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Papers by Khawla Qamhieh

Effect of Dielectric Constant on the Zeta Potential of Spherical Electric Double Layers

Effect of discrete macroion charge distributions in solutions of like-charged macroions

Journal of Chemical Physics, Sep 8, 2005

The effect of replacing the conventional uniform macroion surface charge density with discrete ma... more The effect of replacing the conventional uniform macroion surface charge density with discrete macroion charge distributions on structural properties of aqueous solutions of like-charged macroions has been investigated by Monte Carlo simulations. Two discrete charge distributions have been considered: point charges localized on the macroion surface and finite-sized charges protruding into the solution. Both discrete charge distributions have been examined with fixed and mobile macroion charges. Different boundary conditions have been applied to examine various properties. With point charges localized on the macroion surface, counterions become stronger accumulated to the macroion and the effect increases with counterion valence. As a consequence, with mono- and divalent counterions the potential of mean force between two macroions becomes less repulsive and with trivalent counterions more attractive. With protruding charges, the excluded volume effect dominates over the increased correlation ability; hence the counterions are less accumulated near the macroions and the potential of mean force between two macroions becomes more repulsive/less attractive.

Electrostatic interactions between cationic dendrimers and anionic model biomembrane

Chemistry and Physics of Lipids, Aug 1, 2022

Effect of Discrete Macroion Charge Distributions on Electric Double Layer of a Spherical Macroion

Journal of Dispersion Science and Technology, Nov 2, 2013

ABSTRACT The effect of replacing the conventional uniform macroion surface charge density with di... more ABSTRACT The effect of replacing the conventional uniform macroion surface charge density with discrete macroion charge distributions on the structure of electric double layer (EDL) of a spherical macroion has been investigated by Monte Carlo (MC) simulations. Two discrete models have been investigated in addition to the central macroion charge: point charges localized on the macroion surface and finite-sized charges protruding into the solution. Both models have been studied with fixed and mobile macroion charges. The radial functions of local densities and electrostatic potential in EDL, are calculated and compared to the results obtained for the central macroion charge distribution. It is concluded that the model of charge distribution significantly affects the EDL structure close to the macroion, while the effect is much weaker at larger distances. With point charges localized on the macroion surface, counterions become stronger accumulated to the macroion, as a result the absolute values of surface potential Φ0 and zeta ξ potential are decreased. With protruding charges, the excluded volume effect dominates over the increased correlation ability; hence the counterions are less accumulated near the macroions and the absolute values of Φ0 and ξ potentials are increased.

Analytical model study of complexation of dendrimer as an ion penetrable sphere with DNA

Colloids and Surfaces A: Physicochemical and Engineering Aspects, Feb 1, 2014

ABSTRACT Poly (amidoamine) (PAMAM) dendrimers, being protonated under physiological conditions, h... more ABSTRACT Poly (amidoamine) (PAMAM) dendrimers, being protonated under physiological conditions, have great potential as nonviral vectors for gene transfection. Many experimental and simulation studies have been performed to study the effect of dendrimer size, charge, and salt concentration on the structure and transfection efficiency of condensed DNA aggregates. In this study a theoretical model describing a linear polyelectrolyte (LPE) and ion-penetrable spheres has been developed and applied to investigate the interaction between linearized DNA and positively charged dendrimer of different generations. Throughout the study, we emphasized on the effect of the medium’s environments on the complexation of LPE chain with one dendrimer. It is found that the wrapping degree of the chain around the dendrimer increases by increasing dendrimer’s charge (decreasing pH), Bjerum length, salt concentration, and decreases by increasing the rigidity of the chain. Also, the effect of 1:1 salt concentration on complexation of DNA plasmids with one dendrimer of different generations has been investigated, and found that the optimal wrapping length of the LPE chain around the dendrimer depends on dendrimer generation.

