56 research outputs found
Interaction Mechanism between Hydrophobic and Hydrophilic Surfaces: Using Polystyrene and Mica as a Model System
The interactions between hydrophobic
and hydrophilic molecules,
particles, or surfaces occur in many biological phenomena and industrial
processes. In this work, polystyrene (PS) and mica were chosen as
a model system to investigate the interaction mechanism between hydrophilic
and hydrophobic surfaces. Using a surface forces apparatus (SFA) coupled
with a top-view optical microscope, interaction forces between PS
and mica surfaces were directly probed in five different electrolyte
solutions (i.e., NaCl, CaCl<sub>2</sub>, NaOH, HCl, and CH<sub>3</sub>COOH) of various concentrations. Long-range repulsion was observed
in low electrolyte concentration (e.g., 0.001 M) which was mainly
due to the presence of microscopic and submicroscopic bubbles on PS
surface. A modified Derjaguin–Landau–Verwey–Overbeek
(DLVO) theory well fits the interaction forces by taking into account
the effect of bubbles on PS surface. The range of the repulsion was
dramatically reduced in 1.0 M solutions of NaCl, CaCl<sub>2</sub>,
and NaOH but did not significantly change in 1.0 M HCl and CH<sub>3</sub>COOH, which was due to ion specificity effect on the formation
and stability of bubbles on PS surface. The range of repulsion was
also significantly reduced to <20 nm in degassed electrolyte solutions.
UV-ozone treatment changed the hydrophobic attraction of the untreated
PS–PS system to pure repulsion between untreated PS and treated
PS, demonstrating the important role of surface hydrophobicity on
the formation and stability of bubbles on substrates. Our results
indicate that DLVO forces dominate the interaction between hydrophilic
surface (i.e., mica) and hydrophobic polymer (i.e., PS), while the
types of electrolytes (ion specificity), electrolyte concentration,
degassing, and surface hydrophobicity can significantly affect the
formation and stability of bubbles on the interacting surfaces, thus
affecting the range and magnitude of the interaction forces
Interfacial Hydrogen Bond-Reinforced Adhesion and Cohesion Enabling an Ultrastretchable and Wet Adhesive Hydrogel Strain Sensor
Historically, research on silicotungstic-acid–based
hydrogels
has primarily focused on their adhesive properties, often at the expense
of mechanical strength (cohesion). In this study, we present a novel
approach to fabricate a polysaccharide hydrogel that harmoniously
balances both adhesion and cohesion via interfacial hydrogen bonds.
This hydrogel, composed of carboxymethyl cellulose (CMC), polyacrylamide
(PAM), silicotungstic acid (SiW), and lithium chloride (LiCl), showcases
a unique combination of properties: strain-responsive ionic conductivity,
superior transparency, remarkable stretchability, and robust adhesion.
Contrary to conventional PAM hydrogels, our PAM-SiW networked hydrogel
addresses the common challenge of achieving good adhesion without
compromising on cohesion. Specifically, our hydrogel demonstrates
a maximum toughness of 20.3 MJ/m3 and a strain of 4079%,
an accomplishment rarely observed in other adhesive hydrogel. Furthermore,
the hydrogel’s adhesion is both reversible and versatile, adhering
effectively to a variety of wet and dry substrates. This makes it
a promising candidate for advanced healthcare applications, particularly
as a mechanically reinforced underwater adhesive with unparalleled
stability. We also provide insights into the role of LiCl in the hydrogel
matrix, emphasizing its influence on electrostatic interactions without
affecting the hydrogen bonds. This study serves as a testament to
the potential of harmonizing adhesive and cohesive properties in hydrogels,
paving the way for future innovations in the field
Understanding Copper Activation and Xanthate Adsorption on Sphalerite by Time-of-Flight Secondary Ion Mass Spectrometry, X‑ray Photoelectron Spectroscopy, and in Situ Scanning Electrochemical Microscopy
In
situ scanning electrochemical microscopy (SECM) was applied
for the first time to study the copper activation and subsequent xanthate
adsorption on sphalerite. The corresponding surface compositions were
analyzed by time-of-flight secondary ion mass spectrometry (ToF-SIMS)
and X-ray photoelectron spectroscopy (XPS). The probe approach curve
(PAC) using SECM shows that unactivated and activated sphalerite surfaces
have negative current feedback and partially positive current feedback,
respectively, suggesting that Cu<sub><i>x</i></sub>S is
formed on the sphalerite after copper activation. The copper activation
of sphalerite strongly depends on the surface heterogeneity (e.g.,
presence of polishing defects, chemical composition), impacting the
subsequent xanthate adsorption process. The SECM, ToF-SIMS, and XPS
analyses show that during the copper activation the polishing defects,
which have high excess surface energy, tend to consume more copper
ions, resulting in Cu-rich regions by forming CuS-like species, while
Fe oxide/hydroxide forms at Fe-rich regions. The XPS spectra further
confirm that the CuS-like species involve CuÂ(I) and SÂ(−I).
