Physical Pharmaceutics-II

Emulsion Formulation by HLB Method selects surfactants based on hydrophilic-lipophilic balance for stability. It ensures proper oil-water blending in pharmaceuticals and cosmetics. HLB (Hydrophilic–Lipophilic Balance) is a numerical scale (0–20) used to select appropriate emulsifying agents based on their balance between hydrophilic and lipophilic portions. HLB Scale Interpretation HLB Value Nature of Surfactant Application 1–4 … Read more

Definition: Rate depends on either: Square of one reactant: $\text{Rate} = k[A]^2$ Product of two reactants: $\text{Rate} = k[A][B]$ Integrated form (when A = B): $\tfrac{1}{[A]_t} = \tfrac{1}{[A]_0} + kt$ Derivation: Separate variables: $\frac{d[A]}{[A]^2} = -k \, dt$ Integrate both sides: $\int_{[A]_0}^{[A]} \frac{d[A]}{[A]^2} = -k \int_{0}^{t} dt$ $\left[ -\frac{1}{[A]} \right]_{[A]_0}^{[A]} = -kt$ $- \frac{1}{[A]} + … Read more

Definition: Photolytic (or photodegradation) is the degradation of drug substances due to exposure to light, especially UV and visible light. It leads to chemical breakdown and loss of potency, or formation of toxic by-products. Mechanism: Light provides energy that excites electrons in drug molecules. Excited-state molecules may undergo bond cleavage, oxidation, or isomerization. Photodegradation can … Read more

Mechanism: Oxidation is a process involving electron loss, often initiated by oxygen or free radicals. It can be accelerated by light, heat, trace metals, or pH. Basic pathway: $RH + O_2 \;\rightarrow\; ROOH \;\rightarrow\; RO\cdot + OH\cdot$ $\mathrm{RH} + \mathrm{O}_2 \rightarrow \mathrm{ROOH} \rightarrow \mathrm{RO}^{\bullet} + \mathrm{OH}^{\bullet}$ Commonly Affected Drug Classes: Phenols: epinephrine, morphine Thiols: captopril … Read more

Mechanism of Hydrolysis: Hydrolysis involves nucleophilic attack of water on labile functional groups, breaking chemical bonds such as esters and amides. Example: $\mathrm{R{-}COOR’} + H_{2}O \;\longrightarrow\; \mathrm{R{-}COOH} + \mathrm{R’OH}$ Commonly Affected Drug Classes: Esters: aspirin, procaine Amides: lidocaine, procainamide Lactams: penicillins, cephalosporins Carbamates, imines Stabilization Strategies for Hydrolysis Strategy Explanation pH control (buffering) Adjust formulation … Read more

Specific Acid/Base Catalysis: Catalysis by hydronium (H₃O⁺) or hydroxide (OH⁻) only. Rate is pH-dependent. Rate law: \(\text{Rate} = k_H [H_3O^+] + k_{OH} [OH^-]\) General Acid/Base Catalysis: Catalysis by any proton donor (acid) or proton acceptor (base), e.g., acetate, phosphate, ammonia. Buffers can contribute to catalysis. Rate law (general form): $\text{Rate} = k + k_{AH}[AH] + … Read more

Definition of Dielectric Constant: Dielectric Constant Measure of a solvent’s polarity. High dielectric constant = high polarity (e.g., water = 80). Effect: High ε (e.g., water) stabilizes ionic species and may increase or decrease the rate depending on the mechanism. Solvents with low dielectric constants may: Promote neutral or non-ionic pathways Inhibit ionization-dependent reactions For … Read more

Definition of Ionic Strength: Ionic strength (μ) is a measure of total ion concentration in solution. $\mu = 12 \sum c_i z_i^2$$\mu = \tfrac{1}{2} \sum c_i z_i^2$$\mu = 21 \sum c_i z_i^2$ Where: ci​ = concentration of ion i zi​ = charge of ion i Effect: Reactions involving charged species are influenced by ionic strengths: … Read more

Effect of Solvent: Solvents can change the reaction medium, affecting: Solubility of the drug Polarity of the medium Hydrolysis or oxidation rate Some solvents may stabilize or destabilize the drug. Co-solvent Systems: Adding solvents like ethanol can: Change solvation Affect reaction kinetics Modify degradation pathways Examples: Hydrolysis is faster in aqueous solutions. Adding organic solvents … Read more