Publications by authors named "Ali Zarbakhsh"

21 Publications

  • Page 1 of 1

Influence of Buffers, Ionic Strength, and pH on the Volume Phase Transition Behavior of Acrylamide-Based Nanogels.

Polymers (Basel) 2020 Nov 4;12(11). Epub 2020 Nov 4.

Department of Chemistry, SBCS, Queen Mary University of London, Mile End Road, London E1 4NS, UK.

The use of covalently crosslinked nanogels for applications in biology and medicine is dependent on their properties and characteristics, which often change because of the biological media involved. Understanding the role of salts, ionic strength and pH in altering specific properties is key to progress in this area. We studied the effect of both chemical structure and media environment on the thermoresponsive behavior of nanogels. A small library of methylenebisacrylamide (MBA) crosslinked nanogels were prepared using -isopropylacrylamide (NIPAM) or -propylacrylamide (NPAM), in combination with functional monomers -hydroxyethylacrylamide (HEAM) and -acryloyl-l-proline (APrOH). The thermoresponsive properties of nanogels were evaluated in phosphate buffer, tris-acetate buffer and Ringer HEPES, with varying concentrations and ionic strengths. The presence of ions facilitates the phase separation of nanogels, and this "salting-out" effect strongly depends on the electrolyte concentration as well as the specificity of individual anions, e.g., their positions in the Hofmeister series. A subtle change in the chemical structure of the side chain of the monomer from NIPAM to NPAM leads to a reduction of the volume phase transition temperature (VPTT) value by ~10 °C. The addition of hydrophilic comonomers such as HEAM, on the other hand, causes a ~20 °C shift in VPTT to higher values. The data highlight the significant role played by the chemical structure of the monomers used, with hydrophobicity and rigidity closely interlinked in determining thermoresponsive behavior. Furthermore, the volume phase transition temperature (VPTT) of nanogels copolymerized with ionizable APrOH comonomer can be tailored by changes in the pH of buffer solutions. This temperature-controlled phase transition is driven by intricate interplay involving the entropy of mixing, electrostatic interactions, conformational transitions, and structural rigidity. These results highlight the importance of understanding the physiochemical properties and behavior of covalently crosslinked nanogels in a biological environment prior to their applications in life-science, such as temperature/pH-triggered drug delivery systems.
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http://dx.doi.org/10.3390/polym12112590DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7694245PMC
November 2020

Interactions of NIPAM nanogels with model lipid multi-bilayers: A neutron reflectivity study.

J Colloid Interface Sci 2019 Feb 28;536:598-608. Epub 2018 Oct 28.

Department of Chemistry, Queen Mary, University of London, Joseph Priestley Building, Mile End Road, London E1 4NS, United Kingdom. Electronic address:

In dermal drug delivery, the influence of the chemical structure of the carriers on their penetration mechanisms is not yet fully understood. This is a key requirement in order to design highly efficient delivery systems. In this study, neutron reflectivity is used to provide insights into the interactions between thermoresponsive N-isopropylacrylamide based nanogels, cross-linked with 10%, 20% and 30% N,N'-methylenebisacrylamide, and skin lipid multi-bilayers models. Ceramide lipid multi-bilayers and ceramide/cholesterol/behenic acid mixed lipid multi-bilayers were used for this work. The results indicated that in both multi-bilayers the lipids were depleted by the nanogels mainly through hydrophobic interactions. The ability of nanogels to associate with skin lipids to form water-dispersible complexes was found to be a function of the percentage cross-linker. An enhanced depletion of lipids was further observed in the presence of benzyl alcohol, a well-known skin penetration enhancer.
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http://dx.doi.org/10.1016/j.jcis.2018.10.086DOI Listing
February 2019

Interaction of thermal responsive NIPAM nanogels with model lipid monolayers at the air-water interface.

J Colloid Interface Sci 2018 Jun 17;519:97-106. Epub 2018 Feb 17.

