James Bowers

Critical adsorption and boundary layer structure of 2-butoxyethanol+D2O mixtures at a hydrophilic silica surface

Neutron reflectivity experiments have been performed to investigate the adsorption behavior of aqueous solutions of n-2-butoxyethanol (n-C4E1) and i-2-butoxyethanol (i-C4E1) in D2O against a hydrophilic silica substrate. The measurements were made in the one-phase region near the lower critical solution point of the aqueous systems. At temperatures removed from the lower critical solution temperature, TLCS, evidence for the existence of a microscopically thick (∼40 A) adsorbed D2O-rich layer is presented along with data suggesting that the amphiphile is strongly depleted from the boundary layer. Experiments using a thick “tuning” layer of silica reinforce these observations. Analysis of the data as T→TLCS reveals that the region beneath the adsorbed D2O-rich layer becomes increasingly enriched with D2O, and the nature of the composition profile normal to the interface has been considered. Analysis identifies the exponent describing the power-law decay part of the critical adsorption profile appended to th...

Structural studies of amphiphiles adsorbed at liquid–liquid interfaces using neutron reflectometry

We report the application and refinement of a recently developed method for structural studies at a liquid liquid interface using neutron reflectometry. The technique involves the entrapment of a thin oil layer between a silicon substrate and an aqueous subphase. The thin oil film is prepared by spin-coating an oil film on to an oleophilically treated silicon substrate. During the reflectivity measurement the sample is maintained in a horizontal position, and the angle of incidence of the neutron beam is varied using a supermirror. Attenuation of neutron reflectivity at the lowest angle of incidence is used to determine the oil-layer thickness. We report information regarding the structure at the interface between hexadecane and a 0.1% w/v aqueous solution of the triblock copolymer Pluronic L64 with EO13PO30EO13 (EO = ethylene oxide; PO = propylene oxide) and the interface between hexadecane and a 3.7 mmol dm(-3) (approximately critical micelle concentration) aqueous solution of the cationic surfactant tetradecyltrimethylammonium bromide (C14TAB). For the C14TAB system, the reflectivity data unambiguously reveal the presence of a region highly concentrated in C14TAB on the oil side of the interface. For the Pluronic L64 system, the data suggest that the polymer adsorbs at the interface occupying both oil and water sides of the interface. Model scattering length density profiles that capture these features are presented and further models that better fit the data are discussed.

Adsorption from alkane+perfluoroalkane mixtures at fluorophobic and fluorophilic surfaces. I. Nature of the noncritical adsorption profiles

Neutron reflection has been applied to probe the nature and extent of adsorption from a mixture of (1−x)n-hexane+xperfluoro-n-hexane against silicon substrates modified with alkylsilane (fluorophobic) or fluoroalkylsilane (fluorophilic) coupled layers. For an equimolar mixture (x=0.5, 60.7 vol %) in the one-phase region at T=30 °C—removed both in temperature and composition from the upper critical point at 22.65 °C and x=0.36—the structure was resolved at both fluorophobic and fluorophilic surfaces. Liquid mixtures with three different refractive index contrasts were used to reduce model ambiguity in the ensuing analysis. For both surfaces the composition profiles of the adsorbed liquids could be represented using two-layer slab models which included interlayer Gaussian roughness. For the fluorophobic surface, the thickness of the layer closest to the substrate is ∼20 A and composed of ∼83 vol % n-hexane, and the second, more dilute layer has a composition profile which decays smoothly into the bulk over ...

