Colloids in Paints: Colloids and Interface Science, Volume 6 (Colloids and Interface Science (VCH))

Colloids and Interface Science Series

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In addition, the adhesion of pollen particles is relevant to topics as varied as pollinator ecology, transport of allergens, and atmospheric phenomena. We report the first observation of structurally-derived pressure-sensitive adhesion of a microparticle by using the sunflower pollen and stigma surfaces as a model. This strong, pressure-sensitive adhesion results from interlocking between the pollen's conical spines and the stigma's receptive papillae. Inspired by this behavior, we fabricated synthetic polymeric patterned surfaces that mimic the stigma surface's receptivity to pollen.

These soft mimics allow the magnitude of the pressure-sensitive response to be tuned by adjusting the size and spacing of surface features. These results provide an important new insight for soft material adhesion based on bio-inspired principles, namely that ornamented microparticles and micro-patterned surfaces can be designed with complementarity that enable a tunable, pressure-sensitive adhesion on the microparticle size and length scale. The article was received on 19 Nov , accepted on 04 Feb and first published on 08 Feb If you are not the author of this article and you wish to reproduce material from it in a third party non-RSC publication you must formally request permission using Copyright Clearance Center.

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Fetching data from CrossRef. This may take some time to load. Jump to main content. Jump to site search. Combining the former two equations Equations 2 and 3 , one can calculate that an uncharged particle with 1 nm in radius will travel 1. The DLS device measures D by looking at the Doppler shift of dispersed particles as they randomly move back and forth with respect to the light source. Distribution of the particle size and its average are calculated from the half-width of the peak on the scattering intensity versus light frequency curve Figure 3.

Smaller particles produce a more intensive Doppler effect than the bigger and less movable ones. Light intensity versus frequency curve with the denoted half-width of the main peak from which the diffusion coefficient is calculated. In general, quasi-random physical events could be described with the use of correlation functions, mathematical constructs designed to reflect the average time spans during which the signals followed remain correlated.

This exponentially decaying curve falls to zero at time when the particle in its movement exceeds the wavelength of the laser light. Cumulants analysis used to calculate the average D and polydispersity index PdI from the logarithmic autocorrelation curve a and a comparison of two exponentially decaying autocorrelation curves, one for a suspension medium pure water and another one for a polymeric dispersion b. Obviously, the scattering from rapidly diffusing solvent molecules remains correlated only for a very short time, inaccessible to the correlator.

Reprinted with permission of Malvern Instruments, Ltd. One of the essential factors that affect the quality and reliability of DLS analyses are impurities.

The following example demonstrates the drastic influence that they may exert. Although the light intensity represented by the former peak is only 3. In general, because the scattering intensity is the function of r 3 , a particle with 1 nm in diameter will scatter 10 6 times less light than a particle with 10 nm in radius. Particle size distribution curves expressed in terms of the intensity of the scattered light for two protein samples forming aggregates in suspension.

This sensitivity of DLS measurements to impurities is a seeming weakness that could be transformed into strength. When it comes to DLS analyses of protein solutions or suspensions, however, this technique can prove to be of great use in evaluating the stability of these systems. Since these large clumps of protein typically grow from trace amounts of markedly smaller precursor aggregates, DLS can present valuable means for detecting such segregation process long before apparent aggregation marked by visible particulates floating in the suspension has taken place.

As such, DLS presents a valuable tool for assessing the stability of protein solutions or suspensions, especially since it allows for studying protein sols directly in their formulation buffers as well as at either very low or very high concentrations 0. The latter effect, common to intracellular environments, describes altering of the properties of macromolecules in highly concentrated solutions. It is associated with the protein molecules capturing solvent molecules and increasing the effective concentration and viscosity of the medium. This, in turn, often modifies the behavior of the proteins compared to the one evidenced in test-tube assays at lower concentrations.

The central fundamental problem that the DLS faces in analyzing the particle size is the spherical particle shape approximation that it employs.