Department of Chemistry and Biochemistry, SBCS, Queen Mary University of London, Mile End Road, London E1 4NS, UK. Electronic address:

Understanding the interaction of nanoparticles (NP) with ceramide lipids is important in developing strategies to overcome the formidable obstacle that is skin. This paper presents studies of interactions between N-isopropylacrylamide nanogels, crosslinked with 30% N,N'-methylenebisacrylamide, and model ceramide lipid monolayers at the air-water interface as a function of temperature. In the case of the mixed ceramide/cholesterol/behenic acid monolayer, the interaction of nanogels with the ceramide was strongly mediated by the fatty acids. This interaction between nanogels and monolayer components is dominated by hydrophobic-hydrophobic binding. The data show the important intermediary role of the fatty acid in facilitating transmembrane transport. For a pure ceramide lipid monolayer, the neutron reflectivity (NR), Brewster angle microscopy (BAM) and surface pressure results showed a lipid-nanogel complex formation and the subsequent depletion/solubilisation of the lipids from the interface when the area per molecule for the lipid was increased from 42 to 44 Å.
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http://dx.doi.org/10.1016/j.jcis.2018.02.050DOI Listing
June 2018

Adsorption versus aggregation of NIPAM nanogels: new insight into their behaviour at the air/water interface as a function of concentration.

Phys Chem Chem Phys 2017 Jul;19(26):17173-17179

Department of Chemistry and Biochemistry, SBCS, Queen Mary University of London, Mile End Road, London E1 4NS, UK.

We have used neutron reflectivity (NR) measurements in combination with dynamic light scattering (DLS), surface tension and ellipsometry, to study the adsorption behaviour at the air/water interface of N-isopropylacrylamide-based nanogels as a function of concentration. The data provide clear evidence that the nanogels are adsorbed at the interface in a strongly deformed shape and forming a multi-layer where the thickness increases with nanogel concentration in the bulk. The combination of surface characterisation techniques and bulk studies indicate that interfacial film formation is preferred over bulk aggregation. This observation at the air/water interface supports the Derjaguin prediction, that a sphere's interaction with a plane (the thick adsorbed nanogel layer at interface) is much larger than nanogel-nanogel (sphere-sphere) association in the bulk. These findings, in particular the changes in conformations and the thick layer adsorption at the interface as a function of concentration, impact significantly on a number of applications for which nanogels are currently being investigated. These results contribute to the understanding of the behaviour of soft colloids at the interfaces.
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http://dx.doi.org/10.1039/c7cp02979aDOI Listing
July 2017

Superamphiphobic Surfaces Prepared by Coating Multifunctional Nanofluids.

ACS Appl Mater Interfaces 2016 Nov 10;8(46):32011-32020. Epub 2016 Nov 10.

School of Biological and Chemical Sciences, Queen Mary, University of London , Joseph Priestley Building, Mile End Road, London E1 4NS, United Kingdom.

Construction of surfaces with the capability of repelling both water and oil is a challenging issue. We report the superamphiphobic properties of mineral surfaces coated with nanofluids based on synthesized Co-doped and Ce-doped Barium Strontium Titanate (CoBST and CeBST) nanoparticles and fluorochemicals of trichloro(1H,1H,2H,2H-perfluorooctyl)silane (PFOS) and polytetrafluoroethylene (PTFE). Coating surfaces with these nanofluids provides both oil (with surface tensions as low as 23 mN/m) and water repellency. Liquids with high surface tension (such as water and ethylene glycol) roll off the coated surface without tilting. A water drop released from 8 mm above the coated surface undergoes first a lateral displacement from its trajectory and shape deformation, striking the surface after 23 ms, bouncing and rolling off freely. These multifunctional coating nanofluids impart properties of self-cleaning. Applications include coating surfaces where cleanliness is paramount such as in hospitals and domestic environments as well as the maintenance of building facades and protection of public monuments from weathering. These superamphiphobic-doped nanofluids have thermal stability up to 180 °C; novel industrial applications include within fracking and the elimination of condensate blockage in gas reservoirs.
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http://dx.doi.org/10.1021/acsami.6b10913DOI Listing
November 2016

Smart nanogels at the air/water interface: structural studies by neutron reflectivity.

Nanoscale 2016 Mar;8(9):4951-60

Department of Chemistry, SBCS, Queen Mary University of London, Mile End Road, London E1 4NS, UK.

