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Protective colloid

A protective colloid is a lyophilic substance, such as a polymer or surfactant, that stabilizes lyophobic colloidal particles by adsorbing onto their surfaces and preventing aggregation or coagulation through mechanisms including steric hindrance, osmotic pressure, and electrostatic repulsion.^[1] These colloids are particularly effective in small quantities, where they form a protective sheath around dispersed particles, enhancing the stability of suspensions, emulsions, and dispersions against coagulating agents like electrolytes.^[1]^[2] The concept of protective colloids originated in early 20th-century colloidal chemistry, with key developments by Richard Zsigmondy, who introduced the term in relation to stabilizing gold sols.^[1] In lyophobic systems, such as metal or inorganic sols, particles tend to aggregate due to weak solvent interactions, but the addition of a protective colloid modifies the particle-solvent interface to mimic lyophilic behavior.^[2] This adsorption often involves hydrophilic groups orienting toward the solvent, creating a barrier that reduces van der Waals attractions between particles.^[1] The protective efficiency of these colloids is quantified by the gold number, defined as the minimum mass (in milligrams) of the protective agent required to prevent coagulation of 10 mL of a standard gold sol upon addition of 1 mL of a 10% sodium chloride solution.^[1] Lower gold numbers indicate higher protective power; for instance, gelatin has a gold number of about 0.005–0.01, making it highly effective, while gum arabic has a value around 0.15–0.25.^[1] This metric, introduced by Zsigmondy in 1901, remains a standard for comparing stabilizers in colloidal systems.^[1] Common examples include gelatin, polyvinylpyrrolidone (PVP), and polyvinyl alcohol (PVA), which are widely used due to their biocompatibility and tunable molecular weights.^[1] In industrial applications, protective colloids play crucial roles in emulsion polymerization (e.g., stabilizing styrene or vinyl acetate latices), paints (dispersing titanium dioxide pigments), detergents (preventing soil redeposition), cosmetics (emulsifying oils), and water treatment (flocculating impurities).^[1]^[3] Their ability to maintain dispersion stability also extends to pharmaceuticals, adhesives, and food products, where they ensure product consistency and shelf life.^[2]^[3]

Colloidal Fundamentals

Definition and Classification of Colloids

A colloid is defined as a heterogeneous mixture in which one substance, known as the dispersed phase, consists of particles with dimensions typically ranging from 1 to 1000 nanometers and is uniformly distributed throughout another substance called the dispersion medium.^[4] This particle size allows colloids to exhibit unique properties, such as the Tyndall effect, where they scatter light due to the presence of larger particles compared to molecular-scale solutes.^[4] The term "colloid" was introduced by Scottish chemist Thomas Graham in 1861, derived from the Greek word for "glue," to describe substances like gelatin and starch that form viscous solutions and do not readily diffuse through semipermeable membranes, distinguishing them from crystalloids.^[5] Colloids differ from true solutions, in which solute particles are smaller than 1 nm and the mixture appears homogeneous without light scattering, and from coarse suspensions, where particles exceed 1000 nm and settle rapidly under gravity due to insufficient Brownian motion.^[6] In colloids, the dispersed particles remain suspended indefinitely because their size enables random thermal motion to counteract gravitational settling, maintaining kinetic stability.^[7] This intermediate scale results in systems that are neither fully dissolved nor fully precipitated, with applications spanning pharmaceuticals, food science, and materials engineering. Colloids are classified based on the physical states of the dispersed phase and dispersion medium, leading to several common types. The following table summarizes key examples:

Type	Dispersed Phase	Dispersion Medium	Example
Sol	Solid	Liquid	Ink
Gel	Liquid	Solid	Agar
Emulsion	Liquid	Liquid	Milk
Foam	Gas	Liquid	Soap suds
Aerosol	Solid or Liquid	Gas	Fog or smoke

This phase-based classification highlights the diversity of colloidal systems, from fluid-like sols to semi-rigid gels.^[8] Another fundamental classification distinguishes colloids by the nature of interaction between the dispersed phase and dispersion medium: lyophilic (solvent-attracting) colloids, which form spontaneously and are stabilized by solvation layers around particles, and lyophobic (solvent-repelling) colloids, which require special preparation methods and are inherently unstable, tending toward aggregation.^[2] When the dispersion medium is water, lyophilic colloids are termed hydrophilic and lyophobic ones hydrophobic.^[9] Lyophobic colloids, in particular, exhibit greater susceptibility to instability mechanisms that promote particle coalescence.^[2]

