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Research paper thumbnail of The Structure of Zwitterionic Phosphocholine Surfactant Monolayers

Langmuir, 2006

The structure of a zwitterionic phosphocholine (PC) surfactant monolayer adsorbed on the surface ... more The structure of a zwitterionic phosphocholine (PC) surfactant monolayer adsorbed on the surface of water has been determined using neutron reflectivity in combination with H/D isotopic substitution. The most significant results of this study are the level of hydration of the PC headgroup and the lack of dehydration with increasing temperature and salt addition. The fraction of the alkyl chain (f(c)) immersed in water for all three chain isomers studied was found to be around 0.15, suggesting that the PC headgroup geometries influenced not only the headgroup hydration but also the degree of immersion of the alkyl chain in water. At the critical micelle concentration (CMC), the number of water molecules associated with the PC headgroup in C(m)PC (m = 12, 14, 16) was on order of 15. This value was significantly greater than that obtained for nonionic and ionic surfactants with similar limiting area per molecule at the CMC (A(cmc)). However, the fraction of the chain immersed in water for the ionic and nonionic surfactants was much greater. This suggests that the unique surface biocompatibility of PC surfactants arises from their strong affinity for water, and the relatively low fraction of mixing with the alkyl chain arises from the higher structural order within the PC monolayer. As surface coverage decreased, the number of water molecules associated with each PC headgroup increased, but f(c) remained constant for all the surfactants. This observation was consistent with the small variation in the thickness of the headgroup region, and the entire layer changed little with surfactant concentration. This is attributed to the role of PC headgroup geometries to maintain the conformational order within the layer as packing density varies. Further structural analysis based on a kinematic approach showed that, as the chain length was increased from C12 to C14 to C16 at the CMC, the angle of tilt for the alkyl chain increased from 40 degrees to 48 degrees to 53 degrees , respectively, whereas the thickness of the whole layer and that of the PC head region was largely constant. The almost vertical projection of the PC headgroup from these single alkyl chain surfactants is in sharp contrast to its strongly tilted conformation, as reported for dichain phospholipids such as dipalmitoyl glycerol phosphocholine (DPPC).

Research paper thumbnail of Neutron Reflection from the Liquid−Liquid Interface: Adsorption of Hexadecylphosphorylcholine to the Hexadecane−Aqueous Solution Interface

Langmuir, 2005

Adsorption of water-soluble, zwitterionic n-hexadecylphosphorylcholine (C(16)PC) amphiphiles has ... more 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.

Research paper thumbnail of Biomimetic amphiphiles: Biosurfactants

Current Opinion in Colloid & Interface Science, 2007

Several types of natural and biomimetic surfactants have recently been reported. These biosurfact... more Several types of natural and biomimetic surfactants have recently been reported. These biosurfactants bear the same common structural features as conventional surfactants, but the presence of bioactive functionalities incorporates new features such as structural complementarity and biological specificity into the common character of self-assembly driven by amphiphilicity. This review provides an outline of trend in recent biosurfactant research. Whilst these

Research paper thumbnail of Molecular self-assembly and applications of designer peptide amphiphiles

Chemical Society Reviews, 2010

XU, H.; YASEEN, M.; SHAN, H.; HAUSER, C. A. E.; ZHANG, S.; LU, J. R.; Chem. ; Biol. Phys. Group, ... more XU, H.; YASEEN, M.; SHAN, H.; HAUSER, C. A. E.; ZHANG, S.; LU, J. R.; Chem. ; Biol. Phys. Group, Sch. Phys. Astron., Univ. Manchester, Manchester M13 9PL, UK; Eng.) -Lindner 49-271

Research paper thumbnail of Surface adsorption of zwitterionic surfactants: n-alkyl phosphocholines characterised by surface tensiometry and neutron reflection

