Propylene Glycol Monocaprylate

This article is compiled based on the United States Pharmacopeia (USP) – 2025 Edition

Issued and maintained by the United States Pharmacopeial Convention (USP)

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C₁₁H2₂O₃                202.29

(propylene glycol monocaprylate)

C₁₉H₃₆0₄                  328.49

(propylene glycol dicaprylate)

Octanoic acid, ester with 1,2-propanediol;

Hydroxypropyl octanoate CAS RN^®: 31565-12-5.

1 DEFINITION

Propylene Glycol Monocaprylate is a mixture of the propylene glycol monoesters and diesters of fatty acids composed predominantly of caprylic acid. Propylene Glycol Monocaprylate contains NLT 90.0% monoesters and NMT 10.0% diesters.

2 IDENTIFICATION

A. SPECTROSCOPIC IDENTIFICATION TESTS (197), Infrared Spectroscopy: 197A

B. THIN-LAYER CHROMATOGRAPHIC IDENTIFICATION TEST (201)

Standard solution: 50 mg/mL of USP Propylene Glycol Monocaprylate RS in methylene chloride

Sample solution: 50 mg/mL of Propylene Glycol Monocaprylate in methylene chloride

Chromatographic system

Application volume: 10 µL

Developing solvent system: Ether and hexane (70:30)

Spray reagent: 0.1 mg/mL of rhodamine 6G in alcohol

Analysis: Develop the chromatogram over a path of 15 cm, and dry the plate in a current of air. Spray the plate with Spray reagent, and locate the spots on the plate by examination under UV light at a wavelength of 365 nm.

Acceptance criteria: The R values of the principal spots from the Sample solution correspond to those from the Standard solution.

C. It meets the requirements in Specific Tests for Fats and Fixed Oils (401), Procedures, Fatty Acid Composition.

3 ASSAY

PROCEDURE

Mobile phase: Tetrahydrofuran

Sample solution: 40 mg/mL of Propylene Glycol Monocaprylate in Mobile phase

Chromatographic system

(See Chromatography (621), System Suitability.)

Mode: LC

Detector: Refractive index

Column: 7-mm x 60-cm; 5-µm packing L21 (100 A). [NOTE-TWO 7-mm x 30-cm; 5-µm packing L21 columns may be used in place of one 60-cm column, provided System suitability requirements are met.]

Temperatures

Column: 40°

Detector: 40°

Flow rate: 1 mL/min

Injection volume: 40 µL

System suitability

Sample: Sample solution

[NOTE-The relative retention times with reference to propylene glycol for diesters and monoesters are about 0.85 and 0.90, respectively.]

Suitability requirements

Relative standard deviation: NMT 2.0% for the monoester peak

Analysis

Sample: Sample solution

Calculate the percentage of monoesters or diesters in the portion of Propylene Glycol Monocaprylate taken:

                         Result = (r_U/r_T) x (100 - D)

r_U = peak response for monoesters or diesters

r_T = sum of the peak responses of the monoesters and diesters

D = sum of the percentage content of propylene glycol and the percentage content of free fatty acids

Calculate the percentage content of free fatty acids:

                         Result = (A/561.1) x 144

A = acid value

Acceptance criteria

Monoesters: NLT 90.0%

Diesters: NMT 10.0%

4 IMPURITIES

LIMIT OF PROPYLENE GLYCOL

Mobile phase, Sample solution, and Chromatographic system: Proceed as directed in the Assay.

Standard stock solution: 4 mg/mL of USP Propylene Glycol RS in Mobile phase

Standard solutions: Into four 5-mL volumetric flasks, introduce 0.25, 0.5, 1.0, and 2.5 mL of Standard stock solution, respectively, and dilute with Mobile phase to volume. In a fifth 5-mL volumetric flask, introduce 5.0 mL of Standard stock solution.

Analysis

Samples: Standard solutions and Sample solution

Prepare a standard curve of peak area versus concentration, in mg/mL, of propylene glycol in the Standard solutions. Obtain the concentration of propylene glycol in the Sample solution from the standard curve.

Calculate the percentage of free propylene glycol in the portion of Propylene Glycol Monocaprylate taken:

                         Result = (C/C_U) x 100

C = concentration of propylene glycol in the Sample solution from the standard curve

C_U = concentration of the Sample solution (mg/mL)

Acceptance criteria: NMT 1.5%

5 SPECIFIC TESTS

FATS AND FIXED OILS (401), Procedures, Acid Value: NMT 1.5

FATS AND FIXED OILS (401), Procedures, Fatty Acid Composition: Propylene Glycol Monocaprylate exhibits the composition profile of fatty acids shown in Table 1.

Table 1

FATS AND FIXED OILS (401), Procedures, Jodine Value: NMT 1.0

FATS AND FIXED OILS (401), Procedures. Saponification Value: 270-295

FATS AND FIXED OILS (401), Procedures, Peroxide Value: NMT 6.0.

WATER DETERMINATION (921), Method I, Method la

Analysis: Use a mixture of methanol and methylene chloride (1:1) in place of methanol in the titration vessel.

Acceptance criteria: NMT 1.0%

ARTICLES OF BOTANICAL ORIGIN (561), Methods of Analysis, Total Ash: NMT 0.1%

6 ADDITIONAL REQUIREMENTS

PACKAGING AND STORAGE: Preserve in well-closed containers and protect from moisture. No storage requirements are specified.

USP REFERENCE STANDARDS (11)

USP Propylene Glycol RS

USP Propylene Glycol Monocaprylate RS

Also known as USP Propylene Glycol Monocaprylate Type II RS