The SECM imaging shows that after xanthate adsorption the current
response at the Cu-rich regions decreases because of the formation
of cuprous xanthate (CuX) and dixanthogen (X<sub>2</sub>) while increases
at the Fe-rich regions mainly due to the chemisorption of xanthate
on Fe oxide/hydroxide. Our results shed light on the fundamental understanding
of the electrochemical processes on sphalerite surface associated
with its copper activation and subsequent xanthate adsorption in flotation
Interfacial Hydrogen Bond-Reinforced Adhesion and Cohesion Enabling an Ultrastretchable and Wet Adhesive Hydrogel Strain Sensor
Historically, research on silicotungstic-acid–based
hydrogels
has primarily focused on their adhesive properties, often at the expense
of mechanical strength (cohesion). In this study, we present a novel
approach to fabricate a polysaccharide hydrogel that harmoniously
balances both adhesion and cohesion via interfacial hydrogen bonds.
This hydrogel, composed of carboxymethyl cellulose (CMC), polyacrylamide
(PAM), silicotungstic acid (SiW), and lithium chloride (LiCl), showcases
a unique combination of properties: strain-responsive ionic conductivity,
superior transparency, remarkable stretchability, and robust adhesion.
Contrary to conventional PAM hydrogels, our PAM-SiW networked hydrogel
addresses the common challenge of achieving good adhesion without
compromising on cohesion. Specifically, our hydrogel demonstrates
a maximum toughness of 20.3 MJ/m3 and a strain of 4079%,
an accomplishment rarely observed in other adhesive hydrogel. Furthermore,
the hydrogel’s adhesion is both reversible and versatile, adhering
effectively to a variety of wet and dry substrates. This makes it
a promising candidate for advanced healthcare applications, particularly
as a mechanically reinforced underwater adhesive with unparalleled
stability. We also provide insights into the role of LiCl in the hydrogel
matrix, emphasizing its influence on electrostatic interactions without
affecting the hydrogen bonds. This study serves as a testament to
the potential of harmonizing adhesive and cohesive properties in hydrogels,
paving the way for future innovations in the field
Impact of cranberry juice on initial adhesion of the EPS producing bacterium <i>Burkholderia cepacia</i>
<div><p>The impact of cranberry juice was investigated with respect to the initial adhesion of three isogenic strains of the bacterium <i>Burkholderia cepacia</i> with different extracellular polymeric substance (EPS) producing capacities, viz. a wild-type cepacian EPS producer PC184 and its mutant strains PC184<i>rml</i> with reduced EPS production and PC184<i>bceK</i> with a deficiency in EPS production. Adhesion experiments conducted in a parallel-plate flow chamber demonstrated that, in the absence of cranberry juice, strain PC184 had a significantly higher adhesive capacity compared to the mutant strains. In the presence of cranberry juice, the adhesive capacity of the EPS-producing strain PC184 was largely reduced, while cranberry juice had little impact on the adhesion behavior of either mutant strain. Thermodynamic modeling supported the results from adhesion experiments. Surface force apparatus (SFA) and scanning electron microscope (SEM) studies demonstrated a strong association between cranberry juice components and bacterial EPS. It was concluded that cranberry juice components could impact bacterial initial adhesion by adhering to the EPS and impairing the adhesive capacity of the cells, which provides an insight into the development of novel treatment strategies to block the biofilm formation associated with bacterial infection.</p>
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Surface Forces and Interaction Mechanisms of Emulsion Drops and Gas Bubbles in Complex Fluids
The interactions
of emulsion drops and gas bubbles in complex fluids
play important roles in a wide range of biological and technological
