Journal of Colloid and Interface Science 252, 365–372 (2002) doi:10.1006/jcis.2002.8478 Dependence of Phase Behavior of Some Non-ionic Surfactants at the Air–Water Interface on Micellization in the Bulk Md. Nazrul Islam and Teiji Kato 1 Department of Applied Chemistry, Faculty of Engineering, Utsunomiya University, Yoto 7-1-2, Utsunomiya 321-8585, Japan Received November 28, 2001; accepted May 9, 2002 The temperature-dependent surface phase behavior of two spar- ingly soluble surfactants, namely, ethylene glycol n-dodecyl ether (EGDE) and ethylene glycol n-tetradecyl ether (EGTE), at the air– water interface was investigated by film balance and Brewster angle microscopy (BAM). A cusp point followed by a pronounced plateau region in the surface pressure–time (π –t) adsorption isotherms of the amphiphiles measured by film balance indicates the first-order phase transition. Bright two-dimensional condensed phase domains in a dark background are observed by BAM just after the phase tran- sition. In both cases the critical surface pressure necessary for the phase transition increases with increasing temperature. The do- mains are found to be circular up to 5 and 27 ◦ C for EGDE and EGTE, respectively, above which they show a fingering pattern. Condensed domains are observed up to 23 and 37 ◦ C for EGDE and EGTE, respectively. The surface properties of the amphiphiles are found to be markedly affected by their tendency to aggregate in the bulk as micelles. The CMC values of both the amphiphiles show a maximum at a definite temperature, T max , that corresponds well to their respective maximum temperatures of domain formation. An increase in temperature beyond T max results in an increasing trend for the formation of micelles. Consequently the system suf- fers from a shortage of two-dimensional surface concentration of the molecules to attain the surface pressure necessary for phase transition. With increasing temperature, the enthalpy, H ◦ m , and entropy, S ◦ m , of micellization change from negative to positive in both cases. An enthalpy–entropy compensation effect is found to hold for both the amphiphiles over the entire temperature range. The thermodynamic quantities reveal that the increase in temper- ature is favorable for micellization when the temperature exceeds the corresponding T max of the amphiphiles. C  2002 Elsevier Science (USA) Key Words: non-ionic surfactant; phase transition; Brewster angle microscopy; adsorbed layers; line tension; critical micelle con- centration. INTRODUCTION Langmuir monolayers at the air–water interface provide a sim- ple model for understanding the existence of various phases in two-dimensional systems. When the monolayer is compressed 1 To whom correspondence and reprint requests should be addressed. Fax: +81-28-689-6179. E-mail: teiji@cc.utsunomiya-u.ac.jp. isothermally, it undergoes a transition from a two-dimensional (2-D) lower density phase to a higher density condensed phase showing a cusp point followed by a plateau region in the π – A isotherm, provided that the van der Waals interaction be- tween the hydrophobic alkyl chains and the repulsive inter- action between the head groups favor the coherence of the molecules. Recent development of Brewster angle microscopy (BAM) (1, 2) allows direct visualization of the change in the surface morphology during the phase transition without using any probe impurity. Moreover, by placing an analyzer in the path of the reflected beam it is possible to detect the internal anisotropy of the molecules in a domain that occurs due to the difference in the surface reflectivity of the p-polarized light (3, 4). Despite extensive work on the adsorption kinetics of water- soluble amphiphiles, the concept of phase transition and struc- ture formation in adsorbed layers has been reported quite re- cently. H´ enon et al. (5–7) first reported such phase transition and structure formation during adsorption from the aqueous so- lution of sparingly soluble fatty acids. Although they reported the striking influence of surface-active impurities in the forma- tion of condensed domains, now a number of sparingly solu- ble highly purified amphiphiles are known to form condensed domains in adsorbed layers (8–15). For a clear understanding of the kinetics and the thermodynamic behavior, the π –t ad- sorption kinetics of Gibbs monolayers has been compared with the π –A isotherm of Langmuir monolayers. It is found that due to the slow and homogeneous growth process in Gibbs monolay- ers, the condensed phase domains are more ordered than those in Langmuir monolaers (9–12). Recently, it has been reported that at a particular temperature a minimum bulk concentration is necessary for the phase transition in Gibbs monolayers (11, 13, 14) and if the bulk concentration is increased, phase transi- tion can occur at higher temperatures (14). However, each am- phiphile has a definite temperature, above which it cannot show any indicative feature of phase transition, no matter whatever the bulk concentration is. In a recent paper we have reported that 2-hydroxyethyllaurate can form a condensed domain max- imum up to 25 ◦ C (14). Similar behavior has also been observed for ethylene glycol n-dodecyl ether (EGDE) and ethylene gly- col n-tetradecyl ether (EGTE), which form condensed domains up to 23 and 37 ◦ C, respectively. Now the question arises why 365 0021-9797/02 $35.00 C  2002 Elsevier Science (USA) All rights reserved.