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Although common sense thinking may prompt us to assume that elongation of particles will produce a bimodal distribution of sizes, any elongation will, in fact, produce a single peak at the sizes that would correspond to the averaged particle dimension. Hence, every elongated particle will not be detected as such, but rather as a spherical one with the diameter averaged over the three axes. For this reason, it cannot be expected from elongated particles to produce multiple peaks, presumably one corresponding to their width and another to length. The aforementioned Stokes-Einstein relationship is thence used to calculate the mean particle size.

For a dispersion of rod-like particles, for example, the translational diffusion coefficient D o of the particle can be represented [ 14 ] by a linear combination of the diffusion coefficients parallel and perpendicular to the long axis of the rod, D II and D I:. For very diluted dispersions, particle interactions become negligible and the overall expression for the diffusion coefficient can be written in the following way: The length of the rod is represented with l; d is the diameter of the rod; and l ef represents an effective average particle length.

For rod-like particles with large aspect ratio, the last three terms within square brackets can be neglected. Hence, by knowing the shape and the dimensions of the particles, one can calculate the expected measured hydrodynamic radius using the spherical approximation applied by the device. Whenever one works with elongated particles, one can also apply the hydrodynamic Brenner theory to estimate the hydrodynamic radius of the particle that will be given as the output by the device. Barbara Jachimska et al. Assuming that a monolayer of water dipoles is adsorbed on each side of the molecule proteins in general are strongly hydrated , and knowing that its thickness is 0.

Then, one can apply Brenner equation: In other words, as previously noticed, by knowing the molecular shape, one can deduce details regarding the shape of the aggregates or molecules in solution, but not vice versa. In order to assess how self-assembly of colloidal entities proceeds over a reaction coordinate time, concentration of a reactant, etc. A mirrored sigmoidal curve reaching a plateau at lower decay rates would, thus, indicate attainment of a state of lower mobility, which the decay rate is the direct indicator of.

As oftentimes fibers are assembled from the initial spheres, one would detect one such drop in the decay rate, that is, the mobility or the diffusion coefficient of the particles. Widening of the fibers or their elongation would correspond to the same drop. In general, translational diffusion coefficient D may be more appropriate to plot against time or concentration while following the protein self-assembly since particle size as a parameter becomes meaningless if we know that the spherical approximation fails.

Colloids in Paints

As already mentioned, owing to charge separation effects, particles in suspension acquire surface charge, which is typically screened at a certain distance from the particle surface. An electron micrograph showing negatively charged nano-sized gold particles adsorbed on electronegative plate-shaped kaolin crystals, and the scheme depicting the negatively charged surface structure of gold nanoparticles in water.

Although kaolin platelets are negatively charged as a whole, their edges are electropositive and as such attract the gold particles onto them. Reprinted with permission from Riddick. The latter is the boundary of the sphere composed of both firmly adsorbed ions Stern layer and some of the diffusing ions surrounding the charged particle.

However, the exact location of the shear plane is an unknown feature of the electric double layer. Experiments have shown that even assuming that these two potentials are identical, especially for lyophobic surfaces, would not produce a significant error. Any differences between the two would most likely be pronounced at high surface potentials and high electrolyte concentrations due to the effect of compression of the diffusion layer of charges that leads to a higher slope of the surface potential versus distance curve between the Stern layer boundary and the slipping plane.

Distribution of counterions in the double layer surrounding a negatively charged colloidal particle. Stern layer of counterions is depicted as tightly bound to the charged particle surface. Strictly speaking, isoelectric potential is, thus, never the measure of the surface potential. Also, sometimes, particularly when polyvalent ions act as counterions, this layer of counterions may be sufficiently strong to induce the reversal of charges Figure 7a.

Examples of polyvalent ions adsorbed onto a charged particle and reversing its effective charge a , and of specific adsorption of ions of the same charge as the particle, contributing to an effective increase of the effective charge b. It is the difference between the potential of the medium considered as neutral and the potential at a certain distance from the particle surface.

It normally, but not necessarily lies further away from the Stern layer and shear plane. Whereas the former quantities correspond to certain distances from the particle surface, the latter are associated with the very particle surface. PZC is taken as equal to IEP only in the absence of ions that screen the charge on the particle surface.

