revised version: 30 November 1999/Accepted: 9 December 1999     

Surface Modification of Fluorinated Polymers by Microwave Plasmas

We developed a new plasma treating method, incorporating the use of microwaves generated by an electronic cooking range. Using this method, polytetrafluorethylene (PTFE) and a copolymer of tetrafluoroethylene and hexafluoropropylene (FEP) were treated. Dialkylphthalates (DAP) were used as the standard liquids of contact angle measurements for evaluation of the wetting properties of plasma treated polymers. The components of surface tension (γ_L) due to the dispersion force (γ^d _L) and the polar force (γ^P _L) of DAP were calculated by Fowkes' equation from the contact angles (θ) on polypropylene. After plasma treatment cos θ of several standard liquids on PTFE and FEP increased. The linear relationship between γ_L(1 + cos θ)/(γ^d _L)^½ and (γ^P _L/γ^P _L)^½ was verified. γ_s and γ^d _s and γ^d _s of the plasma treated PTFE and FEP also increased. From the results of ESCA analysis, it was found that a significant amount of oxygen was introduced to the polymer surface by the plasma treatment. Peel strengths of a pressure sensitive adhesive bonded to PTFE and FEP increased approximately two-to threefold if the plasma treatment was used prior to bonding.     

Surface energy and adsorption energy distribution measurements on some carbon blacks

The dispersive component of the surface energy (γ_s^d) of various carbon blacks and of nitric acid or heat-treated samples has been measured by inverse gas chromatography (IGC). It is shown that IGC, at infinite dilution, applies poorly to carbon blacks and provides only apparent and excessively high values of γ_s^d (a few hundreds of mJ/m²). IGC, at finite concentration, readily allows the obtention of n-alkanes, benzene and chloroform adsorption isotherms from which various thermodynamic values are computed (spreading pressure π, isosteric enthalpy of adsorption, surface energy characteristics). The γ_s^d values calculated from π_e are in the 80–120 mJ/m² range (i.e. values which are in fair agreement with published data on graphite or graphitized carbons). Further, the specific interaction potential of polar probes is in relation, as expected, with the oxygen content of the carbons. Finally, from the adsorption isotherms, the adsorption energy distribution function is computed, supposing a patchwise distribution of adsorption sites. The results confirm the high surface heterogeneity of carbon blacks and the influence of surface treatments, but indicate also major differences between carbon blacks from different preparation processes.     

Critical Surface Tension of Wetting and Flotation Separation of Hydrophobic Solids

Abstract

Wetting characteristics of a number of minerals including layer-type hydrophobic minerals as well as common sulfides were investigated. For the majority of the minerals, the critical surface tension of wetting, γ_c determined using Zisman's technique was in the range of 40 to 45 mN/m. Surface pressures of water, II_e, on molybdenite and coal samples were determined from adsorption isotherms. The dispersion component of the surface-free energy, γ_s ^d, for molybdenite was estimated to be 113 ± 3 mJ/m² as compared to the γ_s ^d value for graphite, 109 mJ/m². The wettability data of aqueous methanol solutions, presented in the form of adhesion tension diagrams, yielded significantly lower γ_c values. Flotation behavior of common sulfides, which was similar to that of inherently hydrophobic polymers and minerals, was attributed to elemental sulfur formation. The relevance of critical surface tension of wetting to selective flotation and separation of hydrophobic solids is discussed.     

A Reinterpretation of Organic Liquid-Polytetrafluoroethylene Surface Interactions

The wettability of polytetrafluoroethylene (PTFE) by organic liquids is reanalyzed in terms of dispersion-polar interactions across the liquid-solid interface. The analysis provides values of γ_s ^d = 19.6 dyne/cm, and γ_S ^D = 2.0 dyne/cm for the respective dispersion and polar parts of the surface tension γ_s = 21.6 for PTFE. The definition of a polar contribution to the surface tension of PTFE clarifies detailed aspects of the wettability of this polymer by different homologous liquid series. A modified analytical definition for work of adhesion is developed and applied to this discussion.     

Cohesive Hamaker Constants and Dispersive Surface Energies of RDX,PETN, TNT,and Ammonium Nitrate‐Based Explosives

Inverse gas chromatography (IGC) was used to characterize dispersive surface energies γ_S^D and cohesive Hamaker constants A_ii for RDX, PETN, TNT, ammonium nitrate (AN), and AN‐based explosives at 303 K. The γ_S^D for RDX at 303 K is compared to previous studies and generally found to be in good agreement, substantiating the use of NESTT training materials to characterize explosives via IGC. Additionally, the effect of the amount of fuel in the AN mixtures on γ_S^D is examined using simple linear regression. Finally, the IGC‐predicted A_ii values are compared to Lifshitz estimations for A_ii of RDX, PETN, TNT, and AN.     