PubMed, Mar 8, 2009

The details of melting of DNA immobilized on a chip or nanoparticle determines the sensitivity an... more The details of melting of DNA immobilized on a chip or nanoparticle determines the sensitivity and operating characteristics of many analytical and synthetic biotechnological devices. Yet, little is known about the differences in how the DNA melting occurs between a homogeneous solution and that on a chip. We used molecular dynamics simulations to explore possible pathways for DNA melting on a chip. Simulation conditions were chosen to ensure that melting occurred in a submicrosecond timescale. The temperature was set to 400 K and the NaCl concentration was set to 0.1 M. We found less symmetry than in the solution case where for oligomeric double-stranded nucleic acids both ends melted with roughly equal probability. On a prepared silica surface we found melting is dominated by fraying from the end away from the surface. Strand separation was hindered by nonspecific surface adsorption at this temperature. At elevated temperatures the melted DNA was attracted to even uncharged organically coated surfaces demonstrating surface fouling. While hybridization is not the simple reverse of melting, this simulation has implications for the kinetics of hybridization.

Microemulsion Phase Behavior of Aerosol-OT Combined with a Cationic Hydrotrope in the Dilute Region

Journal of Dispersion Science and Technology, Jan 21, 2010

The phase behavior of systems containing minimum amounts of sodium bis (2-ethylhexyl) sulfosuccin... more The phase behavior of systems containing minimum amounts of sodium bis (2-ethylhexyl) sulfosuccinate with equimolar ratio of tetra ethyl ammonium chloride were studied as a function of salt concentration and alkane carbon number at ambient temperature. Visual inspection as well as cross polarizers were used to detect anisotropy. Solubilization ratios for oil and brine in the middle phases were measured

Physical Chemistry Chemical Physics, 2014

Please note that technical editing may introduce minor changes to the text and/or graphics, which... more Please note that technical editing may introduce minor changes to the text and/or graphics, which may alter content. The journal's standard Terms & Conditions and the Ethical guidelines still apply. In no event shall the Royal Society of Chemistry be held responsible for any errors or omissions in this Accepted Manuscript or any consequences arising from the use of any information it contains.

Journal of Dispersion Science and Technology, Feb 10, 2014

Bicontinuous and water-in-diesel microemulsions were formulated using single nonionic alkyl poly ... more Bicontinuous and water-in-diesel microemulsions were formulated using single nonionic alkyl poly glycol ethers combined with hydrophilic alcohol ethoxylates. The phase behavior at temperatures ranging from 0 C to 50 C was investigated. Visual inspection as well as cross-polarizers were used to detect anisotropy. The fish phase diagrams were determined. The presence of the hydrophilic alcohol ethoxylates was necessary to initiate both types of microemulsions. Increasing the hydrophobic chain length of the surfactant led to a wider range of temperature stability at lower surfactant concentration. Meanwhile, increasing the ethylene oxide units in the headgroup by two units led to a phase diagram that is dominated by lyotropic liquid crystal. The formulated water in diesel microemulsions were tested experimentally in a 4-cylinder diesel engine. From this it is observed that the emissions of NO x , soot, and CO 2 were reduced substantially compared to neat diesel, while for the CO the reduction occurs just at low load.

Biomacromolecules, May 13, 2009

The interaction between positively charged poly(amido amine) (PAMAM) dendrimers of generation 4 a... more The interaction between positively charged poly(amido amine) (PAMAM) dendrimers of generation 4 and DNA has been investigated for two DNA lengths; 2000 basepairs (bp; L) 680 nm) and 4331 bp (L) 1472.5 nm) using a theoretical model by Schiessel for a semiflexible polyelectrolyte and hard spheres. The model was modified to take into account that the dendrimers are to be regarded as soft spheres, that is, the radius is not constant when the DNA interact with the dendrimer. For the shorter and longer DNA, the estimated optimal wrapping length, l opt is ≈15.69 and ≈12.25 nm, respectively, for dendrimers that retain their original size (R o) 2.25 nm) upon DNA interaction. However, the values of l opt for the dendrimers that were considered to have a radius of (R) 0.4R o) 0.9 nm were 9.3 and 9.4 nm for the short and long DNA, respectively, and the effect due to the DNA length is no longer observed. For l opt) 10.88 nm, which is the length needed to neutralize the 64 positive charges of the G4 dendrimer, the maximum number of dendrimers per DNA (N max) was ≈76 for the shorter DNA, which is larger than the corresponding experimental value of 35 for 2000 bp DNA. For the longer DNA, N max ≈ 160, which is close to the experimental value of 140 for the 4331 bp DNA. Charge inversion of the dendrimer is only observed when they retain their size or only slightly contract upon DNA interaction.