Journal of Colloid and Interface Science, 2005

The surface adsorption of n-dodecyl phosphocholine (C 12 PC) has been characterised by a combined... more The surface adsorption of n-dodecyl phosphocholine (C 12 PC) has been characterised by a combined measurement of surface tension and neutron reflectivity. The critical micellar concentration (CMC) was found to be 0.91 mM at 25 • C in pure water. At the CMC, the limiting area per molecule (A cmc ) was found to be 52 ± 3 Å 2 and the surface tension (γ cmc ) to be ca. 40.0 ± 0.5 mN/m. The parallel study of chain isomer n-hexadecyl phosphocholine (C 16 PC) showed a decrease of the CMC to 0.012 mM and a drop of γ cmc to 38.1 ± 0.5 mN/m. However, A cmc for C 16 PC was found to be 54 ± 3 Å 2 , showing that increase in alkyl chain length by four methylene groups has little effect on A cmc . The almost constant A cmc suggested that the limiting area per molecule was determined by the bulky PC head group. It was further found that the surface tension and related key physical parameters did not vary much with temperature, salt addition, solution pH or any combination of these, thus showing that surface adsorption and solution aggregation from PC surfactants is largely similar to the zwitterionic betaine surfactants and is distinctly different from ionic and non-ionic surfactants. The thickness of the adsorbed monolayers measured from both dC 12 hPC and dC 16 hPC was found to be 20-22 Å at the CMC from neutron reflectivity. Neither A cmc nor layer thickness varied with alkyl chain length, indicating that as the alkyl chain length became longer it was further tilted away from the surface normal direction and the layer packing density increased. It was also observed that the thickness of the layer varied little with surfactant concentration, indicating that the average conformational orientation of the alkyl chain remained unchanged against varying surface coverage.  2005 Elsevier Inc. All rights reserved.

Research paper thumbnail of Surface structural conformations of fibrinogen polypeptides for improved biocompatibility

Biomaterials, 2010

This work reports on how incorporation of silica nanocages into poly(urethane) copolymers (PU) af... more This work reports on how incorporation of silica nanocages into poly(urethane) copolymers (PU) affects conformational orientations of adsorbed fibrinogen and how different surfaces subsequently influenced HeLa cell attachment and proliferation. Incorporation of 2 wt% silica nanocages into poly(urethane) (PU4) substantially altered the surface topography of the films and some 50% of the surface was covered with the nanocages due to their preferential exposure. AFM studies revealed the deposition of a dense protein network on the soft polymeric domains of PU4 and much reduced fibrinogen adsorption on the hard nanocage domains. As on the bare SiO 2 control surface, fibrinogen molecules adsorbed on top of the hard nanocages mainly took the dominant trinodular structures in monomeric and dimeric forms. In addition, net positively charged long a chains were prone to being hidden beneath the D domains whilst g chains predominantly remained exposed. Dynamic interfacial adsorption as probed by spectroscopic ellipsometry revealed fast changes in interfacial conformation induced by electrostatic interactions between different segments of fibrinogen and the surface, consistent with the AFM imaging. On the PU surfaces without nanocage incorporation (PUA), however, adsorbed fibrinogen molecules formed beads-like chain networks, consistent with the structure featured on the soft PU4 domains, showing very different effects of surface chemical nature. Monoclonal antibodies specific to the a and g chains showed reduced a but increased g chain binding at the silicon oxide control and PU4 surfaces, whilst on the PUA, C18 and amine surfaces (organic surface controls) the opposite binding trend was detected with a chain binding dominant, showing different fibrinogen conformations. Cell attachment studies revealed differences in cell attachment and proliferation, consistent with the different polypeptide conformations on the two types of surfaces, showing a strong preference to the extent of exposure of g chains.

Research paper thumbnail of Adsorption of single chain Zwitterionic phosphocholine surfactants: Effects of length of alkyl chain and head group linker

Biophysical Chemistry, 2005

The adsorption of a range of single chain zwitterionic phosphocholine surfactants (C(n)P(m)C) at ... more The adsorption of a range of single chain zwitterionic phosphocholine surfactants (C(n)P(m)C) at the air/liquid interface has been studied by a combination of surface tension and neutron reflectivity. The critical micellar concentration (CMC) for C(n)PC (or C(n)P(2)C), where n varied from 12, 14 to 16, was found to be 0.91, 0.14, and 1.2 x 10(-2) mM respectively, and followed the same trend as observed for other zwitterionic and non-ionic surfactants. The area per molecule at the CMC, A(cmc), for C(n)PC was found to remain constant between 50 and 53 A(2), indicating that the increase in the alkyl chain length had little effect on A(cmc) at the interface. The neutron reflection measurement also showed an almost constant layer thickness (tau) of 20+/-2 A from all the alkyl chain deuterated PC surfactants (dC(n)hPC) in null reflecting water (NRW), suggesting that the alkyl chains of the surfactant responded to changes in either chain length or solution concentration by varying their angle of tilt. In contrast, increasing the length of head group linker between P and N atoms in C(12)P(m)C, where m=2, 4, to 6, resulted in a much slower decrease of CMC from 0.91, 0.7, to 0.5 mM, consistent with a different contribution to the free energy of micellization. A(cmc) for C(12)P(m)C did not vary when m was increased from 2 to 4, and this observation together with the thickness of the head group region indicated an almost perpendicular projection of the head group in C(12)P(2)C and C(12)P(4)C. A further increase in m to 6 resulted in an A(cmc) of 70 A(2). This increase in A(cmc) however did not result in any change in either the total layer thickness or the fraction of the head group region submerged in the aqueous subphase, suggesting that the head group in C(12)P(6)C was bent away from the surface normal direction. Both increase in temperature from 25 to 40 degrees C and the addition of 0.1 M NaCl had little effect on the area per molecule or the thickness of C(12)P(m)C surfactant layer, showing that the C(12)P(m)C series behaved like C(n)P(2)C series. The main conclusion from this study is that for all the C(n)P(m)C surfactants studied, change in m or n has little effect on the total thickness, the thickness of the alkyl chain or that of the head group region.