The development of effective transdermal drug delivery systems based on nanosized polymers requires a better understanding of the behaviour of such nanomaterials at interfaces. N-Isopropylacrylamide-based nanogels synthesized with different percentages of N,N'-methylenebisacrylamide as cross-linker, ranging from 10 to 30%, were characterized at physiological temperature at the air/water interface, using neutron reflectivity (NR), with isotopic contrast variation, and surface tension measurements; this allowed us to resolve the adsorbed amount and the volume fraction of nanogels at the interface. A large conformational change for the nanogels results in strong deformations at the interface. As the percentage of cross-linker incorporated in the nanogels becomes higher, more rigid matrices are obtained, although less deformed, and the amount of adsorbed nanogels is increased. The data provide the first experimental evidence of structural changes of nanogels as a function of the degree of cross-linking at the air/water interface.
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http://dx.doi.org/10.1039/c5nr07538fDOI Listing
March 2016

Interfacial equation of state for ionized surfactants at oil/water interfaces.

Soft Matter 2015 Aug 17;11(32):6482-91. Epub 2015 Jul 17.

Institute of Petroleum Engineering, University of Tehran, Tehran, Iran.

A new mathematical approach has been developed for describing the interfacial behaviour of oil/water interfaces in the presence of ionic surfactants. The approach relies on the ideal behaviour of ionized surfactants at oil/water interfaces, which is previously demonstrated by Lucassen-Reynders (J. Phys. Chem., 1966, 70, 1777-1785). The new derived equation simply relates the interfacial tension to the surfactant molecular size and the cmc value of the surfactant in the aqueous phase. The predicted values are in a reasonable agreement with the measured experimental data. Formation of complex multi-layers is considered and the related development is performed. It is shown that, assuming a multi-layer interface, the proposed model gives an area per surfactant molecule similar to the values obtained by techniques such as neutron reflectivity (NR), while a monolayer assumption yields about half the value. The discussion describes the impact of dissolved oil and ionic components on the interfacial tension of the ionized surfactants at oil/water interfaces.
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http://dx.doi.org/10.1039/c5sm01406aDOI Listing
August 2015

Polarized Neutron Reflectometry of Nickel Corrosion Inhibitors.

Langmuir 2015 Jun 17;31(25):7062-72. Epub 2015 Jun 17.

†Department of Chemistry and BP Institute, Cambridge University, Cambridge CB2 1EW, U.K.

Polarized neutron reflectometry has been used to investigate the detailed adsorption behavior and corrosion inhibition mechanism of two surfactants on a nickel surface under acidic conditions. Both the corrosion of the nickel surface and the structure of the adsorbed surfactant layer could be monitored in situ by the use of different solvent contrasts. Layer thicknesses and roughnesses were evaluated over a range of pH values, showing distinctly the superior corrosion inhibition of one negatively charged surfactant (sodium dodecyl sulfate) compared to a positively charged example (dodecyl trimethylammonium bromide) due to its stronger binding interaction with the surface. It was found that adequate corrosion inhibition occurs at significantly less than full surface coverage.
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http://dx.doi.org/10.1021/acs.langmuir.5b01718DOI Listing
June 2015

Neutron reflection study of the adsorption of the phosphate surfactant NaDEHP onto alumina from water.

Langmuir 2015 Mar 11;31(11):3377-84. Epub 2015 Mar 11.

†BP Institute and Department of Chemistry, University of Cambridge, Madingley Rise, Madingley Road, Cambridge, UK.

The adsorption of a phosphorus analogue of the surfactant AOT, sodium bis(2-ethylhexyl) phosphate (NaDEHP), at the water/alumina interface is described. The material is found to adsorb as an essentially water-free bilayer from neutron reflection measurements. This is similar to the behavior of AOT under comparable conditions, although AOT forms a thicker, more hydrated layer. The NaDEHP shows rather little variation with added salt, but a small thickening of the layer on increasing the pH, in contrast to the behavior of AOT.
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http://dx.doi.org/10.1021/la504837sDOI Listing
March 2015

Structural studies of nonionic dodecanol ethoxylates at the oil-water interface: effect of increasing head group size.