Lyophilic vs. Lyophobic Colloids

Lyophilic colloids, also known as solvent-loving colloids, exhibit a strong affinity for the dispersion medium, leading to spontaneous formation upon simple mixing of the dispersed phase and solvent.^[10] This affinity results in the particles being enveloped by a robust solvation shell, where solvent molecules tightly bind to the particle surface through mechanisms such as hydrogen bonding, providing a physical barrier that prevents aggregation and confers high inherent stability.^[10] In contrast, lyophobic colloids, or solvent-repelling colloids, show little to no attraction to the medium, necessitating mechanical dispersion methods like grinding or condensation processes for preparation, as they do not form spontaneously.^[10] Their stability relies primarily on electrostatic repulsion arising from charged particles, but this makes them highly sensitive to electrolytes, which can compress the electric double layer and promote coagulation.^[11] A classic example of a lyophilic colloid is gelatin dispersed in water, where the protein chains interact strongly with water molecules to form a stable sol.^[10] For lyophobic colloids, gold sol in water exemplifies this type, prepared by reducing gold salts and stabilized by the negative charge on the gold nanoparticles acquired through selective adsorption of ions.^[10] The particle charge in lyophobic colloids originates from surface ionization, ion adsorption, or frictional effects during dispersion, creating an electric double layer that generates repulsive forces between particles.^[11] The zeta potential serves as a key indicator of this electrostatic stabilization in lyophobic colloids, defined as the electric potential at the slipping plane (the boundary between the tightly bound solvent layer and the mobile diffuse layer) relative to the bulk medium.^[12] A high absolute zeta potential (typically greater than 30 mV) signifies strong repulsion and thus greater stability, while values near zero indicate potential aggregation due to insufficient electrostatic barriers.^[12] These differences in stability highlight why lyophobic colloids often require additional protective agents to mimic the inherent resilience of lyophilic systems.^[10]

Colloidal Instability

Mechanisms of Coagulation

Lyophobic colloids exhibit inherent instability due to their poor interaction with the dispersion medium, rendering them highly susceptible to aggregation when stabilizing forces are perturbed. The primary mechanisms of coagulation in these systems are governed by the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory, which posits that colloidal stability results from a delicate balance between long-range attractive van der Waals forces and short-range repulsive electrostatic forces arising from the charged electrical double layers on particle surfaces.^[13] Coagulation ensues when environmental changes tip this balance, enabling van der Waals attractions to dominate and drive irreversible particle collisions and fusion into larger aggregates.^[14] A key trigger for coagulation is the addition of electrolytes, which induces compression of the electrical double layer by screening surface charges with counterions, thereby diminishing the repulsive potential barrier between particles and facilitating their close approach.^[14] This process is particularly pronounced in lyophobic systems, where the absence of solvating layers exacerbates the dominance of attractive forces once repulsion is weakened. Coagulation is distinguished from flocculation in that it involves irreversible aggregation leading to compact, stable structures, typically via direct interparticle forces like those in DLVO interactions, whereas flocculation entails reversible bridging mechanisms that form loose, open networks prone to redispersion.^[7] The critical coagulation concentration (CCC) represents the threshold electrolyte concentration at which rapid, diffusion-limited aggregation occurs, as the repulsive energy barrier effectively vanishes, providing a quantitative measure of a colloid's sensitivity to destabilization.^[15] This concept, rooted in experimental observations of salt-induced instability, underscores the valence-dependent efficacy of ions in promoting coagulation, with higher-valence cations exhibiting lower CCC values.^[16]

