In the small intestine, bile salt is responsible for decreasing the surface interfacial tension of lipid droplets, which promotes the emulsification of oil droplets. Bile salts are planar surfactants with hydrophilic and hydrophobic regions, and their self-assembly and orientation at interfaces are discussed in detail in this review.

Bile is a greenish-yellow fluid that contains bile salts. Bile salts make fats easier to digest by **emulsifying large fat globules into tiny micelles**, increasing the surface area on which the enzyme lipase can act. These micelles are then broken down by pancreatic lipase, releasing fatty acids and monoglycerides for absorption.

“Crude emulsions of lipids enter the duodenum as fine lipid droplets and then mix with bile and pancreatic juice to undergo marked changes in chemical and physical form.” “The predominant PL in the lumen of the small intestine is PC, which is found in mixed micelles that also contain cholesterol and bile salts.” “Cholesterol is only minimally soluble in an aqueous environment and thus must be partitioned into bile salt micelles prior to absorption. It usually enters these micelles along with TAGs and PLs, ionized and nonionized FAs, MAGs, and lysophospholipids to form mixed micelles.”

“In intestinal aqueous environments, a myriad of colloidal species may exist such as vesicles, micelles, liquid crystalline phases as well as the colloidal fat digestion products and undigested lipid droplets.” “Bile micelles are thought to mediate intestinal absorption, in part by providing a phase into which compounds can partition. Solubilizing capacity of bile micelles is generally positively correlated to the amount of bile salt present, i.e. bile salt concentration.” “After digestion, mixed micelles consisting of bile salts and digestion products (e.g. fatty acids and monoglycerides) are formed… These micelles coexist with remaining emulsion droplets and other colloidal structures.”

Within the intestinal lumen, lipolysis of dietary triglyceride by pancreatic lipase generates fatty acids and monoacylglycerols that are incorporated into **bile salt–phospholipid mixed micelles**. These micelles deliver their lipid cargo to the brush border membrane for uptake. The mixed micelles are composed of bile salts and phospholipids with their hydrophobic surfaces facing inward and their hydrophilic surfaces exposed to the aqueous phase, surrounding the lipid components; they are not simply droplets containing only neutral fat.

“The lipolysis of lipids in the gastrointestinal tract is greatly influenced by the colloidal structures formed by bile salts, phospholipids, fatty acids, monoacylglycerols and other amphiphilic components.” “Even at a low concentration of <0.1 wt%, BE was able to form finely dispersed emulsion droplets that showed no visible droplet aggregation and coalescence.” “The present study showed that the impact of added BE on the kinetics of in vitro lipid digestion in emulsions was largely dependent on its presence in the unadsorbed phase rather than the adsorbed phase. Clearly, the bile salts adsorbed onto the interface had relatively less influence on lipid digestion kinetics than the bile salts that were present in the aqueous phase.” “The formation of mixed micelles containing bile salts and lipolysis products is believed to be the main mechanism by which lipolysis products are removed from the droplet surface and transported to the intestinal wall.”

Bile salts are amphipathic molecules, meaning they have both hydrophilic (water-attracting) and hydrophobic (water-repelling) regions. During lipid digestion, they help emulsify fats into smaller droplets so digestive enzymes can act more efficiently.

“An emulsion is a mixture of two immiscible liquids, most commonly oil and water, where one liquid (discontinuous phase) is dispersed as droplets throughout the other (continuous phase), with droplet diameters usually in the range of 0.1–100 µm.” “As the oil enters the small intestine, the pH is neutralized and digestive enzymes and bile salts are secreted. When emulsions and bile salts are mixed, the bile salts will adsorb onto the oil–water interface, potentially displacing existing emulsifiers.” “The primary enzyme for digestion of TG is pancreatic lipase. With the help of the co-enzyme colipase, it adsorbs to bile salt covered emulsion droplets and starts the process of lipolysis. Pancreatic lipase breaks each TG down into 2 fatty acids and 1 monoglyceride. These digestion products are continuously removed from the droplets through formation of aggregate particles called mixed micelles. A mixed micelle consists primarily of bile salts and TG digestion products along with other surface-active compounds, such as phospholipids… The inner lipophilic part of the micelles can solubilize lipid-soluble compounds such as cholesterols and fat-soluble vitamins.”

Bile salts act like detergents and **emulsify large fat globules into smaller droplets**, which makes it easier for lipase to access the triglycerides. The lipase breaks these down into free fatty acids and monoglycerides, which then **associate with bile salts to form micelles**. These micelles are tiny, soluble aggregates that can diffuse to the intestinal epithelium; they are not just pure fat droplets but complexes of bile salts and the products of fat digestion suspended in water.

The hydrophobic part of the bile salt interacts with lipids while the hydrophilic part interacts with water. This creates small lipid droplets with a larger surface area, making them more accessible to digestive enzymes. Micelles are formed when bile salts and phospholipids surround the lipids, forming a hydrophilic outer layer and a hydrophobic core.

Bile salts are biosurfactants produced in the liver and released into the small intestine, which play key roles in lipid digestion and absorption. Their amphiphilic character allows them to adsorb at interfaces between oil and water and reduce interfacial tension.

Since fatty acids are hydrophobic, the bile phospholipids or bile salts can surround the fatty acids and form tiny spherical structures called micelles. These structures help keep lipids in the watery environment of the intestine.

The lecture explains that bile contains two key emulsifying components: “phospholipids, particularly one called lecithin” and “bile salts.” It describes how these molecules arrange at the surface of fat droplets. Around timestamp 1411–1432, the presenter notes that small fatty droplets are “still have my phospholipids or that lecithin surrounding them” and then bile salts come in as a second layer, further stabilizing and breaking down the droplets. Earlier, the video states that dietary lipids include triglycerides, phospholipids, and cholesterol esters, which are all present in the lipid phase undergoing digestion, not only pure triglyceride.

The presenter explains that bile salts are amphipathic, with “a hydrophilic water-loving end and a hydrophobic water-fearing end.” She states that bile acids “can break down a large lipid droplet into smaller lipid droplets by orienting the hydrophilic ends towards the outside and the hydrophobic ends towards the center lipid,” describing the structure of emulsified droplets in the intestinal lumen. Later she distinguishes micelles from emulsion droplets: “Micelles… are created by one layer of phospholipids with their fatty acid tails oriented towards the fatty acid core and their hydrophilic heads towards the outside watery environment. The micelles can travel through the unstirred water layer by the brush border of the intestine for absorption.” She also notes that digested fatty acids “get transferred into micelles” from the emulsion droplets, indicating that micelles contain bile salts plus digested lipids, not just pure fat in the interior.

During emulsification, bile salts coat fat droplets with their hydrophobic face toward the lipid and hydrophilic face toward the aqueous intestinal fluid. The droplet interior is lipid, while water remains outside the droplet in the surrounding intestinal contents; however, the system is a dispersion of lipid droplets in water rather than a completely isolated, sealed structure.

The lecture lists the components of dietary fat as “triglycerides… phospholipids in glycerophospholipid form… [and] cholesterol esters,” emphasizing that multiple lipid classes enter digestion together, not just triglycerides. At around 135–149 seconds, it is stated that bile acids have a “detergent action on lipid droplets” and that “this emulsification by bile salt breaks these big fat droplets into minute particles or droplets called emulsification droplets,” implying bile salts coat the droplet surface interacting with both lipid and aqueous phases. Later, in describing micelles, the lecturer notes that monoglycerides, fatty acids, cholesterol, and some fat‑soluble vitamins form a lipid core that is “surrounded by bile salts,” showing bile salts localized at the interface rather than exclusively outside in bulk water.