Langmuir 2014 Sep 19;30(34):10241-7. Epub 2014 Aug 19.

School of Physics and Astronomy, University of Manchester , Oxford Road, Manchester M13 9PL, United Kingdom.

The conformation of charged surfactants at the oil-water interface was recently reported. With the aim to assess the role of the head group size on the conformation of the adsorbed layer, we have extended these studies to a series of nonionic dodecanol ethoxylate surfactants (C12En, ethylene oxide units n from 6 to 12). The study was performed using neutron reflectometry to enable maximum sensitivity to buried interfaces. Similarly to charged surfactants, the interface was found to be broader and rougher compared to the air-water interface. Irrespective of the head group size, the tail group region was found to assume a staggered conformation. The conformations of the head group were found to be significantly different from those of the air-water interface, moving from a globular to an almost fully extended conformation at the oil-water interface. The stretching of the head groups is attributed to the presence of some hexadecane oil molecules, which may penetrate all the way to this region. It is proposed here that the presence of the oil, which can efficiently solvate the surfactant tail groups, plays a key role in the conformation of the adsorbed layer and is responsible for the broadening of the interface.
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http://dx.doi.org/10.1021/la502559rDOI Listing
September 2014

Hexadecylamine adsorption at the iron oxide-oil interface.

Langmuir 2013 Nov 28;29(45):13735-42. Epub 2013 Oct 28.

Department of Chemistry and BP Institute, Cambridge University , Cambridge CB2 1EW, United Kingdom.

The adsorption behavior of a model additive, hexadecylamine, onto an iron surface from hexadecane oil has been characterized using polarized neutron reflectometry, sum-frequency generation spectroscopy, solution depletion isotherm, and X-ray photoelectron spectroscopy (XPS). The amine showed a strong affinity for the metal surface, forming a dense monolayer at relatively low concentrations; a layer thickness of 16 (±3) Å at low concentrations, increasing to 20 (±3) Å at greater amine concentrations, was determined from the neutron data. These thicknesses suggest that the molecules in the layer are tilted. Adsorption was also indicated by sum-frequency generation spectroscopy and XPS, the latter indicating that the most dominant amine-surface interaction was via electron donation from the nitrogen lone pair to the positively charged iron ions. Sum-frequency generation spectroscopy was used to determine the alkyl chain conformation order and orientation on the surface.
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http://dx.doi.org/10.1021/la4018147DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3850247PMC
November 2013

Adsorption of sodium hexanoate on α-alumina.

J Colloid Interface Sci 2013 Oct 27;407:348-53. Epub 2013 Jun 27.

BP Institute and Department of Chemistry, University of Cambridge, Madingley Rise, Madingley Road, Cambridge, UK.

Neutron reflection and adsorption isotherm measurements have been used to study the adsorption behaviour of hexanoic acid onto α-alumina surfaces. Importantly, the pH dependence of the behaviour has been characterised with a pronounced maximum in adsorption identified at a pH of approximately 5, close to the pKa of the acid. The adsorbed layer is identified as a bilayer, which is reasonable given the hydrophilic nature of both side of the layer, and has a thickness of 13 Å, suggesting significant extent of interdigitation. At pH 5, the layer has much lower extent of hydration relative to the higher pH of 7, consistent with the increased total adsorption at pH 5. A number of different mechanisms for the binding of the hexanoic acid to the surface are considered. The experimental data, combined with calculations using equilibrium/binding constants of the surface and ligands, indicates that a ligand exchange reaction may be the most significant mechanism.
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http://dx.doi.org/10.1016/j.jcis.2013.06.019DOI Listing
October 2013

Surfactant mixtures at the oil-water interface.

J Colloid Interface Sci 2013 May 4;398:126-33. Epub 2013 Mar 4.

School of Biological & Chemical Sciences, Queen Mary, University of London, London, United Kingdom.