Factors Influencing Stability

The stability of colloidal dispersions, particularly lyophobic sols, is profoundly influenced by the type and concentration of electrolytes, as governed by the Schulze-Hardy rule. This rule posits that the critical coagulation concentration (CCC) required for coagulation decreases inversely with the sixth power of the counterion valence (CCC ∝ Z⁻⁶), rendering higher-valence ions far more effective coagulants.^[17] For instance, trivalent ions coagulate at concentrations roughly 60-600 times lower than monovalent ions, depending on the system.^[18] In lyophobic sols, divalent ions such as Ca²⁺ coagulate approximately 60 times more efficiently than monovalent ions like Na⁺ by more effectively neutralizing surface charge and compressing the electrical double layer.^[19] Solution pH modulates colloidal stability primarily through its impact on particle surface charge and zeta potential, which dictate electrostatic repulsive forces. At low ionic strengths, lower pH values (e.g., pH 5 versus pH 7) can enhance stability in certain protein colloid systems by increasing the magnitude of surface charge (often positive) and long-range repulsion, reducing aggregation propensity.^[20] Conversely, near the isoelectric point, minimized charge leads to rapid coagulation across various colloids.^[21] Temperature exerts dual effects on colloidal stability via changes in medium viscosity and particle dynamics. Elevated temperatures decrease viscosity, thereby augmenting Brownian motion and particle collision rates, which can accelerate coagulation in flocculated states while simultaneously promoting redispersion in stable dispersions through heightened kinetic energy.^[22] In titanium dioxide dispersions, for example, temperature-dependent shifts in the balance between Brownian diffusion and shear-induced flow alter rheological stability, with higher temperatures generally favoring deflocculation under shear.^[22] The presence of polymers in the dispersion medium can either stabilize or destabilize colloids through non-specific interactions, such as entropic depletion attractions or surface adsorption that modifies interparticle forces. Adsorbed polymers may bridge particles at high concentrations, promoting flocculation, whereas free polymers in solution often induce net repulsion via osmotic effects.^[23] These influences are system-dependent, with polymer molecular weight and concentration playing key roles in modulating overall stability without direct charge neutralization.^[24] Particle size and concentration contribute to inherent self-stability in colloidal systems. Smaller particles (e.g., <100 nm) exhibit enhanced stability due to intensified Brownian motion, which counters sedimentation, and higher CCC values—up to 3-4 times greater than for larger fractions (e.g., <2 µm) in soil colloids under NaCl electrolyte.^[25] Elevated particle concentrations, however, increase collision frequencies, fostering aggregation unless balanced by repulsive forces, though dilute systems benefit from entropic stabilization that resists phase separation.^[26]

Concept of Protective Colloids

Definition and Role

A protective colloid is defined as a lyophilic colloid added in small quantities to a lyophobic sol to prevent coagulation and precipitation of the dispersed particles.^[27] These substances, often polymers or surfactants that exhibit affinity for the dispersion medium, adsorb onto the surface of lyophobic particles, thereby imparting stability to the overall colloidal system.^[1] The primary role of protective colloids is to enhance the longevity of dispersions such as sols and emulsions by shielding them from coagulating agents, including electrolytes that would otherwise neutralize particle charges and promote aggregation. This stabilization maintains the dispersed state under conditions that would lead to phase separation or flocculation, allowing for sustained utility in various chemical and industrial processes.^[28] The concept of protective colloids emerged in early 20th-century colloid chemistry, closely tied to Richard Zsigmondy's investigations of gold hydrosols, where he demonstrated how trace amounts of stabilizing agents could prevent color changes indicative of coagulation upon electrolyte addition.^[29] Zsigmondy's work, including the introduction of the gold number in 1901 as a quantitative measure of protective efficacy, laid foundational principles for understanding and applying these stabilizers.^[1]

Mechanism of Protection

The primary mechanism of protection by protective colloids involves the adsorption of lyophilic macromolecules, such as polymers or surfactants, onto the surfaces of lyophobic colloidal particles. This process creates a thin, hydrophilic sheath that envelops each particle, preventing direct contact and aggregation. The adsorbed layer acts as a physical barrier, primarily through steric hindrance, where the extended solvophilic chains repel approaching particles via entropic effects and osmotic pressure when their solvated regions overlap. This steric stabilization differs fundamentally from electrostatic stabilization, which depends on Coulombic repulsion between charged particles to maintain dispersion. In contrast, protective colloids rely on the mechanical and solvational properties of the adsorbed layer rather than charge interactions, although some electrostatic contributions may arise from charged groups in the polymer. The hydrophilic sheath enhances solvation by the continuous medium, forming a solvated boundary that increases the effective particle volume and reduces solvophobic interactions between bare surfaces. Solvophilic segments of the macromolecule extend outward, promoting hydration or solvation shells that generate additional repulsive forces, while the overall layer configuration diminishes the range and magnitude of attractive van der Waals forces by enforcing a minimum separation distance between particle cores.^[30]^[31] This protective mechanism confers resistance to coagulation induced by electrolytes, as the sheath shields particles from counterion adsorption that would otherwise neutralize surface charges.