An inverse gas chromatographic study of the PMMA / conducting polypyrrole interface

The poly(methyl methacrylate)/chloride-doped polypyrrole (PMMA/PPyCl) interface in PMMA-coated PPyCl powders has been characterized by inverse gas chromatography (IGC). The study is based on the determination of the surface energy of the various materials tested. It is shown that γ^d _s, the dispersive contribution to the surface energy of PPyCl, decreases following adsorption of PMMA onto PPyCl, a high surface energy material. The overall acid-base character of the coated powders shifts from a predominant acidity to a predominant basicity following PMMA adsorption. However, the nature of the casting solvent strongly influences the morphology of the PMMA coatings, as judged from their surface thermodynamic properties. Indeed, for approximately the same relative surface proportion of PMMA, both dispersive and acid-base properties of the coated PPyCl differ, depending on whether PMMA is cast from a good or a poor solvent. The results obtained suggest that from poor solvents, PMMA adsorbs onto PPyCl and leads to homogeneous coatings (provided that the amount of adsorbed PMMA is large enough to reach at least a monolayer), whereas from good solvents, the adsorption of PMMA results in inhomogeneous coatings.     

Contact angle,wetting, and adhesion: a critical review

The theory of the contact angle of pure liquids on solids, and of the determination of the surface free energy of solids, γ_s, is reviewed. The basis for the three components γ^LW _s, γ⊕_s, and γ?_s is developed, and an algebraic expression for these properties in terms of measured contact angles is presented. The inadequacy of the 'two-liquid' methodology (which yields a parameter, 'γ^p') is demonstrated. Attention is given to contact angle hysteresis and to the film pressure, π_e. Some recommendations are made with regard to contact angle measurements. A new treatment of hydrophilicity, and of the scale of hydrophobic/hydrophilic behavior, is proposed. It is shown that there are two kinds of hydrophilic behavior, one due to Lewis basicity (electron-donating or proton-accepting structures) and the other due to Lewis acidity (electron-accepting or proton-donating structures). The properties γ^? and γ^⊕ are the quantitative measures of these types of behavior and they are structurally independent of each other. A triangular diagram, with γ^LW at the hydrophobic corner, and γ^⊕ and γ^? at the two hydrophillic corners, is suggested.     

Studies on the surface free energy and surface structure of PTFE film treated with low temperature plasma

The surface free energy and surface structure of poly(tetrafluoroethylene) (PTFE) film treated with low temperature plasma in O₂, Ar, He, H₂, NH₃, and CH₄ gases are studied. The contact angles of the samples were measured, and the critical surface tension γ_c (Zisman) and γ_c (max) were determined on the basis of the Zisman's plots. Furthermore, the values of nonpolar dispersion force γ^a_s, dipole force γ^b_s, and hydrogen bonding force γ^c_s to the surface tensions for the plasma-treated samples were evaluated by the extended Fowkes equation. Mainly because of the contribution of polar force, the surface free energy and surface wettability of PTFE film which was treated with H₂, He, NH₃, Ar, and CH₄ for a short time increased greatly. Electron spectroscopy for chemical analysis (ESCA) shows that the reason was the decrease of fluorine and the increase of oxygen or nitrogen polar functional group on the surface of PTFE. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 63: 1733–1739, 1997     

Examination of the surface properties of kaolinites by inverse gas chromatography: Dispersive properties

The application of inverse gas chromatography (IGC) for the examination of the surface properties of untreated kaolinite and kaolinites surface-treated with titanate coupling agents is discussed. This paper presents and discusses the dispersive properties expressed by γ_S^D, the dispersive component of the surface free energy, as determined at various temperatures. The γ_S^D values of all the samples were negatively correlated with the temperature of IGC measurement. At the same temperature of IGC measurement, the values of γ_S^D determined by IGC were significantly lower for the surface-treated kaolinites than for the untreated ones. The γ_S^D value of the kaolinite surface-treated with isopropyl triisostearoyl titanate (ITT) at 150°C exhibited a surface energy close to that (28.3 mJm^-2) of polyethylene with no surface polarity.     

Studies on adhesion of polyethylene. Part I. Influence of functionality and phase transfer catalyst

Low-density polyethylene (LDPE) has been oxidized using phase transferred permanganate as an oxidant. The resulting surface modifications have been characterized by different methods and the consequent adhesion promotion has been characterized in terms of contact angle and peel strength measurements. From contact angle measurements using water and formamide liquid drops, reversible work of adhesion, and thence γ^p _s and γ-^d _s, the contributions of polar and dispersion components, respectively, have been calculated. The polar contributions increased in each oxidized LDPE relative to untreated LDPE and the surface energies also increased appreciably. The adhesion strengths between aluminium and untreated LDPE, as well as those between aluminium and oxidized LDPEs, have been examined by peel strength measurements. We found that the adhesion strength increased about 8-28 times in the case of oxidized LDPEs. Maximum strength was observed when benzyl triphenyl phosphonium permanganate was the oxidant. Both the peel force and the thermodynamic work of adhesion increase sharply with an increase in the carboxyl content, the total number of oxo groups, and the combined CO and COOH content. The dependence of these quantities on carbonyl content is either weak or even follows a reverse trend. It is proposed that the adhesion proceeds primarily through hydrogen bonds involving carboxyl group and/or coordinative bond formation between aluminium and epoxy and/or hydroperoxo groups.     