First of all, it is mainly applicable for medium thicknesses of the double layer and low-to-medium ionic strengths. It also applies only when the ratio of the double layer curvature to its thickness is so high that it can be assumed that the surface is almost flat. Ionic strength plays a role in screening ion-ion interactions as well as the ones between charged colloidal particles in the solution. Namely, surface charge density is equal to: The slipping plane separates the mobile fluid from the fluid that remains attached to the particle surface and moves together with it, similar to a cloud of charges, a part of which is bound to the particle and a part of which is of diffuse character.

It can be represented as:. A simpler expression for Debye length when water is used the dispersion medium is the following:. The initially proposed model of charged particle surface in solution was by Helmholtz in , and it merely described the charge on the particle surface as balanced by an equal charge in the liquid phase. Only the subsequent upgrade of this model by invoking the diffuse nature of layer of ions due to thermal motion and its double layer structure yielded the so-called Gouy-Chapman model.

The addition of inert electrolyte compresses the diffuse layer of charged ions and co-ions around each of the dispersed and charged particles. Namely, a higher concentration of co-ions and counterions implies screening of the particle surface charge at a distance closer to the particle. As shown in Figure 8a , increasing ionic strength causes the double layer of ions around the charged particle surface to shrink owing to a higher concentration of ions in the dispersion. IEP, on the other hand, is supposed to remain the same, provided the electrolyte is inert.

However, as shown in Figure 9 , van der Waals forces dominating at shorter particle-particle distances are not influenced by the change in the ionic strength. Van der Waals force remains unchanged while electrostatic field gets suppressed in the vicinity of the particle following an increase in the ionic strength of the dispersion medium.

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The local energy maximum existing at low ionic strengths a disappears at high ionic strengths b , yielding an unstable colloidal or cellular as in this example suspension as the result. It is well known that Nature disperses colloids almost exclusively on the negative side. Most cells and biological surfaces are, thus, negatively charged. The effect of the ionic strength on destabilization of biological colloids is, therefore, such that they are primarily sensitive to the valence of the cation rather than to that of the anion.

Critical coagulation concentration ccc of the cation can be calculated using the following empirical formula, where v is the valence of the cation:. Hence, 10 mM is the ionic strength high enough to induce rapid coagulation of colloidal dispersions if multivalent ions are present. Thus, for example, in the case of negatively charged As 2 S 3 sols where valence of the cation is decisive , ccc for NaCl is 50 mM, for ZnCl 2 it is 0. For the As 2 S 3 sol, ccc values are in the ratio 1: Schulze-Hardy rule has accordingly stated that ccc for a typical lyophobic solvent-fearing, as opposed to lyophilic-solvent-loving sol is extremely sensitive and inversely proportional to the valence of the counterion.

Note that adding two or more electrolytes together to a colloid may have different effects on the colloidal stability. Note also that the addition of small amounts of a hydrophilic colloid to a hydrophobic sol may make the latter more sensitive to flocculation by electrolyte, which is the phenomenon known as sensitization. The former were in his classification named as 2: Reprinted with permission from Zeta-Meter Inc. If DNA coats the particles, they carry a strong negative charge; if calcium phosphate is uncoated, the particle charge is only slightly negative.

This section will deal with the main practical and fundamental challenges in the analysis of ceramic particles using DLS microelectrophoresis, and hydroxyapatite HAP , the main mineral constituent of hard tissues, will be used as the example. For more alkaline solutions the surface should be negatively charged, whereas for more acidic solutions it should be positively charged.

Consequently, whereas some studies report negatively charged particles in the entire pH range in which HAP is the stable phase 4—11 , [ 81 ] others report IEP values at anywhere between 5 and 7. In addition, HAP, like many other materials, has two types of crystal planes, each one of which carries a different charge: Zeta potential analyses are not able to differ between the two and often it happens that an electrostatic attraction that leads to specific adsorption is observed between two types of particles carrying the same net charge.

In such cases, planes or edges of one of the particles carry the opposite charge compared to the particles as wholes, inducing the electrostatic attraction and adsorption to occur. One such example is shown in Figure Similarly, streaming potential measurements of f potential of sapphire monocrystals yielded a difference equivalent to 2 pH units between the IEPs of different crystal planes, indicating the existence of a pH window within which different crystal planes would carry opposite charges. Buffers do not necessarily need to be used when proteins are analyzed because proteins act as buffers per se through their titratable residues.