Types and Examples

Natural Protective Colloids

Natural protective colloids are naturally occurring hydrophilic substances, primarily biopolymers derived from animal, plant, or microbial sources, that stabilize colloidal dispersions through adsorption on particle surfaces. These materials exhibit high molecular weight and possess multiple hydrophilic groups, such as hydroxyl, carboxyl, and amino groups, which facilitate strong solvation and prevent coagulation by creating a steric or electrostatic barrier around dispersed particles.^[32] Gelatin, derived from collagen through hydrolysis of animal connective tissues like skin and bones, serves as an effective protective colloid due to its polypeptide structure and low gold number, indicating high stabilizing efficiency. It adsorbs onto hydrophobic particles via its hydrophilic segments, enhancing colloidal stability in various systems. Notably, gelatin protects silver halide sols in photographic films by forming a protective layer around the microcrystals, preventing aggregation and maintaining emulsion integrity during processing.^[33]^[34] Gum arabic, also known as acacia gum, is a branched polysaccharide exudate from the stems and branches of Acacia senegal and Acacia seyal trees, primarily sourced from sub-Saharan Africa. Its high molecular weight (up to 2 million Da) and abundance of hydrophilic sugar residues enable it to act as a protective colloid, stabilizing emulsions by adsorbing at oil-water interfaces and providing steric hindrance.^[35]^[36] Starch, a plant-derived polysaccharide composed of amylose and amylopectin, is extracted from sources such as potatoes, corn, and rice. As a protective colloid in food systems, it forms viscous colloidal solutions upon heating in water, with amylose contributing to stabilization through its linear chains that adsorb onto particles and impart protective hydration layers.^[37]^[38] Casein, the primary protein in milk comprising about 80% of bovine milk proteins, functions as a strong protective colloid in dairy colloids due to its amphiphilic nature and high molecular weight (around 20,000–25,000 Da per subunit). It forms micelles where hydrophilic phosphoserine groups extend outward, adsorbing onto surfaces to shield against electrolytes and maintain dispersion stability.^[39]^[40]

Synthetic Protective Colloids

Synthetic protective colloids are artificially engineered polymers designed to stabilize lyophobic colloidal dispersions by adsorbing onto particle surfaces, providing steric and electrostatic barriers against aggregation. These materials emerged prominently in the mid-20th century, coinciding with advances in polymer chemistry that enabled the synthesis of water-soluble macromolecules with controlled structures. For instance, the development of polyvinyl alcohol through the saponification of polyvinyl acetate in the 1930s and 1940s laid the foundation for its industrial application as a stabilizer in emulsion systems.^[41]^[1] Key examples of synthetic protective colloids include polyvinyl alcohol (PVA), which is widely used in the production of paints and coatings. PVA acts as a protective colloid during the emulsion polymerization of vinyl acetate, forming a stabilizing layer around dispersed particles to prevent coagulation and ensure uniform latex formation suitable for paint formulations.^[42] Another example is sodium carboxymethyl cellulose (CMC), employed in detergents to enhance suspension stability. In detergent compositions, CMC functions as a protective colloid by generating anions that form a barrier around soil particles, preventing their redeposition on fabrics during washing.^[43] Polyvinylpyrrolidone (PVP), a water-soluble polymer, is commonly used as a protective colloid in pharmaceuticals and adhesives due to its ability to form hydrogen bonds and provide steric stabilization for drug particles and emulsions.^[1] Polyethylene oxide (PEO) serves as a protective colloid in pharmaceutical applications, particularly in stabilizing emulsions and colloidal drug delivery systems. PEO adsorbs onto particle interfaces to induce steric stabilization, improving the longevity and bioavailability of dispersed therapeutic agents.^[1]^[44] A primary advantage of synthetic protective colloids lies in their customizable properties, such as molecular weight and charge distribution, which can be precisely tailored to meet specific stability requirements in various media. For example, varying the molecular weight of PVA influences the thickness of the adsorbed layer and the kinetics of polymerization, allowing optimization for enhanced protection in high-shear environments like paint mixing.^[45] Similarly, introducing charged groups into polymers like polyethylenimine enables electrostatic repulsion alongside steric effects, outperforming natural colloids in tunable responsiveness to pH and ionic strength.^[46]^[47] This design flexibility contrasts briefly with natural protective colloids, which offer less precise control over such parameters. Overall, these attributes make synthetic variants indispensable in industrial processes demanding consistent colloidal stability.