Surface energy characterization of modified carbon blacks and its relationship to composites tearing properties

The effects of three types of chemical treatments, i.e. as polar acidic, polar basic, and nonpolar oxidations, on virgin carbon blacks have been studied in terms of pH, acid-base surface values, specific surface area, X-ray diffraction analysis, and surface free energy. The acidic chemical treatment leads to significant changes in surface and adsorption properties, surface free energy, and microstructures. The increased acidic surface functional groups on carbon blacks result from reaction between the basic carbon and the acidic chemical solution. Also, based on the determination of surface free energy from contact angle measurements, a good correlation between the London dispersive component or apolar (γ^d _s) of surface free energy and specific surface area (S_BET) (or crystalline size S along the c-axis, L_C) is shown in this work. Particularly, it is found that the γ^d _s of the carbon blacks studied is highly correlated with the mechanical tearing test results based on hydrocarbon rubber compound composites.     

A model for transitional breakage probability of droplets in agitated lean liquid-liquid dispersions

A model for transitional breakage probability of droplets in agitated lean fiquid-liquid dispersions is proposed based on the mechanism of breakage of droplets due to their oscillations resulting from relative velocity fluctuations. A universal transitional breakage probability in terms of non-dimensionalized drop diameter is derived for all dispersed phases whose density and viscosity are almost the same as that of continuous phase. The maximum stable drop diameter d_s derived from the model, shows a dependence of N_We^?0.6. It is shown that a “power law” approximation Kvⁿ is valid for transitional breakage probability for d/d_s up to 2. The exponent 2.67, predicted by this model corresponds rather well with an estimate of 2, obtained from experimental observations. A functional relation for the rate constant K in terms of the parameters and physical properties of the system is derived. A universal non-dimensionalized equilibrium drop-size distribution for agitated lean liquid-liquid dispersions is derived by analytical solution of a population balance equation simplified by order of magnitude estimates. Interestingly enough, this analytical solution is the same as the Gaussian distribution suggested empirically by Chen and Middleman.     

Effects of magnetron arrangement and power combination on temperature field uniformity of microwave drying of carrot

The effects of magnetron arrangement and power combination on the temperature field uniformity of microwave (MW) drying of carrot were investigated in this study. Using one magnetron resulted in poor temperature field uniformity. However, when two magnetrons were applied, the T_d (temperature dispersion between the maximum temperature value and the minimum temperature value among detectors) at different hot spots was lower than that obtained for the single magnetron. T_d was 3–10°C when a combination of three magnetrons (α, β, and γ) was used at powers of 400, 500, and 600 W (except for the hot spot of “the highest temperature point”). This method of infrared temperature control-intermittent MW heating could reduce T_d significantly in different hot spot positions and improve the uniformity of the temperature field.     

Early Events in the Precipitation Fouling of Calcium Sulphate Dihydrate under Sensible Heating Conditions

Supersaturated aqueous solutions of calcium sulphate, an inverse solubility salt, were circulated through a 9 mm i.d. stainless steel tube in which they were subjected to a constant and uniform heat flux at Reynolds numbers ranging from 2100 to 36 000. Precipitation fouling of the tube surface, which was monitored and measured thermally, resulted in calcium sulphate dihydrate (gypsum) scales. Measured delay times decreased with increasing bulk solute concentration, C_b, in accord with classical nucleation kinetics, decreased with increasing surface temperature T_s, its reciprocal correlating with T_s in the Arrhenius manner, and decreased with increasing fluid velocity V up to V ≈ 0.5 m/s before flattening out. Initial linear fouling rates,R_fo, at C_b = 3400 ppm, also yielded Arrhenius plots, with fouling activation energies increasing almost eight‐fold over the 0.1–1.6 m/s velocity range. This result implies a shift in the mechanism of fouling rate control with increasing velocity from mass transfer, which is weakly temperature dependent, to surface integration, which is strongly temperature dependent. The “Initial Fouling Rate Model,” which simulates this mechanistic shift with a unique dependence of the surface‐integration step on the residence time of the fluid at the tube wall, was used to represent, by least‐squares fitting, 84 fouling rate data points at different values of V and T_s, with three adjustable parameters. Observed trends of R_fo with V and clean wall T_s were qualitatively captured by the model, but quantitative agreement was imperfect. Substantial quantitative improvement was effected by introducing into the model an additional temperature dependence of the surface‐integration term—resulting in one additional adjustable parameter—rationalized as a method of accounting for the previously neglected temperature‐dependent nucleation that accompanies crystal growth. The fouling investigation was supplemented by an experimental study of crystal growth kinetics.     

Inverse gas chromatographic study of polymer blend interactions and their temperature dependence

The temperature dependence of the thermodynamic interactions in blends of polypropylene and a fluorochemical was studied by inverse gas chromatography (IGC). A modified procedure has been proposed to evaluate the effective surface composition in polyblends. This shows that the low surface energy component preferentially concentrates in the surface region of blends. Moreover, the new procedure greatly reduces the probe-to-probe variation of χ₂₃, a problem frequently encountered in thermodynamic applications of the IGC method. As a result, IGC may now be used with increased confidence for the determination of interaction thermodynamics in polymer systems over broad temperature ranges. © 1992 John Wiley & Sons, Inc.