Research paper thumbnail of Protein adsorption studied by neutron reflection

Current Opinion in Colloid & Interface Science, 2007

This article reviews recent development in the use of neutron reflection to study protein adsorpt... more This article reviews recent development in the use of neutron reflection to study protein adsorption. The phenomenon of protein adsorption will be introduced, followed by the outline of the technological relevance of the interfacial molecular events. As lysozyme and ...

Research paper thumbnail of ALL TII-11-811

This paper presents a new digital control scheme for a standalone photovoltaic (PV) system using ... more This paper presents a new digital control scheme for a standalone photovoltaic (PV) system using fuzzy-logic and a dual maximum power point tracking (MPPT) controller. The first MPPT controller is an astronomical two-axis sun tracker, which is designed to track the sun over both the azimuth and elevation angles and obtain maximum solar radiation at all times. The second MPPT algorithm controls the power converter between the PV panel and the load and implements a new fuzzylogic (FLC) based perturb and observe (P&O) scheme to keep the system power operating point at its maximum. The FLC-MPPT is based on a voltage control approach of the power converter with a discrete PI controller to adapt the duty cycle. The input reference voltage is adaptively perturbed with variable steps till the maximum power is reached. The proposed control scheme achieves stable operation in the entire region of the PV panel and eliminates therefore the resulting oscillations around the maximum power operating point. A 150-Watt prototype system is used with two TMS320F28335 eZdsp boards to validate the proposed control scheme performance.

Research paper thumbnail of The Structure of Zwitterionic Phosphocholine Surfactant Monolayers

Langmuir, 2006

The structure of a zwitterionic phosphocholine (PC) surfactant monolayer adsorbed on the surface ... more The structure of a zwitterionic phosphocholine (PC) surfactant monolayer adsorbed on the surface of water has been determined using neutron reflectivity in combination with H/D isotopic substitution. The most significant results of this study are the level of hydration of the PC headgroup and the lack of dehydration with increasing temperature and salt addition. The fraction of the alkyl chain (f(c)) immersed in water for all three chain isomers studied was found to be around 0.15, suggesting that the PC headgroup geometries influenced not only the headgroup hydration but also the degree of immersion of the alkyl chain in water. At the critical micelle concentration (CMC), the number of water molecules associated with the PC headgroup in C(m)PC (m = 12, 14, 16) was on order of 15. This value was significantly greater than that obtained for nonionic and ionic surfactants with similar limiting area per molecule at the CMC (A(cmc)). However, the fraction of the chain immersed in water for the ionic and nonionic surfactants was much greater. This suggests that the unique surface biocompatibility of PC surfactants arises from their strong affinity for water, and the relatively low fraction of mixing with the alkyl chain arises from the higher structural order within the PC monolayer. As surface coverage decreased, the number of water molecules associated with each PC headgroup increased, but f(c) remained constant for all the surfactants. This observation was consistent with the small variation in the thickness of the headgroup region, and the entire layer changed little with surfactant concentration. This is attributed to the role of PC headgroup geometries to maintain the conformational order within the layer as packing density varies. Further structural analysis based on a kinematic approach showed that, as the chain length was increased from C12 to C14 to C16 at the CMC, the angle of tilt for the alkyl chain increased from 40 degrees to 48 degrees to 53 degrees , respectively, whereas the thickness of the whole layer and that of the PC head region was largely constant. The almost vertical projection of the PC headgroup from these single alkyl chain surfactants is in sharp contrast to its strongly tilted conformation, as reported for dichain phospholipids such as dipalmitoyl glycerol phosphocholine (DPPC).