Evaluation Methods

Gold Number Measurement

The gold number, introduced by Richard Adolf Zsigmondy in 1901, quantifies the protective efficiency of a lyophilic colloid against the coagulation of a lyophobic sol, specifically using gold hydrosol as the test system. It is defined as the minimum mass, in milligrams, of the protective colloid required to prevent 10 mL of a standard gold sol from coagulating when 1 mL of a 10% sodium chloride (NaCl) solution is added.^[48] The measurement procedure starts with the preparation of a standard gold sol, a ruby-red dispersion of gold nanoparticles typically obtained by reducing an aqueous solution of gold(III) chloride (HAuCl₄) with trisodium citrate as the reducing and stabilizing agent under reflux conditions. Serial dilutions of the protective colloid (e.g., in water or buffer) are then added in increasing volumes to separate 10 mL aliquots of the gold sol, ensuring the total volume remains consistent before the electrolyte challenge. To each aliquot, 1 mL of 10% NaCl is added, and the mixtures are gently agitated and allowed to stand for several minutes. Coagulation is observed as a color shift from the characteristic red of the dispersed sol to a blue-violet hue due to particle aggregation and precipitation; the absence of this change indicates successful protection by the colloid through adsorption on the gold particles. The gold number is taken as the minimum mass of colloid required to prevent coagulation, often determined by interpolation between tested concentrations.^[48] A lower gold number signifies greater protective power, as less colloid is needed to stabilize the sol against electrolyte-induced coagulation. For instance, gelatin exhibits a gold number of approximately 0.005–0.01 mg, highlighting its strong protective action compared to colloids like gum arabic (around 0.15–0.25 mg).^[1]

Other Assessment Techniques

Beyond the empirical gold number, which serves as a historical benchmark for protective efficacy, several instrumental techniques provide quantitative insights into the stabilizing action of protective colloids by assessing aggregation resistance and surface modifications in colloidal systems.^[49] Turbidity measurements offer a straightforward optical method to evaluate aggregation kinetics, where the protective colloid's ability to inhibit particle coalescence is monitored through changes in solution opacity over time. As particles aggregate under induced stress such as electrolyte addition, increased turbidity reflects destabilization; effective protective colloids maintain low turbidity by slowing this process, enabling kinetic rate constants to be derived from time-resolved data. This technique is particularly useful for real-time assessment in dilute dispersions, correlating protective action with the onset and extent of flocculation.^[50] Zeta potential shifts following adsorption of protective colloids quantify electrostatic contributions to stability, revealing how the colloid alters the effective surface charge of dispersed particles. Adsorption of charged protective agents, such as polyamino acids, typically increases the magnitude of the zeta potential, enhancing repulsive barriers and preventing coagulation; for instance, values exceeding ±30 mV indicate robust stabilization in aqueous media. Post-adsorption measurements, often via electroacoustic methods, allow direct comparison of bare versus protected particles, with shifts serving as a proxy for protective coverage and ionic layer thickness.^[51] Dynamic light scattering (DLS) assesses particle size stability by tracking the hydrodynamic radius of colloidal particles under stress conditions, providing a sensitive indicator of protective colloid performance. In protected systems, DLS detects minimal radius increases over time, as stabilizers like polymers hinder diffusion-limited aggregation; for example, exposure to shear or pH changes might cause unprotected particles to show radius doubling within minutes, while effective protection preserves monodispersity. This non-invasive method excels in monitoring subtle early-stage instabilities in submicron colloids.^[52] Electrophoretic mobility tests gauge charge modification induced by protective colloids, measuring particle velocity in an electric field to infer surface potential alterations. Protective adsorption often shifts mobility toward higher absolute values due to enhanced charge density, promoting electrostatic repulsion; in polyelectrolyte-stabilized colloids, this can reverse bare particle mobility signs, confirming multilayer formation and improved dispersion longevity. Such tests, typically conducted via phase analysis light scattering, complement zeta potential data by validating charge-based protection mechanisms in complex ionic environments.^[53]

