Journal of Colloid and Interface Science 277 (2004) 221–229 www.elsevier.com/locate/jcis Favorable interactions of amine- and ester-terminated PAMAM with cationic surfactants: photophysical and transport studies Mandeep Singh Bakshi ∗ , Rohit Sood Department of Chemistry, Guru Nanak Dev University, Amritsar 143005, Punjab, India Received 29 February 2004; accepted 20 April 2004 Available online 7 June 2004 Abstract Conductivity (κ ), turbidity (τ ), and fluorescence (I 1 /I 3 ) studies of hexadecyltrimethylammonium bromide (HTAB), hexadecylpyridinium bromide (HPyBr), and hexadecylpyridinium chloride (HPyCl) in aqueous poly(amido amine) (PAMAM) dendrimers of generations 0 to 2.5 G have been carried out. The complexation of surfactant monomers with the PAMAM surface groups is demonstrated by the critical aggregation concentration (cac), which is two to three orders of magnitude less than the micellization of cationic surfactants in aqueous PAMAM and denoted by critical micelle concentration (cmc ∗ ). In the presence of aqueous amine-terminated PAMAM, the cmc ∗ value for each surfactant was much lower than the cmc in pure water, while they remain close to each other in the presence of aqueous ester-terminated PAMAM for each surfactant. The fluorescence studies demonstrated that both amine- and ester-terminated PAMAM interact with the cationic surfactants, though the mode of interaction varied due to the different nature of surface groups.  2004 Elsevier Inc. All rights reserved. Keywords: Cationic surfactants; Poly(amido amine) dendrimers; Aggregated assemblies; Favorable interactions 1. Introduction In continuation of our efforts to study the physicochemi- cal aspects of surfactant–poly(amidoamine) (PAMAM) den- drimer interactions [1,2], we herein report another such study based on transport and spectroscopic measurements. The PAMAM are the dendritic macromolecules consisting of a core with two or more functional groups surrounded by several layers of repeating units in a ball-shaped arrange- ment [3]. The size of the ball grows as the number of layers increases [4,5]. Among the water-soluble PAMAM, amine- and ester-terminated dendrimers have frequently been used due to their readily water-soluble nature. Recent studies have demonstrated that a change in generation alternatively from amine- to ester-terminated PAMAM changes its interactions with surfactants significantly [1,6]. Such interactions, which are primarily electrostatic in origin [7], are responsible for host–guest type association. Small surfactant molecules act as guests for a treelike [8] large PAMAM dendrimer, which * Corresponding author. Fax: 91-183-2258820. E-mail address: ms_bakshi@yahoo.com (M.S. Bakshi). paves the way for a new field of dendrimer–surfactant in- teractions with extraordinary applications in nanostructural materials [9–11] and drug delivery vehicles [12,13]. Recently, a few studies have appeared in the literature related to surfactant–dendrimer interactions in the aqueous phase. Interactions between sugar-persubstituted PAMAM and anionic surfactants [4] have been found to be cooper- ative with generation 5 rather than 3. Similar interactions of hydrophobically modified PAMAM with sodium dode- cyl sulphate (SDS) and dodecyltrimethylammonium bro- mide (DTAB) produced aggregations of lower micropolar- ity in the former case than in the latter case [14]. These studies demonstrated that surfactant monomers initially ad- sorb on the PAMAM surface through electrostatic interac- tions, leaving hydrophobic tails in the bulk aqueous phase, which in turn further trigger self-association of such ag- gregates. This mechanism is different from the traditional mode of association between water-soluble linear polymers and surfactants, where two kinds of aggregation processes have generally been observed [15,16]. The first one, where polymer–surfactant complexation starts, is known as the crit- ical aggregation concentration (cac). The other one, where 0021-9797/$ – see front matter  2004 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2004.04.058