Thermodynamics of linear dimethylsiloxane–perfluoroalkane mixtures

Measurements of the liquidliquid coexistence curve for hexamethyldisiloxane, octamethyltrisiloxane or decamethyltetrasiloxanetetradecafluorohexane near the upper critical endpoint and of the upper coexistence temperatures T UCX for equivolume mixtures of 21 other mixtures of this kind confirm the basic similarity of the thermodynamics of such mixtures to those of alkaneperfluoroalkane mixtures. The upper critical solution temperatures T UCS increase linearly within the range studied with perfluoroalkane chain length and nearly so with dimethylsiloxane chain length. The coexistence curves have been fitted to Wegner extended-scaling expressions in order to investigate the universality of the shape. The temperaturevolume-fraction (T−φ) representation is more symmetrical than that in mole fraction and with a reduced temperature =(T/T UCS ), withthe upper critical solution temperature T UCS as the reduction factor, the (−φ) representation is universal for all threemixtures as well as nearly so for the comparison mixture C 6 H 14 C 6 F 14 . T UCS (C 6 H 14 C 6 F 14 )=295.80 K and T UCS (hexamethyldisiloxaneC 6 F 14 )=296.95 K are unsurprisingly close, in view of the near-equal temperature reduction factors of hexamethyldisiloxane and hexane. The effect of this correspondence is extended to the analysis of the T UCX of the other mixtures. The results are discussed in terms of the solubility parameter version of the regular solution theory incorporating the Flory–Huggins entropy of mixing term. With a correction of nearly 10% on the geometric-mean combining rule for the cohesive energy density T UCS is reasonably well predicted. For these mixtures, the critical mole fraction is also surprisingly well predicted from the ratio of component molar volumes.

The structure of the liquid-vapour interface of n-hexane + perfluoro-n-hexane along the path of critical composition through the critical end-point by neutron reflectivity

The surface structure ofthe mixture n-hexane+perfluoro-n-hexane has been determined using specular neutron reflection.Above the upper criticalend point (UCEP) at 22•65°C the preferential adsorption of the perfluoro-n-hexane is present as a macroscopic layer, becoming richer in perfluoro-n-hexane as the UCEP is approached. This adsorbed layer is capped by a monolayer of perfluoro-n-hexane. The monolayer of perfluoro-n-hexane exists in the approximate concentration range x(perfluoro-n-hexane) = 0•2–0•7 as reported in a previous paper. Just below the UCEP the surface is composed of a monolayer of perfluoro-n-hexane capping a macroscopically thick layer of the perfluoro-n-hexane-rich phase. Further below the UCEP the surface consists of the perfluoro-n-hexane-rich phase, the disappearance of the monolayer being attributed to its composition-dependent existence.

Adsorption from alkane+perfluoroalkane mixtures at fluorophobic and fluorophilic surfaces. II. Crossover from critical adsorption to complete wetting

Using neutron reflectometry, adsorption from an equimolar mixture of hexane + perfluorohexane to a fluorophobic, octadecyl-coated, silicon substrate has been investigated as a function of temperature in the one-phase region upon approach to liquid-liquid coexistence. The composition of the investigated mixture, x(F) = 0.50, is well removed from the critical composition of x(F) = 0.36, where x(F) is the perfluorohexane mole fraction. To aid the modeling, mixtures with three different neutron refractive index contrasts have been used: namely, mixtures of C(6)H(14) + C(6)F(14) (H-F), C(6)D(14) + C(6)F(14) (D-F), and a mixture of C(6)H(14) + C(6)D(14) + C(6)F(14) which has been adjusted to have the same refractive index as silicon (CMSi). For all three contrasts, the principal features of the composition profile normal to the interface follow similar trends as the temperature T is reduced towards T(0), the coexistence temperature. These features consist of: (i) a hexane-rich primary adsorption layer appended to the octadecyl coupled layer. This primary layer is 22 +/- 5 A thick and becomes increasingly enriched in hexane as T(0) is approached. (ii) A tail that decays exponentially towards the bulk composition with a characteristic decay length zeta. As T(0) is approached, zeta increases. The scattering length density profiles have been converted to volume fraction profiles and the surface excess of hexane Gamma has been determined as a function of temperature for all three contrasts. As T(0) is approached Gamma increases, and its behavior can be represented using the scaling law Gamma approximately |T - T(0)|(-m). The resulting values of m are 0.71 +/- 0.09, 0.68 +/- 0.04, and 0.68 +/- 0.06 for the D-F, H-F, and CMSi contrasts, respectively. The behavior of Gamma with temperature does not adhere to the Gamma approximately |T - T(0)|(-1/3) law expected for complete wetting in systems with van der Waals interactions nor does it correspond to Gamma approximately |T - T(c)|(-0.305) expected for critical adsorption. The magnitude of the exponent m indicates that the adsorption resides in the crossover region between critical adsorption and complete wetting.