We report the structural study of mixed monolayers of partially deuterated N,N'-di-hexadecyl-(d33)-4,13-diaza-18-crown-6 ether (d-ACE16) and palmitic acid (PA) at the oil-water interface, in order to understand the mechanism of metal ion transport through Permeation Liquid Membrane (PLM) devices. The composition of the mixed monolayers remains constant with increasing spread amount and the saturation of the interface is achieved at a relatively low spread amount. The excess PA material is accommodated in the oil phase, playing an important role in equilibrating the interfacial concentration of ACE-16. The presence of PA increases the surface concentration of ACE-16 at low spread amount and facilitates its dissolution into the oil phase at the high spread amount. The result suggests a dynamic exchange between the bulk phase and the interface ensuring a continuous turnover which reflects their relevance in PLM devices. The conclusions regarding the role of a fatty acid in regulating the surface concentration of the alkylated azacrown ether and its dominant role in the bulk transport of metal ions through the membrane are consistent with the results of macroscopic studies reported earlier.
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http://dx.doi.org/10.1016/j.jcis.2013.01.069DOI Listing
May 2013

Surfactant adsorption at the metal-oil interface.

Langmuir 2011 May 20;27(10):6085-90. Epub 2011 Apr 20.

School of Biological & Chemical Sciences, Queen Mary, University of London, Joseph Priestley Building, Mile End Road, London E1 4NS, United Kingdom.

The structure of the adsorbed palmitic acid at the iron oxide/oil interface has been investigated using polarized neutron reflectometry. The palmitic acid was found to be strongly adsorbed at the oxide/oil interface resulting in a monolayer of thickness 16 ± 4 Å for 150 and 500 ppm palmitic acid concentrations (16 ± 5 Å for the 1000 ppm solution). These layer thicknesses suggest tilt for the palmitic acid molecules with respect to the interface. The model also requires a second diffuse layer extending in the bulk oil. The thickness of this diffuse layer was 35 ± 17 Å for the 150 ppm solution and 45 ± 22 Å for 500 and 1000 ppm solution. The composition profiles at the interface suggest a depletion of the oil in the vicinity of the interface as the concentration of palmitic acid increases.
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http://dx.doi.org/10.1021/la200670wDOI Listing
May 2011

Stabilization of alkylated azacrown ether by fatty acid at the air-water interface.

Langmuir 2010 Dec 4;26(23):18194-8. Epub 2010 Nov 4.

School of Biological & Chemical Sciences, Queen Mary, University of London, Joseph Priestley Building, Mile End Road, London E1 4NS, United Kingdom.

The adsorbed amount of partially deuterated dihexadecyl-diaza-18-crown-6 ether (d-ACE16) in the presence of different chain length fatty acids as a function of surface pressure was determined by neutron reflectometry technique. The highest adsorbed amount of the azacrown ether was observed for the mixture of ACE16 with hexadecanoic (palmitic) acid, pointing to the importance of chain length matching between the two species for optimum stabilization of the mixed monolayer. The contrast variation technique was used to estimate the contribution to the total adsorbed amount from stearic acid and ACE16. It was found that the mixed Langmuir monolayer is stable against dissolution up to a surface pressure of 20 mN m(-1). Above this pressure, however, the spread and adsorbed amounts start to deviate, indicative of partial dissolution into the aqueous subphase. The consequences of this behavior for the transport of metal ions through the interfaces of permeation liquid membranes (PLMs) are discussed.
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http://dx.doi.org/10.1021/la103620bDOI Listing
December 2010

Structural studies of aliphatic substituted phthalocyanine-lipid multilayers.

Langmuir 2010 Oct;26(19):15383-7

School of Biological & Chemical Sciences, Queen Mary, University of London, Joseph Priestley Building, Mile End Road, London E1 4NS, United Kingdom.

A Langmuir-Blodgett film of aliphatic substituted phthalocyanines on a C18 silane supporting layer coupled onto a silicon substrate has been investigated using neutron reflectometry. This multilayer structure is seen as a possible candidate for phthalocyanine-lipid biosensor devices. The results show the suitability of the C18 ligands as an anchoring layer for the phthalocyanines. The scattering length density profiles demonstrate the effectiveness of a lipid monolayer in partitioning the composition of phthalocyanine layers from that of the bulk liquid. The effectiveness of this barrier is a critical factor in the efficiency of such devices.
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http://dx.doi.org/10.1021/la102876rDOI Listing
October 2010

Neutron reflectivity studies of critical adsorption: behavior of the surface scaling function.