If the experiments are carried out under atmospheric conditions, an amount of CO 2 will be dissolved in the solution and will enter the equilibrium with carbonate ions that CO 2 forms upon dissolution in water. Buffers are mixtures of weak acids or bases and their salts and their purpose is to resist drastic changes in pH produced by adding a strong acid or base to the system. The cause of their acting in such a way can be seen from the titration curve of a weak acid with a strong base Figure 12a.

When HAP is analyzed, however, the use of buffers is recommended in order to eliminate the measurement error due to extensive pH fluctuations and instability. Tris and Bis-Tris are commonly used to stabilize the pH of organic and biological systems, and could be used in the 8. However, it is vital to remember that components of buffers usually show specific adsorption and were thus, for example, excluded from the most extensive compilation of PZCs and IEPs up to date.

In the case of surfaces that undergo selective dissolution and possibly phase transitions of surface layers, as is the case with calcium phosphates, longer equilibration times may be required. The procedure used by Somasundaran et al. By definition, inert electrolyte is composed of ions that react with the surface only by a Coulombic force and do not get specifically adsorbed to it. Still, to these rules there are exceptions since a particular combination of the particle surface composition and the double layer ions may trigger specific adsorption and other counterintuitive effects that may follow.

This can be exemplified by the case of dispersions of ice in water. The complexity and versatility of water as a dispersing medium with respect to the surface charge properties of the interfaces in it is, thus, also implicit. Amino acids as basic building blocks of proteins are normally negatively charged at high pH values and positively charged at low pH values.

In an acidic environment, the free ions are predominantly protons. Hence, the net charge of the molecule will be positive. At high pH values, hydroxyl ions will be the dominant ions in the solution. So, the net charge will be negative Figure The latter is given as:. So, as the concentration of protons is increased by lowering the pH, the concentration of COOH has to go up in order for the K to remain constant. To calculate the isoelectric point pI of an amino acid, the following equation can be used in case when no protonation of the side chain takes place:.

The same equation Equation 19 could be applied for estimating the pI of a protein molecule as a whole. In the case of polypeptides, pK a and pK b of amino acids other than the ones on terminals do not contribute to pI because their COOH and NH 2 groups form peptide bonds.

Note that pK of a compound is equal to pH at which one half of its molecules are protonated and the other half are not. The general trend is that pK values of amino acids decrease with temperature: Empirically speaking, pK b decreases in all cases, whereas pK a may decrease, stay constant or increase following an increase in temperature. Thus, the overall effect of increasing temperature is typically a lower combined pK of the amino acid or a protein. And as pK values get lower, so does pI. In case of Tris and other organic buffers, one often detects a change in the pH by 0.

Thus, pure aqueous Tris buffer at pH 6. In fact, as the temperature increases, pK a of Tris decreases at an approximate rate of 0. In the case of phosphate buffers, this change is almost negligible. Thus, pH 7 buffer has pH 7. However, notice also that a phosphate buffer with pH 7. The effect of this is an increase of the dissociation constant of water, K w , as temperature increases. Table 1 shows K w values for water at different temperatures:. Equilibrium constants for dissociation of pure water and the neutral pH values at different temperatures.

This does not mean, however, that water with pH 6. It means that neutral pH is 6. Namely, in the case of pure water, the number of free hydrogen ions and hydroxide ions is always balanced. Hence, pure water remains pH neutral at all temperatures, even though its pH changes with temperature. In view of this, note that the real concentration of free protons as defined by K w is subject to change even for the phosphate buffers following a change in temperature. Namely, although pH of the pH 7 phosphate buffer changes from 7. Hence, it can be said that concentration of free protons, in fact, decreases with increasing the temperature by about 0.

On the other hand, Tris buffers exhibit a drop of about 0. In view of the mentioned change in the pH of neutral water following this change in temperature, this drop accounts for only 0.

Zeta potential analyses of proteins may be significantly hampered due to high ionic strengths of the conductive media in which they are dissolved or suspended. The consequence of this is that destabilization and disintegration of the protein structure often occurs during the analysis owing to the electric field effect. Essentially, a tradeoff between the intensity of the electric field in which the charged particles or molecules move and susceptibility of the macromolecules analyzed has to be ensured.