Research paper thumbnail of Neutron Reflection from the Liquid−Liquid Interface: Adsorption of Hexadecylphosphorylcholine to the Hexadecane−Aqueous Solution Interface

Langmuir, 2005

Adsorption of water-soluble, zwitterionic n-hexadecylphosphorylcholine (C(16)PC) amphiphiles has ... more 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.

Research paper thumbnail of Biomimetic amphiphiles: Biosurfactants

Current Opinion in Colloid & Interface Science, 2007

Several types of natural and biomimetic surfactants have recently been reported. These biosurfact... more Several types of natural and biomimetic surfactants have recently been reported. These biosurfactants bear the same common structural features as conventional surfactants, but the presence of bioactive functionalities incorporates new features such as structural complementarity and biological specificity into the common character of self-assembly driven by amphiphilicity. This review provides an outline of trend in recent biosurfactant research. Whilst these

Research paper thumbnail of Molecular self-assembly and applications of designer peptide amphiphiles

Chemical Society Reviews, 2010

XU, H.; YASEEN, M.; SHAN, H.; HAUSER, C. A. E.; ZHANG, S.; LU, J. R.; Chem. ; Biol. Phys. Group, ... more XU, H.; YASEEN, M.; SHAN, H.; HAUSER, C. A. E.; ZHANG, S.; LU, J. R.; Chem. ; Biol. Phys. Group, Sch. Phys. Astron., Univ. Manchester, Manchester M13 9PL, UK; Eng.) -Lindner 49-271

Research paper thumbnail of Surface adsorption of zwitterionic surfactants: n-alkyl phosphocholines characterised by surface tensiometry and neutron reflection

Journal of Colloid and Interface Science, 2005

The surface adsorption of n-dodecyl phosphocholine (C 12 PC) has been characterised by a combined... more The surface adsorption of n-dodecyl phosphocholine (C 12 PC) has been characterised by a combined measurement of surface tension and neutron reflectivity. The critical micellar concentration (CMC) was found to be 0.91 mM at 25 • C in pure water. At the CMC, the limiting area per molecule (A cmc ) was found to be 52 ± 3 Å 2 and the surface tension (γ cmc ) to be ca. 40.0 ± 0.5 mN/m. The parallel study of chain isomer n-hexadecyl phosphocholine (C 16 PC) showed a decrease of the CMC to 0.012 mM and a drop of γ cmc to 38.1 ± 0.5 mN/m. However, A cmc for C 16 PC was found to be 54 ± 3 Å 2 , showing that increase in alkyl chain length by four methylene groups has little effect on A cmc . The almost constant A cmc suggested that the limiting area per molecule was determined by the bulky PC head group. It was further found that the surface tension and related key physical parameters did not vary much with temperature, salt addition, solution pH or any combination of these, thus showing that surface adsorption and solution aggregation from PC surfactants is largely similar to the zwitterionic betaine surfactants and is distinctly different from ionic and non-ionic surfactants. The thickness of the adsorbed monolayers measured from both dC 12 hPC and dC 16 hPC was found to be 20-22 Å at the CMC from neutron reflectivity. Neither A cmc nor layer thickness varied with alkyl chain length, indicating that as the alkyl chain length became longer it was further tilted away from the surface normal direction and the layer packing density increased. It was also observed that the thickness of the layer varied little with surfactant concentration, indicating that the average conformational orientation of the alkyl chain remained unchanged against varying surface coverage.  2005 Elsevier Inc. All rights reserved.

Research paper thumbnail of Surface structural conformations of fibrinogen polypeptides for improved biocompatibility

Biomaterials, 2010

This work reports on how incorporation of silica nanocages into poly(urethane) copolymers (PU) af... more This work reports on how incorporation of silica nanocages into poly(urethane) copolymers (PU) affects conformational orientations of adsorbed fibrinogen and how different surfaces subsequently influenced HeLa cell attachment and proliferation. Incorporation of 2 wt% silica nanocages into poly(urethane) (PU4) substantially altered the surface topography of the films and some 50% of the surface was covered with the nanocages due to their preferential exposure. AFM studies revealed the deposition of a dense protein network on the soft polymeric domains of PU4 and much reduced fibrinogen adsorption on the hard nanocage domains. As on the bare SiO 2 control surface, fibrinogen molecules adsorbed on top of the hard nanocages mainly took the dominant trinodular structures in monomeric and dimeric forms. In addition, net positively charged long a chains were prone to being hidden beneath the D domains whilst g chains predominantly remained exposed. Dynamic interfacial adsorption as probed by spectroscopic ellipsometry revealed fast changes in interfacial conformation induced by electrostatic interactions between different segments of fibrinogen and the surface, consistent with the AFM imaging. On the PU surfaces without nanocage incorporation (PUA), however, adsorbed fibrinogen molecules formed beads-like chain networks, consistent with the structure featured on the soft PU4 domains, showing very different effects of surface chemical nature. Monoclonal antibodies specific to the a and g chains showed reduced a but increased g chain binding at the silicon oxide control and PU4 surfaces, whilst on the PUA, C18 and amine surfaces (organic surface controls) the opposite binding trend was detected with a chain binding dominant, showing different fibrinogen conformations. Cell attachment studies revealed differences in cell attachment and proliferation, consistent with the different polypeptide conformations on the two types of surfaces, showing a strong preference to the extent of exposure of g chains.