Controlling microarray DNA hybridization efficiency by probe-surface distance and external surface electrostatics

Nucleation and Atmospheric Aerosols, 2015

DNA microarrays are analytical devices designed to determine the composition of multicomponent so... more DNA microarrays are analytical devices designed to determine the composition of multicomponent solutions of nucleic acids, DNA or RNA. These devices are promising technology for diverse applications, including sensing, diagnostics, and drug/gene delivery. Here, we modify a hybridization adsorption isotherm to study the effects of probe-surface distance and the external electrostatic fields, on the oligonucleotide hybridization in microarray and how these effects are varies depending on surface probe density and target concentration. This study helps in our understanding on-surface hybridization mechanisms, and from it we can observe a significant effect of the probe-surface distance, and the external electrostatic fields, on the hybridization yield. In addition we present a simple new criteria to control the oligonucleotide hybridization efficiency by providing a chart illustrating the effects of all factors on the DNA-hybridization efficiency.

Formulating Middle Phase Microemulsions Using Extended Anionic Surfactant Combined with Cationic Hydrotrope

Journal of Dispersion Science and Technology, Dec 21, 2010

The phase behavior of systems containing sodium alkyl polypropylene oxide sulfate with equimolar ... more The phase behavior of systems containing sodium alkyl polypropylene oxide sulfate with equimolar ratio of polypropylene oxide ammonium chloride was determined as a function of salt concentration and alkane carbon number at ambient temperature. Visual inspection as well as cross polarizers were used to detect anisotropy. Solubilization ratios for oil and brine in the middle phases were measured and used to calculate the interfacial tension (IFT) between the microemulsion and oil or brine. The fish diagram is presented showing a minimum amount of 0.20 wt% surfactant needed to form the middle phase. Swelling increases with decreasing the alkane carbon number of the oil. Ultra low IFT values were predicted for the systems investigated.

Soft Matter, 2018

The osmotic pressure of polyelectrolyte solutions as a function of concentration has been calcula... more The osmotic pressure of polyelectrolyte solutions as a function of concentration has been calculated by Monte Carlo simulations of a spherical cell model and by molecular dynamics simulations with periodic boundary conditions. The results for the coarse-grained polyelectrolyte model are in good agreement with experimental results for sodium polyacrylate and the cell model is validated by the bulk simulations. The cell model offers an alternative perspective on osmotic pressure and also forms a direct link to even simpler models in the form of the Poisson-Boltzmann approximation applied to cylindrical and spherical geometries. As a result, the non-monotonic behaviour of the osmotic coefficient seen in simulated salt-free solutions is shown not to rely on a transition between a dilute and semi-dilute regime, as is often suggested when the polyion is modelled as a linear flexible chain. The non-monotonic behaviour is better described as the combination of a finite-size effect and a double-layer effect. Parameters that represent the linear nature of the polyion, including an alternative to monomer concentration, make it possible to display a generalised behaviour of equivalent chains, at least at low concentrations. At high concentrations, local interactions become significant and the exact details of the model become important. The effects of added salt are also discussed and one conclusion is that the empirical additivity rule, treating the contributions from the polyelectrolyte and any salt separately, is a reasonable approximation, which justifies the study of salt-free solutions.

Journal of Dispersion Science and Technology, Jul 29, 2016

Electrical conductivity of fuel microemulsion composed of diesel, pentanol, water and sucrose lau... more Electrical conductivity of fuel microemulsion composed of diesel, pentanol, water and sucrose laurate as surfactant was investigated over a wide range of water content varying from 0 to 90 wt.% and temperature varying from 10 to 50ºC. Conductivity measurements were performed on samples, the composition of which lie along the one phase channel using conductivity meter. Activation energy of conduction flow was evaluated. The hydrodynamic radius as a function of temperature in the aqueous phase rich region (90 wt.%) was measured using dynamic light scattering method (DLS). The microstructure of microemulsion was further investigated by NMR diffusometry by which the selfdiffusion coefficients for water were determined at 25 o C. Electrical conductivity increases with water content up to 40 wt.% and percolation threshold was observed, and then stabilizes between 40 and 80 wt.% then decreases. Percolation threshold temperature at constant composition was monitored as 36ºC for water contents below 80 wt.% and 34ºC for water contents above. As predicted by the conductivity measurements, the determined self-diffusion coefficients of water confirmed the structural transition from discrete water in oil droplets to bicontinuous phase and finally to oil in water droplet microemulsion.