applications, such as programmable drug and gene delivery, emulsion
and foam formation, and froth flotation of mineral particles. In this
feature article, we have reviewed our recent progress on the quantification
of surface forces and interaction mechanisms of gas bubbles and emulsion
drops in different material systems by using several complementary
techniques, including the drop/bubble probe atomic force microscope
(AFM), surface forces apparatus (SFA), and four-roll mill fluidic
device. These material systems include the bubble–self-assembled
monolayer (SAM), bubble–polymer, bubble–superhydrophobic
surface, bubble–mineral, water-in-oil and oil-in-water emulsions
with interface-active components in oil production, and oil/water
wetting on polyelectrolyte surfaces. The bubble probe AFM combined
with reflection interference contrast microscopy (RICM) was applied
for the first time to simultaneously quantify the interaction forces
and spatiotemporal evolution of a confined thin liquid film between
gas bubbles and solid surfaces with varying hydrophobicity. The nanomechanical
results have provided useful insights into the fundamental interaction
mechanisms (e.g., hydrophobic interaction in aqueous media) at gas/water/solid
interfaces, the stabilization/destabilization mechanisms of emulsion
drops, and oil/water wetting mechanisms on solid surfaces. A long-range
hydrophilic attraction was found between water and polyelectrolyte
surfaces in oil, with the strongest attraction for polyzwitterions,
contributing to their superior water wettability in oil and self-cleaning
capability of oil contamination. Some remaining challenges and future
research directions are discussed and provided
Asialoglycoprotein Receptor-Mediated Gene Delivery to Hepatocytes Using Galactosylated Polymers
Highly
efficient, specific, and nontoxic gene delivery vector is
required for gene therapy to the liver. Hepatocytes exclusively express
asialoglycoprotein receptor (ASGPR), which can recognize and bind
to galactose or N-acetylgalactosamine. Galactosylated polymers are
therefore explored for targeted gene delivery to the liver. A library
of safe and stable galactose-based glycopolymers that can specifically
deliver genes to hepatocytes were synthesized having different architectures,
compositions, and molecular weights via the reversible addition–fragmentation
chain transfer process. The physical and chemical properties of these
polymers have a great impact on gene delivery efficacy into hepatocytes,
as such block copolymers are found to form more stable complexes with
plasmid and have high gene delivery efficiency into ASGPR expressing
hepatocytes. Transfection efficiency and uptake of polyplexes with
these polymers decreased significantly by preincubation of hepatocytes
with free asialofetuin or by adding free asialofetuin together with
polyplexes into hepatocytes. The results confirmed that polyplexes
with these polymers were taken up specifically by hepatocytes via
ASGPR-mediated endocytosis. The results from transfection efficiency
and uptake of these polymers in cells without ASGPR, such as SK Hep1
and HeLa cells, further support this mechanism. Since in vitro cytotoxicity
assays prove these glycopolymers to be nontoxic, they may be useful
for delivery of clinically important genes specifically to the liver
Structural Evolutions of ZnS Nanoparticles in Hydrated and Bare States
Suitable optoelectronic properties
and the nontoxic nature of ZnS
quantum dots capacitate exciting applications for these nanomaterials
especially in the field of biomedical imaging. However, the structural
stability of ZnS nanoparticles has been shown to be challenging since
they potentially are prone to autonomous structural evolutions in
ambient conditions. Thus, it is essential to build an understanding
about the structural evolution of ZnS nanoparticles, especially in
aqueous environment, before implementing them for in vivo applications.