Research paper thumbnail of Adsorption of single chain Zwitterionic phosphocholine surfactants: Effects of length of alkyl chain and head group linker

Biophysical Chemistry, 2005

The adsorption of a range of single chain zwitterionic phosphocholine surfactants (C(n)P(m)C) at ... more The adsorption of a range of single chain zwitterionic phosphocholine surfactants (C(n)P(m)C) at the air/liquid interface has been studied by a combination of surface tension and neutron reflectivity. The critical micellar concentration (CMC) for C(n)PC (or C(n)P(2)C), where n varied from 12, 14 to 16, was found to be 0.91, 0.14, and 1.2 x 10(-2) mM respectively, and followed the same trend as observed for other zwitterionic and non-ionic surfactants. The area per molecule at the CMC, A(cmc), for C(n)PC was found to remain constant between 50 and 53 A(2), indicating that the increase in the alkyl chain length had little effect on A(cmc) at the interface. The neutron reflection measurement also showed an almost constant layer thickness (tau) of 20+/-2 A from all the alkyl chain deuterated PC surfactants (dC(n)hPC) in null reflecting water (NRW), suggesting that the alkyl chains of the surfactant responded to changes in either chain length or solution concentration by varying their angle of tilt. In contrast, increasing the length of head group linker between P and N atoms in C(12)P(m)C, where m=2, 4, to 6, resulted in a much slower decrease of CMC from 0.91, 0.7, to 0.5 mM, consistent with a different contribution to the free energy of micellization. A(cmc) for C(12)P(m)C did not vary when m was increased from 2 to 4, and this observation together with the thickness of the head group region indicated an almost perpendicular projection of the head group in C(12)P(2)C and C(12)P(4)C. A further increase in m to 6 resulted in an A(cmc) of 70 A(2). This increase in A(cmc) however did not result in any change in either the total layer thickness or the fraction of the head group region submerged in the aqueous subphase, suggesting that the head group in C(12)P(6)C was bent away from the surface normal direction. Both increase in temperature from 25 to 40 degrees C and the addition of 0.1 M NaCl had little effect on the area per molecule or the thickness of C(12)P(m)C surfactant layer, showing that the C(12)P(m)C series behaved like C(n)P(2)C series. The main conclusion from this study is that for all the C(n)P(m)C surfactants studied, change in m or n has little effect on the total thickness, the thickness of the alkyl chain or that of the head group region.

Research paper thumbnail of Protein adsorption studied by neutron reflection

Current Opinion in Colloid & Interface Science, 2007

This article reviews recent development in the use of neutron reflection to study protein adsorpt... more This article reviews recent development in the use of neutron reflection to study protein adsorption. The phenomenon of protein adsorption will be introduced, followed by the outline of the technological relevance of the interfacial molecular events. As lysozyme and ...

Research paper thumbnail of ALL TII-11-811

This paper presents a new digital control scheme for a standalone photovoltaic (PV) system using ... more This paper presents a new digital control scheme for a standalone photovoltaic (PV) system using fuzzy-logic and a dual maximum power point tracking (MPPT) controller. The first MPPT controller is an astronomical two-axis sun tracker, which is designed to track the sun over both the azimuth and elevation angles and obtain maximum solar radiation at all times. The second MPPT algorithm controls the power converter between the PV panel and the load and implements a new fuzzylogic (FLC) based perturb and observe (P&O) scheme to keep the system power operating point at its maximum. The FLC-MPPT is based on a voltage control approach of the power converter with a discrete PI controller to adapt the duty cycle. The input reference voltage is adaptively perturbed with variable steps till the maximum power is reached. The proposed control scheme achieves stable operation in the entire region of the PV panel and eliminates therefore the resulting oscillations around the maximum power operating point. A 150-Watt prototype system is used with two TMS320F28335 eZdsp boards to validate the proposed control scheme performance.