Thermodynamics of lineardimethylsiloxane–perfluoroalkane mixtures Part3.—Orthobaric surface tensions ofhexamethyldisiloxane–, octamethyltrisiloxane– ordecamethyltetrasiloxane–tetradecafluorohexane near the uppercritical endpoint

The orthobaric surface tensions of three dimethylsiloxane–perfluoroalkane mixtures: hexamethyldisiloxane–, octamethyltrisiloxane– and decamethyltetrasiloxane–tetradecafluorohexane have been measured at temperatures close to the upper critical solution temperatures. All three mixtures are normal in the sense that the more volatile component has the lower surface tension. The surface tension isotherm closest to the critical endpoint displays a characteristic horizontal inflection at a composition very close to the critical composition previously determined from the liquid–liquid coexistence curve. The incidence of negative aneotropy is distinct in the hexamethyldisiloxane–tetradecafluorohexane mixture and wholly absent in the decamethyltetrasiloxane–tetradecafluorohexane mixture with the mixture containing octamethyltrisiloxane intermediate in behaviour. The first mixture displays both azeotropy (predicted via the application of the regular solution theory of mixtures) and aneotropy, the second azeotropy and rather indistinct aneotropy and the third mixture neither azeotropy nor aneotropy; these observations emphasise that although vapour pressure and surface tension extrema often occur in mixtures simultaneously there is no theoretical necessity that they should do so. As in many other mixture properties there are great similarities between dimethylsiloxane–perfluoroalkane and alkane–perfluoroalkane mixtures, particularly those containing hexamethyldisiloxane or hexane.

Wetting observed by evanescent-wave-generated fluorescence spectroscopy III: Fluorescence lifetimes: n-Heptane+perfluorohexane at the upper critical endpoint

The technique of evanescent-wave-generated-fluorescence spectroscopy EWGFS has been applied here to the investigation of the critical-point wetting in the phase-separated mixture (n-heptane+perfluoro-n-hexane). The discrimination between the two coexisting liquid phases was achieved by exploiting the fluorescence lifetime difference of the flurophor 1,6-diphenylhexatriene in each of the phases. The wetting transition temperature TW is clearly identified as being (31.5 ± 0.2) °C, in relation to the upper consolute temperature of ca. 42°C. This version of the EWGFS technique for studying critical-point wetting may be useful for the future determination of composition and relative thickness of the wetting layer, in addition to the elucidation of the order of the transition. Other related application of the technique include the study of critical-point wetting and critical adsorption at modified solid surfaces.

Neutron reflectivity studies of the free liquid surface of methylcyclohexane–perfluoromethylcyclohexane near the critical endpoint

The surface structure of the liquid mixture methylcyclohexane (MCH)–perfluoromethylcyclohexane (PFMCH) has been investigated by specular neutron reflectometry from 42 K below to 10 K above the upper critical solution temperature TUCS=46.13°C. The results have been interpreted in the light of the Abele′s layer model which indicates that the reflectivity throughout the experimental temperature range is satisfactorily represented by a model with two relatively thin layers interposed between the vapour phase and a well-characterised subphase. The twin-layer region is everywhere richer in PFMCH, the component of lower surface tension, than the immediate subphase and is thus regarded as an adsorbed layer. For TUCS<T, in the region of one-liquid phase coexistence, the composition of the subphase is that of the critical mixture. The divergence of the composition, thickness and surface excess of the adsorbed layers in the very near vicinity of TUCS is indicative of critical adsorption. In the region of two-liquid phase coexistence, i.e. T<TUCS, the experimental data can be further divided into two separate categories depending on whether the temperature was rising or falling. For both categories the interposed adsorption region is again modelled by a minimum of two slab layers of similar thickness while the subphase composition varies with temperature and is well represented by the composition of the lower PFMCH-rich phase for temperatures just falling below TUCS and by that of the upper MCH-rich liquid phase for much lower temperatures rising from around the measured wetting transition temperature TW. The subphase scattering length density is calculated at the experimental temperature from the known liquid/liquid phase diagram. The vapour/liquid wetting phase is thus confirmed directly as the lower bulk phase, while the identification of TW is indeterminate from the measurements due to hysteresis effects associated with the interface.