Alginates are natural polysaccharide polymers isolated from brown seaweed (Phacophycae) and is classified as dietary fiber. Their ability to form viscous solutions and gels have led to their extensive use in foods, cosmetics, and pharmaceutical products for over one century.
As it pertains to reflux, perhaps the most interesting study on alginate was published in 1988 whereby alginate was labelled with 111In radioisotope in order to "see" what happens in the stomach.
"acid concentrations less than 0.05 N diminished raft formation. In vivo, raft formation was significantly better in normal subjects who ingested dilute acid with the labeled alginate/antacid than in subjects who ingested the labeled alginate/antacid with plain water. Gastric emptying of the labeled alginate was also slowed by the presence of acidified gastric contents. These results suggest that the formation of an effective alginic acid antireflux barrier requires acidic gastric contents." [link]As such, if alginate is used, a patient should probably not ingest any proton pump inhibitors (nexium, prevacid, prilosec, etc) or H2 blockers (zantac, pepcid, axid, etc) for best effect which may seem counterintuitive.
In the presence of gastric acid, alginate has been found to precipitate forming a thick gel. Such alginate formulations usually contain sodium or potassium bicarbonate; in the presence of gastric acid, the bicarbonate is converted to carbon dioxide which becomes entrapped within the gel precipitate converting it into a foam which floats on the surface of the gastric contents, much like a raft on water. Both in vitro and in vivo studies have demonstrated that alginate-based rafts can entrap carbon dioxide, as well as antacid components contained in some formulations, thus providing a relatively pH-neutral barrier.
Another factor that affects the alginate raft is the presence of calcium and aluminum. Calcium increases raft strength (becomes more solid) while aluminum reduces raft strength (becomes more gel-like).
To summarize, for effective alginate treatment of reflux, acid as well as bicarbonate needs to be present. The level of solidity is determined by calcium and aluminum concentrations.
Alginate is found in Gaviscon Advance as well as Esophageal Guardian, both of which can be found on Amazon.
You can also make your own Gaviscon Advance from its component ingredients commonly found in the United States here.
Use of 111In-labeled alginate to study the pH dependence of alginic acid anti-esophageal reflux barrier. Int J Rad Appl Instrum B. 1988;15(5):563-71.
Review article: alginate-raft formulations in the treatment of heartburn and acid reflux. Aliment Pharmacol Ther. 2000 Jun;14(6):669-90.
An alginate-antacid formulation (Gaviscon Double Action Liquid) can eliminate or displace the postprandial 'acid pocket' in symptomatic GERD patients. Aliment Pharmacol Ther. 2011 Jul;34(1):59-66. doi: 10.1111/j.1365-2036.2011.04678.x. Epub 2011 May 3.