Practical Applications

Industrial Uses

Protective colloids play a vital role in the paints and coatings industry, where they stabilize emulsions to prevent particle aggregation and ensure product durability. Polyvinyl alcohol (PVA), a synthetic protective colloid, is commonly employed in latex paints to inhibit flocculation during storage and application, maintaining uniform viscosity and enhancing adhesion properties.^[54] This stabilization is critical for water-based formulations, which dominate the market due to their environmental benefits and ease of use.^[55] In the production of synthetic rubber, protective colloids are integral to emulsion polymerization processes, facilitating the creation of stable latex dispersions for materials like styrene-butadiene rubber (SBR). Agents such as PVA or hydroxyethyl cellulose (HEC) form protective layers around growing polymer particles, preventing coagulation and controlling particle size distribution, which directly impacts the final rubber's mechanical properties.^[56] These colloids enable efficient, large-scale manufacturing while minimizing defects in end products used in tires and automotive components.^[57] Food processing relies on natural protective colloids like gum arabic to stabilize emulsions in beverages and confectionery, where it prevents phase separation and extends shelf life by adsorbing onto oil droplets.^[58] In textile dyeing, protective colloids such as carboxymethyl cellulose (CMC) disperse dyes evenly, averting aggregation and promoting uniform coloration on fabrics.^[59] Overall, these applications provide benefits including superior viscosity control and prolonged product stability, contributing to the efficiency of key industries such as paints and coatings, valued at over $200 billion globally as of 2023.^[60]

Biological and Pharmaceutical Uses

In biological systems, albumins serve as natural protective colloids that maintain the colloidal osmotic pressure of blood plasma, preventing the aggregation of blood cells such as erythrocytes by preserving intravascular volume and endothelial integrity.^[61] This stabilizing function is critical for hemodynamic balance, as albumin constitutes about 50% of plasma proteins and contributes significantly to oncotic pressure, thereby reducing the risk of hemoconcentration and cellular clumping during physiological stress or hypovolemia.^[62] For instance, in conditions like sepsis or hemorrhage, endogenous albumin helps shield colloidal particles from coalescence, supporting overall vascular stability without synthetic intervention.^[63] In pharmaceutical applications, protective colloids enhance drug formulation stability and biocompatibility, particularly in delivery systems where they prevent particle aggregation and improve bioavailability. Gelatin, a natural polymer derived from collagen, acts as a protective colloid in capsule shells and liposomal coatings, increasing physical stability against environmental stressors like pH changes and enzymatic degradation while facilitating controlled release.^[64] Similarly, synthetic polymers such as polyethylene glycol (PEG) are conjugated to liposomes or nanoparticles to provide steric stabilization, extending circulation time and enabling targeted delivery by evading immune clearance—a key advancement in post-2010 nanomedicine.^[65] Hydroxyethyl starch (HES), a modified starch colloid, is incorporated into intravenous solutions to expand plasma volume effectively, offering a biocompatible alternative for fluid resuscitation in hypotensive patients, though its use requires careful monitoring due to potential renal effects.^[66] Chitosan, a biocompatible polysaccharide, exemplifies protective colloids in wound healing dressings, where it forms hydrogels that stabilize colloidal dispersions of bioactive agents, maintain a moist environment, and promote cell migration while exhibiting antimicrobial properties to accelerate tissue repair.^[67] Clinical studies demonstrate that chitosan-based hydrocolloid dressings reduce pain and enhance healing rates in chronic wounds by preventing bacterial aggregation and supporting granulation tissue formation.^[68] These applications underscore the role of protective colloids in bridging biological mimicry with therapeutic efficacy, prioritizing materials that integrate seamlessly with physiological processes.

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Poly(vinyl alcohol) is often used in vinyl acetate emulsion polymerization as a protective colloid, but its role is complex and controversial.
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Albumin plays a crucial role in the transport of various ions, electrically neutral molecules and in maintaining the colloidal osmotic pressure of the blood.
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