In this study we compared the structure of ZnS nanoparticles relaxed
in a vacuum and in water using a classical molecular dynamics method.
Structural analyses showed that the previously observed three-phase
structure of bare nanoparticles is not formed in the hydrated state.
The bulk of hydrated nanoparticles has more crystalline structure;
however, the dynamic heterogeneity in their surface relaxation makes
them more polar compared to bare nanoparticles. This heterogeneity
is more severe in hydrated wurtzite nanoparticles, causing them to
show larger dipole moments. Analyzing the structure of water in the
first hydration shell of the surface atoms shows that water is mainly
adsorbed to the nanoparticles’ surface through Zn–O
interaction, which causes the structure of water in the first hydration
shell to be discontinuous
A Molecular Dynamics Study of the Effect of Asphaltenes on Toluene/Water Interfacial Tension: Surfactant or Solute?
A series of molecular dynamics simulations
were performed to investigate
the effects of model asphaltenes on the toluene/water interfacial
tension (IFT) under high temperature and pressure conditions. In the
absence of model asphaltenes, the toluene/water IFT monotonically
decreases with increasing temperature, whereas, with the presence
of model asphaltenes, especially at high concentrations, such monotonic
dependence no longer holds. Furthermore, in contrast with the decreasing
trend of IFT with increasing model asphaltene concentration at low
temperature (300 K), increasing concentration at high temperature
(473 K) leads to increasing IFT. This relation can even be nonmonotonic
at moderate temperatures (373 and 423 K). Through detailed analysis
on the distribution of model asphaltenes with respect to the interface,
such complex behaviors are found to result from the delicate balance
between miscibility of toluene/water phases, solubility of model asphaltenes,
and hydrogen bonds formed between water and model asphaltenes. By
increasing the temperature, the solubility of model asphaltenes in
toluene is enhanced, leading to their transition from being a surfactant
to being a solute. The effect of pressure was found to be very limited
under all model asphaltene concentrations. Our results here present,
for the first time, a complete picture of the coupled effect of (high)
temperature and asphaltene concentration on IFT, and the methodology
employed can be extended to many other two-phase or multiphase systems
in the presence of interface-active chemicals
Understanding the Effect of Secondary Structure on Molecular Interactions of Poly‑l‑lysine with Different Substrates by SFA
Nonspecific
adsorption of proteins on biomaterial surfaces challenges
the widespread application of engineered materials, and understanding
the impact of secondary structure of proteins and peptides on their
adsorption process is of both fundamental and practical importance
in bioengineering. In this work, poly-l-lysine (PLL)-based
α-helices and β-sheets were chosen as a model system to
investigate the effect of secondary structure on peptide interactions
with substrates of various surface chemistries. Circular dichroism
(CD) was used to confirm the presence of both α-helix and β-sheet
structured PLL in aqueous solutions and upon adsorption to quartz,
where these secondary structures seemed to be preserved. Atomic force
microscopy (AFM) imaging showed different surface patterns for adsorbed
α-helix and β-sheet PLL. Interactions between PLL of different
secondary structures and various substrates (i.e., PLL, Au, mica,
and polyÂ(ethylene glycol) (PEG)) were directly measured using a surface
forces apparatus (SFA). It was found that β-sheet PLL films
showed higher adsorbed layer thicknesses in general. Adhesion energies
of β-sheet versus Au and β-sheet versus β-sheet
were considerably higher than that of α-helix versus Au and
α-helix versus α-helix systems, respectively. Au and β-sheet
PLL interactions seemed to be more dependent on the salt concentration
than that of α-helix, while the presence of a grafted PEG layer
greatly diminished any attraction with either PLL structure. The molecular
interaction mechanism of peptide in different secondary structures
is discussed in terms of Derjaguin–Landau–Verwey–Overbeek
(DLVO) theory, Alexander-de Gennes (AdG) steric model and hydrogen
bonding, which provides important insight into the fundamental understanding
of the interaction mechanism between proteins and biomaterials
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