Krafft Points and Cloud Points of Polyoxyethylated Nonionic Surfactants: Part 1 Krafft Points in Binary Surfactant-Water Systems

H. Schott

doi:10.3139/113.100275

Published by De Gruyter May 8, 2013

Krafft Points and Cloud Points of Polyoxyethylated Nonionic Surfactants

Part 1 Krafft Points in Binary Surfactant-Water Systems

Krafft- und Trübungspunkte von polyoxyethylierten nichtionischen Tensiden

H. Schott

From the journal Tenside Surfactants Detergents

https://doi.org/10.3139/113.100275

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Abstract

The solubility of polyoxyethylated nonionic surfactants (POSs) in water is limited by an upper and a lower critical temperature. The former, called cloud point (CP), is the temperature at which POSs precipitate from solution on heating because of excessive dehydration of their polyoxyethylene (POE) moieties. The latter, called Krafft point (KP), is the temperature at which POSs crystallize from solution on cooling because of increased alignment and attraction between their hydrocarbon chains. It occurs less frequently: Even if the POSs were capable of crystallizing, ice often crystallizes first.

Aqueous solutions of over 100 POSs were examined by obtaining their CPs and ascertaining whether they exhibit KPs. KPs were found in polyoxyethylated primary alcohols and fatty acid amides and in polyethylene glycol esters of fatty acids. Only surfactants with strictly linear hydrocarbon chains crystallize, exhibiting KPs. Any branching, including that in polyoxyethylated linear secondary alcohols, prevents crystallization. Crystallization only occurs when the number mof carbon atoms in the hydrocarbon chains equals or exceeds 12. The KPs are then proportional to m. In homologous series, the surfactants most prone to crystallizing are those with borderline solubility, which possess just enough oxyethylene groups to promote solubility in cold water. Large POE moieties depress or eliminate KPs.

Kurzfassung

Die Löslichkeit von polyoxyethylierten nichtionischen Tensiden (POSs) in Wasser ist durch eine obere und untere kritische Temperatur begrenzt. Die erstere, der sogenannte Trübungspunkt (CP), ist die Temperatur, bei der beim Erhitzen POSs aus der Lösung ausfallen, aufgrund überhöhter Dehydratisierung ihrer Polyoxyethylen (POE) Reste. Die letztere, der sogenannte Krafftpunkt (KP), ist die Temperatur, bei der beim Abkühlen POSs aus der Lösung auskristallisieren, aufgrund zunehmender Anordnung und Anziehungskräfte zwischen den Kohlenwasserstoffketten. Es tritt weniger häufig auf: Selbst wenn die POSs zur Kristallisation fähig sind, kristallisiert Eis oft zuerst.

Wässrige Lösungen von über 100 POSs wurden untersucht, indem ihre CPs bestimmt und ermittelt wurde, ob sie KPs aufweisen. KPs wurden bei polyoxyethylierten primären Alkoholen, Fettsäureamiden und Polyethylenglykolestern von Fettsäuren gefunden. Nur Tenside mit streng linearen Kohlenwasserstoffketten weisen KPs auf. Jede Verzweigung, einschließlich die in polyoxyethylierten linearen sekundären Alkoholen, verhindert die Kristallisation. Eine Kristallisation tritt nur dann auf, wenn die Anzahl m der Kohlenstoffatome in den Kohlenwasserstoffketten gleich oder größer als 12 ist. Die KPs sind dann proportional zu m. In homologen Reihen sind Tenside mit der größten Neigung zur Kristallisation diejenigen mit Grenzlöslichkeit, die gerade genug Oxyethylengruppen aufweisen um die Löslichkeit in kaltem Wasser zu begünstigen. Große POE-Reste erniedrigen oder unterbinden KPs.

Keywords:: Nonionic surfactants; polyoxyethylated surfactants; Krafft points; cloud points

Schlüsselwörter: Nichtionische Tenside; polyoxyethylierte Tenside; Krafftpunkte; Trübungspunkte

¹Hans Schott, 801 W. Beaver Hill Apts. 100 West Ave. Jenkintown, PA 19046 U.S.A. Telephone: (2 15)8 87-01 16

Prof. Dr. Hans Schott obtained an MS in colloid chemistry from the University of Southern California and a Ph.D. in physical chemistry from the University of Delaware. After performing and directing research at the Du Pont, Olin, and Lever Brothers companies, he joined the School of Pharmacy of Temple University. He is presently Professor Emeritus of Pharmaceutics and Colloid Chemistry. His research deals with the application of colloid and polymer chemistry to pharmaceutical, cosmetic and industrial systems, especially nonionic surfactants, gelatin, cholesterol deposits, and suspensions of hydrous oxides, clays, drugs and bacteria.

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Received: 2005-8-16

Published Online: 2013-05-08

Published in Print: 2005-12-01