Phys Rev E Stat Nonlin Soft Matter Phys 2005 Oct 28;72(4 Pt 1):041606. Epub 2005 Oct 28.

Department of Chemistry, University of Exeter, Stocker Road, Exeter EX4 4QD, United Kingdom.

Neutron reflectometry has been employed to examine the nature of the critical adsorption surface scaling function for a near-critical mixture of hexane-d14+perfluorohexane adsorbing to a solid substrate from the liquid one-phase region. The analysis method of Dietrich and Schack has been applied to examine the nature of the power-law part of the critical adsorption surface scaling function, which has been found to behave as m(z) approximately P0z(-mu) as the critical point is approached. Values of mu = 0.514+/-0.018 and P0 = 0.90+/-0.04 have been obtained. These values are consistent with theoretical expectations (mu(th) = 0.516+/-0.004; P0(th) = 0.94+/-0.05), the value determined from Monte Carlo simulations (P(MC)0 = 0.866, and other experimental determinations (P(ex)0 = 0.955=/-0.08).
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http://dx.doi.org/10.1103/PhysRevE.72.041606DOI Listing
October 2005

Neutron reflection from the liquid-liquid interface: adsorption of hexadecylphosphorylcholine to the hexadecane-aqueous solution interface.

Langmuir 2005 Dec;21(25):11704-9

Centre for Materials Research (Chemistry Department), Queen Mary, University of London, UK.

Adsorption of water-soluble, zwitterionic n-hexadecylphosphorylcholine (C(16)PC) amphiphiles has been examined at the hexadecane-aqueous solution interface using neutron reflectivity (NR) and interfacial tension measurements. The results of both methods indicate that the limiting area per surfactant molecule at the interface at the critical micelle concentration (cmc) is 40 +/- 5 Angstroms(2). In the NR measurements, two isotopic contrasts have been employed to determine the adsorption isotherm and to explore the structure of the interfacial region. Single-layer model fitting to both isotopic contrasts was only possible for the single sub-cmc concentration studied, where a film thickness of 60 +/- 5 Angstroms was obtained; consistent single-layer model fits to both contrasts for concentrations greater than the cmc were not possible, leading to the requirement of a two-layer model with an overall film thickness close to 60 +/- 2 Angstroms. This film thickness is appreciably greater than the fully extended C(16)PC molecular length and cannot be explained purely in terms of thermal broadening. A further result is that the reflectivity data indicate that, as the C(16)PC concentration increases, the amount of water on the hexadecane side of the interfacial region increases, in contrast to intuitive expectation. These findings are interpreted by conjecturing a structural model in which a trilayer of C(16)PC molecules is formed at the interface with the water concentrated in the region occupied by the headgroups.
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http://dx.doi.org/10.1021/la0518086DOI Listing
December 2005

Width of the hexadecane-water interface: a discrepancy resolved.

Langmuir 2005 Dec;21(25):11596-8

School of Biological and Chemical Sciences, Walter Besant Building, Queen Mary, University of London, UK.

An existing discrepancy in the determined values of the width of the hexadecane-water interface as determined from X-ray reflection (XR) and recent neutron reflection (NR) measurements has been resolved. The NR-determined width reported here is 6.0 +/- 1.0 Angstroms, which compares extremely well with the XR-determined value of 6.0 +/- 0.2 Angstroms. The origin of the discrepancy has been attributed to the thermal history of the interface rather than sample purity or other technique-dependent reasons.
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http://dx.doi.org/10.1021/la051809yDOI Listing
December 2005

Structural studies of amphiphiles adsorbed at liquid-liquid interfaces using neutron reflectometry.

Faraday Discuss 2005 ;129:155-67; discussion 179-92

Centre for Materials Research (Chemistry Department), Queen Mary, University of London, London, UK.

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.
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http://dx.doi.org/10.1039/b404732jDOI Listing
March 2005

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

J Chem Phys 2004 Nov;121(18):9058-65

Department of Chemistry, University of Exeter, Stocker Road, Exeter EX4 4QD, United Kingdom.

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.
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http://dx.doi.org/10.1063/1.1805501DOI Listing
November 2004