The higher the intensity of the field, the more precise is the measurement, but the higher are the chances that the protein will disintegrate too. As with every other method, a compromise between powerfulness and sensitivity needs to be found. Thus, many advanced measurement settings offer very intensive fields, thereby increasing the measurement precision for the relatively robust inorganic particles, but present imperfect solutions for the organics.

High ionic strengths essentially either increase the surface tension of the solvent and intensify the hydrophobic forces, or increase the solubility of the nonpolar protein residues, thus, unbalancing the protein secondary and tertiary structures by different mechanisms.

According to the Hofmeister series, [ 94 — 96 ] which was confirmed as valid for inorganic particles as well, [ 97 ] the following ions are arranged in the direction on their increasing destabilization aggregation or unfolding of proteins:. Be that as it may, to take pK values of all amino acids that comprise a protein into account when calculating its pI would be a mistake.

Namely, as all amino acid residues except those at the C- and N-terminals have their amino and carboxyl groups involved in forming peptide bonds, only those with free acidic or basic side chains should be included in the intrinsic charge calculations. On the other hand, taking into account only these amino acids is again an approximation. Strictly speaking, a deeply buried residue may not necessarily undergo dissociation and, if that is so, it should be excluded from contributing to the intrinsic molecular charge.

Since water is mostly excluded from the predominantly hydrophobic and nonpolar interior of the protein, and whenever present it occupies rigid positions in space where it contributes in highly directional hydrogen bonds, it is difficult to predict the extent of dissociation of buried charged side chains. This is however thought to be a rare case since the crucial role of hydrophobic forces in determining the active conformation of the protein dictates that it is energetically unfavorable to have a charged residue buried in the deep interior of the protein.

The protein core is typically made up only of non-titratable, hydrophobic and neutral amino acids.

Still, the energetically unfavorable packing of charges may happen under certain conditions. Namely, charged residues could be packed inside of the protein as covalent bonds between -SH groups of two cystein residues, forming an S-S bond disulfide bridge ; as salt bridges where opposite charges neutralize each other; or as metal coordination complexes. Furthermore, pK values for individual amino acid residues are empirically determined for amino acids alone and not as parts of a peptide chain, let alone folded into specific three-dimensional confirmations where they may be forming weak bonds with other residues.

Just as increasing the length of an aliphatic chain decreases the frequency of C-C vibrations, a similar synergetic effect occurs herein as well. Amine groups in a chain would not possess the same pK values as in isolation; surrounding cationic amine groups will, for example, electrostatically suppress the protonation of the neighboring amines. Hence, pI values of proteins are subject to significant variations and are intrinsically determined by the content of the titratable residues and extrinsically by the adsorption of ionic species and the effect of the diffuse double layer of charges.

Hence, pI values range from as low as less than 1. Zeta potentials and electrophoretic mobility of different compounds versus pH: From the raw electrophoretic mobility data, one can also estimate the effective uncompensated charge on molecules N e as a function of pH and ionic strength, which is of interest in predicting the deposition propensity and interaction energy, kinetics and mechanisms on various interfaces.

For that purpose, one uses Lorenz-Stokes relationship: This equation is considered to be more reliable than the titration method which typically tends to overestimate charge on molecules, presumably owing to ion exchange processes. Also, it may imply that a significant rearrangement of the flexible parts of the tertiary structure of the molecule is expected to occur, in analogy with the behavior of polyelectrolyte chains.

Representing each single curve of this type as a narrow line, as is often seen in the literature, instead of a spread of values around each data point is, thus, quite misleading. When standard deviations of the error are not denoted in the graphs, one should imagine them in both X and Y directions for each data point.

The DLS device, therefore, takes the average mobility of an ensemble of particles into account, which implies that the measurement error becomes more pronounced at high polydispersities. Spherical approximation mentioned in the previous section accounts for an additional source of uncertainty. However, an orientationally averaged mobility could be considered by assuming that the electric field is too low to align the rod-like particles. The bar along X axis, on the other hand, would originate from the error of pH measurements, which are said to be the main source of uncertainty in microelectrophoretic analyses.

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