Differences between glycolic acid and lactic acid
Fuglycolic acid and lactic acid are both organic acids and are widely used in chemical, pharmaceutical, daily chemical and other fields. Although the two are both organic acids, there are many differences in structure, properties and applications.

First, structural differences
Glycolic acid, its molecular structure is $HOCH_2COOH $, is the simplest hydroxy acid containing hydroxyl and carboxyl groups. In its molecule, the hydroxyl group and the carboxyl group are directly connected to the same carbon atom, and this unique structure gives it special chemical activity.

Lactic acid, chemical name is 2-hydroxy propionic acid, structural formula is $CH_3CH (OH) COOH $. Compared with glycolic acid, lactic acid molecules contain more than one methyl group on the carbon atom of the carboxyl group ortho-position. The presence of this methyl group significantly affects the spatial structure and electron cloud distribution of lactic acid, which in turn makes its properties different from glycolic acid.

Second, property differences
(1) Physical properties
1. ** Solubility **: Glycolic acid has good water solubility because both hydroxyl and carboxyl groups in the molecule can form hydrogen bonds with water molecules. And it also has a certain solubility in organic solvents such as ethanol and acetone. Lactic acid is also easily soluble in water. Due to the lipophilicity of methyl groups, the solubility of lactic acid in organic solvents is slightly better than that of glycolic acid. In some organic solvents with less polarity such as ether, the solubility of lactic acid is higher than that of glycolic acid.
2. ** Melting point and boiling point **: Glycolic acid has a melting point of about 75-80 ° C and a boiling point of about 100 ° C (2kPa). The melting point of lactic acid is 18 ° C and the boiling point is 122 ° C (2kPa). Glycolic acid has a higher melting point than lactic acid due to strong hydrogen bonding between molecules; while lactic acid has a relatively complex intermolecular force due to the influence of methyl groups, and its boiling point is slightly higher than that of glycolic acid.

(2) Chemical properties
1. ** Acidic **: Both are acidic, and hydrogen ions can be ionized due to the presence of carboxyl groups. However, glycolic acid is slightly more acidic than lactic acid. This is because the electronic effect of hydroxyl groups on carboxyl groups in glycolic acid molecules is different from that of lactic The hydroxyl group has an electron-absorbing induction effect. The hydroxyl group in glycolic acid is directly connected to the carboxyl group, which attracts the carboxyl group electron cloud more strongly, which enhances the polarity of the hydrogen-oxygen bond in the carboxyl group and makes it easier to ionize hydrogen ions, so the acidity is stronger.
2. ** Reactivity **: Glycolic acid is more prone to intramolecular or intermolecular dehydration due to the adjacent hydroxyl group and carboxyl group, forming lactate or polyester. Lactic acid is relatively difficult to dehydrate and form lactate due to the influence of methyl steric hindrance, and more intermolecular polycondensation reactions occur to form polylactic acid. And in the esterification reaction, the reactivity of glycolic acid is also slightly higher than that of lactic acid. Due to the relatively simple molecular structure of glycolic acid and small steric hindrance,

III. Application Differences
(I) Cosmetic Field
1. ** Glycolic Acid **: Often used as an exfoliant and moisturizer for cosmetics. Because of its acidity, it can gently remove aging keratinocytes on the surface of the skin, promote skin metabolism, and make the skin smooth and delicate. At the same time, hydroxyl and carboxyl groups can combine moisture and have a moisturizing effect. Suitable for oily and acne skin care, it can improve skin oil secretion and reduce acne formation.
2. ** Lactic Acid **: It is also a commonly used moisturizer, and has high safety and low irritation. Lactic acid can adjust skin pH and maintain skin microecological balance. Commonly used in sensitive skin care products, can enhance the skin barrier function, relieve dry skin, itching and other symptoms.

(2) Industrial field
1. ** glycolic acid **: used as a chelating agent in industrial processes such as metal cleaning and electroplating. Can form a stable complex with metal ions to remove rust and dirt on metal surfaces. In the textile printing and dyeing industry, it can be used as a fabric treatment agent to improve the feel and dyeing properties of fabrics.
2. ** lactic acid **: an important biodegradable material, polylactic acid raw material. Polylactic acid has good mechanical properties and biocompatibility, and is widely used in packaging, medical and other fields. Such as disposable tableware, sutures, etc. Lactic acid can also be used in leather tanning to improve leather softness and durability.

In summary, although both glycolic acid and lactic acid are organic acids, their properties and applications are different due to structural differences. It is of great significance to clarify the differences between the two for rational selection, optimized application and in-depth research and development of related products.