Journal of Physical Chemistry B, Jun 28, 2003

We study electrostatic mechanisms of destabilization of highly asymmetric electrolytes. For this ... more We study electrostatic mechanisms of destabilization of highly asymmetric electrolytes. For this purpose, we perform primitive model Monte Carlo simulations of charged macroions immersed in multivalent salt solution. At low salt concentration, the macroion effective charge is reduced due to multivalent counterion adsorption. At high salt concentrations, the macroions become overcharged so that their apparent charge has the opposite sign to the stoichiometric one. The inverted charge is growing up to a saturation value upon further increasing the salinity. The system remains stable at low as well as at very high salt concentrations. In the intermediate region, close to the macroion isoelectric point, we observe macroion aggregation. The obtained phase behaviour closely resembles polyelectrolyte-induced instability of colloidal dispersions.

Journal of Dispersion Science and Technology, Apr 1, 2012

Alcohol-free microemulsions were formulated using mixtures of extended surfactant (C 12-14-PO 14-... more Alcohol-free microemulsions were formulated using mixtures of extended surfactant (C 12-14-PO 14-EO 2 SO 4 Na), sodium dodecyl benzene sulfonic acid and cationic hydrotropes with equal amounts of water and diesel. The cationic hydrotropes had short hydrocarbon or propylene oxide chain. The formulation included sodium carbonate to convert naphthenic acids in diesel to soaps. The phase behavior at ambient temperature of oil-free mixtures as a function of NaCl concentration was investigated. Visual inspection as well as cross polarizers were used to detect anisotropy. The microemulsion fish phase diagram and solubilization ratios for diesel and brine in the middle phases were determined. The minimum surfactant concentration needed to initiate middle phase formation was 0.10 wt%. Salinity scans revealed that optimal salinity can be adjusted according to the hydrophilic/ lipophilic nature of the hydrotrope used. Interfacial tension measurements using a spinning drop tensiometer showed a minimum value of 0.0015 mN/m between middle phase microemulsion and excess brine and a value of 0.032 mN/m between diesel and brine.

Effect of Type of Fragrance Compounds on Their Location in Hexagonal Liquid Crystal

Journal of Dispersion Science and Technology, Dec 1, 2006

... Three fragrances, limonene, benzaldehyde, and phenethylalcohol (that differ in their polariti... more ... Three fragrances, limonene, benzaldehyde, and phenethylalcohol (that differ in their polarities), were solubilized in hexagonal liquid crystal using the polyethylene oxide‐polypropylene oxide‐polyethylene oxide, (EO 27 PO 61 EO 27 ), P 104 triblock copolymer, and water. ...

The retention of multivalent pollutants in mineral layers

Colloids and Surfaces A: Physicochemical and Engineering Aspects, 2022

Journal of Analytical & Pharmaceutical Research

Gene therapies are receiving a revival, especially because of the potential for use in cancer and... more Gene therapies are receiving a revival, especially because of the potential for use in cancer and infectious disease vaccination. However, effectively condensing and maintaining deoxyribonucleic acid (DNA) carriers for gene therapy are used, such as dendrimers. The penetrable sphere model developed by Qamhieh et al., explain the interaction between linear polyelectrolyte (LPE) and an ion-penetrable sphere was employed to study the complexation of negatively charged DNA and acetylated dendrimer. Throughout the study, we emphasized the effect of acetylation on two types of complexations: single dendrimer-DNA chain complex and multiple dendrimers-DNA chain complex. The interaction between three different DNA lengths: L = 90 nm of 265; L = 184 nm of 541 bp; and L = 680 nm of 2000 bp individually with the poly (amido amine) (PAMAM) dendrimer of generation 5 was studied. For a single dendrimer-DNA complex, the number of condensed monomers around the dendrimer and the fraction of optimal l...

Journal of Physics: Conference Series, 2017

Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licenc... more Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.